The continued increase in semiconductor device sophistication, and resultant decrease in line widths, require cleaner components throughout process gas delivery systems. Properly designed high purity gas filter assemblies are transparent to homogeneous constituents in the gas stream, i.e., to remove particulate contamination with no other effect on the inlet gas purity. In order to accomplish this goal, the assemblies used to house the filter devices are most often constructed of smooth, clean and electropolished 316 stainless steel.
“Smooth and clean” is a relative term, requiring more explicit definition. Specification requirements are established whenever performance limits for filter assemblies are clearly understood and measurable. For example, moisture contribution and surface roughness are reasonably well defined. However, these criteria are based on measurements limited by the sensitivity of the best available instrumentation. In order to meet these criteria, the “best” available raw materials from which to fabricate components are often specified. The logic in simply picking the best material follows only if the material ultimately meets the intended specification requirements and if the filter assembly manufacturer understand which material properties are governing.
Yaang carefully evaluates all raw materials used to fabricate semiconductor gas filter assemblies. There are many important characteristics of 316L stainless steel that must be considered in designing these high purity assemblies. Type 316L stainless steel is a well characterized and understood alloy. Its good corrosion resistance is a result of both careful formulation of the major alloying elements (Fe, Ni, Cr, Mn, and Mo), as well as the control of critical trace constituents (C, S, and Si). Similar to other finished components, filter assemblies utilize stainless steel which has been further worked, i.e., rolled, drawn, machined, or welded.
In these cases, it is not sufficient to merely specify material chemistry. Each intended use of the stainless steel may entail slight modification of the basic specification, but the most commonly specified requirements resulting in a fine, electropolished surface finish are:
- balanced 316L chemistry.
- homogeneous austenitic microstructure.
- very few inclusions.
- fine grain structure.
Materials Comparison: Stainless Steel Bar
* = Maximum
1 316L VAR does not specify (or result in) chemical composition which is of higher performance than SA479 with the noted exception of reduced sulfur.
2 Typical measured values were within a window of 5-10.
3 Not specified. Typical indicated.
In order to remove microscopic voids in the melt, as well as to further purify the metal by reducing certain contaminant impurities, many high purity stainless steels undergo a secondary melting process (e.g. vacuum arc remelt, known as VAR). This process reduces sulfides, silicates, phosphates, aluminas, and other impurities. Of these impurities, sulfur is the most detrimental to the achievement of fine surface finishe (assuming the other impurities are within normally specified limits).
These trace impurities tend to disturb (“break”) the stable austenitic structure and form inclusions which are less noble than the parent metal. When the inclusions are infrequent and very small in size, this may not be a problem. However, when very fine surfaces (<10 microinch Ra) are desired, all inclusions become apparent after final mechanical polishing or electropolishing. It is for this reason that 100X microscopic inspection after fine mechanical polishing is commonly used to quantify the inclusion content. Electropolishing often accentuates the defects in the surface by removing the less noble inclusions first, via preferential dissolution.
Grain structure must also be considered when choosing high purity stainless steel. Fine grained materials are preferable since they tend to be more homogeneous. As grains grow (e.g during slow cooling), there is more time for the austenite breakers to precipitate at the grain boundaries. This has the effect of transforming a clean dislocation of the structure (“healthy” grain boundary) into a site for inclusions. In addition, fine grained materials conduct electric current more uniformly. Uniform conductance is an important characteristic during the electropolishing process.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)