Materials and Processes for Deep-UV Lithography - ACS Publications

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for Deep-UV Lithography Takao Iwayanagi1, Takumi Ueno1, Saburo Nonogaki1, Hiroshi Ito , and C . Grant W i l l s o n 2

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1Central Research Laboratory, Hitachi, L t d . , Kokubunji, Tokyo 185, Japan 2IBM Almaden Research Center, San Jose, CA 95120-6099

This chapter provides a comprehensive review of the resist materials and processes that have been designed and developed to support high-resolution, deep-UV (DUV) lithography (i.e., lithography using radiation in the 200-300 - nm wavelength range). Special emphasis is placed on materials, their lithographic performance, and the chemistry responsible for their function. Topics include the fundamental relationships between resolution and exposure wavelength, sources of DUV radiation, and progress in the development of DUV exposure equipment. Unique applications including multilayer patterning schemes and excimer laser lithography are also discussed.

J L H E F A B R I C A T I O N O F I N T E G R A T E D CIRCUITS involves a series o f steps that defines insulator, conductor, a n d s e m i c o n d u c t o r structures i n a n d o n single crystals o f s i l i c o n o r g a l l i u m arsenide (I). A s p r a c t i c e d today, the c i r c u i t e l e m e n t s are as s m a l l as 1 m i c r o m e t e r (1 jxm) i n d i m e n s i o n . R e p r o d u c i b l e device performance a n d y i e l d issues r e q u i r e c o n t r o l o f b o t h the d i m e n s i o n and p l a c e m e n t o f these 1-u.m structures to tolerances o f fractions o f 1 jxm. T h e n u m b e r o f s u c h c i r c u i t e l e m e n t s p e r c h i p has steadily increased d u r i n g the past three decades, m a i n l y t h r o u g h a decrease i n the size o f the e l e m e n t s . T h i s r e d u c t i o n i n t h e feature size a n d the increase i n c i r c u i t c o m p l e x i t y a n d integration that it allows is largely responsible for the d r a m a t i c i m p r o v e m e n t i n the performance a n d c o s t - p e r f o r m a n c e ratio that has o c c u r r e d a n d is expected to c o n t i n u e to occur. C i r c u i t e l e m e n t s are p a t t e r n e d t h r o u g h a series o f sophisticated i m a g i n g processes c o l l e c t i v e l y c a l l e d lithography. P h o t o l i t h o g r a p h i c processing, 0065-2393/88/0218-0109$22.60/0 © 1988 American Chemical Society

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

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w h i c h is the f o r m of l i t h o g r a p h y u s e d to delineate the c i r c u i t elements i n today's large-scale i n t e g r a t e d devices, is, h o w e v e r , a p p r o a c h i n g its p h y s i c a l resolution l i m i t . T h e r e s o l u t i o n l i m i t of an i m a g i n g system is a f u n c t i o n of the w a v e l e n g t h of the exposing radiation (i.e., h i g h e r resolution can be attained w i t h shorter w a v e l e n g t h radiation). C o n s e q u e n t l y , o n the basis of this s i m p l e p r i n c i p l e , l i t h o g r a p h i c systems i n c l u d i n g the exposure tools, resist systems, a n d the associated processes have b e e n u n d e r i n t e n s i v e d e v e l o p m e n t since the 1960s for short-wavelength radiation i n c l u d i n g • i o n a n d electron beams, • X-ray, and • short-wavelength U V (2). E l e c t r o n b e a m l i t h o g r a p h y e m p l o y s finely focused electron beams that are rasterdd or v e c t o r e d a n d m o d u l a t e d u n d e r c o m p u t e r c o n t r o l to delineate patterns i n resist films (3). Because the de B r o g l i e w a v e l e n g t h of electrons is e x t r e m e l y s m a l l at c o m m o n accelerating potentials ( 1 0 - 5 0 k V ) , these beams can b e focused to diameters of several tens of angstroms. Because this t e c h nology is based essentially o n serial d e l i n e a t i o n of the pattern p i x e l b y p i x e l (4), these systems have l o w t h r o u g h p u t . T h e c o m p l e x i t y of the exposure system a n d its associated electronics a n d s u p p o r t i n g c o m p u t e r h a r d w a r e makes these systems q u i t e expensive. D e s p i t e these limitations, the p r i m a r y pattern-generation capability of e l e c t r o n b e a m systems has f o u n d w i d e use i n the fabrication of masks for o p t i c a l l i t h o g r a p h y a n d , i n c e r t a i n cases, for p r o d u c t i o n o f c u s t o m logic devices. Because of its resolution c a p a b i l i t y , electron b e a m l i t h o g r a p h y has b e e n e x p l o i t e d i n laboratories to p r o d u c e e x t r e m e l y s m a l l structures i n c l u d i n g active devices w i t h c o n d u c t o r d i m e n sions of a few h u n d r e d angstroms i n l i n e w i d t h (5). X - r a y l i t h o g r a p h y is based o n the use of radiation i n the soft X - r a y r e g i o n of w a v e l e n g t h r a n g i n g from about 0.1 to 0.4 n m (6). T h i s technology is capable of r e s o l v i n g v e r y s m a l l features i n resist films. X - r a y lithography a n d , i n particular, X - r a y l i t h o g r a p h y based on s y n c h r o t r o n o r "storage r i n g " X - r a y sources is c u r r e n t l y a n e x t r e m e l y active area of research. T h i s technology is a d v a n c i n g r a p i d l y b u t has not yet r e a c h e d the stage of practical i m p l e m e n tation because o f a variety of t e c h n i c a l p r o b l e m s r e l a t e d to sources, mask fabrication, a l i g n m e n t , a n d resist materials. T h e use of short-wavelength U V radiation to e x t e n d the r e s o l u t i o n of p h o t o l i t h o g r a p h y was first r e p o r t e d b y M o r e a u a n d S c h m i d t i n 1970 (7), a n d L i n (8) further r e f i n e d this t e c h n i q u e . T h e w a v e l e n g t h of radiation u s e d i n what is n o w generally accepted as d e e p - U V ( D U V ) l i t h o g r a p h y a n d that w h i c h w e w i l l consider for the purposes of this r e v i e w is b e t w e e n 200 a n d 300 n m . T h i s range can b e c o m p a r e d to the w a v e l e n g t h of the c o m m o n " g l i n e " (436-nm) step-and-repeat p r i n t i n g tools that are the workhorses of today's factories a n d to the " i - l i n e " (365-nm) p r i n t i n g tools that are j u s t


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b e c o m i n g c o m m e r c i a l l y available. D U V l i t h o g r a p h y , s u p p o r t e d b y the d e v e l o p m e n t of n e w exposure sources, advances i n lens design, a n d n e w resist materials, is d e s t i n e d to b e c o m e a p o w e r f u l tool for the fabrication of s e m i c o n d u c t o r devices. W e expect that d u r i n g the next decade, the majority of p r o d u c t i o n w i l l migrate to some v e r s i o n of this p a t t e r n i n g technology. T h u s , this r e v i e w of the resist materials a n d processes d e v e l o p e d for D U V l i t h o g r a p h y is t i m e l y . T h e special emphasis i n this chapter is o n materials, t h e i r performance, a n d the c h e m i s t r y responsible for t h e i r l i t h o g r a p h i c response.

3.1 Deep-UV Lithography 3.1.1

Historical Development

M o r e a u a n d S c h m i d t (7) d e m o n s t r a t e d the sensitivity of p o l y ( m e t h y l m e t h acrylate) ( P M M A ) to D U V radiation i n 1970 a n d d e s c r i b e d most of the features of D U V lithography. T h e m a i n objective of t h e i r w o r k was to e m p l o y D U V radiation to allow larger mask-to-wafer separation at constant resolution i n p r o x i m i t y p r i n t i n g . T h e i r w o r k demonstrated the u t i l i t y of P M M A as a D U V resist, a n d they p r e d i c t e d that D U V radiation w o u l d be e m p l o y e d w i t h an acrylate p o l y m e r of some type i n future projection p r i n t i n g systems that s h o u l d allow reliable p r i n t i n g of s u b m i c r o m e t e r features. T h e t e r m " D U V l i t h o g r a p h y " was c o i n e d b y B . J . L i n i n his p i o n e e r i n g paper that appeared i n 1975 (8). L i n demonstrated the h i g h - r e s o l u t i o n p o tential of D U V lithography b y contact p r i n t i n g 0.5-|xm " T - b a r " (T-shaped) bubble-propagation patterns separated b y 0.25-|xm gaps i n 1.78-u.m-thick P M M A films. S h o r t l y after L i n ' s report, F e l d m a n et a l . (9) p u b l i s h e d the results of t h e i r experiments i n v o l v i n g exposure of p o l y ( b u t e n e - l sulfone) to 185-nm radiation. S i n c e these early reports, a variety of resist materials a n d processes have b e e n d e v e l o p e d a n d are the subject of this r e v i e w . B u t the application of D U V lithography to practical d e v i c e fabrication requires d e v e l o p m e n t of both the exposure system a n d the resist materials. Significant changes i n c u r r e n t exposure e q u i p m e n t are r e q u i r e d to r e alize the potential for resolution i m p r o v e m e n t that shorter w a v e l e n g t h can p r o v i d e . T h e materials u s e d for fabrication of optical e l e m e n t s , i n c l u d i n g lenses, m u s t be r e p l a c e d b y materials that have acceptable transmission i n the D U V . T h i s step represents a major challenge, p a r t i c u l a r l y for lens d e signers, because they have few transparent materials from w h i c h to choose a n d yet r e q u i r e an i n v e n t o r y of transparent materials w i t h v a r y i n g refractive indices from w h i c h to construct lenses that are corrected for c h r o m a t i c a b errations. T h e alternative is to use v e r y n a r r o w band-pass filtering of the source that, for the usual high-pressure m e r c u r y l a m p , corresponds to an intolerable r e d u c t i o n i n source brightness. M i r r o r - l e n s - b a s e d projection systems have less c r i t i c a l d e m a n d s on materials because c h r o m a t i c aberration c o r r e c t i o n does not r e q u i r e refractive elements, b u t filters a n d coatings


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d e s i g n e d to isolate the d e s i r e d D U V w a v e l e n g t h range m u s t be d e v e l o p e d . E v e n the standard h i g h - p r e s s u r e m e r c u r y l a m p sources m u s t b e fabricated w i t h special attention g i v e n to the transmission characteristics of the e n velope material. M o s t o f these challenges, m a n y o f w h i c h are q u i t e f o r m i d a b l e , have b e e n m e t at some l e v e l . C o m m e r c i a l l y available contact p r i n t e r s a n d m i r r o r based p r o j e c t i o n p r i n t e r s are n o w b e i n g u s e d i n d e v i c e m a n u f a c t u r i n g . R e fracting-lens-based p r o j e c t i o n p r i n t e r s (steppers) that use D U V l i g h t sources are c u r r e n t l y u n d e r d e v e l o p m e n t b y t o o l i n g manufacturers a n d have b e e n d e m o n s t r a t e d i n laboratory e n v i r o n m e n t s (10). A p p a r e n t l y , m a n u f a c t u r i n g engineers w i l l have a range of D U V exposure e q u i p m e n t from w h i c h to choose a n d a r o u n d w h i c h to d e v e l o p processes before the e n d of the decade. I n contact p r i n t i n g , D U V radiation p r o v i d e s i m p r o v e d r e s o l u t i o n a n d the a b i l i t y to p a t t e r n t h i c k e r resist films because of a r e d u c t i o n i n the extent of diffraction-based d i s t o r t i o n of the i n t e n s i t y function (aerial image) w i t h i n the resist film (11). I n p r o x i m i t y p r i n t i n g , w h e r e the mask is d e l i b e r a t e l y separated from the resist film surface b y a s m a l l b u t c o n t r o l l e d distance, a change to shorter w a v e l e n g t h can a l l o w , as M o r e a u a n d S c h m i d t r e c o g n i z e d , e i t h e r a larger gap at constant r e s o l u t i o n o r i m p r o v e d r e s o l u t i o n at constant gap. A d e t a i l e d analysis of the effect o f w a v e l e n g t h o n the i n t e n s i t y f u n c t i o n i n the resist film for p r o x i m i t y p r i n t i n g was p r o v i d e d b y L i n (II), a n d a m o r e qualitative d e s c r i p t i o n is also available (lb). T h e resolution i n b o t h contact a n d p r o x i m i t y p r i n t i n g is p r o p o r t i o n a l to the V% p o w e r of the w a v e l e n g t h . T h e effect o f r e d u c i n g the exposure w a v e l e n g t h is greatest i n p r o j e c t i o n p r i n t i n g , w h e r e the r e s o l u t i o n 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 lens n u m e r i c a l aperture ( N A ) . F o r a fixed N A , r e d u c i n g the exposure w a v e l e n g t h can b e e x p l o i t e d to p r o v i d e an increase i n the d e p t h of focus at constant resolution. T h e s e relationships have b e e n d e r i v e d (lb, 11).

