Polymers for High Technology - American Chemical Society

Chapter 8 E l e c t r o n - B e a m Sensitivity of A c r y l a t e Resists

Cross-Linked

Nigel R. Farrar and Geraint Owen

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch008

Hewlett-Packard Laboratories, Palo Alto, CA 94303-0867

Acrylate resists such as PMMA have excellent resolution and contrast but poor sensitivity. Improved contrast or sensitivity can be achieved by forming radiation sensitive crosslinks in copolymers of PMMA by thermal treatment on the wafer. In this work, we have evaluated the performance of a commercial resist which is a mix of two copolymers, and have explored methods for opti mizing its use. We have examined the effect of altering the crosslink density by controlling softbake tempera ture, using a flood exposure in addition to the patterning exposure, diluting the resist with PMMA and changing the copolymer mix ratio. All of these techni ques lead to improvements in sensitivity at the expense of contrast, with the most promising results being shown by the mix variations. The optimum crosslink density for acceptable contrast with maximum sensitivity has been determined. However, the most satisfactory method for achieving the reduced crosslink density is by con trolling the chemical structure of the resist, since all the methods explored in this work involve additional process complexity. Acrylate resists such as polymethylmethacrylate (PMMA) have been used extensively in electron beam lithography because of their ex cellent resolution and contrast, despite their limited dry etch re sistance and low sensitivity (1). Copolymers of PMMA, containing chemical groups more sensitive to radiation induced degradation, have also been studied and have shown up to a four-fold improvement in sensitivity (2). One approach has been to form a crosslinked gel, in-situ on the wafer, which contains radiation sensitive crosslinks and leads to improved sensitivity and improved contrast during development (3-7). Various crosslinked acrylate resists have been reported in the literature, mainly based on methacrylic acid anhydride (ΜΑΝΗ) cross link units. In the simplest case, the anhydride crosslinks may be formed by baking methacrylic acid (MAA) homopolymer, although this

0097-6156/87/0346-0086$06.00/0 © 1987 American Chemical Society

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

8.

FARRAR A N D OWEN

Electron-Beam

Sensitivity of kcrylate

87

Resists


r e q u i r e s a temperature o f 230°C and i s not s u i t a b l e f o r a l l s i t u a tions (8). One approach has been t o c a r r y o u t the d e h y d r a t i o n r e a c t i o n i n the s o l i d phase ( 9 ) . The r e s u l t i n g m a t e r i a l c o n t a i n s both i n t e r - and i n t r a - m o l e c u l a r a n h y d r i d e . The c r o s s l i n k e d m a t e r i a l can be f i l t e r e d out o f s o l u t i o n l e a v i n g a s e n s i t i v a t e r p o l y m e r which i s completely s o l u b l e . T h i s m a t e r i a l i s d i f f i c u l t t o p r e p a r e and does not o f f e r the advantage o f an i n s o l u b l e unexposed phase t o g e n e r a t e high c o n t r a s t . M A A - m e t h a c r y l o y l c h l o r i d e (MAC1) r e a c t i o n s have been shown t o form a n h y d r i d e c r o s s l i n k s a t lower t e m p e r a t u r e s , a l t h o u g h the s t a r t i n g m a t e r i a l i s more complex as i t must comprise two copolymers (3-4), o r a copolymer and t e r p o l y m e r ( 5 - 6 ) . Both m a t e r i a l s make use o f the g r e a t e r s e n s i t i v i t y o f the a c i d a n h y d r i d e group but d i f f e r i n the c o n t r a s t mechanism i n v o l v e d . Roberts (3) shows t h a t a n h y d r i d e i s d e s t r o y e d d u r i n g exposure and c o r r e l a t e s s e n s i t i v i t y w i t h a n h y dride content. K i t a k o h j i e t . a l . (5) a l s o a s s o c i a t e t h e a n h y d r i d e group w i t h the m a t e r i a l s e n s i t i v i t y but r e t a i n e x c e s s a c i d as a means of improving c o n t r a s t . MMA-MAA and MMA-MAC1 copolymers have a l s o been used s e p a r a t e l y t o form c r o s s l i n k e d r e s i s t s by r e a c t i o n w i t h a d i f u n c t i o n a l monomer ( 1 0 - 1 2 ) . Of the r e p o r t e d m a t e r i a l s , t h e o n l y c o m m e r c i a l l y a v a i l a b l e r e s i s t s are one o f the MMA-MAA/MMA-MAC1 c o m p o s i t i o n s and a t - b u t y l m e t h a c r y l a t e copolymer m a t e r i a l ( 1 3 ) . In t h i s work, we have chosen to examine the copolymer m i x t u r e , which c r o s s l i n k s a t a temperature o f 160°C compared t o 250°C f o r the s i n g l e c o p o l y m e r . We have c a r r i e d out e x p e r i m e n t s t o o p t i m i z e i t s use f o r d i r e c t w r i t e e l e c t r o n beam l i t h o g r a p h y . Theory The absorbed energy d e n s i t y , E g , r e q u i r e d t o d e s t r o y the gel com p l e t e l y i n p r e - c r o s s l i n k e d r e s i s t s can be p r e d i c t e d as f o l l o w s . Assume t h a t the d e n s i t y o f the m a t e r i a l i s p k g / m , the monomer m o l e c u l a r weight i s Mo and t h a t A v o g a d r o ' s number i s N / ( k g . m o l e ) . The number o f monomer u n i t s p e r m^ i s N . p / M . I f the c r o s s l i n k d e n s i t y ( i . e . the f r a c t i o n o f monomer u n i t s which a r e c r o s s l i n k e d ) i s d , then the number o f c r o s s l i n k s per u n i t volume i s 3

