Practical approach and problems in in-situ RGA calibration

Transcription

1 Practical approach and problems in in-situ RGA calibration Oleg Malyshev and Keith Middleman Vacuum Science Group, ASTeC Accelerator Science and Technology Centre STFC Daresbury Laboratory UK Workshop on measurement characteristics and use of quadrupole mass spectrometers for vacuum applications, EMRP IND12. Bled, Slovenia, April 10 13,

2 ASTeC Vacuum Science group main interests Vacuum in particle accelerators: Achieving, measuring, modelling, designing... Vacuum related studies in the VS lab: Thermal outgassing Gauge and RGA calibration Pumping property measurements Electron stimulated desorption Surface coatings and analysis Photocathode development 2

3 What RGAs used Make: MKS (Microvision) 8 Hiden 1 SX200 (head) + VGQ 15 Dyson 1 Prisma 3 Modes used: FAR and SEM Profile, trend and MID Leak detection Total pressure range: from 10-5 down to below mbar Other requirements: Bakeability, Stability, including XHV Traceability Low outgassing A wish: No setting change after initial calibration check and necessary adjustments 3

4 Why we need the RGA calibration A need of quantitative partial pressure measurements Lack of space for two instruments: gauge + RGA Outgassing of gauge + RGA is greater than RGA only Strange experimental results received with RGA output data Example: sticking probability >1?! Pimping speed S > S ideal = A v/4 It does not take too long to find that: RGAs are not calibrated they have a number of factory set parameters I(m/e=28) usually correspond to P(N 2 ) measured with a UHV gauge RGAs are adjusted with injection of noble gases not for residual gases in UHV Generally, as-received it is a qualitative (not a quantitative) device 4

5 Definitions of RGA calibration (1) (2) Peak alignment Accurate partial pressure Width alignment measurement Use of noble gas Different type of gases mixture Questions: Influence of cracking pattern Influence of RGA s gas factory An influence of a large peak on a neighbour small peaks Influence of ESD in the ion source 5

6 Set-up for NEG pumping evaluation O.B. Malyshev and K.J. Middleman. In situ ultrahigh vacuum residual gas analyzer calibration. J. Vac. Sci. Technol. A 26 (2008), p Test chamber 1 (option) 6

7 Main steps of in-situ calibration on the research rig Choice of injected gases Cleaning of injected gases (if necessary) Filling the gas chamber with known volume and high accuracy Baratron gauge If necessary, checking the Extractor gauge calibration vs High Accuracy Baratron gauge FS=1.3 mbar; Res = 10-6 mbar Calibrating RGA vs Extractor gauge by gas injection Analysis of calibration data to obtain calibration coefficients. 7

8 Injected gases H 2, CH 4, CO, CO 2, same as in the residual gas spectrum, N 2, Ar, O 2 LN 2 trap for cleaning of injected gases Very useful to reduce an impurity of injected gases (even for class 9999 gases) H 2 O is present for calibration before a bakeout or by heating a small part of vacuum chamber, or switching on a filament 8

9 Checking the extractor gauge ex-situ The extractor gauge is calibrated on the secondary calibration facility in ASTeC Vacuum Laboratory against two primary calibrated (at PTB and NPL) extractor gauges Advantage: accurate calibration to the secondary standard Disadvantage: requires a lot of work: removing the gauge from an experimental installation, installing it on the calibration facility, perform bakeout, calibration, transfer it back to the experimental installation Traceability against N 2 only... 9

10 Checking the extractor gauge calibration in-situ If there are any doubt about the gauge error in factor 2 or more: Gas expansion from the 0.15-l chamber initially filled with N 2 at about 10-3 mbar to the 1.5-l chamber Pumping gas out from the 1.5-l chamber Expansion to the 1.5-l chamber repeated. Pressure at 0.15-l chamber is a few 10-6 mbar (not accurately measurable by Baratron gauge but well calculated). Expansion to the test chamber (valve to the pumps closed). Equilibrium pressure is in the range of 10-7 mbar Comparison of calculated and measured pressures. Advantage: Time saving Disadvantage: Low accuracy (compared to ex-situ calibration) Accuracy of volume measurement and Baratron calibration 10

11 Calibrating RGA vs Extractor gauge by gas injection Pressure in RGA port and the gauge port should be the same due to test chamber symmetry Pressure in the test chamber varied between ~10-10 mbar to 10-6 mbar. Pressure recorded for the gauge and RGA peaks At mbar to 10-8 mbar the injected gas might be not dominant in gas spectrum 11

