Agilent Technologies - High and Ultra-High Vacuum For Science Research PDF

Seminar Handbook
HIGH AND ULTRA-HIGH VACUUM

FOR SCIENCE RESEARCH
HIGH/ULTRA-HIGH VACUUM
Seminar Outline
Vacuum introduction
Cryopump
Applications of physics
High vacuum pump

comparison
Materials selection
System pumping speed
Vacuum pumps
UHV applications
selection criteria
Gauges
System operation
Case studies
Pumpdown calculations
Turbo pump comparisons Q & A session

Compression ratio
Vacuum Introduction
Overview
Pressure
Levels of vacuum
Gas characteristics in vacuum
Flow regimes: viscous/molecular
Agilent Technologies 1
Notes
Gas Composition
Pressure (torr)
Major Gas Load
10-3
10-6
10-9
10-10
10-11
Water vapor (75% 85%)

H2O, CO
H2O, CO, N2, H2
CO, H2
H2
Rough
High
Ultra High
High/Ultra-High Vacuum Seminar
Atm
Wet Air
Characteristics of Rough Vacuum
Notes
Roughing removes original atmosphere

Gases move in viscous flow
Chamber volume and pump speed determine pumpdown

time
Comparison is same as original environment
(80% N2 :20% O2)
Characteristics of High Vacuum
Gases originate from walls/surfaces

Gases are in molecular flow
MFP > chamber dimensions

Gases are at thermal speeds
Particles move randomly
Surface area, material type and pump speed determne

pressure and pumpdown times
Comparison is constant through high vacuum
(80% H20 and 20% N2, CO, H2, CO2)
Characteristics of Ultra High Vacuum

Gases originate from walls/surfaces
diffuse out of vessel materials

permeate through vessel materials
released within UHV pumps
Gases are in molecular flow
Surface area, material type, pump speed and temperature determne ultimate pressure and pumpdown times
Primary source of gas is H2 (below 1 x 10-10 torr)
Notes
Flow Regimes
Viscous flow:
Distance between particles is small; collisions between
gas particles dominate; flow through momentum transfer;
generally P greater than 0.1 torr (100 millitorr)
Transition Flow:
Region between viscous and molecular flow
Molecular flow:
Distance between particles is large; collisions between
the gas particles and wall dominate; flow through random
motion; generally P smaller than 10-3 torr
Note: A system is in molecular flow when the mean path is

larger than the diameter of the tube or chamber.
Notes
Moving Particles
We use a vacuum when we want to move
a particle through space in a specific way.
In a television set cathode ray tube, electrons
generated at the back of the tube in the electron gun have to move to a particular spot on
the phosphor screen that coats the front of
the tube. If there was air in the tube, the electrons would be deflected and blocked from
reaching the screen.
A very common application is to deposit a material on a substrate. In a vacuum system, a
material is heated and evaporated. The atoms
have to move from a source to a
substrate over a certain distance. If there was
gas in the chamber, the gas particles (atoms
and/or molecules) would prevent
the evaporated material from reaching the
substrate. Also, if the gas in the chamber contains oxygen, it could oxidize the material we
are depositing.
Notes
Mean Free Path
As we lower the pressure in the vacuum
chamber, the amount of space between the
gas molecules increases. The particles bump
into each other less frequently. The average
distance a molecule moves before it bumps
another particle is called the mean free path.
At atmosphere, the mean free path is extremely short, about 2.5 millionths (2.5 x 10-6)
of an inch. Under vacuum, fewer molecules
remain, and the mean free path is longer. Its
length depends on the number of molecules
present, and therefore on the pressure. When P
is expressed in torr, the mean free path for air
can be estimated from the relationship:
Mean Free Path = 5______
x 10-3 (cm)
P
From this we can see that as the pressure gets
lower the mean free path gets longer. Likewise,
as the pressure gets lower, there are fewer
molecules of gas present, so there is less
chance of them striking each other.
In 1 cc of gas at standard conditions (760 torr
at 0OC), there are about 3 x 1019 gas molecules and the mean free path is about
2.5 x 10-6 cm (a few millionths of an inch).
At 1 x 10-9 torr, there are about 4 x 107 (ten
million!) molecules per cc, and the mean free
path is about 31 miles or 50 kilometers. The
number of molecules per unit volume (in this
example cubic centimeters) is called the gas
density or molecular density.
Notes
Adsorbed Gas
The diagram above illustrates the fact that
once the pressure in a high vacuum system
has reached high vacuum levels, most gas resides on the walls of the system. At a pressure
of 1 x 10-6 torr there are 500 molecules residing on the walls of the system for every molecule moving through the system.
This highlights the fact that at high vacuum
and ultra-high vacuum levels, the pressure
in the system is determined by the surface gas
in the system. The right column in the diagram
shows that it only takes 2.2 seconds to coat
a perfectly clean system (a system without
a single molecule in it!) with a monolayer of
gas when it exposed to that gas at a pressure
of 1 x 10-6 torr. At 1 x 10-9 torr it will take
2,200 seconds to coat the system with one
monolayer. This explains why surface analysis
equipment usually operates at ultra-high
vacuum pressures.
Notes
Vacuum system gas load results from:
Surface Condition (outgassing/
desorption)
System Materials (diffusion and
permeation)
Leaks (real and internal/virtual leaks)
Pumps (backstreaming)
Practical Application
of Physics
Overview
Processes that use vacuum
Basic HV/UHV system
System pressure
Gas load
Material permiation
Notes
High Vacuum
Chamber
Throughput
Throughput is the actual amount of gas
or the number of atoms and/or molecules
moving through or being removed from
a vacuum system. This is the work really
being done by a vacuum system. Throughput is expressed by the letter Q.
The flow of gas through a pipe is described as the amount of gas (Q) flowing
through a pipe is equal to conductance (C)
of the pipe times the pressure (P1 P2)
over the pipe.
Or: Q = C x (P1 P2)
For the case where a pump is removing
gas from a chamber at pressure P, we can
look at how throughput is related to pumping speed (S) by taking another look at the
definition of speed
Pumping Speed: amount of gas flowing
into a chamber
pressure in the chamber
or: S = Q
_ (liters/second)
P
reworking this formula:
Q = P x S (torrliters/second)
10 High/Ultra-High Vacuum Seminar
Or in words: the amount of gas being

