RSC Advances REVIEW. Nanoscale deposition of chemically functionalised films via plasma polymerisation. Introduction

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1 REVIEW Cite this: RSC Advances, 2013, 3, Nanoscale deposition of chemically functionalised films via plasma polymerisation Andrew Michelmore,* a David A. Steele, a Jason D. Whittle, a James W. Bradley b and Robert D. Short a Received 2nd April 2013, Accepted 21st May 2013 DOI: /c3ra41563e Plasma polymerisation is a technologically important surface engineering process capable of depositing ultra-thin functionalised films for a variety of purposes. It has many advantages over other surface engineering processes, including that it is completely dry, can be used for complex geometries, and the physico-chemical properties of the film can be tailored through judicious choice of processing conditions. Despite this, the mechanisms of film growth are largely unknown, and current models are based on purely chemical arguments. Consideration of some basic plasma physics shows that some species can arrive at surfaces with energies greater than 1000 kj mol 21 (.10 ev), and thus open a range of surface reactions that have not been considered previously. This review aims to close the gap between the physics and chemistry of reactive plasma systems. Introduction Synthetic polymers can be used in surface engineering as thin film coatings, providing the surface with desirable physical and/or chemical properties such as scratch resistance, wetting behaviour or chemical functionality, without changing the bulk properties of the material. They are important in new and emerging technologies; for example, applications under development include flexible polymer-based substrates for a Mawson Institute, University of South Australia, Mawson Lakes, 5095, Australia. andrew.michelmore@unisa.edu.au; Fax: ; Tel: b Department of Electrical Engineering and Electronics, University of Liverpool, Brownlow Hill, Liverpool, L69 3GJ, United Kingdom electronic displays and improved time-release and targeted drug delivery. 1,2 This progress became possible because of the significant effort and achievements made in understanding polymer molecular structure and the mechanisms by which polymers form. 3 However, the use of traditional synthetic polymers for thin film coatings has some drawbacks. 1. Surface preparation is usually required. 2. Complex geometry of the substrate may affect film thickness. 3. Processing difficulty increases as the film thickness decreases. A niche, but technologically important type of polymer is the plasma polymer. 4 These polymers are grown from the gasphase using plasma (electrically excited gas) and can be used Andrew Michelmore Andrew Michelmore obtained his B.E. (Chem) from the University of Adelaide before starting his PhD in Physical Chemistry at the Ian Wark Research Institute, University of South Australia (UniSA). Following completion of his thesis, he worked in industry for 4 years, before rejoining UniSA at the Mawson Institute as a Research Fellow where his research interests are the mechanisms of plasma polymerisation, surface interactions and surface analysis. David A. Steele David Steele obtained his degrees from the University of Sheffield (UK) before emigrating to Australia in Following positions at the University of Melbourne and Monash University he joined the University of South Australia in He seeks to better control and direct the complex interaction between the synthetic (biomaterial) and natural (biological) world as mediated at surfaces RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

2 to endow surfaces with ultra-thin film coatings down to a few nanometers thick; little or no surface preparation is required, the process is solvent-free, usually conducted at ambient temperature, and the deposited plasma polymer forms a conformal layer over complex geometries. The current value of products which incorporate surface engineering during processing is estimated to be in excess of $10 trillion US annually, 5 and the properties of plasma polymerization make it highly desirable for application in a wide range of industries. From the 1960s onwards plasma polymer thin films have found applications in a range of diverse technologies. Early applications included protective coatings for food packaging and clothing. Many of these applications made use of the deposited film simply as a physical barrier. Of particular interest since the 1980s has been the deposition of functionalized plasma polymer films. For example, functionalized films are used as a means of improving biocompatibility for biological implants 6 and for Jason D. Whittle Jason Whittle completed his PhD in Biomaterials Engineering at the University of Sheffield, UK and spent eight years developing surface treatments for biomedical applications in industry before joining the University of South Australia in His research is focussed on the interaction between engineered surfaces and biological molecules and on the development of plasma processes. Review producing super-hydrophobic coatings. 7,8 They have also been extensively employed in biomaterials for cell attachment, protein binding and as anti-fouling surfaces. 9 Through the use of low power and pressure plasma, high functional retention can be achieved 10 which has led to substantial improvements in the biocompatibility of some products, a simple example being the development of extended wear contact lenses. 11 Due to these successes, the huge potential of functional plasma polymers is slowly being realised by workers in previously unrelated fields such as water treatment 12 and wound management. 13 Emerging technologies such as nanopatterning, 14 3D scaffolds, 15 micro-channel coating 16 and microencapsulation 17 are now also utilizing functionalized plasma polymers, areas for which traditional polymers are often unsuitable. Producing functionalized plasma polymer films represents a particular technological challenge, as retaining the functionality of the monomer while achieving the desired mechanical properties is not trivial. Despite this, industrial uptake has surged ahead in the last 25 years with product development being led by trial-and-error. Fragmentation of the monomer in the plasma phase and ion bombardment or ablation of the film can result in low functional group retention which may lead to poor performance of the final product. To address these issues, functionalized films are usually produced using specifically designed systems which employ low pressure and low RF power. Significant challenges remain in understanding the fundamental physics and chemistry of the process, knowledge which has greatly lagged industrial demand. Reactor design The basic requirements for plasma reactor design are an enclosed chamber, a means of introducing monomer (either volatile liquid or gas) into the chamber, and a means of igniting and maintaining the plasma. Generation of low temperature plasma can be created by use of DC or AC fields, generally at reduced pressures although pressures as high as James W. Bradley James Bradley became Chair in Plasma and Complex Systems at the University of Liverpool UK in Prior to this he was lecturer and senior lecturer at UMIST (Manchester UK) from His main interests are in the diagnosis and development of technological plasmas for materials processing applications. He has over 130 peerreviewed journal paper publications in experimental and modelling studies of low temperature plasmas operating at both low and high pressures. Robert D. Short Rob Short obtained his BSc and PhD from the University of Durham before joining the University of Sheffield in 1988 where he held the Chair of Material and Biomaterial Chemistry from He has worked in the field of plasma polymerisation for 20 years, including being a founding Director of two University spin-out companies. In 2006, he was appointed as Director of the Mawson Institute at the University of South Australia and is now Pro Vice Chancellor and Vice President (Engineering). This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

