Photoacid and Photobase Generators: Arylmethyl Sulfones and

Chapter 9

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Photoacid and Photobase Generators: Arylmethyl Sulfones and Benzhydrylammonium Salts 1

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J. E. Hanson , Κ. H. Jensen , N. Gargiulo , D. Motta, D. A. Pingor , Anthony E. Novembre , David A. Mixon , J. M. Kometani , and C. Knurek 2

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Department of Chemistry, Seton Hall University, South Orange, NJ 07079-2694 AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 Studies of photoacid and photobase generators of interest for microlithography are reported. The efficiency of arylmethyl sulfone photoacid generators was studied as a function of internal hydrogen abstraction and polar substituents. In solution, benzyl phenyl sulfone derivatives with a methyl group as a hydrogen donor substituent on the benzyl side do not show an enhanced quantum yield. This contrasts with their behavior in polymerfilms.The difference can be explained by reference to the viscosity of the medium and the lifetime of the caged radical pair. A methyl substituent at the 2 position on the phenyl side, however, enhances the quantum yield both in solution and in polymer films. For this case, diffusion of the radical pair does not separate the abstracting sulfonyl radical from the hydrogen donor methyl group. Polar effects were studied by examining a series of benzyl phenyl sulfones with electron donor and acceptor substituents at the 4 and 4' positions. These produced only weak effects on photoacid generating efficiency as determined by deprotection of poly(t-butoxycarbonyloxystyrene - sulfur dioxide) films. On the benzyl side, electron donors (less electronegative substituents) appear to increase efficiency. On the phenyl side, the opposite is true: electron acceptors (more electronegative substituents) appear to increase efficiency. The effects are small and in some cases are masked by other influences. A new photobase generating molecule is trimethylbenzhydrylammonium iodide. Photolysis of this substance in the ultraviolet leads to formation of trimethylamine. This is the first example of a photogenerator of tertiary amines. Photogeneration of acid and base catalysts is important to the improvement of chemically amplified photoresists and other imaging technologies.

Photochemically generated catalysts and initiators have a long history, with numerous examplesfromphotography and other imaging techniques.(l) The recent invention and development of chemically amplified photoresists(2-5) has produced a new demand for these materials, and so there has been an increase in reported 0097-6156/95/0614-0137$12.00/0 © 1995 American Chemical Society

In Microelectronics Technology; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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studies, particularly for photoacid generators.(6) There are many important design considerations for photocatalyst generators, which apply to both photoacid and photobase generators. These include high sensitivity (photospeed or quantum yield), good chemical stability for long term storage, good thermal stability for processing considerations, strength of the acid or base catalyst, low cost, and low toxicity. The number of design variables is sufficiently large that superior materials with regard to one or two variables may not be superior with regard to others. The importance of each variable may differ for different applications. We have previously reported on arylmethyl sulfones as photoacid generators.(7-10) These materials have certain advantages in their high chemical and thermal stability, low cost, and low toxicity. Their major disadvantage has been relatively low sensitivity, related to the photochemical quantum yield and the acidity of the photogenerated acid, most likely a sulfinic acid. We report here new studies on a variety of arylmethyl sulfone derivatives, which address the issue of quantum yield. We also report on a new family of photobase generators we have recently developed.(l 1) These are quaternary ammonium salts of benzhydrylamines, photogenerators of tertiary amines. Previously reported amine photobase generators have been capable of photogenerating primary and secondary amines, but not tertiary amines.(12-15) Arylmethyl Sulfone Photoacid Generators. The published photochemistry of arylmethyl sulfones is dominated by photodesulfonylation, often related to the synthesis of strained hydrocarbons such as cyclophanes.(16) This photochemistry is typically carried out in poor hydrogen donor solvents such as benzene. In good hydrogen donor solvents, such as 2propanol or ethanol, a different photochemistry can predominate. Langler et al observed yields of sulfinic acid products as high as 54% when irradiating arylmethyl sulfones in 2-propanol.(18) The literature of this alternate photochemistry of arylmethyl sulfones is radier sparse(17-18), and was unknown to us when we made our first observations of arylmethyl sulfones as photoacid generators.(7) This earlier work did not directly inspire our development of arylmethyl sulfone photoacid generators, but it has provided a mechanisticframeworkin which to understand their photochemistry, as shown in Scheme 1. The initial photochemical event is carbonsulfur bond cleavage to give an arylmethyl radical and a sulfonyl radical. The path taken by the sulfonyl radical is the critical step, as the productive chemistry (for a photoacid generator) is hydrogen abstraction to give a sulfinic acid, but unproductive recombination and desulfonylation pathways are also available. (Under certain R—S0 H 2

SO2

hv

•S0 -R 2

-SO2 +

R«

SOj Scheme 1. Photochemistry of arylmethyl sulfones.


