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Application of ion chromatography in the semiconductor industry L.E. Vanatta*
Air Liquide America, P.O. Box 650311 MS 301, Dallas, TX 75265 USA This review discusses ion-chromatographic advances in the semiconductor industry during the past 5 years. Through the development of more selective and sensitive instruments and columns, plus better attention to contamination control, the detection limits for many analyses are now in the low-ppt ( w / w ) range. The primary area of interest continues to be anion analysis, since spectroscopy remains the industry's preferred technique for quantifying cationic species. Noteworthy ion-chromatographic methods for analyzing semiconductor chemicals, gases, and solid samples are discussed and evaluated critically. Thirty-eight references are included. z2001 Elsevier Science B.V. All rights reserved. Keywords: Ion chromatography; Trace analysis; Semiconductor chemicals; Semiconductor gases; Semiconductor materials
1. Introduction In the semiconductor industry, one of the main concerns is possible contamination of the product during manufacturing. With the hundreds of steps that go into the manufacture of devices there are countless chances of introducing unwanted substances onto the wafer or device. The various process chemicals and gases, the room air, and the materials used to handle the product all may contain detrimental material. As geometries continue to shrink, it is no wonder that more and more substances are being tested to ensure they meet increasingly stringent speci¢cations. This review reports on such analyses that employ ion chromatography. The past 5 years are covered. The discussion has been structured to cover traceanalysis considerations, the analysis of chemicals ( deionized ( DI ) water, organic solvents, concen*Corresponding author. Tel.: +1 (972) 995-7541. E-mail: [email protected]
trated acids, and hydrogen peroxide ), the analysis of cleanroom air and process gases, and the analysis of wafers, devices, and materials. No attempt has been made to be exhaustive in the coverage. Examples that are cited are representative of the work being done in that area. Unless otherwise noted, Dionex Corp. was the manufacturer of all the speci¢c columns and modules mentioned in this paper.
2. Trace-analysis considerations In all of the analyses discussed below, control of contamination is a common theme. Most analytes are present in low-ppb ( w / w ) amount or less, so precautions must be taken in constructing chromatographic systems, and when handling samples and standards. Kaiser et al. [ 1 ], Sanders [ 2 ], and Vanatta [ 3 ] have addressed these considerations, as detailed below. DI water should be of the highest quality, having been passed through a suitable polishing station before use. When DI H2 O is used on-line to rinse columns and tubing loops, an ion-trap column should be used after the water pump to remove any residual contamination. Running blanks is a necessity at each start-up of the chromatograph. If carry-over is a problem, rinse procedures and additional blanks must be performed after a certain number of samples have been analyzed. Instrumental dead volumes should be kept to a minimum. When concentrator columns are employed, the loading system should be chosen carefully. A low-blank, maintenance-free approach is a pressurized reservoir that has a single piece of PEEK ( polyether ether ketone ) tubing going from the sample container to the load / inject valve. Sampling pumps can be used successfully, but achieving acceptable blanks can be dif¢cult and time-consuming. Vinyl gloves should be worn during all critical operations. Sample containers should be of high-
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quality plastic and should be rinsed thoroughly before use. In some instances, such bottles should be left soaking in DI water when not in use. Borosilicate glass is notoriously dirty with respect to ions and should be avoided. Transfer pipets cause contamination at low-ppb levels, so standard preparation at those levels should be accomplished by pouring and weighing accurately.
3. Analysis of chemicals 3.1. DI water Recently, Dionex Corp. introduced two products to improve the selectivity and sensitivity of ionchromatographic methods. These devices have had a large impact in the analysis of DI water, since speci¢cations for ions are now in the lowppt range. The IonPac AS15 [ 4 ] column is a highcapacity, hydroxide-selective separator for quantifying common anions and small organic acids in ultrapure water. All analytes are well separated from each other, thereby overcoming various resolution problems of other columns. The AS15 works well both with concentrator columns and with large-volume ( i.e., 1-ml ) sample loops. The main drawback to this column is that it, the conductivity cell, and the suppressor, must be heated in a special chamber for optimum stability. To achieve a baseline with very low noise, the EG40 Eluent Generator [ 5 ] was designed to produce carbonate-free KOH; only water is needed to operate the unit, which can deliver eluent either isocratically or for gradients. Kaiser et al. [ 6 ] analyzed DI water using the EG40 and large-loop injections with two 2-mm separators: the AS11 and the AS15. For both systems, seven common anions ( £uoride, chloride, nitrite, bromide, nitrate, sulfate, and phosphate ) and small organic acids ( acetate, formate, and oxalate ) were quanti¢ed in under 35 min at sub-ppb concentrations; in addition, glycolate was measured on the AS11. Detection limits ( in ppb ) were lower with the AS11, ranging from 0.008 for chloride to 0.04 for acetate; the range on the AS15 was 0.04 for £uoride to 0.20 for phosphate. (Note: unless otherwise indicated, detection limits were calculated by analyzing a blank or low-level standard in replicate and multiplying the standard deviation by the appropriate value of Student's t. ) However, as can be seen from Fig. 1a,b, the front-end resolution, plus the separation of bromide and nitrate, is better
with the AS15. Thus, the choice of columns will depend on the user's quality objectives. This study also compared two AS11 systems, one with an EG40 Eluent Generator and the other with manually prepared eluent. With the EG40, retentiontime stabilities were signi¢cantly better at the front end ( £uoride, acetate, and formate ); relative standard deviation values with the EG40 were 0.5%, but ranged from 1.5% to 4.7% with the conventional system. In addition, detection limits were higher without the EG40, rising to values between 0.02 ppb ( bromide ) and 0.10 ( phosphate ). Clearly, then, the AS11 and AS15 columns, plus the EG40, give the chromatographer improved capabilities in the analysis of anions in DI water. The sensitivity could be increased by using a concentrator column instead of a loop. However, the penalties would be the need for a pump or pneumatic system to load the sample, an increase in run time, and a larger carbonate peak. Vanatta et al. [ 7 ] developed a new technique for quantifying borate, which is monitored in DI water plants to determine when resins are close to exhaustion. The method was used to quantify borate in discrete samples, using a polyol-based concentrator ( IonPac TBC-1 ), an ion-exclusion column ( IonPac ICE-borate ), and an AMMS-ICE II MicroMembrane Suppressor. The eluent was a solution of methanesulfonic acid and mannitol, and the regenerant was a mixture of tetramethylammonium hydroxide and mannitol. Chromatograms for a 50-ppt boron standard and the accompanying blank are shown in Fig. 2a,b, respectively; in each case, 250 g of solution was concentrated, using an external pump at a £ow rate of 8 mL / min. The Hubaux^Vos detection limit was 25 ppt ( with false-negative and false-positive probabilities both held to 2.5%). This system provides a rapid, lessexpensive alternative to the typical spectroscopic analysis of boron. However, this ion-chromatographic system will not measure any boron that is not converted to the borate form. Other advances included research by Chollet et al. [ 8 ]. They compared DI-water results from two 4-mm columns: the AS4A-SC ( with a tetraborate gradient ) and the AS17 ( with an EG40-generated gradient ). Lower limits of detection could be obtained on the AS4A-SC for £uoride and chloride. In designing and building an ultrapure water system for bench-top use, Darbouret and Kano [ 9 ] monitored anion levels using ion chromatography. They were able to determine which processing
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Fig. 1. Trace-anion determination in DI water using: ( a ) the IonPac AS11 column and ( b ) the IonPac AS15 column. Reprinted from [ 6 ] with permission.
steps were critical to producing the desired water quality. In addition, they could evaluate various types of plastic parts for their suitability in a puri¢cation unit. Similarly, Johnson and Somerville [ 10 ] conducted an on-line study to select the proper polishing resins for their deionization system.
