Polymers for High Technology - American Chemical Society

Chapter 17 Thermally Stable, D e e p - U V Resist Materials 2

Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 25, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch017

S. Richard Turner1, K. D. Ann2, and C. G. Willson

1Corporate Research Laboratories, Eastman Kodak Company, Rochester, NY 14650 Almaden Research Center, IBM, San Jose, CA 95120-6099 2

A new, thermally stable resist based on styrene copolymers with various photochemical acid generators is described. The title copolymers were prepared either by polymer modification or by copolymerization from monomers. Exposure of a resist formulated from the protected copolymers and a photoacid generator followed by baking results in efficient deprotection to produce the free phenolic copolymer in the exposed areas. The resulting latent image can be developed in either positive or negative tone upon proper choice of developer. The resulting relief images exhibit no detectable change in size or shape after extended heating at 200°C. The increasing complexity and sophistication involved in integrated circuit manufacturing places rigorous performance demands on the polymeric resist materials used to delineate the circuit patterns. These demands have led to the development of new resist chemistry that allows improved resolution because of sensitivity to short wave length, deep UV light while at the same time providing greatly improved sensitivity. These new materials derive their sensitivity from a process that the authors term chemical amplification. One example of such a system is based on poly(p-t-butyloxycarbonyloxy)styrene (t-BOC styrene) (1). The resist community has also continued attempts to improve more traditional systems. Resistance to deformation due to flow during high temperature processing is a property that is deficient in most currently available photoresists. The best of the widely used diazoquinone-novolac materials, for example undergo deformation at approximately 150°C (2-3-4). This deficiency in thermal stability is primarily due to the relatively low Tg of the novolac matrix resin. The Tg of these materials ranges between 70 and 120°C depending on structure and molecular weight {5). Image profiles that are stable to 200°C are desired for several processes used in semiconductor manufacturing. Several post exposure treatment schemes that improve the thermal stability of the novolac based resists have been proposed (6-7) but all of these schemes involve increased process complexity and higher cost. Recently, a new family of high Tg, base soluble, phenolic copolymers based on N-(p-hydroxyphenyl)maleimide was reported (8). These copolymers were found to 0097-6156/87/0346-0200$06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Materials

201

serve as replacements for novolac in diazoquinone based resists.

Such formulations

17.

TURNER ET A L .

Thermally

Stable, Deep-UV

Resist

yield images that exhibit remarkable thermal resistance.

The materials show no

deformation upon exposure to 200°C after patterning. In this report, we describe a new, thermally stable, deep U V resist that combines the thermal stability of the new, high T g , phenolic copolymers with the high sensitivity of the chemical amplification deprotection concept. This is accomplished by blocking the phenolic group of the high Tg,

N-(p-hydroxyphenyl)maleimide

polymers


t-butyloxycarbonyl ( t - B O C ) protecting group.

and

copolymers

with

the

When these materials are imagewise

exposed in the presence of photoacid generating compounds such as sufonium salts, and then baked, the phenolic groups are deprotected via a thermolysis reaction that is acid catalyzed in a fashion analogous to the key process responsible for the function of t - B O C styrene (1-9). H i g h resolution images have been obtained in either positive or negative tone depending on the developer.

The resulting images exhibit no

deformation upon heating to 200°C. A recent report has described a similar approach based on t - B O C protection of an unsubstituted maleimide copolymer (10). Experimental Instruments. Infrared spectra were obtained with an I B M IR/32 F T I R Spectrometer. NMR

spectra

were recorded

on

Varian

EM390

and

Brucker

WP200

NMR

Spectrometers in deuteriochloroform except as indicated. U V spectra were recorded on a Hewlett Packard Model 8450A U V / V I S Spectrometer using thin films cast on quartz plates. Mass spectra were obtained with a Hewlett-Packard 5995A G C / M a s s Spectrometer. 150°C

GPC

G e l permeation chromatogrames were obtained on a Waters M o d e l equippped

with

μ-styragel

columns

using T H F

and

polystyrene

calibration. Thermal analyses were carried out with a DuPont 951 and 1090 Thermal Analyzer at a heating rate of 10°C/min for D S C and 5°C/min for T G A measurement. Thermal analysis was carried out under inert atmosphere. Preparation of N-(p-t-butyloxycarbonyloxyphenyl)maleimide I. The t - B O C monomer I was prepared by a reaction of p-t-butyloxycarbonyloxyaniline (11)

with maleic

anhydride according to the procedure of S. R. Turner £8). The detailed preparation method will be published elsewhere (12). Copolymerization of I with styrene. A glass ampoule was charged with 1.45g (5m mol) of 1,0.53g (5m mol) of styrene, and 33 mg of A I B N (2 mol %) dioxane.