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Arylmethyl Sulfones and Benzhydrylammonium Salts

HANSON ET AL.

conditions, the S0 lost on desulfonylation might combine with water to give sulfurous acid, H S0 , providing another source of photogenerated acid.) Since our first report, we have studied several series of sulfones, investigating the influence of structure on the efficiency of photoacid generation and the sensitivity of resist formulations.(7-10) Here we report on two distinct series of arylmethyl sulfones which were designed to reveal different structural effects. The first series of arylmethyl sulfones was designed to investigate the effect of internal hydrogen donors. These compounds are shown in Scheme 2. An enhancement of quantum yieldfroman internal hydrogen donor had been observed in our earlier work for 2-methylbenzyl phenyl sulfone.(7-10) The series examined here all share the benzyl phenyl sulfone skeleton with methyl substituents at different positions. Our initial studies of resist formulations based on these photoacid generators has been published.(10) These earlier results, using 1.4 nm X-ray exposures in polymerfilms,suggested that the quantum yields of the derivatives with the methyl (Me) substituent on the benzylfragmentare ordered 2-Me > 4-Me » 3-Me > unsubstituted benzyl phenyl sulfone. The derivative with the methyl group at the 2 position on the phenyl side, benzyl-2-methylphenyl sulfone, was found to have the highest efficiency in these studies, suggesting it has a higher quantum yield than any of the benzyl substituted derivatives. We have now measured solution quantum yields of acid generation for these molecules at 248 nm. Three solvents were employed: 2-propanol (a good hydrogen donor), glycerol (a similar solvent of much higher viscosity), and Freon-113 (1,1,2-trichlorotrifluoroethane, which lacks hydrogens and therefore cannot be a hydrogen donor). The measured quantum yields are listed in Table I. 2


2

3

R

l

R

R

2

R4

3

R

5

H H H H H CH H H H H H CH H H H H H CH H H H H H CH H H H H H CH 3

3

3

3

3

Scheme 2. Methyl derivatives of benzyl phenyl sulfone. In 2-propanol, benzyl-2-methylphenyl sulfone again stands outfromthe rest The other sulfones' quantum yields all fall in a narrow rangefrom0.08 to 0.13. These values are probably the same within experimental error, although the standard deviation of these measurements is typically ±0.01 (average of four or five measurements). Benzyl-2-methylphenyl sulfone has a significantly higher Φ + of Η


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0.28. Our interpretation of these results and the differences between polymer film and solution results is based primarily on viscosity. In solution, low viscosity allows rapid escape of the photgenerated radical pairfromthe solvent cage. Recombination and "intra-pair" abstraction are minimized, and any abstraction that occurs is primarily intermolecular. Polymers are essentially "infinite viscosity" solvents below their T , so the radicals remain in the solvent cage for a much longer time. This increases recombination, which results in overall lower quantum yields for acid formation in polymers. "Intra-pair" abstraction also becomes important for a caged radical pair, so a hydrogen donor substituent on the other radical partner becomes significant for abstraction. So while the unsubstituted and benzyl substituted sulfones are essentially identical in their solution behavior, differences are observed in resist films.(lO) Benzyl-2-methylphenyl sulfone is unique among this set of molecules, as the Η-donor methyl group necessarily remains in proximity to the sulfonyl radical even after cage escape, and abstraction can be truly intramolecular. Glycerol, a relatively high viscosity solvent, was employed as an intermediate step between 2-propanol and polymer films. The measurements in glycerol are more difficult to make so that the results should not be overinterpreted, but the observed trends are suggestive. A l l of the Φ + values decrease, as expected since recombination increases in the higher viscosity solvent. The decline in quantum yields suggests that the actual quantum yields in "infinite viscosity" polymer films are at best 5 -10% of their value in 2-propanol.


