Chemically Amplified Resists - ACS Symposium Series (ACS

Chapter 3

Chemically Amplified Resists

Downloaded by UNIV OF ROCHESTER on June 5, 2018 | https://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch003

Effect of Polymer and Acid Generator Structure Francis M . Houlihan, Elsa Reichmanis, Larry F. Thompson, and Regine G. Tarascon AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

Deep-UV photolithography will have an important place in the semiconductor manufacturing arena by the mid 1990's. For this lithographic technology to achieve its ultimate capability, it will be necessary to have non-novolac based resists and new resist processes. Resist materials based on chemical amplification have been reported and shown to have most of the resist proper ties needed for deep-UV photolithography. The polymer and acid generator structures are critical issues in the design of an optimum chemically amplified deep-UV resist. This paper reports on the effect of polymer and acid generator structures on the lithographic performance of three polymers; poly(4-t -butoxycarbonyloxystyrene) (TBS), poly(4-t-butoxycarbonyloxy-α -methylstyrene) (TBMS), and poly(4-t-butoxycarbonyloxystyrene sulfone) (TBSS) and three photoactive acid generator materials; 2,6 dinitrobenxyl tosylate (Ts), triphenylsulfonium hexafluoroars enate (Af), and triphenylsulfonium trifluoromethonesulfonate (Tf) The polymer TBSS is known to undergo radiation induced chain scission and provides an improvement in the sensitivity com pared to resists formulated with polymers which do not undergo chain scission. The lithographic performance of a resist formu lated from this polymer and 2,6-dinitrobenzyl tosylate acid gen erator is reported. "The more things change, the more they stay the same" is a well-known quote that is very applicable to photolithography. For nearly four decades, integrated cir cuits have been patterned with photolithography. The complexity and speed o f IC's have increased by several orders o f magnitude with a concomitant decrease i n feature size and cost per bit. During these dramatic changes, photolithography has 0097-6156/89/0412-0039$06.00/0 © 1989 American Chemical Society

Reichmanis et al.; Polymers in Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

40

POLYMERS IN MICROLITHOGRAPHY


remained the dominant imaging technology. The cost and sophistication o f expo sure tools have been increasing, and their performance has kept up with device process demands for resolution and registration accuracy. These improvements have been accomplished through the availability of more precise tools and a decrease i n the exposure wavelength. The most advanced i line (365 nm) stepper w i l l produce features as small as 0.5 μπι; but a shorter wavelength source w i l l be required for the production o f smaller features. T o meet these future requirements, equipment manufactures have already made available prototype 5 X reduction steppers with refractive optics based on 248 n m radiation produced by eximer lasers (1,2), i n addition to I X and 4 X reflective optics system using a H g arc source (3,4). During these decades o f change and evolution i n photolithography, novolacdiazoquinone chemistry remained the basis of conventional positive photoresists (5). However, preliminary results indicate that the traditional positive resists are too optically opaque to be useful in the D U V wavelength regime where the novo lac matrix resin has significant absorption (6). For the first time, new resists must be introduced into manufacturing i n conjunction with a new photolithographic tech nology. Based on initial evaluations, we have established some performance cri teria for an optimum deep-UV resist, and they are given i n Table I. When one ex-

T A B L E I: Performance Criteria for a Deep-UV Resist Sensitivity

4

Resolution

400 C. e

W h e n films o f the substituted styrene and styrene-sulfone polymers contaning 5 wt% 2-nitrobenzyl tosylate were heated to 1 3 0 C , only products arising from the decomposition o f the protecting group (carbon dioxide, isobutene, oligomers of iso butene) were detected. Films cast from the α-methylstyrene analog gave products arising not only from the deprotection process, but also from a depolymerization reaction. T w o products of depolymerization were observed, the expected 4hydroxy-a-methylstyrene, and an unknown compound subsequently identified as 5-hydroxy-l,l,3,3-tetramethylindan. Similar results were observed when tosic acid hydrate was used as source o f acid, indicating that the process is acid initiated. In the case o f tosic acid hydrate, a third compound, 4-isopropylphenol, was also observed as a depolymerization product. e

The G C / M S spectra were used to estimate the % depolymerization of T B M S under various conditions (see Table I V ) . The % depolymerization of polymer sam ples containing 2-nitrobenzyl tosylate is dependent upon the solvent used to cast films. When xylene is used, a higher % depolymerization is observed than with cyclohexanone. Moreover, the ratio of depolymerization products also varies with casting solvent. Specifically, the indan thermolysis product is more predominant when films were cast from xylene. The reaction with tosic acid hydrate resulted in a high degree o f depolymerization; this is most likely a consequence of depolymer ization occurring during sample preparation.

