Vacuum Measurement
Amir Danak
AUT
Winter 2004
Vacuum Measurement – Units / Ranges:
Vacuum Applications:
Industry: Rough-vacuum range near atmospheric to about 1 mbar:
Mechanical handling
Vacuum packing & forming
Gas sampling
Filtration
Degassing ( Removing gases ) of oils
Impregnation of electrical components
Semiconductor devices fabrication
6
At lower pressure down to about 10 mbar:
Refrigeration dehydration
Metallurgical processes : i.e. Melting, Casting, Sintering, Brazing
Chemical processes: vacuum distillation ( as a means of purification ) and freeze drying
Semiconductor device fabrication
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Pressure down to about 10 mbar:
Cryogenic ( low-temperature ) & electrical insulation
Lamps, television tubes, X-ray tubes, etc.
Decorative, optical & electrical thin-film coating
Mass spectrometer leak detectors
Research:
Pressure down to about 10 9 mbar:
Electron microscopes
Analytical mass spectrometers
Particle accelerators
Large-space simulation equipment
Pressure region down to & bellow 10 9 mbar:
Thermonuclear experiments
Field ion & field emission microscopes
Storage rings for particle accelerators
Clean surface studies
Specialized space simulator experiments
Applications in industry:
Electrical engineering industry:
Manufacturing of electric light bulbs ( First usage : about 1900 )
Manufacturing power transformers & cables :
( Rem. large quantities of water from the core & windings, using a combination of heating & vacuum )
( Also Removing air trapped in the windings / Finally being impregnated by oil )
Manufacturing of certain types of HV or HCC switches :
Improvement in electrical insulation ( By Reducing their arcing or flashover in a LP env. )
Semiconductor Industry
Chemical & allied industries:
Lowering the boiling point ( to enable compounds to be separated into their ind. chem. compounds )
Purging Pipelines
Production of reactive metals ( such as titanium )
Freeze drying:
Dehydration by sublimation ( Changing the state without becoming water ) from the frozen surface
Advantages : Minimal product spoilage ( ruin ) due to elim. of liquid & processing at sub-0 temp.s
Using in :
Reactive powders, Museum specimens, Tissue for microscopy, Preparation of blood plasma
Product treatment : Coffee, Fruit juices, Vegetables, meat, Blood Plas., Antibiotics ( penicillin )
even Bone & Arteries have been preserved for long periods
Vacuum coating:
Vaporizing metals & salts under high vacuum , Condensing on any solid surface
Producing : Mirrors, Ophthalmic lenses, Antiglare & antistatic glasses, Tel. tubes, Decorative Pls.
Architectural glass ( multiple layers deposited, both transparent & heat reflecting )
Blooming of lens surfaces ( to increase light transmission )
Vacuum leak testing:
Finding holes of app. one-millionth of cent. in diameter in a comp. having a wall thickness of 2mm.
Aerospace, Electronics, Atomic energy, Cryogenics & Refrigeration industries, Missile const.
Checking sealed devices ( such as transistors, crystals & relays ), U-Enrichment
Vacuum Measurement:
Total Pressure Measurement:
No distinction is made between the permanent gases ( hydrogen, nitrogen, etc. )
and vapors ( oil vapor, water vapor, etc. )
Coming on the next page
Partial Pressure Measurement:
Also known as:
Vacuum Analyzers
Residual gas analyzers
Mass spectrometers
Vapors contained in the gases may be partially condensed, depending upon the
comparison ratio & the degree to which the vapors are saturated
Measurement:
Determining the partial pressure of separate atomic
& molecular species in the vacuum.
Objective: Measuring the number of density of a particular species of molecule
Results: Information more detailed than a rough estimate of total pressure
Difficulty: The absorption rate & the desorption signal are dependant on the magnitude
& location of the pumps
Mass-Spectrometers:
Identification of molecules by separating ions according to their charge to mass ratio
Spectrometers:
Detecting the release of chemisorbed & physisorbed gases upon heating the absorbent
Using field emission or work function changes:
Estimating the adsorbed gas coverage ( extent ) on surfaces
Vacuum Total Pressure Measurement – Gauges:
Direct:
Mechanical phenomena gauges:
Depend on the actual force exerted by the gas
Measurement:
The displacement of an elastic material
The force required to compensate its disp.
