A Sensitive Positive-Working Cross-Linked Methacrylate Electron

1 A Sensitive Positive-Working Cross-Linked Methacrylate Electron Resist E. D. ROBERTS

Downloaded by 80.82.77.83 on February 27, 2018 | https://pubs.acs.org Publication Date: April 7, 1982 | doi: 10.1021/bk-1982-0184.ch001

Philips Research Laboratories, Redhill, Surrey, England

In earlier publications (1,2), a cross-linked methacrylate positive-working electron resist was described. The resist consists of a mixture of two copolymers- poly-(methyl methacrylate -co-methacrylic acid) and poly-(methyl methacrylate-co-methacryloyl chloride), both of which, having straight chain structures, are soluble in organic solvents. A solution of the mixture is used to apply a film to a substrate by the normal spinning process. When the film is subsequently baked, the carboxyl groups react with the chlorocarbonyl groups to form carboxylic acid anhydride cross-links.

The cross-linked polymer is, like all cross-linked materials, completely insoluble in a l l organic solvents. The chemical bonds joining the anhydride group to the main methacrylate chain are particularly susceptible to rupture by ionising radiations, so upon exposure to an electron beam they are broken and the straight chain structure is restored. The irradiated polymer thus becomes soluble again and can be selectively dissolved to give a positive image in the resist film. Methyl isobutyl ketone is normally used as the developing solvent. The system is capable of almost infinite variation, for the proportion of cross-links in the structure can be varied over a wide range. Firstly, it can be changed by altering the proportion of potential cross-linking groups (carboxyl and 0097-6156/82/0184-0001$05.00/0 © 1982 American Chemical Society Feit and Wilkins; Polymer Materials for Electronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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chlorocarbonyl) i n the copolymers and choosing c o n d i t i o n s to ensure that most or a l l of these r e a c t . A l t e r n a t i v e l y , i n a system of p a r t i c u l a r composition, the p r o p o r t i o n of p o t e n t i a l c r o s s - l i n k i n g groups which a c t u a l l y react can be changed by a l t e r i n g the c o n d i t i o n s under which the c r o s s - l i n k s are formed that i s , by changing the c u r i n g c o n d i t i o n s . The curves i n Figure 1 show some of the v a r i e t y of p r o p e r t i e s which can be obtained from these systems. I t was shown e a r l i e r (JL,2) that these r e s i s t s are more s t a b l e to thermal deformation than i s poly-(methyl methacrylate)(PMMA), at l e a s t f o r f i l m s cured at 175°C. At t h i s c u r i n g temperature, presumably, the maximum p o s s i b l e degree of c r o s s - l i n k i n g has been achieved and the s e n s i t i v i t y i s a minimum. The r e l a t i v e l y low s e n s i t i v i t y i s not n e c e s s a r i l y disadvantageous i n r e s i s t s intended f o r use i n equipment such as e l e c t r o n image p r o j e c t o r systems (_3,40 . For use with p a t t e r n generators, (5,6) , however, i t i s d e s i r a b l e to have a r e s i s t of higher s e n s i t i v i t y (7). The e a r l i e r p u b l i c a t i o n s (1,2) described mainly the examination and use of r e l a t i v e l y h i g h l y c r o s s - l i n k e d f i l m s which had been cured at 175°C. This paper d e s c r i b e s some of the p r o p e r t i e s obtained when the r e s i s t s are very l i g h t l y c r o s s - l i n k e d , by c u r i n g at temperatures i n the range 100-125°C. Under these c o n d i t i o n s , much greater s e n s i t i v i t y i s obtained, though more c a r e f u l c o n t r o l of operating c o n d i t i o n s i s needed to o b t a i n reproducible r e s u l t s . P r o p e r t i e s of s e n s i t i v e c r o s s - l i n k i n g r e s i s t s The copolymer system used i n the work described here i s our standard c r o s s - l i n k i n g r e s i s t , comprising the mixture of the two copolymers s p e c i f i e d above, each c o n t a i n i n g 10 mol% of the comonomer which provides the p o t e n t i a l c r o s s - l i n k i n g groups. The degree of c r o s s - l i n k i n g introduced has been r e s t r i c t e d by c u r i n g at temperatures w i t h i n the range 100-125°C, and Table I shows the s e n s i t i v i t i e s of f i l m s cured under these c o n d i t i o n s . At these r e l a t i v e l y low c u r i n g temperatures, the s e n s i t i v i t y changes f a i r l y r a p i d l y with c u r i n g temperature, so i t i s necessary to maintain that temperature w i t h i n about h a l f a degree of the intended value i f r e p r o d u c i b l e r e s u l t s are to be obtained. However, t h i s does not present an insuperable d i f f i c u l t y . I t i s apparent from Table I that the s e n s i t i v i t y of l i g h t l y c r o s s - l i n k e d r e s i s t f i l m s i s more dependent upon f i l m thickness than i s that of f u l l y - c u r e d f i l m s ( i . e . f i l m s cured at 150-175°C). I t seems that the solvent may have some i n f l u e n c e upon sensitivity. The values i n Table I were obtained on f i l m s spun from s o l u t i o n i n ethoxyethyl acetate and even f u l l y c r o s s - l i n k e d f i l m s show some v a r i a t i o n of s e n s i t i v i t y with f i l m thickness. This i s i n contrast to our e a r l i e r work i n which f i l m s spun from s o l u t i o n i n methyl i s o b u t y l ketone had s e n s i t i v i t y 40yC/cm at any f i l m thickness from about 0.3 to 2ym. I t must be more 2

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


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Methacrylate

Electron

3

Resist

Films insoluble in acetone I

100

i

I

200

i

i

300

Curing t e m p e r a t u r e (°C) Figure 1.

Variation of sensitivity of cross-linked methacrylate resists with curing temperature ($).


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Table I V a r i a t i o n of P r o p e r t i e s of Cross-Linked Methacrylate R e s i s t s with Curing Conditions

Curing Temp. ; °C


150-200 Optimum 175

Curing Dev.Time Time at Mins 22-23°C Mins

2 S e n s i t i v i t y uC/cm ^l.Oum M).5um thickness thickness lOkV

20kV

10kV|

20kV 1 i

35-40 110-115

Typical loss j of thickness | during development at 22-23°C.um

50 1

None

- i

None

15

2-4

123

45

4

8

-

120

45

4

6

26

115

45

4

6

-

- !

0.04

110

60

5

3

16

111

0.05

105

60

5

3

12

7;

100

60

5

2

4

0.03

6

1

26

0.12 0.29

d i f f i c u l t to e l i m i n a t e ethoxyethyl acetate from the f i l m but i t i s not c l e a r why t h i s should a f f e c t the s e n s i t i v i t y i n the way i t appears t o . The increase i n s e n s i t i v i t y i s not obtained without some s a c r i f i c e i n other p r o p e r t i e s . During the development process, there i s some l o s s of thickness i n unexposed areas of the f i l m , and t h i s l o s s becomes greater as the s e n s i t i v i t y i s increased. The a c t u a l l o s s i n thickness i s almost independent of the o r i g i n a l f i l m thickness and the curves i n Figure 2 show the r a t e at which t h i s thickness changes, together with the r a t e at which PMMA d i s s o l v e s . Of course, the c r o s s - l i n k e d f i l m s never d i s s o l v e completely, and during a normal development process i n v o l v i n g f i v e minutes immersion, the l o s s i s g e n e r a l l y too small to cause any d i f f i c u l t y , at l e a s t with f i l m s cured at 105°C or at higher temperatures. Many users appear to achieve s a t i s f a c t o r y r e s u l t s even with f i l m s cured at 100 C. The l a s t column of Table I shows t y p i c a l values f o r the thickness l o s s during development of f i l m s cured at d i f f e r e n t temperatures. Both the thickness l o s s and s e n s i t i v i t y depend upon developing time and temperature, and the c o n d i t i o n s s p e c i f i e d i n Table I were chosen to give g e n e r a l l y a reasonable compromise between two opposing requirements. Table I I shows the kind of v a r i a t i o n s which can occur i n l i g h t l y c r o s s - l i n k e d r e s i s t s when the developer temperature i s changed, so i t i s c l e a r that t h i s temperature a l s o needs to be c o n t r o l l e d to w i t h i n about 1 C to achieve good r e p r o d u c i b i l i t y when using them.


