Hiroshi Ito and Mark Sherwood. IBMAlmaden Research Center 650 Harry Road San Jose, CA 95120, U S. A ibm. com.

Journal of Photopolymer Science and Technology Volumel2,Number4(1999) 625-636 1999TAPJ Investigation of Deep UV Solvents, Chemistries, Resists by NMR: Residual and PAG Decomposition in Casting Film Hiroshi Ito and Mark Sherwood IBMAlmaden Research Center 650 Harry Road San Jose, CA 95120, U S. A HIROSHI@almaden. ibm. com. Inverse gated 'H-decoupled carbon-13 nuclear magnetic resonance (NMR) spectroscopy has been employed in the investigation of ESCAP-related deep UV resist films. The concentrations of residual casting solvents were determined as a function of the bake temperature and of storage conditions. In addition to quantification of deprotection, side reactions such as C- and 0-alkylation of the phenol unit in the resist film were quantitatively analyzed while varying the exposure dose, bake temperature, resin structure, and acid generator. Furthermore, photochemical decomposition of several acid generators in the resist film was monitored in a quantitative fashion. This paper demonstrates that the 13C NMR technique can provide a wealth of quantitative and indispensable information about constituents and chemistries in resist films. Keywords: nuclear magnetic resonance spectroscopy, chemical amplification, acidcatalyzed deprotection, photochemical acid generators, hydroxystyrene copolymers, protected polyhydroxystyrene, castingsolvents 1. Introduction The advanced Chemical amplification deep UV positive resists, initially designed for the 0.25 µm resolution by deep UV lithography employing a KrF excimer laser,' are capable of resolving 175 nm line/space patterns2 and more recently, they have been extended to resolutions of <150 nm.3 To further advance chemical amplification resists, we believe that resist films must be studied at a molecular level in an effort to gain a much more fundamental and deeper understanding of the physics and chemistry of imaging processes in resist films. Acid diffusion is an important process in the chemical amplification resist systems and it is affected by residual casting solvents in the baked resist film. However, there has been only a scattered effort to determine the concentration of residual casting solvents. The most systematic investigation of this kind is the quantification of residual propylene glycol methyl ether acetate (PMA or PGMEA) in several polymer films using a 14C-labeling technique.4,5 Acid-catalyzed deprotection chemistry is a platform common to all advanced positive resist systems. However, this acidolysis reaction has been rather casually and somewhat semiquantitatively studied by infrared spectroscopy on resist films and by NMR spectroscopy on model systems in solution. The use of inverse gated ' H- decoupled 13C NMR to accurately determine the degree of deprotection has been reported for the ESCAP resist films.6'' This NMR technique has enabled observation of as little as a few percent of deprotection at the threshold dose at which the exposed film begins to dissolve in aqueous base and also of incomplete deprotection (95 %) even under overexposure. IR spectroscopy fails to provide such detailed information regarding deprotection. Existence of side reactions and the cleanness of deprotection reactions have been largely ignored, although some resist systems have been shown, through model reactions in solution, to produce byproducts.8 One reason for the lack of such information is that the commonly employed IR Received Accepted April 17, 1999 June 1, 1999 625

J. Photopolym. Sci, Technol., Vol.12, No.4, 1999 technique is too insensitive to detect small amounts of foreign structures. Side reactions in the resist film could reduce the dissolution rate, contrast, sensitivity, etc. We have found that 13C NMR is uniquely suited for the detection of minute amounts of side reactions in the resist film. Furthermore, this 13C NMR technique is capable of monitoring the photochemical decomposition of acid generators in the resist film in a quantitative fashion. Photolysis of acid generators has been previously studied in solution or else the acids generated in resist films have been UV-spectroscopically titrated.9 The 13C NMR procedure described in this paper allows quantification of several important parameters such as the concentration of residual casting solvent, the degrees of deprotection and side reactions, and the photodecomposition of acid generators all at once in a single measurement, although it is still desirable to carry out a specific and separate analysis for detection/determination of each unknown parameter. 'H NMR can be performed on much smaller samples, can provide high signal/noise spectra in a significantly shorter period of time, and thus can be highly useful in detection/determination of certain structures. However, its narrower range of chemical shifts and 'H -1H coupling result in severe overlapping of resonances, making its use in quantitative analysis difficult for complex samples. We have therefore focused our attention on '3C NMR. The polymer/resist systems studied included ESCAP10,11 consisting of a copolymer of 4- hydroxystyrene (HOST) and t-butyl acrylate (TBA), the tboc resist12 based on poly(4-tbutoxycarbonyloxystyrene) (PBOCST), poly(4- hydroxystyrene) (PHOST), and poly(methyl methacrylate) (PMMA). The casting solvents examined were PMA, ethyl lactate (EL), ethyl 3- ethoxypropionate (EEP), and cyclohexanone (CH). The photochemical acid generators (PAGs) employed were triphenylsulfonium trifluoromethanesulfonate (triflate) (TPSOTO, di-tbutylphenyliodonium camphorsulfonate (TBIC), di-t-butylphenyliodonium perfluorobutanesulfonate (nonaflate) (TBPIONO, and 1V camphorsulfonyloxynaphthalimide (CSN). The formulations were kept free of any other additives for this round of the investigation. 2. Experimental 2.1. General Procedure of 13C NMR Analysis Polymer films were spin-cast on Si wafers and baked on a hotplate at a temperature ranging from 100 to 150 C for 1 min to give 700-1400 nm thickness. After exposure/bake or without any further processing, the films were scraped off the wafers and dissolved in 500 mg of acetone-d6 together with 25 mg of Cr(acac)3 which was added as a spin relaxation agent.13 Two to three 8" and five 5" wafers were required to produce 40-80 mg of scraped polymer films for 13C NMR measurements. The solution was transferred into a 5 mmd NMR tube. Inverse gated 1H-decoupled14 13C NMR was run at room temperature on a Bruker AM500 (125 MHz for carbon) spectrometer. The number of scans ranged from 24,000 to 170,000. 2.2. Residual Casting Solvents A copolymer containing 61.5 mol% HOST and 38.5 mol% TBA was dissolved in a casting solvent at about 17 wt%. The polymer solution was filtered to 0.2.tm and cast on Si wafers. After baking at a given temperature, the polymer film was subjected to inverse gated 13C NMR analysis. The concentration of residual casting solvents in the ESCAP resist film was also determined. Although ESCAP was selected and studied extensively as an example, PBOCST, PHOST, and PMMA were also included for comparison. The ESCAP copolymer films cast on two 8" Si wafers were baked on a hotplate at 135 C for 1 min and stored for 1 month in 1) a wafer boat, 2) house vacuum, and 3) a desiccator containing a water vessel. The concentration of the residual casting solvent was determined and compared with the value for a freshly baked film. The influence of the storage conditions on the casting solvent residue was studied for EL and PMA. 2.3. Chemistries: Deprotection, Side Reactions, and PAG Photolysis ESCAP resists were formulated using poly(host0.815-co-tba0.3s5) and 3.0 wt% of an acid generator without any other additives. EL was used with TBIC and TBPIONf, PMA with TPSOTf, and CH with CSN. The resist films were spin-cast on five 5" Si wafers for each measurement, baked at 130 or 150 C for 1 min, exposed through a 6" 248 nm Melles Griot filter (8.9 nm bandwidth for 50 % cutoff) to a radiation from an I SA deep UV exposure station, postexposure-baked (PEB) at 130 or 150 C for 90 sec. A tboc resist was formulated by dissolving PBOCST and 3.0 wt% of TPSOTf in PMA. After postapply-bake (PAB) at 130 C for 60 sec, the 626

