an experimental and analytical study of an asymmetric … · 2018. 11. 8. · global model...
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![Page 1: An Experimental and Analytical Study of an Asymmetric … · 2018. 11. 8. · Global model (Asymmetric) •Assume Z plasma ~ 0 and discharge is two capacitors in series •Plasma](https://reader034.vdocuments.net/reader034/viewer/2022051921/600dd42175d3bb037c138e75/html5/thumbnails/1.jpg)
A. Michelmore, J.D. Whittle, R.D. Short, R.W. Boswell and
C. Charles
Plasma Processes Polym., 2014, DOI:10.1002/ppap.201400026
An Experimental and Analytical Study of an Asymmetric Capacitively
Coupled Plasma Used for Plasma Polymerization
Mawson Institute
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Project Aims • To develop a simple model of a capacitively coupled
plasma
• To verify the model experimentally using a simple, commercially available device (OctIV probe)
• Parameters of interest – Ion flux
– Bias voltages
– Electrode separation
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Experimental setup and model
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Reactor check (OctIV pre-match)
Load impedance and Q factor relatively constant with applied RF power
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Model development (symmetric)
• For all plasma discharges: – mi = nsheaths uB (Bohm flux)
– Pi = e nsheaths uB DV (Ion Energy Flux) • Where nsheaths is the plasma density at the sheath edge
for a symmetrical discharge
• Power balance gives:
– 𝑛sheath𝑠=
𝑃𝑟𝑚𝑠
𝑒𝑢𝐵 𝐴𝑙𝑖𝑣𝑒 𝐸𝑐+𝐸𝑒+𝑉𝑝𝑠 +𝐴𝑒𝑎𝑟𝑡ℎ 𝐸𝑐+𝐸𝑒+𝑉𝑝𝑠
– =𝑃𝑟𝑚𝑠
2𝑒𝑢𝐵𝐴𝑙𝑖𝑣𝑒 𝐸𝑐+𝐸𝑒+𝑉𝑝𝑠 for Alive = Aearth (symmetric)
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Global model (Asymmetric)
• Assume Zplasma ~ 0 and discharge is two capacitors in series
• Plasma potential is affected by asymmetry
• 𝑉𝑝 𝑡 = 𝑉𝑝𝑑𝑐+ 𝑉𝑝𝑟𝑓
sin 𝜔𝑡 and 𝑉𝑝𝑟𝑓=
𝐶live𝐶live+𝐶earth
𝑉𝑟𝑓 ~ 𝐴live
𝐴earth𝑉𝑟𝑓
• 𝑉𝑙𝑖𝑣𝑒 𝑡 = 𝑉𝑑𝑐 + 𝑉𝑟𝑓 sin 𝜔𝑡 and 𝑉𝑑𝑐 = 𝑉self−bias
• Vself-bias can be measured using OctIV probe, or estimated by:
• Vself-bias = -[0.83x1.4xQ(ZPrms)1/2 + 5 kTe] (assume Te = 3eV)
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Power Balance
• For asymmetric discharge, ion flux and sheath edge equations must be modified
• 𝑛sheath𝐴=
𝑃𝑟𝑚𝑠
𝑒𝑢𝐵 𝐴live 𝐸𝑐+𝐸𝑒+ 𝑉𝑝𝑑𝑐−𝑉self−bias +𝐴earth
′ 𝐸𝑐+𝐸𝑒+𝑉𝑝𝑑𝑐
• And mi = nsheathA uB
• Ion Energy Flux equations must also be modified for live and earthed electrodes
• 𝑃𝐴live= 𝑒𝑛sheath𝐴
𝑢𝐵(𝑉𝑝𝑑𝑐− 𝑉self−bias)
• 𝑃𝐴earth= 𝑒𝑛sheath𝐴
𝑢𝐵𝑉𝑝𝑑𝑐
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Comparison with Ar data (electrode separation 4cm)
Solid lines are model predictions Good agreement observed
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Comparison with Ar data (electrode separation 22cm)
Measured self bias voltage (red squares) Solid line is model prediction Measured ion energy (blue diamonds) Dashed line assumes Vp = 5kTe
Dotted line assumes Vp = 8kTe
Measured ion flux Solid line is model prediction
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Comparison with a range of depositing monomers (amines, acids, HMDSO etc)
Measured self bias voltage
Measured ion flux (note RF Power = power deposited into plasma)
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Power transfer efficiency
Oxygen
Argon
Allylamine
Power transfer efficiency is affected by gas chemistry
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Conclusions
• Simple model developed for plasma systems
• Comparison with experimental data shows good agreement
• Modelling with Argon can be used to predict plasma parameters for other gases if power transfer efficiency is taken into account.