Vacuum systemsElectron beam – mean free path:Gun – column - sample

Signal detection – electron and X-ray collection:Scattering of emitted electrons and X-rays reduces signal/noise…

λ = A / N0 ρ Q A = atomic wt.

N0 = Avogadro’s number (6.02 x1023 atoms/mol)

ρ = densityQ = cross section (probability of an event)

Q = N/ntni N = events/vol.

nt = target sites

ni – incident particles

Smaller cross section & lower density = greater mean free path

Essential system:1) High vacuum pump (s)

Establishes and maintains high vacuum in gun, column, and sample chamber

Oil diffusion pump

Turbo-molecular pump

2) Mechanical pump (s)

Backing high vacuum pump

“Rough” pumps entire system when required

Rotary direct and indirect drive pumps

Dry pumps (scroll, piston, claw, diaphragm)

High-vacuum pump

Mechanical pump 1Mechanical pump 2

Ion pump

Gun

Column lensesSample chamber

spectrometer

Sample exchange airlock

Air inlet valves

High vacuum valve

Gun valve“manual” valve

Backing line (and valve)

Complete vacuum system:

Ion pump

Mechanical pumpsHigh-vacuum pump

At 1 atm (760 torr, 105 pa):~1019 molecules / cm3

mostly N2 and O2

molecule-molecule distance ~ 5nmmolecular mean free path ~ 0.1μm

At 10-2 torr:~1014 molecules / cm3

mostly H2O vapor, N2 and O2

molecule-molecule distance ~ 0.2μmmolecular mean free path ~ 1cm

At 10-7 torr:~109 molecules / cm3

mostly H2O vapormolecule-molecule distance ~ 10μmmolecular mean free path ~ 105cm (about 0.5 miles)

At 10-10 torr:~105 molecules / cm3

mostly H and He (can diffuse through walls of system)molecule-molecule distance ~ 100μmmolecular mean free path ~ 106m (about 50 miles)

Free molecules

Surface desorption

diffusion

permeation

10-1

101

10-5

10-9

10-13

105 109 1013

Pressure (torr)

Time (sec)

Mechanical pump operation

Mechanical pump (gas transfer pump)1) Rough pumps system from 1 atmosphere2) Backs high-vacuum pump

can use one pump for both purposes, or use two pumps3) Three general types

Indirect drive (Welch)Direct drive (Alcatel, Edwards, etc.)Dry pumps (Edwards, Varian, Pfeiffer, Leybold, Anest Iwata)

Indirect drive (belt drive)

Direct drive (rotary-vane)

Mechanical pumpTypical rotary pump creates low pressure by rotating cam or vane in oil

Rotates away from inlet, compressing air on other side and forcing through the outlet port

Kurt J. Lesker Co.

Dry scroll pumpUse one fixed and one orbiting scroll to create crescent-shaped gas pockets

Gas pockets are compressed and air is forced through central exit port

No oil used for sealing or lubrication

completely dry and contamination-free

Edwards XDS10

Varian Triscroll 300

High vacuum pumps1) Oil diffusion pumps (gas transfer)

10-3 to 10-10 torr

2) Turbomolecular pumps (gas transfer)

10-4 to 10-10 torr

3) Gas capture pumps

ion pumps

must operate in conjunction with other high vacuum

pumps

to 10-11 torr

Oil diffusion pump

1) Oil heated – boils

2) Vapor streams up and is deflected out and down through baffles

3) The large oil vapor molecules transfer momentum to air molecules that randomly enter pump (3-stage stack at right)

4) Oil re-condenses on side of pump that is actively cooled by circulating chilled water

5) Air molecules build up at base of pump

6) Mechanical backing pump removes air from base (4th stage)

500 - 1000 l/s pump rate

Can’t operate above ~ 10-2 torr (can “crack” the oil)

Kurt J. Lesker Co.

Foreline

Pump oil

Heater

Water cooling coils

Turbomolecular pumpsPurely mechanical

Very clean, fast

Stack of rotors which deflect incoming gas molecules with rotating-angled blades

Molecules hit underside of blade and are driven in direction of exhaust

~60,000 rpm

Back with mechanical pump

Two basic types of turbomolecular pumps:

Kurt J. Lesker Co.

SNECMA (Société Nationale d'Etude et de Construction de Moteurs d'Aviation)

Inlet at one end, exhaust at the other

PfeifferInlet between two rotor sets and exhaust at both ends

Ion pump (gas capture)Principal: gases are taken up by reaction with fine

particles of metal, or by ion implantation

Use parallel array of short stainless steel tubes (anode)

Plates of Ti (or Ta) near ends of tubes (cathode)

Generate strong magnetic field parallel to tubes

1) Gas is ionized in tubes by electrons released from cathode

2) Ions strike cathode and sputter Ti

3) Results in chemical reactions and ion burial

Generally used around electron gun

Kurt J. Lesker Co.

Measuring Pressure – Vacuum Gauges:Low vacuum

Thermocouple gauge (to ~10-3 torr)

Pirani gauge (to ~10-5 torr)Two filaments, one measurement, one reference

Thermocouple welded to filament, filament temperature dependent on thermal loss to gas. Thermocouple voltage responds to gas pressure

Filaments heated and the difference in temperature causes change in resistance of Wheatstone bridge (4 resistors, three known value). The current required to rebalance circuit is, therefore a measure of pressure.

High vacuumCold cathode (Penning) gauge

Inverted magnetron gauge

(to ~ 5 X 10-9 torr)

Hot Filament (Bayard-Alpert) gauge

(to ~ 10-11 torr)

+ ions released by HV discharge bombard metal cathode, releasing secondary electrons, which can, in turn, ionize gas atoms, adding to the discharge. Measure ion current and/or electron current.

Essentially an electron gun. Thermionic emission of electrons ionizes gas. Read ion current (function of pressure).

Measuring Pressure – Vacuum Gauges:

103 102 10-4 10-5 10-6 10-7 10-8 10-9 10-10101 100 10-1 10-2 10-3

Pressure (torr)

Bourdon (dial)

Piezo

Diaphragm manometer

McLeod

Pirani

Capacitance manometer

Thermocouple

Hot cathode ionization (Bayard-Alpert)

Cold cathode – (inverted magnetron, Penning)

Residual gas analyzer (RGA)

High-vacuum pump

Mechanical pump 1Mechanical pump 2

Ion pump

Gun

Column lensesSample chamber

spectrometer

Sample exchange airlock

Air inlet valves

High vacuum valve

Gun valve“manual” valve

Backing line (and valve)

Complete vacuum system:

Sample exchange sequence: