Penning Gauge

                                                        Ionization Gauges:

Cold Cathode Type : The cold cathode gauge uses a confined discharge to sustain a circulating electron current for the ionization of gases. The absence of a hot cathode provides a far more rugged gauge, the discharge requires less power, outgassing is much reduced,and the sensitivity of the gauge is high, so that simpler electronics can be used. Based on these facts, the gauges would appear ideal substitutes for the hot-cathode type of gauge. The fact that this is not yet so where precise measurement of pressure is required is related to the history of cold cathode gauges. Some older designs of such gauges are known to exhibit serious instabilities (Lange et al., 1966). An excellent review of these gauges has been published (Peacock
et al., 1991), and a recent paper (Kendall and Drubetsky, 1997) provides reassurance that instabilities should not be a major concern with modern gauges.

Operating Principles: The first widely used commercial cold cathode gauge was developed by Penning (Penning,1937; Penning and Nienhuis, 1949). It uses an anode ring or cylinder at a potential of 2000 V placed between cathode plates at ground potential. A magnetic field of 0.15 tesla is directed along the axis of the cathode. The confined Penning discharge traps a circulating cloud of electrons,substantially at cathode potential, along the axis of the anode. The electron path lengths are very long, as compared to the hot cathode gauge, so that the pressure measuring sensitivity is very high, permitting a simple microammeter to be used for readout. The gauge is known variously as the Penning (or PIG), Philips, or simply cold cathode gauge and is widely used where a rugged gauge is required. The operating range is from 10¯2 to 10¯7 torr.
Note that any discharge current in the Penning and other cold cathode discharge gauges is extinguished at
pressures of a few torr. When a gauge does not give a pressure indication, this means that either the pressure is below 10¯7 torr or at many torr, a rather significant difference.
Thus, it is necessary to use an additional gauge that is responsive in the blind spot of the Penning—i.e.,
between atmospheric pressure and 10-2 torr.
The Penning gauge has a number of limitations which preclude its use for the precise measurement of pressure.It is subject to some instability at the lower end of its range, because the discharge tends to extinguish. Discontinuities in the pressure indication may also occur throughout the range. Both of these characteristics were also evident in a discharge gauge that was designed to measure pressures into the 10¯10 torr range, where discontinuities were detected throughout the entire operating range (Lange et al., 1966). These appear to result from changes between two or more modes of discharge. Note, however, that instabilities may simply indicate that the gauge is dirty.
The Penning has a higher pumping speed (up to 0.5 L/s) than a Bayard-Alpert gauge, so it is even more essential to provide a high-conductance connection between gauge and the vacuum chamber.
A number of refinements of the cold cathode gauge have been introduced by Redhead (Redhead et al., 1968), and commercial versions of these and other gauges are available for use to at least 10¯10 torr. The discontinuities in such gauges are far less than those discussed above so that they are becoming widely used.

Pressure Measurement Gauges

Gauges Using Thermal Conductivity for the Measurement of Pressure :

Applications: Thermal conductivity gauges are relativelyinexpensive. Many operate in a range of ~1 X 10¯3
to 20 torr. This range has been extended to atmospheric pressure in some modifications of the ‘‘traditional’’ gauge geometry. They are valuable for monitoring and control, for example, during the processes of roughing down from atmospheric pressure and for the cross-over from roughing pump to high-vacuum pump. Some are subject to drift over time, for example, as a result of contamination from mechanical pump oil, but others remain surprising stable under common system conditions.

Operating Principles: In most gauges, a ribbon or filament serves as the heated element. Heat loss from this element to the wall is measured either by the change in element temperature, in the thermocouple gauge, or as a change in electrical resistance, in the Pirani gauge.
Heat is lost from a heated surface in a vacuum system by energy transfer to individual gas molecules at low pressures (Peacock, 1998). This process has been used in the ‘‘traditional’’ types of gauges. At pressures well above 20 torr, convection currents develop. Heat loss in this mode has recently been used to extend the pressure  measurement range up to atmospheric. Thermal radiation heat loss from the heated element is independent of the presence of gas, setting a lower limit to the measurement of pressure. For most practical gauges this limit is in the mid- to upper-10¯4 torr range.