pumped from a chamber is equal to the
pressure in the chamber multiplied by the
speed of the pump attached to the chamber.
Flow Regimes
Notes
Pressure is the force exerted by gas particles on a unit of

surface area by momentum transfer during collisions
Pressure is a measure of the number of gas particles per

unit volume (density) at a fixed temperature
At constant temperature
lower pressure <> lower density <> higher vacuum
higher pressure <> higher density <> lower vacuum
At constant pressure
higher temperature <> lower density
lower temperature <> higher density
Pressure is expressed in units of force per unit area

psig, psia, torr, bar, pascal (Pa), in Hg, in H2O

Surface Condition (outgassing/desorption)
System Materials (diffusion and permeation)
Definitions
Outgassing (desorption): Release of gas that has

accumulated on system surfaces when they are
exposed to atmosphere
Diffusion: Gas particles present in the vessel walls at the

start of (initial) pumpdown and released into the system
during operation
Permeation: Gas migrating into the system through the
vessel walls from atmosphere and released into the system during operation
Outgassing (general): Any gas from the above sources
released into the vacuum system during operation
Process: Gas introduced during process
Gas Load (Q)
The gas load is the rate gas enters the system volume
Gas entering the volume through external and
internal leaks
Gas entering the volume through external and
internal leaks
Gas emanating from or passing through materials
by diffusion and permeation
At a known temperature, the gas load is the amount of gas
particles entering the system volume per unit time
Q (gas load, throughput, leak rate) is expressed in units
of pressure volume/time
torrliters/sec, atmcc/sec, sccm, mbarliters/sec,
Pam3/hr
Notes
Notes
Notes
Total Gas Load
QVOLUME is always negligible in UHV
QLEAK may appear following bakeout. tighten bolts following

bakeout. Mass spectrometer indicats peaks at 28, 32 and
also 14 and 18 (m/e) with real external leak.
QOUTGAS can be hastened and QDIFFUSION can be reduced by bakeout, minimuzing time open to atmosphere,
and by purging with a dry gas while system is open.
QPERMEATION can only be reduced by using less permeable

materials
Q PROCESS = process gas in system
Outgassing
QOUTGAS = qOUTGAS x A
Where qOUTGAS is the rate of outgassing per unit area and

A is the geometric surface area exposed to the vacuum
Minimize the total microscopic system surface area in

order to reduce the total gas load from surface desorption
Outgassing
Notes
Rate of outgassing dependent upon the base material,

temperature and time
General outgassing rates are in torrliters sec-1 cm-2 or

in mbarliters sec-1 cm-2 at a defined temperature
Detailed consideratins require the knowledge of the rate

for a specific gas species from an understood surface
Surface state is important

Untreated (as received)
Machined (cutting oil used, etc...)
Degreased (method and solvents)
Post fabrication treatment (baking, degassing)
Material Permeation
Permeation is the ability of a gas to pass through

solid materials
Materials have permeation rates for different gases

specific to that material
Examples
steels have higher permeation rates with higher
carbon content
copper has low permeation for all gases
aluminum has low permeation for hydrogen
Polymers are permeable to all gases
Notes
Materials
Overview
Basic HV/UHV system
Origins of gas
Materials selection
Surface preparation and cleaning
Outgassing rates
Mechanical joining
Valves and seals
Notes
High Vacuum
Chamber

Notes
Materials Selection
General engineering properties
Fabrication processes available and their influence on

the vacuum environment
Chemical compatibility
Cost versus performance
Notes
Notes
Ceramics
Alumina (Al2O3)
Max temperature 1800C

Can be brazed
Tensile strength 25k psi (96% density)
Steatite (MgO-SIO2)
Max temperature (1000C)

Tensile strength 15k psi
Brass (avoid in UHV)
Commonly used for many components in vacuum

chambers and fixtures
Used in vacuum systems where temperatures above

100C are found
Easily machined
Joining techniques include:

Brazing, soldering, welding (not used extensively)
Outgassing rate ranges from 10-5
to 10-7 torrliter/sec cm2
Notes
Surface Preparation and Cleaning Techniques
Notes
Techniques
Physical wiping/polishng
Detergent washing
Chemical cleaning (i.e., acids, solvents/degreasers)
Sand/bead blasting
Ultrasonic bath
Electropolishing
Nickel plating
Most manufacturere and labs have devised their own

proprietary methods, often combining these methods
Avoid fingerprints and dust at all times
Notes
Mechanical Joining Vacuum Systems

Demountable; Conflat or wheeler flange
Valves: all metal valves
Cleanliness and roughness of all sealing seats

very critical
Use of proper sealing methods make real leakage

negligible
O-rings: permeation contributes much to the gas load
Notes
KF Flange
Another popular type of elastomer flange is
the KF flange, also known as an NW flange.
The flange is of standard ISO 2861/1 design
consisting of two symmetrical flanges, a center ring to support and position an o-ring, and
a clamp that allows assembly without any
tools. KF flanges are quite convenient to use
in rough and high vacuum systems.
O-Ring Seals
Below are useful suggestions for working
with o-rings:
1. When preparing to make a flange
connection, be sure to clean and dry
the groove and the flat mating surfaces.
Check the sealing surfaces for scratches
that cross the seal area.
2. Lightly lubricate the o-ring with a vacuum
grease such as Apiezon-L, then wipe off
most of the grease with lint-free paper before making the connection. Keep in mind
that the o-ring makes the seal not the
grease. The grease makes the o-ring slip
and helps it to conform to its groove.
Notes
Joining Vacuum Systems: Welding
Notes
Tungsten and metal inert gas welding (TIG & MIG) are
the most widely used techniques for vacuum systems
The most critical component of welding s the design.

If design is not done properly, acceptable welds cannot
be made
Critical vacuum systems require the joint be cleaned

prior to welding to prevent baked-on oil residue
from machining. Joint also to be cleaned after welding
Welding with flux coated rod should be avoided

to prevent residual contamination
Joining Vacuum Systems: Soldering and Brazing

Soldering and brazing of vacuum components is
a common and cost effective method of joining
Both techniques use a flux to prepare the surface. It can

be difficult to remove the flux completely
Soft soldering < 300C uses filler materials such as lead,

tin, zinc, bismuth, which have high vapor pressures and
are often not compatible with ultra-high vacuum systems
Joining Vacuum Systems: Soldering and Brazing
Silver soldering or torch brazing, accomplished at higher

temperatures, generally uses copper and silver alloys.
these require flux in most cases. Alloys not requiring a
wet flux are available.
In the case of silver soldering where strength is required,

joint design is important
Notes
Notes
Valves
A variety of valves are manufactured for various vacuum requirements. Each of these
takes into consideration factors such as operating vacuum levels, degree of cleanliness
needed, need for bakeout, and materials construction. Above, a small right angle block
valve made of aluminum and using o-ring seals is used as an illustration.
Valves can be separated into several types based first on whether they are elastomersealed or all-metal; then whether they are small or large valves. Valves also come in
hand-operated, pneumatically-operated, and solenoidoperated varieties. Small valves
will be defined as valves with inside diameters less than 2 inches. Large valves will be
defined as valves with more than 2 inches inside diameter. Valves can also be classified
as rightangle, tee, straight-through, back-to-air, variable-leak, gate and slide valves.
Bellows Sealed Valves
Elastomer-Sealed Small Valves

In an elastomer-sealed bellows valve, all o-ring seals are static seals, meaning that they
do not move. These valves are much more reliable than an o-ring shaft-sealed valve.
This type of valve has bellows made of brass, aluminum or stainless steel. The bellows
can be formed or welded. The price, of course, varies accordingly. The choice of material
depends upon the use of the valve.
The stainless steel bellows is generally used in high and ultrahigh vacuum systems. An
example where a stainless steel bellows is not chosen is on valves used with sorption
pumps. The hot steam that is produced during regeneration corrodes the stainless steel
bellows rather quickly. Inconel bellows are therefore recommended rather than the stainless steel bellows.
Viton is generally the elastomer of choice in valves, although other elastomers are also
used. Polyimide finds use in special applications requiring higher temperatures or better
chemical resistance.
Large Elastomer
Sealed Valves
Elastomer-Sealed Large Valves

Most industrial applications require isolation of the work chamber from the system
pumps. Many processes require that the chamber be alternately cycled from vacuum
to atmosphere. Without a valve between the chamber and pump, the cycle time might
be too long, or even physical damage to the pump or system components might result.
Valves for these uses are usually of the sliding gate or swing gate design. Typical port
diameters of this type are 4, 6, or 8 inches, although much larger valves are available
for specialized pumps and applications.
Since the seals in these valves are usually made of Viton o-rings, heat ranges and
operating pressures are about the same as those for small elastomer-sealed valves.
The valve bodies are usually made of cast aluminum or stainless steel.
Valve Design
Valve Design
The valve seal plate opens and closes as
with a gate. That is, the seal plate drops and
retracts from the port. In some designs, the
plate does not fully clear the port and, therefore, does not give maximum conductance.
Also, debris can fall on the seal and
cause leaks.
Valve Operation 2
Valve Operation
After forward motion stops, further driving
motion moves the seal plate up into the
sealed position.
The over-center mechanism and second
mechanical stop insure that the seal plate is
positively locked in the sealed position.
Valves are often air-operated and close with
considerable speed and force. It is important
to remember to disconnect both air and electricity when maintenance has to be done.
Dynamic Seal Wears
O-Ring Seals
In a valve having an o-ring-sealed shaft,
the seal is usually quickly rolled and
scuffed. This seriously reduces its life. The
o-ring must be lubricated in order to minimize wear.
Valves with this type of design should be
one of the first items to check when
troubleshooting a vacuum system.
Differential Pumping
Double O-Ring Seals

The shaft in an o-ring sealed valve is often equipped with a double
o-ring seal. This double seal provides better separation
between the vacuum chamber and atmosphere. However, it also
creates a trapped volume which may result in a virtual leak.
The volume between the two o-rings may also be connected to a
roughing pump. This is to improve vacuum separation of the work
chamber even further. The driving pressure over the o-ring on the
vacuum side of the seal will be significantly reduced. Instead of
760 torr forcing gas through the o-ring, a force of only several hundred millitorr will be driving the gas through the seal. This will reduce the leakage rate by a factor of 1,000 or more!
When leak checking a double shaft seal, the line to the rough
pump is disconnected. Then both the outer and inner seal can be
checked by inserting helium into the space between the o-rings.
Bellows Sealed Seing Gate Valve
Bellows Sealed Swing Gate Valve

To eliminate contamination problems of the sliding gate valve
design, bellows made of brass or stainless steel are often used.
An 8-inch swing gate valve is shown as illustration above.
The main flanges are sealed with Conflat flanges, but an o-ring
seal is used on the seal plate. All seals are static seals.
Debris Is Kept Away from Seal Area
Elastomer-Sealed Large Valve

