Particle measurement in semiconductor process gases
08/01/1999
A variety of technologies now exist to measure particle levels reliably in gas distribution systems with different optimal technologies for each combination of inert
eactive gas and bulk/cylinder distribution system. The proper selection of measurement hardware and sampling procedures allows for detection of particles as small as 0.003µm.
Measurement of contaminant particles in semiconductor processing gases is technically challenging. Accurate sampling and detection of submicron particles is required in large-volume bulk gas systems, high-pressure storage cylinders, in-fab gas distribution lines near the processing tool, and tool vent lines. Particles ranging in size from larger than a micron down to a few nanometers must be accurately detected in gas supply and semiconductor processing equipment.
Particle detectors are required to withstand highly reactive or toxic gas mixtures as well as inerts. Furthermore, sampling is required from systems ranging in pressure from >1.3 x 107Pa in storage cylinders to <1torr in processing equipment vent lines. In some cases, gas samples are extracted from large-volume systems flowing over 105stdl (standard liters)/min. While at the processing tool, samples must be obtained from systems flowing less than 1stdl/min.
The concentration of suspended particles in the system may be uniform and steady in continuously flowing systems, or may be dominated by settling and vary with time in pressurized storage cylinders. Such measurements must be performed with high accuracy even when the concentration of particles is <1 /stdl of gas.
 Figure 1. Typical cylinder gas system layout. |
A number of gas-sampling techniques and instruments have been developed to meet these challenges. Such developments include low-noise and high-sensitivity particle counters, novel gas dilution techniques, and carefully designed low-bias sampling systems. Nevertheless, further advances in particle measurement techniques are required to meet the evolving needs of the semiconductor industry. The most significant advances will permit more routine particle monitoring in reactive gas supply systems, including storage cylinders and gas distribution systems.
Gas-handling options
The assurance of high cleanliness in semiconductor processing gases requires high-accuracy measurements. In clean gas systems, high accuracy can be achieved using large-sample gas volumes, low-bias sampling systems, and low-noise instruments. The methods available for making these measurements depend on such factors as gas properties (i.e., inert or reactive), and sample point pressure and flow rate. These factors, which influence deleterious processes such as corrosion and particle shedding within gas distribution lines, can have a strong effect on the actual particle level in the system.
Gas systems. Processing gases for semiconductors include various inert, flammable, corrosive, toxic, and oxidizing gases. The method by which the gas is supplied to the semiconductor fabrication facility varies among the gases. Gases may be delivered in bulk or in individual transportable cylinders. Bulk delivery systems may include on-site liquid or gas storage systems, dedicated cryogenic plants, or pipeline gas generated in an off-site plant. Transportable cylinders may contain a single gaseous phase or a condensed liquid (Fig. 1). Both bulk and cylinder systems must be designed to provide pressure and flow control, filtration, and (in some cases) purification at the point of entry to the wafer fabrication plant.
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Gases flow to the semiconductor processing tools through a branching piping system within the fabrication plant. Final pressure and flow control are located at the tool point of use. Numerous drop lines provide gas to the tool. Additional filtration and purification are frequently located near the tool inlet.
Gas pressures and flow rates vary greatly throughout each type of distribution system. Typical system pressures and flow rates range over orders of magnitude and can vary greatly with time within an individual distribution system. Due to the cyclic operation of processing tools, the flow rate becomes more variable as the tool is approached. The gas pressure decreases as it flows through the system; in some cases the gas is under partial vacuum at the tool interface, and can be as low as 1.3 x 102Pa (1torr) at the reactor entry point. At the tool vent, where flows are intermittent, gas pressures are usually below 1.3 x 104Pa (100torr) and may approach high vacuum. Each of these factors presents a challenge in efforts to provide tight control and accurate measurement of particulate contamination.
Metrology. Present capabilities in particle metrology for semiconductor processing systems depend on the sample point conditions as well as the gas composition. Table 1 shows the approximate smallest particle size that can be detected at several general system locations using available metrology.
Particle-counting instruments are generally classed into two groups: optical particle counters (OPCs) and condensation nucleus counters (CNCs). OPCs determine the equivalent optical diameters of contaminant particles through a process of light scattering from individual particles [1]. Such instruments have been developed for use with reactive or toxic gases. Some OPCs can be used for either in-line or external sampling, and for pressures ranging from vacuum to 2.5 x 107Pa (3600psig). OPCs typically function with low background noise for particles >0.1µm, but are noise-limited in lower-size detection capability because of light scattering from sub-range particles and gas molecules. Consequently, such instruments cannot detect contaminant particles smaller than about 0.1µm. Reactive gas-compatible OPCs generally have a detectable size limit of about 0.2µm.
