What is vacuum?

Vacuum = empty space, from vacuus= [Latin] empty. However, there does notexist a totally empty space in nature, there is no "ideal vacuum". Vacuum is only a partially empty space, where some of the air and other gases have been removed from a gas containing volume.•A pressure lower than atmospheric, in an enclosed area.•A space in which the pressure is significantly lower than atmospheric pressure.•A condition in which the quantity of atmospheric gas present is reduced to the degree that, for the process involved its effect can be considered negligible.

Important factors to consider while designing and building a vacuumSystem includes:

1. The required system operating pressure and the gaseous impurities that must be avoided;2. The frequency with which the system must be vented to the atmosphere, and the required recycling time;3. The kind of access to the vacuum system needed for the insertion or removal of samples

Why vacuum?

In condensed matter physics and materials science, vacuum systems are used in many surface processing steps. Without the vacuum, such processes as sputtering, evaporative metal deposition, ion beam implantation, and electron beam lithography would be impossible.A high vacuum is required in particle accelerators, from the cyclotrons used to create radio nuclides in hospitals up to the gigantic high-energy physics colliders such as the LHC.

• Anything cryogenic (or just very cold) needs to deal with the air– eliminate thermal convection; avoid liquefying air• Atomic physics experiments must get rid of confounding air particles– eliminate collisions• Sensitive torsion balance experiments must not be subject to air– buffeting, viscous drag, etc. are problems• Surface/materials physics must operate in pure environment

– e.g., control deposition of atomic species one layer at a time

Measures off pressure

• The “proper” unit of measure for pressure is Pascals(Pa), or N·m--2• Most vacuum systems use Torr instead– based on mm of Hg• Atmospheric pressure is: =760 Torr= 101325 Pa=1013 mbar=14.7 psi• So 1 Torr is 133 Pa, 1.33 mbar; roughly one milliatmosphere

The terms “high vacuum”, “ultrahigh vacuum”, “low vacuum” and the like are often used in vacuum textbooks and laboratory discussions. The borders between the regions aresomewhat arbitrary, but a general guideline is given in Table.

Vacuum rangesAtmosphere 760 mm of Hg = 760 TorrRough vacuum ~ down to 10^-3 THigh vacuum ~ 10^-3 – 10^-8 TUltra high vacuum ~10^-9 – 10^-12 T

Metric unit is Pascals, 1 Pascal = 1 N/m21 Torr = 133 Pascals

Factors that govern the ultimate, or base, pressure

The pressure can be calculated fromP =Q/S

where P is the pressure in torr, Q is the total flow, or throughput of gas, in torr-L/s, and S is the pumping speed in L/s.The influx of gas, Q, can be a combination of a deliberate influx of process gas from an exterior source and gas originating in the system itself. The most important internal sources of gas are outgassing from the walls and permeation from the atmosphere, most frequently through elastomer O-rings. There may also be leaks, but these can readily be reduced to negligible levels by proper system design and construction.Don’t confuse throughput and pumping speed. Q depends on p while S does not.S is defined as the volume of gas/unit time which the pump removes from theSystem with pressure p at the inlet of the pump

The Problem of Outgassing

Origin of outgassing may be due to:

1. The manufacture of the materials used in construction,2. In handling during construction,3. And in exposure of the system to the atmosphereThe outgassing load is highest when a new system is put into service. Every time a system is vented to air, the walls are exposed to moisture and one or more layers of waterwill adsorb virtually instantaneously.One way of accelerating the removal of adsorbed water is by purging at a pressure in the viscous flow region, using a dry gas such as nitrogen or argon. Under viscous flow conditions, the desorbed water molecules rarely reach the system walls, and readsorption is greatly reduced. A second method is to heat the system above its normal operating temperature.

Characterization of pumps

Pumps can be broadly categorized according to three techniques:

Positive displacement pumps use a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.

Momentum transfer pumps, also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas molecules out of the chamber.

Entrapment/capture pumps capture gases in a solid or adsorbed state. This includes cryopumps, getters, and ion pumps.