3.2. Organic solvents Although the purity of organic solvents is not as high as with DI water, the development of reliable testing procedures is often dif¢cult because of the sample matrices. Researchers have been meeting
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Fig. 2. ( a ) Chromatogram of a 50-ppt boron standard, where 250 g has been concentrated. ( b ) Chromatogram of the accompanying system blank ( 250 g concentrated ). Boron ( as borate ) is peak 1 in both cases. Reprinted from [ 7 ] with permission.
this challenge either by on-column matrix-elimination ( for water-miscible chemicals ) or by extraction procedures ( for water-immiscible solvents ). Kaiser and Rohrer [ 11 ] worked with isopropanol, acetone, and N-methylpyrrolidone, using a matrixelimination procedure. To increase sensitivity, they concentrated 5 mL of sample on an AG9-HC ( 4 mm ) column and performed the separation on a 2-mm AS9-HC system. The hardware schematic is shown in Fig. 3. After the sample loop was loaded, it was £ushed with DI water. This procedure moved the solvent onto the concentrator column and then washed the solvent off the resin, leaving any anions trapped on the concentrator. To achieve the desired separation, a Na2 CO3 /NaOH eluent was used with the separator. Because retention times of some analytes varied, depending on the solvent, quanti¢cation was performed using the method of standard
addition. Detection limits ( for isopropanol samples ) were less than 1 ppb for chloride, sulfate, phosphate, and nitrate. This procedure has the advantage of being sensitive and relatively rapid ( 35 to 40 min in total ) for water-miscible solvents. However, it does require extra valves and pumps, plus the more complicated quanti¢cation technique of standard addition. Sanders [ 2 ] developed an automated, robust, low-cost method for analyzing cations ( Li , Na , K , Mg2 , and Ca2 ) in three water-immiscible solvents: methyl n-amyl ketone, n-butyl acetate, and propylene glycol monomethyl ether acetate. The hardware schematic is shown in Fig. 4. Analytes were extracted in autosampler-compatible vials, using a solution of 0.6 mM acetic acid that contained 140 ppb of Rb internal standard. A Dionex CG12A concentrator column and heated ( 50³C ) CS12A
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Fig. 3. Matrix-elimination instrument con¢guration for determining anions in organic solvents. Valves are in the sample-loading position. Reprinted from [ 11 ] with permission.
Fig. 4. Valve-con¢guration diagram for determining cations in organic solvents. Valves are in the sample-loading position. Reprinted from [ 2 ] with permission.
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Fig. 5. Chromatographic-hardware con¢guration for trace-anion analysis in concentrated weak acids. Valves are in the sampleloading position. Reprinted from [ 1 ] with permission.
analytical column were employed to analyze 2 mL of extract. The working range of the calibration curve was 1 to 20 ppb; detection limits were between 0.2 and 1.0 ppb. For sodium, chromatographic results compared well with those from inductively coupled plasma mass spectrometry. This protocol is noteworthy for its automation and durability. Others who have worked recently in this ¢eld include Viehweger et al. [ 12 ], who used the concentrating / rinsing approach on a variety of solvents. However, they automatically changed the size of the sampling loop ( for loading the concentrator column ), depending on the sensitivity needed for the particular chemical. Also, Wang et al. [ 13 ] developed anion procedures for some 16 solvents of both types of water solubility. They quanti¢ed F3, NO2 3 , Cl3 , Br3, NO3 3 , SO4 23 , and PO4 33 , with detection limits in the low- to sub-ppb range.
3.3. Concentrated acids One of the most dif¢cult projects has been the development of methods for the determination of anionic impurities in concentrated acids. The overwhelming presence of the acid's anion must be addressed, or this species will interfere with the detection of the analytes. Recently, progress has been made in this area, especially for weak acids. Kaiser et al. [ 1 ] studied hydro£uoric, glycolic, and phosphoric acids. The ¢rst was diluted 1:1, the second was diluted 1:1000, and the third was
left in concentrated form. Methods were developed for Cl3 , NO3 3 , SO4 23 , and PO4 33 , using an ionexclusion column ( ICE-AS6 ) to separate the acid's anion from the unretained analytes. The ions of interest were collected on a concentrator column and then chromatographed on an analytical column. An AS9 column ( with Na2 CO3 /NaOH eluent ) was used for the ¢rst two analyses; an AS11 column ( with NaOH eluent ) was used for phosphoric acid. The plumbing diagram for this procedure is shown in Fig. 5. Detection limits for the acids ( in the dilution used in the analysis ) were between 0.1 and 30 ppb in all cases. Spiking studies yielded recoveries between 84 and 111%. This method, then, achieves quanti¢cation in 45 to 60 min, about the minimum time that can be expected for this type of matrix. Extra valves and columns, plus optimization of valve timing, are also needed. Robust solutions to the problem of determination of anions in strong acids remain elusive. Since these chemicals ionize completely, the dominant ion cannot be removed by ion exclusion. Kaiser et al. [ 14 ] developed a method for 0.7% HNO3 , using an AS15 column and an EG40 Eluent Generator. Their gradient program began at 48 mM KOH, a concentration that resolved £uoride, chloride, sulfate, and phosphate; at this eluent strength, nitrate was retained on the column. Subsequently, the eluent concentration was increased to 100 mM to remove the excess of nitrate ions. Run times were approximately 1 h and detection limits were 41, 104, and 120 ppb for chloride, sulfate, and phosphate, respectively. Spike recoveries for these three ions
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were 86, 109, and 102%, respectively. Quanti¢cation by standard addition was used to deal with matrix effects. This method is a step in the right direction, but still is not capable of analyzing the 70% nitric acid used in the semiconductor industry. Wahl [ 15 ] also investigated the strong-acid issue. A variety of columns ( AS10 and AS11 ) and detectors ( conductivity, UV, amperometric ) were used to quantify common anions in HCl, HNO3 , and H2 SO4 . Sample dilution was necessary and sample pretreatment was employed in some cases to remove the matrix anion. Anion contamination ( speci¢cally, chloride and sulfate ) is a concern in the copper / H2 SO4 baths now being used in processing. Palmans and Vervoort [ 16 ] developed a method for these two analytes, using an AS12A column. For chloride quanti¢cation, a masking technique was used to remove interfering copper ions. Ion chromatography has also been used to assay semiconductor etches, which are mixtures of concentrated acids. Vanatta and Coleman [ 17 ] compared the AS11 and the AS16 columns for quantifying the composition of HF / CrO3 / HNO3 etches. Both columns displayed the necessary precision and were able to withstand hundreds of injections. Although the peak shapes were better on the AS16, the þ prediction intervals were slightly better for all anions on the AS11.
