20 Polymers of α-Substituted Benzyl Methacrylates and Aliphatic Aldehydes as New Types of Electron-Beam Resists
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K . H A T A D A , T. K I T A Y A M A , Y. O K A M O T O , and H . Y U K I Department of Chemistry Faculty of Engineering Science Osaka University, Toyonaka, Osaka 560, Japan
H . A R I T O M E and S. N A M B A Department of Electrical Engineering Faculty of Engineering Science Osaka University Toyonaka, Osaka 560, Japan
K. N A T E , T. I N O U E , and H . Y O K O N O Production Engineering Research Laboratory Hitachi, Ltd., 292 Yoshida-machi Totsuka-ku, Yokohama 244, Japan
Many papers have been published on positive electron-beam resists. These resists are mostly polymers which are degraded upon electron-beam irradiation. The resulting lower molecular weight polymer in the exposed area can be selectively removed by a solvent under certain developing conditions. The development is accomplished by the difference in the rate of dissolution between the exposed and unexposed areas, which is a function of the molecular weight of the polymer. Recently, Willson and his co-workers reported the new type of positive resist, poly(phthalaldehyde), the exposure of which in the presence of certain cationic photoinitiators resulted in the spontaneous formation of a relief image without any development step (1). In this article we will describe two different types of positive electron -beam resists, which were briefly reported in our previous communications (2,3). One is the homopolymer or copolymer with methyl methacrylate and αsubstituted benzyl methacrylate, which forms methacrylic acid units in the polymer chain on exposure to an electron-beam and can be developed by using an alkaline solution developer. In this case, the structural change in the side group of the polymer effectively alters the solubility properties of the exposed polymer, and excellent contrast between the exposed and unexposed areas is obtained. The other is a self developing polyaldehyde resist, which is depolymerized into a volatile monomer upon electron-beam exposure. The sensitivity was extremely high without using any sensitizer. 0097-6156/84/0266-0399$07.00/0 © 1984 American Chemical Society
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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MATERIALS FOR MICROLITHOGRAPHY
Experimental Materials. Benzyl methacrylate was obtained commercially. ex-Methylbenzyl (4) and α,α-dimethylbenzyl (5) methacrylates were prepared in diethyl ether from methacryloyl chloride and the corresponding alcohols in the presence of triethylamine. α,α-Diphenylethyl methacrylate was synthesized from methacryloyl chloride and lithium α,α-diphenylethoxide in tetrahydrofuran (6). Triphenylmethyl methacrylate was prepared from silver methacrylate and triphenylmethyl chloride in diethyl ether (7). The first three methacrylates were purified by fractional distillation under reduced nitrogen pressure. The monomers thus purified were dried over calcium dihydride and then distilled under high vacuum just before use. α,α-Diphenylethyl methacrylate and triphenylmethyl methacrylate were purified by recrystallization from hexane. Ethanal, propanal, butanal, heptanal and 3-phenylpropanal were obtained commercially. 3-Trimethylsilylpropanal was prepared from acrolein and trimethylsilyl chloride in tetrahydrofuran at about — 50 °C (8). Polymerization. Poly (methyl methacrylate) was obtained commercially. The polymers of other methacrylates and their copolymers were prepared in toluene with 2,2 -azobisisobutyronitrile (AIBN) at 60 °C. A l l the polymers prepared free radically were syndiotactic or atactic. Isotactic poly(a,a-dimethylbenzyl methacrylate) was obtained using C H M g B r as the initiator in toluene at 0 ° C . Poly(methacrylic acid) was prepared in water using potassium persulfate at as the initiator 60 °C. The molecular weights, glass transition temperatures and tacticities of the polymethacrylates are summarized in Table I. ,
6
Table I.
5
a
Polymethacrylates Used for
Electron-Beam Resists Τ M
Ester group -CH
n
C 3
-CH C H 2
6
5
-CH(CH )C H 3
6
-C(CH ) C H 3
2
6
5
-C(CH ) C H 3
2
5
6
3
6
5
n
(°C)
/
125000
3.25
123
8
37
55
529000
2.34
63
5
34
61
460000
3.13
106
5
35
60
847000
2.07
101
10
47
43
15000
3.47
85
86
9
5
43000
3.33
98
31
42
27
d 5
-C(CH )(C H )
MjM
Tacticity(%) S H
2
a Polymerization conditions: monomer lOmmol, A I B N O.lmmol, polymn temp. 60 °C, polymn time 24 hr. b The data are taken from the Table I of Ref. 2. c Commercial product. d Isotactic polymer. Polymerization conditions: monomer lOmmol, C H M g B r 0.25mmol, polymn temp. 0 ° C , polymn time 24 hr. 6
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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20.