Types of Pumps

1.) Rough/medium vacuum

a.) Piston pumps (not used much due to particle problems)b.) Rotary vane pumps (majority of cheap applications)c.) Dry pumps (no oil back streaming)

2.) High and Ultrahigh Vacuum

a.) Diffusion pump (not used much due to oil contamination)b.) Turbopump (oil, grease and oil free lubrication of bearings)c.) Cryopump (can be dangerous in certain processes)d.) Ion pump (very clean but low pumping speed and capacity)e.) Titanium Sublimation Pump (evaporate Titanium)

1.) Roughing and Backing pumps

Two classes of roughing pumps are in use. 1. The oil-sealed mechanical pump2. ‘‘dry’’ pump

1.1.) Oil-Sealed Pumps

The oil-sealed pump is a positive displacement pump, of either the vane or piston type, witha compression ratio of the order of 10^5:1

Applications: The pumps are widely used as a backing pump for both diffusion and turbomolecular pumps; in this application the backstreaming of mechanical pump oil is intercepted by the high vacuum pump, and a foreline trap is not required.

1.1.1) Rotary vane pump

For low to medium vacuum applications, a mechanical pump may be sufficient and mechanical pumps in various forms are used as roughing and backing pumps in many systems. Very common fore vacuum-and general vacuum pump.

Appications:Common uses of vane pumps include high pressure hydraulic pumps and automotive uses including, supercharging, power steering and automatic transmission pumps.

Operating Principles:The simplest vane pump is a circular rotor rotating inside of a larger circular cavity. The centers of these two circles are offset, causing eccentricity. The rotor has two spring-loaded vanes mounted opposite each other which separate the stator into two moving sections. Gas is moved by rotating vanes. A blade sweeps along the walls of a cylinder, pushing air from the inlet to the exhaust The pump uses oil to maintain sealing, and to

provide lubrication and heat transfer, particularly at the contact between the sliding vanes and the pump wall.

For “roughing,” or getting the bulk of the air out, one uses mechanical pumps – usually rotary oil-sealed pumps these give out at ~ 1–10 ^-3Torr.

Advantages:High capacity from ~10^-2 torr.

Disadvantages:Potential backstreaming of oil into vacuum chamber.

How to Avoid Oil Contamination from an Oil-Sealed Mechanical Pump

Using foreline traps:1. A liquid nitrogen_cooled trap is always in place between a forepump and the

vacuum chamber, cleanliness is assured. But the operative word is ‘‘always.’’ If the trap warms to ambient temperature, oil from the trap will migrate upstream, and this is much more serious if it occurs while the line is evacuated.

2 Trap using an adsorbent for oil. Typicaladsorbents are activated alumina, molecular sieve (a synthetic zeolite), a proprietary and metal wool

1.2.) Oil-Free (‘‘Dry’’) Pumps

1.2.1.) Diaphragm Pumps

The diaphragm pump is designed to eliminate the need for sealing oils, which makes for a cleaner vacuum system.

Applications:Diaphragm pumps are increasingly used where the absence of oil is an imperative, for example, as the forepump for compound turbomolecular pumps thatincorporate a molecular drag stage. They are adequate once the system pressure reaches the operating range of a turbomolecular pump; usually well below 10^2 torr, but not for rapidly roughing down a large volume.

Operating Principles:Four diaphragm modules are often arranged in three separate pumping stages, with the lowest-pressure stage served by two modules in tandem to boost the capacity. Single modules are adequate for subsequent stages, since the gas has already been compressed to a smaller volume. Each module uses a flexible diaphragm of Viton or other elastomer, as well as inlet and outlet valves. The major required maintenance in such pumps is replacement of the diaphragm after 10,000 to 15,000 hr of operation.

Advantages: 1. Oil free2. Reliable, low maintenance

Disadvantages: 1. Low capacity

1.2.2.) Scroll pump:

Applications: Scroll pumps are used in some refrigeration systems, air conditioning equipment, as an automobile supercharger and as a vacuum pump. Compressed gas volume pushed towards center outlet. Speeds on the order of 10 L/s and base pressures below 10^2 torr make this an appealing combination. Speeds decline rapidly at pressures below10^2 torr.

Operating Principles:Scroll pumps use two enmeshed spiral components, Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid between the scrolls. Another method for producing the compression motion is co-rotating the scrolls, in synchronous motion, but with offset centers of rotation. The relative motion is the same as if one were orbiting.Successive segments of gas are trapped between the two scrolls and compressed from the inlet (vacuum side) toward the exit, where they are vented to the atmosphere. The life of the seals is reported to be in the same range as that of the diaphragm in a diaphragm pump.