films were exposed in a similar fashion, and postexposure-baked at 110 or 130 C for 90 sec. 3. Results and Discussion 3.1. Residual Casting Solvents The importance of the PAB step has been well demonstrated by the annealing concept15 for environmental stabilization as embodied in the ESCAP resist.1o,11"6 Casting solvents which can hydrogen-bond with the phenolic OH group of the resist resin can remain in the film in significant amounts (as much as 20 wt%) after PAB.4,5 The residual casting solvent could reduce the glass transition temperature of the resist film and also to PMA. A more universal method readily applicable to various casting solvents is gas chromatographic (GC) analysis of head space gasses generated by heating a cast polymer film in a closed vial." The GC method may underestimate the residual concentration as the solvent may not completely escape from the film. We propose inverse gated ' H-decoupled '3C NMR as a simple versatile method for quantification of residual casting solvents. This technique can be applied to various casting solvents and polymers without any modifications to the materials or measurement procedures. An inverse gated 'H-decoupled 13C NMR spectrum of an ESCAP resist film, cast from EL Figure 1 125.8 MHz Inverse gated 'H-decoupled 13C NMR spectrum of an ESCAP resist cast from EL and baked at 100 C for 60 sec, in acetone-d6 (64 mg in 500 mg, number of scans=38,400, integration values in parentheses) film, promote the diffusion of a photochemicallygenerated acid, affecting the sensitivity and image profile. It is therefore desirable to determine the concentration of residual casting solvents for each resist system. To our best knowledge, however, only a scattered effort has been directed toward quantification of residual casting solvents in resist films. The most extensive investigation has been the use of 14C-labeled PMA.4'S This technique requires synthesis of a casting solvent labeled with a radioactive 14C isotope and has been applied only and baked at 100 C for 60 sec, in acetone-d6 is shown in Figure 1. The nuclear Overhauser effect (NOE) is essentially eliminated, enabling reliable quantification of the polymer composition, casting solvent concentration, and PAG loading. The carbonyl resonance of EL overlaps with that of the ESCAP copolymer but the other four signals are well separated and can be used in quantification. The contribution of EL to the total carbonyl area 627

can be calculated by subtracting the area under the quaternary carbon resonance (10.00) from the total carbonyl area (11.35). Each carbon resonance of the casting solvent has essentially the same intensity (1.40±0.04), proving the absence of NOE and an excellent quantitative nature of the analysis. The areas under the carbonyl and quaternary carbon resonances of the TBA unit in the copolymer are nearly equal and the HOST C4 and C2 (and C3) resonances have relative areas very close to the expected ratio of 1:2 (16.27/33.21, 33.06). From these integration values, the composition of the copolymer can be determined as TBA/HOST =38/62 mol/mol. The broad peaks due to the main chain methylene and methine carbons have an integration value of 53.08, which agrees with the expected composition. The molar concentration of the residual EL relative to the polymer, which is directly determined from the NMR spectrum, can be readily converted to weight % (4.6 wt% in this case). In the case of PMA, all six carbon resonances are observed well separated from each other and from the peaks ascribed to the ESCAP copolymer. Depending on the resin and casting solvent The concentrations of residual casting solvents in the ESCAP copolymer films were determined by this 13C NMR technique as a function of the bake temperature for PMA, EL, CH, and a mixture of EL with EEP (7/3) as shown in Figure 2. The PMA concentration is about twice as high as that of EL in the ESCAP film at the same bake temperature although PMA has a lower boiling point than EL (145-146 vs. 154 C). While the ESCAP