METHODS FOR IN SITU QMS CALIBRATION FOR PARTIAL PRESSURE AND COMPOSITIONAL ANALYSIS

Robert E. Ellefson REVac Consulting

Dayton, OH 45459 USA robert.ellefson@sbcglobal.net

ERMP IND12 – QMS Workshop Bled, Slovenia

April 10-13, 2012

What do Industrial & R/D Users

Need and Expect?

• Partial Pressures or Composition of Process Gas

• Expect Stable Operation to Trust the Data

– How do they Know it is Stable? Accurate?

• Process Control practices typically call for a Verification of Performance

– Test Stand Calibration of RGA may Qualify the MS

– In Situ Measurement of Reference Mixture is needed to provide Verification that the RGA is “In Control”

Define a MS Measurement and Calibration Method

for a Process or Vacuum System

Logic

Plan:

Application

Sampling

Design

Select and

Qualify MS

Calibration

Design

Quality

Assurance

CVD, Etch, ALD, Degas

0.01 mb < PProcess< 100 mb

Vis., Trans. or Molecular

Process Gas Species(Any Condensibles?)

Viscous, Transition or

Molecular Flow [PProcess ]

Orifice / Small Tube toMS

(Heated Inlet and QMS)

Closed Ion Source (CIS)

Low eV: e.g. 40 eV

Low Emission: e.g. 0.2 mA

Mass Range for Species

Detector: FC & EM(?)

Demo: Linearity, S vs t, A-B

Std Mix: Process Species

Reservoir: PV for 1 year

Cal Flow Std: 1x10-4

mb-l/s

Flow Mix into CIS

Use Std Mix for QA Data

Log Data & Plot PPi vs Time

Plot Xi =PPi / Sum PPi vs Time

Establish Action Limits for Sens or Gain

Atmospheric Processes

100 mb < PProcess < 2 bar

Viscous Flow

Process Gas Species(Any Condensibles?)

Viscous Flow

Capillary Sampling with

Sample Pump and

Molecular Leak to MS

(Heated Inlet and QMS)

Std Mix: Process Species

Volume: 5-10 L@ 1 Atm

Capillary Sampling of

Standard Mixture

Vacuum: PVD- UHV -XHV

PProcess < 0.01 mb

Molecular Flow

Process Gas Species(Any Condensibles?)

Molecular Flow

Direct Insertion of QMS

(Isolation Valve)

Bakeout Provision

Open Ion Source (CIS)

High eV: 70 - 100 eV

High Emission: e.g. 2 mA

Mass Range for Species

Detector: FC & EM(?)

Demo: Linearity, S vs t, A-B

RGA Conditions for Accurate Partial Pressure and Compositional Analysis begins in the Ion Source

• Vacuum System Base Pressure PP Measurements Requires Sensitivity

– OIS: 70 – 110 eV e-; 1-2 mA Emission

• Process Gas Composition allows Lower Sensitivity operation which minimizes Fragmentation

– CIS: 35-40 eV e-; 200 μA Emission

• Ion Extraction, Focus and Ion Energy Potentials affect Space Charge, Ion Residence Times and Linearity

Evidence of Gas Scattering Loss at Higher Pressure is seen as Ions Traverse the Mass Analyzer to the Detector.

XPR Correction for Gas Scattering is I = Imeas eKP ; Gives PPi’s

The Potential Well formed by the Ionizing Electron Beam Lengthens Ion Residence Time (Longer Path) in Ion Source

CIS equipotentials

1V well

(SIMION)

10V

70V

75V

79V

79.9V

E-Beam Well Depth For the Geometry of this CIS:

Vwell = - 3800 ie / (Ve)1/2

Vwell(40eV/200uA) = - 0.12 V

Vwell(70eV/2000uA) = - 0.90V

Note for a given ie the well gets deeper

for lower Ve (slower electrons; higher ρe)

R Ellefson and M Vollero, AVS-57, 2010

The Same Open Ion Source can be Linear or Non-Linear depending on Operating Potentials

The Ar and O2 sensitivity decreases in OIS with time due to O2 Oxidation of Ion Source Grids changing surface potentials.