Advantages1. Oil free2. Reliable, low maintenance

Disadvantages:1. Low to medium capacity

1.2.3.) Screw Compressor

A rotary screw compressor is a gas compressor which uses a rotary type positive displacement mechanism

Applications:Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery. Pumps based on the principle of the screw compressor, such as that used in supercharging some high-performance cars.

Operations:A rotary screw compressors use two helical screws, known as rotors, to compress the gas. The timing gears ensure that the male and female rotors maintain precise alignment.

Advantages:1. Pumping speeds in excess of 10 L/s,2. Direct discharge to the atmosphere, and3. Ultimate pressures in the 10^-3 torr range.4. High reliability in diverse applications5. the closest alternative, in a single unit‘‘dry’’ pump, to the oil-sealed mechanical

pump.

2.) Medium Vacuum Pumps

2.1.) Molecular Drag Pumps

A vacuum pump in which pumping is accomplished by imparting a high momentum to the gas molecules by impingement of a body rotating at very high speeds, as much as 16,000 revolutions per minuteMolecular drag pumps are can achieve 10^−7 torr level vacuums and freedom from organic contamination.

Operating Principles:

Molecular drag pumps work by imparting momentum to the gas molecules through collision with a quickly spinning rotor. The pump is designed so the momentum transfer tends to send the molecules towards the exhaust. The pump uses one or more drums rotating at speeds as high as 90,000 rpm inside stationary, coaxial housings. The clearance between drum and housing is of the order of 0.3 mm. Gas is dragged in the direction of rotation by momentum transfer to the pump exit along helical grooves machined in the housing. It must be supported by a backing pump, often of the diaphragm type, that can maintain the forepressure below a critical value, typically 10 to 30 torr.

Advantages:1.) Less leakage,2.) High compressing ratio and pressure, which can be combined with other

molecular pumps or rotating machinery vacuum pumps by mounting on a common axe to form a group of combined vacuum pumps with good performance.

Disadvantages:1.) The much lower compression ratio for hydrogen.

2.2.) Sorption Pumps

Sorption refers to the action of absorption or adsorption:

Absorption is the incorporation of a substance in one state into another of a different state (e.g. liquids being absorbed by a solid or gases being absorbed by a liquid).

Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another phase (e.g. reagents adsorbed to solid catalyst surface).

The sorption pump is a vacuum pump that creates a vacuum by adsorbing molecules on a very porous material like molecular sieve which is cooled by a cryogen, typically liquid nitrogen. The ultimate pressure is about 10-2 mbar. With special techniques this can be lowered till 10-7 mbar.

Applications:

Sorption pumps were introduced for roughing down ultrahigh vacuum systems prior to turning on a sputter-ion pump.

Operating Principles:A typical sorption pump is a cannister containing about 3 lb of a molecular sieve material that is cooled to liquid nitrogen temperature. Under these conditions the molecular sieve can adsorb 7.6 x 10^4 torr liter of most atmospheric gases; exceptions are helium andhydrogen, which are not significantly adsorbed, and neon, which is adsorbed to a limited extent.

The sorption pump is a cyclic pump and its cycle has 3 phases:

1.) Sorption,2.) Desorption and3.) Regeneration

1.) In the sorption phase the pump is actually used to create a vacuum. This is achieved by cooling the pump body to low temperatures, typically by immersing it in a Dewar flask filled with liquid nitrogen. Gases will now either condense or be adsorbed by the large surface of the molecular sieve.

2.) In the desorption phase the pump is allowed warm up to room temperature and the gases escape through the pressure relief valve or other opening to the atmosphere. If the pump has been used to pump toxic, flammable or other dangerous gasses one has to be careful to vent safely into the atmosphere as all gases pumped during the sorption phase will be released during the desorption phase.

3.) In the regeneration phase the pump body is heated to 300 °C to drive off water vapor that does not desorb at room temperature and accumulates in the molecular sieve. It takes typically 2 hours to fully regenerate a pump.

The pump can be used in a cycle of sorption and desorption until it loses too much efficiently and is regenerated or in a cycle where sorption and desorption are always followed by regeneration.

After filling a sorption pump with new molecular sieve it should always be regenerated as the new molecular sieve is probably saturated with water vapor. Also when a pump is not in use it should be closed off from the atmosphere to prevent water vapor saturation.

