Microelectronics Processing - ACS Publications - American Chemical

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Resists in Microlithography Michael J. O ' B r i e n and D a v i d S. Soane 1

1 2

2

Silicone Products Division, General Electric Company, Waterford, N Y 12188 Department of Chemical Engineering, University of California, Berkeley, C A 94720

The drive toward increased circuit density in microelectronic devices has prompted significant efforts aimed at improving the resolution capabilities of lithographic equipment, materials, and processes. This chapter provides an overview of the various microlithographic strat egies currently in use, with a special emphasis on resist materials, chemistry, and processing schemes. Emerging technologies are also described, which, although not yet implemented, may hold the key to future progress.

T H E

D E M A N D F O R I N C R E A S E D C I R C U I T D E N S I T Y o n s i l i c o n c h i p s o v e r the

last 25 years has c o n t i n u e d to p u s h u p the l e v e l of i n t e g r a t i o n . P h o t o l i t h ography has r e s p o n d e d to this d e m a n d b y i m p r o v e m e n t s i n exposure a n d a l i g n m e n t systems, p r o d u c t i o n of n e w materials, a n d innovative fabrication methods. Today, 1-2-μπι features are typical of critical geometries for most p r o d u c t i o n devices, whereas i n state-of-the-art processes, s u b m i c r o m e t e r features are b e c o m i n g m o r e c o m m o n . I n this chapter, various m i c r o l i t h o graphic strategies a n d the k e y role of p o l y m e r s i n this technology w i l l be discussed. Inorganic resist materials, w h i c h are still i n a h i g h l y exploratory stage o f d e v e l o p m e n t , w i l l not be c o v e r e d . L i t h o g r a p h i c processes are based o n r a d i a t i o n - i n d u c e d alteration of h i g h l y s p e c i a l i z e d photosensitive p o l y m e r i c films, w h i c h are c a l l e d pho toresists or, s i m p l y , resists. T h e photoresists u s e d i n s e m i c o n d u c t o r m i crolithography w e r e o r i g i n a l l y d e v e l o p e d for the p r i n t i n g i n d u s t r y (I). F o r a t y p i c a l process, the resist is a p p l i e d onto a substrate to f o r m a t h i n u n i f o r m film. Irradiation t h r o u g h a glass plate o r " m a s k " coated w i t h an opaque m a t e r i a l (usually c h r o m i u m ) b e a r i n g an array of c i r c u i t patterns allows se0065-2393/89/0221-0325$13.80/0 © 1989 American Chemical Society

Hess and Jensen; Microelectronics Processing Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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MICROELECTRONICS PROCESSING: C H E M I C A L ENGINEERING ASPECTS

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l e c t e d areas o f the photoresist to b e exposed ( F i g u r e 1). T h e m o d i f i e d o r exposed regions of the p o l y m e r e x h i b i t an a l t e r e d rate of r e m o v a l or d e v e l o p m e n t i n certain c h e m i c a l reagents (developers), w h i c h results i n the form a t i o n o f a p o l y m e r i c r e l i e f image o f the mask pattern. O n the basis o f the c h e m i c a l nature o f the photoresist, e i t h e r a positive o r a negative image o f the o r i g i n a l mask is f o r m e d . Resists that p r o d u c e negative-tone images u n d e r g o c r o s s - l i n k i n g u p o n irradiation. C r o s s - l i n k i n g renders these resists less soluble i n the d e v e l o p e r solvent. C o n v e r s e l y , positive resists undergo m o l e c u l a r changes that enhance t h e i r s o l u b i l i t y i n the d e v e l o p e r such that exposed regions are p r e f e r e n t i a l l y r e m o v e d . T h e p a t t e r n e d resist image thus o b t a i n e d delineates the areas i n w h i c h subsequent modification o r r e m o v a l o f the u n d e r l y i n g substrate w i l l take place. T h r o u g h e i t h e r c h e m i c a l or p h y s i c a l processes, the substrate is a l t e r e d i n the u n m a s k e d regions, whereas the r e m a i n i n g resist protects the areas

Exposure

Incident Radiation

Will

llli

min

Mask Photoresist

Development

Positive-tone Resist

Negative-tone Resist

Figure 1. Diagram showing how irradiation through a mask allows selected areas of the photoresist to be exposed. In positive resists, the exposed areas become more soluble in the developer and, therefore, can be selectively removed. In negative resists, the exposed areas become less soluble in the developer, and thus, unexposed material is selectively dissolved.