However Ratio O2 /[O2 +Ar] remains Nearly Constant

Data provided by Wm Sproul

0.0

1.0

2.0

3.0

4.0

5.0

0.0 50.0 100.0 150.0 200.0 250.0

Time, minutes

O2 a

nd

Ar

Pa

rtia

l P

res

su

res

an

d

(O2/O

2+

Ar)

*10

0 R

ati

o

Ratio = 0.0348 + 0.4%

O2+ (E-10 A)

Ar+ (E-9 A)

• Conditioning • RGA exposed to pure Ar

for many hours Reduces the Surface Oxide:

H(Diff) + OH(S) H2O

• Exposure to Ar +3% O2

changes the sensitivity for both Ar and O2 by surface re-oxidation with a

time constant of ~ 2hrs.

• Practical Solution for PVD Process Control:

PP (O2) = R(O2)*P(CDG)

Where R (O2) = O2 /[O2 +Ar]

Recommended Cal Reference: 3% O2 in Ar

Examples of In Situ RGA Calibration Methods

• In Situ Calibration for an Open Ion Source (OIS) RGA

• In Situ Calibration for a Closed Ion Source (CIS) RGA

– System Calibration using a Fixed Reference Supply with Sampling equivalent to the Process Sampling

– Portable Calibration Reference Source introduced directly into the CIS

In Situ Calibration: A Reference Pressure & Composition is established at the IG and RGA when

the Valve to the Gas Mixture is Open

• Pumping is provided by the Vacuum System

• A Calibrated Fixed-Flow Rate produces a Reproducible Pressure and Composition at IG and RGA Ionizers

• The Pressure is

Pcal = Qcal / Ccal

where CCal can be Calculated

from Geometry.

• Mixture Composition chosen for Application

• Vacuum System must tolerate the Qcal Flow Rate

For UHV and XHV Systems, a Lower Calibration Pressure can be created with a Q vs PFill Calibration

The Plot shows Flow Rate, Q; The Pressure generated in the example is ~ Q/10. PFill can be adjusted without altering Composition using the Gas Pipettes. Suggested UHV Composition: 90% H2; 9% CO; 1% CO2

Q = A Pfill2

A Two-Stage Pressure Reduction with multiple probe types enables sampling at a representative

point over a wide range of process pressures

• Process Gas Species in CIS reflect Process: Molecular Flow into CIS; Molecular Flow out • Calibration Reference Mix Species are Altered by Mass:

Viscous Flow into CIS; Molecular Flow out (Addressed Later)

Sensitivity Monitoring Data shows Shifts

in Sensitivity Related to Process Changes

MASS(40) Sensitivity vs Daysfor CIS2 [W Filament; 40eV/200uA]

Reference Gas: VTI Flow Std with INFICON Mix

2.0E-04

2.2E-04

2.4E-04

2.6E-04

2.8E-04

3.0E-04

3.2E-04

3.4E-04

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00

Time (Days)

Ar-

40

Se

ns

itiv

ity

(A

/To

rr)

Event: Accidental Air Venting of

CIS2 Vacuum due to Power Failure

+ 4.8% 2-Sigma

+ 5.5% 2-Sigma

Tritium Fume Hood

Process Capillaries

Tritium Glove Box

Tritium Glove Box

Standards Capillaries

D/TPump

V

T

T

P

D/TPump

D/T/He - 1

D/T/He - 4

D/T/He - 2

D/T/He - 3

30 – 60 m

CDG

3 L

Magnetic SectorMass

Spectrometer

Process Room Analytical Lab

<< 1 mbSample

Pressure

Remote Process Monitoring: D/T/He Gas Mixture Compositional Measurements

Sampling Capillaries 30 – 60 m long Capillaries to Standard Mixtures (Similar) Capillary Sample Purged 3 – 5 Capillary Vol’s ~ 0.1 mb sample P in 3 L volume

