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2.7.3 Sputter-Ion Pumps 237

pump

itself.

The pump runs fairly warm during starting, providing some out-

gassing, and heat from baking out the rest of

the

system results in some further in-

crease in the pump temperature. If the lowest possible system pressure is required

in a reasonably short period of

time,

the entire pump and vacuum chamber can be

baked simultaneously, using a supplementary pumping system whose cleanliness

mirrors that desired in the UHV system. If

bakeout requires a temperature above

250°C, it may be necessary to remove the magnets and high-voltage power cable.

In the discussion that follows, we assume that only the vacuum chamber will be

baked, while using the sputter-ion pump as the pump.

Once a sputter-ion pump has been roughed down, the pump can be turned on.

At the highest operating pressure, ~1 X 10"^ torr, the electrical impedance of

the pump is very low and exceedingly large currents would flow if the power sup-

ply maintained a voltage in the kilovolt range. This could result in serious damage

to the pump. The power supplies are deliberately designed to limit the available

power. The voltage and current are severely limited, and consequently, the pump-

ing speed is initially very low. If the pressure begins to fall when the ion pump is

turned on, the roughing pump can be immediately valved off, proceeding slowly

enough to confirm that the pressure continues to fall as the speed of the roughing

pump is reduced. As pressure continues to fall, the sputter-ion pump voltage will

rise steadily, reaching the maximum, design level when the pressure is around

10 ~^

torr. With a pump in good condition, the pumpdown will proceed smoothly,

at a rate determined by the gas load from the vacuum system. With a well-used

pump, the sound of momentary electrical arcing may be detected, accompanied

by a groan from the power supply, and a sudden pressure burst. Arcs may be ini-

tiated at sharp edges of flakes of titanium, or at whiskers on the cathodes. The lo-

cation of the discharges can be readily seen if a window provides a view of the

pump electrodes. In many cases, arcing episodes gradually diminish as the pres-

sure falls, and the pumps perform satisfactorily at the system operating pressure,

but the arcs give fair warning that the pump should be reconditioned, or replaced

at a convenient opportunity. On other occasions, a flake of titanium may bridge

the electrodes in a pump, either partially or totally shorting out the pump. A smart

blow on the side of the pump can sometimes resolve the problem satisfactorily, at

least on a temporary basis, and operation can continue without problem until the

next time the system is pumped down following venting to atmosphere.

It is appropriate in the present context to note the advantage to be gained by al-

ways venting a vacuum chamber to atmosphere using a dry, gas, nitrogen being

the most conmion choice. This minimizes the exposure of the system to atmo-

spheric moisture, which will tenaciously adsorb on the system walls to a depth of

one or more monolayers, the actual thickness depending on the humidity of the

air. Water is always the last gas to be removed from a chamber during pumpdown

and is normally the cause of slow attainment of the ultimate pressure in a vacuum

system. To maximize the advantage gained by nitrogen venting, minimize the

time that the chamber is exposed to the atmosphere.

238 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

Once the pressure in the vacuum system falls into the

10 ~^

torr range the vac-

uum chamber can be baked out. Few problems are associated with this process,

assuming that the system is free from leaks. The major gases to be removed are

water, carbon monoxide, and hydrogen. Organics, such as methane may well be

generated in the pump, but the rate of production is usually small, and should not

affect the proper operation of the system. 0-rings should not normally be used to

seal against atmosphere, and those used as internal valve seals should be Viton,

Kakez®, or polyimide, which are thermally stable and permit bakeout to approx-

imately 200, 250, and 300°C, respectively.

The type of sputter-ion pump selected for a new vacuum system will depend on

the major gases which will be present. The pumping speeds of three types of

pump, for common gases, is given in Table 1. The size of pump must be selected

with reference to the throughput that is expected for each gas and the pumping

speed for that gas at the operating pressure. The major common gases of concern

are argon and hydrogen. Experience shows that unstable pumping may develop

with pure argon, or with an argon-containing mixture, for example, air. In some

studies, argon instabilities occurred periodically whenever a particular quantity

of gas had been pumped. Once instabilities developed, the time between succes-

sive instabilities was short at high throughput, but increased on lowering the gas

flow. If argon is to be consistently used in a system, it is prudent to choose the

most reliable pump type. The DI type of diode pump or the StarCelF^ triode are

probably the best choices for pumping substantial quantities of argon, because

the quantity of titanium in these pump types is considerably greater than in a con-

ventional triode pump, and therefore better operating life can be expected.