ROBERTS

Methacrylate

Electron

Resist

Normal development time


2-0

oc

i

»-o-

E 1.5 -

40 60 Minutes Time of immersion in methyl isobutyl ketone Figure 2.

Rate of dissolution of methacrylate films. Key: • - • -, VMM A; — cross-linked resist cured 1 h at 105°C.


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Table I I V a r i a t i o n of C r o s s - l i n k e d Methacrylate R e s i s t P r o p e r t i e s with Developing Conditions 1 Developer | Temperature

1

2

°c

M).5ym thickness 20 Downloaded by 80.82.77.83 on February 27, 2018 | https://pubs.acs.org Publication Date: April 7, 1982 | doi: 10.1021/bk-1982-0184.ch001

Loss of thickness during development ym

Sensitivity at lOkV yC/cm

25

5

j

VL.Oym M).5ym thickness thickness

^1 .Oym thickness

!

8

0.11

0.08

i

7

0.13

0.13

A l l r e s i s t s were cured f o r 1 hour at 105°C and developed i n methyl i s o b u t y l ketone f o r 5 minutes. There i s one other source of p o t e n t i a l d i f f i c u l t y , though t h i s has only a r i s e n when seeking to o b t a i n the highest s e n s i t i v i t y by c u r i n g a t 100°C or l e s s , and even then only when using f i l m s at l e a s t lym t h i c k . This i s the appearance during the p a t t e r n development stage of cracks i n the f i l m . During a normal development process, the cracks appear only i n the immediate v i c i n i t y of the exposed area and almost i n v a r i a b l y o r i g i n a t e from the corners of patterns as shown i n F i g u r e 3. I f the f i l m i s l e f t to soak i n the developer f o r some time, during which more of i t d i s s o l v e s (Figure 2 ) , the cracks disappear again. This suggests that they are only surface cracks. No c r a c k i n g has been observed i n Jym t h i c k f i l m s cured at 100 C nor i n f i l m s of any thickness cured a t 105 C or above. These observations are c o n s i s t e n t with the cracks being produced by surface s w e l l i n g during development followed by shrinkage during the drying stage. The ultimate cause of the e f f e c t i s b e l i e v e d to be p a r t i a l breakdown of the c r o s s - l i n k e d s t r u c t u r e by s c a t t e r e d e l e c t r o n s a c t i n g upon the surface of the f i l m , making i t s surface l a y e r s more s u s c e p t i b l e to s w e l l i n g during development. Thicker f i l m s need higher exposures than t h i n ones do, so the exposure and consequent breakdown by s c a t t e r e d e l e c t r o n s i s p r o p o r t i o n a t e l y higher, and presumably i s s u f f i c i e n t to make the f i l m prone to s w e l l i n g during development i n t h i s case. I f the exposure i s increased f u r t h e r , the c r a c k i n g becomes more extensive though at s t i l l higher exposures i t i s not apparent at a l l . In t h i s case, i t appears that the surface c r o s s - l i n k s have been broken s u f f i c i e n t l y to render the surface l a y e r t r u l y s o l u b l e . Indeed, the r e s i d u a l f i l m i s thinner i n the immediate v i c i n i t y of the exposed area. This explanation i s supported by the f a c t that i f a more powerful solvent such as acetone i s used as the developer,


Methacrylate

Electron

Resist


ROBERTS

Figure 3. Cracks in thick cross-linked methacrylate resist films cured at 100°C for 1 h, exposed at 7 pC/cm at 10 kV and developed for 5 min in methyl isobutyl ketone. Key: a, optical micrograph and b, scanning electron micrograph (viewed at 60°). 2


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e x a c t l y analogous behaviour i s observed, though corresponding e f f e c t s occur a t r a t h e r higher curing temperatures. This r e f l e c t s the a b i l i t y of the stronger solvent to produce s w e l l i n g i n more h i g h l y c r o s s - l i n k e d s t r u c t u r e s than the weaker s o l v e n t , methyl i s o b u t y l ketone, can.


