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METHODS FOR IN SITU QMS CALIBRATION FOR PARTIAL PRESSURE AND COMPOSITIONAL ANALYSIS
Robert E. Ellefson REVac Consulting
Dayton, OH 45459 USA [email protected]
ERMP IND12 – QMS Workshop Bled, Slovenia
April 10-13, 2012
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What do Industrial & R/D Users
Need and Expect?
• Partial Pressures or Composition of Process Gas
• Expect Stable Operation to Trust the Data
– How do they Know it is Stable? Accurate?
• Process Control practices typically call for a Verification of Performance
– Test Stand Calibration of RGA may Qualify the MS
– In Situ Measurement of Reference Mixture is needed to provide Verification that the RGA is “In Control”
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Define a MS Measurement and Calibration Method
for a Process or Vacuum System
Logic
Plan:
Application
Sampling
Design
Select and
Qualify MS
Calibration
Design
Quality
Assurance
CVD, Etch, ALD, Degas
0.01 mb < PProcess< 100 mb
Vis., Trans. or Molecular
Process Gas Species(Any Condensibles?)
Viscous, Transition or
Molecular Flow [PProcess ]
Orifice / Small Tube toMS
(Heated Inlet and QMS)
Closed Ion Source (CIS)
Low eV: e.g. 40 eV
Low Emission: e.g. 0.2 mA
Mass Range for Species
Detector: FC & EM(?)
Demo: Linearity, S vs t, A-B
Std Mix: Process Species
Reservoir: PV for 1 year
Cal Flow Std: 1x10-4
mb-l/s
Flow Mix into CIS
Use Std Mix for QA Data
Log Data & Plot PPi vs Time
Plot Xi =PPi / Sum PPi vs Time
Establish Action Limits for Sens or Gain
Atmospheric Processes
100 mb < PProcess < 2 bar
Viscous Flow
Process Gas Species(Any Condensibles?)
Viscous Flow
Capillary Sampling with
Sample Pump and
Molecular Leak to MS
(Heated Inlet and QMS)
Std Mix: Process Species
Volume: 5-10 L@ 1 Atm
Capillary Sampling of
Standard Mixture
Vacuum: PVD- UHV -XHV
PProcess < 0.01 mb
Molecular Flow
Process Gas Species(Any Condensibles?)
Molecular Flow
Direct Insertion of QMS
(Isolation Valve)
Bakeout Provision
Open Ion Source (CIS)
High eV: 70 - 100 eV
High Emission: e.g. 2 mA
Mass Range for Species
Detector: FC & EM(?)
Demo: Linearity, S vs t, A-B
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RGA Conditions for Accurate Partial Pressure and Compositional Analysis begins in the Ion Source
• Vacuum System Base Pressure PP Measurements Requires Sensitivity
– OIS: 70 – 110 eV e-; 1-2 mA Emission
• Process Gas Composition allows Lower Sensitivity operation which minimizes Fragmentation
– CIS: 35-40 eV e-; 200 μA Emission
• Ion Extraction, Focus and Ion Energy Potentials affect Space Charge, Ion Residence Times and Linearity
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Evidence of Gas Scattering Loss at Higher Pressure is seen as Ions Traverse the Mass Analyzer to the Detector.
XPR Correction for Gas Scattering is I = Imeas eKP ; Gives PPi’s
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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
Note for a given ie the well gets deeper
for lower Ve (slower electrons; higher ρe)
R Ellefson and M Vollero, AVS-57, 2010
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The Same Open Ion Source can be Linear or Non-Linear depending on Operating Potentials
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The Ar and O2 sensitivity decreases in OIS with time due to O2 Oxidation of Ion Source Grids changing surface potentials.
However Ratio O2 /[O2 +Ar] remains Nearly Constant
Data provided by Wm Sproul
0.0
1.0
2.0
3.0
4.0
5.0
0.0 50.0 100.0 150.0 200.0 250.0
Time, minutes
O2 a
nd
Ar
Pa
rtia
l P
res
su
res
an
d
(O2/O
2+
Ar)
*10
0 R
ati
o
Ratio = 0.0348 + 0.4%
O2+ (E-10 A)
Ar+ (E-9 A)
• Conditioning • RGA exposed to pure Ar
for many hours Reduces the Surface Oxide:
H(Diff) + OH(S) H2O
• Exposure to Ar +3% O2
changes the sensitivity for both Ar and O2 by surface re-oxidation with a
time constant of ~ 2hrs.
