Polymers for Electronic and Photonic Applications - American

1 Polymers for Electronic and Photonic Applications Downloaded by UNIV OF MEMPHIS on May 27, 2014 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch001

M u r r a e J. Bowden Bell Communications Research, 331 Newman Springs Road, Red Bank, NJ 00701

The application of polymers to selected areas of electronics and photonics is reviewed. These areas include microlithography, packaging, conducting polymers, molecular electronics, optical fiber coatings, integrated optics, nonlinear optics, and optical recording. This chapter provides an overview of the various technologies and highlights the advantages offered by the unique properties of polymers in meeting the material requirements of each technology.

P O L Y M E R S A R E I N C R E A S I N G L Y B E I N G U S E D i n a w i d e variety of applications i n electronics a n d photonics, most of w h i c h use p o l y m e r s i n t h e i r traditional role as e n g i n e e r i n g materials (e.g., c i r c u i t boards, m o l d e d products, w i r e and cable insulation, encapsulants, a n d adhesives). I n a d d i t i o n , m a n y other u n i q u e applications r e q u i r e m a t e r i a l properties that o n l y p o l y m e r s can p r o v i d e . E x a m p l e s i n c l u d e resist materials for the l i t h o g r a p h i c fabrication of integrated circuits (IC) a n d p o l y m e r s for optical r e c o r d i n g . T h e s e types o f applications m a y be c o n s i d e r e d " p a s s i v e " i n the sense that the p o l y m e r does not p l a y an active role i n the operation of the device or c i r c u i t . R a t h e r , it serves some other function such as m e c h a n i c a l support, electrical i n s u l a t i o n , or i n the case of resists, some i n t e r m e d i a t e function i n the fabrication of the device. T e c h n o l o g y is r e a c h i n g a p o i n t , h o w e v e r , w h e r e the u n i q u e properties of p o l y m e r s m a k e t h e m suitable not o n l y for these so-called passive a p p l i cations, b u t also for " a c t i v e " applications, w h e r e i n the p o l y m e r plays an active role i n the f u n c t i o n i n g of the device. E x a m p l e s of s u c h applications i n c l u d e n o n l i n e a r optics, m o l e c u l a r electronics, a n d conductors: electronic

0065-2393/88/0218-0001$16.95/0 © 1988 American Chemical Society

In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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ELECTRONIC & PHOTONIC APPLICATIONS O F POLYMERS

(conducting polymers) a n d p h o t o n i c (waveguides). I n this chapter, various applications o f p o l y m e r s i n electronics a n d photonics w i l l b e b r i e f l y r e v i e w e d w i t h emphasis o n the subject areas just m e n t i o n e d . B r i e f descriptions o f the various technologies are i n c l u d e d so that the advantages o f p o l y m e r s m i g h t b e b e t t e r a p p r e c i a t e d i n terms o f m a t c h i n g t h e i r u n i q u e properties to the r e q u i r e m e n t s of the technology. T h i s chapter is i n t e n d e d to p r o v i d e sufficient b a c k g r o u n d to p u t the rest o f the book i n perspective. A m o r e d e t a i l e d treatment o f the various topics can be f o u n d i n subsequent chapters.

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I . I Polymers for Electronic Applications T h e m o d e r n electronics e r a began at B e l l T e l e p h o n e L a b o r a t o r i e s i n 1948 w i t h the i n v e n t i o n of the solid-state transistor, w h i c h r e p l a c e d the large t h e r m i o n i c v a c u u m t u b e , the mainstay of the electronics i n d u s t r y for the p r e v i o u s 40 years. Transistors w e r e smaller a n d m u c h m o r e robust t h a n t h e i r v a c u u m t u b e counterparts a n d r e q u i r e d m u c h less p o w e r to operate. E l e c t r o n i c circuits o f the 1950s a n d early 1960s w e r e a s s e m b l e d from discrete transistors, diodes, a n d resistors, for e x a m p l e , b u t r a p i d advances i n c i r c u i t c o m p l e x i t y a n d d e n s i t y , d r i v e n b y d e v e l o p m e n t s i n c o m p u t e r technology, soon l e d to a n impasse, n a m e l y , h o w to approach the p r o b l e m o f i n t e r c o n n e c t i n g h u n d r e d s , perhaps thousands, (some visionaries w o u l d have said millions) of discrete devices into a c o m p l e x c i r c u i t . T h i s H e r c u l e a n p r o b l e m was solved i n 1960 w i t h the i n v e n t i o n o f the I C , w h i c h u s h e r e d i n the microelectronics r e v o l u t i o n whose i m p a c t w e c o n t i n u e to experience. T h i s technology a l l o w e d i n d i v i d u a l solid-state c o m p o nents to b e fabricated o n a single wafer of crystalline s i l i c o n — t h e c h i p — b y a mass p r o d u c t i o n t e c h n i q u e a n d i n t e r c o n n e c t e d v i a a n e t w o r k of conductors o n the surface o f the c h i p . T h e first I C s c o n t a i n e d o n l y a few transistors o n a c h i p , whereas the n u m b e r of c o m p o n e n t s integrated into a single c i r c u i t today n u m b e r i n the h u n d r e d s of thousands, e v e n m i l l i o n s . A l o n g w i t h this r e v o l u t i o n have c o m e m a n y o p p o r t u n i t i e s for innovative materials t e c h n o l ogy. P o l y m e r s possess u n i q u e properties that have e n a b l e d t h e m to m e e t m a n y o f the materials r e q u i r e m e n t s o f microelectronics e n g i n e e r i n g . S o m e o f these applications i n microelectronics w i l l be discussed i n the f o l l o w i n g section. L a t e r sections w i l l b r i e f l y r e v i e w c o n d u c t i n g p o l y m e r s a n d also m o l e c u l a r electronics, w h i c h is b e l i e v e d b y m a n y to r e p r e s e n t the next r e v o l u t i o n i n electronics.

1.1.1 F r o m Silicon to Circuit Board T h e m o d e r n - d a y c i r c u i t b o a r d is testimony to the pervasive i n t r u s i o n of p o l y m e r s into the electronics i n d u s t r y . A s seen i n F i g u r e 1.1, a t y p i c a l m o u n t e d c i r c u i t b o a r d contains a large n u m b e r of packaged devices, p r i n -


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c i p a l l y I C s , w h i c h are s o l d e r e d into a plastic l a m i n a t e d c i r c u i t b o a r d a n d i n t e r c o n n e c t e d , t y p i c a l l y o n b o t h sides of the board, w i t h p r i n t e d w i r i n g . T h e b o a r d itself m a y contain u p to six layers of i n t e r c o n n e c t i o n w i t h l i n e w i d t h s of 6 m i l s . T o appreciate the role p l a y e d by p o l y m e r s i n c o n s t r u c t i n g a n d a s s e m b l i n g s u c h a c i r c u i t b o a r d , i t w i l l be i n s t r u c t i v e to start at the b e g i n n i n g (i.e., w i t h the fabrication of the i n d i v i d u a l I C itself) a n d follow the various applications of p o l y m e r s i n the e v o l u t i o n from silicon to c i r c u i t board.


1.1.1.1 R E S I S T M A T E R I A L S

A l l silicon integrated circuits (SICs) b e g i n life as a silicon wafer, w h i c h has b e e n cut from a silicon ingot a n d p o l i s h e d o n one side to p r o v i d e a smooth surface u p o n w h i c h thousands, e v e n m i l l i o n s , of c i r c u i t elements s u c h as transistors, diodes, a n d resistors w i l l be fabricated a n d i n t e r c o n n e c t e d to y i e l d a set o f c o m p l e t e circuits. C i r c u i t fabrication requires the selective diffusion of t i n y amounts of i m p u r i t i e s i n t o specific regions of the silicon substrate to p r o d u c e the d e s i r e d electrical characteristics of the c i r c u i t . T h e s e regions are d e f i n e d b y lithographic processes that consist o f two steps: 1. d e l i n e a t i o n o f the d e s i r e d c i r c u i t pattern i n a resist layer (usually a p o l y m e r i c film that is spin-coated onto the substrate), and 2. transfer of that p a t t e r n v i a processes such as e t c h i n g i n t o the u n d e r l a y i n g substrate. T h e " s u b s t r a t e " i n this case is an o x i d i z e d s i l i c o n wafer w i t h an oxide thickness t y p i c a l l y 0 . 1 - 0 . 5 fim. T h e p r i m a r y d e f i n i t i o n o f the c i r c u i t pattern i n the resist is a two-stage process consisting of the formation of a latent image b y exposure to some f o r m of p a t t e r n e d radiation f o l l o w e d b y d e v e l o p m e n t of that image to p r o d u c e a t h r e e - d i m e n s i o n a l r e l i e f structure. I n the case of photolithography or X - r a y l i t h o g r a p h y , the latent image is f o r m e d b y exposure of the resist t h r o u g h a mask that contains clear a n d o p a q u e features o u t l i n i n g the c i r c u i t p a t t e r n . T h e basic steps o f the process are o u t l i n e d i n F i g u r e 1.2. I n e l e c t r o n b e a m l i t h o g r a p h y , p a t t e r n information is stored d i g i t a l l y i n a c o m p u t e r that controls the b e a m b l a n k i n g a n d deflection function. T h e wafer surface is d i v i d e d into a g e o m e t r i c array of pixels, a n d each p i x e l is e i t h e r exposed (beam on) or not exposed (beam off or blanked) as the b e a m is scanned across the resist surface ( F i g u r e 1.3). I n this way, the pattern is " w r i t t e n " i n serial fashion as o p p o s e d to a mask process, w h i c h " w r i t e s " i n p a r a l l e l . O f course, the e n d result is the same, n a m e l y , formation of an exposed latent image i n the resist. T h e next step i n the process is the d e v e l o p m e n t of the latent image to



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Figure 1.2. Basic steps of the photolithographic process: (I) oxidizing, (2) spin coating, (3) exposing, (4) developing, (5) etching, and (6) stripping.

give a t h r e e - d i m e n s i o n a l r e l i e f image of the mask p a t t e r n i n the resist. E x p o s u r e to actinic radiation causes c h e m i c a l changes i n the resist that enable the exposed a n d unexposed areas to b e differentiated t h r o u g h differences i n s o l u b i l i t y or p l a s m a e t c h resistance. D e p e n d i n g o n the c h e m i c a l nature of the resist, the exposed areas may b e r e n d e r e d m o r e soluble i n the d e v e l o p e r than the unexposed areas, t h e r e b y p r o d u c i n g a positive-tone image of t h e mask. C o n v e r s e l y , the exposed areas may b e r e n d e r e d less soluble, p r o d u c i n g a negative-tone image. E i t h e r way, the result is a wafer w i t h sections of the substrate exposed a n d the r e m a i n d e r still c o v e r e d w i t h resist. T h e r e m a i n i n g resist m u s t n o w w i t h s t a n d the w i d e variety o f chemicals u s e d to etch the exposed substrate. T h e resist m u s t possess excellent adhesion to p r e v e n t u n d e r c u t t i n g d u r i n g e t c h i n g (and developing) that w o u l d result i n loss of r e s o l u t i o n a n d l i n e w i d t h c o n t r o l . T h e e n d objective is a perfect t h r e e d i m e n s i o n a l r e p l i c a t i o n of the t w o - d i m e n s i o n a l mask p a t t e r n i n the wafer surface.



DATA INPUT

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Figure 1.3. Electron beam system and exposure process.

ELECTRON BEAM EXPOSES RESIST SURFACE PATTERNS

D/A CONVERTERS AND BEAM CONTROL ELECTRONICS


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T h i s b r i e f o v e r v i e w of the l i t h o g r a p h i c process is sufficient to appreciate the u n i q u e properties r e q u i r e d of the resist: • It m u s t be capable of b e i n g u n i f o r m l y deposited as a t h i n (typically 0 . 5 - 2 . 0 |xm) film across a wafer, w h i c h m a y be as m u c h as 8 i n . (20.3 cm) i n d i a m e t e r .


• It m u s t b e sensitive to specific actinic radiation ( U V l i g h t i n the case of photolithography) a n d m u s t undergo a c h e m i c a l transformation that w i l l enable exposed a n d unexposed areas to b e differentiated b y an appropriate d e v e l o p i n g s c h e m e such as solvent o r p l a s m a d e v e l o p m e n t . • It m u s t adhere to a v a r i e t y of substrates a n d w i t h s t a n d the various e t c h i n g e n v i r o n m e n t s e n c o u n t e r e d i n I C processing. T h e s e e n v i r o n m e n t s i n c l u d e h i g h temperatures, exceedingly corrosive e t c h i n g chemicals such as strong acids, a n d reactive plasmas. A n o t h e r v e r y i m p o r t a n t r e q u i r e m e n t of the resist is r e s o l u t i o n . L i t h o graphic tools are capable of i m a g i n g s u b m i c r o m e t e r features i n the resist. T h e resist m u s t b e able to " r e s o l v e " such features. I n some cases, the resist may e v e n b e expected to do m o r e , such as compensate for the loss of mask resolution that has b e e n d e g r a d e d i n the aerial image because of f u n d a m e n t a l limitations associated w i t h the physics of image generation. I n projection p h o t o l i t h o g r a p h y , for e x a m p l e , the aerial image of an object s u c h as a mask c o n t a i n i n g an e q u a l l i n e a n d space p a t t e r n corresponds to a sinusoidal p a t t e r n of l i g h t i n t e n s i t y w h e n o p e r a t i n g at the diffraction l i m i t of the projection tool. S u c h a p a t t e r n is i n m a r k e d contrast to the square-wave p a t t e r n of l i g h t intensity that w o u l d c o r r e s p o n d to perfect image transfer (J). (In e l e c t r o n b e a m l i t h o g r a p h y , the " a e r i a l " image q u a l i t y is d e g r a d e d b y scattering p r o cesses w i t h i n the resist.) T h e net result of processes such as these is the d e p o s i t i o n of energy i n regions of the resist not specified i n the o r i g i n a l c i r c u i t (mask) layout. Ideally, one w o u l d l i k e to use c h e m i s t r y to transform this sinusoidal aerial image into a square-wave latent image i n the resist ( F i g u r e 1.4). P o l y m e r s can b e d e s i g n e d w i t h a u n i q u e b l e n d of properties to m e e t most i f not a l l of these r e q u i r e m e n t s . T h e i r m a c r o m o l e c u l a r architecture p e r m i t s t h e m to b e d e p o s i t e d as u n i f o r m t h i n films b y s p i n coating a n d also provides for s o l u b i l i t y differentiation b y means of r a d i a t i o n - i n d u c e d changes i n the m o l e c u l a r w e i g h t . R a d i a t i o n - i n d u c e d scission a n d c r o s s - l i n k i n g are p a r t i c u l a r l y i m p o r t a n t for those l i t h o g r a p h i c technologies s u c h as e l e c t r o n b e a m or X - r a y l i t h o g r a p h y , w h e r e the exposing radiation has sufficient e n e r g y to break c a r b o n - c a r b o n bonds. B y i n c o r p o r a t i n g groups that r e n d e r the m a i n chain susceptible to cleavage u p o n exposure to radiation, the m o l e c u l a r w e i g h t can be r e d u c e d , t h e r e b y m a k i n g the exposed area soluble i n solvents



Figure 1.4. Degradation of the projected image at the diffraction limit of the lens. Diffraction effects transform an initially square-wave image into a sinusoidal pattern of intensity at the image plane.


