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

Подождите немного. Документ загружается.

2.4.7 Other Performance Aspects

147

Rg.21.

r^\

(a)

(b)

(c)

ri\/n\#M

(d)

Evolution of cold cap design.

(e)

(f)

Trap defrosting

6. Explosive breakup of a frozen layer in the trap

The sources inside the pump are

Eruptive or unstable boiling

Boiling outside the jet assembly

Low pressure ratio, light gases

Liquid droplets in the nozzles

Cold top nozzle

6. Leaks near the boiler

Vigorous boiling in vacuum causes droplets to splash upward and be blown out

through the nozzle opening, causing momentary blockage as well as wet running

in the nozzle. A splash baffle located inside the boiler is used to eliminate this

effect. Low pressure ratio for light gases can cause pressure instabilities. An

increase in the heater power, as well as an increase of the number of stages, is

beneficial.

Outside the pump, gas bubbles from the elastomer seals are the major and fre-

quent source of pressure bursts. Gas evolved from and permeating through the

elastomer seals is sometimes trapped by oil films. As the pressure builds up in

the bubbles, they break through the oil film and release the gas into the system.

The inlet flange of the pump is the most vulnerable location. Careful design of the

0-ring groove can minimize this instability. Fluid dripping from the baffles on the

hot surface of the top jet cap can evaporate producing vapor and temporarily

148 Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]

affect the pumping speeds. Pumps having pressure fluctuations for light gases of

at the pump inlet have been reported [19, 20].

REFERENCES

R. Jaeckel, KleinsteDriicke (Springer-Verlag, Beriin and New York, 1950).

H. G. Noller, Physical events in diffusion pumps.

Vacuum,

5 (10) (1955) 59.

K. C. D. Hickman,

Trans.

Am.

Vac.

Soc. (Pergamon, New York, 1961), p. 307.

D. J. Crawley, E. D. Tolmie, and A. R. Huntress,

Trans.

Am.

Vac.

Soc. (AIP for AYS, New York,

1967),

p. 67.

5. H. Okamoto and Y. Murakami,

Ahs.

Am.

Vac.

Soc. Symp. (AIP for AVS, New York, 1967), p. 67.

6. G. M. V^ood, Jr. and R. J. Roenigk, Jr.,

Vac.

Sci.

Technol.,

6 (1969) 871.

7. J. S. Cleaver and R N. Fiveash,

Vacuum,

20 (1970) 49.

8. B. D. Power, High

vacuum pumping

equipment (Van Nostrand-Reinhold, New York, 1966).

9. M. H. Hablanian, /

Vac.

Sci.

Techno!.,

6 (1969), p. 265.

10.

G. Rettinghaus and

K. Huber,

Vacuum,

24 (1974) 249.

11.

F. M. Fowkes, Contact angle. Adv.

Chem.

Sen 43, Am. Chem. Soc. (1964).

12.

D. H. Holkeboer, D.

Jones, F. Pagano, and D.

Santeler,

Vacuum Engineering

(Technical Pub-

lishers, Boston, 1967).

13.

J. Moenich and R. Trcka,

Trans.

Am. Vac. Soc. and

Int.

Congr. Vac. Sci.

Technol.

(Pergamon,

New York, 1961), p. 1133.

14.

B. D. Power and D. J. Crawley,

Vacuum,

4 (1954) (published 1957) 415.

15.

S. A. Vekshinsky, M. I. Menshikov, and I. S. Rabinovich, Proc.

Int.

Cong.

Vac.

Tech.

1st. (Perga-

mon,

London, 1958), p. 63.

16.

N. Milleron and L. Levenson,

Trans.

Am.

Vac.

Soc. (Pergamon, New York, 1960), p. 213.

17.

M. H. Hablanian and H. A. Steinherz,

Trans.

Am.

Vac.

Soc. (Pergamon, New York, 1961), p. 333.

18.

M. H. Hablanian and A. A. Landfors,

Trans.

Am.

Vac.

Soc. (Pergamon, New York, 1960), p. 555.

19.

M. H. Hablanian and A. A. Landfors,

Vac.

Sci.

Technol.,

13 (1976) 494.

20.

J. D. Buckingham and N. Dennis,

Vacuum,

21, 5 (1975).

CHAPTER 2.5

Cryogenic Pumps

Gary

Ash

CTI'Cryogenics Division

Helix

Technology

Corporation

2.5.1

INTRODUCTION

Cryogenic high-vacuum pumps provide clean, high-speed pumping of all gases.

