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

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2.6.4 Evaluation of Combinations of Backing Pumps and TMPs, Etc. 207

• Pumping of corrosive vapors (purge gas)

• Abrasive media: dust, granules (membrane pumps not possible)

• The backing pumps must have no oil in the pumping system (chemical reac-

tions,

waste problems).

Table 8 shows the evaluation of pump combinations for application to semi-

conductor manufacturing processes in form of a matrix.

Table 8

Pump Combination Selection for Semiconductor Processes

Membrane

Pump

MDP 1 HV/FV:

<10-5 J 0/AD

"S:No""T' HVr$"'

1 FV:$

TMP

(mechanical bearing) n.a.

TMP

(magnetic bearing) n.a.

TMP + MDP ] HV/FV:

<10-9 1 0/AD

(mechanical bearing)

S^:

No 1 HV:$$f

1 FV:$

TMP + MDP ] HV/FV:

<10-^

1 AD/AD

(magnetic bearing) S: No "1 HV:$$$$

1 FV:$

Rotary Vane

Pump (oil)

1 HV/FV:

<io-5 1 0/0

S":No 1 HV:$'

1 FV:$$

1 HV/FV:

<io-9 1 o/o

S:~No 1 HV.$$~

1 FV:$$

1 HV/FV:

<10-^

1 AD/O

S":No "1 HV:$$$$

1 FV:$$

] HV/FV:

<io-^

1 O/O

S":No 1 HV:$$$"

1 FV:$$

1 HV/FV:

<10-9 1 AD/O

S~:No "1 HV:$$$$

1 FV:$$

Multi-Roots

Roots-Claw

1 HV/FV:

<10-5 ' 0/TD

S':

No , HV:$

FV:

$$$

1 HV/FV:

<10-8 • OA^D

S:Part

H"V:

1 FV:$$$

1 HV/FV:

<10-« J AD/TD

S: Yes I HV:$$$$

FV:

$$$

1 HV/FV:

<10-9 1 0/TD

S:Part

"1 HV:$$$

FV:

$$$

1 HV/FV:

<10-9 I AD/TD

S:Yes

1 HV:$$$$

FV:

$$$

Legend for the matrix elements:

Ultimate Pressure in ton-

Quality of Vacuum Produced

High-/Forevacuum: HV/FV:

TD:

AD:

Contamination by hydrocarbons possible

Technically dry (no oil vapors in the vacuum

area of

the

vacuum pump, however, grease or

oil in bearings, etc., close to the vacuum area)

Absolutely dry (no oil vapors, etc., at all

present)

For Use in Semiconductor Manufacturing: S

Yes:

Pump combination can be used

Partial: Pump combination can be used

with minor risk of contamination

No:

Pump combination is not recom-

mended for the use in this

application

Price:

$ Low price

$$ High price

$$$ Very high price

$$$$ Extremely high price

High-vacuum

pump:

Fore-vacuum

pump:

208

Chapter 2.6: Turbomolecular Pumps

2.6.5

THE USE OF TMP IN APPLICATIONS:

SPECIFIC EFFECTS AND DEMANDS

Many applications for TMPs require special measures to attain the requested qual-

ity of the vacuum and a safe and reliable operation of the pumping system. The

following section gives some specific effects of the most important applications

for TMPs, etc., and the measures to cope with them.

Application

Elec- Fusion

Mass Leak tron Particle Instal- Load-

Spec- Detec- Micro- Accel- lations. Etchers

lock/

trometers tors scopes erators R&D CVD Transfer

Specific

Effects

• = does apply

(•) = may apply

Measures

High Inlet

Pressures

Aggressive

Gases

Solid Particles,

Dust

Deposition

High Voltage

Magnetic

Field

Mechanical

Shocks

Vibration

Sensitivity

(•)

•

(•)

•

(•)

Throttle valves,

adequate fore-

vacuum system,

combination,

pumps.

Purge gas, anti-

corrosive coat-

ing, magnetic

suspension.

Inlet

filter,

sep-

arators, purge

gas,

venting

procedure.

Magnetic suspen-

sion,

heating,

inlet baffle.

Electronic mea-

sures for pump

and electronic

drive system.

Magnetic shield-

ing (/x-metal).

Magnetic suspen-

sion,

damping

elements, avoid-

ing of

hard

im-

pacts (valves).

Magnetic suspen-

sion,

damping

elements.

