On-line Analysis of Process Chemicals by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Yoko Kishi*, Katsu Kawabata*, David Palsulich** and Dan Wiederin*** *PerkinElmer Life and Analytical Sciences, Concord, Ontario, L4K4V8, Canada **Micron Technology, Inc., Boise, ID 83707-0006, USA *** Elemental Scientific Inc., Omaha, NE 68131, USA Abstract. An initial study of on-line remote system for monitoring of trace metals using the inductively coupled plasma mass spectrometer equipped with dynamic reaction cell system is described. The three types of cleaning solutions (HF, SC-1 and SC-2) used in the traditional RCA clean procedure are analyzed periodically with one set of simple calibration curves to simulate the on-line monitoring. A remote sampling system for on-line monitoring with the combination of the ICP-MS is also demonstrated. reduction of the interferences, and several technical papers on semiconductor application have been published [5-9]. The DRC technique is independent from the plasma temperature and can eliminate variety of interferences with normal or hot plasma condition. Since the DRC does not require the cool plasma condition for the determination of various chemicals used in the semiconductor industry, matrix suppression can be minimized. Another practical feature of the DRC technique is elimination of Cl related interferences. Since the cool plasma technique cannot avoid Cl related interferences, elimination of Cl matrix by evaporation has been required prior to the ICP-MS analysis. However, some of volatile elements such as B, Ge and As are lost during the evaporation, therefore it has been troublesome for the analysis of one of the most commonly used cleaning solution, standard solution-2 (SC-2), which is consisted of H2O2 and HC1. INTRODUCTION As integration of semiconductor devices continues to increase and junctions become shallower, contamination control of chemicals and materials used in the semiconductor industry is becoming more critical and important. The International Technology Roadmap for Semiconductors [1] suggested that critical metallic impurity levels in the ultrapure water (UPW) and the process chemicals should be lower than 20 and 10 ppt, respectively, and that required impurity level of critical surface metals on Si wafers should be lower than IE 10 atoms/cm2. In order to determine such a low concentration of impurities in chemicals, inductively coupled plasma spectrometry (ICP-MS) has been commonly used. However, the ICP-MS has not been applied for on-line monitoring of chemicals used in FABs due to some critical issues such as interference and matrix suppression. Although a cool plasma technique [2] has been widely used to overcome some of interference problems, it cannot eliminate some of critical interferences such as C1H2, CIO, C1OH, C12, ArCl, PO2, SO2 and PO2H on K, V, Cr, Ge, As, Cu and Zn, respectively in HC1, H3PO4 and H2SO4 matrices. In addition, the cool plasma suffers from severe matrix suppression, which requires cumbersome matrix elimination or matrix matched calibration solutions. Using the benefit of hot plasma and the DRC technology, a possibility of on-line monitoring for three cleaning chemicals commonly used in the traditional RCA cleaning procedure was investigated using an autosampler and a lwt% nitric acid calibration curve. Another critical issue of on-line monitoring was how to take samples from chemical baths to the ICP-MS instrument. A sampling length between a bath to the instrument might be up to 30 meters or so, which might cause an adsorption problem of elements in a transfer tube and a long sample uptake time. A feasibility study of remote Recently, ICP-MS with a dynamic reaction cell (DRC) technique has been developed [3, 4] for CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference, edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula © 2003 American Institute of Physics 0-7354-0152-7/03/$20.00 606 EXPERIMENTAL Perfluoroalkoxy (PFA) transfer tube by Ar gas flow. The sample flow was switched from one to another by the PFA switching valves located near the ICP-MS. This system enables to transport even limited amount of samples very quickly minimizing the sample uptake time. Instrumentation Standards and Reagents The experiment was carried out on an ELAN DRC II ICP-MS (PerkinElmer Sciex, ON, Canada), equipped with several types of introduction system (Elemental Scientific, Inc., NE, USA). The operating conditions were determined using a 1 jig/L (ppb) standard solution in lwt% HNO3, which are listed in Table 1. Three types of cleaning solutions were prepared by simply diluting high purity grade chemicals (Tama Chemicals, Inc., Tokyo, Japan): lwt% HF, SC-1 (2.5wt% NH4OH + 3.5wt% H2O2) and SC-2 (3.5wt% HC1 + 3.5wt% H2O2) with high purity water obtained from a Milli-Q Element system (Millipore Corp., MA, USA). A beta version of remote sampling device (Elemental Scientific, Inc., NE, USA) which was equipped with nebulizers, spray chambers, transfer lines and switching valves as shown in Figure 1 was evaluated in this experiment. One set of a nebulizer and a spray chamber was placed close to each sample (it could be a rinse bath in the FABs). In this experiment, three sets were placed close to HNO3, SC1 and SC-2 solutions in the bottles. The sample solution was self-aspirated to the nebulizer where aerosol is generated, and this aerosol was transported to the ICP-MS through a 30 m x 8 mm I.D. TABLE 1. Operating Conditions.__________ Parameter/System Setting/Type Nebulizer PFA concentric type Spray chamber PFA Scott type /Quartz cyclonic type Torch injector Pt Sampling/Skimmer cones Pt RF power 1550 W 18L/min Plasma gas flow Aux. Gas flow 1.8L/min Nebulizer gas flow 1.0- UL/min NH3 Cell gas for DRC sampling system in order to minimize these issues was investigated. 