They function through a combination of condensation and adsorption of gases

and vapors on surfaces held at very low temperatures. Most cryopumps employ

sets of surfaces held at two specific temperature ranges by a reciprocating me-

chanical refrigerator using helium gas as a refrigerant. The helium is supplied at

high pressure and room temperature by a remote compressor, and returns to the

compressor at low pressure but only slightly warmer than room temperature. All

the cold parts of the refrigerator lie within the vacuum environment of the pump,

although there are no moving parts within the vacuum. This approach produces a

rugged, cost-efficient vacuum pump that is suitable for many applications.

The housing of

cryopump, such as the one in Figure 1, is typically a stainless

steel or aluminum vacuum vessel with a high-vacuum flange for mounting to the

high-vacuum valve on the system. A two-stage cryogenic refrigerator is bolted or

welded into the vessel. The refrigerator surfaces exposed to the vacuum volume

are stainless steel. Massive copper heat stations, sometimes plated with nickel for

corrosion protection and appearance, are brazed to the cylinder of

the

refrigerator

for efficient heat transfer to the condensing arrays. A copper thermal radiation

shield, like a large bucket painted black on the inside, is attached to the first-stage

ISBN

0-12-325065-7

$25.00 All rights of reproduction in any form reserved.

149

150

Chapter 2.5: Cryogenic Pumps

Rg.l.

8-inch

(200-mm) diameter cryogenic high-vacuum pump. (Photo of On-Board 8 cryopump

courtesy of CTI-Cryogenics.)

heat station. This thermal radiation shield, kept at 65 K or so by the refrigerator,

absorbs incoming heat from the system on the black paint. The outside of the

shield is nickel plated to reduce the thermal radiation transfer from the room-

temperature vacuum

vessel.

nickel-plated copper inlet

array,

commonly

circu-

lar chevron, attaches to the opening of

the

thermal

radiation shield

and is

also

main-

tained near 65 K. The inlet array acts as a pump for water vapor (see Figure 2).

Within the volume enclosed by the inlet array and radiation shield, a second-

stage array set is located. Attached to the lower temperature (10-20 K) heat sta-

tion, a number of copper

disks,

plates, or cones are used as a second

pump

for ar-

2.5.1 Introduction

Rg.2.

151

MOUNTING FLANGE

80K CONDENSING

ARRAY

ROUGHING VALVE

Components of

cryopump. (Line drawing of cut-away view of On-Board 8 cryopump cour-

tesy of CTI-Cryogenics.)

gon, oxygen, nitrogen, and other gases by condensation. These surfaces are also

usually nickel plated. Over much of the second-stage array surface is a layer of

activated carbon or charcoal. These provide the third type of pumping within the

cryopump: a sorption pump for the light gases hydrogen, helium, and neon. The

size,

shape, and number of these plates determine the speed and capacity for all

the gases to be pumped on the second stage.

2.5.1.1 History

Early cryogenic pumps from the 1930s onward used liquid cryogens, such as liq-

uid nitrogen at 77 K and liquid helium at 4.2 K to produce refrigeration. Although

152

Chapter 2.5: Cryogenic Pumps

simple to construct, they were limited to a single orientation and required con-

stant replenishment of cryogens [1]. The most significant breakthroughs in cryo-

pumping technology were the development of the Gifford-McMahon refrigera-

tion cycle and the use of activated charcoal for the sorption of hydrogen, helium,

and neon in the 1960s. Originally developed at Arthur

Little, Inc. for condens-

ing hydrogen at 20 K and for cooling microwave receivers for satellite communi-

cation ground stations, the Gifford-McMahon refrigerators proved their capabil-

ity of running for many years with little or no maintenance in remote sites around

the world. Other refrigerator versions were initially developed for cooling infra-

red detectors and solid-state lasers for military programs.

2-5.1.2 Refrigerator Operation

The Gifford-McMahon cycle is a "no work" thermodynamic expansion cycle us-

ing regenerative heat exchangers. In cryogenic vacuum pumps, the refrigerator

configuration is usually of two stages of different diameters, one above the other.

(See Figure 3.) A pistonlike displacer, with stepped diameters, fits within the cyl-

inder of the refrigerator [2]. The larger diameter portion is packed with hundreds

of disks made of metal screen. The smaller-diameter section of the displacer is

packed with lead balls or similar materials. The screens and balls are the heat ex-

changer matrix. The displacer body itself is made of a rugged plastic with low

Rg.X

2nd STAGE

1st STAGE

15K

65K

lOOpsig

300K

300psig

EXHAUST

VALVE

INLET

VALVE

A Gifford-McMahon refrigerator.