Advantages:

1.) The absence of oil or other contaminants,2.) Low cost and3.) Vibrations free operation because there are no moving parts.

Disadvantages:

1.) It cannot operate continuously and

2.) Cannot effectively pump hydrogen, helium and neon, all gases with lower condensation temperature than liquid nitrogen.

3.) High vacuum pumps

Four types of high-vacuum pumps are in general use:

1.) Diffusion,2.) Turbomolecular,3.) Cryopump, and4.) Sputter-ion

3.1.) Diffusion pumps

The practical diffusion pump was invented by Langmuir in 1916, are one of the oldest and most reliable ways of creating a vacuum down to 10^-10 Torr, or even lower, at 25° C. A vacuum diffusion pump cannot begin its work with full atmospheric pressure inside the chamber. Instead, an ancillary mechanical roughing pump (or forepump), capable of a modest level of pumping, first brings the pressure inside the vacuum diffusion chamber down to about 10^-3 Torr. At this point, the vacuum diffusion pump takes over to create a vacuum ranging from 10^-3 to 10^-10 Torr. Since the diffusion pump cannot exhaust directly to atmospheric pressure, the forepump is used to maintain proper discharge pressure conditions.

Applications:A diffusion pump can give satisfactory service in a number of situations. One such case is in a large system in which cleanliness is not critical. The diffusion pump is widely used in both industrial and research applications. Most modern diffusion pumps use silicone oil as the working fluid.

Operating Principles:A vacuum diffusion pump is basically a stainless steel chamber containing three or more oil jets operating vertically in series. Typically there are three jet assemblies of diminishing sizes, with the largest at the bottom. At the base of the chamber is a pool of a specialized type of oil having a low vapor pressure. The oil is heated to boiling by an electric heater beneath the floor of the chamber. The high velocity jet collides with gasmolecules that happen to enter it due to their thermal motion. This typically imparts a downward motion on the molecules and transports them towards the pump outlet,creating higher vacuum. At the base of the chamber, the condensed molecules of atmospheric gases are removed by the forepump, while the condensed oil begins another cycle. The vaporized oil moves upward and is expelled through the jets in the various assemblies. Water circulated through coils on the outside of the chamber cool the chamber to prevent thermal runaway and permit operation over long periods of time. There are several types of oil, based variously on silicones, hydrocarbons, esters, perfluorals, and polyphenyl ethers that can be used. The polyphenyl ether (Santovac® 5) fluid or oil has been a worldwide standard for more than 25 years and combines high molecular weight, low reactivity and exceptional vapor pressure.To prevent or minimize backstreaming the selection of the oil used is important. If minimum backstreaming is essential, one can select an oil that has a very low vapor pressure at room temperature. A polyphenyl ether, such as Santovac 5, or a silicone oil, such as DC705, would be appropriate. However, for the most oil-sensitive applications, it is wise to use a liquid nitrogen (LN2) temperature trap between pump and vacuum chamber.

Justification for calling them diffusion pumps is the observation that the molecules of the pumped gas penetrate some distance into the vapor jet in a manner resembling diffusion of one gas into another.

Advantages:

1.) Simple pump without moving parts2.) They are reliable they are simple in design, they run without noise or vibration, and

they are relatively3.) Inexpensive to operate and maintain

Disadvantages:1.) major disadvantage of diffusion pumps is the tendency to backstream oil into the

vacuum chamber.2.) Needs cooled baffle to reduce oil contamination of vacuum chamber

3.2) Terbomolecular pumpsA turbomolecular pump is a type of vacuum pump, superficially similar to a turbopump, used to obtain and maintain high vacuum. These pumps work on the principle that gas molecules can be given momentum in a desired direction by repeated collision with a moving solid surface. In a turbopump, a rapidly spinning turbine rotor 'hits' gas molecules from the inlet of the pump towards the exhaust in order to create or maintain a vacuum.

Appications:Turbomolecular pumps were introduced in 1958 (Becker, 1959) and were immediately hailed as the solution to all of the problems of the diffusion pump.

