Pumping high vacuum cryogenic

Metal films (Aesar, 99.999%) were thermally evaporated from tungsten baskets at 0.1 to 5 A/s in a cryogenically pumped high-vacuum system with base pressure below 10" Torr Au films utilized a 10 - 15 nm-thick Cr ac esion underlayer. SAW devices were masked to expose between 50 and 75% of the region between input and output IDTs for metal deposition. 

Two vacuum systems are used to provide both the high vacuum needed for the mass spectrometer and the differential pumping required for the interface region. Rotary pumps are used for the interface region. The high vacuum is obtained using diffusion pumps, cryogenic pumps, or turbo pumps. 

An extremely high vacuum (10 Torr), free of any electrically active impurity (see Chapter 20), is required for successful production of good superlattices. The inner walls usually have a cryogenic liner filled with LN2 to trap any condensable residual gases. Since pumping such a system down to the vacuum levels is a time[Pg.269]

The evacuated powder type insulations considered were perlite, silica-aerogel, and peach pit charcoal. While the evacuated powder provides excellent insulation in certain cryogenic applications, it was not best adapted for this specific application due to very high installation cost. The factors responsible for these high costs are the additional number of filling and vacuum pumping ports which would be needed, plus the additional vacuum manifold which would be necessary for the full length of the transfer line. 

In the first prototypes, the pinhole inner diameter was only 25 pm, while in the Sciex TAGA a larger pinhole could be applied, because of the high pumping efficiency of the cryogenic vacuum-pumping system available in this instrument. 

Thermal isolation. If the detector is used in a hot environment or is cooled, then it must have some thermal isolation from the environment. The required thermal impedance depends on the temperature difference between the cold-sink and the environment and, if cooled, the available cooling power. To achieve that isolation may require the use of special high-thermal-impedance materials (see Chapter 12, Cryogenics) in some cases, the package may have to be evacuated or backfilled with a low-thermal-conductivity gas. Both of these impose special cleaning ( outgassing ) requirements, and necessitate an evacuation ( pump-out ) port, and hermetic seals for any windows and electrical leads. These vacuum issues are described in Chapter 13 (Vacuum). 

Instrumentation. Metal evaporations were carried out in a cryogenically pumped vacuum system with a base pressure of 5 x 10" torr (Edwards Auto 306A). Au (99.99%) was thermally evaporated at a rate of 0.1 A s l onto the Si substrates which had been affixed to a large aluminum holder with Ag paste (SPI Products). The temperature during the depositions, measured by a thermocouple imbedded in the aluminum holder, did not rise more than l C. Thicknesses were monitored with a quartz crystal microbalance. Once evaporations were complete, the chamber was back-filled with high purity N2 and the samples immediately transferred to the SFM for imaging. Au(lll)/mica substrates were prepared as previously described (30). 

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