Assessing the Cleanliness of Ultrapure Gas Supply Components
Preparing and delivering ultraclean process gases is essential to prevent contamination in the final product. This article discusses a testing method to assess cleanliness of ultrapure gas supply components.
By Johann Dorner
As a result of the scaling-down of semiconductor technology, the critical size of particles that can cause irreparable damage to components has become smaller. Also, the number of gas process steps in the manufacture of microelectrical components has increased. Therefore, error-causing contamination from unclean materials has increased enormously, leading to higher production costs. It is not only essential that process gases be prepared and delivered ultra-clean, but also that impurities in the supply system between point of supply and point of use be either eliminated or minimized. To reduce particle entry into the supply system, a testing method was developed that would provide a guaranteed assessment of the cleanliness of ultrapure gas supply components (UPGSC).
User target groups for the testing method are semiconductor manufacturers and manufacturers of UPGSC (Fig. 1). The operator of a semiconductor fab is responsible for carrying out comparative tests on UPGSC from various manufacturers (e.g., valves, pressure-reducers, filters, etc.), and for determining the optimal operating point (lowest particle entry into the supply system). The test method provides UPGSC manufacturers vital information concerning the effects of changes on the purity of components within the areas of development, production and assembly. As a result, UPGSC particle behavior can be improved.
Testing/Measuring Method
The UPGSC testing method was specifically designed for use in industrial production with the following requirements: The testing method must be standardized, favorably priced, and easy to operate. The results must also be qualitatively assured and provide conclusive results. In addition, the testing method must be able to simulate various operating conditions, i.e., stationary and instationary phases. Parameters such as pressure and flow velocity must be variable and adjustable for specific test phases. Moreover, the testing method should provide for maintenance of specific physical parameters during the measurement and for controls and/or recordings. Only in this way is it possible to obtain a clear correlation between a parameter and its contamination behavior on UPGSC.
A further important demand is reproducible conditions for carrying out comparative testing of different UPGSC manufacturers. In the operational behavior section of the test, the actual particle behavior of the test object during operation is assessed. To do this, it is necessary to simulate conditions occurring during operation that could lead to the generation of particles. For this purpose, stationary and instationary test cycles were set up. The effects of the UPGSC on particle behavior must be measured each time there is variation in a single parameter, while the remaining settings are kept constant. The procedure has an advantage in that a clear correlation exists between the behavior of the particles and the changing of the parameters.
An analysis of the optical particle counters used showed that a minimum testing time of 30 seconds was required. However, to achieve an improved level of reliability in measurement results, a measuring time of 1 min. was selected. To prevent previous test sections from influencing measurement results, free-rinsing phases were planned between each test section. Table 1 shows the structure of the UPGSC test method. Fig. 2 shows the ranges and procedures of the individual testing phases of the standard testing method.
The method should include an option for particle analysis, in addition to particle counting. From the chemical composition of particles, it is possible to obtain better conclusions as to the cause of contamination. For this particle analysis, a scanning electron microscope with an EDX analysis system can be used. Because of the numerous influences on exact particle measurement, an economically viable method could not be devised, and therefore a qualitative measuring method was selected. A further requirement was the ability to integrate the test method into the global running of the development, production and quality control of a company.
Test Set-up
An unpressurized sampling was used for particle measuring. This system, when compared to particle measuring under pressure, has the advantage that pressure fluctuations occurring, for example, under instationary running conditions have little or no effect on particle counters. Pressure-free sampling also permits the use of CNC-particle counters, which are able to measure very small particle sizes of 0.01 µm upwards. To prevent influence on measurement results, Class 1 cleanroom conditions are necessary. Furthermore, particular attention was given to a high level of cleanliness and the preparation of particle-free gases during design and assembly of the test setup over short pathlengths.
The completed test setup allows for altering and controlling important operational parameters, such as pressure and flow velocity, while measurements are being carried out. Because all parameters are either known, or at least able to be constantly maintained during the measuring procedure, the most important criteria for reliable results are fulfilled. To maintain assured test results, only calibrated particle counters can be used. In this way, qualitative, comparative experiments can be carried out on ultrapure gas supply systems. Test costs can also be kept to a minimum, since the test setup has a universal field of application on various types of UPGSC.