Two common sources of drift in the pressure indication are changes in ambient temperature and contamination of the heated element. The first is minimized by operating the heated element at 300°C or higher. However, this increases chemical interactions at the element, such as the decomposition of organic vapors into deposits of tars or carbon; such deposits change the thermal accommodation coefficient of gases on the element, and hence the gauge sensitivity. More satisfactory solutions to drift in the ambient temperature include a thermostatically controlled envelope temperature or a temperature-sensing element that compensates for ambient temperature changes. The problem of changes in the accommodation coefficient is reduced by using chemically stable heating elements, such as the noble metals or gold-plated tungsten.Thermal conductivity gauges are commonly calibrated for air, and it is important to note that this changes significantly
with the gas. The gauge sensitivity is higher for hydrogen and lower for argon. Thus, if the gas composition
is unknown, the gauge reading may be in error by a factor of two or more.

Thermocouple Gauge:
 In this gauge, the element is heated at constant power, and its change in temperature, as the pressure changes, is directly measured using a thermocouple.In many geometries the thermocouple is spot welded directly at the center of the element; the additional thermal mass of the couple reduces the response time to
pressure changes. In an ingenious modification, the thermocouple itself (Benson, 1957) becomes the heated element,and the response time is improved.

Pirani Gauge:
In this gauge, the element is heated electrically,but the temperature is sensed by measuring its resistance. The absence of a thermocouple permits a faster time constant. A further improvement in response results if the element is maintained at constant temperature, and the power required becomes the measure of pressure.
Gauges capable of measurement over a range extending to atmospheric pressure use the Pirani principle. Those relying on convection are sensitive to gauge orientation,and the recommendation of the manufacturer must be observed if calibration is to be maintained. A second point, of great importance for safe operation, arises from the difference in gauge calibration with different gases. Such gauges have been used to control the flow of argon into a sputtering system measuring the pressure on the highpressure side of a flow restriction. If pressure is set close to atmospheric, it is crucial to use a gauge calibrated for argon, or to apply the appropriate correction; using a gauge reading calibrated for air to adjust the argon to atmospheric
results in an actual argon pressure well above one atmosphere, and the danger of explosion becomes significant.
A second technique that extends the measurement range to atmospheric pressure is drastic reduction of
gauge dimensions so that the spacing between the heated element and the room temperature gauge wall is only 5 mm (Alvesteffer et al., 1995).

Sublimation pumps

       -: Sublimation pumps :-

Applications: Sublimation pumps are frequently used in combination with a sputter-ion pump, to provide highspeed pumping for reactive gases with a minimum investment (Welch, 1991). They are more suitable for ultrahigh vacuum applications than for handling large pumping loads. These pumps have been used in combination with turbomolecular pumps to compensate for the limited hydrogen-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 atomically clean metal onto a surface, commonly the wall of a vacuum chamber. In the simplest version, a wire, commonly 85% Ti/15% Mo (McCracken and Pashley, 1966; Lawson and Woodward, 1967) is heated electrically; typical filaments deposit ~1 g before failure. It is normal to mount two or three filaments on a common flange for longer use before replacement. Alternatively, a hollow sphere of titanium is radiantly heated by an internal incandescent lamp filament, providing as much as 30 g of titanium. In either case, a temperature of ~1500°C is required to establish a useable sublimation rate. Because each square centimeter of a titanium film provides a pumping speed of several liters per second at room temperature (Harra, 1976), one can obtain large pumping speeds for reactive gases such as oxygen and nitrogen. 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.With higher gas loads the sublimation cycles become more frequent, and continuous sublimation is required to achieve maximum pumping speed. 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. Do not heat a sublimator when the pressure is too high, e.g., 10¯3 torr; pumping will start on the heated surface, and can suppress the rate of sublimation completely. In this situation the sublimator surface becomes the only effective pump, functioning as a nonevaporable getter, and the effective speed will be very small (Kuznetsov et al., 1969).

Getter Pump

                             :Getter Pumps:

Getter pumps depend upon the reactionof gases with reactive metals as a pumping mechanism;such metals were widely used in electronic vacuum tubes, being described as getters (Reimann, 1952). Production
techniques for the tubes did not allow proper outgassing of tube components, and the getter completed the initial pumping on the new tube. It also provided continuous pumping for the life of the device.
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. This provided a gettering surface that operated close to ambient temperature. 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 nonevaporable getter (NEG).

Sputter Ion Pump

                                       Sputter-Ion Pumps

Applications: These pumps were originally developed for ultrahigh vacuum (UHV) systems and are admirably suited to this application, especially if the system is rarely vented to atmospheric pressure. Their main advantages are as follows.
1. High reliability, because of no moving parts.
2. The ability to bake the pump up to 4008C, 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 capacity by 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.

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.