g

Η

Table I. Quantum Yields of Acid Generation for Methyl Derivatives of Benzyl Phenyl Sulfone Φ Compound 2-Propanol Glycerol 1χ10· Μ IxlO- M Benzyl Phenyl Sulfone 0.12 ±0.01 2-Methylbenzyl Phenyl Sulfone 0.05 ±0.02 0.08 ±0.01 0.05 ±0.02 3-Methylbenzyl Phenyl Sulfone 0.09 ±0.01 4-Methylbenzyl Phenyl Sulfone 0.02 ±0.02 0.08 ±0.01 Η+

3

3

-

Benzyl 2-Methylphenyl Sulfone Benzyl 4-Methylphenyl Sulfone

Φ Freon-113 5χ1(ΗΜ 0.03 ±0.02 Η+

0.02 ±0.02 0.04 ±0.02 0.03 ±0.02

0.28 ±0.03

0.14 ±0.02

0.15 ±0.02

0.13 ±0.01

-

0.10 ±0.02

We recognized that intermolecular abstraction in 2-propanol need not necessarily be from solvent. The arylmethyl sulfones all have benzyl hydrogens and may serve as intermolecular as well as internal donors. To investigate this, the photochemistry in Freon-113 was studied. The sulfones are not highly soluble in this solvent, limiting the experiments that could be done. All of the sulfones were studied at 5 χ 10* M , and all of the quantum yields decreased. The quantum yield in Freon-113 is expected to represent a bimolecular event involving two molecules of sulfone: one as the source of the sulfonyl radical and one as the hydrogen donor. Again, all of the benzyl substituted sulfones gave essentially identical results, although the numbers are low enough that error might mask any weak effects. Benzyl-2-methylphenyl sulfone was considerably higher, as expected, since it should retain a strong internal hydrogen abstraction component. Surprisingly, benzyl-4methylphenyl sulfone also had a considerably higher Φ Η + · A S evidence for 4



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Arylmethyl Sulfones and Benzhydryfammonium Salts

intermolecular abstraction from other molecules of sulfone, a significant dependence of quantum yield on concentration was noticed. For example, 4-methylbenzyl phenyl sulfone gave a quantum yield of 0.01 when the concentration was cut in half to 2.5 χ 10-4 M , compared to 0.04 at 5 χ 10-4 M . The low solubility and low quantum yields in Freon-113, coupled with the inherent errors and difficulties in making quantum yield measurements make such measurements difficult, however. However, the fairly large difference between die quantum yields in 2-propanol (~0.1) and in Freon-113 (-0.02) suggests that the chief abstraction route in 2-propanol is indeed abstractionfromthe solvent. The other series of sulfones we have synthesized are also based on the benzyl phenyl sulfone skeleton, but these compounds carry electron donor or acceptor groups

Series 1: R = H, CH , OCH , Cl, F; R = H x

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Series 2: Rj = H; R = H, CH , OCH , Br, Cl, F 2

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Scheme 3. Benzyl phenyl sulfone derivatives used in study of polar effects. at the 4 position of either the benzyl or phenyl group, as shown in Scheme 3. The primary purpose of these compounds is to investigate polar effects on the photochemistry. Such polar effects are not expected to be large, but have been observed in other radical reactions. (19-25) Polar effects could occur in the photohomolysis step due to stabilization or destabilization of the photogenerated radicals. For substituents on the phenyl side, polar effects could alter the reactivity of the sulfonyl radical toward hydrogen abstraction and desulfonylation as well. We have used lithographic evaluations to obtain the trends in the photoactivity of these compounds. This data is reported in Table II. The results were obtained for photoresists composed of .075 molar equivalents of the sulfone photoacid generators per t-BOC group in a 3:1 t-BOC-styrene/sulfur dioxide copolymer. The processing conditions are described in the experimental section. For these t-BOC-styrene/sulfur dioxide copolymer based photoresists, solubility in aqueous base developers usually becomes significant at ~90% deprotection. Minimum dissolution doses (D ) are reported for each sulfone, and should represent the radiation dose necessary to generate sufficient acid to result in 90% deprotection. With two exceptions, these show little variation, confirming the expected small size of any polar effects. A more sensitive measure of the acid generating efficiency is the percent deprotection at an intermediate dose.(10) In Table Π, percent deprotection is also reported at 11 mJ/cm . In analyzing this data, several of the sulfones appear to behave anomalously. In several studies of polar effects on radical reactions the unsubstituted derivative is unusually unreactive.( 19-25) This appears to be the case here as well, where the parent benzyl phenyl sulfone is almost the least efficient sulfone. The methoxy derivatives appear to be unusually efficient. This may represent a difference in mechanism for the interaction of die methoxy derivatives s