U-tube manometer
Capsule dial gauge
Strain gauge
Capacitance manometers
McLeod gauge
Indirect: A particular physical property of the gas is measured
Transport phenomena gauges:
Measurement:
The gaseous drag on a moving body
Thermal conductivity of the gas
Spinning rotor gauge
Pirani, Thermistor & Thermocouple gauges
Ionization phenomena gauges:
Ionize the gas
Measurement:
Total ion current
Cold cathode ionization gauge
Hot cathode ionization gauge
Mechanical phenomena gauges:
U-tube manometer:
Range: atmospheric pressure to 1 mbar
Theory: System pressure = atmospheric pressure - pressure due to height of liquid ( h )
Problem: Dependent on atmospheric pressure, which varies from day to day
Reason: Modifying by sealing one end
More precision: Inclining the U-tube, thereby increasing the scale length
Capsule dial gauge:
Range: 1000 to 1 mbar
Theory: Bordon
Problem: Becoming contaminated, dirt or oil in the vacuum system
Rapid rise in pressure caused by the system vacuum
Not suitable for long distance measurement, due to the pressure drop along the line
Mechanical phenomena gauges: ( Continued )
Strain gauges:
Range: 1000 to 1 mbar
Theory: Chg. in the Pressure causing a force, chg. the form of gauge, causing a change in R
Advantage:
Fast response time, Remote reading is available
Capacitance manometers:
Range: atmospheric pressure to 10-6 mbar
Theory: Changing the capacitance between an electrode ( or electrodes ) & the diaphragm
Problem:
Sensitive to the temperature, oil …
Advantage:
Excellent zero stability, High signal-to-noise ratio
Fast response, High precision
Transport phenomena gauges:
Spinning rotor gauge:
Range: 0.1 to 10-7 mbar
Theory: Slowing down of a levitated ball-bearing caused by molecular drag effects
between the ball surface & the gas
depends on: pressure, gas molecular weight & temperature of the gas
& the surface state of the ball
Advantage:
Very accurate gauge
Problem:
Expensive
Pirani, Thermistor & thermocouple gauges:
Range: 1013 to 10-4 mbar ( Bellow that, the thermal conductivity principle is insensitive )
Theory: Pressure dependency of the ability of a gas to conduct heat
Transport phenomena gauges: ( Continued )
Pirani: Filament = Sensor + Bridge
Constant-voltage Pirani:
Voltage exerted to the filament is constant, temperature is variable
Range: 10 to 10-3 mbar
Constant-temperature Pirani:
Range: 1000 to 10-3 mbar
Problem:
Contamination ( by oil, etc. )
Thermocouple:
Change of temperature of the filament is monitored by a thermocouple
Range: 5 to 10-3 mbar ( Above 5, the filament temperature changes very little )
Advantage:
Using very low power
Ionization phenomena gauges:
Only the ionization gauges, have proved really practical for UHV pressure measurement.
Hot cathode ionization ( ion ) gauges: ( HCG )
Range: 10-3 to 10-10 mbar
Bayard-Alpert gauge ( BAG )
Suppressor gauge
Extractor gauge
Orbitron gauge
Gauges with magnetic fields
Crossed-Field, Cold cathode gauges: ( CCG )
Range: 0.01 to 10-7 mbar
Penning gauge
Magnetron gauge
Inverted magnetron
Double inverted magnetron
Ionization:
Bombardment of a gas with electrons, remaining a positive ion
Residual ( Remainder ) Currents:
Establish a lower limit to the pressure measurable
Soft x-ray ( Low-energy = No danger to health ) photo-emission:
As electrons from the filament strike the grid, some of their energy is converted into X-rays.
Many of these X-rays strike the collector & cause further electrons to be released from it.
So some positive charge produced. Thus even if the pressure is below 10-10 mbar,
the X-ray limit due to bombardment causes the gauge to register a steady 10-10 mbar.
Hot cathode ionization gauges – Bayard-Alpert Type: ( BAG )
Theory: Thermionic emission
Range:
Boils electrons form a hot filament & accelerates them toward a cylindrical grid cage
Electrons collide with gas molecules ionizing some of them
A fine wire located at the center of the ionization volume collects ions, producing current
Power requirement of a typical filament for 1mA emission is 10-15 W
gas dependent ( Varying ionization efficiencies ) ( Normally nitrogen, argon )
0.001 to 10-10 Torr.
