Introduction of Vacuum Science & Technology Diffusion pumps used on the Calutron mass spectrometers during the Manhattan Project. 1
What is a vacuum? 760 mm Hg Vacuum ATM A vacuum is defined as less than 1 Atmosphere of pressure. 1 Atm = 10 5 Pa = 10 3 mbar = 760 Torr Below 10-3 Torr, there are more gas molecules on the surface of the vessel than in the volume of the vessel. High Vacuum < 10-3 Torr Very High Vacuum < 10-6 Torr Ultra High Vacuum < 10-8 Torr
Pressure Units The SI unit for pressure is the pascal (Pa), equal to one newton per square meter (N/m 2 or kg m -1 s -2 ). 3
Conversion of Common Units 1 atmosphere = 1.0133 bar 1 atmosphere = 760 torr 1 atmosphere = 1.0133 10 5 Pa 1 torr = 1 mm Hg 1 micron Hg = 1 millitorr 1 millibar = 100 Pa 1 torr = 133.32 Pa 1 millibar = 0.75 Torr
Monolayer Time Sticking Coefficient S = # adsorbed / # incident Impingement rate for air: Z = 3 x 10 20 cm -2 s -1 Area of an adsorption site: A 1 Å 2 = 10-16 cm 2 We define the monolayer time as the time for one atomic layer of gas to adsorb on the surface: = 1 / (SZA). At 3 x 10-5 Torr, it takes about one second for a monolayer of gas to adsorb on a surface assuming a sticking coefficient, S = 1. At 10-9 Torr, it takes 1 hour to form a monolayer for S = 1.
Vacuum Range
Why do we need a vacuum? Keep surfaces free of contaminants. Process films with low density of impurities. Maintain plasma discharge for sputtering sources*. Large mean free path for electrons and molecules. *Discharge is maintained by secondary electrons emitted from cathode, with energetic Secondary electrons providing the ionization required to maintain the discharge
How does one achieve vacuum? Pumping Two types Transfer relies on moving molecules from low to high pressure regions Trapping makes use of chemistry to trap or bury gas particles
Types of pumps Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere. Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas molecules out of the chamber. Entrapment pumps capture gases in a solid or adsorbed state. This includes cryopumps and ion pumps. 9
Pressure Ranges Rough vacuum >1 mtorr Rotary vane pump Medium Vacuum 10-8 Torr < P < 1 mtorr Cryo pump, Diffusion Pump, Turbo Pump, Ion pump High to Ultra High Vacuum 10-10 Torr < P < 10-8 Torr Turbo, Ion, Titanium Sublimation Pump, Cryo pump.
Positive displacement pumps are the most effective for low vacuums. Momentum transfer pumps in conjunction with one or two positive displacement pumps are the most common configuration used to achieve high vacuums. In this configuration the positive displacement pump serves two purposes. First it obtains a rough vacuum in the vessel being evacuated before the momentum transfer pump can be used to obtain the high vacuum, as momentum transfer pumps cannot start pumping at atmospheric pressures. Second the positive displacement pump backs up the momentum transfer pump by evacuating to low. Entrapment pumps can be added to reach ultrahigh vacuums, but they require periodic regeneration of the surfaces that trap air molecules or ions. 11
Vacuum Systems A vacuum system consists of chamber, pumps and gauges. Chambers are typically made of glass or stainless steel and sealed with metal gaskets. Pumps include mechanical, turbomolecular, diffusion,cryogenic.
Mechanical pump designs Scroll pump Rotary pump Reciprocating piston pump Diaphragm pump
Diffusion Pumps Rely on jets of boiling fluid (usually silicone oil) to force air particles out of the region being evacuated. Cold traps prevent back streaming.
Turbomolecular Pumps Similar in design to a jet engine. Alternating rotor and stator blade assemblies turn at 20,000-90,000 rpm to force out molecules. Requires a region of low or medium vacuum behind and in front of pump. Pfeiffer Vacuum GmbH
Ion Pumps
Cryopumps It uses a cryogenically cooled surface by helium cryocompressor attached to the pump to condense and trap gas molecules. Cryopumps are particularly suited to pumping atmospheric gases and high melting point vapors (H 2 O) 3.3 K- Temp of liquid helium Kurt J. Lesker Vacuum Technology
Cryopumps Regeneration of a cryopump is the process of evaporating the trapped gases. This can be done at room temperature and pressure, or the process can be made more complete by exposure to vacuum and faster by elevated temperatures. Best practice is to heat the whole chamber under vacuum to the highest temperature allowed by the materials, allow time for outgassing products to be exhausted by the mechanical pumps, and then cool and use the cryopump without breaking the vacuum.
Titanium sublimation pump It consists of a titanium filament through which a high current (typically around 40 Amps) is passed periodically. This current causes the filament to reach the sublimation temperature of titanium, and hence the surrounding chamber walls become coated with a thin film of clean titanium. Since clean titanium is very reactive, components of the residual gas in the chamber which collide with the chamber wall are likely to react and to form a stable, solid product. Thus the gas pressure in the chamber is reduced. a chemical reaction occurring at the exposed surface.
Vacuum flange technology A vacuum flange is a flange at the end of a tube used to connect vacuum chamber, tubing and vacuum pumps to each other. CF (ConFlat) flanges use a copper gasket and knife-edge flange to achieve an ultrahigh vacuum seal.
Measuring the vacuum Ion gauges: is the most widely used low pressure (vacuum) measuring device for the region from 10-3 to 10-10 torr. Filament emits electrons which circle the grid, bombard with gas molecules to create ions, which are subsequently accelerated toward the collector. the ion current from a gas constant composition will be directly proportional to the molecular density of the gas in the gauge. MDC Vacuum, Inc.
Materials in Vacuum Outgassing of materials can be the limiting factor in achieving good vacuum. It is usually best to use all stainless steel, aluminum, glass and copper. NEVER USE: Brass, zinc, or other alloys without first looking up the outgassing rate