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Radical detection in deposition plasmas by threshold ionization mass spectroscopy. P é ter Horv á th. Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology K á roly R ó zsa Research Institute for Solid State Physics and Optics of the - PowerPoint PPT Presentation
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Alan GallagherJILA, University of Colorado and National Institute of Standards and Technology
Károly RózsaResearch Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences
Péter Horváth
Radical detection in deposition plasmas by threshold ionization mass spectroscopy
Sun
Motivation:
Territory of Hungary: 93,000 km2
Solar irradiation: ~ 1 kW/m2
Sunny hours/year: ~1000 h
Solar energy could provide 10,000 GW(Total electricity production is 4 GW)
Primary production concerns:1. Price (1/production time)2. Product lifetime (stability)3. Efficiency (10-20%)
Photovoltaics are primarily made ofamorphous or microcrystalline siliconwhich are produced in Plasma Enhanced Chemical Vapor Deposition systems
Other applications:Large-area semiconductor devices:eg. Thin Film Transistor Liquid Crystal Displays(that’s what you are watching right now)
Introduction
It’s a 2 billion dollar industry, growing 25 % a year.
Introduction
Amorphous or microcrystalline?
Amorphous silicon Microcrystalline silicon
Easier and faster productionHigh film growth ratesPure SiH4
Less stable:Light irradiation decreases efficiency
Slower, more expensive productionLow film growth ratesR>15:1 H2-SiH4 mixtures
More stable
Two goals:• Understanding of the amorphous-microcrystalline transition• Increasing the growth rate of the microcrystalline film
How are these devices produced?
Plasma Enhanced Chemical Vapor Deposition
SiH4
Low pressure glow discharge(usually RF)
Substrate
Ions: SixHy+
Electrons: e-
Neutral radicals: SixHy
Higher silanes: SixH2x+2 (publ.)
Hydrogen: H2
Silicon particles (publ).
Sili
con
film
Discharge chemistry:Optical detection methods
Film properties:optical methods,
diffraction methods device testing
Samplingorifice
Massspectrometer
Particle fluxes:Mass spectrometry
Atomic Hydrogen: H
Introduction
H2-SiH4
or
Mass spectrometer:Breakup pattern
Ele
ctro
n be
am
Ion optics
QuadrupoleMass selector
ChanneltronElectron multiplier
Source
Ar Ar+ Ar++, 36Ar+e-beamSiH4
Si+
SiH+
SiH2+
SiH3+
Si++
SiH++
SiH2++
SiH3++
29Si+29SiH+
29SiH2+
29SiH3+
Si2+
Si2H+
Si2H2+
Si2H3+
Si2H4+
Si2H5+
Si2H6+
e-beam
neutral ion
Exp. Method
28Si 92 %29Si 5 %30Si 3 %
Mass spectrometer:What do we actually measure?
Discharge
Ele
ctro
n be
am
Ion optics
QuadrupoleMass selector
ChanneltronElectron multiplier
?30Ion+ SiH2
+
or 29SiH+
SiH4
SiH3
SiH2
SiH2+
29SiH29SiH+
Satellite peak of SiH3+
(depending on MS resolution)
To make things even worse, the radicaldensities are less than 10-4 of silane density!
ionsneutrals
and ions
Exp. Method
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
Principles of Threshold Ionization Mass SpectrometryThreshold energies (eV) for SiHn ions from neutrals
TIMS requires precise control of electron energy (±0.1 eV)Low temperature, indirectly heated cathode is necessaryto reduce the thermal energy spread of electrons
Exp. Method
The main trick is to tune the energy of the electrons in the ionizer
Mass spectrometer:Improving measurement accuracy
Discharge
Ion deflectors
Ele
ctro
n be
am
Ion optics
QuadrupoleMass selector
ChanneltronElectron multiplier
RF Discharge is modulated
(on/off)
Reactive species can be separated
from stable molecules
Deflectors canbe turned on/off
We can measurethe ion signal orreduce it by a factor of 104
Electron energyand current can
be changed
We can perform athreshold scan or
eliminate the ion currentby measuring at diff.
electron currents
Can operate at2 or 5 MHz
5 MHz is used for0-5 AMU range
Channeltrongain can be setby setting HV
Ampl.