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with the radical center (resonance vs. inductive), or it could represent another instance of internal hydrogen donation as described above, with the methoxy groups as internal donors in place of the methyls. The phenyl substituted bromo derivative is unusually inefficient. This has been observed for other bromo substituted sulfones and may represent alternate pathways available to these compounds, such as homolysis of the carbon - bromine bond. In any event, it seems wise to exclude these anomalous values (parent, methoxy, bromo) until further data can be gathered. The remaining derivatives are the methyl, chloro, and fluoro for both benzyl and phenyl substitution. The results do not correlate well with any of the widely used substituent constants ( σ ^ , o , σ , or σ·). They do establish trends based roughly on the electronegativity of the substituents. On the benzyl side, more electronegative (inductively withdrawing) groups decrease the efficiency of acid generation and therefore the percent deprotection: methyl is die most efficient, fluoro the least. On the phenyl side, more electronegative groups increase the efficiency of acid generation and therefore the percent deprotection: fluoro is the most efficient, methyl die least. The effects are at best moderate in size, as expected. The very small data set makes intensive analysis suspect, therefore we are currently synthesizing more derivatives to increase the size of the data set +

meta

Table Π. Polar Effects in Arylmethyl Sulfone Photoacid Generators Compound

D (milcm ) s

2

% deprotection at 11 mJIcm 2

Unsubstituted Benzyl Phenyl Sulfone

20

51

Benzyl substituted 4-Methoxybenzyl Phenyl Sulfone 4-Methylbenzyl Phenyl Sulfone 4-Chlorobenzyl Phenyl Sulfone 4-Ruorobenzyl Phenyl Sulfone

17 20 20 20

79 58 57 53

Phenyl substituted Benzyl 4-Methoxyphenyl Sulfone Benzyl 4-Methylphenyl Sulfone Benzyl 4-Bromophenyl Sulfone Benzyl 4-Chlorophenyl Sulfone Benzyl 4-Fluorophenyl Sulfone

8 17 26 17 17

99 62 39 66 70


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Quaternary Ammonium Salt Photobase Generators. Photochemical base generation has not been widely investigated. Tertiary amines are in some instances better basic catalysts than primary and secondary amines, as these may be incompatible with certain resist chemistries.(27-28) Photogeneration of tertiary amines also appears to be a relatively difficult problem, however. Our approach has been to build on the work of Kochi in photosolvolysis of benzylammonium salts(29) and Steenken in the photolysis of diphenyl- and triphenylmethane derivatives.(30-32) Conceptually, the photolysis of a quaternary ammonium salt where one substituent forms a stable cation should result in heterolysis to give a tertiary amine and a relatively stable cation. The cation, however, must not be able to eliminate H to give an alkene, and the system must contain a nucleophile capable of competing with the tertiary amine to prevent recombination. TTiis nucleophile also should not be protic (i.e., alcohols), since the product of reaction with the cation will subsequently eliminate H and protonate the photogenerated amine. Ideally, the nucleophilic agent should be the counterion of the original quaternary ammonium salt. Counterions such as cyanide, thiocyanate, and iodide would appear to best meet this set of criteria. A photobase generator system that follows this pattern is trimethylbenzhydrylammonium iodide, as shown in Scheme 4. +

