Tutorial on Deposition of Functionalized Films

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

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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 = , , radicals, neutrals (and photons) • Particles are not in equilibrium • Two important concepts: unit of energy (eV) and average energy per , Emean • 1 eV is KE gained by when loses 1V of PE and conversion to K: 1.6 헑 10⁻¹⁹J 1eV = = 11,600K 1.38 푥 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:

퐸푚푒푎푛=훾 푃/휙

where 훾 is the duty cycle for pulsed plasmas, given by: 훾=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 density compared to neutral/ density in the plasma ….WRONG!

• We need some basic plasma physics to proceed

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Imagine a space plasma, with an imaginary plane

A Net flux of charged particles through an imaginary plane (left)

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A

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Now imagine putting a solid surface in the plasma (e.g. like a chamber wall or substrate)

B Net flux of charged particles to a solid surface (right)

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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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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(푥)²=½M v²-eV(푥)

Schematic of the sheath and pre-sheath adjacent to a wall in contact with a plasma A Michelmore et al , RSC Advances, 2013, 3, 13540

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Presheath – Between the plasma and the sheath – For sheath to be stable region of positive space charge: • Local < local ion density – But at the sheath edge • ion density = electron density (Boundary condition)

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The Bohn Criterion

Solution for these conditions to exist: D. Bohm (1949) ions enter sheath with velocity > acoustic velocity

ퟏ ퟐ ퟏ 풌푻풆 풗풊 = 풌푻풆 풎 풂풏풅 푱풊 = 풆풙풑 − 풏풊 ퟐ 풎풊

푱풊 ퟏ 푻풆 Ion flux increased by = ퟐπ 풆풙풑 − due to the surface! 푱풕 ퟐ 푻풊

So, if Ti ~300K, enhanced ion flux proportional to Te!

If Te = 30,000K, ion flux increased ~15x due to the surface!

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Measuring Ion Flux

• At equilibrium, ion flux = electron flux – No net current • Need to exclude electron current to measure ion current – Apply negative voltage

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Measuring Ion Flux • 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 t V RF Pulse on

RF chopped and measure V vs time

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Measuring Ion Flux

• Sobelewski method (1998) – Uses internal RF electrode – Measure electrode current at bottom of RF sweep

RF Voltage

Measure current at min. V and average

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Measuring Ion Flux - HMDSO

9

s)

8 2

7 ions/m

6 18 5 0.5mT 4 1mT 3 1.5mT 2

1 Positive Ion Ion Flux (10 Positive 0 0 10 20 30 40 50 RF Power (W)

Ion flux increases with RF power, and decreases with

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Ion Energy

Energy of ions arriving at a grounded surface can be measured with Plasma Mass Spectrometers

Ions undergoing collisions

in the sheath, lose energy Counts

0 5 10 15 20 25 30 35 40 45 50 Ion energy (eV)

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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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