 Figure 2. Schematic sampling for 105Pa particle counters. |
CNCs provide low noise detection of particles as small as about 0.01µm [2], and some specially designed CNCs can detect particles as small as 0.003µm [3]. CNCs flow the sample gas over a heated pool of working fluid to create a saturated mixture. Continuous conductive cooling is then used to create a supersaturated aerosol mixture. The fine particles act as nucleation sites for vapor condensation and subsequent droplet growth. Droplets grow to sufficient size to permit detection by conventional light scattering. Although CNCs provide better size sensitivity than OPCs, they are limited to operation at a pressure of about 105Pa (0psig), cannot be used for in-line sampling, and have (until recently) been limited to use with nonreactive gases.
Particle counters generate spurious counts known as "noise." Such counts must be compensated for in any statistical analysis of particle data [4]. Keeping the noise level low is desirable when measuring particles in high-cleanliness gases, where the particle count rate is low and where a strict particle specification must be met. OPC noise is typically on the order of 0.035 count/stdl (1 count/standard ft3) of sample gas. CNC noise is about 3.5 x 10-4 count/stdl (0.01 count/standard ft3) of sample gas.
Particle concentrations are computed by dividing the total number of detected particles by the total volume of sampled gas; thus the sampling volume strongly affects the accuracy of the measurement. The necessary sample volume is determined from statistical considerations and a knowledge of the required accuracy, and the method for calculating accuracy has been widely published [4]. In general, a larger sample gas volume tends to result in a smaller standard deviation for a measurement. Consequently, instruments having high sample flow rates tend to provide a more accurate measurement in a shorter time. Such high-flow instruments can also detect system upsets more rapidly when used in continuous monitoring applications.
Sampling methods. The particle-sampling system must provide a representative sample of the process gas to the particle counter. The sampling system must neither add nor remove a significant number of particles from the sample stream. Sampling bias is minimized by the use of electropolished tubing and high-cleanliness valves. The system must also be designed to minimize transport losses resulting from gravitational settling or diffusion to tube walls [5].
Several sampling system designs can be used for high-accuracy particle measurement. The sampling method used depends on the sample point conditions, the gas composition, and the particle counter design limitations. The external sampling system shown in Fig. 2 uses a clean flow restrictor [6] to reduce the sample stream pressure to about 105Pa (0psig). This system is used when the particle counter cannot withstand the process gas line pressure. The pressure reducer in Figure 2 is located downstream of an in-line isokinetic probe, which minimizes probe tip effects that tend to distort the measured particle concentration. The pressure reducer vents relatively large volumes of sample gas. Such venting is not practical for hazardous or expensive process gases. Therefore, this type of system is typically used only to sample from N2, Ar, He, O2, or H2 process gas lines at pressures <1.1 x 106Pa (150psig).
 Figure 3. In-line sampling system schematic. |
An external sampling system can be much simpler when the particle counter can withstand the process gas line pressure. Such a system can also be used to sample directly from pressurized gas cylinders. No pressure reducer is located upstream of the particle counter, and the system vents only the relatively small volume of sample gas flowing through the instrument. Therefore, this system is more practical for use with hazardous or expensive process gases, as well as inert and bulk gases. When reactive gases are sampled, the system must include provision for inert purging, evacuation, and emission control. Heat tracing of the sample line may also be necessary when sampling easily condensed gases. Heat tracing with inert purging and pressure cycling should be used for initial system drying when the gas may react with trace residual moisture in the line.
The sampling system shown in Fig. 3 is used for in-line OPC particle monitoring of process gas lines. This arrangement does not vent the sample stream and is best suited for hazardous and reactive process gases. OPCs require flow control within a specified range for proper operation. Therefore, two flow controllers must be used to direct the correct amount of flow through the bypass line and OPC. The balance of the gas flows through
Particle levels
Low levels of particulate contamination can be obtained in process gas systems by using careful system design, high-quality compatible materials, minimum dead legs and leak rates, careful start-up and operating procedures, etc. Low particle levels can also be obtained in gas cylinders through careful selection of cylinder materials, surface treatment and preparation, and through close attention to gas fill system design and operation [7].
Particle levels in flowing gas systems may be steady or (as in tool vent lines) cyclic over time. In tool feed lines, the gas is usually well mixed and particles are uniformly distributed. However, particle levels in gas cylinders can vary by orders of magnitude over time due to such effects as liquid boiling, gravitational settling, and diffusion to internal surfaces. Such effects may also produce nonuniform particle distributions, including stratification, in gas cylinders [8]. Levels of suspended particles in filled cylinders can be measured with a high-pressure OPC. Data obtained directly from cylinders show that careful attention to quality can result in low cylinder particle concentrations.
Cylinder and bulk gases are frequently reduced in pressure with an automatic regulator before entering the flowing distribution system. Automatic regulators may produce increased particle levels (through regulator shedding, impurity nucleation, and condensational droplet formation) that are sometimes followed by system corrosion [9] or suspended nonvolatile residue formation. Gases are therefore filtered after pressure reduction and before entering the distribution system.
