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

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)

Schematics of TIMS apparatus

Mass spectrometer and ionizer Mass spectrometer assembly Ionizer assembly

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

Electronics and computer control LaserPD RF Gen Ion deflectors Magnets CathodeAnode F-Cup Ion lenses Quadrupole Channeltron 45 V amplifier Mass spec. Control unit Electrometer Rect. Pressure Labview

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

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 Improvements on Electron gun: system is baked lower cathode temp

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

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

Mass spectrometer: threshold scan

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

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

Observations in silane discharge Discharge was operated for about an hour on 5/17/04. Pressure: 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) Mass spectrometer signal is completely disappeared I was not able to record any signal ever since Observations: 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) Vacuum in ionizer region improved to 7x10 -6 while the pressure in the discharge chamber was kept constant Pumping speed from the discharge chamber is reduced to 1/5 of the original A big chunk of material can be observed at the sampling orifice inside the discharge chamber The pumping orifices are probably blocked by deposited silicon

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…