Amplifier gainis controlled by
computer
106 dynamicrange for ion
detection
Exp. Method
Scanning electron energy below the dissociative ionization threshold. Note that the actual electron energy is 2.6 eV lower than the cathode voltage.
Sample threshold scan on SiH3+ for detecting SiH3 radicals
Exp. Method
9 10 11 12 13 14 15 16 17 181E-4
1E-3
0.01
0.1
1
10
100
1000
10000D
ete
cto
r si
gn
al [
nA
@ 2
30
0 V
]
Cathode voltage [VK]
Discharge On 1.1uA Discharge On 0.1 uA Discharge Off 1.1 uA Discharge Off 0.1 uA
Ions
Radicals
Background Gas
Radical threshold
Dissociative ionizationthreshold
Ionization due to high energy tail of electronenergy distribution
80 counts/s
Experimental ApparatusApparatus
Film growth rate measurement setup
Laser PhotoDiode
RF Gen
Ion deflectors
Magnets
Cathode
Anode
Faraday Cup
Ion lensesQuadrupole
ChanneltronPressuregauge
Electron beam
RF Plasma
Pumpingorifice
Samplingorifice
+
–
Turbo pump 1Turbo pump 2
Turbo pump 3
Sparker
Electro-meter
Gas inlet
0.3 Torr
1.6x10-4
Torr
1.5x10-6
Torr3x10-5
Torr
Film growth rate measurement setup
Laser PhotoDiode
RF Gen
Ion deflectors
Magnets
Cathode
Anode
Faraday Cup
Ion lensesQuadrupole
ChanneltronPressuregauge
Electron beam
RF Plasma
Pumpingorifice
Samplingorifice
+
–
Turbo pump 1Turbo pump 2
Turbo pump 3
Sparker
Electro-meter
Gas inlet
0.3 Torr
1.6x10-4
Torr
1.5x10-6
Torr3x10-5
Torr
• High vacuum (~ 10-8 Torr) to keep the e-gun cathode surface clean• Pressures: 0.3 Torr (pure silane) and up to 2 Torr (H2-SiH4) during operation• Gas flow: 5 sccm in pure silane and up to 100 sccm in hydrogen mixture• Vacuum: < 10-5 Torr during operation
Apparatus
Apparatus
Mass spectrometer and ionizer head
Apparatus
The mass spectrometer head was designed and built to minimize the discharge-ionizer distance in order to maximize the radical density in the ionizer. Effective differential pumping was also important to minimize the background pressure in the ionizer.
Si SiH SiH
2SiH
3Si
2Si
2H Si
2H
2Si
2H
3Si
2H
4Si
2H
5
0
10
20
30
40
50
60
70
80
90
100
110R
el.
De
nsi
ty
Species
5<0.1 <1
100
0.3 Torr SiH4
1 A/s film growth rate
<0.11.4
18
1.66.3
2.5
ResultsRadical densities in pure silane discharge
Measured relative radical densities in pure silane discharge agree with the results of Robertson and Gallagher (J. Appl. Phys. 59 (10) 3402, 1986)
Experimental investigation of radicals so far concentrated mainly on low pressure (<0.1 Torr) SiH4 and Ar-SiH4 discharges. This time, we also investigate the H2-SiH4 discharges.
Si SiH SiH
2SiH
3Si
2Si
2H Si
2H
2Si
2H
3Si
2H
4Si
2H
5
0
10
20
30
40
50
60
70
80
90
100
110
0.1 A/s film growth rate
<6<4<3<3
Re
l. D
en
sity
Species
100
<3
15
<4 <6
1.5 Torr R=40 H2-SiH
4 mixture
<8
ResultsRadical densities in hydrogen-silane discharges
The measured radical densities for 40:1 and20:1 H2-SiH4 discharges have similar behavior: SiH3 is by far the most important radical; Si2H2 is smaller, but comparable, while the rest of the radicals can not be positively identified. (Only R=40 is shown here.)
These results are new and provide importantinformation about the mechanism of film growthin the ever more important hydrogen-silane discharges.