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that do not dissolve the unexposed, larger molecules. F o r e x a m p l e , the alternating c o p o l y m e r s o f olefins a n d sulfur d i o x i d e contain a r e l a t i v e l y weak c a r b o n - s u l f u r b o n d i n the m a i n c h a i n . W h e n exposed to h i g h - e n e r g y r a d i a t i o n , this b o n d is p r e f e r e n t i a l l y b r o k e n , l e a d i n g to a r e d u c t i o n i n the m o l e c u l a r w e i g h t a n d consequent positive resist action (2). A l t e r n a t i v e l y , i n c o r p o r a t i o n of c r o s s - l i n k i n g groups such as epoxy, v i n y l , o r h a l o m e t h y l groups i n t o the p o l y m e r c h a i n w i l l have the opposite effect, causing t h e p o l y m e r to b e c o m e totally i n s o l u b l e i n a l l solvents. M a n y s u c h chain-sciss i o n i n g a n d c r o s s - l i n k i n g p o l y m e r s have b e e n d e v i s e d a n d t h e i r l i t h o g r a p h i c properties r e v i e w e d i n the l i t e r a t u r e . Reference 3 is an i n - d e p t h r e v i e w o f resist materials for fine-line l i t h o g r a p h y before the year 1985. T h e m o r e recent l i t e r a t u r e is c o v e r e d i n C h a p t e r s 2 a n d 3 of this book. I n cases w h e r e the exposing radiation does not have sufficient e n e r g y to r u p t u r e bonds d i r e c t l y (e.g., at the wavelengths e n c o u n t e r e d i n traditional photolithography), o t h e r differential s o l u b i l i t y schemes have b e e n d e v i s e d o n the basis of p o l a r i t y changes i n the matrix r e s u l t i n g from p o l a r i t y changes of the sensitizer (e.g., novolac-based photoresists) or of the m a t r i x itself. A n example of m a t r i x p o l a r i t y changes is the extensive w o r k b y W i l l s o n a n d cow o r k e r s (4, 5) o n a c i d - i n d u c e d cleavage of protective groups s u b s t i t u t e d o n a suitable p o l y m e r i c b i n d e r . I n this case, the a c i d is generated from different o n i u m salts acting as sensitizers. Cleavage of tertiary butoxycarbonyloxy groups, for e x a m p l e , s u b s t i t u t e d o n the aromatic r i n g of p o l y s t y r e n e converts the p o l y m e r from the h y d r o p h o b i c poly(ferf-butoxycarbonyloxystyrene) ( f - B O C ) to the h y d r o p h i l i c poly(hydroxystyrene). E i t h e r positive o r negative tone can b e generated d e p e n d i n g o n the p o l a r i t y of the d e v e l o p e r . I n cases w h e r e the p o l y m e r i c c o m p o n e n t functions p u r e l y as a b i n d e r , its c h e m i c a l c o m p o s i t i o n depends o n factors s u c h as dissolution r e q u i r e m e n t s a n d etch resistance. O n e l i m i t a t i o n o f solvent d e v e l o p m e n t is s w e l l i n g of the r e m a i n i n g resist. S w e l l i n g is p a r t i c u l a r l y a p r o b l e m w i t h r u b b e r y negative resists w h e r e it l i m i t s resolution of the r e s i d u a l p a t t e r n to about 1.5 |xm. T h e r e s o l u t i o n of negative resists m a y b e i m p r o v e d b y u s i n g glassy p o l y m e r s , b u t i n g e n e r a l , resolution is not as h i g h as that w h i c h can b e o b t a i n e d w i t h positive resists. Today, a l l s u b m i c r o m e t e r i m a g i n g i n p h o t o l i t h o g r a p h y is d o n e w i t h positive photoresists. T h e s e materials are t w o - c o m p o n e n t systems based o n d i a z o n a p h t h o q u i n o n e - s e n s i t i z e d novolac resins. I n this case, the d i s s o l u t i o n m e c h a n i s m depends o n a change o f p o l a r i t y of the i r r a d i a t e d matrix (see C h a p t e r 2) r e s u l t i n g from c h e m i c a l changes i n the sensitizer that r e n d e r the novolac b i n d e r soluble i n aqueous base. Because the p o l a r i t y of the u n e x posed resist is u n c h a n g e d , r e s o l u t i o n is not l i m i t e d b y s w e l l i n g of the u n e x posed resist d u r i n g d e v e l o p m e n t i n aqueous base. Schemes have b e e n d e v i s e d to reverse the tone of these resists, e n a b l i n g negative-tone patterns to b e generated w i t h r e s o l u t i o n comparable to positive-tone patterns. T h i s approach also has a n a d d e d advantage over the c o r r e s p o n d i n g positive p r o -


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cess i n that it p e r m i t s b e t t e r c o n t r o l of l i n e w i d t h s o v e r topographic features o n the wafer surface (see C h a p t e r 2). M a n y c h e m i c a l modifications have b e e n d e v i s e d to i m p r o v e resistance to specific etchants. M u l t i l e v e l resist processing schemes, for e x a m p l e , r e q u i r e materials that are h i g h l y resistant to d r y processing e n v i r o n m e n t s such as oxygen reactive i o n e t c h i n g (3). T h e resist c h e m i s t has r e s p o n d e d to this d e m a n d b y i n c o r p o r a t i n g a n o x i d e - f o r m i n g e l e m e n t such as silicon or t i n i n the p o l y m e r structure. S i l i c o n has b e e n extensively u s e d i n this w a y because of the availability of a w i d e variety of organosilicon m o n o m e r s . I n a n oxygen plasma, the s i l i c o n m o i e t y i n the p o l y m e r is c o n v e r t e d to silicon d i o x i d e at the surface of the resist ( F i g u r e 1.5). A l t h o u g h o n l y t y p i c a l l y a few h u n d r e d angstroms t h i c k , the s i l i c o n dioxide essentially passivates the surface a n d thus protects the u n d e r l y i n g resist against further erosion. M u c h of the research o n resists over the past few years has b e e n d i r e c t e d t o w a r d organ o s i l i c o n resists, s p u r r e d b y the g r o w i n g i m p o r t a n c e o f m u l t i l e v e l processing schemes for fabricating devices w i t h feature sizes less than 1.0 (xm. T a b l e 1.1 lists several of these resists that have b e e n p r i n c i p a l l y d e s i g n e d for d e e p U V ( D U V ) a n d e l e c t r o n b e a m lithography. I n the processes d e s c r i b e d so far, the resist is r e m o v e d after e t c h i n g , b a r i n g the p a t t e r n e d oxide that serves as a mask d u r i n g subsequent h i g h t e m p e r a t u r e diffusion of dopants into the exposed s i l i c o n substrate. T h e r e are examples, h o w e v e r , w h e r e the resist m a t e r i a l is left b e h i n d to b e c o m e

Figure 1.5. Surface passivation

of silicon-containing plasma.

resists in an oxygen


1.

BOWDEN


and Photonic

11

Applications

an i n t e g r a l part of the I C (e.g., an i n n e r layer d i e l e c t r i c i n m u l t i l a y e r e d I C s ) . P o l y i m i d e s have b e e n p r o p o s e d for such applications (17). P o l y i m i d e s e x h i b i t excellent d i e l e c t r i c a n d t h e r m a l properties (the transition glass t e m p e r a t u r e (T ) —400 °C), are available w i t h h i g h p u r i t y , a n d may b e p a t t e r n e d p h o tolithographically, e i t h e r d i r e c t l y o r i n d i r e c t l y (e.g., to fabricate contact holes). I n i n d i r e c t p a t t e r n i n g , the p o l y i m i d e is spin-coated o n the substrate, partially c u r e d , a n d t h e n overcoated w i t h a conventional photoresist. T h e photoresist is i m a g e d i n the u s u a l way b y exposing selected areas of p o l y i m i d e that are r e m o v e d b y d e v e l o p m e n t i n a solvent such as h y d r a z i n e . T h e p h o toresist is t h e n s t r i p p e d , a n d the r e m a i n i n g p o l y i m i d e p a t t e r n fully c u r e d (imidized). T h i s process has b e e n s i m p l i f i e d t h r o u g h the i n t r o d u c t i o n o f photosensitive p o l y i m i d e s , w h i c h facilitate d i r e c t p a t t e r n i n g o f the f i l m (Scheme 1.1).


g

T h e microelectronics r e v o l u t i o n is d u e , i n part, to the advances i n d e s i g n and operation o f the l i t h o g r a p h i c h a r d w a r e . B u t the h i g h - r e s o l u t i o n devices that characterize this r e v o l u t i o n w o u l d not have b e e n possible w e r e i t not

o

R

Soluble Photoreactive Precursor

o S r Y * -HN^^SÔ q

s

N

H

_

0 -

0

" 0 "

w

r

hoo

Insoluble Photocross-linked Intermediate

L 0

0

TV^NH-O "^") -0

-HNsÂ^sÔ o R

w

R

O

I Heat

tant

Ei?

Final Product

0

o

-

w

A

_

- XK n

n

O

0

o

O R = - O C H C H OCC(CH) = C H 2

2

3

2

Scheme 1.1. Chemistry of photosensitive polyimides: exposure and cure.



2

3

CH

3

CH

3

CH = C H

S i - O - Si-0-4I I

CH

3

SKCHJ,

0

2

^

a

2

CH C/

R R I I —Si—Si—V I I R R

Ph

2

CH C/

(CH —CH)-

3

o

2

Ph Ph I I -f-Si-0—Si-0->-

2

-(CH —CH)

^Si(CH )

6.

2

^CH -CH)—(CH -CHh

Structure

Positive-tone, D U V , mid-UV

Negative-tone, electron-beam, X-ray, D U V

Negative-tone, electron-beam, D U V

Negative-tone, electron-beam, D U V

Negative-tone, electron beam

Type

I B M (1983) Sandia (1983)

N T T (1983)

I B M (1983)

N E C (1982)

I B M (1981)

Company


10 11

Refe\


C

2

2

3

COOH

3

CH I C-h

3

3

Si(CH )

3

OH

I

2

2

3

I Si(CH )

2

(CH )

3

2

I SKCH,),

CH Br

2

4—CH — CH—S0 -

+ Quinone Diazide

QH

k

CT ÔR R R = —0(CH ) —Si—OSi(CH )

2

2

CH I ^CH —C-CH

3

Positive-tone, D U V

Positive-tone, electron-beam

Positive-tone, near-UV

Positive-tone, electron-beam, D U V

Positive-tone, D U V

Bellcore (1987)

Bellcore (1986)

A T & T (1984)

A T & T (1984)

Hitachi (1984)


14


for the i n g e n u i t y o f the synthetic p o l y m e r c h e m i s t a n d the versatility afforded b y m a c r o m o l e c u l a r architecture. 1.1.1.2 E L E C T R O N I C S P A C K A G I N G


T h e I C is fabricated b y a series of l i t h o g r a p h i c processes similar to that d e s c r i b e d i n the p r e v i o u s section. E a c h i n d i v i d u a l step constitutes a " l e v e l " i n the d e v i c e , the final l e v e l b e i n g a metalization pattern to i n t e r c o n n e c t the c i r c u i t elements that have b e e n fabricated i n the surface of the s i l i c o n wafer. T h e c o m p l e t e d wafer is t h e n d i c e d , a step that involves c u t t i n g the wafer, t y p i c a l l y w i t h a d i a m o n d saw, to separate the i n d i v i d u a l I C chips. T h e next step is to package the chips i n some way, attach the devices along w i t h other components to the p r i n t e d w i r i n g b o a r d ( P W B ) , a n d i n t e r c o n n e c t t h e m to p r o d u c e the c o m p l e t e d c i r c u i t b o a r d . P o l y m e r s are p l a y i n g an increasingly i m p o r t a n t role i n the p r o t e c t i o n a n d i n t e r c o n n e c t i o n of the chips. S I C s are exceedingly fragile, a n d any n u m b e r of e n v i r o n m e n t a l factors can cause the device to fail unless the c h i p is p r o t e c t e d i n some way. T h u s , the I C s h o u l d b e s h i e l d e d from m e c h a n i c a l damage, h u m i d i t y , corrosion, a n d other d e t r i m e n t a l e n v i r o n m e n t a l effects. O n e approach is to h e r m e t i c a l l y seal the c h i p i n some k i n d of i n e r t package. I n d e e d , h e r m e t i c a l l y sealed ceramic-based packages, for example, are v e r y r e l i a b l e . T h e y are, h o w e v e r , e x t r e m e l y costly, a n d g i v e n the b i l l i o n s of I C s p r o d u c e d w o r l d w i d e , there is clear i n c e n t i v e to d e v e l o p a low-cost package, w h i c h is w h e r e p o l y m e r s again c o m e to the forefront. F i g u r e 1.6 shows a h y p o t h e t i c a l I C packaging structure that makes w i d e use of p o l y m e r s i n a n u m b e r of components. F i r s t , the c h i p is b o n d e d to the base of the c h i p c a r r i e r or l e a d frame b y means of an adhesive that m a y also b e r e q u i r e d to p r o v i d e e l e c t r i c a l c o n d u c t i v i t y a n d heat dissipation. P o l y m e r i c adhesives offer v e r y large materials cost savings over g o l d eutectic b o n d i n g . T h e adhesive may b e a p p l i e d to the b o n d i n g p a d b y d i s p e n s i n g d i r e c t l y , transfer p r i n t i n g , or screen p r i n t i n g . S c r e e n p r i n t i n g is p a r t i c u l a r l y s u i t e d to h y b r i d c i r c u i t assembly, w h e r e m a n y chips may n e e d to b e attached to a c e r a m i c substrate. T h e chips are t h e n p u t i n place a n d the assembly is c u r e d at elevated t e m p e r a t u r e , t y p i c a l l y 150 °C. M a n y factors m u s t b e taken i n t o consideration i n d e s i g n i n g an adhesive. T h e r e q u i r e m e n t s i n c l u d e l o w l e v e l of i o n i c i m p u r i t i e s , no voids u n d e r the c h i p caused b y evaporation of solvent or other volatiles, no r e s i n b l e e d d u r i n g c u r e , a n d t h e r m a l expansion properties that m a t c h those of the substrate a n d c h i p . A significant m i s m a t c h i n the t h e r m a l expansion coefficient can l e a d to d e v e l o p m e n t of t h e r m a l stresses that can result i n c r a c k i n g or d i s t o r t i o n of the c h i p . T h i s p r o b l e m is b e c o m i n g m o r e a n d m o r e i m p o r t a n t as d i e sizes c o n t i n u e to increase. E p o x y resins are almost exclusively u s e d for such chip-attach purposes; p o l y i m i d e s are u s e d to a lesser extent. P o l y i m i d e s p r o v i d e excellent p r o tection i n accelerated h u m i d i t y tests, b u t it is difficult to obtain void-free


BOWDEN


Applications


1.

15

IX.

5Si

I 3


16


bonds b e t w e e n the substrate a n d c h i p . P o l y i m i d e s also t e n d to b e m o r e r i g i d a n d b r i t t l e than epoxy resins, a n d this characteristic causes lifting a n d c r a c k i n g of the c h i p as the size of the c h i p increases. E p o x y resins, o n the other h a n d , p r o v i d e b e t t e r b o n d strengths, are tougher, a n d possess greater elongation u n d e r stress. T h e y are easier a n d faster to process than the p o l y i m i d e s ; t h e y p r o v i d e one-step cures w i t h n o e v o l u t i o n of volatile products. T h e o v e r a l l cost of materials a n d processing is also l o w e r . I n the case o f c o n d u c t i v e adhesives, m e t a l l i c fillers such as s i l v e r or gold are m i x e d w i t h the adhesive; for those applications w h e r e c o n d u c t i v i t y is not r e q u i r e d , S i O is the filler o f choice. Downloaded by UNIV OF MEMPHIS on May 27, 2014 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch001

£

O n c e the c h i p has b e e n attached to the b o n d i n g p a d of the lead-frame o r c e r a m i c substrate, it m u s t b e c o n n e c t e d electrically to the i n d i v i d u a l leads or c o n d u c t o r paths o n the substrate, respectively. S u c h connections are often made b y u s i n g t h i n g o l d w i r e s i n a process k n o w n as wire bonding. A s s h o w n i n the h y p o t h e t i c a l package i n F i g u r e 1.6, the c h i p is b o n d e d face-up, a n d the g o l d w i r e s connect the b o n d i n g pads at the edges of the I C to the i n d i v i d u a l leads o n the frame. I n o t h e r processes, notably flip-chip b o n d i n g or b e a m - l e a d b o n d i n g , the c h i p is attached face-down to the substrate t h r o u g h solder b u m p s o n the p e r i p h e r y of the c h i p , o r v i a b e a m leads, w h i c h e x t e n d from the edges o f the c h i p . B o t h processes leave a gap b e t w e e n the active c h i p surface a n d the substrate. T h e c h i p m u s t n o w b e p r o t e c t e d i n some w a y from the w i d e v a r i e t y of e n v i r o n m e n t a l factors e n c o u n t e r e d d u r i n g b o t h subsequent assembly a n d real-life operation that c o u l d cause the c h i p to fail. T h e s e e n v i r o n m e n t a l conditions m a y i n c l u d e : • t h e r m a l shock (e.g., d u r i n g s o l d e r i n g , w h i c h subjects the d e vice to t e m p e r a t u r e differentials of several h u n d r e d degrees), • m o i s t u r e (board c l e a n i n g a n d h u m i d environments), • chemicals a n d salts (fluxes, b o a r d - c l e a n i n g solutions, i n d u s t r i a l fumes, a n d sea air), • m e c h a n i c a l shock (test a n d assembly, h a n d l i n g , a n d careless use), a n d • e x t e n d e d t h e r m a l c y c l i n g (e.g., i f the c h i p heats u p d u r i n g operation). A l t h o u g h the surface of most I C chips has b e e n passivated w i t h a l a y e r of inorganic d i e l e c t r i c m a t e r i a l such as s i l i c o n dioxide o r s i l i c o n n i t r i d e (polyimides have also b e e n u s e d as final passivating layers), the p r o t e c t i o n p r o v i d e d b y such layers is not sufficient to ensure reliable operation t h r o u g h out the l i f e t i m e o f the device. T h e three basic methods o f p r o t e c t i o n are 1. i n c o r p o r a t i o n into a p r e f o r m e d m e t a l , glass, plastic, o r c e r a m i c package;


1.

BOWDEN


Applications

17

2. e m b e d m e n t of the d e v i c e i n a h a r d plastic (epoxy, s i l i c o n e epoxy, o r o t h e r t h e r m o s e t t i n g resin) b y transfer or i n j e c t i o n molding; and 3. the a p p l i c a t i o n of flowable materials (barrier coatings) to the assembly.


W h i c h t e c h n i q u e to use generally depends o n the d e s i r e d r e l i a b i l i t y d u r i n g end-use operation. C e r t a i n m i l i t a r y applications, for e x a m p l e , w i l l d e m a n d expensive h e r m e t i c a l l y sealed packages, whereas an i n e x p e n s i v e calculator may m a k e do w i t h a s i m p l e " g l o b " of encapsulant o n top of the chip. B a r r i e r coatings are often a p p l i e d p r i o r to total encapsulation i n transferm o l d e d packages. T h e coating p r o v i d e s a d d i t i o n a l p r o t e c t i o n against m o i s ture a n d gases a n d also p r o v i d e s r e l i e f from stresses that can d e v e l o p d u r i n g the subsequent t r a n s f e r - m o l d i n g operation. F i g u r e 1.7 shows a d e v i c e e n capsulated b y a silicone elastomer. Silicones are w i d e l y u s e d for this p u r p o s e because t h e y offer excellent p r o t e c t i o n against m o i s t u r e a n d t h e i r h y d r o p h o b i c nature l i m i t s w a t e r absorption. R o o m - t e m p e r a t u r e - v u l c a n i z i n g ( R T V ) silicone r u b b e r is a o n e - c o m p o n e n t c u r e system frequently u s e d i n such b a r r i e r - c o a t i n g applications as s h o w n i n the h y p o t h e t i c a l package i n F i g u r e

Figure 1.7. An encapsulated chip prior to transfer molding. In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

18


1.6. T h e R T V silicones contain methoxy groups s u b s t i t u t e d along the polysiloxane c h a i n , w h i c h react w i t h m o i s t u r e to form silanol groups, w h i c h u n d e r g o self-condensation to f o r m cross-links. M e t h a n o l is p r o d u c e d as a byproduct.


A b a r r i e r coating may also be r e q u i r e d for p r o t e c t i o n against a l p h a particles. H i g h - d e n s i t y devices can suffer soft errors w h e n a l p h a particles e m i t t e d from trace quantities of t h o r i u m or u r a n i u m i n packaging materials strike the active surface. T h u s , the surface m u s t be p r o t e c t e d against this possibility. P o l y i m i d e s are again r e c o m m e n d e d for such applications. I n m a n y cases, the p r o t e c t i o n p r o v i d e d b y a s i m p l e b a r r i e r coating such as R T V silicone w i l l b e sufficient to ensure l o n g - t e r m r e l i a b i l i t y of the d e v i c e . S m a l l , ceramic-based, h y b r i d I C s d e s i g n e d to be i n s e r t e d into a P W B are often p r o t e c t e d i n this m a n n e r . H o w e v e r , for other applications, a d d i t i o n a l p r o t e c t i o n is r e q u i r e d to p r o v i d e i m p a c t a n d shock resistance ( R T V silicones are v e r y soft a n d offer little m e c h a n i c a l protection), better e n v i r o n m e n t a l p r o t e c t i o n , a n d a w e l l - d e f i n e d structure for subsequent assembly a n d h a n d l i n g . S u c h p r o t e c t i o n is p r o v i d e d b y e m b e d m e n t i n a h a r d plastic b y transfer or injection m o l d i n g as s h o w n i n F i g u r e 1.6. T h e m o l d i n g process leaves two p a r a l l e l rows of p i n s exposed for subsequent c o n n e c t i o n to the P W B . C h o i c e of the t r a n s f e r - m o l d i n g c o m p o u n d is again dictated b y the v a r ious e n v i r o n m e n t a l factors l i s t e d p r e v i o u s l y . T h e m a t e r i a l s h o u l d b e of sufficiently l o w viscosity d u r i n g m o l d filling to m i n i m i z e stresses o n the delicate w i r e bonds. T h e m a t e r i a l s h o u l d also c o m p l e t e l y fill the m o l d before g e l l i n g o r setting to m a x i m i z e the y i e l d of encapsulated devices. M o r e o v e r , the c u r e cycle s h o u l d b e repeatable. T h e m o l d i n g c o m p o u n d s expansion coefficient s h o u l d m a t c h that o f the l e a d frame to m i n i m i z e stresses d u r i n g t h e r m a l c y c l i n g , a n d it s h o u l d e x h i b i t l o w shrinkage on c u r i n g to f u r t h e r m i n i m i z e stresses. T h e m a t e r i a l s h o u l d contain a v e r y l o w l e v e l of i o n i c i m p u r i t i e s that m i g h t diffuse to the d e v i c e a n d cause c i r c u i t failure. I n a d d i t i o n , m a n y r e q u i r e m e n t s are i m p o s e d b y the n e e d to ensure l o n g - t e r m p r o t e c t i o n . T h e s e r e q u i r e m e n t s i n c l u d e h i g h T , heat stability, fire retardancy, h y d r o l y t i c stab i l i t y , l o w m o i s t u r e / v a p o r diffusion, good d i e l e c t r i c p r o p e r t i e s , a n d h i g h m e c h a n i c a l strength. g

E p o x y p o l y m e r s ( i n c l u d i n g epoxy novolacs) have b e e n d e s i g n e d to m e e t most of these r e q u i r e m e n t s a n d are almost u n i v e r s a l l y u s e d i n s u c h e n c a p sulant applications. E p o x y p o l y m e r s e x h i b i t superior adhesion that i n m a n y cases eliminates the n e e d for a b a r r i e r o r j u n c t i o n coating. T h e y have a l o w coefficient of t h e r m a l expansion; l o w shrinkage; a n d l o w i n j e c t i o n v e l o c i t y , w h i c h means that l o w transfer o r injection pressures can be u s e d . T h e s e p o l y m e r s also possess excellent m e c h a n i c a l properties c o u p l e d w i t h l o w m o i s t u r e a n d gas p e r m e a b i l i t y . A b o v e a l l , t h e y are cheap a n d r e a d i l y a v a i l able. O t h e r t r a n s f e r - m o l d i n g materials u s e d to a l i m i t e d extent i n c l u d e s i l icones, p h e n o l i c materials, a n d e v e n polyesters. M o s t m o l d i n g formulations are h i g h l y filled (70-75%) w i t h materials such as q u a r t z , fused silica, short


1.

BOWDEN


and Photonic

Applications

19


glass fibers, a n d o t h e r m i n e r a l s . S u c h fillers m i n i m i z e the t h e r m a l expansion m i s m a t c h b e t w e e n the t r a n s f e r - m o l d i n g c o m p o u n d a n d the c h i p substrate m a t e r i a l , t h e r e b y r e d u c i n g stresses o n the c h i p a n d its delicate w i r e bonds. T h e existence of t h e r m a l stresses m a k e it e x t r e m e l y difficult to transfer m o l d large c e r a m i c packages such as h y b r i d integrated circuits ( H I C s ) . A s m e n t i o n e d p r e v i o u s l y , these devices are most often p r o t e c t e d w i t h a s i m p l e b a r r i e r coating such as R T V . A l t e r n a t i v e l y , t h e y m a y b e coated i n a fluidizedb e d process i n w h i c h epoxy materials are again t y p i c a l l y u s e d . T h e h y p o t h e t i c a l package i n F i g u r e 1.6 is representative of the plastic d u a l i n - l i n e package ( D I P ) that has b e e n the mainstay of the s e m i c o n d u c t o r i n d u s t r y for m a n y years. A l t h o u g h not as r e l i a b l e as the h e r m e t i c a l l y sealed c e r a m i c package, it nevertheless offers a l e v e l of r e l i a b i l i t y sufficient for a w i d e v a r i e t y of applications. T h e plastic D I P does, h o w e v e r , have a n u m b e r of limitations. T h e i n c r e a s i n g c o m p l e x i t y of I C s , w h i c h is c h a r a c t e r i z e d i n part b y the large n u m b e r of i n p u t - o u t p u t leads r e q u i r e d for m a n y of today's state-of-the-art devices, has caused research efforts i n I C packaging t e c h nology to b e i n t e n s i f i e d . H e r e again, p o l y m e r s p l a y an i n t e g r a l role i n the d e v e l o p m e n t o f low-cost packaging methods. O n e of the most p r o m i s i n g n e w technologies, for example, is tape automated b o n d i n g ( T A B ) (18). S u p p o r t e d b y fully automated processing a n d assembly t e c h n i q u e s , this approach involves p a t t e r n i n g o f c a n t i l e v e r e d m e t a l l i c leads o n a continuous p o l y i m i d e film i n r e e l format ( F i g u r e 1.8). A l l o f the leads are b o n d e d s i m u l t a n e o u s l y to the c h i p at contact b u m p s located a r o u n d the p e r i p h e r y of the active c h i p surface. A f t e r testing, a n epoxy encapsulant is d e p o s i t e d o n t h e c h i p to create a m i n i a t u r e , surface-mountable c h i p packaged o n tape. T h e m e t h o d is r e p o r t e d to offer r e l i a b i l i t y e q u a l to that of D I P packaging. T h e c h o i c e o f p o l y i m i d e s as the base film reflects the c h e m i c a l stability a n d heat stability of these p o l y m e r s as r e q u i r e d d u r i n g subsequent c i r c u i t b o a r d m o u n t i n g .

Figure 1.8. TAB packaged chips.


20


1.1.1.3 P R I N T E D W I R I N G

BOARDS


T h e P W B (also c a l l e d p r i n t e d c i r c u i t board) is the m o u n t i n g platform for a l l o f the e l e c t r o n i c components s u c h as packaged I C s , H I C s , a n d resistors. It p r o v i d e s a r i g i d m e c h a n i c a l support for the various components as w e l l as a c o n v e n i e n t means for i n t e r c o n n e c t i n g the components v i a t h i n c o n d u c t i n g lines that have b e e n " p r i n t e d " o n the b o a r d . T h e m o d e r n P W B dates back to the mid-1950s, w h e n i t was i n t r o d u c e d b y R C A . T h e s e o r i g i n a l boards w e r e based o n c o p p e r - c l a d , epoxy, glass-reinforced laminates o n w h i c h the c o n d u c t o r p a t t e r n was d e f i n e d i n a s c r e e n - p r i n t i n g process a n d the c o p p e r e t c h e d away w i t h ferric c h l o r i d e . O v e r the years, several refinements to this process have b e e n i n t r o d u c e d , b u t the essentials have changed v e r y little. P W B s are u b i q u i t o u s , b e i n g f o u n d i n applications r a n g i n g from c o n s u m e r electronics to m i l i t a r y h a r d w a r e . A g a i n , t h e i r w i d e s p r e a d use provides m o t i v a t i o n for d e v e l o p i n g a cheap, r e l i a b l e substrate for w h i c h p o l y m e r s are e m i n e n t l y suitable. P o l y m e r s possess good d i e l e c t r i c characteristics a n d can be d e s i g n e d w i t h acceptable m e c h a n i c a l a n d t h e r m a l properties as w e l l . T h e r m o s e t t i n g epoxy resins, t y p i c a l l y c o n t a i n i n g a r e i n f o r c i n g m a t e r i a l such as c h o p p e d o r w o v e n glass, are b y far the most c o m m o n l y e n c o u n t e r e d substrate m a t e r i a l . T h e w i d e l y u s e d F R - 4 l a m i n a t e d P W B , for example, consists of a b r o m i n a t e d epoxy r e s i n r e i n f o r c e d w i t h glass cloth. O t h e r substrate materials i n c l u d e p h e n o l i c materials, epoxy novolacs, polyesters, p o l y i m i d e s , a n d b i s m a l e i m ides. P o l y i m i d e s a n d b i s m a l e i m i d e s are m a i n l y u s e d i n m i l i t a r y applications, w h e r e continuous h i g h - t e m p e r a t u r e performance is r e q u i r e d . P o l y i m i d e s are also u s e d as substrates i n flexible c i r c u i t r y applications. S u c h circuits are u s e d to p r o v i d e i n t e r c o n n e c t i o n b e t w e e n r i g i d boards, as w e l l as i n spacesaving specialty applications s u c h as cameras a n d telephones ( F i g u r e 1.9). O t h e r substrate materials c o n s i d e r e d for flexible c i r c u i t r y i n c l u d e polyesters, fluorocarbons, a n d epoxy-glass composites. T h e manufacture of a P W B begins w i t h coating the w o v e n reinforcement w i t h the r e s i n . T h i s coating is d o n e i n a continuous operation ( F i g u r e 1.10), i n w h i c h glass c l o t h , for e x a m p l e , is fed t h r o u g h the r e s i n b a t h a n d t h e n passed t h r o u g h a c u r i n g o v e n w h e r e the epoxy m a t e r i a l is partially c u r e d i n a process r e f e r r e d to as B staging to p r o d u c e w h a t is c a l l e d a prepreg. T h e p r e p r e g is t h e n cut to the d e s i r e d size, several layers are stacked together, a n d c o p p e r foil (typically 36 |xm thick) is p l a c e d o n b o t h sides to p r o d u c e a l a m i n a t e d l a y - u p . T h i s laminate assembly is t h e n c u r e d i n a press at elevated t e m p e r a t u r e (~350 °F) a n d pressure to p r o d u c e a fully c u r e d laminate. T h e c l a d laminates c o m e i n almost any thickness b u t are most c o m m o n l y 1.5 m m thick. T h e next step i n the process is to create the intricate i n t e r c o n n e c t i o n patterns o n one o r b o t h sides of the b o a r d . T o create patterns o n b o t h sides of the b o a r d , holes must first be d r i l l e d t h r o u g h the b o a r d t h r o u g h w h i c h the electronic components (e.g., the leads of the plastic D I P ) , w i l l e v e n t u a l l y


BOWDEN


Applications


1.

