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

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2.1.5 Pumpdown Time

Rg.3.

f\-h

A chamber with a pumping system showing three typical locations of vacuum gauges.

significant and the evacuation period is extended. Generally, it is recommended

to multiply the values obtained from Equation (5) by about 1.5 between 10 and

0.5 torr, by 2 between 0.5 and 5 X

10 "^

torr, and by 4 between 5 X

"^ and

10 ~^

torr in order to accommodate the outgassing load.

For rapid pumping of small volumes (such as locks), the equation will not give

accurate results due to complications of geometry, conductance and volume of

pumping ducts, and transient effects within the pump

itself.

If the desired pump-

ing time is less than 10 sec, experimental measurements may be advisable.

2.1.5.2 Constant Throughput Case

When constant throughput is used in Equation (4), the solution becomes

t = (V/Q)(Po - P)

(6)

This is not an interesting case technologically, but it occurs in a narrow pressure

region, 10~^-10~-^ torr, in some pumping systems. This period is usually short,

for example, less than 10 sec for a typical bell jar system.

2.1.5.3 In the Presence of a Leak

Equation (3) disregards a possibility of leaks and desorption gas loads. If Q- =

Sp^

is the gas load after "infinite" pumping time, adding this term into Equa-

tion (3) leads to

-Vdpldt+ Q^ = Sp^

(7)

68 Chapter

2.1:

Technology of

Vacuum

Pumps—An Overview

with a corresponding solution

t = (V/S)\n(Po - P^ViP - P^) (8)

The gas load after a long pumping time Q- may be due to an actual leak, out-

gassing of almost constant rate, a finite permeability of the vessel walls, or any

combination of these factors.

If, after a long period of pumping, the external leak is much greater than the

surface outgassing, the pressure established in the chamber is simply given by the

ratio of the leakage throughput divided by the pumping speed

P = Gieak/S torr L/s / L/s (9)

2.1.5.4 In the Presence of Outgassing

For qualitative purposes, the outgassing rate of a surface in high vacuum can be

represented as

G = Goexp(-f/T) (10)

where

is an initial outgassing rate, t the time, and r is associated with the slope

(decay) of the outgassing function relative to time and is assumed to be a constant

[5].

If necessary, for proper matching of experimental outgassing curves, two su-

perimposed exponential terms can be used, such as

Ae~^^

+ BQ~^\ Q is the total

outgassing rate and can be expressed as qA, where A is the area of the outgassing

surface. The general differential equation describing the evacuation of a vessel

can be then written as follows:

- Vdp/dt + (2~ + Go exp

(-t/r)

= Sp (11)

The expression for the evacuation time becomes

= Zi

(PQ

- P~) - Qp/jS - Vlr)

^ S ""(P,- P^) - [Qo/(S - V/r)] exp(-r/r)

The expression for pressure decay relative to time will be

P = (Po~ P~) exp{-St/V)

+ [Qo'iS - Wr)][exp(//T) - exp(-5r/V)] 4- P.

(13)

The first parts of the last two equations can be recognized as solutions when out-

gassing is disregarded.

In the high-vacuum region there does not exist a simple inverse relationship

between pumping speed and time of evacuation, which is obtained at higher pres-

sures.

Using Equation (13) it can be demonstrated that for a conmion bell jar sys-

tem, the pumping speed may have to be increased six or seven times in order to

2.1.6 Ultimate Pressure

cut the time in

half.

However, when process outgassing occurs, the large pump

will maintain a lower process pressure inversely proportional to speed.

The outgassing rates of various materials can vary by many orders of magnitude.

Adsorbing species such as water may exist in ordinary surfaces in the amounts of

the order of 10 to 100 monolayers. The behavior of such films is highly dependent

on temperature and binding energies involved. Outgassing rates of many sub-

stances may be considered to vary exponentially with temperature. The most dis-

turbing species are those with desorption energies from 15 to 25 kcal/mole. Below

that value, the outgassing proceeds rapidly. Above 25 kcal/mole, the desorption

of the material in the vacuum space is likely to give partial pressures below

~ ^ ^

torr. The ordinary conductance relationships cannot be applied to gases in the

temperature regimes in which they are adsorbed.