Sputter-ion pumps are very efficient for pumping hydrogen, with the exception

of very low pressure operation where the hydrogen pumping speed can drop off

even more precipitously than for other gases [14]. Serious problems occur when

pumping so much hydrogen that distortion of the cathodes occurs. For example,

at sustained pressures in the 1 X 10"^ torr range. Pumps have been built using

extra-thick titanium cathodes, reinforced to resist warping and fitted with water

cooling, to avoid problems of dissociation of the titanium hydride that can occur

if the cathodes run too hot in a heavily loaded pump. One additional approach to

limit pump overheating at high hydrogen loads is to operate the pump at a lower

voltage (<4.8 kv) [25]. If standard pumps must be used, it should be noted that

the capacity for pumping hydrogen in a tantalum cathode is substantially less than

in a titanium cathode, so the conventional titanium cathode diode pump is likely

to perform better than the DI pump when large throughputs of hydrogen are antic-

ipated. However, getters are now available with outstanding hydrogen-pumping

capacity, and they should certainly be considered to supplement a sputter-ion

pump; such getters are discussed in Section 2.7.4.4.

Although the sputter-ion pump is an effective general-purpose device, it has a

relatively high capital cost and is often combined with a relatively low-cost getter

2.7.3 Sputter-Ion Pumps 239

pump, most commonly a titanium sublimation pump. Sublimation pumps provide

very high pumping speeds for reactive gases, including for example, oxygen, ni-

trogen, hydrogen, carbon monoxide, and water vapor. They are particularly use-

ful for sustained operation in the UHV region. The sublimation pump can be

started after the vacuum chamber has reached a pressure in the

"^ torr range, or

less,

and can readily provide sufficient speed to reduce the system ultimate pres-

sure by a decade or more. Alternately, a sublimation pump can provide reserve

pumping capacity when a deliberate influx of a reactive gas is to be introduced

into the vacuum system. The characteristics of sublimation pumps are described

in detail in Section 2.7.4.

For any process that requires the maintenance of very low pressures and low

organic impurity levels, a sputter-ion pump can give excellent performance and

long operating life. When samples must be transferred between atmosphere and

the system, on a regular basis, the use of a load-lock chamber for the transfer

avoids any venting of the main vacuum chamber, permitting one to retain most of

the advantages inherent in these pumps.

If a sputter-ion pumped system must be vented to atmosphere on a regular ba-

sis,

it is advantageous to provide a valve to isolate the pump from the chamber, so

that it can be left evacuated, and even running, while the chamber is vented using

dry nitrogen gas. In a further refinement of this approach, the vented vacuum

chamber is continuously purged using dry nitrogen, to minimize air intrusion to

the greatest extent possible; any components or samples added to the system are

pre-baked and kept meticulously clean. Such stringent measures can result in a

far more rapid pumpdown of

the

vacuum chamber, limiting the extent of exposure

to the atmosphere and the necessity for a subsequent severe bakeout.

An operating sputter-ion pump can generate a copious source of electrons and

ions during the initial pumpdown, inducing unexpected system behavior, for ex-

ample, erroneous ion gauge pressure measurements. At lower pressures the pump

can introduce electrons, ultraviolet radiation, and X-rays into the vacuum cham-

ber. Fortunately, if the pump is mounted or shielded so that there is no direct line

of sight into the system, these problems become negligible. A further perturbation

introduced by the pump is stray magnetic fields; the use of better-confined mag-

net assemblies has reduced this problem to a relatively low level.

The power supply of a pump is designed for a particular size of pump, and is

designed to limit the power output under the high-load conditions that prevail

during startup. It is unwise to substitute either a larger or smaller power supply,

without consulting with the manufacturer. The dissipation of too much energy

during the early stages of pumpdown could lead to destructive overload, and the

use of too low a voltage will reduce the pumping speed of the pump.

