Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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2.7.2 The Selection of Pumps for Ultra-High Vacuum Applications 217

2.7.2.1 Diffusion Pumps

Pressures in the low 10 ' '^ or high

"'' torr range can be obtained using a three-

or four-stage pump in conjunction with an efficient liquid-nitrogen-cooled trap.

To prevent oil backstreaming, such a trap must ensure that any oil molecule will

make two or more collisions on a surface that is continuously maintained at liquid

nitrogen temperature. Pressures in the

10 ~^

torr range can be obtained using only

a water-cooled baffle and a low-vapor-pressure pumping fluid of either the poly-

phenyl ether or silicone type (see Chapter 2.4). Diffusion pumps are not selective,

being effective for all gases, and are commercially available with very large

pumping speeds, and therefore are a good general-purpose pump for all system

sizes.

However, satisfactory operation requires continuous maintenance of the

electrical heating, water or air cooling, a forepressure below some critical level

(often —0.5 torr), and liquid nitrogen cooling of the trap. Failure of any of these

items can result in oil contamination of the system. Clearly, preventive mainte-

nance is crucial.

Oil contamination of diffusion-pumped systems is most frequently a result of

initially roughing the system to too low a pressure using an untrapped oil-sealed

mechanical

pump.

It can be readily avoided by crossing over to the diffusion pump

when the foreline pressure is still in the viscous flow region, or alternatively, by

using a properly maintained, effective foreline trap.

The limitations on bakeout described earlier, are particularly severe for a diffu-

sion pump/liquid nitrogen trap assembly, so achievement of the ultimate pressure

may be an unacceptably long process.

Recommendation: Because of the multiplicity of factors that can affect the per-

formance of this pump, the difficulty in outgassing the pump and trap, and the se-

rious potential for oil contamination, diffusion pumps are not recommended for

UHV applications. Exceptions to this recommendation might include the require-

ment for low capital expense (noting that running expense is high) or very large

pumping speed.

2.7.2.2 Cryogenic Pumps

A cryopump provides a nonselective, intrinsically clean method for UHV pump-

ing. Very large pumping speeds are readily available, and the simplicity of the de-

vice permits reliable operation. The capacity for pumping helium, hydrogen, and

neon is very limited, compared to that for all other gases, but this is rarely of con-

cern in a UHV system. The main drawback of such pumps is that the pumped

gases are released very rapidly within 10 minutes of a power interruption, so that

218 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

a rapid-acting, leak-tight, bakeable valve is essential to isolate the system during

such an emergency, and for use during a scheduled regeneration of the pump.

The limitations of bakeout, described earlier, apply also to the cryopump, al-

though the problem is far less severe than for the diffusion pump. The fact that a

large part of the cryopump assembly operates at low temperatures automatically

assures that the outgassing rate decreases once the pump is started.

The procedures used to rough out both cryopump and vacuum system must

avoid contamination, so that the same considerations already described in Sec-

tion 2.7.2.1 apply.

Recommendation. This pump is of value where very large speeds are essential.

The main problems are associated with the limited ability for bakeout, and the

rapid release of previously pumped gas in the event of a power failure. A cryo-

pump can pump only small quantities of helium before regeneration is required

and should never be used if a substantial influx of this gas is ever present.

2.7.2.3 Turbomolecular Pumps

A turbomolecular pump provides a nonselective, clean method for UHV pump-

ing, combining simplicity of operation and high reliability. Current pumps are

limited to a maximum speed of 10,000 L/s, but this is not a limitation for most

UHV applications. Bakeout of the vacuum system is much less of a limitation

than for the two preceding pumps. For example, for one commercial pump a tem-

perature of 160''C is permissible at the mounting flange.

Although the pumps are effective for all gases, a limitation should be noted in

the pumping of hydrogen, one of the principal residual gases in a UHV system.