2.5.1 Introduction 153

thermal conductivity. A number of holes in the displacer body direct the flow of

helium gas through the displacer. The screens and balls are packed so as to en-

courage the relatively free flow of gas through the matrix while exposing the sur-

faces of the balls and screens to the gas for good heat exchange.

A slow-speed motor or pneumatic drive moves the piston up and down within

the cylinder bore at a nominal rate of 60-72 strokes per minute. When the refrig-

erator is operating, the end of the large-diameter, first-stage cylinder near the

drive mechanism is essentially at room temperature. The other end of the first-

stage cylinder, to which the first-stage heat station is brazed, is at approximately

65 K. The smaller-diameter, second-stage cylinder is welded to the 65 K end of

the first stage. The opposite end of the second-stage cylinder achieves a tempera-

ture of about 10-15 K. The exact stage temperatures are determined by the ther-

mal loads placed on them, the design of the displacers in the refrigerator, the sup-

ply and return pressures of the helium gas from the compressor, and a number of

other variables.

Gas supplied from the compressor at about 20 atmospheres pressure (300 psig)

is admitted to the warm end of the cylinder by a mechanically timed inlet valve.

The helium flows through the displacer matrices and fills the entire cylinder vol-

ume to the supply pressure. As the gas passes through the screens and balls, it is

cooled as it goes. In turn, each screen or ball becomes slightly warmer. The mov-

ing displacer passes through the gas, eventually reducing the volume of gas at the

warm end to nearly zero. This also produces small volumes of gas at 65 K and

10 K at the other end of

the

cylinders as the displacer reaches the end of

its

stroke.

At this point, the inlet valve closes and the exhaust valve opens to the return line

to the compressor. Because the return line is at about 6-7 atmospheres (100 psig),

gas within the cylinders expands, cools, and flows back through the displacers.

On each stroke, a little heat is removed from each heat station (and attached ar-

ray) to maintain the operating temperatures. The cold gas flowing back over the

balls and screens cools them back to their previous temperatures. As the displacer

continues its reciprocating motion, the small cold volumes are reduced, forcing

the remaining cold gas back through the displacer matrix. The gas going out the

exhaust valve of the refrigerator is slightly warmer than the gas that came into the

cylinder. Because the refrigerator cylinders are welded tubing, none of the helium

gets into the vacuum space. Similarly, none of the gases condensed in the cryo-

pump ever enter the helium stream.

2.5.1.3 Compressors

The compressor performs one major function: It takes the return gas from the

refrigerator at 100 psig and boosts the pressure to 300 psig before returning it to

the refrigerator. To accomplish this, it must also perform some related functions.

Compressing helium generates lots of heat, so the heat of compression must be

154

Chapter 2.5: Cryogenic Pumps

removed from the helium and transferred to either cooling water or air supplied

from the outside. Since the compressor is a mechanical device requiring lubrica-

tion, oil is injected into the returning helium and carried to the compressor pump.

The oil also serves as an efficient cooling and heat transfer medium. After com-

pression and cooling through

heat exchanger, the helium stream is stripped of the

oil by a mist separator. An additional adsorber cartridge provides a final cleanup

of the helium before it goes out to the refrigerator. A typical piping diagram of a

compressor is shown in Figure 4. Gas purity is held at the level of tens to hun-

dreds of ppm.

As the compressor only takes in and sends out a relatively constant volume of

helium, there is no limit to the separation distance of the compressor and attached

Fig.

HeuOM SUPPLY LINE

(TO THE COLO HCAO) ^-^

-< c&<_>>

METERING

, COOUN6 _0IL OmFjCE

GAS FLOW

—C£- SELF-SEALING

COUPLING

SOLENOID-CONTROLLED VA.Vt

(OPEN *H£N COMPRESSOR

IS IN OPERATION)

Piping circuit of compressor for cryopump showing flow of helium, oil, and cooling water.

(Piping diagram of helium compressor courtesy of CTI-Cryogenics.)

2.S.1 Introduction 155

cryopumps. Helium line lengths of 10-60 feet are common, but a compressor

may be placed hundreds or thousands of feet from the pumps when lines of ade-

quate diameter are used. No loss of refrigerator performance is created by separa-

tion distance.

