METHODS FOR IN SITU QMS CALIBRATION FOR PARTIAL PRESSURE AND COMPOSITIONAL ANALYSIS Robert E. Ellefson REVac Consulting Dayton, OH 45459 USA robert.ellefson@sbcglobal.net ERMP IND12 QMS Workshop Bled, Slovenia April 10-13, 2012
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
Logic Plan: Define a MS Measurement and Calibration Method for a Process or Vacuum System Application Vacuum: PVD- UHV -XHV P Process < 0.01 mb Molecular Flow Process Gas Species (Any Condensibles?) CVD, Etch, ALD, Degas 0.01 mb < P Process < 100 mb Vis., Trans. or Molecular Process Gas Species (Any Condensibles?) Atmospheric Processes 100 mb < P Process < 2 bar Viscous Flow Process Gas Species (Any Condensibles?) Sampling Design Molecular Flow Direct Insertion of QMS (Isolation Valve) Bakeout Provision Viscous, Transition or Molecular Flow [P Process ] Orifice / Small Tube toms (Heated Inlet and QMS) Viscous Flow Capillary Sampling with Sample Pump and Molecular Leak to MS (Heated Inlet and QMS) Select and Qualify MS Calibration Design 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 Std Mix: Process Species Reservoir: PV for 1 year Cal Flow Std: 1x10-4 mb-l/s Flow Mix into CIS 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 Volume: 5-10 L@ 1 Atm Capillary Sampling of Standard Mixture Quality Assurance Use Std Mix for QA Data Log Data & Plot PP i vs Time Plot X i =PP i / Sum PPi vs Time Establish Action Limits for Sens or Gain
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
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 = I meas e KP ; Gives PP i s
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: V well = - 3800 i e / (V e ) 1/2 V well (40eV/200uA) = - 0.12 V V well (70eV/2000uA) = - 0.90V Note for a given i e the well gets deeper for lower V e (slower electrons; higher ρ e ) R Ellefson and M Vollero, AVS-57, 2010
The Same Open Ion Source can be Linear or Non-Linear depending on Operating Potentials
O 2 and Ar Partial Pressures and (O 2 /O 2 +Ar)*100 Ratio The Ar and O 2 sensitivity decreases in OIS with time due to O 2 Oxidation of Ion Source Grids changing surface potentials. However Ratio O 2 /[O 2 +Ar] remains Nearly Constant Conditioning RGA exposed to pure Ar for many hours Reduces the Surface Oxide: H(Diff) + OH(S) H 2 O 5.0 4.0 Ar + (E-9 A) Ratio = 0.0348 + 0.4% Exposure to Ar +3% O 2 changes the sensitivity for both Ar and O 2 by surface re-oxidation with a time constant of ~ 2hrs. 3.0 2.0 O 2 + (E-10 A) Practical Solution for PVD Process Control: Where PP (O 2 ) = R(O 2 )*P(CDG) R (O 2 ) = O 2 /[O 2 +Ar] 1.0 0.0 0.0 50.0 100.0 150.0 200.0 250.0 Recommended Cal Reference: 3% O 2 in Ar Time, minutes Data provided by Wm Sproul
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
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 P cal = Q cal / C cal where C Cal can be Calculated from Geometry. Mixture Composition chosen for Application Vacuum System must tolerate the Q cal Flow Rate
For UHV and XHV Systems, a Lower Calibration Pressure can be created with a Q vs P Fill Calibration Q = A P fill 2 The Plot shows Flow Rate, Q; The Pressure generated in the example is ~ Q/10. P Fill can be adjusted without altering Composition using the Gas Pipettes. Suggested UHV Composition: 90% H 2 ; 9% CO; 1% CO 2
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)
Ar-40 Sensitivity (A/Torr) Sensitivity Monitoring Data shows Shifts in Sensitivity Related to Process Changes 3.4E-04 MASS(40) Sensitivity vs Days for CIS2 [W Filament; 40eV/200uA] Reference Gas: VTI Flow Std with INFICON Mix 3.2E-04 3.0E-04 + 4.8% 2-Sigma 2.8E-04 Event: Accidental Air Venting of CIS2 Vacuum due to Power Failure 2.6E-04 2.4E-04 2.2E-04 + 5.5% 2-Sigma 2.0E-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)