3.4. Hydrogen peroxide Traditional methods for determining anions in 30 to 35% H2 O2 involve decomposing the chemical, usually with UV light or platinum. This technique is prone to contamination and loss of analyte. In a detailed, thorough study, Kim et al. [ 18 ] dealt with these problems by designing an on-line decomposer. The device was made from gas-permeable Gore-Tex membrane tubing and ¢lled with Pt catalyst. A water-cooling jacket was added to control the reaction. After evaluating several forms of Pt ( e.g., on activated carbon, on glass beads, in wire form ), it was found that a three-section decomposer was the best, with the sections containing 0.25-mm Pt wire, 0.125-mm Pt wire, and Pt-coated silica gel, respectively. This decomposer brought the peroxide level down to less than 0.01% at £ow rates up to 0.4 mL / min and was not susceptible to contamination. Post-reaction analyses were performed on an AS14 column with suppressed-conductivity detection. Acceptable recoveries were
achieved for the anions tested ( chloride, nitrate, and sulfate ). Detection limits were between 2 and 4 ppb. The only major omission in this study was a discussion of any clean-up procedures that might be needed for the catalyst, either in the short or long term. Also, for those who prefer to buy all analytical equipment from recognized vendors, this decomposer is not available commercially. Advances have been made in analyzing peroxide directly, with Collard et al. [ 19 ] using an on-line matrix-elimination approach. The analyte anions were trapped on a TAC-LP1 concentrator column, which was then washed with water to remove the peroxide. Samples were loaded with an autosampler and analyses were performed on an AS15, in the 4-mm format. Vanatta and Coleman [ 20 ] expanded on the AS15 research by working with the 2-mm version of the column. They also studied the microbore AS11. The same sample-introduction and matrix-elimination techniques were used. With these smaller-diameter separators, the retention times of all analytes ( £uoride, chloride, bromide, nitrate, sulfate, and phosphate ) decreased as more peroxide samples were tested. However, peak areas remained stable, as did within-day retention times for each analyte. Hubaux^Vos detection limits ( with false-negative and false-positive probabilities both held to 2.5%) were 0.2 to 2.2 ppb on the AS11, and 1.1 to 3.0 on the AS15. This microbore method has the advantage of increased sensitivity, but is hampered by the gradual shifts in retention time. Statistically, the results are quite reliable. Some work has been done in the cation area. These analytes were investigated by Jensen et al. [ 21 ], using a CG12A concentrator column and CS10 separator. The concentrator was £ushed with water before the analysis was begun. They reported determinations in the single-digit-ppb range for lithium, sodium, ammonium, and potassium ions.
4. Analysis of cleanroom air and process gases The study of ionic contamination in cleanroom air has been neglected until recently, when researchers began to realize that such species could be detrimental to product yields. Lue et al. spent considerable time investigating the best way to collect these species for analysis. Their initial work involved anions ( Cl3 , NO3 3 , SO4 23 , and
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F3 ) [ 22 ]. Glass impingers ¢lled with a trapping solution ( carbonate / bicarbonate eluent, DI water, or 0.1 M NaOH ) were tried initially, with the air sample's being pumped through these devices at 0.2 L / min for 24 h. However, £uoride was not trapped well, presumably because this analyte adhered to the glass. Tubes ¢lled with silica-gel were found to be the answer to this issue. These devices had two chambers, each ¢lled with adsorbent. The above pumping protocol was used again. Subsequently, each silica-gel portion was extracted separately by placing it in a centrifuge tube with 10 mL of eluent and heating the mixture for 10 min in a boilingwater bath. Analysis of the extraction solution was performed using an AS4A-SC column and suppressed-conductivity detection. For all analytes, the ¢rst section of silica gel was found to contain all the extractable anions. These sampling tubes had the advantages of being inexpensive and readily available, convenient for sampling in small spaces, easy to transport, and suitable for storage after sample collection. This approach has potential, but the paper leaves some questions unanswered. First, no discussion of silica-gel blanks was included. Second, possible loss of analyte during the 100-degree extraction was not investigated. Third, no spiking studies were conducted to see if the silica gel trapped all anions quantitatively from air. Fourth, the AS4A-SC is not the best column for quantifying £uoride, since that anion is eluted very close to the water dip. Separators such as the AS11 or AS15 are better alternatives. If these issues could be resolved successfully, this procedure would be very useful, although the sampling time is quite lengthy. Lue and co-workers published a similar study [ 23 ] that addressed cations (Na , NH4 , K , Mg2 , and Ca2 ) in air. Analytes were removed from acid-treated silica gel with DI water and analyzed on a CS15 column. The procedure has the same advantages as above. Again, blanks were not mentioned and spiking studies were not performed. Other researchers sought to improve the speed and ef¢ciency of air-analysis methods. Watanabe and Sekiguchi [ 24 ] used acid or alkaline solutions to trap analytes more effectively, and then neutralized the liquids via electrodialysis before chromatographing the samples. Mayama et al. [ 25 ] designed a high-speed Automatic Air Sampler to reduce the collection time necessary. Besides cleanroom air, process gases are also becoming a target of anion analysis. Brzychcy et
al. [ 26 ] have done extensive, thorough methoddevelopment work on anions ( F3, Cl3 , Br3, SO4 23 ) and ammonia in various semiconductor gases ( HBr, HI, N2 O, SiH4 , SiF4, SF6 ). Two different sampling approaches were used, depending on the gas involved and the contamination levels involved. Whenever possible, a `bubble method' was used. Gas was bubbled through a series of impingers that contained the desired collection £uid. The £ow was metered to no more than 1 L / min and the volume of gas tested was determined either by a meter or by monitoring the weight of the gas cylinder. With some gases or when small sample sizes were needed, a `bulb method' was employed. This approach used a bulb ( of known volume ) that had two stopcocks and a septum-covered port. A sample of gas at atmospheric pressure was trapped in the bulb and dissolution of the analytes was accomplished by injecting aliquots of trapping liquid through the septum. Details were given on precautions for assembling the apparatus and handling the various gases, and on how to purge the vessels and check blanks before collecting samples. Also included were instructions for performing the various calculations. For known gas mixtures, extraction ef¢ciencies and sampling reproducibilities were reported as well. When possible, chromatographic results were compared with alternative procedures ( e.g., wet methods or spectroscopy ). Lastly, a small section of the paper discussed how to assay a water-soluble gas ( H2 Se or NH3 or HCl ) in an inert balance gas. The only major drawback of the report is the lack of information on the speci¢c analytical column used for the analyses. Additional work with process gases has been conducted recently. Ruimei [ 27 ] built a membrane-¢ltration device to trap anions ( Cl3 , NO3 3 , SO4 23 , PO4 33 , and F3 ) from a variety of compressed gases.
