Polymers in Electronics - American Chemical Society

8

Downloaded by CORNELL UNIV on October 21, 2016 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch008

Effect of Composition on Resist Dry-Etching Susceptibility Novel Vinyl Polymers J. N. HELBERT, M. A. SCHMIDT, and C. MALKIEWICZ Process Technology Lab/SRDL, Motorola SPS, Phoenix, AZ 85008 E. WALLACE, JR., and C. U. PITTMAN, JR. University of Alabama, University, AL 35486 In an effort to devise a quick screening test for vinyl polymer resist dry-etch susceptibility as part of a total resist design criteria, we have adopted a simple and relatively easy CF4/O2 plasma etch test. This test is empirically found to be a good indicator for predicting etch resistance to even more harsh and anisotropic etch processes, such as reactive-ion etching and ion-milling. Using this test, the plasma etch rates of a large number of novel vinyl polymers have been measured and correlated with vinyl polymer composition. The effect of vinyl resist polymer composition upon plasma etch rate is found to range over a factor of 50. The polymer thermal stabilities have also been measured as a further part of the overall design criteria, because polymer thermal flow is also a vital issue to dry-etching technology. Although no correlation is found between pasma etch rate and polymer thermal stability, the polymers with lowest thermal stability were also most susceptible to detrimental resist image thermal flow. Polymeric resists (1) are utilized widely in the electronics industry to lithographically delineate circuit patterns onto normally nonpatternable inorganic substrates. As device dimensions shrink, an increasing number of the levels of these devices will have to be etched by dry-process (e.g., rf plasma) etching techniques (2). As a 0097-6156/ 84/ 0242-0091 $06.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

P O L Y M E R S IN E L E C T R O N I C S


92

r e s u l t , new p o l y m e r i c resists must be able to w i t h s t a n d these harsh e n v i r o n m e n t s i n order t h a t the necessary d i e l e c t r i c , g a t e and m e t a l m a t e r i a l l e v e l s may be p a t t e r n e d d u r i n g d e v i c e f a b r i c a t i o n . Thus, r e s i s t c o m p a t i b i l i t y w i t h these processes must be e s t a b l i s h e d before lithographic utilization. It has been shown t h a t p l a s m a e t c h i n g ( P E ) , r e a c t i v e - i o n e t c h i n g (RIE) and i o n - m i l l i n g (IM) e q u i p m e n t subject the r e s i s t i n the e t c h m a s k i n g a p p l i c a t i o n to r e a c t i v e f r e e r a d i c a l s , a t o m s or ions w h i c h are c a p a b l e of d e c o m p o s i n g p o l y m e r i c r e s i s t s (3). H a r a d a (3) has d e m o n s t r a t e d that poly(methyl methacrylate) ( P M M A ) d e c o m p o s e s i n a C F 4 / O 2 p l a s m a by a f r e e r a d i c a l - i n d u c e d random chain scission mechanism. Furthermore, reference 2 e s t a b l i s h e s good t r a c k i n g or c o r r e l a t i o n b e t w e e n r e l a t i v e e t c h r a t e r a t i o s as the e t c h t e c h n i q u e is m a d e m o r e harsh and a n i s o t r o p i c , and t h a t these r e l a t i v e v a l u e s a r e i n f a c t dependent upon p o l y m e r composition. T h e d e p e n d e n c e of e t c h r a t e r a t i o upon p o l y m e r c o m p o s i t i o n c a n be as l a r g e as 100X (2). E t c h r a t e r a t i o s v a r y i n g o v e r a f a c t o r of 10 are m o r e c o m m o n , and r e s u l t s of other r e s e a r c h e r s (3,^,5) have r e c e n t l y appeared as f o c u s upon p o l y m e r resist dry-process compatibility intensifies. S t r i c t l y s p e a k i n g , the d r y - p r o c e s s c o m p a t i b i l i t y for a r e s i s t is v e r y process d e p e n d e n t , and must be m e a s u r e d for t h e s p e c i f i c p r o c e s s i n v o l v e d . A g e n e r a l i d e a of c o m p a t i b i l i t y , h o w e v e r , c a n be o b t a i n e d by doing a C F 4 / O 2 p l a s m a e t c h test vs S1O2 a n d / o r P M M A r e f e r e n c e s , and this test has b e e n adopted as a q u i c k s c r e e n t e s t f o r d r y - p r o c e s s c o m p a t i b i l i t y for new r e s i s t s . T h e r e l a t i v e e t c h r a t i o s vs these r e f e r e n c e s u s u a l l y , but not a l w a y s , r e m a i n c o n s t a n t when the process r e q u i r i n g r e s i s t m a s k i n g is c h a n g e d (e.g., P E t o RIE), thus, what is m e a s u r e d is r e s i s t c o m p o s i t i o n a l l y - d e p e n d e n t . In this w o r k , we p r o v i d e d r y - p r o c e s s p l a s m a e t c h r a t e r a t i o s versus a s t a n d a r d for an expanded l i s t of n o v e l v i n y l p o l y m e r i c r e s i s t s and c o m m e r i c a l p h o t o r e s i s t s . T h e n o v e l v i n y l r e s i s t s have been s y n t h e s i z e d as p a r t of a l a r g e r r e s i s t d e v e l o p m e n t p r o g r a m a i m e d t o w a r d s the d e v e l o p m e n t of i m p r o v e d x - r a y and e - b e a m l i t h o g r a p h i c r e s i s t s (6). The v a r i e t y of v i n y l h o m o p o l y m e r s and c o p o l y m e r s , most c o n t a i n i n g q u a r t e r n a r y c e n t e r s and s y n t h e s i z e d for p l a s m a e t c h r a t e r a t i o e v a l u a t i o n , c o n t a i n the f o l l o w i n g m o n o m e r s to v a r y i n g proportions: CH2=C(CH3)C02CH2CH CN CH2=C(CH3)C02CH2CN CEMA CMMA 2

