Polymers in Microlithography - American Chemical Society

Chapter 17

Characterization of a Thiosulfate Functionalized Polymer A Water-Soluble Photosensitive Zwitterion

Downloaded by YORK UNIV on July 7, 2012 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch017

C. E . Hoyle, D. E . Hutchens, and S. F. Thames Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 49406-0076

The characterization of a water-soluble zwitterionic polymer derived from aminoethane thiosulfuric acid (AETSA) and a diglicydyl ether of bisphenol A (DGEBA) has been accomplished. The polymer has been shown to form an associative network in water at high concentrations. The gel network disassociates at higher temperatures or under a shearing stress. Upon heat treatment above 165°C, the polymer sustains a measurable weight loss and is crosslinked to form a water-insoluble film presumably with disulfide formation. Photolysis of coalesced films, either direct or sensitized, results in sulfur-sulfur bond cleavage with subsequent disulfide formation and loss of water sensitivity. Initial imaging studies demonstrate the potential of the thiosulfate functionalized polymer as a photoresist.

A large number of polymeric materials have been developed over the past two decades which are photochemically reactive. In many cases, such polymers are initially soluble in organic solvents prior to exposure with insolubilization accompanying ultraviolet radiation. This often presents a problem in practical applications where handling of organic solvents is objectionable or expensive. A need exists to develop functional polymers which are both water soluble and photochemically labile. A number of reports in the literature describe the use of alkyl thiosulfates to modify reactive vinyl type monomers and/or preformed polymers with the expressed goals of producing polymers with enhanced water solubility The alkylthiosulfate modified polymers have been shown to be thermally and photochemically reactive and capable of producing crosslinked films with varying degrees of stability (5). This paper describes the synthesis and characterization of a new zwitterionic water-soluble thiosulfate polymer (Poly[7-(amino ^-thiosulfate) ethe^-PATE) via chemical reaction of a diglicydyl ether of bisphenol A (DGEBA) with aminoethane thiosulfuric acid (AETSA) as a reactive 0097-6156/89AM12-0280$06.75/0 o 1989 American Chemical Society

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


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nucleophile. Due to the presence of the ionizable thiosulfate moiety, the resulting polymer is water soluble. Further, the ability of the water soluble polymer PATE to form an inner zwitterionic salt allows for significant stability in water at near neutral pH. Since AETSA itself, as well as cured D G E B A resins, are known to be of negligible toxicity, such polymers are also most likely non-toxic. The associative, thermochemical, and photochemical properties of P A T E polymer will be described and evidence will be provided supporting the thesis of labile sulfur-sulfur bonds. Photolysis of a model compound and a polymer similar to PATE but lacking thiosulfate functional groups has been conducted to provide supportive evidence for the proposed sulfur-sulfur bond cleavage reactions. Finally, a preliminary investigation of the imaging characteristics of the P A T E polymer is presented in order to place the photochemical investigation in perspective. Experimental Materials. Reagent grade solvents, dimethyl formamide (DMF), dimethyl acetamide (DMAC), dimethyl sulfoxide (DMSO) and methanol were purchased from Baker, stored over molecular sieves once opened, and used without further purification. Aminoethane thiosulfuric acid (AETSA) purchased from Kodak, and Taurine, purchased from Alfa were purified by recrystallization. Each was thrice recrystallized from hot, deionized water. The crystalline precipitate was dried (48 hours at 40 °C) in-vacuo and subsequently stored in a desiccator. Benzophenone (BP) was purchased from Aldrich Chemical Company. Q U A N T A C U R E BTC (BTC), (4-benzolybenzyl) trimethylammonium chloride, was used as supplied by Aceto, Inc., Flushing, New York. Phenyl glycidyl ether (PGE) was purchasedfromMCB, distilled in-vacuo. and stored at -15 °C. Epon 828* was used as supplied bv Shell Chemical Company. The epoxy equivalent weight (EEW) for Epon 828 determined by an appropriate titration, was found to be 187.7. Elemental Analysis. All samples submitted for elemental analysis were dried at 40 ° C in a vacuum oven at less than 1 torr pressure for 24 hours and then sealed in ampoules. Elemental analyses were performed by MHW Laboratories of Phoenix, Arizona. Instrumentation. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 5DX using standard techniques. Spectra were measuredfromvarious sample supports, including KBR pellets, free polymer films and films cast on NaCl windows. Spectra for quantitative analysis were recorded in the absorbance mode. The height of the 639 cm" absorbance was measured after the spectrum was expanded or contracted such that the 829 cm' absorbance was a constant height. In some spectra an artifact due to instrumental response appeared near 2300 cm" . Proton-decoupled C NMR spectra were obtained using a JEOL FX90Q spectrometer. Polymer solutions for analysis were 5 to 15 weight percent. All 1

