Leaky Silane Fitting Seal
|Figure 1. 300X scanning electron microscope micrograph.|
The manufacture of many semiconductors involves the use of silane gas. This gas is pyrophoric – it ignites spontaneously when released into air. A fire broke out at a semiconductor fabrication facility, and the initial investigation traced the origin of the fire back to one of many fittings which joined various sections of the 3/8 inch diameter steel tubing of a newly installed 1250 pound per square inch pressurized silane gas distribution system.
These fittings were specially designed for this service given the reactivity of the silane gas; the mating surfaces were sealed with a deformable metal gasket, and the fitting nuts tightened to a specified gap. Testing of the fittings found near the fire origin did in fact confirm that one of them was leaking; however the fitting nuts were not loose. In fact, the leaking fitting was one of the two tightest of the four tested.
Figure 1 is a 300X magnification scanning electron micrograph showing the sealing surface of the gasket. The debris here is silicon and is residue of the silane passing the sealing surface. Because the debris here appears to be a result of the leak rather than a cause, and because the fitting was so tight, it was concluded that this fitting was over tightened, damaging the gasket and preventing proper sealing.