Operating Principles: The operating mechanisms of sputter-ion pumps are very complex indeed (Welch, 1991). Crossed electrostatic and magnetic fields produce a confined discharge using a geometry originally devised by Penning (1937) to measure pressure in a vacuum system.A trapped cloud of electrons is produced, the density of which is highest in the 10¯4 torr region, and falls off as the pressure decreases. High-energy ions, produced by electron collision, impact on the pump cathodes, sputtering
reactive cathode material (titanium, and to a lesser extent, tantalum), which is deposited on all surfaces within line-of sight of the impact area. 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. This is the source of the ‘‘memory’’ effect in diode ion pumps; previously pumped species show up as minor impurities when a different gas is pumped.
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 (Jepsen, 1968).
4. Dissociation of molecules by electron impact. This is the mechanism for pumping methane and other organic molecules.
The pumping speed of these pumps is variable. Typical performance curves show the pumping of a single gas under steady-state conditions.

The pumping speed of hydrogen can change very significantly with conditions, falling off drastically at low pressures and increasing significantly at high pressures (Singleton, 1969, 1971; Welch, 1994). The pumped hydrogen can be released under some conditions, primarily during the startup phase of a pump. When the pressure is 10¯3 torr or higher, the internal temperatures can readily reach 5008C (Snouse, 1971). Hydrogen is released, increasing the pressure and frequently stalling the pumpdown.
Rare gases are not chemisorbed, but are pumped by burial (Jepsen, 1968). Argon is of special importance, because it can cause problems even when pumping air.The release of argon, buried as atoms in the cathodes, sometimes causes a sudden increase in pressure of as much as three decades, followed by renewed pumping, and a concomitant drop in pressure. The unstable behavior
is repeated at regular intervals, once initiated (Brubaker, 1959). This problem can be avoided in two ways.
1. By use of the ‘‘differential ion’’ or DI pump (Tom andJames, 1969), 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 the
anode and walls are maintained at ground potential. This pump has the additional advantage that the ‘‘memory’’ effect of the diode pump is almost completely suppressed.
The operating life of a sputter-ion pump is inversely proportional to the operating pressure. It terminates when the cathodes are completely sputtered through at a small area on the axis of each anode cell where the ions impact. The life therefore depends upon the thickness of the cathodes at the point of ion impact. For example, a conventional triode pump has relatively thin cathodes as compared to a diode pump, and this is reflected in the expected life at an operating pressure of 1x10¯6 torr, i.e., 35,000 as
compared to 50,000 hr. The fringing magnetic field in older pumps can be very significant. Some newer pumps greatly reduce this problem.
A vacuum chamber can be exposed to ultraviolet and x radiation, as well as ions and electrons produced by an ion pump, so appropriate electrical and optical shielding may be required.

Operating Procedures: A sputter-ion pump must be roughed down before it can be started. Sorption pumps or any other clean technique can be used. For a diode pump, a pressure in the 10¯4 torr range is recommended, so that the Penning discharge (and associated pumping mechanisms) will be immediately established. A triode pump can safely be started at pressures about a decade
higher than the diode, because the electrostatic fields are such that the walls are not subjected to ion bombardment (Snouse, 1971). An additional problem develops in pumps that have operated in hydrogen or water vapor. Hydrogen accumulates in the cathodes and this gas is released when the cathode temperatures increase during startup. The higher the pressure, the greater the temperature; temperatures as high as 900ºC have been measured at the center of cathodes under high gas loads (Jepsen, 1967).An isolation valve should be used to avoid venting the pump to atmospheric pressure. The sputtered deposits on
the walls of a pump adsorb gas with each venting, and the bonding of subsequently sputtered material will be reduced, eventually causing flaking of the deposits. The flakes can serve as electron emitters, sustaining localized (non-pumping) discharges and can also short out the electrodes.

Cryopumps

                     Cryopumps

Applications: Cryopumping was first extensively used in the space program, where test chambers modeled the conditions encountered in outer space, notably that by which any gas molecule leaving the vehicle rarely returns.This required all inside surfaces of the chamber to function as a pump, and led to liquid-helium-cooled shrouds in the chambers on which gases condensed. This is very effective, but is not easily applicable to individual systems, given the expense and difficulty of handling liquid helium. However, the advent of reliable closed-cycle mechanical refrigeration systems, achieving temperatures in the 10 to 20 K range, allow reliable, contamination-free pumps, with a wide range of pumping speeds, and which are capable of
maintaining pressures as low as the 10-10 torr range (Welch, 1991).

