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Cylinder Package effects on the purity of the electronic specialty gases

Cylinder package effects on the purity of electronic specialty gases

Philip B. Henderson, Ronald M. Pearlstein, Air Products and Chemicals Inc., Allentown, Pennsylvania

The composition of electronic specialty gases (ESGs) will remain unchanged for over two years in a well-prepared and passivated cylinder. Nevertheless, the cylinder package can affect the purity of an ESG if improper materials of construction, preparation, or handling techniques are employed. This report discusses these effects and how they can be avoided.

As device geometries of ICs continue to shrink below 0.25 ?m, more stringent control of individual processing steps used in manufacturing is necessary. Semiconductor manufacturers are demanding higher-purity ESGs and consistent purity of an ESG over time, both when a gas cylinder is in storage and in use. One concern of the process engineer is the degrading purity of the ESG from the time of original analysis, which may be several weeks or months before the ESG is used. In previous studies [1], we have found that more than two years after cylinder fill, the composition of ESGs remains unchanged, provided the cylinder is well prepared and passivated, and the analytical samples are withdrawn under controlled conditions using proper handling techniques. This is true not only for nonreactive gases such as nitrogen, but for reactive (e.g., SiH4) and corrosive gases (e.g., HCl, HBr) also. This report discusses the deleterious effects of ESG cylinder packages and how they can be avoided.

Materials of construction

Proper choice of wetted materials in storage containers and distribution systems used with specialty gases ensures reliable contamination-free delivery to the point of use. Storage and distribution sometimes have different requirements and should be considered separately. Table 1 summarizes the materials compatibility for storage of common ESGs. There are often several acceptable materials for a given gas; thus, the choice for a preferred package may be based on other considerations as well.

Many of the specialty gases used in microelectronics processes are stored under pressure in a liquefied form. Since the solvation power of liquids generally exceeds that of gases, the choice of material for a compressed gas cylinder may sometimes differ from that of the gas-phase distribution system. For example, liquid tungsten hexafluoride (WF6) readily attacks the chromium in stainless steel (SS), but gaseous WF6 can be satisfactorily delivered to a chemical vapor deposition (CVD) tool through properly passivated, electropolished type-316L SS tubing [2]. This highly reactive, liquefied specialty gas is therefore typically stored in nickel cylinders to preclude the dissolution of metallic contaminants.