Returning to equation (13), it may be observed that in many common systems,

for the part of the evacuation in the high-vacuum region,

and V/T « S

and the expression for the pressure reduction can be simplified to

P-P^ =

P^e-^^""^'

+ {QQlS){e-'" - ^-(^/^>0 (14)

When the time of interest is typically between 1 and 10 hours of continuous

pumping, we may say that ^""^'^^ « e~'^^, and the solution simplifies further to

P = {QQlS)e-"' + P^ (15)

This illustrates that the main effect on the rate of pressure reduction is due to

the decay of

the

outgassing rate in respect to time.

is the initial outgassing rate,

which can be associated with some significant starting time. One possibility is to

use the time where the pressure versus time curve (on a log-linear plot) begins to

deviate from a straight line. The onset of this deviation depends on the specific

outgassing rate and the surface area exposed to vacuum. Thus, the evacuation

process can be divided into two parts: the rough vacuum period, where the major

influence is due to volume, and the high-vacuum period, where the dependence is

mainly due to the internal surface area.

2.1.6

ULTIMATE PRESSURE

The ultimate pressure for a high-vacuum pump in a system is reached when fur-

ther pumping will result in negligible reduction of pressure. The total ultimate

pressure consists of partial pressures of various gases present in the system. In

positive displacement pumps such as forevacuum pumps, the pumping speed for

70 Chapter

2.1:

Technology of

Vacuum

Pumps—An Overview

various gases

approximately

the

same. However, under molecular flow condi-

tions

in a

high-vacuum pump each species

of gas is

pumped with

different

pumping speed,

and the

ultimate pressure

given

by the sum of the

individual

partial pressures obtained

dividing

the gas

load (throughput)

of the gas by its

pumping speed.

addition,

the

ultimate pressure includes

the

component arising

the

pump such

pumping fluid

lubricant vapors. Generally,

the

ultimate

pressure

of a

pump

obtained when

equilibrium

established between

the

amount

of gas

flowing into

the

pump

and the

amount flowing

reverse (back-

streaming).

2.1.6,1 Pump

Limit Versus

System Limit

A clear distinction should

made between

ultimate pressure attained

by a

pump alone

and

that attained when connected

to a

system. System gas loads often

are orders

magnitude higher than

the

internal pump

gas

evolution,

so the

sys-

tem

may

never reach

the

ultimate pressure capability

of the

pump. Normally,

pump performance data

are

given

for a

minimum volume

and

surface area

at the

inlet

of the

pump.

addition

to the

amount

of gas

loads,

the

conductance

of the

inlet ducts will also influence

the

system ultimate pressure.

Ultimate pressure

in a

system

rarely

an end in itself. In

some work,

mini-

mum partial pressure

of one

species, such

as oil, may be

critical, while much

higher pressures

other species

may be far

less important.

certain work

is to

be performed inside

the

vessel after evacuation,

usually results

additional

gas

evolution. Therefore,

if a

process

or an

experiment

is to be

performed

at a

given

pressure, the ultimate system pressure should

lower.

convenient

and

very

ap-

proximate rule

for

work that produces

low gas

evolution

is to

specify ultimate

system pressure

times lower than

the

desired process environment.

In general,

the

inlet pressure

of a

high-vacuum pump

can be

expressed

as a

sunmiation

effects

various

gas

flows

and

compression ratio limits,

*(^f)

-4

where

is the

inlet pressure,

g, is the gas

flow rate (throughput)

for a

particular

gas,

P2i

is the

partial pressure

of the

particular

gas at the

discharge

end of the

pump,

and

Ki is

the

limiting compression ratio

for

the particular

gas;

the

first term

represents external

gas

loads;

the

second term,

gas

loads from sources inside

the inlet sections

of the

pump

itself.

What

generally taken

to be the

ultimate

pressure

of the

pump

(not the

system!)

given

by the

last

two

terms

the sum-

mation. However,

cannot

emphasized

too

strongly that

the

manufacturers

and users

vacuum pumps traditionally have ignored

the

last term

for

nearly

•,=(2^1

+|S^| +2^ (16)

2.1.7 Forevacuum

and

High-Vacuum Pumping

100 years. This is certainly true for mechanical pumps and vapor jet pumps (so-

called diffusion pumps). The compression ratio effects and limits are typically

only discussed with the more recently developed turbomolecular

pumps.

For cap-

ture pumps, the compression ratio term must be replaced by a term associated

with the number of desorbing previously pumped molecules.