a

a

0

0

N .P.d /2M Q

a

0

The f a c t o r o f two appears s i n c e one c r o s s l i n k j o i n s two monomer u n i t s . At the p o i n t a t which the gel i s d e s t r o y e d , the c r o s s l i n k d e n s i t y i s 1/yw where y i s the w e i g h t a v e r a g e degree o f p o l y m e r i z a t i o n o f the r e s i s t (14). I t i s assumed t h a t i r r a d i a t i o n d e s t r o y s o n l y c r o s s l i n k s , not main c h a i n bonds, and t h a t y remains c o n s t a n t . I f the c r o s s l i n k d e n s i t y i n the unexposed r e s i s t i s d o , then the number o f c r o s s l i n k s per u n i t volume which must be broken t o d e s t r o y the gel i s w

w

P.N .(d -l/y )/2.Mo a

I f the energy sorbed energy g i v e n by

0

w

r e q u i r e d to d e s t r o y a c r o s s l i n k i s ε χ , then the a b p e r u n i t volume, E g , r e q u i r e d t o d e s t r o y the gel i s

Eg = p . N . e . ( d - l / y ) / 2 . M a

x

0

0


(1)

POLYMERS FOR HIGH T E C H N O L O G Y

88


T h i s e q u a t i o n i s o f the same form as one proposed by Suzuki and Ohnishi (12). T h u s , s e n s i t i v i t y depends s t r o n g l y on the c r o s s l i n k d e n s i t y , clu, which i s c o n t r o l l e d by the f r a c t i o n o f c r o s s l i n k a b l e u n i t s i n the m a t e r i a l and t h e e x t e n t o f the c r o s s l i n k i n g r e a c t i o n d u r i n g baking. In the p r e s e n t work, the number o f c r o s s l i n k a b l e s i t e s on each copolymer was f i x e d so d i f f e r e n t methods o f changing the c r o s s l i n k d e n s i t y were e x p l o r e d . Three approaches were used: 1) the e x t e n t o f the c r o s s l i n k r e a c t i o n was c o n t r o l l e d by v a r y i n g the bake c o n d i t i o n s , 2) the c r o s s l i n k r e a c t i o n was c a r r i e d out to c o m p l e t i o n and then the c r o s s l i n k d e n s i t y was m o d i f i e d by a subsequent p r o c e s s s t e p , 3) the t o t a l number o f c r o s s l i n k s i t e s was a l t e r e d and the r e a c t i o n was a l l o w e d to proceed to c o m p l e t i o n . Experimental The r e s i s t used i n most o f t h e s e e x p e r i m e n t s was I s o f i n e E-B P o s i t i v e R e s i s t PM-15 purchased from Microimage I n c o r p o r a t e d , Orange, C o n n e c t i c u t . The two copolymers which c o m p r i s e t h i s m a t e r i a l are MMA-MAA and MMA-MAC1, w i t h an MMA c o n t e n t o f 90% i n both c a s e s , and a r e mixed i n equal p r o p o r t i o n s . The copolymers were a l s o p r o v i d e d as s e p a r a t e s o l u t i o n s by Microimage T e c h n o l o g y L t d . , Riddings, Derbyshire, England. Both copolymers were r e p o r t e d , by the m a n u f a c t u r e r , t o have a number average m o l e c u l a r weight o f about 2 5 , 0 0 0 . R e s i s t f i l m s o f a p p r o x i m a t e l y 0.5ym t h i c k n e s s were spun on s i l i c o n wafers and c r o s s l i n k e d by baking e i t h e r i n an oven o r on a hotplate. Incremental exposures were made by a JE0L JBX6A2 e l e c t r o n beam machine a t 20 keV. The UV f l o o d exposures were c a r r i e d out under n i t r o g e n u s i n g a 185nm UV lamp. UV d o s i m e t r y was c a r r i e d out on the b a s i s o f exposure time which had p r e v i o u s l y been c o r r e l a t e d w i t h the e q u i v a l e n t e l e c t r o n beam exposure by measuring d i s s o l u t i o n rates. C l e a r i n g doses were d e t e r m i n e d by immersing t h e wafers i n a s t r o n g s o l v e n t ( a c e t o n e ) f o r two m i n u t e s . S t a n d a r d d i p development was c a r r i e d o u t by immersion i n MIBK ( m e t h y l - i s o b u t y l - k e t o n e ) at 21°C f o r time i n c r e m e n t s from one minute to t e n m i n u t e s . The d i s s o l u t i o n r a t e was c a l c u l a t e d from t h i c k n e s s l o s s , measured u s i n g a Nanospec, and development t i m e . SEM e x a m i n a t i o n o f t e s t s t r u c t u r e s was used to e v a l u a t e the r e s o l u t i o n o f the r e s i s t under the d i f f e r e n t processing conditions. U n l e s s s p e c i f i c a l l y m e n t i o n e d , each e x p e r i ment showed t h a t 0.5ym f e a t u r e s c o u l d be r e s o l v e d i n the r e s i s t . Results

