Vybrané kapitoly z elektronové mikroskopie
část 4 – Vakuové systémy pre elektronovou
mikroskopii
Marek Talába
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Content
• Introduction
• Why we need vacuum in electron microscopes?
• Pumping of vacuum system - Basic vacuum terms and relations
• Vacuum pumps
• Vacuum gauges
• Design of vacuum system in electron microscopes
• Leak checking and Residual Gas Analysis (RGA)
• Summary
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Introduction
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What is vacuum?
• Vacuum is space that is devoid of matter.
• An approximation to such vacuum is a region with a gaseous pressure
much less than atmospheric pressure.[Wikipedia]
• The quality of a vacuum is measured by the amount of matter remaining
in the system.
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Vacuum ranges
• No defined borders, only consensus
• Sometimes in a broader sense
High Vacuum = any environment < 0.1 Pa
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Vacuumrange Pressurein Pa
Low vacuum 103 – 102
Medium vacuum 102 – 0.1
High vacuum 0.1 – 10−5
Ultra high vacuum 10−5 – 10−10
Extremely high vacuum <10−10
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Why we need vacuum?
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Why we need vacuum in electron
microscopes?
There are three main reasons why the electron microscope must be operated
under vacuum:
• to ensure straight particle movement without collisions
• to protect electron emitter from oxidationand damage by ion sputtering
• to keep clean environment and sample surface
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Avoiding Collisions
• Basic parameter describingprobability of collision between particles in
vacuum is mean free path (MFP).
• The MFP of an electron is the average distance an electron travels
betweentwo consecutive scatteringevents
𝜆 = ( 2𝜎𝑛)−1
l is MFP, 𝜎 is Collision cross-section and n is particle density
• Approximation MFP = 6.4*10-3 / p [m; Pa]
• Example: 1 cm at 0.6 Pa
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Avoiding Collisions
• Basic parameter describingprobability of collision between particles in
vacuum is mean free path (MFP).
Vybrané kapitoly z elektronové mikroskopie
Vacuumrange Pressurein Pa Molecules / m3 Mean free path
Ambient pressure 105 2.7 × 1025 ~100 nm
Low vacuum 103 – 100 1025 – 1022 0.1 – 100 μm
Medium vacuum 100 – 0.1 1022 – 1019 0.1 – 100 mm
High vacuum 0.1 – 10−5 1019 – 1015 10 cm – 1 km
Ultra high vacuum 10−5 – 10−10 1015 – 1010 1 km – 105 km
Extremely high
vacuum <10−10 <1010 >105 km
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Electron and ion beam scattering
• Collisions with residual gas deflects particles from straight movement
• Expected beam trajectory (optical path) not valid anymore
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Emitter Tip Environment
• Oxygencan damage thermionic cathodes
• Suppress ion bombardment of the FEG tip
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• Contamination influences high resolution even at high accelerating
voltage like on the example below (30 kV)
Sample contamination
Clean sampleContamination grown during single scan!
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Pumping of vacuum system
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• The vacuum achieved at steady state is the result of a dynamic balance
betweenthe total gas load Qtot and the ability of the pump to remove gas
from the volume – effective pumping speed Seff. The ultimate pressure pu
is then given by:
Pumping of Vacuum System
𝑝 𝑢 =
𝑄𝑡𝑜𝑡
𝑆 𝑒𝑓𝑓
𝑄 𝐺 = ෍
𝑖
𝑄𝑖
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We can divide these sources into several categories:
• Leaks (real or virtual)
• Outgassing (surface)
• Diffusion (volume)
• Vaporization
• Permeation
• Back-streaming
• Process generatedgases
Gas Load Sources
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We can divide these sources into several categories:
• Leaks (real or virtual)
• Outgassing (surface)
• Diffusion (volume)
• Vaporization
• Permeation
• Back-streaming
• Process generatedgases
Gas Load Sources
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We can divide these sources into several categories:
• Leaks (real or virtual) - no trapped volumes, effective sealing method
• Outgassing (surface) - keep system in evacuated state, venting with dry
nitrogen or inert gas
• Diffusion (volume) - selection of suitable materials for chamber
construction
• Vaporization - materials (e.g. lubricants) with low vapor pressure
• Permeation - suitable materials, sufficient wall thickness
• Back-streaming - use low backing pressure
• Process generated gases - design system with high enough effective
pumping speed
Gas Load Reduction
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• Viscous Flow - collisions between molecules dominate, generally
p > 10 Pa.
• Molecular Flow - collisions between molecules and wall
dominate, generally p < 0.1 Pa.
• Transition Flow - region between viscous and molecular flow.
Types of gas flow
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The conductance for an aperture of surface area A (in cm2) in the
case of laminar flow of air is given by
The conductance for an aperture of surface area A (in cm2) in the
case of molecular flow of air is given by
Conductance of Aperture
𝐶𝐿 = 20 ∙ 𝐴 𝑙 ∙ 𝑠−1
𝐶 𝑀 = 11.6 ∙ 𝐴 𝑙 ∙ 𝑠−1
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The conductance for long pipe (L >> D, p2 > p1, p = average value)
in the case of laminar flow is given by
The conductance for long pipe (L >> D, p2 > p1, p = average value)
in the case of molecular flow is given by
Conductance of pipe
𝐶𝐿 = 137 ∙ 𝑝 ∙
𝐷4
𝑙
𝑙 ∙ 𝑠−1
𝐶𝐿 = 12,1 ∙
𝐷3
𝑙
𝑙 ∙ 𝑠−1
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Analogy with electric circuit
• Vacuum resistance W = 1 / C
• Pressure ~ Voltage
• Flow ~ Current
Vacuum system calculations
W = W1 + 1/(1/W2 + 1/W3) = W1 + W2W3/(W2+W3)
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• Diaphragm (membrane) pump 1 l/s, ultimate pressure ~1000 Pa
• Hose length = 3 m
Pumping speed vs. hose diameter ?
Example 1 - Vacuum oven
50 l
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Pumping speed vs. hose diameter ?
• ConductancePipe: GP = 137*D^4*p/L (aperture effect negligible)
Example 1 - Vacuum oven
50 l
D [mm] p [kPa] GP [l/s]
G total
[l/s]
25
100 1.8E+06 1.00
1 1.8E+04 1.00
10
100 4.6E+05 1.00
1 46 1.00
5
100 2.9E+04 1.00
1 29 0.97
2
100 7.3 1.00
1 0.73 0.42
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• Typical HV pressure range 10-2 – 10-4 Pa
• Maximum pumping speed for selected valve ?
• Effective pumping speed ?
• Valve diameter 10 cm
• Tube length 0,5 m
• TMP shield 80 % open
• TMP pumping speed 300 l/s
Example 2 – high vacuum chamber
50 l
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1. Valve (aperture) conductance
GV = 11.6*3.14*102/4 = 911 l/s
= maximum pumping speed for selected port
2. Tube conductance
GT = 12.1*103/50 = 242 l/s
3. TMP shield = GV * 0.8 = 730 l/s
Effective pumping speed:
TMP inlet = 300 l/s
TMP + shield = 213 l/s
TMP + tube = 113 l/s
At chamber port = 100 l/s
Example 2 – high vacuum chamber
50 l
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Vacuum pumps
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• Gas transfer pumps: compress and move gas out
• Gas capture pumps: entrap gas inside the pump
Vacuum pumps
Gas transfer pumps Gas capture
pumps
Gas displacement
pump
Momentum transfer
pump
Membrane pump Turbomolecular pump Ion getter pump
Rotary vane pump Molecular pump Cryo pump
Scroll pump Diffusion pump
Roots pump
Screw pump
Piston pump
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• Requires inlet and outlet valve to achieve aligned gas displacement
• Oil free, corrosion resistant
• Only about 7 kPa ultimate pressure for single stage
• Multiple stages – ultimate pressure up to 50 Pa
• Well suited for low pumping speeds up to 10 m3/h
Diaphragm pump
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Rotary Vane Pump
• Enclosed gas is compressed until the dischargevalve opens against
atmosphere
• Pump oil is essential for the function – lubricates and seals
• Single and two stage versions – higher flow x higher compression ratio
• Pumping speed 2 – 200 m3/h
• Ultimate pressure <1 Pa
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• Lubricant-free withinthe vacuum envelope
• No need of shaft seals
• Pumping of corrosive and oxidizinggases
• Pumping speed 5 – 100 m3 / h
• Ultimate pressure <10 Pa
Scroll pump
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• High speed moving blades transfer momentum to molecules
• Can not aperate at atmospheric pressure - Requires backingpump
• Backing pressure ~1 kPa
• Ultimate pressure 10-6 Pa (ISO-K) or 10-8 Pa (CF)
Turbomolecular pump
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Ion getter pump
• Gas ionizationby free acceleratedelectrons
• Ion accelerates to cathode
• Neutral Ti atoms sputtered
• Gas atoms chemically bounded or “buried”
• For noble gasses
• Triode version
• Ta cathode
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Vacuum gauges
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Direct measurement of force = gas independent
• Piezo sensors
• Capacitance sensors
Indirect measurement = gas dependent
• Pirani and thermocouple sensors
• Cold cathode (Inverted magnetron, Penning, Philips)
• Hot cathode (Ionisation, Bayard-Alpert) sensors
Pressure measurement
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Measurement ranges
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• Usually measures over 4 decades, usable 3 decades
• Very high accuracy 0.2% and stability
Capacitance diaphragm gauges
p p0
p0 << p
Condenser capacity C = ε . S / d
-thermal effects
-usually linear output
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• Measurement of heat los through conduction
• Radiation and convection must be minimized
• Accuracy 10-15% (50% near to atmopshere), repeatability 2%, gas
dependent, low price
Pirani gauge
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• Measures current in glowing discharge
• Sputtering effects at higher pressures
• Accuracy 30%, repeatability 5%, gas dependent.