Operating Principles: The pump is a multistage axial compressor, operating at rotational speeds from around 20,000 to 90,000 rpm. Gas captured by the upper stages is pushed into the lower stages and successively compressed to the level of the fore-vacuum (backing pump) pressure. As the gas molecules enter through the inlet, the rotor, which has a number of angled blades, hits the molecules. Thus the mechanical energy of the blades is transferred to the gas molecules. With this newly acquired momentum, the gas molecules enter into the gas transfer holes in the stator. Because of the relative motion of rotor and stator, molecules preferably hit the lower side of the blades. Because the blade surface looks down, most of the scattered molecules will leave it downwards.This leads them to the next stage where they again collide with the rotor surface, and this process is continued, finally leading them outwards through the exhaust.At high speeds, the dominant problem is maintenance of the rotational bearings. Careful balancing of the rotor is essential; in some models bearings can be replaced in the field, if rigorous cleanliness is assured, preferably in a clean environment such as a laminar-flow hood.

In the standard construction, it cannot exhaust to atmospheric pressure, and must be backed at all times by a forepump. The critical backing pressure is generally in the 10^-1 torr, or lower, region, and an oil-sealed mechanical pump is the most common choice.

Different types of bearings are common in turbomolecular pumps:1.) Oil Lubrication: All first-generation pumps used oil-lubricated bearings that

often lasted several years in continuous operation. Using a wick as the oil reservoir both localizes the liquid and allows more flexible pump orientation.

2.) Grease Lubrication: A low-vapor-pressure grease lubricant was introduced to reduce transport of oil into the vacuum chamber and to permit orientation of the pump in any direction. Grease has lower frictional loss and allows a lower power drive motor, with consequent drop in operating temperature.

3.) Ceramic Ball Bearings: Most bearings now use a ceramic-balls/steel-race combination; the lighter balls reduce centrifugal forces and the ceramic-to steel interface minimizes galling. There appears to be a significant improvement in bearing life for both oil and grease lubrication systems.

4.) Magnetic Bearings: Magnetic suspension systems have two advantages: a non-contact bearing with a potentially unlimited life, and very low vibration. First-generation pumps used electromagnetic suspension with a battery backup. A second generation using permanent magnets was more reliable and of lower cost. Some pumps now offer an improved electromagnetic suspension with better active balancing of the rotor on all axes. In some designs, the motor is used as a generator when power is interrupted, to assure safe shutdown of the magnetic suspension system. Magnetic bearing pumps use a second set of ‘‘touch-down’’ bearings for support when the pump is stationary. The bearings use a solid, low-vapor-pressure lubricant (O’Hanlon, 1989) and further protect the pump in an emergency. The life of the touch-down bearings is limited.

5.) Combination Bearings Systems: Some designs use combinations of different types of bearings. One example uses a permanent-magnet bearing at the high-vacuum end and an oil-lubricated bearing at the forevacuum end. A magnetic bearing does not contaminate the system and is not vulnerable to damage by aggressive gases as is a lubricated bearing. Therefore it can be located at the very end of the rotor shaft, while the oil-fed bearing is at the opposite forevacuum end. This geometry has the advantage of minimizing vibration.

Problems with Pumping Reactive Gases: Very reactive gases, common in the Semiconductor industry, can result in rapid bearing failure. A purge with non reactive gas, in the viscous flow regime, can prevent the pumped gases from contacting the bearings.

Operation of a Turbomolecular Pump System: In the standard construction, it cannot exhaust to atmospheric pressure, and must be backed at all times by a forepump. The critical backing pressure is generally in the 10^-1 torr, or lower, region, and an oil-sealed mechanical pump is the most common choice.

1.) Startup: Begin roughing down and turn on the pump as soon as is possible without overloading the drive motor. A rapid startup ensures that the turbomolecular pump reaches at least 50% of the operating speed while the pressure in the foreline is still in the viscous flow regime, so that no oil backstreaming can enter the system through the turbomolecular pump. Before opening to the turbomolecular pump, the vacuum chamber should be roughed down using a procedure to avoid oil contamination.

2.) Venting: When the entire system is to be vented to atmospheric pressure, it is essential that the venting gas enter the turbomolecular pump at a point on the system side of any lubricated bearings in the pump. This ensures that oil liquid or vapor is swept away from the system towards the backing system. Never vent the system from a point on the foreline of the turbomolecular pump; that can flush both mechanical pump oil and turbomolecular pump oil into the turbine rotor and stator blades and the vacuum chamber. Ventingis best started immediately after turning off the power to the turbomolecular pump and adjusting so the chamber pressure rises into the viscous flow region within a minute or two. Too-rapid venting exposes the turbine blades to excessive pressure in the viscous flow regime, with unnecessarily high upward force on the bearing assembly (often called the ‘‘helicopter’’ effect).