• Practical Solution for PVD Process Control:
PP (O2) = R(O2)*P(CDG)
Where R (O2) = O2 /[O2 +Ar]
Recommended Cal Reference: 3% O2 in Ar
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Examples of In Situ RGA Calibration Methods
• In Situ Calibration for an Open Ion Source (OIS) RGA
• In Situ Calibration for a Closed Ion Source (CIS) RGA
– System Calibration using a Fixed Reference Supply with Sampling equivalent to the Process Sampling
– Portable Calibration Reference Source introduced directly into the CIS
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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.
• Mixture Composition chosen for Application
• Vacuum System must tolerate the Qcal Flow Rate
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For UHV and XHV Systems, a Lower Calibration Pressure can be created with a Q vs PFill Calibration
The Plot shows Flow Rate, Q; The Pressure generated in the example is ~ Q/10. PFill can be adjusted without altering Composition using the Gas Pipettes. Suggested UHV Composition: 90% H2; 9% CO; 1% CO2
Q = A Pfill2
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A Two-Stage Pressure Reduction with multiple probe types enables sampling at a representative
point over a wide range of process pressures
• Process Gas Species in CIS reflect Process: Molecular Flow into CIS; Molecular Flow out • Calibration Reference Mix Species are Altered by Mass:
Viscous Flow into CIS; Molecular Flow out (Addressed Later)
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Sensitivity Monitoring Data shows Shifts
in Sensitivity Related to Process Changes
MASS(40) Sensitivity vs Daysfor CIS2 [W Filament; 40eV/200uA]
Reference Gas: VTI Flow Std with INFICON Mix
2.0E-04
2.2E-04
2.4E-04
2.6E-04
2.8E-04
3.0E-04
3.2E-04
3.4E-04
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00
Time (Days)
Ar-
40
Se
ns
itiv
ity
(A
/To
rr)
Event: Accidental Air Venting of
CIS2 Vacuum due to Power Failure
+ 4.8% 2-Sigma
+ 5.5% 2-Sigma
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Tritium Fume Hood
Process Capillaries
Tritium Glove Box
Tritium Glove Box
Standards Capillaries
D/TPump
V
T
T
P
D/TPump
D/T/He - 1
D/T/He - 4
D/T/He - 2
D/T/He - 3
30 – 60 m
CDG
3 L
Magnetic SectorMass
Spectrometer
Process Room Analytical Lab
<< 1 mbSample
Pressure
Remote Process Monitoring: D/T/He Gas Mixture Compositional Measurements
Sampling Capillaries 30 – 60 m long Capillaries to Standard Mixtures (Similar) Capillary Sample Purged 3 – 5 Capillary Vol’s ~ 0.1 mb sample P in 3 L volume
Species deplete through Molecular Leak Record time for peak for t=0 Composition Analysis Time: ~ 20 minutes
Qualify MS with Std Mix each AM + Run Time Enables < 0.5% relative accuracy for D/T/He-3
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The D/T/He-3 mixtures were used to Assess Accuracy and do Daily Qualification of the MS’s
• A daily routine for operators was to run an analysis of a known mixture to check performance
• Call Customers “We are ‘up’ today” so they could run a Test, etc.
• If a MS was “Out of Control”, a re-run was done to note permanent shifts in calibration (due to power failures, etc)
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RGA Calibration Source
•Fixed Flow Rate Gas Source •Pumping by Process through a Fixed Conductance (~ 10 L/s)
Process Monitor
Calibration Gas Source
•Fixed Flow Rate Gas Source •Pumping by Monitor Vacuum System through the Closed Ion Source (~ 1 L/s)
Can we define a Multi-Application,
Portable InSitu Calibration Gas Source?
