Effect of Composition on Resist Dry-Etching Susceptibility - ACS

5 Effect of Composition on Resist Dry-Etching Susceptibility Vinly Polymers and Photoresists J. N . H E L B E R T and M. A. S C H M I D T

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: April 7, 1982 | doi: 10.1021/bk-1982-0184.ch005

Process Technology Laboratory, S R D L , Motorola, S G , Phoenix, AZ 85008

Plasma and reactive-ion etching and ion-milling etch rates for a group of vinyl polymer resists and photoresists have been determined and found to vary by over a factor of 20 as the vinyl side-group substituents were altered synthetically or the photoresist varied. Lower etch rates or better etch compatibilities are observed for vinyl polymer resists containing multiply-bonded and unsaturated side-groups, as well as for lithographically negative behaving systems. Generally, better etch compatibilities were observed for the photoresist systems although a couple of the v i n y l systems did perform as well. The etch selectivities and trends, measured versus PMMA or S i 0 2 , are reasonably constant for the three etch techniques, except for the negative photoresists. Polymer resists (1) serve as masking layers in the patterning of the dielectric and conducting layers encountered i n integrated circuit (IC) fabrication. When IC device geometries were greater than 5 micrometers, the c r i t i c a l dimensions of these IC patterns could be controlled reasonably well--even though the wet chemical etching processes employed were purely isotropic. As IC geometries are shrunk from 3 to 1 micrometer and below, established wet isotropic techniques w i l l have to be abandoned i n favor of more anisotropic ones, in order to maintain etched geometry c r i t i c a l dimension control. As a result, the electronics industry has moved heavily into the area of dry-etching using plasma techniques (2) which are capable of achieving the desired etching anisotropy and dimensional-control. Obviously, polymer resists with good plasma etch resistance, or more generally, with "dry-process" compatibility, are in demand. This situation has evolved to the point that dry-etch resistance or compatibility has become the most important design criterion for new r e s i s t s . It is this resist property that w i l l determine resist polymer applications in the near future. Etch resistance data for several polymer resist systems have been reported by Taylor and Wolf (3) and Moreau (4). Some of the results are tabulated in Table I. While Taylor and Wolf (3) have 0097-6156/82/0184-0061 $05.00/0 © 1982 American Chemical Society Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

62

POLYMER

Table I.

CF^/Oa PE Rate Ratio vs S i 0 carbazole)

Poly(styrene) AZ

FOR

ELECTRONIC

-

0.43

0.3

b

Poly(vinylidene flouride)

0.5

b

Poly(methyl

1.0

methacrylate)

a

0.25

5* Data of G. T a y l o r and T. Wolf, r e f e r e n c e Data of W. Moreau, r e f e r e n c e 4.


0 PE Rate Ratio vs. S i 0 2 2

2

0.1

1350

APPLICATIONS

O2 Plasma E t c h ( P E ) r e s u l t s .

CF^/0;z versus

System Poly(N-vinyl

MATERIALS

0.83 1.0 3.