Cryopumps are general purpose and available with very high pumping speeds (using internally mounted cryopanels),so they work for all chamber sizes. These are capture pumps, and, once operating, are totally isolated from the atmosphere. All pumped gas is stored in the body 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. They must be captured by adsorption, for which the capacity is far smaller. Indeed, if pumping any significant quantity of helium, regeneration would have to be so frequent that another type of pump should be selected. 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. An external compressor, incorporating a heat exchanger that is usually water-cooled, supplies helium at ~300 psi to the cold head, which is mounted on the vacuum system. The helium is cooled by passing through a pair of regenerativeheat 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.The second stage should typically remain in the range 10 to 20 K, low enough to pump most common gases
to well below 10-10 torr. In order to remove helium, hydrogen, and neon the modern cryopump incorporates a bed of charcoal, having a very large surface area, cooled by the second-stage array. This bed is so positioned that most gases are first removed by condensation, leaving only these three to be physically adsorbed. As already noted, the total pumping capacity of a cryopump is very different for the gases that are condensed, as compared to those that are adsorbed. The capacity of a
pump is frequently quoted for argon, commonly used in sputtering systems. For example, a pump with a speed of ~1000 L/s will have the capability of pumping ~3 X 10­­ 5 torr-liter of argon before requiring regeneration. This implies that a 200-L volume could be pumped down from a typical roughing pressure of 2.5 X 10¯1 torr ~6000 times.The pumping speed of a cryopump remains constant for all gases that are condensable at 20 K, down to the 10¯10 torr range, so long as the temperature of the second-stage array does not exceed 20 K. At this temperature the vapor pressure of nitrogen is ~1 X 10­11 torr, and that of all
other condensable gases lies well below this figure.
The capacity for adsorption-pumped gases is not nearly so well defined. The capacity increases both with decreasing temperature and with the pressure of the adsorbing gas. The temperature of the second-stage array is controlled by the balance between the refrigeration capacity and generation of heat by both condensation and adsorption of gases. Of necessity, the heat input must be limited so that the second-stage array never exceeds 20 K, and this translates into a maximum permissible gas flow into the pump. The lowest temperature of operation is set by the pump design, nominally ~10 K. onsequently the capacity for adsorption of a gas such as hydrogen can vary by a factor of four or more when between these two temperature extremes. For a given flow of hydrogen, if this is the only gas being pumped, the heat input will be low, permitting a higher pumping capacity, but if a mixture of gases is involved, then the capacity for hydrogen will be reduced,
simply because the equilibrium operating temperature will be higher. A second factor is the pressure of hydrogen that must be maintained in a particular process. Because the adsorption capacity is determined by this pressure, a low hydrogen pressure translates into a reduced adsorptive capacity, and therefore a shorter operating time before the pump must be regenerated. The effect of these factors is very significant for helium pumping, because the adsorption capacity for this gas is so limited. A cryopump may be quite impractical for any system in which there is a deliberate and significant inlet of helium as a process
gas.

Operating Procedure: Before startup, a cryopump must first be roughed down to some recommended pressure,
often `1 X 10¯1 torr. This serves two functions. First, the vacuum vessel surrounding the cold head functions as a Dewar, thermally isolating the cold zone. Second, any gas remaining must be pumped by the cold head as it cools down; because adsorption is always effective at a much higher temperature than condensation, the gas is adsorbed in the charcoal bed of the 20 K array, partially saturating it, and limiting the capacity for subsequently adsorbing helium, hydrogen, and neon. It is essential to avoid oil contamination when roughing down, because oil vapors adsorbed on the charcoal of the second-stage array cannot be removed by regeneration and irreversibly reduce the adsorptive capacity. Once the required pressure is reached, the cryopump is isolated from the roughing line 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. This cross-over pressure can readily be calculated from the figure for the impulse gas load, specified by the manufacturer, and the volume of the chamber. The impulse load is simply
the quantity of gas to which the pump can be exposed without increasing the temperature of the second-stage array
above 20 K. 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 (~60°C) dry gas, such as nitrogen, at atmospheric pressure. Internal heaters were deliberately avoided for many years, to avoid an ignition source in the event that explosive gas mixtures, such as hydrogen and oxygen, were released during regeneration. To the same end, the use of any pressure sensor having a hot surface was, and still is, avoided in the  regeneration procedure. Current practice has changed, and many pumps now incorporate a means of independently heating each of the refrigerated surfaces. This provides the flexibility to heat the cold surfaces only to the extent that adsorbed
or condensed gases are rapidly removed, greatly reducing the time needed to cool back to the operating temperature. Consider, for example, the case where argon is the predominant gas load. At the maximum operating temperature of 20 K, its vapor pressure is well below 10¯11 torr, but warming to 90 K raises the vapor pressure to 760 torr, facilitating rapid removal.
In certain cases, the pumping of argon can cause a problem commonly referred to as argon hangup. This occurs after a high pressure of argon, e.g., >1 X 10¯3 torr, has been pumped for some time. When the argon influx stops, the argon pressure remains comparatively high instead of falling to the background level. This happens when the temperature of the pump shroud is too low. At 40 K, in contrast to 80 K, argon condenses on the outer shroud instead of being pumped by the second-stage array. Evaporation from the shroud at the argon vapor pressure of 1X10¯3 torr keeps the partial pressure high until all of the
gas has desorbed. The problem arises when the refrigeration capacity is too large, for example, when several pumps are served by a single compressor and the helium supply is improperly proportioned. An internal heater to increase the shroud temperature is an easy solution. A cryopump is an excellent general-purpose device. It can provide an extremely clean environment at base pressures in the low 10¯10 torr range. Care must be taken to ensure that the pressure-relief valve is always operable, and to ensure that any hazardous gases are safely handled in the event of an unscheduled regeneration. 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 nonreactive 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.
Maintenance of a cryopump is normally minimal. Seals in the displacer piston in the cold head must be replaced as required (at intervals of one year or more, depending on the design); an oil-adsorber cartridge in the compressor housing requires a similar replacement schedule.

