Master Class: Electronegative Plasmas September 28-30
Diagnostics for Electronegative Plasmas
Winfred Stoffels
Master Class: Electronegative Plasmas September 28-30
Outline
Introduction
Standard diagnostics
Specific diagnostics
Using negative ion diagnostics for neutrals
Dusty plasma
Master Class: Electronegative Plasmas September 28-30
Electronegative plasmas
Master Class: Electronegative Plasmas September 28-30
Example plasma: capacitively coupled
radiofrequency low pressure plasma
Master Class: Electronegative Plasmas September 28-30
Properties of electronegative plasmas
Negative ions
Charged
Heavy
Trapped in the discharge polymerization
Ne + N_ = N+
Changed ionization and loss rates plasma
quenching (switches)
Changed transport different I-V characteristics
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Standard diagnostics show influence of negative ions
Approach
Use electropositive plasma as comparison
Measure as a function of dilution
Beware that transition electropositive to electronegative can occur at small dilutions
Master Class: Electronegative Plasmas September 28-30
We can use traditional diagnostics to follow negative ions: resistance
Master Class: Electronegative Plasmas September 28-30
We can use traditional diagnostics to follow negative ions: optical emission
Master Class: Electronegative Plasmas September 28-30
We can use traditional diagnostics to follow negative ions:
Charge neutrality
Master Class: Electronegative Plasmas September 28-30
We can use traditional diagnostics to follow negative ions:
Transport
Master Class: Electronegative Plasmas September 28-30
We can use traditional diagnostics to follow negative ions:
Plasma structure and current
Master Class: Electronegative Plasmas September 28-30
Conclusion
Transition from electropositive to electronegative plasma
occurs at small dilution
Visible in:
Resistance
Voltage
Current
Emission
Charge balance
Plasma and sheath structure
Measurable by traditional plasma diagnostics
Master Class: Electronegative Plasmas September 28-30
Outline
Introduction
Standard diagnostics
Specific diagnostics
Using negative ion diagnostics for
neutrals
Dusty plasma
Master Class: Electronegative Plasmas September 28-30
OutlineIntroduction
Standard diagnostics
Specific diagnostics
Probes
Mass spectrometry
Photo detachment
Using negative ion diagnostics for neutrals
Dusty plasma
Master Class: Electronegative Plasmas September 28-30
Direct negative ion measurement with Probes(Amemiya)
+ simple and cheap setup
- “not” species selective
- difficult to measure
Negative ion current is visible in the second derivative of the probe characteristic as a small peak just above 0 V.
Master Class: Electronegative Plasmas September 28-30
Ion Mass Spectrometry (QMS, TOF, …)
+ Well known technique
+ Direct ion measurement
+ Mass selective most often chemical identification possible
+ Sensitive
- Detects a flux not a density plasma model is needed
- Mass dependent transmission model is needed
- Poor spatial and temporal resolution
Master Class: Electronegative Plasmas September 28-30
Ion Mass Spectrometry (negative ions)
- Negative ions are trapped in glow
Solutions:
positive bias disturbs plasma
typical approach in ion sources
Pulse plasma
» Afterglow measurement
» Diffusion model needed
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Pulse plasma.
Negative and positive ions in SiH4 plasma
(after Howling)
Master Class: Electronegative Plasmas September 28-30
direct measurement : Surface produced negative ions
Example:
O- produced at kathode and measured at anode
Master Class: Electronegative Plasmas September 28-30
OutlineIntroduction
Standard diagnostics
Specific diagnostics
Probes
Mass spectrometry
Photo detachment
Using negative ion diagnostics for neutrals
Dusty plasma
Master Class: Electronegative Plasmas September 28-30
Photodetachment
X- + hv X + e The negative ion is transformed into a free electron
Lasers provide the needed photon
Electron can be measured easily
Optogalvanic
Probe
Microwave resonance
Electron and ion kinetics can be measured
Master Class: Electronegative Plasmas September 28-30
Photodetachment
X- + hv X + e+ Local measurement
+ High time resolution (pulsed laser)
+ Kinetic information
+/- Specific on species depends on threshold
- Electron diagnostic needed (and its minus points)
- Need to check for other laser induced effects (ionization of
neutrals)
Master Class: Electronegative Plasmas September 28-30
Selectivity depends on electron affinity EA of F- 3.4 eV (364nm)
(Haverlag)
Master Class: Electronegative Plasmas September 28-30
•Photodetachment can be done in two regimes
linear +less disturbing- cross section must be known
saturation+ independent of cross section-hard to acchieve
After Bacal
Master Class: Electronegative Plasmas September 28-30
+ easy
+/- spatial information modified by transport true plasma
- difficult to make quantitative
Photodetachment in combination with Optogalvanic method
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Photodetachment in combination with probe(Bacal)
+ easy
- need theory
- laser hits probe and can result in photo electrons
from probe
Master Class: Electronegative Plasmas September 28-30
Photodetachment in combination with probe(Bacal)
Master Class: Electronegative Plasmas September 28-30
+ quantitative
+ no model needed
+ gives information on ion kinetics
+/- line of sight
- complex
Photodetachment in combination with microwave resonance(Eindhoven)
Master Class: Electronegative Plasmas September 28-30
Photodetachment in combination with microwave resonance
Master Class: Electronegative Plasmas September 28-30
negative charge in dusty plasma
Beginning: only negative ions
ne = 1013 m-3
Slow electron capture
After 1 sec:
charged particles
ne = 1014 m-3
Fast electron capture
Master Class: Electronegative Plasmas September 28-30
particle charge: measured by photodetachment
As particles grow:
-Charge on particles increases
-Recharging time decreases
Master Class: Electronegative Plasmas September 28-30
OutlineIntroduction
Standard diagnostics
Specific diagnostics
Probes
Mass spectrometry
Photo detachment
Using negative ion diagnostics for neutrals
Dusty plasma
•Optogalvanic
•Probe
•Microwave resonance
Master Class: Electronegative Plasmas September 28-30
Electron Attachment Mass Spectrometry
Use electronegative character to analyze neutrals
XY + e XY- or X- + Y
+ negative ions can be mass specific detected by QMS
+ lower electron energy needed less fragmentation
+ resonant process possible to selectively create negative
ions
Master Class: Electronegative Plasmas September 28-30
Electron Attachment Mass Spectrometry
Master Class: Electronegative Plasmas September 28-30
Polymerization in C2F6 plasma:
Negative ion fragments Positive ion fragments
Master Class: Electronegative Plasmas September 28-30
Combinations of Mass spectrometry and photodetachment
After
Boesl, Muenchen
Master Class: Electronegative Plasmas September 28-30
Combinations of Mass spectrometry and photodetachment
Master Class: Electronegative Plasmas September 28-30
Figure 1: Partial cross section for the process K- + γ - K(5s) + e- in the photon energy range from
4.19 eV to 4.26 eV. Curves: present results in dipole velocity (solid) and dipole length
(dotted) approximations
After Chien-Nan Liu
Master Class: Electronegative Plasmas September 28-30
OutlineIntroduction
Standard diagnostics
Specific diagnostics
Probes
Mass spectrometry
Photo detachment
Using negative ion diagnostics for neutrals
Dusty plasma
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Master Class: Electronegative Plasmas September 28-30
Solar Cells
m etalnegatively doped layer
intrinsic a-S i:H
positively doped layertransparent conductive oxide
glass
-
+
incom ing light
Master Class: Electronegative Plasmas September 28-30
a solar cell
Consists of multiple layers with different functions
Produced in low pressure RF plasma
Layer is an amorphous hydrogen rich silicon layer
wall
Amorphous H-Si
rf
C
Master Class: Electronegative Plasmas September 28-30
The Staebler-Wronski effect
0 50 100 150 2000
2
4
6
8
10
pm Si
Eff
icie
ncy
(%
)
Time (hours)
He 250oC Std 250oC
Std 150oC
Std 100oC
After Roca i Cabarrocas et al, Ecole Polytechnique, Palaiseau, France
• Solar cell degradation induced by exposition to sun light
• Initial degradation during the first 200 kWh/m2
• Embedded nanocrystalline structures increase solar cell lifetime and efficiency (pm-Si)
Master Class: Electronegative Plasmas September 28-30
Solar Cells
Roca i Cabarrocas et al, Thin Solid Films 403-404 (2002) 39-46
cr:Si a:Si
pm:Si
m etalnegatively doped layer
intrinsic a-S i:H
positively doped layertransparent conductive oxide
glass
-
+
incom ing light
Master Class: Electronegative Plasmas September 28-30
Dust Particles
0.1nm 1nm 10nm 100nm 1μm
molecule macro-molecule nano-particle agglomerate powder
α-regime γ´-regime
amorphous not usable
Master Class: Electronegative Plasmas September 28-30
•Neutral chemistry
•Particles
•Electrons and ions
Internal plasma parameters:
Master Class: Electronegative Plasmas September 28-30
•Neutral chemistry
•Emission spectroscopy: ASDF, dissociation
•RGA, FTIR, LIF: dissociation, polymerization, ASDF
•Infrared absorption spectroscopy: dissociation, polymerization, temperature
•Infrared CRD: radical densities
•Ellipsometry: surface chemistry
•Particles
•Electrons and ions
Results available
Available if needed
Under construction
Internal plasma parameters:
Master Class: Electronegative Plasmas September 28-30
•Neutral chemistry
•Particles
•Electrons and ions
•Microwave resonance: electron density
•Photodetachment: negative ion density
•Doppler resolved LIF, energy resolved QMS: IEDF, E-field
•Langmuir probe, Thomson scattering: EEDF
Results available
Available if needed
Under construction
Internal plasma parameters:
Master Class: Electronegative Plasmas September 28-30
•Neutral chemistryResults available
Available if needed
Under construction
•Particles
•Mie Scattering: density, size, spatial distribution
•Laser heating, LIPEE: presence, size
•Photodetachment: particle charge; charging kinetics
•FTIR: particle composition
•Infrared CRD: particle formation
•Electrons and ions
Internal plasma parameters:
Master Class: Electronegative Plasmas September 28-30
Experimental setup
• rf CCP plasma
•Homogeneous gas flow though top rf showerhead and bottom grid in closed configuration
•Controled temperature using heater/cooling system
•Variable gas: today only 5% SiH4 in Ar
Master Class: Electronegative Plasmas September 28-30
The “neutral chemistry” setup
Infrared CRDS
FTIR
OES
The “electrons and ions” setup
Microwave resonance
Photodetachment
PIM
Master Class: Electronegative Plasmas September 28-30
Emission increases with time
After some time particles are ejected
Time delay depends on temperature
video
14.39
15.40
16.19
Master Class: Electronegative Plasmas September 28-30
Plasma impedance monitor•a small capacitance and a pickup coil in rf line
Determines:
•Voltage, current en phase between them•For the fundamental frequency and the first 6 harmonics
•With a time resolution of 0.1 s
V I
RF
Master Class: Electronegative Plasmas September 28-30
Measurement Scheme
Particle detectionDissociation processes
Electric parameters
He-Ne laser beam
OMA-
PIM
Master Class: Electronegative Plasmas September 28-30
0.8
0.9
1
1.1
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1.4
0 20 40 60 80 100Time [s]
Fu
nd
am
en
tal cu
rre
nt
[A]
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80
120
35
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0 20 40 60 80 100Time [s]
Fu
nd
am
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tal V
olt
ag
e [
V]
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120-95
-93
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-81
0 20 40 60 80 100Time [s]
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nd
am
en
tal P
has
e [
de
gre
e]
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80
120
p=0.133 mbar f=10 sccm 5%SiH4 in Ar V=155 mV (P=8 W)
current voltage phase
353K
393K
290K
3rd h
arm
onic
1st
har
mon
ic
fund
amen
tal
0
0.005
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0.025
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st
harm
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ic [
A]
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A]
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ic [
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V]
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0 20 40 60 80 100Time [s]
Fir
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on
ic [
de
gre
e]
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-50
0
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250
0 20 40 60 80 100
Time [s]
Th
ird
harm
on
ic [
de
gre
e]
17
80
120
Master Class: Electronegative Plasmas September 28-30
Si-H line
0
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He-Ne laser
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Halfa line
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Si-H line
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HE
-NE
Ha
SiH
170C= 290KHe-Ne laser
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Halfa line
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He-Ne laser
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Halfa line
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Si-H line
0
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0 50 100 150 200 250
800C= 353K 1200C= 393K 100 secp=0.133 mbar f=10 sccm 5%SiH4 in Ar P=8 W
Master Class: Electronegative Plasmas September 28-30
Same behavior in electron density measured by microwave resonance
-fast decay with several phases
-oscillations as several generations grow
-electron density growth as particles are expelled
Master Class: Electronegative Plasmas September 28-30
5 mm
Master Class: Electronegative Plasmas September 28-30
10 mm
500 nm
Master Class: Electronegative Plasmas September 28-30
The End
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