Partial Pressure Analysis for Large Vacuum Systems

Robert E. Ellefson

REVac Consulting

Dayton OH 45459 USA

OLAV IV

NSRRC

Hsinchu, Taiwan

4 April, 2014

REVac Consulting Ph: 1 937 435 2559 Cell: 1 315 247 3679

E-mail: Robert.Ellefson@sbcglobal.net

Outline • Partial Presssure Measurement Systems vs Pressure

– Focus on UHV/XHV Pressures

• Residual Gas Analyzers (RGA) [QMS and Ion Trap]

– Ion Formation / Filaments / Spectral Artifacts

– Ion Transmission

– Ion Detection

• Calibration of RGAs

– Initial Calibration

– In Situ Calibration / Verification

– Traceability to National Measurement Standards

• How to Qualify a RGA for UHV/XHV

– Cleanliness/Outgassing Properties

– Some Ideas for Discussion

There are Many Analytical Tools Designed for Processes

Two Types of RGAs are available for

UHV/XHV Measurements

Quadrupole Mass Spectrometer

Ion Source Operation

Filament: 70 eV / 1-2 mA

Ion Energy = Vanode – Vaxis = 8 eV

Mass Filter: Quadrupole with

VRF/VDC = Constant for ΔM = 1

Ion Detection: Faraday Plate

or Electron Multiplier

Auto-Resonant Ion Trap Mass Spectrometer

Ion Source Operation

100 eV/ 0.070 mA

Mass Filter:

Ion Trap

Ion Detection:

Electron Multiplier

An additional SS Surface is the Vacuum Housing provided for the RGA

All Ions (and Problems) begin in the Ion Source…

Ii+ = ie σi A de (Pi/kT) Ex T(M) D(M)

Ion Current = e Current, Cross Section, Ion Volume, Density, Extraction, Transmit, Detection

Filament: Y2O3/Ir or Re (Oxygen Cmpd); W for H & UHV

ie as small as practical for application

A de Ion Formation Volume: “Large” to increase sensitivity

Ex Ion Extraction and Coupling to Mass Analyzer

T(M) Transmission Factor as a function of Mass

D(M) Ion Detection Efficiency [For Electron Multiplier (EM)]

For an QMS, the result of the Mass Analysis is ~ 10% of

Ions Formed are Mass Separated and Detected

* Tungsten Filaments Survive in Well-Managed Vacuum Systems.

* 3% Re in W Does not Warp with use (Change filament position).

* New Y/Re Alloy (SIS™) Filaments are O2 Tolerant and Don’t Warp.

Power Failure Venting Day 26

Erodes W Filament

Tfil goes Up; SAr Goes Down

W Filament on CIS

The Work Function for Yttria (2.8 eV) Requires Less Power

than W (5 eV) and Operates at Lower Temperature

Y2O3/Ir: (2.8 eV) ~ 1650 C 1 mA

• Less Thermal Outgassing

•Surface Area (Ceramic)

•Prep Method Important

Re: (4.7 eV) ~ 2100 C @ 1 mA

•Pure Re Warps; Alloy Better

•Evaporation Limits Lifetime

•Not a Good Choice for UHV

W: (4.3-5.2 eV) ~ 2100 C

@ 1 mA

•Can Warp; W(1% Re) Good

•Carbides and Oxides

Replacement Filaments at

www.sisweb.com/filaments Curves from Scientific Instrument Services website

Artifacts that Appear in a Mass Spectrum come

from Multiple Origins

A Specification for Outgassing of RGA is Needed to Minimize the Following Occurances:

• Electron Stimulated Desorption (Surfaces near Ion Source) – O+, F+,Cl+, (H3O+ ? with H2)

– What is the Cleaning History? Freon Cleaning or Dusting?

• Thermal Desorption (~ 5 W @ 2 mA Emission)

Physisorbed and Chemisorbed H2O, HCs, CO2

Minimize SS Surfaces within RGA

Filament Reactions (@ 1650 to 2100 C):

CxHy + H2O or O2 CO + CO2 (+ CH4 ?)

WC + H2 CH4 and WC + H2O CO + CO2

2 Y2O3 + H2 2 Y2O2 + O2 Reduction to Sub-Oxide; Chronic in UHV

How do you reduce Artifacts in a Mass Spectrum ?

• Lower Power to Filament – Lower Emission Current : 500 uA or 1 mA instead of 2 mA

– Smaller Diameter Filament Wire [0.003” (2 W) rather than 0.005” (5 W)]

– Conduct away Heat from Filament: Watanabe Source (WatMass)

• Avoid C or HC Contaminants on a W filament or within an Y2O3 or ThO2 Coating – UHV: Reduce WC with Pure H2 exposure as “CleanUp” or

“Conditioning”

– HV: Reduce/Sinter Y2O3 or ThO2 Coating with H2 after Electrophoresis rather than stabilize with methyl methacrylate or other binder

• Consider a Cold Electron Emitter (with External heating to Degas)

– CNT have a Large Surface Area to Degas

– Spindt Diode type might work better at UHV (A more open structure)

– Graphene on Metal Substrate (Ir?)

The Potential Well formed by the Ionizing Electron Beam Lengthens Ion Residence Time (Longer Path) in Ion Source

CIS equipotentials

1V well

(SIMION)

10V

70V

75V

79V

79.9V

E-Beam Well Depth For the Geometry of this CIS:

Vwell = - 3800 ie / (Ve)1/2

Vwell(40eV/200uA) = - 0.12 V

Vwell(70eV/2000uA) = - 0.90V

R Ellefson and M Vollero, AVS-57,

2010

UHV/XHV RGA Ion Source is similar to the

Successful Extractor Gauge

UHV/XHV Extractor Gauge UHV/XHV RGA

Increase Anode Diameter for large ion volume to increase Sensitivity to ~ 5E-4 A/Torr Ions Extracted from Source to a detector to minimize X-Ray (false) ion currents. W filament with 1 mA Emission minimizes Heating of low surface area ion source. Pt Anode Grid to minimize ESD (Optional) Focus Plate to improve ion focus, transmission and detection

Anode (100 V) [Pt or Ir Grid]

Filament (50-90 eV) [W]

Focus Plate (~35 V)

Ion Source Exit(0 V)/Quad Ion Entrance

Molecule e-Cross Sections at http://physics.nist.gov/PhysRefData/Ionization/molTable.html

Atom e-Cross Sections inferred from Wutz Handbuch Vacuumtechnik Ed 9 - 2006, Bild 12.42

Ionization Cross Sections(Å2) differ substantially with e- Energy

• Ion Gauges use 150 eV

• INFICON OIS uses 105 eV

• VQM Ion Trap uses 100 eV

• Most OIS and CIS’s use

70 eV for high Sensitivity

• Some OIS and CIS use 40 eV for reducing Fragmentation and Multi-Charge peaks e.g. [36Ar++ at M/e = 18]

• IG Sensitivity ratios to N2 are at best a guess for RGAs

• For Accurate Partial Pressures, Sensitivity Measurements are Required for Species of interest for a RGA

Relative Cross Sections vs eV σrel(40eV) σrel(70eV) σrel(100eV)σrel(150eV)

Ar 2.11 1.56 1.42 1.35

O2 0.94 1.00 1.00 1.04

N2 1.00 1.00 1.00 1.00

H2 0.50 0.40 0.37 0.33

He 0.06 0.12 0.15 0.14

The Same Open Ion Source can be Linear or Non-Linear depending on Operating Potentials

Mass Dependence of Ion Transmission for

QMS and ARTMS is quite Different

TQMS(M) = Area(ΔM) / Area of Stable Trajectories (M)

TQMS(M) = K/Ma where a is 0.4 < a < 1 (By Calibration)

QMS Sensitivity is higher for low Mass Ions

QMS Ion Transmission ARTMS Ion Transmission

Resonant Ion Pumping ≈ fR

Ion Acceleration: VRF = 50 mV

Duration of Pumping ≈ 1 / fR

The result is Each Ion is Accelerated for the

Same number of RF Cycles .

So Ion Ejection is Mass Independent

and ARTMS Sensitivities reflect σe(E)

Pi = Xi ∙PGauge Pi = Ii / SQMS

Recent Data from Mark Pendleton, Daresbury comparing 2 QMS’s

and Ion Trap RGAs validates the previous modeling*

* www.rgausers.org/ Choose “Previous Meetings” RGA11

Courtesy of Granville-Phillips

FC EM

Anode 100 V

Filament 30 V

Focus 30 V

Ion Exit 0 V

Quad Axis 92 V The Electron Multiplier has a Mass Dependence too.

D(M) = α(M) ∙ Gain(VEM)

Where α(M) is Secondary

Electron Yield of Ion at EM

Entrance Surface and

Gain(VEM) is electron current

Gain for VEM applied

Partial Pressure Measurement Detection Limit could be

lowered by Increasing Ion Source Sensitivity and Ion Counting:

N(Ions/s) = 10-13 mb* 2x10-4A/mb = 2x10-17 A ~ 100 Ions/s

Analog Measurement (Electrometer) Data Ion Counting Projection

• EM with Gain =1000 improves S/N over FC by a factor of 100 • Detection Limit is lowered with longer measurement times: Selected Peaks at 1024 ms recommended

• Pulse Counting EM with Gain =10,000 generates ~ 5 nsec pulse/ion for counting • Detection Limit is limited by Dark Current at low P and counting dead time at high P • Available from Hiden on a UHV RGA

The Dynamic Range of VQM is limited by Ion Statistics

Comparison of RGA Features for QMS and Ion Trap

Feature QMS Ion Trap MS

Partial Pressure Measurement Direct: Pi = Ii/Si Pi = PTotal ∙ Xi

Partial Pressure Range 10-14<Pi<10-4 mb 10-14<Pi<10-6 mb

Dynamic Range at a Pressure 7 Decades 3 Decades

Linear Response ~ 10 % < 10-5 mb PTotal Response

MDPP [Noise (3σ) / Sensitivity] 10-13 mbar 5x10-14 mbar

2-50 AMU Scan Time 400 - 1000 ms 85 ms

10 Selected Peaks Scan Time > 100 ms 85 ms

Outgassing

Complicated

Structure

Simpler, Open

Structure

Sensor Bakeout Temperature 300 C (EM) 200 C (EM)

Radiation Protection Orientation Remote Electronics

RGA Calibration

Initial RGA Calibration can be done on a

Test Stand with Pure Gases *

Distributed RGAs can be used to determine

localized Leaks in an accelerator

But, Is it a Leak or Calibration Drift?

A Method for in situ RGA Species Calibration ** would clarify a real Leak from Cal Drift

* Malyshev OB, Middleman KJ. J Vac Sci Technol A 2008;26:1474-9.

** R.E. Ellefson / Vacuum (In Press) (2013) 1-10

In Situ Calibration: A Reference Pressure & Composition is established at the IG and RGA when

the Valve to the Gas Mixture is Open

• Pumping is provided by the Vacuum System

• A Calibrated Fixed-Flow Rate produces a Reproducible Pressure and Composition at IG and RGA Ionizers

• The Pressure is

Pcal = Qcal / Ccal

where CCal can be Calculated

from Geometry or Measured

• Mixture Composition chosen for Application

• Vacuum System must tolerate the Qcal Flow Rate

Mass Spectrum of a PVD Mixture

Calibration Reference Source

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

0 10 20 30 40 50 60 70 80 90 100

Mass

Ion

Cu

rre

nt

(A)

INFICON PVD Mixture

Calibration Reference Source Flow RATE 1x10-4 mbar-l/s @ 23.5 oC

Transpector CIS: 70eV/2000uA/EM

200 ppm

H 2 +

1000 ppm

He

Ar +

Xe ++

Kr +

40 Ar

++

Composition of the Gas Mixture in the Ionizer is

altered by the RGA’s molecular flow pumping

• To assure a >1 year Supply of Calibration Gas, Fill Pressure is 2.8 bar

•The Mixture is in viscous flow from the Calibration Reference Source.