Species deplete through Molecular Leak Record time for peak for t=0 Composition Analysis Time: ~ 20 minutes

Qualify MS with Std Mix each AM + Run Time Enables < 0.5% relative accuracy for D/T/He-3

The D/T/He-3 mixtures were used to Assess Accuracy and do Daily Qualification of the MS’s

• A daily routine for operators was to run an analysis of a known mixture to check performance

• Call Customers “We are ‘up’ today” so they could run a Test, etc.

• If a MS was “Out of Control”, a re-run was done to note permanent shifts in calibration (due to power failures, etc)

RGA Calibration Source

•Fixed Flow Rate Gas Source •Pumping by Process through a Fixed Conductance (~ 10 L/s)

Process Monitor

Calibration Gas Source

•Fixed Flow Rate Gas Source •Pumping by Monitor Vacuum System through the Closed Ion Source (~ 1 L/s)

Can we define a Multi-Application,

Portable InSitu Calibration Gas Source?

2x10 -5 mbar

Vacuum Process

2x10 -7 mbar

Q=2x10 -4 mbar-l/s

Process

Q=2x10-4 mbar-l/s

2x10-4 mbar

1x10-5 mbar

Portable Calibration Sources have

been used for many years

• Crimped-Capillary Leak Standards (Boeckmann, et al.)

• VTI Positive Shut off Flow Standards (Traceable to NIST) • Sandia Flow Standards [Chamberlin, et al. JVST A7 (1989)]

• High accuracy primary flow references (Traceable to NIST)

INFICON has used their compact Reference Gas Source

for InSitu Calibration of OIS and CIS products for over

10 years

• SS Frit Flow Element For Stable Flow

• Low dead volume valve to minimize turn-on bursts

• Measured Flow Rate stated on Label (+ 10% of Rate)

• Predictable Flow Rate (Depletion < 10%/Yr normal use) • Gas mixture composition is constant (viscous flow)

• Gas fill Pressure < 2.8 bar (non-compressed gas for simple shipping)

• Flow Rate can be independently Certified at a Standards Lab

INFICON Calibration Reference Source

• 4-VCR Male Connection • Air Operated Valve • 38mm- (+VCR) x 270mm • 132 cc Volume

Flow Through Element goes as Pfill

2

RGA Calibration Pressure vs Fill Pressure of Reference

0.0E+00

2.0E-06

4.0E-06

6.0E-06

8.0E-06

1.0E-05

1.2E-05

1.4E-05

1.6E-05

0 0.5 1 1.5 2 2.5 3

Fill Pressure (Ar) [bar]

RG

A C

alib

rati

on

Pre

ss

ure

: Q

(P

) / C

[m

bar]

PP(Ar) = Q(P) / C

Q(P) / C = A P2

Vacuum Process

Q=2x10 -4 mbar-l/s

RGA Calibration Pressure Depletion is

Predictable with Time of Flow

0.0E+00

5.0E-06

1.0E-05

1.5E-05

2.0E-05

2.5E-05

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Elapsed Flow Time (Days)

PC

al[A

r-4

0]

(mb

ar)

Model for Depletion of

Calibration Reference

Pressure:

PCal(t) = Pcal(0) / [1+Qo t/PoV]

Qo = 2.8x10-4

T-L/s

Fill Parameters:

Po = 2053 Torr [2.7 bar]

V = 0.135 L

PCal[Ar-40] (t)

Expected 10% Depletion in 1 year of automated use

(5 minutes of flow/day )

Composition Ratio of 50/50 He/Ar Mix does not change with

30% Depletion of Fill Gas

Ratio 40/4 = -0.0041 t + 1.1627

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0.00 2.00 4.00 6.00 8.00 10.00 12.00

Time (Days)

Ion

Cu

rre

nt

(A -

Co

rre

cte

d t

o R

ef

mix

) &

Ra

tio

40

/4

Flow Rate Depletion: Ar-40

(Data Sensitivity Corrected)

Flow Rate Depletion: He-4

(Data Sensitivity Corrected)

Pcal produced from Cal Reference Source changes

with Ambient Temperature by + 0.15 %/oC

4.06E-09

4.08E-09

4.10E-09

4.12E-09

4.14E-09

4.16E-09

4.18E-09

4.20E-09

4.22E-09

4.24E-09

4.26E-09

20 25 30 35 40 45 50

Average Temperature [Valve & Tank] (C)

Ion

Cu

rre

nt [A

r-40

] (A

) fr

om

PC

al

Temperature Coefficient of Flow Rate

0.15% / oC

INFICON Gas Mixture options allow

choice of gas pumped by the process

Component Argon (99.995%) Ar with 5% Impurities PVD Mix

Argon 99.995 % 95 % Balance

H2 - 1.0 % 200 ppm

He - 1.0 % 1000 ppm

N2 - 1.0 % 50 ppm

CO2 20 ppm

Kr - 1.0% 1000 ppm

Xe - 1.0 % 1000 ppm

Composition of the Gas Mixture in the Ionizer is

altered by the RGA’s molecular flow pumping

• The Mixture is in viscous flow from the Calibration Reference Source. The composition entering the vacuum chamber is the stated Mixture. The partial flow rate qi of a species is qi (in) = Xi (Ref) Qo (mbar-L/s) • The partial flow rate out of the chamber depends on the mass of the species, Mi and the pumping system conductance, CN2 at the ion source: qi (out) = PPi Ci = PPi CN2 [28 / Mi]

1/2

But qi (in) = qi (out)

So at the ionizer: PPi (Ion Source) = [Mi / 28]1/2 Xi (Ref) Qo / CN2

From which a Sensitivity Factor can be calculated:

SFi (mb/A) = PPi (Ion Source) / [ Ii – Interference Contributions ]

Composition of Mixture at the MS ionizer shows

depleted light gases and enhance heavy gases

CEM 1:1:1 Ar-He-N2 Mix

Component Tank Mix Xi -Ion Source

Ar 33.33 46.4492392

He 33.33 14.6885391

N2 33.33 38.8622217

Mass Spectrum of a PVD Mixture

Calibration Reference Source

1.0E-14

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

0 10 20 30 40 50 60 70 80 90 100

Mass

Ion

Cu

rre

nt

(A)

INFICON PVD Mixture

Calibration Reference Source Flow RATE 1x10-4 mbar-l/s @ 23.5 oC

Transpector CIS: 70eV/2000uA/EM

200 ppm

H 2 +

1000 ppm

He

Ar +

Xe ++

Kr +

40 Ar

++

Mass Scale Calibration with

Calibration Reference Source

H2 & He 36Ar & 38Ar Kr

Electron Multiplier Gain Adjustment

• Adjust RGA EM (HV) to give Ion Current Level:

• e.g.: PCal = 2x10-5 mbar

SFC = 2x10-4 A/mbar

Gain = 200

Species Abundance Ion current (EM)

Ar-36 3370 ppm 8.0x10-7 A

Ar-38 630 ppm 1.5x10-7 A

Ar-40 99.6% Off Scale

1% N2 1% 2.4x10-7 A

• Measure Gain of the EM at a value of High Voltage

Gain = I(Ar-38: EM)

I(Ar-38: FC)

Sensitivity and Gain can be quickly measured with sequential

FC/EM scans with Selected Ion Monitoring of Ar species.