copolymer film becomes almost free of the residual EL when baked at 150 C, a higher temperature of 170 C is required for complete removal of PMA. Also shown in Figure 2 are the concentrations of PMA remaining in the ESCAP copolymer film after 5 min bake as determined by the 14C-labeling techniquel.4 The agreement between the values obtained by two different methods is excellent, proving that 13C NMR can be made truly quantitative. When a mixture of EL with EP is used as a casting solvent, the major component EL is completely removed by baking at 135 C for 60 sec, leaving behind about 2 wt% of EEP (bp > 166 C). Concentrations (wt%) of residual casting solvents for select polymer-casting solvent pairs Figure 2 Concentration of residual casting solvents in ESCAP film as a function of bake temperature structures, some resonances might overlap but at least one solvent peak is quite likely to be well separated for direct comparison with at least one polymer resonance. Small 1 H NMR signals of these aliphatic casting solvents would be obscured by broad polymer resonances. are presented in Figure 3. The films were baked at 130 C for 60 sec and for additional 90 sec (PEB) in a couple of cases. Although attempts were made to keep the film thickness between 1.3 and 0.8 µm, the PBOCST film was measured to be 1.8 µm thick. Nevertheless, the PBOCST film dries 628

J. Photopolym. Sci. Technol., Vo1.12, No.4, 1999 Figure 3 Residual casting solvents (wt%) remaining in polymer films baked at 130 C for 60 sec and additional 90 sec out very quickly, containing no detectable amount of PMA after 60 sec at 130 C. According to the 14C -labeling experiment,4 a PBOCST film baked at 100 C for 5 min contained 0.6 wt% PMA. PHOST can retain more casting solvents (7.3 wt% PMA and 6.3 wt% EL) than ESCAP due to the higher concentration of the hydrogen-bonding phenolic OH group. In contrast, PMA evaporates out of the nonpolar PMMA film more rapidly than out of the phenolic resins but at a much slower rate than from PBOCST. While the PMA concentration in the PMMA film is a function of the bake time, the effect of the bake time on the EL and PMA concentrations in the ESCAP (and perhaps PHOST) film is insignificant, indicating that the solvent remaining after a 60 sec bake is strongly hydrogen-bonded with the phenolic OH group and thus cannot be readily baked out by additional heating at the same temperature. CH (bp 155 C) tends to stay in the ESCAP film at a higher concentration (4.9 wt% after a 150 sec bake at 130 C) than PMA (and EL) perhaps due to a stronger hydrogen-bonding interaction. 3.2. Residual Casting Solvents vs. Storage Conditions Casting solvents remain in a phenolic resist film in significant quantity. The fate of the residual casting solvent upon storage of coated wafers is an important (but neglected) subject particularly for electron-beam mask making where coated quartz blanks are stored for a long period of time before they are exposed to electron beams. We studied the influence of the storage conditions on the coated/baked ESCAP resist film using the inverse gated ' H-decoupled 13C NMR technique. Two ESCAP resists were formulated using PMA and EL as a casting solvent. Two 8" Si wafers were coated and baked at 135 C for 60 sec for each measurement. Each set was stored at room temperature for one month in (1) a wafer boat, (2) continuously evacuated chamber, and (3) closed desiccator containing a water vessel. The highly robust ESCAP resist does not exhibit any evidence of decomposition of the polymer or acid generator even when the formulated resist solution is stored at room temperature for one year as studied by the 13C NMR technique. There was no decomposition of the polymer or acid generator in the coated films after one month of storage under these conditions. As summarized in Figure 4, the change (aging) was very small in many cases. The concentration of the casting solvents, both EL and PMA, essentially remained constant. Continuous application of a house vacuum for one month did not remove the casting solvent at all! This indicates that the residual casting solvent is strongly bound to the polymer through hydrogenbonding as mentioned earlier. However, when the coated wafers were stored in a closed vessel saturated with water. EL was almost completely removed (from 1.6 to 0.01 wt%) from the polymer film and the PMA concentration was greatly reduced (from 3.8 to 0.12 wt%). Only one week was needed to reduce the EL concentration to N0. Rapid replacement of the casting solvent with water molecules was suspected. Two 8" Si wafers were coated with the ESCAP resist and baked at 629

