Vacuum Systems
- Requirements
- at 1 X 10-4 torr e-s travel 1 meter
- most EMs operate at 2 X 10-4 torr
- contaminants
- block passage of e- beam
- degrade imaging quality of column
- charged contaminants affect e- path
- necessitates:
- aperture cleaning, column cleaning
- column tube replacement
- more frequent filament changes
- degrades sample
- causes arcing
- Rotary pump
- rates 100 - 1000 liters/min
- ultimate 5 X 10-5 torr
- inefficient below 10-2 torr
- based an eccentric pin and oil bath
- inlet connected to microscope
- air is based through oil bath
- outlet connected to mist filter
- present on all EMs
- maintenance medium; need to be rebuilt every year or two ($150)
- cost: $1500, but rarely need replacement
- Oil diffusion pump
- rates: 500-1000 liters/ sec
- ultimate 5 X 10-7 torr
- cannot operate above 10-2 torr
- inefficient
- oil will crack
- "backstreaming:" oil will enter column
- based on streaming of vaporized oil droplets
- oil heated to boiling by heating pad
- vapor rises rapidly in vacuum
- baffles redirect oil stream downwards
- oil droplets cause molecular convection
- oil condenses on water-cooled jacket
- pressure gradient generated by oil stream
- rotary pump is present at outlet
- maintenance annually; replacement of oil
- cost: $3000; power proportional to throat width
- Turbo-molecular
- rates: 500 - 1000 liters/ sec
- ultimate: 5 X 10-7 torr
- cannot operate above 10-2 torr
- rotor will bind
- no hydrocarbons introduced into column
- based on high-speed rotor
- 50,000 rpm turbine
- bearing acts in friction-less bearing
- inlet located at column
- rotary pump is present at outlet
- maintenance low; replacement cost high
- pump must be replaced every 5 to 7 years of use
- approx. $15,000
- Ion pump or Sputter ion pump (SIP)
- ultimate: 10-11 torr
- cannot operate at above 10-5 torr
- arcing will occur
- over-loads the ion gun
- based on molecules adhering to ionized metals - high voltage source ionizes gas
- metal sputtered from target
- adheres to opposite charged surface
- must minimize charged contaminants
- must be "baked-out routinely
- replacement every two years in "dirty" environment
- approx. $15,000
- maintenance high, replacement cost high
- Cryogenic pump
- ultimate: 10-8 torr
- cannot operate at above 10-2 torr
- ice forms on cryohead, insulating the head
- pumping must be minimized because of backstreaming
- based on molecules ceasing movement and adhering to cryohead
- single stage helium compressor removes heat from cryohead
- cryohead operates at 10 to 20°K
- cryohead freezes higher weight elements and compounds to head
- lighter contaminants are trapped by activated charcoal bed
- must be "baked-out" routinely with dry hot nitrogen gas
- replacement of charcoal bed when temperature of head rises
- approx. $15,000 cost
- low maintenance (replacement of compressor: ~10 years)
- Ultimate vacuum
- all O-rings will leak
- rubber or viton for >10 -7 torr
- copper for 10-8 torr ion gun systems
- Virtual leaks
- adherent gas molecules
- contaminants on surfaces
- specimens
- fingerprints
- photographic film
- Ultimate vacuum is an asymptotic function
- Pumping efficiency goes to nearly zero
- Leaks balance removal of molecules, so no progress is made