CH =C(CN) VDCN

CH =C(CH3)CN MCN

CH =C(C1)CN ACAN

CH =C(CF )CN TFMAN

2

2

2

2

2

3

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

8.

Resist Dry-Etching

HELBERT ET AL.

CH2=C(C1)C02CH CF3 ACTFEMA

CH2=C(Cl)C0 CH CCl3 TCECA

CH2=C(CH )C0 CCl3 TCEMA

CH =C(Br)C0 CH MBA

CH =C(C02CH2CH )2 BCEE

CH =C(C0 CH3) BCME

C H = C ( H ) C 6 H 5CH2CI CMS

CH2=C^2Z/° AMBL

2

3

2

2

2


Susceptibility

3

2

2

2

2

2

2

3

2

In a d d i t i o n , e i g h t a r o m a t i c - l i k e n o v o l a c resin c o n t a i n i n g f i r s t and second g e n e r a t i o n photoresist systems h a v e been e v a l u a t e d . Experimental P l a s m a e t c h m e a s u r e m e n t s w e r e c a r r i e d out a t 6 0 ° C i n a T e g a l M o d e l 421 b a r r e l p l a s m a r e a c t o r (see r e f 2 for c o n d i t i o n s ) . To d e t e r m i n e the e t c h r a t e of the p o l y m e r r e s i s t , a r e s i s t c o a t e d o x i d i z e d s i l i c o n substrate is exposed to the p l a s m a for a s p e c i f i e d t i m e i n t e r v a l (10-20 m i n u t e s ) , t h e n the o r i g i n a l or p r e - e x p o s u r e resist step and a new p o s t - e x p o s u r e step are measured w i t h a T e n c o r a l p h a - s t e p p r o f i l o m e t e r . These steps a r e made by r e m o v i n g the resist by m e c h a n i c a l l y s c r a t c h i n g or solvent d i p p i n g the s u b s t r a t e . The change in the resist is o b t a i n e d by s u b t r a c t i n g the p o s t - e x p o s u r e f r o m the p r e - e x p o s u r e step h e i g h t (see F i g u r e 1). T h e S 1 O 2 s u b s t r a t e loss is measured by stepping down f r o m the p o s t - e t c h step to the o r i g i n a l resist s t e p , where the o x i d e had been p r e v i o u s l y exposed to the p l a s m a . The o x i d e loss i n these e t c h tests is u s u a l l y 1000-2000Â, a v a l u e t y p i c a l l y e n c o u n t e r e d i n r e a l device fabrication. T h e e t c h r a t e r a t i o of the resist to the S 1 O 2 r e f e r e n c e is r e f e r r e d to as the r e s i s t p r o c e s s s e l e c t i v i t y , and c o u p l e d w i t h r e s i s t t h e r m a l s t a b i l i t y d a t a is r e p r e s e n t a t i v e of the o v e r a l l resist process c o m p a t i b i l i t y . The l o w e r the s e l e c t i v i t y r a t i o (i.e., «1), the b e t t e r the resist p o l y m e r d r y - p r o c e s s c o m p a t i b i l i t y . PMMA, for e x a m p l e , has a m a r g i n a l s e l e c t i v i t y of 0 . 9 - 1 . 2 , or e t c h e s a t the undesirably same r a t e as S 1 O 2 . In a d d i t i o n , P M M A is also v e r y s u s c e p t i b l e to t h e r m a l l y - i n d u c e d i m a g e f l o w due to low t g and T G A p a r a m e t e r s , and also undergoes s u r f a c e " f r y i n g " phenomena (see F i g u r e 2). T h e r e f o r e , P M M A has v e r y poor o v e r a l l d r y - p r o c e s s compatibility. R e s u l t s and D i s c u s s i o n N o v e l V i n y l P o l y m e r s . P o l y m e t h a c r y l o n i t r i l e ( P M C N ) was found to possess a P E r a t e r a t i o vs S 1 O 2 of 0.3, and this r e l a t i v e l y low r a t i o



