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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