Cold Cathode / Penning Gauges
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• Heated cathode emits constant electron current
• Accuracy 10%, repeatability 5%
•Sensitive to pressure burst
Hot cathode (Bayard-Alpert) gauge
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Vacuum systems
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Vacuum systems consists of several stages:
• Low or roughing vacuum – achieved with either rotary vane pump, scroll
pump or diaphragm pump
• High vacuum – achieved with turbomolecularpump – vacuum in
microscope chamber.
• Ultra high vacuum – achieved ion getter pump – vacuum in electron/ion
column.
Pressure ranges
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SEM/SDB vacuum systemPressures
Gun: 10-7 – 10-8 Pa
Column: 10-5 – 10-7 Pa
S. Chamber: 10-2 – 10-5 Pa
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SEM/SDB vacuum systemPressures
Gun: 10-7 – 10-8 Pa
Column: 10-5 – 10-7 Pa
S. Chamber: 103 – 10-5 Pa
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• Vacuum system of EM has several stages with different vacuum level
• Stages are isolated by valves or differential pumping apertures (DPA)
• DPA prevents diffusionof gas molecules into the higher vacuum area faster
than they can be pumped out.
Differential Pumping – ESEM Mode
PLA
Set of 5 apertures
4 kPa
TMP-interstage ~ 100 Pa
TMP-main ~ 3e-2 Pa
aperture
IGP 2 ~ 1.5e-6
Pa
aperture
~ 4e-4 PaIGP 1
Specimen chamber
FEG module
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SEM/SDB vacuum systemPressures
Gun: 10-7 – 10-8 Pa
Column: 10-5 – 10-7 Pa
S. Chamber: 10-2 – 10-5 Pa
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TEM vacuum system
Pressures
10-6 – 10-8 Pa
10-4 – 10-6 Pa
10-2 – 10-5 Pa
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Leak checking and
Residual gas analysis
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Leak detection
• Two types leaks – real and virtual
• Large vs. Small leaks – selecting a leak detection method
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Leak rate [mbar.l.s-1]
Test Method 100
10-2
10-4
10-6
10-8
10-10
10-12
Acoustic Leak Detection
Bubble Testing
Pressure Decay
HalogenGas Detection
Helium Leak Detection
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Helium is a superior choice of tracer gas used to find leaks for a
multitude of reasons. Helium is:
• Non-toxic
• Inert and non-condensable
• Normally not present in the atmosphere at more than trace amounts
• Relatively inexpensive
• Readily passes through leaks due to its small atomic size
• Non-flammable
• Available in various size cylinders
• Available in purities appropriate for medical usage
Helium leak detection
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• Helium counter-flowleak detectors are designedin accordance with the
schematic diagram in figure below.
• A mass spectrometer (MS) is mounted on the intake flange of a
Turbomolecular pump.
• A backing pump Sv evacuates the Turbomolecular pump via valve V2.
Helium leak detection
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• Typpical vacuum system without leaks
• To reach <10-6 Pa, bakeout is used
• Metal seals in UHV region
Residual gas composition
Pressure [Pa] Major constituents
atm. wet air
0.1 water vapor
10-4 H2O, CO
10-7 CO, N2, H2
10-8 CO, H2
10-9 H2
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• Most frequent– quadrupole mass spectrometer
• Analysis whole residual gas composition
• Reveals leaks
• Evaluates contamination
Mass Spectrometer for RGA
Spectrum
1.00E-14
1.00E-13
1.00E-12
1.00E-11
1.00E-10
1.00E-09
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76
Mass [au]
Intensity[A]
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Mass Spectrometers for RGA
1. Sector Field MS 2. Quadrupole MS
Vybrané partie z elektronové mikroskopie
From: www.pfeiffer-vacuum.com
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Design of vacuum system in electron microscopes
• Consider basic vacuum rules
• Calculate dimensions of pipes and apertures
• Select suitable sealingfor each part (minimize number of joints)
• Choose active components (pumps, gauges) with respect to specific EM
requirements
• Verify assembly of whole system
• Measure leak rates and RGA
Summary
Vybrané kapitoly z elektronové mikroskopie