pre-etch


RESIST Si0

o

///////////

Si

post-etch

77777777777 Δ resist =

Δ OX = F i g u r e 1.

A-B

C

Etch rate ratio measurement method.


Resist Dry-Etching

Susceptibility


HELBERT ET AL.

F i g u r e 2.

R I E r e s i s t " f r y " e x a m p l e s for A Z - 2 4 0 0 (top) a n d P M M A (bottom) v s . n o n - f r y i n g P C - 1 2 9 .



96


was a t t r i b u t e d to the a l p h a - c y a n o group w h i c h is a s t r o n g l y bonded side group (2). To a s c e r t a i n the e f f e c t of f u r t h e r C N i n c o r p o r a t i o n (i.e., a t the CH3 s i t e of P M C N ) the h o m o p o l y m e r P V D C N was s y n t h e s i z e d for e t c h r a t e m e a s u r e m e n t . Unfortunately, this p o l y m e r is insoluble i n a l l k n o w n spinning s o l v e n t s , t h e r e f o r e , m e a s u r e m e n t a t t e n t i o n was focused upon the m o r e soluble M M A / V D C N copolymers. R e s u l t s for a l l the C N - c o n t a i n i n g s y s t e m s are t a b u l a t e d i n T a b l e I. A l i n e a r l e a s t squares f i t of the V D C N / M C N c o p o l y m e r d a t a y i e l d s an e x t r a p o l a t i o n v a l u e a t 100% of V D C N of 0.3, a v a l u e v e r y c l o s e to t h a t o b t a i n e d for P M C N . L i k e P V D C N , P A C A N is also s p a r i n g l y s o l u b l e . A s a r e s u l t , the A C A N / M C N c o p o l y m e r s w e r e focused upon for P E r a t e r a t i o m e a s u r e m e n t (see T a b l e I). A l i n e a r l e a s t squares f i t y i e l d s a P E r a t e r a t i o for P A C A N of 1.3. F o r P A C A N , the a l p h a - C l appears t o d o m i n a t e the P E b e h a v i o r over t h a t of the a l p h a - C N s t a b i l i z i n g group. The P T F M A N h o m o p o l y m e r , «(CH2-C(CF3)CN)-, is d i f f i c u l t to s y n t h e s i z e , t h e r e f o r e , a t t e n t i o n was focused upon the m o r e soluble M C N and M M A / T F M A N c o p o l y m e r s . A s T a b l e I shows, the T F M A N c o p o l y m e r s e x h i b i t i m p r o v e d d r y - p r o c e s s c o m p a t i b i l i t y over t h a t of the P M M A r e f e r e n c e , a g a i n , due to the C N group s t a b i l i z a t i o n effect. The C M M A and C E M A p o l y m e r s w e r e i n c l u d e d to d e t e r m i n e if the C N p r o t e c t i n g group c o u l d i n f l u e n c e the P E r a t e a t the a l k y l ester site. D a t a for both s y s t e m s is l o c a t e d