5. Analysis of wafers, devices, and materials Researchers have been utilizing ion chromatography more and more to characterize contamination on solid surfaces. The ultimate goal, of course, is to prevent anions from reaching wafers and devices. Yanagi and colleagues [ 28 ] built a device that has ingeniously solved a wafer-analysis problem: how to extract the ions on each side of the wafer sepa-
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rately. The technique used commercially available polypropylene bags that are manufactured for device extractions. The sample was placed in the bag as usual. Then the halves of a `sandwich-type' tool were placed around the wafer / bag. Each side of the tool had an O-ring ( of diameter equal to that of the wafer ), which pressed against the bag. When the tool was clamped together securely, the O-rings sealed off each side of the wafer from the rest of the bag. At the top, the O-ring had a small open section, thereby allowing the introduction of DI water into each chamber. After the wafers were extracted in a temperature-controlled bath, the solutions were removed from each side of the wafer and analyzed by ion chromatography. A TAC-2 concentrator column and AS12A separator were used with Na2 CO3 / NaHCO3 eluent to quantify anions; a TCC-LP1 and CS12A with methanesulfonic-acid eluent were employed to measure cations. This research also reported studies on extraction-time optimization and the integrity of the O-ring seal. The main drawback to the protocol is that the O-ring tool is not commercially available. Thompson et al. [ 29 ] addressed the problem of measuring contaminants on very small devices. They developed a microextraction procedure to quantify anions on thin-¢lm sliders for hard disk drives. The cell and its O-ring were made of PEEK and were sized to contain solids with surface areas less than 0.1 cm2 . The cell contents were extracted on-line and then concentrated onto an AMC-1 column, which was used in tandem with an AS12A ( 2-mm ) separator. Detection limits ranged from ca. 2 to ca. 21 ng / cm2 for the various analytes. Comparison with microscope and X-ray data gave complementary results. Two main limitations exist with this technique. First, the cell is not commercially available. Second, the AS12A may not be the best separator for low-level work. Increased sensitivity and better front-end resolution probably could be achieved with an AS11 or AS15 column. Jarvis [ 30 ] investigated cations, developing a reliable method to determine the ammonium content in adhesives. Samples were extracted by heating them for 5 h in a vessel that was purged with nitrogen gas. The gas that exited from the chamber was sent to impinger tubes that contained 20 mL of DI water. The ammonium ions in the solution were analyzed using a TCC-LP1 column to concentrate the analyte and a CS12A ( 2-mm ) separator to perform the chromatography. For the 20-mL sample
size, the detection limit was 500 mg of NH4 . Data from these studies were used to work with vendors in setting speci¢cations for the adhesives. Other researchers have been active in this area. Tan et al. [ 31 ] have used the technique to quantify both anions and cations on silicon wafers. Munson [ 32 ] has applied the instrumentation to conduct failure analysis on devices at all levels of their manufacturing process. Bombien and Bahten [ 33 ] have evaluated polyvinyl-alcohol sponge material ( used to clean wafers and disks ) and gloves for their ionic content. Jiranut [ 34 ] studied the formate level in the electro-deposition coating of hard disk-drive components. Ju et al. [ 35 ] constructed a special scanning tool and used a glove box to improve sensitivity of ionic determinations on wafers and glasses. Bhattacharjee and Atterbury [ 36 ] utilized both ion chromatography and capillary electrophoresis to quantify anions and cations in cleanroom wipers. Finally, two ion-chromatographic studies have been performed to assess the ions that are leached from PVDF resins. Henley [ 37 ] looked at anions in both plastic and stainless-steel. Dennis and Seiler [ 38 ] quanti¢ed both anions and cations in PVDF resins.
6. Conclusion In the past 5 years, ion chromatography has increased in usefulness in detecting ionic levels in semiconductor-related solids, liquids, and gases. No longer is the technique used simply to verify the cleanliness of DI water. Improvements in column ef¢ciency and eluent cleanliness have resulted in lower detection limits. More stable column resins and better concentrator columns have meant that challenging matrices such as corrosive and oxidative chemicals, small parts, and gases are now being examined. Contamination control will continue to be a major challenge in the semiconductor industry, and ion chromatography no doubt will remain a signi¢cant technique for meeting that goal.
Acknowledgements The author would like to acknowledge Bev Newton of Dionex Corp. for her assistance is searching the literature; Dan Cowles and Todd Baldwin of Air Liquide America for their helpful discussions regarding spike-recovery studies with gases.
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References [ 1 ] E. Kaiser, J.S. Rohrer, K. Watanabe, J. Chromatogr. A 850 ( 1999 ) 167. [ 2 ] J.K. Sanders, J. Chromatogr. A 804 ( 1998 ) 193. [ 3 ] L.E. Vanatta, J. Chromatogr. A 739 ( 1996 ) 199. [ 4 ] Dionex Product Information Bulletin, LPN 1000-02 4M, July 1999. [ 5 ] Dionex Product Information Bulletin, LPN 0506-04 7M, June 1998, p. 4. [ 6 ] E. Kaiser, J. Rohrer, Y. Liu, presented at the International Ion Chromatography Symposium, paper 146, Nice, France 2000. [ 7 ] L.E. Vanatta, D.E. Coleman, R.W. Slingsby, J. Chromatogr. A 850 ( 1999 ) 107. [ 8 ] C. Chollet, L. Doyen, T. Hazard, M. Carre, J. Dollet, presented at the International Ion Chromatography Symposium, paper 51, Nice, 2000. [ 9 ] D. Darbouret, I. Kano, presented at the International Ion Chromatography Symposium, paper 50, Nice, France, 2000. [ 10 ] E. Johnson, K. Somerville, presented at the International Ion Chromatography Symposium, paper 90, San Jose, CA, 1999. [ 11 ] E. Kaiser, J. Rohrer, J. Chromatogr. A 858 ( 1999 ) 55. [ 12 ] K. Viehweger, H. Schaëfer, P. Walter, U. Waldburger, presented at the International Ion Chromatography Symposium, paper 88, San Jose, CA, 1999. [ 13 ] K. Wang, R. Wang, J. Kuang, S. Tan, presented at the International Ion Chromatography Symposium, paper 15, San Jose, CA, 1999. [ 14 ] E. Kaiser, J. Rohrer, R. Kiser, presented at the International Ion Chromatography Symposium, paper 145, Nice, France, 2000. [ 15 ] F. Wahl, presented at the International Ion Chromatography Symposium, paper 46, Nice, France, 2000. [ 16 ] R. Palmans, I. Vervoort, presented at the International Ion Chromatography Symposium, paper 150, Nice, France, 2000. [ 17 ] L.E. Vanatta, D.E. Coleman, J. Chromatogr. A 884 ( 2000 ) 151. [ 18 ] D.-H. Kim, B.-K. Lee, D.S. Lee, Bull. Korean Chem. Soc. 20 ( 1999 ) 696. [ 19 ] J. Collard, T. Harris, C. Costanza, B. Newton, presented at the International Ion Chromatography Symposium, paper 92, San Jose, CA, 1999. [ 20 ] L.E. Vanatta, D.E. Coleman, J. Chromatogr. A, in press. [ 21 ] D. Jensen, J. Kerth, C. Rattmann, presented at the International Ion Chromatography Symposium, paper 148, Nice, France, 2000. [ 22 ] S.J. Lue, T. Wu, H. Hsu, C. Huang, J. Chromatogr. A 804 ( 1998 ) 273. [ 23 ] S.J. Lue, C. Huang, J. Chromatogr. A 850 ( 1999 ) 283.
