A Test for Correlation between Residual Solvent and Rates of

Chapter 7

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 16, 2016 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch007

A Test for Correlation between Residual Solvent and Rates of N-Methylpyrrolidone Absorption by Polymer Films 1

1

2

1

W. D. Hinsberg , S. A. MacDonald , C. D. Snyder , H . Ito , and R. D. Allen 1

1

IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120 IBM Corporation, 5600 Cottle Road, San Jose, CA 95119 2

Resist systems based on chemical amplification can exhibit very high radiation sensitivity, but this is accompanied in most cases by an extreme susceptibility to adventitious basic airborne contaminants. Recent work in our laboratory has shown that the rate of uptake of such contaminants by thin spin-cast films can vary widely depending on the structure of the polymer. In the present study we probe whether those variations are attributable to differences in levels of residual casting solvent entrapped in the films. Radiochemical methods are used to quantify solvent retention and to measure the rate of uptake of N-methylpyrrolidone, a representative organic contaminant. A series of polymerfilmsof differing structure are examined. In this series the residual solvent levels range from 0.05 to 21 wt %. The measured rates of absorption of NMP vapor are not correlated with residual solvent content. However, small changes in polymer structure cause a large change in the rate of NMP uptake.

Resist systems based on acid-catalyzed chemical amplification (CA) can exhibit very high radiation sensitivity, but this is accompanied in most cases by an extreme susceptibility to adventitious airborne contaminants which interfere with the resist chemistry (2, 2). Airborne amines (2, 2) and N-methylpyrrolidone (5) (NMP) have been reported to cause image degradation characterized by linewidth shifts and, in the case of positive-tone C A resists, by formation of a thin, poorly-soluble "skin" on the resist film. These basic substances are damaging even at extremely low vapor concentrations on the order of 15 ppb (2, 2). The practical application of such C A resists is simplified if the interfering substances can be rigorously excluded from the resist film. Two means of achieving this are by application of a protective overcoat (2) or by purification of the enclosing atmosphere using activated carbon filtration (2). If it is assumed that an absorbed contaminant degrades the performance of a C A resist by simple acid-base neutralization of the catalytic species, then the degree of degradation should be related to the relative basicities of the contami0097-6156/94/0537-0101$06.00/0 © 1994 American Chemical Society Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


102

POLYMERS FOR MICROELECTRONICS

nating species versus the resist components, the catalytic chain length, and the rate at which the contaminant enters the resist film. Recent work at our laboratory has demonstrated that the rate of contaminant uptake varies sharply as the structure of the polymer comprising the film is changed (4). In that study, the rate of uptake of radiolabeled NMP vapor by spin-cast polymer films varied by a factor of 40 depending on the polymer structure. No simple relation linking polymer structure and NMP uptake was evident in these data. By understanding the causes of this variation, it may prove possible to design resist materials with improved tolerance toward airborne basic contaminants. We know from past work that the amount of residual casting solvent in thin resist films varies significantly depending on the characteristics of the polymer and the solvent, and it is well-accepted that residual solvent can influence diffusion processes in polymer films. Therefore, one possible factor influencing contaminant uptake is that the films studied have differing levels of retained casting solvent. Polymer permeation by vapors and gases is generally treated in terms of the solution diffusion model (J), in which the vapor is first sorbed onto the surface of the film in a rapid equilibration step and then slowly diffuses through the polymer (Figure 1). The rate of uptake is determined in part by the ease with which the contaminant diffuses in the film. It is well-known that the mobility of small molecules in polymeric materials can be strongly affected by the concentration of solvent in the polymer matrix (6) Presumably the solvent acts to disrupt interactions between polymer chains and thereby enhances motion within the matrix. Recent reports from other workers have described residual solvent effects in lithographic materials. In one study, significant differences in image profiles were observed with one chemically amplified deep-UV resist depending on the choice of casting solvent (7) Of particular note was the appearance of a poorly-soluble surface skin when the resist film was cast from diglyme. This effect was attributed to the influence of residual solvent on acid diffusion within the film. Another group has provided more direct evidence that residual casting solvent influences mobility in chemically amplified resists (8). Figure 2 shows data from their study. In Figure 2a is plotted the acid diffusion range in two resist films of identical composition which were post-apply baked at different temperatures. The film baked at the higher temperature exhibits a much decreased diffusion range, attributed to a lower level of residual solvent. Figure 2b shows a similar result for resist films of different polymer composition which were post-apply baked using identical conditions. The different compositions retain casting solvent to different degrees. Again the acid diffusion range is much lower in the film with less solvent residue. The goal of the present study is to establish experimentally whether the differences in contaminant uptake such as those seen in our previous study (4) can be similarly attributed to variations in levels of residual casting solvent. Since the final shape of a lithographic relief image is determined by a large set of interdependent variables, it is difficult to characterize the effect of residual solvent on susceptibility to airborne contamination in an unambiguous way by SEM examination of resist patterns. In our prior work on NMP uptake (4), films

Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


TO EXHAUST

Figure 1. Schematic diagram of polymer permeation by a vapor.

A



Figure 2. Influence of (a) post-apply bake temperature and (b) polymer structure on acid diffusion range. These data are taken from reference 8.



7.

HINSBERG ET A L .

N-Methylpyrrolidone Absorption by Polymer Films

were prepared by spin-casting from various solvents (diglyme, cyclohexanone, toluene or propylene glycol monomethyl ether acetate (PGMEA), depending on solubility characteristics of the polymer). In the present work we have selected for study a series of polymers soluble in a single casting solvent (PGMEA) but with widely varying structural characteristics. The methodology used here is first, to accurately quantify residual casting solvent in films of this set of polymers, and second, to test for a correlation between the level of residual solvent and the uptake rate of NMP, a representative organic contaminant. EXPERIMENTAL Quantitation of Residual Casting Solvent A radiochemical method was used to determine solvent retention after a typical spin-coat/post-apply bake cycle. Carbonyl- C-propylene glycol monomethyl ether acetate (PGMEA*) casting solvent was prepared by reaction of propylene glycol monomethyl ether with carbonyl- C-acetyl chloride in pentane solution in the presence of triethylamine (Figure 3), followed by an aqueous extractive workup, drying over MgS0 , and distillation. The final material (98.7% purity by G C analysis, with pentane the only volatile impurity) had a specific activity of 57.7 /LtCi/mmol. A series of polymer solutions were prepared by combining an aliquot of PGMEA* with a non-labeled P G M E A solution of the polymer. The final solutions had specific activities of ca. 5 fiCi/mmol PGMEA. These solutions were applied as ca. 1 /im thick films to tared 25 mm diameter silicon wafers, weighed to ±0.01 mg and post-apply baked on a hotplate at 100 °C for 300 seconds. After a final weighing the films were stripped with 5 ml unlabeled P G M E A and rinsed twice with Scintiverse II liquid scintillation cocktail (LSC) (Fisher Scientific). The combined washings were assayed for C using a Packard Tri-Carb 460 Liquid Scintillation System. All analyses were performed in duplicate; results from duplicate runs generally agreed within 5 %, and at worst agreed within 10 % of their average value. 14

14

4

1 4

Quantitation of NMP Uptake The apparatus shown in Figure 4 was employed for this measurement. Its construction and use have been previously described (2, 4). In the present work the airborne substance was methyl- C-N-methylpyrrolidone, mixed into the purified airstream to a final concentration of 15 ppb. A series of polymer solutions identical to those prepared above but containing only unlabeled P G M E A were applied as ca. 1 fim films to 125 mm dia silicon wafers and post-apply baked on a hotplate at 100 °C for 300 seconds. Immediately after coating, the wafers were immersed in the C-NMP-doped airstream (for a period of 60 minutes unless otherwise specified). The films were then stripped using 5-7 ml of unlabeled PGMEA, twice rinsed with LSC solution, and the combined washings assayed for C content as above. Results on duplicate wafers typically agreed within 10%. 14

14

14


105


106


CI-'IC-CH* ii O 3

CHt-0 I CHJ-CH-CHJ

CH3-O I m

O-H

E t

3

N

CHJ-CH-CHJ O-^-CH, 11

pentane

0

0

14

Figure 3. Synthesis of carbonyl- C-propylene glycol monomethyl ether acetate.

