Vacuum Fundamentals

1 atmosphere = 760 mm Hg = 101.3 kPa1 torr = 1 mm Hg

vacuum range pressure range low 760 ~ 25 torr medium 25~ 10-3 high 10-3 ~ 10-6

very high 10-6 ~ 10-9

ultrahigh 10-9 ~ 10-12

extreme high 10-12 ~ less

Gas law:P=nkT, p: pressure in Pascal, n: molecular density (m-3), k: Boltzmans’ constant, T: temperature (k) Gas load: after initial pump down, gases continue to enter into chamber from leaks and outgassing

True leaks: leaks from an infinite source such as air outside the chamber walls

Trapped volumes:gases from trapped volumes (internal finite leaks), e.g. cracks in inside chamber walls, sandwiched gaps in screws and other internal fixtures

Adsorption: when exposed to ambient atmosphere, gases physisorbed or chemisorbed on the surface

ofSolids. These adsorbed layers consists mostly of water molecules with carbon oxides,

oxygen, and nitrogen. (a monolayer forms in about half an hour even in 10-9 torr)

Desorption: a strong function of temperature (temperature accelerates the desorption process)

Permeation :-a gas migrates from the outer wall of a vacuum vessel to the inner wall (the gas adsorbs

on the outer wall, and diffuses through the bulk of the solid, then desorbed from the interior

wall) -For metal solids (not for glass, ceramic, and polymeric solids), hydrogen is one of the few gases that can measurably permeate metals

Evaporation:release of molecular material from the same solid or liquid material to the gaseous phase

Backstreaming:migration of molecules from the pump back into the pumped volume

Metals: - Better suited for vacuum systems then glasses, ceramics, and polymers (due to metal’s low absorption and permeation coefficient).- Alloys containing zinc, lead, cadmium, selenium, and sulfur should be avoided (high vapor pressures) - The most commonly used stainless steels are the 304 and 316 types

4 ways vacuum cross

Glasses and Ceramics:Borosilicate glasses are used whenever baking temperature exceeds 350 ºC Fused silica is required for temperatures higher than 500 ºC.

Glass viewport

Materials

Polymers:the least desired material for vacuum applications due to their large porosity and

their release of particles during compression

Vacuum components

Del Seal 2-3/4 inch flange

Copper gasket

Kwik flange

4 inch gate valve

Water pump

- Ionize gas, under strong electrical potentials (3~7 kV) accelerate them into a solid electrode. (swirling electrons in plasma ionizes incoming gas atoms and molecules while they are trapped in a strong magnetic field) - The swirling ions strike the chemically active cathode inducing sputter and are then pumped by chemisorption which effectively removes them from the vacuum chamber, resulting a net pumping action - capable to reach <10-11 mbar

Ion pump

Diffusion pump

-capable to reach 10-9 mbar-high speed, low cost

Viewport forOptical Excitation

Ion Gauge

Chamber

Ion PumpGate Valve

Ion Pump

ChamberGate Valve

N2

RoughingPump

PurgeValve

RoughingPump Valve

ThermocoupleGauge Electrical feedthrough

pins/SMA

Illustration of an ultra-high vacuum system