3 Review atmospheric may be used. For functionalized coatings, vacuum conditions with pressures from 0.75 mtorr (,1 Pa) to 100 s of mtorr (.100 Pa) are usually employed so that plasma ignition occurs at reasonable input power. Excitation methods range from DC through radio frequency (RF usually at MHz) to microwave (MW at 2.45 GHz). Additionally the excitation source may be continuous wave or pulsed. The power source may be coupled directly, with the electrodes within the plasma chamber, indirectly with the electrodes external to the chamber or by combination. Power is coupled to the electrons, which gain energy (are heated) from the electric fields and in turn distribute some of this to other species within the chamber. RF offers some significant advantages in the context of depositing soft, functionalized polymers over DC and AC sources: 1. RF power is deposited by displacement rather than particle currents. This offers easier coupling through the chamber walls, which are often dielectric materials such as glass. 2. The use of glass vessels can provide lower losses and wall recombination. 3. Generally RF-generated plasmas are more stable, and tend to have higher electrical efficiency than equivalent DC or AC plasmas. 4. RF plasmas have electrons with higher temperatures for the same plasma densities as equivalent DC or AC plasmas. This can be beneficial where an increased number of free radicals, plasma-chemical reactions or dissociation and ionisation reactions are desired. 5. RF plasma can be used to process insulating materials without sputtering of the electrodes and hence, can be used for deposition from organic monomers. Consequently, the majority of recent studies with functional organic monomers have been carried out using MHz RF power in glass reactor vessels with external coils or bands to couple the power. Over the past three decades various designs have been developed by researchers for functional plasma polymer deposition, some of which are shown in Fig Other important contributors in the field include Griesser, 26 Knoll, 27 Cooper, 28 Badyal, 29 Goeckner 30 and Timmons. 31 A matching network (MN) is used to match the impedance of the generator to that of the plasma (see Fig. 1 for examples). It is important to note that the MN itself, the cables and radiation of RF power into the air give rise to considerable losses. In a typical coil wound device, from a power balance, only 20% 50% of the power on the dial is transmitted to the plasma. 32 The design in Fig. 1(a), used at Durham University since 1970s 18 and by Ward 19 in 1989 is often known as the Clark reactor. The powered electrode usually consists of number of turns of sheathed wire wrapped around the dielectric glass vessel or is in the form of external copper bands. In the coil configuration, the coil itself can be terminated at ground, or left un-terminated. In the latter set-up there is no conduction path to ground and no DC currents can flow. In the absence of a conduction current the coupling is purely capacitive. In almost all cases even with grounded end coils the plasma is capacitively coupled, as the electromagnetic skin RSC Advances depth of the plasma greatly exceeds the chamber size. An important point is that the excitation wire passes over the chamber and the oscillating electric fields (driven at MHz) heat and sustain the plasma. The position of the substrate in the chamber is important. In some situations, the substrate is placed on the vessel floor and coupling of the RF through the wall and substrate may induce large (self-bias) potentials on the substrate itself. The origin of these is described later. The result of these potentials is that they can lead to ions bombarding the substrate with high energy. For substrates mounted in the plasma bulk, the RF modulation of the plasma potential can also give rise to self-bias conditions and energetic ions, however these potentials are usually much lower than when directly mounted above the coil (effectively on the driver electrode). External parameters vs. intrinsic properties What characterizes the majority of studies in the field is their phenomenological nature. Whilst identifying the external parameters, such as power, pressure and flow rate, which affect the process properties such as deposition rate and functional group retention, they do not provide a direct insight into how films grow at the molecular level. Indeed, there has been paucity of effort put into exploring intrinsic properties (those that properly define plasma) and linking them to polymer formation. It should be noted that in the closely related field of hard, carbon and silicon plasma deposits, this is not the case. 33,34 Investigations which have considered intrinsic properties have focused on the bulk plasma parameters of electron temperature (T e ), density of charged species (n i ) and of radicals in plasma etc., ignoring the basic fact that where the plasma meets any surface (where the film actually grows) there is necessarily a sheath region (a region of space charge and an associated electric field), which creates a boundary problem which we will later describe and which has a major influence on how polymer films grow. External plasma parameters are often used as quasi-substitutes for factors which affect conventional polymerization reactions; for example, RF power (as read from an external dial) for temperature, and flow-rate for concentration. These parameters and others, such as whether the RF power is applied continuously or pulsed, the monomer pressure and reactor geometry, are generally not correlated with intrinsic properties of the plasma. Some of the monomers that have been deposited by plasma would be considered polymerizable in a conventional sense, as they contain a carbon carbon double bond the prerequisite for most conventional polymerization; however, many compounds deposited by plasma are not polymerizable in a conventional sense, as they do not contain sites of unsaturation. This indicates the uniqueness of the plasma environment to promote polymerization, and highlights that it is highly unlikely that there is a single, ubiquitous polymerization pathway. Identifying which pathways lead to film growth is important, as is establishing the site of polymer growth (plasma phase vs. the plasma substrate interface). These represent significant scientific challenges. There are a number of possible con RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

4 Review Fig. 1 (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 [ref. 19] (c) Lopez et al [ref. 20] (d) O Toole et al [ref. 21] (e) Favia et al., 1996 [ref. 22] (f) Candan et al [ref. 23] (g) Alexander et al [ref. 24] (h) Voronin et al [ref. 25]. This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

