Electronic and Photonic Applications of Polymers - American

2 Organic Resist Materials C . Grant Willson and M u r r a e J. Bowden

Downloaded by TUFTS UNIV on November 22, 2016 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch002

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1IBM Almaden Research Center, San Jose, CA 95120 B e l l Communications Research, R e d Bank, N J 07701

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Recent developments in applications of polymers as resists in microlithography are reviewed. Emphasis is placed on the advances in materials and processes associated with current photolithographic resists designed to extend the utility of photolithography into the submicrometer regime and to ensure continued dominance of photolithographic technology in commercial manufacture of integrated circuits.

O R G A N I C I M A G I N G M A T E R I A L S have u n d e r g o n e a p e r i o d of r a p i d change i n the past decade. A d v a n c e s i n organic a n d p o l y m e r c h e m i s t r y a n d i n innovative processing techniques have l e d to r e m a r k a b l e i m p r o v e m e n t s i n the resist materials a n d processes available for m i c r o l i t h o g r a p h i c a p p l i c a tions. T h i s area of research is e x p a n d i n g at an e v e r - i n c r e a s i n g pace a n d has b e c o m e the focus of groups w o r k i n g i n academic laboratories as w e l l as those of the microelectronics a n d c h e m i c a l industries throughout the w o r l d . S e v eral excellent reviews of this topic have b e e n p u b l i s h e d (1-3). T h e goal of this chapter is to h i g h l i g h t i m p o r t a n t d e v e l o p m e n t s that have o c c u r r e d i n the resist area i n recent years a n d to p r o v i d e some insight i n t o the trends of c u r r e n t research activities.

2.1 Diazoquinone-Novolac Resists D u r i n g the early years of the microelectronics i n d u s t r y , the i m a g i n g process u s e d to define the layers o f p a t t e r n e d conductor, insulator, a n d s e m i c o n d u c t o r materials that constitute active devices was a c c o m p l i s h e d b y u s i n g contact p r i n t i n g i n c o n j u n c t i o n w i t h resists based o n p h o t o - i n d u c e d crossl i n k i n g to generate differential s o l u b i l i t y . T h e resolution of the processes i n

0065-2393/88/0218-0075$09.50/0 © 1988 American Chemical Society

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

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use at that t i m e was l i m i t e d b y the ability to c o n t r o l mask-to-wafer separation a n d the pervasive use of isotropic, wet e t c h i n g techniques. T h e resist m a terials that f o u n d w i d e s t acceptance w e r e based o n bisazide sensitization of partially c y c l i z e d synthetic r u b b e r (1,2) (Chart 2.1). A s the i n d u s t r y m i g r a t e d from contact (or p r o x i m i t y ) p r i n t i n g to projection p r i n t i n g a n d started to i n t r o d u c e anisotropic e t c h i n g technology based o n reactive i o n e t c h i n g ( R I E ) , the l i m i t to r e s o l u t i o n b e c a m e m o r e a n d m o r e a n issue of the resist m a t e r i a l itself. T h e r e l a t i v e l y l o w contrast of the b i s a z i d e - r u b b e r resists; t h e i r p r o p e n s i t y for s w e l l i n g d u r i n g d e v e l o p m e n t ; a n d t h e i r r e l a t i v e l y p o o r resistance to plasma-based, anisotropic e t c h i n g processes l e d to the i n t r o d u c t i o n of positive resist c h e m i s t r y that is i n pervasive use today. T h e microelectronics m a n u f a c t u r i n g process is at present o p e r a t i n g at a resolution of 1 |xm that allows efficient p r o d u c t i o n of, for e x a m p l e , 1-

CH

3

H

CH

3

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cyclization to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create intermolecular cross-links that render the exposed areas insoluble.


