Vacuum Measurement Amir Danak AUT Winter 2004. Vacuum Measurement – Units / Ranges:

Vacuum Vacuum MeasurementMeasurement

Amir DanakAUT

Winter 2004

Vacuum Measurement – Units / Ranges:

Vacuum Applications:Industry: Rough-vacuum range near atmospheric to about 1 mbar:

Mechanical handlingVacuum packing & formingGas samplingFiltrationDegassing ( Removing gases ) of oilsImpregnation of electrical componentsSemiconductor devices fabrication

At lower pressure down to about mbar:Refrigeration dehydrationMetallurgical processes : i.e. Melting, Casting, Sintering, BrazingChemical processes: vacuum distillation ( as a means of purification ) and freeze dryingSemiconductor device fabrication

Pressure down to about mbar:Cryogenic ( low-temperature ) & electrical insulationLamps, television tubes, X-ray tubes, etc.Decorative, optical & electrical thin-film coatingMass spectrometer leak detectors

Research:Pressure down to about mbar:

Electron microscopesAnalytical mass spectrometersParticle acceleratorsLarge-space simulation equipment

Pressure region down to & bellow mbar:Thermonuclear experimentsField ion & field emission microscopesStorage rings for particle acceleratorsClean surface studiesSpecialized space simulator experiments

610

610

910

910

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 plasmaProduct treatment : Coffee, Fruit juices, Vegetables, meat, Blood Plas., Antibiotics ( penicillin )

even Bone & Arteries have been preserved for long periodsVacuum coating:

Vaporizing metals & salts under high vacuum , Condensing on any solid surfaceProducing : 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 analyzersMass spectrometers

Vapors contained in the gases may be partially condensed, depending upon thecomparison 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 moleculeResults: Information more detailed than a rough estimate of total pressureDifficulty: 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 gasMeasurement: The displacement of an elastic material

The force required to compensate its disp.U-tube manometerCapsule dial gaugeStrain gaugeCapacitance manometersMcLeod gauge

Indirect: A particular physical property of the gas is measuredTransport phenomena gauges:

Measurement: The gaseous drag on a moving bodyThermal conductivity of the gas

Spinning rotor gaugePirani, Thermistor & Thermocouple gauges

Ionization phenomena gauges:Ionize the gasMeasurement: Total ion currentCold cathode ionization gaugeHot cathode ionization gauge

Mechanical phenomena gauges:

U-tube manometer: Range: atmospheric pressure to 1 mbarTheory: System pressure = atmospheric pressure - pressure due to height of liquid ( h )Problem:Dependent on atmospheric pressure, which varies from day to dayReason: Modifying by sealing one endMore precision: Inclining the U-tube, thereby increasing the scale length

Capsule dial gauge:Range: 1000 to 1 mbarTheory: BordonProblem:Becoming contaminated, dirt or oil in the vacuum system

Rapid rise in pressure caused by the system vacuumNot suitable for long distance measurement, due to the pressure drop along the line

Mechanical phenomena gauges: ( Continued )Strain gauges:

Range: 1000 to 1 mbarTheory: Chg. in the Pressure causing a force, chg. the form of gauge, causing a change in RAdvantage: Fast response time, Remote reading is available

Capacitance manometers:Range: atmospheric pressure to 10-6 mbarTheory: Changing the capacitance between an electrode ( or electrodes ) & the diaphragmProblem: 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 variableRange: 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 thermocoupleRange: 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 gaugeExtractor gaugeOrbitron gaugeGauges with magnetic fields

Crossed-Field, Cold cathode gauges: ( CCG )Range: 0.01 to 10-7 mbar

Penning gaugeMagnetron gaugeInverted magnetronDouble inverted magnetron

Ionization: Bombardment of a gas with electrons, remaining a positive ionResidual ( 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

Boils electrons form a hot filament & accelerates them toward a cylindrical grid cageElectrons collide with gas molecules ionizing some of themA 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 Wgas dependent ( Varying ionization efficiencies ) ( Normally nitrogen, argon )

Range: 0.001 to 10-10 Torr.Lower limit:X-ray emission from the gridUpper 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 pressureMore 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 CCGsCurrent 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 systemActing 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 filamentTheory: 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 outMaking outgassing much less of a problemFree of X-ray effectDegassing is not necessary

Problems:The ion-induced current is not linearly related to the pressure, rather, the relationship is

exponentialLess accurate than HCGsCleaning 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.comwww.lesman.com/index.htmlwww.vacuumlab.comwww.aip.org/tipwww.activac-technology.com/index.htmlwww.bocedwards.comwww.inficon.comwww.lacotech.com/index.htmlwww.thinksrs.comwww.helixtechnology.com/index.htmlwww.mksinst.com/hpshome.htmlwww.myers-vacuum.comwww.jccontrols.net

Intelligent:

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