HATADA ETAL.
401
New Types of Electron-Beam Resists
Copolymerizations of aldehydes were carried out in toluene at — 78 °C using diethylaluminum diphenylamide as an initiator (9). Some of the results are shown in Table II. Unlike the homopolymers of aliphatic aldehydres, the copolymers were soluble in organic solvents such as toluene, xylene or chloroform in a certain range of copolymer composition (10-12). The soluble copolymers containing approximately equal amounts of both monomer units were used for making the resist films. The weight average molecular weights of the poly(ethanal-co-butanal)s prepared in toluene at — 78 °C by diethylaluminum diphenylamide were reported to be more than 1,000,000 (13). It is reasonable to assume that the molecular weight of the aldehyde copolymers used in this work is at least the order of 10 . The compositions of copolymers of methacrylates or aldehydes were determined by *H N M R spectroscopy or elemental analysis. 6
Table II. Copolymerization of Aldehydes by Diethylaluminum Diphenylamide in Toluene at — 78 °C for 24 hr Monomer
a
[M]
b 0
Initiator Toluene
M -M
(mmol/ mmol)
AA-PA
(25/25)
2
AA-BA
(50/50)
4.5
AA-HA
(25/25)
2
0.25
PA-PhPA
(15/15)
1.3
0.30
l
2
(mol/1)
a A A : ethanal, P A : propanai, phenylpropanal.
Polymer
(mmol)
(ml)
Yield (%)
(mol%)
(dl/g)
0.25 1.00
21
52.4
51.2
—
e
15
72.9
50.0
14.3
e
19
58.3
54.4
18.2
20
43.1
52.0
10.3
B A : butanal,
H A : heptanal,
PhPA: 3-
b Initial concentration of total monomer. c Determined from elemental analysis. d Measured in toluene at 30.0 °C. e The solutions of initiator and monomer were mixed at 0 ° C and then the mixture was cooled to — 78 °C. Lithographic Measurements. The polymers and copolymers of methacrylates were dissolved in toluene and the solution was spun onto a silicon substrate. Poly(methacrylic acid) was used in a pyridine solution. The thickness of the resist films was about 0.3 μπ\. The resist films of polyaldehydes were made from xylene, toluene or chloroform solutions by spin-coating or dip-coating, and were 0.04 ~~ 3.4 μ m in thickness. The coated silicon wafer was prebaked and then exposed to 20 K V electron-beam using J E O L JBX-5B or Elionix ERE-301 electron-beam
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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MATERIALS FOR MICROLITHOGRAPHY
exposure system. The exposed polymethacrylates were developed using a mixture of methyl isobutyl ketone (MIBK) and isopropyl alcohol (IPA) or a dilute solution of sodium methoxide in methanol, and subsequently rinsed with IPA or methanol, respectively. In the case of the polyaldehyde resists almost complete development was accomplished by exposure alone and the development process was not needed. The film thickness was measured on a Talystep instrument or by optical interference. The molecular weights of the polymethacrylates were measured with a J A S C O F L C - A 1 0 G P C chromatograph with a Shodex G P C column A - 8 0 M (50 cm) with a maximum porosity of 5 x 10 using tetrahydrofuran as a solvent. The chromatogram was calibrated against standard polystyrene samples. Glass transition temperatures of the polymers were measured with a Rigaku Denki Calorimeter, Model 8001 S L / C , at a heating rate of 10°C/min using an aluminum sample pan with lid. Infrared spectra were recorded on the resist film spun onto a silicon wafer using a J A S C O IR-810 spectrometer equipped with a J A S C O BC-3 beam condenser or a J A S C O A-3 spectrometer. In the measurements on the latter spectrometer an uncoated silicon wafer was placed in the reference beam in order to balance the silicon absorption band. The subtraction between the spectra was carried out on a built-in micro-processor attached to the IR-810 spectrometer, and the resulting difference spectrum was used to detect structural changes in the polymer molecule upon exposure. The subtraction technique was also used to balance the silicon absorption band. The *H N M R spectra were taken on a J E O L J N M - M H - 1 0 0 (CW) spectrometer using tetramethylsilane as an internal standard. C spin-lattice relaxation time of the polymer was measured by the inversion-recovery Fourier transform method on a J N M - F X 1 0 0 FT N M R spectrometer operating at 25 M H z . 7
1 3
Results and Discussion Positive Electron-beam Resist of Poly (a-substituted Benzyl Methacrylate). The electron-beam resist behaviors of poly (α-substituted benzyl methacrylate) s are given in Table III. When the exposed resist films were developed with a mixture of M I B K and IPA, the sensitivities of these polymers were on the order of 10~ C / c m . When a dilute solution of sodium methoxide in methanol was used as a developer, the sensitivity was enhanced as compared with the former case, and increased with an increase in the bulkiness of the ester group of the polymer except for poly(a,a-diphenylethyl methacrylate). Figure 1 shows the exposure characteristics of atactic and isotactic poly(a,a-dimethylbenzyl methacrylate) resists with C H O N a development together with those of the poly(methyl methacrylate) resist with M I B K / I P A development. Poly(a,a-dimethylbenzyl methacrylate) s showed high sensitivity and very good contrast between exposed and unexposed areas. The atactic polymer with alkaline development was improved in the sensitivity and γ-value by a factor of more than three over poly(methyl methacrylate) with M I B K / I P A development. Infrared spectra of unexposed and exposed poly(a,a-dimethylbenzyl methacrylate) are given in Figure 2 together with the spectrum of 4
2
3
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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403
New Types of Electron-Beam Resists
HATADA ET AL.
Table III.
Electron-beam Exposure Characteristics a b of Polymethacrylates ' CH ONa/CH OH (1/20 w/v)
MIBK/IPA (1/5 v/v)
3
3
Prebake Ester group -CH
CO
3
-CH C H 2
6
5
-CH(CH )C H 3
6
-C(CH ) C H
5
-C(CH ) C H
5
3
3
2
2
6
6
5
C
-C(CH ) ( C H ) 3
6
5
2
170
167 x l O "
6
170
172 x l O "
6
170
250X 1 0
142
85X10"
142
l l O x 10"
120
2
Sens. (C/cm )
2
Sens. (C/cm )
150xl0"
3.6
226 x l O "
6
103 x l O "
6
2.1
- 6
2.4
97X10"
6
4.4
36 x l O "
6
11.3
4.8
28 x l O "
6
7.4
150 x l O "
6
6
6(j
6
a Film thickness 0.3 μπι, prebake 1 hr, development time 30 sec, rinse time 30 sec. b The data are taken from the Table II of Ref. 2. c Isotactic polymer d Developer M I B K / I P A =1/1 (v/v).
EXPOSED CHARGE DENSITY
(C/cm~ ) 2
Figure 1. Exposure characteristics of poly (methyl methacrylate) (PMMA), and atactic and isotactic poly(a,a-dimethylbenzyl methacrylate)s (PDMBMA). Reproduced with permission from Ref 2. Copyright 1983, "Springer Verlag".
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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MATERIALS FOR MICROLITHOGRAPHY
(D)
i
.
i
1800
.
I
l
1700
1600
ι
L_
800
700
WAVE NUMBER (Cm" ) 1
Figure
2. Infrared
spectra
of atactic poly(a,a-dimethylbenzyl
unexposed(A)
and
dimethylbenzyl
methacrylate)
Exposure 142°
C.
charge
exposed(B)
density
Reproduced
to
electron-beam,
exposed(C)
1.6xl0~
4
2
C/cm ,
with permission
from
and film
poly(a,a-
poly(methacrylie
acid)(D).
thickness
Ref. 2.
methacrylate)s
isotactic
0.5 μm, prebake
Copyright
1983,
at
"Springer
Verlag".
poly(methacrylic acid). The exposure of the polymers resulted in a decrease of the absorption at 1729, 764, and 700 c m and the appearance of an absorption at 1700 c m . The first three bands are the characteristic absorptions of poly(a,a-dimethylbenzyl methacrylate) and the last one coinsides with the carbonyl stretching band of poly (methacry lie acid). A small shoulder at 1760 c m may be due to the formation of acid anhydride groups in the polymer chain, as mentioned below. When the atactic poly(a,a-dimethylbenzyl methacrylate) was heated at 170°C for 30 min under vacuum, it decomposed into volatile and nonvolatile components. The former was found to be α-methylstyrene and the latter was to be very similar to poly (methacrylie acid) as determined by H N M R spectroscopy. Figure 3 shows the infrared spectra of atactic and isotactic poly(a,a-dimethylbenzyl methacrylate) s heated at 174°C under vacuum for various times. In the spectra of the atactic polymer, the absorption of the ester carbonyl at 1729 c m " decreased and that of the acid carbonyl at 1700 c m increased as the heating time increased. After heating for a period of 30 min - 1
- 1
- 1
l
1
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
- 1
20.
HATADA ET AL.