Turbomolecular Pumps

                                            Turbomolecular Pumps

Applications: Turbomolecular pumps were introduced in 1958 (Becker, 1959) and were immediately hailed as
the solution to all of the problems of the diffusion pump.Provided that recommended procedures are used, these pumps live up to the original high expectations. These are reliable, general-purpose pumps requiring
simple operating procedures and capable of maintaining clean vacuumdown to the 10-10 torr range.Pumping speeds up to 10,000 L/s are available.

Operating Principles: The pump is a multistage axial compressor, operating at rotational speeds from around
20,000 to 90,000 rpm. The drive motor is mounted inside the pump housing, avoiding the shaft seal needed with an external drive. Modern power supplies sense excessive loading of the motor, as when operating at too high an inlet pressure, and reduce the motor speed to avoid overheating and possible failure. Occasional failure of the frequency control in the supply has resulted in excessive speeds  and catastrophic failure of the rotor.
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 other designs, the pump must be returned to the manufacturer for bearing replacement and rotor rebalancing. This service factor should be considered in selecting a turbomolecular pump, since few facilities can keep a replacement pump on hand.

Several 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. These pumps were mounted horizontally with the gas inlet between
two sets of blades. The bearings were at the ends of the rotor shaft, on the forevacuum side. This
type of pump, and the magnetically levitated designs discussed below, offer minimum vibration.
Second-generation pumps are vertically mounted and single-ended. This is more compact, facilitating
easy replacement of a diffusion pump. Many of these pumps rely on gravity return of lubrication oil to the
reservoir and thus require vertical orientation.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 (Osterstrom, 1979) and to permit orientation of the pump in any direction.
Grease has lower frictional loss and allows a lowerpower 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-tosteel 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. When nickelcadmium batteries were used, this backup was not continuously available; incomplete discharge before recharging cycles often reduces discharge capacity.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, and their replacement may be a nuisance; it is, however, preferable to replacing a shattered pump rotor and stator assembly.
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 nonreactive gas, in the viscous flow regime, can prevent the pumped gases from contacting the bearings. To permit access to the bearing for a purge, pump designs move the upper bearing below the turbine blades, which often cantilevers the center of mass of the rotor beyond the bearings. This may have been a contributing factor to premature failure seen in some pump designs.
The turbomolecular pump shares many of the performance characteristics of the diffusion pump. 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. Failure to recognize the problem of oil contamination from this pump was a major factor in the problems with early applications of the turbomolecular pump. But, as with the diffusion pump, an operating turbomolecular pump prevents significant backstreaming from the forepump and its own bearings. A typical turbomolecular pump compression ratio for heavy oil molecules,  ~10-12:1, ensures this. The key to avoiding oil contamination during evacuation is the pump reaching its operating speed as soon as is possible.
In general, turbomolecular pumps can operate continuously at pressures as high as 10-2 torr and maintain constant pumping speed to at least 10-10 torr. As the turbomolecular pump is a transfer pump, there is no accumulation of hazardous gas, and less concern with an emergency shutdown situation. The compression ratio is ~108:1 or nitrogen, but frequently below 1000:1 for hydrogen.
Some first-generation pumps managed only 50:1 for hydrogen.Fortunately, the newer compound pumps, which add an integral molecular drag backing pump, often have compression ratios for hydrogen in excess of 105:1. The large difference between hydrogen (and to a lesser extent helium) and gases such as nitrogen and oxygen leaves the residual gas in the chamber enriched in the lighter species.If a low residual hydrogen pressure is an important consideration, it may be necessary to provide supplementary pumping for this gas, such as a sublimation pump or nonevaporable getter (NEG), or to use a different class of pump.
The demand for negligible organic compound contamination has led to the compound pump, comprising a standard turbomolecular stage backed by a molecular drag stage, mounted on a common shaft. Typically, a backing pressure of only 10 torr or higher, conveniently provided by an oil-free (‘‘dry’’) diaphragm pump, is needed (see discussion of Oil-Free Pumps). In some versions, greased or oil-lubricated bearings are used (on the high-pressure side of the rotor); magnetic bearings are also available.