1.0E-13

1.0E-12

1.0E-11

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

00:00.0 00:08.6 00:17.3 00:25.9 00:34.6 00:43.2 00:51.8 01:00.5 01:09.1 01:17.8 01:26.4

Time (m:s)

Ion

Cu

rre

nt

(A)

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

CIS

To

tal

Pre

ss

ure

(m

b)

Ar-40 EM (Off Scale)

FC FCEM

Pressure: Calibration Reference ON

Gain = 445 + 8

Gain = 443 + 38

S(Ar-40) = 1.1E-5 A/mb

Ar-36 FC

Ar-38 FC

Ar-40 FC

Ar-38 EM

Ar-36 EM

Sensitivity Monitoring Data shows Shifts

in Sensitivity Related to the Sensor

MASS(40) Sensitivity Vs TimeFor CIS2 (W filament; 40eV/200uA)

Reference Gas: Inficon/VTI Calibration Standard

2.20E-04

2.25E-04

2.30E-04

2.35E-04

2.40E-04

2.45E-04

2.50E-04

2.55E-04

2.60E-04

0 2 4 6 8 10 12 14 16

Time (Days)

Ar-

40

Se

ns

itiv

ity

(A

mp

/To

rr)

2-Sigma Deviation 5% of Average over 15 days

5% Shift in Average Sensitivity on Day 7

2-Sigma: 2% over 7 days

2-Sigma: 3% over 8 days

Composition Calculations: Accurate Compositional Analysis is the Goal for Process Monitoring

• QMS measures the Ion Current vs Time for the Gas being sampled

• Partial Pressures of Species Present are Calculated Ion Currents with Corrections for Interferences, and Sensitivities

• Mol-% Composition is defined as

Xi(mol-%) = 100 * PPi / ∑ PPj

For Quality Assurance, Accuracy is measured with known mixtures:

ΔXi = Xi - Xi (Std) [for Major Components]

TMPTMP

DragDrag

8 CapillariesPprocess = 0.5 – 2 Atm

i.d. = 0.25 mmL = 1.5 m

Diaphragm Pump

35μ Leak CIS 100 amu QMS w FC

Ion Currents vs Time from 2 Gas Streams

How do we relate Ion Current to Partial Pressure?

Calibration

Molecule e-Cross Sections at http://physics.nist.gov/PhysRefData/Ionization/molTable.html Atom e-Cross Sections inferred from Wutz Handbuch Vacuumtechnik Ed 9 - 2006, Bild 12.42

Ionization Cross Sections(Å2) differ substantially with e- Energy

• Ion Gauges use 150 eV

• INFICON OIS uses 105 eV

• Most OIS and CIS’s use

70 eV for high Sensitivity

• Some OIS and CIS use 40 eV for reducing Fragmentation and Multi-Charge peaks e.g. [Ar++]

• IG Sensitivity ratios to N2 are at best a guess for RGAs

• Sensitivity Measurements are Required for Species of interest for a RGA

Partial Pressures are Calculated from Ion Current in Real Time using a Previous Calibration

Partial Pressure = Sens Factor * Trans(M) * [Ion Current – Interferences]

PPi = SFi * MZ * [ Ii - Σk≠i Aik * Ik ]

The Result of the Partial Pressure Calculation is Real Time Display of the Partial Pressures and Clearing Times when Gas Comp Changes

4m-% H2 in Ar 4m-% H2 in Ar Air

More informative is the Composition of the Gas Species showing Stable Composition after Conditioning the Inlet System

5 Min Elapsed Time Composition Table at the Time with the Yellow Cursor

4m-% H2 in Ar 4m-% H2 in Ar Air

“Safe” Hydrogen [4 mol-% H2 in Ar] is a Low-Cost Reference Mixture

• Initial Calibration for H2 was done with 2.0% H2 in Synthetic Air (N2 & O2) in August 2011 • Data for 4% H2 in Ar (at right) began a month later. Data shown are values from a single scan sampled at random during the sampling of the 4 mol-% H2 in Ar from the Trend Data over 12 days • The Mean Value for H2 over 12 days is 3.88 + 0.34 Mol-%

