Vacuum Techniques
•Gauges
Vacuum Gauges• Thermal Conductivity Gauges: (Pirani, TC, Convection)
– Describe the operating principle for each gauge– Explain the use of the Wheatstone bridge– Compare the constant current and constant temperature modes of
operation– Explain why the TC gauges typically have an upper limit of 1 torr– Relate the lower pressure limit to thermal radiative heat transfer
• Ionization Gauges: (Hot Cathode, Cold Cathode)– Describe the operating principle and electrical configuration for each
gauge– Relate the sensitivity of the HCG to the geometry and gas properties– Explain what is meant by the ‘soft x-ray’ limit– Describe the role of magnetic field for the CCG
• Absollout gauges??/
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.
• Mechanical guagesMechanical: liquid column, diaphragm, etcReal pressure from force measurement
Mechanical phenomena Pressure Measurement
Mechanical phenomena Pressure Gauge
Bourdon Gauge
Mechanical phenomena How the gauge works
Capacitance (diaphragm) gauge: measure diaphragm bending by capacitance change
d
d
AC
pressure
Absolute pressurereference to a vacuum cell
Full range: 0.02 torr, 1 torr, 1000 torr,
10000 torr
Accuracy ~ 0.1 %
real species independent
linear, accurate
zero point is not absolute
• Transport phenomena
Transport phenomena(1)
• Gas molecules colliding with the heated filament take kinetic energy away from the filament’s atoms. This cools the filament down.
• A higher pressure means that none molecules collide with the filament per second, so a larger cooling effect occurs.
Transport phenomena(2)
• When the filament is cooler its resistance is lower.
• The resistance can be measured automatically.
• One way of doing this is to use a Wheatstone Bridge.
Transport phenomena(3)
Pirani Gauge
Filament
-+
Thermal conductivity gauges mechanisms
– Four possible mechanisms to take heat away from sensing element:
• Radiative heat transfer to surroundings
• Conduction along the sensing element
• Conduction through the residual gas
• Convective heat flow to the residual gas
Hot wire
1/2 WC1/2 WC
Thermocouplejoint
WR
WG
WCNV
Heatingcurrent
Thermal conductivity gauges mechanisms
1-Radiative heat transfer transfer:
WR 1 2r1L T14 T2
4
L
T1
T2r1
• Conduction along the wire
1
2WC r1
2kwiredT
dl
L
T1 T2
• Conduction through the residual gas:
as properties determine variation in sensor response to pressure
WG 2r12L F 1
1
P
R '
8MT2
1/2
T1 T2 (M, )
R`: wire resistance,
P: pressure ,
L, r1: Wire length and diameter,
M, , F: Gas properties.
T1 and T2 Wire and gas temperature respectively
Thermal Conductivity Gauges• Pirani calibration (and low pressure limit):
• Platinum wire with r1 = 0.0127 mm, L = 150 mm
• At very low P, Vbr = 0.3907 V, and Rs = 43.61 WR+WC =8.75x10-4 W
• Subtracting WR+WC from WT yields WG for N2 as shown below:
WT WG WR Wc Vbr
2
4RS
Thermal Conductivity Measurment Constant current mode
• Constant current mode:– As pressure decreases, less heat is
removed by conduction through the gas from the sensing element, and temperature increases.
– Measure temperature dependent change of resistance (or TC voltage).
Thermal Conductivity MeasurmentConstant temperature mode:
• Constant temperature mode:– Adjust current to heater to maintain constant
sensor property (resistance or voltage)– Use Wheatstone bridge arrangement to
optimize sensitivity– Correlate current flow against pressure– Extends high pressure range (used in Pirani,
not common for TC)
Pressure GaugesGauge
Compensator to system
milliammeter
Power Supply
Thermocouple gauge
For roughing vacuum measurements
• Ionization gauges
Ionization phenomena(1)
• Electrons emitted from the heated filament are attracted to the positive grid. Many electrons follow long looped paths before striking the grid.