The compression ratio for this gas is always less than for all other gases, fre-

quently being as low as 600 in a standard turbomolecular pump, resulting in a

limit to the minimum residual hydrogen pressure that can be achieved. For ex-

ample, if it is essential to maintain a hydrogen partial pressure of

X 10"^^ torr

in the vacuum chamber the foreline partial pressure of hydrogen must be main-

tained at ~6 X

"^ torr or less. Some oil-sealed mechanical pumps cannot meet

this requirement, and in fact generate hydrogen by degradation of the oil. In crit-

ical cases, the addition of a titanium sublimator pump (see Section 2.7.4.1) to the

vacuum chamber will resolve the problem. An alternate and preferable approach

is to use a wide-range turbopump, where the turbo stage is backed up by a molec-

ular drag stage, providing compression ratios for hydrogen as high as 10'^; an ad-

ditional advantage is that such pumps can use an oil-free diaphragm forepump,

thus eliminating any possibility of oil contamination from that source.

Recommendation. For general UHV applications, the turbomolecular pump is

probably the best choice among pumps described in Chapters 2.4, 2.5, and 2.6.

The growing reliability of the bearings systems, the availability of magnetic bear-

2.7.2 The Selection of Pumps for Ultra-High Vacuum Applications 219

ing pumps, and the combination turbomolecular drag pumps are important fac-

tors in this recommendation. The ability to subject the pump to even a limited

bakeout permits better outgassing of the entire system, especially because this

bakeout can be performed with the pump in operation. The major difficulty to be

considered is that a power failure results in loss of pumping within a minute or

two,

as the pump rotor slows down, and a rapid-acting, leak-tight, bakeable valve

must be present to isolate the UHV chamber from the pumping line in such an

eventuality.

2.7.2,4 Sputter-Ion (Getter-ion) Pumps

Sputter-ion pumps, which are discussed in detail in Section

2.7.3,

are particularly

suited to UHV applications provided that the throughput of gas is relatively low.

Because of their simple construction, and lack of moving parts, they are highly

reliable, easily degassed, and provide virtually fail-safe operation.

Their disadvantages include high selectivity in pumping different gases, a seri-

ous decrease in pumping speed at the lowest pressures, and high initial expense.

They are not available in very large pumping speeds, and are not appropriate if

very large quantities of gas must be pumped, primarily because such applications

result in short operational life.

Recommendation. The sputter-ion pump is probably the best choice for general

UHV applications if it is essential that a low pressure be maintained at all times.

It provides by far the closest approach to fail-safe operation, and the operating life

is very long at low pressures. However, if a system must be frequently cycled to

atmospheric pressure, or if a significant gas load must be handled by the pump—

for example, argon in a sputtering system—the operational life will be reduced,

and a turbomolecular pump may be a better choice.

2.7.2.5 Sublimation Pumps

These pumps are discussed in detail in Section

2.7.4.1.

They provide very-high-

speed pumping of chemically reactive gas at a relatively low capital and operating

cost. They do not pump the rare gases or methane and similar highly stable or-

ganic molecules, and must therefore be used in conjunction with a second pump,

most commonly a sputter-ion pump, which is effective for pumping such gases.

Recommendation. The sublimation pump is invaluable in providing very high

speeds for reactive gases. The pump contains no moving parts, which provides

high reliability. It is normally an integral part of

the

vacuum chamber and is there-

fore effectively outgassed during bakeout. Most frequently used in conjunction

220 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

with a sputter-ion pump, it is also useful in conjunction with a standard turbo-

molecular pump when a very low residual hydrogen partial pressure is required.

2.7.2.6 Nonevaporable Getter (NEG) Pumps

These pumps are discussed in detail in Section 2.7.4. They find application both

in very large systems, and in very small, sealed-off devices. If the gas influx is

primarily hydrogen, deuterium, or tritium, high-speed pumping can be achieved

at ambient temperature. They are becoming widely used as distributed pumps in

very large high-energy physics vacuum systems. The pumps are useful for all

chemically reactive gases.

Recommendation. NEGs are excellent for pumping hydrogen and its isotopes

and all reactive gases, particularly where a simple, reliable device, operating at

relatively low or, in some cases, ambient temperature, is required. The combina-

tion of a NEG with a sputter-ion pump can maintain hydrogen pumping to very

low pressures. The total quantity of gas that can be pumped before a NEG is sat-

urated is quite low as compared to most pumps previously discussed; they are not

suitable for high-throughput applications.

