Chemical Vapor Deposition Enhanced by Atmospheric Pressure Non-thermal Non-equilibrium Plasmas**

Transcription

1 Review Chemical Vapor Deposition Enhanced by Atmospheric Pressure Non-thermal Non-equilibrium Plasmas** By Sergei E. Alexandrov and Michael L. Hitchman* This review gives an overview of the characteristics of various non-thermal, non-equilibrium plasmas and discusses applications of AP-PECVD with dielectric barrier discharges, corona discharges, RF discharges, and microwave discharges. Keywords: Non-thermal plasmas, non-equilibrium plasmas, AP-PECVD principles and applications 1.Introduction ± [*] Prof. M.L. Hitchman Department of Pure and Applied Chemistry, University of Strathclyde 295 Cathedral Street, Glasgow G1 1XL (UK) Prof. S.E. Alexandrov Department of Electronic Materials Technology St Petersburg State Polytechnical University St Petersburg, Polytechnical Str 29 (Russia) [**] We thank the European Commission - Project G5RD-CT (Activated CVD for in-line coating of temperature sensitive parts, at atmospheric pressure) ± for financial support and the project partners for interesting discussions. Plasma enhanced chemical vapor deposition (PECVD) has been widely used for many different applications. These processes are distinguished from conventional, heat activated CVD processes by electrical energy rather than thermal energy being used for initiating homogeneous reactions for the production of chemically active ions and radicals that can participate in heterogeneous reactions which, in turn, lead to layer formation on a substrate. The main purpose of using a plasma as a source of energy for CVD processes instead of heat is to overcome high deposition temperatures that can degrade many substrate materials. This goal is achieved through the use of non-equilibrium, nonthermal plasmas that are typically generated by electrical discharges in the gas phase at low pressure (LP). In such plasmas, the electron temperature is much higher than the gas temperature and inelastic collisions of the electrons with precursor molecules form chemically active species that participate in the reactions leading to film formation. In addition, surfaces in the plasma can be bombarded with the active species, such as ions, electrons and photons, leading to changes in surface chemistry. PECVD processes based on this approach have been widely used for the deposition of a wide range of materials with standard and novel properties. Inorganic elements and compounds as well as organic polymers have been deposited by PECVD. [1] The use of LP-PECVD has limitations for various industrial applications, though. For many industrial products it is not practicable to use vacuum technology with load locks for large scale processes, and even if it were practicable the high capital and running costs of vacuum equipment can become prohibitive. As a result of these limitations, there has been considerable interest in recent years in the development of atmospheric pressure, non-thermal plasma sources suitable for use with CVD technologies. [2] In this paper we review some of the results achieved with atmospheric pressure PECVD (AP-PECVD) processes based on the use of non-thermal plasmas. 2.Sources of Atmospheric Pressure, Non-thermal Plasmas The various types of electrical discharge that can be used to generate non-thermal plasmas at atmospheric pressure have been recently reviewed. [3±6] These discharges occur in an appropriate gaseous atmosphere and are, typically, low frequency, dielectric barrier glow or filamentary discharges, as well as types of corona discharge, radio-frequency (RF) discharges in narrow gaps (which may include a dielectric barrier), or microwave (MW) discharges. The main aspects of these discharges and their common features are that the electrical input power generates plasmas with highly energetic electrons, yet the gas molecules passing through the discharge generation region remain ªcoldº. Inelastic collisions of the energetic electrons with gas molecules produce chemically active species such as free radicals, atoms and ions that can be involved in the chemical reactions leading to layer deposition. Chem. Vap. Deposition 2005, 11, 457±468 DOI: /cvde WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 457

2 There are several approaches for maintaining a low temperature of neutral species in a gas passing through an electrical discharge at atmospheric pressure. [3] d Limiting the input energy absorbed in the discharge by restricting the voltage applied or the discharge current d Providing efficient cooling of the plasma stream by increasing the rate of heat transfer through turbulent flow, or cooled electrodes or sidewalls d Decreasing the residence time of the gas stream in the discharge generation region Dielectric barrier, surface and corona discharges usually occur under conditions with limited absorbed power, with the equipment being designed to restrict the applied voltage or discharge current; the design can also help prevent arcing. Because of this limitation on input energy the absorbed power density (i.e. the watts per unit volume of the discharge generation region) is generally rather low, being, typically, 50 W cm ±3, and heavy particle temperatures do not usually exceed 200 C. The mechanism of energy absorption in the case of RF discharges is, however, quite different. These types of discharge provide highly efficient energy absorption in the volume of the discharge and this feature, coupled with the low thermal conductivity of plasmas at atmospheric pressure, requires cooling of the plasma stream in order to obtain non-thermal plasma characteristics. Such plasmas generated by RF gas discharges are usually sustained in narrow gaps between cooled electrodes or in narrow capillaries. And even though direct current (DC) discharges have a different ignition mechanism to RF discharges, non-thermal plasmas can also be obtained by using intensive cooling of the sidewalls of the plasma source. For all these types of discharge, high flow rates of gas streams through the discharge can assist in keeping the gas temperature low. Notwithstanding the various types of gaseous discharge at atmospheric pressure that can be used to generate nonthermal plasmas, the discharges are characterized by rather similar properties ± Table 1. For