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64th IUVSTA Workshop, May 16-19, 2011
Problems of vacuum metrology for industrial applications that call for solutions by rarefied gas dynamics
Karl Jousten, PTB, Berlin
1. Applications of vacuum and leak detection with conclusions for rarefied gas dynamics
2. Present state: Measurement standards for vacuum and low flow rates (leaks)
3. Four problems for rarefied gas dynamics
4. Conclusion
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 2
Applications of vacuum in science
Gravitational wave detectors
Elementary particle physics: Accelerators, KATRIN
Fusion: ITER
Surface physics
Conclusions
In most cases just sufficiently low gas density
In some cases complicated and extensive design calculations of vacuum system (ITER, KATRIN)
Reliable pumping speed values needed!
Applications of vacuum and leak detection
Virgo-Detector, near Pisa with 3 km long vacuum tubes
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 3
Pumping speed measurement
p
qS pV
Turbomolecular pump, Pfeiffer Vacuum
Applications of vacuum and leak detection
From science to industry, standardization:
How and where to measure p?
This (and hence S) is defined by international standards, but is it a physical quantity for design and theoretical calculations?
Conclusions: Physical relevance of standardized quantities needs to be adressed.
pVq p ?
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 4
Microelectronic industry
Applications of vacuum and leak detection
MOCVD: Reactor forferroelectric films
Cluster toolAIS, Dresden
Typical: fast processes
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 5
Industrial applications: CD/DVD metallization
Type Cathode Pumps Cycle-time
SINGULUS V Focus 1
Focus Cathode 1 Turbo Molecular Pump
2,7 s
SINGULUS V Focus 2
Focus Cathode 2 Turbo Molecular Pumps
1,5 s
SINGULUS V Smart 1
SMART CATHODE®
1 Turbo Molecular Pump
2,5 s
SINGULUS V Smart 2
SMART CATHODE®
2 Turbo Molecular Pumps
1,9 s
Applications of vacuum and leak detection
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 6
Applications of vacuum and leak detection
Other examples of fast processes:
Leak tests of rims for cars (mainly aluminum for light wheels)
Coating of bottles (food industry)
1.2 s … 2.5 s
PET- bottle coating Fa. Sidel company
Conclusions: Fast changes of pressure and gas flows need to be calculated for engineering design.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 7
Example of non-detructive leak test: pacemaker and air bag
Required tightness:< 10-7 Pa L/s !
Required tightness:< 10-5 Pa L/s
Applications of vacuum and leak detection
Source: St. Jude Medical, Sweden
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 8
Airconditioning in cars, refrigerators etc. : Environmental issues
Required tightness:< 10-3 Pa L/s (1 g/a)
Applications of vacuum and leak detection
Conclusions: Leak testing is normally performed with helium, but neededare leak rates for other gases and even liquids.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 9
For a long time until about 1990:Magnetic sectors are used for leak detection and quadrupole mass spectrometers (QMS)for both leak detection and analysis of background residual gas level causing the name Residual gas analyzers.
Nowadays in addition QMS for:Gas purity, in-situ analysis for reagent gasesand low-level components in semiconductorIndustry.• Sputter process control• CVD monitoring, gas abatement analysis• MBE source control• End point detection (etching)• Gas chromatography
Applications of vacuum and leak detection
Partial pressuremeasurement
Etching: end point detection
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 10
Applications of QMS < 1E-2 Pa:Direct installation of QMS to process chamber.
1E-2 Pa:Differential pumping necessary with manifold, conductance, high vacuum pump, and total pressure gauge.Conductance for sputtering pressures PVD ( 0.1 Pa): Orifice, Dual inlet (RGA + process)for MOCVD (up to 100 kPa): Capillary
Conclusions: Mass (gas species) discrimination within the measuring device?