focused upon measuring polymer r e s i s t etching r a t e s (or constants) f o r O2 etched exposed polymers, Moreau 04) has focused upon determining r e s i s t etch r e s i s t a n c e s to the CFi*/02 plasma system, which i s used i n a c t u a l d i e l e c t r i c l a y e r etching processes as opposed to r e s i s t ashing removal a p p l i c a t i o n s f o r the O2 system. I t i s easy to see that the observed etch r a t e s tabulated are i n fluenced s i g n i f i c a n t l y by the r e s i s t polymer composition. The aromatic polymers at the top of Table I are c l e a r l y most r e s i s t a n t to e t c h i n g . C u r i o u s l y , the etch r e s i s t a n c e trend i s maintained r e g a r d l e s s of plasma type or plasma r e a c t i v e s p e c i e s , and depends more upon the r e s i s t polymer composition or s t r u c t u r a l formula. In t h i s work, we were p a r t i c u l a r l y i n t e r e s t e d i n expanding Moreau s l i m i t e d l i s t of CFit/02 etch t e s t e d samples with a group of polymer r e s i s t s , where the composition i s known and s i d e chain groups have been s y s t e m a t i c a l l y a l t e r e d to determine chemical moiety e f f e c t s upon plasma etch r e s i s t a n c e . We were a l s o i n t e r e s t ed i n determining the e f f e c t upon the r e s i s t polymer etch s e l e c t i v i t i e s and s e l e c t i v i t y trends caused by dry-etching with more a n i s o t r o p i c and p o t e n t i a l l y damaging systems, namely r e a c t i v e - i o n etching and i o n - m i l l i n g . A l l three of the chosen dry etching techniques are capable of a n i s o t r o p i c e t c h i n g ; the s p e c i f i c CF4/O2 plasma e t c h i n g (PE) system employed happens to be i s o t r o p i c , but i t i s the most representa t i v e of the e x i s t i n g halogen-based plasma etching techniques. Plasma and r e a c t i v e - i o n e t c h i n g (RIE) are governed p r i m a r i l y by halogen f r e e r a d i c a l chemistry ( i . e . , the halogen r a d i c a l s or other halocarbon moieties produced i n the plasma r e a c t with the samples to produce v o l a t i l e halogen compounds). R e a t i v e - i o n etching cond i t i o n s d i f f e r from those of PE as RIE i s c a r r i e d out at lower pressures and higher e l e c t r i c a l power; thus, RIE etching i s aided and a s s i s t e d by i o n bombardment. RIE and i o n - m i l l i n g (IM) are a l s o more a n i s o t r o p i c , due to the p a r a l l e l nature of the e l e c t r o d e s employed and reduced gas pressures. Ion m i l l i n g i s c a r r i e d out at lower pressures and the etching species i s u s u a l l y A r , not h a l o gen f r e e r a d i c a l s or fluorocarbon i o n s . IM etching i s a p h y s i c a l f

+

Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

5.

H E L B E R T A N D SCHMIDT

Vinyl

Polymers

and

Photoresists

63

process and i s governed l e s s by chemical r e a c t i v i t y , unless the i o n employed i s a l s o r e a c t i v e . We r e p o r t here plasma etch r a t e data f o r a s e r i e s of v i n y l r e s i s t polymers w i t h a wide range of s i d e c h a i n s u b s t i t u e n t s . The r e s u l t s of t h i s study are v a l u a b l e because they p r o v i d e , when combined with other r a d i a t i o n chemical t e s t data, improved design c r i t e r i a f o r making improved high performance r a d i a t i o n r e s i s t s . S t r u c t u r a l fomulae and chemical nomenclature plus acronyms f o r the v i n y l polymer systems s t u d i e d are compiled below: X,Y CH ,C02CH - poly(methyl methacrylate) (PMMA) C1,C0 CH - poly(methyl a l p h a - c h l o r o a c r y l a t e ) (PMCA) F,C0 CH - poly(methyl a l p h a - f l u o r o a c r y l a t e ) (PMFA) 3

3

2

3


2

X (CH -C) 2

3

X,Y n

R

^

C

6

H

5

C

H, C H 6

_ poly(chlorostyrene)

1

- poly(styrene)

5

CI, CN

-

(PCS)

(PS)

poly(MCA-co-methacrylonitrile)(P(MCA-co-MCN))

X=CH ,Y C0 CH - poly(methyl methacrylate) (PMMA) C0 CH CH - p o l y ( e t h y l methacrylate) (PEMA) C 0 C H C H ( C H ) - p o l y ( i s o b u t y l methacrylate) (PIBM) C 0 C ( C H ) - p o l y ( t e r t - b u t y l methacrylate) (PTBM) C0 CH CC1 - p o l y ( t r i c h l o r o e t h y l methacrylate) (PTCEM) C 0 C H C F - p o l y ( t r i f l u o r o e t h y l methacrylate) (PTFEM) C 0 C H ( C F ) - p o l y ( h e x a f l u o r o i s o p r o p y l methacrylate)(PHFIM) CN - poly(methacrylonitrile)(PMCN) 3