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megabit d y n a m i c access r a n d o m m e m o r i e s ( D R A M s ) , a n d is r a p i d l y m o v i n g t o w a r d p r o d u c t i o n of 4-megabit D R A M s w i t h s u b m i c r o m e t e r design rules (4) ( F i g u r e 2.1). A l l of the s e m i c o n d u c t o r d e v i c e fabrication lines have e v o l v e d t o w a r d the c o m b i n a t i o n of projection p r i n t i n g i n positive resists a n d plasma-based, anisotropic e t c h i n g techniques. T h e resist materials that s u p port this technology are a l l versions of the 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 r e s i n system ( D Q N ) first i n t r o d u c e d b y the K a l l e C o . m a n y years ago (Chart 2.2). T h e s e materials (1-3) are positive i n tone a n d p r o v i d e b o t h h i g h e r resolution a n d greater e t c h resistance to plasma-based e t c h i n g p r o cesses than the b i s a z i d e - r u b b e r resists. T h e i m p r o v e d e t c h resistance is a manifestation of the aromatic character of the novolac matrix r e s i n (5-8) as c o m p a r e d to the aliphatic nature of the c y c l i z e d r u b b e r materials. T h e i m p r o v e d r e s o l u t i o n e x h i b i t e d b y the positive D Q N materials relative to the negative b i s a z i d e - r u b b e r resists results from t h e i r s u p e r i o r contrast a n d the fact that the d e v e l o p m e n t process does not cause s w e l l i n g of the unexposed areas of the resist film. T h e D Q N resist system is able to resolve images m u c h smaller than 0.5 |xm. T h u s , the resists available today are not the l i m i t i n g factor i n d e f i n i n g d e v i c e d i m e n s i o n s . I n fact, the resolution l i m i t that can b e reached i n a m a n u f a c t u r i n g e n v i r o n m e n t is l i m i t e d not b y the i n t r i n s i c p r o p e r t i e s of the resist materials available b u t b y the p h y s i c a l limitations of the exposure e q u i p m e n t a n d b y practical issues that i n c l u d e c o n t a m i n a t i o n c o n t r o l , l e v e l t o - l e v e l a l i g n m e n t capability, etc. C o n s e q u e n t l y , resist materials research has b e e n , a n d w i l l c o n t i n u e to b e , focused o n d e v i s i n g m a t e r i a l approaches to e x t e n d i n g the resolution l i m i t s i m p o s e d b y the physics of the available exposure e q u i p m e n t . T h e factors that c o n t r i b u t e to l i m i t r e s o l u t i o n i n p r o j e c t i o n optics are w e l l k n o w n (2) a n d are manifested i n the form of a degradation i n the "free space" image as the exposure systems are operated near t h e i r diffraction l i m i t . T o some extent, this lens-generated degradation of image q u a l i t y can b e c o m p e n s a t e d b y d e s i g n i n g resist systems w i t h a n o n l i n e a r dissolution response to dose. M a n y positive resist materials, such as p o l y ( m e t h y l methacrylate) ( P M M A ) have l i n e a r dissolution k i n e t i c s . T h e rate of change i n thickness of such materials as a function of t i m e i n the d e v e l o p i n g solvent is constant w i t h respect to d e p t h into the film. T h i s rate is d i r e c t l y p r o p o r t i o n a l to the exposure a n d increases w i t h i n c r e a s i n g dose. P r o p e r l y f o r m u l a t e d D Q N resists can e x h i b i t h i g h l y n o n l i n e a r dissolution kinetics such that the d i s solution rate i n the surface regions of the film is substantially l o w e r t h a n it is i n the b u l k of the film. T h e consequence of this n o n l i n e a r rate f u n c t i o n is i m p r o v e d contrast ( F i g u r e 2.2). N o n l i n e a r dissolution kinetics is a c h i e v e d b y p r o p e r choice of the m o n o m e r c o m p o s i t i o n of the novolac (9), the p o l y d i s p e r s i t y of the r e s i n , a n d the processing conditions. E x a m i n a t i o n of F i g u r e 2.2 shows that the n o n l i n e a r materials e x h i b i t a greatly r e d u c e d


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Figure 2.1. IBM's 4-megabit DRAM is a retrograde n-well CMOS device with Q.8-\xm minimum features. The access time of this chip is 65 ns, and the chip size is 78 mm . 2