Figure 4 Influence of storage conditions on residual casting solvents in ESCAP baked at 135 C (60 sec) 135 C for 60 sec. The wafers were stored in a closed container saturated with water for one week and then in EL vapor for another week. A large amount (10.8 wt%) of EL was absorbed back into the ESCAP resist film. The humidity in the wafer storage environment clearly has a large impact on evaporation of the residual casting solvent out of coated wafers. 3.3. Chemistries in ESCAP Resist Film ESCAP resists were formulated with P(HOST0.615-co-TBA0.3s5) using 3.0 wt% of a sulfonium salt (TPSOTf), two iodonium salts (TBIC and TBPIONf), and a non-ionic iminosulfonate (CSN) as acid generators and PMA, EL, and CH (with CSN) as casting solvents. No other additives were employed in this round of investigations. The film thickness was 1.1 to 1.6 µm. No special attempts were made to keep the thickness exactly the same because the main purpose of this study was to investigate what can be learned about chemistries in resist films using ' 3C NMR. Inverse gated ' H-decoupled ' 3C NMR spectra were obtained after PAB, after W exposure, and after PEB. Since the degree of deprotection can be easily determined by this technique, more focus was placed on more subtle chemical changes such as side reactions and photochemical decomposition of acid generators in the film. 3.3.1. Deprotection IR spectroscopy is widely used to investigate resist chemistry in films and it can provide important information about gross chemical changes. However, as pointed out earlier, the IR technique fails to offer truly quantitative information and to detect small chemical changes. Conversion of carbonate to phenol can be readily monitored by observing disappearance of the carbonate carbonyl IR absorbance in the case of PBOCST (homopolymer) but the situation becomes more complex in the presence of a phenolic group due to appearance of a hydrogenbonded carbonyl absorption at a slightly lower wavenumber. Conversion of ester to carboxylic acid is observed as a decrease in the ester carbonyl absorption and the appearance of an acid carbonyl peak within a very narrow wavenumber range of the IR spectrum and it is thus difficult to quantify this change by monitoring the carbonyl region. The IR absorptions at 1150 and 1375 cm' are therefore employed to monitor the t-butyl ester acidolysis. However, accurate determination of the end point is still difficult as a fully deprotected polymer has a residual absorption at these wavenumbers. Inverse gated 'H-decoupled 13C NMR is an excellent method for quantification of the deprotection reaction as Figure 5 demonstrates. The ESCAP resist formulated with 3.0 wt% of TBIC in EL was prebaked at 130 C for 60 sec, exposed to 7 mj/cm2 of 248 nm radiation, and postbaked at 130 C for 90 sec. The spectrum exhibits a resonance due to a polymer end CN group, which arises from 2,2'- azobis(isobutyronitrile) (AIBN) used as the polymerization initiator. A carboxylic acid 630

