Threshold ionization mass spectroscopy of radicals in RF silane discharge

1. Threshold ionization mass spectroscopy ofradicals in RF silane discharge Progress of experiments Peter Horvath 5/19/2004

2. Overview Experiment is fully assembled Electronics and computer control work Electron gun seems to work Mass spectrometer works (most of the time ) RF generator can operate discharge both in argon and silane Film growth rate measurement setup is built by Alan, experiment is ready to start Discharge and mass spectrometer do not work together  (yet)

3. Schematics of TIMS apparatus

4. Mass spectrometer and ionizer Ionizer assembly Mass spectrometer assembly

5. Electronics and computer control Mass spectrometer unit and step-up transformer head

6. Electronics and computer control Quadrupole Magnets F-Cup Pressure RF Gen Rect. Laser PD Ion deflectors Cathode Anode Ion lenses Channeltron Mass spec. Control unit Electrometer 45 V amplifier Labview

7. Problems with the electron gun The current arriving to the Faraday cup is very small, Only 1% of the total emission Magnet Cathode Screen F-Cup • Possible reasons: • misalignment (--) • magnetized SS (?) • weak B field (+) • inhom. E field (?) The total emission is well below space charge limit. Anode Magnet • reflecting anode (+) Limited cathode lifetime • contamination • operating temp. too high • 7V offset in the emission vs. voltage curve • the anode may be covered by an insulating layer

8. Improvements on electron gun Improvements on Electron gun: • moly anode and screen • bigger hole on anode • double magnets • F-cup is painted black • anode is painted black • plate in front of the • cathode • system is baked • lower cathode temp

9. Performance of electron gun • Typical operating conditions: • cathode voltage: -10 V • anode-cathode voltage: 2.4 V • emission current: 4.4 uA • anode current: 0.9 uA • screen current: 2.5 uA • F-cup current: 1.0 uA

10. Mass spectrometer: mass spectrum • Mass spec. works: • good resolution • reliable mass setting • (no drift) • huge dynamic range: • 1pA - 1uA • works well with • high silane pressure

11. Mass spectrometer: threshold scan

12. Mass spectrometer: optimization of ion optics Water (background) Silane (beam) Entrance E-beam Screen Ion2 Ion3 (exit)

13. Test of mass spectrometer with discharge Black curve is the initial spectrum (without discharge)

14. Observations in silane discharge Discharge was operated for about an hour on 5/17/04. Pressure: 250-300 mTorr silane Flow: 5 sccm. Peak-to-Peak RF voltage: 170 V Vacuum in the ionizer chamber: 3x10-5 Torr (2x10-6 in the mass. spec chamber) Silicon deposition on windows: ~500 nm (estimation from interference rings) Observations: Mass spectrometer signal is completely disappeared I was not able to record any signal ever since Channeltron may be coated with silicon? Pressure started to increase at constant flow; Flow is reduced when I adjusted the pressure (flow is measured by foreline pressure) Pumping speed from the discharge chamber is reduced to 1/5 of the original Vacuum in ionizer region improved to 7x10-6 while the pressure in the discharge chamber was kept constant The pumping orifices are probably blocked by deposited silicon A big chunk of material can be observed at the sampling orifice inside the discharge chamber

15. Conclusions • Electronics (including mass spec.) work reliably • Electron gun is ok. • Hopefully the cathode lifetime can be increased by baking the system • The system works well without discharge • Silicon deposition from the discharge will cause a lot of problems… Plans • Make the channeltron work again • Deal with the silicon deposition • Film growth rate measurements • Deal with ions from the discharge • Radical measurements • Start Labview in the morning and go hiking… 