The conical shape of the seal plate causes process debris to fall
away from the seal area. Also, the seal is located in the valve body
instead of in the seal plate. This location further minimizes the possibility of seal leaks.
Maintenance
The valve should be in the open position for maintenance and
cleaning. Air and electrical lines should then be removed. Next, remove
the whole actuator sealing-plate assembly by taking off the body
flange and its o-ring.
The body flange o-ring is held in its position over a large, rectangular
o-ring retainer. Re-assembly of this flange can be time-consuming if
the o-ring continues to slip off its retainer during the bolt-on process.
Bolting on is made much easier by using a seal positioning tool.
Once out of the valve body the mechanism can be cleaned, adjusted,
or repaired. Full removal of the valve and valve body from the vacuum
system is usually required to perform proper cleaning and refurbishing. See instruction manual for proper maintenance procedures.
The cleaning procedure is similar to the small elastomer-sealed
valves. Do not attempt to spray cleaning solutions or solvents into
the valve body or port area while the valve is on the vacuum system. Serious pump or system contamination could result.
Notes
Small Metal-Sealed Valves
Small valves are 3 4 in. to 21 2 in. in diameter.
The construction of the valves is entirely
metal. There are no elastomers used. The
seal is accomplished using a copper gasket.
Therefore, metal-sealed valves provide reliable seals under repeated bakeout conditions.
A cross-section of an all-metal valve, made
entirely of stainless steel and copper is
shown above.
The sealing surface inside the valve has a
knife edge which cuts into the copper button
to seal the valve closed. The same capturing
principle is used as described previously.
This type of valve can be baked to 450C,
if necessary.
All Metal Valves

All metal valves use the gasket-capturing
design discussed in the flange section. This
insures that the gasket material will not flow
away from the seal area, even under bakeout temperatures of 450C.
Metal-sealed valves are used in ultrahigh
vacuum systems or for high-purity gas systems. Because of the UHV requirements,
these valves are baked. The temperature at
which they keep their sealing integrity is
usually related to their size and seal design.
These valves, when operated at room temperature, will perform well up to 100 cycles.
When baked out to maximum temperatures,
however, valve seal life is about one order
of magnitude less. The sealing torque must
also be increased after each bakeout.
The drive mechanisms (outside the vacuum
system) must be lubricated with an appropriate high-temperature grease after each
bakeout. This prevents galling of the threads
and early wear of the valve components.
Pressure ranges of these valves are usually
from about atmosphere to 10-11 torr. Typical
leak rates are less than 10-10 std cc/sec.
Notes
Sealing Torque
Knife edge grooves can produce vacuum
leaks where the grooves crisscross if the sealplate position is not exactly maintained. If the
grooves do crisscross, a leak can be avoided
by using greater sealing pressure each time a
closure is made. This limits the life of the gasket, however. The torque used to seal the
valve must be increased with each closing as
closing repeatability has some slight tolerance.
The initial sealing torque ranges from about
1 to 13 ft-lbs to a maximum of 6 to 46 ft-lbs,
again depending on valve size.
Maintenance
Some maintenance suggestions for all-metal
valves are outlined below:
1. Metal sealed valves require more sealing
force than elastomer valves. When the valve
is opened or closed, support the valve so the
attached plumbing is not bent or kinked.
2. After baking at high temperatures, the
threads on the valve need to be lubricated
to prevent galling. Use a suitable high- temperature lubricant such as Fel-Pro C-100.
3. A new seal needs to be installed after a
maximum of 300 closures. While the procedure is quite simple, it might be necessary
to remove the valve from the vacuum system
to carry out the seal change. The valves require a torque wrench to increase the torque
by about 1 2 ft-lb per closure.
4. Keep a closure log!
Conical Plate
Another solution to the sealing force requirement is to use a thin lightweight conical
plate. The basic design resembles the elastomer-sealed gate design described earlier.
The seal plate in this valve is also conical,
thin and lightweight. These characteristics
are used to good advantage.
Notes
Seal Flexing
When moving into the sealing position, the
seal plate drives against a stop built into
the valve body. When this occurs, further
upward force spreads the plate outward.
This flexing of the seal plate actually multiplies the driving force. It also reduces the
load on the drive mechanism. A goldplated
ridge machined into the seal-plate edge
makes a tight leak-free seal when the plate
flattens out against the seat.
The other two ridges protect the center or
sealing ridge against mechanical damage.
This valve is bakeable in the open or closed
position. Perhaps the greatest advantage
of this metal-seal design is that it isnt at all
sensitive to position repeatability. That is,
there is no possibility of indenting eccentric
circles into the seat and causing leaks as in
the case of other metal seals.
Maintenance
Maintenance should be carried out according to the manufacturers instructions. The
exact dimensional tolerances needed in reassembly require specialized tools and
proper training whenever possible.
Cleaning of the valve body and actuator
assembly is usually the same as discussed
earlier in this chapter. Again, UHV usage
demands that good vacuum practice be
strictly followed. Lint, grease and cleaning
residue can cause excessively long pumpdown time. This is related to the outgassing load produced by contaminants in
the system.
System Pumping Speed
Overview
Definition
Ohms Law correlation
Delivered (net) speed
Throughput vs pressure
Effect of conductance
Notes
Throughput
Throughput is the actual amount of gas, or
the number of atoms and/or molecules,
moving through or being removed from a
vacuum system. This is the work really being
done by a vacuum system. Throughput is expressed by the letter Q.
The flow of gas through a pipe is described
as the amount of gas (Q) flowing through a
pipe is equal to conductance (C) of the pipe
times the pressure (P1 P2) over the pipe.
Or: Q = C x (P1 P2)
In the case where a pump is removing gas
from a chamber at pressure P, we can look
at how throughput is related to pumping
speed (S) by taking another look at the
definition of speed
into a chamber
Pressure in the chamber
or: S = Q_ (liters/second)
P
reworking this formula: Q = P x S
(torrliters/sec.)
System Pumping Speed (S)
System pumping speed is a measure of:

the rate system pressure decreases in time with no
gas load, or
the change in system pressure per change
in throughput
System base pressure (when Snet = 0) is determined by:

limiting compressin ratio of the pump with no gas load, or
the minimum achievable gas load (permeation)
Pumping speed is expressed in units of volume/time
liters/sec, liters/min, m3/hr, cfm
Or in words: the amount of gas being

pumped from a chamber is equal to the
pressure in the chamber multiplied by the
speed of the pump attached to the chamber.
Notes
When we talk about moving a gas through

an opening or tube, we use the term conductance (C). Conductance is the ability of
an opening or tube to allow a given volume
of gas to pass through in a given time. It is
expressed in such units as liters per second,
cubic feet per minute or cubic meters per hour.
A good conductance path is wide and short.
It has few turns, thus allowing free gas flow.
This is important for molecular flow. In viscous
flow, these conditions for good conductance
are not so important. This is because the molecules tend to push one another along under
the influence of a pressure difference.
Conductance is also defined as the amount
of gas per unit time (Q) that flows through
an orifice or tube divided by the pressure
over the tube (P1 P2), or:
Conductance: gas flowing through an opening or tube
pressure over the opening/tube
Or: C = _____
.Q (liters per second)
P1 P2
This formula is usually expressed as follows:
Q = C (P1 -P2) (torrliters per second)
Notes
Notes
Notes
Notes
Notes
Addendum B
Conductance Formulas
Notes
Notes
Notes
Notes
Notes
Vacuum Pumps
Overview
Operating ranges
Rough pumps
Scroll pumps
Diaphragn pumps
Blower/booster pumps
Screw pumps
Pump performance
Pump precautions
Pump comparison
Notes
Notes
High Vacuum
Chamber
Roughing Pump Uses for HV/UHV
Remove volume gas (air) from chamber(s)
Keep foreline of high vacuum pump at acceptable

pressure level
Oil Sealed Rotary Vane Pump
Notes
Scroll Pumps
Notes
Dry (no oil in vacuum

portion of pump)
High pump speed
Compact size
Base pressure low mtorr
Notes
Principle of Operation
Gas is transferred through the pump in three
phases similar to gas transfer occurring in
other types of mechanical pumps shown before. The circular motion of the orbiting scroll
forms crescent shaped spaces into which
the gas enters (1 left) and becomes
isolated (2,3 and 4). The orbiting motion
moves the gas towards the center. The volume of the gas becomes smaller and the
pressure increases. Finally the crescent is
connected to the exhaust port (5 left) and
moved to the second set of scrolls. The gas
is then compressed in an identical way and
exhausted from the the pump
Diaphragm
Notes
Blower/Booster Pumping System
Notes
Notes
Notes
Rough Pump Comparison

An overview of rough pump advantages and
disadvantages is shown above. Wet pumps
tend to be reliable, have long life and are
relatively inexpensive. Their major disadvantage is that fluids used in the pump can
backstream into the vacuum system.
Dry pumps are clean. They are used in applications where backstreaming of pump fluid
can not be tolerated. They are more complex
and expensive than the equivalent wet pump.
Also, in many cases preventative maintenance has to be performed more frequently.
High Vacuum Pumps
Overview
Basic system
Operating ranges
Vapor jet pumps
Molecular pumps
Historical perspective
Commercial solutions
Turbo pump comparisons
Compression ratio
Cryo pump
High vacuum pump comparison
Notes
High Vacuum
Chamber
Vapor Jet (Diffusion) Pump
Notes
Notes
Notes
While turbomolecular pumps achieve high
pumping speeds, drag pumps have high
compression ratios (especially for light
gases) and can therefore discharge against
pressures of up to several torr. A compound
pump design provides both capabilities from
a single pump. The Agilent compound pump
(Macrotorr) configuration is shown. It consists of 8 to 10 turbomolecular stages
(Rotor/Stator) followed by several drag
stages (MacroTorr Rotor/Stator) combined
in a single rotor-stator assembly.
The turbo rotor-stator blade combinations at
the low-pressure side (inlet) of the compound
pump have compressed the gas. So the volume of gas being moved at the high-pressure
side (foreline) of the pump has become much
smaller, and high pumping speed is no longer
needed. Due to the increased compression
ratio provided by the drag stages of a compound pump, the discharge pressure against
which it can operate is higher than that of a
traditional turbomolecular pump. Compound
pump can overlap the pressure range of dry
roughing pumps, such as a scroll or a diaphragm pump, allowing for a totally dry
vacuum system to be built.
Notes
Notes
Some manufacturers have developed
magnetic bearings for rotor suspension.
Magnetic bearings use electromagnets to
lift the rotor and hold it in place during operation. Sensors detect any off-center movement of the rotor and, through feedback
circuitry, the magnetic fields are adjusted
to recenter the rotor. A so-called crash
bearing is used to handle inadvertent contact between rotor and stator during power
failures or when the pump is accidentally
vented to atmosphere ("dumped").
Notes
The atoms and molecules in a gas are in
constant high-speed, straight-line motion
in random directions. This is called thermal
motion (or the Kinetic Energy of Motion) and
is associated with the temperature of the
molecules. The higher the temperature, the
higher the speed. Thermal motion will tend
to move molecules away from each other
until they collide with something, usual the
walls of the container and with one another.
For a given amount of thermal energy (temperature), the speed of an atom will depend
upon its Mass (or weight) - lighter elements
will travel faster than heavier elements.
The atomic motion can be seen (we cannot really see atoms move) by watching the
collective effect of atoms colliding with a
dust particle, that is, we may watch the dust
particle move and extrapolate what the surrounding atoms must have been doing. The
balance between the electrical dispersion
forces holding molecules together and the
thermal motion which tends to move molecules apart, is very important in a vacuum
system. If the dispersion forces are strong,
as they are with some molecules, the molecule will stick to a wall when it collides with
it. With weak dispersion forces, the molecule
will stay on the wall for a very short period.
A water molecule is surrounded by strong
dispersion forces. When it collides against
a wall, it will stick and its movement through
the system will be delayed.
Gases such as helium, nitrogen and oxygen
have relatively weak dispersion forces.
When they move through a vacuum system
and periodically collide against a wall, they
will not stick and there will essentially be no
delay in their movement through the system.
This results in helium, nitrogen and oxygen
being pumped away much faster than water.
The balance between thermal motion and
dispersion forces can be changed by changing the temperature of the vacuum system,
For instance, in order to speed up removal
of water, vacuum systems are often heated
or baked. This increases the energy of thermal motion, resulting in shorter dwell time
of the water molecule on a wall.
Notes
Notes
Ultra-High Vacuum Pumps
Notes
High Vacuum
Chamber
Throughput
Throughput is the actual amount of gas or
the number of atoms and/or molecules
moving through or being removed from a
vacuum system. This is the work really being
done by a vacuum system. Throughput is expressed by the letter Q.
The flow of gas through a pipe is described
as the amount of gas (Q) flowing through a
pipe is equal to conductance (C) of the pipe
times the pressure (P1 P2) over the pipe.
Or: Q = C x (P1 P2)
In the case where a pump is removing gas from
a chamber at pressure P, we can look at how
throughput is related to pumping speed (S) by
taking another look at the definition of speed
into a chamber
Pressure in the chamber
or:
S = _Q (liters/second)
P
reworking this formula: Q = P x S (torrliters/sec.)

Or in words: the amount of gas being pumped
from a chamber is equal to the pressure in the
chamber multiplied by the speed of the pump
attached to the chamber.
Notes
Overview
Theory of Ion Pumps

Basic pumping mechanism
Pumping elements
TSP
How to choose a pump
Ion Pump Controller

Variable voltage
The ion pump as a gauge
Best Practice
Bakeout procedure
High potting
Theory of Ion Pumps

Basic Pumping Mechanism
There are three (3) cases to be considered:
1. Chemically active gases

2. Hydrogen
3. Noble gases
Notes
Notes
Notes
Notes
Notes
Notes
Notes
Titamium Sublimation Pumps
Designed to reach very low pressures
Provide very high speed at low pressures for all

getterable gases
Zero pumping speed for noble gases and methane
Limited use (life) at pressure higher than 10-7 mbar
Notes
Notes
Titamium Sublimation Pumps
Designed to reach lower pressures
Provide very high speed at low pressures for all

getterable gases
Zero pumping speed for noble gases and methane

Limited use (life) at pressure higher than 10-7 torr
Selection Criteria
for UHV Applications
Overview
RGA scan
Selection criteria
Performance vs pressure
Ion pump selection guide
Ion pump characteristics
Leakage
Baking
Potting
Comparison of high vacuum pumps
Notes
Notes
Summary of Ion Pumps Selection Guidelines
Diode best for UHV where 98% of gas is hydrogen.

Diodes have the highest hydrogen pump speed.
StarCell good overall performance and best choice

for applications with P > E-8 torr. Good H2 speed, best
for pumping noble gas (Ar) or air (air influx, small leaks,
frequent vent/pump, air = 1% Ar)
Nobel Diode compromise of H2 speed with limited

argon stability
TSP used with ion pump to achieve very low pressures
Diode Ion Pumps Best Used When
DIODE ION PUMPS: best in UHV applications, where:
Ion pumps are started below 1 E-6 mbar
The system is rarely vented to air
There ae no air leaks
The ion pump is used to pump the outgassing

of the chamber
The operating pressure is below 1 E-8 mbar
Operated @ UHV conditions. Diode ion pumps can work for

20 years before reaching the maximum capacity for Argon
Diode Ion Pumps Characteristics

DIODE ION PUMPS vs others
Highest pumping speed for all getterable gases

(N2, O2, H2O, CO, CO2, H2)
Highest pumping speed at low pressures
Limited speed and stability when pumping noble gases

such as Argon and Helium and non-getterable methane
The only reason to use different and more expensive

ion pumps is to improve pumping speed and stability
for noble gases
Notes
Ion Pumps Other Selection Factors
Notes
Diode ion pumps work great on properly operated UHV

systems. However, in real life:
Air leaks may be present
Venting to air may be more frequent than desired
Working pressures may be higher than design values

then more Argon has to be pumped
Noble gas-stable ion pumps/StarCell ion pumps may

offer safer approach when system operating conditions
are unknown
StarCell Ion Pump Characteristics

StarCell Ion Pump vs Diode
Best stability and speed for noble gases
Lower pumping speed for all getterable gases
Comparable speed for Hydrogen
Slightly lower speed at low pressure
Measured pump current @ UHV might not be the

best pressure indicator because of potentially higher
leakage current
Notes
Combination Ion Pumps TSP Guidelines
Notes
Use Ion Pump + TSP combo when a higher pumping speed

is needed:
At low pressures
For H2, CO, CO2
Ion Pump + TSP combo benefits:
TSP provides much higher pumping speed for

getterable gas
Pumping speed independent from pressure
Ion pump mainly used to pump CH4 and noble gases
Notes
Notes
The Ion Pump As A Gauge
The current in an ion pump is linearly proportioned

to the pressure
The ion pump can so be used as a pressure gauge
The limitation at low pressures is given by the

leakage current
Notes
Notes
Baking of an Ion Pump
Extremely important for pressures less than 10-8 mbar

