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

1 Workshop on measurement characteristics and use of quadrupole mass spectrometers for vacuum applications,

EMRP IND12. Bled, Slovenia, April 10–13, 2012.

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

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

3

Total pressure range: from 10-5 down to below

10-12 mbar

Other requirements: Bakeability,

Stability, including XHV

Traceability

Low outgassing

A wish: No setting change after

initial calibration check and necessary adjustments

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 > Sideal = 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(N2) 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

Definitions of RGA calibration

(1)

Peak alignment

Width alignment

Use of noble gas

mixture

(2)

Accurate partial pressure

measurement

Different type of gases

5

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

Set-up for NEG pumping evaluation

Test chamber 1

(option)

6

O.B. Malyshev and K.J. Middleman.

In situ ultrahigh vacuum residual gas

analyzer “calibration”.

J. Vac. Sci. Technol. A 26 (2008), p. 1474.

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

Injected gases

H2, CH4, CO, CO2, same as in the

residual gas spectrum,

N2, Ar, O2

LN2 trap for cleaning of injected

gases

Very useful to reduce an impurity of

injected gases (even for class 9999

gases)

H2O is present for calibration

before a bakeout or by heating a

small part of vacuum chamber, or

switching on a filament

8

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 N2 only...

9

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 N2 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

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 10-10 mbar to 10-8 mbar the injected gas might be not dominant in gas spectrum

11

NEG-1 rig test chamber

RGA

Extractor

Gauge Gas

injection

Pumping

12

Analysis of the measurements

RGA spectrum: there is always a mixture of gases

Assuming that the calibration coefficients ai for the gauge and

bi for the RGA, the measured currents are:

Then

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. 1474.

( ) ; ( )g i i RGA i iI i a P I i b P

( )ig i i RGA

i i i

aI a P I i

b

13

Typical results

Gas Type H2 CH4 CO N2 Ar CO2

Gauge

ai /aN2

0.44 1.5 1.04 1 1.3 1.6

RGA-FAR

bi /bN2

2.5 1.4 1.05 1 1.3 1.4

RGA-SEM

bi /bN2

4 - 7 1.7 1.05 1 1.3 1.4

The RGA calibration coefficients are normalised to a Nitrogen

coefficient and compared with referenced coefficients for an

ionisation gauge

14

RGA calibration check vs. extractor gauge

15/40 WS-63, 14-19 September 2010, Ávila, Spain

Injection: H2 CO CO2 CH4 H2 CO CO2 CH4

0 100 200 300 4001 10

10

1 109

1 108

1 107

1 106

1 105

Pext measured

Pext calculated

H2

mass15

mass 16

CO

Ar

CO2

mass 12

mass 14

16

Calibration of Extractor Gauge and RGA‟s

Compared to our

measurements

H2 differs by a factor of 4.3

CO differs by a factor of 2.4

CH4 differs by a factor of 3.0

CO2 differs by a factor of 4.7

Ar differs by a factor of 3.3

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Probability of

Ionisation for ionisation

gauges

H2 = 0.44

CO = 1.05

CH4 = 1.6

CO2 = 1.4

Ar = 1.2

Linearity over pressure ranges

2 RGAs and extractor gauge measured simultaneously

Injection of H2, CH4, CO and CO2

Relative sensitivity normalised to one measured at 10-7 mbar

+5% -10% -10% +60%

17

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 > 10-10 mbar

Checked during each experiment when both SEM and FAR

used

Residual sensitivity coefficients need to be corrected every

~3 months

WS-63, 14-19 September 2010, Ávila, Spain 18

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+, 35Cl+ and 37Cl+

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 < 10-14 mbar for

such species is required.

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Influence of ESD Peaks in RGA Data

10-13

10-12

10-11

10-10

10-9

0 20 40 60 80 100

RGA Scan from Outgassing System

Pa

rtia

l P

ressu

re (

mb

ar)

Mass

10-13

10-12

10-11

10-10

10-9

0 20 40 60 80 100

RGA Scan from Outgassing System

Pa

rtia

l P

ressu

re (

mbar)

Mass (amu)

ESD Peaks only

Gas phase and ESD species have different energies which allow separation between

the two.

Mass 19 is the dominant peak

20

10-13

10-12

10-11

10-10

10-9

0 20 40 60 80 100

RGA Scan showing the influence of ESD peaks.P

artia

l Pre

ssur

e (m

bar)

Mass (amu)

GaAs photocathode studies

0

5E-13

1E-12

1.5E-12

2E-12

2.5E-12

3E-12

0 10 20 30 40 50 60

Before injection

RGA scan showing ideal vacuum system

P [

mb

ar]

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GaAs lifetime studies, purposely poisoning the cathode

0 500 1000 1500 2000 25000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

QE

Pressure

Elapsed time (s)

QE

(a

.u.)

5.0x10-11

1.0x10-10

1.5x10-10

2.0x10-10

2.5x10-10

3.0x10-10

Pre

ss

ure

(m

bar)

0 500 1000 1500 2000 25000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

QE

Pressure

Elapsed time (s)

QE

(a

.u.)

5.0x10-11

1.0x10-10

1.5x10-10

2.0x10-10

2.5x10-10

3.0x10-10

Pre

ss

ure

(m

bar)

0 500 1000 1500 2000 25000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

O2 exposure

CO exposure

CO2 exposure

QE

Pressure

Elapsed time (s)

QE

(a

.u.)

5.0x10-11

1.0x10-10

1.5x10-10

2.0x10-10

2.5x10-10

3.0x10-10

Pre

ss

ure

(m

bar)

-5E-13

0

5E-13

1E-12

1.5E-12

2E-12

2.5E-12

0 10 20 30 40 50

-1E-12

0

1E-12

2E-12

3E-12

4E-12

5E-12

6E-12

7E-12

-10 10 30 50

-2E-12

0

2E-12

4E-12

6E-12

8E-12

-10 0 10 20 30 40 50

CO

injection

CO2

injection

O2

23

Conclusions

Vacuum science requires:

a quantitative RGA:

Pi = f (Ii) = Ii /C(Ii,Ptot,Ie,Eion,...)

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

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