and D i s c u s s i o n

(a) Performance o f A s - r e c e i v e d M a t e r i a l . The commercial PM-15 r e s i s t was p r o c e s s e d a c c o r d i n g t o the m a n u f a c t u r e r ' s specifications w i t h a 30 m i n u t e , 160°C s o f t b a k e and development i n MIBK. I t was found t h a t the dose r e q u i r e d t o d e s t r o y the gel i n the exposed r e g i o n s was 5 0 - 8 5 y C / c m . The dose i s f e a t u r e dependent due t o the p r o x i m i t y e f f e c t , caused by e l e c t r o n b a c k s c a t t e r i n g from t h e s u b strate. An i s o l a t e d exposed l i n e r e q u i r e s 85uC/cm2 to c l e a r because 2


FARRAR A N D OWEN

8.

Electron-Beam

Sensitivity of Acrylate

i t r e c e i v e s v i r t u a l l y no b a c k s c a t t e r e d energy w h i l e a l a r g e pad r e q u i r e s o n l y 50yC/cm2 because i t r e c e i v e s s u b s t a n t i a l a d d i t i o n a l d e p o s i t e d energy from b a c k s c a t t e r e d e l e c t r o n s . For c r o s s l i n k e d p o s i t i v e r e s i s t s , the i s o l a t e d l i n e s a r e t h e most d i f f i c u l t f e a t u r e s t o c l e a r and r e q u i r e the g r e a t e s t d o s e . T h e r e f o r e , the s e n s i t i v i t y o f t h e r e s i s t w i l l be g i v e n as the c l e a r i n g dose f o r an i s o l a t e d exposed l i n e . A t the c l e a r i n g d o s e , the v a l u e o f r e l a t i v e d i s s o l u t i o n r a t e , S g , i s very l a r g e , i n d i c a t i n g e x c e l l e n t c o n t r a s t . The S parameter (15) i s a measure o f the r a t e a t which d i s s o l u t i o n r a t e i n c reases with dose. I t s v a l u e a t any dose i s d e f i n e d as the d i s s o l u t i o n r a t e at t h a t dose d i v i d e d by t h e r a t e a t 40% o f t h a t d o s e , which i s e q u i v a l e n t t o the r e l a t i v e energy d e n s i t y between an i s o l a t e d unexposed r e g i o n and the l a r g e a d j a c e n t exposed p a d s . This r e p r e s e n t s the worst c a s e s i t u a t i o n f o r r e s i s t c o n t r a s t i n e l e c t r o n beam l i t h o g r a p h y . The v a l u e o f Srg can be r e l a t e d t o t h e f e a t u r e p r o f i l e o f the r e s i s t and p r e v i o u s work has i n d i c a t e d t h a t a v a l u e o f S g = 8 . 7 c o r r e s p o n d s t o t h e good l i n e p r o f i l e c h a r a c t e r i s t i c o f PMMA exposed a t 80yC/cm2 ( 1 5 ) . F o r PM-15, v a l u e s w e l l o v e r 100 were o b t a i n e d , i n d i c a t i n g a l e v e l o f c o n t r a s t much g r e a t e r than r e q u i r e d . I t was c l e a r t h a t the i n i t i a l c r o s s l i n k d e n s i t y o f t h e r e s i s t was much h i g h e r than n e c e s s a r y t o g e n e r a t e a c c e p t a b l e c o n t r a s t and was l e a d i n g t o reduced s e n s i t i v i t y . The subsequent e x p e r i m e n t s were d e s i g n e d t o improve s e n s i t i v i t y by r e d u c i n g t h e i n i t i a l c r o s s l i n k density. A l t h o u g h t h i s was e x p e c t e d to degrade c o n t r a s t , i t was f e l t t h a t an adequate l e v e l o f c o n t r a s t c o u l d be m a i n t a i n e d w h i l e the o t h e r r e s i s t parameters were o p t i m i z e d . r