2.7.3

SPUTTER-ION PUMPS

In this section, two types of pumps that are particularly well suited for UHV ap-

plications, sputter-ion and getter pumps, will be described in detail. They are

classified as capture pumps, because the gases that are pumped are trapped within

the pump structure, most of them being irreversibly immobilized as stable chem-

ical compounds. The pumps can be baked to at least as high a temperature as a

turbomolecular pump, while operating, and their simple structure permits effec-

tive,

fairly rapid outgassing under these conditions. For the ultimate in rapid cy-

cling from atmospheric pressure, they can be baked as high as 450°C, simultane-

ously with the vacuum chamber,* so that very rapid outgassing of all surfaces

within the vacuum system can be achieved. At the higher bakeout temperatures,

neither sputter-ion or getter pumps are normally operated, so that the system must

be pumped using a subsidiary turbomolecular or similar "clean" pump. The ad-

vantage of such baking is clearly illustrated by the fact that routine operation of

a new system in the

10 ~^^

torr range has frequently been demonstrated after a

single 16-hour bake at 400°C.

' For bakeout above 250°C, the magnet and connecting cable may have to be removed (see section

2.7.3.2).

2.7.3 Sputter-Ion Pumps 221

When sputter-ion and getter pumps are used, the system is fully isolated from

the atmosphere, and this, coupled with the fact that a UHV system must be free

from significant leaks, ensures a degree of fail-safe operation that is unmatched

by any other type of pump. If a power failure shuts off the pumps, only a rela-

tively modest pressure rise will normally occur, and the system will rapidly return

to its normal operating pressure once power is restored.

2.7.3.1 Operating Mechanism of Sputter-Ion Pumps

The first sputter-ion pump, which became commercially available in 1958, was

the so-called diode type

[2,3].

Figure

is a drawing of a small single-cell sputter-

ion pump. It has a pumping speed of about 0.2 liter/sec, and is typical of an "ap-

pendage" pump, which is used to maintain a low pressure in a sealed-off elec-

tronic device throughout its useful life. A cylindrical stainless steel anode, about

1 cm in diameter and 1 cm long, is maintained at a potential of ~2.3 kv with re-

spect to a pair of cathodes, which are positioned a few millimeters from each end

of the cylinder, in contact with the pump envelope, at ground potential. A perma-

nent magnet provides a field of about 0.1 tesla, directed along the axis of the

cylinder. Any electron that is produced in the body of the pump, perhaps by field

emission or cosmic radiation, is accelerated toward the anode, but is constrained

to move in a circular orbit by the magnetic field, and thus is trapped and cannot

reach the anode. If the electron collides with a gas molecule, causing ionization, a

second electron is produced and a cascade process quickly generates an electron

density of the order of 10^^ electrons/cm^, which is sustained over a wide pres-

sure range [4]; this high density of electrons provides efficient ionization of gas

entering the pump.

The electrical and magnetic fields, for the electrode geometry shown in Fig-

ure 1, appear to be oriented parallel to each other, but in fact the space charge

Fig.l.

MAGNETIC FIELD

0.1 TESLA

+ 2.2 kV

TITANIUM

CATHODES

STAINLESS STEEL

ANODE

TITANIUM

CATHODES

Appendage single-cell sputter-ion pump.

222 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

caused by the high density of electrons has the effect of depressing the potential

along the axis of the anode to that of the cathode, so that a radial electrical field

results. This particular geometry, with the electrical and magnetic fields orthogo-

nal to each other, is known as a Penning Discharge, and was described by Pen-

ning, in 1937, in a paper [5] discussing a cold cathode discharge device for the

measurement of pressure (the Philips gauge).

The collision of an electron and a gas molecule generates both an electron and

a positive ion. The ions are accelerated in the electrical field to a high energy, col-

lide with the cathodes, which are typically a chemically reactive metal such as ti-

tanium, and are neutralized. Many are buried in the cathodes to a maximum depth

of about a micron, and this burial process provides one primary pumping mecha-

nism. A smaller number of the ions are reflected, with some loss of energy, as en-

ergetic neutrals, which are no longer constrained by the electrical and magnetic

fields and bury themselves in any surface within the pump that is in line of sight.