+

Scheme 4. Photochemistry of trimethylbenzhydrylammonium iodide. The synthesis of trimethylbenzhydrylammonium iodide was accomplished by exhaustive methylation of benzhydrylamine with methyl iodide.(ll, 33-35) Other counterions can be exchanged for die iodide (for example, we have exchanged for a completely nonnucleophilic counterion by reaction with silver trifluoromethanesulfonate). Unfortunately, few other alkyl groups can be added to benzyhydrylamine by this technique: reactions with ethyl iodide, propyl iodide, and butyl iodide all fail. A molecular mechanics study suggested that there is considerable steric hindrance around the nitrogen lone pair of benzhydrylamine. The most stable conformation appears to be one where the amine lone pair is "guarded" by ortho hydrogens on the two phenylrings,as shown in the Newman projection in Scheme 5. This apparently limits the nucleophilicity of the lone pair such that it can only react with relatively unhindered electrophilic centers. Even ethyl iodide appears to be too hindered.(36) While trimethylamine might be a useful catalyst due to its expected high mobility in polymer systems, volatility could be a problem since the boiling point is only 2.9°C. The ability to generate various amines with different mobilities, volatilities, and base strengths would be preferable. We have recently begun to experiment with the aminofluorene system, where the extra ring fusion forces near coplanarity and reduces the steric demand around the nitrogen. This amine is also easily exhaustively methylated to the trimethylfluorenylammonium salt, and we have been


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able to alkylate amines using 9-bromofluorene as well. This system promises to allow variation in the substitution of both the tertiary amine and the stabilized cation. This should provide a greater measure of control over the materials properties of these photobase generators.

H Scheme 5. Newman projection of calculated favored conformation of benzhydrylamine. The nitrogen is in the foreground (with lone pair). We have demonstrated the solution photochemistry of the quaternary ammonium photobase generators using N M R . ( l l ) Solutions of trimethylbenzhydrylammonium iodide in deuterated acetonitrile show changes in peak heights with UV exposure. The peak due to the methyl groups of the ammonium salt and the peak due to the benzhydryl proton decrease, while a peak due to free trimethylamine and a peak due to die benzhydryl proton of benzhydryl iodide grow in. For irradiation at 254 nm in a Hanovia photochemical apparatus, photochemical conversion is essentially complete in 60 minutes. Similar changes are observed for the trimethylfluorenylammonium iodide. Figure 1 shows the photolysis of the benzhydryl derivative. The thermal properties of these compounds are quite good, as both quaternary ammonium iodides have decomposition temperatures above 185 C. We are beginning to explore the use of these photogenerated amines in photoresist and polymer curing applications, with several promising results.(37) A number of organic transformations are catalyzed by tertiary amines, so the ability to generate these catalysts should open new avenues for resist chemistry. e

Conclusions. Advances in the photogeneration of catalysts are important in the development of new photoresists. Photoacid generation is a relatively mature field, and we are working on understanding the details of the photochemistry of the arylmethyl sulfones. We have established the importance of internal hydrogen donors, particularly in viscous solvents such as polymer films. Polar effects appear to be much less important, although there are measurable differences that correlate with substituent electronegativity. Photobase generation is far less developed, and we have reported a new type of photobase generator, where photochemical dissociation of a quaternary ammonium salt gives a tertiary amine. This appears to be the first tertiary amine photogenerator, which should open new possibilities for resist chemistry.


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HANSON ET AL.


a) Before photolysis. 1

CH.

CHJ-N-CHJ-


Il I

1

1

9.0 B.0 7.0 6.0 5.0 .4.0 3.0 ,1X> 0.0 7-0 > 2.0 ; I ι ι • • I ι ι ι ι I ι t • ι I ι t ι ι I ι ι ι ι I ι ι ι ι I ι ι ι ι I ι ι t t 1ι ι ι ι I ιι 1

b) After 60 minutes photolysis at 254 nm.

CH3-N: I

CH

3

-

CH

ατο il

9.0 8.0 7X> 6.0 5.0 jl I ι ι ι ι I ι ι ι -ι I ι ι ι ι I ι ι ι ι I ι ι ι ι I

3.0

2.0

P.0

.1.0

Figure 1. Photolysis of trimethylbenzhydrylammonium iodide in

CD3CN.