example, as can be seen, the electron temperature (T e ) for the various types of nonthermal plasma lies in relatively narrow range of 1±10 ev, while the temperature of heavy particles (i.e. the gas temperature T g ) is relatively low and does not exceed 1000 K. Also, on the whole, the parameters of many types of nonthermal plasmas generated at atmospheric pressure are similar to those for LP-PECVD processes which have an average electron temperature of 1±10 evand an electron concentration of 10 8 ±10 12 cm ±3. [1] Therefore one can expect some similarities between many of the characteristics of LP-PECVD and AP- PECVD processes. At the same time, though, the higher pressure associated with AP-PECVD processes would be Michael Hitchman has worked in the area of CVD for more years than he cares to remember. However, he recalls it all started some 30 years ago in the RCA Laboratories, Zurich, when his workon the electrochemistry of electrochromic displays ground to a halt and he had to find alternative research interests. So he turned his attention to another system involving homogeneous chemistry, heterogeneous process, and mass transport, namely CVD, and found that rotating disks were as useful and powerful for CVD as for electrochemistry. Since that time he has studied, and published extensively on, a wide range of CVD systems and materials. In 1993 the edited volume (with Klavs Jensen) on Chemical Vapor Deposition ± Principles and Applications appeared and he was also awarded the British Vacuum Council Medal and Prize for his workon CVD. Since 1995 he has been Editor of the CVD journal. More recently he retired from university life and founded two companies, one of which ± Thin Film Innovations Ltd ± seeks to capitalize on his knowledge of materials science using CVD, electrochemistry, and a variety of other deposition techniques. That enterprise has yet to achieve commercial success! Sergei E. Alexandrov received his Ph.D. in Technology of Electronic Materials in He has been studying various CVD processes for more than 25 years starting from his diploma research workdedicated to the development of CVD technology of barrier layers on the inner surfaces of quartz tubular reactors. In 1999 he was awarded Dr.Sc. Degree in Chemistry for his extensive research directed to the study of low temperature CVD of nitride and oxynitride dielectric layers. He has been a professor at St.-Petersburg State Polytechnical University since He is currently Head of the Department of Electronic Materials Technology and Dean of the Faculty of Materials Research and Technology. His research activities involve the development of a range of CVD processes including low and atmospheric pressure plasma enhanced CVD systems, studies of mechanisms of CVD processes using in situ diagnostics of the gas and plasma phases, development of CVD technology of nanoparticles and nanostructures, and technologies of new smart materials and MEMS WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2005, 11, 457±468

3 expected to lead to some specific differences from LP processes. For example, for AP-PECVD: d Partial pressures of precursors are about three orders of magnitude higher than in LP systems and this can lead to a significant increase in the rate of homogeneous reactions; in many cases these reactions are responsible for the formation of powders deposited at the same time as layers grow heterogeneously. d Thesignificantlyhigherprecursorpartialpressurescanlead to higher film growth rates compared with LP processes, which in turn can cause depletion of the precursor (or of an intermediate active species) in the gas stream over the substrate resulting in poor thickness uniformity. d Mass transport limitations are much more significant than at lower pressures and this can cause poor thickness uniformity as well. d For the case of remote processes, where the plasma stream exiting the discharge generation region is mixed with a gas stream containing the precursor, the fast homogeneous reactions of the active plasma species necessary for layer growth mean that the good intermixing required for uniform layer distribution over the substrate surface can be difficult to achieve. These particular features of AP-PECVD process need to be taken into account when developing equipment and choosing precursors. However, notwithstanding the potential problems arising from the use of high reaction pressures, AP-PECVD techniques present considerable opportunities for simple, cost effective depositions. In particular, the absence of vacuum equipment means there are the possibilities of cost savings and significant simplifications for continuous sample treatment. Some particular examples of the promising benefits of AP-PECVD processes are discussed in the following section. 3.Applications of Various Electrical Discharges for AP-PECVD 3.1. Dielectric Barrier Discharges (DBDs) General Principles of DBD Dielectric barrier discharges (DBDs) are one of the most promising sources for the generation of non-equilibrium plasmas used in AP-PECVD processes. These discharges are characterized by the presence of one or more insulating layers (which prevent DC discharges) in the current path through the discharge space between two metal electrodes [7] (Fig. 1) with planar or cylindrical configurations having been described. [8±10] Fig. 1. Schematic representation of a DBD discharge. A typical range of operating frequencies lies between 50 Hz and 500 khz, with gap spacings typically of a few mm, and AC driving voltages usually in the range 5±15 kv. For a DBD generated in this way, breakdown is initiated in most gases by a large number of independent filaments of microdischarges, or streamers, with diameters of about 0.1 mm. At the dielectric surface the microdischarges spread out into a surface discharge, covering the whole region. Typical microdischarge properties for a 1 mm air gap at 1 bar have been summarized in Table 1, and further characteristics are given in Table 2. Under certain conditions pulsed DBD discharges can show a homogeneous normal glow, a so-called atmospheric pressure glow discharge (APGD). [11] This can be achieved by appropriate choice of the frequency of power supply (usually in the khz range), power of excitation and gas type. [4,6] The discharge is most stable in He, although an APGD