21

o

•3 H O

ft



22


TRIM AND SIZE

PACKAGE AND SHIP

Figure 1.10. Prepreg process in the manufacture

ofPWBs.

b e i n s e r t e d . T h e c o n d u c t o r paths are t h e n d e f i n e d l i t h o g r a p h i c a l l y . T h e o r i g i n a l screen p r i n t i n g process d e s c r i b e d p r e v i o u s l y c a n resolve 1 0 - m i l features, b u t s u c h feature sizes are insufficient for m a n y o f today's c i r c u i t boards. A n e w technology based o n a d r y - f i l m photoresist (19) was i n t r o d u c e d i n the late 1960s a n d is n o w u s e d to fabricate most o f the h i g h - d e n s i t y c i r c u i t boards for c o m p u t e r , t e l e c o m m u n i c a t i o n , a n d aerospace applications. A d r y - f i l m photoresist has a t h r e e - l a y e r structure consisting o f a clear strong polyester support film, a p h o t o p o l y m e r i z a b l e resist l a y e r , a n d a p o l y o l e f i n separator sheet ( F i g u r e 1.11). T h e polyester film is s i m i l a r to that u s e d as the base for m a n y photographic films a n d is o n l y about 2 5 |xm t h i c k . T h e m i d d l e l a y e r is t h e p h o t o p o l y m e r resist that has b e e n coated o n t h e polyester film b y t h e d r y - f i l m manufacturer. T h i s layer, w h i c h ranges i n thickness f r o m 17 to 100 |xm d e p e n d i n g o n t h e i n t e n d e d a p p l i c a t i o n , consists p r i n c i p a l l y of a p o l y m e r i c b i n d e r , a m o n o m e r , a n d photoinitiator. T h e b i n d e r consists o f a t o u g h film-forming m a t e r i a l such as a methacrylate p o l y m e r . T h e resist is a p p l i e d to t h e c o p p e r - c l a d l a m i n a t e d substrate i n a l a m i n a t i o n process a n d t h e n exposed t h r o u g h a photographic mask o u t l i n i n g t h e c i r c u i t p a t t e r n . P r o c e s s i n g is s i m i l a r to that discussed e a r l i e r i n Section 1.1.1.1.


1.

BOWDEN


Applications

23


Lamination

Exposure

Development

Figure J . I I . Dry-film photoresist process. E x p o s u r e to U V l i g h t p o l y m e r i z e s the m o n o m e r to a c r o s s - l i n k e d m a t r i x a n d enables a negative image o f the mask to b e f o r m e d b y solvent d e v e l o p m e n t . S e v e r a l techniques can b e u s e d to p r o d u c e the actual c o n d u c t o r paths o n the P W B substrate. T h e s e techniques are d e p i c t e d i n F i g u r e 1.12. S u b tractive processing starts w i t h a P W B coated o n one or b o t h sides w i t h c o p p e r foil r a n g i n g i n thickness b e t w e e n 5 a n d 70 |xm. I n the case o f d o u b l e s i d e d P W B s c o n t a i n i n g p l a t e d - t h r o u g h holes, the holes are first d r i l l e d a n d



E L E C T R O N I C & PHOTONIC APPLICATIONS OF POLYMERS

a

•I e

§


1.

BOWDEN


and Photonic

Applications

25

c o p p e r is d e p o s i t e d i n the d r i l l e d hole b y electroless p l a t i n g , w h i c h is a process that deposits c o p p e r o n a l l surfaces i n c l u d i n g the i n n e r surface of the n o n c o n d u c t i n g hole. N e x t , the c o p p e r thickness is b u i l t u p b y e l e c t r o p l a t i n g to the r e q u i s i t e thickness w i t h i n the h o l e . A t this p o i n t , the d r y - f i l m resist is a p p l i e d , i m a g e d , d e v e l o p e d , a n d the exposed c o p p e r is e t c h e d away. T h e p l a t e d - t h r o u g h holes are p r o t e c t e d b y the resist d u r i n g the e t c h i n g


process. O t h e r variations of this t h e m e i n c l u d e partially a d d i t i v e a n d f u l l y a d d i t i v e processing. I n partially a d d i t i v e processing, an u n c l a d c i r c u i t b o a r d is p u n c h e d (to f o r m the holes), a n d a t h i n l a y e r of electroless c o p p e r is d e p o s i t e d o n a l l surfaces. T h e substrate is t h e n i m a g e d l i t h o g r a p h i c a l l y , a n d the exposed c o p p e r surface is p l a t e d to the d e s i r e d thickness. T h e final step involves r e m o v a l of the t h i n flash coat of c o p p e r from the p r o t e c t e d areas; this process also removes a s i m i l a r thickness from the p l a t e d - u p areas. T h e fully a d d i t i v e process also starts w i t h an u n c l a d b o a r d to w h i c h a n adhesion p r o m o t e r a n d activator (usually a P d salt) is a p p l i e d o n the surface. T h e resist is a p p l i e d , i m a g e d , a n d p a t t e r n e d , t h e r e b y exposing the activated surface u p o n w h i c h c o p p e r is p l a t e d i n an electroless process. T h e advantages of this process i n c l u d e d e p o s i t i n g c o p p e r o n l y w h e r e it is r e q u i r e d as o p p o s e d to subtractive e t c h i n g , w h e r e 8 0 % of the c o p p e r is discarded. P r i n t e d c i r c u i t r y today is at the crossroads. T h e density of components o n the P W B continues to increase a n d is a c c o m p a n i e d b y shrinkage of the d i m e n s i o n s of the c o n d u c t o r paths o n the surface of the b o a r d . F u r t h e r m o r e , the P W B s frequently contain several layers of metalization. T h e r e is a m a r k e d t r e n d t o w a r d surface-mounted devices, w h i c h are i n c r e a s i n g i n c o m p l e x i t y a n d generating m o r e heat. T h e s e trends are c r e a t i n g a d e m a n d for boards that e x h i b i t better t h e r m a l p r o p e r t i e s , h i g h e r d e n s i t y (resolution), a n d l o w e r t h e r m a l expansion coefficients (facilitating d i r e c t m o u n t i n g of I C s o n the board). P o l y m e r s w i l l play an i m p o r t a n t part i n this c o n t i n u i n g e v o l u t i o n . A l r e a d y , considerable interest has b e e n generated i n d i r e c t l y m o l d e d c i r c u i t boards (20), w h i c h use the n e w e r h i g h - t e m p e r a t u r e resins s u c h as p o l y e t h e r i m i d e s , polysulfones, p o l y e t h e r e t h e r ketones ( P E E K ) , a n d p o l y p h e n y l e n e sulfide reinforced w i t h c h o p p e d glass fibers. D i r e c t m o l d i n g offers the a d vantage that features such as spaces, stand-offs, a n d brackets can b e d i r e c t l y fabricated. H o l e s can b e m o l d e d rather than p u n c h e d , a n d boards can b e m o l d e d to suit specific s t y l i n g r e q u i r e m e n t s . T h e l i q u i d crystalline p o l y e s ters, s u c h as p o l y ( h y d r o x y b e n z o i c acid) a n d various derivatives a n d m o d i fications thereof, h o l d p r o m i s e for future applications, p a r t i c u l a r l y for specialized uses. T h e s e materials are characterized b y v e r y h i g h m o d u l u s a n d t e m p e r a t u r e stability, l o w t h e r m a l expansion coefficients, a n d h i g h r e sistance to chemicals. S u c h specialty materials are v e r y expensive at the m o m e n t , a n d the h u m b l e epoxy l a m i n a t e m a y w e l l r e m a i n the mainstay of the i n d u s t r y for m a n y years to c o m e . O n e final area that is h a v i n g a significant i m p a c t o n P W B technology



26


is that of h i g h - s p e e d electronics (21). S i l i c o n a n d G a A s technologies are d r i v i n g d e v i c e speeds i n t o the gigahertz frequency range w i t h rise t i m e s i n the o r d e r o f fractions of a nanosecond. A t s u c h signal frequencies, e l e c t r i c a l e n e r g y no l o n g e r resides as voltage a n d c u r r e n t w i t h i n the conductors. E n e r g y flows as electromagnetic waves outside of a n d b e t w e e n conductors, w h o s e propagation v e l o c i t y is d e t e r m i n e d b y the d i e l e c t r i c constant of the s u r r o u n d i n g m a t e r i a l . G e n e r a l l y speaking, the h i g h e r the d i e l e c t r i c constant, the greater the p e n a l t y i n t e r m s of propagation delay. F o r most P W B s , the p o l y m e r i c substrate has a d i e l e c t r i c constant from 3 to 5, w h i c h translates i n t o a s p e e d p e n a l t y of 5 0 % for most P W B s a n d flexible c i r c u i t s . A n o t h e r factor that m u s t b e t a k e n i n t o consideration is c o n t r o l of the p h y s i c a l d i mensions of the conductors a n d spacing b e t w e e n conductors, w h i c h d e t e r m i n e s i m p e d a n c e a n d cross-talk characteristics. T h e s e r e q u i r e m e n t s w i l l place increasingly stringent d e m a n d s o n the l i t h o g r a p h y a n d o n the materials components of the P W B itself. Poly(tetrafluoroethylene) a n d its derivatives, for e x a m p l e , are b e i n g u s e d as P W B substrate materials because o f the l o w e r d i e l e c t r i c constant (e) of these p o l y m e r s c o m p a r e d w i t h epoxy materials a n d p o l y i m i d e s (21). W i t h € values o f 2 . 2 - 2 . 8 , propagation delays are r e d u c e d b y as m u c h as 3 0 % c o m p a r e d w i t h the woven-glass, fabric-reinforced epoxy a n d p o l y i m i d e materials, a n d b y as m u c h as 1 0 % o v e r the n e w e r p o l y i m i d e laminates r e i n f o r c e d w i t h q u a r t z or p o l y ( p - p h e n y l e n e terephthalamide) (Kevlar) fabric.

1.1.2 Conducting Polymers E l e c t r i c a l c o n d u c t i v i t y refers to the transport of charge carriers t h r o u g h a m e d i u m u n d e r the influence of an electric field or t e m p e r a t u r e gradient a n d is thus d e p e n d e n t o n the n u m b e r of charge carriers a n d t h e i r m o b i l i t y . T h e charge carriers m a y b e generated intrinsically or from i m p u r i t i e s , i n w h i c h case t h e y m a y b e electrons, holes, or ions. A l t e r n a t i v e l y , electrons o r holes m a y b e i n j e c t e d from electrodes. C o n d u c t i o n may therefore b e of two t y p e s — i o n i c a n d e l e c t r o n i c — b o t h of w h i c h have b e e n the focus of intense research, p a r t i c u l a r l y i o n i c c o n d u c t i o n , w h i c h has b e e n s t u d i e d for m a n y years a n d has b e e n the subject o f several books. Advantage is t a k e n of i o n i c c o n d u c t i o n i n c e r t a i n p o l y m e r s , for e x a m p l e , i n the design of p o l y m e r i c electrolytes for solid-state batteries. Poly(ethylene oxide) has b e e n e x t e n sively s t u d i e d i n this regard. E l e c t r o n i c c o n d u c t i o n i n p o l y m e r s , o n the o t h e r h a n d , is a r e l a t i v e l y n e w p h e n o m e n o n , h a v i n g b e e n d i s c o v e r e d o n l y about 10 years ago a n d is the subject of this section. I n g e n e r a l , p o l y m e r s are i n s u l a t i n g materials h a v i n g conductivities r a n g i n g from 1 0 " (ft c m ) " for p o l y v i n y l chloride) to 1 0 " (ft c m ) " for poly(tetrafluoroethylene), w h i c h are m a n y orders o f m a g n i t u d e b e l o w the c o n d u c t i v i t i e s associated w i t h metals ( F i g u r e 1.13). I n d e e d , l o w c o n d u c t i v i t y (and consequent l o w d i e l e c t r i c constant) is one of the major reasons p o l y m e r s have f o u n d w i d e s p r e a d acceptance i n a m y r i a d of i n s u l a t i n g a n d structural 1 0

1

1 8


1

In Electronic and Photonic Applications of Polymers; Bowden, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. T

m

S

,

U

C

0

N

SODA-LIME GLASS _

POLYACETYLENE BORON

TRANS-

GERMANIUM

-10"

-10"

-10"

1

10

10

10 6 \ LEAD/1 BISMUTH + 10

COPPER^ SILVER I GOLD J

8

Figure 1.13. Conductivity

POLY(PHENYLENE SULFIDE)

?

DOPED POLYMERS

and insulating

materials.

NOTE: Vertical scale shows conductivity at room temperature in (ohm-cm)1

DOPED POLYACETYLENES

POLYPHENYLENE, POLYPYRROLE

f

j

of various metallic, semiconductor,

POLYETHYLENE + 10 -16 SULFUR TEFLON 10 •18

-10 -14

10" CIS-POLYACETYLENE FUSED QUARTZ POLYVINYL CHLORIDE 3 - 1 0 " WHITE PHOSPHORUS INSULATORS -10 -12

SEMICONDUCTORS

METALS -

"

CONDUCTIVITY (ohm-cm) -1


28


applications t h r o u g h o u t the electronics i n d u s t r y . N e v e r t h e l e s s , the discove r y i n 1973 (22) that poly(sulfur nitride) ( S N ) was intrinsically c o n d u c t i n g p r o v i d e d p r o o f that p o l y m e r s c o u l d be c o n d u c t i n g a n d greatly s t i m u l a t e d the search for o t h e r c o n d u c t i n g p o l y m e r s . X


1.1.2.1 B A N D T H E O R Y O F S O L I D S

T o p r o v i d e a framework for discussing c o n d u c t i o n i n p o l y m e r s a n d w h y p o l y m e r s are n o r m a l l y classified as insulators, some o f the basic ideas of b a n d t h e o r y i n solids s h o u l d b e r e v i e w e d . C o w a n a n d W l y g u l (23), i n t h e i r r e v i e w of the organic s o l i d state, discussed this topic from the organic v i e w p o i n t a n d p r o v i d e d a useful f r a m e w o r k w i t h i n w h i c h chemists can appreciate the salient features o f the theory. W h e n a large n u m b e r of atoms (e.g., as i n metals or semiconductors) or molecules (e.g., organic metals) are b r o u g h t together i n the crystalline state, the e l e c t r o n i c states m i x so as to f o r m bands, each b a n d consisting of e l e c t r o n i c states w h o s e energies form a continuous range. T h i s situation is analogous to the s p l i t t i n g o f a t o m i c energy levels as two atoms are b r o u g h t together to f o r m a m o l e c u l e . F o r example, the e t h y l e n e m o l e c u l e consists o f two s p - h y b r i d i z e d c a r b o n atoms, each c o n t a i n i n g an u n p a i r e d e l e c t r o n i n a p o r b i t a l ; the t w o orbitals overlap to f o r m a i r b o n d . A c c o r d i n g to H u e k e l t h e o r y , the i n t e r a c t i o n o f these two p orbitals forms two m o l e c u l a r orbitals c o r r e s p o n d i n g to the IT b o n d i n g a n d TT a n t i b o n d i n g orbitals ( F i g u r e 1.14), separated b y an e n e r g y A . I f these orbitals are a l l o w e d to interact w i t h the IT a n d IT* orbitals o f a second e t h y l e n e m o l e c u l e stacked d i r e c t l y above t h e first, two sets o f t w o m o l e c u l a r orbitals are f o r m e d that are separated b y energy 28, w h e r e 8 is the resonance or transfer i n t e g r a l . L i k e w i s e , i f n e t h y l e n e m o l e c u l e s are a l l o w e d to interact, n states from each o f the TT a n d IT* orbitals are f o r m e d . F o r large values o f n , the e n e r g y states are close e n o u g h together to c o r r e s p o n d to a continuous b a n d . T h e 2 n electrons are t h e n a l l o w e d to fill the bands i n a m a n n e r analogous to the A u f b a u p r i n c i p l e for atoms (i.e., electrons are p l a c e d i n these states i n pairs starting w i t h the lowest energy state a n d filling the h i g h e r energy states successively). T h e highest o c c u p i e d state is c a l l e d the F e r m i l e v e l . 2

A s seen i n F i g u r e 1.14, t h e b a n d f o r m e d from the highest o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O ) i n the stack of e t h y l e n e molecules is e n t i r e l y f u l l , b u t the b a n d f o r m e d from the lowest u n o c c u p i e d m o l e c u l a r o r b i t a l ( L U M O ) is e n t i r e l y e m p t y . A c c o r d i n g to b a n d t h e o r y , i f the highest filled b a n d (ref e r r e d to as the valence band) is o n l y partially f u l l , the e m p t y states w h i c h exist close to the F e r m i l e v e l w i l l facilitate c o n d u c t i o n . I n the case o f the h y p o t h e t i c a l stack o f e t h y l e n e m o l e c u l e s , the H O M O b a n d is c o m p l e t e l y f u l l . F o r the stack to b e c o n d u c t i v e , e n e r g y m u s t be s u p p l i e d (either t h e r m a l l y o r photolytically) to m o v e a n e l e c t r o n i n t o the next lowest state, w h i c h i n this case happens to b e the lowest energy l e v e l i n the L U M O b a n d (also c a l l e d the c o n d u c t i o n band). T h i s energy gap separating the t w o bands is



1.