2x10 -5 mbar
Vacuum Process
2x10 -7 mbar
Q=2x10 -4 mbar-l/s
Process
Q=2x10-4 mbar-l/s
2x10-4 mbar
1x10-5 mbar
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Portable Calibration Sources have
been used for many years
• Crimped-Capillary Leak Standards (Boeckmann, et al.)
• VTI Positive Shut off Flow Standards (Traceable to NIST) • Sandia Flow Standards [Chamberlin, et al. JVST A7 (1989)]
• High accuracy primary flow references (Traceable to NIST)
INFICON has used their compact Reference Gas Source
for InSitu Calibration of OIS and CIS products for over
10 years
• SS Frit Flow Element For Stable Flow
• Low dead volume valve to minimize turn-on bursts
• Measured Flow Rate stated on Label (+ 10% of Rate)
• Predictable Flow Rate (Depletion < 10%/Yr normal use) • Gas mixture composition is constant (viscous flow)
• Gas fill Pressure < 2.8 bar (non-compressed gas for simple shipping)
• Flow Rate can be independently Certified at a Standards Lab
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INFICON Calibration Reference Source
• 4-VCR Male Connection • Air Operated Valve • 38mm- (+VCR) x 270mm • 132 cc Volume
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Flow Through Element goes as Pfill
2
RGA Calibration Pressure vs Fill Pressure of Reference
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
1.0E-05
1.2E-05
1.4E-05
1.6E-05
0 0.5 1 1.5 2 2.5 3
Fill Pressure (Ar) [bar]
RG
A C
alib
rati
on
Pre
ss
ure
: Q
(P
) / C
[m
bar]
PP(Ar) = Q(P) / C
Q(P) / C = A P2
Vacuum Process
Q=2x10 -4 mbar-l/s
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RGA Calibration Pressure Depletion is
Predictable with Time of Flow
0.0E+00
5.0E-06
1.0E-05
1.5E-05
2.0E-05
2.5E-05
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Elapsed Flow Time (Days)
PC
al[A
r-4
0]
(mb
ar)
Model for Depletion of
Calibration Reference
Pressure:
PCal(t) = Pcal(0) / [1+Qo t/PoV]
Qo = 2.8x10-4
T-L/s
Fill Parameters:
Po = 2053 Torr [2.7 bar]
V = 0.135 L
PCal[Ar-40] (t)
Expected 10% Depletion in 1 year of automated use
(5 minutes of flow/day )
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Composition Ratio of 50/50 He/Ar Mix does not change with
30% Depletion of Fill Gas
Ratio 40/4 = -0.0041 t + 1.1627
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
0.00 2.00 4.00 6.00 8.00 10.00 12.00
Time (Days)
Ion
Cu
rre
nt
(A -
Co
rre
cte
d t
o R
ef
mix
) &
Ra
tio
40
/4
Flow Rate Depletion: Ar-40
(Data Sensitivity Corrected)
Flow Rate Depletion: He-4
(Data Sensitivity Corrected)
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Pcal produced from Cal Reference Source changes
with Ambient Temperature by + 0.15 %/oC
4.06E-09
4.08E-09
4.10E-09
4.12E-09
4.14E-09
4.16E-09
4.18E-09
4.20E-09
4.22E-09
4.24E-09
4.26E-09
20 25 30 35 40 45 50
Average Temperature [Valve & Tank] (C)
Ion
Cu
rre
nt [A
r-40
] (A
) fr
om
PC
al
Temperature Coefficient of Flow Rate
0.15% / oC
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INFICON Gas Mixture options allow
choice of gas pumped by the process
Component Argon (99.995%) Ar with 5% Impurities PVD Mix
Argon 99.995 % 95 % Balance
H2 - 1.0 % 200 ppm
He - 1.0 % 1000 ppm
N2 - 1.0 % 50 ppm
CO2 20 ppm
Kr - 1.0% 1000 ppm
Xe - 1.0 % 1000 ppm
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Composition of the Gas Mixture in the Ionizer is
altered by the RGA’s molecular flow pumping
• 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 ]
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Composition of Mixture at the MS ionizer shows
depleted light gases and enhance heavy gases
CEM 1:1:1 Ar-He-N2 Mix
Component Tank Mix Xi -Ion Source
Ar 33.33 46.4492392
He 33.33 14.6885391
N2 33.33 38.8622217
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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
++
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Mass Scale Calibration with
Calibration Reference Source
H2 & He 36Ar & 38Ar Kr
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Electron Multiplier Gain Adjustment
• Adjust RGA EM (HV) to give Ion Current Level:
• e.g.: PCal = 2x10-5 mbar
SFC = 2x10-4 A/mbar
Gain = 200
Species Abundance Ion current (EM)
Ar-36 3370 ppm 8.0x10-7 A
Ar-38 630 ppm 1.5x10-7 A
Ar-40 99.6% Off Scale
1% N2 1% 2.4x10-7 A
• Measure Gain of the EM at a value of High Voltage
Gain = I(Ar-38: EM)
I(Ar-38: FC)
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Sensitivity and Gain can be quickly measured with sequential
FC/EM scans with Selected Ion Monitoring of Ar species.