Take adequate precautions to ensure safe operation of a sputter-ion pump, be-

cause of the potentially lethal currents and voltages involved. A damaged power

cable should never be used, and it is strongly recommended that an independent

240 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

electrical ground be provided for the pump body (and for the rest of the vacuum

chamber, if there is any insulating barrier between the pump and chamber). It

should be recognized that, when a diode pump is first started, the general glow

discharge may spread into the rest of the vacuum system. Electrons can travel

remarkable distances and if there is any wire or structure in the chamber that is at-

tached to an electrical feedthrough, that feedthrough should never be left electri-

cally "floating." It can accumulate an electrical charge quite high enough to pro-

vide a nasty electrical shock when touched. Ground all unconnectedfeedthroughs!

2.7.3.9 Troubleshooting and Maintenance

of Sputter-Ion Pumps

Since there are no moving parts in a sputter-ion pump, there are only a few com-

mon problem areas. Failure of the power supply is relatively uncommon. Most

supplies have automatic overload protection when the power drawn by the pump

exceeds the design value, as for example would result if a pressure surge above

10 "-^

torr initiated a general high-current glow discharge. When the pump is

started, this overload protection must be turned off, but should be restored as soon

as the pressure begins to fall, say into the

10 ""^

torr range. In a pump that is mar-

ginally operational, momentary arcs within the pump could trigger the protective

circuit, which should therefore be left off when these problems arise, but the

pump must be watched carefully through this startup phase, until arcing ceases

once the pressure has fallen far enough.

After long use, many pumps develop whiskers, which serve as field emission

sources for electrons in the presence of high electrical fields. The conditions for

the formation of these whiskers are not well defined, but they produce localized

discharges, giving higher than expected pump currents. The presence of discharges

can be inferred by comparing the pressure in the vacuum chamber, using an ion

gauge, with that indicated by the pump current. When the pump is operating nor-

mally, its indicated pressure will be lower than that in the system, but if localized

discharges have developed, the reverse will be true. Discharges do not provide

significant pumping, and if the current draw is high, may result in the voltage ap-

plied to the pump falling below its design value.

Two procedures have been used to remove whiskers in an ion pump. The first

is known as "Hi-Potting," a procedure widely used in the conditioning of high-

voltage vacuum devices. It consists of the momentary application of a current-

limited potential on the order of 20 kv to the pump, while under vacuum. This

procedure can remove whiskers by ion bombardment. A second procedure is to

run the ion pump at an oxygen pressure in the

10 ~^

"^ torr range in the hope

of destroying the whiskers by chemical reaction. The procedures are not always

2.7.3 Sputter-Ion Pumps 241

successful and should be used as a last resort if replacement of the pump is not

possible.

The problems that arise from the flaking of thick titanium deposits from the

pump walls have already been alluded to. Denison [26] found that flaking prob-

lems in sublimation pumps could be greatly reduced by using a 300°C bakeout

after each exposure to atmosphere, presumably removing adsorbed water. A bake-

out is not feasible with a sputter-ion pump unless a supplementary pumping sys-

tem can be employed, but the precautions just outlined, of either leaving the pump

valved off during system venting, or of venting the pump with dry nitrogen, should

reduce flaking problems.

When whiskers or flakes are present, they can produce an electrical short. Also,

if the pump has been run at high pressures, there may be titanium deposits on the

high-voltage feedthrough or anode insulators. The pumping efficiency will usu-

ally be reduced if electrical leakage problems cause excessive current draw from

the power supply. In extreme cases, this may result in an obviously low pump

voltage, while in other cases the pressure indication from the pump current will

be higher than normal. The problem may be identified by disconnecting the power

supply and measuring the electrical resistance from the high-voltage terminal to

ground. Check the resistance with both polarities of the meter, since it may be low

in one direction and high in the other. Note that exposure of the pump to atmo-

spheric pressure will often increase the resistance dramatically, so resistance

checks are best made while the pump is still under vacuum.

A series of sharp knocks on the side of the pump will sometimes displace a

short, and whiskers can be removed by the procedures just described. In many

cases,

however, the pump must be either replaced or cleaned. The former is the

preferred route assuming that time and money are available, but an on-site cleanup

is possible provided adequate precautions are taken. Details of recommended

procedures can be found in old sputter-ion pump manuals dating from the 1960s.