7.

O'BRIEN & S ο AN Ε

Resists in

Microlithography

w h e r e m i n i m a l change is i n t e n d e d . A s the final step i n this process,

327 the

r e m a i n i n g resist is s t r i p p e d b y w e t - or p l a s m a - e t c h i n g methods. T h i s p h o tolithographic sequence is repeated for e v e r y p a t t e r n e d c i r c u i t layer o n the s e m i c o n d u c t o r device. E a c h t i m e , the appropriate mask is p r e c i s e l y a l i g n e d to the previous p a t t e r n o n the wafer. Resist materials u s e d i n this a p p l i c a t i o n m u s t m e e t stringent resolution a n d sensitivity r e q u i r e m e n t s . T h e y m u s t also possess excellent

film-forming

properties a n d d u r a b i l i t y to w i t h s t a n d the


h i g h l y cojrosive chemicals, p l a s m a treatments, a n d t e m p e r a t u r e cycles e n c o u n t e r e d d u r i n g substrate e t c h i n g , d o p i n g , a n d deposition processes.

Exposure Techniques Optical Lithography.

L i t h o g r a p h i c processes can be classified ac

c o r d i n g to the energy u s e d to expose the resists a n d the e q u i p m e n t necessary to a c c o m p l i s h the process. Image q u a l i t y depends o n the exposure m e t h o d , h a r d w a r e , a n d resist m a t e r i a l . I n optical l i t h o g r a p h y , the resist is exposed to radiation w i t h i n the near- to d e e p - U V region (200-450 nm). Near-UV

Lithography.

N e a r - U V l i t h o g r a p h y , i n w h i c h resists are ex

posed to radiation i n the 3 5 0 - 4 5 0 - n m n e a r - U V range, is b y far the most c o m m o n l y u s e d o p t i c a l l i t h o g r a p h i c m e t h o d i n p r o d u c t i o n . E x p o s u r e systems d e s i g n e d for this p u r p o s e are e q u i p p e d w i t h h i g h - i n t e n s i t y m e r c u r y - x e n o n lamps as radiation sources a n d a variety of lenses a n d m i r r o r s for l i g h t c o l l i m a t i o n . T h e spectral output of the m e r c u r y - x e n o n l a m p i n the 3 5 0 - 4 5 0 n m range has several strong peaks, the most i m p o r t a n t of w h i c h are at 365 n m (i line) a n d 436 n m (g line). S e v e r a l methods are available to image photoresists. I n contact p r i n t i n g , the mask a n d substrate are b r o u g h t into h a r d contact u n d e r v a c u u m . E x posure occurs t h r o u g h a mask, w i t h the c i r c u i t pattern r e p r o d u c e d m a n y times i n an array. T h i s p r o c e d u r e results i n a 1:1 image of the entire mask o n each wafer. U n f o r t u n a t e l y , several major faults of this scheme offset the advantage of excellent resolution a n d p r e c l u d e its use i n the fabrication of h i g h - d e n s i t y devices. F i r s t , scratches r e s u l t i n g from surface contact l e a d to w e a r a n d p r e m a t u r e degradation of the mask. S e c o n d , unacceptably h i g h levels of resist damage a n d particulate defects occur. T h i r d , the i n h e r e n t lack of absolute substrate a n d mask flatness precludes perfect contact a n d gives rise to d i s t o r t i o n . T h e s e p r o b l e m s have p r o m p t e d the d e v e l o p m e n t of more-sophisticated a l i g n m e n t tools, a n d n o w , contact p r i n t i n g is relegated p r i m a r i l y to the p r o d u c t i o n of inexpensive chips w i t h large device g e o m e tries. P r o x i m i t y p r i n t i n g , a variation of contact p r i n t i n g , preserves a m i n i m u m gap of approximately 1 0 - 3 0 μπι b e t w e e n the silicon wafer a n d the mask. A l t h o u g h the p r o b l e m of particulate c o n t a m i n a t i o n is a v o i d e d , light d i s t o r t i o n is e n h a n c e d , a n d a loss i n r e s o l u t i o n results.


