6 Vacuum Gauges

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 guages Mechanical: liquid column, diaphragm, etc Real 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

A C=ε d

pressure Absolute pressure reference to a vacuum cell

Full range: 0.02 torr, 1 torr, 1000 torr,

real species independent

10000 torr

linear, accurate

Accuracy ~ 0.1 %

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 WG

– Four possible mechanisms to take heat away from sensing Heating current 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 WC

1/2 WC

WCNV

Thermocouple joint

WR

Thermal conductivity gauges mechanisms 1-Radiative heat transfer transfer:

(

WR = ε1 (2π r1 L )σ T − T T2 T1 L

4 1

4 2

r1

)

• Conduction along the wire

1 dT 2 WC = π r1 kwire 2 dl

T1

T2 L

• Conduction through the residual gas: 1/2

 γ + 1  R'  WG = 2π r L Fα  P (T1 − T2 )    γ − 1  8π MT2  as properties (M, γ, α) determine variation in sensor response to pressure

(

2 1

)

R`: wire resistance, P: pressure , L, r1: Wire length and diameter, M, γ, Fα: Gas properties. T1 and T2 Wire and gas temperature respectively

Thermal Conductivity Gauges • • • •

Vbr2 WT = WG + WR + Wc = 4RS

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 Ω WR+WC =8.75x104 W Subtracting WR+WC from WT yields WG for N2 as shown below:

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 Measurment Constant 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 Gauges Gauge 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+ + 2eI+ = 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

+ve Ion Collector

Heated Filament Grid

A.C. Emission Current

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

σ LA ⋅ n

– Ion collision cross section given by • σ = cross section per molecule • n = molecule density = P/(kT) • L = length of ionizing space • A = cross sectional area of electron beam

HCG(6) Ion current – Ion current is then:

P = Nq ⋅ σ LA kT • N = arrival rate of electrons • q = charge per electron – With Nq = ie, we have i+

= P K ie + ir,

– Gauge sensitivity S = K ie

σ LA K= kT

– Typical range for K: 0.02 Pa-1 to 0.2 Pa-1 (bigger is better)

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 selfsustaining 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:

ig = KP n ; n = 1.05 − 1.2

•

Typical anode voltage +3 kV • Typical current 0.1 mA

Pressure Measurement 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)

Ionization of Gas

RGA: A simple mass spectrometer

Ion Gauge: Sensitive to gas composition, but a good choice for base pressures

Vacuum Gauge Selection adapted from Lesker.com

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)

Analyzer: Quadrupole Mass Filter Ion source: Tungsten filament Gas inlet

Detector: Faraday/ Channeltron

Gas outlet

Diaphragm Vacuum Pump

Turbomolecular Drag Pump

Mass Spectrometer Basics Vacuum Requirements Ion Creation Ion Filtering Ion Detection

Mass Spectrometer Basics Ion Source

e-

Quadrupole Mass Filter

Ion Detector

Vacuum Requirements e-

• Filament Longevity H

H

 Ion Mobility  Detector Operation

Typical Vacuum ~ 1 x 10-6 Torr Gas Density ~ 1010 molecules /cm3 (@ 760 Torr (1 atm) ~ 1019 molecules /cm3 )

+

The Atomic Model 12

C = 12 A.M.U. = Proton ~ 1AMU , +1e charge = Neutron ~ 1 AMU , no charge = Electron ~ 0 AMU, -1e charge

1 AMU = 1.66 X 10 -27 kg

The Periodic Table

Ionization e-

Ion

Atom 12

C + 1e-

12

C+ + 2e-

Ionization Source - Open Gas Atom Electron Gas Ion

Ionization Source - Closed or Gas Tight Filaments Neutral Gas Atom/Molecule Electron Ion

Gas In

Ions Out

10-05

10-04

10-03

Pressure (mBar)

10

Electron Emission The emission of electrons from the filament is carefully controlled to provide consistent ion formation. IF eIon Formation Camber

IE

SET

IF =Filament Current ~ 3 Amps d.c. IE =Electron (emission) Current ~ 1 mAmp d.c.