The process starts with disassembly, followed by mechanically cleaning the pump

walls.

At this stage some parts, such as insulators, should be replaced. If the cath-

odes are seriously eroded, they should be replaced. Take care when removing any

sputtered titanium films; they may contain a great deal of unreacted metal, which

may glow or burn when scraped free from the wall. Chemical etching of pump

parts is commonly required, involving the use of very aggressive etching baths,

including nitric and hydrofluoric acids. All necessary precautions to protect the

eyes and skin are essential in these procedures, which are not reconunended for

the uninitiated.

Finally, it is important to be aware that sputter-ion pumping falls off to negli-

gible values very rapidly at pressures above

10 ~-^

torr. Consequently, if the pres-

sure in a system is measured using the ion pump current, very low readings may

indicate either a very low or a very high pressure.

242 Chapter 2.7: Pumps

for

Ultra-High Vacuum Applications

2.7.4

GETTER PUMPS

The term gettering has its origin in the techniques used in early radio tubes to ob-

tain and maintain an acceptable degree of vacuum for the operation of the tubes.

These devices had to be manufactured on a time scale that did not permit ade-

quate outgassing, and so a reactive metal surface was included in the tube to react

with any residual gases, essentially getting them out of the way. Commonly used

getters included heated filaments of reactive metals such as titanium or zirco-

nium, and evaporated films of the alkaline metals, particularly barium. The one

thing that all these

"getters"

have in common is their rapid reaction to form a stable

chemical compound with conmion residual gases, such as oxygen, nitrogen, and

water vapor. It is interesting to note that the much smaller residual amounts of

argon and helium, which cannot be pumped by chemical reaction, are undoubt-

edly pumped as ions, after ionization by electron bombardment. Such processes

were widely studied in attempts to explain the pumping of these gases in ioni-

zation gauges [27]. In current vacuum technology, barium gettering remains a

crucial step in the production of TV picture tubes and cathode-ray tubes, and

evaporated films of titanium find wide use in UHV pumping systems; bulk or

nonevaporable getters, commonly known as NEGs, are finding applications in

general vacuum use and in sealed-off

devices.

Note, however, that getters are sup-

plementary pumps that can only remove chemically reactive gases; they must be

combined with another pump that can handle any influx of noble gases, or of

methane and other chemically stable organic compounds.

2.7.4.1 Sublimation Pumps

Sublimation pumps are devices that deposit a film of a reactive metal on a se-

lected substrate either continuously or periodically. Titanium is the preferred metal

and is deposited from a high-temperature source. The titanium sublimes from the

source, going directly from the solid to the vapor phase. This fortunate circum-

stance permits the direct electrical heating of a titanium wire in order to provide

an appropriate sublimation rate, without melting the wire.

The removal of

gas

by a sublimation pump involves a process of chemisorption

on a titanium surface. Most gases are sorbed only on the surface (the one excep-

tion being hydrogen). As the surface coverage increases, the rate of pumping de-

creases, becoming negligible once the surface is fully covered by a single ad-

sorbed layer. The probability of a reactive gas atom or molecule being adsorbed

on first impact, on an atomically clean titanium surface, is between 10 and 100%,

2.7.4 Getter Pumps 243

depending on the particular gas. As the surface becomes covered, this probability

drops by one or two orders of

magnitude!

When a vacuum chamber is in the ultra-

high vacuum range, say 1 X

10 ~^

torr, it will take two to three hours for a mono-

layer to form, so that a brief period of sublimation each hour will replenish the

clean titanium layer, maintaining a high average rate of pumping. The time nec-

essary to replenish the film depends on the rate of sublimation, and on the area

to be covered. A larger surface area translates into higher pumping speeds, but

requires a larger total quantity of titanium to be deposited each time. If a subli-

mation pump is operated at higher pressures, the periods between operation of

the sublimator must be decreased, and at the highest practical rate, the sublima-

tor must operate continuously, so as to maintain the rate of sublimation slightly

higher than the rate at which the gas reacts with the titanium. It is important

to note that, even at the highest pressures at which a sublimator is likely to be

operated, say 1 X

10 ~^

torr, the mean free path of a nitrogen molecule is around

500 cm, so that the probability of nitrogen reacting with titanium in the gas phase

is quite negligible.

