Threshold ionization mass spectroscopy ofradicals in RF silane discharge

Progress report 11/19/2003

Contributors:Alan GallagherPeter HorvathKaroly Rozsa : Design of apparatusDamir Kujundzic : Forevacuum assembly and RF Power SupplyWengang Zheng : Design of QMS for threshold ionization

Schematics of TIMS apparatus

D ischarge cham ber Ionization cham ber

M ass Spectrom etercham ber

TP1

TP2

TP3

R ough Pum ps

Pyolizer

Turbo Pum ps

Diagram of the vacuum system

Current status:• The system is built• Vacuum in the chambercan reach 3x10-7 Torr• Argon and silane lines are built• Pyrolizer is tested at900 OC with silane flow• Gauges and leak valvesare calibrated for 0-10 sccmair, argon and silane flow

• Hydrogen line is to bebuilt

Schematics of the vacuum chamber assembly

Current status:• All parts are manufactured• Discharge chamber is assembled (windows are needed)• Discharge was operated in argon flow

• Ionizer and mass spectrometer are to be assembled

0 100 200 300 400

0

1x10-5

2x10-5

3x10-5

4x10-5

5x10-5

6x10-5

7x10-5

8x10-5 Flow test with argondischarge chamber assembled

Pre

ssu

re in

oth

er

cha

mb

ers

[To

rr]

Pressure in the discharge chamber [mTorr]

Buffer chamber Ionizer chamber Mass spec. chamber

1.7 sccm

3.0 sccm

4.3 sccm

5.8 sccm

Flow test for the differential pumping

Flows are obtainedby previous calibrationof foreline pressures

The ionizer & massspec. chambers are notseparated yet.

Electronics

Current status:• New RF Power Supply for the discharge is under constructionFully computer controllable (power supply works, minor components are needed)• Data acquisition computer boards are acquired (installation is needed)

QMS electronics will be identical to Wengang Zheng’s TIMS apparatusfor Hot Wire Silicon Deposition experiment

• The commercial Ametek QMS needed to be modified (will be discussed later)• The modified apparatus works satisfactory in the HWSD experiment• An other unit need to be modified for the discharge experiment

• Software is written for HWSD experiment (Labview)• Software need to be adapted for discharge experiment

Rectifier(unknown)

Comp

Reference

RFAmplifier

ProgrammableOscillator

Commercial Ametek QMS

Internalmicrocomputer

Problems:• No coupling capacitor for rods• Programmable frequency(internally controlled)• RF amplitude is controlled byinternal microcomputer• Unable to lock on given massRF amplitude is tested against 4 values to get the mass peak int.• Cannot be controlled from our own computer (& unit is broken )

± ½ AMU

Rectifier(our)

Comp

Reference from our own computer

RFAmplifier

CrystalOscillator

Modified QMS

Modifications:• tuning capacitor• fixed frequency oscillator• our own rectifier• our own feedback loop• external DC voltages

TIMS-related modifications:• Own ionizer, variable energy• Our own ion optics• Electron Multiplier

Advantages:• Full control over RF andDC voltages• Full control of ionizer energy• High stability quartz oscillator• Amplitude stability by feedback

Tunablecapacitor

5 10 15 20 25 30 35 40 450.0

0.5

1.0

1.5

2.0

Am

plitu

de

Mass

5 10 15 20 25 30 35 40 450

1

2

3

4

5

6

Am

plitu

de

Mass

Results of modificationMass scan in HWSD experiment

Original unitWith broken frequency control 1

and without feedback for RF amplitude 2

Modified unitWith stable frequency andown feedback for RF amplitude

1 by accident, IC cannot be replaced2 removed to gain control over RF amplitude

Neutral Parents Si+ SiH+ SiH2+ SiH3

+

Si 8.2

SiH 11.2 9.5

SiH2 10.4 13.1 9.7

SiH3 13.1 11.3 12.5 8.4

SiH4 12.5 15.3 11.9 12.3

Priciples of Threshold Ionization Mass SpectrometryThreshold energies (eV) for SiHn ions from neutrals

8 9 10 11 12 13 14 15 16

0.0

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

1.2x10-3A

mp

litu

de

Voltage applied on the cathode(-volts)

Operation of TIMSSample threshold (energy) scan in HWSD experiment

In silane w/o radicals

(cold filament)

W/ radicals(hot filament)

To do in the near future…

• Assemble and wire the ionizer and mass spectrometer• Finish computer controlled RF Power Supply• Modify commercial QMS unit• Build our own step-up transformer and rectifier for QMS• Build computer control of MS, ionizer and ion optics• Optimize ion optics• Calibrate MS• Measurements…