Glossary of Terms

• Absolute melting point: The melting point of a substance measured in degrees Kelvin (absolute temperature).
• Absolute pressure: See Pressure, absolute.
• Absolute temperature: The temperature scale which starts at "true" or absolute zero. It is often called the Kelvin scale.
• Absorption: The binding of a gas in the interior of a solid or liquid.
• Adsorption: The condensing of a gas on the surface of a solid.
• Amorphous: Usually refers to a particle or grain size smaller than 30 A that does not show a crystal structure when using x-ray diffraction techniques. atom The smallest identifiable part of an element. An atom has a nucleus with particles called protons and neutrons. Under normal conditions, it is surrounded by a number of electrons equal to the number of protons.
  Neutrons are neutral, protons are positively charged, and electrons are negatively charged.
• Atom: The smallest particle of an element that can exist either alone or in combination
• Atomic mass unit: A way of classifying atoms according to their weight, or mass. Atoms of the different elements have different weights, or masses.
• Avogadro's Law: The gas law that states that one mole of any gas has 6.023 x 1023 particles and under standard conditions occupies 22.4 liters.

• Backing pump: See Forepump.
• Backstreaming: The small amount of pump fluid vapor that moves in the wrong direction, i.e., toward the work chamber.
• Bakeout: The degassing of a vacuum system by heating during the pumping process.
• Bar: Unit of pressure measurement. There are 1.010 bar in one standard atmosphere. One bar equals 1 x 106 dynes per square centimeter.
• Base pressure: That pressure which is typically reached with your system when it is clean, empty, and dry.
• beam density Describes what is scientifically the power density of electron beam striking a target to be melted or evaporated.
• Belljar: A container open at the bottom and closed at the top which is used as a vacuum chamber or test vessel. Also called a work chamber.
• Bellows-sealed valve: A valve type in which the stem seal is accomplished by means of a flexible bellows, one end of which is attached to the sealing disk, the other end to either the bonnet or the body.
• Binary alloy: A composition of two or more metals. An alloy may a compound, a solid solution, a heterogeneous mixture, or any combination of these.
• Bleed: A colloquial term in vacuum technology referring to the admission of a small quantity of gas into a vacuum system.
• Blower pump: A type of vacuum pump which functions from l0 torr to 0.0001 torr. Also called a booster or Roots pump.
• Body: That part of a valve which contains the external openings for entrance and exit of the controlled fluid.
• Bomb test: A form of leak test in which enclosures are immersed in a fluid. The fluid is then pressurized to drive it through possible leak passages and thus into the internal cavities. The enclosures are then placed in a leak detector to detect the escaping fluid.
• Bonnet: In general, that part of the valve through which the stem enters the valve, and which is rigidly attached to the valve body.
• Bourdon gauge: A roughing gauge that responds to the physical forces that a gas exerts on a surface.
• Boyle's Law: The gas law that states PIV, = P2V2, or original pressure times original volume equals new pressure times new volume. This equation predicts new pressure or new volume whenever the other is changed by any amount (providing that the temperature is unchanged).