There is an upper-pressure limit of around

10 ""^

torr for the effective operation

sublimator. On exposure to the atmosphere, a sublimator surface is almost in-

stantaneously covered with a strongly bound, chemisorbed layer, mainly of oxy-

gen and nitrogen. This layer is not desorbed at the operating temperature but

dif-

fuses into the bulk titanium, leaving a clean surface on the titanium filament on

which further chemisorption can occur. When the chemisorbed layer is present,

sublimation of titanium is strongly inhibited. Consequently, if the filament is

heated when the system pressure is too high, the chemisorption of gas on the hot

surface will exceed the rate of diffusion into the bulk, thus maintaining a full sur-

face layer [28]. A similar phenomenon has been reported for yttrium sublimation

[29].

Because sublimation is suppressed, a layer of titanium is not deposited on

the system walls, and essentially all the gettering of gas must occur on the small

area of the hot sublimator. Consequently, the effective pumping speed remains

very low. In contrast, if the system pressure is in the low

10 "^

"^ torr region,

the rate of diffusion into the filament will exceed the rate of gettering on its sur-

face,

with the result diat sublimation proceeds. The surface area of deposited tita-

nium soon far exceeds that of the hot filament, and the bulk of the gettering is

transferred from the hot-filament surface to the deposited titanium surface. Get-

tering always occurs on the hot filament, but at lower operating pressures, it is too

small to sustain significant surface coverage on the sublimator, and proper opera-

tion is assured.

A typical titanium sublimation temperature is around 1550°C, giving a vapor

pressure of about 1 X 10""^ torr and a rate of sublimation of around 0.05 g/hour

for every square centimeter area of hot titanium; this rate is very roughly equiva-

lent to the pumping (or gettering) capacity required to handle a throughput of

244 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

1 X

10 ~-^

torr liter/sec. Most sublimators are only used at operating pressures

below 10"^ torr, thus assuring a reasonable operating life before the sublimator

must be replaced. The construction of practical sublimators is discussed later.

2.7.4.2 The Practical Application of Sublimation Pumps

The selection of a titanium sublimation source is determined by the requirements

for both pumping speed and throughput. If the system always operates in the

UHV range, and the throughput of gas is small, then the simplest type of device

and manual control will give adequate life even at high pumping speeds. Where

the throughput is substantial, then a device capable of dispensing appropriate

quantities of titanium, preferably with automatic control of the sublimation rate,

may prove essential.

When the titanium is deposited on a surface at room temperature, it provides

maximum pumping speeds of the order of

and 9 liter/sec.cm^, for hydrogen and

oxygen, respectively [30]. Cooling the titanium film to liquid nitrogen tempera-

ture increases the pumping speeds by a factor between 2 and 5, depending on the

gas.

Higher pumping speeds can be easily achieved by increasing the area on which

the titanium is deposited, but note that a larger surface area will necessitate an in-

crease in the rate of sublimation to maintain the higher rate of pumping.

The maximum pumping speed is obtained when the titanium surface is clean.

In practical applications, the surface will always be partially covered by an ad-

sorbed layer, so the pumping speed will be only some fraction of this maximum

value, determined by the frequency at which the surface is replenished by subli-

mation. Harra [30] provides guidance for the case where continuous sublimation

is used. For the case where the pressure is very low, and periodic sublimation is

used, it is necessary to assume some effective pumping speed of the titanium,

which is less than the speed for the clean film [30], say 20 to 30%.

The required area of titanium in a particular system can be calculated from the

equation

Titanium area = Q/P X 5 cm^

where Q is the estimated influx of gas from all sources, including outgassing

and leakage, in torr liter/sec

P is the requires pressure, in torr

S is the gettering speed per square centimeter of titanium for the gas

being pumped

Particular care must be taken to assure that the conductance between the pump

and the vacuum chamber does not seriously limit the effective speed of the pump.