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POLYMERS IN MICROLITHOGRAPHY


chemical shifts are referenced externally to trimethyl silane. Ultraviolet spectroscopy was performed with a Perkin-Elmer model 330 spectrophotometer, using double-beam, background-cancelling techniques. HPLC analysis was performed using an L D C Minipump, a Rheodyne 7125 sample injector and a Perkin-Elmer LC-75 variable wavelength U V detector operating at 245 nm. The detector used air as a reference with offset background cancelling. The mobile phases were mixtures of acetonitrile and water, most commonly 85 parts water and 15 parts acetonitrile by volume. The columns were a Waters microbondapack CN alone or in series with a Waters C-18 column (3.9 mm i.d. X 30 cm.). The flow rate varied between 0.9 and 1.0 mL/min to generate a pressure of less than 2000 psi at the pump exit. Synthesis of PolylWamino ^-thiosulfate^ ether] (PATEV Polyfr-amino ^-sulfonic acid] (PASEV and Hydroxy-3-aminoethane thiosulfuric acid (AETSAPPE). Details of the synthesis of these three compounds are given elsewhere (7). Preparative Photolysis. The preparative photolysis of an aqueous solution (pH=8.5) of AETSAPPE (2.5 M) was conducted in a 1-inch diameter quartz test tube in a Rayonet Reactor (Southern New England Radiation Co.) fitted with 254 nm lamps. Within two hours the solution gelled and the reaction was terminated. Upon acidification the solution cleared, and the product could be re-precipitated by addition of base. This indicates loss of the thiosulfate functionality. The product was dissolved in dilute HC1, precipitated with acetone, and filtered. This process was repeated three times, and the final precipitate was washed with water. The product (20 to 30 mg) was dried in vacuo for 24 hours and stored in a dessicator until use. Comparison of the C NMR spectrum of the product with the starting AETSAPPE C NMR spectrum clearly shows that the thiosulfate methylene peak shifted upfield, from 39 ppm to 35 ppm. The complete C NMR and IR analysis of the product were consistent with the disulfide product. Further, elemental analysis of the product confirmed that the product was the desired disulfide product 2-amino (2-hydroxy 3-(phenyl ether) propyl) ethyl disulfide (AHPEPED): Expected C: 58.39, H : 7.08, N: 6.20, S: 14.18; actual C: 58.26, H : 7.22, N: 6.06, S: 14.28. 13

1 3

1 3

Quantitative Photolysis. Photolysis experiments were performed by exposing samples to a 450 Watt medium pressure mercury lamp. A shutter was placed between the samples and the lamp so that the exposure time could be accurately controlled. Unless otherwise stated, the samples were placed 4 inches from the lens. Filters (254 nm, 280 nm and 366 nm) when used, were placed immediately in front of the shutter. Sample holders were available for 1" χ 1" quartz plates, NaCl windows and quartz U V cuvettes, and samples of each type were utilized. U V intensity measurements were made with an International Light 700A Research Radiometer. The measuring head was tightly covered with aluminum foil for zeroing, and then exposed to the lamp output under exactly the same conditions as the actual samples (i.e., same distance, angle, elevation, etc.). The results of these experiments were used to evaluate the quantum yield or efficiency of the photochemical process. Specifically, photolysis of AETSAPPE