{continues)

2.6.5 The Use of TMP in Applications: Specific Effects and Demands 209

Application

Specific

Effects

Hydrocarbon

Sensitivity

Pressure

Shocks

Ultimate

Pressure

Ionizing

Radiation

Electro-

magnetic

Compatibility

Elec-

Fusion

Mass Leak tron Particle Instal- Load-

Spec-

Detec- Micro- Accel- lations. Etchers lock/

trometers tors scopes erators R&D CVD Transfer

• = does apply

(•) = may apply

(•)

Measures

Magnetic suspen-

sion, grease-

lubricated bear-

ings,

"dry"

backing pump.

Fast forevacuum

valves, combina-

tion pumps,

magnetic suspen-

sion, buffer vol-

ume,

venting

procedures.

Baking, CF-

flanges, high

compression

forH2.

Lead shielding,

radiation-resis-

tant materials.

Electronic

measures.

Application

Evacua-

lon Ion tion of

mplanter Implanter TV

Source Beamline Tubes

Aluminiz-

ingof

TV Coating,

Tubes PVD Sputtering

Specific Effects

High Inlet

Pressures

Aggressive

Gases

(•)

•

(•)

•

Measures

Throttle valves,

adequate fore-

vacuum system,

combination

pumps.

Purge gas, anti-

corrosive coat-

ing, magnetic

suspension.

(continues)

210

Chapter 2.6: Turbomolecular Pumps

Application

Evacua- Aluminiz-

lon Ion tion of ing of

Implanter Implanter TV TV Coating,

Source Beamline Tubes Tubes PVD Sputtering

Specific Effects

Measures

Solid

Particles,

Dust

Deposition

High Voltage

Magnetic

Field

Mechanical

Shocks

Vibration

Sensitivity

Hydrocarbon

Sensitivity

Pressure

Shocks

Ultimate

pressure

Ionizing

Radiation

Electro-

magnetic

Compatibility

(•)

(•) (•)

Inlet

filter,

sepa-

rators, purge

gas,

venting

procedure.

Magnetic suspen-

sion,

heating,

inlet baffle.

Electronic mea-

sures for pump

and electronic

drive system.

Magnetic shield-

ing (/x-metal).

Magnetic suspen-

sion,

damping

elements, avoid-

ing of hard im-

pacts (valves).

Magnetic suspen-

sion,

damping

elements.

Magnetic suspen-

sion,

grease-

lubricated bear-

ings,

"dry"

backing pump.

Fast forevacuum

valves, combina-

tion pumps,

magnetic suspen-

sion,

buffer vol-

ume, venting

procedures.

Baking, CF-

flanges, high

compression

forH2.

Lead shielding,

radiation-resis-

tant materials.

Electronic

measures.

2.6.6 Avoiding Operational Mistakes 211

2.6.6

AVOIDING OPERATIONAL MISTAKES

While operating TMPs, etc., the following rules, if not in contradiction to direc-

tions from the manufacturer's manuals, must be observed carefully, to take full

advantage of the features of these pumps.

2.6.6.1 Entering of Solid Particles

Solid particles entering the inlet of

TMP may damage rotor and bearings. A pro-

tection screen should be used. However, these screens reduce the pumping speed

by approximately 20 to 30%.

Large quantities of dust entering the TMP can block the gaps between blades

and pump body. Therefore, it is advisable to use dust filters at the inlet of

the

TMP.

Here too, pumping speed losses caused by the low flow conductance of these

filters will occur.

2.6.6.2 Inclined or Horizontal Position of the Pump

If the pump is mounted in a horizontal position, the outlet flange (forevacuum

side) should point downward, to avoid condensates (water vapor, oil vapor from

the forevacuum tubulation) from accumulating in the bearing area of the pump

(risk of damage).

2.6.6.3 Baking of the Pump

While baking a pump to attain low pressures, the maximum permissible tempera-

tures,

according to the manufaturer's specification, must be respected. Overheat-

ing can lead to bearing and rotor failures (rotor material loses its strength).

Do not bake the pump when the system is not baked. The gases or vapors re-

leased from the pump during bakeout will condense on the cooler surfaces of the

system.

2.6.6.4 Replacement of Lubricant

The time intervals for replacing the lubricant, indicated by the manufacturer,

should be respected to assure full performance and lifetime of the bearings.

212 Chapter 2.6: Turbomolecular Pumps

2.6.6.5 Backing Pressure

With conventional TMPs the backing pressure should typically not exeed 0.5 torr.

With higher foreline pressures, the pump heats up (gas friction) and the rotor

and/or the bearings could be damaged. In addition, if the rotor is not in the mo-

lecular flow range, the pumping speed and the compression are reduced.

