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442

Chapter 4.3: Magnetic-Fluid-Sealed Rotary Motion Feedthroughs

Fig.

Quartz Crucible-

Moiten Silicon-

Vacuum

SiO evaporates from

surface

molten silicon

Lip Seal Protects Fluid

Seal from Dust

Atmosphere

Magnetic Fluid Seal—'

Silicon Crystal Growth System

isolates the magnetic fluid seal from the process, increasing the number

pro-

cess cycles that can

run before the seal must

serviced. This

is a

custom-

designed product.

4.3.6.4 Coaxial Feedthrough

for

Robotic Arm

Robotic arms are widely used to handle silicon wafers and

flat-panel

displays

cluster tools and other semiconductor processing equipment. Two independent

motions are required. One method

achieving this is to nest one feedthrough in-

side another, as illustrated in Figure 6. This

a custom-designed product.

4.3.7

COMPARISON TO OTHER TYPES OF FEEDTHROUGHS

Table

compares magnetic fluid feedthroughs with three other types. Magnetic

fluid feedthroughs provide very high reliability and require no maintenance be-

4.3.7 Comparison

Other Types

Feedthroughs

443

Rg.6.

• Atmosphere

Outer Fluid Seal

Outer Shaft-

Outer Shaft

Inner Shaft

Inner Fluid Seal

Coaxial Feedthrough for Robotic Arm

Inner Bearing

Outer Bearing

cause the liquid sealing element is not subject to any inherent wearout mecha-

nism. Shafts are continuous from atmosphere to vacuum, resulting in high torque

and zero backlash. Auxiliary devices (differential pumping, guard vacuums) are

not normally required. Because magnetic fluids are viscous, the feedthroughs ex-

hibit noticeable drag, a factor that becomes significant at high speeds or with low-

powered drive systems.

Table 3

Comparison of Four Types of Feedthroughs

Reliability

Maintenance

Torque Capacity

Backlash

Guard Vacuum

Bakeouttol50°C

Internal Drag

Magnetic

Fluid

Seals

High

None

High

None

Required only

for hazards

Only if

water-cooled

Moderate to high

Lip Seals

and

O-Ring

Seals

Low

High

None

Frequently

required

for

Viton®

0-rings

Low

Bellows

Seals with

Bent Shaft

Moderate

Low

Yes

Low

Magnetically

Coupled

Seals

High

None

Slippage is possible

May be large

Yes

Low

CHAPTER

4.4

Viewports

Chuck Kraft

Larson

Electronic

Class

4.4.1

MATERIALS

The three most common viewport materials used in high- and ultra-high-vacuum

systems are glass, quartz, and sapphire. The choice of which to use often depends

on cost, transmission characteristics, bakeout temperature, style, and materials

used for the viewport seal.

Glass viewports, the most widely used and least expensive of the three choices,

are available in a wide variety of diameters. The transmission is approximately

90%

between 0.32 and 2.6 microns. Glass viewports are bakeable to

400""

Celsius

(centigrade). Zero-length viewports, where the surface of the glass is below the

surface of the flange, are Kovar®' sealed. Also available are nonmagnetic view-

ports with the glass sealed to the end of

stainless steel tube. The viewport is nor-

mally mounted with the tube extending out from the vacuum chamber. In diame-

ters up to 4 inches, the tube can be inverted to place the glass close to an object

within the chamber. Tubular Kovar® viewports can be made the same way.

Quartz viewports have a transmission of approximately 90% in the range of 0.3

to 2.5 microns and 0.2 to 2.0 microns for UV-grade materials. They are available

in both zero-length and standard-length. A standard-length quartz viewport has

the quartz disk mounted on a tube. Quartz viewports are sealed to stainless steel

' Kovar® is a registered trademark of Carpenter Technology. Kovar is an iron, nickel, and cobalt al-

loy used for glass sealing.

$25.00 All rights of reproduction in any form reserved.

444

4.4.1 Materials 445

frames with a low-temperature brazing alloy. The bakeout temperature is limited

to 200° Celsius.

Sapphire viewports have the broadest transmission range. The transmission is

above 80% at 0.3 to 4.0 microns and 0.25 to 4.5 microns for UV-grade sapphire.

These viewports are sealed with a high-temperature braze material and can with-

stand bakeout temperatures as high as 450° Celsius. The frame is Kovar®.

4.4.2

MOUNTING SYSTEMS AND PRECAUTIONS

Viewports of all three types are available with a variety of mounting flanges, and,

with the exception of the glass and quartz zero-length types, without flanges. In

use,

it is important not to induce stress into the seal, which could cause cracking

or breakage. The two types of stress are mechanical and thermal. Mechanical

stress is created from uneven pressure applied to the viewport frame. Flange bolts

should be tightened in a cross pattern, and foreign objects should not be allowed

to make contact with the frame or the glass. Thermal stress is the result of uneven

heat being applied to the frame and/or the glass during welding, bakeout, or in ac-

tual use on the system. Viewports are designed for vacuum use. The zero-length

glass viewports should be mounted in the normal configuration. Nonmagnetic

glass viewports up to 4 inches in diameter, and quartz and sapphire viewports can

be mounted with vacuum on either

side.

Precautions should be taken to avoid pres-

sure differentials greater than

atmosphere. In some applications, safety pressure

valves or shielding should be considered to protect operators and equipment.

CHAPTER 4.5

Construction Materials

janda K. Panitz

Sandi3

National

Laboratories

4.5.1

PROPERTIES DEFINING MATERIAL PERFORMANCE

The gas load in a vacuum system critically depends on the materials used to

build the system and how they are prepared and maintained. Materials add to gas

load in two ways: (1) evaporation, sorption and outgassing, and (2) diffusion and

permeation

4.5.1.1

Evaporation, Adsorption, Absorption, and Outgassing

EVAPORATION

Vapor pressure can be defined as the pressure at which a material's evaporation or

sublimation rate equals its condensation

rate.

If you pump a system below a mater-

ial's vapor pressure, the material sublimes or evaporates. As a general rule, vapor

pressure increases with temperature. At a given temperature and pressure, the sub-

limation or evaporation rate can be predicted from vapor pressure using thermo-

dynamic theory [1].

It is desirable to use low-vapor-pressure materials, e.g., stainless steel, glass,

and polyimide, as opposed to high-vapor-pressure materials, e.g., zinc, ice cubes,

and camphor, at practical operating temperatures. Selected information is pre-

ISBN

0-12-325065-7

$25.00 All rights of reproduction in any form reserved.

446

4.5.1 Properties Defining Material Performance 447

sented in Tables

to 4. These data can be used for guidance in selecting or reject-

ing metals, alloys, and selected compounds.

ADSORPTION, ABSORPTION; AND OUTGASSING

Adsoq)tion can be defined as the condensation of molecularly thin films on a

solid surface. Absorption is condensation that extends into the near-surface re-

gion of a solid. Outgassing occurs when high-vapor-pressure-adsorbed or -ab-

sorbed species evaporate as the vacuum system is pumped down and/or heated.

Sometimes, high-vapor-pressure solid, liquid, or gas films chemically bond to a

surface ("chemisorption"). If this occurs, the pumpdown rate to the high-vacuum

regime and below can be substantially slowed as the film is slowly wrested from

its substrate.

Water, which readily chemically bonds to many metal and ceramic surfaces

when vacuum system components are exposed to lab air, is a particularly ubiqui-

tous and persistent absorbed species. Water is an especially troublesome species

when it outgasses because low partial pressures of water strongly interfere with

many processes and measurements performed in practical vacuum systems. The

problem can be reduced by venting a system with dry nitrogen to blanket surfaces

with a gas species that adheres less tenaciously than chemisorbed water during

pumpdown.

Using smooth, nonporous, relatively chemically inert materials that are easy to

clean and adapting appropriate cleaning techniques will reduce the gas load added

by outgassing. A material's performance depends not only on its intrinsic proper-

ties but also on on how well it is cleaned. Section 4.9, "Preparation and Cleaning

of Vacuum Systems," is critically linked to this section on vacuum material se-

lection. Depending on whether high, ultra-high, and extra-ultra-high vacuums are

desired, progressively aggressive cleaning and system maintenance procedures can

be used to remove high-vapor-pressure contaminants. (Tools, including gloved

hands,

coming into contact with vacuum materials must also be clean.)

4.5.1.2 Solubility, Permeation, and Diffusion

Solubility, diffusion, and permeation relate to a material's chemical properties,

lattice spacing, and microstructure. Gases and certain other high-vapor-pressure

species readily dissolve in many solids. If these materials are used in a vacuum

system, dissolved high-vapor-pressure species can diffuse to the surface and add

to the gas load. If these materials are used to build a vacuum chamber, soluble

gasses can permeate through the walls and add to the gas load [2]. For example,

large amounts of hydrogen can be dissolved in palladium, platinum, and nickel.

Significant amounts of hydrogen can diffuse out of vacuum fixturing made from

44«

Chapter 4.S: Construction Materials

Table I

Selected Properties of Polymeric Materials Discussed in This Section

Material

Maximum Fabri- Outgas

Operating cation Rate

Cost Temperature Ease (20-25°C)

Vapor

Pressure

at Room

Temperature

Comments

Silicone

Rubber

hr pumping

PTFE

(Teflon®)

Medium

150°C

Fair

10"

T liter

cm-

sec

Medium 150-200°C Good

APIEZON

Glyptar

Varnish

Polymide

[W Medium

Low

Medium

80°C

SOX

150-200°C

Good

—

, T liter

5 X 10-«—-,

cm-

sec

@ 4 hr pumping

Polymethyl

Methacrylate

(PMMA,

Lucite,

Plexiglas®)

Polyvinyl

Chloride

(rigid)

(PVC)

Low

SOT

Good 120 X 10"

T liter

10-5-10-^T

cm-

sec

@ 3 hr pumping

Low 80°C Good 40 X 10"^

T liter

cm-

sec

@ 3 hr pumping

<10-^T

2X lO'^T

• Avoid formula-

tions with acetic

acid & acetic

anhydride

• High thermal con

ductivity (for a

polymer)

• Chemically inert

• Low friction

coefficient

• Useful dielectric

• Vapor pressure de

pends on thermal

history

• Brittle, fractures

easily

• Time-honored

standard

• Properties depend

on thermal his-

tory—outgassing

can be comparable

to stainless steel

<200°C

• Useful dielectric,

adhesive

• Absorbs 2-3%

water

• Releases water

• Requires vacuum-

compatible

adhesives

• Outgasses water

4.5.1 Properties Defining Material Performance

449

Table 2

Selected Properties of Ceramics Discussed in This Section

Material

Maximum

Operating Fabrication

Cost Temperature Ease

Temperature

Where

V.P.

10"^

T=*

Comments

SiO.

Silica,

Glass,

Quartz

AI2O3

• Alumina

• Alpha Phase

(Sapphire or

Corundum)

Low-

Medium

High

500°C

(Soda Glass)

1600°C

(Quartz)

2000°C

Special

Training

Needs

special

techniques

~800"C

• Anodic

aluminum

oxide

(gamma

phase)

Zr02

Zirconia

BeO

Beryllia

Medium

200°C

2700°C

2500°C

-lOOOX

Outgasses

. T liter

10-6—^

cm" sec

(after 3 hr

pumping

@ 20°C)

~1100°C

~1350°C

• Withstands

thermal shock

• Good thermal

conductor (for

a ceramic)

• Hard, chemi-

cally inert.

transparent

mto i.r.

• Porous—

releases

trapped

water, etc.

• Properties

vary depend-

ing on forming

process

• Dust poisonous

V.P.

is vapor pressure.

raw stock that contains dissolved hydrogen. Considerable amounts of hydrogen

can pass from

lab

air into the vacuum system v^hen these materials are incorporated

into a vacuum chamber's walls. Similarly, significant amounts of helium can pass

though glass and vitreous quartz. Water and air permeate through most polymers.

Permeation and diffusion rates increase with temperature. Permeation occurs

more readily through thin-walled chambers. Pick's laws predict how the diffusion

and permeation rates of a material change as a function of pressure differentials,

temperature, and wall thickness [3].