89

Resists

r g

r

(b) E f f e c t o f Bake T e m p e r a t u r e . The PM-15 r e s i s t was baked a t t e m p e r a t u r e s between 110°C and 160°C i n o r d e r t o v a r y the e x t e n t o f the c r o s s l i n k i n g r e a c t i o n . The bake time was h e l d c o n s t a n t a t 30 minutes a t each t e m p e r a t u r e . Above 160°C the c r o s s l i n k i n g r e a c t i o n appeared to have reached c o m p l e t i o n and the r e s i s t performance was not s e n s i t i v e t o changes i n bake t e m p e r a t u r e and t i m e i n t h i s region. However, a t lower t e m p e r a t u r e s than 160°C, the d i s s o l u t i o n r a t e o f m a t e r i a l exposed a t a g i v e n dose d e c r e a s e d w i t h i n c r e a s i n g bake t e m p e r a t u r e , as shown i n f i g u r e 1. A l s o , the g r a d i e n t o f the c u r v e s , which d e t e r m i n e s the S parameter, decreased with decr e a s i n g bake t e m p e r a t u r e . The minimum bake t e m p e r a t u r e r e q u i r e d f o r the r e s i s t t o have an e q u i v a l e n t c o n t r a s t t o PMMA was between 120°C and 130°C, as seen when comparing t h e d i s s o l u t i o n r a t e c u r v e s t o the Srg=8.7 l i n e i n f i g u r e 1. However, the r e s i s t t h i c k n e s s l o s s i n u n exposed r e g i o n s became g r e a t e r a t lower t e m p e r a t u r e s , see f i g u r e 2 , and the optimum bake t e m p e r a t u r e was found t o be 130°C. At t h i s t e m p e r a t u r e , the c l e a r i n g dose was 20-35PC/cm (depending on f e a t u r e ) , as shown i n f i g u r e 3 , and t h e s e n s i t i v i t y was 35uC/cm^. r g

2

S i n c e the c r o s s l i n k i n g r e a c t i o n a t t h e s e temperatures had not proceeded t o c o m p l e t i o n the r e s i s t c h a r a c t e r i s t i c s were p o t e n t i a l l y more s u s c e p t i b l e t o bake f l u c t u a t i o n s . A l t h o u g h o u r r e s u l t s were q u i t e c o n s i s t e n t , a p r o c e s s i n which t h e m a t e r i a l was baked a t 160°C was f e l t t o be more d e s i r a b l e . (c) Changing t h e C r o s s l i n k D e n s i t y by F l o o d I r r a d i a t i o n . Wafers were p r e p a r e d as i n s e c t i o n (a) and then f l o o d exposed by deep UV o r e l e c t r o n beam i r r a d i a t i o n to reduce the c r o s s l i n k d e n s i t y b e f o r e the




90

O.lJ 10

• 1001 Dose,

uC/cm2

F i g u r e 1. D i s s o l u t i o n Rate vs Dose f o r Various Temperatures.

PM-15 R e s i s t Baked

at

__ 1.0+

0.8

0.6

2 0.4

I

0.2

0.0Ί 100

120

140 Bake Temperature,

F i g u r e 2. Resist.

160

180|

C

T h i c k n e s s Remaining vs Bake Temperature f o r

Unexposed


8.

FARRAR A N D OWEN

Electron-Beam


91

Resists

p a t t e r n i n g exposure. T h i s p r o c e d u r e reduces t h e p a t t e r n i n g dose r e q u i r e d but degrades t h e c o n t r a s t o f t h e r e s i s t . There a r e optimum v a l u e s o f t h e f l o o d e x p o s u r e , Q f , and p a t t e r n i n g e x p o s u r e , Q , such t h a t Qf i s as l a r g e as p o s s i b l e and Q as s m a l l as p o s s i b l e w i t h o u t d e g r a d i n g t h e c o n t r a s t t o an u n a c c e p t a b l e l e v e l . The type o f f e a t u r e r e q u i r i n g t h e g r e a t e s t exposure t o d e s t r o y the g e l i s an i s o l a t e d l i n e , because i t r e c e i v e s l e s s b a c k s c a t t e r e d energy than any o t h e r type o f f e a t u r e . From t h e r e s u l t s o f s e c t i o n (a) i t i s known t h a t t h e r e q u i r e d d o s e , Q , i s 85yC/cm i n t h e absence o f f l o o d e x p o s u r e . F o r a f l o o d exposure d o s e , Q f , t h e new p a t t e r n i n g dose, Q , r e q u i r e d to c l e a r the l i n e i s p

p

c

2

p


Qp = Q

c

-

Q (l+n ) f

e

where ne i s t h e b a c k s c a t t e r e d energy c o e f f i c i e n t f o r t h e s u b s t r a t e . For s i l i c o n and 20keV e l e c t r o n s , n = 0 . 7 3 . Hence e