• During this time they collide with gas molecules, thus creating positive ions.
Ionization phenomena(2)
• These ions are attracted to the negative collector and constitute a current into the gauge circuit.
• A higher pressure results in a higher gauge current.
• The emission current must be kept within strict limits as it too affects the gauge current.
Ionization phenomena(3)
• Thermionic/hot cathode ionization gauges.• Energetic beam of electrons (constant I-) used
to ionize gas molecules and produce ion current.
• e- + M → M+ + 2e-
I+ = p KI -, K: ion gauge sensitivity• 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.
Hot Catode Ionization Gauges – HCG(1)
• Hot cathode gauge (HCG)– Thermionic source (electrons)
cause inelastic collisions with gas, producing ions
– Ions collected at (-) biased surface– Electrons travel to (+180 V) grid
• Pre-1950 lower limit of 10-6 Pa– Soft x-rays produced at grid result in
photo-electron flow away from collector (same as ion flow to collector)
– Bayard-Alpert modification: fine wire collector surrounded by grid range extended to 10-9 Pa
+
-
-
collector
grid
filament
HCG(2)
Ionization Gauge
Heated Filament
Emission Current
A.C.
+ve
Grid
Ion Collector
Pressure Gauge
HCG(3)
HCG(4)
• 10-11 ~ 10-3 torr• linear sensitivity• absolute zero point • species dependent
Th on Ir has a lower work function,
so works at a lower temperature
sensitivity ∝ ionization cross section
air, N2, O2 1.0
He 0.15
Ne 0.3
H2 0.4
CH4 1.4
HCG(5) Gauge sensitivity
• Gauge sensitivity:– Dependent on rate of ionizing
collisions
– Ion collision cross section given by• = cross section per molecule• n = molecule density = P/(kT)• L = length of ionizing space• A = cross sectional area of
electron beam
LAn
HCG(6) Ion current
– Ion current is then:
• N = arrival rate of electrons• q = charge per electron
– With Nq = ie, we have i+ = P K ie + ir,
– Gauge sensitivity S = K ie
– Typical range for K: 0.02 Pa-1 to 0.2 Pa-1 (bigger is better)
NqLAP
kT
K LAkT
Cold Cathode Ion Gauges: CCG(1)
• Cold Cathode Gauge (CCG) Invented by Penning in 1937 - electrical configuration like diode SIP
• 1952 design of wire-in-cylinder with reverse polarity (+ wire) provides key performance improvement
• “Inverted Magnetron” - Magnetic field produces long helical path for electrons & ions many collisions
• Ionizing electrons part of self-sustaining gas discharge
• No background current to mask the ion current
• Power into CCG ~ 0.1 W
Cold Cathode Ion Gauges: CCG(2) similar to hot cathode ion gauge
more robust
less accurate
hard to ignite at low pressure
magnet
Hot Cold
CCG(3)
• “Striking time” is delay before discharge starts. At 10-4 Pa, ts ~ seconds; at 10-8 Pa, ts ~ hours or days
• Nearly linear dependence of ion current with pressure:
• Typical anode voltage +3 kV
• Typical current 0.1 mA
ig KPn; n 1.05 1.2
Pressure Measurement
Vacuum Gauge Selection adapted from Lesker.com
Convectron Gauge:Initial pumpdown from
1 atm, and as a foreline monitor
Thermal Conductivity of Gas
Baratron:Insensitive to gas
composition,Good choice for
process pressures
True Pressure (diaphragm displacement)
Ion Gauge:Sensitive to gas composition, buta good choice for base pressures
Ionization of Gas
RGA:A simple mass spectrometer
Residual gas analyzers
• More compact with higher sensitivity.• Gaseous ions formed in ion source box by
electron bombardment, extracted with suitable fields, separated in analyzer and then collected and measured.
• Magnetic sector analyzer: masses separated by static magnetic and electric fields.
• Quadrupole mass analyzer: masses separated in oscillating quadrupolar electric field.