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OH

OH


Novolac Resin

Diazonaphthoquinone Chart 2.2. This DQN system employs a cresol-formaldehyde novolac resin as the matrix material. The resin is rendered photosensitive by addition of a diazonaphthoquinone that undergoes photolysis to produce a ketene intermediate that rapidly reacts with water present in the resin to yield an indenecarboxylic acid. The lipophilic diazoquinone serves to reduce the solubility of novolac films in aqueous base. Photolysis leads to production of an acidic photoproduct that renders exposed areas of the film soluble in aqueous base. Hence, this system functions as a positive resist. dissolution rate o f the u n e x p o s e d film c o m p a r e d w i t h that o f l i n e a r materials w i t h the same photospeed. T h e r e d u c e d attack o n the u n e x p o s e d resist represents an i m p r o v e m e n t i n contrast. Resist contrast i n D Q N systems is also d e t e r m i n e d to some extent b y the structure o f the d i a z o n a p h t h o q u i n o n e sensitizer. A c c o r d i n g to p u b l i s h e d reports (JO), the use of d i a z o q u i n o n e structures b e a r i n g m u l t i p l e c h r o m ophores o n a single m o l e c u l e p r o v i d e s h i g h e r contrast resist formulations than formulations based o n diazoquinones w i t h fewer c h r o m o p h o r e s p e r m o l e c u l e . P r e s u m a b l y , some t h r e s h o l d n u m b e r of c h r o m o p h o r e s m u s t b e p h o t o c o n v e r t e d to a c i d to r e n d e r the m u l t i f u n c t i o n a l molecules soluble i n base. H e n c e , the d i s s o l u t i o n rate response to dose o f resist f o r m u l a t e d from m u l t i f u n c t i o n a l diazoquinones s h o u l d be n o n l i n e a r . T h i s a r g u m e n t assumes that the major c o n t r i b u t i o n to the change i n dissolution rate i n D Q N systems is the change i n p o l a r i t y o r base s o l u b i l i t y associated w i t h c o n v e r s i o n o f the d i a z o q u i n o n e i n t o the c o r r e s p o n d i n g i n d e n e c a r b o x y l i c acid. T h e r e are c o n f l i c t i n g reports o n this m e c h a n i s m . O n e group c o m p a r e d the d i s s o l u t i o n rate of D Q N systems i n w h i c h the d i a z o q u i n o n e was p h o tolytically c o n v e r t e d to the c o r r e s p o n d i n g a c i d to that of resists f o r m u l a t e d


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en in CD C

J* o

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DC

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\

\\ \\

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Time in Developer Figure 2.2. This plot depicts the time evolution of the development process for two resist films with identical starting thickness and identical overall development rates. The nonlinear system (dashed lines) has approximately 80% of the unexposed film remaining at the time when the exposed film is developed to the substrate. The linear system (solid lines) has only approximately 50% of the unexposed film remaining at the time when the exposed film is developed to the substrate. This difference in the extent of unexposed film loss represents a difference in contrast. The nonlinear system has higher contrast than the linear system. from novolac a n d the p u r e a c i d p h o t o p r o d u c t (JI). T h e result was m u c h h i g h e r rates for films i n w h i c h the acid was photogenerated, l e a d i n g to the c o n c l u s i o n that n i t r o g e n e v o l u t i o n , w h i c h is a b y p r o d u c t of the photolysis of the d i a z o q u i n o n e , is a major c o n t r i b u t o r to photogenerated changes i n dissolution rate i n these systems. P r e s u m a b l y , the n i t r o g e n e v o l v e d u p o n photolysis causes an increase i n free v o l u m e that is k n o w n to have a p r o f o u n d effect o n the dissolution rate of p o l y m e r films (12). A n o t h e r group s t u d y i n g a closely related system r e p o r t e d conflicting results, w h i c h l e d to the c o n c l u s i o n that the p o l a r i t y change was the o v e r r i d i n g c o n t r i b u t o r to the p h o t o i n d u c e d changes i n dissolution rate (13). F u r t h e r e x p e r i m e n t a t i o n is r e q u i r e d to resolve this controversy, b u t the ability to formulate positive resist m a terials that have a t h r e s h o l d l i k e n o n l i n e a r response to dose is a valuable s k i l l that resist chemists use to i m p r o v e the resolution a n d latitude of the i m a g i n g process.