i n T a b l e I. S u r p r i s i n g l y , the C N does i n f l u e n c e the P E r a t e r a t i o for C M M A b u t not for the C N - e t h y l s y s t e m , w h i c h appears to be m o r e i n f l u e n c e d by the e t h y l group e f f e c t . This e f f e c t was f i r s t observed when ΡΕΜΑ, " ( C ^ - C i C ^ C C ^ C r ^ C r ^ ) ; was found t o e t c h 1.7X f a s t e r t h a n P M M A ( 2 ) . A s i n r e f e r e n c e 2, the C N group is found to be a d r y - p r o c e s s c o m p a t i b i l i t y enhancer or found to d e c r e a s e the P E r a t e r a t i o when i t is a l p h a to the m a i n c h a i n or one c a r b o n r e m o v e d . E a r l i e r , (2) P M C A , - ( C r ^ - C f C D C C ^ C r ^ ) ; was r e p o r t e d t o p l a s m a e t c h a t a r a t e g r e a t e r t h a n 1.8X S1O2, and t h i s was a t t r i b u t e d to the weak C - C l bond. P T C E M A has a P E e t c h r a t e r a t i o of 2.3, t h u s , the presence o f the weak C - C l bond a t e i t h e r the a l p h a - p o s i t i o n or on the e s t e r a l k y l group a p p a r e n t l y enhances the P E d e c o m p o s i t i o n of these r e s i s t s . To f u r t h e r study t h i s phenomenona, the obvious s y s t e m w i t h b o t h a l p h a - C l and the t r i c h l o r o e t h y l e s t e r g r o u p , P T C E C A , was s y n t h e s i z e d and s t u d i e d . A s p r e d i c t e d , (see T a b l e I) P T C E C A and the 29/71 T C E C A / M M A c o p o l y m e r both e t c h f a s t e r t h a n P M M A r e f e r e n c e . These v a l u e s a r e s i g n i f i c a n t l y l a r g e r t h a n the d e s i r e d less t h a n one v a l u e . E B R 9, the T o r a y Japanese Ε-beam r e s i s t , is s t r u c t u r a l l y s i m i l a r to


8.

HELBERTETAL.

Resist Dry-Etching

T a b l e I: P l a s m a e t c h r a t e p o l y m e r s and c o p o l y m e r s .

ratios

Susceptibility

vs

97

S1O2 r e f e r e n c e

for

vinyl

a P o l y m e r or Copolymer PMCN2 PVDCN PVDCN-CO-MMA

TGA °C

£c 120 165 155 135 107 131 128 123

PACAN PCMMA PCEMA PCEMA-CO-MMA PTFMAN-CO-MCN PTFMAN-CO-MMA

100 100 52/48 38/62 100 50/50 39/61 11/89 100 100 100 50/50 12/88 32/68

PMCA PTCEMA PTCEMA-CO-MCA PTCEMA-CO-MCN PTCEMA-CO-TCECA PTCECA PTCECA-CO-MMA P A C T F E M A (EBR-9) PMFA PMBA

100 100 66/34 50/50 90/10 100 29/71 100 100 100

130;151 123 137 130 123 147 130 133 131 130

PBCEE PBCEE-CO-MMA

100 25/75 50/50 100 50/50 100

PAMBL P C M S (Toyo Soda)