3.1.2 D e e p - U V Light Sources T h e source of radiation for p h o t o l i t h o g r a p h y has traditionally b e e n a H g o r H g - r a r e gas discharge l a m p (12). T h e c o n v e n t i o n a l discharge l a m p consists of a q u a r t z e n v e l o p e that encloses two refractory m e t a l (usually W ) electrodes that are c h a r g e d w i t h a carefully m e t e r e d a m o u n t of e l e m e n t a l H g a n d a rare gas (usually X e ) . T h e s e lamps are excited b y external p o w e r supplies i n the range of 0 . 5 - 2 . 0 k W . T h e e q u i l i b r i u m pressure of H g a n d the rare gas d e t e r m i n e the spectral d i s t r i b u t i o n of the light output. T h i s o u t p u t is h i g h i n the n e a r - U V r e g i o n (350-450 n m ) , l o w e r i n the m i d - U V r e g i o n ( 3 0 0 - 3 5 0 n m ) , a n d v e r y l o w i n the D U V r e g i o n (200-300 nm). T h e l o w efficiency of these lamps i n the D U V r e g i o n can b e e x p l a i n e d i n t e r m s of the e n e r g y l e v e l d i a g r a m s h o w n i n F i g u r e 3.1. T h e transitions i n v o l v i n g the g r o u n d state of the atom (resonance lines) have the greatest


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p r o b a b i l i t y o f occurrence. H g has o n l y one resonant l i n e at 253.7 n m , corr e s p o n d i n g to the e m i s s i o n associated w i t h transition from the excited state Px to the g r o u n d state S . T h i s resonant l i n e is the strongest or characteristic o u t p u t of a l o w - p r e s s u r e discharge, b u t , w i t h i n c r e a s i n g pressure, it b e c o m e s almost c o m p l e t e l y reabsorbed. 3

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F i g u r e 3.1 shows that e m i s s i o n b e t w e e n 200 a n d 300 n m occurs from transitions b e t w e e n h i g h l y e x c i t e d states to the group of 6 p t r i p l e t levels l y i n g a r o u n d 5 eV. T h u s , to e m i t D U V photons, H g atoms m u s t b e e x c i t e d to a state w i t h e n e r g y 8 - 1 0 e V above the g r o u n d state. T h e major m e c h a n i s m for excitation i n these lamps involves inelastic c o l l i s i o n o f H g atoms w i t h electrons from the plasma. T h e average energy o f these electrons is o n l y 0.5-1.0 e V ; thus, i n d i v i d u a l electrons do not, o n the average, have sufficient 3


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e n e r g y to excite the r e q u i r e d transitions. O n e obvious m e t h o d for i n c r e a s i n g the average e n e r g y o f the p l a s m a electrons w o u l d b e to increase the discharge t e m p e r a t u r e . H o w e v e r , these lamps are already o p e r a t e d near the t h e r m a l stability l i m i t of quartz i n steady-state discharge applications. D e s p i t e t h e i r l o w o u t p u t , discharge lamps have the attractive feature of s m a l l source size that makes c o n d e n s e r design s i m p l e a n d operation stable. D e u t e r i u m lamps have b e e n c o n s i d e r e d for D U V sources because these lamps have a b r o a d - b a n d e m i s s i o n i n the d e s i r e d spectral r e g i o n . H o w e v e r , they are difficult to r u n at h i g h p o w e r because of out-diffusion of d e u t e r i u m , p l a s m a instability, a n d short lifetime (13). O n e m e t h o d of i m p r o v i n g the efficiency of the H g discharge l a m p is to d o p e it w i t h Z n or C d ( I I , 12). T h i s approach p r o b a b l y deserves m o r e attention. A n o t h e r m e t h o d is to s i m p l y a p p l y h i g h e r p o w e r b u t i n short pulses. T h i s m e t h o d allows a c h i e v e m e n t of the h i g h e l e c t r o n velocities r e q u i r e d for excitation to the r e q u i r e d e n e r g y levels w i t h o u t o v e r h e a t i n g the q u a r t z e n v e l o p e . T h i s m e t h o d is u s e d i n p u l s e d X e lamps. T h e s e lamps operate w i t h h i g h c o n v e r s i o n efficiency a n d pulse w i d t h s of m i l l i s e c o n d s . H o w e v e r , the p u l s e d nature of these l a m p s , t h e i r l o w r e p e t i t i o n rate, a n d special c o o l i n g r e q u i r e m e n t s have p r e c l u d e d t h e i r use, particularly i n m i r r o r based scanning exposure systems. M i c r o w a v e - p o w e r e d H g discharge lamps have v e r y h i g h source b r i g h t ness i n the D U V r e g i o n . T h e i r efficiency stems from the fact that the m i c r o w a v e - i n d u c e d p l a s m a forms i n a n a r r o w s h e l l v e r y near the l a m p w a l l . A s a result, most of the l i g h t is e m i t t e d near the l a m p w a l l a n d t h e r e is less reabsorption. T h e efficiency of these lamps is h i g h because the average e l e c t r o n e n e r g y is h i g h e r than that i n an arc l a m p . T h e m a i n drawback of these sources is t h e i r large source size that greatly complicates condenser optics design. T h e s e lamps are e x t r e m e l y useful i n applications of b l a n k e t D U V exposure. A n o t h e r i n t e r e s t i n g source of D U V radiation for m i c r o l i t h o g r a p h y is e x c i m e r lasers (13). T h i s r e l a t i v e l y n e w class of v e r y efficient a n d e x t r e m e l y p o w e r f u l p u l s e d lasers b e c a m e c o m m e r c i a l l y available i n 1978. T h e y operate at several characteristic wavelengths r a n g i n g f r o m less than 200 n m to greater than 400 n m . T h e o u t p u t is t y p i c a l l y 10-20-ns w i d e pulses w i t h r e p e t i t i o n rates from t e n to several h u n d r e d H e r t z . J a i n (14) p r o v i d e d a recent r e v i e w of laser a p p l i c a t i o n to m i c r o l i t h o g r a p h y . A n i m p o r t a n t characteristic of e x c i m e r lasers that sets t h e m apart from traditional U V lasers is t h e i r lack of spatial coherence. T h e interference p h e n o m e n a that result from the h i g h spatial coherence of traditional singlem o d e continuous wave lasers produces a r a n d o m i n t e n s i t y variation i n p r o j e c t e d patterns c a l l e d speckle. T h i s speckle p h e n o m e n o n has historically made use of lasers i n h i g h - r e s o l u t i o n l i t h o g r a p h y v e r y difficult. T h e b e a m of e x c i m e r lasers is so h i g h l y m u l t i m o d e that speckles are, for a l l practical purposes, nonexistent i n p r o j e c t e d patterns. T h e application of e x c i m e r laser



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sources to D U V l i t h o g r a p h y has b e e n d e m o n s t r a t e d for contact p r i n t i n g (15), m i r r o r p r o j e c t i o n p r i n t i n g (16), a n d , most r e c e n t l y , for r e d u c t i o n p r o j e c t i o n p r i n t i n g i n step-and-repeat fashion b y u s i n g specially c o n s t r u c t e d refracting lenses (JO). T h e use o f e x c i m e r lasers i n p r o d u c t i o n has b e e n h a m p e r e d b y the h i g h cost a n d i n c o n v e n i e n c e o f o p e r a t i n g these devices i n a p r o d u c t i o n e n v i r o n m e n t a n d difficulty i n d e s i g n i n g optical couplers that w o u l d b e the analog o f c o n v e n t i o n a l condenser systems that efficiently fill the entrance p u p i l o f the p r o j e c t i o n lens w i t h u n i f o r m i l l u m i n a t i o n o f appropriate partial c o h e r e n c e . F u r t h e r m o r e , the lasers c u r r e n t l y available suffer f r o m p u l s e - t o p u l s e p o w e r i r r e p r o d u c i b i l i t y a n d l i m i t e d operational stability o n a single gas fill. H o w e v e r , significant progress has b e e n made i n a l l o f these areas. I m p r o v e m e n t s i n electrode materials, gas h a n d l i n g systems, a n d p u l s e - c o n t r o l electronics have d e m o n s t r a t e d major extensions i n p o w e r stability. W h e n the e n g i n e e r i n g difficulties have b e e n o v e r c o m e , e x c i m e r lasers w i l l b e c o m e an i m p o r t a n t part o f the l i t h o g r a p h i c process. I n fact, one c o m m e r c i a l contact p r i n t e r can b e p u r c h a s e d w i t h an e x c i m e r laser source (16).

3.1.3 D e e p - U V Printer Systems 3.1.3.1 D E E P - U V C O N T A C T / P R O X I M I T Y P R I N T E R S

I n D U V contact p r i n t i n g , a photomask a n d a resist-coated wafer are b r o u g h t into tight contact. T h e wafer is t h e n exposed to D U V r a d i a t i o n of c o n t r o l l e d c o l l i m a t i o n t h r o u g h the mask. I n p r o x i m i t y p r i n t e r s , some m e c h a n i s m is p r o v i d e d that allows exposure w i t h a s m a l l , c o n t r o l l e d gap b e t w e e n the mask a n d the wafer surface. O p e r a t i o n o f such systems i n the D U V r e q u i r e s c e r t a i n modifications. T h e s e modifications are e x e m p l i f i e d i n the C a n o n P L A 5 2 0 F A p r i n t e r (17). T h e o p t i c a l system o f this p r i n t e r is s h o w n i n F i g u r e 3.2. T h e transmission e l e m e n t s o f the system are o f h i g h q u a l i t y q u a r t z to m i n i m i z e absorption i n t h e D U V . T h e system has a specifically d e s i g n e d d i c h r o i c , d i e l e c t r i c stack " c o l d m i r r o r " that has h i g h reflectivity i n the D U V r e g i o n b u t transmits v i s i b l e a n d I R radiation from the source o n to a heat sink. F i g u r e 3.3 shows the spectral o u t p u t o f this system w i t h t w o different c o l d m i r r o r s . T h e C a n o n P L A 5 2 0 F A can b e operated i n e i t h e r the contact o r p r o x i m i t y m o d e . T h e t i m e r e q u i r e d to expose P M M A has b e e n r e p o r t e d to b e 4 0 s w h e n c o l d m i r r o r C M 2 5 0 is u s e d (17c).

3.1.3.2 D E E P - U V P R O J E C T I O N P R I N T E R S

T h e a l l - r e f l e c t i n g 1:1 p r o j e c t i o n system was first r e p o r t e d b y M o l l e r i n 1973 (18). T h e o p t i c a l a n d scanning configuration o f such a system is i l l u s t r a t e d i n F i g u r e 3.4. I n these systems, a set of spherical m i r r o r s is u s e d to generate a n a r r o w , r i n g - s h a p e d aberration-free arc o f light. Wafers are scanned past this arc to p r o d u c e a n image as the mask is m o v e d s y n c h r o n o u s l y i n t h e object p l a n e . A n arc o f the mask is t h e r e b y i l l u m i n a t e d a n d i m a g e d onto


116


500W Xe-Hg lamp .Ellipsoidal mirror


A l u m i n u m mirror

Cold mirror

C o n d e n s e r lens

Detector M e r c u r y l a m p screen f

o r

|jg^

t

integrator

Mask surface ( 1 1 0 m m ) Figure 3.2. Optical layout of the illuminator in the Cannon PLA520FA contact/ proximity printer. (Reproduced with permission from reference 17a.)

Cold mirror C M 2 5 0 Xe-Hg lamp / C o l d mirror C M 2 9 0

250

300 mm Wavelength

Figure 3.3. Spectral deflection of cold mirrors. (Courtesy of Cannon.)


3.

IWAYANAGI E T A L .

Deep-UV

117

Lithography


Folding

Figure 3.4. All-reflecting

Mirror

1:1 projection optical system.

the wafer so that scanning once across the wafer a n d mask achieves exposure of the e n t i r e wafer surface. O n e of the o u t s t a n d i n g features of this system is the flexibility i t offers. I n p a r t i c u l a r , the m i r r o r lens system i n this configuration has essentially n o c h r o m a t i c aberration. H e n c e , one can vary the exposure w a v e l e n g t h s i m p l y b y i m p o s i n g the appropriate transmission filter b e t w e e n the condenser syst e m a n d the mask. M a n y i m p r o v e m e n t s have b e e n m a d e i n the d e s i g n of these tools since t h e i r first c o m m e r c i a l i n t r o d u c t i o n b y P e r k i n - E l m e r (19). T h e projection optics o f the P e r k i n - E l m e r M i c r a l i g n 500 are s h o w n i n F i g u r e 3.5. T h e N A of this system is 0.16. T h e refracting elements (shells) are a l l single elements a n d fabricated of h i g h q u a l i t y quartz. A n o t h e r example of such a system is the C a n o n M P A 5 2 0 F A ( F i g u r e 3.6). T h e i l l u m i n a t i o n source i n these p r i n t e r s is a h i g h - p r e s s u r e H g - X e l a m p . T h e spectral irradiance of the P e r k i n - E l m e r l a m p is s h o w n i n F i g u r e 3.7. P e r k i n - E l m e r p r o v i d e s band-pass filters for operation i n the " U V - 2 " ( D U V ) , " U V - 3 " ( m i d - U V ) a n d " U V - 4 " ( n e a r - U V ) spectral regions as s h o w n i n F i g u r e 3.8. T h e o u t p u t i n the U V - 4 m o d e is s i m i l a r to P e r k i n - E l m e r ' s earlier p r o j e c t i o n p r i n t e r s , the P E 1 0 0 a n d P E 2 0 0 series, a n d passes the spectral lines at 365, 405, a n d 436 n m . T h e U V - 3 m o d e transmits b e t w e e n 290 a n d 340 n m i n the m i d - U V r e g i o n , a n d the U V - 2 filter transmits i n the D U V r e g i o n b e t w e e n 220 a n d 290 n m . T h e relative o u t p u t of the M i c r a l i g n 500 at the wafer plane i n the absence o f any filtering is s h o w n i n F i g u r e 3.8. I n F i g u r e 3.8, the area u n d e r the c u r v e i n the D U V is a v e r y s m a l l p r o p o r t i o n of that u n d e r the c u r v e i n the n e a r - U V ( U V - 4 ) r e g i o n of the s p e c t r u m because of the o u t p u t inefficiency of the H g discharge source. A l t h o u g h these systems have b e e n d e s i g n e d a n d e n g i n e e r e d such that a shift into the D U V w i l l p r o v i d e n e a r l y a twofold i m p r o v e m e n t i n r e s o l u t i o n , e i t h e r a m u c h b r i g h t e r l i g h t source or a far m o r e sensitive resist is r e q u i r e d




Su c/3

κ

Ο

Ης)

ο

ζ

Ω

Ζ

ι

ο

o ζ

m r

M

H00

3.

