Vacuum Engineering

Start here: ten words every newcomer needs

Overheard on the job

Real-sounding lines from the floor, translated into plain English.

• Base is up a decade and it's wet, the chamber sat up to air all weekend, give it a bake before you leak-check.

  TranslationThe lowest pressure the chamber reaches is ten times worse than normal because the walls soaked up water while the chamber was open for days; heat it under vacuum first, then test for leaks if it is still high.

• Rough it out to crossover, open the gate, and don't touch the ion gauge till the turbo's at speed.

  TranslationPump the chamber from atmosphere with the roughing pump to the switch-over pressure, open the valve to the turbo pump, and wait for the turbo to reach full speed before switching on the filament gauge.

• RGA's showing 28 and 32 at four to one, so it's a real leak, not a virtual, go spray the new feedthrough.

  TranslationThe mass spectrometer shows nitrogen and oxygen in the ratio found in air, so outside air is getting in rather than gas trapped inside; check the newly installed feedthrough with helium.

• Somebody vented with the gate open and crashed the turbo, there's blade debris down its throat.

  TranslationAir was let into the chamber while the turbo pump was still connected and spinning, the rotor hit the stator, and fragments are lying in the pump inlet.

• Mech pump oil's milky, crack the gas ballast and let it run hot overnight.

  TranslationThe rotary vane pump's oil has absorbed water; open the ballast valve and let the warm pump purge the water out of the oil over several hours.

• Foreline Pirani's pegged at atmo but the turbo's still at speed, so it's the gauge, not the backing pump.

  TranslationThe gauge on the backing line shows full atmospheric pressure, yet the turbo pump is running normally, which would be impossible if the line were really at atmosphere, so the gauge has failed.

• Cryo's full, second stage is at 20 K, regen it tonight and cross over at a tenth of a torr tomorrow.

  TranslationThe cryopump has collected all the gas it can hold and its coldest stage is warming; warm it up and empty it overnight, then switch from roughing to the cryopump at 0.1 Torr on the next pump-down.

• Spray's just giving hang-up, bag the bottom flange and let the background settle before you chase it further.

  TranslationSpraying helium has left so much helium in the system that small leaks are hidden; enclose the suspect flange in a helium-filled bag instead and wait for the detector signal to fall before continuing the search.

The pressure ranges, units and reference pressures that every reading and specification is written in.

• Atmoalso atm, air
  Shop clipping of atmospheric pressure, used for a chamber that is vented and open to air. Shop talk
• Base pressurealso base
  The lowest pressure a particular chamber reaches after pumping for a set time with no process gas flowing.
• High vacuumalso HV, hi-vac
  The range from about 10⁻³ down to 10⁻⁷ mbar, where molecules mostly hit the walls rather than each other.
• Medium vacuumalso fine vacuum
  The band between about 1 and 10⁻³ mbar, where gas flow shifts from fluid-like to molecule-by-molecule.
• Micronalso millitorr, mTorr
  Shop name for a millitorr, one thousandth of a Torr, used in US furnace shops, labs and refrigeration work. Shop talk
• Millibaralso mbar, hPa
  One thousandth of a bar, equal to 100 Pa, and the default pressure unit in European vacuum work.
• Partial pressurealso partial
  The share of the total pressure contributed by one gas species in a mixture.
• Pascalalso Pa
  The SI unit of pressure, one newton per square metre, used in standards and on calibration certificates.
• Rough vacuumalso low vacuum, coarse vacuum
  The pressure range from atmosphere down to about 1 mbar, where gas still flows like a fluid.
• Torralso torr
  Pressure unit equal to 1/760 of a standard atmosphere, about 1.333 mbar, and the default in US labs and fabs.
• UHValso ultra-high vacuum, ultrahigh vacuum
  Ultra-high vacuum: pressures below about 10⁻⁷ mbar, reached only with baked, metal-sealed systems.
• Ultimate pressurealso ultimate, ultimate vacuum
  The lowest pressure a pump can reach on a sealed, clean test dome, as stated on its data sheet.
• Vacuumalso vac
  Any enclosed space where the gas pressure sits below the surrounding atmosphere, graded by how far below it goes.

How gas moves through pipes and pumps at low pressure, and the numbers used to size a system.