G as
To vacuum pum ps
To Q M SProcess G asC ham ber
Ulvac Co
Applications of vacuum and leak detection
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 11
1. Fast processes: dynamics
2. Physical relevance of standardized quantities: helium leak rate and S
3. QMS as process tool: Discrimination for partial pressure measurement
4. Design of complex vacuum systems
Summary from applications
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 12
2. Present state: Measurement standards for vacuum and low flow rates (leaks)
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 13
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 14
Pressure balance as primary standard
effA
Fp
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 15
Static or series expansion system as primary standard
21
112 VV
Vpp
V1,p1V2,p2
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 16
V4
100 L
V6
100 L
V3
V2
0,1 L
1 L
1 L
1 L
GAS INLET
UUC
V1
V7
V5
Example forstatic expansion
PTB, SE2
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 17
Measurement standards for vacuum and low gas flow
Static expansion system SE2, PTB, Berlin
Accurate pressures from 10-2 Pa to 103 Pa
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 18
Continuous expansion system as primary standard
p2 p3
gas flow
C1<< C2
p1
flowmeter2
112 C
Cpp
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 19
p=const
t
p
Gas Flow
C
V
Gas Inlet
V1
V2
V3
"Leak"
CDG
Measurement standards for vacuum and low gas flow
t
Vpq pV
Range: 10-8 Pa L/s … 10-1 Pa L/s
Gas flow meter FM3, PTB, Berlin
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 20
FM3
FLOW DIVIDERp0,V0
UHV-VESSELp1, V1
XHV-VESSELp2, V2
GAS FLOW
KP2
KP1
C02 C01
C2 C1
Accurate pressures from 10-9 Pa to 10-2 Pa
Measurement standards for vacuum and low gas flow
Primary standard CE3, PTB, Berlin
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 21
Relative uncertainties of pressures in primary standards
2E-021E-02
5E-034E-03
2E-03
3E-04
5E-05
2E-05
6E-06 5E-06
1E-06
1E-05
1E-04
1E-03
1E-02
1E-01
1E+00
1E-10 1E-08 1E-06 1E-04 1E-02 1E+00 1E+02 1E+04 1E+06
p in Pa
Re
lati
ve
Un
ce
rta
inti
es
(k=
2)
Mercury Manometer
Continuous expansion Series expansion
p atm
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 22
Valve 1
Testleak
Valve 3
Waterbath T=const.
Flowmeter
Valve 2
QMS
Traceability for leak measurements against vacuum
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 23
Testleak
Needle
CDG 133 Pa FS
CDG 133 kPa FS T1
T3
T2
T4
V1V2
V3
V = 5.1 cm³
V = 6.1 cm³Thermal insulation
Leak measurements against atmosphere
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 24
Measurement standards for vacuum and low gas flow
Conclusions from present state measurement standards
• Accurate vacuum gauge calibration is possible from 10-9 Pa to 105 Pa with uncertainties ranging from 0.001% up to 10%
• Accurate flow rate calibration is possible from 10-8 Pa L/s up to 0.1 Pa L/s against vacuum, 10-4 Pa L/s to 0.1 Pa L/s against atmosphere, with uncertainties ranging from 0.5% to 10%
• Only steady state conditions (constant pressure, equilibrium)
• In some ranges only some gas species (non-adsorbing) and pure gases
• Stable environmental conditions
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 25
Measurement standards for vacuum and low gas flow
A comment on traceability
• Whenever experimental results are being compared with a physical model or theory (pressures, mass flow rate, conductance, accommodation coefficient etc.), „true“ values of the experimental results are necessary.
•Characteristic for a true value is the number and the uncertainty of the value. Uncertainty is the interval in which the true value lies with a specified confidence limit (68%, 95%, …) around the given value.
•For true values and the respective uncertainty you need to have traceability to the SI.
•Traceability to the SI is given by complete calibration chain to a national primary standard for the given quantity (vacuum pressure, flow rate etc.)
•National primary standards are regularly checked internationally.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 26
Measurement standards for vacuum and low gas flow
Gap from present state measurement standards to applications
To close this gap there will be a new project IND12 within the EMRP (European metrology research programme) funded by the EU.
