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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
Mawson Institute
“Trial and error”
• Industrial uptake surged over past 25 years
• Product development lead by “trial and error”
• Significant challenges remain in understanding physics and chemistry of processes
Mawson Institute
Scale of problem: international round robin
Deposition rates for plasma polymerized acrylic acid at different nominal plasma powers (2 sccm nominal flow rate). Error bars on reactor N represent the standard deviation of six repeats each carried out by a different operator J D Whittle et al, PPP, DOI 10.1002
Mawson Institute
Deposition rate for different reactors. Acrylic acid plasma. These were “nominally” the same experiment!
How do films grow: back to the future!
• Much of what is cited (about mechanism) today comes from Yasuda 1985 “Plasma Polymerisation”
• Cited +2000 times
• “Folk Law”
Mawson Institute
What Yasuda said:
– Polymerization is predominately radical (gas phase) – WRONG!
– Films pinhole free/substrate does not influence process – WRONG!
– Can describe deposition through W/F parameter
– WRONG!
Early stage of film deposition non-trivial
AFM images (4mm x 4mm) with profiles showing the evolution of a plasma deposit from heptylamine plasma. Times are 7s (left), 15s, 25s and 40s. (Reproduced with permission from [100])
A Michelmore et al, Plasma Processes Polym., 2011, 8, 367
Mawson Institute
Processes: gas phase • Excitation of monomer
- Ionisation (minimally)
- Fragmentation
- Rearrangement
- Combination of fragments
• If monomer is vinylic - Radical fragments can initiate conventional polymerisation
- Rate is limited at low pressure
• Large fragments may become negatively charged and grow (dusty plasmas)
Mawson Institute
Surface processes • Adsorption of vapour
• Ion bombardment - Transfer of energy to surface
- Sputtering and damage
- Activation of surface / growing film
• Photon activation
• Radical Reactions
• If monomer is vinylic - Activated surface sites can initiate polymerisation
- Likely to be rapid if vapour is strongly adsorbed
Mawson Institute
Why do plasmas only treat the surface?
• Low penetration depth (nm scale)
- Ions
- Electrons
- Activated / reactive gas
• Medium penetration depth (m scale)
- Photons
Mawson Institute
What happens at the surface?
Starting Gas
Substrate
Effluent
Reaction, Etching or ablation
Chemical modification or Roughening
Mawson Institute
Are ions important in film growth? We can compare ion mass flux to total deposition
Mawson Institute
A.J. Beck et al, Chem. Comm, 1998, 1221
Are ions important in film growth?
Table 2 Comparison of total and charged mass deposition rates at closest available P/Ø
Deposition rate/μg m⁻² s ⁻¹
P/Ø Measured Calculatedᵃ
2.3 ± 0.2 52 10.3
3.6 ± 0.2 58 13
7.3 ± 0.2 56 54
10.9 ± 0.2 58 80
ᵃ Calculated using ion flux and average m/z at closest available P/Øs; e.g. for first row (P/Ø = 2.3) we have used the ion flux at 2 W (P/Ø = 2.5) and average m/z at 3 W (P/Ø = 2.14).
Mawson Institute
A.J. Beck et al, Chem. Comm, 1998, 1221
Are ions important in film growth?
• At low W - contribute (directly) no more than 20% of deposit mass
• At high W – potentially contribute all mass!
• Ions found to be more important in fully saturated compounds
Mawson Institute
PP of acrylic acid
CH2 CH C
O
OH
Mawson Institute
Probably the most studied plasma polymers are from Acrylic acid
Contains a C=C bond
Role of carbon double bond? Allows another route to deposition
Deposition Rate (QCMB)
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6 7 8 9
De
po
siti
on
ra
te n
g s-1
P /W
= acrylic acid
= propionic acid
(saturated)
Mawson Institute
L O’Toole et al, J Chem Soc., Faraday Trans., 1995, 91, 1363
BUT
• Acrylic acid is just one of (probably) hundreds of organic compounds that have been deposited by plasma!
• Importance of ions depends upon chemistry of starting compound and plasma parameters
Mawson Institute
W/F parameter
• Common way of reporting plasma conditions:
- Power input (W)
- Flow rate of monomer (F)
• Power is energy / time
• Flow rate is number of molecules / time
• W/F is energy / molecule
Mawson Institute
Deposition rate
W/F
Dep
osi
tio
n R
ate
OIL
FILM
POWDER
Mawson Institute
Broadly, W/F can be used to predict physical properties of the film.
Dangers of the W/F parameter
• (chemical perspective..) assumes mechanism depends only on average amount of energy available to each molecule
- Energy distribution
(Energy barrier)
- Doesn’t take into account time energy is delivered over
• Ignores pressure
• Ignores role of surfaces
Mawson Institute
Dangers of the W/F parameter
• (practical….) ignores how effectively power is coupled with the plasma
• Power transfer efficiency can vary between 5% and 90% depending on: – Chamber geometry/material
– RF electrode type
– Pressure
– Gas
Mawson Institute
External parameters (and limitations)
• External = power, pressure, flow rate, (geometry)… How do these affect deposition rate, deposit chemistry?
• They have no direct link to: - Degree of ionisation, i.e. ion density, electron density - Temperature of ions, electrons, neutrals - Electric and magnetic fields
• They provide an inadequate description (in plasma
polymerization) of the processes leading to film formation
Mawson Institute
What Yasuda said:
– In polymerization of monomers containing O or N (or O and N) functional groups
• Can’t retain functionality
• Do not incorporate O, N or O and N
– Very different technological applications; operating parameters
Mawson Institute
Yasuda (orthodox)
Mi
Mi•
Mi• + M Mi - M•
Mi• + Mj
Mi•
Mi - Mj
+•Mk - Mj
•Mk•
• Mk• + Mj•
• Mk• + M • Mk - M•
• Mk•
• Mk - Mj•
Cross-cycle reaction
Plasmaexcitation
Mawson Institute
Functional group retention: • Many emerging
technologies depend upon precise surface chemistries
• Power no 1 parameter in preservation of functionality of starting compound
Mawson Institute
Finally, pulsed plasmas
• Reduce average power by pulsing the plasma
• Reduces deposition rate
• Greater retention of functional groups
• Works well with vinylic monomers
• Great for making PEO like surfaces (non-fouling…)
• During ‘off’ time conventional polymerisation?
• Physics of these plasmas very different to continuous wave
Mawson Institute