dissolved in 2.0 m l of

The ampoule was sealed under vacuum after a freeze-thaw cycle and the

copolymerization was carried out at 5 8 ° C for 3 hours. The jelly-like polymer mixture was dissolved in N M P and the polymer was isolated by precipitation into methanol. After drying in vacuo, 1.76g (89%)

of a white fibrous polymer were collected. The

polystyrene equivalent molecular weight (Mw) is 1.3 χ 1 0 by G P C . 6

t-Butyloxycarbonylation of poly(styrene-co-N-(p-hydroxyphenyl)maleimide). synthesis of the starting copolymer has been previously described (8).

The

The t - B O C

protecting groups were introduced on the precursor polymers, two different molecular weights, using di-t-butyl dicarbonate (9-11-12). A solution of 5.87g of the copolymer (Mw

= 16,100) in 70 m l of T H F was cooled in ice water and 2.24g (20m mol) of

potassium t-butoxide were added under nitrogen atmosphere. The pink solution was stirred for 20 min. at room temperature then 4.80g (22m mol) of di-t-butyldicarbonate were added and the cooled polymer mixture was

stirred for 4 hours at room

temperature. The t - B O C protected polymer was obtained by precipitating into water. Filtration and drying under vacuum afforded 6.35g ( 8 1 % yield) of t-BOC-protected

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

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

202

polymer. N M R , IR and U V were consistent with the proposed structure and identical to the copolymer prepared by direct copolymerization of the monomer. Results and Discussion The t - B O C protected copolymers were prepared both by copolymerization of the t - B O C protected hydroxyphenylmaleimide monomer with styrene and by modification of preformed phenolic copolymers of various molecular weights as shown in Scheme


I.

In both cases the copolymer compositions were found to be 1:1 based on N M R

results and elemental analyses. The N M R and IR spectra obtained from copolymers from both routes were identical.

The 13C and 1 H N M R spectra of the modified

polymer are shown in Figures 1 and 2. These data substantiate the completeness of the protection reaction of the preformed phenolic copolymer.

The copolymers are

presumed to be predominately alternating since these comonomers represent an example of the classic general alternating copolymerization case of an electron rich comonomer (styrene) and an electron poor comonomer (N-substituted

maleimide)

(13L D S C analysis (Figure 3) of the copolymers shows a large, sharp endotherm at 152°C during the first heat. This endotherm and the concomitant mass loss are associated with the threshold like thermolysis of the t - B O C protecting group. A second heating of the sample shows a T g at 235°C. The original substrate phenolic copolymers have been shown in a previous study to have high Tg's of similar values (8). The deprotected polymer undergoes thermal degradation beginning at about 300°C. Thermal gravimetric analysis of the t - B O C protected copolymers shows a precipitous loss of 2 5 % of the sample mass between 150 and 180°C then a plateau followed by slow decomposition above 300°C (Figure 4). These results mirror the D S C results. The first weight loss agrees well with that calculated for loss of C 0 and isobutene (25.4%) and occurs coincident with loss of the 1755 cm-1 carbonate absorbance in the infrared and the appearance of the broad phenolic O H absorbance (Figure 5). 2

The thermolytic deprotection reaction is extremely clean. The infrared spectrum of the deprotected polymer is identical to that of the phenolic precursor as is the ultraviolet spectrum. The molecular weight of the phenolic copolymer precursor is unchanged by the t - B O C protection reaction/thermolysis cycle based on G P C data. The t - B O C protected copolymers have good solubility in common organic solvents such as acetone, T H F , chloroform and D M F but are insoluble in water, aqueous base and methanol. The deprotected, phenolic copolymers (or precursor phenolic copolymers) are very soluble in aqueous base, dioxane, T H F , acetone and D M F and are insoluble in water and solvents of lower polarity such as chlorobenzene and toluene. Clearly the presence or absence of the t - B O C protecting group has a large effect on the solubility of the copolymer. Imaging studies were done on copolymers prepared by the polymer modification route because of the availability of the precursor polymers of various molecular weights. The protected copolymers were compounded with triphenylsulfonium hexafluoroantimonate (13% w/w) in cyclohexanone. One micron thick films were spin coated on N a C l plates, baked at 140°C for 5 minutes to expel solvent and then subjected to infrared spectroscopic analysis before and after exposure. Exposure to 18 mJ/cm2 at 254 nm caused no change in the infrared spectrum. However, when the films were baked at 140°C for 120 sec. following exposure, deprotection was quantitative based on loss of the characteristic carbonate C = Ο absorption and


17.