Among others the tasks are:
Dynamic vacuum standard
Leak rate conversion from calibrated rate (for gas species, environmental conditions)
Joint research project (JRP) IND12:
Duration 3 years, begins Sept 2011, 2.8 M€
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 27
3. Four problems for rarefied gas dynamics
3.1 Dynamic vacuum standard
3.2 Predictable leak (flow) rate from secondary standards
3.3 QMS as process tool: Mass discrimination for partial pressure measurement
3.4 Physical relevance of standardized quantity S
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 28
3. Problems for rarefied gas dynamics
3.1 Dynamic vacuum standard
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 29
Dynamic vacuum pressures
How fast are vacuum gauges?
Which measurement principle is fast?
Which electronic is needed? Resolution?
Hystereses effects of gauges?
→ Establish well defined dynamic pressures to test gauges
Measurement standards for vacuum and low gas flow
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 30
Dynamic vacuum standard
Goal:Pressure reduction from 100 kPato 100 Pa within 1000 ms.Predictable on the time scale of ms and with an of u(p)/p(t) < 50% at all times.Extendible to fast pressure changes down to 0.1 Pa.Tests for optical method possible.
100000
600200
100
80
60
40
20
t in ms
p in
kPa
0.003 kPa
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 31
Dynamic vacuum standard
Idea for realization:Expand the gas from a small volume into a large one by a duct or orifice of calculable conductance.Calculate p(t), T(t).Compare with pressure and temperature measurement.
Conductance of fast valve >> conductance of orifice or duct
Fast valve must open within about 10 ms … 50 ms.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 32
Dynamic vacuum standard
Rough estimate for necessary duct or orifice:
teptp 0 S
V
001.0 te
7s1
t ms145
1-Ls1.2L3.0
C
In the case of orifice and molecular flow:
mm8.4d
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 33
Dynamic vacuum standard
Problems to be solved:
Conductance must be known for any pressure between 100 kPa and 100 Pa, non-stationary flow.Which is best to calculate conductance for viscous flow: orifice or duct?Shall we generate conditions for choked flow?Temperature change, velocity of sound change, choked flow condition permanent?Can we calculate T(t) on the ms scale and test calculations?Can we calculate p(r) in V1?Is fast opening valve (DN40) available?If not, are their alternatives?Can we extend to lower pressures (< 100 Pa) and include desorption?
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 34
3. Problems for rarefied gas dynamics
3.2 Predictable leak elements
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 35
Used for:
Calibrating leak detectors (linearity tests), partial pressure analyzers
Gases and gas mixtures for leaks
Most leak rate measurements are performed with helium, but the tightness for other gases, gas mixtures, even liquids is required.
+ Test conditions are different than the calibration conditions.
→ Establish procedures to convert the quantities.
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 36
Secondary standard for leaks:
Permeation leak (see figure), temperature dependent, gas specific
Capillaries (less temperature dependent), crimped capillaries,
Porous plugs (sintered material)
Permeation leak type
Crimped capillary leak type
Gas flow
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 37
permeation porous capillary crimped
Temperature dependence
strong weak weak weak
Stability under rough cond
medium bad medium good
Operability under rough condition
good medium medium bad
Gas species flexibility
no yes yes yes
Rate easily changeable (T, p)
no yes yes yes
Predictable (gas, T, p)
medium medium yes yes
size large medium medium-large small
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 38
Choice for industry:
Permeation leak: stable, but not flexible, strong T-dependence, slow
crimped capillaries: small, large flexibility, work (good result) or do not work, fast, geometry is poorly defined
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 39
Possible equations of gas flow rate or conductance for capillaries (Tison, 1993)
2 Knudsen equation
1 Slip-flow equation
3 Linearized Boltzmann equationLoyalka, 19904 Guthrie, 1949, Steckelmacher, 1951:
Leak elements for industry
Empirical, shows minimum
EmpiricalKnudsen successivemonotonic
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 40
Stuart Tison, Vacuum 44 (1993), 1171-1175F.Sharipov, Handbuch Vakuumtechnik, Bild 5.25
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 41
Stuart Tison, Vacuum 44 (1993), 1171-1175
p: 20 kPa … 2 MPa
Crimped capillary
Agreement slip-flow and exp fortuitous?