Compound pumps provide an extremely low risk of oil contamination and significantly higher compression ratios for light gases.
Operation of a Turbomolecular Pump System: Freedom from organic contamination demands care during both the evacuation and venting processes. However, if a pump is contaminated with oil, the cleanup requires disassembly and the use of solvents.
The following is a recommended procedure for a system in which an untrapped oil-sealed mechanical roughing/
backing pump is combined with an oil-lubricated turbomolecular pump, and an isolation valve is provided between the vacuum chamber and the turbomolecular pump.
1. Startup: Begin roughing down and turn on the pump as soon as is possible without overloading
the drive motor. Using a modern electronically controlled supply, no delay is necessary, because the
supply will adjust power to prevent overload while the pressure is high. With older power supplies,
the turbomolecular pump should be started as soon as the pressure reaches a tolerable level, as given by
the manufacturer, probably in the 10 torr region. 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, as was described for diffusion pump startup (see discussion
above).
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. Some
pumps have a vent midway along the turbine blades, while others have vents just above the upper, system-
side, bearings. If neither of these vent points are available, a valve must be provided on the vacuum chamber itself. 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. Venting is 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). When venting frequently,
the turbomolecular pump is usually left running, isolated from the chamber, but connected to the forepump.
The major maintenance is checking the oil or grease lubrication, as recommended by the pump manufacturer,
and replacing the bearings as required. The stated life of bearings is often ~2 years continuous operation, though an actual life of ~5 years is not uncommon. In some facilities, where multiple pumps are used in production, bearings are checked by monitoring the amplitude of the vibration frequency associated with the bearings. A marked increase in amplitude indicates the approaching end of bearing life, and the pump is removed for maintenance.

Diffusion Pump

                                               Diffusion Pumps

Applications: The practical diffusion pump was invented by Langmuir in 1916, and this is the most common
high-vacuum pump when all vacuum applications are considered. It is far less dominant where avoidance of organic contamination is essential. Diffusion pumps are available in a wide range of sizes, with speeds of up to 50,000 L/s; for such high-speed pumping only the cryopump seriously competes.

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. Contamination problems of diffusion-pumped systems have actually been somewhat overstated. Commercial processes using highly reactive metals are routinely performed using diffusion pumps.When funds are scarce, a diffusion pump, which incurs the lowest capital cost of any of the high-vacuum alternatives,is often selected. The continuing costs of operation, however, are higher than for the other pumps, a factor not often considered.
An excellent detailed discussion of diffusion pumps is  available (Hablanian, 1995).

Operating Principles: A diffusion pump normally contains three or more oil jets operating in series. It can be
operated at a maximum inlet pressure of ~1 X 10-3 torr and maintains a stable pumping speed down to 10-10 torr or lower. As a transfer pump, the total amount of gas it can pump is limited only by its reliability, and accumulation of any hazardous gas is not a problem. However, there are a number of key requirements in maintaining its operation.
 First, the outlet of the pump must be kept below some maximum pressure, which can, however, be as high as
the mid-10-1 torr range. If the pressure exceeds this limit,all oil jets in the pump collapse and the pumping stops. Consequently the forepump (often called the backing pump) must operate continuously. Other services that must be maintained without interruption include water or air cooling, electrical power to the heater, and refrigeration,if a trap is used, to prevent oil backstreaming. A major drawback of this type of pump is the number of such criteria. The pump oil undergoes continuous thermal degradation. However, the extent of such degradation is small, and an oil charge can last for many years. Oil decomposition products have considerably higher vapor pressure than their parent molecules. Therefore modern pumps are designed to continuously purify the working fluid, ejecting decomposition products toward the forepump. In addition, any forepump oil reaching the diffusion pump has a much higher vapor pressure than the working fluid, and it too must be ejected. The purification mechanism primarily involves the oil from the pump jet, which is cooled at the pump wall and returns, by gravity, to the boiler.
The cooling area extends only past the lowest pumping jet, below which returning oil is heated by conduction from the boiler, boiling off any volatile fraction, so that it flows toward the forepump. This process is greatly enhanced if the pump is fitted with an ejector jet, directed toward the foreline; the jet exhausts the volume directly over the boiler,where the decomposition fragments are vaporized. A second step to minimize the effect of oil decomposition is to design the heater and supply tubes to the jets so that the uppermost jet, i.e., that closest to the vacuum chamber,is supplied with the highest-boiling-point oil fraction. This oil, when condensed on the upper end of the pump wall, has the lowest possible vapor pressure. It is this film of oil that is a major source of backstreaming into the vacuum chamber.

