1.3. General Vacuum Chamber Features

1.3. General Vacuum Chamber Features

Several different types of pumps (different operating pressures) Pressure measurement (multiple gauges)

Sample movement, heating, cooling, cleaning, viewing Gas/vapor admission system (controlled "leak")

Multiple analytical techniques (neutral or charged particles or radiation)

1.4. Construction and Design

Stainless steel construction

1. Modular (metal-to-metal seals)

Elastomer o-ring seals

Weld Buna N or Viton O Ring

Centering Flange

Clamp

Reuseable, inexpensive

atm to ~10

(permeation) torr

Elastomer (Viton, Buna) limited to <150 °C, certain solvents Metal knife-edge seals

ConFlat (CF) Fitting

Flange

Nut and Bolt Weld

304 Stainless Steel Tube

OFHC Cu Gasket Knife Edge Vertical Wall

1/8"

Durable, atm to 10

torr

1.5. Vacuum Pumps

1.5.1. Pressure Regimes A. Viscous Flow

Pressures >10

torr Mean free path short

Molecule-molecule collisions more frequent than molecule-wall Gaseous momentum transfer

Net pressure gradient

Flow laminar, Poiseuille, turbulent B. Molecular Flow

Pressure <10

torr Mean free path long

Molecule-wall collisions more frequent than molecule-molecule No gaseous momentum transfer

Almost no net pressure gradient

Molecules removed by chance collision with pumping surface

K

= λ

a Knudsen Number

λ = mean free path a = diameter of tube K

< 0.01 viscous flow

0.01 < K

< 1 transitional (Knudsen) flow

K

> 1 molecular flow

1.5.2. Rotary Vane (Mechanical) Pump

Atmosphere to >10

torr Robust, inexpensive Single- or two-stage

Belt-drive or direct-drive electric motor

Oil lubricated

1.5.3. Sorption Pump

LN

cooled molecular sieve with large surface area (2500 m

g

) condenses (sorbs) many gases

Quickly becomes saturated with gas

Must be baked at >200 °C to remove adsorbed gases Atm to 10

torr (two units working alternately) Simple, inexpensive, oil-free

Poor pumping of non-condensable gases (noble gases, H

, O

)

1.5.4. Turbomolecular Pump

No physical barrier between high and low pressure side

Directed molecular momentum through collision with high speed angled turbine blade (rotor)

Fixed stator blade produces additional momentum exchange

Net molecular flow small but several rotor/stator pairs arranged in series Oil/grease/electromagnetic bearings

Atmosphere to 10

torr when backed with rotary vane pump Moderate pumping speed

Fragile, expensive, poor pumping of light gases

1.5.5. Diffusion Pumps

Momentum transfer to gas molecules through collision with directed jet of oil molecules

Require cooling water, backing pump

10

to 10

torr* without LN

baffles (to 10

torr* with LN

baffles)

*depends on fluid

Robust, high pumping speed, inexpensive, reliable

Oil decomposition, "dirty"

1.5.6. Sublimation/Getter pump

Heated Ti filament evaporates Ti film onto cooled surface Ti getters reactive gases by reaction

10

-10

torr

Inexpensive, reliable

Molecule specific pumping speed

Periodic operation - not primary pumping mechanism

1.5.7. Sputter-Ion Pumps

High voltage between anode and cathode (<10 kV)

Gas molecules ionized near anode 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

Magnetic field applied to create spiral ion trajectories (longer pathlength) 10

torr to 10

torr

Reliable, oil-free, no moving parts

Pumping speed varies with molecule, rapid pumping of H

, O

, poor pumping

of noble gases

1.6. Attaining Ultrahigh Vacuum

Rapid initial followed by slower pressure drop A. Evacuation (e

) - removal of gas volume

B. Surface desorption (1/t) - adsorbed water on onner surfaces C. Wall diffusion (1/t

) - gases dissolved in walls

D. Permation limit (ultimate pressure) - gas diffusion through walls

Baking entire chamber at 100-200 °C for 24 hours increases rate B and C.

RGA of turbomolecular-pumped system