The use of rotary vane pumps (oil-sealed) should be restricted to two-stage

pumps only, since single-stage pumps of this type with opened gas ballast will

bring the risk of not attaining the necessary vacuum level in the backing line.

2.6.6.6 Dimensions of Inlet Connections

It is recommended that the high-vacuum connection have at least the same diam-

eter as the inlet flange of the pump. Smaller diameters lead to significant pumping

speed losses caused by the low conductances.

For example, for a TMP with a nominal pumping speed of 500 L/s for N2 and

an inlet flange of 150 mm I.D., the pumping speed losses for

using a 300-mm-

long connection of different inner diameter are as follows:

High-Vacuum

Connection

I.D.

150

100

40 mm

Length

300 mm

Effective Pumping

Pumping Speed Speed Loss

370 L/s 26%

100

L/s 80%

20 L/s 96%

REFERENCES

W. Becker,

Vak.

Tech. 7 (1958) 149.

W. Gaede, Ann. Phys. 41 (1913) 337.

M. Holweck, C.R.

Acad.

Sci. Paris 111 (1923) 43.

M. Siegbahn, Arch. Math. Astr. Fys. 30B (1944) 17.

5. C. H. Kruger and A. H. Shapiro, in Transactions of

the

7th National

Vacuum

Symposium,

AVS.

New York, 1960, pp. 6-12.

6. L. Maurice, S. Sagot, Le Vide 111 (1964) 109.

K. H. Bernhardt, J.

Vac.

Sci. Technol. Al (1983) 136.

8. J. Henning, Vacuum 21 (1971) 523.

9. Produkt Leitfaden, Alcatel Hochvakuumtechnik GmbH, Annecy 1995.

References 213

10.

Komponenten fiir die Vakuumtechnik, Balzers-Pfeiffer GmbH, Balzers 1996.

11.

Vakuum Katalog, Edwards Hochvakuum GmbH, Marburg 1993.

12.

Katalog HV 300, Teil B, Leybold AG, Cologne 1994.

13.

Vacuum Products Catalog, Varian Ass. Inc., Lexington 1995/1996.

14.

K. H. Mirgel, J.

Vac.

Sci Technol 9 (1972) 408.

15.

L. Maurice,

Jap.

J. Appl Phys. Suppl. 2 (1975) 21.

16.

Y. Murakami, T. Abe, S. Mori, N. Nakaishi, and S. Hata, J.

Vac.

Sci. Technol. A5 (1987) 2599.

17.

A. Conrad and O. Ganschow, R&D Magazine, April 1994, p. 35.

18.

J. E. de Rijke and W. A. Klages, Jr., Solid State Technology, April 1994, p. 63.

CHAPTER 2.7

Pumps

for

Ultra-High

Vacuum Applications

Jack H. Singleton

Consultant

An ultra-high vacuum (UHV), typically defined as a pressure below 10"^ torr,

can be obtained using many of the high-vacuum pumps already described in

Chapters 2.4, 2.5 and 2.6, but the successful application of any pump for UHV

use requires that the influx of gas into the system from all sources—including out-

gassing, permeation through the walls, and leakage from the atmosphere—be

rigidly controlled.

The equilibrium pressure, P (torr), in a system is the result of the balance be-

tween the rate of influx of gas from all sources, Q (torr liter/sec), and the effective

pumping speed of the pumps, S (liter/sec):

P = Q/S

In a typical, leak-tight, unbaked system using a cryopump, turbomolecular pump,

or liquid-nitrogen-trapped diffusion pump, pressures in the low (10~^) to high

(10 "^ torr) range can readily be obtained after a few days pumping. Over longer

pumping periods, the base pressure continues to fall, but at a much slower

rate,

and

pressures below the

10 ~^

torr range are hardly ever attained in an unbaked sys-

tem. Outgassing from the system

walls,

typically in the range of

"^ to

~ ^^

torr

liter/sec cm^, is the limiting factor. The achievement of UHV conditions, say in

the

10 ~^^

torr range, simply by using a bigger pump would require an increase in

speed of the order of 10^, which is generally impractical. The simplest practical

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1998 by Academic Press

$25.00 All rights of reproduction in any form reserved.

214

2.7.1 System Design

for

Ultra-High Vacuum

215

approach is to design a vacuum system that can be baked out at a temperature of

at least 200°C higher than its normal operating temperature so as to reduce the

rate of outgassing by a factor of about 10^.

