Rob Short Mawson Institute University of South Australia

1. Plasma Processing and Deposition of Polymers: Rob Short Mawson Institute University of South Australia Mawson Institute

2. Presentations covers: • Plasma processing of polymers – technological importance + examples • External parameters and limitations • Plasma phase and chemical processes • Plasma – linking chemistry and physics • Film Growth - The orthodox view - Newer ideas Mawson Institute

3. Renaissance in plasma polymerisation • Traditionally, scratch resistance, wetting, new chemistry without changing bulk properties • Recent-cited applications: - Coating of tissue engineering scaffolds (Adv. Mater., 2006, 18,1406) - Functionalisation of nanotubes for covalent coupling of quantum dots (Adv. Mater., 2007, 19, 4003) - Fabrication of a microcantilever fast humidity sensor (Adv. Mater., 2007, 19, 4248) - Micro- and nano-engineering of surface structures, (Adv. Mater.,2006, 18, 1406; Adv. Mater., 2007, 19, 1947; Adv. Mater., 2010, 22, , 1451) - Surfaces for high-throughput screening devices (Adv. Mater., 2008, 20, 116; Lab on a Chip, 2011, 11, 541) Mawson Institute

4. A a) b) c) d) B e Figure 2. XPS C1s core levels from the top surface of a) scaffold, b) ppAAm-coated scaffold, and c) the ppAAm- and ppHex-coated scaffold. Components have been included to represent the plasma polymer environments C-C, C-N, C=N/C-OX, and the environments C-C, C(=O)O-C, and C(=O)O-C. A single-component N1s core level was observed at 399.8 eV for all ppAAm deposits, consistent with amine and/or imine functionally. d) Schematic of the ppAAm/ppHex coated surface structure indicated by the XPS data at the outer scaffold surface and e) CT image of a whole scaffold. Figure 4. A) Scaffolds cultured with 3T3 fibroblasts for 24h with mild agitation (the scale bars are 1mm and cells are color-coded in red): a) , b) /ppAAm, and c) /ppAAm/ppHex. The lower images show X-ray CT images from approximately 2mm slices through the centers of the scaffolds. B) Cumulative cell area in the 0.01 mm slices through the centers of the scaffold within the core and the sheath denoted by the black dotted lines in (A). the error bars are the standard error in the mean where n = 20 John J. A. Barry et al, Advanced Materials, DOI: 10.1002 Mawson Institute

5. My own work: • Treatment of severe burns • Not enough skin to graft • Have to grow up ‘more” skin • Culture cells directly on plasma polymer (pp)-coatedbandage • Delivery of cells “off” pp-coated bandage • Highly effective way of getting cells to patients rapidly • Wider range of potential applications Mawson Institute

6. Applications in burns since 2008 – used in over 200 patients with severe burns (Europe) Mawson Institute

7. Anterior trunk Day 10 Day 0 N Zhu, Eur J Plastic Surg, 2005, 26 (3) 319 Mawson Institute

8. Extended wear contact lenses • Functional plasma polymer layer • One of Australia’s most successful exports • A$2.5 bn sold annually! Mawson Institute

9. Google “Plasma” Mawson Institute

10. PDP with up to 120" diagonal screens and millions of individual colour elements Mawson Institute

11. Plasma – what is it? • Often referred to as the 4th state of matter • 99% of the observable universe is plasma • PLASMA The name of "plasma" comes from the Greek plasma, which means something moulded or fabricated Mawson Institute

12. Plasma – what is it? • - Sir Humphry Davy 1808 - d.c. arc discharge • - Michael Faraday 1830’s • Sir William Crookes in 1839, heated a solid melting  vaporising  breakdown • - Irvin Langmuir in 1928 introduced the term “plasma” and described it as “fourth state of matter” alongside gases, liquids and solids. • Plasma consists of free moving electrons and ions • Energy needed to strip electrons from atoms • Thermal, electrical, or light (UV or laser) • With insufficient sustaining power, plasmas recombine into neutral gas • Plasma can be accelerated and steered by electric and magnetic fields - allowing it to be controlled Mawson Institute