can be obtained in other gases, such as Ar and N 2. [4] The shape of the electrodes has also been found to influence the stability of APGD; for example, a glow regime was reached even at 50 Hz by putting a thin mesh between the insulators and the metal electrodes. [12] Some authors have suggested that perfect matching of the power supply and discharge assembly is the main condition leading to the appearance of a glow discharge. [13] It seems that the mechanisms responsible for this glow regime of DBD are still not clear yet. Table 1. Overview of parameters for various atmospheric pressure discharges [2, 6]. Discharge Type Discharge Frequency [MHz] Operating Parameter Power Input [W] Power Density [W cm ±3 ] Discharge Gas Flow Rate [slm] Electron Temperature (T e ) [ev] Plasma Characteristic Electron Density (n e ) [cm ±3 ] Gas Temperature (T g )[K] RF non-thermal 1±40 20±500 3±30 0.5±50 1± ±7x ±700 Glow discharge between two electrodes ± ± ±1 1±10 3± ±5x ±500 with dielectric DBD ± ±1000 1±50 0.2±5 1± ± ±400 Corona 0.001±1 1±50 0.5±4 3± ± ±400 Chem. Vap. Deposition 2005, 11, 457± WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 459

4 Table 2. Characteristics of DBD microdischarges. Operating Parameter Discharge Characteristic Current density [A cm ±2 ] 100±1000 Filament duration [ns] 1±10 Peak current [A] 0.1 Filament radius [mm] 0.1 Total charge [nc] 0.1±1 Electron density [cm ±3 ] ±10 15 Electron energy [ev] 1±10 Gas temperature [K] 300±400 APGDs are considered further in Section 3.3. in the context of AP-PECVD with RF discharges. The most widely used arrangement for DBD AP plasma processes is with the substrate having its surface orthogonal to the filamentary discharge. However, with this substrate orientation undesirable surface damage or surface heating can occur. To overcome these drawbacks, planar type DBD systems with the substrate and discharge in a parallel orientation have been proposed. [14] This type of DBD system has been realized with coplanar, surface, and insulated surface barrier discharges [14] and is promising for extended area AP-PECVD processes since these configurations can allow for the design of large, PECVD reactors with uniform plasma densities. Another approach for minimizing surface damage is to employ remote AP-PECVD, as discussed in Section Application of DBD for the CVD of Polymer Layers Polymerization of hydrocarbons in a DBD at atmospheric pressure occurs spontaneously and rapidly [15,16] and therefore is promising for the industrial growth of polymeric materials for protection, lubrication, etc. One of the first attempts using DBD discharges for polymerization processes was for the AP-PECVD of polyethylene coatings. [17] A uniform glow discharge was sustained by a 60 Hz power supply in an ethylene-he gas mixture introduced between two flat electrodes covered with insulator layers. Uniform films were deposited on glass substrates oriented parallel to the surface of the electrodes. Later filamentary types of discharge were also successfully used for deposition of polymeric thin films on glass substrates with deposition rates of up to 40 lm min ±1 being achieved. [18] A typical system for the study of plasma polymerization processes at atmospheric pressure is shown in Figure 2. [19,20] The discharge arrangement consisted of two diskshaped Al electrodes both covered with Al 2 O 3 plates. The gap between the un-cooled electrodes was typically a few millimeters to ensure stable plasma operation. The DBD was produced by a 20 kv/ 200 ma AC power source with a frequency between 1 and 4 khz. Polyethylene was deposited on substrates of silicon, glass or stainless steel placed on the lower, high voltage electrode from a plasma of He or Ar mixed with ethylene. It was found that the ethylene polymerized in He gave a low density, sticky, opaque polymer while the Ar plasma gave a clear and more solid polymer with good adhesion to all of the substrates. Also the polymer from the He plasma dissolved quite easily in chloroform, but that obtained from the Ar plasma was almost insoluble. These observations clearly indicate a difference in polymer' structure. Conventional LP-PECVD has been used extensively for the deposition of a wide range of organic polymers. [1] Examples include coatings for tribological, optical, electrical, and barrier applications. [2] The use of AP-PECVD for the growth of polymer layers is still at an early stage. However, the promising results obtained from the very limited studies made so far with DBDs indicates that AP-PECVD, with the inherent advantages mentioned earlier over LP- PECVD, has exciting potential for the formation of polymeric deposits, especially for deposition over large areas. An interesting design of the reactor for AP-PECVD based on the use of surface planar DBD discharge, in which an APGD is initiated at a dielectric surface as a result of strong electrical fields generated by embedded metal electrodes, has been described [14] and has been used for the formation of polymer films on the inner surface of PVC tubes. [21] The special electrodes used for the APGD consisted of two electrically conductive parallel structures helically wound around a glass cylinder into which the PVC sample tube was inserted. A 5kVsupply with a frequency of 20 khz was applied across the electrodes and the He carrier gas and the fluorinated monomer ± tetrafluoroethylene (TFE) or hexafluoropropylene (HFP) ± flowed into the PVC tube resulting in the APGD being restricted to the inner surface of the PVC tube. The optimum conditions gas for polymer deposition were determined in a separate study. [22] The fluoropolymer layers deposited using AP- PECVD have been shown to have similar properties to conventional LP-PECVD fluoropolymers. The low surface Fig. 2. Schematic experimental set-up for plasma polymerization by a DBD process (after [19,20]) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2005, 11, 457±468

