PROCESS ANALYZERS FOR PURIFIED BULK GAS QC/QA: ENTERING THE PPT RANGE

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1 JPACSM 27 PROCESS ANALYZERS FOR PURIFIED BULK GAS QC/QA: ENTERING THE PPT RANGE Marco Succi SAES Getters S.p.A. Milano, Italy James G. Moncur and William L. Robinson Independent Consultants KEYWORDS: TRACE GAS ANALYSIS, PPB, PPT, REDUCTION GAS DETECTOR, FLAME IONIZATION DETECTOR, HYDROGEN, CARBON MONOXIDE, CARBON DIOXIDE, METHANE ABSTRACT The Reduction Gas Detector (RGD) is routinely used for gas chromatographic analysis of volatile compounds that are readily oxidized. Compounds quantified at part-per-trillion (PPT) and higher levels include hydrogen and carbon monoxide in nitrogen, oxygen, argon, helium, air, and carbon dioxide as the RGD does not react with these relatively stable bulk gases. An optimized GC-FID-methanizer is used to analyze CH 4, CO, CO 2, and non-methane hydrocarbons in bulk gases even at sub part-per-billion (PPB) levels. The paper will discuss trace and ultra-trace bulk gas applications using GC-RGD and GC-FID analyzers. INTRODUCTION The electronics industry requires strictly controlled gas purity for the production of more and more integrated solid state devices. Gaseous impurities, especially in the bulk gases which are used in large quantities, can have a significant impact on final device performances if not maintained within very strict specification 1-5. Currently, semiconductor fabs specify gas purity in the single part-per-billion (ppb) range for every impurity; specifications of less than one ppb are not uncommon. Improper supply of gas purity due to a bad batch of gas or the malfunctioning of the air separation unit or gas purifier can have a dramatic impact on production quality. Trace gas analysers are normally used to continuously monitor gas quality; however, the higher gas purities required for the new generation of devices with 0.13 micron geometry requires analytical tools with improved detection limits to track impurity trends. Some situations where impurity levels exceeded specifications are described in this paper, along with a description of a newly developed analyzer with sub-ppb detection capability for H 2, CO, CO 2, and CH 4. A comparison between this new analyzer and APIMS in nitrogen is also briefly discussed. CASE STUDIES Nitrogen Analysis An RGA5 analyzer, suitable for trace analysis of H 2, CO, CH 4, and CO 2 (Trace Analytical) was mounted downstream of a nitrogen catalytic purifier. It detected periodic spikes of H 2 up to few hundreds of ppbs while all the other impurities remained below the 20 ppb specification 6. As shown in Figure 1, the H 2 concentration between the spikes also dropped to two different and distinct concentrations. The problem was discovered in the clean room and traced back to the purifier. Considering the operation of a catalytic nitrogen purifier, it is relatively easy to understand the observed behavior. Catalytic purifiers have two sets of columns operating in parallel. While one set is in purifying the feed gas, the other set is being regenerated. This regeneration process requires adding a small concentration of H 2 to the purifier column undergoing the regeneration in order to reduce the catalyst. At the end of the regeneration process, the regenerated column is placed into operation and the other column is regenerated. The problem, illustrated in Figure 1, was the result of two factors: 1. Insufficient purging time to remove H 2 after the regeneration. The H 2 decay following the spike was H 2 purging from the purifier, and; 2. The difference in the two H 2 concentrations after the spike indicated a leaking H 2 valve on one column set. It is important to note that even if H 2 is not normally present at high concentration in the source nitrogen, it reached a dangerously high concentration at the delivery point. The presence of an online analyzer capable of continuously monitoring H 2 allowed the immediate identification and repair of the problem.

