MBE Frequently Asked Questions for non-MBE Folks

Q: If the toxic gas alarm goes off (blue lights), is it a false alarm?

A: Don't gamble with your life. Get out of the building!

Q: What is MBE?

A: http://en.wikipedia.org/wiki/Molecular_beam_epitaxy.

Q: Why do you do MBE? 

A: It lets us grow perfectly crystalline semiconductors and other layers with atomic control.

Q: If MBE users are wearing respirators, is it safe to walk through? 

A: Yes, but keep 6 feet distance, and don't enter the curtain if there is one installed. The major hazard of MBE maintenance is arsenic dust from all the GaAs deposited inside the machine. For small MBE openings, when there is little chance of arsenic dust, we wear respirators as a backup safety measure if we're working close to the MBE. For large openings which are likely to generate arsenic dust, we wear full disposable bunnysuits and erect a curtain to prevent arsenic dust from entering the rest of the room. We also clean the MBE, floors, & tools after each opening, and carefully dispose of the curtain and bunnysuits as hazardous waste. 

Q: I heard there is toxic gas in the MBEs. Is that true? 

A: One of the MBEs has digermane and similar gases installed, and these are toxic and pyrophoric. But there are similar gases installed in the ALDs, the ICP, and other fab tools. We go through extensive planning to minimize any risks from these tools, even in the event of multiple failures. Also, the toxic gases are purged and pumped out before any part of the machine is opened to air, and provide local exhaust. Toxic gas sensors near each tool will trigger the building toxic gas alarm in the event of a gas leak. 

Q: Is it true arsine and phosphine will kill you before you smell them?

A: Quote (DABM): "The olfactory threshold is very near the lethality threshold. In other words, everyone who knows what they smell like... is dead." Arsine smells like metallic garlic. Phosphine smells like fish. If you smell food in the cleanroom, either it's a bad sign, or someone just forgot to eat a breath mint after lunch. Better have someone check it out right away. Oh, and we don't intentionally use arsine or phosphine in the MBEs. 

Q: What do you mean by intentionally?

A: When the MBE is opened to air (a few times per year), a small amount of arsine is generated as arsenic reacts with water vapor in the air. To reduce this, we minimize the exposure of the MBE to air. The toxic gas monitoring system can detect arsine and phosphine as well as other gases.

Q: Can MBEs catch fire? I heard about this fire in Santa Rosa... 

A: The main fire risk in MBE comes from exposing phosphorous to air. We only use a small amount of phosphorous at Notre Dame, so we're not expecting any fires. 

Q: Why do you bake the MBEs?

A: To remove contamination, mostly from oxygen and water vapor. MBE requires ultrahigh vacuum (UHV) in the range of 1E-10 Torr, or 1/10000000000000th of an atmosphere. The pressure inside the MBE is 1000x lower than the "air" outside the space shuttle in low earth orbit. 

Q: Why do you bake them so often?

A: We have to bake whenever we have a big leak, and 2-3 times after a big opening. We try to keep the openings few and far between... like once/year. 

Q: Is MBE practical for real devices?

A: Your cell phone has parts grown by MBE. Probably your CD/DVD player, too. And mini satellite dishes. And GPS receivers. And...

Q: Anything else?

A: The world record for photovoltaics has been held by MBE for years. Nothing comes close.