5 energies of fluorocarbon thin films have led to important applications for bearings or sliding seals, while their chemical inertness provides resistance to chemical attack in corrosive and strongly reactive environments. Therefore, the production of such polymers by AP-PECVD with properties comparable to those produced by LP-PECVD could allow more economic layer deposition since no vacuum technology is required. By combining APGD technology with a unique precursor delivery system, Dow Corning Plasma has developed a new coatings approach ± Atmospheric Pressure Plasma Liquid Deposition. [23,24] This technique gives high coating rates onto flexible substrates and operates at atmospheric pressure and ambient temperature with a wide range of liquid precursors. For example, polypropylene film with a line speed of 1 m min ±1 was coated with highly hydrophilic, polyacrylic acid by introducing liquid acrylic acid precursor at 30 slm into a He APGD with a power of 0.4 W cm ±2. Oleophobic fluorocarbon coatings have also been produced under similar conditions, as well as silica layers; the latter type of material is the subject of the paper by O'Neill et al. in this issue. [24] The potential economic and technical benefits of AP-PECVD processes are again apparent. Very recently barrier coatings have been obtained from hybrid organic±inorganic precursors as well as from purely organic compounds. [25,26] This work is described in the paper by Vangeneugden et al. elsewhere in this volume. [26] In summary here it can be said that the hybrid precursors lead to coatings with excellent barrier properties due to the unique synergistic effect of the organic and the inorganic network structures that are formed in the plasma. Barrier coatings are useful in preventing the loss of volatile materials, such as petroleum products and VOCs, from containers or tubing. In addition, they can prevent corrosion and chemical attack of non-noble metals, and the promising possibilities of AP-PECVD for making a significant contribution to barrier coating technology are evident Application of DBD for the CVD of Inorganic Substances Layers of various inorganic materials can be successfully deposited by AP-PECVD based on the use of DBD discharges as well. [27±30] The most widely studied inorganic layers have been oxides of silicon, in both stoichiometric and non-stoichiometric form. Precursors commonly employed have been hexamethyldisiloxane (HMDSO), hexamethyldisilazane (HMDS) and tetraethoxysilane (TEOS), with TEOS being the most frequently used. [27,31±33] An example of the deposition of SiO x films from TEOS by DBD under atmospheric pressure is a study using two rectangular shaped Al 2 O 3 electrodes with the powered electrode moving over and along the substrate on the grounded electrode. [33] The typical operation conditions during plasma treatment were a voltage of 15 kvand 6.6 khz, a power density of 1±1.6 W cm ±2, and a gas flow of 1 slm. The mean residence time of the substrate was only 0.1 s and so there is the potential for coating on a continual basis at a reasonable rate. Indeed, the deposition of SiO x on a substrate moving at 4 m min ±1 has been achieved by others using a different arrangement, [24] so the industrial viability of AP-PECVD with DBD is apparent. Pulsed DBD has been used for the deposition from TEOS of thin films containing Si, O and C on polycarbonate sheet. [32] The influence of plasma parameters, such as the energy of a single discharge pulse and the position of the polycarbonate samples on the electrodes, on the deposition rate were investigated. The deposition rate increased nonlinearly with increasing pulse energy; for example, with 6.2 mj the deposition rate was 4.6 nm min ±1 while with 9.1 mj it was 37 nm min ±1. The technique was shown to produce transparent, smooth homogeneous films. In general, deposition rates by DBD with TEOS are not as high as 37 nm min ±1. Higher rates can be obtained if HMDSO is used as a precursor. For example, a comparison between TEOS and HMDSO with comparable deposition conditions showed <5.5 nm min ±1 and ~10 nm min ±1 for the two precursors, respectively. [34] Hence there has been considerable interest in HMDSO. Indeed, very high growth rates in the range 12±120 nm min ±1 of silicon oxide like films have been obtained by DBD of low concentrations (<0.1 %) of HMDSO in He. [28] In this case the DBD discharge was maintained by a 100 khz supply at 1.4±3.2kVat a power of 1±4 W; the low power input is an attractive feature of non-thermal AP-PECVD processes in general and DBD systems in particular. Interestingly, in this work a study of the gas phase composition by mass-spectrometry showed m/z values of 15, 16, 25, 26 with peaks also in the range m/z 35±47, indicating the formation of hydrocarbons such as CH 4,C 2 H 2, and C 3 H x. An enhanced signal of m/z 73 was attributed to the formation of tetramethylsilane. This chemistry might be expected to lead to organic fragments in the deposited films, and this has indeed been found with FTIR absorption bands corresponding to Si±CH n groups. [35] The carbon content of films is, though, reduced for films obtained from mixtures of HMDSO and an inert gas, e.g., argon, with oxidants such as oxygen and nitrous oxide. In traditional forms of CVD the importance of the C' (i.e. the chemistry) cannot be overemphasized. The results highlighted above show that for AP-PECVD this is equally true. Also, as with other types of CVD, the results illustrate the range of possibilities for controlling and modifying the deposition processes. Much more work obviously needs to be done along the lines indicated above if the latent capabilities of AP-PECVD are to be fully exploited. Other chemistries that have been investigated for AP- PECVD of SiO x layers have used siloxanes, such as tetramethylcyclotetrasiloxane and octamethylcyclotetrasiloxane, which are particularly useful for the preparation of hydrophilic coatings. [23,24] Still other organosilicon precursors, namely, hexamethyldisilane (HMDS) and tris(trimethylsilyloxy)vinylsilane (TTMSVS) have been used with Chem. Vap. Deposition 2005, 11, 457± WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 461

