UMEL FEKT VUT V BRNĚ J.Boušek / Vacuum technology / P6

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Vacuum Technology J.Boušek, FEEC, BUT Brno 1

Vacuum measurements are performed in great range of pressure. Different types of gauges and many principles are used: - Ion Gauge and Penning Gauge for high vacuum - Pirani, Thermocouple or Viscous Gauges for medium vacuum - Capacitance Gauge in medium and rough vacuum (high precision) - Mechanical gauges in some cases for rough vacuum Absolute gauges: - value indicated is gas independent - exact value by measurement and some computation - many types of absolute vacuum gauges - Diaphragm Vacuum Gauge,Torricelli tube, Mc Leod vacuum gauge

Vacuum measurement

Vacuum Technology J.Boušek, FEEC, BUT Brno 2

Vacuum measurement

Torricelli tube:

S

ghS

S

gV

S

gm

S

Fp

...... ρρ==== hgp ..ρ=

p – pressure; F - a force to surface S; m – mass; g – gravitation constant; V-volume; ρρρρ - liquid density; h height of liquid column in the tube;

For given liquid ρρρρ is always the same and g is constant: - pressure can be measured by means of height of liquid column - for mercury the pressure is in mm of mercury column = Torr

U-tube : Pressure difference Torricelli tube

Vacuum Technology J.Boušek, FEEC, BUT Brno 3

Vacuum measurement

Higher sensitivity of measurement by liquid column: - using the liquid with lower density (1 mm of water column) - in vacuum technology the water must not be used - the vacuum pumps oils with very low vapor pressure can be used Diaphragm vacuum gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 4

Capacitance gauge (Baratron): - diaphragm gauge but with electrical transducer - extremely precise above 10-2 Pa - requires different heads for different pressure ranges - Baratron is the trade name of the dominant company - moderatly complex, stable - used very widely for precise pressure measurement - process and flow control for plasma deposition and plasma etching

Capacitance Manometer

Vacuum Technology J.Boušek, FEEC, BUT Brno 5

Bourdon vacuum gauge: - oval cross section copper tube connected to the vacuum - atmospheric pressure outside bends it to greater or lesser degree - mechanical force moves an indicator through geared linkage - used primarily in high pressure measurement - from atmospheric pressure to about 100 Pa - whether the vacuum exist (not accurate)

Bourdon vacuum gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 6

Principle of measurement: - pressure dependence of the thermal conductivity of the gas - by low pressure the temperature of sensor (wire) grow - possibility to use very thin filament for the gauge sensor - larges range of pressure measurement (Mean Free Path limit) Sensor temperature is given by: 1. Thermal conductivity of the gas 2. Thermal conductivity of the sensor holder inclusive current wires 3. Thermal radiation from the sensor surface Low pressure limit: - is given by thermal conductivity of the sensor holder - is given by thermal radiation from the sensor surface - depends on the wire thickness and sensor holder design - is about 10-1÷10-2 Pa /10-3÷10-4 Torr

Thermal vacuum gauges

UMEL FEKT VUT V BRNĚ J.Boušek / Vacuum technology / P6

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Vacuum Technology J.Boušek, FEEC, BUT Brno 7

Principle and use: - utilizes the thermal conductivity of the gas - works over a relatively narrow range above 10-1 Pa - typically used for qualitative monitoring of the fore-vacuum. - measure of pressure is change of wire resistance - resistance is measured by usually used methods - simple, reliable and inexpensive - gas dependence (as by all thermal vacuum gauges)

Filament surface: - must not be reactive otherwise chemical components can origin - change of the molecules stay time on filament surface - the exchange mechanism of thermal energy can be affected - most used material is platinum - two identical heated filaments or electronic calibration

Measuring range: 104 Pa to 10-2Pa (100 Torr to 10-4 Torr )

Thermal vacuum gauges - Pirani Vacuum Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 8

- Non-linear when pressure difference is indicated by electric device. - Electronic linearization gives possibility of larger measuring range.

Thermal vacuum gauges - Pirani Vacuum Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 9

Principle and use: - thermocouple is attached to filament or heated directly - heating is usually by constant current power supply - at high pressure filament is cooled by gas molecules collisions - by low pressure less collisions = higher filament temperature - simple, reliable, inexpensive - gas dependence (as by all thermal vacuum gauges)

Measuring range: 103 Pa to 10-2Pa (10 Torr to 10-4 Torr)

Thermal vacuum gauges - Thermocouple Vacuum Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 10

Gas:CH4 H2 H2S SO2 C2H2 CO2 CO Air He Ne Ar R: 0,61 0,67 0,71 0,77 0,79 0,86 0,94 1,00 1,12 1,31 1,56

