Advanced Technology Center1861 Lefthand CircleLongmont, CO 80501Phone: (303)678-0700FAX: (303) 442-0711

Measurement of Trace Oxygen in Corrosive Gases by GC-DID

Mark Raynor, Terry Gerhart, Jon Welch hans, Brad Grissom, Clark McGrew and

Virginia Houlding

Matheson Tri-Gas Inc., Advanced Technology Center, 1861 Lefthand Circle,Longmont CO 80501

Venue:Gas Workshop:Date:Time:Place:

SEMICON West, 2001Determination of Low Levels of O2 in Specialty GasesTuesday, July 17, 20011:30 pm -4:00 pmSan Francisco Marriott Hotel55 4th StreetSan Francisco, California

Measurement of Trace Oxygen in Corrosive Gases by GC-DID

Mark Raynor, Terry Gerhart, Jon Welch hans, Brad Grissom, Clark McGrew and

Virginia Houlding

Matheson Tri-Gas Inc., Advanced Technology Center, 1861 Lefthand Circle,Longmont CO 80501

The device yield and performance characteristics of many microelectronic products arecritically dependent on the presence of trace impurities in the semiconductor processgases used in their manufacture. In particular, atmospheric impurities, such as oxygen,in corrosive gases can alter certain semiconductor manufacturing processes. In plasmaetching of polysilicon films, for example, O2 has been reported to affect the CI2 dischargeproperties and hence the etch rate of the process (1). A reliable analytical method istherefore necessary to monitor and control the level of this important impurity.

Typically, oxygen in corrosive gases such as C12, HCI and HBr, is analyzed by gaschromatography with discharge ionization detection (GC-DID). With this technique, thecorrosive matrix gas is prevented from entering the detector by using a pre-column thatpreferentially retains the corrosive matrix peak, but allows the elution of atmosphericimpurities on to the analytical column. The pre-column is then back-flushed to vent thecorrosive gas matrix, while the impurities of interest on the analytical column areseparated and detected by the DID.

Unfortunately, difficulties are often encountered with oxygen impurity analysis. GC-DIDsystems that show good performance for measurements made with standards in an inerthelium matrix, may not necessarily perform in the same way when exposed to corrosivegas matrices. Furthermore, instruments that detect O2 at high concentrations may notdetect the impurity at ppb levels. The choice of pre-column is critical to the detection ofoxygen at ppb and ppm levels. Porous polymer GC columns that are commonly used foranalysis have been found to absorb trace oxygen in some corrosive gases. Columnpassivation, involving a single injection of a high concentration oxygen gas standard iscommonly employed to prevent oxygen absorption. However, this is not a permanentsolution, as continued heating of the packing material or exposure to corrosive gas mayresult in a gradual increase in oxygen absorption over time, affecting instrumentcalibration. This paper focuses on the analysis of O2 impurity in corrosive gases. TheGC-DID technique, as well as factors influencing detection of O2 in HCI, HBr and CI2, willbe discussed and results obtained with different pre-columns will be presented.

H.H. Sawin, L.D. Baston, D. Gray, L. Tepermeister, M.T. Mocella and G.C. Zau,Threshold levels and effects of feed gas impurities on plasma etching processes, JElectrochem. Soc., 137, 3526 (1990).

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MEASUREMENT OF TRACE OXYGEN INCORROSIVE GASES BY GC-DID

Mark Raynor, Terry Gerhart, Jon Welch hans, BradGrissom and Virginia Houlding

Advanced Technology Center, Matheson Tri-Gas, Inc.Longmont, CO 80501

SEMICON West 2001, Gas Workshop, July 17, 2001

A MATHESON..TRI'GAS

Semi-Gas Oivision

Introduction

.Micro-electronic product performance can beeffected by the presence of trace impurities in theprocess gases used in manufacture

.Oxygen is particular problem in corrosive gasessuch as HCI, HBr and CI2

-In plasma etching of polysilicon films, O2 hasbeen reported to affect the CI2 dischargeproperties and hence the etch rate (1)

.A reliable analytical method is necessary tomonitor and control O2 levels

(1) H.H. Sawin et al., J. Electrochem. Soc.. 137. 3526 (1990)