Without knowing the gas composition and the compression ratio limits for

each gas present in the system, it is generally not possible to know what deter-

mines the ultimate pressure in a vacuum system.

Because of the enormous range of densities involved in evacuating a vacuum

chamber from atmospheric pressure to ultra-high vacuum, it is not possible, for

practical reasons, to do the required pumping with one integral pump. Typically,

at least two pumps are connected in series. To understand the interaction between

those pumps in regard to its effects on inlet pressure, it is necessary to know not

only the total pressure established in the conduit between the two pumps but also

the partial pressures of various gases. Yet even if a partial pressure analyzer were

used at the discharge side of the pump, it is not simple to make definitive conclu-

sions from the known pump performance parameters.

Most high-vacuum pumps do not operate entirely in the molecular flow range.

Therefore, gas interactions can occur in the downstream parts of the pump, which

are not easily predictable from the primary performance data. Even if compres-

sion ratios are known for the particular pump and gas, the presence of a second

gas can alter the expected compression limit values. Therefore, it is very impor-

tant for a high-vacuum technologist to be aware of various effects not normally

given in the technical bulletins or textbooks on high-vacuum technology, and to

understand qualities and peculiarities of a particular pump and its construction

details. The traditional display of pump performance does not represent the entire

story. It does not, for example, give any information about the influence of the

discharge pressure on the performance of the pump.

2.1.7

FOREVACUUM AND HIGH-VACUUM PUMPING

High-vacuum technology deals with a very wide range of pressure or particle den-

sity conditions. Usually, the process of evacuation begins at atmospheric pressure

and then proceeds to high or ultra-high vacuum. In a multistage pumping system,

the type of pumping mechanism employed is different at atmosphere and at high

vacuum. Even if, in principle, a given pumping method could be used throughout

the entire pressure range, such an attempt would be impractical in regard to size,

weight, or cost of equipment, as illustrated in Figure 4.

72 Chapter

2.1:

Technology of

Vacuum

Pumps—An Overview

Fig.

liJ

-I

4»

¥~

t/i

-100

'CA.

\\ \^

X'V'v^ \\

C^^^'^^O

PUjA^

DIFFUSION

\^^^

PUMPS

\MECH"

:f^

\BLOWERS

J L

>^MECH

(" PUMPS

10 10

-9

lO'^

10"

PRESSURE,TORR'

10"

qualitative

illustration

of the

pressure

ranges

which

the use of

various

pumps

most

economical.

The most common pumping arrangement for production of high vacuum con-

sists of a positive displacement mechanical pump for initial evacuation followed

by a high-vacuum pump. Often parallel and series arrangements are used. For ini-

tial evacuation, the mechanical pumps are used alone, and for obtaining high vac-

uum, the two types of pumps are connected in series. In such a system, the gas en-

ters the pumping train at high vacuum and is exhausted at atmospheric pressure

by the last pump. In some cases, the device may be evacuated and sealed, termi-

nating the pumping process; in others, the pumping is continuously applied to

compensate for the gas evolution in the vacuum chamber.

In capture pumps (sorption pumps, and particularly in getter-ion pumps), the

pumped gas is not exhausted to atmosphere. This has an obvious advantage of

isolation from high-pressure environment and the disadvantage of limitation in

gas load capacity or the necessity for periodic regeneration or replacement, in the

case of getter-ion pumps.

The basic performance of pumps and compressors can be associated with flow

and pressure factors. With appropriate allowance for size, power, pumped fluid

characteristics, etc., all such devices generally behave according to the pressure

flow graph shown in Figure 5.

For high-vacuum work, both high-volume flow rate and high-pressure ratio

(inlet to discharge) are necessary. The flow rate is associated with the size of the

pumping device. Thus, for a given size, the pressure ratio needs to be increased as

much as possible. Mechanical vacuum pumps, vapor jet pumps, and turbomolecu-

2.1.8 Pump System Relationships

Fig.

0 FLOW

The general qualitative relationship between pressure and flow for most compressors or

pumps.

lar pumps produce pressure ratios of over

million to 1. This can be compared to

industrial plant air compressors, automobile engines, or aircraft compressors not

exceeding a compression ratio of 10 to 1. Obviously, high-vacuum pumps require

very special designs, and familiarity with their design, construction, operation,

and maintenance is an important ingredient of success in producing and using

high-vacuum environments.