Remote Process Monitoring: D/T/He Gas Mixture Compositional Measurements Process Room 30 60 m Analytical Lab T P T V Process Capillaries D/T Pump Tritium Glove Box Standards Capillaries CDG << 1 mb Sample Pressure 3 L D/T Pump Tritium Fume Hood Magnetic Sector Mass Spectrometer D/T/He - 1 D/T/He - 2 Tritium Glove Box D/T/He - 3 D/T/He - 4 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
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)
Can we define a Multi-Application, Portable InSitu Calibration Gas Source? Vacuum Process 2x10-7 mbar Process Q=2x10-4 mbar-l/s 1x10-5 mbar Q=2x10-4 mbar-l/s 2x10-5 mbar 2x10-4 mbar 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)
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
INFICON Calibration Reference Source 4-VCR Male Connection Air Operated Valve 38mm- (+VCR) x 270mm 132 cc Volume
Flow Through Element goes as P fill 2 RGA Calibration Pressure vs Fill Pressure of Reference 1.6E-05 RGA Calibration Pressure: Q (P) / C [mbar] 1.4E-05 1.2E-05 1.0E-05 8.0E-06 6.0E-06 4.0E-06 2.0E-06 Vacuum Process Q=2x10-4 mbar-l/s PP(Ar) = Q(P) / C Q(P) / C = A P 2 0.0E+00 0 0.5 1 1.5 2 2.5 3 Fill Pressure (Ar) [bar]
P Cal [Ar-40] (mbar) RGA Calibration Pressure Depletion is Predictable with Time of Flow 2.5E-05 Expected 10% Depletion in 1 year of automated use (5 minutes of flow/day ) 2.0E-05 P Cal [Ar-40] (t) 1.5E-05 1.0E-05 5.0E-06 Model for Depletion of Calibration Reference Pressure: P Cal (t) = P cal (0) / [1+Q o t/p o V] Q o = 2.8x10-4 T-L/s Fill Parameters: P o = 2053 Torr [2.7 bar] V = 0.135 L 0.0E+00 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 Elapsed Flow Time (Days)
Ion Current (A - Corrected to Ref mix) & Ratio 40/ Composition Ratio of 50/50 He/Ar Mix does not change with 30% Depletion of Fill Gas 10.0 9.0 8.0 Flow Rate Depletion: Ar-40 (Data Sensitivity Corrected) 7.0 6.0 5.0 4.0 Flow Rate Depletion: He-4 (Data Sensitivity Corrected) 3.0 2.0 1.0 Ratio 40/4 = -0.0041 t + 1.1627 0.0 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Time (Days)
Pcal produced from Cal Reference Source changes with Ambient Temperature by + 0.15 %/ o C 4.26E-09 Ion Current [Ar-40] (A) from P Cal 4.24E-09 4.22E-09 4.20E-09 4.18E-09 4.16E-09 4.14E-09 4.12E-09 4.10E-09 Temperature Coefficient of Flow Rate 0.15% / o C 4.08E-09 4.06E-09 20 25 30 35 40 45 50 Average Temperature [Valve & Tank] (C)
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 H 2-1.0 % 200 ppm He - 1.0 % 1000 ppm N 2-1.0 % 50 ppm CO 2 20 ppm Kr - 1.0% 1000 ppm Xe - 1.0 % 1000 ppm
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 q i of a species is q i (in) = X i (Ref) Q o (mbar-l/s) The partial flow rate out of the chamber depends on the mass of the species, M i and the pumping system conductance, C N2 at the ion source: q i (out) = PP i C i = PP i C N2 [28 / M i ] 1/2 But q i (in) = q i (out) So at the ionizer: PP i (Ion Source) = [M i / 28] 1/2 X i (Ref) Q o / C N2 From which a Sensitivity Factor can be calculated: SF i (mb/a) = PP i (Ion Source) / [ I i Interference Contributions ]
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 X i -Ion Source Ar 33.33 46.4492392 He 33.33 14.6885391 N2 33.33 38.8622217
Ion Current (A) Mass Spectrum of a PVD Mixture Calibration Reference Source 1.0E-08 1.0E-09 1.0E-10 200 ppm H 2 + 40 Ar ++ Ar + INFICON PVD Mixture Calibration Reference Source Flow RATE 1x10-4 mbar-l/s @ 23.5 o C Transpector CIS: 70eV/2000uA/EM 1.0E-11 1000 ppm He Xe ++ Kr + 1.0E-12 1.0E-13 1.0E-14 0 10 20 30 40 50 60 70 80 90 100 Mass
Mass Scale Calibration with Calibration Reference Source H 2 & He 36 Ar & 38 Ar Kr
Electron Multiplier Gain Adjustment Adjust RGA EM (HV) to give Ion Current Level: e.g.: P Cal = 2x10-5 mbar S FC = 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)