1.0E-13
1.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
00:00.0 00:08.6 00:17.3 00:25.9 00:34.6 00:43.2 00:51.8 01:00.5 01:09.1 01:17.8 01:26.4
Time (m:s)
Ion
Cu
rre
nt
(A)
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
CIS
To
tal
Pre
ss
ure
(m
b)
Ar-40 EM (Off Scale)
FC FCEM
Pressure: Calibration Reference ON
Gain = 445 + 8
Gain = 443 + 38
S(Ar-40) = 1.1E-5 A/mb
Ar-36 FC
Ar-38 FC
Ar-40 FC
Ar-38 EM
Ar-36 EM
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Sensitivity Monitoring Data shows Shifts
in Sensitivity Related to the Sensor
MASS(40) Sensitivity Vs TimeFor CIS2 (W filament; 40eV/200uA)
Reference Gas: Inficon/VTI Calibration Standard
2.20E-04
2.25E-04
2.30E-04
2.35E-04
2.40E-04
2.45E-04
2.50E-04
2.55E-04
2.60E-04
0 2 4 6 8 10 12 14 16
Time (Days)
Ar-
40
Se
ns
itiv
ity
(A
mp
/To
rr)
2-Sigma Deviation 5% of Average over 15 days
5% Shift in Average Sensitivity on Day 7
2-Sigma: 2% over 7 days
2-Sigma: 3% over 8 days
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Composition Calculations: Accurate Compositional Analysis is the Goal for Process Monitoring
• QMS measures the Ion Current vs Time for the Gas being sampled
• Partial Pressures of Species Present are Calculated Ion Currents with Corrections for Interferences, and Sensitivities
• Mol-% Composition is defined as
Xi(mol-%) = 100 * PPi / ∑ PPj
For Quality Assurance, Accuracy is measured with known mixtures:
ΔXi = Xi - Xi (Std) [for Major Components]
TMPTMP
DragDrag
8 CapillariesPprocess = 0.5 – 2 Atm
i.d. = 0.25 mmL = 1.5 m
Diaphragm Pump
35μ Leak CIS 100 amu QMS w FC
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Ion Currents vs Time from 2 Gas Streams
How do we relate Ion Current to Partial Pressure?