• Compression ratioalso K0, compression
  Ratio of a pump's outlet pressure to its inlet pressure for a given gas, measured at zero net flow.
• Conductancealso C
  How easily a pipe, orifice or valve lets gas through, expressed in litres per second.
• Effective pumping speedalso net pumping speed, S_eff
  The pumping speed actually delivered at the chamber after losses in the connecting pipes and valves.
• Gas loadalso load
  Total gas flow a pumping system must remove, summed from process gas, leaks, outgassing and permeation.
• Knudsen numberalso Kn
  Ratio of the mean free path to a characteristic size such as pipe diameter, used to name the flow regime.
• Mean free pathalso MFP, lambda
  Average distance a gas molecule travels before it collides with another molecule.
• Molecular flowalso free molecular flow
  Flow regime in which molecules bounce between the walls almost without meeting each other.
• Pumping speedalso speed, S
  Volume of gas a pump removes from its inlet per unit time, measured at the inlet pressure.
• sccmalso standard cubic centimetres per minute, SCCM
  Standard cubic centimetres per minute: a mass flow unit for process gases, referred to 0 °C and 1 atm.
• Throughputalso Q, gas throughput
  How much gas flows past a point each second, counted as pressure times volume, in units such as mbar·L/s.
• Transitional flowalso Knudsen flow, intermediate flow
  Intermediate flow regime between viscous and molecular, where wall and intermolecular collisions both matter.
• Viscous flowalso continuum flow
  Flow regime in which molecules collide mostly with each other, so the gas moves as a continuous fluid.

Roughing, backing and high vacuum pumps, how each one works and how each one fails.

• Argon instabilityalso argon pumping instability
  Periodic pressure bursts in a diode ion pump as buried argon is released again from the cathodes.
• At speedalso up to speed, normal speed
  Said of a turbo pump that has finished spinning up and reached its normal rotational speed. Shop talk
• Backing pumpalso forepump, fore pump
  Pump that keeps the exhaust of a turbo, diffusion or Roots pump below its maximum allowed pressure.
• Cold trapalso LN2 trap, trap
  Chilled surface in a pumping line that freezes out water and oil vapour before it reaches the chamber or the pump.
• Cryopumpalso cryo, cryogenic pump
  Capture pump that freezes and adsorbs gas onto surfaces cooled to about 10 to 20 K by a closed-cycle refrigerator.
• Diffusion pumpalso diff pump, DP
  High vacuum pump with no moving parts that drives gas toward the exhaust with downward jets of hot oil vapour.
• Dry pumpalso dry vacuum pump, dry roughing pump
  Vacuum pump with no oil or liquid in its pumping chamber, used where oil vapour would contaminate the process.
• Gas ballastalso ballast, GB
  Controlled bleed of air into an oil-sealed pump's compression stage that keeps vapours from condensing in the oil.
• Getteralso getter pump
  Reactive material that removes gas from a vacuum by binding it chemically to a fresh metal surface.
• Ion pumpalso sputter ion pump, SIP
  Capture pump that ionizes gas in a high-voltage magnetic discharge and buries the ions in titanium.
• Mech pumpalso mechanical pump
  Lab slang for the oil-sealed rotary vane pump that sits under the bench or behind the tool. Shop talk
• Milky oilalso milky, cloudy oil
  Pump oil turned cloudy or creamy by an emulsion of water, a sign the pump has been swallowing wet loads. Shop talk
• NEGalso non-evaporable getter, NEG pump
  Non-evaporable getter: a porous alloy that pumps active gases once it has been activated by heating in vacuum.
• Regenalso regeneration, regen cycle
  Clipped form of regeneration: warming a cryopump to drive off the gas it has captured, then cooling it again. Shop talk
• Roots pumpalso Roots blower, booster
  Rotary lobe booster with two figure-eight rotors that adds large pumping speed between about 10 and 10⁻³ mbar.
• Rotary vane pumpalso RV pump, rotary pump
  Oil-sealed positive displacement pump in which sliding vanes sweep gas around an off-centre rotor.
• Roughing pumpalso rougher, rough pump
  Pump that takes a chamber from atmosphere down to the pressure where the high vacuum pump can take over.
• Scroll pumpalso scroll, dry scroll
  Dry pump that compresses gas between a fixed spiral and an orbiting spiral, with no oil in the gas path.
• Throatalso pump throat, inlet
  Inlet opening of a high vacuum pump, the flange where gas enters a turbo, cryo or diffusion pump. Shop talk
• Tip sealalso tip seals
  Polymer strip along the edge of a scroll pump's spiral that seals each gas pocket against the facing plate.
• TSPalso Ti-sub, titanium sublimation pump
  Titanium sublimation pump: a heated titanium filament that coats nearby walls with a fresh gettering film.
• Turbomolecular pumpalso turbo, TMP
  High vacuum pump whose rotor blades spin at tens of thousands of rpm and bat molecules toward the exhaust.

Instruments that read total and partial pressure, from atmosphere down to ultra-high vacuum.