328


P r o j e c t i o n p r i n t i n g uses a series of h i g h l y refined reflecting lenses to project the mask image onto the wafer over a distance of m a n y inches. T h i s m e t h o d allows tight c o n t a m i n a t i o n c o n t r o l a n d p r o l o n g e d mask life b u t is p r o n e to optical aberrations. T h e necessity of u s i n g h i g h l y sophisticated optical systems a n d the mechanics r e q u i r e d to achieve adequate a l i g n m e n t dramatically increase the cost of projection aligners. T h e q u a l i t y o f p a t t e r n transfer differs greatly a m o n g the three m o d e s of


p r i n t i n g . A s an example, a mask w i t h p a r a l l e l b u n d l e s of slits a n d spaces b e t w e e n slits w i t h d i m e n s i o n s comparable w i t h the slits can b e c o n s i d e r e d . I n this case, o p t i c a l interference results i n d i s t o r t e d images. T h e theoretical m i n i m u m d i m e n s i o n (for b o t h space a n d slit) that allows resolvable i n t e r ference peaks for contact o r p r o x i m i t y p r i n t i n g is a p p r o x i m a t e d b y :

I n e q u a t i o n 1, b

min

is the m i n i m u m feature size transferable, λ is the w a v e

l e n g t h o f light, s is the separation b e t w e e n the mask a n d the substrate, a n d d is the thickness of the resist layer. I n p r o j e c t i o n p r i n t i n g , a series o f u n d u l a t i n g m a x i m a a n d m i n i m a are p r o d u c e d . Because of m u t u a l i n t e r f e r e n c e , the dark r e g i o n is n e v e r c o m p l e t e l y dark, a n d the m a x i m u m brightness does not c o r r e s p o n d to 1 0 0 % transmission. T h e q u a l i t y of transfer can b e c o n v e n i e n t l y i n d i c a t e d b y the m o d u l a t i o n i n d e x , M , w h i c h is d e f i n e d as follows:

M =

/ m a x

I x

max

~ + I

I m i n

1

(2)

* mm

I n e q u a t i o n 2, J and Z are the peak a n d t r o u g h intensities, respectively. I d e a l optics w o u l d give a n i n d e x e q u a l to u n i t y . H o w e v e r , i n practice, a l l exposure systems behave less than i d e a l l y (i.e., M < 1). m a x

m i n

E v e n t h o u g h p r o j e c t i o n optics e m b o d i e s the i n h e r e n t l i m i t a t i o n of pat t e r n transfer j u s t m e n t i o n e d , this t e c h n i q u e has b e c o m e a d o m i n a n t approach i n h i g h - r e s o l u t i o n w o r k . A k e y reason for this success is the a b i l i t y o f p r o j e c t i o n p r i n t i n g to use r e d u c t i o n refraction optics w i t h h i g h n u m e r i c a l a p ertures. T h e r e s o l v i n g p o w e r of projection systems can be a p p r o x i m a t e d b y :

ΝΑ I n equation 3, W is the m i n i m u m feature size, k is an e m p i r i c a l l y d e t e r m i n e d constant that depends o n resist processing, λ is the w a v e l e n g t h of the i n c i d e n t


7.

O ' B R I E N & SOANE

Resists in

Microlithography

329

radiation, a n d N A is the n u m e r i c a l aperture of the optical system. T h u s , resolution can be increased b y u s i n g shorter w a v e l e n g t h r a d i a t i o n o r b y increasing the n u m e r i c a l a p e r t u r e . I n a d d i t i o n , some i m p r o v e m e n t i n reso l u t i o n can be a c h i e v e d b y adjusting processing conditions to m i n i m i z e k. U n f o r t u n a t e l y , r e s o l u t i o n gains t h r o u g h the use of h i g h - N A optics or shorter wavelengths have a deleterious effect o n the d e p t h of focus ( D O F ) ,


as s h o w n b y e q u a t i o n 4:

