8.2 Gen II MBE System Modifications and Substrates - NH_3 Compatibility

The most common materials attacked by NH_3 in a traditional MBE system are copper, especially copper gaskets, and most O-rings. Copper gaskets usually pose no big problems, since NH_3 seems to only attack the surface without contaminating the system. So far no leaks in our system could be traced to leaky copper gaskets. If the need arises, the copper gaskets can be replaced by aluminum gaskets.

The biggest problem are O-rings, which are readily attacked by NH_3, causing F and C contamination, as well as component failure. The only material known to be comfortably resistive to NH_3 is called Kalrez, manufactured only by DuPont, and about 3 orders of magnitude more expensive than Viton. Before replacing most Viton O-rings with Kalrez we had several NH_3 related O-ring failures, most severely the O-rings in the gate valves to the cryopumps. Regenerating the pumps causes the pressure of NH_3 to reach atmospheric pressure, causing the O-rings to fail. Replacing the 75¢ Viton O-ring with Kalrez cost about $500 each.

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