The Principles of Vacuum Technology Vacuum Terminology Vacuum units Vacuum regimes How to measure vacuum. Gauge designs. How to create vacuum Pump classifications and designs UHV compatibility considerations Materials to use in UHV and those to be avoided.
Units of Pressure Atmospheres (atm): Scale relative to our atmospheric pressure as 1 atm Pascal (Pa): SI unit equal to N/m 2 mbar Equal to 1x10-8 Pa. Torr or mmhg: Most commonly used pressure unit, based on mercury vacuum gauges.
Vacuum Regimes Atmospheric: 760 Torr Low Vacuum: 1 to 1x10-3 Torr Medium Vacuum: 1x10-3 to 1x10-5 Torr High Vacuum (HV): 1x10-6 to 1x10-8 Torr Ultra-High Vacuum (UHV): < 1x10-9 Torr Maximum acceptable base pressure for UHV experiments: 2x10-10 Torr
Why do we need UHV? Nearly all surface science experiments are carried out in UHV. Why? UHV is required primarily for two reasons: To obtain an atomically clean surface that will remain free of contaminates for the duration of the experiment. Gas scattering interferes with electron and ion based instrumentation.
Monolayer Coverage Time For an ideal gas, the time needed for monolayer coverage is given by: t ML 10 19 / Z A where Z A is impingement rate. Impingement Rate (Flux) For an ideal gas, the impingement rate is given by: Z A = ρ v / 4 where, ρ - the number density of gas molecules v - the mean velocity
Degree of Vacuum Pressure (Torr) Gas Density, ρ (molecules m -3 ) Mean Free Path (m) Time / ML, t ML (s) Atmospheric 760 2 x 10 25 7 x 10-8 10-9 Low 1 3 x 10 22 5 x 10-5 10-6 Medium 10-3 3 x 10 19 5 x 10-2 10-3 High 10-6 3 x 10 16 50 1 UltraHigh 10-10 3 x 10 12 5 x 10 5 10 4 Collision Free Conditions: Maintain a Clean Surface: P ~ 10-6 Torr P ~ 10-10 Torr
Thermocouple Gauge Heat filament with a constant current. Measure filament temperature with thermocouple. Gas molecules collide with and cool the filament. Voltage increases to keep filament at constant current. Atm to 10-4 Torr Fast, simple, inexpensive.
Pirani Gauge Two identical heated filaments; one sealed at HV, one exposed to system. Current flows through Wheatstone bridge circuit. Pressure difference indicated by meter (non-linear). Atm to 10-4 torr. Simple, reliable, inexpensive.
Ion Gauge (Bayard-Alpert) Heated filament produces electrons via thermionic emission. Electrons are accelerated towards anode grid. Many electrons pass through the grid and create positive ions from collisions with gas molecules. Ions are accelerated to collector wire. Measure the current between anode and collector. Operate at 10-4 to 10-11 Torr Sensitive, high accuracy, widely used.
Mass Spectrometer Quadrupole mass spectrometer - RGA (residual gas analyzer) 10-4 to <10-14 torr Total pressure mode integrates all ion intensities Partial pressure mode indicates residual vacuum composition Highly accurate, precise Complex, expensive.