2

3

2

2

2

2

2

3

3

2

2

2

2

2

3

2

3

3

2

3

2

These v i n y l systems were chosen a l s o because they f u n c t i o n as h i g h r e s o l u t i o n e l e c t r o n beam r e s i s t s and deep UV r e s i s t s a t A130°C), and t h e r e f o r e , are more thermally s t a b l e than the polymers of Table I I . S u r p r i s i n g l y , PMFA etches slower and i s more r e s i s t a n t , while PMCA i s s i g n i f i c a n t l y l e s s r e s i s t a n t . The a - c h l o r i n e i s known t o enhance the r a d i a t i o n degradation s u s c e p t i b i l i t y f o r PMCA v s PMMA as v e r i f i e d by a higher G v a l u e . (5) Since the C-Cl bond i s r e a d i l y cleaved, (5,6) i t i s easy t o e n v i s i o n enhanced PE and RIE g

s

s

g

s


66

MATERIALS

FORELECTRONIC

APPLICATIONS


POLYMER

Figure 1.

PE rate in A/min vs. G for the vinyl polymer resist systems. The etching conditions are found in Table II. s


5.


Table I I I .

Vinyl

2

a

P E Rate (X/min)

X (PMMA) (PMCA) (PMFA)

3

a

67

3

b

R I E Rate PE Rate R a t i o vs Si02 Ratio vs Si02 1.2 1.2 0.6

0.9-1.2 1.8 0.4

93-110 186 40

s 1.3 6.0 0

G X 0 0.9 1.0

See PE c o n d i t i o n s of Table I I See RIE c o n d i t i o n s o f Table I I

b


and Photoresists

PE and RIE r e s u l t s f o r a l p h a - s u b s t i t u t e d p o l y a c r y l a t e s «CH -C(X)C0 CH >. 2

-CH -CI -F

Polymers

degradation s u s c e p t i b i l i t y f o r PMCA, v i a a b s o r p t i o n of hv by the polymer and cleavage of the C-Cl bond i n the plasma. The C-F bond i n PMFA, on the other hand, i s not e a s i l y cleaved,(7) henPe, PMFA would be l e s s PE and RIE degradable. The G v a l u e s of Table I I I and the data o f the c i t e d r e f e r e n c e s support t h i s r a t i o n a l i z a t i o n . PMFA i s a l s o s i m i l a r i n s t r u c t u r e t o that of p o l y ( v i n y l i d e n e f l u o r i d e ) , -(CH2-CF2)-, which has been reported i n r e f e r e n c e s 3 and 4 t o be h i g h l y r e s i s t a n t t o PE degradation (see a l s o Table I ) . PVDF has not been i n v e s t i g a t e d as a r e s i s t due t o poor s o l u b i l i t y , but i t i s known t o predominantly c r o s s l i n k when i r r a d i a t e d by i o n i z i n g r a d i a t i o n l i k e PMFA. (8) The e t c h r e s i s t a n c e s of PVDF and PMFA are most c e r t a i n l y governed by the strong C-F s i d e c h a i n bond(s). The polymer r e s i s t e x h i b i t i n g the lowest PE r a t e or highest etch r e s i s t a n c e versus PMMA o r oxide i s p o l y ( s t y r e n e ) (see Table IV). T h i s system, l i k e the others of Table IV, i s r e p r e s e n t a t i v e of a v i n y l polymer with g e n e r a l s t r u c t u r a l formula of 4CH2-CXY)-. P o l y ( c h l o r o s t y r e n e ) , a c h l o r i n a t e d d e r i v a t i v e of the aromatic p o l y ( s t y r e n e ) , e x h i b i t s equal r e s i s t a n c e towards a l l three d r y etch processes. Here halogenation has not enhanced the etch r a t e or reduced the r e s i s t a n c e as seen b e f o r e f o r PTECM, PTFEM, and PMCA nonaromatic systems. T h e r e f o r e , t h e aromatic s i d e group must g

Table IV: PE, RIE and IM r e s u l t s f o r d i - s u b s t i t u t e d polymers {CH ~CXY>.