PMMA PACAN-CO-MCN 2


Mole % ratio

2

2

PBCME PBCME-CO-MMA ΡΕΜΑ 2

PS a

c d

2

-

115 105 102 123 98

Plasma etch Rate Ratio

a

335 260/500° 330/500° 270/400° 275 210/50θ£ 223/500° 216/500°

-

250 250/400° 293 357 295

0.3 insoluble 0.4 1.0 0.9-1.2 1.0 0.5 0.4 I.3 0.4-1.1 2.1 1.4 0.3 0.3

C

-

1.8 2.3 3.5 0.4 2.0 2.0 1.6 2.1 0.4 2.0

> 100 78 92 72 100 65

293 275 292 320 310

1.7 1.0 1.1 *2 0.9 1.7

100

83

363

0.3

100

105

350

£0.06

305 286 310 290 290 325 277 404 170

-

0.1 322 103 100 M e a s u r e d by D u P o n t M o d e l 900 D S C / T G A . b T G A shows t w o d i s t i n c t b r e a k s : the f i r s t b r e a k is a t t r u b u t e d to the l a d d e r l i k e r e a c t i o n t h a t o c c u r s b e t w e e n a d j a c e n t a l p h a - C N groups. Extrapolated value from the F i g u r e . 0.06 is the l o w e s t m e a s u r e a b l e v a l u e of t h i s t e c h n i q u e .


d

98


P T E C A , ^ C H 2 - C ( C 1 ) C 0 C H 2 C F ) - ( A C T F E M A ) , and e t c h e s a t a r a t i o o f 2.1. A g a i n , the v a l u e is l a r g e but not as l a r g e as the 3.1 v a l u e o b t a i n e d by adding the P M C A a n d P T F M A P E r a t e r a t i o s . C u r i o u s l y , the observed e t c h r a t e r a t i o for E B R - 9 is v e r y c l o s e t o the sum of the P M C A and P T C E M A P E r a t e r a t i o v a l u e s minus the r a t i o for P M M A . T h i s a d d i t i v i t y r e l a t i o n s h i p holds t r u e for b o t h P T E C A and E B R - 9 , and m a y be a v a l i d w a y t o p r e d i c t P E r a t e r a t i o s for doubly s u b s t i t u t e d p o l y m e r s , w h e r e the r e s p e c t i v e m o n o substituted P E rate ratio values are known. When the c h l o r i n e of P M C A is r e p l a c e d by F , the P E r a t e r a t i o decreases f r o m 1.8 t o 0.4 (2). This large difference was a t t r i b u t e d to the d i f f e r e n c e i n bond d i s s o c i a t i o n e n e r g i e s b e t w e e e n C - C l and C - F ; the C - F bond d i s s o c i a t i o n energy is 107 k c a l / m o l e c o m p a r e d to t h a t for C - C l o f 80 k c a l / m o l e . To c h e c k t h i s hypothesis f u r t h e r , the a l p h a - B r a n a l o g , -{CH^-CfàOCC^CH^); was s y n t h e s i z e d , t e s t e d and found to e t c h a t a r a t i o of 2.0. Thus r e p l a c e m e n t of the C - F bond w i t h an e v e n w e a k e r C - B r bond (D=67 k c a l / m o l e ) has l e a d to a less d r y - p r o c e s s c o m p a t i b l e p o l y m e r i c r e s i s t , c o n s i s t e n t w i t h the e a r l i e r work of r e f e r e n c e 2. P B C E E l i k e ΡΕΜΑ e t c h e s 1.7 t i m e s t h a t of P M M A r e f e r e n c e . A s discussed e a r l i e r and p r e v i o u s l y i n r e f e r e n c e 2, this e f f e c t is a t t r i b u t e d to the a l p h a - e t h y l e s t e r g r o u p . A s found for P V D C N , - ( C H 2 - C ( C N ) 2 H the i n f l u e n c e of the second s u b s t i t u e n t t o the q u a r t e r n a r y c a r b o n is n e g l i g i b l e . B C M E , on the other h a n d , e t c h e s a t « 2 , but this p o l y m e r has a t g o f 7 2 ° C , w h i c h is v e r y c l o s e to the m e a s u r e m e n t t e m p e r a t u r e of the p l a s m a e t c h i n g t o o l e m p l o y e d . H e a t i n g a b o v e 6 0 ° C d u r i n g e t c h i n g , h o w e v e r , has been o b s e r v e d , and e v i d e n c e o f t h e r m a l f l o w was observed on the t e s t w a f e r a f t e r e t c h i n g . The m o r e t h e r m a l l y s t a b l e 5 0 / 5 0 B C M E / M M A c o p o l y m e r w i t h a to o f 1 0 0 ° C e t c h e d a t the same r a t e as P M M A , t h e r e f o r e , the P B Q V I E v a l u e must be biased h i g h due t o t h e r m a l f l o w or i n s t a b i l i t y . T h e r e f o r e , the e f f e c t of a d i - e s t e r q u a r t e r n a r y c a r b o n is n e g l i g i b l e upon the P E r a t e r a t i o , but does t e n d t o reduce the polymer flow resistance significantly.


