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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“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

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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

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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”

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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

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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)

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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

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Why do plasmas only treat the surface?

• Low penetration depth (nm scale)

- Ions

- Electrons

- Activated / reactive gas

• Medium penetration depth (m scale)

- Photons

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What happens at the surface?

Starting Gas

Substrate

Effluent

Reaction, Etching or ablation

Chemical modification or Roughening

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Are ions important in film growth? We can compare ion mass flux to total deposition

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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).

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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

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PP of acrylic acid

CH2 CH C

O

OH

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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)

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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

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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

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Deposition rate

W/F

Dep

osi

tio

n R

ate

OIL

FILM

POWDER

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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

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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

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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

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Functional group retention

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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

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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

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Functional group retention: • Many emerging

technologies depend upon precise surface chemistries

• Power no 1 parameter in preservation of functionality of starting compound

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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

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Conclusions:

• Yasuda written for another era

• W/F crude parameter

– Better to measure intrinsic plasma parameters

• Films do not grow (necessarily) in uniform way across surfaces

• Ions can be important

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