vinyl

2

PE

a

Etch Rate Ratio vs Si0

X> Y

2

-CH , -C0 CH -CH ,-CN -H,-C H -H^CeH^Cl 3

2

b

c

c

G

T

x

2

2

0.9-1.2

1.2

d

1.2

105

1.3

0

0.3 0.1 0.1

0.4 0.1 0.3

0.9 0.7 0.8

120 100

3.3 0

0 0.05

-

-

3

6

a

b

R I E Etch I o n - m i l l i n g Rate Rate Ratio Ratio g vs S i 0 vs S i 0

5

-

See PE c o n d i t i o n s of Table I I See RIE c o n d i t i o n s of Table I I IM c o n d i t i o n s ; A r , 0.9 x 10 * t o r r , 600 v o l t s , 15° angle, on Veeco 3" system RD = r e s i s t s u r f a c e s e v e r e l y degraded _i



68

POLYMER

MATERIALS

FOR ELECTRONIC

APPLICATIONS

dominate and cause the reduced PE degradation s u s c e p t i b i l i t y of these v i n y l polymers. These r e s u l t s a r e c o n s i s t e n t with those l i s t e d i n Table I taken from the l i t e r a t u r e . I t a l s o i s evident that negative-behaving e-beam r e s i s t s , l i k e PS, PCS, PMFA and the others are g e n e r a l l y more PE and RIE r e s i s t a n t than the other v i n y l polymers. T h i s may be a t t r i b u t e d t o the f a c t that when the a-hydrogens are a b s t r a c t e d by r a d i c a l s p e c i e s , a c r o s s l i n k i n g s i t e i s created and not an unstable degradation intermediate. Of the polymer r e s i s t s with s t r u c t u r a l formula {CH2-CXY> from Table IV or of the group of p o s i t i v e behaving e-beam v i n y l r e s i s t polymers, PMCN i s the most r e s i s t a n t (see Table IV and V ) . T h i s etch r e s i s t a n c e i s a t t r i b u t e d t o the CN s i d e c h a i n group, which i s a s t r o n g l y bonded group that cannot be r e a d i l y cleaved from the polymer backbone by r a d i a t i o n . I t i s notable that PMCN does not thermally degrade to monomer l i k e other v i n y l p o l y mers, except a t temperatures g r e a t e r than 270°C; below 270°C the polymer i s more thermally s t a b l e than the other v i n y l s . Thus, the same s t a b i l i z i n g r e a c t i o n , as that which occurs thermally over the temperature range o f 100-270°C to produce a l a d d e r - l i k e p o l y mer c o n t a i n i n g -(N=C-N=C)- u n i t s p a r a l l e l t o the main c h a i n , may be o c c u r r i n g during d r y - e t c h i n g .

Table V:

E l e c t r o n beam p o s i t i v e (top) and negative (bottom) r e s i s t e t c h i n g r a t e r a t i o s f o r three sets of e t c h i n g conditions. 1

3

CFijOzPE* Rate RIE* Rate Ion M i l l i n g Ratio v s Si02 R a t i o vs Si02 Rate v s Si02

Resist PBS PHFIM PTFEM PMCN PMMA AZ2400 PC 129 PS PMFA COP (KTI) SEL-N OEBR-100

^9 - 10 2.4 2.2 0.3 0.9 - 1.2 0.5 0.2 0.1 0.4 0.7 0.6 0.6

_ RD RD 0.4 1.2 0.6 0.2 0.1 0.6 0.2 0.6 0.6 a

0.9 1.2 0.5 0.5 0.7

1.6 1.2 1.1

0.7 1 1 2 5 1 2 1.5 2 0.8 0.8

E-Beam Q,C/cm

2

_ 1 x 1010" io10" 1510"* io-* 10"* IO" - 1 x IO" X io-

X X X X X X X X

6

5

5

5

5

5

6

6

-

See etching c o n d i t i o n s of Table IV Consistent with the PMCN homopolymer r e s u l t s above, the MCN copolymers of Table VI e x h i b i t PE etch r a t e s intermediate t o those of the two r e s p e c t i v e homopolymer v a l u e s . When MCN i s copolymeri z e d with MCA, the r e s u l t i n g copolymers etch f a s t e r than PMCN and slower than PMCA (see Table V I ) . The etch r a t e i s approximately l i n e a r with mole % MCA content; t h i s data i s p l o t t e d i n F i g u r e 2.