[ 24 ] K. Watanabe, Y. Sekiguchi, presented at the International Ion Chromatography Symposium, paper 37, Osaka, Japan, 1998. [ 25 ] K. Mayama, M. Maki, S. Sakai, presented at the International Ion Chromatography Symposium, paper 115, Osaka, Japan, 1998. [ 26 ] A.M. Brzychcy, B.P. Twombly, L. Zhang, in: J.D. Hogan ( Editor ), Specialty Gas Analysis. A Practical Guidebook, Ch. 7, Wiley, New York, 1997. [ 27 ] W. Ruimei, J. Chromatogr. Sci. 36 ( 1998 ) 579. [ 28 ] K. Yanagi, H. Shibata, K. Nagai, M. Watanabe, Electrochem. Soc. Proc. Vol. 2000-17 ( 2000 ) 561. [ 29 ] J. Thompson, T. Prommanuwat, A. Siriraks, S. Heberling, presented at the International Ion Chromatography Symposium, paper 13, San Jose, CA, 1999. [ 30 ] M. Jarvis, presented at the International Ion Chromatography Symposium, paper 47, Nice, France, 2000. [ 31 ] S. Tan, R. Liu, H. La, presented at the International Ion Chromatography Symposium, paper 46, Santa Clara, CA, 1997. [ 32 ] T. Munson, presented at the International Ion Chromatography Symposium, paper 14, San Jose, CA, 1999. [ 33 ] C. Bombien, K. Bahten, presented at the International Ion Chromatography Symposium, paper 87, San Jose, CA, 1999. [ 34 ] K. Jiranut, presented at the International Ion Chromatography Symposium, paper 113, Osaka, Japan, 1998. [ 35 ] J.-H. Ju, S.-C. Kang, S.-M. Chon, presented at the International Ion Chromatography Symposium, paper 44, Santa Clara, CA, 1997. [ 36 ] H.R. Bhattacharjee, O. Atterbury, presented at the International Ion Chromatography Symposium, paper 49, Nice, France, 2000. [ 37 ] M. Henley, Ultrapure Water 14 ( 10 ) ( 1997 ) 16. [ 38 ] G.M. Dennis, D.A. Seiler, Ultrapure Water 16 ( 10 ) ( 1999 ) 27. Lynn Vanatta is the manager of chromatography research and statistics at Air Liquide America's Dallas Technical Center, where she has worked for 9 years and authored 13 publications. She holds a bachelor's degree in chemistry from Indiana University / Bloomington. Her research focuses on statistically sound anion methods for analyzing semiconductor-grade water and chemicals. Ms. Vanatta is a member of the American Chemical Society, the American Society for Mass Spectrometry, the American Statistical Association, and the American Society for Testing and Materials. Within this last organization, she is chairman of a committee on precision and bias, and also a De¢nitions Advisor. She is active in the International Ion Chromatography Symposium, where she organizes an annual session for the semiconductor / pure-chemicals industry, teaches a short course in statistics, and is the Program Chairman for the 2001 meeting. In addition, she co-instructs a detection / quantitation short course at the Pittsburgh Conference.
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Application of ion chromatography in the semiconductor industry L.E. Vanatta*
Air Liquide America, P.O. Box 650311 MS 301, Dallas, TX 75265 USA This review discusses ion-chromatographic advances in the semiconductor industry during the past 5 years. Through the development of more selective and sensitive instruments and columns, plus better attention to contamination control, the detection limits for many analyses are now in the low-ppt ( w / w ) range. The primary area of interest continues to be anion analysis, since spectroscopy remains the industry's preferred technique for quantifying cationic species. Noteworthy ion-chromatographic methods for analyzing semiconductor chemicals, gases, and solid samples are discussed and evaluated critically. Thirty-eight references are included. z2001 Elsevier Science B.V. All rights reserved. Keywords: Ion chromatography; Trace analysis; Semiconductor chemicals; Semiconductor gases; Semiconductor materials
1. Introduction In the semiconductor industry, one of the main concerns is possible contamination of the product during manufacturing. With the hundreds of steps that go into the manufacture of devices there are countless chances of introducing unwanted substances onto the wafer or device. The various process chemicals and gases, the room air, and the materials used to handle the product all may contain detrimental material. As geometries continue to shrink, it is no wonder that more and more substances are being tested to ensure they meet increasingly stringent speci¢cations. This review reports on such analyses that employ ion chromatography. The past 5 years are covered. The discussion has been structured to cover traceanalysis considerations, the analysis of chemicals ( deionized ( DI ) water, organic solvents, concen*Corresponding author. Tel.: +1 (972) 995-7541. E-mail: [email protected]
trated acids, and hydrogen peroxide ), the analysis of cleanroom air and process gases, and the analysis of wafers, devices, and materials. No attempt has been made to be exhaustive in the coverage. Examples that are cited are representative of the work being done in that area. Unless otherwise noted, Dionex Corp. was the manufacturer of all the speci¢c columns and modules mentioned in this paper.
2. Trace-analysis considerations In all of the analyses discussed below, control of contamination is a common theme. Most analytes are present in low-ppb ( w / w ) amount or less, so precautions must be taken in constructing chromatographic systems, and when handling samples and standards. Kaiser et al. [ 1 ], Sanders [ 2 ], and Vanatta [ 3 ] have addressed these considerations, as detailed below. DI water should be of the highest quality, having been passed through a suitable polishing station before use. When DI H2 O is used on-line to rinse columns and tubing loops, an ion-trap column should be used after the water pump to remove any residual contamination. Running blanks is a necessity at each start-up of the chromatograph. If carry-over is a problem, rinse procedures and additional blanks must be performed after a certain number of samples have been analyzed. Instrumental dead volumes should be kept to a minimum. When concentrator columns are employed, the loading system should be chosen carefully. A low-blank, maintenance-free approach is a pressurized reservoir that has a single piece of PEEK ( polyether ether ketone ) tubing going from the sample container to the load / inject valve. Sampling pumps can be used successfully, but achieving acceptable blanks can be dif¢cult and time-consuming. Vinyl gloves should be worn during all critical operations. Sample containers should be of high-
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quality plastic and should be rinsed thoroughly before use. In some instances, such bottles should be left soaking in DI water when not in use. Borosilicate glass is notoriously dirty with respect to ions and should be avoided. Transfer pipets cause contamination at low-ppb levels, so standard preparation at those levels should be accomplished by pouring and weighing accurately.
3. Analysis of chemicals 3.1. DI water Recently, Dionex Corp. introduced two products to improve the selectivity and sensitivity of ionchromatographic methods. These devices have had a large impact in the analysis of DI water, since speci¢cations for ions are now in the lowppt range. The IonPac AS15 [ 4 ] column is a highcapacity, hydroxide-selective separator for quantifying common anions and small organic acids in ultrapure water. All analytes are well separated from each other, thereby overcoming various resolution problems of other columns. The AS15 works well both with concentrator columns and with large-volume ( i.e., 1-ml ) sample loops. The main drawback to this column is that it, the conductivity cell, and the suppressor, must be heated in a special chamber for optimum stability. To achieve a baseline with very low noise, the EG40 Eluent Generator [ 5 ] was designed to produce carbonate-free KOH; only water is needed to operate the unit, which can deliver eluent either isocratically or for gradients. Kaiser et al. [ 6 ] analyzed DI water using the EG40 and large-loop injections with two 2-mm separators: the AS11 and the AS15. For both systems, seven common anions ( £uoride, chloride, nitrite, bromide, nitrate, sulfate, and phosphate ) and small organic acids ( acetate, formate, and oxalate ) were quanti¢ed in under 35 min at sub-ppb concentrations; in addition, glycolate was measured on the AS11. Detection limits ( in ppb ) were lower with the AS11, ranging from 0.008 for chloride to 0.04 for acetate; the range on the AS15 was 0.04 for £uoride to 0.20 for phosphate. (Note: unless otherwise indicated, detection limits were calculated by analyzing a blank or low-level standard in replicate and multiplying the standard deviation by the appropriate value of Student's t. ) However, as can be seen from Fig. 1a,b, the front-end resolution, plus the separation of bromide and nitrate, is better
with the AS15. Thus, the choice of columns will depend on the user's quality objectives. This study also compared two AS11 systems, one with an EG40 Eluent Generator and the other with manually prepared eluent. With the EG40, retentiontime stabilities were signi¢cantly better at the front end ( £uoride, acetate, and formate ); relative standard deviation values with the EG40 were 0.5%, but ranged from 1.5% to 4.7% with the conventional system. In addition, detection limits were higher without the EG40, rising to values between 0.02 ppb ( bromide ) and 0.10 ( phosphate ). Clearly, then, the AS11 and AS15 columns, plus the EG40, give the chromatographer improved capabilities in the analysis of anions in DI water. The sensitivity could be increased by using a concentrator column instead of a loop. However, the penalties would be the need for a pump or pneumatic system to load the sample, an increase in run time, and a larger carbonate peak. Vanatta et al. [ 7 ] developed a new technique for quantifying borate, which is monitored in DI water plants to determine when resins are close to exhaustion. The method was used to quantify borate in discrete samples, using a polyol-based concentrator ( IonPac TBC-1 ), an ion-exclusion column ( IonPac ICE-borate ), and an AMMS-ICE II MicroMembrane Suppressor. The eluent was a solution of methanesulfonic acid and mannitol, and the regenerant was a mixture of tetramethylammonium hydroxide and mannitol. Chromatograms for a 50-ppt boron standard and the accompanying blank are shown in Fig. 2a,b, respectively; in each case, 250 g of solution was concentrated, using an external pump at a £ow rate of 8 mL / min. The Hubaux^Vos detection limit was 25 ppt ( with false-negative and false-positive probabilities both held to 2.5%). This system provides a rapid, lessexpensive alternative to the typical spectroscopic analysis of boron. However, this ion-chromatographic system will not measure any boron that is not converted to the borate form. Other advances included research by Chollet et al. [ 8 ]. They compared DI-water results from two 4-mm columns: the AS4A-SC ( with a tetraborate gradient ) and the AS17 ( with an EG40-generated gradient ). Lower limits of detection could be obtained on the AS4A-SC for £uoride and chloride. In designing and building an ultrapure water system for bench-top use, Darbouret and Kano [ 9 ] monitored anion levels using ion chromatography. They were able to determine which processing
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Fig. 1. Trace-anion determination in DI water using: ( a ) the IonPac AS11 column and ( b ) the IonPac AS15 column. Reprinted from [ 6 ] with permission.