D

0

F

-

« b

( 4 )

Because D O F is d i r e c t l y p r o p o r t i o n a l to 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 tional to the square of N A , the use of shorter w a v e l e n g t h radiation has less effect than the increase i n the n u m e r i c a l aperture. D e s p i t e i m p r o v e m e n t s i n p r o j e c t i o n optics, interference p h e n o m e n a u n r e l a t e d to m e c h a n i c a l design c o n t i n u e to l i m i t lithographic r e s o l u t i o n . O n e s u c h example is the standing-wave effect (2). D u r i n g exposure, the i n c i d e n t l i g h t is o n l y partially absorbed b y the resist, a n d u n a b s o r b e d r a diation can u n d e r g o partial reflection at the r e s i s t - s u b s t r a t e interface. T h e r e s u l t i n g reflected b e a m t h e n sets u p an interference p a t t e r n w i t h the u n absorbed i n c i d e n t b e a m . A resist near a constructive node reacts m o r e extensively than does a m a t e r i a l near a destructive node. T h e u n e v e n structural alteration manifests i t s e l f as scalloped edge profiles after resist d e v e l o p m e n t , w h i c h c o m p r o m i s e s p a t t e r n r e s o l u t i o n . O n e solution to this p r o b l e m is the use o f an antireflective coating to r e d u c e reflective waves (3, 4). A n o t h e r approach involves the use o f a postexposure bake step that smoothes the resist edges b y diffusion of the reacted species (5). M u l t i l e v e l resists, w h i c h w i l l b e discussed i n a later section, offer still another r e m e d y to this p r o b l e m . T h e f u n d a m e n t a l l i m i t a t i o n s o f optical interference can b e suppressed greatly i f the w a v e l e n g t h of the source radiation is shortened. Because patt e r n d i s t o r t i o n is severe w h e n feature r e s o l u t i o n approaches the exposure w a v e l e n g t h , the use of short-wavelength radiation pushes the r e s o l u t i o n towards finer features. T h u s , the i n c r e a s i n g t r e n d is to explore d e e p - U V sources a n d to i m p r o v e u p o n the existing n e a r - U V hardware. T h e desire to r e d u c e feature size has also generated m u c h interest i n X - r a y s a n d e l e c t r o n beams as alternative radiation sources. Deep-UV Lithography. T h e i m p o r t a n t issues for d e e p - U V l i t h o g r a p h y ( 2 0 0 - 2 5 0 nm) are a l i g n e r optics a n d resist materials. P r o b l e m s i n a l i g n e r optics stem from the decreased transparency of standard lens materials i n this frequency range, w h i c h necessitates the use of more-expensive c o n struction materials such as q u a r t z . T y p i c a l n e a r - U V positive resists are not useful for d e e p - U V l i t h o g r a p h y because of unacceptable absorption at


330

MICROELECTRONICS PROCESSING: C H E M I C A L E N G I N E E R I N G ASPECTS

2 0 0 - 2 5 0 n m . Resists t a i l o r e d for i m p r o v e d performance i n the d e e p - U V r e g i o n , h o w e v e r , are n o w b e c o m i n g available i n n e w products (6). N e v e r theless, d e e p - U V l i t h o g r a p h y remains i n the d e v e l o p m e n t phase a n d is not c u r r e n t l y u s e d i n i n t e g r a t e d - c i r c u i t (IC) p r o d u c t i o n . D e e p - U V source brightness is another issue, because the p o w e r o u t p u t of a 1 - k W m e r c u r y - x e n o n l a m p i n the 2 0 0 - 2 5 0 - n m range is o n l y 3 0 - 4 0 m W . F o r this reason, e x c i m e r lasers (such as K r C l a n d K r F ) , w h i c h can d e l i v e r


several watts of p o w e r at the r e q u i r e d wavelengths, are b e i n g c o n s i d e r e d as alternatives (7). I n fact, a d e e p - U V step-and-repeat projection system w i t h an a l l - q u a r t z lens a n d a K r F e x c i m e r laser w i t h an o u t p u t at 248 n m has b e e n r e p o r t e d (8). E v e n the laser-based systems r e q u i r e resists w i t h a s e n sitivity of 3 0 - 7 0 m j / c m . 2