Vinyl

Polymers

and

Photoresists



Figure 2.

PE rate in A/min vs. mole % MCA for the MCN/MCA mer. The etching conditions are found in Table II.

vinyl copoly-


70

POLYMER

MATERIALS

FOR ELECTRONIC

APPLICATIONS

When MCA i s copolymerized with MMA, the r e s u l t i n g copolymer etches f a s t e r than PMMA, c o n s i s t e n t with the PMCA and MCN/MCA r e s u l t s . Incorporation of MCA, a monomer with ot-chlorine, has the e f f e c t of decreasing plasma etch r e s i s t a n c e , as i s observed f o r the MCA homopolymer. S e n s i t i z a t i o n by c h l o r i n e appears to be general (see Tables I , I I I , and V I ) , except f o r the case where the c h l o r i n e i s incorporated onto the aromatic s i d e chain group. T h i s e f f e c t was observed p r e v i o u s l y by T a y l o r and Wolf f o r other polymer systems. (3) The C-Cl bond strength i s lower than that f o r C-H and C-F, and there i s much evidence that t h i s bond can be e a s i l y cleaved, even i n the s o l i d s t a t e . 05,6) T h i s weaker s i d e chain group leads to lower dry-etch r e s i s t a n c e . Table VI;

Plasma etch r a t e r a t i o s f o r copolymers.


PE System

a

Rate Ratio vs SiO 2

PMFA P(MFA-CO-MCN) P(MFA-CO-MCN) PMCN P(MCN-CO-MCA) P(MCN-CO-MCA) PMCA P(MCA-CO-MMA) PMMA P(MMA-CO-MFA) PMFA a

(20/80) (57/43) (49/51) (32/68) (46/54) (78/22)

0.4 0.4 0.3 0.3 0.6 1.2 1.8 1.1 0.9-1.2 0.7 0.4

T

e 131 115 124 120 120 130 151,130 125 105 106 131

See PE c o n d i t i o n s of Table I I .

The etch r a t e measurements f o r p o s i t i v e and negative-behaving e-beam r e s i s t s a r e found i n Table V. I t i s apparent that the etch r e s i s t a n c e i s lower the more s e n s i t i v e the p o s i t i v e r e s i s t . The exception would be PMCN, which e x h i b i t s b e t t e r dry-etch r e s i s t a n c e than that which would be p r e d i c t e d based on e-beam s e n s i t i v i t y alone. Where e-beam s e n s i t i v i t y and etch r e s i s t a n c e are needed, copolymerization becomes very important. This has been demons t r a t e d f o r the MCN/MMA and MCA/MCN model copolymer systems i n references 9 and 10, r e s p e c t i v e l y . Dry-etch s e l e c t i v i t i e s f o r s e v e r a l negative e-beam r e s i s t s are a l s o l i s t e d i n Table V. They a r e more r e s i s t a n t than the p o s i t i v e e-beam r e s i s t s of the Table except PMCN and the p o s i t i v e p h o t o r e s i s t s , AZ2400 and PC 129. The p o s i t i v e - b e h a v i n g v i n y l polymer r e s i s t s t e s t e d a r e g e n e r a l l y l e s s r e s i s t a n t than the negative-behaving systems. T h i s g e n e r a l i t y , however, does not hold f o r the p h o t o r e s i s t s t e s t e d , as the data of Table V I I v e r i f i e s . In general, the p h o t o r e s i s t s e x h i b i t greater dry-process r e s i s t a n c e than the v i n y l polymers of Table I I . The greater d r y etch r e s i s t a n c e s of p h o t o r e s i s t s i s a t t r i b u t e d t o the aromatic nature of the c r o s s l i n k i n g agents, photoactive components, and novolac r e s i n s ( p o s i t i v e p h o t o r e s i s t s o n l y ) . In a d d i t i o n , the


5.


Vinyl

Polymers

and

71

Photoresists

negative p h o t o r e s i s t r e s i n s a r e known t o be of the c y c l i z e d p o l y (isoprene) type w i t h v a r y i n g degrees of u n s a t u r a t i o n . I t i s t h i s same compositional e f f e c t that leads t o high thermal degradation r e s i s t a n c e f o r the polymer r e s i n s . In the case of novolacs, thermal degradation does not y i e l d monomer as f o r many of the v i n y l r e s i s t s . Thus, there i s a good c o r r e l a t i o n between v a r i o u s types of data.

Table V I I :

Commercial p h o t o r e s i s t etch r a t e r a t i o s . a

P E Rate Ratio vs Si02


Resist PMMA Kodak 809 AZ2400/1350J KTI I I PC 129 HPR 204 Merck S e l e c t i l u x Cop (Hunt) Kodak 747 HNR 80

0.9 - 1.2 0.3 0.5 0.4 0.2 0.5 0.3 0.6 0.2 0.7

a

R I E Rate Ion M i l l i n g Rate R a t i o Ratio vs Si02 vs Si02 1.2 0.2 0.6 0.5 0.2 0.3 0.3 0.6 0.1 0.1

3

1.2 0.8 0.6 0.7 0.5 0.4 1.5 0.8 0.8 0.6

Resist Tone

+ + + + + +

-

-

See etching c o n d i t i o n s of Table IV.

Summary I t i s evident that dry-etch r a t e s or t h e i r etch r a t i o s can vary s i g n i f i c a n t l y f o r v i n y l polymers with d i f f e r e n t s i d e chain s u b s t i t u e n t s . The aromatic v i n y l polymer r e s i s t s a r e the most r e s i s t a n t ; t h i s grouping a l s o i n c l u d e s the novolac-based p o s i t i v e p h o t o r e s i s t s . Polymer r e s i s t s with s t r o n g l y bonded s i d e chain groups l i k e the a - f l u o r i n e or ^--cyano a c r y l a t e s , are a l s o h i g h l y r e s i s t a n t . Halogenated p o l y ( m e t h a c r y l a t e s ) , on the other hand, are s i g n i f i c a n t l y l e s s r e s i s t a n t , except when the halogen i s i n corporated i n t o the aromatic part of the polymer. Greater gene r a l e t c h r e s i s t a n c e i s observed f o r negative than f o r p o s i t i v e e-beam polymer r e s i s t s . Greater dry-etch r e s i s t a n c e i s d i s p l a y e d by the p h o t o r e s i s t systems.

Acknowledgment The authors g r a t e f u l l y acknowledge t e c h n i c a l d i s c u s s i o n s concerning t h i s work with Dr. J . L a i .


72

POLYMER MATERIALS FOR ELECTRONIC APPLICATIONS

Literature Cited 1. 2. 3. 4. 5.


6. 7. 8. 9. 10.

Thompson, L . F . ; Kerwin, L . E . , Ann. Rev. of Materials Sci., 1967, 6, 267. Kirk, R.W. Chap. 9 in "Techniques and Applications of Plasma Chemistry"; Hollahan, J.R.; Bell, A.T. eds., Wiley, New York 1978. Taylor, G.N.; Wolf, T.M. Polym. Eng. and Sci., 1980, 20, 1087. Moreau, W.M. Mohawk Photopolymer Conference, June 1979. Helbert, J.N.; Chen, C-Y.; Pittman, C.U.; Hagnauer, G.L. Macromolecules 1978, 11, 1104. See references 20-23 of reference 5. Pittman, C.U.; Chen, C-Y.; Ueda, M.; Helbert, J.N.; Kwiatkowski, J.H., J. of Polym. Sci: Chem Ed. 1980, 18, 3413. Florin, R.F. in "Fluoropolymers", Wall, L.A. ed., Wiley, New York, Chap. 11 1972. Stillwagon, L . E . ; Doerries, E.M.; Thompson, L . F . ; Bowden, M.J. Org. Coatings and Plastics Preprints 1977, 37, (2) 38. Lai, J.H.; Helbert, J.N.; Cook, C.F.; Pittman, C.U. J. Vac. Sci. Technol. 1979, 16, 1992.

RECEIVED

October 19, 1981.


Effect of Composition on Resist Dry-Etching Susceptibility - ACS

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