5 Review tributing reasons for our ignorance of these in the context of plasma-deposited functionalized films: 1. Plasma polymerization cuts across traditional disciplinary areas of study (engineering, chemistry, physics, materials science, etc.) and therefore is not the preserve of any one community. 2. The complexity of the problem and lack of tools to study the problem may have been considered too daunting. 3. The technological application of these thin films has raced ahead and generally coatings with the appropriate properties can be obtained by trial-and-error. 4. Over simplification: studies which have been conducted to investigate the effects of the external parameters (such as dial power) on the final coating provide simple explanations, but do not take into account the physical processes taking place within the plasma environment. 5. And finally, the erroneous perception that the mechanisms have already been studied and adequately explained. All of the above have led to the situation that persists today, where we believe there is an inadequate description of the processes leading to film formation, particularly in the context of functionalized plasma polymer films. This is in contrast to conventional polymerization, where the pathways by which polymer chains grow are largely known and utilized in the design of new polymers; in plasma polymerization, coatings of the required properties are obtained by trial-and-error. The lack of an adequate description means that researchers are restricted in that they (i) can t design films a priori and (ii) cannot reproduce the work of others using the same parameters. Whilst (i) may never be entirely possible, (ii) should be and this has significantly limited exploitation of plasma polymerization. There is also inconsistency between work undertaken in different laboratories and reactors, and issues with both scaling-up and scaling-down plasma. Since no two experimental systems are exactly the same and small differences in design can lead to large changes in, for example, the coupled power, presently it is not possible to directly cross correlate process or external parameters such as applied power between systems. In the semi-conductor field the Gaseous Electronics Conference (GEC) RF reference cell was developed to eliminate such variation for workers using etching plasmas. 35 While this would seem to be a straightforward exercise, even this proved to be non-trivial. 36 This review concerns the basic principles of plasma polymerization specifically in the context of functionalized films. The most widely studied plasma polymers are those arising from acrylic acid and its saturated analog propionic acid 37 which can be used to produce functionalized organic films, in which the carboxylic acid group is retained through the plasma polymerization processes and manifests in the final product, a thin-film coating. Other commonly produced functionalities of plasma polymer films include amines (e.g. allylamine 38 ), alcohols (allyl alcohol 39 ), fluorocarbons (hexafluoroethane 40 ) and silicates (hexamethyl disiloxane 41 ). The following discussion is divided into three parts. In the first part, we consider the likely chemical processes that take place within the bulk of the plasma. We introduce some of the basic concepts of plasmas, the range of chemical species which may be encountered in plasmas, explain how they form after ignition of the plasma and their typical properties. In the second, we consider the physics, and relevant physical processes that take place when a collecting surface is placed in contact with plasma. We investigate the processes that occur at the plasma phase surface interface, and introduce the concept of formation of a sheath between these two regions. We also describe a range of experimental techniques which may be used to probe the intrinsic properties of the plasma phase, and show some experimental data demonstrating the essential properties of some typical plasmas. In the third, we describe some common surface analysis techniques used to characterise plasma polymer films, and discuss how typical results can be reconciled with some proposed models for plasma polymer growth. The plasma phase and chemical processes Plasma consists of a mixture of electrons, ions, radicals, neutrals and photons. Some of these species are in local thermodynamic equilibrium, while others are not. Even for simple gases like argon this mixture can be complex. For plasmas of organic monomers, the complexity can rapidly increase as some components of the plasma fragment, while others interact and form larger species. Two important concepts are the unit of energy and the average amount of energy that is adsorbed per molecule in plasma, E mean. An electron volt (ev) is defined as the amount of kinetic energy gained by an unbound electron when it loses 1 V of electrical potential energy. This can be converted to a temperature using Boltzmann s constant, k, giving: 1eV~ 1:6 10{19 J ~11600K (1) 1:38 10 {23-1 JK For many of the charged species in plasmas, this unit of energy is convenient as it not only defines their temperature, but also the potential difference the species have enough energy to overcome. The amount of energy absorbed per molecule is related to the monomer flow rate, w, by eqn (2). E mean ~c P w Where c is the duty cycle for pulsed plasmas, given by: c~ t on t on zt off RSC Advances For continuous wave plasma, this term reduces to 1. (2) (2a) Ignition If we consider a gas at low pressure and ambient temperature, the gas molecules will typically be moving at m s 21.A low degree of ionization may occur due to absorption of RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

6 cosmic radiation 42 and other random processes, giving rise to ion-electron pairs. In the atmosphere at sea level, the electron density is approximately m 23, or 1 ion electron pair per neutral gas molecules; 43 so under typical vacuum conditions used in plasma polymerization, the electron density, n e, is in the range of m 23 prior to igniting the plasma. When RF power is supplied to the system, the much lighter electrons will be accelerated much more quickly than the ions, and will gain kinetic energy (and heat) more readily. Typically the ions will remain close to ambient temperature, but the electrons will quickly reach temperatures greater than K (y1 ev), with a very small proportion being in excess of K (.10 ev). Thus the electrons are not in thermal equilibrium with the ions and neutrals. High energy electrons can ionize neutral species in the gas phase by colliding with them, producing an ion and another free electron. For a diatomic molecule X 2, the reaction can be written as e 2 +X 2 A 2e 2 +X 2 + Ionization of the molecule and release of a second electron results in a cascade of electrons being introduced to the gas phase which in turn may lead to further ionization reactions. Of course ions and electrons may also recombine forming neutral molecules or be lost to the walls of the chamber, and eventually a steady state will be reached. As long as the RF power is maintained, the population of ions and electrons will remain and a plasma phase will persist. Plasma composition Reduced pressure, low power plasmas used in the polymerization of volatile organic compounds comprise a weakly ionized gas which is overall electrically neutral but contain positivelyand negatively-charged particles, as well as neutrals and excited state species, metastables and photons. Here we will describe the components of plasmas, how they arise and what properties they possess. Electrons As described in the previous section, free electrons are produced and heated by the application of RF power. An important property of a plasma is the electron energy distribution function (EEDF). This is because electrons are the engines of plasmas; they acquire kinetic energy from the RF electric fields, and then distribute this energy through collisions with other species in the plasma. Heating of the electrons can be via two mechanisms; Ohmic heating due to electron neutral collisions in the bulk of the plasma, and Stochastic heating due to momentum transfer across electric fields. 44 Of critical importance to the ignition and maintenance of the plasma is the ability of electrons to break bonds, giving rise to radicals, ions and more electrons. The electron energy required for dissociation of chemical species is usually around 3 5 ev, and the ionisation energy is of the order of 10 ev. Typical plots of the EEDF are given in Fig. 2 assuming the electron-molecule collision frequency is constant (Maxwellian distribution). (For the purist, Dreyvesteyn 45 distributions are more appropriate in plasma systems but the (3) Review Fig. 2 Maxwellian Electron Energy Distribution Functions (EEDF) for plasmas of different average electron energy. two are similar.) Even with an average electron temperature of 4 ev, it can be seen that only a very small fraction of the electrons have enough energy to ionize molecules, and only around half have enough to dissociate molecules. For this reason, the density of radical species is much higher than the density of ions in the plasma phase, as discussed in following sections. With a mixture of electrons with a distribution of energies, and neutral monomers in the gas phase, a number of collision processes are possible which are characterized by the energy with which they occur. The lowest energy collisions result in elastic scattering, with some kinetic energy being lost by the electron and gained by the molecule. By conservation of kinetic energy and momentum, the maximum amount of energy that may be transferred in an elastic collision is given by: E max ~ 4m em neutral (m e zm neutral ) 2 (4) where m is the atomic mass. As m e % m neutral, this term reduces to 4m e /m neutral. The result is that typically only about 0.01% of the electrons energy is transferred to the neutral molecule. 46 Radicals Electron impacts with energies in the range of 3 5 ev are sufficient to break bonds in neutral molecules, following eqn (5). For example, Bouchoux 47 has measured the energy required to homolytically remove a proton from CH 3 CH 2 X molecules. For X = NH 2 and OH, the energies required are 3.7 and 4.0 ev respectively. Font et al. 48 have also identified a range of radical products resulting from c-c 4 F 8 plasmas having energy thresholds below 5 ev. e 2 +X 2 A e 2 +? X+X? (5) For plasmas of organic monomers, these dissociation collisions result in fragmentation of the monomer and low molecular weight species will be formed. Being neutral species, radicals do not gain energy from the applied RF This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