Results
160 170 180 190 200 210 220 230 240 2500.00
0.05
0.10
0.15
0.20
0.25R
ad
ica
l sig
na
l [n
A @
23
00
V]
Discharge voltage [Vpp]
SiH3
Si2H
2 x10
1.5 Torr, R=20 H2-SiH
4 discharge
Major radical densities vs. discharge voltage
27 28 29 30 31 32 330
2
4
6
8
10
12
14
16
Sig
na
l [n
A @
13
00
V]
Mass [amu]
Silane Ions from the discharge
54 55 56 57 58 59 60 61 62 63 640
2
4
6
8
10
12
14 Disilane Ions from the discharge
Sig
na
l [n
A @
13
00
V]
Mass [amu]
27 28 29 30 31 32 330.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5Silane Ions from the discharge
Sig
na
l [n
A @
13
00
V]
Mass [amu]54 55 56 57 58 59 60 61 62 63 64
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45 Disilane Ions from the dischargeS
ign
al [
nA
@ 1
30
0 V
]
Mass [amu]
ResultsMass spectra of discharge ions
Pure silane:mostly stripped ions (less hydrogen)
Hydrogen-silane(R=40):hydrogenated ions
SiH+
SiH3+
Si2H2+
Si2H2+
SiH+
SiH3+
Si2+
Si2H5+
Si2H5+
ResultsConclusions
We measured the mono- and disilicon radical fluxes in R=20 and 40 hydrogen-silane discharges and compared the results to the previously known pure silane results. All of the above cases show similar behavior, SiH3 and Si2H2 being the major radicals contributing to film growth. The similar radical chemistry does not explain the difference in resulting film quality and growth rate.
The measured ion spectra is shifted towards the hydrogenated ions in the highly diluted case. The different ion bombardment can result in a different film structure, explaining the amorphous-microcrystalline transition.
Growth rate measurements in highly diluted (R>80) case show negative growth rates. (Not shown here.) This observation indicates a possible atomic hydrogen and/or hydrogen-ion etching of the film. This can also explain the transition between amorphous and microcrystalline film.
Articles:
P. Horvath, K. Rozsa, A. Gallagher: Production of higher silanes in radiofrequency SiH4 and H2-SiH4 plasmas. J. Appl. Phys. Vol. 96, 7660 (2004)
G. Bano, P. Horvath, K. Rozsa and A. Gallagher: The role of higher silanes in silane-discharge particle growth J. Appl. Phys. Vol. 98, 013304 (2005)
P. Horvath and A. Gallagher: Threshold ionization mass spectroscopy of radicals in RF SiH4 and H2-SiH4 plasmas (in preparation)
Publications in this topicPublications
Conference Contributions:
P. Horvath, K. Rozsa, A. Gallagher: Polysilane production in RF SiH4 and H2-SiH4 plasmas XXVIth International Conference on Phenomena in Ionized Gases, Greifswald 15-20 July, 2003
P. Horvath, A. Gallagher: Threshold ionization mass spectroscopy of radicals in a radiofrequency SiH4 plasma, XXVIIth International Conference on Phenomena in Ionized Gases, Eindhoven 17-22 July, 2005
Acknowledgements
I would like to acknowledge the help of my supervisors, Alan Gallagher and Károly Rózsa. I would like to thank my collagues, Wengang Zheng and Damir Kujundzič for valuable discussions. I also thank the JILA instrument shop personel for the construction of the apparatus.
Thank you for your attention!
Film growth rates
Film growth rates are measured by laser inter-ferometry on the grounded electrode. A 780 nmdiode laser was used in a perpendicular setup.
Film growth rate is one of the most importantparameters in industrial systems, and helps usto relate our results to other experiments.
Apparatus
780 nm
diode laser
PD
Polishedelectrode
Growing film
Ground electrode with ~100 nm amorphous silicon film
time
ResultsFilm growth rates
100 150 200 250 300 350 400 450 5000.00
0.05
0.10
0.15
0.20
0.25
0.30
Film
Gro
wth
Rat
e [n
m/s
]
RF peak-to-peak voltage [V]
1.0 Torr 36:1 1.5 Torr 36:1 2.0 Torr 36:1 1.0 Torr 16:1 1.5 Torr 16:1 2.0 Torr 16:1
Film Growth Rates in H2-SiH
4 plasmas
100 150 200 250 300 350 400 450 5000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9Film Growth Rates in SiH
4 plasmas
Film
Gro
wth
Ra
te [n
m/s
]
RF peak-to-peak voltage [V]
230 mTorr 250 mTorr 272 mTorr 300 mTorr 323 mTorr 337 mTorr 355 mTorr
Film growth rates are measured at different pressures, hydrogen dilution ratios and discharge voltages.