2

3

P A M B L , a s p e c i a l v i n y l p o l y m e r s y s t e m due t o the f i v e m e m b e r e d e s t e r r i n g e d m a i n c h a i n q u a t e r n a r y c a r b o n , has a s u r p r i s i n g l y l o w P E r a t e r a t i o o f 0.3. A t f i r s t g l a n c e , one w o u l d p r e d i c t an e t c h r a t e r a t i o f o r P A M B L s i m i l a r to t h a t of P M M A , due t o the f a c t b o t h s y s t e m s c o n t a i n a a l p h a - e s t e r group. T h i s r e s u l t may i n d i c a t e t h a t the a l p h a - m e t h y l group (i.e., i n P M M A ) p a r t i c i p a t i o n i n the p l a s m a e t c h d e g r a d a t i o n m e c h a n i s m has been underestimated. U n f o r t u n a t e l y , the good P E r a t e r a t i o is s o m e w h a t o v e r s h a d o w e d by the l o w o b s e r v e d to v a l u e of 8 3 ° C , a v a l u e 2 4 ° C l o w e r t h a n the m a r g i n a l v a l u e o f 1O7°C observed f o r P M M A reference. T h e r e f o r e , o v e r a l l t h i s r e s i s t is c o n s i d e r e d t o possess only m a r g i n a l o v e r a l l d r y - p r o c e s s c o m p a t i b i l i t y . The l a s t v i n y l s y s t e m i n v e s t i g a t e d is P C M S (7). T h i s r e s i s t has H a t the a l p h a - p o s i t i o n , w h i c h is w e l l k n o w n to e s t a b l i s h the e b e a m or x - r a y r e s i s t tone as n e g a t i v e (1). L i k e o t h e r n e g a t i v e e b e a m r e s i s t s such as P S and P M F A , P C M S r e s i s t is also d r y - p r o c e s s c o m p a t i b l e , t h a t i s , has a n e t c h r a t e r a t i o 4 1 . 0 . T h e P C M S r e s i s t is



8.

HELBERT ET AL.

Resist Dry-Etching

99

Susceptibility

even m o r e c o m p a t i b l e t h a n the p a r e n t P S s y s t e m . This d a t a i m p l i e s t h a t the same t y p e of r e a c t i o n s m a y o c c u r i n a gas p l a s m a as o c c u r d u r i n g i o n i z i n g r a d i a t i o n e x p o s u r e ( i . e . , a b s t r a c t i o n of alpha-Η a t o m s l e a d i n g to the same c r o s s l i n k i n g s i t e r a d i c a l p r e c u r s o r s as a r e k n o w n to be c r e a t e d by r a d i a t i o n exposures). F o r e a c h v i n y l p o l y m e r resist l i s t e d i n T a b l e I, a v a l u e for t g , the glass t r a n s i t i o n t e m p e r a t u r e , and T G A t e m p e r a t u r e , w h i c h is the t e m p e r a t u r e for e a c h s y s t e m w h e r e s i g n i f i c a n t t h e r m a l d e g r a d a t i o n and w e i g h t loss begins to o c c u r , are l i s t e d . T h e r e is no a p p a r e n t c o r r e l a t i o n b e t w e e n these t h e r m a l c h a r a c t e r i s t i c s and the P E rate ratio data. P e d e r s o n has also r e p o r t e d a s i m i l a r l a c k o f c o r r e l a t i o n (4). S t i l l , this t h e r m a l d a t a is i m p o r t a n t because i t aids the l i t h o g r a p h y and p r o c e s s i n g engineers i n the s e l e c t i o n of the r e s i s t p r e b a k e t e m p e r a t u r e ^ t g ) and e t c h i n g t e m p e r a t u r e (

Polymers in Electronics - American Chemical Society

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