Thermocouple Vacuum Gauge – gas dependence

Vacuum Technology J.Boušek, FEEC, BUT Brno 11

Pressure dependence of viscosity: - based on the Mean Free Path. change with pressure - in principle is the same as by Thermal Gauges Vibrating sensor (filament or diaphragm): - measure of the pressure is energy needed for the constant amplitude - problem with mode of vibration and its change with pressure - vibrating viscous vacuum gauges were not used in large scale. Spinning Rotor Gauge: - mechanical friction force is large in case of rotational sensors - to perform the measurement in low pressure region - solution - spinning ball held in magnetic field – no friction force - ball rotation is impeded only by collisions with the gas molecules - ball is accelerated to defined velocity, then the drive is switched off - measure of the pressure is time for defined drop of turns number

Viscose gauges - Spinning Rotor Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 12

Principle of measurement: - measure of pressure is a concentration of ions in ionized gas - ion concentration is very low in comparison with neutral particles - ion to neutral ratio should be the same in whole pressure range - onization efficiency is different for different gases - measured value is always gas dependent

Penning Vacuum Gauge (Could Cathode Ionization Gauge): - ionisation is made by gas discharge - the discharge is maintained by means of magnetic field - strength lines of B and E should be perpendicular each to other - electron trajectories are bended to spirals or cycloids - probability of electron – neutral particle collision is much larger - the resulting plasma is held together (lower ambipolar difussion) - discharge ignition voltage is driven by Paschen Law - ignition voltage dependence on the product p*d (Paschen curve)

Ionization Vacuum Gauges

UMEL FEKT VUT V BRNĚ J.Boušek / Vacuum technology / P6

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Vacuum Technology J.Boušek, FEEC, BUT Brno 13

Low Pressure Limit - power supply voltage is not large enough to maintain the discharge - depending on the gauge design it is about 10-4-10-5Pa (10-6-10-7 Torr)

Old design; large current New design; low memory effect

Penning Vacuum Gauge (Could Cathode Ionization Gauge)

Vacuum Technology J.Boušek, FEEC, BUT Brno 14

Memory effect: - by high pressure the sputtering process is significant - in sputtered layers large amount of gas is absorbed - by low pressure absorbed gas particles are freed again - vacuum gauge reading is much higher then is the right value - above 10-2 Pa switch on only for time needed for pressure reading !!!!

New device design: - field strength lines of B and E are perpendicular in whole volume - anode has significantly lower area (memory effect is less significant) - the discharge voltage is lesser Un < 2kV - discharge current is very low (electronic amplifier must be used) - electronic linearization of the scale is possible

Drawback of Penning vacuum gauges: - pressure dependence of reading is nonlinear - a calibration in whole pressure range must be done - accuracy worse than about 10% (device+pressure depending)

Penning Vacuum Gauge (Could Cathode Ionization Gauge)

Vacuum Technology J.Boušek, FEEC, BUT Brno 15

Pump design: - cylindrical canister or body is filled with zeolite - canister is placed in a Dewar container cooled by liquid nitrogen - array of aluminum fins inside the pump maximize thermal contact

Cryosorption pumps

Vacuum Technology J.Boušek, FEEC, BUT Brno 16

Advantage: - ion current – pressure dependence is linear - calibration only for one pressure value is sufficient - the same (linear) scale is used for whole measuring range - only the orders of pressure are to switch (x10) - for low pressure the electron current is set about one order larger

High pressure limit is about 10-1 Pa:

- multiple ions per electron creation - risk of filament burnout !!!!!

Low pressure limit depends on gauge design: - electrons impacting on the electron collector originate X-ray emission - X-ray photons can be catch by ion collector - energy is more than sufficient for photoemission - photocurrent from ion collector adds to measured ion current - photocurrent can be higher then ion current by very low pressure

Ionisation Ion Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 17

Ionisation Ion Gauge

Ion collector with very small area: - small area for X-ray capture; pressure limit up to 10-10 Pa - Alpert-Bayard Ion Gauge

Ion collector does not see electron colector: - electrostatic deflection of ion current - no X-ray capture; pressure limit up to 10-13 Pa - Helmer-Hayward Ion Gauge

Vacuum Technology J.Boušek, FEEC, BUT Brno 18

Basic gauge for high-vacuum and ultra-vacuum processes: - high sensitivity and high accuracy - moderately complex and reliable - typically calibrated for N2 - by other gases conversion or recalibration must be done Hot cathode filament: - can be chemically poisoned - can produce substances with high vapor pressure (wolfram oxides) - by long run by high pressure cathode can burn out Pressure above 10

-1Pa or with reactive gases:

- sputtering or plasma processes - rhenium cathode - short time measurement only

Ionisation Ion Gauge - summary