Electron Beam Lithography. T h e e v e r - d i m i n i s h i n g I C feature size has m o t i v a t e d the d e v e l o p m e n t of exposure techniques w i t h h i g h - e n e r g y sources. O n e such radiation source is the e l e c t r o n b e a m . T h i s technology is r o u t i n e l y u s e d to generate masks for p h o t o l i t h o g r a p h y a n d is foremost i n e x p e r i m e n t a l applications of c o m p l e x - d e v i c e fabrication. Its two major d r a w backs are l o w t h r o u g h p u t a n d h i g h capital cost. F o r direct wafer w r i t i n g or mask fabrication, sensitive resists are n e c essary to ensure a reasonable t h r o u g h p u t . A second p r o b l e m is e l e c t r o n back scattering caused b y collisions of electrons w i t h atoms w i t h i n the resist a n d substrate. I n this situation, the electronic stopping p o w e r of organic resists is l i m i t e d , a n d a large fraction of the i n c i d e n t electrons is a l l o w e d to reach the u n d e r l y i n g substrate. C o l l i s i o n w i t h the substrate causes r a n d o m scatt e r i n g a n d secondary a n d back electron generation. H e n c e , the resist is s h o w e r e d w i t h electrons from the substrate, a n d the total energy d e p o s i t e d w i t h i n the resist has a s m e a r e d d i s t r i b u t i o n , w i t h a b r o a d base near the b o t t o m (9). I f a low-contrast resist is u s e d , these scattered electrons may have sufficient energy to cause degradation. T h i s effect lowers the l i n e w i d t h r e s o l u t i o n a p p r e c i a b l y . F o r reasonable t h r o u g h p u t s , sensitive resists are r e q u i r e d , a n d for better l i n e w i d t h control a n d r e s o l u t i o n , a high-contrast resist is r e q u i r e d . X-ray Lithography. X - r a y l i t h o g r a p h y is similar to optical l i t h o g r a p h y i n that flood exposure of the e n t i r e wafer t h r o u g h a p a t t e r n e d mask is possible. T h u s , the p o t e n t i a l for p r o d u c t i o n applications is greater. X - r a y lithography also has the advantages of an essentially infinite d e p t h of field, a h i g h tolerance to dust a n d c o n t a m i n a t i o n , a n d the absence of standing waves. Because the radiation w a v e l e n g t h varies from about 0.5 to 3 n m , diffraction is not an issue. O n e challenge of X - r a y l i t h o g r a p h y is the f a b r i cation of h i g h - q u a l i t y masks. H i g h - a t o m i c - n u m b e r metals, such as g o l d , are opaque to X - r a y s ; thus they p r o v i d e a shadow for pattern d e f i n i t i o n b y the masks. G o l d patterns are f o r m e d b y e l e c t r o n b e a m (e-beam) l i t h o g r a p h y o n


7.


Resists in

331

Microlithography

substrates such as b o r o n n i t r i d e , silicon c a r b i d e , or silicon n i t r i d e m e m b r a n e s (JO). O r g a n i c films s u c h as p o l y i m i d e have b e e n used also ( I I , 12). Because of the m i s m a t c h b e t w e e n the t h e r m a l expansions of the different materials used i n mask m a k i n g , stress-related pattern d i s t o r t i o n is possible. O t h e r major practical p r o b l e m s must b e o v e r c o m e before X - r a y l i t h o g raphy is accepted i n p r o d u c t i o n . F o r e m o s t is the availability of sensitive X ray resists. T o effect structural changes i n the p o l y m e r , the i n c i d e n t radiation


must be effectively absorbed. H y d r o c a r b o n - b a s e d organic resists are often transparent to X - r a y s , a n d hence, X - r a y resists m u s t be made sensitive b y the i n c o r p o r a t i o n of X - r a y - a b s o r b i n g h i g h - a t o m i c - n u m b e r atoms. T h i s p r o b l e m is a great challenge i n the synthesis or f o r m u l a t i o n of resists. A l t e r n a t i v e l y , a h i g h e r i n t e n s i t y X - r a y source can be d e v e l o p e d , so that the total exposure t i m e can be shortened. O n e such p o w e r f u l source is s y n c h r o t r o n radiation; h o w e v e r , c o m m e r c i a l i m p l e m e n t a t i o n of this costly source has not b e e n r e a l i z e d yet.

Resist Characterization T o accommodate the diverse needs of l i t h o g r a p h i c processes a n d device design specifications, resist properties vary. H o w e v e r , a few p r i m a r y characteristics c o m m o n to a l l resists can b e used to gauge t h e i r performance. T h e s e characteristics i n c l u d e sensitivity, contrast, r e s o l u t i o n , a n d e t c h i n g resistance. Because resist performance is strongly operation d e p e n d e n t , c o m p a r i s o n b e t w e e n materials m u s t be made u n d e r i d e n t i c a l conditions. Analysis by Dissolution Curves. M o s t performance indicators r e q u i r e o n l y an operational d e f i n i t i o n ; these concepts are e x p l a i n e d b y a film dissolution c u r v e . F i g u r e 2 shows a family of such curves, i n w h i c h the i n d i v i d u a l curves c o r r e s p o n d to resist b e h a v i o r i n d e v e l o p e r solution after exposure to the i n d i c a t e d radiation dose l e v e l . F i g u r e 2 is constructed for

T i m e in D e v e l o p e r

(s)

Figure 2. Dissolution curves for positive resists after exposure. The doses are designated by the numbers accompanying the traces. A stronger dose leaves a thinner film at a fixed development time.


332


positive resists, for w h i c h h i g h e r doses lead to faster dissolution. W i t h n e g ative resists, h i g h doses t e n d to decrease t h e dissolution rate. F u r t h e r m o r e , such a f a m i l y of curves is h i g h l y system specific. T h e same p o l y m e r d e v e l o p e d i n solutions o f different strengths w o u l d give different sets o f d i s s o l u t i o n curves. S i m i l a r l y , w i t h i d e n t i c a l developers a n d e v e n t h e same m a n n e r o f agitation, p o l y m e r s dissolve at different rates i f t h e y are b a k e d (annealed) a n d c o o l e d at different temperatures a n d rates (13). I f the starting m a t e r i a l


has a s l i g h t l y different c o m p o s i t i o n o r m o l e c u l a r w e i g h t d i s t r i b u t i o n , again, these curves w o u l d b e shifted. M a n y e x p e r i m e n t a l techniques exist to d e t e r m i n e dissolution curves b y i n situ m o n i t o r i n g o f film d e v e l o p m e n t . T h e simplest t e c h n i q u e is t h e laser e n d - p o i n t - d e t e c t i o n system. I n this system, m o n o c h r o m a t i c l i g h t from a H e - N e laser is d i r e c t e d at a resist layer f r o m a n e a r - n o r m a l d i r e c t i o n (14). T h e reflected l i g h t is p i c k e d u p b y an adjacent optical fiber, a n d the i n t e n s i t y is analyzed b y a d i o d e detector. T h e o u t p u t is a smooth trace w i t h p e r i o d i c oscillations. T h e peaks a n d valleys c o r r e s p o n d to successive constructive a n d destructive interference nodes, w h i c h result f r o m film thickness changes as the resist is e t c h e d away. F r o m these p e r i o d i c o u t p u t traces o b t a i n e d w i t h resists exposed to v a r y i n g degrees o f radiation, t h e family o f characteristic curves s h o w n i n F i g u r e 2 c a n b e constructed. O t h e r more-sophisticated techniques exist for this p u r p o s e , i n c l u d i n g i n situ e l l i p s o m e t r y a n d t w o w a v e l e n g t h i n t e r f e r o m e t r y . T h e s e techniques w i l l b e discussed i n a later section. A n a l y s i s o f S e n s i t i v i t y . F r o m t h e characteristic dissolution curves, a cross plot c a n b e m a d e o f the n o r m a l i z e d r e m a i n i n g film thickness (ratio of c u r r e n t thickness to o r i g i n a l thickness) as a function of c u m u l a t i v e dosage. F i g u r e 3 gives such curves for a positive a n d a negative resist. T h e s e curves are referred to as sensitivity o r exposure response curves for resists. F o r positive resists, the g o v e r n i n g p h e n o m e n o n is film disappearance, whereas for negative resists, t h e i m p o r t a n t c r i t e r i o n is the film r e m a i n i n g . T h e m i n i m u m dose n e e d e d to cause t h e relevant p h e n o m e n o n to emerge, as measu r e d b y the d e v e l o p m e n t p r o c e d u r e , is k n o w n as t h e incipient dose for t h e particular resist u n d e r study. T h e i n c i p i e n t dose corresponds to the intercept f o r m e d b y the t w o extrapolated regions o f the c u r v e , d e n o t e d as D ° a n d D.° i n t h e figure. T h e completion dose is d e n o t e d b y t h e same symbols b u t w i t h o u t the superscript o. F o r positive resists, t h e c o m p l e t i o n dose c o r r e sponds to t h e p o i n t at w h i c h the film is c o m p l e t e l y d i s s o l v e d , whereas for negative resists, the c o m p l e t i o n dose designates the p o i n t at w h i c h t h e film is c o m p l e t e l y intact. T h e s e c o m p l e t i o n doses m a y b e c a l l e d t h e resist s e n sitivity; h o w e v e r , these doses are not necessarily those r e q u i r e d to y i e l d a lithographically useful image a n d are h i g h l y d e p e n d e n t o n t h e processing conditions chosen. p