Mass Spectrometry
• Quadrupole Mass Spectrometry– Most commonly used in laboratories
• Least expensive of commercially available mass specs
• Detects up to 100 AMU commonly
• Can be extended easily to 300 AMU detection
• Downside: fragmentation patterns of molecules can be complex
• Upside: the simplest and least expensive mass spec (typically $15,000 to $35,000 depending on attachments; other types of mass specs can easily cost a few million)
Ion source: Tungsten filament
Gas inlet
Diaphragm Vacuum Pump
Turbomolecular Drag Pump
Analyzer: Quadrupole Mass Filter
Detector: Faraday/ Channeltron
Gas outlet
Mass Spectrometer Basics
Vacuum Requirements
Ion Detection
Ion Filtering
Ion Creation
Mass Spectrometer Basics
e-
Ion Source
Quadrupole Mass Filter
Ion Detector
Vacuum Requirements
• Filament Longevity
Typical Vacuum ~ 1 x 10-6 Torr
HH
e -
Gas Density ~ 1010 molecules /cm3
(@ 760 Torr (1 atm) ~ 1019 molecules /cm3 )
+
Ion Mobility
Detector Operation
The Atomic Model
12 C = 12 A.M.U.
1 AMU = 1.66 X 10 -27 kg
= Electron ~ 0 AMU, -1e charge
= Neutron ~ 1 AMU , no charge
= Proton ~ 1AMU , +1e charge
The Periodic Table
Ionization
12 C 12 C+ + 2e-
Atom Ion
+ 1e-
e-
Gas Atom
Gas Ion
Electron
Ionization Source - Open
Filaments
Ions Out
Neutral Gas Atom/Molecule
Electron
Ion
Gas In
Pressure (mBar)
Filaments
10-05 10-03 1010-04
Ionization Source - Closed or Gas Tight
Electron EmissionThe emission of electrons from the filament is carefully controlled to provide consistent ion formation.
e-IF
IE
IF =Filament Current ~ 3 Amps d.c.
IE =Electron (emission) Current ~ 1 mAmp d.c.
SET
IonFormationCamber
Ionization
1 e- + 12C12C++ + 3e-
1 e- + 18H2O 1 e- + 17OH 1 e- + 17OH 17OH+ +2 e-
Double Ionization
Fragmentation - Ionization
m/e = 6
m/e = 17
Ionization Products
Ionization can produce more than
one ion type.
E.g. Argon , 40Ar
40Ar + 1e- 40Ar+ + 2e-
40Ar2+ + 3e-
40Ar3+ + 4e-
(m/e = 40)
(m/e = 13.3)
(m/e = 20)
Ionization - Isotopes
Argon Isotopic Masses
P=Proton, N=Neutron
18P+22N=40AMU
18P+20N=38AMU
18P+18N=36AMU M/Z
Log
(Inte
nsity
)
-1e
-1e
-1e
Mass scan of ultra pure argon - showing singly and doubly charged isotope peaks
Ionization Products – UHP Argon
Ionization Products
Molecules can fragment in the ionization process.
ABC + e- ABC+ + 2e-
AB+C+ + 2e-
AB+ + C + 2e-
A + BC+ + 2e-
A+ + BC + 2e-
AC+ + B + 2e-
AC + B+ + 2e-
In addition to the above it is possible for other combinations to be formed
Typical Ion Formation
Spectrum of CO2 showing the 11 mostintense ions
NaturalAbundances
18O = 0.2%
13C = 1.1%
Mass Filter
• Cylindrical Rods.• Stainless Steel or Molybdenum.•Opposite Rods are Connected Electrically.•Alignment is Critical not adjustable.
U + V cos wt
+
+
-
-
Mass Filter
++ +++
++
I i
e-++
++
+
y
x
z
SEM
ION
SO
UR
CE
QUADRUPOLE ROD
QUADRUPOLE ROD
QUADRUPOLE ROD
Selected m/e ion
Higher m/e ion
Lower m/e ion - deflected in y-axis
- deflected in z-axis
- reaches detector
Resolution
Over Resolution
10
1
1 AMU
10% Valley Resolution
Under Resolution
1 AMU 1 AMU
Valley height
Ion Detectors - Faraday
I i
e-
I i ~ 10-14....10-9 A
= Selected ion - positive charge
Indestructible Detector but gain = unity.