Photochemically Initiated Depolymerization Having established the tendency of T B M S to undergo acid catalyzed depoly merization, the propensity for photochemically generated acid to effect the same phenomenon was evaluated. The resistance of T B S and T B S S to acid catalyzed depolymerization was also evaluated. Onium salts are well known photogenerators of acid, (16,17) while 2,6-dinitrobenzyl tosylate has been shown recently (15) to efficiently generate acid upon irradiation. Unirradiated samples of all the polymers


3. HOULIHAN ET AL.


45

containing either 2,6-dinitrobenzyl tosylate or onium salt showed no detectable sign of polymer depolymerization or deprotection when subjected to a P E B temperature of 120°C.


TABLE IV: Depolymerization of TBMS Initiated by Thermal and Photochemical Generators of Acid at 120*C

a. b.

c. d.

0

Acid Generator (5wt%) Nitrobenzyl Tosylate

Mode

Casting Solvent*

Atmosphere

Thermal

C

AD

11

Composition Mole% I II III 87 13

Nitrobenzyl Tosylate

Thermal

X

AD

16

64

36

-

Tosic Acid Hydrate

Thermal

c

AD

77

45

45

11

Dinitrobenzyl Tosylate

hv

c

AD

16

88

12

-


hv

c

AS

4

66

34

-


hv

c

AP

10

83

17

-

Triphenylsulfonium Hexafluoroarsenate

hv

c

AD

14

77

23

Triphenylsulfonium Hexafluoroarsenate

hv

c

AW

8

31

37

5

d

%Depolymerization

32

C denotes cyclohexanone and X denotes xylene. Experiments done on days o f high or l o w atmospheric moisture are denoted as air(AW) or air(AD), respectively. Samples stored during transit on high humidity days i n a P2O5 glove bag are denoted A P and samples stored dur ing transit i n a glove bag saturated with water are denoted A S . The products are identified as follows: I is 4-hydrxy-a-methylstyrene, II is 5-hydroxy-l,1,3,3- tetramethylindan and ΙΠ is 4-isopropyl phenol. Substantial amounts o f isobutene were detected on pyrolysis o f these samples at 725°C.

Irradiation followed by P E B o f T B M S , T B S and T B S S polymer films contain ing acid photogenerators, resulted i n complete deprotection o f the t-butoxycarbonyl group providing sufficient dose was used for the exposure. In agreement with the thermal experiments using 2-nitrobenzyl tosylate, depolymerization resulting from acid catalyzed chain scission is observed only i n the case o f T B M S . Both depro tection and depolymerization o f the α-methylstyrene polymer occur upon irradia tion and P E B i n the presence o f both the dinitro and onium salt compounds (Table I V ) . The deprotection reaction is essentially quantitative as evidenced by the fact



46

that only trace amounts of isobutene are detected during the final pyrolysis o f the samples at 7 2 5 C . It was found that the extent of depolymerization, and product ratio is dependent upon atmospheric humidity. The presence o f the moisture tends to give a higher ratio o f the indan compound, and for films containing the onium salt, reaction in a humid atmosphere results i n the appearance of a third depoly merization product, 4-isopropylphenol. Additionally, samples prepared under high humidity resulted i n substantial amounts of isobutene during the final pyrolysis. This may be due to either incomplete removal o f the protecting group upon P E B , or, alternatively, to the generation o f polyisobutylene during the P E B step. Since carbon dioxide was not detected, and substantial quantities of oligomers derived from isobutylene were observed, the latter hypothesis appears more likely. A d d i tionally, IR analysis o f films treated i n a similar manner detected the absence of the t - B O C group.