1
405
New Types of Electron-Beam Resists 1
,
,
1
J
1
ISOTACTIC
I
I
Γ
ATACTIC
WAVE
NUMBER
1
(CITT )
Figure 3. Infrared spectra of isotactic and atactic poly(a,a-dimethylbenzyl methacrylate)s heated at 174° C under vacuum for various times.
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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MATERIALS FOR MICROLITHOGRAPHY
- 1
two new bands appeared at 1805 and 1760 c m , and their intensities increased with an increase in time. A concomitant decrease in the absorption of the acid carbonyl group at 1700 c m was observed. Glutaric anhydride shows two carbonyl stretching bands at nearly the same positions as 1805 and 1760 c m , while the bands for isobutyric anhydride are slightly different (Figure 4). It - 1
- 1
Τ
2000
1900
1800 WAVE
1700
NUMBER
1600 1
(cnrr )
Figure 4. Infrared spectra of (A) isobutyric anhydride, (B) glutaric anyhdride, and (C) isotactic and (D) atactic poly(a,a-dimethylbenzyl methacrylate)s heated at 174° C under vacuum for 2 and 3 hr, respectively.
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
20.
407
New Types of Electron-Beam Resists
HATADA ET AL.
has been shown that the separation of two carbonyl frequencies in acid anhydrides varies from 40 to 80 c m " and those with six-membered rings usually have the least separation. The higher frequency peak is more intense for open chain anhydrides and this reverses for the five- or six-membered ring anhydrides (14,15). Therefore, the two peaks at 1805 and 1760 c m " should be assigned to the six-membered cyclic acid anhydride, which was formed intramolecularly from neighboring methacrylic acid units. Autocatalytic character was observed in the ester decomposition of poly(t-butyl methacrylate) into poly (methacrylic acid) and isobutene, for which a mechanism of olefin elimination involving an adjacent acid unit was proposed (16). The results mentioned here clearly indicate that the enhancement in the sensitivity and γ-value of the poly(a,a-dimethylbenzyl methacrylate) resist over poly (methyl methacrylate) is mainly due to facilitated formation of methacrylic acid units on electron-beam exposure. The exposed area, which contains 1
1
methacrylic acid units, easily dissolves in the alkaline developer but the unexposed area does not. In this case, the factor which determines the resist properties, is the change in solubility characteristics upon exposure, and not the rate of dissolution during the development process. This is the reason why high sensitivity and contrast are obtained with poly(a,a-dimethylbenzyl methacrylate). Some of the adjoining methacrylic acid units form acid anhydride groups, which exhibit a small shoulder at 1760 c m in the spectrum of exposed resist film. The formation of acid anhydride may decrease the sensitivity and this will be discussed later. Spectral subtraction usually provides a sensitive method for detecting small changes in the sample. Figure 5 shows the difference spectra between the atactic poly(a,a-dimethylbenzyl methacrylate) s unexposed and exposed to electron-beam at several doses. The positive absorption at 1729 c m is due to the ester carbonyl group consumed on the exposure and the negative ones at 1700 and 1760 c m to the acid and acid anhydride carbonyl groups formed, respectively. The formation of methacrylic acid units was more easily detected using the difference spectrum However, these difference spectra could not be used for the quantitative determination because the absorptions overlap somewhat. In order to estimate the amount of methacrylic acid units formed on the exposure, the infrared spectra for the mixtures of known amounts of poly(a,adimethylbenzyl methacrylate) and poly(a,e*-dimethylbenzyl methacrylate-co- 1
- 1
- 1
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
408
MATERIALS FOR MICROLITHOGRAPHY
1900
1800 WAVE NUMBER
1700
1600
- 1
(cm )
Figure 5. Difference infrared spectra between the atactic poly(a,adimethylbenzyl methacrylate)s unexposed and exposed to electron-beam of several doses. (A) 4 x 10~ , (Β) 1.6 x 10" , (C) 7 x 10" C/cm . 5
4
4
Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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20.
HATADA ET A L .
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New Types of Electron-Beam Resists
methacrylic acid) were measured on films cast from chloroform solution. The relative intensities of A -i/A - i were plotted against the contents of the acid units. The copolymer was prepared in dimethylformamide with A I B N at 60°C and contained 15.0 mol% of methacrylic acid units. The results of this plot are shown in Figure 6. Using this calibration curve and the spectra of the exposed polymer, the methacrylic acid content in the atactic polymers exposed to electron-beam doses of 4 x 10~ and 1.6 x 10~ C / c m were estimated to be 5.5 and 15.2 mol%, respectively. 1 7 0 0 c m
1 7 2 9 c m
5
4
2
0.3 ι £