1 sec/mass Integration time mm i Oralri FIGURE 1. Schematic Diagram of Remote Sampling System. 607 used to monitor the signals, and re-calibration and the QC sample check were performed every 30 samples. Since the self-aspiration was used to introduce the samples into the nebulizer, it was considered the sample uptake rate would be varied due to the variance of each chemical viscosity. Besides, there would be unevenness on the nebulizers used for remote sampling system. To compensate these issues, internal standard correction by 5 |o,g/L (ppb) Be, and 1 |0,g/L (ppb) Sr, In and Tl was used in this experiment. Standard solutions were made from three types of 10 mg/L (ppm) multi-element standard solutions (PerkinElmer Life and Analytical Sciences, CT, USA) by serial dilution with the high purity water. The spike recovery obtained from the unspiked sample and the 100 ng/L (ppt) spiked sample for each sample type showed 90-110% and its stability over 15 -48 hours was less than 1% for most of the elements. The detection limits calculated by dividing the standard deviation of blank solution with the sensitivity were below 10 ng/L (ppt). The stability of internal standard elements in the SC-2 sample, which indicates the real instrument stability before the internal standard correction, is shown in Figure 2. All the signals were very stable over 48 hours, 2.1 -4.3%. RESULTS AND DISCUSSION -*— Be (4.3%) -A—Sr(2.1%) Recovery and Stability -+--- In (2.6%) * As a first step, the ICP-MS performance for the analysis of three types of rinse solutions was verified without the remote sampling device. The standard solutions, 0 and 1 |0,g/L (ppb), in lwt% HNO3 were used to establish one set of calibration curves in the beginning, followed by a QC sample, 500 ng/L (ppt) in lwt% HNO3. Then, HF, HF + 100 ng/L (ppt) spike, SC-1, SC-1 + 100 ng/L (ppt) spike, SC-2 and SC-2 + 100 ng/L (ppt) spike samples were alternately analyzed for 15-48 hours by the DRC-ICP-MS with an Autosampler, AS-93 Plus (PerkinElmer Life and Analytical Sciences, CT, USA). More than 20 elements including critical elements in the semiconductor field such as Na, Al, K, Ca, Fe, Cu and Zn were analyzed in one run within 5 minutes. The QC software equipped with the ELAN system was 200 11 Tl (3.6%) 31 21 Measurement number FIGURE 2. 48 Hours Stability for Internal Standard Elements in SC-2. 800 500 561 700 Fe 400 600 100 500 300 400 200 300 200 100 10 20 30 ppt 40 50 60 10 20 30 ppt 40 50 60 FIGURE 3. Calibration Curves for Na, K and Fe. 608 10 20 30 ppt 40 50 60 Na(3.2%) 1.20 Mg(3.0%) 1.00 15 0.80 c 0) "55 | 0.60 0.40 0.20 0.00 Mo (1.8%) 0 100 200 300 400 500 600 700 800 Time (min) Pb(1.4%) FIGURE 4. Stability of Analytes with 30 m Transfer Tubing for 13 Hours. sampling system. The samples were nebulized with the PFA concentric nebulizer and transported to the ICP-MS through the 30 m transfer tubing which was connected to the torch of the ICP-MS system. One series of standard solutions, from 0 to 50 ng/L (ppt), were analyzed sequentially to check the capability to analyze ultra trace level. Calibration curves obtained for some critical elements in the semiconductor industry are shown in Figure 3. As can be seen, very good linearity was obtained even at ultra trace level. 0 Using the same system, the long-term stability test was performed for 39 elements with a 1 (ig/L (ppb) multi-element standard solution for 13 hours. Excellent stability was obtained even without the internal standard correction: the RSDs of the signals over that period were around 1 -2% for most of the elements. The results of some key elements are shown in Figure 4. 100 200 300 400 500 600 700 Time (min) FIGURE 5. System. Sample Switching with Remote Sampling The performance of switching valve was studied with a 10 jig/L (ppb) Y in SC-1, and 1 (ig/L (ppb) In in SC-2 and Rh in HNO3. Transient signals were monitored to check the effect of switching on the signals. Each sample was transported with the remote sampling device shown in Figure 1. Remote Sampling Device In the beginning, standard solutions in lwt% HNO3 was used to evaluate the performance of remote 609 The valve was switched every 100 sec, and three types of samples were introduced into the ICP-MS alternatively. As can be seen in Figure 5, the signal response was very quick: the signals were stabilized in 20 sec after switching the valve, and three orders of magnitude signal reduction could be achieved in 50 sec when the valve was switched to another samples. 9. Kawabata K., Kishi Y. and Thomas R., Spectroscopy, 18 (1), 2-9 (2003) CONCLUSIONS The data presented in this work demonstrate that the ICP-MS equipped with the DRC system can analyze typical cleaning solutions used in the FABs just after simple dilution. It is also shown that the DRC-ICP-MS with the combination of innovative remote sampling system would be applicable to real time on-line monitoring of chemicals used in the semiconductor FABs. ACKNOWLEDGMENTS The authors would like to thank Matt ew Knapp (PerkinElmer Life and Analytical Sciences) for his technical support. REFERENCES 1. International Technology Roadmap for Semiconductors 2001 Edition, 2. Sakata K., and Kawabata K., Spectrochimica Acta, 49B, 1027-1038(1994). 3. Baranov V. I. and Tanner S. D., J. Anal At. Spectrom., 14,1133-1142(1999). 4. Tanner S. D. and Baranov V. I., J. Am. Soc. Mass Spectrom., 10, 1083-1094(1999) 5. Vollkopf U., Klemm K., and Pfluger M., Atomic Spectroscopy, 20 (2), 53-59 (1999) 6. Bollinger D. S. and Schleisman A. J., Atomic Spectroscopy, 20 (2), 60-63 (1999) 7. Collard J. M., Kawabata K., Kishi Y., Thomas R., Micro, January, 39-46 (2002) 8. Kishi Y. and Kawabata K., Atomic Spectroscopy, 23 (5), 165-169(2002) 610
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