The selection of the oil used is important (O’Hanlon,1989). 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. Any cold trap will reduce the system base pressure, primarily by pumping water vapor, but to remove oil to a partial pressure well below 10-11 torr it is essential that molecules make at least two collisions with surfaces at LN2 temperature. Such traps are thermally isolated from ambient temperature and only need cryogen refills every 8 hr or more. With such a trap, the vapor pressure of the pump oil is secondary, and a less expensive oil may be used.
If a pump is exposed to substantial flows of reactive gases or to oxygen, either because of a process gas flow
or because the chamber must be frequently pumped down after venting to air, the chemical stability of the oil
is important. Silicone oils are very resistant to oxidation, while perfluorinated oils are stable against both oxygen and many reactive gases. When a vacuum chamber includes devices such as mass
spectrometers, which depend upon maintaining uniform electrical potential on electrodes, silicone oils can be a problem,because on decomposition they may deposit insulating films on electrodes.

Operating Procedures: A vacuum chamber free from organic contamination pumped by a diffusion pump
requires stringent operating procedures. While the pump is warming, high backstreaming occurs until all jets are in full operation, so the chamber must be protected during this phase, either by a LN2 trap, before the pressure falls below the viscous flow regime, or by an isolation valve. The chamber must be roughed down to some predetermined pressure before opening to the diffusion pump. This cross-over pressure requires careful consideration. Procedures to minimize the backstreaming for the frequently used oil-sealed mechanical pump have already been discussed (see Oil-Sealed Pumps). If a trap is used, one can safely rough down the chamber to the ultimate pressure of the pump. Alternatively, backstreaming can be minimized by limiting the exhaust to the viscous flow regime. This procedure presents a potential problem. The vacuum chamber will be left at a pressure in the 10-1 torr range, but sustained operation of the diffusion pump must be avoided when its inlet pressure exceeds 10-3 torr. Clearly, the moment the isolation valve between diffusion pump and the roughed-down vacuum chamber is opened, the pump will suffer an overload of at least two decades pressure.
In this condition, the upper jet of the pump will be overwhelmed and backstreaming will rise. If the diffusion
pump is operated with a LN2 trap, this backstreaming will be intercepted. But, even with an untrapped diffusion pump, the overload condition rarely lasts more than 10 to 20 s, because the pumping speed of a diffusion pump is very high, even with one inoperative jet.
Consequently, the backstreaming from roughing and high-vacuum pumps remains acceptable for many applications. Where large numbers of different operators use a system, fully automatic sequencing and safety interlocks are recommended to reduce the possibility of operator error.

Diffusion pumps are best avoided if simplicity of operation is essential and freedom from organic contamination is paramount.

High Vacuum Pumps

                                                High-Vacuum Pumps

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

Diffusion, Turbomolecular, Cryosorption, and Sputter-ion.

Each of these classes has advantages, and also some problems, and it is vital to consider both sides for a particular application. Any of these pumps can be used for ultimate pressures in the ultrahigh vacuum region and to maintain a working chamber that is substantially free from organic contamination. The choice of system rests primarily on the ease and reliability of operation in a particular environment, and inevitably on the capital and running costs.

we wil discuss all High Vacuum Pumps in detail.

Oil free ( Dry ) Pumps

            Oil-Free (‘‘Dry’’) Pumps

Many different types of oil-free pumps are available. We will emphasize those that are most useful in analytical
and diagnostic applications.