Lower limit: X-ray emission from the grid
Upper limit: The response is non-linear & the danger of filament burning out
Hot cathode ionization gauges – Bayard-Alpert gauge: ( Continued )
Advantages:
Strict linear dependence of collector current on pressure
More accurate, stable & reproducible than CCGs
Problems:
Reactions of the gas mols with hot filament, seriously affects the composition of the gas, reliability
( Typical operating temperature is 1700 ºC, hot enough to break down many mols into smaller fragments )
Filament lifetime ( ion bombardment, high pressure operation or chemical effects )
Delay ( Thermal equilibrium ) ( minutes to weeks! )
More complicated, require more power, bigger size than CCGs
Current leakage through conducting layers, deposited on the inside of the gauge head
Factors affecting the accuracy:
Vacuum pumps resulting: water vapor, oil vapors, etc. changing the composition of gases.
Depending on the past history of operation & the precise atmosphere in the vacuum system
Acting either a source ( out gassing ) or sink ( pumping ) of gas:
Whenever the gauge is exposed to atmosphere, gases are sorbed on all the interior surfaces.
Releasing slowly when under vacuum, causing the gauge to read falsely high pressure.
Reason: Degassing facility, causing the metal electrodes to be heated to 900 ºC.
Immediately after degassing, the gauge acts like a pump, trapping gases to very clean surfaces.
Reason: Using ‘Nude’ type of gauge head: a gauge head mounted on a flange, rather than in a body
Cold cathode ionization gauges – Penning:
Cold:
Means No filament
Theory: Discharge tubes: Two ways for indication of pressure
a vacuum discharge tube is a glass vessel into which metal electrodes have been sealed and from
which the air has been removed by a vacuum system.
The probability of collision is proportional to gas density
Range: 0.01 to 10-9 Torr.
Lower limit: MFP of the electrons = very great, they travel straight to the electrodes, No colliding
Upper limit: Current becomes so large, heating & sputtering from the electrodes becomes a problem
Cold cathode ionization gauges – Penning: ( Continued )
Advantages:
Faster than HCGs ( Respond very quickly to pumpdowns )
There is no filament to burn out
Making outgassing much less of a problem
Free of X-ray effect
Degassing is not necessary
Problems:
The ion-induced current is not linearly related to the pressure, rather, the relationship is exponential
Less accurate than HCGs
Cleaning is necessary ( Oil vapors )
Starting of the CCGs can be delayed ( LP ), can be turned on at higher pressure during a pumpdown
Uncertainty surrounding the accuracy of measurement: ( Price Comparison )
Lack of knowledge of the mixture of gases existing in the gauge head at any particular time
Capsule dial
±5%
of full-scale deflection
Capacitance manometer
±1%
of reading or better
McLeod
±10%
between 10-4 & 0.05 mbar
Spinning Rotor
±1 – 2½%
Pirani
±6%
between 0.01 & 10 mbar
Thermocouple
±10%
between 0.01 & 1 mbar
Penning
+100% to –50 %
e.g. at 10-4 it can be 2*10-4 or 5*10-5 mbar
Bayard-Alpert
±10%
between 10-7 & 10-4 mbar
±20%
at 0.001 & 10-9 mbar
±100%
at 10-10 mbar
Vacuum Gauges – Companies:
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Other companies: USG, Pascal technologies, VRC, JC Controls, …
Vacuum Measurement – Active gauges – Inficon:
Vacuum Measurement – Active gauges – Boc Edwards:
Vacuum Intelligent ( Smart ) gauges:
These electronic vacuum and pressure sensors are the way of the future
Refrences:
Books:
‘Modern vacuum practice’ by ‘Nigel Harris’ , McGraw-Hill
‘Scientific foundation of vacuum technique’ by ‘Saul Dushman’ , John Wiley & Sons
‘The physical basis of ultra-high vacuum’
by ‘P.A. Redhead, J.P. Hobson, E.V. Kornelsen’ , Chapman & Hall LTD
Web:
www.modernvacuumpractice.com
www.lesman.com/index.html
www.vacuumlab.com
www.aip.org/tip
www.activac-technology.com/index.html
www.bocedwards.com
www.inficon.com
www.lacotech.com/index.html
www.thinksrs.com
www.helixtechnology.com/index.html
www.mksinst.com/hpshome.html
www.myers-vacuum.com
www.jccontrols.net
Intelligent:
www.meriam.com
www.festo.com
www.hotektech.com/index.html
www.sunx-ramco.com/index.htm
www.vaccon.com
www.fipa-online.com/language.asp
www.ifm-electronic.com