Air Components Altered by Seasons and Breathing

Sampling Room Air is a Convenient Reference Gas

• Capillary Sampling and CIS Analysis of Room Air over 160 Days

• Calibration for this data done in August 2011; No Sensitivity Drift Trend

• Mean Value of N2, O2, Ar is within 1 σ

• Data displayed is a randomly selected, single measurement value of Xi

• O2 and Ar have σ ~ 2 %-RSD

• H2O and CO2 are Biased due to Seasonal ΔRH and Breathing in Lab

Now to Summarize…

In Situ Calibration by Two Methods Provides Accurate

Partial Pressures and/or Composition to a RGA / IG

• Calibrated Flow Reference Gas Mixtures Deliver Known PPi’s and Xi’s to the Ionization Region

• Method of Introduction is Designed for a Particular Application

• Regular Measurements for PPi’s and Xi’s Plotted over Time Reveals Accuracy, Drift Trends, Time for Recalibration and Data for Sensitivity or Gain Changes

• Flow Standards should be designed to last > 1 year or Flow Rate Re-Calculated Based on Time Measured Use

XHV UHV Low Pressure Processes Atmospheric Processes

Tritium Fume Hood

Process Capillaries

Tritium Glove Box

Tritium Glove Box

Standards Capillaries

D/TPump

V

T

T

P

D/TPump

D/T/He - 1

D/T/He - 4

D/T/He - 2

D/T/He - 3

30 – 60 m

CDG

3 L

Magnetic SectorMass

Spectrometer

Process Room Analytical Lab

<< 1 mbSample

Pressure

• A Tank of Reference Gas is Sampled by the Same Means as Process Gas

• Certified Gas Mixture should be Pertinent to the Process

• Do QA Measures and Plot Difference Results

• Reference Gas Supply should last > 1 Year in planned useage

Summary: Operation of RGAs for Analytical Measurements

• Choose Ion Source potentials that produce linear stable operation – Electron Energy, Emission Current, Extraction/Focus Voltages and Ion Energy

• Choose a Calibration Reference Gas for Calibration Verification (QA) – For UHV/XHV: Consider a Viscous Leak with Mix Pressure defining Flow Rate.

Needs Flow Rate vs Pmix Calibration + Conductance (Calc/Measure); or Total P @ RGA

– For UHV: Fixed Flow Rate Calibration Reference Mix (INFICON or VTI) + Conductance @ OIS

– 0.01 mb<Pprocess<100 mbar: Calibration Reference Mix into CIS; Conductance of CIS

– In each application, calculate Composition in Ion Source given Viscous in – Molecular Out

– For Atm Processes: Tank with Gas Mix representing the Process; Sample with Capillary [In this case, the composition in Ion Source is the same as Tank due to Mol. In - Mol. Out]

• Calculate Sensitivities from Partial Pressures at Ion Source from Cal Gases – Initial Calibration requires known gases in the list of Species to be analyzed.

– Determine Fragmentation Factors during these initial calibrations

– If a gas species not available, estimates of the Sens can come from Gauge Constants or σ(E)

– Choosing a model for Ion Transmission, e.g. T(M) = Mz where 0 < Z < 1 to make SF ≈ σ(E)

• For High Ion Source Pressure, Xi -mol-% more stable than PPi for species

• QA: Plot differences , Xi –Xi(Std) vs Time to monitor when to Re-Calibrate

Acknowledgements

• INFICON RGA Engineers (Developed the Calibration Reference Source)

– Tim Karandy, Michael Vollero, Peter Schubert, Ken Rosys, Lou Frees

• Mound Technical Solutions, Inc (MTS) – Doug McClelland, Steve Huff

• AVS/IUVSTA Colleagues

• EMRP IND12 Chairs for the Invitation to Speak – Janez Šetina and Karl Jousten

And to All of You for Listening:

Hvala (Thank You)