3. 3
Why is a Vacuum Needed
• Evacuation increases the mean free path of electrons. The
mean free path of electrons at atmospheric pressure is only
1 cm. At 10-6 Torr they can travel several meters (about 6.5
m) and eliminate electron scattering
• No interaction of electron beam and gas molecules.
Eliminate electrical discharges, particularly between anode
and cathode and in area around field emitters
• It increase Filament’s life. Elimination of oxygen prevents
“burning out” of filament
[1]
4. 4
Why we need Vacuum?
To move a particle in a (straight) line over a large distance
[2]
5. 5
Why we need Vacuum?
Contamination
(usually water)
Clean surface
Atmosphere (High)Vacuum
To provide a clean surface
[2]
11. Rotary Pump Precautions
• Two aspects that are often overlooked are the use of
the ballast valve and replacement of oil at the
correct intervals.
• Ballast valve allows the ingress of air from the
atmosphere into the pump, purging volatiles from
the oil
• While ballasting, vacuum will be limited to ~10-2
mmHg in place of normal 10-3 mmHg
• Normally, ballasting for ~15 minutes each week is
adequate or consult to manufacturers’ manual
11
12. Rotary Pump Precautions
• Always use high quality oil i.e., a low
vapour-pressure oil such as PFPE
(Perfluoropolyether)
• When a RP is switched off, it should be let
up to atmosphere (without letting air into a
hot diffusion pump) otherwise the oil will
be sucked into any lines or chambers
connected.
12
13. Rotary Pump Oil
• Oil exhausted due to vapour saturation/impurities pick up
• Normally it may be used for years
• Fresh oil is clear and color less
• Signs for degradations are:
– Oil level is low
– Oil is a dark-brown /yellow colour
– Oil is turbid due to a build-up of water /vapours
– Pump becomes slow
• If oil looks OK but the pump is slow then ballasting may
improve performance
13
14. Oil Change Precautions
• Treat as hazardous waste (samples/compound used have been
concentrated)
• Use PPE/if possible replace in fume hood.
• Turn off and Isolate pump, allow oil to cool down
• Take out oil and rinse it with fresh oil for 1-2 mins
• Fill fresh oil to 75% of the full mark or as indicated by min-
max lines
• Change oil-mist filter
• Run the pump for 5-10 minutes and check the level and
refill/top-up if needed
• Retain the equipment used for oil drainage; it is impossible to
clean it for any other use.
14
15. gurgling noiseBallasting The
RP
• Pump out normally, then slightly open the ballast-
valve until one hear the low-frequency gurgling
noise.
• This will allow vapours in the oil to easily escape the
pump
• Normally 15mins of ballasting once in a week is
sufficient
• May be done for a day in this condition – but don’t
use machine/EM
15
20. 20
Boil the oil
Condense Oil Vapor (cooling coils)
Condensing vapor sweeps gas
molecules down
Reboiling releases gas molecules
which are then removed by
mechanical pump
Diffusion Pump Cont..
[4]
26. 28
Must be used in conjunction with another (usually rotary) pump
Can’t be used at greater than 10-2 Torr.
Hot oil will deteriorate “crack” and form tar. Diffusion oil is
VERY expensive ($1-2 per ml.)
If cooling system or backing pump fails oil will “backstream” into
the system by way of diffusion
Needs time to heat up and cool down (~30 min)
Diffusion Pump Considerations
Diffusion Pump Cont..
[4]
27. 29
Diffusion Pump
Advantages
Simple design
Relatively cheap
No moving parts
No vibration
Pumps light gasses well
Tolerant of particles
Disadvantages
Oil Vapor Can “crack”
Time to heat up/cool down
Needs coolant
Can overheat
If lose RP, will have oil
throughout system
Diffusion Pump Cont..
[4]
29. Turbomolecular Pump
• Like a jet engine that pulls air instead of pushing
it.
• Turbine spins ~ 20-50,000 rpm creating
“downdraft” which sweeps out gas molecules.
Multiple stages of rotating blades (rotors) spaced
between fixed blades (stators).
• Usually requires rough (backing) pump although
in theory can go from atmosphere
31
Turbomolecular Pump Cont..
[4]
33. 35
Turbomolecular Pump
Advantages
Very high Vacuum
10-7 Torr.
Very clean (no oil)
Relatively fast
Disadvantages
Must be vibration
damped
Sensitive to movement
Moving parts
Very expensive
Turbomolecular Pump Cont..
