tutorial on plasma polymerization deposition of ... · a. michelmore, d.a. steele, j.d. whittle,...
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A. Michelmore, D.A. Steele, J.D. Whittle, J.W. Bradley,
R.D. Short
University of South Australia Based upon review article
RSC Advances, 2013, 3, 13540-13557
Tutorial on Plasma Polymerization
Deposition of Functionalized Films
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Plasma – Surface Interactions
• For plasma polymerisation, what happens at the surface is key.
• This is the intersection of plasma physics and plasma chemistry.
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Some basic terms and concepts • Plasma = electrons, ions, radicals, neutrals (and photons)
• Particles are not in equilibrium
• Two important concepts: unit of energy (eV) and average energy per molecule, Emean
• 1 eV is KE gained by electron when loses 1V of PE and conversion to K:
1.6 𝗑 10⁻¹⁹J 1eV = = 11,600K 1.38 𝑥 10⁻²³ J K ⁻¹
• eV useful as not only defines temperature, but also DV species have energy to overcome
• Amount of energy per molecule:
𝐸𝑚𝑒𝑎𝑛=𝛾 𝑃/𝜙 where 𝛾 is the duty cycle for pulsed plasmas, given by: 𝛾=ton /((ton + toff )
For continuous wave plasma, this term reduces to 1
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What happens at a surface?
• Does a surface affect the plasma …… YES!
• First described by David Bohm in 1949
• Often not even considered in depositing plasmas.
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Surfaces change everything!
• Traditional view of plasma polymerization does not account for plasma physics at surfaces
• Assume ions not important because low ion density compared to neutral/radical density in the plasma ….WRONG!
• We need some basic plasma physics to proceed
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A Net flux of charged particles through an imaginary plane (left)
Imagine a space plasma, with an imaginary plane
A
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B Net flux of charged particles to a solid surface (right)
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Now imagine putting a solid surface in the plasma (e.g. like a chamber wall or substrate)
B Formation of (charge density) sheath • There is a net flow of negative charge to the surface
– Initially much higher electron flux at surface (hotter and lower mass)
– The surface develops a negative potential compared to the plasma
– All surfaces in contact with the plasma develop a sheath
Electrons start to be repelled from surface
Positive ions start to be attracted to surface
No glow in this region
Extends up to a few mm from surface
– Surface charges negatively until ion flux = electron flux (steady state)
– Typical potential difference of ~10 – 50V
- Positive ions accelerated across sheath to the surface
- Ion energies quite large when striking surface (>10eV)
- Electrons decelerated (only high energy e-s get through)
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Schematic of the sheath and pre-sheath adjacent to a wall in contact with a plasma phase
Within the sheath, ions convert electrical potential energy into kinetic energy as they approach the negatively charged surface. For ion energy conservation:
½ M v(𝑥)²=½M v²-eV(𝑥)
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A Michelmore et al , RSC Advances, 2013, 3, 13540
Presheath – Between the plasma and the sheath
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– For sheath to be stable region of positive space charge: • Local electron density < local ion density
–But at the sheath edge • ion density = electron density (Boundary condition)
The Bohn Criterion
Solution for these conditions to exist: D. Bohm (1949) ions enter sheath with velocity > acoustic velocity
𝒗𝒊 = 𝒌𝑻𝒆 𝒎 𝟏 𝟐 𝒂𝒏𝒅 𝑱𝒊= 𝒆𝒙𝒑 −𝟏
𝟐𝒏𝒊
𝒌𝑻𝒆𝒎𝒊
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𝑱𝒊𝑱𝒕= 𝟐π𝒆𝒙𝒑 −
𝟏
𝟐
𝑻𝒆𝑻𝒊
Ion flux increased by due to the surface!
So, if Ti ~300K, enhanced ion flux proportional to Te!
If Te = 30,000K, ion flux increased ~15x due to the surface!
Measuring Ion Flux
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• At equilibrium, ion flux = electron flux
– No net current
• Need to exclude electron current to measure ion current
– Apply negative voltage
Measuring Ion Flux
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• Braithwaite ion flux probe design
– Apply RF pulse to a surface (~10ms)
– Surface develops negative bias
– Chop RF pulse, and measure probe voltage vs time
– Slope proportional to ion current
V
t
RF Pulse on
RF chopped and measure V vs time
Measuring Ion Flux
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• Sobelewski method (1998)
– Uses internal RF electrode
– Measure electrode current at bottom of RF sweep
RF Voltage
Measure current at min. V and average
Measuring Ion Flux - HMDSO
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0
1
2
3
4
5
6
7
8
9
0 10 20 30 40 50
Po
siti
ve Io
n F
lux
(10
18 io
ns/
m2 s
)
RF Power (W)
0.5mT
1mT
1.5mT
Ion flux increases with RF power, and decreases with pressure
Ion Energy
0 5 10 15 20 25 30 35 40 45 50
Co
un
ts
Ion energy (eV)
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Energy of ions arriving at a grounded surface can be measured with Plasma Mass Spectrometers
Ions undergoing collisions in the sheath, lose energy
Summary
• Neutrals/radicals diffuse to surfaces by thermal motion
• Only hot electrons can impact surface, with reduced energy
• Ions are accelerated to surfaces by the sheath – Increased flux (approx. 15x higher than thermal flux)
– Increased ion energy (typically 20eV)
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