Typically 0.1-0.3 nm/s growth rates are used for high quality film production. Highergrowth rates result in unstable films.
Microcrystalline silicon films produced in H2-SiH4 has better quality and long-term stability, but the growth rates are limited to < 0.1 nm/s.
IntroductionSilicon based photovoltaic devices
Light
a-Si p
a-Si i
a-Si n
Glass
TCO
Metal
e-
hole
Typical p-i-n structure of a PV device
Photoelectric effect works in every semiconductor material. Silicon is commonly used, because it is easy to dope to both p and n-type.
Amorphous silicon is preferred over single crystals, because it absorbs light 40 times better. → device can be 1 μm thin → cheaper to produce → can be produced on top of flexible materials
Electron-hole recombination rate in pure a-Si is very high due to dangling bonds → + hydrogen → Hydrogenated amorphous silicon (a-Si:H)
a-Si:H devices suffer from the Staebler-Wronski Effect:Device efficiency decreases by ~20% after exposure to sunlight
Microcrystalline silicon (μC-Si:H ) can be produced by adding hydrogen to the feed gas during production → more stable film
• Film growth rate is decreased as dilution ratio (R) is increased• Transition from amorphous (R<10) to microcrystalline film (R>20) is not fully understood
0.0 0.5 1.0 1.5 2.0 2.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0Io
n S
ign
als
[arb
. no
rm. u
nits
]
Time [s]
Silane Disilane Molecular hydrogen Atomic hydrogen Silyl radical
Discharge
Time dependence of densities in a pulsed dischargeExtra
27 28 29 30 31 32 330
20
40
60
80
100 Silane spectrum
Sig
nal [
nA @
130
0 V
]
Mass [amu]
54 55 56 57 58 59 60 61 62 63 640.0
0.2
0.4
0.6
0.8
1.0 Disilane spectrum (produced by the discharge)
Sig
na
l [n
A @
13
00
V]
Mass [amu]
Typical mass spectra of stable gases in the discharge
Silane Disilane
Extra
26 27 28 29 30 31 32 33 34 35
1E-4
1E-3
0.01
0.1
1
10
100
1000Io
n S
ign
al [
nA
]
Mass [amu]
DC/RF: 0.1600 0.1610 0.1620 0.1630 0.1640 0.1650 0.1660 0.1670 0.1680 0.1690 0.1700
Silane Peaks vs. DC/RF setting
Mass_spec_resulotion_03-29-05.OPJ
ExtraMass spec. resolution vs. DC/RF setting
34 35 36 37 38 39 40 41 421E-5
1E-4
1E-3
0.01
0.1
1
10S
ign
al [
nA
@ 1
30
0 V
]
Mass [amu]
Properly aligned quadrupole Misaligned quadrupole
Satellite_peaks_03-29-05.OPJ
Extra
Effect of quadrupole misalignment on mass peaks
36Ar
38Ar
40Ar
LG Phillips cathode used for the measurementsExtra
Extra
Apparatus
-40 -30 -20 -10 0 10 20 30 40
-40
-30
-20
-10
0
10
20
30
40
Deflector_optimization_2D_28-13-05.OPJ
H+
3 1400 V
Defl+ [V]
De
fl- [V
]
-3.900
-3.035
-2.170
-1.305
-0.4400
0.4250
1.290
2.155
3.020
Logarithmic scale
Optimizing deflector voltages for maximum ion rejection
Defl+
Defl-
Extra
Design of ion optics:calculation of ion trajectories
Courtesy of Wengang Zheng
Extra
-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
3.000
6.000
9.000
12.00
15.00
18.00
21.00
24.00
Optimizing ion optics to collect the best possible signal
Silane beam
EntranceE-beam
Screen
Ion2
Ion3 (exit)
Extra