Q: What is MBE?

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

Q: Why do MBE?

A: It lets us grow perfectly crystalline semiconductors and other layers with atomic control. The original nanotechnology! MBE originally grew only flat planes, but our group developed techniques for lateral growth and regrowth for nano-scale devices.

Q: Why do you bake the MBEs?

A: We bake to remove contamination, mostly from water and oxygen, which are bad for common semiconductors. 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 International Space Station in low earth orbit. At these pressures, a single fingerprint left inside the machine would be detectable for months due to the skin oils, and they would contaminate the semiconductors we grow. We go through great pains to make sure the machines stay ultraclean inside–far cleaner than a cleanroom.

Q: Is MBE practical for real devices?

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

The economics of MBE have changed since the 1990’s. With robotic operation, MBE is now cheaper as a production tool–growing around the clock–than MOCVD. For example, one major wafer production company does very good growth using both MBE and MOCVD, and they prefer to grow new batches of wafers by MBE. One of the biggest expenses in MBE is the liquid nitrogen, which is running all the time and is a constant expense whether you grow 1 wafer/month or 1000.

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

A: Yes, but keep 6 feet distance, and don’t enter the curtain if there is one installed. The major hazard of GaAs MBE maintenance is dust from excess arsenic that gets 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 MBE 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 may have digermane and similar gases installed, and these are toxic and pyrophoric. But there are similar gases installed in cleanroom tools as well. 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 we provide local exhaust. Toxic gas sensors near each tool would trigger the building fire 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 needed a breath mint after lunch. Better have someone check it out right away. 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 days per year), a small amount of arsine is generated when 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?

A: The main fire risk in MBE comes from exposing phosphorous to air. We use almost no phosphorous in the EPEE group at Texas State, so we’re not expecting any fires. If we do start using larger amounts of phosphorous, we have plans for a phosphorous recovery system to minimize any possibility of fire.

Q: What do the names of the MBE machines mean?

A: The arsenide-antimonide MBE (a Veeco Gen 930) is named Firehole, after the river in Yellowstone National Park that Old Faithful Geyser flows into: the Firehole River. Also, due to all the hot springs, the river is full of arsenic. (Great fly fishing, but catch and release only!) The Group IV MBE (a Varian Mod Gen II with hybrid solid+gas sources for Si, Ge, C, and Sn) is named Green, after the Green River in Colorado. It also grows new green technologies for solar power and low-power electronics and silicon photonics.