Ion Current (A) CIS Total Pressure (mb) Sensitivity and Gain can be quickly measured with sequential FC/EM scans with Selected Ion Monitoring of Ar species. 1.0E-05 1.0E-06 Pressure: Calibration Reference ON Ar-40 EM (Off Scale) 1.0E-03 1.0E-04 1.0E-07 1.0E-05 1.0E-08 S(Ar-40) = 1.1E-5 A/mb 1.0E-09 1.0E-10 Ar-40 FC Gain = 445 + 8 Ar-36 EM Ar-38 EM 1.0E-06 1.0E-07 1.0E-11 Ar-36 FC Gain = 443 + 38 1.0E-08 1.0E-12 Ar-38 FC 1.0E-09 1.0E-13 FC EM FC 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) 1.0E-10
Ar-40 Sensitivity (Amp/Torr) Sensitivity Monitoring Data shows Shifts in Sensitivity Related to the Sensor MASS(40) Sensitivity Vs Time For CIS2 (W filament; 40eV/200uA) Reference Gas: Inficon/VTI Calibration Standard 2.60E-04 2.55E-04 2.50E-04 2.45E-04 2.40E-04 2-Sigma Deviation 5% of Average over 15 days 2-Sigma: 2% over 7 days 5% Shift in Average Sensitivity on Day 7 2.35E-04 2.30E-04 2.25E-04 2.20E-04 0 2 4 6 8 10 12 14 16 Time (Days) 2-Sigma: 3% over 8 days
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 X i (mol-%) = 100 * PP i / PP j For Quality Assurance, Accuracy is measured with known mixtures: ΔX i = X i - X i (Std) [for Major Components] 8 Capillaries P process = 0.5 2 Atm i.d. = 0.25 mm L = 1.5 m 35μ Leak CIS 100 amu QMS w FC TMP Drag Diaphragm Pump
Ion Currents vs Time from 2 Gas Streams How do we relate Ion Current to Partial Pressure? Calibration
Ionization Cross Sections(Å 2 ) 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 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
Partial Pressures are Calculated from Ion Current in Real Time using a Previous Calibration Partial Pressure = Sens Factor * Trans(M) * [Ion Current Interferences] PP i = SF i * M Z * [ I i - Σ k i A ik * I k ]
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 Air 4m-% H2 in Ar
More informative is the Composition of the Gas Species showing Stable Composition after Conditioning the Inlet System 4m-% H2 in Ar Air 4m-% H2 in Ar 5 Min Elapsed Time Composition Table at the Time with the Yellow Cursor
Safe Hydrogen [4 mol-% H 2 in Ar] is a Low-Cost Reference Mixture Initial Calibration for H 2 was done with 2.0% H 2 in Synthetic Air (N 2 & O 2 ) in August 2011 Data for 4% H 2 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-% H 2 in Ar from the Trend Data over 12 days The Mean Value for H 2 over 12 days is 3.88 + 0.34 Mol-%
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 N 2, O 2, Ar is within 1 σ Data displayed is a randomly selected, single measurement value of X i O 2 and Ar have σ ~ 2 %-RSD H 2 O and CO 2 are Biased due to Seasonal ΔRH and Breathing in Lab Air Components Altered by Seasons and Breathing
Now to Summarize
In Situ Calibration by Two Methods Provides Accurate Partial Pressures and/or Composition to a RGA / IG Process Room 30 60 m Analytical Lab T P T V Process Capillaries D/T Pump Tritium Glove Box Standards Capillaries CDG 3 L << 1 mb Sample Pressure D/T Pump Tritium Fume Hood Magnetic Sector Mass Spectrometer D/T/He - 1 D/T/He - 2 Tritium Glove Box D/T/He - 3 D/T/He - 4 XHV UHV Low Pressure Processes Atmospheric Processes Calibrated Flow Reference Gas Mixtures Deliver Known PP i s and X i s to the Ionization Region Method of Introduction is Designed for a Particular Application Regular Measurements for PP i s and X i 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 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
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 P mix Calibration + Conductance (Calc/Measure); or Total P @ RGA For UHV: Fixed Flow Rate Calibration Reference Mix (INFICON or VTI) + Conductance @ OIS 0.01 mb<p process <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) = M z where 0 < Z < 1 to make SF σ(e) For High Ion Source Pressure, X i -mol-% more stable than PP i for species QA: Plot differences, X i X i (Std) vs Time to monitor when to Re-Calibrate
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)