Deep-UV

IWAYANAGI E T A L .

119

Lithography 2 kW X e - H g lamp


Spherical m i r r o r

Spherical m i r r o r jA\ m i r r o r \ Mask \ A r c u a t e slit

Primary m i r r o r

Figure 3.6. Illumination and projection systems of the Cannon (Courtesy of Cannon.)

MPA520FA.

to m a i n t a i n the wafer t h r o u g h p u t (productivity) that can b e a c h i e v e d i n the U V - 4 mode.

3.2 Deep-UV Resist Materials 3.2.1 P os i t i v e Resists 3.2.1.1 D I S S O L U T I O N - I N H I B I T O R S Y S T E M S

C o n v e n t i o n a l positive photoresists consist o f a m a t r i x r e s i n a n d a photoactive c o m p o u n d . T h e m a t r i x r e s i n is a c r e s o l - f o r m a l d e h y d e novolac r e s i n (struct u r e 3.1) that is soluble i n aqueous base s o l u t i o n , a n d the photoactive c o m p o u n d is a s u b s t i t u t e d d i a z o n a p h t h o q u i n o n e (structure 3.2) that functions as a d i s s o l u t i o n i n h i b i t o r for the matrix r e s i n . A s o u t l i n e d i n S c h e m e 3.1 (20), the photoactive c o m p o u n d undergoes a structural transformation u p o n U V radiation, k n o w n as Wolff rearrangement, f o l l o w e d b y reaction w i t h w a t e r


120


50 r-

365

CD

O

c


CO

o o Q. CO CD >

CD

DC

200

300

500

400

Wavelength

(nm)

Figure 3.7. Spectral irradiance of the Perkin-Elmer of Perkin-Elmer.)

Xe-Hg

lamp. (Courtesy

£ c CN £ 5

D +a-» 3

o

200

300

400

500

600

Wavelength in n m Figure 3.8. Optical output at wafer plane of Perkin-Elmer (Courtesy of Perkin-Elmer.)

Micralign


500.


3.

IWAYANAGI ET AL.

121

Deep-UV Lithography

0

R

0

R

Base S o l u b l e Photoproduct Scheme 3.1. Photochemical transformation of diazonaphthoquinone

sensitizer.

to f o r m a base-soluble i n d e n e c a r b o x y l i c a c i d that no l o n g e r i n h i b i t s dissol u t i o n o f the novolac matrix r e s i n i n alkaline d e v e l o p e r . C o n s e q u e n t l y , the exposed regions o f the film are r e n d e r e d m o r e soluble t h a n the u n e x p o s e d regions, a n d the p h o t o c h e m i c a l l y i n d u c e d differential s o l u b i l i t y rate is u s e d to generate positive-tone images o f the mask. T h e positive photoresists based o n a novolac m a t r i x r e s i n a n d a d i a z o q u i n o n e sensitizer e v o l v e d f r o m materials o r i g i n a l l y d e s i g n e d b y K a l l e C o r p o r a t i o n i n G e r m a n y to p r o d u c e photoplates u s e d i n the p r i n t i n g i n d u s t r y . T h e s e positives photoresists have b e c o m e the " w o r k h o r s e s " o f the m i c r o electronics i n d u s t r y because o f t h e i r h i g h r e s o l u t i o n a n d d r y e t c h resistance. In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

122



3.2.1.1.a Mid-UV

Resists Based on

Diazoquinone-Novolac

B i n n i n g a n d co-workers (21-23) d e m o n s t r a t e d that the theoretical i m p r o v e m e n t i n r e s o l u t i o n can b e a c h i e v e d b y r e d u c i n g the exposing w a v e l e n g t h from the n e a r - U V to the m i d - U V . T h e y exposed a positive photoresist, H P R 2 0 4 , coated o n T a - m e t a l i z e d wafers t h r o u g h a m e a n d e r p a t t e r n mask o n a m o d i f i e d P e r k i n - E l m e r 111 projection p r i n t e r o p e r a t i n g i n the n e a r U V a n d m i d - U V ranges. A f t e r p l a s m a e t c h i n g of the substrate, the T a p a t t e r n integrity was e x a m i n e d b y e l e c t r i c a l p r o b i n g . T h e percentage of good patterns is p l o t t e d as a function of l i n e size i n F i g u r e 3.9. T h i s figure indicates that m i d - U V exposure results i n a n i m p r o v e m e n t i n b o t h resolution a n d y i e l d . T h i s i m p r o v e m e n t is r o u g h l y p r o p o r t i o n a l to the m e a n w a v e l e n g t h of the exposing r a d i a t i o n . H o w e v e r , c o m m e r c i a l l y available positive photoresists such as A Z 1 3 5 0 J a n d H P R 2 0 4 demonstrate greatly r e d u c e d sensitivity i n the m i d - U V r e g i o n i n c o m p a r i s o n to t h e i r performance i n the n e a r - U V r e g i o n . T h e reasons for this loss i n s e n s i t i v i t y are as follows: • T h e m o l a r e x t i n c t i o n coefficient of the l - o x o - 2 - d i a z o n a p h t h o quinone-5-arylsulfonate (structure 3.2) sensitizers that are u s e d to formulate most c o m m e r c i a l photoresists is v e r y l o w at 313 n m c o m p a r e d to that at 405 n m .

100

S =

l

2 1.0

0.5

l

0.86

I

l

1.5

2.5

2.0

Meander Width and Space (pm) Figure size

3.9. and

Electrical

wavelength. wafer

tested.

probe The

yield vertical

(Reproduced

of

meander

bars with

indicate permission

patterns the

versus total

from

yield reference

design range

feature for 23.)


the

3.

IWAYANAGI E T A L .

Deep-UV

123

Lithography

• T h e s e materials u n d e r g o p h o t o c h e m i s t r y , u l t i m a t e l y l e a d i n g to a p h o t o p r o d u c t that is transparent at 405 n m b u t absorbs at 313 n m . • T h e p h e n o l i c resins u s e d i n most o f the c o m m e r c i a l resists have a significant u n r e a c h a b l e absorbance at 313 n m b u t are essentially transparent above 350 n m .


T h e s e a c c u m u l a t e d u n d e s i r a b l e optical characteristics are s h o w n i n F i g u r e 3.10 for A Z 1 3 5 0 J a n d F i g u r e 3.11b for H P R 2 0 4 . A Z 2 4 0 0 is different from most other c o m m e r c i a l positive photoresists i n b o t h f o r m u l a t i o n a n d response to m i d - U V radiation. T h i s resist is f o r m u l a t e d w i t h a r e s i n that is r e l a t i v e l y transparent i n the m i d - U V a n d l-oxo-2-diazonaphthoquinone-4-arylsulfonate (structure 3.3) rather t h a n the 5-arylsulfonate (structure 3.2) that is c o m m o n l y u s e d i n most c o m m e r c i a l photoresists (24). A transmittance s p e c t r u m for A Z 2 4 0 0 is p r o v i d e d i n F i g u r e 3.11a. T h i s

1.0 0.9 0.8 0.7

S

c CO

0.6

•S 0.5 o _Q

5) (40). A n e a r l y t h r e s h o l d l i k e exposure-dissolution response has b e e n o b s e r v e d i n this u n u s u a l l y h i g h contrast resist. T h e resist f o r m u l a t i o n is essentially aliphatic a n d w o u l d be less stable i n d r y e t c h i n g e n v i r o n -


ments t h a n resists based o n aromatic resins (43,

44).

3.2.1.2 POSITIVE R E S I S T S B A S E D O N M A I N - C H A I N SCISSION

M o r e a u a n d S c h m i d t (7) s h o w e d that the e l e c t r o n b e a m positive resist, P M M A , responds to D U V r a d i a t i o n . L i n d e m o n s t r a t e d its h i g h - r e s o l u t i o n capability b y D U V contact p r i n t i n g 0.5-u,m bars separated b y 0.25-|xm gaps i n 1.78-jxm-thick films of P M M A as s h o w n i n F i g u r e 3.19 (8). Photolysis of P M M A leads to c h a i n scission (Scheme 3.4): homolysis of the side c h a i n is followed b y decarbonylation to f o r m a stable tertiary radical o n the m a i n c h a i n , w h i c h , i n t u r n , undergoes cleavage t h r o u g h ^-scission of the c h a i n to generate an a c y l - s t a b i l i z e d t e r t i a r y r a d i c a l . T h i s process generates fragments of carbon m o n o x i d e , c a r b o n d i o x i d e , a n d m e t h y l a n d m e t h o x y radicals (45-50). T h e c h a i n scission i n i t i a t e d b y the photolysis of P M M A results i n the r e d u c t i o n of the m o l e c u l a r w e i g h t , w h i c h is p r i m a r i l y responsible for i n creased s o l u b i l i t y rate of the exposed r e g i o n . I n a d d i t i o n to the m o l e c u l a r

Figure 3.17. Computer-simulated resist profiles (SAMPLE). Operating input parameters include matched substrate, AZ1350J resist, 4358 A, 90 mj/cm , NA = 0.35, a = 9.99, defocus 0.0, development 80 s. The open image (B = 0.058) simulates AZ1350J performance. The shallow profile (B = 1.96) was generated from identical input parameters with the exception that the unbleachable absorbance (B) was adjusted to the value corresponding to the absorbance of 1 jxm of novolac at 254 nm. (Reproduced with permission from reference 37. Copyright 1981 Institute of Electrical and Electronics Engineers.) 2


132

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

1.0 "I

1

i t

0.9 i i 0.8

Exposed Resist \

Unexposed Resist


\

0.7

\

V

\

\

0.6

1

0.5

-

0.4

-

0.3 -

0.2

-

0.1

-

\ \

^

''r\

\ / \ / V S ^—

V V V u H U \\ U

1 250

300

350

Wavelength (nm) Figure 3.18. Absorbance spectra of Meldrums diazo resist before and after exposure. (Reproduced with permission from reference 37. Copyright 1981 Institute of Electrical and Electronics Engineers.)

o

°

x

°

5

•

No

hv

•

n

^ . 3 CO +

Scheme 3.2. Photochemical decomposition of 5-diazo-Meldrum's acid.


O


3.

IWAYANAGI E T AL.

Deep-UV

Lithography

Scheme 3.3. Photochemical decomposition

133

of o-nitrobenzyl

ester.

w e i g h t r e d u c t i o n , factors c o n t r o l l i n g solvent m o b i l i t y i n the P M M A m a r i x strongly affect the d i s s o l u t i o n kinetics of P M M A (51, 52). O u a n o (51) f o u n d that the increase i n the dissolution rate (S) w i t h decreasing m o l e c u l a r w e i g h t ( M W ) is m u c h faster i n electron-beam-exposed P M M A film t h u i i n u n e x posed films. T h e slopes o f the l o g S - l o g M W plots for electron-beam-exposed P M M A a n d unexposed P M M A are r e p o r t e d to be 2 a n d 0.5, respectively, i n a m y l acetate. T h e slope for D U V - e x p o s e d P M M A has b e e n r e p o r t e d to be 1.7 i n m e t h y l i s o b u t y l ketone ( M I B K ) (53). T h e greatly i n c r e a s e d d i s solution rate o f the exposed film c o m p a r e d to unexposed film o f the same m o l e c u l a r w e i g h t has b e e n ascribed to the formation o f increased free v o l u m e or d e n s i t y r e d u c t i o n d u e to gaseous p r o d u c t generation d u r i n g the r a d i o c h e m i c a l degradation o f P M M A a n d a c o r r e s p o n d i n g increase i n the rate at w h i c h the d e v e l o p e r solvent diffuses into the films i n the exposed regions. P M M A offers several advantages as a resist. T h e s e i n c l u d e e x t r e m e l y h i g h r e s o l u t i o n , ease o f h a n d l i n g , excellent film-forming characteristics, w i d e processing l a t i t u d e , a n d ready availability. U n f o r t u n a t e l y , P M M A is a r e l -



134


Figure 3.19. Y - I bars (Y- and I-shaped patterns) (0.5 \im) separated by 0.25|xm gaps, printed in 1.78 |xm of PMMA2041. (Reproduced with permission from reference 8. Copyright 1975 American Institute of Physics.) atively i n s e n s i t i v e m a t e r i a l a n d r e q u i r e s 0 . 5 - 1 . 0 J / c m o f D U V dose for w o r k a b l e processing. C o n s e q u e n t l y , m a n y analogs of P M M A have b e e n evaluated as D U V resists. T h e goal o f these studies has b e e n to p r e s e r v e the desirable properties of P M M A w h i l e i m p r o v i n g its sensitivity. T a b l e 3.1 s u m m a r i z e s the l i t h o g r a p h i c properties of methacrylate p o l y m e r s (54). T h e s e polymethacrylates a l l e x h i b i t similar U V absorptions w i t h optical densities of 0 . 2 7 - 0 . 4 7 | x m at 215 n m , w h i c h is c o n s i d e r a b l y less t h a n that of A Z 1 3 5 0 J i n the n e a r - U V r e g i o n (0.87 p - r n " at 405 n m ) . T h i s l o w absorption represents inefficient use of flux a n d is one reason for the l o w sensitivity o f the system. 2

_ 1

1

P o l y ( g l y c i d y l methacrylate) ( P G M A ) , a w e l l - k n o w n negative e l e c t r o n b e a m resist first r e p o r t e d b y H i r a i et al. (55), actually functions as a p o s i t i v e tone resist u p o n D U V exposure (Table 3.1) (56). T h e epoxide functionality responsible for c r o s s - l i n k i n g u n d e r electron b e a m exposure does not absorb i n the D U V r e g i o n , a n d the response o f P G M A to D U V radiation is d e t e r m i n e d b y the absorption d u e to the n — IT* transition of the c a r b o n y l c h r o m o -


3.