J. Photopolym. Sci. Technol., Vo1.12, No.4, 1999 Figure 5 125.8 MHz Inverse gated 'H-decoupled13C NMR spectrum of ESCAP film consisting of P(HOST-co-TBA) and 3.0 wt% TBIC (cast from EL, PAB:130 C/60 sec, 7.0 mj/cm2, PEB:130 C/90 sec, integration values in parentheses) carbonyl resonance is observed well separated from the ester carbonyl resonance. However, when EL is used as a casting solvent, its carbonyl absorption overlaps with that of the polymer and therefore its area must be subtracted from the ester carbonyl integration. The ester carbonyl resonance or the t-butyl ester quaternary carbon resonance at 80 ppm can be compared with the acid carbonyl resonance for determination of the acrylic acid (AA) yield (55 % in this case). Use of PMA as a casting solvent provides straightforward quantitative 13C NMR analysis, as the solvent carbonyl carbon resonates at a significantly higher field than the polymer carbonyl. 3.3.2. Side Reactions The NMR spectrum in Figure 5 further reveals at a closer inspection that the exposed/baked resist film contains small amounts of foreign structures bound to the polymer (78.0, 1 and 153.9 ppm). These small broad resonances are ascribed to a t-butyl phenyl ether produced by O-alkylation of the HOST unit by the t-butyl cation (Figure 6). The assignments of these small resonances have been accomplished by comparison with an authentically prepared poly(4-t-butoxystyrene). About 2.9 mol% of the HOST was alkylated to the t-butyl ether in this case. Thus, while the yield of carboxylic acid was calculated to be about 55 %, the degree of deprotection as calculated on the basis of a loss of the t-butyl group from the ester comes out a little higher (57 %) because the 0- alkylation must be taken into consideration. 0- Alkylation was also observed when the ESCAP copolymer was treated with triflic acid in acetoned6 at room temperature. In addition, the t-butyl cation released by acidolysis of the t-butyl ester can react with the phenol ring itself before it undergoes 3-proton elimination (Figure 6). Both 0- and C-alkylation processes regenerate a proton. In fact, C-alkylation has been clearly observed in the exposed/baked ESCAP resist film especially when a strong acid such as triflic or nonaflic acid is generated. An inverse gated ' H-decoupled 13C NMR spectrum of an ESCAP resist formulated with P(HOST0 815-co-TBAo.385) and 3.0 wt % of TBPIONf in EL, postapply-baked at 130 C for 60 sec, exposed to 7.0 mj/cm2 of 248 nm radiation, and postexposure-baked at 130 C for 90 sec, is presented in Figure 7. The t-butyl ester is almost completely removed under these process conditions, leaving only very small signals at 80 631

Figure 6 Acidolysis in ESCAP (deprotection, 0-alkylation, and C alkylation) Figure 7 125.8 MHz Inverse gated 'H-decoupled 13C NMR spectrum of ESCAP resist film composed of P(HOST-co-TBA) and 3.0 wt% TBPIONf (cast from EL, PAB:130 C/60 sec, 7.0 mj/cm2, PEB:130 C/90 sec) and 175 ppm. The sharp resonance on top of the small polymer carbonyl peak is due to the residual EL. The absence of a peak at 78 ppm indicates that there is no t-butyl ether. Therefore, the small resonances at 126 and 154 ppm are not due to the t-butyl ether but due to the C-alkylation product. Note that the 0-alkylation gives a resonance at a slightly higher field than the CN group in the 125 ppm range while the C-alkylation process results in generation of a 13C absorption at a slightly lower 632

field than CN. When both 0- and C-alkylated side products co-exist, the peak at 154 ppm is larger than that at 78 ppm and a subtraction provides the concentration of the C-alkylation product. The compositions of the polymers in the exposed/baked ESCAP resist films processed under different conditions are summarized in Figure 8. The PAG loading was 3.0 wt%. When only the PAB temperature was changed for the TBPIONf system, the compositions of the polymers produced were essentially identical to each other, indicating excellent reproducibility of the analysis. Stronger acids (triflic and nonaflic acids) tend to promote C-alkylation while only a small amount of t-butyl ether is produced with camphorsulfonic acid. However, the weak camphorsulfonic acid can also induce C-alkylation when present in high concentration (high dose). The t-butyl ether is acid-labile and therefore whether C-alkylation directly takes place or goes through 0-alkylation first is not clear. These side reactions could lower the resist contrast (and sensitivity) because the C- and 0-alkylation processes reduce the dissolution rate of PHOST in aqueous base. However, the small degree of side reactions and the extremely