Ionization m/e = 6

Double Ionization 1 e- +

12

12

C

C++ + 3e-

Fragmentation - Ionization

1 e- +

18

H2O

1 e- +

17

OH

1 e- +

17

OH

17

OH+ +2 e-

m/e = 17

Ionization Products Ionization can produce more than one ion type. E.g. Argon ,

Ar + 1e-

40

Ar

40

Ar+ + 2e-

40

(m/e = 40)

Ar2+ + 3e-

(m/e = 20)

Ar3+ + 4e-

(m/e = 13.3)

40 40

Ionization - Isotopes Argon Isotopic Masses P=Proton, N=Neutron

-1e

18P+20N=38AMU

-1e

18P+18N=36AMU

-1e

Log (Intensity)

18P+22N=40AMU

M/Z

Ionization Products – UHP Argon Mass scan of ultra pure argon - showing singly and doubly charged isotope peaks

Ionization Products Molecules can fragment in the ionization process. ABC + e-

ABC+ + 2eAB+C+ + 2eAB+ + C + 2eA + BC+ + 2eA+ + BC + 2eAC+ + B + 2eAC + 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 most intense ions

Natural Abundances 18

O = 0.2%

13

C = 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 y

x

ION SOURCE

QUADRUPOLE ROD

+ + + + +

QUADRUPOLE ROD

+

+

++

z

e-

+ +

QUADRUPOLE ROD SEM

Lower m/e ion - deflected in y-axis

+

Higher m/e ion - deflected in z-axis Selected m/e ion - reaches detector

Ii

Resolution 10

Valley height

1 1 AMU

Over Resolution

1 AMU

10% Valley Resolution

1 AMU

Under Resolution

Ion Detectors Faraday Ii 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 MASS FILTER

= Selected ion - positive chargeattracted into SEM by -ve dc volts.

GAIN ~ 100 106 set by SEM VOLTS

e-

SEM VOLTS ~ - 1500V dc

Ii Can be destroyed by high currents >10-5 Amps , or by operation at high pressure.

e-

I i ~ 10-14....10-5 A

SEM Detector – Discrete Dynode Deflection Unit Faraday signal

MASS FILTER

Vdef

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 diagnostics and 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 mass peak is found - this reduces the data for simplicity. This mode is mainly used for scanning for unknown components.

Data acquisition modes - MID, MCD MID = Multiple Ion Current Determination. Ion current vs. time MCD = Multiple Concentration Determination. Concentration vs. time NOTE: Requires calibration gas mixes MID/MDC - here the masses to be monitored are known and the system will only report back these points. It is the fastest mode and very useful for trending data over time

Residual Gas Analyzer

QUADRUPOLE HEAD

CONTROL UNIT

How the RGA works

(A)

RELATIVE INTENSITY

RGA SPECTRUM

H2 O H2

NORMAL (UNBAKED) SYSTEM

N2,, CO CO2 MASS NUMBER (A.M.U.)

(B)

RELATIVE INTENSITY

RGA SPECTRUM N2 SYSTEM WITH AIR LEAK H2 O O2 H2

CO2 MASS NUMBER (A.M.U.)

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

•

5­15 V quadrupole, 100­1000 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

Relative Intensity

• 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

M/q

•

Mass Analyzers

RF Quadrupole – Two pairs of rods, one (+) pair, one (-) pair – DC plus AC(rf) potentials applied to all rods: • (+) pair: U + V cos ω t • (-) pair: −(U + V cos ω t) – (+) 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

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 peak width at 10% height – 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

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: S’n = 5x10­6 A/Pa • 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

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 n G = G1G2 Gain # electrons generated by ion impact – G1 = # electrons generated by electron impact at each stage

– 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-ofphase secondary electrons

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 peak Pc

=

I peakVc PiVi

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 70 volts – Ion detector inlet and ground 5 ­ 15 volts ­2000 volts

Gauge Operating Ranges Ultra High Vacuum

Rough Vacuum

High Vacuum

Bourdon Gauge Capacitance Manometer Thermocouple Gauge Pirani Gauge Hot Fil. Ion Gauge Cold Cathode Gauge Residual Gas Analyzer McLeod Gauge Spinning Rotor Gauge

10-12

10-10

10-8

10-6 10-4 P (mbar)

10-2

1

10+2

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.