Advantages:1.) High capacity2.) Low maintainance

Disadvantages:1.) Sudden large gas loads may cause severe, expensive damage.

3.3.) CryopumpsA cryopump is a vacuum pump that traps gases and vapours by condensing them on a cold surface.

Applications:Cryopumping was first extensively used in the space program, where test chambers modeled the conditions encountered in outer space. These are capture pumps, and, once operating, are totally isolated from the atmosphere. All pumped gas is stored in thebody of the pump. They must be regenerated on a regular basis, but the quantity of gas pumped before regeneration is very large for all gases that are captured by condensation.Only helium, hydrogen, and neon are not effectively condensed. If the refrigeration fails due to a power interruption or a mechanical failure, the pumped gas will be released within minutes. All pumps are fitted with a pressure relief valve to avoid explosion, but provision must be made for the safe disposal of any hazardous gases released.

Operating Principles: A cryopump uses a closed-cycle refrigeration system with helium as the working gas. Compressed helium refrigerators take advantage of the cooling effect of expanding gases to produce extremely cold surface onto which gas molecules may be captured. The helium is cooled by passing through a pair of regenerative heat exchangers in the cold head, and then allowed to expand, a process which cools the incoming gas, and in turn, cools the heat exchangers as the low-pressure gas returns to the compressor. Over a period of several hours, the system develops two cold zones, nominally 80 and 15 K. The 80 K zone is used to cool a shroud through which gas molecules pass into its interior; water is pumped by this shroud, and it also minimizes the heat load on the second- stage array from ambient temperature radiation. Inside the shroud is an array at 15 K, on which most other gases are condensed. The energy available to maintain the 15 K temperature is just a few watts.

In fact, a cryopump is really three pumps in one. The inlet array of the pump operates at 60 to 100 K to condense water vapor and heavy hydrocarbons on metal surfaces. Behind the inlet array, a condensing array operates at 10 to 20 K to capture argon, nitrogen, oxygen and most other gases. At these temperatures, nearly all gases form dense, ice-like solids with low vapor pressures. Hydrogen, neon and helium do not form solids at these temperatures and must be held by adsorption into activated carbon at 10 to 12 K.

Operating Procedure:

Before startup, a cryopump must first be roughed down to some recommended pressure,often of the order of 1 x 10^-1 torr.Once the required pressure is reached, the cryopump is isolated from the roughingline and the refrigeration system is turned on. When the temperature of the second-stage array reaches 20 K, the pump is ready for operation, and can be opened to the vacuum chamber, which has previously been roughed down to a selected cross-over pressure.When the quantity of gas that has been pumped is close to the limiting capacity, the pump must be regenerated. This procedure involves isolation from the system, turning off the refrigeration unit, and warming the first- and second-stage arrays until all condensed and adsorbed gas has been removed. The most common method is to purge these gases using a warm (about 608 °C) dry gas, such as nitrogen, at atmospheric pressure.There is some possibility of energetic chemical reactions during regeneration. For example, ozone, which is generated in some processes, may react with combustible

materials. The use of a non reactive purge gas will minimize hazardous conditions if the flow is sufficient to dilute the gases released during regeneration. The pump has a high capital cost and fairly high running costs for power and cooling.

Advantages:1.) Very High capacity.2.) No contamination

3.) There are no moving parts in the pump and no vibrations

Disadvantages:1.) Pump saturates if exposed to high pressure or continuous gas flow.2.) Need periodic regeneration of cool head.

3.4.) Sputter ion pumpsThe performance of getter pumps is very dependent on the chemical activity of the gasesin the volume, so they tend to work extremely well for chemically active gases such as hydrogen, oxygen, and nitrogen, but rather poorly for noble gases. Sputter Ion pumps function much like getter pumps, but add an additional capture mechanism.

Applications:It is commonly used in ultra high vacuum (UHV) systems, as they can attain ultimate pressures less than 10−10 torr. Sputter ion pumps have no moving parts and use no oil, and are therefore clean and low-maintenance, and produce no vibration, which is an important factor when working scanning probe microscopy.