A MATHESON~. TRI'GAS

Semi.Ga, Di,,;,ion

.Galvanic cell

-Very sensitive technology but not compatible with

corrosive gases

.Mass spectrometry

-50 ppb MDL in helium possible. El sensitivity dependant

on matrix. difficult to calibrate, detector drifts

.APIMS

-very sensitive. requires special source for corrosive gas

analysis, expensive

.Paramagnetic susceptibility

-sensitivity limited to ppm levels

.GC-DID

-low ppb sensitivity. but need to ensure matrix does not

enter detector

A. MATHESON..TRI.GAS

Semi-Ga, Division

GC-DID Requirements

.Discharge ionization detector -Universal response dueto the high ionization potential of Helium (19.8 eV). Hecapable of ionizing all compounds except for Ne (21 eV)

.DID requires gettered high purity helium carrier gas

.Leak free connections

.Important to keep corrosive matrix from entering DID

.Various Separation Strategies

-Series by-pass

-Heartcut

-Fore-flush

-Back-flush

A MATHESON

..:r:~r~f:t.n~"-,--

..

Sequence of Events in GC-DID Analysis with Back-flushof Corrosive Gas Matrix

Pre-column Analytical Column

He -+1 -Ii t==j ~

Matrix Impurities

He 1 -~III ~

Scrubber .-1 -~ III~

He

Scrubber .- ..-£::a.- ~DIDI ,

He

GC-DID Flow Schematic

~ MATHESON

..TRI'GASSemi-Gils Oivision

5

Valve 1.

Injection/Back-flushPre-column

Valve 2 -AnalyticalColumn Selection

Valve 3.DID/Bypass

A MATHESON~. TRI-GAS

Sem;-G.. D;,,;,lon

~

Calibration and Validation

.Many GC calibrations are performed in inert gas matrix

.Important to perform the cali~ration in the same matrixgas as that of the sample

-Retention time changes ~ay result in incorrectidentification of impurity

-Changes in peak shape may affect quantification

-Reactions that don't occur in inert matrix may occurin corrosive gas matrix a~d affect detection limits

.With the use of a dilution manifold, the corrosive matrixcan be spiked dynamically with known concentrationsof standard gas

.Standard in corrosive matrix used to calibrate GC-DID

A MATHESON~ .TRJoGAS

Semi-Gos Dil/ision7

GC-DID Sampling and Dilution System

tielium Purge

Standard Gas in

~I~

HCI

[1-~.:-.~~~---1: i ReOOout :

:'::::::::::::::i i: :, ,: ,: i.Valve :l ~~ ,

~

Rotameter

C¥J+1To Scrub~r

HelIumPurge

A MATHESON

..TRI-GASSem;.tia, D;,,;,ion

8

"¥ ,

--,

Detection of Oxygen in HBr: Matrix Spiked with Gas Standard

WXX),

&XXXJ

..4(XXX) -!

<:mX>~.200!) -..

1(XXX)-

0 2

0. C...'.nIr3Uon

Porapak R precolumn, Porapak Qand Mol Sieve 13X analyticalcolumns in parallel

Based on five replicate injections ateach concentration

Linear response (R2 = 0.9999)2.. 5.0

Time (mln)

7.5

A MATHESON..TRI.GAS

Semi-Go. Divi.;ol1

11

Absorption of Trace Oxygen by Worous Polymer ColumnI

c

0.

j

~

00 2.' ,.. 7.'n-Imln)

10 ppm standard In He afterexposure to atmosphereand reinstallation

,., 2.' '0 7.5"-(m'")

10 ppm standard in He priorto atmospheric exposure

00 25 5.0 7.5"-(mln)

10 ppm standard in Heusing new or reconditionedcolumn

A MATHESON

...TRI'GASSem;G.. Div;,lon

12

GC-DID Response to O2 in He After Exposure to CI2

10 ppm Gas standardcontaining H2, O2, N2, CO andCH4 in Helium

Pre-column: Porapak R at 40°C

Analytical column: MolecularSieve 13X at 30°C

A MATHESON~ .TRI'GAS

Semi-Gas Oivis;on13

Porous Polymer Pre-column

.O2 analysis possible using polrous polymer pre-columnin HCI and HBr matrices

.Reactivation or column passivation techniques may berequired

-Injection of high concentration of O2

-Recondition column in dryl O2 at 200°C

-Continuous addition of O2 ~o carrier gas

.In CI2 matrix, low levels of O2 is absorbed and/or reactswith porous polymer packing i

-Detector response for all analytes is reduced in CI2matrix I

.SiJica-based pre-column investigated

A MATHESON

..TRpGASSem"G.. Division

14

~