fast dissolution rate achieved by generation of carboxylic acid make the ESCAP resist rather insensitive to such effects. We have taken a closer look at the deprotection chemistry in the classical tboc resist.12 PBOCST and 3.0 wt% of TPSOTf were dissolved in PMA. Spin-cast films were baked at 130 C for 60 sec, exposed, and baked at 130 or 110 C for 90 sec. One film was further baked at 180 C for 2 min without exposure. The attack of the t-butyl cation onto the aromatic ring occurred more extensively than in the ESCAP system with the more dominant C-alkylation amounting to 14 % in acidolysis (Figures 9-11). Thermolysis at 180 C resulted in formation of ca. 4 % t-butyl ether. Furthermore, some other thermally-induced side reactions have been detected. The sharp resonances at 145, 144, 132, 126, and 114 ppm in Figure 9 (and 7) appear only after PEB, independent of the casting solvent (PMA or EL), polymer (ESCAP or PBOCST), and PAG (TPSOTf or TBPIONf). The sharpness of these resonances suggests that the products are small molecules that are not bound to the polymer. Further investigation is underway to identify these side products. Figure 8 Polymer compositions (mol%) in exposed/baked ESCAP resists formulated with TPSOTf, CSN, TBPIONf, and TBIC in PMA, CH, or EL (PAB/dose/PEB) 633

Figure 9 125.8 MHz Inverse gated 'H-decoupled 13C NMR spectrum of tboc resist film composed of PBOCST and 3.0 wt% TPSOTf (cast from PMA, PAB:130 C/60 sec, 1.75 mj/cm2, PEB:110 C/90 sec) Figure 10 Deprotection and side reactions in PBOCST Figure 11 Compositions (mol%) of exposed/baked tboc resist formulated with 3.0 wt% TPSOTf (PAB/dose/PEB) 634

3.3.3. Photolysis of Acid Generators Small amounts of acid generators (1.5 wt% TPSOTf in Figure 1, for example) in phenolic resins can be detected and quantified by 13C NMR. The triphenylsulfonium cation gives rather strong C2 and C3 resonances (6 carbons each) at ca. 133 ppm between the C3 and C 1 resonances of the HOST unit (see Figures 1 and 8). The di-tbutylphenyliodonium cation shows a distinct resonance due to the aromatic carbon adjacent to the iodonium cation at about 112 ppm, which is well separated from the aromatic C2 resonance of the phenolic polymer (see Figures 5 and 7). These absorptions can be used in quantification of PAG photolysis. When the aromatic region of interest in terms of PAG quantification becomes too complex due to build-up of C- and 0-alkylation products, inverse gated 1H-decoupled 13C NMR measurements without PEB can provide more accurate concentrations of the remaining PAG. In the case of the di-t-butyiphenyliodonium system, a photolysis product, 4-t-butyliodobenzene, is clearly detected (the resonances at 91 and 153 ppm in Figure 7). In the solid state, photolysis of TPS salts is known to produce thiophenoxybiphenyls,'8 which we have been unable to detect in films of ESCAP, PHOST, PBOCST, or PMMA by the 13C NMR technique. This is probably due to the very low intensity of each resonance. 19F NMR spectroscopy on the exposed ESCAP resist containing TBPIONf did not distinguish between the fluorine groups on the PAG and photochemically generated nonaflic acid. Furthermore, only one set of 19F resonances was observed in a mixture of TPSOTf and triflic acid in acetone-d6. Thus, unfortunately 19F NMR does not seem to be useful in monitoring photolysis of fluorine-containing PACs. Figure 12 shows the degrees of PAG decomposition in several polymer films as a function of exposure dose. The naphthalimide PAG exhibited the lowest efficiency of decomposition. The iodonium PAGs appear to be more efficiently decomposed by UV irradiation. To assess the possible sensitization from a phenolic resin in comparison with PMMA, further investigations must be carried out. The reproducibility and consistency of the measurements are quite high, though there is some scatter in the data,. The degree of TBIC decomposition calculated from the integration of the 4-t-butyliodobenzene peaks is 62 % at 35 mj/cm2, which agrees well with the value determined from the concentration of unphotolyzed TBIC. In order to rigorously compare the PAG efficiencies, the molar concentration of PAG, film thickness, or absorption at the exposing wavelength must be kept constant. Since this study was meant to be a preliminary investigation on the feasibility of the NMR technique to monitor PAG decomposition in polymer films, no special efforts were directed toward keeping the thickness or absorption constant. This NMR technique may not have high accuracy in quantification of PAG decomposition, especially when the change is small, but it can Figure 12 Photochemical decomposition of PAGs in polymer films 635