2.7.1

SYSTEM DESIGN FOR ULTRA-HIGH VACUUM

To select an appropriate pump for ultra-high vacuum use, it is useful to provide a

few basic rules for the design of an entire system that will achieve the ultimate

pressure within 24 to 48 hours of startup.

As much as possible of the system should be bakeable to at least 200°C

higher than the normal operating temperature, so that materials used for construc-

tion must be stable at the bakeout temperature.

Minimize the surface area of the system, because the outgassing rate is pro-

portional to area. The use of porous materials, such as anodized aluminum,

should be avoided.

Wall materials that are permeable to atmospheric gases should be elimi-

nated, or used very sparingly. This particularly applies to the use of 0-ring seals,

which provide reliable and simple construction for unbaked systems, ^uch seals

not only limit the temperature to which the system can be baked, but they also

permit permeation of atmospheric gases through the elastomer into the vacuum

system, providing a rate of influx that is second only to that from outgassing. Dur-

ing a bakeout, the rate of permeation actually increases, falling to its original

value as the system cools to normal operating temperature. Conversely, the rate of

permeation can be reduced by cooling the seals, although this is generally not a

practical procedure. Demountable 0-ring sealed flanges can be easily replaced by

metal-gasketed flanges such as the Conflat® geometry, eliminating permeation.

The selection of valves is particularly important; permeation from atmosphere

can be avoided by using a metal-gasket bonnet seal and a bellows-sealed shaft

seal. For the most rigorous conditions, the internal seal on the nose of the valve is

a metal seal, such as copper, silver, or gold. Elastomers may be used to provide

somewhat easier sealing (lower force) but at the expense of a limitation in the

bakeout temperature: Viton, Kakez®, and polyimide elastomers have been used,

with usable temperatures of approrfmately 200, 250, and 300°C, respectively,

with the valve baked in the open position.

The efficacy of the procedures just recommended is dramatically illustrated by

the work of Alpert and his colleagues [1] in developing practical techniques for

achieving UHV. Their original glass vacuum systems were outgassed by baking

overnight at ~400°C, while pumping with a liquid-nitrogen-trapped diffusion

216 Chapter 2.7: Pumps for Ultra-High Vacuum Applications

pump, then isolated from that pump, so that the only remaining pumping action

was that of the Bayard-Alpert ionization gauge, which provided a speed of

~0.1 liter/sec when operated at an electron emission of 10 mA; pressures in the

low

10 ~^^

torr range were routinely achieved. Note that in this work, the entire

system on which the gauge was pumping was baked at 400°C so that very low to-

tal outgassing rates were achieved. In subsequent work, the vacuum systems were

continuously pumped by a liquid-nitrogen-trapped glass diffusion pump; this pro-

vided a very limited speed of about 0.1 liter/sec, largely limited by the small con-

ductance of the all-metal Alpert valve [1] that was used as the system isolation

valve. The connection from that valve to the diffusion pump could not be effec-

tively baked out, and therefore had a much higher outgassing rate than the baked

part of the system; the fact that UHV was achievable despite this outgassing load

must

ascribed

relatively high speed of the diffusion pump, typically ~30 liter/

sec.

Clearly the greater part of the pumping capacity was used to control the out-

gassing load from a small section of the entire system.

2.7.2

THE SELECTION OF PUMPS FOR

ULTRA-HIGH VACUUM APPLICATIONS

All the high-vacuum pumps described in Chapters 2.4, 2.5, and 2.6 can be suc-

cessfully used to exhaust UHV systems. However, it must be noted that many

commercial versions of the pumps cannot be baked to temperatures that are re-

quired for rapid outgassing. It is therefore essential to minimize the section of the

system that cannot be effectively outgassed, and to provide adequate pumping

speed to handle the higher outgassing influx from this section. These pumps are

increasingly used for UHV applications, especially in systems that are rarely

vented to atmospheric pressure. In such applications, the initial pumpdown of the

system may be protracted because surfaces in and adjacent to the pump remain at

too low a temperature for rapid outgassing, but once a satisfactory ultimate pres-

sure has been reached, the system remains continuously in the UHV range until a

failure occurs, or routine maintenance is required. For example, in thin-film de-

position production systems and surface analysis systems, samples are introduced

to and removed from the vacuum chamber by use of

load-lock system, thus min-

imizing gas influx at all times.

The following brief comments discuss the use and suitability of the pumps,

described in Chapters 2.4, 2.5, and 2.6 and in Sections 2.7.3 and 2.7.4, for UHV

applications.