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ANALYTICAL SCIENCES VOL. 7 SUPPLEMENT 1991 1265

DETERMINATION OF TRACE ELEMENTS IN PHOTORESISTS USED IN THE FABRICATION OF SUBMICRON VLSI

TSU-YENMaterialsChutung,

CHANG*, LIAN-SHERN CHEN, MEEI-YUN SHIUE AND YA-CHUN SUN,Research Laboratories, Industrial Technology Research Institute,Hsinchu, Taiwan R.O.C.

Abstract Chloride ion and several metals in submicron VLSI grade photoresistshave been determined at their trace levels. Chloride ion was determined by poten-tiometric argentometric titration of standard silver nitrate solution in acetone.Good recovery was obtained for as low as 2,ug of total chloride, including thoseattributed to blank and sample, in the titration cell. In the determinationof trace metals, we made comparison between two methods, namely, flame AAS pre-ceded by dry ashing of the photoresist and the direct analysis using graphitefurnace AAS without sample digestion. In the latter method, oxygen ashing wasemployed. Analytical results for iron and zinc by the two methods were comparable.Key words photoresist, metals, chloride ion, trace analysis.

INTRODUCTIONTrace contaminants in the chemicals used in microelectronic industry are usually of highinterest to both manufacturers and end users. The contaminants not only change theelectrical properties of integrated circuits but also corrode their metallic parts andshorten their lifetime [1,2]. In this study, we investigated the determination ofchloride ion and several metals in photoresists used in the fabrication of submicronVLSI. Because manufacturers generally desire less than 0.5ug/g each of elements inphotoresists, highly sensitive techniques are necessary. Photoresists are dark-colored,highly viscous organic materials. Normal techniques for the determination of tracechloride ion [3-5] are not directly applicable to such materials. The dark color ofphotoresists forbids the proper detection of end point in mercuric nitrate titration andinterferes strongly in spectrophotometric technique, while their viscous organic matricesforbid the use of ion chromatographic or ion selective electrode techniques.Potentiometric argentometric titration in organic solvents [6,7] on the other hand seemsfeasible, and thus are investigated in this study. For the determination of tracemetals in organic materials, direct analysis of samples are highly desirable becauseof its high speed, low risk of contamination and low loss of interested metals [8-10].We therefore used the direct analysis approach for the analysis of photoresists usinggraphite furnace atomic absorption spectrometry. Because of the lack of proper standardreference materials for the evaluation of accuracy, we compared the results with thoseof conventional dry-ashing pretreatment of samples followed by flame AAs. The resultsfrom calibration curve method and standard addition method were also compared.EXPERIMENTALApparatus For the determination of chloride ion in photoresists, a Kyoto AT-118po entTometric automatic titrator was used, with a Metrohm 6.0404.100(OC) combinedsilver electrode. For the determination of metals, a Varian 875 flame atomic absorptionspectrometer and a Perkin-Elmer 5100 atomic absorption spectrometer were used. ThePerkin-Elmer 5100 instrument was equiped with a model HGA 600 graphite furnace and wascapable of performing Zeeman background correction. Manual injection by micropipettewas used for analyzing photoresist samples. Pyrolytically coated graphite tubes withL'vov platforms were used throughout the study. The preparation of all standard solutionsand samples were performed in a clean room to minimize airborne contamination. Alllabwares were soaked in (1:4) nitric acid (v/v) overnight and rinsed by deionzed water,Reagents All the reagents used were OR grade chemicals. Silver nitrate stock solution ofOTIN was prepared from E.Merck volumetric solution. Chloride and metal stock solutionsof 1000 pug/ml were prepared from E.Merck Titrisol solutions. Chloride-free acetone wasdistilled from GR grade acetone with added silver nitrate crystallines. Deionized doubledistilled water and subboiled acids were used throughout the experiment for metaldetermination. Oxygen gas as matrix modifier was of a purity greater than 99.99% andargon gas was of the similar purity.