Qp = 85 -

1.73Qf

and i s p l o t t e d as l i n e A i n f i g u r e 4 . F o r t h e exposed r e g i o n s t o c l e a r d u r i n g development Qf and Q must be chosen such t h a t Q l i e s above t h i s l i n e . An i s o l a t e d unexposed space i s t h e type o f f e a t u r e w i t h t h e worst p o s s i b l e c o n t r a s t i n e l e c t r o n beam l i t h o g r a p h y s i n c e i t r e c e i v e s more b a c k s c a t t e r e d energy than any o t h e r t y p e o f f e a t u r e (16). F o r such a f e a t u r e t h e r a t i o o f t h e energy d e p o s i t e d i n t h e a d j a c e n t exposed r e g i o n t o t h a t d e p o s i t e d i n t h e n o m i n a l l y unexposed r e g i o n , a f t e r both f l o o d and p a t t e r n i n g e x p o s u r e s , i s p

p

(l+n ).(Qf+Qp) e

(l+n ).Q e

f

+ n .Q e

m {

p

ά

)

From t h e e x p e r i m e n t a l d a t a o f s e c t i o n ( a ) , i t i s known t h a t t h i s ratio must have a minimum v a l u e o f 1 . 6 3 , e q u i v a l e n t t o S g = 8 . 7 . E q u a t i o n 2 thus reduces t o r

Qp = 1.97Qf and g i v e s t h e r e l a t i o n s h i p between t h e f l o o d and p a t t e r n i n g exposures for acceptable contrast. T h i s i s p l o t t e d i n f i g u r e 4 as l i n e B. T h u s , t h e f l o o d and p a t t e r n i n g exposure doses must be chosen such t h a t Qp l i e s above both l i n e s A and B. The a l l o w e d w o r k i n g r e g i o n i s shown i n f i g u r e 4 and i n d i c a t e s t h a t t h e minimum p o s s i b l e p a t t e r n i n g exposure dose i s 4 5 y C / c m , a t which t h e f l o o d exposure r e q u i r e d i s 23uC/cm2. T h i s p r e d i c t i o n was c o n f i r m e d by e x p e r i m e n t , w i t h a 45yC/cm p a t t e r n i n g exposure r e q u i r e d when a 25yC/cm2 ( o r e q u i v a l e n t UV dose) f l o o d exposure was used. However, d u r i n g t h e s e e x p e r i m e n t s , i t was o b s e r v e d t h a t , f o r a g i v e n t o t a l dose ( f l o o d + p a t t e r n i n g ) , t h e d i s s o l u t i o n r a t e depended on t h e f l o o d / p a t t e r n i n g exposure dose r a t i o . T h i s e f f e c t appears t o o c c u r i n s e v e r a l a c r y l a t e r e s i s t s and i s i l l u s t r a t e d f o r PMMA r e s i s t i n f i g u r e 5. S h i r a i s h i e t . a l . (17) o b s e r v e d a s i m i l a r e f f e c t i n a two component r e s i s t and e x p l a i n e d t h e r e s u l t on t h e b a s i s o f two 2

2



92


120T

0+— 100

'

1

•

1

120

•

1

140 Bake Temperature,

Figure

3.

C l e a r i n g dose v s .

Figure

4.

Relationship

bake

1 180

C

temperature

between f l o o d

•

160

for

PM-15

and p a t t e r n i n g

resist.

exposures.


8.

FARRAR A N D OWEN

Electron-Beam


Resists

93

competing r e a c t i o n s t a k i n g p l a c e a f t e r e x p o s u r e . They p o s t u l a t e d t h a t the e f f e c t o f i r r a d i a t i o n i s to produce f r e e r a d i c a l s and t h a t t h e i r c o n c e n t r a t i o n d e c r e a s e s w i t h time due t o both a f i r s t o r d e r and second o r d e r r e a c t i o n . These r e a c t i o n s a r e a s s o c i a t e d w i t h f r e e r a d i c a l s t a b i l i z a t i o n and f r e e r a d i c a l r e c o m b i n a t i o n r e s p e c t i v e l y . T h i s c o n c e p t may be a p p l i e d t o bond s c i s s i o n r e s i s t s s i n c e permanent bond s c i s s i o n i s a f i r s t o r d e r p r o c e s s and r e c o m b i n a t i o n i s a second order process. A f t e r a l l the r a d i c a l s have r e a c t e d , s e v e r a l hours a f t e r e x p o s u r e , the number o f bond s c i s s i o n s per u n i t volume, Ν| is 5

Ni = K . l n ( l + k . C ) 0

where C i s the i n i t i a l c o n c e n t r a t i o n o f f r e e r a d i c a l s and Κ and k a r e r e l a t e d t o the r a t e c o n s t a n t s o f the f i r s t and second o r d e r reactions respectively. T h i s r e s u l t may be extended t o the case o f two o r more e x posures s e v e r a l hours a p a r t . I f the i n i t i a l f r e e r a d i c a l c o n c e n t r a t i o n s a r e a C and ( l - a ) C > f o r two e x p o s u r e s , then the number o f bond s c i s s i o n s , Ν > s e v e r a l hours a f t e r both exposures i s