steps were critical to producing the desired water quality. In addition, they could evaluate various types of plastic parts for their suitability in a puri¢cation unit. Similarly, Johnson and Somerville [ 10 ] conducted an on-line study to select the proper polishing resins for their deionization system.
3.2. Organic solvents Although the purity of organic solvents is not as high as with DI water, the development of reliable testing procedures is often dif¢cult because of the sample matrices. Researchers have been meeting
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Fig. 2. ( a ) Chromatogram of a 50-ppt boron standard, where 250 g has been concentrated. ( b ) Chromatogram of the accompanying system blank ( 250 g concentrated ). Boron ( as borate ) is peak 1 in both cases. Reprinted from [ 7 ] with permission.
this challenge either by on-column matrix-elimination ( for water-miscible chemicals ) or by extraction procedures ( for water-immiscible solvents ). Kaiser and Rohrer [ 11 ] worked with isopropanol, acetone, and N-methylpyrrolidone, using a matrixelimination procedure. To increase sensitivity, they concentrated 5 mL of sample on an AG9-HC ( 4 mm ) column and performed the separation on a 2-mm AS9-HC system. The hardware schematic is shown in Fig. 3. After the sample loop was loaded, it was £ushed with DI water. This procedure moved the solvent onto the concentrator column and then washed the solvent off the resin, leaving any anions trapped on the concentrator. To achieve the desired separation, a Na2 CO3 /NaOH eluent was used with the separator. Because retention times of some analytes varied, depending on the solvent, quanti¢cation was performed using the method of standard
addition. Detection limits ( for isopropanol samples ) were less than 1 ppb for chloride, sulfate, phosphate, and nitrate. This procedure has the advantage of being sensitive and relatively rapid ( 35 to 40 min in total ) for water-miscible solvents. However, it does require extra valves and pumps, plus the more complicated quanti¢cation technique of standard addition. Sanders [ 2 ] developed an automated, robust, low-cost method for analyzing cations ( Li , Na , K , Mg2 , and Ca2 ) in three water-immiscible solvents: methyl n-amyl ketone, n-butyl acetate, and propylene glycol monomethyl ether acetate. The hardware schematic is shown in Fig. 4. Analytes were extracted in autosampler-compatible vials, using a solution of 0.6 mM acetic acid that contained 140 ppb of Rb internal standard. A Dionex CG12A concentrator column and heated ( 50³C ) CS12A
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Fig. 3. Matrix-elimination instrument con¢guration for determining anions in organic solvents. Valves are in the sample-loading position. Reprinted from [ 11 ] with permission.
Fig. 4. Valve-con¢guration diagram for determining cations in organic solvents. Valves are in the sample-loading position. Reprinted from [ 2 ] with permission.
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Fig. 5. Chromatographic-hardware con¢guration for trace-anion analysis in concentrated weak acids. Valves are in the sampleloading position. Reprinted from [ 1 ] with permission.
analytical column were employed to analyze 2 mL of extract. The working range of the calibration curve was 1 to 20 ppb; detection limits were between 0.2 and 1.0 ppb. For sodium, chromatographic results compared well with those from inductively coupled plasma mass spectrometry. This protocol is noteworthy for its automation and durability. Others who have worked recently in this ¢eld include Viehweger et al. [ 12 ], who used the concentrating / rinsing approach on a variety of solvents. However, they automatically changed the size of the sampling loop ( for loading the concentrator column ), depending on the sensitivity needed for the particular chemical. Also, Wang et al. [ 13 ] developed anion procedures for some 16 solvents of both types of water solubility. They quanti¢ed F3, NO2 3 , Cl3 , Br3, NO3 3 , SO4 23 , and PO4 33 , with detection limits in the low- to sub-ppb range.
3.3. Concentrated acids One of the most dif¢cult projects has been the development of methods for the determination of anionic impurities in concentrated acids. The overwhelming presence of the acid's anion must be addressed, or this species will interfere with the detection of the analytes. Recently, progress has been made in this area, especially for weak acids. Kaiser et al. [ 1 ] studied hydro£uoric, glycolic, and phosphoric acids. The ¢rst was diluted 1:1, the second was diluted 1:1000, and the third was
left in concentrated form. Methods were developed for Cl3 , NO3 3 , SO4 23 , and PO4 33 , using an ionexclusion column ( ICE-AS6 ) to separate the acid's anion from the unretained analytes. The ions of interest were collected on a concentrator column and then chromatographed on an analytical column. An AS9 column ( with Na2 CO3 /NaOH eluent ) was used for the ¢rst two analyses; an AS11 column ( with NaOH eluent ) was used for phosphoric acid. The plumbing diagram for this procedure is shown in Fig. 5. Detection limits for the acids ( in the dilution used in the analysis ) were between 0.1 and 30 ppb in all cases. Spiking studies yielded recoveries between 84 and 111%. This method, then, achieves quanti¢cation in 45 to 60 min, about the minimum time that can be expected for this type of matrix. Extra valves and columns, plus optimization of valve timing, are also needed. Robust solutions to the problem of determination of anions in strong acids remain elusive. Since these chemicals ionize completely, the dominant ion cannot be removed by ion exclusion. Kaiser et al. [ 14 ] developed a method for 0.7% HNO3 , using an AS15 column and an EG40 Eluent Generator. Their gradient program began at 48 mM KOH, a concentration that resolved £uoride, chloride, sulfate, and phosphate; at this eluent strength, nitrate was retained on the column. Subsequently, the eluent concentration was increased to 100 mM to remove the excess of nitrate ions. Run times were approximately 1 h and detection limits were 41, 104, and 120 ppb for chloride, sulfate, and phosphate, respectively. Spike recoveries for these three ions
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were 86, 109, and 102%, respectively. Quanti¢cation by standard addition was used to deal with matrix effects. This method is a step in the right direction, but still is not capable of analyzing the 70% nitric acid used in the semiconductor industry. Wahl [ 15 ] also investigated the strong-acid issue. A variety of columns ( AS10 and AS11 ) and detectors ( conductivity, UV, amperometric ) were used to quantify common anions in HCl, HNO3 , and H2 SO4 . Sample dilution was necessary and sample pretreatment was employed in some cases to remove the matrix anion. Anion contamination ( speci¢cally, chloride and sulfate ) is a concern in the copper / H2 SO4 baths now being used in processing. Palmans and Vervoort [ 16 ] developed a method for these two analytes, using an AS12A column. For chloride quanti¢cation, a masking technique was used to remove interfering copper ions. Ion chromatography has also been used to assay semiconductor etches, which are mixtures of concentrated acids. Vanatta and Coleman [ 17 ] compared the AS11 and the AS16 columns for quantifying the composition of HF / CrO3 / HNO3 etches. Both columns displayed the necessary precision and were able to withstand hundreds of injections. Although the peak shapes were better on the AS16, the þ prediction intervals were slightly better for all anions on the AS11.