7 Review power, and therefore remain at or near ambient temperature. However, they are highly reactive as they have a spare electron with which to covalently bond to other radical species, or neutral molecules containing a double bond. The degree of fragmentation is an important consideration in plasma polymerization, particularly when the goal is to coat a surface with a specific functional chemistry. For example, acrylic acid plasmas are frequently used to provide surfaces with functional carboxylic acid groups. It has been convincingly shown that using low power acrylic acid plasma results in good retention of the carboxylic acid group. 21 However, as the RF input power is increased, fragmentation of the monomer species increases and functionality of the species in the plasma may be lost resulting in plasma polymers with little to no carboxylic acid groups. 49 Radical species are of particular interest as they are the building blocks in traditional polymerization. They are also relatively abundant in the plasma phase. Agarwal et al. 50 measured the radical density of oxygen plasmas at around radicals m 23, or approximately 1 for every 200 gas molecules in the chamber. This value is probably an overestimate in the context of functionalized films as the power per molecule was quite high, but demonstrates the relative abundance of radicals in plasma. It has therefore long been assumed that along with neutrals, these radicals are the only species which contribute to plasma polymerization at surfaces. While these traditional routes to forming plasma polymers are important, as we will discuss later they are not the only species capable of contributing mass to the plasma polymer. Excited states, metastables and VUV Higher energy collisions between electrons and neutral species can result in some kinetic energy being transferred from the electron to the neutral molecule. 46 This generally takes the form of an electron in the molecule being excited to a higher energy orbit, as shown in eqn (6) e 2 +X 2 A e 2 +X 2 * where X 2 * represents an excited molecule. Excited molecules are inherently unstable, and the electron will quickly fall back to its 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, (6) X 2 * A h 0 +X 23 (7) where X 23 represents a lower level excited state (not necessarily ground state), and h 0 a photon of energy equal to the difference between the two states. Some of these photons will be in the visible region of the electromagnetic spectrum, and give rise to the characteristic glow of the discharge. Higher energy photons are also possible resulting in vacuum ultraviolet (VUV). VUV radiation may then also dissociate and ionize molecules if the photons are of sufficient energy. These excited molecules are typically not very long lived, and will return to the ground state in a time of the order of 10 ns. RSC Advances However, some of these molecules may reach a metastable state which may last for 1 ms or longer. Ions The traditional view is that ions are created by high energy collisions between electrons and molecules, above approximately 10 ev, which result in ionization of the molecules and liberation of secondary electrons as shown in eqn (3). It has also been shown recently in Selected Ion Flow Tube (SIFT) experiments that ionization may result from collisions between neutral molecules and H 3 O + ions. 51 For this reaction scheme, a proton is transferred from the H 3 O + ion to the neutral molecule, N. H 3 O + +NA H 2 O+NH + (8) The source of H 3 O + is adsorbed water on the walls of the chamber being removed by ion bombardment which is described in detail later. Thus, overall charge neutrality is conserved but local space charges may develop. While ions are charged and therefore affected by the electric fields produced by the RF power used to ignite and maintain the plasma, they are massive relative to electrons. Consequently ions do not accelerate and gain energy in the way that electrons do, and so remain roughly in thermodynamic equilibrium with the radical and metastable species. An important property of the plasma is the plasma density, which is the density of ionized species in the plasma phase, n i. The number of charged species per unit volume and the temperature of these species is influenced by the plasma reactor design, the operating pressure, power source and mode of power coupling. Typically for the RF plasma systems used in plasma polymerization, the plasma density is in the range of ions m This compares with the neutral density of the plasma phase which is usually molecules m 23. Therefore ions are relatively scarce in the plasma phase, with typically gas molecules for each ion particle. As a consequence, the role of ions in plasma polymer formation has historically been discounted. More recently, experimental data has become available which indicates that ions may play a much greater role in both the plasma phase and surface polymerization than has been accepted. Reasons for greater ion involvement are discussed later. Reactions in the plasma phase Once the various species mentioned above have been initiated in the plasma, reactions may take place between these species. The obvious candidates are the excited molecules, radicals and ions. Previous textbooks and reviews of plasma, for instance Chapman 46 and Lieberman and Lichtenberg, 52 and of plasma polymerization 53 have considered the likely chemical species within plasmas that give rise to film growth. Notwithstanding the influence of plasma power (power in the plasma and its distribution) power source (usually radio frequency at MHz in the context of the functional film growth), power coupling, gas flow rate and pressure and finally reactor design, it is believed that in the plasma there will a complex mixture of RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

8 species including the starting compound, larger species built up from the starting compound and fragments of the starting compound. These species will be neutral (ground or excited state), radical or charged (cations, anions and electrons). Below, we briefly consider the likely types of reactions that take place within the plasma bulk, and whilst others have ascribed film growth (exclusively) to radical processes, we start from the premise that we should examine (again) the relative importance of any particular pathway in film growth of functionalised plasma polymers. Within the bulk of the plasma we would consider five general 2-body reactions which may occur (excluding neutralneutral reactions). In order of decreasing cross sectional area they are:? R+NA? R 2 N (radical) (9a)? R+R? A N 2 N (neutral) or? R 2 R? (diradical) (9b) I+NA I 2 N (ion) I+R? A I 2 R? (ion) I+IA N (neutral) (9c) (9d) (9e) where? R = any radical species, N = any neutral species (ground state or excited) and I = any ionic species (positively or negatively charged). 3-body collisions are also possible, where the involvement of the third body allows any excess energy to be dissipated. 46 The third body is often the walls of the chamber (which obviously has a large cross sectional area relative to the molecules!) In the bulk of the plasma which we are considering here however, the likelihood of 3-body collisions occurring is very low. The reaction between? R + N (irrespective of the nature of N, unsaturated or saturated) results in a further radical, and where N contains a carbon double bond in some instances there is the possibility of chain growth polymerization. However, care is required in assigning polymerization to chain growth just because there is a carbon double bond, as for example allylic compounds will not readily polymerize in this fashion. 3,54 In the case of? R+R?, the resultant species will either be a diradical (presumably still reactive) or the radical centres combine and the result is a neutral, in which case for further reactions to occur the resultant species will need to be reactivated by the plasma. Both? R + N and? R + R? are exothermic and readily proceed with high rate constants. For example, the radical neutral rate constant for acetylene plasmas has been estimated at cm 3 s As ions are scarce in low temperature, low pressure plasmas it has been generally accepted until recently that reactions involving ions in the plasma phase could be discounted. However, given that greater than 99% of the molecules in the plasma chamber are neutral species, the cross sectional area for I + N collisions is relatively high. It is also important to note I + N has a very high rate constant. Indeed, O Toole et al. 39 have measured the (M H + + M) rate constant for allyl alcohol as being cm 3 s 21, similar to the collisional rate coefficient. Reactions I + N and I + R? result in larger ionic species, while I + I may result in neutral species if the ions are of opposite charge. In this case, while the collision cross section is extremely low, the rate constant is very high. Stoykov et al. 56 have measured the ion-ion recombination rate constant at around cm 3 s 21 for acetylene plasmas. As will be discussed later, there is mounting experimental evidence that reactions involving ions occur in the plasma phase, leading to molecular rearrangements which manifest themselves in the plasma polymer formed on a surface. Plasma analysis Review Therefore, from the above discussion, it can be seen that the plasma phase is a complex mixture of electrons (with various energies), metastables, excited molecules, photons, radicals and ions (of various molecular weights). A comprehensive description of the state of the plasma is therefore difficult and requires a great deal of information. In this section we point to the important parameters that may be measured in the plasma, and outline the techniques available for obtaining these data. The electrons are the workhorses of the plasma, and therefore are an important consideration in describing plasmas. Of interest are the electron density, n e, and electron temperature, T e. For most plasma systems, the ions that are produced are positively charged and so the electron density is the same as the ion density, n i. It should be noted this is not always the case, as for example monomers containing fluorine can produce negative ions. 57 However, assuming all negative charges in the plasma are electrons, measuring the electron density gives the positive charge density. As the electrons are not in thermodynamic equilibrium with the rest of the species in the plasma, the electron temperature must be independently measured. The fragmentation of species in the plasma phase is also critical. This is particularly so when retention of functional groups is important and therefore measuring the degree of fragmentation and oligomerization in the plasma affords some insight into the plasma chemistry. Langmuir probe In its simplest form, a Langmuir probe consists of an electrode placed in contact with a plasma. 58 The voltage (V) of the electrode relative to the surrounding vessel can be varied and the resulting current (I) can be measured. From the V I characteristic of the probe, various parameters of the plasma can be calculated, including the electron temperature, plasma density and the plasma potential (V p, discussed later). 59 Care must be taken, as the probe draws electrons from the plasma, and therefore may affect the plasma locally. This effect is usually minimized by keeping the probe area small, although it is sometimes overcome by using a double probe. 60,61 Geometrical effects are also known as the collection area of the probe is actually the sheath area surrounding the probe (sheaths are discussed later) which depends on the This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