• Calibrated leak: An external reference standard that permits calibration of a helium leak detector.
• Capacitance manometer: A vacuum gauge which senses pressure by the change in capacitance between a diaphragm and an electrode.
• Charles'Law: The gas law that describes what happens to the volume of gas as the temperature is changed. As a gas is cooled, its volume gets smaller. As a gas is heated, its volume increases (at constant pressure).
• Chemical pumping: The removal of gas from a vacuum system by acting it with another to form a compound of very low vapor pressure. See gettering.
• Chemisorption: The binding of a gas on or in a solid by chemical action. See gettering.
• Chip: A term used in the microelectronics industry to describe a self-contained circuit on a wafer. A wafer may contain many chips on its surface.
• Closed-loop refrigeration system: A refrigeration system in which the coolant is recycled continuously.
• Cold cap: A component mounted on top of the jet assembly in a diffusion pump. This cap helps to keep pump fluid vapor out of the work chamber.
• Cold cathode discharge: A visible glow caused by the recombination of electrons and ions. The color is characteristic of the gas species present.
• Cold cathode gauge: See Ionization gauge.
• Cold trap: See Cryotrap
• Compound: Two or more elements combined chemically in specific proportions. condensation The process of a gas turning back into a liquid.
• Conductance: A term used to indicate the speed with which atoms and molecules can flow through a particular region such as an orifice or pipe.
• Conductance limited: The inability to make use of the rated speed of a pump due to the use of an opening or pipe smaller than the inlet diameter of the pump.
• Conduction: The transfer of energy (heat, light, etc.) by direct contact. In the case of gaseous conduction, the transfer of energy by molecules directly contacting surfaces and other molecules.
• Convection: The transfer of heat from one place to another by the circulation of currents of heated gas or other fluid.
• Cosine law: The intensity of light from a point source impinging on a flat surface is proportional to the cosine of the angle subtended by the source at the plane surface. The thickness of material deposited by physical vapor deposition from a point source on a plane is also related to the cosine of the angle subtended by the source at the plane surface.
• Critical forepressure: See Maximum tolerable foreline pressure.
• Crossover: The pressure at which a vacuum chamber is changed from being pumped by a roughing pump to being pumped by a high vacuum pump.
• Cryocondensation: The pumping of gases that are condensed at cold temperatures. For example, water vapor on a liquid nitrogen trap at -1960C.
• Cryogenic pump: By reducing the temperature of a surface to very low values, all vapors can be condensed and removed from a vacuum. If all gases, including helium, are to be pumped, temperatures below 12 [UNITS????] must be reached.
• Cryosorption: The pumping of gases that are not readily condensed (or pumped) at cold temperatures, by the process of sticking onto a cold surface.
• Cryotrap: A device usually placed before the inlet of a high vacuum pump to "trap" or freeze out gases such as pump oil vapor and water vapor. Cryotraps commonly use liquid nitrogen as the coolant. Also called cold trap or liquid nitrogen trap.

• Dark space: Loosely applied to the faraday dark space which occurs between a cathode and plasma in a glow discharge.
• Degassing: The removal of gas from a material, usually by application of heat under high vacuum. See Bakeout.
• Desorption: See Outgassing.
• Diffusion: (1) The flow of one substance through another by random molecular motion.
  (2) The process by which molecules intermingle as a result of their thermal motion.
• Diffusion pump: A vapor pump having boiler pressures of a few torr and capable of pumping gas continuously at intake pressures not exceeding about 2 mtorr and discharge pressures (forepressures) not exceeding about 500 mtorr. The term diffusion should be applied only to pumps in which the pumping action of each vapor jet occurs as follows: The gas molecules diffuse through the low-density scattered vapor into the denser, forward-moving core of a freely expanding vapor jet. Most of the gas molecules are then driven at an acute angle toward the wall and on into the fore vacuum.
• Dynamic seat: A seal that moves. (See Static seal)

• Electron: A negatively charged particle. (See Atom.)
• Electronic structure: A portion of an atom where electrons are located in their respective orbits or shells.
• Element: A substance entirely consisting of atoms having the same atomic number.
• Epitaxy: The growth of one substance upon another in which the crystal structure of the substrate is copied by the growing substance and substituted for its natural structure.
• Evaporation: The process that happens when a liquid or solid becomes a gas.

• Feedthrough: A device used to allow some sort of utility service to go from the outside world to the inside of a vacuum system while maintaining the integrity of the vacuum; for example, an electrical feedthrough.
• Foreline: The section of a pump through which the gases leave.
• Foreline valve: A vacuum valve placed in the foreline to permit isolation of the pump from its forepump.
• Forepump: The pump which is used to exhaust another pump which is incapable of discharging gases at atmospheric pressure. Also called the backing pump.
• Fractionation: A process that helps to purify the condensed fluid in a diffusion pump. This process distills out contaminants produced by decomposition of pump fluid.

