Threshold ionization mass spectroscopy ofradicals 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 assemblyIonizer assembly
Electronics and computer control
Mass spectrometer unit and step-up transformer head
Electronics and computer control
Laser PD
RF Gen
Ion deflectors
Magnets
Cathode Anode
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 theFaraday 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
MagnetScreenF-Cup Cathode
AnodeMagnet
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
0 2 4 6 8 100
10
20
30
40
50
Anode_scan_051804.OPJ
Em
issi
on c
urre
nt [
A]
Anode-Cathode voltage [V]
10V 9V 8V 7V
Emission current
0 2 4 6 8 100
2
4
6
8
Anode_scan_051804.OPJ
Far
aday
Cup
cur
rent
[A
]
Anode-Cathode voltage [V]
10V 9V 8V 7V
F-Cup current
1 A
-8 -6 -4 -2 0 2 4 6 80
1
2
3
4
5
6
7
16 %
13 %
F-C
up c
urre
nt [
A]
F-Cup bias voltage [V]
12 eV 11 eV 10 eV 9 eV 8 eV 7 eV
13 %
18 %
24 %
31 %
4.6 eV
3.6 eV
3.5 eV
3.4 eV
3.3 eV3.7 eV
Change of currentbw/ 0 & 9 V bias
With of stepbw/ 10 & 90%
Effect of biasing on F-Cup current
Test_e_gun_F_Cup_biased_04-28-04.OPJ
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
10 15 20 25 30 35 40 45
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
SiH++
2
SiH++
Si+
Ion
sig
na
l [n
A]
Mass [amu]
SiH+
2
SiH+
3
O+
2
Ar+
CO+
2 x20
SiH+
Si++
H2O+
OH+
H3O+
O+
3.9 mTorr SiH4
3.1 mTorr Ar
Residual water x20
Sample mass spectrum
Sample_mass_scan_051604.OPJ
Mass spec. works:• good resolution• reliable mass setting (no drift)• huge dynamic range: 1pA - 1uA• works well with high silane pressure
10 12 14 16 18 200.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
15.23
Data: Thresh042804_B
y=A*exp((x-x0)/t
0)
A 0.10656 ±0.00475x
015.23 ±0
t0
0.41661 ±0.02772
Ion
sig
na
l [n
A]
Electron energy [eV]
Threshold scan with water(ionization energy: 12.61 eV)
Mass spectrometer: threshold scan
Mass spectrometer: optimization of ion optics
-6 -5 -4 -3 -2 -1 0-40
-35
-30
-25
-20
-15
-10
-5
0
Ion3 voltage [V]
Ion
2 v
olta
ge
[V]
-0.005000
0.1887
0.3825
0.5762
0.7700
0.9637
1.157
1.351
1.545
-6 -5 -4 -3 -2 -1 0-40
-35
-30
-25
-20
-15
-10
-5
0
Ion3 voltage [V]
Ion2
vol
tag
e [V
]
0
3.000
6.000
9.000
12.00
15.00
18.00
21.00
24.00
EntranceE-beam
Screen
Ion2
Ion3 (exit)
Silane (beam)
Water (background)
25 26 27 28 29 30 31 32 33 34 35
1E-4
1E-3
0.01
0.1
1
10
100Io
n s
ign
all
[nA
]
Mass [amu]
0 min 3 min 7 min 10 min 16 min 22 min
Discharge_n_mass_spec_051804
Evolution of mass spectrum during discharge
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: 250-300 mTorr silaneFlow: 5 sccm.Peak-to-Peak RF voltage: 170 VVacuum 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 disappearedI was not able to record any signal ever since
Observations:
Channeltron may becoated 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 chamberwas kept constant
Pumping speed from the discharge chamberis reduced to 1/5 ofthe original
A big chunk of material can be observed at thesampling orifice inside the discharge chamber
The pumping orificesare probably blockedby 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…
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