Effectively removes water vapor, slowly reduces hydrogen
Does not remove hydrocarbon contamination, HCs may
crack and burn into chamber surface this can ruin the
vacuum chamber!
Result varies with time (linear) and temperature
(exponentially)
Minimum 150C, best at 300+C, 200C is typical
Heating must be even for all surfaces
Best results if system is pumped to base pressure
before bake
Best if each pump is processed following its own outgassing rate-pressure, not time control
Baking of an Ion Pump
Rough pump system with dry or oil-trapped roughing pump

Pump to 10-4 or 10-5 mbar with turbo pump
Best to start bake with ion pump off; use turbo pump
to remove gas
After bake curve approaches flat, start ion pump
Check that the maximum current of the ion pump doesnt

exceed maximum baking current and eventually switch
oil heating
When ion pump reaches full voltage @ stable current

close turbo valve
When pressure curve is flat, outgas filaments & turn off heat
Allow system to reach base pressure
Notes
When High-Pot?
Notes
When the pump is old (some years)
When the pump has worked at high pressure for long period (e.g., during a bakeout)
When we are sure that the leakage current is not coming

from the controller or the cable (disconnect the cable
from the pump and switch the controller on; read the
current on the controller display)
When the leakage current is of the same order of magnitude (or higher) of the current the customer runs the
pump at
High Potting Procedure?
The output of an appropriately sized AC transformer

may be applied to the pump (preferably without the
magnets installed)
High-potting should be done carefully and in voltage steps

since uncontrolled arcing inside the pump can cause
permanent damage
Slowly increase the applied voltage and watch the current
meter for indication of arcing inside the pump as whiskers
are burned away
If arcing occurs, wait at this voltage until the current is stable

Then slowly increase voltage again in steps up to a maximum voltage (depending on the F/T and cables)
The current should never exceed 50 mA
Notes
Notes
Notes
Gauges
Basic system design
Pressure ranges
Heat transfer gauges
Pirani transducer
Capacitance manometer
Hot filament ion gauge
Gauge maintenance
Gauge sensitivity
UHV ion gauge
Cold catheter gauge
Notes
High Vacuum
Chamber
Notes
Vacuum Pumps
Q (Gas Load) = Pressure x Speed
Notes
Heat Transfer Gauges
Thermocouple Gauge
5 torr to 1 x 10 10-3 torr
Convection Gauge
atmosphere to 1 x 10 10-3 torr
Notes
Notes
Ionization Gauge Maintenance
Degas when using below 10-5 torr

Degas longer at lower pressure
Degas more frequently at lower pressure
Adjust the control unit

Gauge calibration
Check sensitivity of gauge/control unit
Notes
UHV Ion Gauge
Operating range:
1 x 10-3 torr to 2 x 10-11 torr
Notes
HV/UHV System Operation
Overview
System operation
Valve maintenance
System troubleshooting
Virtual leaks
Cleanliness
Vacuum pumpdown
Notes
High Vacuum
Chamber
Notes
Valve maintenance
1. Look at the seals whenever maintenance on
a valve is done. Inspect the seals to see if they
have been warped, or are nicked or scratched.
2. The pistons on pneumatically-actuated
valves need to be lubricated yearly. Most
pneumatically-actuated valves are of the airopen, spring-closed variety. Be careful when
disassembling the valve so that it does not
come apart during disassembly.
3. Lubricate the piston on pneumatic valves
with a lubricant recommended for use with
compressed-air lines. Please remember that
this is not exposed to the vacuum system. This
lubricant, which works well for compressedair pistons, will cause problems (high outgassing) if used inside the vacuum system.
4. Inspect the bellows for dents or cracks.
The brass variety is easy to dent. The dent
will cause the bellows to work-harden and
crack at that location. It should be replaced
while it is disassembled on the bench.
5. Only lubricate the o-rings when necessary
with a vacuum grease such as Apiezon-L or
other lubricant. Grease is a contaminant! Remember that only a very thin film is needed.
6. The valve should be leak-checked on a
helium mass spectrometer leak detector. Do
not forget to check the bellows as well as
the valve seal and bonnet seal. Leak checking before reinstallation on the system can
save a lot of disassembly/re-assembly time
should a leak be present.
Notes
Notes
Notes
Notes
Notes
Notes
Notes
UHV System Operation
Overview
Basic UHV system design
UHV system cleanliness
UHV system operation
Notes
High Vacuum
Chamber
Notes
Notes
Case Studies
Case Study 1:
High/UHV Vacuum Pumpdown
Overview
Basic system design
System parameters
Calculations worksheet
High vacuum pumpdown
Outgassing rates stainless steel
Pumpdown pressure
Pumpdown calculations
Notes
High Vacuum
Chamber
Notes
Notes
Notes
Notes
Addedum A
Pumpdown Calculations
Notes
Notes
Notes
Notes
Notes
Case Study 2:
Backing Pump
Overview
Selection of forepump
System pressure rough vacuum
Confirmation of ion pump selection/size
Notes
Notes
Agilent Technologies Vacuum Products Division Around the World