This process provides a second pumping mechanism, which is particularly im-

portant as the dominant process for pumping noble gases, such as argon.

A third consequence of the collision of high-energy ions with the cathode is

that cathode material is sputtered from the point of impact, producing an atomi-

cally clean layer on all surfaces that are within line of sight. Neutral molecules of

reactive gases, such as nitrogen and oxygen, which collide with these sputtered

films have a high probability (0.1 to 1) of being chemisorbed (gettered) on the

first impact, and this process is the third primary pumping process in the sputter-

ion pump. Note that this process is restricted to the pumping of chemically reac-

tive gases, and is not effective for the noble gases, such as argon and helium, or

for methane.

In a new pump, the trapping of gases by burial in the cathodes, the first process

just described, is initially a major contributor to the overall pumping mecha-

nisms. The ions are neutralized at the surface and penetrate a micron or less. The

ion impact area is the central portion of the cathodes, and becomes more tightly

focused as the pressure falls below the

10 ~^

torr range, covering 20% or less of

the cathode. Consequently, the volume of the cathodes in which the atoms are

buried is limited, and with one exception, hydrogen, the buried atoms are immo-

bile.

With continued pumping, the concentration of buried gas atoms in the im-

pact area steadily increases, and thus these atoms become an increasing fraction

of the sputtered species. With a constant gas influx, the rate of resputtering of the

gas approaches its rate of burial, and at steady state there will be no net pumping

by this process. The time taken to reach steady state is not well defined but is on

the order of hours at a pressure in the 10"^ Torr range. Clearly, the high initial

pumping speed in a pump with new cathodes will decrease toward a long-term

equilibrium speed.

The process just described is responsible for an important problem of the diode

sputter-ion pump. This is the memory effect. Sputtering from the cathodes re-

2.7.3 Sputter-Ion Pumps 223

leases smaU amounts of previously pumped gases, accounting for the presence of

impurities in a vacuum chamber that can be quite unexpected, especially when

the previous pumping history of an ion pump is not known. Such impurities are,

of course, more significant in a low-impurity UHV environment. Impurities will

be most prevalent whenever a system is first pumping down, since the initial

higher operating pressure results in a higher sputtering rate from the cathodes. If

pumping is continued at relatively high pressure, the impurities rapidly disappear,

being replaced by the new gas species that is being pumped. However, in the more

typical operating procedure for a UHV system the gas influx is not sustained be-

cause, as the pressure falls steadily toward the ultimate operating level, the previ-

ously pumped species in the cathodes are not completely displaced, remaining as

a continuing source of impurities.

A second problem of the diode pump relates to the pumping of argon. This is

an important gas because it is frequently used in sputtering processes in the vac-

uum chamber

itself.

In addition, the low level of argon in the atmosphere, ^7 torr,

can occasionally lead to serious pumping problems when pumping air. Argon is

not a chemically reactive gas and cannot be pumped by chemisorption (i.e., chem-

ical reaction at the surface) on the sputtered titanium film. The sole mechanism

for pumping is by burial in any surface, and the primary location in a new pump

is in the cathodes. The fact that the pumping of argon, in a diode pump with tita-

nium cathodes, is limited to burial, is reflected in a low pumping speed, typically

1 %, for

this

gas as compared to the speed for reactive

gases.

Early experiences with

the diode sputter-ion pump uncovered a problem when pumping argon, either

alone or in mixtures [6].

After some critical amount of argon has been pumped, the pumping process

sometimes fails dramatically, with a rapid release of argon into the system, which

is then followed by a resumption of pumping. Once initiated, this regurgitation

process is repeated at regular time intervals. Clearly, the purely physical contain-

ment of argon within the bulk of the cathodes sometimes reaches an unstable

level. One successful solution to the instability of diode pumping is the substitu-

tion of one of the titanium cathodes by tantalum, producing the so-called differ-

ential ion, or DI pump^ [7]; this substitution also results in an increase in the

pumping speed for argon, up to -^20% of that for reactive gases. Clearly, if argon

is an important gas load in a system, the use of a DI pump should be considered.