7.


Resists in

Microlithography


Positive Resist

Figure 3. Response curves for positive and negatives resists. Marked on the curves are incipient and completion doses, which indicate the onset and completion of observable events. The variables monitored are film attrition for positive resists and film remaining for negative resists. These traces can be affected by a number of process parameters, particularly development conditions.

Analysis o f Contrast.

T h e contrast,

7 , o f a g i v e n resist, is d e f i n e d

mathematically for positive a n d negative resists b y equations 5 a n d 6, r e spectively.


334

MICROELECTRONICS PROCESSING: C H E M I C A L E N G I N E E R I N G ASPECTS

I n general, the h i g h e r the contrast, the sharper are the edge profiles of d e v e l o p e d l i n e s . C o n t r a s t is also a resist q u a l i t y that can b e fine t u n e d b y j u d i c i o u s c h o i c e of processing parameters. I n the case of p o l y m e r i c e-beam a n d X - r a y resists, w h i c h u n d e r g o b o n d breakage u p o n i r r a d i a t i o n , followed b y c h a i n scission (positive resists) o r c r o s s - l i n k i n g (negative resists), resist sensitivity can be r e p r e s e n t e d b y a s t r u c t u r e - d e p e n d e n t constant c a l l e d a G value. G is a measure of scission efficiency, a n d G is a measure o f c r o s s - l i n k i n g efficiency. G values for resists that u n d e r g o o n l y c h a i n scission can b e d e t e r m i n e d e x p e r i m e n t a l l y b y p l o t t i n g the i n v e r s e o f the n u m b e r - a v e r a g e m o l e c u l a r w e i g h t ( M * ) o f the p o l y m e r versus the exposure dose (D). A s s h o w n b y e q u a t i o n 7, such a p l o t gives a straight l i n e w i t h a slope that is d i r e c t l y p r o p o r t i o n a l to G . s


x

s

n

s

I n e q u a t i o n 7, M ° is the i n i t i a l number-average m o l e c u l a r w e i g h t a n d A N n

is Avogadro's n u m b e r . W h e n b o t h c h a i n scission a n d c r o s s - l i n k i n g occur, the G values for b o t h processes (i.e., G

s

a n d G J can b e d e t e r m i n e d . T h i s

is a c c o m p l i s h e d after m e a s u r i n g changes i n the number-average m o l e c u l a r w e i g h t ( M ) a n d t h e weight-average m o l e c u l a r w e i g h t ( M j a n d t h e n s o l v i n g n

the f o l l o w i n g equations s i m u l t a n e o u s l y :

à

-

έ

+

κ

-

έ

+

( c

-

-

< - G

G

4

-

)

G

D

J

( 8 )

D

Microelectronics Processing - ACS Publications - American Chemical

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