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

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2.6.3 Combination of Pumps [TMP + MDP] 197

Table 3

Compression for Commercial Molecular Drag Pumps

^max

Gas Pumped

Manufacturer

[9,

10]

2 X

10'...

1 X 10^

1 X 10^..2X 10^

1 X 10\..

X 10^

Table 4

Pumping Speed for Molecular Drag Pumps

Pumping Speed

Gas Pumped

H. (%)

He (%)

(%)

Manufacturer

[9,

10]

40...

53 ... 67

100

2.6.3

COMBINATION OF PUMPS (TMP + MDP)

Turbomolecular pumps have a good pumping speed but an upper limit of backing

pressure near 0.1 mbar. Compared with that, molecular drag pumps may operate

at much higher backing pressure of about 10 torr and above but with clearly lower

pumping speed.

A combination of the two types of pumps will offer the advantage of using the

high pumping speed of the turbomolecular pump in the first stage, followed by a

molecular drag pump with lower speed but high compression ratio as a second

stage. As a result, a backing pressure two orders of magnitude higher than with

common turbomolecular pumps is obtained.

2.6.X1 Design

This capability allows the use of simple small, dry, and inexpensive backing pumps

(e.g., membrane pumps) and therefore permits a dry evacuation from atmospheric

pressure down to the ultimate pressure.

198

Fig.

Chapter 2.6: Turbomolecular Pumps

Combination pump.

Turbomolecular part:

1 high-vacuum flange

2 first rotor disc stage

3 first stator disc dtage

Rotor suspension:

6 magnetic bearing

7 magnetic radial bearing

10 safety ball bearing

Courtesy of Pfeiffer Vacuum

Molecular drag part:

4 cylindrical rotor stage

5 threaded stator stage

8 backing vacuum flange

11 position sensors

12 electric connection

It is common practice to place all rotating elements of the combination pump

on a single shaft as shown in Figure 8. If the components are arranged very close

together, the gas duct from one to the following will be short, to assure good flow

conductance. For the components of the combination pumps, design considera-

tions and performance data as given in paragraphs 2.6.1.3, 2.6.1.4, and 2.6.2 are

valid. For most applications of these pumps, a high gas throughput is usual. It

is important to avoid any pressure rise caused by conductance limited apertures

inside and between the pump components. Similarly, the temperature rise from

heating by gas friction must be controlled. The admissible backing pressure is

2.6.3 Combination of Pumps [TMP + MDP] 199

higher if mechanical spaces in the drag channels are quite narrow. This creates the

requirement to limit temperature increases. On the other hand, high operational

temperatures are used to reduce undesired reactions or deposits from the pumped

media. Thus, careful temperature control is an important design feature of combi-

nation pumps.

These combination pumps are available in different design configurations:

• TMP stages -f multistage MDP (disk type, derived from Gaede): high fore-

vacuum tolerance

• TMP stages -f multistage MDP (drum type, derived from Hoi

week):

high

forevacuum tolerance

• complete TMP + multistage MDP (drum type, derived from Holweck): high

forevacuum tolerance and very low ultimate pressure.

2.6.3.2

Typical Vacuum Data of Commercial

Combination Pumps

Typical data of conmiercial combination pumps are given in Tables 5 and 6.

Table 5

Compression Data for Commercial Combination Pumps

*^max

Gas Pumped

Manufacturer

[9,10,11,13]

2X 10^..2X 10^

7 X 10^..2X 10^

5 X 10\..

X 10'2

Table 6

Pumping Speed Data for Commercial

Combination Pumps

Pumping Speed Ratios

Gas Pumped

(%)

He (%)

(%)

Manufacturer

[9,

10, 11, 13]

24 ...133

50 ...133

100

200 Chapter 2.6: Turbomolecular Pumps

ULTIMATE PRESSURE

Depending on how many TMP and MDP stages are used in series, the attainable

ultimate pressure is in the range of

10 ~ ^ ^

torr to 1 X

10 ~^

torr.