13. Examples of plasma: Mawson Institute

14. The Sun – UV light Coronal loops are found around sunspots and in active regions Associated with the closed magnetic field lines that connect magnetic regions on the solar surface Many coronal loops last for days or weeks but most change quite rapidly These loops contain denser material than their surroundings Naturally occurring plasmas Mawson Institute

15. Aurora Charged particles (electrons) guided by the earths magnetic field, spiral around field lines. Electrons accelerate along field magnetic field lines into the upper atmosphere, collide with gas atoms, causing the atoms to give off light Lightning For example, one lightning bolt can generate more than 30 million volts and 250,000 amperes Magnetosphere The Earth's field is compressed on the day side, where the solar wind flows over it. It is also stretched into a long tail Mawson Institute

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17. Plasma exist over a massive range in temperatures and densities Mawson Institute

18. The technological plasma environment Low pressure - Plasma constituents are not in thermal Equilibrium kTe >> kTi kTgas kTsurface • Electrons • Positive ions ( atomic and molecular) • Negative ions ( atomic and molecular) • Excited atoms, molecules and ions • Dissociation products (atomic,molecular, neutral,positive, negative) • Process products (atomic,molecular, neutral,positive, negative} • Clusters/dust • Changing contact surfaces Mawson Institute

19. Remove material • Add material • Change chemical or physical nature of the surface Mawson Institute

20. Surface engineering and plasma technology is part of any “advanced” economy Mawson Institute

21. Market for surface engineering processes in the UK(1995 prices) Market Value Value of manufactured products in £billions critically affected by SE processes 1995 2005 2010 1995 2005 2010 Engineering coatings £4.5 £7.0 £8.9 £82.9 £117.3 £141 Semiconductors £3.0 £9.0 £15.6 £3.3 £9.9 £17.1 Other functional coatings £2.5 £5.4 £7.8 £9.2 £16.6 £22.6 Totals £9.6 £21.3 £32.3 £95.5* £143.9 £180.7 * ~7% of UK GDP Matthews, Artley and Holiday, ‘2005 revisited: the UK surface engineering industry to 2010.’ NASURF, 1998. Mawson Institute

22. UK surface engineering market & value 2005*: Industry Total (£m) Share (%) % industry affected by coatings Automotive 12155.780% Construction 8854.230% Electronic 212610.0100% Retail consumer 12215.740% Aerospace 354 1.7 80% Other Eng. 1166 5.5 51% Optical & display 1892 8.9 100% Data storage 2397 11.2 100% Semiconductor 8990 42.2 100% Decorative 299 1.4 100% Other functional 771 3.6 100% Total coating market 21316 71.1% Engineering sector Functional coatings sector * 1995 prices. Matthews, Artley and Holiday, ‘2005 revisited: the UK surface engineering industry to 2010.’ NASURF, 1998. Mawson Institute

23. Technological plasmas and polymers • Plasma treatment Reactive or inert gas Modify surface • Plasma polymerisation Deposition of ultra-thin film Functionalised Mawson Institute

24. “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

25. Reactor design • Enclosed chamber - Means to introduce gas (treatment), monomer (polymerisation) • Reduced pressure (0.75 mTorr, < 1 Pa → 100mTorr, > 100 Pa) • Method of excitation DC or AC(RF→MW), CW or pulsed Mawson Institute

26. RF preferred for polymers • Displacement rather than particle currents • Stability • Electrons higher temperature • Process insulating materials without sputtering at electrodes Mawson Institute

27. (a) Clark and Dilks reactor design 1977 [ref 18] and three decades of reactor design evolution since, illustrating a variety of electrode configurations, power supplies and diagnostic tools (b) Ward 1989 [19] (c) Lopez et al. 1992 [20] (d) O’Toole et al. 1995 [21] (e) Favia et al., 1996 [22] (f) Candan et al. 1998 [23] (g) Alexander et al. 1998 [24] (h) Voronin et al. 2006 [25] A. Michelmore et al, RSC Advances, DOI 10.1039 Mawson Institute

28. “Clark” Reactor Capacitive No current flows Potentially large self-bias on substrate Mawson Institute