12 NEG-1 rig test chamber Pumping RGA Gas injection Extractor Gauge 12

13 Analysis of the measurements RGA spectrum: there is always a mixture of gases Assuming that the calibration coefficients a i for the gauge and b i for the RGA, the measured currents are: Then I ( i) a P; I ( i) b P g i i RGA i i a I a P i I () i g i i RGA b i i i Details of calibration coefficient matrix calculation was covered by B. Jenninger in his talk in Session 2 and in our paper in J. Vac. Sci. Technol. A 26 (2008), p

14 Typical results The RGA calibration coefficients are normalised to a Nitrogen coefficient and compared with referenced coefficients for an ionisation gauge Gas Type H 2 CH 4 CO N 2 Ar CO 2 Gauge a i /a N2 RGA-FAR b i /b N2 RGA-SEM b i /b N

15 RGA calibration check vs. extractor gauge Injection: H 2 CO CO 2 CH 4 H2 CO CO2 CH P ext measured P ext calculated H2 mas s15 mass 16 CO Ar CO2 mass 12 mass 14 WS-63, September 2010, Ávila, Spain 15/40

16 Calibration of Extractor Gauge and RGA s Probability of Ionisation for ionisation gauges H 2 = 0.44 CO = 1.05 CH 4 = 1.6 CO 2 = 1.4 Ar = 1.2 Compared to our measurements H 2 differs by a factor of 4.3 CO differs by a factor of 2.4 CH 4 differs by a factor of 3.0 CO 2 differs by a factor of 4.7 Ar differs by a factor of

17 Linearity over pressure ranges 2 RGAs and extractor gauge measured simultaneously Injection of H 2, CH 4, CO and CO 2 Relative sensitivity normalised to one measured at 10-7 mbar +5% -10% -10% +60% 17

18 SEM vs. FAR Calibration with a Faraday cap is quite stable no drift over ~3 years detected. SEM is calibrated against a Faraday cap at P > mbar Checked during each experiment when both SEM and FAR used Residual sensitivity coefficients need to be corrected every ~3 months WS-63, September 2010, Ávila, Spain 18

19 Electron Stimulated Desorption (ESD) Another factor to consider with RGA data at low pressure is the influence of ESD from the ion source. Typical ESD generated peaks include: H +, O +, F +, 35 Cl + and 37 Cl + If unaccounted for it can lead to false conclusions in interpretation of RGA data. This is particularly important when considering the influence of Oxygen containing species when activating GaAs photocathodes. These species are considered a contaminant and can kill the QE of a GaAs surface. Suggestions are that partial pressures of < mbar for such species is required. 19

20 Partial Pressure (mbar) Partial Pressure (mbar) Influence of ESD Peaks in RGA Data Gas phase and ESD species have different energies which allow separation between the two. RGA Scan from Outgassing System RGA Scan from Outgassing System Mass 19 is the dominant peak ESD Peaks only Mass Mass (amu) 20

21 Partial Pressure (mbar) 10-9 RGA Scan showing the influence of ESD peaks Mass (amu)

22 P [mbar] GaAs photocathode studies 3E-12 Before injection 2.5E-12 2E-12 RGA scan showing ideal vacuum system 1.5E-12 1E-12 5E

23 QE (a.u.) Pressure (mbar) QE (a.u.) Pressure (mbar) QE (a.u.) Pressure (mbar) GaAs lifetime studies, purposely poisoning the cathode QE Pressure Elapsed time (s) CO exposure 3.0x x x x x x10-11 CO injection 7E-12 6E-12 5E-12 4E-12 3E-12 2E-12 1E E QE Pressure CO 2 exposure 3.0x x x x x10-10 CO 2 injection 2.5E-12 2E E-12 1E-12 5E x E Elapsed time (s) QE Pressure Elapsed time (s) O 2 exposure 3.0x x x x x x10-11 O 2 8E-12 6E-12 4E-12 2E E-12 23

24 Conclusions Vacuum science requires: a quantitative RGA: P i = f (I i ) = I i /C(I i,p tot,i e,e ion,...) Stabile, including XHV With high traceability Low outgassing In-situ RGA calibration can be performed against a total pressure UHV/XHV gauge when gas injection is available In-situ RGA calibration check might be performed without a gas injection Ex-situ RGA calibration would bring more confidence in the RGA performance 24