Determination of Minimum Detectable Partial Pressure (MDPP) of QMS and its Uncertainty

Workshop on Towards more reliable partial and outgassing rate measurements Berlin, Germany, January 30 - February 1, 2017 Determination of Minimum Detectable Partial Pressure (MDPP) of QMS and its Uncertainty Hajime Yoshida National Institute of Advanced Industrial Science and Technology (AIST) National Metrology Institute of Japan (NMIJ) 1

2 Outline 1. Background 2. Example of the determination of minimum detectable partial pressure (MDPP) according to ISO TS WD 20175 3. Discussion on the uncertainty of MDPP 4. Summary

1. Background (1) Industry Examples of catalog spec of MDPP Manufacture Minimum detectable partial pressure (Pa) Faraday cup Electron multiplier A 5 x 10-10 1 x 10-12 B 4 x 10-11 7 x 10-13 C 2 x 10-9 3 x 10-12 D 1 x 10-9 2 x 10-11 E 10-8 10-12 F 5 x 10-10 1 x 10-12 G 5 x 10-6 - MDPP Faraday Cup SEM 10-11 Pa 10-6 Pa 10-13 Pa 10-11 Pa 3

(2) Metrology BIPM key comparison data base National standards (SI traceable calibration) are limited down to 10-9 Pa 10-5 Pa at present. 4

5 (3) ISO 14291:2012 2.2 Definitions of physical parameters

6 (3) ISO 14291:2012 5 Specifications for a QMS to be provided by manufacturers No description on procedures / conditions to determine MDPP

7 (4) ISO TS WD 20175 Today s topic 7 Characterization and calibration procedures 7.3 Minimum detectable partial pressure (MDPP) 8 Measurement uncertainties 8.2 Uncertainty of MDPP Procedure and uncertainty evaluation are described in this draft.

8 Summary of background - According to the catalog, MDPP of QMS is in the range from 10-13 Pa to 10-6 Pa. - National standards of pressure and vacuum are limited down to 10-9 Pa 10-5 Pa at present. - ISO 14291:2012 defines MDPP = 3s/S. Current noise Sensitivity - ISO TS WD 20175 contains the procedure to determine MDPP and its uncertainty.

9 2. Example of the determination of MDPP according to ISO TS WD 20175

Apparatus and setting of QMS Standard Conductance Element (SCE) C = 3.33 10-11 m 3 /s for He CDG QMS Procedure 1. Select the detector and the settings of the QMS He p R RP TMP RP Effective pumping speed EXG SRG Gate valve with an orifice Operational mode Emission current (ma) Electron energy (ev) 50 N 2 0.285 m 3 /s Integration time (s) Conductance He 0.296 m 3 /s modulation method Un-baked (CM method) Typical Background pressure ~ 1 x 10-6 Pa Analog 0.5 (fixed) Mass range (m/z) 1 50 Step width (m/z) Scan speed (s/u) SEM Voltage (kv) 1.0 0.05 (fixed) Unchangeable Unchangeable 10

11 Procedure 2 Determine the sensitivity S for helium at one partial pressure value in the range between 10-9 Pa and 10-5 Pa Peak signal Background signal I He S = I He I 0 p He P He Partial pressure of He p He = Q SCE S eff Flow rate through SCE Effective pumping speed of TMP 0.296 m 3 /s for He SEM 1.0 kv I 0 Average value

Sensitivity of QMS (10-6 A/Pa) Sensitivity of EXG 12 Sensitivity of QMS for helium 3 1 SEM 1.0 kv 2.5 0.8 2 1.5 1 0.5 0.6 0.4 0.2 0 0 0 0.5 1 1.5 Pressure (10-6 Pa) S of QMS is determined to be 2.5 x 10-6 A/Pa.

13 Procedure 3 Measure the noise at m/z = 5 with 100 values at residual pressure, each with an integration time of 1s, and determine the sample standard deviation s I of these values P He s I in the range from m/z 4.5 to m/z 5.5 20 data points per one scan 6 scans => total 120 data points s I = 5.4x10-14 A

14 Procedure 4 The MDPP is given by the following equation MDPP = 3s/S MDPP = 3 5.4x10-14 (A) 2.5 10-6 (A/Pa) = 6.4 x 10-8 Pa 1/3