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HOYLE ET AJL

Thiostdfate Functionalized Polymer

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to y i e l d A H P E P E D was followed quantitatively by H P L C (Waters) using a 50:50, watenacetonitrile m o b i l e phase and a C - 1 8 / c y a n o c o l u m n c o m b i n a t i o n i n series. A E T S A P P E eluted at a retention v o l u m e o f 5.9 m L a n d A H P E P E D at a n elution time o f 3.2 m L . T h r e e u n k n o w n products o f relatively l o w amounts also eluted w i t h retention volumes o f 3.0 mL, 5.3 mL> a n d 7.9 m L . F o r the sensitized photolysis, a 85:15 watenacetonitrile m o b i l e phase was used to separate A H P E P E D and B T C from A E T S A P P E . Photolysis o f P A T E F i l m s . P A T E films (5 μ to 20 μ thick) were obtained by casting o n glass 10-20% solids aqueous solutions containing 0.02% wetting agent. T h e resultant films, heated i n a drying oven at 100 ° C for four minutes, were quite water soluble. F o r I R studies, films o f 5 to 10 microns were a p p l i e d to N a C l disks by injecting k n o w n , s m a l l volumes o f dilute solutions of P A T E i n D M S O . The solutions were spread to cover a 1-cm diameter area o n the N a C l disk, and subsequently d r i e d 10 minutes at 105 ° C followed by photolysis. Results and D i s c u s s i o n Synthesis o f P o l y | V ( a m i n o 0-thiosulfate) ether] ( P A T E ) . P o l v [Ύ-(amino Bsulfonic acid) ether] ( P A S E ) . and Hydroxy-3-(aminoethane thiosulfuric acid) ( A E T S A P P E ) . T h e general scheme for synthesis o f b o t h the P A T E a n d P A S E polymers is shown i n Scheme I. I n the case o f the P A T E polymer, the m i l d conditions employed (40-60° C ) insure that crosslinking o f the resultant p o l y m e r by disulfide linkages is m i n i m i z e d . Specific details o f the synthesis are given elsewhere (7). D e p e n d i n g o n the exact reaction conditions as w e l l as the nature (value o f η may vary but is close to 0.1 i n Scheme l a ) o f the starting diglycidal ether o f bisphenol A , the molecular weight (determined by viscometry i n D M S O and the a m i n o equivalent weight) o f the P A T E p o l y m e r v a r i e d between about 3000 a n d 9000. I n the polymer synthesis employed the p r i m a r y amine o f aminoethane thiosulfate ( A E T S A ) reacts as a difunctional m o n o m e r , since it possesses two active hydrogen atoms and reacts w i t h two epoxy groups. A E T S A can also act as a monofunctional end-capping agent, and the c o m b i n e d effect o f these two reactions is the formation o f the P A T E polymer. T h e value o f such a synthetic scheme is that the desired thiosulfate-functional p o l y m e r is formed directly f r o m stable m o n o m e l i c materials. It is therefore unnecessary to subject a thiosulfate-functional vinyl m o n o m e r to p o l y m e r i z a t i o n conditions (i.e. anionic, cationic or radicals) w h i c h c o u l d result i n premature degradation o f the thiosulfate bond, w i t h a subsequent high probability o f disulfide crosslinks. A d d i t i o n a l l y , this synthesis does not rely o n the availability o f a linear (uncrosslinked) prepolymer and therefore represents a n entirely new m e t h o d for introducing the thiosulfate (Bunte salt) functionality into a p o l y m e r m o l e c u l e . T h e synthesis o f hydroxy-3-aminoethane thiosulfuric a c i d ( A E T S A P P E ) is shown i n Scheme II. T h e same basic conditions used for the p o l y m e r synthesis were employed to synthesize the m o d e l c o m p o u n d ( A E T S A P P E ) although the work-up conditions were less stringent. T h e structure was confirmed by carbon13 N M R and elemental analysis.


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300


approximate dry film thickness was less than 25 microns. However, it was found that thicker films (10 to 25 microns) were more difficult to develop, and gave poor line resolution, so efforts were confined to films less than 10 microns. Typically, 8 to 15 minutes of photolysis under the full arc of a 450 W medium pressure mercury lamp from a distance of 4 inches were required to crosslink the films. A thin metallic strip machined as a support for silicon microchips was clamped onto the surface of the circuit board for masking purposes. An initial attempt was made to etch the copper immediately after photolysis using a Keprrf™ BTE-202 bench-top etcher containing a strong FeC^ solution (3.03 M). Unfortunately, the etchant bath crosslinked the unphotolyzed resin, so a developing step in neutral pH water was required. Photocured P A T E resins were thus developed for 10 to 30 seconds under warm tap water: the boards were immediately placed in the etchant for 2 minutes, then washed and dried. Figure 13 shows a section of one of the circuit boards produced by this technique. In order to establish the ultimate resolution possible for the P A T E polymer, aqueous solutions of PATE (a different batch from that used in the quantitative photolysis investigation) were spin cast to give thin films (less than 1 micron) on a silicon wafer, exposed to a 240-260 nm source (PE 600; Scanspeed 50,000; Aperture 3; UV-2, 240-260 nm) and subsequently developed by rinsing with a neutral pH water stream. Figures 14a and 14b show the resultant electron micrographs corresponding to mask line resolutions of 2.5 and 1.5 microns. At 1.5 micron resolution, a "snaking" or swelling of the pattern is noted. Attempts to generate higher resolution patterns resulted in an increased tendency to swelling, therefore defining the ultimate resolution of the P A T E system tested at about 1.5-2.0 microns.

Figure 13. Image of mask produced by exposure of a PATE film followed by appropriaterinsing/etchingprocedure on copper board.


Thiosulfate Functionalized Polymer

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17. HOYLEETAK

Figure 14. Image generated by exposure of spin watered PATE film on silicon wafer: (a) 2.5 micron line resolution (b) 1.5 micron line resolution.


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Conclusions


This paper describes the successful synthesis and examination of polyfr(amino /Mhiosulfate) ether] (PATE), a water soluble photolabile polymer. Evidence has been presented that the PATE polymer is zwitterionic and forms weak associations in aqueous solutions. Heat treatment of PATE films result in extensive crosslinking, presumably through a disulfide bond. This work presents strong evidence that PATE is activated by deep U V radiation, and that a disulfide crosslink is formed. Sensitization experiments demonstrate that the crosslinking reaction can be induced by a triplet sensitizer. Finally, preliminary results point out the potential for application of PATE films as active photoimaging systems. Acknowledgments We thank C. G . Willson for help with the imaging investigations which were performed at IBM. We also gratefully acknowledge the efforts of E . Bernardo and S. Buckley for their assistance in preparation of certain PATE samples. Literature Cited 1. 2. 3.

4. 5. 6. 7.

Beerman, C. German Patent 1 143 330, 1963. Feldstein, R., Bunte Salt Polymers: Synthesis, Reactivity and Properties. Ph.D. Dissertation, The American University, 1971. Harris, J. R., Investigation of Bunte Salts as Potential Polymeric Emulsifying and Crosslinking Agents, Ph.D. Dissertation, The University of Southern Mississippi, 1986. Vandenberg, E., U. S. Patent 3,706,706, 1972. Okawara, M. and Ochiai, Y., ACS Symposium Series 121, 41 (1980). Stewart, M. and Dawson, J., U.K. Patent 2 050 438 A, 1979. Hoyle, C. E.; Hutchens, D. E.; and Thames, S.F.,Macromolecules (1989).

RECEIVED July 31, 1989


Polymers in Microlithography - American Chemical Society

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