Ceramic, metal, or polymer membrane filters are selected for compatibility with the process gas. Such filters can produce a low particle level as well as a low degree of variability in contamination over time. Table 2 shows typical particle levels in specialty process gases after pressure reduction and filtration; in each case at least 57stdl (2 standard ft3) of gas were sampled. A reactive gas CNC, recently developed by Air Products and Chemicals Inc., was used to detect particles as small as 0.01µm for the gases listed in Table 2.
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Particle levels can also be well controlled in O2 and H2 using filtration. External sampling systems of the type shown in Fig. 2 were used to obtain levels for particles as small as 0.01µm (Table 3). CNC data for particles as small as 0.003µm in O2 and H2 can also be obtained using an inert gas CNC with a special sample dilution device developed by Air Products [11]. These data show that membrane filters can be used to produce high-cleanliness gases to 0.003µm in large-volume gas systems.
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Well-designed distribution systems should contribute a minimum of additional particulate contamination to the flowing gas. For example, Table 3 also shows data obtained further downstream in a gas system, near the tool point of use. The sample points were hundreds of meters downstream of the filters, and in each case at least 850stdl (30 standard ft3) of gas were sampled. These data indicate particle concentrations similar to those obtained far upstream, following filtration.
Conclusion
High-quality semiconductor gas systems and in-line filters can produce nearly particle-free gas with minimal variability. This gas cleanliness can be maintained over large distances within the gas system. However, an accurate assurance of such cleanliness requires careful measurement techniques.
Many particle measurement methods (i.e., for bulk inert gas systems) have been well established and standardized by the semiconductor industry. However, some methods that are still being developed (i.e., for reactive and high-pressure gas systems) should also be standardized to assure uniform application. Standardization of procedures for particle measurement should also continue to follow the steady advances in available metrology. These efforts will lead to a tighter control of contamination in semiconductor processing gases.
References
- R.G. Knollenberg, D.L. Veal, "Optical Particle Counters and Spectrometers: Performance Characterization, Comparison, and Use," Proceedings of the Institute of Environmental Sciences, Annual Technical Meeting, pp. 751-771, 1991.
- P.B. Keady, F.R. Quant, G.J. Sem, "A Condensation Nucleus Counter for Clean Rooms," Proceedings of the Institute of Environmental Sciences, Annual Technical Meeting, pp. 445-451, 1986.
- M.R. Stolzenburg, P.H. McMurry, "An Ultrafine Aerosol Condensation Nucleus Counter," Aerosol Sci. Technol., Vol. 14, pp. 48-65, 1991.
- R.A. Van Slooten, "Statistical Treatment of Particle Counts in Clean Gases," Microcontamination, Vol. 4, No. 2, pp. 32-38, 1986.
- D.Y.H. Pui, Y. Ye, B.Y.H. Liu, "Sampling, Transport, and Deposition of Particles in a High-Purity Gas Supply System," 9th ICCCS Proceedings, Institute of Environmental Sciences, pp. 287-293, 1988.
- J-K. Lee, K.L Rubow, D.Y.H. Pui, B.Y.H. Liu, "Comparative Study of Pressure Reducers for Aerosol Sampling from High-Purity Gases," Aerosol Sci. Technol., Vol. 23, pp. 481-490, 1995.
- J. Hart, A. Paterson, "Evaluating the Particle and Outgassing Performance of High-Purity, Electronic-Grade Specialty Gas Cylinders," Microcontamination, Vol. 12, No. 7, pp. 63-67, 1994.
- J. Hart, W. McDermott, A. Holmer, J. Natwora, "Particle Measurement in Specialty Gases," Solid State Technology, Vol. 38, No. 9, pp. 111-116, 1995.
- N.M. Chowdhury, "Designing a Bulk Specialty Gas System for High-Purity Applications," Proceedings of the Institute of Environmental Sciences, Annual Technical Meeting, pp. 65-72, 1997.
- W.T. McDermott et al., "30-Angstrom Particle Measurement in a Manufacturer's Ar, N2, and O2 Systems," Proceedings of the Institute of Environmental Sciences, Annual Technical Meeting, pp. 328-332, 1992.
- W.T. McDermott, "A Gas Diluter for Measuring Nanometer-Size Particles in Oxygen or Hydrogen," Proceedings of the Institute of Environmental Sciences, Annual Technical Meeting, pp. 26-33, 1997.
About the Author
Wayne T. McDermott received his PhD in mechanical engineering from the University of California, Berkeley. He has published 29 technical papers, and has registered seven patents. McDermott conducts studies on the measurement and control of particulate contamination, surface cleaning, and multiphase fluid flow at Air Products and Chemicals Inc., 7201 Hamilton Blvd., Allentown, PA 18195; ph 610/481-4911, fax 610/481-5361.