2 JPACSM 28 Oxygen Analysis After three months of normal operation, a catalytic oxygen purifier suddenly stopped removing CO 2, yet continued to remove all other impurities 6. As shown in Figure 2, the CO 2 concentration increased from less than 10 ppb to several hundreds of ppb over a period of 12 hours. An analysis of the oxygen at the inlet of the purifier showed a CO 2 concentration within specification, indicating the purifier as the source of the problem. Catalytic oxygen purifiers are designed to remove impurities through use of a catalyst and adsorber column located in series. The catalyst column converts hydrocarbons, carbon monoxide, and hydrogen to carbon dioxide and water vapor, which are removed by the absorption column located downstream of the catalyst. The absorption columns have a very high capacity for removing moisture and a lower capacity for CO 2 removal; two columns are located in parallel and are alternated between operating and regeneration modes. Regeneration mode involves heating the column to a temperature that allows the absorbing material to release the trapped impurities to a gas stream that goes to vent. The problem with the oxygen purifier was due to a failure of the regeneration cycle timer to allow the correct regeneration cycle. The CO 2 breakthrough would have eventually been followed by water vapor breakthrough. The above examples show the importance of monitoring the complete spectrum of impurities in a gas stream, not only the ones that are potentially present as impurities in the source gas. In the attempt to reduce costs, some purifier manufacturers are compromising in regard to component selection and engineering, and thus risk the loss of purifier reliability. As a result, impurities normally not present in the source gas can be introduced, as in the case of H 2 in N 2, or converted, as in the case of CH 4 in O 2. Online trace gas analyzers are an essential tool to control gas purity and quickly diagnose problems. With the ever increasing integration of solid state devices, analyzers capable of measuring sub-ppb trends are becoming essential to protect the high value of the final Si-based devices. Figure 1. H 2 trend at the outlet of a nitrogen catalytic purifier. Figure 2. CO 2 trend at the outlet of an oxygen purifier.

3 JPACSM 29 THE ta7000 Trace Analytical has developed a new trace gas analyzer to satisfy the most advanced market needs in terms of sensitivity, reliability, and easy of use. The ta7000 is a GC with improved detectors: a new long cell Reduction Gas Detector with enhanced sensitivity to reducing compounds, such as H 2 and CO, and an optimized design of the Flame Ionization Detector (FID) for superior detection of carbon-based compounds, such as CH 4 and CO 2. Both detectors have advanced electronics for enhanced signal-to-noise at extremely low sample concentrations. Different from the old RGA5, the ta7000 s two detectors have been engineered as two separate units to provide more flexibility and better reliability. The ta7000 has been designed as a process gas monitor to work fully unattended and without the control of an analytical expert. The unit is engineered with the features necessary for simple operation and optimal instrument performance. Among the built-in features are: gas regulators to control operating gases; a carrier gas purifier; calibration system; automatic fail safe mode for hydrogen flows; and auto shutdown when carrier gas is depleted. Designed for dedicated duty on a single gas stream, chromatographic data is processed using an advanced algorithm (EDL Enhance Detection Limit) to isolate measurement signals while filtering out random electronic noise. This feature further improves the instrument s detection capability. RESULTS Figure 3 shows the improvement in the stability of H 2 readings in N 2 during the continuous monitoring of a few ppb using the ta7000 with EDL mode disabled and enabled. Table I compares the old RGA5 unit and the new ta7000 when reading a few ppbs of H 2 in nitrogen. It clearly indicates a dramatic reduction of the standard deviation between the RGA5 and the ta7000. Table II shows a typical result obtained during 10 hours of continuous monitoring of purified nitrogen and 10 hours of continuous monitoring of nitrogen containing 1-2 ppb each of the detectable impurities H 2 Response Single Mode Time (minutes) EDL Mode Figure 3. H 2 trend in nitrogen using the ta7000 without (single) and with EDL mode. Table I Comparison of the Standard Deviations of H 2 in N 2 at a Low PPB Level RGA5 ta7000 Single Mode ta7000 EDL Mode Average 4.27 ppb 2.60 ppb 2.57 ppb Standard Deviation 2.27 ppb 0.37 ppb 0.1 ppb Table II ta7000 Results at Low Contaminant Levels in N 2 ta7000: reading at zero (ppb) Average Standard Deviation ta7000: reading at 1-2 ppb (ppb) Average Standard Deviation H CO CH CO The improved stability is, of course, essential to improving the signal-to-noise ratio and thus the detection sensitivity. Figure 4 shows the trend of the continuous monitoring of CH 4 in nitrogen, initially without methane and then introducing 1.1 ppb. The 1.1 ppb increase is clearly visible, demonstrating the instrument s ability to reliably detect concentrations below 1 ppb. Figure 5 is a chromatogram of the analysis of 1.1 ppb of each impurity with the FID detector. A typical trend of CO 2 in hydrogen is shown in Figure 6. The statistics during the zero analysis show an average value of 0.15 ppb and a standard deviation of 0.17 ppb and an average of 1.58 ppb and standard deviation of 0.29 ppb respectively during CO 2 generation. A typical CO trend in oxygen obtained with the RGD detector is shown in Figure 7. It clearly demonstrates the outstanding capability of this detector. The average reading during zero analysis was ppb with a standard deviation of ppb. During the 1.6 ppb generation, the average was 1.61 ppb and the standard deviation ppb.