Thermal

s t a b i l i t y of l i g h t l y c r o s s - l i n k e d

films

The patterns which are shown i n Figures 3 to 7, were made by exposing the f i l m s to a f l o o d beam of e l e c t r o n s o f energy lOkV, using a f i n e copper gauze as a contact mask. A f t e r development, the f i l m p a t t e r n s were heated to v a r i o u s temperatures, cleaved and examined f o r signs o f deformation. Figure 4 shows that the c h a r a c t e r i s t i c undercut p r o f i l e , which i n s i m i l a r l y t r e a t e d PMMA disappears on heating between 90 and 100 C ( 2 ) , i s s t i l l present i n the l i g h t l y c r o s s - l i n k e d f i l m which has been heated f o r 15 minutes a t 130 C. At higher temperatures the undercut p r o f i l e does disappear but there does not seem to be any change i n l a t e r a l dimensions and edge d e t a i l s are not destroyed to the extent that they are i n PMMA, Figure 5a i s a p a t t e r n i n a f i l m cured f o r 1 hour at 100 C and exposed at 2uC/cm , the r e s i d u a l f i l m thickness being about a t h i r d o f a micron. F i g u r e 5b i s the same f i l m a f t e r being heated f o r 20 minutes at 175°C and the edge d e t a i l appears to be unchanged a f t e r t h i s treatment. Figures 6a and 6b are patterns i n a PMMA f i l m which i s a l s o one t h i r d of a micron t h i c k a f t e r development and has been heated i n the same way (Figure 6b). The flow o f the polymer f i l m i s much more extensive and has destroyed a l l the d e t a i l i n the p a t t e r n edges. I f the user i s prepared to s a c r i f i c e some s e n s i t i v i t y , much greater thermal s t a b i l i t y i n developed patterns can be a t t a i n e d . F i g u r e 7 shows a f i l n ^ w h i c h was cured f o r 45 minutes at 125 C, and exposed a t 12uC/cm at lOkV. Under these c o n d i t i o n s , unexposed p a r t s are completely i n s o l u b l e during the development process (Table I ) . The p i c t u r e s show the edge p r o f i l e s o f a p a t t e r n which, a f t e r development, has been heated at v a r i o u s e l e v a t e d temperatures. During t h i s heat treatment, no flow o f the undercut edge has occurred. D i s c u s s i o n on thermal s t a b i l i t y . The most unexpected feature o f t h i s i n v e s t i g a t i o n i s the good thermal s t a b i l i t y of these l i g h t l y c r o s s - l i n k e d r e s i s t s , and i t i s p a r t i c u l a r l y s t r i k i n g i n the f i l m p a t t e r n which has been heated a t 250 C (Figure 7c) apparently without changing. The same area o f a number o f samples has been examined a f t e r s u c c e s s i v e l y increased heat treatment. The only change which can be seen i s the appearance of undulations i n the surface, l i k e that which can be seen i n F i g u r e 7b. However, these occur only w i t h i n areas which have been scanned during the previous e l e c t r o n microscope examination and so c e r t a i n l y r e s u l t from



ROBERTS

Methacrylate

Electron

Resist

Figure 4. Edge profile of a pattern in cross-linked methacrylate resist film cured at 105°C and exposed at 8 pC/cm at 10 kV after heating for 15 min at 130°C (S). 2


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10

Figure 5. Optical micrographs of patterns in cross-linked methacrylate resist film cured at 100°C for 1 h, exposed at 2 pC/cm at 10 kV, and developed for 5 min in methyl isobutyl ketone (&). Key: a, as developed and b, after heating for 20 min at!75°C. 2


Methacrylate

Electron

Resist


ROBERTS

Figure 6. Optical micrographs of patterns in PMMA exposed at 50 iiC/cm' 10 kV (8). Key: a, as developed and b, after heating for 20 min at 175°C.