6 a DBD discharge to improve polyethylene terephthalate (PET) fabric coloring effects arising from the deposition of an anti-reflective coating. [36] FTIR of the deposited films showed the presence of the Si±O±Si peak at 1050 cm ±1.It was suggested that the Si±O bonding process is enhanced by more oxygen being present during deposition, which is consistent with the fact that films obtained from TTMSVS, a precursor with intrinsic Si±O bonds, showed a higher intensity of the Si±O±Si peak than films deposited from HMDS. It was also demonstrated that PET with SiO x deposited from TTMSVS had better anti-reflection properties than a coating prepared from HMDS; this is probably associated with the greater Si±O bonding when using TTMVS, which, in turn, leads to structures characterized by lower reflection characteristics. Requirements for coated precision optical components are changing rapidly, driven by the expanding growth in markets such as optical telecoms, imaging systems, data storage, sensors and displays. In addition there is a general trend in these markets towards miniaturization, leading to reduced size and higher functionality and performance of components and sub-systems. These needs are pushing conventional technologies based on physical vapor deposition (PVD) to higher levels of control and throughput, but there are limitations arising from the tooling required for the line-of-sight PVD methods. CVD, in general, offers the possibility of a non-line of sight deposition method and is well suited to economical coating of 3D micro-optical components. From what has been said above, it is clearly the reactive process which distinguishes CVD from PVD and if the deposition process occurs under kinetic control on the substrate surface this means that it is possible to coat intricate solid shapes, such as spherical and cylindrical lenses, with layers that are of uniform thickness and composition. With AP-PECVD there is the prospect of not only doing this, but also of achieving it in relatively simple and cost-effective reactor systems. HMDS can be used to deposit silicon oxide-like layers. [37] However, it is probably more widely used for the deposition of silicon nitrides. A reactor used previously for the deposition of silicon oxide layers [27] was used to study the possibility of depositing silicon nitride with the gas mixture HMDS±N 2 ±NH 3. [29] Absorption peaks corresponding to Si±N and Si±NH±Si groups were observed in FTIR spectra, but in addition peaks related to incorporated organic and other fragments (e.g. Si±CH n, Si±H) were observed. Furthermore, Si±O±Si absorption bands were often seen as well. XPS analysis confirmed the presence of carbon and oxygen in some films with typical compositions beings Si: 51±54, O: 15±17, N: 20±28, and C: 4±6 at.-%. In addition, the refractive index of the films varied between 1.23 and 1.68 showing their very non-stoichiometric nature. Clearly, careful regulation of the reactor environment is required, and the role of the chemistry of the process is again emphasized, if uncontrollable doping of the deposited films with oxygen and other elements is to be prevented. Other materials to be deposited by AP-PECVD with DBD have included titanium dioxide and diamond-like carbon (DLC) films. For titanium oxide deposition a laminar flow reactor has been used with the reaction gases being introduced between parallel plate electrodes placed inside a silica tube and powered by a 10 kv, 33 Hz supply. [38] The gases used were titanium tetrachloride picked up from a bubbler with helium (0.5 slm) and mixed with additional helium (3 slm) and a small amount of oxygen (5 sccm). Growth rates were quite high at 60±70 nm min ±1. The films were amorphous and of uncertain quality. Titanium dioxide thin films have a number of important applications, including as optical and photocatalytic coatings. For these uses it could be beneficial to be able to deposit titania layers on large areas at low cost. AP-PECVD provides the possibility for achieving this, but obviously considerable more work needs to be done in order for the technique to be commercially interesting and viable. A very simple design of a DBD reactor consisting of two parallel plate electrodes with a glass dielectric barrier has been successfully employed for the deposition of DLC films using CH 4 and C 2 H 2 as precursors. [30] The authors reported that successfully deposited hydrogenated DLC films with extremely high growth rates of tens of micrometers per hour. As for the case of titania, DLC layers have wide ranging applicability; e.g. for optical, mechanical, electronic chemical functions. So, again, AP-PECVD is a prospective technology for industrial production Remote AP-PECVD Based on DBD As indicated earlier, the conventional approach of AP- PECVD based on DBD where substrates are placed on one of the electrodes has drawbacks because of the possible generation of defects and pinholes in the deposited films, particularly when the discharge is filamentary. As a result of these problems, investigations have been made of remote AP-PECVD processes, where the plasma stream exiting the discharge generation region is mixed with a gas stream containing the precursor. A systematic study of remote AP-PECVD of silicon oxide films using DBD has been carried out. [37,39±41] Two experimental AP-PECVD arrangements, one using a horizontal reactor and one with a vertical reactor, have been developed, but most of the studies have been with the vertical reactor. The glass reaction chamber consisted of two zones. The top zone of the reactor was used for the generation of the barrier discharge between two coaxial insulated copper electrodes. To generate and sustain a barrier filamentary discharge with this electrode design an alternating 50 Hz voltage with an amplitude of at least 3 kvwas required. Voltages in the range 3.5±15 kv were investigated, with a value of about 9 kvgenerally being used. Typical values of electrical power were 40±90 W. Electronic grade argon and oxygen were introduced into the top of the barrier discharge gap and passed through it in the direction of WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2005, 11, 457±468