9 Review Fig. 3 Typical V I characteristic for a Langmuir probe in contact with a plasma phase (Adapted with permission from ref. 32). plasma density. Finally, for depositing plasmas, an insulating deposit quickly reduces the effective area of the probe and therefore its effectiveness. This may be overcome by heating the probe to burn off any deposit as it forms. Fig. 3 shows typical Langmuir probe current voltage characteristic plots for an argon plasma at 10 W input RF power. 32 The voltage at which the current drawn is zero is known as the floating potential V f. Above this voltage, the current increases with a slope which is proportional to the electron temperature T e. The point at which the electron current saturates is the plasma potential V p. Typical results for polymerizing plasmas have been obtained by Dhayal and Bradley 62 using acrylic acid with a heated Langmuir probe. The electron temperature was found to be between 1 and 1.5 ev over a wide range of RF power inputs. The plasma density varied between ions m 23 over the same power range. For argon plasmas, the electron temperature is usually a little higher at 2 3 ev, while the plasma density is y10 15 ions m It should be noted that these measurements were taken at different pressures (5.2 and 1.3 Pa, respectively), which may account for the differences here. Mass spectrometry Mass spectrometry may be used to determine the spectrum of both neutral and ionic species in the plasma. The technique relies on separating species based on their mass-to-charge ratio by applying an electric field. RSC Advances For neutral species, after entering the instrument via a small orifice, molecules (or atoms) are converted to ions by electron impact (typically at 70 ev). For ionic species, this step is not required. Ions are then subjected to an electric field in an analyser which mass selects the species, which are then measured by a detector in the form of a current and the signal is converted to a mass spectrum. Quadrupole mass analysers are most commonly used, which consist of 4 parallel electrodes which are oppositely paired. Under the influence of an RF signal between the electrodes, the ions oscillate proportional to their mass-to-charge ratio. The RF signal is then alternated between the electrode pairs such that only the selected mass-to-charge ratio species may pass through the analyser and be recorded. Ions with different mass-to-charge ratios collide with the electrodes before exiting the analyser. To date, experimental evidence shows that the neutral mass spectrum of polymerizing monomers rarely exhibit species of higher mass than the starting compound. For example, acrylic acid plasmas show a dominant neutral peak at 72, the intact monomer, and only smaller neutral species are observed. Similar findings are shown for allyl alcohol in Table 1. Methyl isobutyrate is one example where neutral oligomerization is observed in the plasma phase. The intact monomer is observed at m/z 103, again with smaller neutral fragments. However a peak is observed at m/z 205, corresponding to the dimeric species. In the case of the ionic species, the dominant ionic species for acrylic acid in the plasma phase is the protonated monomer at m/z 73 (M H + ). Smaller species are also observed at m/z 1, 18, 28, 39 and 55 showing that the monomer is fragmented into smaller ionic fragments in the plasma phase. However, oligomers of acrylic acid are also observed, with higher m/z species seen at 127, 145 (2M H + ), 199 and 217 (3M H + ). Similar oligomers are observed for both allyl alcohol and methyl isobutyrate. Importantly, there is a general observation that ionic species larger than the monomer are formed in the plasma phase. The degrees of fragmentation and oligomerization are linked to the applied RF power and chamber pressure 41 and shows that simply measuring the plasma density and ion flux (discussed later) is not enough to fully describe the state of plasmas formed from organic monomers; the presence of charged oligomers in the plasma phase can result in an increase in ion mass flux which would not be detected by simply measuring the ion flux to a surface. These results and others demonstrate that while ionic species are able to form clusters/oligomers in the plasma Table 1 Plasma Phase Mass Spectrum results for three monomers, neutral and positive ions, given as percentage of total counts Monomer Allyl alcohol [ref. 39] Acrylic acid [ref. 63] Methyl isobutyrate [ref. 21] Neutral Ion Neutral Ion Neutral Ion m/z, M 85% 21% 61% 33% 74% 74% m/z = M 15% 17% 39% 34% 16% 23% M, m/z, 2M 47% 21% m/z =2M 8% 7% 11% 3% 2M, m/z, 3M 5% 3% m/z =3M 2% 1% RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

10 phase, generally neutral and radical species do not react to form larger molecules. It should be noted that we cannot preclude these reactions from occurring at the surface. Optical emission spectroscopy (OES) OES is a non-invasive technique for probing the plasma phase, and can be used to obtain information about the plasma in real-time, 67 and has been proposed as a semi-quantitative technique for process control. 68,69 The electron temperature and density may also be calculated from the optical emission spectra. 70 As discussed above, excited species in the plasma emit photons as they relax to their ground state. Some of these photons are in the visible part of the spectrum, and typically spectra are recorded for wavelengths between nm. There is extensive data available for transitions of neutral argon species, 71,72 but the most intense peaks occur in the red/ near-infrared region between 690 and 900 nm relating to the transitions 3p 5 4p to 3p 5 4s. 73 For depositing plasmas, the emission spectra may be quite complex. For example, Pappas and Hopwood 74 measured the spectra of methane plasmas and observed many peaks between nm assigned to various states of carbon and hydrogen. The relative intensities of these peaks changed markedly with applied RF power. Booth and Corr 75 measured the spectra of CF 4 plasmas and again observed a rich chemistry of carbon and fluorine. They also observed that many of these species persisted after the plasma was turned off for up to 1.5 ms for some species. Similarly, Corr et al. 76 measured the spectra of downstream nitrogen/trimethylgallium plasma and observed strong peaks associated with Ga and CN species. Physics at the plasma surface interface Now we have described the components of the plasma phase and some of their properties, we can move onto how these components interact with a surface placed in contact with the plasma. Generation of electrical potentials The fluxes, J, due to thermal motion of all species in the plasma phase to an imaginary plane from one side only are given by: J i = 0.25 n i v i J e = 0.25 n e v e J rad = 0.25 n rad v rad J neutrals = 0.25 n neutrals v neutrals (10a) (10b) (10c) (10d) where the thermal velocity of the particles, v, is given by rffiffiffiffiffiffiffiffiffi 8kT n~ (10e) pm and T is the absolute temperature. 46 For the radical and neutral species, eqn (10c) and (10d) hold true in plasma. Ions and electrons on the other hand obviously react to electric fields which can alter both their density and velocity. Any insulating or electrically isolated surface will assume a potential such that the fluxes of positive (ions) and negative (electrons) charge carriers arriving at the surface will be equal. As discussed previously, when the plasma is ignited, the electrons gain significant kinetic energy due to the applied electric fields, however the ions remain relatively cold. If we consider just the ions and electrons, overall electroneutrality is conserved, n i = n e, however the electron velocity is far greater than the ion velocity as they are both hotter and lighter. Therefore, J e & J i. If we consider an imaginary plane in Fig. 5, the flux of particles to the plane from one side is balanced by the flux of particles from the other, and the net current is zero. This is the case in the bulk of the plasma. If the imaginary plane is replaced with a solid surface (such as a wall of the chamber), the particles cannot pass through the plane, and there is a net flow of negative charge as the electron flux is greater than the ion flux. This causes the plasma to develop a positive potential relative to ground, known as V p, and a surface placed in contact with the plasma will develop a negative potential relative to the plasma. The negative potential developed at the surface will then begin to attract positive ions from the plasma (J i increases), and simultaneously repel electrons (J e decreases). This process will continue until equilibrium between the ion flux and electron flux is achieved. The potential of the surface is then known as the floating potential, V f, and for a DC discharge the potential between the plasma and the surface is given by V p {V f ~ kt e e " rffiffiffiffiffiffiffiffiffi# 1 2 z ln M 2pm Review (11) The questions that remain are how the potential varies between the plasma bulk and the surface, and how this affects the various components of the plasma phase. We shall see that there are two regions which must be addressed; a region adjacent to the surface called the sheath, and a region between the sheath and the bulk plasma known as the pre-sheath. The sheath As the surface develops a negative potential, close to the surface the electron density will be greatly reduced compared to the bulk plasma, with only high energy electrons able to reach the surface (Fig. 6). Therefore the space adjacent to the surface will develop a net positive charge. This space is known as the sheath. The density of this net positive charge decreases as we move further from the surface. At some point, the electron and ion densities become equal and the net space charge is zero, which marks the outer edge of the sheath. The electron distribution with respect to the distance from the wall, x, in the sheath is given by the Boltzmann distribution n e (x)~n e exp ev(x) (12) kt e This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

11 Review RSC Advances Poisson s eqn (13) can be used in electrostatics to determine the variation of potential in regions of space charge. + 2 Q~{ r e 0 (13) J i ~ exp { 1 2 n i sffiffiffiffiffiffiffiffi kt e m i (16) where Q is the potential, r is the density, and e 0 is the permittivity of a vacuum. The important outcome though is obtained by combining this with the Boltzmann distribution, giving: DV(x)~DV 0 exp {x (14) l D sffiffiffiffiffiffiffiffiffiffiffi kt e e 0 where l D ~ n e e 2 (14a) l D is known as the Debye length, and determines the length scale over which the voltage drops with the distance from the surface. This sheath region usually extends between 20 and 100 mm from the surface into the plasma. Once positive ions are in the sheath, they are trapped by the negative potential of the surface and cannot escape unless they collide with other atoms. Pre-sheath Within the sheath, ions convert electrical potential energy into kinetic energy as they approach the negatively charged surface. For ion energy conservation: 1 2 Mv(x)2 ~ 1 2 Mv2 {ev(x) (15) As the positive ions accelerate, they spread out and their density decreases. Electrons on the other hand are repelled from the surface and ejected from the sheath, and also decrease in density following the Boltzmann distribution. However, for the sheath to remain a stable region of positive space charge, the local electron density must always be less than ion density. Also, at the sheath edge, by definition, the ion and electron densities must be equal. The solution for these conditions was identified by David Bohm in 1949 and is that the ions must enter the sheath with a speed greater than (kt e /m) 1/2, known as the Bohm velocity. 77 Therefore, there exists a region between the sheath and the bulk plasma where ions are accelerated to the Bohm velocity, known as the presheath. In this region, the ion and electron densities are equal, but lower than the bulk plasma as ions are accelerated and low energy electrons are repelled from the surface. This is known as the Bohm Sheath Criterion. The Bohm Sheath Criterion has a number of consequences for the plasma system. The most important for low temperature plasma polymerization is an increase in the flux of ions to the surface. If we consider the ions enter the pre-sheath with negligible energy, it can be shown that the ion flux at the sheath edge is: Note that the ion flux to the surface is determined by the electron temperature, not the ion temperature. This is because the flux of electrons to the surface is determined by the number of electrons with enough energy to overcome the negative surface potential. For charge equilibrium to be maintained, this electron flux at the surface must be balanced by the flux of ions into the sheath. For typical values of plasma density ( ions m 23 ) and electron temperature (2 4 ev), typical ion fluxes are in the range of ions m 22 s 21. This result is quite different to the thermal ion flux of 0.25n i v i, often being at least an order of magnitude larger, an effect which has often been neglected when considering plasma polymerisation. If we calculate the ratio of the ion flux to a surface in eqn (16), to the thermal flux of ions to an imaginary plane (from one side only, eqn (10a)), we get: J i ~ ffiffiffiffiffi rffiffiffiffiffi p 1 T e 2p exp { (17) J t 2 T i This shows that assuming the ion temperature remains close to ambient, the ion flux is enhanced by a factor proportional to the electron temperature due to the presence of the surface. For typical plasma systems used for polymerisation, T e /T i y 100, and so the flux of ions to a surface is enhanced by a factor of y15 times. To illustrate this point, taking an average value of plasma density in Fig. 4 of ions m 23, the thermal flux of ions to a plane would be ions m 22 s 21. However as the measured electron temperature was y2.5 ev (29000 K), the ion flux to a surface is ions m 22 s 21. This value is consistent with the measured ion flux for a number of plasma systems including acrylic acid and hexamethyl disiloxane. 78 Measuring ion flux Measuring the ion flux to a surface in contact with the plasma is non-trivial, as at equilibrium the ion and electron fluxes are equal, and the net current flow is zero. Therefore, to exclude the electrons from impacting upon an electrode surface, we must apply a negative voltage of sufficient magnitude to repel even the highest energy electrons. One method was developed Fig. 4 Electron temperature and plasma density for argon plasmas (Reproduced with permission from ref. 32) RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

12 Review Fig. 5 Net flux of charged particles through an imaginary plane (left) and to a solid surface (right). by Braithwaite et al. 79 and involves applying a pulsed RF signal to the electrode. As the electrons react to the RF signal much more quickly than the ions, the electrode will develop a negative self-bias when the RF signal is applied. The RF signal is then chopped for approximately 5 10 ms and the change in voltage is measured with time as the ions impact with the negatively biased electrode. It is important to note that before the RF signal is applied, the probe is already negatively charged. Thus the ion flux is not altered by the probe as long as the RF power is low compared to the RF power maintaining the plasma. Typically the self bias voltage must exceed 20 V in order to repel all electrons in the plasma. The collection area of the probe is actually the sheath surrounding the probe. In order to eliminate non-planar geometrical effects at the edge of the probe electrode, a guard ring is employed which surrounds the probe electrode and is tuned to mimic the behaviour of the electrode. This ensures the sheath remains planar above the probe. The ion flux can then be calculated from eqn (18) below. J i ~ dv bias Cp (18) dt ea Fig. 6 Schematic of the sheath and pre-sheath adjacent to a wall in contact with a plasma phase. potential in the presence of RF sheath potentials and hence the maximum energy may be expressed as " E i max ~Vp{Vsb~ kt rffiffiffiffiffiffiffiffiffi# e 1 e 2 z ln M z kt e 2pm e ln I ev RF 0 (19) kt e where I 0 is the modified Bessel function of zero order and V p is the mean local plasma potential. 60 Eqn (19) shows that as the RF potentials increase, the self-bias voltage becomes more where C p is the capacitance of the circuit and A the collection area. Typical results for hexamethyl disiloxane plasmas (HMDSO) are presented in Fig A second method described by Sobelewski, 80 uses an internal RF electrode and the current is measured at the minimum voltage for each cycle of the RF. Ion energy Also of interest is the ion energy upon impact with the surface. As discussed later, the ion energy affects the sticking and ablation probabilities of the ion. In an RF discharge the insulating substrate potential is usually more negative than the classical floating potential V f acquired in a DC discharge. We call this the self-bias potential V sb. This potential is significant because ions crossing the plasma-polymer sheath (in low density, and therefore collisionless, conditions relevant to the discharges here) gain a maximum energy. The self-bias Fig. 7 Positive Ion flux vs. Power for HMDSO plasmas at 0.5 mtorr ( ) 1 mtorr (%) and 1.5 mtorr (m) (Reproduced with permission from ref. 78). This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

13 Review RSC Advances Fig. 8 Ion energy of acrylic acid plasma m/z 73 ions arriving at grounded and self-biased surfaces. (Reproduced with permission from ref. 49). Fig. 9 Deposited mass per ion for allylamine plasma as a function of power. (Reproduced with permission from ref. 54). negative. Furthermore, for large RF amplitudes, (V RF & kt e /e) ln I 0 (x) is almost a linear function and the second term on the right-hand side of eqn (19) approximates to V RF and the expression reduces to: E i max = V p 2 V sb = V RF (20) The potential that the ions fall through to an insulated surface (i.e. a growing polymer film) in this case is governed by the RF potential in the plasma and not T e. Fig. 8 shows the energy of ions arriving at a surface from acrylic acid plasmas as a function of power. 49 One must note that on arrival at the substrate ions also give up their recombination energy (ionisation potential) which can up to 20 ev depending on the species and this is in addition to the kinetic energy described here. Film characterisation and growth mechanisms The question that arises is which components of the plasma contribute to the deposition process. Ionic and radical species are the most likely candidates, and both were considered likely to provide mass to the growing deposit in early investigations. 81,82 More recently, ions have been described as providing energy to the surface through high energy collisions creating sites for neutral and radical species to chemically adsorb, but have been discounted in providing mass to the surface due to low abundance. 83 The majority of the recent growth models proposed attribute growth to radical chemistry. However films grown solely from hyperthermal ions (ions derived from plasmas) have been observed and shown to be chemically very similar to plasma polymer. 84 Indeed these hyperthermal films are instructive, as measuring the adsorbed mass compared to the total mass flux to the surface shows that the sticking probability of ions to a surface is of the order of 20 50%, depending on the ion energy upon arrival at the surface. When similar values are estimated for neutrals and radicals, the sticking probability is less than 0.1%. Also recall that ion-molecule reactions in the plasma phase may produce ions that are quite large, which may increase the ion mass flux. Fig. 9 shows the mass deposited per ion reaching the surface for allylamine plasma. 54 At low power, the deposited mass per ion increases rapidly with power, but then decreases rapidly, probably due to sputtering of the surface by high energy ions. Electron impact and VUV may also provide a means of polymerizing neutral molecules grafted to the surface. In the context of promoting polymerization, electrons with energies below 20 ev (as are typical in plasma systems) are considered extremely low energy. 250 ev electrons have been used to polymerize butadiene gas on surfaces, but the deposition efficiency was shown to be extremely low (y0.1%). 85 In this study it was noted that electron impact was a likely cause of cross-linking once the surface polymer was formed. Wertheimer and co-workers have shown that VUV can cause radical polymerization of nitrogen containing monomers, with deposition rates similar to those encountered in plasma polymerization. However, the VUV flux during these experiments was photons cm 22 s 21, compared to typical fluxes of VUV in low pressure, low temperature plasmas of around photons cm 22 s Therefore, while we should not discount these mechanisms entirely, they probably make only a minor contribution. Once a deposit has been formed from plasma, a number of techniques may be used to characterize the processing parameters and final surface chemistry. In the context of functionalized films, the surface chemistry is particularly important as this directly affects the performance of the material. Over the past 50 years there have been a large number of possible growth mechanisms proposed for plasma polymerization. 90 Here, we discuss some of the more commonly used characterization techniques and show how typical results may be reconciled with different growth mechanisms RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013