2.1.8

PUMP SYSTEM RELATIONSHIPS

There are two basic ways of reducing the pressure or time needed for evacuation

chamber: increase the pumping speed or decrease the outgassing

rate.

In some

cases,

it may be less expensive to provide additional pumps rather than attempt to

reduce gas evolution. This depends on the degree of vacuum and other process re-

quirements. For very-low-pressure or ultra high vacuum systems, degassing is the

only approach possible. Checking the following points is useful before specifying

or designing a vacuum installation: The gas-handling capacity of the pumping

system at a given pressure is usually more important than the ultimate pressure;

the net pumping speed at the chamber is more significant than the nominal speed

of the pump; increasing the number of pumps in the high vacuum region does

not necessarily decrease the evacuation time in the same proportion; well-chosen

pumps, working fluids, baffles, and traps can reduce or eliminate contamination

problems: rapid pumping requirements increase system cost; long exposure of the

chamber to atmospheric conditions will increase the subsequent outgassing rate;

temperature history and distribution in the chamber before and during evacuation

should not be neglected; the amount of surface area present in the vacuum cham-

ber is more significant (at high vacuum) than the volume.

74 Chapter

2.1:

Technology of

Vacuum

Pumps—An Overview

2.1.8.1 Surface-to-VoIume Ratio

Generally, in high-vacuum conditions more gas is adsorbed on the walls of the

vacuum chamber than present in the space. This, of

course,

depends on the size of

the chamber. For small devices, the surface-to-volume ratio is higher, and small

vacuum chambers tend to have a higher ratio of surface area to pumping speed.

Therefore, generally 5-cm-diameter high-vacuum pump systems, for example, do

not produce low ultimate pressure as easily as 10- or 20-cm-diameter pumping

systems.

A similar comment may be made about other sources of gas. Pumping speed

grows approximately with the square of the pump diameter, but the exposed area

of gaskets, and therefore the associated outgassing load, increases closer to a lin-

ear rate. Thus it is relatively easy to produce high vacuum in large chambers (as

high as 10 m in diameter). In such cases, the rough vacuum system may be the ex-

pensive part, because of its dependence on volume.

In laboratory system, pumps of 15-20 cm diameter are usually more conve-

nient for low-ultimate-pressure work even if the high-speed requirements are not

necessary. In normal practice, pumps are described by the size of the flange at the

opening.

2.1.8.2 Evacuation or Process Pumping

Vacuum pumps are used for two somewhat distinct applications. The first in-

volves evacuation of gas (usually air) from a vessel, the other maintaining process

pressure with a given gas load. It is difficult to choose the correct pump size with

any degree of precision for both applications when required pressure is in the

high-vacuum region. In the first case, this is due to the uncertainty of surface out-

gassing rates, which depend on temperature, humidity during atmospheric expo-

sure,

and the presence of gas adsorbing deposits on the chamber walls. The water

vapor is the usual cause of concern. In the second case, the process gas evolution

is uncertain, due to variation of gas content in materials and the variations in the

process parameters. For example, when electron beams are used for evaporation

the power distribution in the focal spot may not be exactly repeatable. This may

produce variations in the degree of heating of the surfaces in the vicinity of the

electron gun.

As a very rough rule for evaporation, melting, welding, or sputtering of metals,

at least 100 L/sec of net pumping speed should be provided for every kilowatt of

power used for the process.

Generally, one can hardly make an error in the direction of using too large a

pump. It is conmion to use 25-cm vapor jet pumps in a large bell jar system to

2.1.8 Pump System Relationships 75

produce a net system speed exceeding 1000 L/sec. Thirty years ago, the net

pumping speed at the base plate was only 500 L/sec. These differences are not

important in improving system pumpdown time but they do help to maintain

lower process pressures. As far as evacuation time is concerned, the difference

between a

15-

or 25-cm pump is hardly noticeable. When outgassing is taken into

account, the evacuation time is not inversely proportional to the pumping speed,

as might be expected from the experience at higher pressures. However, during

rapid process gas evolution or introduction, the process pressure and the pumping

speed are often related inversely.