Figure 4. Schematic diagram of the air handling apparatus used in this study. (Reprinted with permission from reference 4. Copyright 1992 Society of PhotoOptical Instrumentation Engineers.)


7.

HINSBERG ET A L .

N-Methylpyrrolidone Absorption by Polymer Films 107


RESULTS AND DISCUSSION Table 1 summarizes the residual solvent contents for a series of polymer films of interest in microlithography, tabulated on a weight percent basis. Depending on the properties of the polymer, the residual solvent contents vary from 0.05 to 21 wt %. The general trend is that the level of solvent residue is related to the overall polarity of the polymer, with polymers of similar structure exhibiting nearly identical solvent levels. One striking aspect of these data is that the amount of residual solvent can be substantial. In the case of the m-cresol novolac film, about one-fifth of the film on a weight basis is retained solvent. We have found in further work on the cresol novolac system that (1) the addition of 23 wt. percent of a typical 5-substituted diazonapthoquinone to the novolac film does not significantly change its retention of PGMEA; (2) the solvent level does not decrease significantly with more extended bake times as long as 30 minutes, suggesting that the P G M E A is relatively strongly bound to the polymer; and (3) that otherwise identical films formulated using ethyl cellosolve acetate as casting solvent retain similar proportions of solvent after baking. These observations demonstrate that residual casting solvent can be a major component of films of DNQ-novolac photoresists, though its presence and its role in influencing resist properties is often disregarded. NMP absorption measurements indicate that the nature of the casting solvent has relatively small influence on NMP uptake. A comparison of NMP absorption by polymer films cast from P G M E A with that of otherwise identical films cast from other solvents reveals similar rates of uptake. For example, 1.0 fim films of cresol-novolac (post-apply baked as above) absorb 165 ng NMP in 15 minutes when cast from a diglyme solution, and 176 ng NMP when cast from P G M E A solution. Using data of Ouano (9), the estimated solvent content of the two films is comparable. Qualitatively similar uptake results are obtained for epoxy-novolac films cast from cyclohexanone or from PGMEA. Figure 5 displays the relative absorption of NMP vapor for a series of polymers, plotted as a function of the residual (PGMEA) solvent content. There is no apparent correlation between solvent content of the film and its propensity to absorb NMP vapor. For example, the residual P G M E A content of P(paraTBOCST) is l/35th that of the cresol novolac film, yet its rate of NMP uptake is about 3.5 times that of novolac. Similarly, compare the relative solvent contents and N M P absorption rates of P(para-TBOCST) and P(TBMA-MMA-MAA). This provides further support that residual solvent has relatively small influence on NMP uptake. Recall that all films in this study and in our previous work were subjected to identical processing conditions. That, in combination with the evidence presented here on the role of residual casting solvent, leads to the conclusion that polymer structural characteristics are the dominant factor controlling contaminant absorption. A closer examination of the data of Figure 5 support this conclusion: • The residual P G M E A contents of films of P(para-HOST) and cresol-novolac are nearly identical. The structural features of both polymers are quite similar Thompson et al.; Polymers for Microelectronics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


108


Table 1. Measured Residual Solvent Levels for a Series of Spin-Coated Polymer Films POLYMER

RESIDUAL SOLVENT (WT %)

STRUCTURE

0.05

Epoxy Novolac CH,

\

Q

/

'

—(CHj-CHh-

P(meta-TBOCST)

0.5 O^Q-BOC

P(para-TBOCST)

0.6

9

O-BOC

P(TBMA-MMA)

r

r

-(CH^C), COOtBu

M*

P(TBMA-MMA-MAA)

5.9

(CH,-C)COOM*

M«

1

M*

1

-(CM..C),

1

( C H . . C ) — ( C H , - ^

COOIBu

COOM«

8.6

COOH

—(CKj-CH^

12.8

P(3-Me-4-H0ST) OH —

A Test for Correlation between Residual Solvent and Rates of

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