An explanation of the argon-pumping mechanism is given in an elegant paper

by Jepsen [8], which suggests that efficient pumping of argon results when an ar-

gon ion, reflected at a cathode as a neutral atom, retains a significant fraction of

the kinetic energy of the incident ion. The energetic neutral so produced is

unaf-

fected by the electrical and magnetic fields, and can bury itself in any surface of

^The differential ion pump was introduced by Ultek under the trademark Ultek D-I Pump. For con-

venience it will be referred to as the DI pump.

224 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

Rg.2.

mSSBBBBSSBBtm

mt 1^ i||i i|p fipi

1^19^^*

ppi ^9 9W*.

Anode and sputter-cathode of

StaiCell^* triode sputter-ion

pump.

Each star-shaped section is

coaxial with

individual anode

cell.

Reprinted with permission from

Varian

Vacuum Products.

the pump within line of sight of

the

ion impact

point.

In contrast to the central ar-

eas of the cathodes, where the initial ion impact occurs, most surfaces available

for

this

burial are not subject

ion bombardment, and re-sputtering is not

prob-

lem. Such a burial is reinforced by subsequent deposition of sputtered titanium

over the "grave

site."

The enhanced pumping, which results from the use of

tan-

talum cathode in the DI pump, is a consequence of the higher atomic mass ratio

for tantalum/argon

(181/40),

compared

that for titanium/argon

(48/40),

which

ensures that

much larger fraction of

the

argon atoms is reflected, at significantly

higher energies. The Jepsen hypothesis has been experimentally confirmed by

Vaumeron and DeBiasio [9].

An important variant of the diode pump is the introduction of

second pair of

electrodes between the anode cell and the pump wall, the so-called triode ion

pump

[6,10].

These additional electrodes are constructed of titanium to form an

open structure, having an "egg crate", mesh, strip, or StarCell® geometry [11].

Figure 2 shows this additional electrode in the StarCell® geometry. These elec-

trodes serve as the cathode of the pump, while the walls of the pump, usually

stainless steel, are operated at anode potential (for safety reasons, the wall of the

pump must always be at ground potential, which means that the polarity of the

power supply for a diode pump is the reverse of that for a triode pump). The ad-

vantages of

the

triode geometry are twofold:

2.7.3 Sputter-Ion Pumps

225

Ions strike the cathode grid at a low angle, which results in a higher sputter-

ing yield. The cathode atoms are mostly sputtered in a forward direction to de-

posit on the wall of the pump. Because the pump wall is maintained at cathode

potential, any ions that pass through the open grid structure of the cathode find

themselves in a retarding potential field that causes them to reverse direction,

back to the cathode; they do not have sufficient energy to reach the wall of the

pump and thus cause no resputtering from that surface. As compared to a diode

pump, the release of previously pumped gas (the memory effect) is negligible.

There is a large increase in the number of ions that are neutralized and scat-

tered in the forward direction, to bury themselves in the pump walls, where they

are subsequently further covered by sputtered titanium. The pumping speed for

argon is high, —30% of that for nitrogen.

The triode pumping configuration provides very effective, stable pumping for

argon and for the other noble gases, so that it provides an alternative to the DI

pump. A disadvantage is that, in many versions, the total quantity of titanium avail-

able for sputtering is substantially lower than in the corresponding diode pump.

Consequently, the operating life is shorter, a factor to be considered if substantial

throughputs of gas must be pumped for sustained periods, as in sputtering appli-

cations. The pumping of hydrogen is of particular interest, first because this is

one of the most prevalent gases in a UHV system, and second, because of the

problems that arise from the pumping characteristics for this gas. The pumping of

hydrogen is very efficient at pressures above ~10~^ torr [12], and essentially all

hydrogen ions that strike the cathodes are pumped, as compared to —30% for

nitrogen ions [12,13]. The hydrogen first forms a solid solution in the titanium

until the solubility limit is reached, at around 0.1 atom % [14]. Additional hydro-

