Download Current status

Threshold ionization mass spectroscopy of
radicals in RF silane discharge
Progress report 11/19/2003
Contributors:
Alan Gallagher
Peter Horvath
Karoly Rozsa : Design of apparatus
Damir Kujundzic : Forevacuum assembly and RF Power Supply
Wengang Zheng : Design of QMS for threshold ionization
Schematics of TIMS apparatus
Diagram of the vacuum system
Discharge chamber
Ionization chamber
Mass Spectrometer
chamber
TP
1
TP
2
TP
3
Turbo Pumps
Rough Pumps
Current status:
• The system is built
• Vacuum in the chamber
can reach 3x10-7 Torr
• Argon and silane lines
are built
• Pyrolizer is tested at
900 OC with silane flow
• Gauges and leak valves
are calibrated for 0-10 sccm
air, argon and silane flow
• Hydrogen line is to be
built
Pyolizer
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
Flow test for the differential pumping
Flow test with argon
discharge chamber assembled
-5
8x10
5.8 sccm
-5
Pressure in other chambers [Torr]
7x10
Flows are obtained
by previous calibration
of foreline pressures
-5
6x10
4.3 sccm
-5
5x10
-5
4x10
3.0 sccm
The ionizer & mass
spec. chambers are not
separated yet.
-5
3x10
1.7 sccm
-5
2x10
Buffer chamber
Ionizer chamber
Mass spec. chamber
-5
1x10
0
0
100
200
300
Pressure in the discharge chamber [mTorr]
400
Electronics
Current status:
• New RF Power Supply for the discharge is under construction
Fully 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 apparatus
for 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
Commercial Ametek QMS
Rectifier
(unknown)
RF
Amplifier
Problems:
• No coupling capacitor for rods
• Programmable frequency
(internally controlled)
• RF amplitude is controlled by
internal microcomputer
• Unable to lock on given mass
RF amplitude is tested against 4
values to get the mass peak int.
• Cannot be controlled from our
own computer (& unit is broken )
Comp
Reference
Programmable
Oscillator
Internal
microcomputer
± ½ AMU
Modified QMS
Modifications:
• tuning capacitor
• fixed frequency oscillator
• our own rectifier
• our own feedback loop
• external DC voltages
Tunable
capacitor
RF
Amplifier
Crystal
Oscillator
Rectifier
(our)
Comp
Reference from
our own computer
TIMS-related modifications:
• Own ionizer, variable energy
• Our own ion optics
• Electron Multiplier
Advantages:
• Full control over RF and
DC voltages
• Full control of ionizer energy
• High stability quartz oscillator
• Amplitude stability by feedback
Results of modification
Mass scan in HWSD experiment
2.0
Original unit
With broken frequency control 1
and without feedback for RF
amplitude 2
Amplitude
1.5
1.0
0.5
0.0
5
10
15
20
25
30
35
40
45
Mass
6
Modified unit
With stable frequency and
own feedback for RF amplitude
5
Amplitude
4
3
2
1
1
0
5
10
15
20
25
Mass
30
35
40
45
by accident, IC cannot be replaced
removed to gain control over RF
amplitude
2
Priciples of Threshold Ionization Mass Spectrometry
Threshold energies (eV) for SiHn ions from neutrals
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
Operation of TIMS
Sample threshold (energy) scan in HWSD experiment
-3
1.2x10
W/ radicals
(hot filament)
-3
1.0x10
-4
Amplitude
8.0x10
-4
6.0x10
-4
4.0x10
In silane
w/o radicals
(cold filament)
-4
2.0x10
0.0
8
9
10
11
12
13
14
Voltage applied on the cathode(-volts)
15
16
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…