• Gas: A state of matter where the individual particles are free to move in any direction and tend to expand uniformly to fill the confines of a container.
• Gas ballast: A method used with any oil-sealed rotary pump which allows a quantity of air to be admitted during the compression cycle to prevent condensation of water vapor. The amount of air admitted is regulated by the gas ballast valve. The use of a gas ballast raises the ultimate pressure of the pump.
• Gas density: The number of molecules per unit of volume.
• Gas load: The amount of gas being removed from a vacuum chamber by the vacuum pumps. Typically measured in torr-liters per second, cubic feet per minute, or cubic meters per hour.
  gauge pressure See Pressure, gauge.
• Gay-Lussac's Law: The gas law that states that if the temperature of a volume of gas at O°C is changed by 1°C, the volume will change (plus or minus, as appropriate) by 1/273 of its original value.
• General gas law: The gas law that covers pressure, volume, and temperature in one single equation, or PlVlT2 = P2V2T1
• Gettering: A method of pumping gases through chemical reaction of a material with gas molecules. The material usually used is an active element such as titanium. See chemisorption.

• Helium mass spectrometer leak detector (HMSLD): See Mass spectrometer leak detector.
• High vacuum: Pressure which ranges from about 10-4 torr (0.0001 torr) to approximately 10-8 torr (0.00000001 torr).
• High vacuum pump: A vacuum pump which will function in the high vacuum range. Common examples are the diffusion pump and the mechanical cryopump.
• High vacuum valve: A large diameter valve usually placed between the vacuum chamber and the vacuum pumps. It is used to isolate the vacuum chamber from the pumps when it is necessary to work on something in the chamber. Also called hi-vac valve, gate valve, or trap valve.

• Implosion: In vacuum work, the inward collapse of the walls of a vacuum system, caused by external pressure.
• Inside-out leak detection technique: A method of leak detection whereby the tracer gas is placed under pressure inside the container to be leak checked. A detector probe attached to a leak detector is used to locate leaks.
• Ion: A charged particle consisting of an atom or molecule which has an excess of positive or negative charge. Typically produced by knocking an electrons) out of an atom or molecule to produce a net positive charge. ion pump An electrical device for pumping gas. The ion pump includes a means for ionizing the gas with a system of electrodes at suitable potentials, and also a magnetic field. The ions formed move toward a cathode or a surface on which they are reflected, buried, or cause sputtering of cathode material.
• Ionization: The process of creating ions. See Ion.
• Ionization gauge: A vacuum gauge that has a means of ionizing the gas molecules, electrodes to enable the collection of the ions formed, and a means of indicating the amount of the collected ion current. Various types of ionization gauges are identified according to the method of producing the ionization. The common types are:
  1. hot cathode ionization gauge The ions are produced by collisions of gas molecules with electrons emitted from a hot filament (or cathode) and accelerated by an electric field. Also called hot-filament ionization gauge, or simply ion gauge.
  2. cold cathode ionization gauge The ions are produced by a cold cathode discharge, usually in the presence of a magnetic field which lengthens the path of the electrons.

• Jet assembly: A nozzle assembly that directs oil vapors in a diffusion pump.

• Latent heat of evaporation: The energy required to convert material from the liquid (or solid interface of subliming materials) to the vapor phase under isothermal conditions.
• Leak: Leaks may be of three different types: (1) a real leak, which is a crack or hole allowing gases to pass through; (2) a virtual leak, which is caused by outgassing of some volatile material inside a vacuum system or trapped volume; and (3) a permeation leak, which consists of atomic-scale holes throughout the material of construction: for example, 0-rings are quite permeable.
• Leak detector: A device for detecting, locating, and/or measuring leakage.
• Leak rate: Mass flow through an orifice per unit time. Vacuum system leakage rates are typically measured in atm cc per second or torr-liters per second.
• Liner: An insert placed in a water cooled copper crucible which is used to contain the material to be evaporated to reduce conductive heat transfer.
• Liquid nitrogen trap: See Cryotrap

