Center for Materials for Information Technology an NSF Materials Science and Engineering Center Vacuum Systems Lecture 6 G.J. Mankey

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Pumping Speed and Throughput The mass flow or throughput of a pump is given by the equation Q = SP where S is the pumping speed and P is the pressure. For a conductive element Q = C(P 1 -P 2 ) For elements of a system connected in series, we must add the conductance of these elements as in an electrical circuit: 1/C = 1/C 1 + 1/C 2 + 1/C 3 +… Conductance depends on pressure in the low to medium vacuum regions, and is independent of pressure in high to ultrahigh vacuum regions. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Calculations of Conductance In the molecular flow region, the conductance of a long straight circular tube is C = 12 d 3 /z liters/sec where d is the diameter(cm) and z is the length(cm). For an orifice C = 12 A liters/sec where A is the area in square centimeters. These equations should be used to estimate the effect of connecting pumps, hoses, etc. to a system to insure the pumps are properly utilized. The effective pumping speed of a system is then given by the equation 1/S eff = 1/S + 1/C where S is the pumping speed of the pump and C is the conductance of the associated flanges and fittings. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Standard Flanges Conflat flanges use viton or copper gaskets with a knife edge for high to ultrahigh vacuum applications. Care must be taken not to damage the flange knife edge. Standard sizes are mini (¾" ID), 2 ¾" (1 ½" ID), 4 ½" (2 ¾" ID), 6" (4" ID), 8" (6" ID) and 10" (8" ID). Medium vacuum applications use ISO and ASA flanges, low vacuum uses KF quik flanges.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Differential Pumping The amount of gas Q is equated: SP 2 = Q = C(P 1 – P 2 ) This trick can be used to maintain a constant pressure difference between two vessels. P1P1 P2P2 C S

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Rotary Vane Pump An oil seal between a phenolic vane and a steel cylinder is used to scavenge gas from the vacuum region and exhaust it to the atmosphere. This pump works from atmosphere to about 0.1 mTorr. Precautions must be taken at low pressures to avoid oil backstreaming into the vacuum vessel. It is also used as a backing pump for compression pumps like a diffusion pump or turbomolecular pump. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Oil Diffusion Pump Oil vapor forced through jets in the stack transfer momentum to gas molecules and force them down through the pump and out the exhaust (must be backed). The pump is characterized by a compression ratio and an ultimate pressure. Economical (no moving parts). If used with a cryogenic trap, UHV can be routinely achieved. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Turbomolecular Pump Turbine blades rotating at high speed transfer momentum to gas molecules to force them out the exhaust (must be backed). The pump is characterized by a compression ratio and ultimate pressure. Expensive (>$10k). UHV can be readily achieved (better if used in combination with a titanium sublimation pump). Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Gas Compression Ratio Since the pump works by momentum transfer, the compression ratio depends on the atomic mass. The thermal velocity of light gas is much greater, so the molecules are pumped less efficiently. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Mass Spectrum of Turbo System The gas composition reflects the difference in compression ration of light gases and the composition dependent outgassing rates of stainless steel. Usually hydrogen is the main constituent of a well- baked system. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Titanium Sublimation Pump High current (50 A) is passed through a titanium impregnated molybdenum filament to sublimate a fresh coating onto the cryoshroud walls. The film is highly reactive to H, CO and O and catalytically converts H 2 and CO to CH 4 which is more readily pumped by a turbo pump. Cooling the cryoshroud with liquid nitrogen goes the extra mile to get into the low mbar range. Pumping speed depends on gas, activated area and wall temperature (can be quite high, i.e. limited by inlet flange size).

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Ion Pump A high voltage combined with a magnetic field causes electrons to travel in a helical path with an energy sufficient to ionize gas atoms. The ions are accelerated so they strike a Ti plate and become buried in the plate. Can be started below mbar. Pumping speed is gas dependent and drops off below mbar. Buries the gas in the plate, so no backing pump is required. A little less expensive than turbo pumps. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Ion Pump Types Diode pump: center Ti electrode is biased positively to accelerate ions toward pump wall. Triode Pump: Intermediate Ti electrode is biased negatively to accelerate ions toward pump wall. Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Cryopump A He refrigerator is used to cool a large-area surface where gas is condensed. The gas absorption depends on the bonding mechanism to the cryopanels. After prolonged use, the pump must be “regenerated.” Ref: Inficon Instrumentation Catalog ( )

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Cryo Pump Speed for Various Gases Pumping speed depends on type of gas and area of selected cryopanel. Ref: Inficon Instrumentation Catalog ( )