Partial Pressure Analysis for Large Vacuum Systems

Transcription

1 Partial Pressure Analysis for Large Vacuum Systems Robert E. Ellefson REVac Consulting Dayton OH USA OLAV IV NSRRC Hsinchu, Taiwan 4 April, 2014 REVac Consulting Robert.Ellefson@sbcglobal.net Ph: Cell:

2 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

3 There are Many Analytical Tools Designed for Processes

4 Two Types of RGAs are available for UHV/XHV Measurements Auto-Resonant Ion Trap Mass Spectrometer Ion Source Operation 100 ev/ ma Quadrupole Mass Spectrometer Ion Source Operation Filament: 70 ev / 1-2 ma Ion Energy = V anode V axis = 8 ev Mass Filter: Quadrupole with V RF /V DC = Constant for ΔM = 1 Mass Filter: Ion Trap Ion Detection: Electron Multiplier Ion Detection: Faraday Plate or Electron Multiplier An additional SS Surface is the Vacuum Housing provided for the RGA

5 All Ions (and Problems) begin in the Ion Source I i + = i e σ i A d e (P i /kt) Ex T(M) D(M) Ion Current = e Current, Cross Section, Ion Volume, Density, Extraction, Transmit, Detection Filament: Y 2 O 3 /Ir or Re (Oxygen Cmpd); W for H & UHV i e as small as practical for application A d e 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

6 * 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 O 2 Tolerant and Don t Warp. W Filament on CIS Power Failure Venting Day 26 Erodes W Filament T fil goes Up; S Ar Goes Down

7 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) ~ ma Pure Re Warps; Alloy Better Evaporation Limits Lifetime Not a Good Choice for UHV W: ( ev) ~ ma Can Warp; W(1% Re) Good Carbides and Oxides Replacement Filaments at Curves from Scientific Instrument Services website

8 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 +, (H 3 O +? with H 2 ) What is the Cleaning History? Freon Cleaning or Dusting? Thermal Desorption (~ 5 2 ma Emission) Physisorbed and Chemisorbed H 2 O, HCs, CO 2 Minimize SS Surfaces within RGA Filament Reactions (@ 1650 to 2100 C): C x H y + H 2 O or O 2 CO + CO 2 (+ CH 4?) WC + H 2 CH 4 and WC + H 2 O CO + CO 2 2 Y 2 O 3 + H 2 2 Y 2 O 2 + O 2 Reduction to Sub-Oxide; Chronic in UHV

9 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 (5 W)] Conduct away Heat from Filament: Watanabe Source (WatMass) Avoid C or HC Contaminants on a W filament or within an Y 2 O 3 or ThO 2 Coating UHV: Reduce WC with Pure H 2 exposure as CleanUp or Conditioning HV: Reduce/Sinter Y 2 O 3 or ThO 2 Coating with H 2 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?)

10 R Ellefson and M Vollero, AVS-57, 2010 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 = i e / (V e ) 1/2 V well (40eV/200uA) = V V well (70eV/2000uA) = V

11 UHV/XHV RGA Ion Source is similar to the Successful Extractor Gauge UHV/XHV Extractor Gauge UHV/XHV RGA Anode (100 V) [Pt or Ir Grid] Filament (50-90 ev) [W] Focus Plate (~35 V) Ion Source Exit(0 V)/ Quad Ion Entrance 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

12 Ionization Cross Sections(Å 2 ) 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. [ 36 Ar ++ 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 σ rel (40eV) σ rel (70eV) σ rel (100eV)σ rel (150eV) Ar O N H He Molecule e-cross Sections at Atom e-cross Sections inferred from Wutz Handbuch Vacuumtechnik Ed , Bild 12.42

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

14 Mass Dependence of Ion Transmission for QMS and ARTMS is quite Different QMS Ion Transmission ARTMS Ion Transmission Resonant Ion Pumping f R Ion Acceleration: V RF = 50 mv Duration of Pumping 1 / f R T QMS (M) = Area(ΔM) / Area of Stable Trajectories (M) T QMS (M) = K/M a where a is 0.4 < a < 1 (By Calibration) QMS Sensitivity is higher for low Mass Ions The result is Each Ion is Accelerated for the Same number of RF Cycles. So Ion Ejection is Mass Independent and ARTMS Sensitivities reflect σ e (E) P i = I i / S QMS P i = X i P Gauge

15 Recent Data from Mark Pendleton, Daresbury comparing 2 QMS s and Ion Trap RGAs validates the previous modeling* * Choose Previous Meetings RGA11

16 Courtesy of Granville-Phillips

17 30 V it 0 V xis 92 V The Electron Multiplier has a Mass Dependence too. D(M) = α(m) Gain(V EM ) FC EM Where α(m) is Secondary Electron Yield of Ion at EM Entrance Surface and Gain(V EM ) is electron current Gain for V EM applied

18 Partial Pressure Measurement Detection Limit could be lowered by Increasing Ion Source Sensitivity and Ion Counting: N(Ions/s) = mb* 2x10-4 A/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

19 The Dynamic Range of VQM is limited by Ion Statistics

20 Comparison of RGA Features for QMS and Ion Trap Feature QMS Ion Trap MS Partial Pressure Measurement Direct: P i = I i /S i P i = P Total X i Partial Pressure Range <P i <10-4 mb <P i <10-6 mb Dynamic Range at a Pressure 7 Decades 3 Decades Linear Response ~ 10 % < 10-5 mb P Total Response MDPP [Noise (3σ) / Sensitivity] mbar 5x10-14 mbar 2-50 AMU Scan Time 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

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

22 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 or Measured Mixture Composition chosen for Application Vacuum System must tolerate the Q cal Flow Rate

23 Ion Current (A) Mass Spectrum of a PVD Mixture Calibration Reference Source 1.0E E E ppm H Ar ++ Ar + INFICON PVD Mixture Calibration Reference Source Flow RATE 1x o C Transpector CIS: 70eV/2000uA/EM 1.0E ppm He Xe ++ Kr + 1.0E E E Mass

24 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 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 So at the ionizer: q i (in) = q i (out) 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 ]

25 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 X i -Ion Source Tank Mix X i -Ion Source Ar H He N CO Kr Xe

26 An In Situ Calibration Method for UHV/XHV with Molecular Flow Into Ionizer and Out Delivers the Tank Composition R G A 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 Extractor Ion Gauge Secure Isolation Valves Bypass Leak UHV/XHV Vacuum System Ion-Getter Pump CDG 1000 Pa 200 cm 3 2 cm 3 20 cm 3 Cal Mixture ~ 1 Bar UHV-Getter Pump The Plot shows Flow Rate, Q; The Pressure generated in the RGA/IG 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 Ellefson RE, Methods for in situ QMS calibration for partial pressure and composition analysis, Vacuum (2013),

27 Establishing a Good Base Pressure is Essential for UHV RGA Choose a Value of q Outgassing Rate/cm 2 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

28 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

29 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

30 Thank You for Your Attention