United States
Agilent Technologies
121 Hartwell Avenue,
Lexington MA 02421, USA
Toll free: +1 800 882 7426
vpl-customerservice@agilent.com
China
Beijing Office
Agilent Technologies (China) Co. Ltd
No.3, Wang Jing Bei Lu, Chao Yang District,
Beijing, 100102, China
Toll free: 800 820 3278
vacuum.cnmarketing@agilent.com
vpc-customerservice@agilent.com
Shanghai Office
2F, Building 4, No.690 Bibo Road,
Zhangjiang Hi-Tech Park, Pudong District,
Shanghai, 201203, China
Toll free: 800 820 3278
Guangzhou Office
Unit 08, 66/F, Citic Plaza, 233 Tian He Bei Rd.
Guangzhou, 510613, China
Toll free: 800 820 3278
Shenzhen Office
3/F Dutyfree Business Bldg.,
No.6, 1st Fu Hua Road, Futian CBD,
Shenzhen, 518048
Toll free: 800 820 3278
Brazil
Agilent Technologies Brasil
Avenida Marcos Penteado de Ulhoa
Rodrigues, 939 6 andar
Castelo Branco Office Park,
Torre Jacarand - Tambor
Barueri, Sao Paulo CEP: 06460-040
Toll free: 0800-728-1405
Benelux
Agilent Technologies Netherlands BV
Groenelaan 5, 1186 AA Amstelveen
The Netherlands
Tel: +31 20 547 2000
France
Agilent Technologies France
7 avenue des Tropiques
Z.A. de Courtaboeuf - B.P. 12
91941 Les Ulis cedex, France
Tel: +33 (0) 1 69 86 38 84
Fax: +33 (0) 1 69 86 29 88
Germany and Austria
Sales & Services GmbH & Co. KG
Lyoner Str. 20
60 528 Frankfurt am Main, Germany
Tel: +49 69 6773 43 2230
Fax: +49 69 6773 43 2250
India
Agilent Technologies India Pvt. Ltd.
G01. Prime corporate Park, 230/231,
Sahar Rd., Opp. Blue Dart Centre,
Andheri (East), Mumbai, 400 099 India
Toll free: 1800 113037
This information is subject to change without notice.
Agilent Technologies, Inc., 2011
Published in USA, September, 2011
Italy
Agilent Technologies Italia SpA
via F.lli Varian 54
10040 Leini, (Torino), Italy
Toll free: 00 800 234 234 00
vpt-customerservice@agilent.com
Japan
Agilent Technologies Japan, Ltd.
8th Floor Sumitomo Shibaura Building
4-16-36 Shibaura Minato-ku
Tokyo 108-0023, Japan
Toll free: 0120 655 040
vpj-customerservice@agilent.com
Korea
Agilent Technologies Korea Ltd.
Shinsa 2nd Bldg. 1F, 966-5 Daechi-dong
Kangnam-gu, Seoul, Korea 135-280
Toll free: 080 222 2452
vpk-customerservice@agilent.com
Mexico
Concepcion Beistegui No 109
Col Del Valle, C.P. 03100, Mexico, D.F.
Tel: +52 5 523 9465
Singapore
Agilent Technologies Singapore Pte Ltd.
No.1 Yishun Avenue 7, Singapore 768923
Toll free: 1 800 2762622
vps-customerservice@agilent.com
Southeast Asia
Agilent Technologies Sales Sdn Bhd
Unit 201, Level 2 uptown 2,
2 Jalan SS21/37, Damansara Uptown
47400 Petaling Jaya, Selangor, Malaysia
Toll free: 1 800 880 805
vps-customerservice@agilent.com
Taiwan
Agilent Technologies Taiwan Limited
20 Kao-Shuang Road Ping-Chen City
Tao-Yuan Hsien,
32450 Taiwan, R.O.C.
Toll free: 0800 018 768
vpw-customerservice@agilent.com
UK and Ireland
Agilent Technologies UK Ltd.
6 Mead Road, Oxford Industrial Park
Yarnton, Oxford OX5 1QU, UK
Tel: +44 (0) 1865 291570
Learn more:
www.agilent.com/chem/vacuum

Agilent Technologies - High and Ultra-High Vacuum For Science Research PDF

Uploaded by

Copyright:

Available Formats

Agilent Technologies - High and Ultra-High Vacuum For Science Research PDF

Uploaded by

Document Information

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Agilent Technologies - High and Ultra-High Vacuum For Science Research PDF

Uploaded by

Copyright:

Available Formats

Seminar Handbook

HIGH AND ULTRA-HIGH VACUUM

High vacuum pump

Turbo pump comparisons Q & A session

Major Gas Load

Water vapor (75% 85%)

High/Ultra-High Vacuum Seminar

Characteristics of Rough Vacuum

Roughing removes original atmosphere

Chamber volume and pump speed determine pumpdown

Characteristics of High Vacuum

Gases originate from walls/surfaces

MFP > chamber dimensions

Particles move randomly

Surface area, material type and pump speed determne

Characteristics of Ultra High Vacuum

diffuse out of vessel materials

Gases are in molecular flow

Primary source of gas is H2 (below 1 x 10-10 torr)

Note: A system is in molecular flow when the mean path is

High/Ultra-High Vacuum Seminar

High/Ultra-High Vacuum Seminar

High/Ultra-High Vacuum Seminar

10 High/Ultra-High Vacuum Seminar

Or in words: the amount of gas being

Pressure is the force exerted by gas particles on a unit of

Pressure is a measure of the number of gas particles per

Pressure is expressed in units of force per unit area

Vacuum system gas load results from:

Outgassing (desorption): Release of gas that has

Diffusion: Gas particles present in the vessel walls at the

Gas Load (Q)

12 High/Ultra-High Vacuum Seminar

Total Gas Load

QVOLUME is always negligible in UHV

QLEAK may appear following bakeout. tighten bolts following

QPERMEATION can only be reduced by using less permeable

Where qOUTGAS is the rate of outgassing per unit area and

Minimize the total microscopic system surface area in

14 High/Ultra-High Vacuum Seminar

Rate of outgassing dependent upon the base material,

General outgassing rates are in torrliters sec-1 cm-2 or

Detailed consideratins require the knowledge of the rate

Surface state is important

Permeation is the ability of a gas to pass through

Materials have permeation rates for different gases

Polymers are permeable to all gases

16 High/Ultra-High Vacuum Seminar

Vacuum system gas load results from:

18 High/Ultra-High Vacuum Seminar

General engineering properties

Fabrication processes available and their influence on

Cost versus performance

20 High/Ultra-High Vacuum Seminar

Max temperature 1800C

Max temperature (1000C)

Brass (avoid in UHV)

Commonly used for many components in vacuum

Used in vacuum systems where temperatures above