RGA’S FOR VACUUM DIAGNOSTICS AND - vacuum-uk.org · 2 Outgassing measurements / Vacuum-Diagnostics PV = Q (amount of gas included in a vacuum-chamber) dQ/dt = - S eff P ressure

RGA’S FOR VACUUM DIAGNOSTICS AND OUTGASSING MEASUREMENTS

2

Outgassing measurements / Vacuum-Diagnostics

P*V = Q (amount of gas included in a vacuum-chamber)

dQ/dt = - Seff*Pressure (Seff effective pumping speed)

P = P0 * exp (- (Seff/V)*t)

[Seff/V] = s-1 vacuum time constant

Start with a very simple picture:

Why Vacuum-Diagnostics?

Dr. Günter J. Peter Coventry Oct 2014

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20 40 60 80 100 120

0.0

2.0x10-5

4.0x10-5

6.0x10-5

8.0x10-5

1.0x10-4

Volume of the chamber = 1'000 liters

Effective pumping speed = 100 l/sec

P = Po * exp(- (V/

Seff)*t)

5*10-10

mbar ???

Pre

ssu

re[m

ba

r]

Time [ seconds]

what happens in reality

Basic calculations are right, every time, however,…


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That’s what we might get in

reality.

both curves start at approx. same time


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Pump-down curve:

time / arb. units

pre

ssu

re /

arb

. u

nit

s l

og s

cale

base pressure of the pump

theory 1st order approximation

desorption, leak, permeation

Preal

Preal = Q(desorption,permeation,leak) / Seff


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Just to save time to start a process

The influence of the residual gas on the

result

Sometimes the residual gas even can have a

negative impact on the whole equippment.

Motivation: Why?


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This is an issue since decades!


I like to show this old equippment. It’s delivering valuable results since decades and physics

doesn’t change.

elevated temperature

possible sensor too.

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1. Introduction

2. RGA’s for UHV / XHV

3. Outgassing Measurements Basics

Partial Pressure Gauges

Calibration, Limitations

4. Examples for routine measurements

(In situ cleaning and endpoint detection are out of the scope

of this talk. We are willing to discuss such applications later.)

Agenda:


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0 20 40

0.00E+000

1.00E-010

2.00E-010

3.00E-010

4.00E-010

C

H2

CO2

CO

pressure = 1.5*10-11

mbar

Residual Gas SpectrumS

ign

al S

EM

-cu

rren

t

mas number [amu]


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0 10 20 30 40 50

0.00E+000

5.00E-011

1.00E-010

1.50E-010

2.00E-010

2.50E-010

3.00E-010

The signals at mass 16 and 19 are clearly identified as EID-Ions

The fact that there is still nitrogen and argon in the residual gas

spectrum indicates a leak!

However at an integral leak test no leak could be detected;

the conclusion is that there is a very small virtuel leak.

N2

+

at a total pressure of 1.5*10-11

mbar

F+

O+

Ar+

N+

Virtuel Leak

CO2

+

CO+

H2

+

Inte

nsity [S

EM

cu

rre

nt]

mass number [amu]


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Specs for such «RGA’s»

• bakeout temperature 4000 C

• detection limit < 5*10-15 mbar

• outgassing rate < 10-10 mbar*l*S-1

• radiation resistivity / screening of the

electronics


more details see, Partial Pressure Gauges CAS proceedings 16-24

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The outgassing rate of the test-chamber has to be

low compared to the outgassing rate to be determined.

clean (UHV) environment without sample is required?

P = Q / Seff

Low pumping speed to achieve a high pressure

mass spectrometer to be operated below 10-5 mbar

adjust the pumping speed?


General Remarks

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Partial pressure = Qpartial / Seff

Example: 1.0*10-4 mbar*l*s-1 and Seff = 500 l*s-1

P = 2.0*10-7 mbar

If the base pressure of the pump << 10-7 mbar,

and if the outgassing of the chamber is

lower than the outgassing of the sample, then

this can be measured.