2.2 Image Reversal T h e c h e m i s t r y of the D Q N materials has b e e n m o d i f i e d such that t h e y function as a h i g h - r e s o l u t i o n , high-contrast, negative-tone resist system that is as d e v o i d of d i s t o r t i o n d u e to s w e l l i n g as the standard, positive D Q N system (14, 15). T h i s tone reversal of the D Q N system is a c c o m p l i s h e d b y a d d i t i o n of an appropriate base to the f o r m u l a t i o n . T h e first step i n the image-reversal process ( F i g u r e 2.3) is p a t t e r n e d exposure. D u r i n g this exposure, a latent image of photogenerated i n d e n e Bowden and Turner; Electronic and Photonic Applications of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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carboxylic a c i d is p r o d u c e d . W h e n the exposed f i l m is treated w i t h base a n d b a k e d to a t e m p e r a t u r e above ~ 7 6 ° C , base-catalyzed d e c a r b o x y l a t i o n o f the i n d e n e c a r b o x y l i c a c i d occurs, a n d a latent image of an i n d e n e d e r i v a t i v e is p r o d u c e d . T h e i n d e n e d e r i v a t i v e also functions as a d i s s o l u t i o n i n h i b i t o r but is not photosensitive. T h e next process step is a flood exposure that


converts the d i a z o q u i n o n e i n p r e v i o u s l y u n e x p o s e d regions of the f i l m i n t o the c o r r e s p o n d i n g i n d e n e c a r b o x y l i c acid, t h e r e b y r e n d e r i n g these regions m o r e soluble i n aqueous base than the p a t t e r n e d regions i n w h i c h the d i a z o q u i n o n e has b e e n c o n v e r t e d i n t o the i n d e n e d e r i v a t i v e . The base r e q u i r e d for catalysis of the decarboxylation can b e a d d e d to the f o r m u l a t i o n p r i o r to spin coating of the resist or can b e a d d e d after coating a n d p a t t e r n e d exposure. A d d i t i o n after exposure is a c c o m p l i s h e d b y i m m e r s i n g the exposed film i n an atmosphere of a volatile base s u c h as a m m o n i a or an a l k y l a m i n e . W h e n most bases are a d d e d to t h e D Q N resist f o r m u l a t i o n , a slow degradation i n the r e s u l t i n g m i x t u r e occurs. T h e for-

Develop Figure 2.3. The base-catalyzed image-reversal process involves a patternwise exposure that converts the diazoquinone to the corresponding indenecarboxylic acid, followed by a bake step that causes base-catalyzed decarboxylation that produces the nonphotosensitive indene derivative. Subsequent flood exposure converts the diazoquinone in the previously unexposed areas of the film into the indenecarboxylic acid. Development then yields a negative image of the mask because the originally patterned areas containing the lipophilic indene derivative are less soluble in base than those containing the acidic photoproduct. Bowden and Turner; Electronic and Photonic Applications of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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m u l a t i o n undergoes spectral changes, a n d t h e viscosity increases w i t h t i m e . T h i s shelf-life p r o b l e m has l e d to a preference, i n m a n y cases, for the gaseous a m i n e postexposure t r e a t m e n t process. Because the latent image p r o d u c e d i n the resist film d u r i n g p a t t e r n e d exposure has a positive w a l l slope angle ( F i g u r e 2.4), the d e v e l o p e d image f o l l o w i n g r e v e r s a l can have walls w i t h a p o s i t i v e , v e r t i c a l , o r negative slope d e p e n d i n g o n the processing a n d d e v e l o p i n g conditions. T h e p o s i t i v e w a l l slope stems from t h e fact that the top o f t h e i n s o l u b i l i z e d area d e t e r m i n e s t h e l i n e w i d t h i n t h e negative m o d e , whereas the foot o f the resist s t r u c t u r e d e t e r m i n e s t h e w i d t h o f t h e image i n t h e positive m o d e . T h e a b i l i t y to generate v e r t i c a l w a l l profiles has i m p o r t a n t i m p l i c a t i o n s for i m a g i n g o v e r topographical features, w h i c h w i l l b e discussed i n S e c t i o n 2.6. T h e u n d e r c u t , o r negatively s l o p e d resist images, are p a r t i c u l a r l y useful as stencils for a d d i t i v e (lift-off) m e t a l l i z a t i o n (15).