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1266Procedure for chloride ion determination Weigh 1 to 10 grams of photoresist sample intolop m beaker. Add 40 m acetone an ml (1:1) nitric acid (v/v). Mix well and thenpotentiometrically titrated with 0.01 to 0.5 mN standard silver nitrate solution. Thechoice of the concentration of silver nitrate solution lies in the chloride content ofsample and the weight of sample used. Usually, the titrant volume exhausted was kept toless than 1 ml i n order to eliminate the error caused by the dissolution of silver chloride.Procedure for metal determination using flame AAS preceded by dry ashing/aciddissolution len grams of photoresist sample was weignea into a piatinum cruciDle.The sample was then dried completely on a hot plate and under an infrared lamp in cleanroom for 10-15 hours, followed by dry ashing in muffle furnace at 500C. The ash wasdissolved in 2 ml (1:1) subboiled hydrochloric acid and diluted to 10 ml with deionizeddouble distilled water. The metal content was then determined by flame AAS.Procedure for metal determination using graphite furnace AAS Both of standard additionme o an ca ira Ton method were emp aye or the de termination of the metals. Photor-esist samples were diluted with GR grade acetone to proper ratio by weight. For standardaddition method, a 25aul aliquot of sample was carefully injected into the graphitetube followed by a 5 ul aliquot of the standard solution of varying concentration. Thesample was then dried, ashed and atomized with a temperature program shown in TABLE 1.

RESULTS AND DISCUSSIONChloride ion determination Chloride is highly prone to contamination, therefore, samplesmust be handled with special care. All the containers must be cleaned thoroughly asdecribed in the experimental section. Electrodes used to potentiometrically detect theend point of argentometric titration must be carefully selected so that there is noleakage of chloride to the sample solution. Regular Ag/AgCI reference electrodes filledwith KC1 solution, thus, are not appropriate. A combined silver electrode filled withKNO3 solution was employed in this study and proved its excellent applicability. Acetonewas chosen as the titration medium because it suppressed the solubility of silverchloride and offered high solubility for photoresists. Chloride concentrations as low as0.04&ug/ml was determined with good accuracy. Recovery study of chloride ion added tochloride-free acetone showed 92 to 103% recovery for 2,ug to 20,ug added chloride. Forthe case of spiked chloride less than 2Mg, recovery dropped dramatically and thus thismethod was not applicable. The accuracy of this method for photoresist analysis was provedby the recovery study of chloride ion spiked to photoresists and GR grade acetone(Fig.1).We found that chloride content in GR grade acetone (blank value) varied widely from0.002 to 0.12 pug/ml. Therefore, blank value must be determined before samples areanalyzed. Standard chloride should be added to make up the total amount of chloridein acetone to greater than 2ug in the case of low blank value. The detection limit ofthis method is dependent upon the precision of the measurements and is estimated as0.02~ug/ml (3 SD). The ratio of acetone to photoresist sample was adjusted according tothe viscosity and the chloride content of the photoresist. For photoresists of lowviscosity and low chloride content, lower ratio was a better choice. As shown inFig. 2, accurate results were obtainable at the ratio as low as 0.4 for typicalphotoresist samples.

TABLE 1. Instrument parameters for graphite furnace AAS.

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ANALYTICAL SCIENCES VOL. 7 SUPPLEMENT 1991 1267

Thethation

analytical results ofamples of different

contents.several commercial photoresists were shown

lot numbers sometimes showed quite largein TABLE 2.deviation

Weon

foundchloride

Metal element determination Commercial photoresists used in the fabrication of submicronVLSI were mixtures o p o osensitizer, binder, solvent, and additives if necessary. Thebinder used are mostly Novolac. The solvent can be xylene, ethoxyethyl acetate (EEA),propylene glycol methyl ether acetate (PGMEA) etc, or mixture of these. The dilutedsample contained a lower b.p. solvent (acetone, b.p. =53C) and a higher b.p solvent(xylene, EEA.., b.p. in the range of 100-150C). For drying process, a stepwise tem-perature program was carefully designed in accordance to the b.p. of solvents to ensureno spattering occurred. Fig. 3 shows that Novolac resists to thermal decompositionin inert atmosphere while it decomposes easily in air at 600C. Therefore, oxygen wereused as matrix modifier in the ashing step to eliminate carbonaceous build-up onplatform and in graphite tube. For some elements, such as zinc and sodium,background absorbance was largely reduced (Fig.4).