0

0

0

2

N

2

= k.lnil

+ k.C

0

+ o(l

-

a)C

0

2

}

I t i s c l e a r t h a t N 2 > N i f o r a l l v a l u e s o f α between 0 and 1. It i s e v i d e n t t h a t N reaches a maximum when a = l / 2 and t h a t the r e l a t i o n s h i p i s symmetrical about t h i s v a l u e . T h i s b e h a v i o r d e s c r i b e s the graph o f f i g u r e 5 q u i t e w e l l s i n c e the d i s s o l u t i o n r a t e o f PMMA depends on the t o t a l number o f bond s c i s s i o n s . The e f f e c t i s a l s o dependent on the time i n t e r v a l s and wafer h i s t o r y between the two exposures. The double exposure c o m p l i c a t i o n s i n t r o d u c e d by the f i r s t and second o r d e r r e a c t i o n mechanisms i n a c r y l a t e r e s i s t s l e d to the c o n c l u s i o n t h a t the f l o o d and p a t t e r n i n g exposure p r o c e s s was not a p r a c t i c a l s o l u t i o n t o the problem o f h i g h c r o s s l i n k d e n s i t y i n PM-15 resist. 2

(d) D i l u t i n g the R e s i s t w i t h PMMA. S o l u t i o n s o f PM-15 r e s i s t and PMMA r e s i s t (M = 496,000) were mixed i n equal p r o p o r t i o n s i n o r d e r t o reduce the t o t a l number o f c r o s s l i n k i n g s i t e s and hence reduce the c r o s s l i n k d e n s i t y a f t e r a 160°C bake. The maximum c l e a r i n g dose was reduced t o 55uC/cm and c o n t r a s t was p r e s e r v e d w i t h Srg >8.7. However, from SEM p i c t u r e s o f the t e s t r e s o l u t i o n p a t t e r n s , i t was c l e a r t h a t the s t r u c t u r e o f the r e s i s t f i l m was not u n i f o r m and t h a t t h e r e were r e g i o n s o f d i f f e r i n g s o l u b i l i t y . T h i s may have been due t o n o n - u n i f o r m c r o s s l i n k i n g o r i n c o m p a t i b i l i t y o f the polymers i n the m i x t u r e . w

2

(e) M i x i n g the Copolymers i n Unequal P r o p o r t i o n s . In o r d e r t o o v e r come the i n c o m p a t i b i l i t y problem o b s e r v e d i n the PMMA m i x t u r e s , the t o t a l number o f c r o s s l i n k s i t e s was reduced by m i x i n g polymers w i t h known c o m p a t i b i l i t y . The MMA-MAC1 and MMA-MAA copolymers were mixed i n d i f f e r e n t r a t i o s ( 1 : 2 , 1:4, and 1:10) and baked a t 160°C. For unequal amounts o f c h l o r i d e and a c i d g r o u p s , the c r o s s l i n k d e n s i t y has a lower v a l u e than t h a t f o r PM-15, i n which the mix r a t i o i s 1:1, and may be c a l c u l a t e d as f o l l o w s . Assume t h a t the



94

r a t i o o f c h l o r i d e t o a c i d copolymer m o l e c u l e s i s l : n , and t h a t n > l . Assume t h a t t h e c h l o r i d e and a c i d copolymers both have a number a v e r a g e degree o f p o l y m e r i z a t i o n , y n , and t h a t t h e y both have a f r a c t i o n , f , o f c r o s s l i n k a b l e u n i t s (f=0.1 f o r PM-15). For a c o l l e c t i o n o f Ν polymer m o l e c u l e s , t h e r e a r e N/(n+l) c h l o r i d e m o l e c u l e s and Nn/(n+l) a c i d m o l e c u l e s . S i n c e t h e r e a r e fewer o f them, i t i s t h e c h l o r i d e m o l e c u l e s which c o n t r o l t h e c r o s s l i n k d e n s i t y because when a l l t h e a v a i l a b l e c h l o r i d e u n i t s have r e a c t e d no further crosslinking i s possible. The t o t a l number o f c h l o r i d e u n i t s i s y n . f . N / ( n + l ) a n d , when a l l o f t h e s e have r e a c t e d , t h e t o t a l number o f c r o s s l i n k e d u n i t s i n t h e Ν m o l e c u l e s w i l l be 2.y .f.N/(n+l)


n

s i n c e each c h l o r i d e u n i t w i l l be c r o s s l i n k e d t o an a c i d u n i t . The t o t a l number o f monomer u n i t s i s y . N and so t h e c r o s s l i n k d e n s i t y , d , w i l l be n

0

d

0

= 2.f/(n+l)

(3)

The p o i n t s i n f i g u r e 6 show t h e v a l u e s o f do f o r t h e d i f f e r e n t mixes p l o t t e d a g a i n s t t h e c l e a r i n g dose f o r a l a r g e exposed p a d . E q u a t i o n 3 was s u b s t i t u t e d i n t o e q u a t i o n 1 and ε was used as a f i t t i n g parameter t o p l o t t h e l i n e s t h r o u g h t h e d a t a shown i n f i g u r e 6. Eg was c o n v e r t e d t o dose assuming t h a t lyC/cm2 · e q u i v a l e n t t o 3 . 6 x l 0 J / m . The dependence o f c l e a r i n g dose on c r o s s l i n k d e n s i t y agreed very well with the t h e o r y . The two s e t s o f d a t a f o r "new m a t e r i a l " and "aged m a t e r i a l " c o r r e s p o n d t o r e s i s t f i l m s which were spun from s o l u t i o n i m m e d i a t e l y a f t e r m i x i n g and 137 days l a t e r , r e s pectively. The p r o p e r t i e s o f t h e spun f i l m change w i t h time and t h e m a t e r i a l appears t o become more s e n s i t i v e . This effect i s discussed f u r t h e r a t t h e end o f t h i s s e c t i o n . χ