29. External parameters (and limitations) • External = power, pressure, flow rate, (geometry)… How do these affect deposition rate, deposit chemistry? • No direct link to: - Degree of ionisation, i.e. ion density, electron density - Temperature of ions, electrons, neutrals - Electric and magnetic fields • Inadequate description (in plasma polymerisation) of processes leading to film formation Mawson Institute

30. 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

31. Plasma phase and chemical processes • 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 p.d. species have 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 Mawson Institute

32. Ignition (all about electrons) Photon Gas Molecules - Electron + + Ionisation Acceleration due to electric field Mawson Institute

33. Why are the electrons ‘hotter’ • For a particle starting at rest - Kinetic energy INVERSELY proportional to mass after any given time • Mass of electron at least 1800 times smaller than an ion - Electric charge the same • Also, ions lose energy by elastic collisions with gas Mawson Institute

34. Electron energies Mawson Institute

35. 3-5 eV electron impacts sufficient to break bonds Based on EEDF most likely event. In bulk, [radicals] >> [ions] [Radical] goes up as E mean increases Estimate Agarwal: 1 radical in 200 molecules* *1019 radicals per m3 in oxygen plasma (Agarwalet al., J. Vac. Sci. A. 2004, 22, 71) Radicals Mawson Institute

36. KE transferred: e-+ X2 e- + X2* Excited molecules are inherently unstable → fall back to initial (ground) state in either one or more transition steps Each transition step from high to low energy states is accompanied by emission of a photon: X2*  h0 + X2† Some metastables last for 1ms! Excited states, metastables and VUV Mawson Institute

37. Ions • Traditional view, high energy collisions with tail-end electrons > 10 eV • Selected Ion Flow Tube (SIFT) experiments show ions result from collisions between neutral molecules and H3O+ ions.*A proton is transferred from the H3O+ ion to the neutral molecule, N H3O+ + N  H2O + NH+ • Estimates1013 -1016 ions m-3 ( 1 in 10,000→100,000) • (*Steele et al, PPP, 2011, 8, 287) Mawson Institute

38. Reactions in plasma phase Mawson Institute

39. Plasma-phase species: • Mass spectrometry (invasive) • Optical emission spectroscopy (non invasive) Mawson Institute

40. Mass spectral study of plasma composition L O’Toole et al, J Chem Soc., Faraday Trans., 1995, 91, 1363 Mawson Institute

41. Three organic compounds Plasma phase mass spectrum results for three monomers, neutral and positive ions, given as percentage of total counts A Michelmore et al, RSC Advances, DOI 10.1039 Mawson Institute

42. Surfaces change everything! Mawson Institute

43. A Net flux of charged particles through an imaginary plane (left) Mawson Institute

44. A Mawson Institute

45. B Net flux of charged particles to a solid surface (right) Mawson Institute

46. B Formation of (charge density) sheath • All surfaces in contact with the plasma develop a sheath - No glow in this region - Extends upto a few mm from surface • Caused by charging of plasma relative to surface • Initially much higher electron flux at surface - Due to higher velocities • Surface charges negatively until ion flux = electron flux • Typical charge of tens of volts - Ions accelerated in sheath - Ion energies quite large when striking surface - Electrons decelerated (only high energy e’s get through) Mawson Institute

47. 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) Schematic of the sheath and pre-sheath adjacent to a wall in contact with a plasma phase A Michelmore et al , RSC Advances, DOI 10.1039 Mawson Institute

48. Presheath • In sheath, as positive ions accelerate (to surface) they spread out; density decreases • Electrons repelled from surface and ejected from sheath; density also decreases • For sheath to be stable region of positive space charge: Local electron density < local ion density • But at sheath edge ion density = electron density • Solution for these conditions to exist: D. Bohm (1949) ions enter sheath with velocity > acoustic velocity Ion Flux >15 thermal Mawson Institute

49. Uses of technological plasmas • Treatment - surface modification of organic surfaces • Wettability • Adhesion • Polymerisation - chemical synthesis of new organic surfaces • Novel reaction pathways = films with unique properties Mawson Institute

50. Surface modification of organic surfaces Mawson Institute