[4]
34. 36
Ultrahigh
(< 10-7 Torr)
High
(10-3 ~ 10-7 Torr)
Medium
(1 ~ 10-3 Torr)
Low
(760 ~ 1 Torr)
Rotary Rotary
Sorption Sorption
Diffusion Diffusion
Turbomolecular
Turbomolecular
Sputter ion Sputter ion
Cryogenic Cryogenic
The different vacuum pumps
[5]
36. 38
Acetone & Alcohol Precautions
• When heated, sprayed or exposed to high temperature, are become
flammable and explosive, causing serious injury or death.
• Acetone or alcohol are 4 to 5 times heavier than air and flows
down, settling in tanks, pits, and low areas, thus displacing air
creating which can kill by asphyxiation (Deficiency of oxygen).
• Acetone, alcohol, and other solvents are irritants, narcotics,
depressants, and carcinogenic.
• Their inhalation and ingestion may produce serious effects.
• Prolonged or continued contact with the skin will result in
absorption through the skin and moderate toxicity.
[10]
37. 39
Acetone & Alcohol Precautions
• Always ensure that cleaning operations are carried out in
large, well-ventilated rooms.
• The use of a self-contained breathing apparatus may
also be necessary.
• Wear eye shields, gloves, and protective clothing.
• Solvents degrade O-ring materials reducing their ability
to hold vacuum.
• If necessary to clean O-rings, wipe them with a clean,
lint-free cloth or use a small amount of DP-oil.
[10]
39. 41
Measurement of pressure
• Mechanical phenomena gauges: measure
actual force exerted by gas (e.g. manometer).
• Transport phenomena: measuring gaseous
drag on moving body (e.g. spinning rotor
gauge) or thermal conductivity of gas (e.g.
thermocouple gauge).
• Ionization phenomena gauges: ionize gas and
measure total ion current (e.g. ion gauge).
• Partial pressure residual gas analyzers: mass
spectrometers.
[5]
43. 45
• As a gas increases in density (indicate an increase in pressure),
its ability to conduct heat increases.
• A wire filament is heated by running current through it.
• A thermocouple or Resistance Temperature Detector (RTD)
is used to measure temperature of filament.
• This temperature is dependent on the rate at which the filament
loses heat to the surrounding gas, i.e. depend on thermal
conductivity.
• A common variant is the Pirani gauge which uses a single
platinum filament as both the heated element and RTD.
• Working range is 10 Torr to 10−3 Torr, but are sensitive to
chemical composition of gases being measured.
Thermal Conductivity based Gauges
[6]
44. 46
• A Pirani gauge is a one wire gauge
• The wire is heated by a current flowing through it and
cooled by the gas surrounding it.
• If pressure is reduced, the cooling effect will decrease,
hence the equilibrium temperature of the wire will
increase.
• The resistance of the wire is a function of its
temperature: by measuring the voltage across the wire
and the current flowing through it, the resistance (and so
the gas pressure) can be determined.
• Thermocouple gauges and thermistor gauges work in
a similar manner, except a thermocouple or thermistor is
used to measure the temperature of the wire.
Thermal Conductivity based Gauges Cont..
[6]
45. 47
Uses a wire (platinum) in a sealed vacuum tube (reference)
and a second wire in specimen chamber.
Apply a constant voltage of 6-12V to heat the wires.
The resistance of the wire is a function of its temperature,
so current through it (or voltage across it) will have
relation with the gas pressure. The higher the temperature,
the greater the resistance and the less the current flow.
Normal working range 0.5 Torr to 10-4 Torr.
Pirani Gauge
Thermal Conductivity based Gauges Cont..
[4,6]
46. 48
The thermal conductivity of the gas may affect the reading. So for more
precise readings in a particular chamber/environment calibration procedure is
executed.
Thermal Conductivity based Gauges Cont..
[4]
47. 49
• Most ion gauges come in two types:
Hot cathode
Cold cathode
Spinning rotor gauge: more sensitive and expensive but
rarely used
• Most sensitive and used for very high vacuum
10-10 - 10-3 torr
Ionization Gauges
[6]
48. 50
• In Hot-cathode gauge, gas is bombarded with high
energy electrons produced by heated filament (thermionic
emission) giving positive ions.
• The ions are attracted towards a charged electrode called
Collector.
• Collector current is proportional to ionization-rate, i.e.
function of pressure
• The principle of Cold-Cathode gauge is the same,
except that electrons are produced by discharge of a high
voltage.
Ionization Gauges
49. 51
• Its Calibration is very sensitive to:
Construction geometry
Chemical composition of gases
Corrosion and surface deposits
Their calibration can be invalidated by activation at
atmospheric pressure or low vacuum
• The composition of gases is usually unpredictable, needing
MS(Mass Spectrometer) with IG for high accurate
measurement.
