Transcript Slide 1
Pressure 1 atmosphere ~ 1 bar ~ 760 mm Hg ~ 760 torr ~ 100,000 Pa Ion gauges read in mbar i.e. 1x10-10 mbar = 1x10-13 atm. Sometimes ion gauges read in torr but ours are set to mbar Lower Pressure 1x10-4 mbar Rough Vacuum 1x10-8 mbar High Vacuum Ultra High Vacuum Viscous vs. Molecular Flow Regimes The gas in a vacuum system can be in a viscous state, in a molecular state or in a state which is intermediate between these two. When a system is brought from the atmospheric pressure to "high vacuum", the gas in the system goes through all these states. The mean free path of the gas molecules is very small at atmospheric pressure so that the flow of the gas is limited by its viscosity. At low pressures where the mean free path of the molecules is similar to the dimensions of the vacuum enclosure, the flow of the gas is governed by viscosity as well as by molecular phenomena; this is the intermediate flow. At very low pressures where the mean free path is much larger than the dimensions of the vacuum enclosure, the flow is molecular. Viscous > 10-4 Molecular < 10-6 Pumps WE WILL ALWAYS MEAN mbar WHEN WE SAY PRESSURES! PUMPS: Rotary goes to ~ 1x10-2 Turbo (depending on size) goes to ~1x10-9 Ion Pump goes to ~1x10-10 TSPs help maintain UHV Rotary Pump http://www.quorumtech.com/Products/RV5PUMP.jpg Turbomolecular Pump http://www.varianinc.com Turbomolecular Pump Turbo pumps utilize a stack of turbine blades which rotate at very high speed (of order 50,000 rpm) to move gas from the inlet port to the exhaust port. Turbo pumps are very effective at low pressures (<10 torr), essentially in the molecular flow regime in which gas densities are so low that the molecules collide with chamber walls far more often than with each other. As a result, turbo pumps can achieve chamber base pressures of 10-9 torr or below. However, the high packing of fan blades and the high rotation speed of the turbo pump make it ineffective at higher pressures, where fluid (viscous) flow dominates. Powering a turbo pump alone at atmospheric pressure will barely cause the blades to rotate. THEREFORE TURBOS ARE BACKED BY ROTARY PUMPS Ion Pumps 1 Permanent magnets 2 Pump envelope 3 Titanium cathodes 4 Anode cell array 5 Positive high voltage lead http://www.thermionics.com/ip_too.htm Ion Pumps Sputter ion pumps operate by ionizing gas within a magnetically confined cold cathode discharge. The events that combine to enable pumping of gases under vacuum are: Entrapment of electrons in orbit by a magnetic field. Ionization of gas by collision with electrons. Sputtering of titanium by ion bombardment. Titanium gettering of active gases. Pumping of heavy noble gases by ion burial. Diffusion of hydrogen and helium into titanium. Dissociation of complex molecules into simple ones for pumping ease, e.g., CH4 breaks down into C and 2H2. Hydrogen is pumped separately. Carbon is no longer part of the residual gas and resides in solid form. http://www.thermionics.com/ip_too.htm Titanium Sublimation Pumps (TSPs) Resistively heat Ti metal Thin layer of Ti on chamber walls This reacts with O2 etc and pumps chamber N.B. Sample areas must be shielded!