Master Class: Electronegative Plasmas September 28-30

Diagnostics for Electronegative Plasmas

Winfred Stoffels

Outline

Introduction

Standard diagnostics

Specific diagnostics

Using negative ion diagnostics for neutrals

Dusty plasma

Electronegative plasmas

Example plasma: capacitively coupled

radiofrequency low pressure plasma

Properties of electronegative plasmas

Negative ions

Charged

Trapped in the discharge polymerization

Ne + N_ = N+

Changed ionization and loss rates plasma

quenching (switches)

Changed transport different I-V characteristics

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

We can use traditional diagnostics to follow negative ions: resistance

We can use traditional diagnostics to follow negative ions: optical emission

We can use traditional diagnostics to follow negative ions:

Charge neutrality

Transport

Plasma structure and current

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

Outline

Introduction

Using negative ion diagnostics for

neutrals

Dusty plasma

OutlineIntroduction

Probes

Mass spectrometry

Photo detachment

Dusty plasma

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.

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

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

Pulse plasma.

Negative and positive ions in SiH4 plasma

(after Howling)

direct measurement : Surface produced negative ions

Example:

O- produced at kathode and measured at anode

OutlineIntroduction

Probes

Mass spectrometry

Photo detachment

Dusty plasma

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

Microwave resonance

Electron and ion kinetics can be measured

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)

Selectivity depends on electron affinity EA of F- 3.4 eV (364nm)

(Haverlag)

•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

+ easy

+/- spatial information modified by transport true plasma

- difficult to make quantitative

Photodetachment in combination with Optogalvanic method

Photodetachment in combination with probe(Bacal)

+ easy

- need theory

- laser hits probe and can result in photo electrons

from probe

Photodetachment in combination with probe(Bacal)

+ quantitative

+ no model needed

+ gives information on ion kinetics

+/- line of sight

- complex

Photodetachment in combination with microwave resonance(Eindhoven)

Photodetachment in combination with microwave resonance

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

particle charge: measured by photodetachment

As particles grow:

-Charge on particles increases

-Recharging time decreases

OutlineIntroduction

Probes

Mass spectrometry

Photo detachment

Dusty plasma

•Optogalvanic

•Probe

•Microwave resonance

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

Electron Attachment Mass Spectrometry

Polymerization in C2F6 plasma:

Negative ion fragments Positive ion fragments

Combinations of Mass spectrometry and photodetachment

Boesl, Muenchen

Combinations of Mass spectrometry and photodetachment

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

OutlineIntroduction

Probes

Mass spectrometry

Photo detachment

Dusty plasma

Solar Cells

m etalnegatively doped layer

intrinsic a-S i:H

positively doped layertransparent conductive oxide

incom ing light

a solar cell

Consists of multiple layers with different functions

Produced in low pressure RF plasma

Layer is an amorphous hydrogen rich silicon layer

Amorphous H-Si

The Staebler-Wronski effect

0 50 100 150 2000

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)

Solar Cells

Roca i Cabarrocas et al, Thin Solid Films 403-404 (2002) 39-46

cr:Si a:Si

m etalnegatively doped layer

intrinsic a-S i:H

positively doped layertransparent conductive oxide

incom ing light

Dust Particles

0.1nm 1nm 10nm 100nm 1μm

molecule macro-molecule nano-particle agglomerate powder

α-regime γ´-regime

amorphous not usable

•Neutral chemistry

•Particles

•Electrons and ions

Internal plasma parameters:

•Emission spectroscopy: ASDF, dissociation

•RGA, FTIR, LIF: dissociation, polymerization, ASDF

•Infrared absorption spectroscopy: dissociation, polymerization, temperature

•Infrared CRD: radical densities

•Ellipsometry: surface chemistry

•Particles

Results available

Available if needed

Under construction

•Particles

•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

•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

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

The “neutral chemistry” setup

Infrared CRDS

The “electrons and ions” setup

Microwave resonance

Photodetachment

Emission increases with time

After some time particles are ejected

Time delay depends on temperature

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

Measurement Scheme

Particle detectionDissociation processes

Electric parameters

He-Ne laser beam

0 20 40 60 80 100Time [s]

tal cu

0 20 40 60 80 100Time [s]

120-95

0 20 40 60 80 100Time [s]

p=0.133 mbar f=10 sccm 5%SiH4 in Ar V=155 mV (P=8 W)

current voltage phase

0 20 40 60 80 100Time [s]

0.0005

0.0015

0.0025

0 20 40 60 80 100TIme [s]

0 20 40 60 80 100Time [s]

0 20 40 60 80 100

Time [s]

Si-H line

0 20 40 60 80 100 120 140

He-Ne laser

0 10 20 30 40 50 60

Halfa line

0 20 40 60 80

Si-H line

0 20 40 60 80

170C= 290KHe-Ne laser

0 20 40 60 80 100 120 140

Halfa line

0 20 40 60 80 100 120 140

He-Ne laser

0 50 100 150 200 250

Halfa line

0 50 100 150 200 250

Si-H line

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

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

500 nm

The End

Master Class: Electronegative Plasmas September 28-30

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