• Mass: A fundamental characteristic of matter which is most closely related to the unit of weight.
• Mass spectrometer (MS): An instrument that is capable of separating ionized molecules of different mass/charge ratio and measuring the respective ion currents. The mass spectrometer may be used as a vacuum gauge that measures the partial pressure of a specified gas, as a leak detector sensitive to a particular tracer gas, or as an analytical instrument to determine the percentage composition of a gas mixture.
• Mass spectrometer leak detector: A mass spectrometer adjusted to respond only to the tracer gas. Helium is commonly used as the tracer gas, and thus the instrument is normally referred to as a helium leak detector.
• Maximum tolerable foreline pressure: A measure of the ability of the diffusion pump to pump gases against a certain discharge pressure. Also called critical forepressure.
• Mean free path: The average distance between molecular collisions. Of importance for vacuum systems where one is interested in getting some particular type of particle from a source to a surface. For example, ion implanters, coaters, or television tubes.
• Microelectronics: Electronics reduced to a very small scale by using integrated circuits.
• Micrometer: A device using a screw thread as its basis to measure lengths accurately.
• Micron: Pressure unit equivalent to 1 mtorr.
• Millibar: Unit of pressure measurement, equal to l/1000 bar.
• Millimeter of mercury: See Torr.
• Millitorr: Unit of pressure measurement, equal to l/1000torr.
• Mole: The number of particles in equal volumes of gases under the same conditions of temperature and pressure. One mole of any gas has 6.023 x 1023 particles.
• Molecular density: The number of molecules in a unit of volume such as a cubic centimeter. There are approximately 3 x 1019 molecules per cc at one standard atmosphere.
• Molecular flow: The type of flow which occurs when gas molecules are spread far apart. There are few collisions so that the molecules tend to act independently of any other molecules which may be present. The molecular directions are completely random.
• Molecular sieve: A very porous material used to contain the pumped gases in sorption pumps. May also be used in a foreline trap to constrain oil molecules.
• Molecular sieve trap: A device used to collect oil vapors backstreaming from oil-sealed mechanical pumps.
• Molecular weight: A way of classifying molecules according to their weight, or mass. Molecular weight or mass is the sum of the individual atomic weights that make up the molecule.
• Molecule: One atom, or two or more atoms joined together and having definite chemical and physical characteristics.

• Neutron: A particle located in the nucleus of an atom which has no electrical charge but does have mass. (See Atom.)
• Nucleus: The dense center portion of an atom containing protons and neutrons. (See Atom.)

• Open-loop refrigeration system: A refrigeration system in which the coolant vents to atmosphere.
• Outgassing: The process in which a gas particle leaves a surface and moves into the volume of a vacuum chamber. This, of course, adds to the gas load and may or may not be desirable. In extreme cases, it prevents "pumping down" a vacuum system to the specified pressure. The system is then said to be "hung up," or outgassing. Also called desorption or virtual leak.
• Outside-in leak detection technique: A leak detection technique where the leak detector senses a tracer gas which passes from the outside of the container to the inside of the container. May be used to determine the size and/or the location of a leak.