Experimental Section. Materials Synthesis and Characterization. Arylmethyl sulfones were synthesized from the arylmethyl halides and the sodium sulfinates by the method of Baldwin and Robinson.(38) The arylmethyl halides and most of the sulfinates are available commercially. Some sodium sulfinates were obtained by reduction of the commercially available sulfonyl chlorides.(39) A l l sulfones were purified by recrystallization from toluene before use. The t-BOC-styrene/sulfur dioxide copolymer was prepared and characterized as previously described. (40) Trimethylbenzhydrylammonium iodide was prepared by exhaustive methylation of benzhydrylamine with methyl iodide and sodium carbonate in methanol as described elsewhere.(ll)


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Photochemistry: Quantum Yields. Commercially available spectroscopy grade solvents were used for the quantum yield determinations. Absorbance measurements were made on a Hewlett-Packard 8452A diode array spectrometer. A stock solution of the indicator bromophenol blue sodium salt was prepared in 2-propanol and used for all experiments. A calibration curve was established by treating 5.00 ml aliquots of this stock solution with 0-100 μΐ of 0.001 M HC1 in methanol, diluting with 2propanol, glycerol, or Freon-113 as appropriate such that each calibration sample had a total volume of 6.00 ml, and measuring the absorbance of these samples at 610 nm. Sulfone samples in the appropriate solvent (2-propanol, glycerol, or Freon113)were prepared and their absorbance measured. The samples were then irradiated for 1 to 60 seconds at 248 nm using a Lambda-Physik KrF excimer laser with an intensity of 1.5 to 2.0 mJ/cm -sec. Malachite green leucocyanide solutions irradiated simultaneously were employed as an actinometer, following the method of Calvert and Rechen.(41) Following irradiation, 1.00 ml of each sulfone solution was combined with 5.00 ml of the bromophenol blue stock solution, and the absorbance at 610 nm measured. The quantum yield can be determined knowing the photon flux from the actinometry, the absorbance of the sample, and the concentration of acid producedfromthe calibrated indicator absorbance. Reported quantum yields are the average of two to four measurements. 2

Photochemistry: NMR Experiments. Trimethylbenzhydrylammonium iodide was dissolved in acetonitrile-dj. The solution was divided among four to six quartz NMR tubes (Wilmad). These were irradiated in a Hanovia photochemical reactor using 254 nm lamps. The tubes were removedfromthe reactor at time intervalsfrom0 to 60 minutes and their NMR spectra obtained on a General Electric QE-300 FT-NMR. X-Ray Lithography. Resist solutions (15 w/v%) were prepared using 3:1 t-BOCstyrene/sulfur dioxide copolymer in ethyl-3-ethoxypropanoate. The arylmethyl sulfones were added such that the ratio of t-BOC groups to sulfone was 100:7.5 (7.5 mol%). The solutions were filtered through 0.2 μπι Millipore filters, then used to coat nominal 1.0 μπι films on hexamethyldisilazane primed 5 inch (130 mm) silicon substrates. The wafers were subsequently baked at 105 C for 2 minutes on a vacuum hotplate. Film thickness was measured using a Nanometrics Nanospec/AFT film thickness gauge. X-ray (λ = 0.8 to 2.0 nm centered at 1.4 nm) exposures in 1.00 atmosphere helium were performed using a Hampshire Instruments Series 5000 point source proximity print stepper(42), operating with a pulse rate of 1.0 Hz and a flux of 0.8 - 1.2 mJ/cm /pulse. An openframepolysilicon mask (1.0 μπι thick) was used in all exposures. Resist films were post-exposure baked on a vacuum hot plate for 140 C for 2.5 minutes. Wafers were customarily split in two, with one half developed and one half used for infrared measurements. Development was performed by immersion in 0.17 Ν tetramethylammonium hydroxide for 30 seconds followed by a de-ionized water immersion rinse for 30 seconds. The minimum exposure dose required to develop a approximately 5x5 mm area was reported as the measure of resist sensitivity D . Infrared analysis was performed on a Mattson Instruments Galaxy Series Model 8020 dual beam FT-IR spectrometer. Deprotection of the t-BOC-styrene/sulfur dioxide copolymer resist material is accompanied by disappearance of the carbonyl band at 1760 cm" . The extent of deprotection at a given exposure dose was determined by comparison of the integrated area of the carbonyl band for an exposal section of the resist film versus the integrated area of the carbonyl band for an unexposed section. e

2

e

s

1


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Acknowledgments. Discussions of this work with G.N.Taylor and E. Reichmanis have been useful. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS; Research Corporation's Partners in Science Program; and the Seton Hall University Research Council for partial support of this research.


References 1. 2. 3.

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