Calibration
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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
• 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. [Ar++]
• IG Sensitivity ratios to N2 are at best a guess for RGAs
• Sensitivity Measurements are Required for Species of interest for a RGA
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Partial Pressures are Calculated from Ion Current in Real Time using a Previous Calibration
Partial Pressure = Sens Factor * Trans(M) * [Ion Current – Interferences]
PPi = SFi * MZ * [ Ii - Σk≠i Aik * Ik ]
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The Result of the Partial Pressure Calculation is Real Time Display of the Partial Pressures and Clearing Times when Gas Comp Changes
4m-% H2 in Ar 4m-% H2 in Ar Air
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More informative is the Composition of the Gas Species showing Stable Composition after Conditioning the Inlet System
5 Min Elapsed Time Composition Table at the Time with the Yellow Cursor
4m-% H2 in Ar 4m-% H2 in Ar Air
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“Safe” Hydrogen [4 mol-% H2 in Ar] is a Low-Cost Reference Mixture
• Initial Calibration for H2 was done with 2.0% H2 in Synthetic Air (N2 & O2) in August 2011 • Data for 4% H2 in Ar (at right) began a month later. Data shown are values from a single scan sampled at random during the sampling of the 4 mol-% H2 in Ar from the Trend Data over 12 days • The Mean Value for H2 over 12 days is 3.88 + 0.34 Mol-%
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Air Components Altered by Seasons and Breathing
Sampling Room Air is a Convenient Reference Gas
• Capillary Sampling and CIS Analysis of Room Air over 160 Days
• Calibration for this data done in August 2011; No Sensitivity Drift Trend
• Mean Value of N2, O2, Ar is within 1 σ
• Data displayed is a randomly selected, single measurement value of Xi
• O2 and Ar have σ ~ 2 %-RSD
• H2O and CO2 are Biased due to Seasonal ΔRH and Breathing in Lab
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Now to Summarize…
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In Situ Calibration by Two Methods Provides Accurate
Partial Pressures and/or Composition to a RGA / IG
• Calibrated Flow Reference Gas Mixtures Deliver Known PPi’s and Xi’s to the Ionization Region
• Method of Introduction is Designed for a Particular Application
• Regular Measurements for PPi’s and Xi’s Plotted over Time Reveals Accuracy, Drift Trends, Time for Recalibration and Data for Sensitivity or Gain Changes
• Flow Standards should be designed to last > 1 year or Flow Rate Re-Calculated Based on Time Measured Use
XHV UHV Low Pressure Processes Atmospheric Processes
Tritium Fume Hood
Process Capillaries
Tritium Glove Box
Tritium Glove Box
Standards Capillaries
D/TPump
V
T
T
P
D/TPump
D/T/He - 1
D/T/He - 4
D/T/He - 2
D/T/He - 3
30 – 60 m
CDG
3 L
Magnetic SectorMass
Spectrometer
Process Room Analytical Lab
<< 1 mbSample
Pressure
• A Tank of Reference Gas is Sampled by the Same Means as Process Gas
• Certified Gas Mixture should be Pertinent to the Process
• Do QA Measures and Plot Difference Results
• Reference Gas Supply should last > 1 Year in planned useage
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Summary: Operation of RGAs for Analytical Measurements
• Choose Ion Source potentials that produce linear stable operation – Electron Energy, Emission Current, Extraction/Focus Voltages and Ion Energy
• Choose a Calibration Reference Gas for Calibration Verification (QA) – For UHV/XHV: Consider a Viscous Leak with Mix Pressure defining Flow Rate.
Needs Flow Rate vs Pmix Calibration + Conductance (Calc/Measure); or Total P @ RGA
– For UHV: Fixed Flow Rate Calibration Reference Mix (INFICON or VTI) + Conductance @ OIS
– 0.01 mb<Pprocess<100 mbar: Calibration Reference Mix into CIS; Conductance of CIS
– In each application, calculate Composition in Ion Source given Viscous in – Molecular Out
– For Atm Processes: Tank with Gas Mix representing the Process; Sample with Capillary [In this case, the composition in Ion Source is the same as Tank due to Mol. In - Mol. Out]
• Calculate Sensitivities from Partial Pressures at Ion Source from Cal Gases – Initial Calibration requires known gases in the list of Species to be analyzed.
– Determine Fragmentation Factors during these initial calibrations
– If a gas species not available, estimates of the Sens can come from Gauge Constants or σ(E)
– Choosing a model for Ion Transmission, e.g. T(M) = Mz where 0 < Z < 1 to make SF ≈ σ(E)
• For High Ion Source Pressure, Xi -mol-% more stable than PPi for species
• QA: Plot differences , Xi –Xi(Std) vs Time to monitor when to Re-Calibrate
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Acknowledgements
• INFICON RGA Engineers (Developed the Calibration Reference Source)
– Tim Karandy, Michael Vollero, Peter Schubert, Ken Rosys, Lou Frees
• Mound Technical Solutions, Inc (MTS) – Doug McClelland, Steve Huff
• AVS/IUVSTA Colleagues
• EMRP IND12 Chairs for the Invitation to Speak – Janez Šetina and Karl Jousten
And to All of You for Listening:
Hvala (Thank You)