BOWDEN


Applications

29

Figure 1.14. Band formation obtained by mixing of electronic states. (Reproduced from reference 23. Copyright 1986 American Chemical Society.) c a l l e d the band-gap energy, a n d its m a g n i t u d e d e t e r m i n e s w h e t h e r s u c h a material is a s e m i c o n d u c t o r or an insulator ( F i g u r e 1.15). T h e i n t r i n s i c c o n d u c t i n g properties o f ( S N ) d e r i v e from the presence of one u n p a i r e d e l e c t r o n for each S - N u n i t . A s a result, the highest o c c u p i e d electronic levels (i.e., the valence band) are o n l y half-occupied. Because no f o r b i d d e n gap exists b e t w e e n the highest o c c u p i e d a n d lowest u n o c c u p i e d levels (both exist w i t h i n the H O M O band), the u n p a i r e d electrons can r e a d i l y m o v e u n d e r the a p p l i c a t i o n o f an electric field a n d give rise to m e t a l l i c l i k e conductivity. X

U n l i k e ( S N ) , most p o l y m e r s c o r r e s p o n d to closed-shell systems w h e r e all the electrons are p a i r e d . S u c h a configuration leads to i n s u l a t i n g o r s e m i c o n d u c t i n g properties as n o t e d p r e v i o u s l y . Polyacetylenes a n d r e l a t e d c o n j u g a t e d p o l y m e r s , for example, have conductivities that classify t h e m as semiconductors. T h e carbon atom i n polyacetylene is sp h y b r i d i z e d , w h i c h leaves one p e l e c t r o n out o f the b o n d - f o r m i n g h y b r i d orbitals. I n p r i n c i p l e , such a structure m i g h t b e expected to give rise to e x t e n d e d e l e c t r o n i c states f o r m e d b y overlap o f the p (IT) electrons a n d thus p r o v i d e a basis for m e t a l l i c behavior in polymers. I n practice, the q u a s i - o n e - d i m e n s i o n a l structure j u s t d e s c r i b e d is not X

2



30


Figure 1.15. The allowed energy states for an insulator, semiconductor, and metal. Blackened regions represent regions filled with electrons. E is the energy gap between filled and empty states. (Reproduced from reference 23. Copyright 1986 American Chemical Society.) G

stable. Instead, the IT electrons o v e r l a p i n a n a l t e r n a t i n g fashion, r e s u l t i n g i n the familiar conjugated TT-bond s t r u c t u r e of p o l y a c e t y l e n e . I n e n e r g e t i c t e r m s , b o n d a l t e r n a t i o n causes a gap ( £ ) to be o p e n e d at the F e r m i l e v e l g

that converts the system from a c o n d u c t o r to a s e m i c o n d u c t o r ( F i g u r e 1.16). Physicists refer to this as a Pierls

transition.

A major b r e a k t h r o u g h i n the search for c o n d u c t i n g p o l y m e r s o c c u r r e d i n 1977 (24) w i t h the d i s c o v e r y that p o l y a c e t y l e n e c o u l d b e r e a d i l y o x i d i z e d (by e l e c t r o n acceptors s u c h as i o d i n e o r arsenic pentafluoride) or r e d u c e d (by donors s u c h as l i t h i u m ) . T h e r e s u l t i n g m a t e r i a l h a d a c o n d u c t i v i t y that was orders o f m a g n i t u d e greater than the o r i g i n a l , u n t r e a t e d sample. T h i s process is often r e f e r r e d to as doping b y analogy w i t h the d o p i n g o f i n o r g a n i c s e m i c o n d u c t o r s , b u t i t contrasts w i t h the i n o r g a n i c s e m i c o n d u c t o r d o p i n g i n that d o p i n g i n p o l y m e r s is a redox process i n v o l v i n g charge transfer w i t h s u b s e q u e n t c r e a t i o n o f c h a r g e d species. T h e redox reaction m a y b e c a r r i e d out i n the v a p o r phase, i n s o l u t i o n , or e l e c t r o c h e m i c a l l y .

1.1.2.2 C O N D U C T I O N

MECHANISMS

S i n c e the first r e p o r t o f m e t a l l i c c o n d u c t i v i t y i n d o p e d p o l y a c e t y l e n e , a n u m b e r o f o t h e r conjugated p o l y m e r s have b e e n s h o w n to possess h i g h



Figure 1.16. Splitting of the valence band caused by overlap of p electrons to form the alternating double bond sequence of polyacetylene.


© o 5*

i

a m z

da o

32



c o n d u c t i v i t y . T h e structures o f the p r i n c i p a l c o n d u c t i n g p o l y m e r systems are s h o w n i n T a b l e 1.2. T h e m a x i m u m dopant concentrations are o n the o r d e r o f several m o l e p e r c e n t , w h i c h is c o n s i d e r a b l y h i g h e r than r e q u i r e d for t h e inorganic semiconductors. T h e o r i g i n o f the c o n d u c t i o n m e c h a n i s m has b e e n a source o f c o n t r o v e r s y e v e r since c o n d u c t i n g p o l y m e r s w e r e first d i s c o v e r e d . A t first, d o p i n g was a s s u m e d to s i m p l y r e m o v e electrons from the top of the valence b a n d (oxidation) or a d d electrons to the b o t t o m o f the c o n d u c t i o n b a n d (reduction). T h i s m o d e l associates charge carriers w i t h free spins (unpaired electrons). H o w e v e r , the m e a s u r e d c o n d u c t i v i t y i n d o p e d polyacetylene (and other c o n d u c t i n g p o l y m e r s such as p o i y p h e n y l e n e a n d p o l y p y r r o l e ) is far greater than w h a t can b e a c c o u n t e d for o n the basis of free spin alone. T o account for this p h e n o m e n o n o f spinless c o n d u c t i v i t y , physicists have i n t r o d u c e d t h e c o n c e p t o f transport v i a structural defects i n the p o l y m e r c h a i n . I n a c o n v e n t i o n a l s e m i c o n d u c t o r , an e l e c t r o n can b e r e m o v e d from t h e valence b a n d a n d p l a c e d i n the c o n d u c t i o n b a n d , a n d the s t r u c t u r e can be a s s u m e d to r e m a i n r i g i d . I n contrast, an e l e c t r o n i c excitation i n p o l y m e r i c materials is a c c o m p a n i e d b y a d i s t o r t i o n or relaxation o f the lattice a r o u n d the excitation, w h i c h m i n i m i z e s the local lattice strain energy. T h e c o m b i n e d Table 1.2. Structures and Conductivity of Doped Conjugated Polymers

Polymer

Structure

Polyacetylene

Typical Methods of Doping

Typical Conductivity (Q cm)~ l

Electrochemical, chemical A s F , I , L i , K) 5

500-1.5 X 10

2

Chemical (AsF , L i , K)

Poiyphenylene

500

5

Poly(phenylene sulfide)

Chemical (AsF )

Polypyrrole

Electrochemical

600

Polythiophene

Electrochemical

100

Poly(phenyl-quinoline)

5

C«H

5

Electrochemical, chemical (sodium naphthalide)


50

5

1.

BOWDEN


Applications

33


structural a n d e l e c t r o n i c excitation w i l l n o w look l i k e a defect o n the c h a i n . F r o m a c h e m i c a l v i e w p o i n t , this defect is i n t e r p r e t e d as a radical cation (or radical a n i o n i n the case of reduction). Physicists refer to i t as a polaron. Because these defects represent localized distortions of the lattice, the associated energy l e v e l m u s t b e split off from the c o n t i n u u m of b a n d states ( F i g u r e 1.17a). T h e two p o l a r o n states c o r r e s p o n d i n g to a radical cation a n d radical a n i o n are s y m m e t r i c a l l y disposed a r o u n d the F e r m i l e v e l (i.e., the m i d p o i n t of the gap). R e m o v a l of an electron leaves an u n p a i r e d s p i n near the valence b a n d edge (p doping), a n d a d d i t i o n of an e l e c t r o n fills the c o r r e s p o n d i n g state near the c o n d u c t i o n b a n d edge (n doping). T h e s e e n e r g y levels are d e p i c t e d i n F i g u r e 1.17b. F u r t h e r oxidation (or reduction) results i n the formation of w h a t p h y sicists c a l l a bipolaron. I n the oxidation case, it is energetically m u c h m o r e favorable to take the second e l e c t r o n from the polaron than to f o r m a second p o l a r o n (25); thus, the oxidation process may b e v i e w e d as l e a d i n g to the formation of a l o c a l i z e d d o u b l y charged species (i.e., a d i c a t i o n , or d i a n i o n i n the case of reduction). T h e b i p o l a r o n is thus i d e n t i f i e d as a dication o r d i a n i o n associated w i t h a strong local lattice d i s t o r t i o n . Because the lattice relaxation a r o u n d the charges is stronger than i n the case of a single charge, the e l e c t r o n i c states a p p e a r i n g i n the b a n d edge are f u r t h e r away f r o m the b a n d edges (closer to the F e r m i level) than they are for polarons ( F i g u r e 1.17c). C o u l o m b i c r e p u l s i o n m i g h t be expected to result i n charge separat i o n , b u t as w i l l b e seen, separation is o n l y feasible i f the p o l y m e r possesses a degenerate g r o u n d state. I n p o l y p y r r o l e , for example, the charges associated w i t h the b i p o l a r o n are separated, b u t o n l y over about four p y r r o l e units. A s seen i n F i g u r e 1.17c, bipolarons contain no free spins. A l l energy levels i n the gap are e i t h e r e m p t y or f u l l . T h e s e species are b e l i e v e d to b e i n v o l v e d i n the c o n d u c t i o n process, t h e r e b y accounting for the o b s e r v e d " s p i n l e s s " c o n d u c t i v i t y . E v i d e n c e for t h e i r existence comes from spectroscopic studies, a l t h o u g h the precise m e c h a n i s m of charge c o n d u c t i o n is not really k n o w n . F o r one t h i n g , the charges s h o u l d b e fixed i n p o s i t i o n along the c h a i n b y the c o u n t e r i o n d e r i v e d from the dopant species. F u r t h e r m o r e , p o l y m e r s themselves contain m a n y defects such as cross-links, c h a i n ends, and b e n d s , a n d it is difficult to see h o w e v e n a m o b i l e b i p o l a r o n or p o l a r o n c o u l d m o v e past such obstacles. C o n d u c t i o n mechanisms have b e e n p r o p o s e d , at least i n the case of polyacetylene itself, that i n v o l v e a different type of defect structure c a l l e d a soliton. t r a n s - P o l y a e e t y l e n e is u n i q u e a m o n g c o n d u c t i n g p o l y m e r s i n that it possesses a degenerate g r o u n d state c o r r e s p o n d i n g to the two g e o m e t r i c forms s h o w n i n F i g u r e 1.18. A s a c o n s e q u e n c e , the two charges associated w i t h the b i p o l a r o n can easily separate a n d b e c o m e i n d e p e n d e n t , c h a r g e d species. N o associated energy p e n a l t y occurs because the g e o m e t r i c s t r u c t u r e a p p e a r i n g b e t w e e n the two charges has the same energy as the configuration




34

Figure 1.17. Electronic energy levels associated with various types of defect structures found in doped polyacetylene.


1.

BOWDEN


Applications

35


t

I

1

I k

Figure 1.18. Energy diagram for ground-state tylene.

I

I—I k geometric isomers of polyace-

o n the o t h e r side o f the charge. O n the contrary, the two g e o m e t r i c isomers of cis-polyacetylene o r p o l y p y r r o l e are not energetically e q u i v a l e n t . I n p o l y p y r r o l e , the q u i n o i d l i k e g e o m e t r i c i s o m e r has a h i g h e r total e n e r g y t h a n the aromatic (ground state) s t r u c t u r e ; h e n c e , charge separation o f the b i p o l a r o n is not favored. T h e fact that l i m i t e d charge separation does o c c u r to the extent o f about four p y r r o l e units is a t t r i b u t e d to the larger e l e c t r o n affinity o f the q u i n o i d structure relative to that o f the aromatic s t r u c t u r e . M a x i m u m stabilization o f the d i c a t i o n occurs at a separation o f about four units. S i m i l a r defect structures m a y b e e n v i s i o n e d as arising d u r i n g i s o m e r i zation o f d s - p o l y a c e t y l e n e to t r a n s - p o l y a c e t y l e n e . I f t w o trans sequences w i t h opposite b o n d alternation approach each other along a c h a i n c o n t a i n i n g an o d d n u m b e r o f conjugated carbon atoms, an u n p a i r e d e l e c t r o n (radical) w i l l b e left at the p o i n t w h e r e the two sequences meet. T h i s defect, w h i c h chemists call a free radical, is s i m i l a r to the solitary charged defect p r o d u c e d b y separation o f a b i p o l a r o n , except that i t is n e u t r a l . Physicists call this type o f defect a soliton, w h i c h is d e f i n e d as a p h a s e b o u n d a r y defect l i n k i n g two energetically e q u i v a l e n t configurations. T h e e l e c t r o n i c energy l e v e l associated w i t h the soliton i n frans-polyacetylene is located i n the m i d d l e o f the gap because the phase b o u n d a r y effectively represents a single n o n b o n d i n g p o r b i t a l that has the energy (£) o f the local atomic o r b i t a l , d e f i n e d to b e at £ = 0. T h e soliton can exist i n any o f t h r e e spin-charge configurations ( F i g u r e 1.17d): n e u t r a l (S = V2), negatively c h a r g e d (S = 0), or p o s i t i v e l y c h a r g e d (S = 0). N e u t r a l solitons are b e l i e v e d


36


to b e the o r i g i n of the free spins (~1 p e r 4000 carbon atoms) f o r m e d d u r i n g t h e r m a l i s o m e r i z a t i o n of d s - p o l y a e e t y l e n e to the trans isomer.


T h e d i a g r a m m a t i c representation of the soliton depicts the defect as b e i n g c e n t e r e d o n a single c a r b o n atom. S p i n density measurements i n d i c a t e , h o w e v e r , that the s p i n is not localized o n a single c a r b o n atom b u t is spread o v e r several (~15) c a r b o n atoms. L i k e w i s e , the charge density associated w i t h charged solitons is spread over a s i m i l a r n u m b e r of carbon atoms. T h e p i c t u r e that emerges is that at l o w d o p i n g levels, charged solitons are f o r m e d , e i t h e r d i r e c t l y from n e u t r a l solitons or b y r e c o m b i n a t i o n of polarons to form bipolarons, w h i c h t h e n m o v e apart to b e c o m e i n d i v i d u a l l y c h a r g e d solitons. B u t the q u e s t i o n of the m e c h a n i s m of actual charge transp o r t still r e m a i n s . C e n t r a l to m a n y of the p r o p o s e d theories is the a s s u m p t i o n that the soliton can m o v e freely along the c h a i n . O n e p r o p o s e d m e c h a n i s m , d u e to K i v e l s o n (26), is d e p i c t e d i n F i g u r e 1.19. A n e u t r a l soliton m o v i n g

Figure 1.19. Kivelson mechanism for charge transport involving mobile neutral solitons.



1.