2.3 Contrast Enhancement A t t e m p t s to compensate for t h e limitations of the p r o j e c t i o n lens systems c u r r e n t l y available have l e d to some o t h e r v e r y i n t e r e s t i n g innovations i n c l u d i n g contrast-enhancement materials. T h e s e materials are, i n essence, dyes that absorb strongly at the exposure w a v e l e n g t h a n d u n d e r g o efficient p h o t o c o n v e r s i o n to products that are transparent at the exposure w a v e l e n g t h . T h e dyes are s p i n coated, i n a n appropriate v e h i c l e , o v e r the photoresist p r i o r to exposure. U p o n exposure, the d y e d layer is b l e a c h e d . T h i s step enables the u n d e r l y i n g photoresist to b e exposed i n a d y n a m i c process. Because t h e dyes b l e a c h most r a p i d l y i n areas of h i g h i n t e n s i t y , t h e c e n t e r areas o f exposed l i n e s b l e a c h m o r e r a p i d l y than the edges; thus, t h e s l o p i n g , low-contrast character o f the edges of the free space image are c o m p e n s a t e d for ( F i g u r e s 2.5 a n d 2.6). T h e dyes that are u s e d for contrast-enhancing materials range f r o m the nitrones (16) to various d i a z o n i u m salts (17,18) a n d

Vertical slope

Positve slope

Figure 2.4. The latent image in the resist has essentially the shape indicated because of the sloping nature of the free space image and the absorption of the resist film. Depending on the processing conditions, either negatively sloped (undercut profiles) vertical profiles or positively sloped profiles can be obtained.


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A

7

Mask

CEL. Resist

Unexposed Ideal Aerial Image


Distance Project Aerial Image

Lightly exposed fifth

(7ft\

Heavily exposed Figure 2.5. The intensity function or aerial image of a mask is ideally a square wave. However, projection optics operating near their diffraction limit degrade this square wave into a sinusoid with a small direct current (dc) term. When this intensity function is imposed on the contrast-enhancement lithographic material, bleaching occurs most rapidly in the high-intensity areas such that the transmitted intensity function that exposes the resist is modified and thus leads to improved contrast.

Figure 2.6. SEM images of 0.8-\xm lines and spaces printed with (right) and without (left) contrast-enhancement lithography. (Courtesy of Cliff Takemoto of National Semiconductor.)


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polysilanes (19) ( C h a r t 2.3). C o n t r a s t - e n h a n c e m e n t l i t h o g r a p h y u n q u e s t i o n ably i m p r o v e s the r e s o l u t i o n l i m i t of a g i v e n lens system. T h e extent o f resolution i m p r o v e m e n t is a c o m p l e x f u n c t i o n of the properties of the d y e , the thickness o f the d y e l a y e r , a n d the characteristics of the u n d e r l a y i n g photoresist. I n g e n e r a l , the i m p r o v e m e n t i n r e s o l u t i o n is a c h i e v e d at the expense of l o w e r p r o d u c t i v i t y (throughput). H e n c e , the process allows a


trade to be m a d e b e t w e e n r e s o l u t i o n a n d t h r o u g h p u t (20). A n o t h e r i n t e r e s t i n g use o f a d y e layer to i m p r o v e the r e s o l u t i o n l i m i t of a lens system is f o u n d i n the b u i l t - o n mask ( B O M ) process (21). T h e B O M m a t e r i a l is coated o v e r the photoresist p r i o r to exposure i n a m a n n e r a n a l ogous to the contrast-enhancing material. T h e B O M m a t e r i a l is, i n essence, p h o t o c h r o m i c i n that exposure at one w a v e l e n g t h generates a p h o t o p r o d u c t that is transparent at the exposure w a v e l e n g t h b u t absorbs strongly i n a different spectral r e g i o n at w h i c h the photoresist is sensitive. T h e o t h e r i m p o r t a n t characteristic o f the d y e is that it can b e " f i x e d " , or r e n d e r e d

General Electric

Polysaccharide + ( C H - C H ) 2

n

+

Matsushita

Hitachi R

R

R

R2

R2

^2

IBM Research Chart 2.3. Various dyes used as contrast-enhancement materials. The polysilanes are useful in the mid-UV region (308-313 nm); the other materials are designed for use in the near-UV region (365-436 nm).


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insensitive to exposure. I m i n e s have b e e n used as the c h r o m o p h o r e for the B O M process, a n d perhalohydrocarbons such as C B r have b e e n u s e d as a source of photogenerated a c i d . T h e i m i n e absorbs strongly at 365 n m b u t not at 436 n m , a n d the p r o t o n a t e d f o r m , the i m i n i u m salt, behaves i n the reverse (i.e., it absorbs strongly at 436 n m b u t not at 365 nm). H e n c e , exposure to essentially any w a v e l e n g t h at w h i c h the p e r h a l o h y d r o c a r b o n absorbs produces p a t t e r n e d areas of local acid concentration that l e a d to i m i n i u m salt formation, t h e r e b y r e n d e r i n g these areas opaque at 436 n m . T h e films are t h e n b a k e d to volatilize unexposed C B r i n the fixing process; thus, the films are r e n d e r e d insensitive to subsequent exposure. T h i s process p h o t o c h e m i c a l l y generates a " b u i l t - o n m a s k " i n perfect contact w i t h the resist for contact p r i n t i n g at 365 n m . S u b s e q u e n t flood exposure at 365 n m f o l l o w e d b y d e v e l o p m e n t leads to a positive-tone image of the mask (Figures 2.7 a n d 2.8). Adaptations of this i n t e r e s t i n g t e c h n i q u e to X - r a y exposure a n d to generation of negative-tone images have b e e n discussed (21). 4


4

2.4 Deep-UV Resists F o r a g i v e n lens system o p e r a t i n g at a specified w a v e l e n g t h , the contraste n h a n c e m e n t l i t h o g r a p h y or B O M techniques represent c h e m i s t s ' c o n t r i butions to i m p r o v e m e n t i n useful or functional r e s o l u t i o n . T h e functional resolution of a g i v e n lens system can also be i m p r o v e d b y r e d u c i n g the exposure w a v e l e n g t h because resolution is d i r e c t l y p r o p o r t i o n a l to the exposure w a v e l e n g t h a n d i n v e r s e l y p r o p o r t i o n a l to the n u m e r i c a l a p e r t u r e . E x p o s u r e tools based o n all-reflecting optics such as the P e r k i n - E l m e r M i cralign 6 0 0 H T are not subject to c h r o m a t i c aberration d i s t o r t i o n of the free space image. H e n c e , the r e s o l u t i o n of such systems can be substantially i m p r o v e d b y r e d u c t i o n of the exposure w a v e l e n g t h . T h e u s u a l exposure w a v e l e n g t h for D Q N resists is i n the r e g i o n of 4 0 5 - 4 3 6 n m . M a t e r i a l s have n o w b e e n t a i l o r e d to allow efficient exposure at 3 0 0 - 3 5 0 n m (22) a n d thus p r o v i d e i m p r o v e d r e s o l u t i o n for exposure tools such as the M i c r a l i g n 6 0 0 H T that can p r o v i d e variable w a v e l e n g t h exposure. A t t e m p t s to tailor D Q N materials to still shorter, d e e p - U V ( D U V ) w a v e lengths (near 250 nm) have b e e n largely unsuccessful because of the strong absorbance of the novolac resins a n d the e x t r e m e l y weak D U V o u t p u t of standard, h i g h - p r e s s u r e , m e r c u r y arc lamps of the sort u s e d i n today's exposure tools. S o m e i n t e r e s t i n g approaches have b e e n made t o w a r d t a i l o r i n g the o p t i c a l properties of b o t h the r e s i n a n d the sensitizer materials for D U V exposure. A system based o n base-soluble acrylic resins that are transparent in the D U V , i n c o m b i n a t i o n w i t h o - n i t r o b e n z y l esters of l i p o p h i l i c carboxylic acids that u n d e r g o photolysis to p r o d u c e carboxylic acids, has b e e n d e s c r i b e d (23) (Chart 2.4). T h e s e materials are r e p o r t e d to have sensitivity comparable to the D Q N systems a n d r e m a r k a b l y h i g h contrast. H o w e v e r , the aliphatic



86


-J

I

I

300

I

1—

400

500

R-N=C-R'+H -> R-N=C-R' ® +

Base F o r m (mine

Protonated F o r m Iminium Salt

• A c i d Generator

I

1 1 50

I

I

150

1

»

250

Dose in m J / c m

2

Figure 2.7. The materials used for the BOM process. Exposure at 365 nm produces acid derived from CBr . This photogenerated acid protonates the imine dye, which causes a dramatic bathochromic shift in the absorbance. As exposure proceeds, the absorbance at 365 nm is bleached and the absorbance at 436 nm increases because of iminium salt formation. 4


2.

WILLSON AND BOWDEN


87

B O M Layer Resist

Si


Expose

Si Fix Image (opaque at 4 3 6 nm)

A

3 6 5 nm /

4\ 4 3 6

nm

Positive

F l o o d Expose & Develop

Negative Figure 2.8. The BOM process.

nature o f the a c r y l i c matrix r e s i n materials leads to resists that have less e t c h resistance i n p l a s m a e n v i r o n m e n t s than the D Q N systems. The synthesis a n d characterization o f a n o v e l class of diazoketones that function i n a m a n n e r analogous to the diazoquinones b u t have bleachable absorbance m a x i m a i n the D U V r e g i o n have b e e n d e s c r i b e d ( F i g u r e 2.9). T h e s e systems are r e p o r t e d to f u n c t i o n w i t h photosensitivity c o m p a r a b l e to that of the D Q N systems, b u t t h e i r u t i l i t y still appears to b e l i m i t e d b y the fact that t h e y are e m p l o y e d i n conduction w i t h a novolac r e s i n , w h i c h has a substantial, u n b l e a c h a b l e absorbance i n the D U V r e g i o n . T h i s u n d e s i r a b l e resin absorbance leads to r e d u c e d contrast (24, 25).

2.5 Chemical Amplification A l t h o u g h the t a i l o r e d sensitizers a n d resins p r e s u m a b l y i m p r o v e D U V p e r formance, systems based o n the D Q N design have a sensitivity l i m i t i m p o s e d b y the q u a n t u m efficiency o f t h e sensitizer to p h o t o p r o d u c t c o n v e r s i o n that


88



Methacrylate resin

O

R—C—O—CH —(' 2

x

)

•

R—C—OH + H O — C H

2

NOs

NO

O-Nitrobenzylcholate Chart 2.4. The o-nitrobenzylcholate DUV resist system uses a copolymer of methyl methacrylate and methacrylic acid as the base-soluble matrix resin. This resin is rendered photosensitive by addition of the o-nitrobenzyl ester of cholic acid. Upon exposure, the ester is photolyzed to yield cholic acid and an o-nitrosobenzyl alcohol. The system functions in a manner analogous to the DQN system and is positive in tone. is s i m p l y too l o w for practical use i n c u r r e n t D U V projection aligners. T h e s e tools are based o n m e r c u r y l a m p light sources, a n d t h e D U V p o w e r that these systems can d e l i v e r at the wafer plane is almost 2 orders o f m a g n i t u d e l o w e r than that available i n the n e a r - U V range ( F i g u r e 2.10). C o n s e q u e n t l y , although r e s o l u t i o n m a y b e extended b y D U V exposure o f such resist systems, t h e p r o d u c t i v i t y loss is intolerable. N e w D U V resist materials w i t h greatly e n h a n c e d sensitivity s e e m to offer t h e p o s s i b i l i t y o f p r o v i d i n g useful p r o d u c t i v i t y i n t h e D U V r e g i o n . T h e s e systems are based o n chemical amplification (26), w h e r e i n a single p h o t o e v e n t generates a catalytic species that acts o n the matrix r e s i n m a t e r i a l to alter its s o l u b i l i t y i n some subsequent process step. O r d e r s - o f - m a g n i t u d e i m p r o v e m e n t i n sensitivity apparently c a n b e r e a l i z e d w i t h systems based o n this sort o f design w i t h o u t significant loss o f resolution. Several examples of resist materials that function o n t h e basis o f c h e m i c a l amplification are d e s c r i b e d i n detail i n C h a p t e r 3. T h e m a t e r i a l p r o b l e m s a n d t h e advances that have b e e n m a d e i n resists for D U V lithography are also d e t a i l e d i n C h a p t e r 3. H o w e v e r , regardless o f t h e exposure w a v e l e n g t h , several i m portant factors l i m i t resolution i n t h e practical application of m i c r o l i t h o g r a p h y to d e v i c e fabrication. T h e s e factors warrant discussion a n d have b e e n the focus o f an i n t e r e s t i n g b o d y o f materials research.


2.

WILLSON AND BOWDEN

89


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II

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Novolac Resist Formulation 1 r

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Electronic and Photonic Applications of Polymers - American

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