    Diaphragm Pumps:

Applications: Diaphragm pumps are increasingly used where the absence of oil is an imperative, for example, as
the forepump for compound turbomolecular pumps that incorporate a molecular drag stage. The combination renders oil contamination very unlikely. Most diaphragm pumps have relatively small pumping speeds. 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. Pumps are available with speeds up to several
liters per second, and base pressures from a few torr  to as low as 10-3 torr, lower ultimate pressures being associated with the lower-speed pumps.

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. In some pumps the modules can be arranged to provide four stages of pumping, providing a lower base pressure, but at lower pumping speed because only a single module is employed for the first stage. The major required maintenance in such pumps is replacement of the diaphragm after 10,000 to 15,000 hr of operation.

Scroll Pumps :

Applications: Scroll pumps (Coffin, 1982; Hablanian, 1997) are used in some refrigeration systems, where the
limited number of moving parts is reputed to provide high reliability. The most recent versions introduced for
general vacuum applications have the advantages of diaphragm  pumps, but with higher pumping speed. Published 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 below ~2 X 10-2 torr.

Operating Principles: Scroll pumps use two enmeshed spiral components, one fixed and the other orbiting. Successive crescent-shaped 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. A sophisticated and expensive version of this pump has long been used for processes where leaktight operation and noncontamination are essential, for example, in the nuclear industry for pumping radioactive gases. An excellent description of the characteristics of this design has been given by Coffin (1982). In this version, extremely close tolerances (10 mm) between the two scrolls minimize leakage between the high- and low-pressure ends of the scrolls. The more recent pump designs, which substitute Teflon-like seals for the close tolerances, have made the pump an affordable option for general oil-free applications. The life of the seals is reported to be in the same range as that of the diaphragm in a diaphragm pump.

Screw Compressor: Although not yet widely used, pumps based on the principle of the screw compressor, such as that used in supercharging some high-performance cars, appear to offer some interesting advantages: i.e., pumping speeds in excess of 10 L/s, direct discharge to the atmosphere,and ultimate pressures in the 10-3 torr range. If such pumps demonstrate high reliability in diverse applications,
they constitute the closest alternative, in a singleunit ‘‘dry’’ pump, to the oil-sealed mechanical pump.


Molecular Drag Pump:

Applications: The molecular drag pump is useful forapplications requiring pressures in the 1 to 10-7 torr range
and freedom from organic contamination. Over this range the pump permits a far higher throughput of gas, compared to a standard turbomolecular pump. It has also been used in the compound turbomolecular pump as an integral backing stage. This will be discussed in detail under Turbomolecular Pumps.

Operating Principles: 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 ~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. The bearings of these devices are similar to those in turbomolecular pumps (see discussion of Turbomolecular Pumps, below). An internal motor avoids difficulties inherent in a high-speed vacuum seal. A typical pump uses two or more separate stages, arranged in series, providing a compression ratio as high as 1:107 for air, but typically less than 1:103 for hydrogen. 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, depending upon the particular design. The much lower compression ratio for hydrogen, a characteristic shared by all turbomolecular
pumps, will increase its percentage in a vacuum chamber, a factor to consider in rare cases where the presence
of hydrogen affects the application.

Sorption Pumps:

Applications: Sorption pumps were introduced for roughing down ultrahigh vacuum systems prior to turning
on a sputter-ion pump (Welch, 1991). The pumping speed of a typical sorption pump is similar to that of a small oilsealed mechanical pump, but they are rather awkward in application. This is of little concern in a vacuum system likely to run many months before venting to the atmosphere. Occasional inconvenience is a small price for the ultimate in contamination-free operation.

Operating Principles: A typical sorption pump is a cannister containing ~3 lb of a molecular sieve material
that is cooled to liquid nitrogen temperature. Under these conditions the molecular sieve can adsorb ~7.6X 104 torrliter of most atmospheric gases; exceptions are helium and hydrogen, which are not significantly adsorbed, and neon, which is adsorbed to a limited extent. Together, these gases, if not pumped, would leave a residual pressure in the 10-2 torr range. This is too high to guarantee the trouble-free start of a sputter-ion pump, but the problem is readily avoided. For example, a sorption pump connected to a vacuum chamber of ~100 L volume exhausts air to a pressure in the viscous flow region, say 5 torr, and then is valved off. The nonadsorbing gases are swept into the pump along with the adsorbed gases; the pump now contains
a fraction (760–5)/760 or 99.3% of the nonadsorbable gases originally present, leaving hydrogen, helium, and
neon in the low 10-4 torr range in the vacuum chamber.
A second sorption pump on the vacuum chamber will then readily achieve a base pressure below 5 X 10-4 torr,
quite adequate to start even a recalcitrant ion pump.