e

The tendency o f the protected styrene and α-methylstyrene homopolymers to depolymerize i n the presence of photogenerated acid was also evaluated using G P C analysis. W h i l e quantitative data regarding deprotection and/or depolymerization could not be obtained using polystyrene equivalent molecular weights, qualitative information, confirming the tendency o f T B M S to depolymerize was obtained. The analyses for T B M S show striking changes i n molecular weight after exposure which are amplified upon P E B to effect a 2.5 fold reduction in molecular weight. This is accompanied by the appearance of low molecular weight peaks (Figure 1) i n the region where the 2,6-dinitrobenzyl tosylate acid generator elutes. T w o of these new peaks differ i n retention time from the ester and they appear to have aromatic groups as judged by their response with the 254nm detector. Extensive depolymerization, with formation of monomelic products is consistent with these results. L o w molecular weight products are also formed when tosic acid is used, but their number increased and molecular weights were higher than with the the photolyzed ester, presumably because of depolymerization occurring during sample preparation. Alternatively, only a slight decrease (-11%) i n the polystyrene equivalent molecular weight was found for the styrene based system after both exposure and P E B and no low molecular weight products were observed. Poly(a-methylstyrene) and poly(styrene-sulfone) have been reported to undergo both chain scission and depolymerization upon irradiation w i t h C o γ-rays (21,24). It was demonstrated that for poly(a-methylstyrene), chain scission and the forma tion of monomer and dimer (l,l,3-trimethyl-3-phenylindan) occur through a cationic mechanism. It follows, that addition of a strong acid to poly(ocmethylstyrene) should promote chain cleavage. The propensity of poly(4hydroxy-a-methylstyrene) to undergo acid catalyzed chain scission i n solution has been reported (12) and the ability o f the parent polymer to depolymerize i n the presence of acid i n the solid-state was confirmed here. Thermolysis of poly(ccmethylstyrene) i n the presence of 2-nitrobenzyl tosylate at 120°C effects 5% depo lymerization and exclusive formation of the dimer, l,l,3-trimethyl-3-phenylindane. Based on this evidence, it is clear that the solid-state depolymerization of T B M S and formation o f 5-hydroxy-l,l,3,3-tetramethylindan observed in this study proceeds through cationic mechanisms as outlined in Schemes I and I I respec tively. W h i l e poly(styrene-sulfone) does not undergo an acid catalyzed chain 6 0



3.

HOULIHAN ET AL.


—

Figure 1:

MOLECULAR WT

— -

G P C chromatograms of poly (t-BOC-oc-methylstyrene) formulated with 2,6-dinitrobenzyl tosylate ( A , B ) and ptoluenesulfonic acid (C). The processing conditions were (A) as coated, (B) after post-exposure-baked and (C) after postexposure-bake. The top, inverted traces were obtained with the U V detector.

American Chemical Society Library 1155 16th St., N.W. Reichmanis et al.; Polymers Washington, DX.in Microlithography 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

47

48



Scheme I




HOULIHAN ET AL.

Scheme II



50

scission process, aromatic sulfones are known to undergo radiation induced chain scission and have been demonstrated to be effective e-beam and deep-UV resists (20).

Process Considerations


The chemically amplified resists reported here for deep-UV applications require a post-exposure thermal treatment process step to effect the deprotection reaction. This step has proven to be critical, and i n order to understand the processing con siderations it is instructive to discuss, qualitatively, the various primary and secon dary reactions that occur with these systems during both exposure and P E B , ie: Δ AG A~ H

+

+ hv

-> A H + G

-»

A" + H

+

+ G

(1)

+ C"

-> A C

(2)

+ D"

-> H D

(3)

4

H+ + p-poly

-> poly-OH

+ H*

(4)