IWAYANAGI ET AL. CH

CH

3

2

c=o


t!iL_^

H

3

OCH

CH

3

3

H - C H ^ C ^ C H ^ - C - ^

c=o

I I

OCH

CH

3

- H C H - C - C H - C - + 2

135

Deep-UV Lithography

c=o

I I

OCH

3

c=o

OCH

3

3

1 CH

CH

3

-f-CH —C=CH + 2

3

-C-+-

2

C=0 I OCH + CO, C 0 Scheme 3.4. Mechanism of radiation-induced

2 >

CH «, 3

chain scission in

3

CH 0 3

PMMA.

Table 3.1. Polymethacrylate Positive D U V Resists a dxm )

Ester Group

Resist Poly(methyl methacrylate) ( P M M A ) Poly(glycidyl methacrylate) ( P G M A )

-1

-CH —CHU-CH—CH2 3

a

Sensitivity (//cm ) 2

0.42 0.33

0.6 0.8

0.29

0.5

0.27

0.094

\/

Poly(butyl methacrylate) (PBMA) Poly(fluorobutyl methacrylate) (FBM)

O n-butyl/ isobutyl (50/50) -CH CF CFHCF 2

2

3

The symbol a is the absorption coefficient at 215 nm. p h o r e that leads to m a i n - c h a i n scission. A m o n g the polymethacrylates, F B M , a poly(fluorobutyl methacrylate)-based resist, seems to be the most sensitive to D U V (54) as w e l l as to electron b e a m a n d X - r a y radiation (57). T h i s m a t e r i a l is c o m m e r c i a l l y available from D a i k i n K o g y o , Japan. T a b l e 3.2 summarizes a variety of the m e t h y l methacrylate ( M M A ) c o p o l y m e r s d e v e l o p e d as D U V resists. C h a n d r o s s et a l . (41, 61), W i l k i n s et al. (58), a n d R e i c h m a n i s et a l . (59, 60) r e p o r t e d that i n c o r p o r a t i o n of 3oximino-2-butanone methacrylate into the P M M A structure [ P ( M M A - O M ) ] i m p r o v e s b o t h the absorption characteristics of the p o l y m e r i n the 2 2 0 - 2 6 0 -


In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988. 110

60

85

50

0.85 2

RS

m a x

max

M M A / I = 97/3 \ = 240, 280 nm M M A / P h l P K = 72/28

a (215 nm) = 0.47 X u,m" M M A / G M A = 70/30 X = 215 nm M M A / O M = 63/37 Xouu = 220 nm MMA/DM/MAN = 69/16/15

Comments

NOTE: RS denotes the sensitivity relative to PMMA, and a denotes the absorption coefficient.

P(MMA-PhlPK)

P(MMA-I)

Phenyl isopropenyl ketone

3-Oximino-2-butanone methacrylate 3-Oximino-2-butanone methacrylate, methacrylonitrile Indenone

P(MMA-OM)

P(MMA-OMMAN)

Methacrylic acid Glycidyl methacrylate

Comonomer

P(MMA-MA) GCM

Resist

Table 3.2. M M A Copolymers as Positive D U V Resists

1

220-360

230-300

240-270

240-270

200-240 200-240

71

62

59

58

54 56

Effective Spectral Range (nm) Reference


3.

IWAYANAGI ET AL.

Deep-UV

137

Lithography

n m range a n d p r o v i d e s an alternate p a t h for scission a n d therefore leads to e n h a n c e d sensitivity. P ( M M A - O M ) (63:37) is 50 times m o r e sensitive than P M M A . I n c o r p o r a t i o n of m e t h a c r y l o n i t r i l e ( M A N ) into P ( M M A - O M ) results i n f u r t h e r increase i n the sensitivity (41, 5 9 - 6 1 ) . P ( M M A - O M - M A N ) is 85 times m o r e sensitive t h a n P M M A to D U V r a d i a t i o n . W h e n s e n s i t i z e d w i t h £er£-butylbenzoic a c i d , i t r e q u i r e s an exposure dose of less t h a n 30 m j / c m at 240 n m (61). C o p o l y m e r s o f i n d e n o n e w i t h M M A , also d e v e l o p e d b y C h a n d r o s s et a l . (41, 61, 62), have a strong U V absorption i n the 2 3 0 - 3 3 0 n m r e g i o n a n d e x h i b i t a m o n o c h r o m a t i c sensitivity of 60 m j / c m at 3 % i n d e n o n e concentration. T h e h i g h sensitivity is e x p l a i n e d i n terms of the steric strain present i n the cyclopentanone m o i e t y . R e s o l u t i o n of 0.75-fxm lines a n d spaces has b e e n a c h i e v e d i n the i n d e n o n e c o p o l y m e r s b y u s i n g a m o d i f i e d P e r k i n - E l m e r M o d e l 111, 1:1 p r o j e c t i o n p r i n t e r .


2

2

P o l y ( d i m e t h y l glutarimide) ( P M G I ) (structure 3.7) was s h o w n b y H i r aoka (63) to u n d e r g o m o l e c u l a r w e i g h t r e d u c t i o n u p o n i r r a d i a t i o n w i t h a sensitivity comparable to P M M A . T h i s p o l y m e r is sensitive to D U V r a d i a t i o n b e l o w 280 n m ; soluble i n aqueous base; resistant to c o m m o n organic solvents; a n d t h e r m a l l y stable to ca. 185 ° C , w h i c h renders the m a t e r i a l v e r y attractive as a t h i c k p l a n a r i z i n g l a y e r i n the e x p o s u r e - P C M scheme as w i l l b e discussed i n a later section. T h i s m a t e r i a l is b e i n g evaluated for c o m m e r c i a l i z a t i o n b y S h i p l e y C o m p a n y (64, 65).

CH

H

3

3.7 A n o t h e r i m p o r t a n t class o f d e g r a d i n g D U V positive resists is based o n i s o p r o p e n y l ketone p o l y m e r s (Table 3.3). T s u d a et a l . (66) r e p o r t e d t h e use of p o l y ( m e t h y l i s o p r o p e n y l ketone) ( P M I P K ) as a positive D U V resist. T h i s p o l y m e r exhibits a weak absorption c e n t e r e d at 285 n m d u e to the c a r b o n y l c h r o m o p h o r e a n d is about 7 t i m e s m o r e sensitive than P M M A . A n exposure t i m e o f 28 s for l-|xm-thick P M I P K films was o b t a i n e d o n a C a n o n P L A 5 2 0 F A contact p r i n t e r e q u i p p e d w i t h a C M 2 9 0 c o l d m i r r o r (67). A d d i t i o n of s e n sitizers such as 3 , 4 - d i m e t h o x y b e n z o i c a c i d results i n a threefold increase i n sensitivity (53, 66, 67). P M I P K a n d sensitized P M I P K are c o m m e r c i a l l y available u n d e r the trade names of O D U R 1 0 1 0 a n d O D U R 1 0 1 3 a n d 1014 from T o k y o O h k a K o g y o , i n Japan. M a c D o n a l d et a l . (68, 69) f o u n d that the q u a n t u m y i e l d of c h a i n scission at 313 n m i n films of p o l y ( i s o p r o p e n y l tertb u t y l ketone) ( P I P T B K ) is 12 times h i g h e r than that of P M I P K . I n P I P T B K ,



138


the c a r b o n y l g r o u p is located b e t w e e n two quaternary centers. T h u s , the N o r r i s h T y p e I a-cleavage o n e i t h e r side o f the c a r b o n y l carbon generates a stable t e r t i a r y radical as d e p i c t e d i n S c h e m e 3.5. T h i s result s h o u l d b e contrasted w i t h the situation i n P M I P K , w h e r e a-cleavage yields e i t h e r a tertiary b u t y l o r m e t h y l radical. A s the stability of these species is significantly different, cleavage o n l y occurs to generate the tertiary radical. T h u s , the r e p l a c e m e n t of the m e t h y l i n P M I P K b y a tertiary b u t y l group increases the p r o p e n s i t y for the N o r r i s h T y p e I p h o t o c h e m i c a l degradation, w h i c h is a p r e d o m i n a n t pathway i n the s o l i d state (70). A r o m a t i c groups p r o v i d e a c o n v e n i e n t c h r o m o p h o r e for U V absorption. C o p o l y m e r s of i s o p r o p e n y l p h e n y l ketone ( I P P h K ) w i t h M M A show spectral sensitivity u p to the m i d - U V range (Table 3.3) (71). P ( I P P h K - M M A ) (28:72) is 10 times m o r e sensitive t h a n P M I P K w h e n exposed to the full o u t p u t of a X e - H g lamp. P o l y ( b u t e n e - l sulfone) ( P B S ) , a sensitive, positive, e l e c t r o n b e a m resist, is h i g h l y sensitive to 185-nm radiation (Table 3.4) (9). H o w e v e r , P B S does not absorb above 200 n m , a n d the sensitization has not b e e n successful. Incorporation o f p e n d a n t aromatic rings into the polysulfone structure extends the photosensitivity to the D U V a n d m i d - U V regions (72). H i m i c s a n d Ross (73) r e p o r t e d that c a r b o n y l - c o n t a i n i n g poly(olefin sulfones) such as poly(5-hexen-2-one sulfone) are sensitive to U V - i n d u c e d degradation a n d Table 3.3. Poly(isopropenyl ketones) as Positive D U V Resists Resist

Comments

Reference

O D U R 1 0 1 0 (Tokyo Ohka) P M I P K + 3,4-dimethoxy benzoic acid P M I P K + sensitizer (ODUR1013) (scission)(solid) = 0.29 at 313 nm 12 times higher than P M I P K Copolymer with MMA(28:72)

66 66

RS

Poly(methyl isopropenyl ketone) (PMIPK)

1 3 3

Poly(isopropenyl tert-butyl ketone) (PIPTBK) Poly(phenyl isopropenyl ketone-co-M MA) [P(PhlPK-MMA)]

10

53 68, 69

71

NOTE: RS denotes the relative sensitivity to P M I P K . Table 3.4. Poly(olefin sulfones) as Positive D U V Resists Resist Poly(butene-l sulfone) (PBS) Polystyrene sulfone) (PSS) Poly(styrene-co-acenaphthalene sulfone)

Effective Spectral Range (nm)

Sensitivity (mjl cm )

Reference

180-200 240-280 250-300

5 (at 185 nm) 1000 (at 265 nm) 500 (200-400 nm)

9 72 72

2


3.

IWAYANAGI E T AL.

139

Deep-UV Lithography CH I -fCH —C> I

3

2

c=o

I CH3—C—CH3 I CH


3

CH

CH

3

-fCH -C-)I

-fCH -C>

2

2

c=o

00

I CH —C—CH3

CH3—C—CH3

3

1 CH

3

1 CH

3

CH I

3

3

-fCH -C-)2

C=0

•

CH3—C—CH

3

1 CH Scheme 3.5. Norrish Type I cleavage in PIPTBK. 3

that the sensitivity can be increased b y a d d i t i o n of c e r t a i n photosensitizers (e.g., b e n z o p h e n o n e ) . T h e i n c o r p o r a t i o n o f aromatic groups i n t o D U V resist systems also offers an increase i n d r y e t c h resistance. A s the m i n i m u m feature size of s e m i c o n d u c t o r devices s h r i n k , anisotropic d r y e t c h i n g is b e c o m i n g

more and

m o r e i m p o r t a n t i n d e v i c e fabrication. H o w e v e r , positive resist materials that efficiently u n d e r g o m a i n - c h a i n scission u p o n i r r a d i a t i o n generally lack d r y e t c h d u r a b i l i t y . T h i s d i c h o t o m y of performance r e q u i r e m e n t s is most c o m m o n l y c i r c u m v e n t e d o n l y i n t w o - c o m p o n e n t resists, b u t r e c e n t l y , 1:1 a l t e r n a t i n g c o p o l y m e r s of styrene a n d olefins that are t r i - or tetrasubstituted w i t h e l e c t r o n - w i t h d r a w i n g groups (74) have b e e n s h o w n to u n d e r g o m a i n - c h a i n scissioning reactions u p o n D U V or e l e c t r o n b e a m i r r a d i a t i o n w i t h a s e n s i -


140


t i v i t y c o m p a r a b l e to that o f P M M A , y e t these p o l y m e r s are as stable i n p l a s m a e n v i r o n m e n t s as p o l y s t y r e n e (75).

3.2.2 Negative Resists 3.2.2.1 A Z I D E SENSITIZER SYSTEMS


3.2.2.1.a Azide-Cyclized

Polyisoprene Photoresists

Photosensitive systems c o m p o s e d o f photoactive aromatic azide c o m p o u n d s a n d a v a r i e t y o f host p o l y m e r s have b e e n w e l l k n o w n since t h e 1930s (la). N e g a t i v e photoresists c o m p r i s i n g a n azide a n d a c y c l i z e d c i s - l , 4 - p o l y i s o p r e n e (structure 3.8) as a host p o l y m e r , s u c h as K o d a k ' s K T F R , have b e e n

3.8 w i d e l y u s e d i n the m i c r o e l e c t r o n i c s i n d u s t r y . T h e most c o m m o n l y u s e d azide sensitizer for c o n v e n t i o n a l n e a r - U V l i t h o g r a p h y is 2,6-bis(4'-azidobenzal)-4m e t h y l c y c l o h e x a n o n e (structure 3.9). O

CH

3

3.9 C y c l i z e d p o l y i s o p r e n e s e n s i t i z e d w i t h a n aromatic bisazide generates an i n s o l u b l e , t h r e e - d i m e n s i o n a l n e t w o r k v i a c r o s s - l i n k i n g u p o n i r r a d i a t i o n . T h e p h o t o i n d u c e d reactions associated w i t h t h e generation o f the n e t w o r k are s h o w n i n S c h e m e 3.6. T h e p r i m a r y event is t h e d e c o m p o s i t i o n o f the arylazide i n t h e e x c i t e d state i n t o a reactive n i t r e n e i n t e r m e d i a t e that c a n u n d e r g o a v a r i e t y o f reactions. T h e n i t r e n e reactions i n c l u d e n i t r e n e - n i t r e n e c o u p l i n g to f o r m azo dyes, i n s e r t i o n into c a r b o n - h y d r o g e n bonds to f o r m secondary a m i n e s , abstraction o f h y d r o g e n from t h e r u b b e r backbone to f o r m a n i m i n o radical a n d a c a r b o n radical that c a n s u b s e q u e n t l y u n d e r g o c o u p l i n g reactions, a n d i n s e r t i o n into t h e d o u b l e b o n d o f p o l y i s o p r e n e to f o r m t h r e e - m e m b e r e d a z i r i d i n e linkages (Scheme 3.6).