TURNER ET A L .

Thermally

Stable, Deep-UV


ο

Resist

ο

ο

ο

1

Scheme 1 I

50 M H z

Ι—ι 200

1—I

203

Materials

1

I

1 3

I

I

1

j

ι

I

I

I

I

I

I

ι 50

•

•

I

I

C Spectrum

1 1 150

1

1

j

I ι 100

,

,

ι 0

I

ppm ( W

C

2 - ?

H

i r

H

cA Ao Downloaded by CALIFORNIA INST OF TECHNOLOGY on December 25, 2017 | http://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch017

N

-L

10

9

Figure

8

2·

7

6

5

1H-NMR

carbonyloxyphenyl)

J

4 3 ppm (δ)

spectrum

of

u

2

1

poly

0

(styrene-co-N-(p-t-butyloxy

maleimide). Τ — ι — Γ

-

1 — I — Γ — ι — ι — ι — ι — ι — ι — ι

ι

ι

Second Heating Tg « 235° ξ Ε,

îο

2

0

co -2 -4 Tdp « 152°C -6 J

-8 Ο Figure 3. copolymer.

40

ι

L

80

J

ι

120

L

J

160 200 240 Temperature (°C)

ΐ-

280

320

360

400

Differential scanning calorimetry analysis on the t - B O C protected The endotherm at

152° on the first heating is associated

thermolysis of the t - B O C group.

The second heating shows a T g for

deprotected, phenolic copolymer at 235°.


with the

17.

TURNER ET A L .

"T

r

1 '

Thermally

1 ' 1

Stable, Deep-UV

ι

ι • ι- ι

Resist

r-|' ι

140


i ' |-' ι

| ι -| Ί

CH -CH-^2

jsX

100

Γ sz

ι y.

120 -

205

Materials

ο

0 V +

80

-

:

1 >

—

60

- ^ ^ ^

40 20

I 40

ι

I ι I ,ι I L-.J... ι I ι ! ι I ι I ι I ι I ι 80 120 160 200 240 280 320 360 400 440

0

480

Température ( ° C )

Figure 4. Thermal gravimetric analysis of the t - B O C protected copolymer. T h e mass loss at 150-160° is associated with thermolysis of the t - B O C group.

%T

4000

3000

2000

1000

Wavelength, cm"

1

Figure 5. Infrared spectra of films of the t - B O C protected copolymer coated on N a C l plates before (a) and after (b) thermolysis of the protecting group. Note the loss of the carbonate carbonyl stretch at 1755 c m " and creation of the phenolic 1

hydroxyl absorbance at 3400-3600 c m " . 1


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

206

appearance of the characteristic phenolic O H absorbance in the IR (Figure 6).

The

deprotected copolymer IR spectrum was identical to the spectrum of the unprotected precursor copolymer.

Unexposed films baked under the same conditions showed no

change in the IR spectrum. The photoacidolysis was also documented by U V spectroscopy.

The protected

polymer has a weak absorbance at 280 nm. After exposure and baking, a more intense and red shifted band is produced (Figure 7). The photoacidolysis is very slow at room


temperature but very fast above 100°C.

The deprotection reaction and the resultant

change in thermal activation for deprotection are shown in Figure 8. Silicon wafers were coated with the formulation and baked at 100°C for 5 min. to give 0.9 micron thick films. These were exposed through a quartz resolution mask to narrow band 254 nm light. After exposure, the wafers were baked at 140°C for 2 minutes. Exposed and baked wafers were developed using organic solvent to give negative tone images and with aqueous base to give positive tone images.

Resolution

of 1.0 micron was achieved at 10 mJ/cm2. Optical micrographs of these images are shown in Figure 9. T h e developed resist patterns were heated at 2 0 0 ° C for 1 hour in a convection oven.

Scanning electron microscopy shows no thermal deformation in the negative

tone images (Figure 10).

The positive tone images lost thickness consistent with

thermolysis of the t - B O C side chain but also do not show evidence of thermal flow deformation. Thus, these new copolymers form the basis of a new, deep U V , thermally stable photoresist.