where A G is the radiation sensitive acid generator, C * is a contaminant cation, ¥t is the proton that is regenerated after each deprotection reaction (4), D ~ is a con taminant anion, p-poly is the protected polymer and poly-OH is the alkali soluble product polymer. The "contaminants" addressed above (C* and D~) are not present i n every system, their concentrations have not been established and their presence is inferred from observed lithographic effects. Even with these uncertain ties, it is the authors experience that the qualitative kinetic equations are useful in process optimization and in guiding fundamental studies to gain insight into indivi dual process steps. The A G molecule is converted to a strong acid ( A H ) upon absorption o f a pho ton and the rate of this reaction is fast, with the extent o f reaction being governed by the quantum effeciency of the particular acid generator and flux. The acid pro ton affects the desired deprotection reaction (4) with a finite rate constant. This rate is a function of the acid concentration, [H*], the temperature and most impor tantly, the diffusion rate of the acid i n the polymer matrix. The diffusion rate in turn, depends on the temperature and the polarity of the polymer matirx. A t room temperature, the rate of this reaction is typically slow and it is generally necessary to heat the film to well above room temperature to increase reaction rates and/or diffusion to acceptable levels. The acid ( H ) is regenerated (reaction 4) and con tinues to be available for subsequent reaction, hence the amplification nature o f the system. +

F r o m these highly idealized reactions, one can gain an understanding o f some potential diffculties and process related concerns. For this system to work satisfac torily, it would be necessary for the radiation generated acid concentration, [H*], to remain constant. However, i n most chemically amplified systems, undesired side reactions occur that prematurely destroy the acid, i.e., reactions with contaminants such as water, oxygen, ions or reactive sites on the polymer (reactions 2 and 3).


3. HOULIHAN ET AL.


51


The rates of these reactions depend upon the contaminant concentration and the inherent rate constants of the reactions. W h i l e the exact nature of these reactions differ for each type o f chemically amplified system and are not fully understood, this generalized discussion is sufficient to understand many o f the process issues. The process control of the post-exposure bake that is required for chemically amplified resist systems deserves special attention. Several considerations are apparent from the previous fundamental discussion. In addition for the need to understand the chemical reactions and kinetics of each step, it is important to account for the diffusion of the acid. Not only is the reaction rate o f the acidinduced deprotection controlled by temperature but so is the diffusion distance and rate of diffusion o f acid. A n understanding o f the chemistry and chemical kinetics leads one to predict that several process parameters associated with the P E B w i l l need to be optimized i f these materials are to be used in a submicron lithographic process. Specific important process parameters include: 1. Time between exposure and post-exposure

bake

The rate and extent of the deprotection reaction (4) is critically dependent on the acid concentration, [ H ] . Side reactions (2 and 3) reduce the effective acid concentration and must be controlled. A l l of these reactions are thermally activated, however they do occur at a finite rate at room temperature. In order to assure a constant total extent of deprotection (reaction 4) it is necessary to control the elapsed time between exposure and P E B . +

2. Temperature

of post-exposure

bake

The P E B temperature and temperature uniformity must be tightly controlled for the same reasons discussed above. It has been found that it is feasible to drive the deprotection reaction in t-butoxycarbonyl protected systems to com pletion, providing the side reactions are minimized or controlled. This is a necessary requirement for satisfactory lithographic performance. 3. Time of post-exposure

bake

Since it is important to control the thermally activated side reactions in addition to the temperature, the time of the P E B must also be controlled. It should be noted that excessive time at an elevated temperature can result in undesirable additional chemical changes i n the polymer such as crosslinking or oxidation that result i n solubility changes o f both the exposed and unexposed regions.

Lithographic Characterization The lithographic performance, as measured by sensitivity and contrast for the T B S , T B M S and T B S S polymers containing two onium salt A G systems and the tosylate A G is given i n Table V . The absorbance of these films at 248nm was - 0 . 3 μ π ι i n each case. - 1


52



T A B L E V: Lithographic Performance of Resist Systems Formulated with TBS, TBSS and TBMS Matrix Polymers and 5 wt% Tf, Ar and TS Acid Generators Sensitivity mJ c m "

Contrast

Polymer

Acid Generator

TBS TBMS TBSS

Tf Tf Tf

3 5 4

3 3 5

TBS TBMS TBSS

Ar Ar Ar

20 18 6

2 5 6

TBS TBMS TBSS

Ts Ts Ts

170 90 65

2 3 6

2

γ

The triphenylsulfonium mfluoromethanesulfonate (Tf) photoactive acid generator affords the highest sensitivity (3-5 m J c m " ) for all polymer systems studied. The contrast for these systems ranged between 2 and 6 and sub-micron resolution was obtained with a l l the materials. Resist systems using the triphenylsulfonium hexafluoroarsenate (Ar) precursor exhibited slightly lower sensitivities (16-20 m J c m " ) while contrast values were similar, i.e., 2-6. Upon formulation with 5 wt% 2,6-dinitrobenzyl tosylate (Ts) the substituted styrènes exhibited still lower sensitivities (65-170 m J c m ) and contrast remained i n the range o f 2-6. 2