gen forms a stable hydride, eventually reaching the composition TiH2. When op-

erating at pressures below 10"^ torr, the rate of hydrogen diffusion into titanium,

even at room temperature, exceeds the rate of arrival of hydrogen ions. Conse-

quently, the buried hydrogen atoms diffuse away from the point of impact, pro-

ducing a more uniform distribution in the pump cathodes.^

Resputtering of hydrogen from the cathodes is therefore of minor importance

until very large quantities have been pumped. For this reason, the pumping of hy-

drogen in the cathodes remains an important pumping mechanism throughout the

life of the pump, in contrast to the limited amount of ion burial for all other spe-

cies.

Over the life of a pump, the capacity for pumping hydrogen is much greater

than for other reactive gases, because it makes use of the entire volume of the

cathode, whereas other reactive gases are predominandy pumped by chemisorp-

tion on the surface of sputtered material, most of which is sputtered from a rela-

tively small area of each cathode. It must be noted that the pumping efficiency for

hydrogen drops significantly as the pressure falls below 10"^ torr [14]. In this re-

^At high hydrogen pressures, the diffusion may be more complex. See the extended discussion by

Welch [3].

226 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

gion, the use of a supplementary pump, such as a titanium sublimation pump or a

non-evaporable getter pump [15], proves beneficial (see Section 2.7.4.).

Hydrogen is also pumped by chemisorption on sputtered titanium or tantalum,

and once adsorbed, diffusion into the bulk metal will occur. However, the strength

of the hydrogen-titanium bond is far less than that for all other reactive gases, in-

cluding nitrogen and oxygen, and hydrogen chemisorption on sputtered titanium

surfaces is greatly inhibited in the presence of these gases. A surface covered with

a monolayer of nitrogen will chemisorb very little hydrogen, effectively blocking

the diffusion of hydrogen into the bulk titanium [12]. For this reason, the pump-

ing of hydrogen in the presence of other reactive gases depends predominantly on

the burial of hydrogen ions in the titanium cathode. These factors account for a

further complication in the pumping of hydrogen. In the unusual condition of

pumping essentially pure hydrogen [12,13], the titanium cathode surfaces are

progressively cleaned as the surface layer of gases such as nitrogen is sputtered

away, permitting hydrogen molecules to first chemisorb on the atomically clean

surface and then to diffuse, as atoms, into the titanium. This direct gettering of

molecules increases the effective pumping speed by as much as a factor of

and

the increase in speed is sustained when the power supply is turned off, slowly de-

creasing as the surface become covered with other chemisorbed gases. This en-

hanced pumping for hydrogen occurs automatically after a few hours pumping

pure hydrogen at pressures in the high

10 ~^

torr range. It can also be achieved by

pumping argon in the pump for a relatively short period of time [13]. Argon has

a much higher sputtering yield than does hydrogen, cleaning the surface more

rapidly, and additionally, argon does not adsorb, thus leaving an atomically clean

surface.

The formation of titanium hydride by extensive pumping of hydrogen, causes

substantial lattice expansion, with severe cracking [13] and warping of the cath-

odes,

which may eventually cause an electrical short circuit to the anode. For hy-

drogen service, it has been customary to use much thicker titanium cathodes to

increase the capacity for hydrogen, and

reinforce the cathodes to minimize warp-

ing. At the present time, the more conmion solution to pumping large quantities

of hydrogen is to use a nonevaporable getter (NEG) as a supplementary pump,

mounted either internal or external to the sputter-ion pump. NEGs are discussed

in Section 2.7.4.4.

There remains yet another problem associated with hydrogen that has been

pumped by a sputter-ion pump. The cathodes become loaded with hydrogen by

pumping hydrogen or water. The hydrogen pumping is a reversible process, be-

cause of the low binding energy for this gas, which can be driven out from the

cathodes by increasing the temperature. At 25°C titanium hydride has a dissocia-

tion pressure of the order of

10 ~^^

torr, but this increases to ~1 torr at 400°C.

Clearly, one can remove the hydrogen from the cathodes, in a controlled way, by

a bakeout at 400°C, but this same process can inadvertently occur when starting a