Fl g.1 Recovery study of chloride ionpiked to photoresists and GRrade acetone.

Fi g.2 The effect of the anaunt of photoresistsPR) to the analytical results. Voltme ofcetone:2D ml.

TABLE . Analytical results of chloride content i n conmerci al photoresists

Fi g.3 Thermal decomposition of ahotoresi st i n (a) air, andb) nitrogen atmosphereFi g. 4 Absarbance profiles for Na and Zn of 201 aliquot of a photoresist sair letomic absorbance profiles.ackground absorbance profiles.a) For Na, ashed in 0 ,b) For Na, ashed in A,c) For Zn, ashed i n 0 ,

d) For Zn, ashed in A.

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The effect of the background reduction is even greater for zinc because the completeoverlap of the background and atomic absorbance profiles. The complete expulsion ofoxygen with argon gas under low temperature before atomization was necessary inorder to avoid the excessive oxidation of graphite tubes. The optimum atomization tem-perature for sodium and iron were higher than those typically suggested in handbook asexpected from the mechanisms proposed by Salmon, Davis and Holcombe [11,12]. Theabsorbance profiles for zinc and iron dropped back to baselines within 5 secondswhile that of sodium showed a bad tailing. Huie and Curran [13] suggested that thiswas a result of strong interactions of sodium vapor with the furnace wall during thecourse of atom removal. Providing internal argon flow during atomization would reducethe tailling but at the same time lower the sensitivity. Nonlinear Curve of multiplestandard additions for sodium was obtained and thus could not be used for thedetermination. The analytical results of iron and zinc in several commercialphotoresists are shown in TABLE 3 and TABLE 4. Data obtained from standard addition bygraphite furnace AAS and dry ashing/flame AAS for both elements showed reasonableagreement. Iron content in PR013 and zinc content in PR014 determined by flame AAS weresuspected to be contaminated. It "s also shown in the tables that calibration curvemethod gave good results for iron but not for zinc. The averaged precision usinggraphite furnace AAS method was about 5 to 10% RSD while that using dry-aching/flame AASmethod varied widely from 10 to 20% RSD.

CONCLUSIONSPotentiometric argentometric titration using acetone as titration medium isbe an accurate method for the determination of trace chloride in photoresists.iron can be directly determined without predigestion by graphite furnaceoxygen as matrix modifier. The experimental conditions for sodium mustinvestigated in order to attain accurate results.

proved toinc andAAS using

be further

TABLE 3. Conpari son of anal yti cal results of iron i n photoresi sts .

TABLE 4. Con ari son of analytical results of zinc i n photoresi sts.

REFERENCES1. E. Sacher, IEEE Trans. Electr. Insul., El-18, 369 (1983).2. H,J.Neuhaus, D.R.Day and S.D.Senturia, J.. hElectr. Materials, 14, 379 (1985).3. K. Tomlinson and K.Torrance, Analyst, 102, 1 (1977). ^'4. H.V.Malmstadt and J.D.Winefordener, Analyti a Chim. Acta, 20, 283 (1959).5. R.J.Bertolacini and J.E.Barney ll , Anal. Chem., 30,202 (1958).6. ASTMD512-81, "Standard Test Methods for Chloride Ion in Water', ASTM, 11.01, 1988.7 T.Y.Chang and D.Wang MRLBull. Res. Dev., 4, 69 (1990).8. C.L.Chakarbarti, R.Karwowska, B.R. Hollebane and P.M.Johnson, Spectrochimic Acta,2B, 1217 (1987).9. 1T asaki, S.Hagi and H.Yamamoto, Sekiyu Gakkai Shi, 23, 210 (1980).10.L.A.Way and B.J.Presley, Anal. Chim. Acta, 83, 385 (T75).11.S.G.Salmon, R.H.Davis and J.Holcombe, J.AnaT: Chem. 53, 324 (1981).12 . S 0 . Salmon and J.Holcombe, Anal. Chem. 54, 630 (1982)13.C,W.Huie and C.J.Curran Jr., Appl. SpecTT, 44, 1329 (1990).