2.6.X3 Split-Flow TMPs

Split-flow TMPs [10, 13] are special multiport (two to three inlet ports) combina-

tion pumps for the application at, for example, multichamber and differentially

pumped systems (e.g., analytical systems: GC-MS/LC-MS, etc.) [17].

Each pumping port conducts the gas at its different pressure separately into the

appropriate stage of the combination pump. Together with akeady compressed

other gas, it is then further pumped and exhausted into the common foreline.

2.6.4

EVALUATION OF COMBINATIONS

OF BACKING PUMPS AND TMPS, ETC.

2.6.4.1 Nomenclature of the Different Types of Pumps

Depending on the manufacturer of these pumps, different designations are used

for the same type of pump. Following is a list of trade names of the different

TMPs, MDPs, and combination pumps:

Molecular Drag Pumps (MDP), Pumping Speed Range 7.5-300 L/s

Trade names:

• Drag pumps

• Molecular pumps

• Molecular drag pumps

• Spiromolecular pumps

Turbomolecular Pumps (TMP), Pumping Speed Range 35 L/s-10.000 L/s

Trade names:

• Turbomolecular pumps

• Turbo pumps

2.6.4 Evaluation of Combinations of Backing Pumps and TMPs, Etc. 201

Combination Pumps (TMP -f- MDP), Pumping Speed Range 30-2.000 L/s

Trade names:

• Hybrid pumps

• MacrotoiT pumps

• Turbodrag pumps

• Turbospiromolecular pumps

• Wide-range pumps

• Hy.cone pumps

2.6.4.2 Typical Data of Commercial MDPs,TMPs,

and Combination Pumps

The following is a sunmiary of the specific features of these pumps which must

be considered when selecting a pump for a certain application.

Molecular Drag Pumps (MDP)

Bearings Type Lubrication

Mechanical Grease

Mechanical/magnetic Oil

Pressure range From <

10 "^

torr to >

torr ... 40 torr

Backing pump required Foreline pressure: <10 torr ... 40 torr

Turbomolecular Pumps (TMP)

Bearings Type Lubrication

Mechanical Grease, oil

Mechanical/magnetic Oil

Magnetic None

Pressure range From <

~' ° torr to >

torr

Backing pump required Foreline pressure: < 10'' torr

Pump Combinations (TMP and MDP)

Bearings Type Lubrication

Mechanical Grease, oil

Mechanical/magnetic Oil

Magnetic None

Pressure range From <

~'° torr to >

torr ... 40 torr

Backing pump required Foreline pressure: <

... 40 torr

202 Chapter 2.6: Turbomolecular Pumps

2.6.4.3 Typical Data of Backing Pumps

As all the TMPs, MDPs, and combination pumps mentioned need

backing pump,

worthwhile to look at the existing types of backing

pumps,

in order

be able to

evaluate which backing pump could be used together with the high-vacuum pump.

In principle, the selection of an adequate backing pump depends on its pump-

ing speed, its pressure range, the quality of the forevacuum produced, its price,

and its size.

Type

Membrane

Rotary vane

Piston

Multi-Roots

Claw-Roots

Ultimate Pressure

<10torr

<10-^torr

<10"2torr

<10-'

torr

<10~'

torr

Quality of Vacuum

Absolutely dry

Oil vapors

Technically dry

Backing pumps producing an "absolutely dry" vacuum do not have any oil or

grease in possible contact with the pumping area. "Technically dry" pumps do not

have any oil or grease in the pumping area; however, they have grease or oil, e.g.,

in the bearing or feedthrough areas, which are kept away from the pumping area

by dynamic seals such as labyrinths, etc.

2.6.4.4 Adaptation to Applicational Requirements

VENTING

After a power shutdown a TMP should be vented, or it will be contaminated by

oil vapors as a result of pressure equalization between exhaust and inlet. By vent-

ing with a dry gas to atmospheric pressure, this effect can be suppressed and a

contamination of the vacuum system avoided. Most TMPs, with the exception of

the larger pumps, may be vented from the high-vacuum side at full rotor speed, if

not otherwise contraindicated in the manufacturer's manual. Too strong a pres-

sure rise will deform the rotor blades and make them contact the stator, damaging

the pump. If the vacuum system cannot be vented directly, it is advisable to vent

the TMP by means of a special vent port that opens into the compression stages.