The composition entering the vacuum chamber is the stated Mixture.

The partial flow rate qi of a species is

qi (in) = Xi (Ref) Qo (mbar-L/s)

• The partial flow rate out of the chamber depends on the mass of the

species, Mi and the pumping system conductance, CN2 at the ion

source:

qi (out) = PPi Ci = PPi CN2 [28 / Mi]1/2

But qi (in) = qi (out)

So at the ionizer: PPi (Ion Source) = [Mi / 28]1/2 Xi (Ref) Qo / CN2

From which a Sensitivity Factor can be calculated:

SFi (mb/A) = PPi (Ion Source) / [ Ii – Interference Contributions ]

Two examples of Tank Mixtures with Viscous Flow into

the Ionization Region and Molecular Flow out

Ar / 5% Impurities Ar / PPM Impurities

Component Tank Mix Xi -Ion Source Tank Mix Xi -Ion Source

Ar 95.00 95.34 99.6730 99.6312

H2 1.00 0.22 0.0200 0.0045

He 1.00 0.32 0.1000 0.0316

N2 1.00 0.84 0.0050 0.0042

CO2 0.00 0.00 0.0020 0.0021

Kr 1.00 1.45 0.1000 0.1449

Xe 1.00 1.82 0.1000 0.1816

An In Situ Calibration Method for UHV/XHV with Molecular

Flow Into Ionizer and Out Delivers the Tank Composition

R

G

A

Extractor

Ion

Gauge

UHV/XHV Vacuum System

Cal Mixture

~ 1 Bar

Ion-Getter

Pump

20

0 c

m3

UHV-Getter

Pump

CDG

1000

Pa

20 cm3

2 cm3

Leak

Bypass

Secure

Isolation

Valves

Controlled Leak Source is Removable for:

* Leak Calibration vs Gas Pressure (Local or NMI)

* Use as a Portable Flow Standard for in situ

Calibration of Multiple RGAs

The Plot shows Flow Rate, Q; The Pressure generated in the RGA/IG is ~ Q/10.

PFill can be adjusted without altering Composition using the Gas Pipettes.

Suggested UHV Composition: 90% H2; 9% CO; 1% CO2

Ellefson RE, Methods for in situ QMS calibration for partial pressure and composition analysis, Vacuum (2013),

http://dx.doi.org/10.1016/j.vacuum.2013.08.011

Establishing a Good Base Pressure is

Essential for UHV RGA

• Choose a Value of q

Outgassing Rate/cm2

• Estimate Surface Area

of RGA, A

• From Q = q A, divide

Q by Conductance to

get Base Pressure

Define a Test to Measure

Base Pressure and

P vs time

Summary

• Minimize Outgassing that raises local measured pressure – Bakeout Protocols for RGA Sensor

– Electron Source choices [Yttria or Tungsten]

– Minimize Material Outgassing by choices, operations and treatments

• Improve Ion Extraction and Transmission to a Detector

• Use Mass Analysis for direct measurement of Partial Pressures and rejection of ESD species

• Shield/Harden Detector Electronics from local Radiation (Orientation)

• Locate Support and Control Electronics away from the Radiation area to avoid failure of expensive electrical components

The RGA Needs of UHV/XHV Users could be presented

to RGA Manufacturers to develop a better RGA

• OLAV Members provide:

– Knowledge

– Have Cooperative Relations

– Multiple Test Facilities

– Motivated to define Next Generation RGA

• Consider:

– Defining a Specification(s) that would meet common needs of OLAV Users

[E.g. An Outgassing Specification and Acceptable Test Methods]

– Contact RGA Manufacturers to propose Partnership(s)

• What do Manufacturers Need to consider a Special Product?

– Market Predictions: How Many? When?

– Price Target vs Performance

Thank You for Your Attention