2.7.4 Getter Pumps

245

Fig.

PRESSURE GAUGE

BAKEABLE ISOLATION

VALVE ^

BAFFLE TO

LIMIT TITANIUM

DEPOSITION

ISOLATION VALVES

•>^^

PRESSURE RELIEF

SORPTION PUMPS

(LN2 COOLED)

BAKEOUT OVEN

SPUTTER

ION PUMP

TITANIUM SUBLIMATION BAKEABLE ISOLATION

PUMP VALVE

Schematic of vacuum system using sputter-ion and titanium sublimation pumps in parallel,

with sorption pump roughing system.

The sublimator is mounted inside the vacuum chamber so that the sublimed

titanium film is deposited on the chamber walls, or on baffle plates, positioned to

protect the working volume of the chamber from titanium deposits. A typical sys-

tem combining a sputter-ion pump and a titanium sublimation pump is shown

schematically in Figure 6.

For the reasons just discussed, do not heat the sublimator until the system pres-

sure is well below 10"'* torr, so as to avoid unnecessary contamination of the

source. This is not a serious limitation, since the sublimation pump is most com-

monly employed as a supplementary pump.

One of the only serious problems of the sublimation pump is the flaking of the

sublimed titanium film from the surfaces on which it is deposited. This is pre-

cisely the same problem already discussed for sputter-ion pumps. It can be mini-

mized by avoiding unnecessary venting to the atmosphere, or, by baking the sub-

limator after such exposure [26], before restarting the sublimation process.

In normal operations, the amount of titanium sublimed is invariably in excess

of that necessary to sustain the required pressure, but this titanium remains acces-

sible for hydrogen pumping. On exposure to the atmosphere, oxygen adsorption

on the surface occurs, effectively blocking access to any unreacted titanium in the

bulk. Steinberg and Alger [31] showed that hydrogen gettering on such titanium

films can be restored by the deposition of a new film of titanium on the exposed

surface. Presumably such a film permits dissociative chemisorption of hydrogen

followed by diffusion of the hydrogen atoms into the bulk.

246

Chapter 2.7: Pumps for Ultra-High Vacuum Applications

Fig.

THREE TITANIUM-MOLYBDENUM ALUMINA INSULATOR

SUBLIMATION FILAMENTS

(Two

shown)

N^ \///A

^>S<i^S>^:^i:^^

2.75" diam.CONFLAT™ FLANGE

Titanium sublimator pump cartridge.

ZZZZ2

WIRE SUBLIMATOR SOURCES

The simplest form of sublimator consists of a feedthrough carrying two or more

replaceable filament sources, which are heated, in turn, until all the titanium has

been sublimed. A drawing of this type of sublimator is shown in Figure 7. Typi-

cally, each filament can sublime over 1 g of titanium before failure. Most fila-

ments are made from a 85% titanium-15% molybdenum alloy. When heated at

constant power, this alloy is more robust, and evaporates titanium at a more con-

stant rate, over its life, than does a comparable pure titanium source [32,33]. A

typical sublimator equipped with three filaments can dispense over 3 g of tita-

nium when each filament is used, in turn, to exhaustion. With a rate of sublima-

tion adjusted so as to just balance the rate of pumping, roughly 210 torr liter of

gas can be pumped before the sublimators must be replaced.

SUBUMATION FROM TITANIUM SPHERES

Substantially greater quantities of titanium can be provided from hollow titanium

spheres using radiant heaters [34]. In one form, the heater is a tungsten filament

mounted inside the sphere, and up to 35 g of titanium can be sublimed from a

sphere of roughly 3.5 cm diameter. With these thin-walled spheres the structural

integrity is a concern, because a phase change in titanium, at around 880°C, de-

velops stresses in the titanium, which, with repeated temperature cycling will

eventually cause failure [34]. For this reason, the power supply for the sublimator

is programmed to hold the titanium at a stand-by temperature in excess of 880°C,

where the sublimation rate is negligible, and to cycle up to a selected temperature

above 1500°C when sublimation is required. Failure occurs when sublimation

produces a hole in the sphere. Radiation losses through the hole cause the sphere

to run at too low a temperature for usable rates of sublimation. One disadvantage