Venting a TMP from the contaminated foreline should be avoided by all means.

BAKING

To attain low ultimate pressures in the high-vacuum and ultra-high-vacuum

range, the internal surfaces of a TMP (rotor, stator, housing) must be baked out.

2.6.4 Evaluation of Combinations of Backing Pumps and TMPs, Etc. 203

Due to the temperature sensitivity of

the

aluminium alloy used for the rotors, there

is a limit to the maximum value of the baking temperature. This maximum must

be well below the critical temperature for the aluminium alloy with respect to its

strength. Typical baking temperatures are in the range of 100°C to 140°C, using

the bakeout systems and following the operation instructions supplied by the

pump manufacturers.

COOLING

To dissipate the frictional heat from the bearing areas, the motor, and the heating

by gas at high pressures, the TMPs must be cooled. Although convection cooling

is sufficient for small pumps, larger pumps are equipped with fans for air cooling.

For many pumps, water cooling is usual.

OPERATION IN MAGNETIC FIELDS

In the presence of magnetic fields, TMPs that have metallic rotors experience in-

duction of eddy currents, which, due to heating, can cause serious problems con-

cerning material strength. Therefore, TMPs with metallic rotors can be used in

magnetic fields only if certain maximum values of the magnetic flux density will

not be exceeded. These maximum values are specified by the manufacturers and,

for static magnetic fields perpendicular to the axis of rotation, typically are in the

range of 10 mT to 30 mT (tesla). For pulsed magnetic fields, different maximum

values are valid. In case of higher flux densities, the TMP must be shielded mag-

netically, which can create problems with the distribution of the magnetic field it-

self.

An experimental TMP with a ceramic rotor has been reported that has been

tested with a magnetic flux density of 460 mT [16].

In this context keep in mind that the electronic drive systems and the control

systems (magnetic suspension) of TMPs create magnetic stray fields in their

vicinity. These stray fields can influence other electronic and magnetic systems.

Depending on the position relative to the TMP, typical values for these stray fields

are in the

range

JULT

to 50

/JLT.

PUMPING CORROSIVE GASES

Modern manufacturing techniques require pumping processes where the ultimate

system pressure must be in the high-vacuum range and corrosive gases must be

handled, e.g., below 10"^ torr. At these pressures the rotor material will not be

attacked by the corrosives. However, the higher gas density at the forevacuum

side in the rotor bearing area requires the use of special lubricants (Fomblin®,

Krytox®, etc.), which are corrosion resistant.

204

Fig.

Chapter 2.6: Turbomolecular Pumps

TMP for pumping corrosive gases (purge

gas).

1 splinter guard

2 pump case

3 rotor

4 venting connection

5 water-cooling circuit

6 VV flange

7 HV flange

8 stator disk

9 spindle unit with ball bearings

10 purge gas entry

11 water-cooling entry

12 electric driver [12]

In the semiconductor industry, Si and Al are etched in plasma etch machines,

e.g., at pressures of above

10 ~^

torr. To avoid etching and corrosion of the TMP

^ s

aluminium rotor, the rotor and all internal parts of the TMP having contact with

the corrosives are either made of corrosion-resistant materials or are specially

coated (e.g., Ni-plated). Such coatings of the rotor can reduce the attainable ulti-

mate vacuum of a TMP to 5 X

10 ~^

torr. Besides using special lubricants, for fur-

ther protection TMPs for plasma etching are equipped with a purge gas system

that admits inert gas (N2, Ar, typically 30 seem) directly into the bearing area and

the forevacuum side of the TMP (Figure 9), where it creates a directed flow to the

exhaust of the pump. This flow prevents the corrosive gases from entering the

bearing area.

Using TMPs with magnetic suspension in these applications will reduce the

problems mentioned.