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Figure 7. Edge profiles of patterns in cross-linked methacrylate resist film cured at 125° C for 45 min and exposed at 12 pC/cm at 10 kV (viewed at 60°). Key: a, after heating for 20 min at 175°C and b, after further heating for 20 min at 200°C. Continued. 2



ROBERTS

Methacrylate

Electron

Resist

Figure 7. Edge profiles of patterns in cross-linked methacrylate resist film cured at 125° C for 45 min and exposed at 12 fiC/cm at 10 kV (viewed at 60°). Key: c, similar pattern as (a) and (b) after heating for 20 min at 250°C (%). 2



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exposure during that examination. In a l l the samples examined, no changes i n p r o f i l e of the patterns were seen to have occurred as a r e s u l t of the heat treatment. I t was shown e a r l i e r (2) that when the same r e s i s t i s f u l l y cured, by baking at 175 C, f i l m patterns show signs of flow as a change i n slope of the edge p r o f i l e a f t e r heating at 175"C Thi s was, and s t i l l i s , b e l i e v e d to be due to breakdown of the c r o s s - l i n k e d s t r u c t u r e j u s t i n the edges of the nominally u n i r r a d i a t e d areas, caused by e l e c t r o n s s c a t t e r e d from adjacent i r r a d i a t e d areas. In p r i n c i p l e , t h i s can s t i l l happen i n the s e n s i t i v e form of t h i s r e s i s t although the exposure l e v e l s are lower. However, because the f i l m i s now only l i g h t l y c r o s s - l i n k e d before exposure, a number of the p o t e n t i a l c r o s s - l i n k i n g groups (carboxyl and chlorocarbonyl) w i l l s t i l l be present i n t h e i r o r i g i n a l forms throughout the f i l m remaining on the substrate a f t e r exposure and development. Even though the o r i g i n a l c r o s s - l i n k e d s t r u c t u r e may have been broken to some extent by s c a t t e r e d e l e c t r o n s , the p o t e n t i a l c r o s s - l i n k i n g groups s t i l l i n the f i l m can r e a c t to form new c r o s s - l i n k s during any subsequent heating process. Presumably, t h i s occurs s u f f i c i e n t l y r a p i d l y and i s extensive enough to enable the f i l m to r e s i s t deformation at higher temperatures. In f i l m s cured i n i t i a l l y at 175°C, determination by i n f r a red spectroscopy of the concentration of anhydride groups i n d i c a t e d that a l l p o t e n t i a l c r o s s - l i n k i n g groups had r e a c t e d . In t h i s case, t h e r e f o r e , no groups remain to form new c r o s s l i n k s upon heating a f t e r i r r a d i a t i o n , and some flow can occur where the s t r u c t u r e has been p a r t i a l l y degraded by s c a t t e r e d electrons. Supporting evidence f o r t h i s hypothesis of post-exposure c r o s s - l i n k i n g came i n other experiments i n which r e s i s t f i l m s cured i n i t i a l l y at 100 C and subjected to various exposures, were then post-baked at s l i g h t l y higher temperatures before development. The purpose of the experiment was a c t u a l l y to d i s c o v e r i f the r a t h e r high l o s s of thickness which occurred during development of patterns i n f i l m s cured at 100 C could be reduced, and indeed i t was. Unfortunately, the s e n s i t i v i t y was a l s o reduced, and the o v e r a l l e f f e c t was as i f the i n i t i a l c u r i n g had been performed a c t u a l l y at the postbaking temperature. In t h i s case, f u r t h e r c r o s s - l i n k i n g was c l e a r l y t a k i n g place during the post-baking process, even i n the i r r a d i a t e d areas, f o r these d i d not develop p r o p e r l y unless the exposure exceeded the s e n s i t i v i t y corresponding to the post-bake temperature. Uses of s e n s i t i v e c r o s s - l i n k e d

resists

The r e s i s t s can be used f o r d e f i n i n g f i n e d e t a i l s , and Figure 8a and 8b show some groduced by a p a t t e r n generator i n a f i l m which was cured at 100 C and exposed at 3.2uC/cm at 20kV. The l i n e s are 1, 1.5 and 3um wide.