Ί

7

δ

3

As w i t h t h e r e s u l t s i n s e c t i o n ( b ) , r e d u c i n g t h e c r o s s l i n k d e n s i t y reduces t h e g e l c o n t e n t i n t h e unexposed r e g i o n s and l e a d s t o g r e a t e r t h i c k n e s s l o s s d u r i n g development. An e s t i m a t e o f t h i s e f f e c t may be made on t h e b a s i s o f a n e a r e s t - n e i g h b o r c r o s s l i n k i n g analysis. Assuming t h a t i ) a l l t h e c h l o r i d e m o l e c u l e s a r e i n c o r p o r a t e d i n t o t h e g e l , i i ) t h e polymer m o l e c u l e s a r e r e p r e s e n t e d as c l o s e - p a c k e d s p h e r e s and hence have t w e l v e immediate n e i g h b o r s , and i i i ) n e i g h b o r i n g a c i d and c h l o r i d e m o l e c u l e s a r e c r o s s l i n k e d by a t l e a s t one bond, i t f o l l o w s t h a t any a c i d m o l e c u l e t h a t has a c h l o r i d e m o l e c u l e n e i g h b o r i s a p a r t o f t h e g e l . T h e r e f o r e , an a c i d molecule i s o n l y a p a r t o f the s o l i f i t i s completely surrounded by o t h e r a c i d m o l e c u l e s . The p r o b a b i l i t y t h a t a m o l e c u l e i s an a c i d m o l e c u l e i s n/(n+l) and t h e p r o b a b i l i t y t h a t i t s n e i g h b o r s a r e a l l a c i d molecules i s [n/(n+l)]12. F o r Ν polymer m o l e c u l e s t h e number o f a c i d m o l e c u l e s c o m p l e t e l y surrounded by o t h e r a c i d m o l e cules i s [N.n/(n+l)].i[n/(n+l)]12} These m o l e c u l e s c o n s t i t u t e t h e s o l , so t h e s o l f r a c t i o n i s [n/(n+l)]!3. Therefore the gel f r a c t i o n i s 1 -

[n/(n+D]

1

3



8.

FARRAR A N D OWEN

Electron-Beam


Dose % i n F i r s t Figure 5. PMMA.

120

9

Resists

Exposure

D i s s o l u t i o n Rate vs Dose R a t i o f o r

Double

Exposed

T

100+

Crosslink Density Figure 6. Clearing Copolymers.

Dose vs C r o s s l i n k D e n s i t y f o r

Unevenly


Mixed

96


T h i s c u r v e i s p l o t t e d as the l i n e i n f i g u r e 7 and shows good a g r e e ment w i t h the e x p e r i m e n t a l p o i n t s . For m i x t u r e s w i t h a u s e f u l g a i n i n s e n s i t i v i t y o v e r s t a n d a r d PM-15 r e s i s t , d i s s o l u t i o n r a t e was measured as a f u n c t i o n o f dose and i s shown i n f i g u r e 8. By comparison w i t h the S g = 8 . 7 l i n e , i t i s seen t h a t the c o n t r a s t i s adequate f o r both the 1:4 and 1:10 mixes a t about 2 0 y C / c m . T h i s dose i s h i g h e r than the c l e a r i n g dose f o r t h e 1:10 m i x t u r e but n e c e s s a r y f o r a c c e p t a b l e c o n t r a s t . Howe v e r , the r e s o l u t i o n and t h i c k n e s s r e m a i n i n g f o r t h i s m i x t u r e a r e i n a d e q u a t e , but may have been degraded by u s i n g an i n a p p r o p r i a t e developer. Because the p o l a r i t y o f t h e m a t e r i a l i n c r e a s e s as t h e e x c e s s MAA c o n t e n t i n c r e a s e s , f u r t h e r work i s r e q u i r e d t o o p t i m i z e the development system f o r each m i x t u r e . The 1:4 m i x t u r e meets t h e c o n t r a s t and r e s o l u t i o n r e q u i r e m e n t s but c o u l d p r o b a b l y be f u r t h e r improved i n terms o f s e n s i t i v i t y . I t appears t h a t a m i x t u r e between the 1:4 and 1:10 r a t i o s would o f f e r the optimum c r o s s l i n k d e n s i t y t o maximize s e n s i t i v i t y and r e t a i n s u f f i c i e n t c o n t r a s t . A c o n c e r n t h a t remains f o r t h e s e m a t e r i a l s i s t h a t the p e r f o r mance o f f i l m s c a s t from t h e mixed s o l u t i o n s changes o v e r t i m e , w i t h a d e c r e a s e i n the c l e a r i n g dose o f about 30% a f t e r 100 days from the p r e p a r a t i o n o f the r e s i s t , as shown i n f i g u r e 9 . However, the p r o p e r t i e s appeared to approach a s t a b l e l e v e l . T h i s e f f e c t had o b v i o u s l y o c c u r r e d i n t h e commercial premixed m a t e r i a l a l s o , s i n c e the c l e a r i n g dose had s t a b i l i z e d c l o s e t o t h a t o f t h e aged 1:1 s o l u t i o n , and was much lower than the c l e a r i n g dose f o r the f r e s h l y mixed s o l u t i o n . H y d r o l y s i s o f the c h l o r i d e groups by a t m o s p h e r i c m o i s t u r e c o u l d e x p l a i n a v a r i a t i o n i n performance but not the apparent s t a b i l i z a t i o n a t longer times. r