3.

IWAYANAGI ET AL.

n

} *-0*0 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch003

141

Deep-UV Lithography

R-N Azide

2

— -

I R-N: + H-C I

R-N =

N-R

I R - N H - C I

I R-N: + H-C I R - N : + |(^

'(

R-N: + N Nitrene + Nitrogen

3

R-N: + R-N:

N 3

I R-NH • + • C -

—-

R-N

Scheme 3.6. Crosslinking reactions in bisarylazide-ruhber

resists.

T h e c y c l i z e d r u b b e r - b i s a z i d e formulations offer h i g h sensitivity, ease o f h a n d l i n g , a n d w i d e process l a t i t u d e . H o w e v e r , the r e s o l u t i o n o f these systems is l i m i t e d b y r e l a t i v e l y l o w contrast a n d the s w e l l i n g - i n d u c e d d e formation of resist patterns d u r i n g d e v e l o p m e n t . A l t h o u g h the c r o s s - l i n k i n g reaction renders the p o l y m e r i n s o l u b l e i n the d e v e l o p e r , the c r o s s - l i n k e d p o l y m e r still has a n affinity for the d e v e l o p e r solvent. T h e r e f o r e , the crossl i n k e d regions absorb solvent d u r i n g d e v e l o p m e n t . T h e r e s u l t i n g increase i n v o l u m e , or s w e l l i n g , causes distortions o f fine patterns < 2 jxm i n the f o r m o f " b r i d g i n g " o r " s n a k i n g " ( F i g u r e 3.20). S e v e r a l bisazides w i t h absorption m a x i m a b e t w e e n 240 a n d 290 n m have b e e n e x a m i n e d as D U V sensitizers for c y c l i z e d p o l y i s o p r e n e (76). T h e sensitivity of D U V resist f o r m u l a t e d w i t h c y c l i z e d p o l y i s o p r e n e a n d 1 w t % o f 3 , 3 ' - b i s a z i d o p h e n y l sulfone (structure 3.10) is 75 times h i g h e r t h a n that o f P M M A . T h e s e azide D U V resists suffer f r o m the resolution l i m i t i m p o s e d b y l o w contrast a n d the s w e l l i n g p h e n o m e n o n , w h i c h is as expected from the p r e c e d i n g discussion.



142


Figure 3.20. Swollen images in a cross-linking negative resist. (Reproduced with permission from reference 105. Copyright 1986 Electrochemical Society.) 3.2.2.l.b

Azide-Phenolic

Resin

System

A n o v e l , n o n s w e l l i n g D U V resist was d e v e l o p e d b y Iwayanagi et a l . (77). T h i s resist, k n o w n as M R S (micro resist for shorter wavelengths), consists o f p o l y ( p - v i n y l p h e n o l ) (structure 3.11), a n a l k a l i n e - s o l u b l e p h e n o l i c r e s i n , a n d a n azide (structure 3.10).

D i s s o l u t i o n i n h i b i t i o n occurs u p o n exposure, a n d the unexposed p o r t i o n o f the resist film is d i s s o l v e d i n an aqueous alkali solution i n an e t c h i n g - t y p e dissolution that is d e v o i d of s w e l l i n g (78, 79). Because the c o m b i n e d a b sorption o f the azide a n d the r e s i n is q u i t e intense i n the D U V r e g i o n , as s h o w n i n F i g u r e 3.21, the p h o t o c h e m i c a l reaction l e a d i n g to the decrease i n s o l u b i l i t y occurs m a i n l y i n the u p p e r regions of the film. T h i s situation is s h o w n i n F i g u r e 3.22a, w h e r e the resist was exposed i n the U V - 2 m o d e ( 2 2 0 - 2 9 0 nm) o n a P e r k i n - E l m e r M 5 0 0 projection p r i n t e r (80). T h e u p p e r layers o f the exposed resist film d e v e l o p slowly, a n d the u n e x p o s e d l o w e r levels d e v e l o p at a constant rate. T h e i n s o l u b i l i z e d surface layer o f the exposed areas acts as a mask w h i l e the alkaline d e v e l o p e r removes u n e x p o s e d




•β

a

(δ

§

> r

H

a

>

$

Η—I



4

M S3

S

Ο

Τ3

*η

Ο

C/5

ΙΖ

Ζ Ω > Ω

§

33

*d

8°

Ω

ο

M

r

M

H-


c o £

-

Least Exposure

0.2

0.4

E 0.8 3 CO 8 0.6

1.0

1.2

V

Development

100

_

Time

200 (seconds)

300

Most , Exposure

Increasing Exposure

V

if •C

Dissolution Rate 22.50 A/secr Exposure 10800 1

Least

Most

10800 9000 7200 6300 5400 4500 3860 3600 3240 2880 2520 2160 1800 900 100

Exposure Table Micralign Scan Setting

Figure 3.22. Resist thickness versus development time of RD2000N in MF312-water (1:4): (a) UV-2 mode and (b) UV-3 mode. (Reproduced with permission from reference 80. Copyright 1983 Society of Photo-Optical Instrumentation Engineers.)

)


146


areas i n an e t c h i n g - t y p e d e v e l o p m e n t process. C o n s e q u e n t l y , resist profiles change from o v e r c u t to u n d e r c u t w i t h i n c r e a s i n g d e v e l o p m e n t t i m e . T h e resist profiles calculated b y the S A M P L E s i m u l a t i o n are c o m p a r e d i n F i g u r e 3.23 w i t h S E M s o f resist images o b t a i n e d b y D U V p r o j e c t i o n p r i n t i n g (81).


T h e resist responds to m i d - U V as w e l l as D U V r a d i a t i o n . F i g u r e 3.22b indicates that the exposure i n the U V - 3 m o d e (290-340 nm) results i n less u n d e r c u t t i n g because of the h i g h e r transparency i n the m i d - U V range (80). H o w e v e r , the resist is less sensitive to the m i d - U V than to the D U V b y a factor o f 2. S c a n n i n g exposure times r e q u i r e d for M R S are ca. 20 a n d 40 s i n the U V - 2 a n d U V - 3 m o d e , respectively, o n a P e r k i n - E l m e r M 5 0 0 p r i n t e r . T o s u m m a r i z e , the resist offers h i g h sensitivity a n d n o n l i n e a r d i s s o l u t i o n kinetics that l e a d to a h i g h p r o p e n s i t y for u n d e r c u t profiles w h e n exposed to the D U V r a d i a t i o n ; the resist offers l i n e a r dissolution kinetics a n d l o w sensitivity i n the U V - 3 m o d e . O n e possible solution to this d i c h o t o m y is the r e p l a c e m e n t of the c o m m e r c i a l p h e n o l i c r e s i n w i t h a r e s i n m o r e transparent i n the D U V r e g i o n . O n e s u c h example is an a c r y l i c acid-based r e s i n (82), w h i c h lacks d r y e t c h i n g resistance. A n o t h e r solution w o u l d b e to expose the M R S resist s i m u l t a n e o u s l y to b o t h D U V a n d m i d - U V radiation w i t h o u t any filter, w h i c h leads to h i g h - r e s o l u t i o n surface i m a g i n g d u e to the d o m i n a n t D U V exposure of the surface a n d h i g h aspect ratio i m a g i n g d u e to the d e e p p e n e t r a t i o n of the m i d - U V l i g h t ( F i g u r e 3.24) (83, 84). T h i s m o d e of exposure offers an a d d e d advantage o f decreased exposure t i m e (ca. 15 s o n a P e r k i n - E l m e r M 5 0 0 scanner). A n S E M p h o t o g r a p h of 1-jxm lines a n d spaces o b t a i n e d b y the b r o a d - b a n d exposure of M R S is g i v e n i n F i g u r e 3.25 (78). L i n (83) d e m onstrated the different responses of the M R S resist to m o n o c h r o m a t i c D U V K r C l (222 nm) a n d m i d - U V X e C l (308 nm) e x c i m e r laser exposures (see S e c t i o n 3.4.2). T h e u n d e r c u t profile available from D U V exposure of M R S is, h o w e v e r , p r e f e r r e d o v e r the overcut profile for anisotropic d r y e t c h i n g o n s t e p p e d surfaces as i l l u s t r a t e d i n F i g u r e 3.26 (83, 8 5 , 86). I n this a p p l i c a t i o n , the d i m e n s i o n of e t c h e d patterns is d e t e r m i n e d b y the top d i m e n s i o n of the i m a g e d resist; thus, the topography effect is e l i m i n a t e d . I n fact, the top edge of the u n d e r c u t M R S resist serves as a mask for anisotropic reactive i o n e t c h i n g ( R I E ) as s h o w n i n F i g u r e 3.27, w h e r e l-|xm-tall a l u m i n u m patterns w i t h v e r t i c a l sidewalls are o b t a i n e d . T h e M R S resist is c u r r e n t l y b e i n g u s e d i n a 1.5-|xm a l u m i n u m m e t a l i z a t i o n process c o m b i n i n g the P e r k i n - E l m e r M 5 0 0 p r o j e c t i o n p r i n t i n g w i t h R I E (87). T h e decrease i n s o l u b i l i t y u p o n exposure i n this type of resist was first a s c r i b e d to the formation of a secondary a m i n e generated from n i t r e n e i n sertion into C - H bonds o f the p o l y m e r (see S c h e m e 3.6) (88). H o w e v e r , g e l p e r m e a t i o n c h r o m a t o g r a p h i c analyses r e v e a l e d that the m o l e c u l a r w e i g h t of p o l y ( p - v i n y l p h e n o l ) increased u p o n i r r a d i a t i o n i n the presence of the azide. H y d r o g e n abstraction from the p o l y m e r b y n i t r e n e a n d subsequent p o l y m e r


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Figure 3.23. Cross sectional view of MRS resist as a function of development time: (a) experiment and (b) simulation. (Reproduced with permission from reference 81. Copyright 1982 Institute of Electrical and Electronics Engineers.)



148


Distance from line edge (urn) Figure 3.24. Simulated MRS profiles: (a) The intensity ratio of 256-300-nm radiation is 1:1. (b) The intensity ratio is 1:3. (Reproduced with permission from reference 84. Copyright 1983 North-Holland.) radical r e c o m b i n a t i o n results i n a n increase i n t h e m o l e c u l a r w e i g h t o f the p o l y m e r , r e n d e r i n g the exposed areas less soluble i n aqueous base solutions. T h e r a p i d decrease i n t h e d i s s o l u t i o n rate o f p o l y ( p - v i n y l p h e n o l ) i n alkaline solution w i t h i n c r e a s i n g m o l e c u l a r w e i g h t has b e e n separately ascertained (89). S e v e r a l D U V resists based o n p h e n o l i c resins sensitized w i t h azide compounds are n o w commercially available from H i t a c h i C h e m i c a l ( R D 2 0 0 0 N ) (90), H u n t C h e m i c a l (WX303) (91), a n d T o k y o O h k a K o g y o (ODUR120). 3.2.2.2 N E G A T I V E RESISTS B A S E D O N P O L Y S T Y R E N E DERIVATIVES

I n c o r p o r a t i o n o f aromatic rings i n t o p o l y m e r structures results i n i m p r o v e m e n t i n d r y e t c h resistance (43, 44). C o n s e q u e n t l y , negative resists based



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Figure 3.25. SEM of l-\im lines and spaces printed in MRS resist by broadband exposure. (Reproduced with permission from reference 78. Copyright 1981 Institute of Electrical and Electronics Engineers.)

/! i\

RTH

_

_

J

Figure 3.26. Line width variation caused by overcut resist profiles. This figure shows images printed in a resist with (left) overcut and (right) undercut resist profiles. The target etch dimension isLi.ln the left figure, topographic features result in variation in line width L > L j . In the figure on the right, with undercut profiles, the anisotropically etched image is controlled by the dimension of the opening at the top of the resist. Hence, both etched dimensions are the same, L j . (Reproduced from reference 83. Copyright 1983 American Chemical Society.) 2

o n c r o s s - l i n k i n g p o l y m e r s w i t h p e n d a n t aromatic groups have b e e n d e v e l o p e d p r i m a r i l y for d r y e t c h i n g applications 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 (92). T h e i n c o r p o r a t i o n o f aromatic rings i n t o p o l y m e r s p r o v i d e s not o n l y d r y e t c h i n g resistance b u t also absorption o f D U V a n d m i d - U V radiation d u e to the I T - I T * transitions o f the aromatic c o m p o u n d s as m e n t i o n e d i n section 3.1.2. T h e D U V l i t h o g r a p h i c properties o f c r o s s - l i n k i n g polystyrenes are listed i n T a b l e 3.5. In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


150

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

Figure 3.27. Reactive ion etching with MRS resist as the mask. (Reproduced with permission from reference 87. Copyright 1985 Society of Photo-Optical Instrumentation Engineers.)