Sensitivity of QMS (A/Pa) 15 Dependence of MDPP on SEM voltage is examined. - Current noise at m/z 5, s I - Sensitivity of helium Current noise at m/z 5, s (A) 10-9 10-10 10-11 10-12 10-13 10-14 Faraday cup 0 0.5 1 1.5 2 2.5 3 3.5 SEM voltage (kv) 10 0 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 0 0.2 0.4 0.6 0.8 1 1.2 Pressure (10-6 Pa) 3 kv 2.5 kv 2 kv 1.5 kv 1 kv Faraday cup Current noise increases > 2 kv Sensitivity increases with SEM voltage

MDPP (Pa) 16 Dependence of MDPP on SEM voltage 10-5 10-6 10-7 Faraday cup MDPP = 3σ I S 10-8 10-9 10-10 10-11 0 0.5 1 1.5 2 2.5 3 3.5 SEM voltage (kv) MDPP has a minimum value against SEM voltage.

17 3. Discussion on the uncertainty of MDPP

Measurement uncertainty of MDPP u MDPP MDPP = u(σ I ) σ I 2 + u S S 2 14 % Relative uncertainty of current noise Relative uncertainty of sensitivity Type A uncertainty ~ 10 % Calibration at 10-6 Pa ~ 10 % Is relative uncertainty of MDPP truly 14 %? 18

Sensitivity of QMS (10-2 A/Pa) Sensitivity of EXG 19 Calibration results at SEM voltage of 2 kv where MDPP is calculated to be 5x10-11 Pa. 1 Sensitivity of helium 1 Mass spectrum 0.8 0.8 0.6 0.4 0.2? MDPP 0 0 10-11 10-10 10-9 10-8 10-7 10-6 10-5 No vacuum standards Pressure (Pa) QMS EXG 0.6 0.4 0.2 1/2000? Is the uncertainty of MDPP truly 14 %?

4. Summary (1) Could you discuss my procedure and results to determine MDPP? Additional comments/questions from 4.5 to 5.5? Why? more than 20? 40? under 10-5 Pa? 20

21 (2) Uncertainty evaluation of MDPP Very difficult. 14 %? In addition, MDPP depends on gas species. - Sensitivity depends on gas species - Existence of residual gas (e.g. H 2, H 2 O, CO) or not (e.g. He, Ar) MDPP is just a rough guideline for users. SI traceable measurements Industry Uncertainty evaluation Vacuum pressure Required Necessary MDPP?? Thank you for your attention

Supplement 22

What is Standard Conductance Element (SCE)? Test gas - Sintered filter made of stainless steel - Porous structure < 1 mm 16CF (Knife edge on the both sides) User s vacuum chamber -Molecular conductance C S is given by calibration certificate. - Molecular flow condition is realized up to 10 kpa in typical. - Available range of C S is from 1x10-11 m 3 /s to 1x10-8 m 3 /s for N 2. - User can calculate the flow rate Q of test gas by using various gas species. NMIJ started supplying and calibration service of SEC from 2011. 23

SCE のコンダクタンス C S (m 3 /s) Molecular conductance of SCE (m 3 /s) 24 Six advantages of SCE Vacuum 85 (2012) 838-842. (1) Introducing various gas with known flow rate is available only with this single leak element. 10 8 Experimental results SCE1 (x10-10 m 3 /s) SCE2 (x10-9 m 3 /s) H 2 (2) Molecular conductance is constant against changing in the pressure Multi-points calibration is easily available. 6 4 2 Ar CO 2 N 2 CH 4 H 2 O Ne He 1 C S M C C 2 H 0 3 H 8 6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ( 分子量 M) の逆数 (u -1/2 ) (Molecular mass) -1/2 (u -1/2 ) (3) Calibration for mixture gases is available. (4) The temperature dependence is small and theoretical. C S T 1/2. (5) Calibration using liquid vaper including water vapor is available. (6) Sufficient long-term stability < 3 %/year.

Conductance modulation method (CM method) K. Terada, T. Okano, Y. Tuzi, J. Vac. Sci. Technol. A7, (1989) 2397. Vacuum chamber TMP RP Flow rate Q IG Gate valve with an orifice Open gate valve Q S eff Effective pumping speed of TMP Close gate valve (pumping through orifice) Q S p O po0 Seff po Pressure ori Combining conductance Background pressure pc pc0 Sori pc Effective pumping speed through orifice 1 S ori 1 C ori 1 S eff -(3) -(1) -(2) Calibrating IG is unnecessary Because the sensitivity of IG is canceled by assuming its linearity. From eq.(1), Eq.(2) and Eq.(3) S eff p C C ori 1 po Conductance of orifice 25