Cannot detect small ion currents <10-14 Amps.(Limit depends on electrometer only)
SEM Detector – Continuous Dynode
I i
GAIN ~ 100 106
set by SEM VOLTS
e-
e-
I i ~ 10-14....10-5 A
= Selected ion - positive charge-attracted into SEM by -ve dc volts.
SEMVOLTS ~ - 1500V dc
Can be destroyed by high currents>10-5 Amps , or by operation at highpressure.
MASS FILTER
SEM Detector – Discrete Dynode
MASS FILTER
Deflection Unit
Vdef
Faraday
signal
HV- Ie
Data acquisition modes - Scan Analog
Here a range of masses are scanned and the data appears as mass vs. intensity. This mode is mainly used for diagnosticsand checking mass spec performance, peak shape etc.
Data acquisition modes - Scan Bargraph
This mode is similar to Analog but it only reports data when a masspeak is found - this reduces the data for simplicity. This mode is mainly used for scanning for unknown components.
Data acquisition modes - MID, MCD
MID/MDC - here the masses to be monitored are known and thesystem will only report back these points. It is the fastest mode andvery useful for trending data over time
Multiple Ion Current Determination.Ion current vs. time
Multiple Concentration Determination.Concentration vs. time
NOTE:Requires calibration gas mixes
MCD =
MID =
Residual Gas Analyzer
QUADRUPOLEHEAD
CONTROL UNIT
How the RGA works
MASS NUMBER (A.M.U.)
RE
LA
TIV
E I
NT
EN
SIT
Y
NORMAL (UNBAKED) SYSTEM
H2
H2O
N2,, CO
CO2
(A)
RGA SPECTRUM
MASS NUMBER (A.M.U.)
RE
LA
TIV
E I
NT
EN
SIT
Y
SYSTEM WITHAIR LEAK
H2
H2O
N2
CO2
(B)
O2
RGA SPECTRUM
The RGA 100 Residual Gas Analyzer
Quadrupole Mass Filter Components
Principles of Filter Operation
Residue Gas Analyzer (RGA)
A small quadrupole mass analyzer with an electron impact ionizer
With electron multiplier, sensitivity ~ 10-15 torr
Residual Gas Analysis
Residual Gas Analyzers
Learning Objectives
• Identify the 3 primary components of an RGA and describe the available options for each component
• Describe three performance parameters of an RGA and factors that influence the performance
• Describe the operation of the RF quadrupole
RGA Components
• Ion source– Open ion source– Closed ion source
• Mass filter (analyzer)– Magnet sector– RF quadrupole
• Ion current detection system– Faraday cup– Electron multiplier
Ionization• Ion production of each
species is proportional to its density or partial pressure – but sensitivity and gain
are not independent of mass
– Linear dependence on partial pressure holds true up to total pressure of ~ 10-3 Pa
• Ionization voltage typically 70 volts
• Ionization products can be single or double ionized molecules, or fragment ions
Ionization Sources
• Open Ion Source– Like the Bayard-Alpert gauge– High conductance of ion formation region to analyzer region for
composition consistency– Grid at (+) potential w.r.t. analyzer: defines ion energy
• 5-15 V quadrupole, 100-1000 V magnet sector
Ionization Sources• Closed Ion Source
– Process gas flows through ionization region
– Higher pressure in enclosure enables increased sensitivity (10 to 100 times higher pressure in ionization vs. analyzer region)
– Avoids effects of electron stimulated desorption (ESD) from walls when sampling high-pressure chambers
Mass Analyzers• Separate ions according to ratio of M/q
40Ar++ and 20Ne+ produce identical signal• Magnetic Sectors
– As seen in the helium mass spectrometer– Separation angles of 60º, 90º, 180º are common– Mass sweep via acceleration voltage change limited to x25– Electromagnetic change (0-0.25T) enables 1-100 amu
sweep
Rel
ati v
e In
t en
sity
M/q
Mass Analyzers• RF Quadrupole
– Two pairs of rods, one (+) pair, one (-) pair– DC plus AC(rf) potentials applied to all rods:
• (+) pair: • (-) pair:
– (+) pair creates ion valley most of the time: high mass, high inertia molecules pass through
– (-) pair creates ion hill most of the time: low mass, low inertia molecules pass through
U V cost (U V cost)
Mass Analyzers
• RF Quadrupole (cont.)– The width of the band pass,
or resolving power (M/M), is adjusted via ratio of U/V, length of the rods, or ion energy
– Resolving power measured by
– Detector on axis at the end of the filter counts the transmitted ions
– Linear sweep of rf & dc potential produces linear sweep of amu’s
peak width at 10% height
Ion Detectors
• Ion current, in, sensitivity S’n and partial pressure Pn are related by in = S’n Pn
– Sensitivity determines smallest detectable signal (peak height)– Typical sensitivity for nitrogen:
• Faraday cup– Measures electric current flowing to neutralize ion arrival– Incorporates means to avoid secondary electron release– Should be followed by stable, low-noise, high-gain FET amplifier– Commercial detector limit of 5x10-14 A (10-8 Pa, or 106 ions/sec)– Higher sensitivity (down to 10-12 Pa) requires an electron
multiplier
S’n = 5x10-6 A/Pa
Ion Detectors: Electron Multiplier• How does it work?
– Secondary electrons from ion impact generate multiple electrons upon each successive impact as they are accelerated toward the anode
• Gain
– G1 =
– G2 =
• Inlet voltage typically -2000 V• Glass tube is curved to prohibit (+) ions generated at anode from
traveling back full length of tube and generating out-of-phase secondary electrons
G G1G2n
# electrons generated by ion impact
# electrons generated by electron impact at each stage
Operation
• Mount a valve between RGA and chamber• Mount the RGA in a non-remote location• If pressure > 10-3 Pa:
– reduce electron multiplier voltage – use simple Faraday cup– use differentially pumped sampling configuration
• If spectrum changes when ion energy is varied, then ESD is present (reduce ion energy)
• Select appropriate filament (ThO2, W, Re) and precondition it by heating above normal operating temp.
• Turn off any Bayard-Alpert gauge• Save background spectrum for later comparison
Calibration• Do not accept partial pressure results
unless instrument has recent calibration for several useful gas mixtures
• Calibration procedure standards available from AVS
• Pulse injection method (quick, in-situ method)
– Isolate small quantity of gas in pipe (volume Vi, pressure Pi ) between two valves
– Calculate instantaneous pressure rise in chamber Pc = (PiVi/Vc)
– Determine sensitivity from peak ion current at t=0
S ' I peakPc
I peakVcPiVi
Did you catch it?
• Identify at least 3 important features of an RGA
» Range (1-800 amu)– Sensitivity (10-5 A/Pa, G ~ 106, 10-12 Pa)– Resolving power (1 amu)
• Identify the approximate voltage between each of these stages:– Electron filament and grid– Grid and quadrupole analyzer– Ion detector inlet and ground
70 volts
5 - 15 volts
-2000 volts
Gauge Operating Ranges
10-12 10-10 10-8 10-6 10-4 10-2 1 10+2
P (mbar)
Rough VacuumHigh VacuumUltra High Vacuum
Bourdon Gauge
Thermocouple Gauge
Cold Cathode Gauge
Capacitance Manometer
Hot Fil. Ion Gauge
Residual Gas Analyzer
Pirani Gauge
Spinning Rotor Gauge
McLeod Gauge
Vacuum gauges must calibrated by
• Comparison with absolute standard calibrated from its own physical properties.
• Attachment to calibrated vacuum system.
• Comparison with calibrated reference
gauge.
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