14 Review Fig. 10 Deposition rate vs. power for acrylic acid (top) and propionic acid plasmas [ref. 91]. Quartz crystal microbalance (QCM) A critical measure of plasma deposition industrially is the deposition rate. Quartz Crystal Microbalances may be used to measure the deposition rate during plasma polymer film deposition. The collecting surface consists of a quartz crystal typically coated with gold electrodes. The crystal is then biased with alternating current and driven at its resonance frequency. The adsorption (or removal) of mass to the surface is then measured as a shift in the resonance frequency. QCM is therefore extremely useful for monitoring the thickness of plasma deposits and calculating the deposition rate. Typically the deposition rate can be reliably measured to within 10 mg m 22 s 21. In Fig. 10, the deposition rate is presented for acrylic acid, and its saturated analog propionic acid. 91 The deposition rate is shown to increase rapidly with power up to a maximum after which there is a slight decrease at high power. The deposition rate for acrylic acid is approximately 2 5 times higher than for propionic acid, showing that subtle changes in monomer chemistry can have a large effect on the plasma. 92 X-ray photoelectron spectroscopy (XPS) X-ray Photoelectron Spectroscopy is a technique that may be used to quantify the elemental composition, and chemical state of a surface. 93 The method involves irradiating the surface with X-rays which then interact with core-level electrons. These electrons are ejected from the surface with a kinetic energy which is characteristic of each core-level/ element combination. The ejected electrons are collected by an analyser which measures the electron current as a function of kinetic energy. The kinetic energies can then be converted to the electron binding energy using eqn (21) E binding = E X-ray 2 E kinetic + W (21) where W is a work function. Thus the elemental composition may be determined. Of greater utility for surface engineers is the ability to determine surface chemistry, and this technique is widely used in plasma polymerization to determine the retention of functional groups on the surface. For example, Fig. 11 XP spectra of the C1s core-level for polyacrylic acid, and acrylic acid plasma polymers at low power and high power. acrylic acid contains a carboxylic acid group which is highly electronegative and the C1s binding energy of the COOH group occurs at ev. 94 As the monomer is fragmented in the plasma phase, the carboxylic acid groups may be broken up into C H, C O and CLO groups which have lower binding energies at 285.0, and ev respectively. This is demonstrated in Fig. 11 below, which shows at low power there is high retention of the carboxylic acid functionality. Note that while the carboxylic acid peak is almost the same intensity as that for polyacrylic acid, additional peaks are evident at lower binding energies, consistent with a low degree of fragmentation. However at higher power, increased fragmentation of the monomer results in lower acid functionality on the surface and further increases in the intensity of lower binding energy peaks. This result correlates well with plasma phase mass spectrometry, which demonstrates that high plasma power results in significant fragmentation of the monomer and loss of the carboxylic acid functionality. At low power however, lower monomer fragmentation in the plasma manifests in the surface as retention of the carboxylic acid group. Functional group retention also demonstrates that growth of the plasma polymer deposit (at low power at least) is due to molecular adsorption, in contrast to some proposed mechanisms which predict atomisation of the monomer in the plasma phase. 53 Infra Red spectroscopy (IR) XPS is a useful technique for determining the chemistry of films, but has a few limitations; the technique is surface sensitive and so only probes the top 5 10 nm of the surface, and some functionalities may be difficult to distinguish from XP spectra. For example, the peaks for amines, amides and This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3,

15 Review RSC Advances Fig. 13 AFM images (4 mm 6 4 mm) with profiles (below) showing the evolution of a plasma deposit from heptylamine plasma. Times are 7s (left), 15s, 25s and 40s. (Reproduced with permission from ref. 100.) Fig. 12 IR spectra of allylamine plasma polymers at different power. (Reproduced with permission from ref. 98). imines overlap. 95 For many biological applications, the density of primary amine functional groups is important. 96,97 To achieve this, allylamine plasma polymers are typically used, however it has been shown that many of the primary amines are converted to imines or nitriles in the plasma phase. IR spectroscopy may also be used to probe the chemistry of films, and does not suffer from these drawbacks. The sample is exposed to infrared radiation, typically between cm 21, and molecules absorb energy at resonant frequencies which are characteristic of their chemical structure. The depth of analysis is.1 mm, meaning that the bulk properties of the film may be determined, rather than just the surface chemistry. Fig. 12 shows the IR spectra of allylamine plasma polymers at different RF power input. It can clearly be seen that the bands at y2200 cm 21 and y3350 cm 21 become more prominent with increasing power. 98 The increase in these bands is indicative of an increase in doubly and triply bonded groups being formed in the plasma and depositing in the film. Based on these IR spectra, combined with deposition rate results, Choukourov et al. proposed a mix of radical/neutral and ion deposition mechanisms. They also noted a large decrease in deposition rate for ethanediamine compared to allylamine, which was attributed to the lack of a double bond, ruling out intact molecule surface radical deposition. Secondary ion mass spectroscopy (SIMS) Secondary Ion Mass Spectrometry is a method for probing the chemical structure of surfaces. 93 Heavy, high energy ions (typically gallium or gold, at y30 kev) are used to sputter the surface which cause secondary particles, (electrons, atoms, radicals and ions) to be ejected from the surface. In static mode, the primary ion current is kept low (,10 12 ions cm 22 ) and the ions are spread over the target region such that the secondary ions remain relatively intact. The secondary ions are then measured with a quadrupole mass spectrometer as discussed earlier, or using a Time-of-Flight (ToF) mass spectrometer. SIMS gives useful information about the chemical structure of the surface deposit. The link between the measured ion fragments and the polymer structure is strong, particularly when specific functionality is observed. It has been shown for HMDSO 41,78 that the positive SIMS of the deposit exhibits the same ion fragments as those measured in the plasma phase, including both monomer fragments and oligomers. This strongly suggests that ions are integral to the growth of these plasma polymer films. Atomic force microscopy (AFM) Atomic Force Microscopy is a high resolution 3D imaging technique, capable of measuring features down to less than 1 nanometre. 99 A cantilever with a sharp tip (tip radius, 10 nm) is brought into contact with the surface, where the force between tip and the surface results in deflection of the cantilever. The cantilever is scanned over the surface, typically over areas of 100 mm mm or less. Deflections of the cantilever as it scans over features on the surface are measured by reflecting a laser off the back of the cantilever onto a four quadrant photodiode. Usually, in order to avoid the tip hitting large features and being damaged, the cantilever is mounted on a piezoelectric tube and a feedback mechanism is used to maintain a constant force between the tip and the surface. AFM can be used to follow the deposition of plasma polymers over time. In Fig. 13 above, the plasma polymer film grown from heptylamine is shown to initially grow as discrete islands, which then coalesce and eventually form a flat, pinhole free film after approximately 30 s. 100 This result is contrary to the often-stated view of plasma deposition that films always grow as a conformal layer. It has also been observed by XPS that the chemistry of the film immediately RSC Adv., 2013, 3, This journal is ß The Royal Society of Chemistry 2013