Table 3.5. Polystyrene-Based Cross-Linking Negative D U V Resists

Resist

M

Chloromethylated polystyrene (CMS) Chlorinated poly(ra,p-methylstyrene) (m-/p-isomer 2:1) (CPMS) Poly(4-chlorostyrene) (PCS)

w

x 10

Comments

4

1.8-56 3-22 70

RS = 3.3-150, chloromethylation 0.13-0.98 4—0.8-s exposure times for a contact printer, chlorine content ~ 20 unit% 50 mj/cm , a (245 nm) = 0.21 j i m 2

Reference 93 97

1

95

NOTE: Abbreviations and symbols are as follows: M denotes the weight-average molecular weight, RS denotes the relative sensitivity to PMMA, and a denotes the absorption coefficient. w


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P o l y s t y r e n e , one of the simplest v i n y l p o l y m e r s w i t h a p e n d a n t b e n z e n e ring, is a n e g a t i v e - w o r k i n g resist b u t is m u c h less sensitive than P M M A (93). C h l o r o m e t h y l a t i o n o r e h l o r i n a t i o n of p o l y s t y r e n e has p r o d u c e d a class of negative D U V resists that exhibits v e r y h i g h sensitivity (Table 3.5). F o r e x a m p l e , c h l o r o m e t h y l a t e d p o l y s t y r e n e ( C M S ) w i t h a m o l e c u l a r w e i g h t of 1.8 X 1 0 is r e p o r t e d to be 40 times m o r e sensitive t h a n P M M A (93). P o l y s t y r e n e has a weak absorption at about 250 n m a n d a strong absorption b e l o w 220 n m . T h e absorption is e n h a n c e d a n d shifted to longer w a v e l e n g t h b y the i n t r o d u c t i o n of the c h l o r o m e t h y l group i n t o the b e n z e n e r i n g . T h e absorption i n the D U V r e g i o n increases as the extent of c h l o r o m e t h y l a t i o n increases, a n d , finally, a n e w peak appears at 230 n m . Increasing the extent of c h l o r o m e t h y l a t i o n is a c c o m p a n i e d b y a deterioration i n r e s o l u t i o n , w h i c h is a t t r i b u t e d to i n e v i t a b l e overexposure of the top l a y e r of the resist film w h e n adequate dose is d e l i v e r e d to ensure c r o s s - l i n k i n g of the b o t t o m l a y e r because of the strong absorption of the c h l o r o m e t h y l a t e d b e n z e n e c h r o m ophore. T h e D U V sensitivity of C M S increases w i t h i n c r e a s i n g m o l e c u l a r w e i g h t ; its contrast decreases w i t h increasing M /M (ratio of weight-average m o l e c u l a r w e i g h t to n u m b e r - a v e r a g e m o l e c u l a r weight) as has b e e n r e p o r t e d for e l e c t r o n b e a m a n d X - r a y exposures (94). T h i s class of c r o s s - l i n k i n g p o l y m e r s suffers from r e s o l u t i o n l i m i t a t i o n s d u e to the 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 . H o w e v e r , use of l o w - d i s p e r s i t y p o l y m e r s (93, 95) a n d j u d i c i o u s choice o f d e v e l o p e r solvents (96) can m i n i m i z e the s w e l l i n g . A t y p i c a l exa m p l e is g i v e n for c h l o r i n a t e d p o l y m e t h y l s t y r e n e ( C P M S ) i n F i g u r e 3.28 (97).


5

w

n

T o elucidate the m e c h a n i s m of c r o s s - l i n k i n g i n C M S , p u l s e radiolysis studies (98) a n d l o w - t e m p e r a t u r e e l e c t r o n s p i n resonance ( E S R ) studies (99)

Figure 3.28. SEM of 0.8-|xm lines and spaces printed in a I-jxm CPMS resist. (Reproduced with permission from reference 97. Copyright 1982 Society of Photographic Scientists and Engineers.)


152



o n substituted p o l y s t y r e n e m o d e l c o m p o u n d s have b e e n c a r r i e d out. O n the basis of t h e i r E S R studies, T a n i g a k i et a l . (99) p r o p o s e d n e w resist materials: a b l e n d of poly( p-methoxystyrene) a n d poly( p - c h l o r o m e t h y l styrene) a n d a n o n s w e l l i n g , aqueous base-developable negative resist c o n sisting of poly( p-hydroxystyrene) a n d a c h l o r i n e - r e l e a s i n g c o m p o u n d . A s i m ilar study was r e p o r t e d b y M a c D o n a l d et a l . (100). P o l y s t y r e n e - t e t r a t h i o f u l v a l e n e ( P S T T F ) (structure 3.12) is a n o v e l n e g ative resist d e v e l o p e d b y H o f e r et a l . (101) that has h i g h contrast a n d is n o n s w e l l i n g . T h i s resist has i n t r o d u c e d a n e w concept i n resist design. T h e differential dissolution is a c h i e v e d not t h r o u g h generation o f a t h r e e - d i m e n sional, c r o s s - l i n k e d n e t w o r k , b u t rather t h r o u g h a change i n the p o l a r i t y of p e n d a n t groups.

-fCH -CH_^ 2

3.12. E x p o s u r e of P S T T F sensitized w i t h a perhaloalkane such as carbon t e t r a b r o m i d e results i n generation of d i m e r i c tetrathiofulvalene b r o m i d e salts (Scheme 3.7) that are polar i n nature a n d , therefore, are i n s o l u b l e i n n o n p o l a r organic solvents (102). C o n s e q u e n t l y , d e v e l o p m e n t w i t h a nonpolar solvent selectively d i s solves the unexposed, n o n p o l a r p o l y m e r , p r o v i d i n g h i g h - r e s o l u t i o n negative patterns w i t h n o e v i d e n c e o f s w e l l i n g - i n d u c e d distortion. T h e P S T T F system is r e p o r t e d to show good sensitivity to A l K a X - r a y (50 m j / c m ) a n d e l e c t r o n beams (10 p , C / c m ) (101). T h e D U V l i t h o g r a p h i c performance of P S T T F has not yet b e e n p u b l i s h e d . 2

2

3.2.3 Dual-Tone Resists T h i s section discusses recent examples o f application of the design c o n c e p t e m b o d i e d i n the P S T T F resist (i.e., differential s o l u b i l i t y generation t h r o u g h

PS^TTF PSTTF

CBr

(Br-)

4

2

PSVTTF Scheme 3.7. Imaging mechanism of PSTTF resist.


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alteration o f the p o l a r i t y o f p o l y m e r side-chain groups rather than t h r o u g h the m o l e c u l a r w e i g h t change v i a backbone scissioning or cross-linking). T h e s e systems do not e x h i b i t r e s o l u t i o n loss d u e to s w e l l i n g d u r i n g d e v e l opment. O n e s u c h D U V resist e m p l o y s the p h o t o - F r i e s r e a r r a n g e m e n t as a m e c h a n i s m for a c h i e v i n g t h e d e s i r e d change i n s i d e - c h a i n p o l a r i t y . T h i s system is based o n poly( p-formyloxystyrene) (103-105) that undergoes a F r i e s - l i k e h o m o l y s i s a n d d e c o m p o s i t i o n u p o n exposure to D U V radiation to p r o d u c e poly( p-hydroxystyrene) a n d c a r b o n m o n o x i d e i n the exposed areas (Scheme 3.8)* T h e p h e n o l i c p h o t o p r o d u c t is soluble i n p o l a r solvents s u c h as alcohol o r aqueous base because o f the a c i d i t y o f the p h e n o l i c functionality, whereas t h e f o r m y l ester p r e c u r s o r i n the u n e x p o s e d areas is c o m p l e t e l y i n s o l u b l e i n these solvents. C o n s e q u e n t l y , d e v e l o p m e n t i n p o l a r solvents generates a positive-tone image of the mask. C o n v e r s e l y , the f o r m y l ester p o l y m e r is soluble i n n o n p o l a r solvents such as c h l o r o b e n z e n e o r anisole, i n w h i c h t h e p h e n o l i c p h o t o p r o d u c t is c o m p l e t e l y i n s o l u b l e . T h e r e f o r e , d e v e l o p m e n t i n these n o n p o l a r solvents generates a negative image o f the mask. T h e S E M s s h o w n i n F i g u r e 3.29 w e r e m a d e from a single wafer o f poly( p-formyloxystyrene) that h a d b e e n exposed o n a P e r k i n - E l m e r M 5 0 0 scanner i n the U V - 2 m o d e a n d t h e n b r o k e n i n half. O n e h a l f was d e v e l o p e d i n a p o l a r solvent to generate a positive-tone image, whereas the o t h e r was d e v e l o p e d i n a n o n p o l a r solvent to p r o d u c e a negative-tone image. T h e avoidance of s w e l l i n g d u r i n g processing is a necessary b u t not sufficient characteristic o f a v i a b l e resist system for use i n m i c r o m e t e r a n d s u b m i c r o m e t e r l i t h o g r a p h y . F o r s u c h systems to have p r a c t i c a l u t i l i t y , t h e y m u s t also f u n c t i o n w i t h e x t r e m e l y h i g h sensitivity to m a x i m i z e p r o d u c t i v i t y . T h e efficiency o f c r u c i a l p h o t o c h e m i c a l transformations is c h a r a c t e r i z e d b y the q u a n t u m y i e l d for the process expressed as molecules transformed p e r photons absorbed. T h e q u a n t u m y i e l d of t y p i c a l d i a z o n a p h t h o q u i n o n e s is from 0.2 to 0.3. T h u s , three or four photons are r e q u i r e d to transform a single m o l e c u l e o f sensitizer. T h i s places a f u n d a m e n t a l l i m i t o n the p h o tosensitivity o f s u c h systems. T o c i r c u m v e n t this i n t r i n s i c sensitivity l i m i t a t i o n that q u a n t u m efficiency

+ OCH II O Scheme 3.8. Photo-Fries

cot

OH

degradation.



154


Figure 3.29. (top) Positive and (bottom) negative images projection printed in a poly(p-formyloxystyrene) resist. (Reproduced from reference 103. Copyright 1984 American Chemical Society.) imposes o n systems that c o n s u m e at least one p h o t o n for every p r o d u c t i v e c h e m i c a l transformation, Ito et a l . (105-108) d e s i g n e d resist systems that incorporate " c h e m i c a l a m p l i f i c a t i o n " . I n such systems, a single p h o t o e v e n t initiates a cascade of subsequent c h e m i c a l reactions that u l t i m a t e l y express the i n t e n d e d f u n c t i o n . T h e y (105-108) d e s i g n e d n e w resist materials b y c o m b i n i n g c h e m i c a l amplification to p r o v i d e v e r y h i g h sensitivity a n d s i d e c h a i n modification to a l l o w processing w i t h o u t resolution loss d u e to s w e l l i n g .


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These systems are based o n acid-eatalyzed thermolysis o f side-chain p r o tecting groups. E x a m p l e s o f p o l y m e r s that function i n this fashion are l i s t e d in Chart 3.1. T h e tertiary b u t y l ester a n d carbonate groups are p a r t i c u l a r l y useful i n this application because they are sensitive to A - 1 hydrolysis that does n o t r e q u i r e a stoichiometric amount o f water. T h e s e materials u n d e r g o a c i d catalyzed thermolysis. C o n s e q u e n t l y , a resist system c a n b e f o r m u l a t e d b y casting these p o l y m e r s from solutions that also contain a substance that Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch003

A L

OCOBut I! o 4CH -CH^ 2

n

C-O-Bu* II

0

4CH -CH^~ 2

-4CH - C H ) -

n

•i Cr^OCr^COBu

0 I II CH OCH COH

1

2

CHQ

I 4CH -CfI C=0 2

n

0 Bu
o

CD

s

o

> r

PI H

I


180


optical p r o j e c t i o n p r i n t i n g offers a freedom from such defects a n d a h i g h a l i g n m e n t accuracy b u t suffers f r o m l o w contrast i n image i n t e n s i t y profiles at the resist surface caused b y diffraction o f light i n p r o j e c t i o n optics ( F i g u r e 3.14). A n elegant c o m b i n a t i o n of the advantages of the two p r i n t i n g m e t h o d s l e d to the e x p o s u r e - P C M t e c h n i q u e , first d e s c r i b e d b y L i n (157). I n this scheme, a t h i n resist layer that is opaque i n the D U V r e g i o n is s p u n o n top o f a t h i c k , p l a n a r i z i n g , D U V resist layer. T h e top layer is first i m a g e d b y a n o p t i c a l p r o j e c t i o n aligner a n d t h e n serves as a mask for the b o t t o m D U V resist that can n o w b e flood exposed to D U V radiation ( F i g u r e 3.45). L i n chose A Z 1 3 5 0 J as a top i m a g i n g resist because o f its h i g h opacity i n the D U V r e g i o n (158) a n d P M M A as a b o t t o m p l a n a r i z i n g l a y e r (257). T h e top l a y e r can b e i m a g e d b y e l e c t r o n b e a m exposure as w e l l as b y o p t i c a l p r o j e c t i o n (159). T h e b o t t o m P M M A image can be d e v e l o p e d i n c h l o r o b e n zene o r t o l u e n e , w i t h the top resist cap r e t a i n e d , or i n M I B K , w h i c h dissolves the cap, to p r o v i d e c a p p e d or u n c a p p e d structures, respectively ( F i g u r e 3.45). T h e c a p p e d configuration is p r e f e r r e d o v e r the u n c a p p e d one for s u b sequent R I E applications because P M M A has less e t c h resistance t h a n a r omatic materials (43, 44). O n the other h a n d , M I B K d e v e l o p m e n t p r o v i d e s h i g h e r contrast t h a n d e v e l o p m e n t i n c h l o r o b e n z e n e or toluene. F i g u r e 3.46 exhibits u n c a p p e d 0.85-u.m lines d e l i n e a t e d i n 1.9-u.m-thick P M M A . F i g u r e 3.47 shows c a p p e d images o b t a i n e d w i t h 0.3-u.m-thick A Z 1 3 5 0 J o n 2-u.mthick P M M A . T h e top resist l a y e r was i m a g e d b y e l e c t r o n b e a m exposure (30 p - C / c m ) i n b o t h cases (159). C o n s i d e r a b l e effort has b e e n d e v o t e d to the d e s i g n a n d a p p l i c a t i o n o f the e x p o s u r e - P C M system since L i n ' s report. T a b l e 3.7 s u m m a r i z e s the e x p o s u r e - P C M systems based o n the d i a z o q u i none r e s i s t - P M M A two-layer scheme. 2