2

2

Differential solubility of chemically amplified resists is the result of two sequential reactions: photoinduced decomposition of the acid generator followed by a thermally driven catalytic reaction that decarboxylates the protected matrix polymer. W h e n comparing different polymers and acid generator resist systems, it is important to ensure that the P E B results in equivalent degrees of deprotection (near complete). W h i l e some effect on the sensitivity and contrast of the styrene based polymers formulated with an onium salt acid generator is observed when an inherently photodegradable polymer is used, the small differences i n sensitivity could easily be attributed to process related variables. Resist exposure dose is reduced (sensitivity increased) by as much as a factor of 2.5 when the tosylate ester is used i n conjunction with T B S S or T B M S , polymer systems known to undergo radiation induced main chain cleavage. This increased sensitivity is likely the result o f an increase i n solubility that arises from a reduction i n polymer molecular weight due to chain scission. This effect would work synergistically with the change i n polymer solubility resulting from the catalytic deprotection of the t-butyloxycarbonyl groups to afford the hydroxystyrene derivative.


3.

HOULIHAN ET A L

53


The sensitivity o f both T B S S and T B M S can be further improved by the addi tion o f higher weight fractions o f Ts (Table V I ) . The absorbance o f the T B M S / T s resist compositions is also given i n Table V I .

TABLE VI: Lithographic Performance of a 2:1 TBSS and TBMS Polymer with the Ts Acid Generator Polymer

% Ts

Absorbance 248nm μιιΓ

Sensitivity mJ c m

Contrast

- 2

γ


1

TBSS TBSS TBSS

5 10 15

.27 .40 .53

65 52 26

6 10 20

TBMS TBMS TBMS

5 10 15

0.24 0.37 0.50

90 38 18

2 4 5

Note that incorporation o f 15 wt% T s i n either polymer affords resists that require an irradiation dose as l o w as 20 m J c m " . Other lithographic properties such as adhesion and crack resistance are improved with T B S S vs. T B M S . T h e best overall lithographic performance was achieved with the poly(4-tbutoxycarbonyloxystyrene-sulfone) polymers. Although the sensitivity o f resist systems formulated with 2,6-dinitrobenzyl tosylate was lower, the contrast, resolu tion and process latitude were superior i n comparison to the onium salt formula tions, and the alternate matrix resins. Figure 2 depicts typical contrast curves for T B S S containing 5, 10 and 15 wt% o f the 2,6-dinitrobenzyl tosylate acid generator, and Table V I lists the sensitivity and contrast values taken from such curves. T h e remarkably high contrast for the 10 and 15 wt% formulations indicates a very non linear relationship between the extent o f deprotection and dissolution rate, a phenomenon not observed with the onium salt systems. Although this is not fully understood, it is perhaps due to an increase i n dissolution inhibition imparted by the nonionic character o f the tosylate acid generator. 2

Scanning electron microscopy confirmed s u b - 0 ^ m resolution capabilities i n T B S S -acid generator resist films (Figure 3). Note that the edge profiles are nearly vertical. Preliminary results indicate that the plasma etching resistance is satisfac tory for semiconductor device processing. SUMMARY D e e p - U V resists comprised o f matrix polymers and a 2,6-dinitrobenzyl tosylate photoactive acid generator have been described and compared to previously reported onium salt systems. Although these resists exhibited lower sensitivity than onium salt-based materials, the contrast and processibility are superior. The use o f a matrix polymer capable o f radiation-induced chain scission improves the sensitivity and allows the 2,6-dinitrobenzyl tosylate acid generator to more nearly



54


1000 EXPOSURE DOSE (mJ/cm ) 2

Figure 2:

Exposure curve for P B S S containing 5 ( · ) , 10(o) and 15(Δ) wt % TS.

Figure 3:

S E M micrographs depicting 0.5, 0.45, 0.4 and 0.35μπι coded line/space images printed i n P B S S - A s resist.