General Remarks

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Model spectrum (Origin of peaks)

0 5 10 15 20 25 30 35 40 45 50

m/e

Hydrogen Nitrogen Oxygen Water

Carbon dioxide Argon Carbon monoxide

nature isn’t always kind!


Some use generic factors for calibration, from where does the computer know that it’s only

N2 detected at mass 28???

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Calibration is required anyway.

Question is what can be calibrated how far?


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Test-Gas

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Test-Gas

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Oops! What happened? Ar/Xe = 174.0 previous and 82.1 now

Test-Gas

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Ar/Xe = 174.0 previous and 82.1 now???

Our mass spectrometer are very stable in terms of mass resolution,

mass scale and sensitivity.

In order to demonstrate what might happen, I just changed the

mass-resolution settings *) for the next spectrum recorded.

-> mass resolution might be (is) rather different from RGA to RGA

and the sensitivity too.

Calibration is essential !

*) The change in resolution settings isn’t obvious from the peak-width at all. However, it caused a factor of

two change.

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Residual Gas:

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Task:

measure outgassing rates Q := mbar*liter*S-1

RGA: used for partial pressure measurement (after clever calibration)

P = Q / Seff effektive pumping speed and pressure have to be known

The base-pressure has to be a factor of 100 lower to avoid systhematic errors.


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Sensitivity of an RGA depends on the gas measured

Interferences (for example m/e= 28 can be N2, CO, fraction of CO2,…

Transmission through the mass-filter

Rem. The outgassing rate of a «normal RGA’s» is about 10-8 mbar*L*S-1


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Seff the effective pumping speed depends on the gas

S is proportional to (m)-1/2 m = mass of the gas, molecular flow

Use an orifice to throttle the (turbo) pump


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Sensitivity?

The ionisation cross section versus electron

energy for various gases can be found in text-

books.

Same problem as with an ionisation gauge to be

corrected for different gases.


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Test-Gas-Mixture : 10% He, 10% N2, 40% Ar, 10% Kr, 10% Xe


He N2 Ar Kr Xe

contend % 10 10 60 10 10

mass number 4 28 40 84 129

(m)1/2 2.00 5.29 6.32 9.17 11.36

expected partial pressure ratio 20.00 52.90 379.20 91.70 113.60

ionisation efficiency N2 set to 1.0 0.15 1.00 1.20 1.00 2.40

expected signal ratio 3 52.9 455.04 91.7 272.64

measured signal ratio ? ? ? ? ?

---> transmission versus mass

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With this information the results achieved with the «green mass spec» can be

compared with the «red one».


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calibration of the mass-spectrometer:

Seff at the place of the result = ampere/mbar*l*s-1

sensor has to be known. and with a different Seff, you’ll

get another result.

Both calibrations are gas-specific and there can be a pressure gradient

Inside the test chamber. Seff has to be known anyway for transferable

results.


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From a test-protocoll of an RGA used

In XHV-applications

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The RGA has to be calibrated:

Sensitivity A/mbar for an inert gas (Argon)

Transmission versus mass number - precisely speaking : The

ratio for He, N2, Ar, Kr and Xe can be determined.

Then the spectra recorded with various instruments can be

compared.

Spectra still have to be interpreted, because of complex

interferences, «fingerprints» of complex molecules.

Recommendation: create own libaries for specific compounds

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Thank you!

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The time intervall after re-calibration of the

RGA is required, rather depends on the specific

instrument used and the application as well.

No general advice can be given.

Best practise: Start with a short intervall.

After some time you know,

whether a longer intervall is ok.

I personally check the instrument sometimes routinely after a few month.

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Thank you again!

Documents

RGA’S FOR VACUUM DIAGNOSTICS AND - vacuum-uk.org · 2 Outgassing measurements / Vacuum-Diagnostics P*V = Q (amount of gas included in a vacuum-chamber) dQ/dt = - S eff *P ressure

RGA’S FOR VACUUM DIAGNOSTICS AND - vacuum-uk.org · 2 Outgassing measurements / Vacuum-Diagnostics PV = Q (amount of gas included in a vacuum-chamber) dQ/dt = - S eff P ressure