Gas cell selection for analysis of electronic gases

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Courtesy of CIC Photonics, Inc.

APIMS and FTIR remain the preferred analytical tools for monitoring impurities in the electronic  specialty gases used by the semiconductor industry, although other tools such as CRDS are currently finding application in selected cases. FTIR spectroscopy offers the advantage of the simultaneous measurement of all gaseous species that exhibit dipole moment changes during vibrations at concentration levels down to the ppb level. FTIR also offers the advantage of transferring all contact with the corrosive and/or toxic gases to a long or short path gas cell that contains the sample gas. Therefore, the design and the  materials of which the gas cell is fabricated become the critical features determining the quality of the impurity analyses. This paper will present and review the advantages and disadvantages of several gas cell configurations on the market today, including the following features: optical design (multipass “white cell,” folded path, direct pass), pathlength, materials of construction (stainless steel, aluminum, glass), mirror substrates and coatings, windows, cell finishes/coatings, cell volumes, measurement wavelength (IR & UV), sample gas composition, cell pre-treatment and  decontamination, energy throughput, and sensitivity.

The uses of electronic specialty gases in semiconductor applications are several, including:

  • Ion Implantation
  • Thin Film Deposition
  • Sputtering Operations
  • Physical Vapor Deposition
  • Chemical Vapor Deposition
  • Wafer Cleaning and Etching
  • Synthesis of Compound Semiconductors

Many of these applications involve the use of very high purity gases that are both corrosive and toxic, such as HCl, HBr, Cl2, WF6, SiH4, SiH2Cl2, NH3, PH3, etc. It is the chemical nature of these gases that creates great problems for most analytical methods. Among the problems for
both the user and the analyst are:

  1. Many of these gases can not be produced at the highest purity required by the semiconductor industry, a major contaminant being moisture;
  2. The moisture content of the halogen-type gases makes them highly acidic;
  3. The chemical reactivity of these gases and their impurities leads to degradation of optics, materials, and seals in both process tools and analytical instruments;
  4. The acid gases react with and destroy the electronic elements in some mass spectrometers, gas chromatographs, and electrical conductivity devices; they also chemically degrade the mirrors and windows in gas absorption cells;
  5. When sampling of a gas stream is required, the expansive cooling of the gas can yield liquid droplets, which are even more chemically destructive of electronic and optical elements because of their highly concentrated nature;
  6. For the process tool operator, these gases are highly toxic if leakage occurs due to any number of equipment failures: breaks in process lines, bad welds, degraded vac-uum connectors or seals, operator error, etc.;
  7. Most critical, perhaps, is the fact that variations in the quality of the process gas, due in many cases to moisture as an impurity, interferes with the kinetics of the semiconductor wafer etching or treatment process.

The analytical tools being used today for monitoring the quality of the electronic specialty gases all have their advantages and disadvantages. Key points for each are given in Table 1.

Both APIMS and FTIR are the standard analytical methods accepted by the semiconductor industry for qualifying the purity of cleaning, etching, and treatment gases. FTIR is broadly used also for PFC emission measurements.

When FTIR is coupled with long path gas cells, it is not only applied to semiconductor gas monitoring but also to a broad spectrum of other analytical applications, including combustion and stack gas compositions, tobacco blen-ding, fire and forestry, hydrogen fuel cells, battery tests, and medical and air reduction gases. This breadth of applications has led to a multiplicity of long path gas cell designs and manufacturers. Illustrated in Figure 1, are a few of these cells, from Infrared Analysis, Gemini Scientific Instruments, Axiom Analytical, Thermo Nicolet, CIC Photonics, among others.

Several different optical designs of long path gas cells exist in the field of analytical applications, each having specific or unique features as shown in Table 2.

Over the years, the white cell has been the most popular and widely used gas cell, because it can be readily fabricated from a variety of construction materials at a broad range of prices. The Folded Path cell offers fewer mirror reflections per unit pathlength, but its vertical height is not conducive to industrial applications. The Herriott cell and the CRDS Cavity cell both achieve PPT sensitivity due to their exceptionally long pathlengths, but a distinct
laser frequency is required for each molecular absorption line to be monitored.

The application of long path gas cells to electronic specialty gases has introduced a number of material and performance problems due to the chemical reactivity of many of these gases. Such problems are listed in the next four tables, Tables 3 – 6. The information in these tables is based upon the experiences of the author and many customers.

Table 3 identifies the principal problems and their frequency of occurrence. With stainless steel cells and nickel- plated stainless steel cells, degradation of the cell body rarely occurs even after years of service. Some prospective users request that the cell body be made from a high nickel-content alloy, such as Monel or Hastelloy, but fabrication from those materials is very expensive. The author has found that nickel-plated stainless steel serves equally well at much lower cost. Contamination from gas-phase produced powders on the mirrors or windows and the failure of O-rings and C-seals are also low in occurrence.

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