Ionization Gauges Cont..
50. 52
• The calibration is unstable and dependent on the
nature of the gases being measured, which is not
always known.
• They can be calibrated against a McLeod gauge
which is much more stable and independent of gas
chemistry.
Calibration of IG
51. 53
Cold Cathode Ionization Gauge
• The major difference between the two is the position of
the anode with respect to the cathode.
• Neither has a filament,
• Both require DC potential of ~ 4 kV for operation.
• Inverted magnetrons can measure down to 10−12 Torr.
• Penning gauge by Frans Michel Penning
• Inverted magnetron or Redhead gauge
Ionization Gauges Cont..
52. 54
Penning Gauge
It is a cold-cathode based.
Get current flow between anode and cathode by applying high
voltage ~4k which ionizes gas molecules
The more gas molecules, the more will be collisions i.e. more
ions or more current in measured
Ionization Gauges Cont..
[4]
53. 55
• There is a lower vacuum limit for IGs, like for Penning gauge is
~ 10−3 Torr
• This is due to shorter mean free path in presence of more gas
molecules, leading to less current by recombining ions before
reaching anode
• Similarly, cold-cathode gauges has problem to start at very high
vacuum (near-absence of a gas molecules), i.e. difficult to
establish an electrode current
• For PG, a symmetric magnetic field is used to create path
lengths for ions
• Additionally, in PG, the electrode is fine-tapered to facilitate the
field-emission of electrons, thus easing the set-up of discharge
path.
Ionization Gauges Cont..
54. 56
• Cold-cathode gauges has less Maintenance-Cycles and works
for years depending on gas type and pressure operated.
• Gases with substantial organic components, such as pump oil,
can result in growth of carbon layers, leading to
• Short-circuit the electrodes of the gauge
• Impede the generation of a discharge path
Ionization Gauges Cont..
[6]
56. 58
Ionization Gauges
(Hot Cathode)
Ionization Gauges Cont..
[2]
• Thermionic/hot cathode ionization gauges.
• Energetic beam of electrons used to ionize gas
molecules and produce ion current.
• Upper pressure limit (10-3 Torr): secondary ion
ionization excitation, filament burn out.
• Lower pressure limit (10-10 Torr): secondary
electron current from X-ray emission.
60. 62
Vacuum Gauges are
Calibrated by
• Attachment to calibrated vacuum system.
• Comparison with calibrated reference
gauge.
• Comparison with absolute standard
calibrated from its own physical properties.
61. 63
Ultrahigh
(< 10-7 Torr)
High
(10-3 ~ 10-7 Torr)
Medium
(1 ~ 10-3 Torr)
Low
(760 ~ 1 Torr)
Manometer Manometer
Thermocouple Thermocouple
Spinning rotor
Ionization
Ionization
Mass
spectrometer
Mass
spectrometer
Spinning rotor
Vacuum gauges used in vacuum
systems
[5]
62. 64
Vacuum Systems in EM
• Different levels of vacuum are
required for different portions of the
microscope
• Gun:10-9 Torr
• Specimen: 10-6 Torr
• Chamber and Camera: 10-5 Torr
Vacuum System Cont..
[4]
63. 65
Pir = Pirani Gauge
V = Valve
ODP = Oil Diffusion Pump
Pen = Penning Gauge
Igp = Ion Getter Pump
PVP = Pressure Variable Pump
(rotary)
Vacuum System Cont..
[4]
64. Getting Better Vacuum
• Use best quality oil for RP/DP
• Prevent back-streaming of oil vapours into the EM,
– Use a bakeable vapour trap in the rough pumping line
– If possible (and it unfortunately may be difficult) fit a
LN trap above DP
• Changing O-rings to ‘Viton’ type will make a
significant improvement
• Viton O-rings do not need greasing and offer
better seals at higher vacuum
• Never grease O-rings until used in moving parts
66
65. Leak Detection
It rarely happen under normal operation, so be
carful for major dismantling
• Remove the specimen; it may be wet and/or
outgassing
• Check the O-ring on the stage, and the
associated sealing surface
• Remember that if you have just cleaned an
internal component then the EM may take
hours to reach a good vacuum
67
66. Detecting Leaks
• Take good sensitive analogue vacuum meter and a high
vacuum gauge head hooked up to an appropriate part of the line
• Freon, Arklone and Helium are common leak sprays that are
harmless but volatile
• Previously Ether (dangerous) was used because this is highly
volatile and it finds its way very quickly through the leak and
vaporizes quickly to give a blip on the vacuum meter.