Porous, open-celled materials, such as many cast metals and hot-pressed or sin-

tered ceramics, allow gas to travel into a vacuum, although this is not permeation

as the word is conventionally defined by vacuum technologists.

Table 3

Selected Properties of Metals Discussed in This Section

Material

Aluminum

Cadmium

Chromium''

Copper'^'^

Gold

Iron''

Lead

Magnesium

Molybdenum^

Nickel^

Rhenium

Silver

Tantalum

Tin^

Titanium

Tungsten

Zinc*^

Cost

Low

Moderate

High

Low

Moderate

Low

High

Moderate

Low

Moderate

Low

Maximum

Operating

Temperature

660°C

320°C

~1850°C

1083°C

i064°C

1535°C

328°C

649°C

2610°C

1455°C

3i80°C

962°C

2996°C

232°C

~1660°C

3410°C

420°C

Fabrication

Ease

Moderate

Soft

Difficult

Moderate

Easy

Moderate

Hard

Easy

Temperature

Where

V.P. = 10-^T"

812°C

119°C

977°C

852°C

947°C

1032°C

429°C

246°C

1822°C

1072°C

2217°C

685°C

2237°C

807°C

1227°C

2407°C

177°C

Comments

• Metal soft and

gummy—more

useful as alloy

• Poisonous, al-

loying constitu-

ent, avoid — bad

vacuum material

• Hard, brittle,

inert

• Inert, soft, good

gasket material,

good tribologi-

cal couple with

hard surfaces

• Excellent ther-

mal conductor

at low temp-

eratures

• Rusts—bad

• Poisonous

oxides, dust

• Used in alu-

minum alloys

• Dust flammable

• Soft and gummy

• Work hardens

readily

• Good tribologi-

cal couple with

hard surfaces

• Beware of

chemical

reactions

• Soft—good

low-temperature

gasket material

•Hard!

•Brittle

• Beward of oxide

• Bad high-

vacuum material

V.P.

is vapor pressure.

^ Alloying constituents — steel.

Alloying constituents—brass.

'^ Alloying constituents—bronze.

4.5.2 Vacuum Chamber Materials

451

Table 4

Selected Properties of Miscellaneous Materials Frequently Used as Alloying Constituents

Material

Carbon

Phosphorous

Silicon

Cost

Graphite — low

Diamond—high

—

Moderate

V.P.

is vapor pressure.

4.5.2

Maximum

Temperature

—

1410°C

Fabrication

Ease

Graphite—easy

Diamond—hard

—

Needs special

techniques

Temperature

Where

VR = 10'^ T^

1860°C

88°C

1147°C

Comments

•~1200°C diamond

turns to graphite

• Carbon sublimes

• Alloying constituent

• Avoid

VACUUM CHAMBER MATERIALS

Glass,

quartz, sapphire, stainless steel, and aluminum are commonly used for high,

ultra-high, and extra-ultra-high-vacuum system chambers. More cost-effective,

more easily machined and manipulated materials such as copper, brass, certain

polymers, and somewhat porous ceramics (Macor®, and sintered, anodic, or

plasma-sprayed aluminum

oxide,

zirconium

oxide,

etc.) are strategically employed

in medium- or low-vacuum environments, such as roughing and forelines as well

as high vacuums present in many practical physical and chemical vapor deposi-

tion systems, microelectronics fabrication tools used for plasma-assisted etching,

freeze-dryers, etc.

4.5.2.1 Glass

During the first half of the twentieth century, well into the 1960s, the majority of

medium-, high-, and ultra-high-vacuum systems were built using glass. Scientific

glass blowing was part of the education of most experimentalists [4]. Valves and

feedthroughs consisted of ground glass joints with stopcock grease and some-

times included

0-ring

seals or creative use of Glyptal® varnish and Apiezon

"black wax"

[5].

At least one company currently markets a fairly extensive line of

glass vacuum components. Glass systems offer a number of advantages. They are

relatively inexpensive (especially if the owner is able to do glass blowing). There

is a clear view of processes occurring within the chamber, e.g., vacuum deposi-

tion, chemical synthesis, or freeze drying. Glass is smooth, relatively chemically

inert, and easy to clean for vacuum applications.