• Partial pressure: See Pressure, partial.
• Pascal: Unit of pressure measurement. There are 101,325 pascals in one standard atmosphere. A pascal equals one newton per square meter.
• Permeation leak: Molecular-scale holes through a material of construction. See leak.
• Physical vapor deposition: Transfer of material via the vapor phase by simple physical changes such as boiling.
• Pirani gauge: A vacuum gauge used to measure pressure in the rough vacuum range.
• Planetary: A device used in metallizing microelectronic devices to produce an even thickness and to assist covering all portions of the steps.
• Plasma: An ionized gas containing approximately equal numbers of positive and negative charged carriers.
• Powers of ten: A convenient way of describing very large and very small numbers. A number is written as some value from 1 and up to 1 0 (but not including 1 0). Then, it is multiplied by either a positive or negative power of ten. Also called exponential notation or scientific notation.
• Pressure: Force per unit area. The force is created when atoms, molecules or "particles" strike the walls of their container. Common pressure units for vacuum work are torr, pounds per square inch relative (psig), inches of mercury, millimeters of mercury, bar, millibar, and pascal.
• Pressure measurement: A measurement of the pressure (the number and intensity of particle impacts) on a given unit of area. There are several different scales for pressure measurement: for example, torr, millitorr, bar, millibar, and pascal. These scales may be used as absolute or relative scales.
• Pressure, absolute: Pressure above zero pressure (corresponding to totally empty space) as distinguished from "gauge" pressure. In vacuum technology, pressure is always measured from zero pressure, not atmospheric pressure, and therefore the term absolute pressure is not required.
• Pressure, gauge: The difference between absolute pressure and atmospheric pressure. The most common unit is probably psig.
• Pressure, partial: A measurement of the pressure of one particular gas in a mixture of gases. For example, the partial pressure of oxygen in air is about 160 torr.
• Pressure, relative: See Pressure, gauge.
• Pressure, total: The sum of all of the partial pressures of every gaseous species. The force exerted by all the gas molecules in any mixture of gases. We commonly assume that a pressure gauge reads total pressure.
• Pressure, vapor: The pressure exerted by molecules after they have escaped from a liquid or solid and formed a vapor (gas). One tries, in general, to put substances of low vapor pressure into a vacuum system so as to decrease the gas load on the vacuum pumps.
• Probe: A tube having a fine opening at one end, used for directing or collecting a stream of tracer gas.
• Probe test: A leak test in which the tracer gas is applied by means of a probe so that the area covered by the tracer gas allows tracer gas to enter and locate the leak.
• Prorilometer: A profile measuring device which drags a fine stylus over a surface and records the deflections of the stylus.
• Proton: A positively charged particle. (See Atom.)
• PSIA: Pounds per square inch absolute, a unit of pressure measurement. There are 14.69 psia in one standard atmosphere.
• PSIG: Pounds per square inch gauge. a unit of pressure measurement. Gauge pressure is the difference between absolute pressure and atmospheric pressure. One standard atmosphere equals 0 psig.
• Pump-down curve: A graphic plot of pressure versus time as a vacuum system is being pumped. Usually plotted on loglog graph paper. Can be used to distinguish real leaks from virtual leaks.
• Pumping speed: A measure of the ability of a vacuum pump to remove gases. It is typically measured in liters per second. cubic feet per minute, or cubic meters per hour.

• Radiation: Heat transfer by energy from infrared light. Radiated heat is the only way to transfer heat inside of a vacuum system at high vacuum.
• Rate of rise: The rate of pressure increase versus time when a vacuum system is suddenly isolated from the pump by a valve. The volume and temperature of the system are held constant during the rate-of-rise measurement.
• Rate-of-rise test: A method of determining whether a leak is present in a system, or of obtaining an estimate of the magnitude of a leak, by observing the rate of rise of pressure in the evacuated system when the system is isolated from the pump. This method also can determine if leakage is real or virtual.
• Real leak: A crack or hole that allows gases to pass through in both directions. See leak.
• Refractive index: Ratio of the sine of the angle of incidence to the sine of the angle of refraction of light as it enters a body. It is also the ratio of the velocity of light in the body compared with the velocity light in vacuum.
• Regeneration: Some vacuum pumps and traps fill up from usage (containment pumps) and must be emptied periodically. The process of emptying the pump is called regeneration.
• Residual gas analyzer: A gauge that measures partial pressure.
• Resistance heating: Heating a material by passing an electric current through the material.
• Roots blower: See Blower pump.
• Rough pump: A vacuum pump which will function in the rough vacuum range.
  A roughing pump is often used to "rough" a vacuum chamber. Typical examples of rough pumps are the mechanical pump and the sorption pump.
• Rough vacuum: Pressure which ranges from just below atmospheric pressure to about 10-3 torr (0.001 torr).
• Roughing: The initial evacuation of a vacuum system.