The test setup (Fig. 3) consists of three system sections: gas preparation, activation, and measuring techniques. The gas preparation system is composed of one or several stop valves, pressure-reducers, and filters. Ultrapure compressed air is used as the test gas. The activation system section is equipped with a stop valve, a pressure-reducer, and a timing unit. For measurements, optical particle counters and CNC particle counters are used. The universal test setup shown in Fig. 3 includes all components necessary for testing a wide variety of UPGSC. But for every individual UPGSC, the test setup has to be specifically modified. For example, the aerosol generator, mix-chamber and heater only need to be installed when testing filters under pressure fluctuations, aerosol concentration, and temperature-loading. For testing valves, these components are not part of the test setup.
Fig. 4 represents the changed test setup for the analysis of pressure-reducers. As far as setup and components, the test corresponds very much to the universal test setup. The use of an aerosol generator is not necessary for the testing of pressure-reducers. The following parameters were used during measurement: total volume flow was 1000 NI/h; pressure in front of the pressure-reducer was 9 bar, with a pressure of 4 bar behind the component being tested; volume flow at the particle counter was set at 0.1 cft/min.
Fig. 5 shows the results from testing of a pressure-reducer. In this instance, the x-axis reproduces the length in time of the test phases, whereas the y-axis demonstrates particle emission. For statistical reasons, at least five specimens in a series must be measured to obtain conclusive results. It is important that the measurements be carried out within the shortest possible time intervals, so that almost identical starting conditions can be maintained. The mean values determined for the testing of pressure-reducers are shown in Table 2.
The results show that in the dynamic preconditioning phase, and in the continual operation phase, the number of rinsed particles is very high. Declining particle concentrations in the two phases are clearer in Fig. 5 and are typical for some tested pressure reducers. The first pressure changes lead to oscillation effects, which explains the initial increase in particle concentration.
Conclusion
The test method described is useful for testing a variety of UPGSC (valves, filters, pressure-reducers, etc.), even under instationary working conditions. A large number of tests were carried out over the last few years by the institute in cooperation with companies such as IBM, Siemens, Linde Messer-Griesheim etc. With the data on lowest possible particle entry, manufacturers of UPGSC were able to improve their products. On the other hand, as a result of comparative studies, semiconductor manufacturers were provided an important assessment criterion for purchasing decisions and a choice of UPGSC. n
References
1. Dorner, 10th ICCCS Symposium 1990, Proposals for the Standardization of Particle Measurements at Gas Supply Components and its Realization in Practice.
2. SEMI Standards: C 13-95; C 14-95.
3. Joint Project Media Supply in the Semiconductor Industry; German- funded project with 36 industry partners.
4. Joint Project Particle Measuring Techniques; German-funded project with twelve industry partners.
5. Wang et. al.: “Establishing a Particle Test Sequence for UHP Gas Valves,” Microcontamination, April 1993.
6. Sematech Inc.: SEMASPEC 90120390A-S
Johann Dorner completed his studies in process engineering at the University of Stuttgart in 1986. He has been working at the Fraunhofer Institute for Manufacturing and Automation (IPA; Stuttgart, Germany) since 1987 in the field of contamination in ultraclean media supply. In addition to his basic research work on particle contamination of media supply components, he has participated in several committees. He was also the leader of the joint project Media Supply. Since May 1990, he has served as head of the Department of Ultrapure Engineering and Microproduction Technology at the IPA.
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Figure 1. Shown are the focal points of the test method applications.
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Figure 2. The individual testing phases of the standard testing method are shown. (Refer to Table 1 for a description of the phases.)
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Figure 3. Three system sections–gas preparation, activation, and measurement–comprise the test method.
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Figure 4. For pressure reducers, the test setup must be changed. The setup does correspond to
the universal test setup, however, the use of an aerosol generator is not necessary.
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Figure 5. The results from testing pressure-reducers are shown. Declining particle concentrations
are easily seen. (Also, refer to Table 2).