3.

HOULIHAN ET AL.


55

meet sensitivity requirements of deep-UV steppers. W o r k is i n progress to further optimize both the molecular properties and processing conditions for this system. ACKNOWLEDGEMENTS The authors would like to acknowledge E . A . Chandross, G . N . Taylor, L . E . Stillwagon and M . Y . Hellman for useful discussions concerning the chemical aspects o f this work and to S. Vaydia, V . P o l and J. T. Clemens for input and con sultation about the deep-UV resist requirements and process evaluation.


LITERATURE CITED [1] Pol, V., Bennewitz, J. H., Escher, G. C., Feldman, M., Firtion, V. Α., Jewell, T. E., Wilcomb, Β. E. Clemens, J. T., Proc. SPIE, 1986, 633, 6. [2] Pol, V., Solid State Technology, 1987, 30 (1), 71. [3] Buckley, J. D., Karatzas, C., Proc. SPIE, 1989, 1088. [4] Ruff, B., Tai, E., Brown, R. Proc. SPIE, 1989, 1088. [5] Willson, C. G. in Introduction to Microlithography. Thompson, L. F., Willson, C. G., Bowden, M. J., eds., ACS Symposium Series, 219, Ameri can Chemical Society, Washington, DC, 1983, pp.111-117. [6] Moreau, W. M., "Semiconductor Lithography, Principles, Practices and Materials", Plenum, NY, 1988, pg.372. [7] Willson, C. G., Bowden, M. J. in Electronic and Photonic Applications of Polymers. Bowden, M. J., Turner, S. R., eds., ACS Advances in Chemistry Series, 218, American Chemical Society, Washington, DC, 1988, p 87. [8] Ito, H., Willson, C. G., in Polymers in Electronics. Davidson, T., ed., ACS Symposium Series, 242, American Chemical Society, Washington, DC, 1984, p. 11. [9] Willson, C. G., Ito, H., Frechet, J. M. J., Tessier, T. G., Houlihan, F. M. J. Electrochem. Soc., 1986, 133(1), 181. [10] Houlihan, F. M., Shugard, Α., Gooden, R., Reichmanis, E., Proc SPIE, 1988, 920, 42. [11] Neenan, T. X., Houlihan, F. M., Kometani, J. M., Tarascan, R. G., Reich manis, E., Thompson, L. F., Proc SPIE, 1989, 1086, 2. [12] Ito, H., Willson, C. G., Frechet, J. M. J., Farrall, M. J., Eichler, E., Macro molecules, 1983, 16, 510. [13] Tarascon, R. G., Reichmanis, E., Houlihan, F. M. Shugard, Α., Thompson, L. F., Polymer Engineering and Science, 1989. [14] Frechet, J. M. J., Eichler, E., Ito, H., Willson, C. G. Polymer, 1983, 24 995. [15] Houlihan, F. M., Shugard, A. Gooden, R., Reichmanis, E., Macromolecules, 1988, 21, 2001. [16] Crivello, J. V., Lam, J. H. W. J. Poly. Sci., Sci., Poly, Chem. Ed., 1979, 17, 977. [17] Crivello, J. V. In "Advances in Polymer Science", Springer-Verlag, Berlin, 1984 pp. 1-48. [18] Matsuda, M., Iino, M., Hirayama, T., Miyashita, T., Macromolecules, 1972, 5(3), 240.



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[19] [20] [21] [22] [23]

Bowden, M. J., Thompson, L. F., J. Electrochem. Soc., 1974, 121, 1620. Bowden, M. J., Chandross, Ε. Α., J. Electrochem. Soc., 1975, 122, 1370. Hayashi, K., Yamamoto, Y., Miki, M., Macromolecules, 1977, 10(5), 1316. Aliev, S. M., and Pokindin, V. K., Vopr. Neftekhim. 1977, 137. Boon, J. L., Wetzel, R. J., and Godshalk, G. L., Limnol. Oceanorg., 1982, 27 (5), 839. [24] Brown, J. R., and O'Donnell, J. H., Macromolecules, 1971, 5, 109.


Received July 13, 1989


Chemically Amplified Resists - ACS Symposium Series (ACS

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