This resist combines

styrene-N-(p-hydroxyphenyl)maleimide

the high T g

and thermal stability of

the

copolymer and the sensitivity of the chemical

amplification design concept. τ

4000

3000

2000

Wavelength, c m "

1000 1

Figure 6. Infrared spectra of films of the resist formulation on N a C l plates a) after coating, b) after exposure to 18 m J / c m of 254 nm radiation and c) after exposure and baking briefly at 140°C. 2



17.

TURNER ET A L .

220

240

Thermally

260

Stable, Deep-UV

280

300

Resist

320

207

Materials

340

360

380

400

Wavelength (nm) Figure 7.

U . V . spectra of polymer films on quartz.

Spectrum (a) is the t - B O C

protected copolymer only, spectrum (b) is the resist formulation and spectrum (c) is the resist film after exposure and baking.


POLYMERS FOR HIGH TECHNOLOGY

208


Chemical Amplification Concept

(O) "Y^

3

SSbF

•

6

^CH —CH-A—

H SbF + Other Products +

6

X .

2

2

CH

3

3

Ô ^ O - Ç - C H

ΤΟ

3

2

X

ÇH

CH ^ C H — C H — ) — + CO? + CH =C

ÔH

3

Γ Η3

C

H

Soluble in Nonpolar Solvents Insoluble in Polar Solvents

I

Soluble in Polar Solvents Insoluble in nonpolar Solvents

w/Acid

Y

\

w/o Acid

\

^

Temperature Figure 8. A c i d catalyzed thermolysis of the t - B O C protected copolymer is responsible for the change in solubility. The quantum efficiency for generation of the phenolic is the product of the efficiency of photoacid generation and the catalytic chain length. Exposure generates a local concentration of acid. Subsequent heating to a temperature below that at which uncatalyzed thermolysis occurs allows local acidolysis of the t - B O C protecting group.



17.

TURNER ET A L .

Thermally

Stable, Deep-UV

Resist Materials

209

Figure 9. Optical micrographs of negative and positive tone resist images. T h e smallest features are 1 micron.

Figure 10. Scanning electron micrographs of negative tone images before (left) and after (right) heating in air at 200° C . for 30 min.


210

POLYMERS FOR HIGH TECHNOLOGY

Acknowledgment The authors acknowledge R. Herbold for NMR spectra, R. L. Siemens for Thermal Analysis and S. A. MacDonald for SEM work and for helpful technical discussions.


References 1. Willson, C. G.; Ito, H.; Fréchet, J. M. M.; Tessier, T. G.; Houlihan, F. M. J. Electrochem. Soc. 1986, 133, 181; Ito, H.; Willson, C. G. In Polymers in Electronics Davidson, T., Ed. ACS Symposium Series 242, Washington, DC, 1984; p. 11. 2. Dill, F. H.; Shaw, J. M. IBM Journal Research and Development 1977, V21, 210. 3. Vazsonyi, E. B.; Vertesy, Z. Microcircuit Eng. 1983, 183, 338. 4. Johnson, D. W. SPIE, 469, Advances in Resist Technology, 1984; pg. 72. 5. Russell, G. Eastman Kodak Company unpublished results. 6. Hiraoka, H.; Pacansky, J. J. Vac. Sci. Tech. 1981, 19, 1132. 7. Allen, R.; Forster, M.; Yen, Y. T. J. Electrochem. Soc. 1982, 129, 1379. 8. Turner, S. R.; Arcus, R. Α.; Houle, C. G.; Schleigh, W. R. Photopolymers: Principles, Processes, and Materials; SPE Reg. Tech. Conf. Proc. 35, Ellenville, New York, October, 1985; Polymer Engineering and Science, 1986, in press. 9. Fréchet, J. M. J.; Eichler, E.; Ito, H; Willson, C. G. Polymer 1983, 24, 995. 10. Osuch, C. E.; Brahim, K.; Hopf, F. R.; McFarland, M. J.; Mooring, A. M.; Wu, C. J.; Advances in Resist Technology and Processing III, SPIE 1986, 631. 11. Houlihan, F.; Bouchard, F.; Fréchet, J. M. J.; Willson, C. G. Canadian J. Chem. 1985, 63, 153. 12. Ann, K. D.; Willson, C. G. manuscript in preparation. 13. Mohamed, A. Aziz; Jebrael, F. H.; Elsabée, M. Z. Macromolecules 1986, 19, 32; Barrales-Rienda, J. M.; Gonzalez DeLa Campa, J. I.; Ramos, J. G. J. Macromol. Sci.-Chem. 1977, A11, 267. RECEIVED

June 1, 1987


Polymers for High Technology - American Chemical Society

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