3.4. Hydrogen peroxide Traditional methods for determining anions in 30 to 35% H2 O2 involve decomposing the chemical, usually with UV light or platinum. This technique is prone to contamination and loss of analyte. In a detailed, thorough study, Kim et al. [ 18 ] dealt with these problems by designing an on-line decomposer. The device was made from gas-permeable Gore-Tex membrane tubing and ¢lled with Pt catalyst. A water-cooling jacket was added to control the reaction. After evaluating several forms of Pt ( e.g., on activated carbon, on glass beads, in wire form ), it was found that a three-section decomposer was the best, with the sections containing 0.25-mm Pt wire, 0.125-mm Pt wire, and Pt-coated silica gel, respectively. This decomposer brought the peroxide level down to less than 0.01% at £ow rates up to 0.4 mL / min and was not susceptible to contamination. Post-reaction analyses were performed on an AS14 column with suppressed-conductivity detection. Acceptable recoveries were
achieved for the anions tested ( chloride, nitrate, and sulfate ). Detection limits were between 2 and 4 ppb. The only major omission in this study was a discussion of any clean-up procedures that might be needed for the catalyst, either in the short or long term. Also, for those who prefer to buy all analytical equipment from recognized vendors, this decomposer is not available commercially. Advances have been made in analyzing peroxide directly, with Collard et al. [ 19 ] using an on-line matrix-elimination approach. The analyte anions were trapped on a TAC-LP1 concentrator column, which was then washed with water to remove the peroxide. Samples were loaded with an autosampler and analyses were performed on an AS15, in the 4-mm format. Vanatta and Coleman [ 20 ] expanded on the AS15 research by working with the 2-mm version of the column. They also studied the microbore AS11. The same sample-introduction and matrix-elimination techniques were used. With these smaller-diameter separators, the retention times of all analytes ( £uoride, chloride, bromide, nitrate, sulfate, and phosphate ) decreased as more peroxide samples were tested. However, peak areas remained stable, as did within-day retention times for each analyte. Hubaux^Vos detection limits ( with false-negative and false-positive probabilities both held to 2.5%) were 0.2 to 2.2 ppb on the AS11, and 1.1 to 3.0 on the AS15. This microbore method has the advantage of increased sensitivity, but is hampered by the gradual shifts in retention time. Statistically, the results are quite reliable. Some work has been done in the cation area. These analytes were investigated by Jensen et al. [ 21 ], using a CG12A concentrator column and CS10 separator. The concentrator was £ushed with water before the analysis was begun. They reported determinations in the single-digit-ppb range for lithium, sodium, ammonium, and potassium ions.
4. Analysis of cleanroom air and process gases The study of ionic contamination in cleanroom air has been neglected until recently, when researchers began to realize that such species could be detrimental to product yields. Lue et al. spent considerable time investigating the best way to collect these species for analysis. Their initial work involved anions ( Cl3 , NO3 3 , SO4 23 , and
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F3 ) [ 22 ]. Glass impingers ¢lled with a trapping solution ( carbonate / bicarbonate eluent, DI water, or 0.1 M NaOH ) were tried initially, with the air sample's being pumped through these devices at 0.2 L / min for 24 h. However, £uoride was not trapped well, presumably because this analyte adhered to the glass. Tubes ¢lled with silica-gel were found to be the answer to this issue. These devices had two chambers, each ¢lled with adsorbent. The above pumping protocol was used again. Subsequently, each silica-gel portion was extracted separately by placing it in a centrifuge tube with 10 mL of eluent and heating the mixture for 10 min in a boilingwater bath. Analysis of the extraction solution was performed using an AS4A-SC column and suppressed-conductivity detection. For all analytes, the ¢rst section of silica gel was found to contain all the extractable anions. These sampling tubes had the advantages of being inexpensive and readily available, convenient for sampling in small spaces, easy to transport, and suitable for storage after sample collection. This approach has potential, but the paper leaves some questions unanswered. First, no discussion of silica-gel blanks was included. Second, possible loss of analyte during the 100-degree extraction was not investigated. Third, no spiking studies were conducted to see if the silica gel trapped all anions quantitatively from air. Fourth, the AS4A-SC is not the best column for quantifying £uoride, since that anion is eluted very close to the water dip. Separators such as the AS11 or AS15 are better alternatives. If these issues could be resolved successfully, this procedure would be very useful, although the sampling time is quite lengthy. Lue and co-workers published a similar study [ 23 ] that addressed cations (Na , NH4 , K , Mg2 , and Ca2 ) in air. Analytes were removed from acid-treated silica gel with DI water and analyzed on a CS15 column. The procedure has the same advantages as above. Again, blanks were not mentioned and spiking studies were not performed. Other researchers sought to improve the speed and ef¢ciency of air-analysis methods. Watanabe and Sekiguchi [ 24 ] used acid or alkaline solutions to trap analytes more effectively, and then neutralized the liquids via electrodialysis before chromatographing the samples. Mayama et al. [ 25 ] designed a high-speed Automatic Air Sampler to reduce the collection time necessary. Besides cleanroom air, process gases are also becoming a target of anion analysis. Brzychcy et
al. [ 26 ] have done extensive, thorough methoddevelopment work on anions ( F3, Cl3 , Br3, SO4 23 ) and ammonia in various semiconductor gases ( HBr, HI, N2 O, SiH4 , SiF4, SF6 ). Two different sampling approaches were used, depending on the gas involved and the contamination levels involved. Whenever possible, a `bubble method' was used. Gas was bubbled through a series of impingers that contained the desired collection £uid. The £ow was metered to no more than 1 L / min and the volume of gas tested was determined either by a meter or by monitoring the weight of the gas cylinder. With some gases or when small sample sizes were needed, a `bulb method' was employed. This approach used a bulb ( of known volume ) that had two stopcocks and a septum-covered port. A sample of gas at atmospheric pressure was trapped in the bulb and dissolution of the analytes was accomplished by injecting aliquots of trapping liquid through the septum. Details were given on precautions for assembling the apparatus and handling the various gases, and on how to purge the vessels and check blanks before collecting samples. Also included were instructions for performing the various calculations. For known gas mixtures, extraction ef¢ciencies and sampling reproducibilities were reported as well. When possible, chromatographic results were compared with alternative procedures ( e.g., wet methods or spectroscopy ). Lastly, a small section of the paper discussed how to assay a water-soluble gas ( H2 Se or NH3 or HCl ) in an inert balance gas. The only major drawback of the report is the lack of information on the speci¢c analytical column used for the analyses. Additional work with process gases has been conducted recently. Ruimei [ 27 ] built a membrane-¢ltration device to trap anions ( Cl3 , NO3 3 , SO4 23 , PO4 33 , and F3 ) from a variety of compressed gases.