J. Photopolym. Sci. Technol., Vo1.12, No.4, 1999 conveniently provide important information on the concentration of residual casting solvent, the degree of deprotection, the extent of side reactions, and the degree of PAG decomposition in a single experiment. 3.3.4. Catalytic Chain Lengths Quantum efficiencies of PAG decomposition and catalytic chain lengths in the deprotection reaction can be calculated from the data generated by the NMR technique. In the case of the ESCAP resists containing 3 wt% of TBIC and TBPIONf, decomposition of 1 mole of PAG results in cleavage of 205 and 658 moles of the t-butyl ester, respectively, when the resists are prebaked at 130 C for 60 sec, exposed to 7.0 mjlcm2, and postbaked at 130 C for 90 sec. The TBIC/ESCAP system prebaked at 150 C (60 sec), exposed to 35 mjlcm2, and postbaked at 150 C (90sec) gave the catalytic chain length of 98. The source of the largest error in the catalytic chain length determination is the accuracy in the quantification of PAG photolysis. Catalytic chain lengths at Eo will be determined for different formulations and under different process conditions. 4. Summary Inverse gated ' H-decoupled 13C NMR spectra of scraped resist films can readily provide valuable quantitative information about the concentration of residual casting solvents, the degree of deprotection, the extent and the nature of side reactions, and the degree of PAG decomposition, in a single measurement. References 1. H. Ito, Solid State Technol. 36(7) (1996)164. 2. W. Conley et al., Proc. SPIE 3049 (1997) 282. 3. W. Conley, C. Babcock, N. Farrar, H.-Y. Liu, B. Peterson, and K. Taira, Proc. SPIE 3333 (1998) 357. 4. W. D. Hinsberg, S. A, MacDonald, C. D. Snyder, H. Ito, and R. D. Allen, ACS Symposium Series 537, "Polymers for Microelectronics: Resists and Dielectrics, " L. F. Thompson, C. G. Willson, and S. Tagawa, eds., American Chemical Society, Washington, D. C., (1993) p. 101. 5. V. Rao, W. D. Hinsberg, C. W. Frank, and R. F. W. Pease, Proc. SPIE 2195 (1994) 596. 6. H. Ito, D. Fenzel-Alexander, and G. Breyta, Proc. 5P1E 3049 (1997) 575. 7. H. Ito, D. Fenzel-Alexander, and G. Breyta, J. Photopolym. Sci. Technol. 10 (1997) 397. 8. T. Sakamizu, H. Shiraishi, H. Yamaguchi, T. Ueno, and N. Hayashi, Jpn. J. Appl. Phys. 31 (1992) 4288. 9. D. R. McKean, U. Schaedeli, and S. A. MacDonald, ACS Symposium Series 412, "Polymers in Microlithography: Materials and Processes, " E. Reichmanis, S. A. MacDonald, and T. Iwayanagi, eds., American Chemical Society, Washington, D. C., (1989) p. 27. 10. H. Ito, G. Breyta, D. Hofer, R. Sooriyakumaran, K. Petrillo, and D. Seeger, J. Photopolym. Sci. Technol. 7 (1994) 433. 11. H. Ito, G. Breyta, D. Hofer, T. Fischer, and B. Prime, Proc. SP1E 2438 (1995) 53. 12. H. Ito and C. G. Willson, ACS Symposium Series 242, "Polymers in Electronics, " T. Davidson, ed., American Chemical Society, Washington, D. C., (1984) p. 11. 13. R. Freeman, K. G. R. Pachler, and G. N. La Mar, I Chem. Phys. 55 (1971) 45 86. 14. R. Freeman, H. D. W. Hill, and R. Kaptein, J. Magn. Reson. 7 (1972) 327. 15. H. Ito, W. P. England, R. Sooriyakumaran, N. J. Clecak, G. Breyta, W. D. Hinsberg, H. Lee, and D. Y. Yoon, Proc. SPIE 1925 (1993) 65; H. Ito, G. Breyta, R. Sooriyakumaran, and D. Hofer, I Photopolym. Sci. Technol. 8 (1995) 505. 16. W. Conley et al., Proc. SP1E 2724 (1996) 34. 17. K. Asakawa, T. Ushirogouchi, and M. Nakase, J. Photopolym. Sci. Technol. 7 (1994) 497. 18. J. L. Dektar and N. P. Hacker, J Amer. Chem. Soc. 112 (1990) 6004. 636