In some cases, such as sputtering, maximum throughput becomes important,

because it may be desirable, from the process viewpoint, to have a certain flow of

argon through the system. A high pumping speed for the initial evacuation of the

system will be of lesser importance. In some instances, a system is pumped with

the pump valve fully open to a given base pressure and then throttled to obtain an

argon pressure and flow rate as required by process specifications, without ex-

ceeding the tolerable throughput of the pumps. As the pump speed has been re-

duced, it is possible that the impurity level in the system has increased.

2.1.8,3 Pump Choice

A variety of mechanical pumps are used at the present time for the initial evacua-

tion of a vacuum vessel. Most common are rotary vane and rotary piston positive

displacement pumps, in which the narrow spaces between moving and stationary

parts are sealed by oil. The presence of sealing and lubricating oil reduces the in-

ternal leakage from discharge side to the inlet side of the pump. The oil also fills

the so-called dead space under the discharge valve. This permits very high com-

pression ratios to be achieved in such pumps. They obtain inlet pressures of

10 "-^

torr in a single stage and better than

10 ""*

torr in two stages. When the presence of

oil is undesirable, a variety of oil-free pumps can be used. These are usually lim-

ited to a lowest inlet pressure of about 10"^ torr. Such pumps can be multistaged

piston pumps, multistaged Roots type (or other cam-type) blowers, orbiting scroll

pumps, or screw pumps.

To achieve high vacuum after the initial evacuation, at the present time four

different type of pumps are used: getter-ion (or sputter-ion) pumps, cryopumps,

oil vapor jet pumps (or diffusion pumps), and turbine-type pumps of various

designs.

Getter-ion

pumps function by providing chemical reaction with freshly evap-

orated or sputtered metallic films (usually titanium) and by ionization and

subsequent capture of the gas molecules in the surface layers of the elec-

trodes. TTiey are most suited for systems that are not frequently opened to

76 Chapter

2.1:

Technology of

Vacuum

Pumps—An Overview

atmosphere. Because they are capture pumps, they cannot accept large gas

loads.

They are extremely useful for pumping of vacuum tubes and long-

term experiments. They are clean and have no oil contamination.

Cryopiimps, as the name implies, immobilize the gas on cold surfaces by con-

densation and cryosorption. Cryopumps are oil free and have high pumping

speeds, particularly for water. They are capture pumps and consequently re-

quire frequent regeneration to dispose of the captured gases. This can pose a

problem when used with hazardous gases, because a significant accumulation

explosive or toxic gas is difficult to deal with in an emergency shutdown.

Vapor jet pumps function by moving gas molecules from the inlet to the dis-

charge side through collisions with a high-velocity stream of vapor (usually a

variety of low-vapor-pressure oil-like substances). The advantage of a vapor

jet pump is that it can pump a very wide variety of

gases,

including rare gases,

such as helium, that are difficult to pump using cryopumps or ordinary turbo-

pumps. Vapor jet pumps with very high pumping speeds can be obtained.

The major disadvantages are that if improperly operated, oil vapor can back-

stream into the system. Cold traps are used to prevent this migration and have

an advantage of increasing the pumping speed for water.

Turbine-type pumps produce the pumping action by collisions with a high-

velocity moving surface (blade rotors or plain disks or cylinders). These

pumps are generally clean and oil free, and provide high pumping speeds for

most gases. Due to their precision construction, these pumps are particularly

susceptible to particle contamination. A turbopump can be turned on or off

faster than both the cryo and diffusion pumps. Hence its use may be preferred

in applications sensitive to pumping system failures.

Figure 6 shows an approximate range of inlet pressures where such pumps can

be used for continuous operation. To achieve oil-free pumping, cryopumps or

grease-lubricated or magnetic bearing turbopumps backed by dry roughing pumps

are normally employed.

The selection of high-vacuum pumps is not as straightforward as selection of

mechanical pumps. The considerations for reaching a given pressure in a short

time may not prove an adequate measure of the system's capability to handle

steady gas load. Characteristics of pumping curves of high-vacuum pumps indi-

cate the proper components. More frequently, gas loads are not known, and the

selection of the pump must be based on experience with similar applications.

The vacuum equipment suppliers generally offer performance based on pump-

down characteristics of an empty chamber. This can be misleading if it is con-

strued to be a measure of pumping efficiency under full load conditions. The vac-

uum equipment designers must relate performance to a set of conditions that are

known and can be defined and duplicated, which may have misled some users to

construe the pumpdown time to be the true indicator of process time.