2

Conclusions The c o m m e r c i a l l y a v a i l a b l e PM-15 r e s i s t c o n t a i n i n g 10% c r o s s l i n k a b l e groups shows no improvement i n s e n s i t i v i t y o v e r PMMA a l t h o u g h , a t the o p e r a t i n g d o s e , the c o n t r a s t i s s u p e r i o r . Reducing the c r o s s l i n k d e n s i t y by changing t h e bake c o n d i t i o n s l e a d s t o a s e n s i t i v i t y improvement o f 60% by s a c r i f i c i n g the p r e v i o u s l y h i g h c o n t r a s t . O t h e r methods o f r e d u c i n g the c r o s s l i n k d e n s i t y have a l s o l e d t o s e n s i t i v i t y improvements, o f which the most p r o m i s i n g i s t h e uneven copolymer m i x t u r e s w i t h a p o t e n t i a l 60-70% r e d u c t i o n i n o p e r a t i n g dose. However, the s o l u t i o n s must be aged a f t e r m i x i n g i n o r d e r t o o b t a i n r e p r o d u c i b l e performance. T h i s t e c h n i q u e a l s o r e q u i r e s work on a d e v e l o p e r s y s t e m . F l o o d exposure y i e l d s a 50% improvement i n s e n s i t i v i t y but c r e a t e s a d d i t i o n a l p r o c e s s i n g d i f f i c u l t i e s due t o double exposure effects. A comparison o f the r e s u l t s i s shown i n T a b l e I. It i s c l e a r t h a t a reduced c r o s s l i n k d e n s i t y i s d e s i r a b l e f o r improved s e n s i tivity. T h i s must be o p t i m i z e d t o ensure a c c e p t a b l e c o n t r a s t and minimal t h i c k n e s s l o s s i n unexposed r e g i o n s . A l t h o u g h a l l the methods e x p l o r e d i n t h i s work gave improved s e n s i t i v i t y and a c c e p t a b l e c o n t r a s t , i t appears t h a t the s i m p l e s t and most r e p r o d u c i b l e method f o r a c h i e v i n g lower c r o s s l i n k d e n s i t y i s t o modify the


8.

FARRAR A N D OWEN

S


Resists

1

'

97

T

0.4»

•ri r-i

ro ε ο 0.2--

O.o-I 0

'

1 2

'

1 4

•

1

1

6

·

8

Mix R a t i o ,

1

I

10

1

'

12

14

η

Figure 7 . T h i c k n e s s Remaining vs Mix R a t i o f o r Unevenly Copolymers.

Mixed

ΙΟΟΊ

Pi: 10

Rate, Ay 'sec

/ y

L s s o l u t:ion


1.2

Electron-Beam

/

/

/

'''

/srg=8.7

10;

/J/

α

Λ

1

Dose, Figure 8. polymers.

100

10

L

Dissolution

uC/cm2

Rate vs Dose f o r Unevenly

Mixed C o




98

Figure

9.

Clearing

Dose vs Age o f Mix f o r

1:4

Mix

Ratio.


8. FARRAR AND OWEN

99 Electron-Beam Sensitivity of Acrylate Resists

structure of the resist by reducing the number of crosslinkable groups in both copolymers. The crosslinking reaction can then be carried to completion, which reduces its sensitivity to bake temperature fluctuations. Also, the gel structure should be more homogeneous and the polarity more uniform than the unevenly mixed copolymers. If such resists were available, as copolymer mixtures or a single terpolymer, they would be very attractive candidates for 20uC/cm2 electron beam lithography.


Table I. Summary of Results Clearing dose, yC/cm^ Material (Isolated line) As received 85 (160°C bake) As received 35 (130°C bake) PMMA mixture 55 1:4 mixture 30 1:10 mixture 15 Flood exposed 45 (25yC/cm)

S Contrast (at clearing dose) 200 9 rg

30 15 6 9

2

Resolution

Polymers for High Technology - American Chemical Society

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