• Scanning electron microscope: It is an electron microscope which projects an image produced from a surface by the secondary electrons derived from a primary electron beam displayed in a raster pattern. The image shows three dimensional topography with high resolution an high magnification.
• Sightports: Holes covered with glass through which the inside of a vacuum system may be observed.
• Silicon wafer: A thin slice of single crystal silicon of it particular crystallographic orientation doped to produce it specific bulk receptivity on which integrated circuits may be formed.
• Sniffer probe See Probe. (More correctly called a detector probe.)
• Spectrophotometer: A device which continually measures light intensity at specific frequencies over a broad band of frequencies.
• Sputtering: The release of one or more molecules from a cathode surface when that surface is struck by a high-energy ion.
• Standard atmosphere: At 45° N latitude, at sea level, and O°C, the average pressure exerted on the earth's surface, This average pressure is 14.69 pounds per square inch (absolute), or 14.69 psia.
• Standard cubic centimeter: The quantity of gas in a volume of 1 cc at standard temperature and pressure (O'C, 760 torr).
• Static seal: A seal that does not move. (See Dynamic seat.)
• Sublimation: The process in which a substance can go directly from the solid state to the vapor state, without passing through a liquid state.
• Sublime: Vaporization which occurs directly from a solid without first transforming to the liquid.
• Sublimes: Changes directly from a solid to a vapor state.

• TC gauge: See Thermocouple gauge.
• Temperature: A qualitative measurement of energy. The hofter something is, the more energy it contains, thus its temperature is higher.
• Thermal expansion rate: Materials change in size as their temperature changes. This size-to-temperature relationship of the material is called its thermal expansion rate.
• Thermocouple gauge: A vacuum gauge used to measure pressure in the rough vacuum range.
• Throughput: Pumping speed times the pressure. It is a term used to measure the quantity of gas per unit of time flowing through a vacuum system or through a component of that system, such as a pump. Typical units are torr-liters per second. It is a unit of power:
  5.70 torr-liters/sec = 1 waft
• Torr: Unit of pressure measurement, equal to the force per unit area exerted by a column of mercury one millimeter high. There are 760 torr in one standard atmosphere.
• Trace element: An element which occurs as an impurity in small amounts; usually less than 1 %.
• Tracer gas: A gas which, passing through a leak, can be detected by a specific leak detector and thus reveal the presence of a leak.
• Transfer pressure: See Crossover pressure.
• Transition range: A range of pressure that cannot be correctly defined as either a viscous flow condition or molecular flow condition.
• Translational energy: Energy associated with the motion of a molecule.
• Trap: A device which will hold selected molecules and not let them pass. Two common types are the molecular sieve trap and the liquid nitrogen trap.
• Tubulation: A pipe or hose used in a vacuum system.
  
  
  
  

• Ultimate pressure: The lowest pressure a vacuum pump or vacuum system can reach when clean and empty. is dependent upon the particular gas species being pumped.
• Ultra high vacuum: Pressure which ranges from about l0-8 torr(O.00000001 torr) to less than 10-14 torr.
• Ultra high vacuum pump: A vacuum pump which will function in the ultra high vacuum range. Typical examples are the ion pump and the TSP (titanium sublimation pump).
• Useful operating range: The pressure range of a vacuum pump between the higher pressure limit where it will begin pumping and the base (or ultimate) pressure, which is the pump's lower operating limit.

• Vacuum: Any pressure lower than atmospheric pressure.
• Vacuum pump A type of pump which is capable of removing the gases in an enclosed volume such as a vacuum chamber. Vacuum pumps are typically divided into three broad categories: (1) roughing pumps, (2) high vacuum pumps, and (3) ultrahigh vacuum pumps.
• Vapor: The gas produced as a result of evaporation.
• Vapor pressure: See Pressure, vapor.
• Vent valve: A valve used for lifting atmospheric air or other gas into a vacuum system. Also called a BTA or back-to-air valve.
• Virtual leak: An apparent leak that is caused by release of gas from a trapped volume or outgassing of some volatile material or trapped gas inside a vacuum system. See leak.
• Viscous flow: The type of flow which occurs when gas molecules are packed closely together and collide with each other quite frequently.

• Work chamber: A contained volume from which some of the air and other gases have been removed. The work chamber separates the vacuum from the outside world. The portion of a vacuum system where the process is performed. See Belljar.