BOWDEN


and Photonic

Applications

37

along the p o l y m e r c h a i n encounters a charged soliton, w h i c h is spatially p i n n e d b y its c o u n t e r i o n . Transfer of an e l e c t r o n results i n the appearance of the soliton o n the n e i g h b o r i n g c h a i n . T h e m e c h a n i s m thus p u r p o r t s to account for b o t h i n t e r c h a i n a n d i n t r a c h a i n c o n d u c t i o n a n d predicts that the c o n d u c t i v i t y s h o u l d v a r y w i t h the n u m b e r of free spins. H o w e v e r , recent e x p e r i m e n t s (27) o n specially p r e p a r e d polyacetylenes, i n w h i c h the s p i n concentration was d e l i b e r a t e l y v a r i e d , have cast doubt o n such theories. T h e s e studies indicate that c o n d u c t i v i t y does not vary w i t h s p i n c o n c e n t r a t i o n i n the m a n n e r p r e d i c t e d b y the K i v e l s o n theory. A d d i t i o n a l studies based o n techniques s u c h as e l e c t r o n nuclear d o u b l e resonance ( E N D O R ) (28) a n d e l e c t r o n spin-echo m u l t i p l e q u a n t u m n u c l e a r magnetic resonance ( E S E M Q N M R ) (29), w h i c h p r o b e the local e n v i r o n m e n t of the soliton, i n dicate that the soliton does not m o v e (i.e., i t is static). M u c h w o r k remains to be d o n e to u n r a v e l the transport m e c h a n i s m responsible for c o n d u c t i o n i n these systems. 1.1.2.3 C O M M E R C I A LAPPLICATIONS

C o n d u c t i n g p o l y m e r s appear to b e o n the b r i n k of c o m m e r c i a l exploitation. Progress i n c o m m e r c i a l i z a t i o n has b e e n h a m p e r e d b y p o o r stability (polyacetylene, for e x a m p l e , is r a p i d l y o x i d i z e d i n air a n d thus loses conductivity) a n d b y p o o r processability. M a n y of the materials are intractable a n d t h e r e fore are not amenable to easy fabrication. N e v e r t h e l e s s , w o r k has c o n t i n u e d o n d e v e l o p i n g a n e w generation of stable, processable c o n d u c t i n g p o l y m e r s . N a a r m a n n a n d coworkers (30) at B A S F m o d i f i e d the standard m e t h o d for p o l y m e r i z i n g acetylene, a n d the result was a m a t e r i a l w i t h a h i g h e r d e g r e e of crystallinity a n d consequent greater t h e r m a l stability t h a n polyacetylene p r e p a r e d b y the o l d e r t e c h n i q u e . Perhaps m o r e i m p o r t a n t l y t h o u g h , the n e w m a t e r i a l r e p o r t e d l y exhibits c o n d u c t i v i t y that, o n a w e i g h t basis, is twice that of c o p p e r . A l t h o u g h the B A S F m a t e r i a l is h i g h l y c o n d u c t i n g a n d m u c h m o r e stable t h a n earlier films, it is still a h i g h l y intractable p o l y m e r a n d thus not processable. T h e Feast t e c h n i q u e (31) for p r e p a r i n g p o l y a c e t y l e n e involves formation of a soluble p r e c u r s o r v i a r i n g - o p e n i n g metathesis. T h e p r e c u r s o r can b e processed (e.g., stretched to orient the chains) a n d t h e n t h e r m a l l y c o n v e r t e d to polyacetylene. D o p e d p o l y p y r r o l e can b e p r o d u c e d i n a continuous process to give films that appear to b e r e l a t i v e l y stable. T h e c o n d u c t i v i t y of these materials is h i g h e r than that of plastics e x t e n d e d w i t h c o n d u c t i v e fillers. T h i s characteristic suggests possible a p p l i c a t i o n as an electromagnetic interference s h i e l d . A major goal of the research o n c o n d u c t i n g p o l y m e r s has b e e n the d e v e l o p m e n t of a rechargeable plastic battery. C e l l s based o n p o l y p y r r o l e a n d l i t h i u m electrodes have b e e n d e v e l o p e d i n w h i c h the energy p e r u n i t mass a n d discharge characteristics are comparable to n i c k e l - c a d m i u m cells. C u r r e n t interest appears to c e n t e r a r o u n d stable, processable p o l y m e r s , s u c h as p o l y t h i o p h e n e a n d its derivatives, a n d p o l y a n i l i n e .


38



1.1.3 Polymers for Molecular Electronics O n e o f the b u z z w o r d s i n the w o r l d of science today is the t e r m " m o l e c u l a r e l e c t r o n i c s " . O n e has o n l y to o p e n the pages o f the p o p u l a r scientific l i t erature to find s u c h headlines as " A n d N o w — t h e B i o c h i p " , or " T h e O r g a n i c C o m p u t e r " . T h e p r e m i s e b e h i n d these articles is the i d e a that a single m o l e c u l e m i g h t f u n c t i o n as a self-contained electronic d e v i c e ; t h u s , the p o s s i b i l i t y arises to d e v e l o p a c o m p u t e r based o n m o l e c u l a r - s i z e d e l e c t r o n i c e l e m e n t s . T h i s p o s s i b i l i t y was first discussed b y C a r t e r i n 1979 a n d was the subject o f t w o i n t e r n a t i o n a l workshops i n 1981 a n d 1983 (32). T h e genesis o f these ideas m a y b e seen i n the relentless progress o f the s e m i c o n d u c t i n g i n d u s t r y i n d i m i n i s h i n g the size o f c i r c u i t features o f m i c r o e l e c t r o n i c devices. C i r c u i t densities have d o u b l e d v i r t u a l l y e v e r y year since the i n v e n t i o n o f the I C i n 1960, p r i m a r i l y t h r o u g h s h r i n k i n g the size of the i n d i v i d u a l c i r c u i t e l e m e n t s . M i n i m u m feature sizes i n today's devices are o n the o r d e r of 1 |xm, a n d 0.5-|xm-sized devices are l i k e l y b y the early 1990s. A s i m p l e l i n e a r extrapolation of the feature size vs. t i m e p l o t ( F i g u r e 1.20) leads to the c o n c l u s i o n that w i t h i n 30 years o r so, the size of electronic components w i l l b e o n the o r d e r o f nanometers (i.e., the size of i n d i v i d u a l molecules).

Figure 1.20. Microlithographic trends in minimum device feature size and imaging technology. The figure illustrates the time evolution of random access memory (RAM) devices.


1.

BOWDEN


Applications

39

1.1.3.1 D E V I C E F A B R I C A T I O N C O N S I D E R A T I O N S


C u r r e n t I C fabrication technology, w h i c h involves s c u l p t u r i n g m i c r o s t r u c tures from single crystalline blocks o f s i l i c o n , is not l i k e l y to fabricate devices at m o l e c u l a r d i m e n s i o n s . M o l e c u l a r electronics, o n the other h a n d , offers the p o s s i b i l i t y o f b u i l d i n g u p c o m p l e x structures b y u s i n g atomic a n d m o lecular forces. D e v i c e s w i l l b e c o n s t r u c t e d b y assembly o f i n d i v i d u a l m o lecular e l e c t r o n i c c o m p o n e n t s i n t o arrays, t h e r e b y e n g i n e e r i n g from s m a l l to large rather t h a n from large to s m a l l as d o c u r r e n t l i t h o g r a p h i c t e c h n i q u e s . A t o u r c u r r e n t l e v e l of technology, m a n y o f these concepts are still i n the r e a l m o f science fiction. Ideas have b e e n suggested for devices such as m o l e c u l a r switches, b u t these devices still await e x p e r i m e n t a l r e a l i z a t i o n and verification. A m o l e c u l a r s w i t c h , for e x a m p l e , m u s t possess a v a r i e t y o f properties for it to b e classified as operational (33): • It m u s t possess b i s t a b i l i t y (i.e., capable of existing i n two or m o r e stable states). • T h e s w i t c h i n g process m u s t b e controllable (i.e., i t m u s t b e possible to u n a m b i g u o u s l y set the state o f the switch). • T h e state o f the s w i t c h m u s t b e readable (i.e., it m u s t b e possible to u n a m b i g u o u s l y sense w h e t h e r the s w i t c h is o n o r off). • T h e two p r e c e d i n g functions m u s t b e executable at the m o lecular l e v e l w i t h full addressability. T h a t is, it m u s t b e possible to selectively s w i t c h o r address an i n d i v i d u a l m o l e c u l e . T h u s , whereas m a n y systems have b e e n suggested that e x h i b i t o p t i c a l bistability, they have yet to b e fabricated i n t o an operational s w i t c h . T h e same a r g u m e n t c o u l d b e made for design o f m o l e c u l a r i n t e r c o n nects. Proposals have b e e n m a d e for m o l e c u l a r " w i r e s " c o n s t r u c t e d from c o n d u c t i n g p o l y m e r s . B u t h o w w o u l d one isolate an i n d i v i d u a l m o l e c u l a r strand, m u c h less attach it to a m o l e c u l a r switch? W o u l d s u c h a o n e - d i m e n sional w i r e e x h i b i t c o n d u c t i o n properties a k i n to b u l k c o n d u c t i o n ? T h e s e questions have yet to b e a n s w e r e d . I n a d d i t i o n , the subject o f assembly looms as an a p p a r e n t l y i n s u r m o u n t a b l e obstacle. It is not at a l l clear h o w researchers w o u l d go about a s s e m b l i n g i n d i v i d u a l m o l e c u l a r components into a f u n c t i o n i n g d e v i c e , a l t h o u g h b i o e n g i n e e r i n g offers a p o t e n t i a l solution to this p r o b l e m . E x a m p l e s of self-assembled structures exist e v e r y w h e r e i n nature from the h e l i c a l secondary structure o f D N A to the h u m a n b r a i n . C u r r e n t k n o w l e d g e of s u c h systems is s i m p l y inadequate to a l l o w scientists to e m p l o y s i m i l a r forces to create synthetic m o l e c u l a r e l e c t r o n i c devices. C l e a r l y , an e n o r m o u s a m o u n t of g r o u n d w o r k needs to b e l a i d i f the concept o f the m o l e c u l a r e l e c t r o n i c device or b i o c h i p c o m p u t e r is e v e r to b e c o m e a reality.



40


T h e field of m o l e c u l a r electronics m a y b e c o n s i d e r e d to encompass m u c h m o r e t h a n m o l e c u l a r electronic devices. I n its broadest context, m o l e c u l a r electronics m a y be r e g a r d e d as s i m p l y the a p p l i c a t i o n of m o l e c u l e s , p r i m a r i l y organic m o l e c u l e s , to electronics. T h i s d e f i n i t i o n w o u l d i n c l u d e such areas as l i q u i d c r y s t a l l i n e materials, p i e z o e l e c t r i c materials such as p o l y ( v i n y l i d i n e fluoride), c h e m i c a l l y sensitive field-effect transistors ( C H E M F E T ) , a n d the w h o l e range o f electroactive p o l y m e r s . T h e s e applications are b e y o n d the scope o f this book a n d are c o v e r e d i n o t h e r reviews (34, 35). H o w e v e r , g i v e n the basic tenet o f m o l e c u l a r electronics, n a m e l y , the ability to e n g i n e e r a n d assemble m o l e c u l a r structures i n t o a useful d e v i c e , the broader d e f i n i t i o n raises the q u e s t i o n of w h e t h e r organic molecules can be specifically assemb l e d or e n g i n e e r e d for u n i q u e applications i n electronics. 1.1.3.2 L A N G M U I R - B L O D G E T T

FILMS

T h e L a n g m u i r - B l o d g e t t ( L B ) t e c h n i q u e is one of the few methods available for m a n i p u l a t i n g the architecture of an assembly of organic molecules (36). It m a y b e r e g a r d e d as the organic analog of m o l e c u l a r b e a m epitaxy, w h i c h is e x t r e m e l y i m p o r t a n t to solid-state electronics as a means of fabricating precise microstructures w i t h n o v e l charge-transport properties. T h e L B t e c h n i q u e offers the means to construct s i m i l a r organic analogs b y b u i l d i n g u p organic layers one m o n o l a y e r at a t i m e a n d enables precise geometries (e.g., m o l e c u l a r o r i e n t a t i o n a n d thickness) to b e constructed. T h e t e c h n i q u e involves s p r e a d i n g some suitable organic m o l e c u l e onto a w a t e r surface, c o m p r e s s i n g the film to f o r m a compact monolayer, a n d t h e n transferring this layer to a suitable, substrate. T h e processing sequence is d e p i c t e d i n F i g u r e 1.21. V e r y few materials are suitable for L B film formation. T y p i c a l m o n o l a y e r - f o r m i n g molecules possess b o t h h y d r o p h o b i c a n d h y d r o p h i l i c e n d groups such as l o n g a l k y l a n d carboxylic groups, respectively. C e r t a i n p o l y m e r s w i l l also f o r m monolayers as w i l l c e r t a i n aromatic macrocycles s u c h as p o r p h y r i n s a n d phthalocyanines. Phthalocyanines t e n d not to have h i g h l y o l e o p h i l i c o r h y d r o p h i l i c parts, b u t consist of a r o u g h l y c i r c u l a r aromatic disk, often w i t h a m e t a l atom at the c e n t e r a n d w i t h a m i n i m u m of side groups. T h e m a t e r i a l is usually d i s s o l v e d i n an organic solvent a n d carefully spread onto the surface o f w a t e r c o n t a i n e d i n a L a n g m u i r t r o u g h . T h e c o n centration is such that the molecules spread to a d e p t h of one monolayer. I f the surface pressure F is s m a l l , the m o n o l a y e r behaves as a t w o - d i m e n sional gas o b e y i n g the equation FA

= kT

(1.1)

w h e r e A is the area p e r m o l e c u l e , k is B o l t z m a n n ' s constant, a n d T is absolute t e m p e r a t u r e . T h e surface pressure is increased b y c o m p r e s s i n g the film b y means of a s l i d i n g b a r r i e r . E v e n t u a l l y , a p o i n t is r e a c h e d w h e r e a l l the


1.