The major factors in cryopump performance controlled by the compressor are

the absolute and differential pressures of the supply and return lines. Other things

being equal, higher refrigeration capacity is produced by higher differential pres-

sure.

Performance is slightly better at lower absolute pressures. Higher refrigera-

tion capacity provides shorter cooldown and regeneration times, as well as greater

ability to handle heat loads at constant temperature. Since compressors are made

with positive displacement mechanical pumps, increased helium output flow re-

duces the differential pressure. This means that when more small pumps are

connected to one large compressor, the refrigeration capacity of each pump de-

creases, increasing cooldown time. To limit the maximum pressure differential

and absolute pressure to a reasonable level, a spring-loaded bypass valve in the

compressor limits the differential pressure when the compressor is on but has no

output (cryopumps in regeneration) or only a low gas output. Depending on the

application, the performance of the cryopumps may be increased by providing as

much compressor capacity as is economically practical. On multipump systems

with automated controls, the compressors may account for only 15-20% of the

total pumping cost, so that there is great leverage on small investments in addi-

tional compressor capacity.

Normally, compressors require little maintenance. At periodic intervals of one to

three years, the adsorber should be changed to continue high gas purity. If some

helium charge has been lost and the static charge pressure (compressor off) has

dropped, 99.999% pure helium gas should be added. In the event of helium gas

contamination, as indicated by the onset of knocking noises on each stroke of the

refrigerator, the compressor and each of the attached cryopumps should be

purged and recharged. In addition, the adsorber should be changed at that time.

Compressors will run for many years without any other maintenance. Operating

lifetimes of 50,000 to 100,000 hours are not unusual.

2.5.1.4 Applications

High-vacuum evaporators were the first commercial application of cryopumps,

where they replaced diffusion pumps. Freedom from oil vapor backstreaming and

the ability to reach pressures in the

10 ~^

torr range and below, make cryopumps

appealing for use in the deposition of

variety of electronic and optical thin films

by evaporation or sputtering. Ultra-high-vacuum versions of cryopumps operate

in the 10"^ to 10~^^ torr range for surface science and molecular beam epitaxy

systems. In the semiconductor industry, cryopumps serve on cluster tool sputter-

ing systems at 5 X 10"^ torr and on ion implanter end stations and beam lines.

1S6 Chapter 2.5: Cryogenic Pumps

Ion milling processes using inert gases such as argon, as well as reactive gases,

are pumped with cryopumps. Industrial equipment such as electron beam welders

and vacuum brazing and heat-treating furnaces rely on the oil-free environment

of cryopumped chambers for high-quality metal processing. NASA and the aero-

space companies use large cryopumps of 36- to 48-inch diameter for evacuat-

ing 100-foot-long space simulation chambers to 10"^ torr, although these larger

pumps may use multiple refrigerators and LN2 for part of the refrigeration. Small

cryopumps of 100 mm diameter are used on load locks of vacuum systems and on

analytical instruments such as secondary ion mass spectrometer (SIMS) systems.

The most popular size of cryogenic vacuum pump is of 200-mm (8-inch) di-

ameter, and most of these are used on commercial semiconductor production sys-

tems.

Typical characteristics for a 200-mm cryopump are given here:

8-inch

(2(X)-mm) Cryopump Characteristics

Water vapor speed 4000-4500 L/s

Nitrogen speed 1500 L/s

Argon speed 1200 L/s

Hydrogen speed 2200-2500 L/s

Argon capacity 1000 standard liters

Hydrogen capacity 12-18 standard liters

Argon throughput 700 std. cm-^/minute

Crossover rating 150 torr-liters

Cooldown time 90-100 minutes

Cold-to-cold full regeneration time 2.5-3 hours

2.5.2

CRYOPUMP BASICS

2.5.2.1 Flow Regimes

Cryopumps can operate over a wide range of pressures, typically from about 5 X

10"^

torr on the high end to 1 X 10~^^ torr on the low end. Laboratory pumps

have reached even lower pressures. This range of pressures is generally in the

molecular flow regime for systems of reasonable dimensions. In molecular flow,

the trajectories of the molecules are straight lines with a mean free path very long

compared to system dimensions. The molecular density is low enough to ignore

all molecule-molecule interactions and scattering. This simplifies the design of

the elements of the cryopump. In addition, heat transfer by gas conduction can be

ignored below

"^ torr. At sputtering pressures in the mid-10

torr

range,

how-

ever, both gas scattering and conduction may be significant [3].