O n e p r o b l e m associated w i t h the P C M scheme is that d u r i n g a p p l i c a t i o n o f a photoresist s u c h as A Z 1 3 5 0 J onto a P M M A film, a t h i n layer of P M M A is r e d i s s o l v e d a n d m i x e d w i t h the photoresist so that a t h i n interfacial l a y e r is f o r m e d that r e m a i n s after d e v e l o p m e n t of the photoresist layer a n d i n h i b i t s p r o p e r exposure a n d d e v e l o p m e n t o f the P M M A layer. Because the P M M A d e v e l o p e r , s u c h as c h l o r o b e n z e n e or toluene, u s e d i n the c a p p e d process is chosen to b e a nonsolvent for the photoresist, such a solvent cannot r e m o v e the interfacial layer. T h e r e f o r e , some process, l i k e p l a s m a t r e a t m e n t , is r e q u i r e d to r e m o v e the interfacial layer p r i o r to the b l a n k e t exposure o f the b o t t o m P M M A layer (83, 85). M I B K u s e d i n the u n c a p p e d process dissolves the interfacial layer. H o w e v e r , attenuation of D U V l i g h t d u e to the novolac r e s i n i n the interfacial layer, c o u p l e d w i t h the i n t r i n s i c , l o w sensitivity of P M M A d e v e l o p e d i n M I B K (see T a b l e 3.6), demands p r o l o n g e d b l a n k e t D U V exposure. T h i s h i g h dose o f D U V leads to excessive c r o s s - l i n k i n g a n d photooxidation of the n o volac resist (53, 166-169) a n d , consequently, to i n c o m p l e t e r e m o v a l of the top resist d u r i n g the d e v e l o p m e n t o f P M M A i n M I B K . L i n et a l . (85) r e p o r t e d that a soak i n m e t h a n o l / w a t e r (1:1) p r i o r to the M I B K d e v e l o p m e n t



3.

IWAYANAGI ET AL.

Deep-UV

Lithography

181

Figure 3.46. Uncapped resist image (0.8-u,m lines separated by 2.4 (xm) printed in 1.9-\km-thick PMMA by DUV blanket exposure with 0.2-\xm-thick AZ1350J as a PCM. (Reproduced with permission from reference 159. Copyright 1979 American Institute of Physics.)

Figure 3.47. Capped resist image obtained with a DUV PCM system. (Reproduced with permission from reference 85. Copyright 1981 American Institute of Physics.)



P M M A + coumarin 30

P M M A + coumarin 6

AZ1350J

Kodak 809 Scanner, stepper

Stepper

Stepper Stepper Stepper

UV E-beam Stepper, E-beam UV

Imaging

" M O S F E T is metal oxide semiconductor field effect transistor.

PMMA PMMA P M M A + coumarin 6

Layer

Hunt M P R AZ1350J Kodak 809

Planarizing

PMMA PMMA PMMA PMMA

Layer

AZ1350J,2400 AZ1350J AZ1350J AZ1350B

Imaging First P C M E-beam imaging Fabrication of l - ( x m M O S F E T C d lamp, acetone-based development P M M A planarization Fabrication of bubble device First application to production dye i n P M M A Submicron contact hole delineation Simulation

Comments

Table 3.7. Two-Layer D U V Flood E x p o s u r e - P C M Systems


f l

164 165

163

160 161 162

157 159 85

Reference

3.

IWAYANAGI E T A L .

Deep-UV

Lithography

183

facilitates the cap r e m o v a l . Bartlett et a l . (163) u s e d K o d a k 809 as the top i m a g i n g resist a n d r e m o v e d it i n a metal-free alkaline d e v e l o p e r after b l a n k e t D U V exposure a n d before P M M A d e v e l o p m e n t i n M I B K ; thus, the i n t e r facial l a y e r p r o b l e m i n the u n c a p p e d process was o v e r c o m e .


W i j d e n e s a n d G e o m i n i (170) e x a m i n e d the effects o f the p h e n o l i c r e s i n c o m p o s i t i o n , its 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 , solvent c o m p o s i t i o n , a n d p r e b a k e t e m p e r a t u r e o n the interfacial l a y e r formation. T h e y f o u n d that c o m b i n e d use o f p o l y ( p - v i n y l p h e n o l ) (structure 3.10) as m a t r i x r e s i n a n d cyclohexanone as the casting solvent i n the d i a z o q u i n o n e resist f o r m u l a t i o n m i n i m i z e s m i x i n g o f the two layers a n d y i e l d s a c a p p e d P C M structure w i t h o u t any p l a s m a treatment. A n o t h e r p r o b l e m w i t h the P C M process stems from reflections f r o m substrate surfaces, w h i c h cause l i n e w i d t h variations i n the m a s k i n g resist exposure. O ' T o o l e et a l . (171) d e m o n s t r a t e d that e v e n the P C M s c h e m e suffers from l i n e w i d t h variations d u e to reflections from the topographic structures o n the wafer. T h e y m i n i m i z e d the effect o f the reflected l i g h t b y d y e i n g the b o t t o m p l a n a r i z i n g layer i n a t h r e e - l a y e r R I E - P C M scheme. H e w l e t t - P a c k a r d manufactures V L S I N M O S (n-channel m e t a l oxide s e m i conductor) chips i n an u n c a p p e d P C M process b y u s i n g K o d a k 809 a n d a d y e d p l a n a r i z i n g l a y e r (163, 172). A c o u m a r i n d y e , c o u m a r i n 6 (structure 3.14), was chosen because this d y e strongly absorbs at the i m a g i n g w a v e -

3.14 l e n g t h , is transparent at the a l i g n m e n t w a v e l e n g t h , a n d does not p r e v e n t exposure o f P M M A ( F i g u r e 3.48). T h e N M O S I I I chips i n H e w l e t t - P a c k a r d ' s H P 9 0 0 0 32-bit c o m p u t e r are m a n u f a c t u r e d b y this b i l e v e l - d y e d D U V P C M . Various modifications o f the basic d i a z o q u i n o n e r e s i s t - P M M A c o m b i n a t i o n i n the b i l a y e r b l a n k e t - e x p o s u r e - P C M system have b e e n p r o p o s e d . M o d i f i e d P C M systems are s u m m a r i z e d i n T a b l e 3.8. T h e b o t t o m P M M A layer can b e r e p l a c e d w i t h m o r e sensitive D U V positive resists s u c h as P ( M M A - O M ) a n d P M I P K (cf. Section 3.1.2) to r e d u c e b l a n k e t - e x p o s u r e t i m e . C o n v e n t i o n a l p o s i t i v e p h o t o r e s i s t s are o p a q u e e n o u g h b e l o w 250 n m to act as an excellent mask for the image transfer exposure o f P M M A , w h i c h is sensitive to the D U V radiation r a n g i n g from 200 to 230 n m . O n the other h a n d , P M I P K is most sensitive i n the range


184



(a)

(b)

200-250

Hg/Xe Lamp Spectrum

CD O C CO +-*

E c

CO

CO

CD O

c

(c)

CO

E C/> c CO

500

600 Wavelength

PMMA Exposure

700

800

(nm)

GCA Exposure

Figure 3.48. UV transmission characteristics of dyed PMMA and Kodak 809 resist as a function of wavelength. (Reproduced with permission from reference 173. Copyright 1984 Society of Photo-Optical Instrumentation Engineers.)



PMMA PMMA

P M G I + coumarin 6

I B M terpolymer

LMR Ag Se/GeSe

Microposit 1470

Photoresists

2

PMMA PMMA

Layer

PSTTF RD2000N(MRS)

Planarizing

Kodak 809 +dye

Layer

Modified P M M A P(MMA-OM), P(MMA-OM-MAN) P M I P K + coumarin 6

HPR204

Imaging

Stepper

E-beam D U V scanner, D U V contact D U V contact U V scanner, stepper Stepper

Stepper

U V scanner

Imaging

A l lift-off High resolution, negligible interfacial layer Negligible interfacial layer, high thermal stability, aqueous base development Higher sensitivity, higher thermal stability, mold hardening

177

174 175 155 176 149 150 64

173

167

Reduction i n flood exposure time Dyes in both imaging and planarizing layers Negligible interfacial layer Negligible interfacial layer

Reference

Comments

Table 3.8. Various Schemes of Two-Layer D U V Flood E x p o s u r e - P C M Systems


186


of 2 9 0 - 3 1 0 n m , w h e r e the novolac resists are transparent e n o u g h to r e n d e r the mask useless for P M I P K exposure (Figures 3.10 a n d 3.11). T h e r e f o r e , i n c o r p o r a t i o n of a d y e w i t h absorption c e n t e r e d at about 300 n m is r e q u i r e d to m a k e the resist film m o r e opaque at the w a v e l e n g t h of P M I P K exposure. B i s p y r i d y l e t h y l e n e (structure 3.15) ( X = 300 nm) has b e e n f o u n d to be m a x


a good d y e i n K o d a k 809 for this P M I P K P C M s c h e m e as s h o w n i n F i g u r e 3.49 (173).

3.15 L i n et a l . (177) r e p l a c e d P M M A w i t h I B M t e r p o l y m e r resist (a t e r p o l y m e r of M M A , m e t h a c r y l i c a c i d , a n d m e t h a c r y l i c a n h y d r i d e d e v e l o p e d as an e l e c t r o n b e a m resist), w h i c h has a h i g h e r t h e r m a l stability a n d a h i g h e r D U V sensitivity. A n o t h e r i n t e r e s t i n g , D U V - s e n s i t i v e , p l a n a r i z i n g layer for the expos u r e - P C M s c h e m e is p o l y ( d i m e t h y l glutarimide) ( P M G I ) (structure 3.7). E x p o s u r e of P M G I to D U V l i g h t o r e l e c t r o n b e a m radiation results i n m a i n c h a i n scission; therefore, this resist is positive w o r k i n g . A totally aqueous-base-developable b i l a y e r P C M system that takes a d vantage of the h i g h t h e r m a l stability, aqueous-base s o l u b i l i t y , a n d h i g h s o l v e n t resistance of P M G I has b e e n r e p o r t e d (64, 65). T h e glass transition t e m p e r a t u r e (Tg) o f P M G I is ca. 189 °C; consequently, the resist image is t h e r m a l l y stable to this t e m p e r a t u r e . T h e acidic N - H groups r e n d e r P M G I soluble i n aqueous base w i t h the dissolution properties s i m i l a r to those of novolac resins. T h i s p o l y m e r is sparingly soluble i n c o m m o n organic solvents. T h i s characteristic results i n m i n i m a l interfacial m i x i n g b e t w e e n a top i m aging l a y e r a n d the b o t t o m p l a n a r i z i n g layer. T h e significant absorption of P M G I i n the D U V r e g i o n to 280 n m necessitates the i n c o r p o r a t i o n of a d y e into a d i a z o n a p h t h o q u i n o n e photoresist to r e n d e r the i m a g i n g l a y e r m o r e opaque i n the 2 5 0 - n m r e g i o n . S E M s d e m o n s t r a t i n g the t h e r m a l flow r e sistance a n d h i g h aspect ratio P C M p a t t e r n i n g are p r o v i d e d i n F i g u r e s 3.50 a n d 3.51. I n the P C M systems j u s t d e s c r i b e d , b o t h top a n d b o t t o m resists are positive w o r k i n g . R e s i d u a l exposure of a b o t t o m positive resist d u r i n g U V o r e l e c t r o n b e a m i m a g i n g of a positive top resist is acceptable. H o w e v e r , w h e n a negative resist is u s e d as the top layer, the r e s i d u a l exposure may r e d u c e the contrast of a b o t t o m positive resist. T h e several P C M systems i n v o l v i n g the use o f a negative resist as a top layer l i s t e d i n T a b l e 3.8 (imaging layers 3-6) indicate that i f there is a sufficient sensitivity difference b e t w e e n the top a n d the b o t t o m resists, the r e s i d u a l exposure can b e tolerated.


3.

Deep-UV

IWAYANAGI E T A L .

Hg/Xe Lamp Spectrum

200-2501 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: October 1, 1988 | doi: 10.1021/ba-1988-0218.ch003

187

Lithography

u

300

500

600

700

800

Wavelength (nm)

PMIPK GCA Exposure Exposure Figure 3.49. UV transmission characteristics of dyed PMIPK and dyed Kodak 809 resist in a mid-UV exposure-PCM scheme. (Reproduced with permission from reference 173. Copyright 1984 Society of Photo-Optical Instrumentation Engineers.)