• Work systematically: Spray on a part and wait for ~30 secs and
then proceed for the next
68
72. 74
• Check the Element / filament. It might be
ruptured or blown out. Should be replaced.
• Check the PCB/preamp section.
• For lower values check the supply voltage.
Gauge shows full range or continuously
lower value:
74. 76
– Cleaning for deposition / contamination
– Supply voltage (usually 24~34 volt) too noisy
• Trace out the noise source, if it is from power supply,
look at their filters.
• Usually a filter capacitor can remove the pickup
noise.
Measuring Signal Unstable
[8]
75. 77
• May be very dirty / Contamination on inside,
– May have got carbon layer due to back-streaming of
pump oil
– May be cleaned with metal polishes.
– Be careful for Trigger-Electrode (present in some
models) not need to be wiped off and bend.
– Give it a soak in acetone, be careful for using more
aggressive caustics or acid base cleaners, unless given for
some specific gauges.
– After washing/cleaning, heat it gently with some heat-gun
or hair dryer till it warms to touch.
Unstable or Erratic Results
Penning Gauge
76. 78
• Fluctuation in the gauge HV circuitry.
– Disconnect the power supply, test the components,
soldering and connections.
– Be careful, the gauge uses several thousand voltages, so
there might be a chance of capacitor charge, so wait for at
least 5minutes when turned off.
• ADC Circuit failure
– Check the conversion circuit for their components,
operating supply and reference voltages for the ADC.
Unstable or Erratic Results
Penning Gauge
78. 80
Changing Heater
Element
• Disconnect the supply
• Remove the polished reflector
cover.
• Remove the block containing the
defective heater.
• Remove the heater from its hole
and replace with a new element.
• Coat the replacement heater with
milk of magnesia before
inserting in the block. This
prevents seizing of the element on
repeated heat up, and make future
servicing easier.
• Reconnect heater and check its
continuity
• Replace the polished reflector.
• Use penetrating oil if necessary.
[9]
79. 81
Precautions for DP
• Do not run pump without cooling water
• Do not run pump with low level of pump Oil
• Do not open drain plug when pump is in vacuum
and especially when heater is hot
• Do not override pressure, thermal switch and
valve sequencer interlocks.
• Do not weld/machine any part without removing
all fluids or solvents. [10]
81. 83
Inlet Pressure Surges
• Incorrect heater voltages
• Fluid out-gassing, condition fluid by operating the
pump for 24 hours.
• Leak in the system inlet.
[10]
82. 84
Poor System Pressure
• Leaks in the system, virtual or real
• Contaminated pump oil
• Low heat/temperature, check input voltage,
heater resistance, poor thermal contact.
• High foreline-pressure
– Check foreline connection
– Poor performance RP, TMP
– Check its oil, oil-breakdown
[10]
83. 85
Cleaning
• Disconnenct power & allow to cool
• Disconnect the inlet-flange and the
foreline connections, be careful for
the O-rings for any scratch or
damage.
• Remove the cold cap and jet
assembly .
• Drain the oil and remove all O-rings
and gaskets.
• Wash the pump interior body and
the jet assembly with acetone.
• Rinse it with isopropyl alcohol
followed by drying with dry
nitrogen.
• Reinstall and fill the pump with oil
up to the level. Jet Assembly [10]
86. 88
Acetone & Alcohol Precautions
• When heated, sprayed or exposed to high temperature, are become
flammable and explosive, causing serious injury or death.
• Acetone or alcohol are 4 to 5 times heavier than air and flows
down, settling in tanks, pits, and low areas, thus displacing air
creating which can kill by asphyxiation (Deficiency of oxygen).
• Acetone, alcohol, and other solvents are irritants, narcotics,
depressants, and carcinogenic.
• Their inhalation and ingestion may produce serious effects.
• Prolonged or continued contact with the skin will result in
absorption through the skin and moderate toxicity.
[10]
87. 89
Acetone & Alcohol Precautions
• Always ensure that cleaning operations are carried out in
large, well-ventilated rooms.
• The use of a self-contained breathing apparatus may
also be necessary.
• Wear eye shields, gloves, and protective clothing.
• Solvents degrade O-ring materials reducing their ability
to hold vacuum.
• If necessary to clean O-rings, wipe them with a clean,
lint-free cloth or use a small amount of DP-oil.
[10]
A substance with a high vapor pressure at normal temperatures is often referred to as volatile.
perfluoropolyether (PFPE), perfluoroalkylether (PFAE) and perfluoropolyalkylether (PFPAE).
Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system
![2
Application
• Electron Microscope
• XRF
• LCMS
• ICPMS
• Centrifuges
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