Operating Principles:The operating mechanisms of sputter-ion pumps are very complex indeed. Ion pumps produce beams of electrons between a cathode and anode, which are constrained by a magnetic field to move in helices, greatly increasing their path length and the chance of ionizing a molecule of the gas. The ionized gas molecules are accelerated to the cathode. Generally the impact of the gas molecules on the cathode will eject cathode atoms which sputter onto the surface of a secondary cathode and bury gas molecules, trapping them there.The pumping mechanisms include the following.

1.) Chemisorption on the sputtered cathode material, which is the predominant pumping mechanism for reactive gases.2.) Burial in the cathodes, which is mainly a transient contributor to pumping. With the exception of hydrogen, the atoms remain close to the surface and are released as pumping/sputtering continues.

3.) Burial of ions back-scattered as neutrals, in all surfaces within line-of sight of the impact area. This is a crucial mechanism in the pumping of argon and other noble gases.4.) Dissociation of molecules by electron impact. This is the mechanism for pumping methane and other organic molecules

Advantages:1.) High reliability, because of no moving parts and oil free.2.) The ability to bake the pump up to 400 °C, facilitating outgassing and rapid attainment of UHV conditions.3.) Fail-safe operation if on a leak-tight UHV system. If the power is interrupted, a moderate pressure rise will occur; the pump retains some pumping capacityby gettering. When power is restored, the base pressure is normally reestablished rapidly.4.) The pump ion current indicates the pressure in the pump itself, which is

useful as a monitor of performance.

Disadvantages:Sputter-ion pumps are not suitable for the following uses:

1.) On systems with a high, sustained gas load or frequent venting to atmosphere.2.) Where a well-defined pumping speed for all gases is required. This limitation can be circumvented with a severely conductance-limited pump, so the speed is defined by conductance rather than by the characteristics of the pump itself.

Rare gases are not chemisorbed, but are pumped by burial. The release of argon, buried as atoms in the cathodes, sometimes causes a sudden increase in pressure of asmuch as three decades, followed by renewed pumping, and a concomitant drop in pressure. The unstable behavior is repeated at regular intervals, once initiated. This problem can be avoided in two ways:

1. By use of the ‘‘differential ion’’ or DI pump, which is a standard diode pump in

which a tantalum cathode replaces one titanium cathode.2. By use of the triode sputter-ion pump, in which a third electrode is interposed between the ends of the cylindrical anode and the pump walls. The additional electrode is maintained at a high negative potential, serving as a sputter cathode, while theanode and walls are maintained at ground potential. This pump has the additional advantage that the ‘‘memory’’ effect of the diode pump is almost completelysuppressed.

3.5.) Getter pumpsGetter pumps are similar to cryopumps, but use chemical action to capture the gas molecules. Getter pumps depend upon the reaction of gases with reactive metals as a pumping mechanism.

Applications:Getters are used in photomultiplier tubes and other sealed systems to remove gases whichpenetrate the system or desorb from the surface after the device is disconnected from active pumps. In the form called the “non-evaporative getter” or NEG, getters are used in particle accelerators where very large but extremely narrow (low flow rate)volumes are evacuated.

Operating Principles:Usually getters are activated by heating, which pumps the gas from the surface into the bulk of the getter. This prevents the getter from being saturated while in storage or during initial pumpdown. Some practical getters used a ‘‘flash getter,’’ a stable compound of barium and aluminum that could be heated, using an RF coil, once the tube had been sealed, to evaporate a mirror-like barium deposit on the tube wall. Such films initially offer rapid pumping, but once the surface is covered, a much slower rate of pumping is sustained by diffusion into the bulk of the film. These getters are the forerunners of the modern sublimation pump.

A second type of getter used a reactive metal, such as titanium or zirconium wire, operated at elevated temperature; gases react at the metal surface to produce stable,low-vapor-pressure compounds that then diffuse into the interior, allowing a sustained reaction at the surface.These getters are the forerunners or the modern nonevaporablegetter (NEG).

Advantages:1.) Large pumping speed,2.) Large pumping capacity, 3.) Unlimited ultimate pressure,4.) Compact, inexpensive and easily operated.

Disadvantages:1.) No pumping for inert gases, and 2.)L ocalised pumping (not very suitable for conductance-limited vacuum systems, as those of particle accelerators).