Pumps Ultimate Pressure Low Vacuum (Rough) Pumps Rotary Vane Pumps Sorption Pumps High Vacuum Pumps Diffusion Pumps Turbo Molecular Pumps Ultra-High Vacuum Turbo Molecular Pumps Ion Pumps Titanium Sublimation Pumps Oil / Oil-Free Oil Rotary Vane Pumps Diffusion Pumps Turbo Molecular Pumps Oil-Free Turbo Molecular Pumps Ion Pumps Titanium Sublimation Pumps
Rotary Vane Pump Gas enters the inlet port and is trapped between the rotor vanes and the pump body. The eccentrically mounted rotor compresses the gas and sweeps it toward the discharge port. When gas pressure exceeds atmospheric pressure, the exhaust valve opens and gas is expelled. Atmosphere to 10-3 torr Robust, inexpensive Oil lubricated
Sorption Pump LN2 cooled molecular sieve with large surface area Atm to 10-3 Torr (two units working alternately) Quickly becomes saturated Must be baked at >200 C to remove adsorbed gases Simple, inexpensive, oilfree
Diffusion Pump Momentum transfer to gas molecules through collision with directed jet of oil molecules Require cooling water, backing pump 10-3 to 10-7 Torr (to 10-9 Torr with LN2 cooling) Advantages Robust High pumping speed for relatively low cost. No vibration or noise. Disadvantages Oil as a pumping medium, high risk of back-streaming oil, cold traps required Potential for serious vacuum problems
Turbo Molecular Pump Molecules mechanically pumped by collision with angled high speed turbine blades (rotor). Several rotor arranged in a series spinning at 30,000-60,000 rpm. Rotor tangential velocity is on the order of the average thermal velocity of molecules. Atmosphere to 10-10 Torr Oil/grease/electromagnetic bearings Most common HV/UHV pump.
Advantages Correctly operated they do not back-stream oil into the vacuum system at any time. They can be started and stopped in a few minutes time. Disadvantage Turbo pump can be noisy and they induce vibration. Turbo pumps are expensive.
Ion Pump High voltage between anode and cathode (~5 kv) Electrons are captured in anode and spiral due the to magnetic field (longer path-length). Gas molecules are ionized by collisions with electrons and are accelerated to cathode. Ions embedded in cathode material (titanium) and sputter titanium atoms from surface. Sputtered Ti atoms act as "getter" for reactive gases. 10-4 Torr to 10-11 Torr
Advantages Clean, oil-free. No moving parts, no vibrations, quiet. Low power consumption and relatively long operating lives Disadvantage Do not pump noble gases well. Requires regeneration of Ti every 4-6 years.
Titanium Sublimation Pump (TSP) Heated Ti filament evaporates Ti film onto cooled surface. Ti getters reactive gases by reaction. Operate at 10-8 -10-11 Torr Inexpensive, reliable Periodic operation - not primary pumping mechanism
Materials Considerations Outgassing rates Producing virtual leaks Mechanical stability Temperature stability Conductivity Chemical inertness Weldability
UHV Compatible Materials Oxygen free high-purity copper (OFHC) Be-Cu alloy Tantalum, Molybdenum, Tungsten Teflon (gassy) MACOR (machinable glass composite) Alumina Quartz, pyrex (gassy) "mu-metal" magnetic shielding (Co, Ni, Fe) Molybdenum disulfide (lubricant)
Materials to be Avoided (high vapor pressures) Zn, Cd: especially be careful of fasteners, bolts Brass Certain solders Any type of grease or oils Common Vacuum Problems Improper cleaning or handling techniques Using incompatible materials Leaks Virtual leaks
Obtaining UHV Pump Down Typically follows a well-defined sequence according to the types of pumps on the vacuum system For UHV systems, typically requires a few hours to reach a medium vacuum after a vent to air Bake Out Heat the chamber to temperatures between 100 o C and 200 o C for 1-2 days. Rapidly remove adsorbed gases from the chamber walls at high temperatures in order to lower the outgassing rates at room temperature. Generally it takes another day to cool and recover base pressure.
Ultimate Base Pressure (Steady State) Leak rates Flanges Virtual leaks Outgassing rates Contaminants Materials Samples Pumping Rates Type of pumps Type of gases being pumped H 2 H 2 O N 2 CO 2 Typical UHV mass spectrum of background gases after bakeout.
Reference and Suggested Reading http://www.uwo.ca/ssw/ (They have an excellent newsgroup for communicating with surface scientists around the world) http://www.uksaf.org/tutorials.html (An excellent resource with many surface related online tutorials) http://www.chem.qmw.ac.uk/surfaces/scc/
Typical UHV mass spectrum of residual gases before bakeout.