7 the substrate which were placed on a heated stainless steel susceptor in the bottom part of the reactor. Deposition temperatures in the range 300±600 C were studied. Four organosilicon precursors ± HMDSO, HMDS, TEOS, and diacetoxydi-t-butoxysilane (DADBS) ± were investigated. It was shown that high quality silica films can be deposited with all three precursors, but that HMDS has the potential for achieving high growth rates (up to 25±30 nm min ±1 ) while DADBS, although giving a substantially lower growth rate (less than 3 nm min ±1 ), allows deposition at lower temperatures ± less than 300 C. The quality and integrity of the layers is illustrated by breakdown voltages as high as 8 MVcm ±1 (Fig. 3), which is comparable to that obtained with a thermal oxide grown on silicon. A more detailed description of results obtained with HMDSO is given in a companion paper in this issue. [41] As already mentioned, silica is probably the most extensively studied oxide produced by CVD because of its widespread applicability. The fact that AP-PECVD can produce layers of comparable quality to those deposited by more conventional CVD processes indicates the inherent usefulness of atmospheric pressure techniques, although a drawback of remote AP-PECVD based on DBD could be the relatively slow growth rates and the elevated substrate temperatures; changing precursor chemistry could help to overcome these problems, to some extent. A rather different type of remote AP-PECVD DBD system has been used for the deposition of polymeric films from two monomers: hexafluoropropene (C 3 F 6 ) or trifluoroethylene (CF 2 CFH). [42] A spray-type remote reactor was used consisting of electrodes covered with Pyrex glass tubes. A monomer, diluted with He or Ar, passed through the discharge region onto the sample. The power supply frequency was 100 khz at a 100 W. Fluorinated polymer films were deposited with rather high growth rates of up to Breakdown Voltage/MV cm nm min ±1 for C 3 F 6 and up to 200 nm min ±1 for the CF 2 CFH/He system. This is an illustration of how modifying precursor chemistry can lead to improved rates of deposition. From this brief survey of AP-PECVD it can be concluded that for processes based on the use of homogeneous and filamentary DBD, layers of a number of polymeric and inorganic materials can be deposited with various material characteristics and over a range of growth rates. The suitability of the techniques for producing commercially interesting layers will need to be assessed for each particular application, and it may be concluded that considerable more effort is needed to make a process viable. However, the particular simplicity of the two techniques and their low power requirements make them an attractive technology for commercial applications, with remote AP-PECVD having the advantage of low damage to the substrate and the depositing layer AP-PECVD Based on Corona Discharge AP-PECVD using a corona discharge has produced similar types of results to those obtained with DBD. One of the first elementary applications of corona discharge to AP- PECVD was in 1988 for the deposition of carbon films. [43] The reactor was a simple design with a brush-style upper electrode consisting of 25 fine wires of stainless steel or tungsten. A continuous stable discharge was created using a 3 khz or an RF voltage. Thin films of carbon were prepared on Pyrex or quartz substrates by decomposition of methane in H 2 -He mixtures. Thyen et al. [44] used a corona discharge for the AP- PECVD of organic and inorganic thin films at low temperatures. The general experimental setup consisted of 4 metal rods used as high voltage electrodes, each covered by an alumina tube as a dielectric barrier. The high-voltage electrodes, operating at 10±20 kvand 20±50 khz, were placed over a grounded and PTFE-covered aluminum plate leaving a gas gap of about 2 mm. Two gas showers, each placed between two high voltage electrodes, HMDSO HMDS Power/W allowed the introduction of the process gases. To obtain a homogeneous deposition the grounded substrate was mechanically moved back and forth beneath the discharge. Using this simple equipment, silicon oxide films have been deposited either from tetramethylsilane (TMS) or from TEOS in oxygen containing atmospheres. The effective deposition rate taking into account the movement of the substrate, and as- Chem. Vap. Deposition 2005, 11, 457± WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 463 TEOS Fig. 3 Dependence on plasma power of breakdown voltage of silicon oxide grown by remote AP-PECVD DBD (data from [40]).

8 suming the use of an array of high-voltage electrodes, could be higher than 500 nm min ±1. With the system described, an increase in discharge power and precursor concentration did lead to an increase of growth rate greater than 100 nm min ±1, but at the same time produced deposits of soft, polymer-like films. Also it was found that without movement of the substrate very rough and powdery deposits were obtained directly below the alumina tubes. For good quality films with a high scratch resistance the results of electron probe micro analysis (EPMA) and secondary ion mass spectrometry (SIMS) showed that the oxygen/silicon ratio was in the range 2.2±2.5, with, not unexpectedly, an increase in content of oxygen in the gas phase leading to deposition of films with lower carbon content. When oxygen was not used then with TMS-Ar or TMS-N 2. polymeric, hydrogenated silicon-carbon films were deposited, as indicated by the appearance in FTIR spectra of the films of absorption bands corresponding to Si-CH 3 vibrations. These films also contained substantial amount of oxygen (up to 27 at.-%) which presumably came from the ingress of atmospheric oxygen from the surroundings. The importance of fluoropolymers has already been mentioned and using C 2 F 4 as a precursor, it has been shown that smooth fluorocarbon, PTFE-like coatings can be deposited by corona discharge with rather high growth rates of 100±200 nm min ±1. The IR spectrum of the deposited films demonstrated intense C-F absorption at 1200 cm ±1 and the C:F ratio was calculated to be 5:8, nearly corresponding to that of PTFE. These results from a simple AP- PECVD system are encouraging. A novel AP-PECVD technique that combines non-equilibrium plasma reactions with template-controlled growth technology has been developed for synthesizing aligned carbon nanotubes at low temperatures. [45,46] A schematic of the reactor is shown in Figure 4. An anodized aluminum template with an area of 7 mm 7 mm was placed on the plate electrode and the wire electrode was positioned with its tip 5 mm above the template surface. The reactant mixture of methane and hydrogen (1:10) was fed into the reactor at a feed rate of 22 sccm. An AC generator operating at 8 kvand 25 khz with power of 40 W was used to initiate corona discharge. With this system multiwalled, aligned carbon nanotubes with diameters of approximately 40 nm formed in the channels