Residuals from Slip Flow Model
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 42
Stuart Tison, Vacuum 44 (1993), 1171-1175
Regularcapillary
Leak elements for industry
1
018.1478.01
ln04.01
3
88.0
7.0tubepG
p: 20 kPa … 2 MPa
In Future?Sharipov, 2010:
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 43
To extend calculations from regular capillary to crimped capillary:
Geometry has to be well determined. Regular capillary: Uniformity of diameter? Advantage of crimped capillary may be that crimped part will dominate the result for mass flow.If geometry not well defined:Prediction from He calibration, pc, for other species, p? or …
Courtesy M. Bergoglio, INRIM
600 µm
Leak elements for industry
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 44
alternatively nano „holes“ made by focused ion beams:
Leak elements for industry
FIB hole L2 80 nm
y = 3,364E-14x - 8,749E-14
0,0E+00
5,0E-12
1,0E-11
1,5E-11
2,0E-11
2,5E-11
3,0E-11
3,5E-11
0 100 200 300 400 500 600 700 800 900 1000p in mbar
q in
mo
l/s
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 45
3. Problems for rarefied gas dynamics
3.3 Predict mass discrimination of „high pressure“ quadrupole mass spectrometers
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 46
Mass discrimination in inlet stages for QMS
G as
To vacuum pum ps
To Q M SProcess G asC ham ber
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 47
3. Problems for rarefied gas dynamics
3.4 Improve standards for pump speed measurements
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 48
Physical relevance of standardized S
Infinite volume
Equilibrium not disturbed by in- or
outflow
Vacuum pump
Vqt
VS
p
qS pV
Mechanical pumps
All vacuum pumps, but
Avn
Avn
qN 44
mkTAS 2
p is not isotropic in molecular range
The concept of pumping speed
intrinsic pumping speed
Infinite volume cannot be realized, reflected particles disturb equilibrium.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 49
Physical relevance of standardized S
Feng Yu-guo and Xu Ting-wei, The appropriate test domes for pumping speed measurement, Vacuum 30 (1980), 377…382.
Concept of tubular test dome with diameter equal to pump inlet flange.
Ideal gauge position: Simulates infinitely large dome
Appropriate test dome: Ideal gauge position is independent of .
„Play“ with d/D and L/D to find appropriate test dome.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 50
Physical relevance of standardized S
Deficencies of calculation of Yu-guo:
Only lower chamber simulated
Transmission probability calculated with 0.1% with a few 10 000 particles only
Optimum L/D could only be calculated with uncertainty of 10 %
Conclusion however:
L/D=1.5 is not optimal, lower values preferable.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 51
Improving standard for pumping speed measurement
Physical relevance of standardized S
Repeat MC calculations to find better L/D and/or gauge position (d/D is less critical)
Extend simulations to transitional flow
Consider non-uniform across pump inlet area D.
Consider disturbed angular distribution of reflected particles inside pump
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 52
Conclusion
• Primary standards for pressure and low flow rates are available world wide
• These standards give traceability to SI. This traceability is needed whenever theory/simulation of rarefied gas flow is compared with experimental result
• Industry quite often needs conditions other than ideal
• Rarefied gas dynamics can help in designing new standards and predict behaviour of standards under industrial conditions: standard for fast changing pressure, predict leak elements, predict mass discrimination, design standard for physical S.
Problems of vacuum metrology for rarefied gas dynamics, IUVSTA WS 64, 2011 53
Dome of the Reichstag in Berlin