BOWDEN


SAMPLE j m IITIHM ^SOLUTION

Applications

41

MOLECULES 1_

WATER SPREADING •Of

WATER

kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk

FILM COMPRESSION UUMt

. | rlLM


rw—o—n

(3rd LAYER) Figure 1.21. The LB sequence: spreading, compression, and deposition. (Reproduced with permission from reference 45. Copyright 1987 Academic Press.)

molecules t o u c h , f o r m i n g a " p e r f e c t " pinhole-free monolayer. T h e change i n surface pressure as the area p e r m o l e c u l e is decreased is s h o w n i n F i g u r e 1.22. T h e correct degree o f c o m p r e s s i o n is d e t e r m i n e d b y m o n i t o r i n g t h e surface tension o f the water, w h i c h starts to d r o p w h e n t h e molecules are nearly dense-packed. A t h i g h e r pressures, t h e m o n o l a y e r w i l l b u c k l e a n d collapse. B e l o w t h e collapse pressure, t h e m o n o l a y e r can b e transferred to a suitable substrate b y l o w e r i n g the substrate carefully t h r o u g h the f i l m i n t o the w a t e r a n d slowly w i t h d r a w i n g i t ( F i g u r e 1.21). A l t e r n a t i v e l y , transfer m a y b e effected h o r i z o n t a l l y b y contacting t h e surface w i t h t h e substrate o r i e n t e d h o r i z o n t a l l y to the film surface. T h e s e procedures m a y b e r e p e a t e d successively u n t i l the r e q u i r e d n u m b e r of monolayers is o b t a i n e d . T h e t h i c k ness w i l l thus b e an i n t e g r a l m u l t i p l e of the l e n g t h of the a m p h i p h i l i c species. I n this way, u l t r a t h i n , compact, pinhole-free (at least i n theory) films of constant, w e l l - c o n t r o l l e d thickness can b e p r e p a r e d . T h e films are solids w i t h a l a m e l l a r s t r u c t u r e , a n d b y p r o p e r l y choosing t h e c h e m i c a l c o m p o s i t i o n , t h e y c a n serve a variety o f useful electronic functions. M o l e c u l e s c a n


42


METASTABLE| .COLLAPSE REGION (BUCKLING)

50 £ 40

3 (0 m

CONDENSED • PHASE (NO VOIDS)

30


Q. LU

O 20 QC D 0) 10

EXPANDED •PHASE (VOIDS)

0

GASEOUS PHASE ) LATERAL COHESION)

0.1 AREA PER MOLECULE, nm

Figure 1.22. A typical monolayer

2

isotherm.

b e d e s i g n e d , for e x a m p l e , w i t h unsaturated groups e i t h e r near t h e h y d r o p h o b i c e n d (e.g., v i n y l stearate), near the h y d r o p h i l i c e n d (e.g., tricosenoic acid), o r a r o u n d t h e c e n t e r o f t h e m o l e c u l a r c h a i n as w i t h diacetylenes. T h e s e molecules c a n b e p o l y m e r i z e d w i t h o u t d i s r u p t i o n o f t h e l a m e l l a r structure a n d thus i m p r o v e t h e m e c h a n i c a l a n d t h e r m a l stability o f t h e L B film. O n e p r o p o s e d application o f these films is as resists for m i c r o l i t h o g r a p h y (37), a l t h o u g h t h e complexities o f film d e p o s i t i o n a n d concerns o f p i n h o l e density i n these u l t r a t h i n films make t h e L B approach a d o u b t f u l p r a c t i c a l t e c h n i q u e for s e m i c o n d u c t o r d e v i c e fabrication. L B films have b e e n p r o p o s e d as passive o p t i c a l waveguides (see S e c t i o n 1.2.2), w h e r e t h e y a l l o w closer c o n t r o l o f t h e w a v e g u i d e parameters (both d i m e n s i o n a l a n d optical) than is possible w i t h evaporated layers. O n e p r o b l e m w i t h L B films i n o p t i c a l applications is that they are n o t t r u l y single-crystalline materials. Instead, t h e y consist o f crystallites separated b y d o m a i n b o u n d a r i e s that act as scatt e r i n g centers a n d l i m i t applications i n n o n l i n e a r a n d g u i d e d - w a v e optics. T h e attractiveness o f silicon as a s e m i c o n d u c t o r material for I C s d e rives i n part from the fact that this i m p o r t a n t m a t e r i a l forms a naturally i n s u l a t i n g surface oxide. U s e is m a d e o f this fact, f o r e x a m p l e , i n m e t a l o x i d e - s e m i c o n d u c t o r ( M O S ) field-effect transistors ( F E T ) , w h e r e t h e oxide serves as t h e gate insulator. N o such naturally i n s u l a t i n g oxide occurs w i t h any o f the c o m p o u n d semiconductors that offer i m p r o v e d performance o v e r silicon i n m a n y d e v i c e applications. R o b e r t s et a l . (38) d e m o n s t r a t e d t h e feasibility o f s u c h m e t a l - i n s u l a t o r - s e m i c o n d u c t o r ( M I S ) structures as F E T s and c h e m i c a l sensors s h o w n schematically i n F i g u r e 1.23. T h e s e researchers



Figure 1.23. Cross-sections of FET structures employing LB films as gate insulators. The top diagram illustrates a thin-film FET using an amorphous silicon source and drain structures. The bottom diagram illustrates a chemically sensitive FET (CHEMFET) in which the electrical characteristics of the LB gate insulator are sensitive to specific chemical species in the environment.


44



s h o w e d that b y a p p l y i n g L B films to the surfaces of amorphous S i , G a A s , G a P , I n S b , C d T e , a n d ( H g C d ) T e , charge-density profiles can also be m o d ified. B y u s i n g films of t u n n e l i n g d i m e n s i o n s , charge injection l e a d i n g to e n h a n c e d e l e c t r o l u m i n e s c e n c e efficiency was achieved. T h e efficiency of devices that incorporate layers of q u a n t u m - m e c h a n i c a l t u n n e l i n g d i m e n s i o n s is critically d e p e n d e n t o n the c o n t r o l of the insulator thickness. T h i s d e p e n d e n c y is w h y the L B approach is e m i n e n t l y suitable for fabricating such devices. T h e s e examples of L B film application give o n l y a cursory v i e w of the m a n y structures a n d processes c u r r e n t l y b e i n g investigated. T h i s field has u n d e r g o n e a m a r k e d resurgence o v e r the last decade as scientists have b e g u n to realize that functional devices can b e constructed b y a m o l e c u l a r e n g i n e e r i n g approach, although as yet, such devices generally r e m a i n an exp e r i m e n t a l curiosity. T h e u l t i m a t e objective, n a m e l y , that of u t i l i z i n g i n d i v i d u a l m o l e c u l e s as functional elements a n d the assembly of such e l e ments into a d e v i c e , is as elusive as ever. N e v e r t h e l e s s , s h o u l d a b r e a k t h r o u g h be a c h i e v e d , it promises to be every b i t as r e v o l u t i o n a r y as was the transistor.

1.2 Polymers for Photonic Applications T h e microelectronics r e v o l u t i o n , w h i c h began w i t h the i n v e n t i o n of the transistor i n 1948 a n d r a p i d l y accelerated f o l l o w i n g the i n v e n t i o n of the I C i n 1960, has h a d a p r o f o u n d influence o n m o d e r n society, p a r t i c u l a r l y i n the processing a n d transport of i n f o r m a t i o n . T h e heart of today's t e l e c o m m u n ications n e t w o r k , for example, is the h i g h l y sophisticated electronic s w i t c h i n g m a c h i n e , w h i c h offers a w i d e variety of service options that cannot b e p r o v i d e d b y e l e c t r o m e c h a n i c a l switches. V o i c e a n d data are s w i t c h e d o v e r a n e t w o r k that ranges from t w i s t e d c o p p e r pairs to m i c r o w a v e c o m m u n i c a t i o n towers. H o w e v e r , two inventions d u r i n g the 1960s have b e g u n to radically change the w a y i n w h i c h information is t r a n s m i t t e d a n d m a y , i n the future, l e a d to a totally n e w s w i t c h i n g technology. T h o s e two inventions are the laser a n d the o p t i c a l fiber, w h i c h together have spawned the photonics r e v o l u t i o n u p o n whose d a w n w e are n o w e n t e r i n g . S i m p l y speaking, photons are to photonics what electrons are to e l e c tronics. I n electronic transmission, i n f o r m a t i o n , e n c o d e d i n the f o r m of d i g i t a l or analog electronic signals, is r o u t e d along a c o n d u c t o r p a t h s u c h as a c o p p e r w i r e . I n an optical transmission system, information is t r a n s m i t t e d i n d i g i t a l format as pulses of l i g h t along glass fibers specifically d e s i g n e d to confine the l i g h t to the i n t e r i o r of the fiber. Lasers a n d l i g h t - e m i t t i n g diodes are i d e a l l i g h t sources for optical transmission. T h e s e solid-state devices can be t u r n e d o n a n d off b i l l i o n s of times p e r second, p r o v i d i n g an i d e a l m e c h a n i s m for c o n v e y i n g digital information w h e r e the presence of l i g h t c o r r e sponds to a " 1 " a n d its absence signifies a " 0 " . T h e d r i v i n g force b e h i n d


1.

BOWDEN


45

Applications

optical transmission is the enormous capacity, o r b a n d w i d t h , available. A laser p u l s e d at 540 megabits p e r second is e q u i v a l e n t to 24,000

telephone

conversations that can b e t r a n s m i t t e d simultaneously over a single h a i r - t h i n optical fiber. N o t o n l y does o p t i c a l transmission offer the p o t e n t i a l o f essentially u n l i m i t e d b a n d w i d t h , b u t i t is also free f r o m interference effects that plague e l e c t r o n i c transmission a n d offers enormous cost savings i n space a n d materials. A s w i l l b e s h o w n i n the f o l l o w i n g discussion, p o l y m e r s play a n e n o r m o u s l y i m p o r t a n t role i n this r a p i d l y e x p a n d i n g technology.


1.2.1 Optical Fiber Coatings T h e glass fibers u s e d i n l i g h t - w a v e telecommunications systems m u s t b e p r o t e c t e d from m e c h a n i c a l abrasion t o preserve t h e i r strength. F l a w s i n the glass surface as s m a l l as 1.0 u-m w i l l r e d u c e the tensile strength o f the fiber from over 1 0 k p s i to less t h a n 1 0 k p s i , a n d l e a d to b r e a k i n g o f the fiber d u r i n g c a b l i n g . A c c o r d i n g l y , o p t i c a l fibers m u s t b e treated w i t h a p r o t e c t i v e coating i m m e d i a t e l y after b e i n g d r a w n i n a furnace. A schematic d i a g r a m o f the process is s h o w n i n F i g u r e 1.24. T h e fiber is d r a w n from a glass p r e f o r m that is f e d at a c o n t r o l l e d rate into the t o p o f a t u b u l a r h i g h - t e m p e r a t u r e (2200 °C) furnace. A fiber d i a m e t e r m o n i t o r provides a n e l e c t r o n i c signal that can b e u s e d to adjust the speed o f a capstan d r i v e , w h i c h p u l l s the fiber at the base o f the m a c h i n e . E q u i p m e n t for coating the fiber a n d s o l i d i f y i n g it is located b e t w e e n t h e d i a m e t e r m o n i t o r a n d t h e capstan. T h i s i n - l i n e coating p r o c e d u r e enables the coating to b e a p p l i e d a n d h a r d e n e d before 6

5

PREFORM FEED MECHANISM ZIRCONIA INDUCTION FURNACE

COATINGAPPLICATOR

. CURING FURNACE OR LAMPS

COATING CONCENTRICITY MONITOR Figure 1.24. A fiber-drawing apparatus. (Reproduced with permission reference 39.)


from

46


the fiber contacts any surface. F o r this reason, the most successful coating techniques i n v o l v e the application of a l o w viscosity ( 1 0 - 5 0 poise) o l i g o m e r i c l i q u i d to the fiber f o l l o w e d b y r a p i d solidification. T h r e e coating processes have b e e n d e v e l o p e d to m e e t this r e q u i r e m e n t of r a p i d solidification. T h e s e processes use 1. U V - r a d i a t i o n - c u r a b l e p o l y m e r formulations, 2. t h e r m a l l y cross-linkable p r e p o l y m e r s capable of b e i n g forcec u r e d at h i g h rates (e.g., silicones), o r


3. h o t - m e l t thermoplastics that can be solidified b y r a p i d c o o l i n g (39). I n a d d i t i o n to p r o v i d i n g m e c h a n i c a l a n d e n v i r o n m e n t a l p r o t e c t i o n to the optical fiber, the p o l y m e r coating s h o u l d reduce m i c r o b e n d i n g losses. A s a result of t h e i r v e r y s m a l l diameters, t y p i c a l l y 1 0 0 - 1 5 0 p-m, o p t i c a l fibers b e n d r e a d i l y . T h i s feature is advantageous i n that the transmission m e d i u m is flexible a n d easily r o u t e d . H o w e v e r , w h e n the spatial p e r i o d of the b e n d i n g becomes v e r y s m a l l , some of the light rays n o r m a l l y g u i d e d b y the fiber a r e lost t h r o u g h radiation. S u c h s m a l l - p e r i o d distortions may occur w h e n a fiber is w o u n d o n a spool u n d e r tension, or w h e n it is p l a c e d i n a cable structure. T h e p h e n o m e n o n is c a l l e d microbending loss, w h i c h i n extreme cases can result i n transmission losses a m o u n t i n g to several decibels p e r k i l o m e t e r , t h e r e b y seriously d e g r a d i n g the performance of an optical fiber transmission l i n k . T h i s p r o b l e m can b e m i n i m i z e d t h r o u g h use of l o w - m o d u l u s coatings. T h e p r e f e r r e d coating design involves a dual-coated fiber consisting o f a soft, l o w - m o d u l u s p r i m a r y coating (to m i n i m i z e m i c r o b e n d i n g losses) a n d a h a r d , h i g h - m o d u l u s secondary coating (for m e c h a n i c a l protection) ( F i g u r e 1.25). T h e second coating can be a p p l i e d e i t h e r i n - l i n e or off-line. T h e r m a l l y

Chief Advantage

Chief Advantage

HIGH STRENGTH

HIGH MICROBENDING RESISTANCE

SINGLE COATING

DUAL COATING

Figure 1.25. Protecting

glass optical fibers by employing coatings.

single and dual


1.

BOWDEN


and Photonic

Applications

47


curable silicones a n d U V - c u r a b l e epoxy acrylates are u s e d c o m m e r c i a l l y for the soft p r i m a r y coating. H i g h - m o d u l u s U V - c u r a b l e epoxy acrylates have b e e n u s e d for the h a r d secondary coating as have extrudable thermoplastics such as n y l o n . P o l y m e r s have b r o a d a p p l i c a t i o n i n c a b l i n g , sheathing, a n d c o n n e c t i n g operations. C a b l i n g of o p t i c a l fibers is q u i t e different from c a b l i n g of c o p p e r conductors. T h e coated fibers are usually assembled o r packaged p r i o r to sheathing. A t y p i c a l fiber-packaging structure u s e d i n cable designs is s h o w n i n F i g u r e 1.26. I n this s t r u c t u r e , k n o w n as a r i b b o n , 12 fibers are packaged i n a l i n e a r array b y s a n d w i c h i n g t h e m b e t w e e n two adhesive-coated polyester tapes. T h e r i b b o n s are t h e n g r o u p e d along w i t h m e t a l l i c strength m e m b e r s a n d enclosed i n a p o l y m e r i c sheath i n a final extrusion process. D e p l o y m e n t o f o p t i c a l fibers i n the field r e q u i r e s the fibers to be s p l i c e d (joined) at various j u n c t i o n points. F u s i o n s p l i c i n g is the most r e l i a b l e m e t h o d for p e r m a n e n t l y j o i n i n g o p t i c a l fibers a n d has b e e n successfully a p p l i e d i n the field. H o w e v e r , the fusion splice needs a protective package because the s p l i c e d section has no p r o t e c t i v e coating a n d its strength is w e a k e n e d b y the s p l i c i n g operation. O n e t y p e of packaging system, s h o w n i n F i g u r e 1.27, consists of a h e a t - s h r i n k a b l e tube of c r o s s - l i n k e d p o l y e t h y l e n e , a h o t m e l t adhesive i n n e r t u b e , a n d a strength m e m b e r r o d of stainless steel. C o n n e c t o r s that operate b y p h y s i c a l a l i g n m e n t have also b e e n d e s i g n e d . A t y p i c a l e x a m p l e is s h o w n i n F i g u r e 1.28. T h e e x t r e m e l y n a r r o w tolerances r e q u i r e d d e m a n d h i g h - d i m e n s i o n a l stability a n d p r e c i s i o n . A s a conseq u e n c e , most connectors are m a d e of metals a n d ceramics b y p r e c i s i o n m a c h i n i n g techniques. Plastic connectors are also available b u t are b e t t e r suited to c o n n e c t i o n of m u l t i m o d e fibers, w h e r e the larger core imposes less severe a l i g n m e n t tolerances t h a n r e q u i r e d for single-mode fibers. O n e a p p l i c a t i o n that has not b e e n m e n t i o n e d is the use o f p o l y m e r s for the o p t i c a l core i n place of silica. A t t e n u a t i o n losses essentially p r e c l u d e such applications i n l o n g - h a u l transmission systems. F o r e x a m p l e , p o l y ( m e t h y l methacrylate) has b e e n s t u d i e d as a p o t e n t i a l plastic o p t i c a l fiber, b u t i n t r i n s i c absorption losses d u e to overtones of the C - H v i b r a t i o n l i m i t attenuation to 55 d B / k m at 570 n m (40). T h e losses caused b y overtone a b sorptions can b e e l i m i n a t e d b y d e u t e r a t i n g the p o l y m e r . T h e overtone absorptions associated w i t h C - D vibrations (where D is d e u t e r i u m ) occur at longer wavelengths a n d are of l o w e r intensity t h a n C - H overtones ( F i g u r e 1.29). A t t e n u a t i o n losses of 20 d B / k m at 65Q-680 n m have b e e n r e p o r t e d for p e r d e u t e r a t e d p o l y ( m e t h y l methacrylate), b u t this value is still o v e r 2 orders of m a g n i t u d e h i g h e r t h a n that o f silica fibers, for w h i c h attenuations of 0.2 d B / k m at 1300 n m have b e e n r e p o r t e d . T h e theoretical m i n i m u m attenuation for plastic fibers is b e l i e v e d to b e 9 d B / k m at 680 n m . T h i s m i n i m u m makes such materials c l e a r l y unacceptable for transmission o v e r l o n g distances. Plastic o p t i c a l fibers m a y , h o w e v e r , find a p p l i c a t i o n i n backplane interconnections, for e x a m p l e , w h e r e transmission distances m a y b e o n the o r d e r of inches.