4 JPACSM CH 4 Response Zero Gas 1.1 ppb Sample Gas Figure 4. CH 4 trend in N 2. FID Signal (counts) 37,000 36,000 Detector Auto Zero 35,000 CH 4 CO 2 34,000 CO 33,000 1 ppb Normal Valve Pressure Switch NMHC Because of space limitations, only a few results have been described in this paper. However, because the matrix of the gas has a minor impact on the detectors, similar results in terms of sensitivity and response time have been obtained for other combinations. COMPARISON WITH APIMS A test was set up to compare responses to H 2 in nitrogen of the ta7000 and two different atmospheric pressure ionized mass spectroscopy (APIMS) detectors: a Sensar TOF2000 and a VG Thermo-Onix Trace+. The three instruments were mounted in parallel and were exposed to an H 2 increase from zero to about 650 ppb and then back to zero 7. Before performing the test, they were calibrated as recommended by the manufacturers using a cylinder containing a known concentration of all impurities of interest, such as H 2 and CH 4. As can be seen by Figure 8, neither of the two APIMS detected a hydrogen increase close to the generated concentration. On the other hand, the ta7000 tracked the generated concentration accurately. H2 IN NITROGEN: response to high H2 concentrations 32, Time (seconds) Figure 5. Typical FID chromatogram of N 2 containing about one ppb of impurities CO2 Response H TA7000R SENSAR TOF2000 VG TRACE zero gas 1.6 ppb Figure 6. CO 2 trend in H CO response zero gas 1.6 ppb TIME (hh.mm) Figure 8. Comparison of the response to H 2 in nitrogen of various instruments. The poor APIMS response can be easily explained considering that N 2 H +, normally used for H 2 quantification, is generated by CH 4 and by the trace of H 2 O always present in the system, but not by H 2. So when only H 2 is present, the formation of the cluster N 2 H+ is a very inefficient process and totally unreliable results are achieved. Interference problems from CO 2 and CH 4 are also present in the quantification of CO by APIMS while the ta7000 did not suffer from their presence in the sample Figure 7. CO trend in O 2.

5 JPACSM 31 CONCLUSIONS Given the materials and techniques used in constructing present and future semiconductor gas distribution systems, the concentration of all impurities in bulk gases is, and will remain, at a low and narrow concentration range. The damage caused by each specific impurity is therefore equivalent, based on the similarity of impurity concentrations in the gas distribution system. Additionally, the damaging effects of impurities are not the same for all processes. Specific impurities cause damage to specific processes and the logical solution to this problem is to monitor all of the damaging impurities rather than just a few. From a historical viewpoint, the need to monitor moisture and oxygen impurities in UHP distribution systems makes perfectly good sense. However, the high cost of the present gas systems is due to the technology that has been developed to reduce the probability and frequency of leakage. In effect, this reduces the concentration of impurities, O 2 and H 2 O, due to leakage to levels that are similar to the concentrations of impurities that may be present from other sources, such as H 2, CH 4, CO, and CO 2. Problems with gas quality can arise from a variety of sources. The case histories reported in this paper are examples of this fact. The operating characteristics of gas purifiers and other components of gas distribution systems determine which impurities are most likely to be present during specific types of failures in the delivery system. Frequently, the concentration of only one impurity may suddenly rise to levels far exceeding the concentration of any other impurity. If the fab is not equipped to monitor this situation, then there is no protection from the resulting damage to wafers in process. The presence of specific impurities can also be used as a diagnostic indicator of the source of the problem. However, this information can only be used effectively if it is easily and quickly available. The new ta7000-r and ta7000-f provide the ability to reliably monitor impurities trends from sub-ppb to several hundreds of ppb concentrations with a sensitive, easy to use, and reliable process unit. REFERENCES 1. T. Ito, 6 th International SAES Pure Gas Workshop, Geneva, March 28, Reed Rosenberg, 7 th SAES Pure Gas Workshop, Geneva, April 16, Sanjay Banerjee, Microcontamination Conference Proceedings, M. Yamazaki, 5 th European Pure Gas Forum, Geneva, April 5, N.K. Verma, A.M. Haider, and F. Shadam, J. Electrochem. Soc., 140, 1459, (1993). 6. C.R. Ostrander and C. Solcia, Fab Tech, 13 th Editions, L. Pusterla et al., PITTCON 99, paper 936.