Vacuum Science and Technology

Why?
1. To remove active atmospheric constituents (e.g. deposition of Al which may end up Al2O3)
2. To decrease energy transfer (so that only transfer by radiation only)
3. To remove occluded or dissolved gas or volatile liquid (clean substrate surface)
4. To extend the long mean free path (prevent collision)
5. Particles move in a straight line over large distance
Gas flow, conductance, pressure ranges
1. Assumption: Gaseous molecules = hard spheres
2. Maxwell speed distribution
3. Gases at atmospheric pressure: - change in velocity is due to gas phase conditions
  1. 1. A molecule with diameter, d moving randomly in a gas
  2. 2. collisions occur
  3. 3. molecule has a collision cross section of pi d square
4. Primary standards to describe gas flow
  1. Throughput
  2. Standard volume or standard liter
5. Flow regimes
  1. Viscous flow
    1. Distance between is small; collisions between molecules dominate, flow through momentum transfer, generally P>0.1mbar
  2. Transition flow
    1. Region between viscous and molecular flow
  3. Molecular flow
    1. Distance between molecules is large; collisions between molecules and wall dominate; flow through random motion, generally P<10^-3mbar
Vacuum pumps
1. Rough vacuum pump
  1. Involves 3 steps: (i) capture of a volume of gas; (ii) compression of the captured gas; (iii) gas expulsion
  2. Low-medium vacuum
    1. Rotary vane pump
      1. Ways to increase thoughout
        increase the volume pumped
        increase the rotational speed
      2. Backstreaming
        Solution: molecular sieve trap, dry pump, inert gas purge flow
        Cause contamination
        As pump operate, oil gets hot
      3. Condensation of vapour
        solution: oil free pump - poor sealing, short lifetime, dusty
        liquids mixed with oil may cause corrosion
        if the pressure of vapour is high (due to compression), gas will condense to liquid droplets
      4. Purpose of oil
        to compress the gas
        to cool the pump
        as lubricant
        to seal the vane
    2. Roots blower
      1. pump without sliding seals
      2. consists of two figure-8 rotors that revolve at high speed in opposite direction
      3. compression is achieved by narrow clearance
      4. normally used as pre-compressor for a conventional rotary vane pump
  3. High vacuum
    1. Momentum transfer pump
      1. gas is pumped by transferring momentum
      2. requires rough pump
      3. pumping corrosive and toxic gases or high flows of gases
    2. Diffusion pump
      1. Working principle
        1. momentum of oil molecules
        2. heating the oil at the bottom of the pump
        3. pump oil vapour rise through the centre stack and are ejected through vents at very high speeds
        4. then strike to cooled walls at the top of the pump, condense and run down the walls
      2. Gas are pumped by:
        momentum transfer between the vapour stream and the gas molecules
        gas molecules may also be transported by dissolving in the vapour droplets
      3. Advantages
        simple, cheap, robust (no moving parts)
      4. Drawbacks
        slow heat up (~0.5 hr) and cool down (~2hr)
        high power consumption
        high pumping speed as long as the inlet pressure is in the molecular flow regime
        low purity, may cause contamination
    3. Turbomolecular pump
      1. working mechanism
        high speed fan
        a large number of stages in series and each stage consists of a fan blade that rotates at extremely high speed (>20,000rpm)
        blade speed ~molecular speed
        stator (stationary set of blades)-to avoid bounce bacl
      2. Advantages
        very clean
        high compression rate
    4. Capture (gas entrainment) pump
      1. working mechanis
        pump that capture and hold all the gaseous
        require forepumps (because limited holding capability)
        need to be generated regularly
        require high purity
Vacuum valves and seals
1. Valves
  1. to isolate high vacuum trapping pumps, allowing chamber to be pumped by medium and rough vacuum pumps when fist pumping down from an atmosphere and once the process gas begins to flow
2. Seals
  1. O-ring
    1. seal the vacuum chamber
    2. elastomers (vulcanized rubber)
    3. begin to leak at pressure below 10^-7 torr
  2. Metal seals
    1. Withstand higher pressure
    2. deform during sealing process
    3. Gold, Cu
    4. Cu/silver alloy
Pressure measurement
1. Bourdon gauge
2. Capacitance manometer
3. Pirani gauge
4. Thermocouple gauge
5. Ion gauge
6. Hot filament
  1. Advantage
    1. simpler to operate as compared to cold cathode
  2. Disadvantages
    1. might face problem in highly corrosive ambient or in ultraclean chamber where filament outgassing is a concern
    2. cannot be operated at pressure >10^-4 torr due to short filament life

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Vacuum Science and Technology

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	Created by Blublue yong about 11 years ago