Figure 1.26. Section o/ a 12-fiber ribbon. (Reproduced with permission from reference 39.)


M

O

3 o •z c/i o

•A

>

2

i

k

R°

g o

i

m r w o

00

1.

BOWDEN

Polymers for Electronic and Photonic Applications

49

HEAT-SHRINKABLE TUBING ADHESIVE


RESISTANCE ROD (a) BEFORE HEATING Figure 1.27. A heat-shrinkahle tubing package for protecting spliced fibers. Shrinking is accomplished by means of an internally heated resistance rod. (Reproduced with permission from reference 40.)

(b) AFTER HEATING

PLUG

ADAPTOR Figure 1.28. A precision-molded plastic connector. (Reproduced with permission from reference 40.)

E m CO CO

4vCH

o

P(MMA-d)

500

600 700 800 WAVELENGTH (nm)

900

Figure 1.29. Transmission spectra of PMMA and perdeuterated PMMA core plastic optical fibers. (Reproduced with permission from reference 40.)


50



1.2.2 Polymers for Integrated Optics I n l o n g - h a u l t r a n s m i s s i o n , the l i g h t i n t e n s i t y decreases w i t h distance a n d must b e regenerated (amplified) at intervals d e t e r m i n e d b y the i n t r i n s i c loss i n the fiber. T h i s regenerative process is a c c o m p l i s h e d electronically. T h e light pulses r e c e i v e d at the e n d o f an optical fiber are c o n v e r t e d to e l e c t r o n i c signals b y a p h o t o d e t e c t o r (photodiode). T h e s e signals are t h e n a m p l i f i e d a n d u s e d to m o d u l a t e the o u t p u t of a laser that forms part of the repeater c i r c u i t . A n e w set o f pulses are therefore generated a n d l a u n c h e d i n t o the fiber. It w o u l d be advantageous to p e r f o r m s u c h functions p h o t o n i c a l l y rather than going to the t r o u b l e of c o n v e r t i n g from photonics to electronics a n d back to photonics. L i k e w i s e , p h o t o n i c s w i t c h i n g has the p o t e n t i a l to be m u c h faster t h a n electronic s w i t c h i n g (with r e s u l t i n g h i g h e r bandwidths) a n d carries w i t h it the p o s s i b i l i t y of p a r a l l e l processing. A l t h o u g h the i m p l e m e n t a t i o n of a n all-optical n e t w o r k lies w e l l i n the future, m u c h o f the technological framework is b e i n g d e v e l o p e d . T h e attendant passive c o m p o n e n t s associated w i t h m u l t i m o d e fibers (e.g., couplers, p o w e r d i v i d e r s , w a v e l e n g t h filters, a n d connectors) m a y be constructed b y u s i n g m i n i a t u r e versions o f familiar b u l k optical c o m p o n e n t s s u c h as lenses, p r i s m s , a n d diffraction gratings. F u r t h e r scaling d o w n of e q u i v a l e n t m u l t i m o d e c o m p o n e n t s for m o n o m o d e applications is not usually practical o w i n g to stringent a l i g n m e n t tolerances b e t w e e n fiber a n d c o m ponent. O n e approach to solve this d i l e m m a is to integrate s u c h devices o n a single c h i p . T h e basic m o t i v a t i o n of this field of integrated optics is to do for o p t i c a l circuits w h a t integrated electronics has d o n e for e l e c t r i c a l c i r c u i t s , n a m e l y , replace a set of large, i n d i v i d u a l l y fabricated elements w i t h i n t e grated, m i n i a t u r i z e d c i r c u i t elements that are i n t e r c o n n e c t e d o n a single chip. A major c o n c e p t of i n t e g r a t e d optics is that i n a l l of the elements a n d interconnections, the o p t i c a l signals are confined i n compact o p t i c a l w a v e guides, a n d thus the field is often r e f e r r e d to as guided-wave optics, a n d the devices are r e f e r r e d to as p h o t o n i c circuits. T h e fabrication o f s u c h circuits again involves l i t h o g r a p h i c processes i n w h i c h p o l y m e r i c resists are u s e d for the i m a g i n g steps. R e l a t i v e l y c o m p l e x assemblies of devices have b e e n d e m o n s t r a t e d (e.g., a n 8 X 8 m a t r i x s w i t c h i n t e g r a t i n g 64 2 X 2 i n t e r c o n n e c t e d s w i t c h e l e m e n t s o n a single chip) (41). T h e c o n s t r u c t i o n o f a n o p t i c a l w a v e g u i d e i n the surface of a substrate r e q u i r e s alteration o f the refractive i n d e x of the substrate along the l e n g t h of the g u i d e . B y i n c r e a s i n g the refractive i n d e x o f the r e g i o n , l i g h t can be g u i d e d o r c h a n n e l e d i n the same w a y as takes place i n the core of a n o p t i c a l fiber. T h e s e guides are t h e n u s e d b o t h to f o r m the various c i r c u i t elements (both active a n d passive) a n d to m a k e interconnections b e t w e e n e l e m e n t s . T h e basic concepts are i l l u s t r a t e d i n F i g u r e 1.30, w h i c h is a schematic d r a w i n g o f a n integrated, o p t i c a l , f o u r - c h a n n e l , w a v e l e n g t h - d i v i s i o n m u l t i plexer. T h e waveguides have b e e n constructed s u c h that l i g h t of a p a r t i c u l a r



1.

BOWDEN

Figure

1.30.


An

integrated,

and Photonic

Applications

optical, four-channel, multiplexer.

51

wavelength-division

w a v e l e n g t h e n t e r i n g the i n t e r a c t i o n r e g i o n of the first two guides w i l l transfer to the second g u i d e , b u t l i g h t at other wavelengths w i l l r e m a i n i n the first guide. F o r waveguides b u i l t i n t o substrates e x h i b i t i n g a large electrooptic effect (e.g., l i t h i u m niobate), one can use an e l e c t r i c field to c o n t r o l the interaction w h e r e i n the w a v e l e n g t h selected b y the c o u p l e r can b e c o n t r o l l e d b y the voltage a p p l i e d to the electrodes. A s w i t h e l e c t r o n i c I C s , the fabrication of optical waveguides is p r e c e d e d b y the d e s i g n a n d c o n s t r u c t i o n o f a mask o u t l i n i n g the p a t h l i g h t w i l l take i n the g u i d i n g layer of the structure. L i t h i u m niobate is the favored d i e l e c t r i c m a t e r i a l for use i n integrated optics. T h i s m a t e r i a l can b e r o u t i n e l y f o r m e d to give large crystals of o p t i c a l q u a l i t y a n d exhibits a reasonably large electrooptic figure-of-merit (i.e., change of refractive i n d e x w i t h change of a p p l i e d field strength). O n c e the p a t h has b e e n d e f i n e d b y l i t h o g r a p h i c t e c h n i q u e s , the waveguides are fabricated b y d e p o s i t i n g a t h i n layer of t i t a n i u m o n the exposed surface, r e m o v i n g the r e m a i n i n g photoresist, a n d finally diffusing the m e t a l i n t o the crystal at temperatures o n the o r d e r of 1000 °C for 6 h . G u i d e s can also b e made b y an ion-exchange process i n w h i c h L i ions i n the substrate are r e p l a c e d b y H ions. I n this process, the surface of the crystal is c o v e r e d b y a g o l d film, except w h e r e one wants to f o r m guides. T h e crystal is t h e n p l a c e d i n an a c i d b a t h for a p e r i o d of +

+



52


t i m e (e.g., b e n z o i c a c i d at 250 °C for a few hours), d u r i n g w h i c h i o n exchange takes place t h r o u g h the openings i n the gold mask. P o l y m e r s are also w e l l s u i t e d for w a v e g u i d e applications. T o t a l b u l k optical losses i n the r e d a n d n e a r - I R spectral regions are t y p i c a l l y < 1 d B / c m . F o r m a n y p o l y m e r s , losses are < 0 . 1 d B / c m . M o s t organic materials have refractive indices i n the range 1 . 4 - 1 . 7 , a n d a n u m b e r of s i m p l e fabrication techniques such as e m b o s s i n g a n d casting a n d p h o t o l o c k i n g have b e e n s h o w n to be effective i n p r o d u c i n g regions of different refractive i n d e x separated b y e x t r e m e l y smooth surfaces (42). I n the case of t h r e e - d i m e n s i o n a l waveguides, the refractive i n d e x differential r e q u i r e d b e t w e e n the g u i d e and its surroundings depends on the sharpest b e n d that m u s t be traversed b y the g u i d e . A 1% i n c r e m e n t is sufficient for most purposes. Refractive i n d e x differentials of this m a g n i t u d e can be generated v i a p h o t o c h e m i c a l l y e n h a n c e d processes. A l t h o u g h changes i n optical p o l a r i z a b i l i t y r e s u l t i n g from p h o t o c h e m i c a l l y i n d u c e d processes i n v o l v i n g b o n d b r e a k i n g result i n m u c h smaller changes i n refractive i n d e x than the 1% n e e d e d for w a v e g u i d e fabrication, advantage can be taken of such reactions to alter the c h e m i c a l c o m p o s i t i o n or density b y subsequent c h e m i s t r y to achieve large refractive i n d e x differences. T h e p h o t o l o c k i n g t e c h n i q u e , shown schematically i n F i g u r e 1.31, has b e e n w i d e l y u s e d to fabricate waveguides. A film consisting of a m a t r i x r e s i n [e.g., polycarbonate (n = 1.59)], volatile m o n o m e l i c dopant, a n d p h o t o s e n -

Figure 1.31. The photolocking process: (a) Initial film, dots represent the dopant, (b) Exposure induces reaction of the dopant illustrated here as dimerization. (c) Heating the film evaporates the unreacted dopant. (Reproduced with permission from reference 42. Copyright 1974 American Institute of Physics.)


1.

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and Photonic

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sitizer is i r r a d i a t e d t h r o u g h a mask w i t h U V l i g h t , w h i c h p o l y m e r i z e s the m o n o m e r i n the i r r a d i a t e d r e g i o n , t h e r e b y effectively i m m o b i l i z i n g i t . T h e m o n o m e r i c dopant is t h e n r e m o v e d from the u n i r r a d i a t e d portions of the film b y evaporation i n a v a c u u m . B y selecting dopants w i t h h i g h e r refractive i n d e x than the m a t r i x m a t e r i a l , the p h o t o l o c k e d region w i l l have a h i g h e r refractive index t h a n the s u r r o u n d i n g matrix. A l t e r n a t i v e l y , u s i n g dopants of l o w e r refractive i n d e x such as m e t h y l acrylate (n = 1.48) results i n regions of l o w e r refractive i n d e x . A d d i t i o n a l m o n o m e r m a y t h e n b e i n t r o d u c e d at the o u t e r edges to p r o d u c e a n enclosed w a v e g u i d e structure ( F i g u r e 1.32).


1.2.3 Polymers for Nonlinear Optics A l l of the applications i n v o l v i n g waveguides discussed i n the p r e v i o u s section may be c o n s i d e r e d " p a s s i v e " . T h e p o l y m e r serves some s t r u c t u r a l , p r o t e c tive, or g u i d i n g f u n c t i o n b u t is not i n t e g r a l to the f u n c t i o n i n g of a d e v i c e . A n u m b e r of p h o t o n i c d e v i c e applications are available, h o w e v e r , w h e r e p o l y m e r s m a y b e useful as active elements. T h e s e applications r e q u i r e some type of n o n l i n e a r o p t i c a l response w h e n the m a t e r i a l is i r r a d i a t e d w i t h l i g h t of v e r y h i g h i n t e n s i t y , usually f r o m a laser. Nonlinear optics is c o n c e r n e d w i t h the interactions of electromagnetic fields w i t h materials to p r o d u c e n e w fields a l t e r e d i n phase, frequency, a m p l i t u d e , or other propagation characteristics f r o m the i n c i d e n t fields. A n example of such an effect is second h a r m o n i c generation ( S H G ) . W h e n c e r t a i n crystalline materials are i r r a d i a t e d w i t h l i g h t of frequency co, t h e r e is g e n erated a second b e a m of frequency 2co that emerges from the crystal at an angle different f r o m that of the e m e r g i n g p r i m a r y b e a m . T h e efficiency of conversion is d e t e r m i n e d b y the optical properties of the m a t e r i a l . O t h e r examples i n c l u d e t h i r d h a r m o n i c generation, optical b i s t a b i l i t y , degenerate four-wave m i x i n g , a n d phase conjugation. T h e o r i g i n of these effects lies i n the polarization P i n d u c e d i n a m o l e c u l e b y a local electric field E (due to the electromagnetic radiation). P is expressed as a p o w e r series of the local field: P = aE

+ P£

2

+ Ê + . . . . 3

(1.2)

T h e coefficients a , (3, a n d y are the second, t h i r d , a n d f o u r t h rank tensors a n d are r e f e r r e d to as the p o l a r i z a b i l i t y , first h y p e r p o l a r i z a b i l i t y , a n d second h y p e r p o l a r i z a b i l i t y , respectively. T h e h y p e r p o l a r i z a b i l i t y terms are r e s p o n sible for the n o n l i n e a r response of the m o l e c u l e to i m p i n g i n g radiation. T h e s e coefficients are not v e r y large, a n d the associated n o n l i n e a r optical effects are usually s t u d i e d b y taking advantage of the h i g h optical field obtainable w i t h laser beams. A n u n d e r s t a n d i n g of these effects can b e s i m p l i f i e d b y a s s u m i n g that polarization is a scalar q u a n t i t y (as o p p o s e d to a vector). F o l l o w i n g the approach of W i l l i a m s (43), w h e n electromagnetic radiation interacts w i t h a




54

Figure 1.32. Formation of an optical waveguide obtained by using the photolocking technique. (Reproduced with permission from reference 40.)



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and Photonic Applications

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m o l e c u l e o r m e d i u m consisting of m a n y molecules, the field polarizes the molecules w h i c h i n t u r n act as oscillating dipoles radiating electromagnetic waves i n a l l directions. I n a n o n l i n e a r m e d i u m , P is a n o n l i n e a r function of the a p p l i e d field. T h i s c o n c e p t is i l l u s t r a t e d i n F i g u r e 1.33, w h e r e P exhibits an a s y m m e t r i c response to the a p p l i e d field E. A m e d i u m e x h i b i t i n g s u c h a response m i g h t be envisaged as a crystal c o m p o s e d of molecules w i t h a s y m m e t r i c charge d i s t r i b u t i o n arranged i n such a way that a polar o r i e n tation is m a i n t a i n e d throughout the crystal. S u c h a crystal is said to b e noncentrosymmetric. M o l e c u l e s e x e m p l i f y i n g n o n c e n t r o s y m m e t r i c c r y s t a l l i n e orientations i n c l u d e m - a n d p - n i t r o a n i l i n e a n d various substituted d e rivatives. S u c h molecules are m o r e easily p o l a r i z e d i n the d i r e c t i o n f r o m the e l e c t r o n - r i c h substituent (donor) to the electron-deficient substituent A (acceptor) (i.e., p o l a r i z a t i o n is m o r e effective i n one d i r e c t i o n than the other). T h i s c o n s e q u e n c e is reflected i n the a s y m m e t r i c response o f P i n F i g u r e 1.33. S u c h a n o n s i n u s o i d a l response can be b r o k e n d o w n i n t o its F o u r i e r c o m p o n e n t s consisting o f the f u n d a m e n t a l frequency a> a n d an appropriate s u m m a t i o n of the e v e n harmonics (i.e., 2

Polymers for Electronic and Photonic Applications - American

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