188


Figure 3.50. SEMsof a PMGI pattern obtained by DUV exposure. (Reproduced with permission from reference 64. Copyright 1985 Society of Photo-Optical Instrumentation Engineers.) L i n et a l . first r e p o r t e d a negative top resist P C M system w i t h P S T T F (structure 3.12; Section 3.2.2.2) as an i m a g i n g layer. T h e P S T T F - P M M A system offers two advantages o v e r the A Z 1 3 5 0 J - P M M A system (174): 1. formation of a c a p p e d structure w i t h the high-contrast d e v e l o p m e n t of P M M A i n M I B K , a n d 2. no interfacial m i x i n g . T h e same advantages have b e e n o b t a i n e d w i t h o t h e r systems that use a negative resist as an i m a g i n g layer. T h e s e systems i n c l u d e M R S - P M M A a n d A g S e - G e S e - P M M A systems (Table 3.8) that are r e p o r t e d l y free from reflection p r o b l e m s d u e to the intense absorption of i m a g i n g U V i r r a d i a t i o n b y the top resist layer. F i g u r e 3.52 demonstrates patterns generated i n the c a p p e d P C M m o d e i n 0.4-u.m-thick M R S a n d 1.2-u.m-thick P M M A layers coated o n a 0.7-u.m-stepped S i 0 substrate (19). 2

2

A n isolation layer can b e i n c o r p o r a t e d b e t w e e n the two layers to alleviate the interfacial m i x i n g p r o b l e m . Table 3.9 summarizes such t h r e e - l a y e r P C M systems c o m p o s e d of a d i a z o q u i n o n e resist, an isolation layer, a n d P M M A . T h e use of a l u m i n u m , amorphous s i l i c o n , a n d a s p i n - o n antireflective coating



3.

IWAYANAGI ET AL.

Deep-UV

Lithography

189

Figure 3.51. SEMs demonstrating thermal stability (180 °C) of a PMGI pattern. (Reproduced with permission from reference 64. Copyright 1985 Society of Photo-Optical Instrumentation Engineers.)

Figure 3.52. Patterns formed in a bilayer resist system consisting of a 1.2-\imthick planarizing layer of PMMA and a OA-pm-thick RD2000N imaging resist. (Reproduced with permission from reference 19. Copyright 1984 Technical Publishing.)


190


Table 3.9. Three-Layer D U V Flood E x p o s u r e - P C M Systems Isolation Layer

Planarizing Layer

AZ1350J AZ1350J

Al a-Si

PMMA PMMA

Microposit 1400-17 Kodak 820

ARC ARC


Imaging

Layer

a

PMMA PMMA

Imaging

Reference

E-beam Stepper, E-beam Stepper Stepper

159 178 179 180

a-Si is amorphous silicon.

fl

( A R C ) as the isolation l a y e r has b e e n r e p o r t e d . A t t r a c t i v e a m o n g the isolation materials is the A R C , because a l l o f the layers can b e o b t a i n e d b y s p i n casting. T h e A R C is a t h i n organic coating c o m p o s e d of a p o l y a m i c a c i d a n d dyes for n e a r - U V p r o j e c t i o n l i t h o g r a p h y that reduces the reflection effect i n d i a z o q u i n o n e positive resists (181). T h e A R C absorbs strongly at the n e a r U V exposure w a v e l e n g t h to e l i m i n a t e d e t r i m e n t a l effects of standing waves a n d scatterings from the surface topography i n the top resist layer. T h e A R C layer can b e e t c h e d away d u r i n g the alkaline d e v e l o p m e n t o f the top resist. Because the p r o p e r l y b a k e d A R C film does not interact w i t h e i t h e r the b o t t o m P M M A or the top photoresist, c h l o r o b e n z e n e d e v e l o p m e n t of P M M A produces a c a p p e d structure desirable for R I E processes w i t h o u t c u m b e r s o m e p l a s m a d e s c u m m i n g . T h e major l i m i t a t i o n of this P C M s c h e m e seems to b e the n a r r o w process w i n d o w o f the A R C layer. C a r e f u l c o n t r o l of b a k i n g conditions is necessary to a v o i d severe erosion of the A R C layer d u r i n g the alkaline d e v e l o p m e n t of the top resist layer because the A R C m a t e r i a l is not photosensitive a n d is e t c h e d isotropically i n base. O v e r b a k i n g renders the A R C totally i n s o l u b l e . 3.3.1.2

DEEP-UV IMAGING AND R I E - P C M

SYSTEMS

A P C M system that uses R I E to define patterns i n a t h i c k p l a n a r i z i n g l a y e r was first r e p o r t e d b y H a v a s for the m e t a l lift-off process (182). M o r a n a n d M a y d a n (183, 184) fully d e m o n s t r a t e d the versatility a n d process c o m p a t i b i l i t y of this system. I n the t r i l e v e l R I E - P C M system, the t h i n top resist is p a t t e r n e d i n the standard fashion. T h e r e l i e f image is t h e n transferred b y R I E t h r o u g h an i n t e r m e d i a t e layer, w h i c h t h e n serves as an oxygen R I E mask for the t h i c k b o t t o m layer ( F i g u r e 3.45). H a r d - b a k e d novolac resists or h a r d - b a k e d p o l y i m i d e s are c o m m o n l y u s e d as the p l a n a r i z i n g layer. I n t e r m e d i a t e layers u s e d i n various t r i l e v e l systems i n c l u d e v a c u u m - d e p o s i t e d films o f S i O , S i N , S i , G e , s p u n - o n films such as "spin-on-glass", a n d soluble t i t a n i u m complexes. M o r a n a n d M a y d a n (184) d e m o n s t r a t e d the use of e l e c t r o n b e a m , X - r a y , refractive n e a r - U V projection, a n d reflective nearU V p r o j e c t i o n for i m a g i n g of the top resist a n d addressed the benefit of the t r i l e v e l R I E s c h e m e i n each l i t h o g r a p h y t e c h n i q u e . £

3

4

D U V l i t h o g r a p h y can also benefit from the R I E - P C M scheme. T a b l e 3.10 s u m m a r i z e s m u l t i l a y e r R I E systems that use D U V exposure to image In Electronic and Photonic Applications of Polymers; Bowden, Murrae J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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

RD2000N (MRS)

"Symbols used are as follows: a denotes a hard-baked diazoquinone resist, and b denotes a hard-baked polyimide.

x

T i O isolation layer, contact 0.4-|xm pattern Spin-on-glass isolation layer, scanner (M500), 0.7-|xm pattern

2

b a

Poly( p-disilanylene phenylene) Poly(Si-substituted M M A - M MA-MA) RD2000N (MRS)

2

Scanner (M500), 50-100 m j / c m (UV-3) 0.75-|xm pattern Contact, 0.7-jxm space pattern 0.5-3.6 J / c m at 260 n m

a

Aliphatic polysilane

2

Contact (PLA520), 60 m j / c m (CM290)

a

2

Scanner (M500), 5 m j / c m at 254 n m

a

2

10 m j / c m at 254 n m

a

2

a

Poly(dimethyl diphenylvinyl siloxane) Chloromethylated polydiphenylsiloxane Poly(trimethylstannylstyreneco-chlorostyrene) Poly(trimethylsilylstyreneco-chloromethylstyrene)

2

Seanner(M500), 100-s exposure (UV-2), 0.75 j x m L / S 200 m j / c m at 254 n m

Comment

a

0

Planarizing Layer

Ag Se/GeSe

Imaging Layer

Table 3.10. D U V Imaging R I E - P C M Resist Systems


87

196

194 195

192, 193

190, 191

188, 189

186, 187

185

152

Reference

192


the top resist. M o s t of the efforts i n this area have b e e n d e v o t e d to the design of s i m p l e , b i l a y e r R I E - P C M systems, as Table 3.10 shows. T h e i n t e r m e d i a t e l a y e r i n the t r i l e v e l scheme can b e e l i m i n a t e d i f the top l a y e r offers sufficient oxygen R I E resistance as w e l l as i m a g i n g capability. Inorganic resist systems are q u i t e suitable i n this respect. F i g u r e 3.53a shows profiles of 0.75-u,m l i n e a n d space arrays d e l i n e a t e d i n a h a r d - b a k e d p h o toresist w i t h A g S e - G e S e _ a mask, w h i c h was exposed i n the U V - 2 m o d e o n a P e r k i n - E l m e r M 5 0 0 scanner (152). A 5 0 % change i n exposure t i m e of this resist still results i n adequate resolution of 0.75-u.m lines a n d spaces w i t h reasonable l i n e w i d t h c o n t r o l . O n g et a l . (152) estimated that 0.65-u.m l i n e a n d space features c o u l d be resolved i n this resist system w i t h the scanner o p e r a t i n g i n the U V - 2 m o d e , although 0.5-u.m l i n e a n d space patterns w e r e not resolved. W h e n the inorganic resist was exposed to the b r o a d b a n d r a d i a t i o n , 0.75-p,m features w e r e resolved ( F i g u r e 3.53b), a n d the exposure t i m e was 4 times faster than i n the U V - 2 m o d e .


2

t

1

x

Resist materials c o m p o s e d o f organic p o l y m e r s do not possess sufficient plasma e t c h i n g d u r a b i l i t y to function p r o p e r l y as m a s k i n g layers for the e t c h i n g o f t h i c k p l a n a r i z i n g p o l y m e r layers (197). T o s i m p l i f y the R I E - P C M scheme, various organometallic resists that offer b o t h i m a g i n g capability a n d oxygen R I E resistance have b e e n d e v e l o p e d (Table 3.10). I n general, a l l of the systems contain a significant w e i g h t percentage of an e l e m e n t that forms a refractory oxide u p o n reaction w i t h oxygen plasma. W h e n these organom e t a l l i c resists are p l a c e d i n an oxygen p l a s m a , a t h i n l a y e r of nonvolatile oxide is generated o n the surface that is i m p e r v i o u s to f u r t h e r oxygen e t c h i n g (198) a n d acts as a mask for the anisotropic etch transfer of the top image into the t h i c k p o l y m e r p l a n a r i z i n g layer. T h e use of organometallic p o l y m e r s i n the b i l e v e l R I E - P C M s c h e m e was first r e p o r t e d b y S h a w et a l . (185). T h e y u s e d polysiloxane (structure 3.16) as a n e g a t i v e - w o r k i n g electron b e a m i m a g i n g layer. T h e etch rate ratio

i

i

1

3

_ ^ S i - 0 - S i - 0 ^ R

R4

2

3.16 of A Z 1 3 5 0 J to polysiloxane i n an oxygen plasma is r e p o r t e d to b e 50:1, i n d i c a t i n g that o n l y a 4 0 - n m - t h i c k polysiloxane layer w o u l d be sufficient to protect a 2-|xm-thick novolac resist. F i g u r e 3.54 shows h i g h aspect ratio 0.4p,m lines i m a g e d i n the p o l y ( d i m e t h y l - d i p h e n y l v i n y l s i l o x a n e ) - h a r d - b a k e d A Z 1 3 5 0 J system. I n c o r p o r a t i o n of a c h l o r o m e t h y l group i n t o the b e n z e n e r i n g of p o l y d i p h e n y l s i l o x a n e i m p r o v e s the D U V sensitivity as d e s c r i b e d i n the case of c h l o r o m e t h y l a t e d polystyrene ( C M S ) (see Section 3.2.2.2). A h i g h sensitivity (10 m j / c m 254 nm) has b e e n r e p o r t e d for c h l o r o m e t h y l a t e d 2


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Figure 3.53. SEMs of 0.75-jxra lines and spaces projection printed with a GeSe bilayer system: (a) UV-2 mode and (b) broad-band exposure. (Reproduced with permission from reference 152. Copyright 1982 Electrochemical Society.)



194


Figure 3.54. High aspect ratio images obtained by oxygen RIE pattern transfer with a polysiloxane as an etch mask. (Reproduced with permission from reference 185.) p o l y d i p h e n y l s i l o x a n e (186, 187). T h i s m a t e r i a l has a T r o o m t e m p e r a t u r e , g

whereas p o l y d i m e t h y l s i l o x a n e is a grease at r o o m t e m p e r a t u r e . T h e h i g h oxygen p l a s m a resistance o f t r i m e t h y l s i l y l - a n d t r i m e t h y l s t a n n y l s t y r e n e p o l y m e r s has b e e n c o m b i n e d w i t h h i g h D U V sensitivity t h r o u g h c o p o l y m e r i z a t i o n w i t h c h l o r i n a t e d o r c h l o r o m e t h y l a t e d styrene p o l y m e r (188-191) to p r o v i d e another example o f an imageable oxygen e t c h b a r r i e r m a t e r i a l . A v e r y h i g h s e n s i t i v i t y o f 5.5 m j / c m

2

254 n m has b e e n r e p o r t e d

for a 1:1 c o p o l y m e r o f t r i m e t h y l s t a n n y l s t y r e n e a n d p-chlorostyrene (189) (structure 3.17). E l e c t r o n spectroscopy for c h e m i c a l analysis ( E S C A ) (189)

- e

C

H

2 - C H ^ _ ^ C

CH3—M—CH I CH

3

H - ^

R

3

M = Si or S n R = CI o r C H C I 2

3.17 a n d A u g e r e l e c t r o n spectroscopic studies (190) s h o w e d that s i l i c o n o r t i n i n a v a r i e t y o f oxidation states generates a n effective oxygen e t c h b a r r i e r . T h e E S C A spectra s h o w n i n F i g u r e 3.55 demonstrate that after oxygen R I E t r e a t m e n t o f p o l y t r i m e t h y l s i l y l s t y r e n e , t h e b i n d i n g e n e r g y o f the S i 2 p t r a n sition increases b y 2.7 e V , i n d i c a t i n g t h e formation o f S i O , w h e r e x has a x

value b e t w e e n 1.5 a n d 2. M a c D o n a l d et al. (189) f o u n d that the i n c o r p o r a t i o n


3.

IWAYANAGI ET AL. 1

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Deep-UV 1

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Lithography I

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Materials and Processes for Deep-UV Lithography - ACS Publications

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