of the aluminum oxide coated template at a temperature below 200 C. The technique allows carbon nanotube devices to be fabricated under relatively mild conditions and has potential for a broad range of practical applications. Also the low synthesis temperature by the corona discharge procedure opens up new prospects for experimental and theoretical studies of growth mechanism for carbon nanotubes. It is interesting to note that the typical main parameters of plasmas generated by DBD and corona discharges are quite similar (cf. Table 1). So the similarities in the results of AP-PECVD processes based on the two types of discharge are not too surprising. However, because a DBD discharge requires a lower voltage than corona discharge and is more flexible from the point of view of reaction chamber design, the use of AP-PECVD with DBD has been more widespread than applications of corona discharges AP-PECVD Based on the use of RF Discharge Notable progress has been made over recent years in the development of various RF atmospheric pressure plasma sources with a planar geometry and this major topic is the subject of another review in this Special Issue of the CVD journal by Moravej and Hicks. [47] That review provides an in-depth analysis of capacitively coupled AP-PECVD RF systems and assesses the properties and characteristics of a variety of layers deposited using the technique. Here, therefore, we only briefly consider the technology. Figure 5 illustrates a schematic arrangement [48] and Figure 6 shows a coaxial electrode configuration, [49] in which 50±300 sccm He gas flowed down the inner space of the RF He cathode oxide layers plasma Fig. 4. Schematic diagram of the template-directed synthesis of carbon nanotubes using atmospheric pressure corona discharge. The diameter of the multiwalled carbon nanotubes was about 40 nm (after [45, 46]). anode Fig. 5.SchematicdiagramofanRFAP-PECVDcoldplasmareactor.(after[48]) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2005, 11, 457±468

9 Fig 6. A coaxial RF plasma torch (after [49]). alumina coated cathode that was powered by 20±50 W to generate a capacitively coupled plasma. Because of their ease of operation jets and torches are the main types of RF plasma sources that have been investigated for AP-PECVD processes. One of the first attempts to use an RF cold plasma torch for AP-PECVD was for the deposition of silicon oxide films. [50] The cathode was a tungsten needle connected to an RF (13.56 MHz) generator. The grounded anode was a stainless steel cylinder of 5 mm inner diameter and was separated from the cathode by an alumina tubing. A mixture of He and O 2 gas was passed through the space between electrodes and a reactive cold plasma was generated by mixing tetramethoxysilane (TMOS) with its carrier gas of Ar and hydrogen. The applied RF power was 80±100 W. It was demonstrated that with this system SiO 2 films could be deposited on various substrates exposed to air at rates exceeding 600 nm min ±1. The characterization of the deposited layers by IR, XPS and SEM revealed that the mixing of hydrogen was quite effective to reduce the carbon contamination and improve the film quality. Later, the same type of cold plasma torches were used for the deposition of TiO 2 films on substrates exposed to air. [51] For example, Ti(OEt) 4 or Ti(OiPr) 4 vapors were introduced into the plasma at the torch nozzle and a homogeneous plasma with an electron temperature of about 1.8 evand a gas temperature of about 200 C led to the decomposition of the precursors. The active products of decomposition were directed on to the substrate surface. The film growth rate for both precursors was very high (up to 1 lm min ±1 ). The deposits were shown to be stoichiometric and amorphous TiO 2, although they contained short-range crystallinity, and the TiO 2 deposited from Ti(OEt) 4 ±H 2 mixtures had better dielectric properties. Substantial contributions to the development of AP- PECVD processes based on the use of RF plasma jets for the deposition of oxide, nitride and amorphous silicon layers have been made by Hicks et al. [49,52,53] These papers are not discussed here, though, since, as already indicated, they are reviewed in another contribution to this Special Issue. [47] However, the most important conclusion that can be drawn from the papers is that RF AP-PECVD provides much higher deposition rates than processes based on DBD or corona discharges, and that the quality of the deposited films is similar to that obtained by traditional LP- PECVD technology. Also because of the high growth rates achieved using RF AP-PECVD this technology is attractive for specific applications where the substrate to be covered spends a limited time in the deposition zone. One of the problems connected with application of RF torches is overheating of the nozzle, which can lead to an arc discharge with hot electrodes. To prevent this undesirable effect a dielectric layer has been used to cover the inner walls of the metallic nozzle. [54, 5] Such a torch has been used for the deposition of CeO 2 thin films on polished aluminum substrates. [54] The cerium precursor was prepared as an aerosol of a water solution of cerium acetate and was carried by an air/he flow to the plasma jet. The plasma in the jet channel was found to be strongly non-isothermal with the temperature of the He being lower than 800 K, but with the vibrational temperature of the atmospheric nitrogen molecules being around 3600 K. Most importantly, the average substrate temperature did not exceed 250 C during the deposition. Preliminary results showed that the formation of CeO x films was only possible when the RF power applied to the plasma jet was greater than 360 W. The stoichiometry of all the CeO x films showed excess of oxygen and they were heavily contaminated by carbon. Further investigations into, and improvements of, the deposition chemistry are required, but the potential for low temperature deposition is nevertheless apparent. The barrier torch discharge has also been used for low temperature deposition of In x O y and SnO x thin films on polymer substrates. [55] Vapors of Sn- and In-acetylacetonate were used as precursors for the deposition process. Transparent films with a conductivity of about 10 S cm ±1 were obtained for SnO x and of 10 2 Scm ±1 for In x O y under conditions where the plasma jet directly interacted with the polymer substrate. SnO x and In x O y films of thickness in the range 200±300 nm were deposited with typical deposition rates in the range 0.4±0.6 lm h ±1. Electron probe analysis showed that all the films had compositions close to stoichiometry and no detectable contamination by carbon and by other measurable elements was found in the In x O y films, but about 2 at.