• Bayard-Alpert gaugealso B-A gauge, BA gauge
  Hot cathode ionization gauge with a fine central collector wire, the standard gauge for high and ultra-high vacuum.
• Capacitance manometeralso CDG, capacitance diaphragm gauge
  Diaphragm gauge that reads true pressure, regardless of gas type, from the capacitance of a flexing diaphragm.
• Cold cathode gaugealso Penning gauge, inverted magnetron gauge
  Ionization gauge with no filament that uses a magnetically trapped discharge to read high vacuum pressure.
• Convection gaugealso Convectron, convection Pirani
  Pirani-type gauge that uses gas convection around the heated wire to read from about 10⁻³ mbar up to atmosphere.
• Cracking patternalso fragmentation pattern, fragment pattern
  Set of fragment ion peaks a molecule produces in a mass spectrometer, used to identify it in an RGA scan.
• Degasalso degassing, gauge degas
  To heat a gauge's electrodes hard in vacuum so they release adsorbed gas that would otherwise skew the reading.
• Gas correction factoralso gas factor, gas correction
  Multiplier that converts a gauge reading calibrated for nitrogen into the true pressure of another gas.
• Ionization gaugealso ion gauge, IG
  Gauge that reads high and ultra-high vacuum by ionizing gas molecules and measuring the resulting ion current.
• Peggedalso railed, pinned
  Said of a gauge stuck at the top or bottom end of its scale, giving no usable reading. Shop talk
• Pirani gaugealso Pirani, thermal conductivity gauge
  Thermal conductivity gauge that reads pressure from how quickly gas carries heat away from a hot wire.
• RGAalso residual gas analyzer, quad
  Residual gas analyzer: a small mass spectrometer that identifies which gases remain in a vacuum chamber.

Finding, measuring and specifying leaks, with helium and without it.

• Baggingalso bag test, hood test
  Enclosing a part or flange in a plastic bag filled with helium to check its overall tightness in one shot. Shop talk
• Calibrated leakalso standard leak, reference leak
  Reference leak with a certified helium flow, used to calibrate and check a leak detector's readings.
• Chase a leakalso leak chasing, leak hunting
  To hunt down a leak of unknown location joint by joint, often over hours, until the source is pinned down. Shop talk
• Counterflowalso counter-flow, reverse flow
  Leak detector layout in which helium travels backward through a turbo pump's exhaust to reach the mass spectrometer.
• Gross leakalso big leak, coarse leak
  Leak big enough to stop a system pumping down properly, or to be found without sensitive instruments.
• Hang-upalso helium hang-up
  Helium that lingers in a leak detector or system after a big hit, holding the signal high and masking small leaks. Shop talk
• Helium bombingalso bombing, pressure-bomb test
  Soaking sealed parts in pressurized helium so any leak lets helium in, then measuring how much comes back out.
• Helium leak detectoralso MSLD, helium mass spectrometer leak detector
  Mass spectrometer tuned to helium that finds and measures leaks by detecting helium drawn into the system.
• Helium spray testalso spray test, outside-in test
  Outside-in leak test: helium is sprayed on an evacuated part while a detector watches for it inside.
• Leak checkalso leak-check, leak test
  To test a system or a joint for leaks, usually with a helium leak detector, after any break of vacuum. Shop talk
• Leak ratealso leakage rate
  Quantity of gas entering a vacuum system through a leak per unit time, usually in mbar·L/s.
• Rate of risealso ROR, leak-up rate
  Test that isolates a pumped chamber and tracks how fast the pressure climbs, to estimate leaks and outgassing.
• Sniffalso sniffer test, sniffer mode
  To hunt for helium escaping from a pressurized part by sweeping a sniffer probe slowly over joints and welds. Shop talk
• Virtual leak
  Gas trapped in a hidden pocket inside the vacuum that bleeds out slowly and behaves like a small real leak.

Where the gas inside a sealed system comes from, and the habits that keep it low.

• Backstreamalso backstreaming, oil backstreaming
  Of pump oil or vapour, to flow the wrong way, from the pump back toward the chamber. Shop talk
• Bake-outalso bake, bakeout
  Heating a vacuum system while pumping to drive off water and other adsorbed gas and lower the base pressure.
• Fingerprintalso finger oil, skin oil
  Skin oil and salt left on a vacuum-side surface by a bare hand, which outgasses for weeks and spoils base pressure. Shop talk
• Oil creepalso creep, oil migration
  Slow spread of a thin oil film along metal surfaces, carrying pump fluid far from where it started.
• Outgassingalso offgassing
  Slow release of gas from the surfaces and bulk of materials inside a vacuum system.
• Permeationalso gas permeation
  Gas passing through the solid walls or seals of a vacuum system by dissolving into and diffusing through them.
• Vapor pressurealso vapour pressure, saturation vapor pressure
  Pressure at which a liquid or solid is in balance with its own vapour, which decides what can go inside a chamber.
• Wetalso wet chamber, water-loaded
  Said of a chamber whose walls still hold adsorbed water, so it pumps down slowly and stalls high. Shop talk

Flanges, gaskets, valves, fittings and the plumbing that holds a vacuum together.

• Bellowsalso flex hose, flex line
  Thin, pleated metal tube that lets a vacuum joint flex or move while staying leak-tight.
• Blankalso blank-off, blank flange
  Solid flange with no hole, used to seal an unused port or to test a pump on its own.
• CF flangealso ConFlat, CF
  All-metal flange that seals by biting knife edges into a soft copper gasket, the standard joint for UHV.
• Copper gasketalso copper, CF gasket
  Flat ring of soft oxygen-free copper crushed between two CF knife edges to make a metal vacuum seal.
• Crossalso 4-way cross, 6-way cross
  Fitting with several ports meeting at a central body, used to hang gauges, pumps and valves off one point.
• Feedthroughalso vacuum feedthrough, feedthru
  Sealed fitting that carries electricity, motion, fluid or light through a vacuum wall without leaking.
• Forelinealso fore, backing line
  Pipe that connects the exhaust of a high vacuum or booster pump to its backing pump.
• Gate valvealso gate, hi-vac valve
  Full-bore valve with a sliding plate that isolates a high vacuum pump or a load lock from the chamber.
• ISO flangealso ISO-K, ISO-F
  Larger elastomer-sealed flange family, clamped with claws or bolted, used for pump and chamber ports.
• KF flangealso KF, ISO-KF
  Quick-release clamp flange with a centring ring and O-ring, the standard small fitting for rough and high vacuum.
• Knife edgealso knife-edge seal, sealing edge
  Sharp circular ridge machined into a CF flange face that cuts into the copper gasket to make the seal.
• Load lockalso loadlock, LL
  Small chamber that is pumped and vented to pass parts in and out without venting the main process chamber.
• O-ringalso Viton, FKM seal
  Elastomer ring with a round cross-section that seals demountable vacuum joints by being squeezed into place.
• Stackalso pump stack, pumping stack
  Shop name for the whole column of high vacuum pump, valves, trap and backing line that pumps a chamber. Shop talk
• Vacuum symbolsalso vacuum schematic symbols, graphical symbols
  Standard graphic symbols for pumps, gauges, valves and traps used on vacuum system schematics.

Day-to-day operation: pumping down, venting, crossing over and the talk that goes with it.

• Backfillalso back-fill, backfilling
  To fill an evacuated chamber with a chosen gas, often to a set pressure, rather than with room air.
• Bleedalso bleed in, leak in
  To admit a small, controlled flow of gas into a vacuum system through a needle or leak valve. Shop talk
• Burpalso pressure burst, spike
  Brief, sudden pressure burst in a vacuum system from trapped gas, a warming trap or a moving part. Shop talk
• Crash a turboalso turbo crash, crashed turbo
  To wreck a turbo pump's rotor by letting it hit the stator, most often by venting it hard while it spins. Shop talk
• Crossoveralso crossover pressure, cross over
  Point in a pump-down where pumping switches from the roughing pump to the high vacuum pump.
• Foggingalso pump-down fog, condensation fog
  Mist that forms inside a chamber during a fast pump-down when expanding air cools and its water condenses. Shop talk
• Hold vacuumalso hold
  Of a sealed system, to keep its pressure low with the pumps valved off; failing to hold means a leak. Shop talk
• Pump downalso pump-down, pumpdown
  To evacuate a chamber from atmosphere to its working pressure, through roughing, crossover and high vacuum. Shop talk
• Pump-down curvealso pumpdown curve, P versus t
  Plot of chamber pressure against time during a pump-down, used as the fingerprint of a healthy system.
• Rough outalso rough down, rough
  To pump a vented chamber from atmosphere to crossover with the roughing pump, before the high vacuum valve opens. Shop talk
• Soft pumpalso slow pump, soft rough
  Slow, throttled roughing from atmosphere that avoids stirring up particles and condensing moisture. Shop talk
• Spin downalso spin-down, coast down
  Of a turbo pump, to coast down from full speed after it is switched off, which takes many minutes on its own. Shop talk
• Up to airalso let up to air, at air
  Said of a chamber that has been vented to atmospheric pressure and is open or ready to open. Shop talk
• Ventalso vent up, let up
  To let gas back into an evacuated chamber, usually dry nitrogen, to bring it up to atmosphere. Shop talk

Frequently asked questions

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