Methacrylate

Electron

Resist


ROBERTS

Figure 8. Fine lines in cross-linked methacrylate resist film cured at 100°C for 1 h, exposed at 3.2 fiC/cm at 20 kV, and developed for 5 min in methyl isobutyl ketone. Key: a, 1.5-yjn and 3-pjn wide lines (8) and b, 1-yjn wide lines. 2


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The r e s i s t has been used as a mask i n wet etching and i n l i f t - o f f processes, and more r e c e n t l y i n etching chromium f i l m s i n a chlorine-oxygen-helium plasma. In the l a t t e r , the etch rates have ranged from 4 to 5.5nm/min at 100W power i n a b a r r e l type r e a c t o r . Chromium etches at about 6.5nm/min under these c o n d i t i o n s . The etch r a t e o f the r e s i s t appears to be independent o f the degree to which i t has been cured before exposure, so the s e n s i t i v e form described here i s j u s t as e f f e c t i v e a mask as the h i g h l y c r o s s - l i n k e d r e s i s t s described e a r l i e r , a t l e a s t i n the chromium etching process.


Conclusions S e n s i t i v e c r o s s - l i n k i n g positive-working r e s i s t s can be made by r e s t r i c t i n g the degree of c r o s s - l i n k i n g introduced i n t o the r e s i s t f i l m s . This can be done by l i m i t i n g the p r o p o r t i o n of p o t e n t i a l c r o s s - l i n k i n g groups i n the c o n s t i t u e n t copolymers, though considerably greater thermal s t a b i l i t y i s obtained i f i t i s done rather by r e s t r i c t i n g the curing temperature of a system c o n t a i n i n g a r e l a t i v e l y high p r o p o r t i o n o f p o t e n t i a l c r o s s - l i n k i n g groups. By s e l e c t i n g s u i t a b l e c u r i n g c o n d i t i o n s , one r e s i s t mixture can be used to o b t a i n s e n s i t i v i t i e s appropriate to the type of e l e c t r o n beam equipment i n which i t w i l l be used - image p r o j e c t o r s or p a t t e r n generators.

Acknowledgments The author thanks the D i r e c t o r s of P h i l i p s Research L a b o r a t o r i e s f o r permission to p u b l i s h t h i s paper. He i s a l s o g r a t e f u l to many o f h i s colleagues i n the L a b o r a t o r i e s f o r s t i m u l a t i n g and h e l p f u l d i s c u s s i o n s , p a r t i c u l a r l y concerning the use o f the m a t e r i a l s described.

Literature Cited 1. 2. 3. 4.

Roberts, E . D . ; Applied Polymer Symposium No.23; John Wiley & Sons Inc., New York, 1974; p.87-97. Roberts, E . D . ; A.C.S. Coatings and Plastics Preprints, 35, No.2, 281-6. O'Keefe, T.W.; Vine, J . , Handy, M.R.; Solid State Electronics, 1969, 12, 841-8. Scott, J . P . ; 6th Int. Conf. on Electron and Ion Beam Science and Technology, 1974, Edited by Bakish, Robert A. Electrochemical Soc. Inc., New York, N.Y. p.123-36.


1. 5. 6. 7. 8.

ROBERTS

Methacrylate Electron Resist

Beasley, J.P.; 4th Int. Conf. on Electron and Ion Beam Science and Technology, 1970. Edited by Bakish, Robert A. Electrochemical Soc. Inc., New York, N.Y. p.515-23. Beasley, J . P . , Squire, D.G.; IEEE Trans. Elect. Devices, ED-22, 1975, (7), 376. Roberts, E . D . ; Vacuum, 1976, 26, (10/11), 459-67. Roberts, E . D . ; A.C.S. Coatings and Plastics Preprints, 1980, 43, 231-5.


RECEIVED October 19, 1981.


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A Sensitive Positive-Working Cross-Linked Methacrylate Electron

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