-% carbon was found in the SnO x films. XRD analysis showed all the films to be amorphous. When suitably doped, both tin and indium oxides are useful transparent electronic conductors that can be employed in intelligent optical structures; e.g. switching mirrors and windows. These applications require large surface coverage and RF AP-PECVD would be an appropriate technique for achieving this. A different application of the technology has been for the deposition of thin layers on pigment particles. [56] The reactor for this consisted of concentric electrodes with the high voltage inner electrode being an aluminum tube (diameter 6.5 mm) and the grounded electrode was a stainless mesh wrapped around a concentric insulating tube (diameter 16 mm). The outer mesh electrode had a glass jacket around it through which cold water was passed to keep the discharge temperature under 100 C. The pigment powder Chem. Vap. Deposition 2005, 11, 457± WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 465

10 held in a reservoir at the bottom of the reactor was blown up by the helium carrier gas together with the precursor TEOS and N 2 O as an oxidizing agent through the inside of the powered electrode tube, and it was subjected to the plasma discharge as it fell back down to the reservoir between the inner electrode and the outer insulating tube. Silica coatings could be deposited on Fe 3 O 4 black, FeOOH yellow and Lithol Rubine BCA RED powders. A similar approach has been used for the deposition of zirconia coatings by RF AP-PECVD on amorphous Co Fe 4.7 Si 10 B 15 powders with Zr(OC 4 H 9 ) 4 as a precursor. [57] In this case, though, an ultrasonic horn was used to keep aggregated powder particles separated and also to assist in blowing the powder up from the reservoir. The powered electrode was a stainless steel rod (diameter 15 mm and length 260 mm) with many minute grooves cut on its surface to form a screw-like pattern. This made it possible to keep a stable glow discharge, with the electric charge being evenly focused on the grooves. Inside the electrode was a water-cooled honeycomb structure that allowed the discharge area to be kept below 100 C. The ground electrode was a copper plate wrapped around a glass tube with an internal diameter 22 mm and a wall thickness of 3 mm. The powder dispersed by the ultrasonic horn was blown up by a He/O 2 flow through the discharge area. The treated powder was fed back into a side reservoir and then re-circulated through the discharge region. The overall treatment time was 20 min. This procedure converted the powder from a conductor to a good insulator. In addition, the treated powder showed stronger resistance to oxidization. Another reason for coating powders is to modify their optical appearance. Thin films with a high refractive index can give interesting interference effects to painted surfaces having coated pigment particles incorporated into them. AP- PECVD processes could allow large quantities of powders to be modified with a high throughput. Very exciting results have been obtained recently on the application of RF AP-PECVD to the rapid growth of epitaxial silicon at the relatively low temperatures of 530± 690 C. [58] The authors developed an original reactor design (Fig. 7) with a cylindrical rotary electrode (300 mm diameter). The substrate was vacuum-chucked onto a SiC coated graphite susceptor and the plasma was generated with a gas mixture of He, H 2 and SiH 4 in the gap (700 lm) between the rotary electrode and the substrate by supplying 150 MHz VHF power through an impedance matching unit. High quality Si films with excellent crystallinity, and with a surface flatness similar to, or better than, that of commercial Si wafers, were deposited on (001) Si wafers in the area where the deposition gap between the substrate and rotary electrode was small. In particular, TEM of epitaxial Si films grown at 610 C with an input plasma power of 2 kw showed no lattice defects. The maximum growth rate depended on substrate temperature and plasma power. For 610 C with 2 kw the rate was 1.2 lm min ±1, and a rate as high as 6.6 lm min ±1 was obtained at 690 C and 1.5 kw; Fig. 7. Schematic illustration of atmospheric pressure plasma CVD with a rotary electrode (after [58]). these are approximately 20±30 and 4±6 times faster, respectively, than those obtained by thermal CVD at a temperature in the region of 1100 C. If the quality of the epitaxial films can be maintained for an upgraded commercial size reactor then the potential of the process is enormous AP-PECVD Based on the use of MW Discharge A type of AP-PECVD that we have not considered so far is that using microwave (MW) discharges, which have some distinctively different operating characteristics and properties from the other AP-PECVD processes discussed up to now. Of course, the operating frequency is very much higher than other discharges, typically being about 2.54 GHz, but in addition electron densities can be as low as 10 7 cm ±3 and gas temperatures as high as 1000K, or even higher. So far there has not been very much use of MW sources for generating non-equilibrium, non-thermal plasmas for AP-PECVD. This is most probably because it is not easy to design simple and convenient equipment for AP-PECVD processes and also because of the difficulties in sustaining MW discharges at low powers. One example where these criteria have been achieved is the for continuous deposition of silica coatings onto carbon fibers in a tow form with a low cost, novel atmospheric pressure microwave plasma technique. [59] The objective was to improve the interfacial properties of low modulus carbon fiber composites. A schematic diagram of the experimental setup is shown in Figure 8. Fig. 8. Schematic diagram of an AP-PECVD microwave reactor (after [59]) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Vap. Deposition 2005, 11, 457±468