5. Analysis of wafers, devices, and materials Researchers have been utilizing ion chromatography more and more to characterize contamination on solid surfaces. The ultimate goal, of course, is to prevent anions from reaching wafers and devices. Yanagi and colleagues [ 28 ] built a device that has ingeniously solved a wafer-analysis problem: how to extract the ions on each side of the wafer sepa-
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rately. The technique used commercially available polypropylene bags that are manufactured for device extractions. The sample was placed in the bag as usual. Then the halves of a `sandwich-type' tool were placed around the wafer / bag. Each side of the tool had an O-ring ( of diameter equal to that of the wafer ), which pressed against the bag. When the tool was clamped together securely, the O-rings sealed off each side of the wafer from the rest of the bag. At the top, the O-ring had a small open section, thereby allowing the introduction of DI water into each chamber. After the wafers were extracted in a temperature-controlled bath, the solutions were removed from each side of the wafer and analyzed by ion chromatography. A TAC-2 concentrator column and AS12A separator were used with Na2 CO3 / NaHCO3 eluent to quantify anions; a TCC-LP1 and CS12A with methanesulfonic-acid eluent were employed to measure cations. This research also reported studies on extraction-time optimization and the integrity of the O-ring seal. The main drawback to the protocol is that the O-ring tool is not commercially available. Thompson et al. [ 29 ] addressed the problem of measuring contaminants on very small devices. They developed a microextraction procedure to quantify anions on thin-¢lm sliders for hard disk drives. The cell and its O-ring were made of PEEK and were sized to contain solids with surface areas less than 0.1 cm2 . The cell contents were extracted on-line and then concentrated onto an AMC-1 column, which was used in tandem with an AS12A ( 2-mm ) separator. Detection limits ranged from ca. 2 to ca. 21 ng / cm2 for the various analytes. Comparison with microscope and X-ray data gave complementary results. Two main limitations exist with this technique. First, the cell is not commercially available. Second, the AS12A may not be the best separator for low-level work. Increased sensitivity and better front-end resolution probably could be achieved with an AS11 or AS15 column. Jarvis [ 30 ] investigated cations, developing a reliable method to determine the ammonium content in adhesives. Samples were extracted by heating them for 5 h in a vessel that was purged with nitrogen gas. The gas that exited from the chamber was sent to impinger tubes that contained 20 mL of DI water. The ammonium ions in the solution were analyzed using a TCC-LP1 column to concentrate the analyte and a CS12A ( 2-mm ) separator to perform the chromatography. For the 20-mL sample
size, the detection limit was 500 mg of NH4 . Data from these studies were used to work with vendors in setting speci¢cations for the adhesives. Other researchers have been active in this area. Tan et al. [ 31 ] have used the technique to quantify both anions and cations on silicon wafers. Munson [ 32 ] has applied the instrumentation to conduct failure analysis on devices at all levels of their manufacturing process. Bombien and Bahten [ 33 ] have evaluated polyvinyl-alcohol sponge material ( used to clean wafers and disks ) and gloves for their ionic content. Jiranut [ 34 ] studied the formate level in the electro-deposition coating of hard disk-drive components. Ju et al. [ 35 ] constructed a special scanning tool and used a glove box to improve sensitivity of ionic determinations on wafers and glasses. Bhattacharjee and Atterbury [ 36 ] utilized both ion chromatography and capillary electrophoresis to quantify anions and cations in cleanroom wipers. Finally, two ion-chromatographic studies have been performed to assess the ions that are leached from PVDF resins. Henley [ 37 ] looked at anions in both plastic and stainless-steel. Dennis and Seiler [ 38 ] quanti¢ed both anions and cations in PVDF resins.
6. Conclusion In the past 5 years, ion chromatography has increased in usefulness in detecting ionic levels in semiconductor-related solids, liquids, and gases. No longer is the technique used simply to verify the cleanliness of DI water. Improvements in column ef¢ciency and eluent cleanliness have resulted in lower detection limits. More stable column resins and better concentrator columns have meant that challenging matrices such as corrosive and oxidative chemicals, small parts, and gases are now being examined. Contamination control will continue to be a major challenge in the semiconductor industry, and ion chromatography no doubt will remain a signi¢cant technique for meeting that goal.
Acknowledgements The author would like to acknowledge Bev Newton of Dionex Corp. for her assistance is searching the literature; Dan Cowles and Todd Baldwin of Air Liquide America for their helpful discussions regarding spike-recovery studies with gases.
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[ 24 ] K. Watanabe, Y. Sekiguchi, presented at the International Ion Chromatography Symposium, paper 37, Osaka, Japan, 1998. [ 25 ] K. Mayama, M. Maki, S. Sakai, presented at the International Ion Chromatography Symposium, paper 115, Osaka, Japan, 1998. [ 26 ] A.M. Brzychcy, B.P. Twombly, L. Zhang, in: J.D. Hogan ( Editor ), Specialty Gas Analysis. A Practical Guidebook, Ch. 7, Wiley, New York, 1997. [ 27 ] W. Ruimei, J. Chromatogr. Sci. 36 ( 1998 ) 579. [ 28 ] K. Yanagi, H. Shibata, K. Nagai, M. Watanabe, Electrochem. Soc. Proc. Vol. 2000-17 ( 2000 ) 561. [ 29 ] J. Thompson, T. Prommanuwat, A. Siriraks, S. Heberling, presented at the International Ion Chromatography Symposium, paper 13, San Jose, CA, 1999. [ 30 ] M. Jarvis, presented at the International Ion Chromatography Symposium, paper 47, Nice, France, 2000. [ 31 ] S. Tan, R. Liu, H. La, presented at the International Ion Chromatography Symposium, paper 46, Santa Clara, CA, 1997. [ 32 ] T. Munson, presented at the International Ion Chromatography Symposium, paper 14, San Jose, CA, 1999. [ 33 ] C. Bombien, K. Bahten, presented at the International Ion Chromatography Symposium, paper 87, San Jose, CA, 1999. [ 34 ] K. Jiranut, presented at the International Ion Chromatography Symposium, paper 113, Osaka, Japan, 1998. [ 35 ] J.-H. Ju, S.-C. Kang, S.-M. Chon, presented at the International Ion Chromatography Symposium, paper 44, Santa Clara, CA, 1997. [ 36 ] H.R. Bhattacharjee, O. Atterbury, presented at the International Ion Chromatography Symposium, paper 49, Nice, France, 2000. [ 37 ] M. Henley, Ultrapure Water 14 ( 10 ) ( 1997 ) 16. [ 38 ] G.M. Dennis, D.A. Seiler, Ultrapure Water 16 ( 10 ) ( 1999 ) 27. Lynn Vanatta is the manager of chromatography research and statistics at Air Liquide America's Dallas Technical Center, where she has worked for 9 years and authored 13 publications. She holds a bachelor's degree in chemistry from Indiana University / Bloomington. Her research focuses on statistically sound anion methods for analyzing semiconductor-grade water and chemicals. Ms. Vanatta is a member of the American Chemical Society, the American Society for Mass Spectrometry, the American Statistical Association, and the American Society for Testing and Materials. Within this last organization, she is chairman of a committee on precision and bias, and also a De¢nitions Advisor. She is active in the International Ion Chromatography Symposium, where she organizes an annual session for the semiconductor / pure-chemicals industry, teaches a short course in statistics, and is the Program Chairman for the 2001 meeting. In addition, she co-instructs a detection / quantitation short course at the Pittsburgh Conference.
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