3.6.) Sublimation pumpsThe titanium sublimation pump is a true UHV pump. It is used to supplement the pumping action of other UHV pumps (normally ion pumps) as it can be more effective at pumping certain gases. Sublimation pumps are frequently used in combination with a sputter-ion pump, to provide highspeed pumping for reactive gases. These pumps have been used in combination with turbomolecular pumps to compensate for the limitedhydrogen-pumping performance of older designs. The newer, compound turbomolecular pumps avoid this need.

Operating Principles:Most sublimation pumps use a heated titanium surface to sublime a layer of atomicallyclean metal onto a surface, commonly the wall of a vacuum chamber.The titanium sublimation pump (TSP) consists of three removable hairpin-shaped filaments. They are made from an alloy of titanium and molybdenum and are mounted in a simple carrier designed to have low electrical resistance. A high current, from an external power supply, is passed through the filament, raising its temperature so that titanium is sublimed directly from the filament (titanium vapour is producedfrom solid without the material going through the liquid phase). This sublimed titanium then coats the nearby walls of the chamber. The resultant clean titanium film reacts with the active gas molecules to form low vapour pressure compounds (i.e. the gasesare effectively removed). Once the film has fully reacted it must be replaced by more sublimed titanium. The time between the regenerative evaporation depends on the pressure of the system and the gas types being pumpedOne can obtain large pumping speeds for reactive gases such as oxygen and nitrogen. To enhance the pumping speed of the titanium film the condensing wall can be cooled to liquid nitrogen temperatures. It should be noted that this reservoir must be carefullydesigned to give a maximum surface area in the region of the titanium film so as to maximize pumping speed.

Advantages: 1.) It is simple, low cost, and 2.) Can give a high pumping speed.

Disadvantages:1.) The speed falls dramatically as the surface is covered by even one monolayer.

Although the sublimation process must be repeated periodically to compensate for saturation, in an ultrahigh vacuum system the time between sublimation cycles can be many hours.

2.) A sublimator can only pump reactive gases and must always be used in combination with a pump for remaining gases, such as the rare gases and methane.

3.7.) Nonevaporable Getter Pumps (NEGs)Non-Evaporable Getter is a powder of an Al-Zr alloy that readily forms stable compounds with active gases. Sintered on the inner surface of high vacuum vessel it acts as a getter pump that is able to reach the pressure of less than 10-9 Torr. The NEG coating

can be applied even to spaces that are narrow and hard to pump out, which makes it very popular in particle accelerators where this is an issue

Applications: In vacuum systems, NEGs can provide supplementary pumping of reactive gases, being particularly effective for hydrogen, even at ambient temperature. They are most suitable for maintaining low pressures. A niche application is the removal of reactive impuritiesfrom rare gases such as argon. NEGs find wide application in maintaining low pressures in sealed-off devices.

Operating Principles:In one form of NEG, the reactive metal is carried as a thin surface layer on a supportingsubstrate. An example is an alloy of Zr/16%Al supported on either a soft iron or nichrome substrate. The getter is maintained at a temperature of around _4008C, eitherby indirect or ohmic heating. Gases are chemisorbed at the surface and diffuse into the interior. When a getter has been exposed to the atmosphere, for example, when initially installed in a system, it must be activated by heating under vacuum to a high temperature, 600 to 800 °C. This permits adsorbed gases such as nitrogen and oxygen to diffuse into the bulk. Hydrogen is evolved during reactivation; consequently reactivation is most effective when hydrogen can be pumped away. In a sealed device, however, the hydrogen is readsorbed on cooling.A second type of getter, which has a porous structure with far higher accessible surface area, effectively pumps reactive gases at temperatures as low as ambient. In many cases, an integral heater is embedded in the getter.

The performance of a given NEG is characterised by activation temperature, stickingprobability, surface capacity, total pumping capacity (for H2 and for heavier gases) and particulateloss.Advantages:

1.) Linear pumping and passive activation. If the activation temperature is compatible with the baking temperature of the chamber where the NEG is inserted, the getter may be activated during bakeout. This feature is particularly attractive because it removes the need of electric feedthroughs and powering/control systems and allows increasing the NEG surface and consequently its pumping speed.

Disadvantages:1.) compared to Ti sublimation pumping, NEG pumping presents the risk of powder peel-off (excessive heating or H2 embrittlement) and a lower pumping capacity.