PUMPING TOXIC OR RADIOACTIVE GASES

An additional range of problems must be solved in the case of a TMP pumping

toxic or radioactive gases, e.g., tritium, with application in plasma fusion installa-

tions.

Because of the safety hazards involved, the TMP must be extremely tight.

With a "tritium" TMP, all seals to atmosphere are made of metal. The integral leak

rate is below 10"^ torr L/s.

2.6.4 Evaluation of Combinations of Backing Pumps and TMPs, Etc. 205

TMP IN COMBINATION WITH OTHER HIGH-VACUUM PUMPS

To increase the pumping speed of a TMP for hydrogen or water vapor and make

use of its constant throughput and pumping speed, TMPs have been combined

with Ti sublimation pumps or, recently, with cryopumps [18].

2.6A5 Precautions for the Operation of Pump

Combinations for Clean Vacuum

In vacuum systems there are several sources of contamination, such as improp-

erly cleaned surfaces or not using the proper materials for seals, feedthroughs,

etc.

Additional sources of contamination can be present in the pumping system.

An oil-filled backing pump and the lubricant for the rotor bearings of a TMP are

potential sources for contamination by oil vapors, which can be avoided with

proper procedures. Depending on what components are in the pumping system,

there are several ways in which a vacuum system may become contaminated un-

less some fundamental rules of operation are followed.

STARTING PROCEDURE

When roughing the system by the backing pump prior to starting the TMP, it

is very easy to reach pressures below

10 ~^

torr in the roughing phase, allowing

molecular backstreaming of roughing pump oil directly into the vacuum system.

When in contrast both pumps are switched on together, by the time the pressure is

low enough to allow this molecular backstreaming from the roughing pump, the

TMP rotor will be accelerated and its compression sufficient to act as an efficient

barrier for any backstreaming oil vapor.

Note: Whenever possible, the TMP and the backing pump should be started si-

multaneously at atmospheric pressure.

In applications that absolutely require a separate roughing phase before switch-

ing on the

TMP,

care should be taken that the roughing line does not enter molecu-

lar flow conditions. This could be achieved by pressure control systems or by gas

purge systems set to pressures above 10"^ torr.

VENTING PROCEDURE

A TMP that is shut down should be vented with a dry, clean gas before its rotor

decelerates so much as to allow backstreaming. This procedure is necessary for

all combinations of lubricated TMPs with oil-sealed backing pumps.

For TMPs with greased bearings, problems of contamination are reduced to

some extent, but care must also be taken while operating these pumps. Table 7

206

Chapter

2.6:

Turbomolecular Pumps

Table 7

Operational Modes that Need Precautions to Avoid Contamination of the Vacuum System

Pump or

Combinations

MDP

(mechanical bearing)

IMP

(mechanical bearing)

TMP

(magnetic bearing)

TMP + MDP

(mechanical bearing)

TMP + MDP

(magnetic bearing)

Membrane

Pump

(absolutely dry)

Venting

n.a.

Venting

Rotary Vane

Pump

(oil vapor)

Start/Venting

Multi-Roots

Claw-Roots

(technically dry)

Venting

shows the modes of operation of several TMP pumping systems for which the

reconmiended precautions must be observed to avoid contamination.

By observing these rules of operation and regarding the specifics of the differ-

ent combinations of TMP and backing pump, these combinations can be used to

produce and maintain a clean vacuum. However, with some of these combinations,

wrong operation can lead to a contamination of the complete vacuum system.

2.6.4.6 Evaluation of Pump Combinations

for a Specific Application

Evaluation of a pump combination for a certain application must consider the

following:

• Special requirements of the application

• Possible pumps

• Attainable vacuum pressures

• "Quality of the vacuum"

• Price

MANUFACTURING PROCESSES FOR SEMICONDUCTORS

In the following, the evaluations of different pump combinations for use in man-

ufacturing processes for semiconductors are given as examples. The typical

manufacturing processes for semiconductors have special requirements concern-

ing the vacuum pumping system: