Polymer-like Films Research Articles

Ion-implantation into amorphous hydrogenated carbon films

Amorphous hydrogenated carbon is being investigated as a possible semiconducting material. A required property of a semiconductor is electronic doping, and this may be achieved either by in situ addition of gaseous precursors during deposition or ex situ ion-implantation. This study shows that resulting ion beam damage produced by ion-implantation may be kept to a minimum as long as the ion dose is less than approximately 10 14 cm −2. Polymer-like carbon films grown using radio frequency plasma enhanced chemical vapour deposition have been implanted with carbon, nitrogen and boron ions, and the electronic and optical properties of the material analysed. An analysis of the electrical and optical data is carried out to distinguish between the physical and chemical changes that occur within the microstructure of the films.

Physical properties of a-C:H films prepared by electron cyclotron resonance microwave plasma chemical vapor deposition

a-C:H films have been deposited on silicon and glass using electron cyclotron resonance microwave plasma decomposition of CH 4 diluted with Ar gas at 0–160 V rf negative bias and 65–250 W microwave power. The deposition rates, absorption coefficients, optical bandgaps, refractive indices and internal stresses of the a-C:H films grown under varying preparation parameters have been measured. The microstructure of the carbon films has been evaluated by Raman spectroscopy. It has been found that the properties and structure of the carbon films are strongly dependent upon the rf substrate bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses, and higher optical bandgaps; at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the properties and structure of the a-C:H films.

Structural and mechanical characterization of fluorinated amorphous-carbon films deposited by plasma decomposition of CF4–CH4 gas mixtures

Fluorinated amorphous-carbon films (a-C:F:H) were deposited by low-power rf capacitively coupled plasma-enhanced chemical-vapor deposition using CH4–CF4 gas mixtures. Different series of films were deposited, changing one parameter at a time: the CF4 partial pressure from 0% to 100%, the self-bias voltage from −50 to −700 V, and the total deposition pressure from 5 to 15 Pa. The composition was determined by ion-beam analysis (IBA): Rutherford backscattering spectrometry, elastic recoil detection analysis, and nuclear reaction analysis. The atomic density of the films was evaluated by combining the IBA results with the thickness value measured by stylus profilometry. Film structure was investigated by infrared transmission and Raman scattering spectroscopies. The internal stress and Vickers hardness were also measured. For a fixed self-bias, the increase of the CF4 partial pressure leads to a higher fluorine incorporation and the decrease of both hardness and internal stress. The film microstructure changes from diamond-like to a polymer-like structure. The fluorine incorporation also increases with the self-bias, and fluorine-poor polymer-like films are deposited at low-bias voltage. Fluorine incorporation occurs at the expense of the hydrogen content in both cases, i.e., increasing the CF4 partial pressure or the self-bias. Finally, the role of ion bombardment during film growth on the mechanical and structure properties of the films is discussed.

Time-resolved study of photoluminescence polarization in a-C:H films

The decays of intensity and polarization anisotropy of the photoluminescence (PL) of polymer-like amorphous carbon films have been measured at 300 and 80 K using 4.13 and 2.15 eV excitation. For emission energies, E em, between 1.8 and 3.5 eV, PL polarization anisotropy decreases within 100 ps and reaches a plateau within 1 ns. The anisotropy plateau increases from 0.02 to 0.12 and the intensity decay time decreases from 1 ns to 40 ps as E em increases. Anisotropy and intensity decays are interpreted within a dipole–dipole energy transfer (Förster) model.

Comparative study of anneal-induced modifications of amorphous carbon films deposited by dc magnetron sputtering at different argon plasma pressures

Abstract This work investigates the evolution of the structural, mechanical and tribological properties of amorphous carbon films (a-C) undergoing thermal annealing. Two different types of a-C film were studied: hard, stressed and dense films, henceforth called ‘diamond-like’, and soft, more relaxed and less dense films, called ‘polymer-like’. The two sets of films were deposited by dc magnetron sputtering at 0.36 and 1.4 Pa argon plasma pressures, respectively, and both were annealed in vacuum for 1 h at temperatures between 300 and 700°C. Raman results provided indications of a remarkable increase of the sp2 hybridized domains, that occurs mainly at temperatures higher than 400°C for the diamond-like films and in a continuous way for the polymer-like films. The friction coefficient of both types of film approaches the same value with increasing temperature, whereas the surface roughness is nearly constant. For diamond-like films, the hardness and compressive internal stress are nearly constant up to 500°C, and then undergo a fast decrease. In contrast, the hardness of the polymer-like film continuously decreases with temperature, whereas the compressive stress remains constant.

The protection of metallic archaeological objects using plasma polymer coatings

One major concern of conservators and archaeologists is to limit the alteration and the corrosion of metallic archaeological objects. Organic coatings are one of the most important ways of protecting metal against corrosive agents, thanks to their diffusion barrier properties. The aim of this work was to develop effective protective coatings fitting with the specific requirements of heritage preservation (in particular the questions of reversibility and aesthetic appearance). Ultrathin (5-50nm) polymer-like films have been deposited by plasma polymerisation in a methane or propane discharge on various metallic samples (stainless steel, Fe, Cu, Ag and Roman iron nails). Some important results are:1.on tested substrates, except Ag, adhesion is very good;2.concerning the reversibility of such layers, they can be totally removed in H2 plasma;3.the coatings are slightly coloured, which seems linked to the amount of unsaturated CC bonds in the C layer and to a finite electrical conductivity;4.they show interesting barrier diffusion properties to H2S, O2 and water vapour molecules; and5.on silver substrates, some problems of adhesion are observed due to the diffusion of Ag ions through the organic films.

ELECTRONIC CONDUCTION IN ION IMPLANATED AMORPHOUS CARBON THIN FILMS

The electronic conduction mechanism occuring in amorphous thin films is quite complicated. In amorphous carbon films it is further exacerbated by the rich diversity of its microstructure as well as the large number of gap states present in the films. One of the main reasons for the tunability of the optical band gaps of these films not being exploited in active devices, has been the inability firstly to reduce the gap states to an acceptable level, and secondly finding suitable dopants that are electrically active at room temperature. The large number of gap states in the films further exacerbates its problems by not allowing suitable barriers (eg. Schottky) to be formed on to the amorphous carbon films. In this paper we hope to first divide the amorphous carbon thin films into two main categories. Namely, diamond-like carbon films which have a high precentage of C-C sp 3 bonding, and polymer-like carbon films that also have a high percentage of sp 3 bonding which is a mixture of C-C and C-H bonds, and have a high percentage of hydrogen as well as large optical band gaps. Recent results based on ion implantation using ions such as N, B, C will be contrasted to in-situ dopant incorporation via a gaseous source., and is shown to be a very powerful technique of modifying the conduction properties. It will be shown that in the diamond-like films the conduction properties are usually controlled via Poole-Frenkel type defect conduction, while for the polymer-like films it is more a space charge based bulk and possibly, barrier controlled process. But, due to the large band gaps of these films it is difficult to distinguish between the bulk effects and the barrier effects.

Effects of plasma power on the properties of low-k polymerlike organic thin films deposited by plasma-enhanced chemical vapor deposition using the toluene precursor

Effects of plasma power on the properties of polymerlike organic thin films deposited by plasma-enhanced chemical vapor deposition using the toluene precursor were studied. As the plasma power was increased from 5 to 60 W, the relative dielectric constant increased from 2.53 to 2.85. The film deposited at higher plasma power showed higher thermal stability. The film deposited at 60 W was stable up to 400 °C. All the films were insulating under applied field ≤1 MV/cm.

Deposition and properties of a-C:H films on polymethyl methacrylate by electron cyclotron resonance microwave plasma chemical vapor deposition method

a-C:H films were deposited on polymethyl methacrylate (PMMA) using electron cyclotron resonance (ECR) microwave plasma (MP) decomposition of CH 4 diluted in Ar gas. The effect of substrate on the deposition process, optical properties and structure of the a-C:H films was studied. A transparent polymer-like carbon film formed on PMMA, while a semi-transparent diamond-like carbon (DLC) formed on silicon under the same growth conditions. The a-C:H films grown under certain conditions were found to substantially improve the wear resistance of PMMA substrates.

Influence of the plasma pressure on the microstructure and on the optical and mechanical properties of amorphous carbon films deposited by direct current magnetron sputtering

The influence of the plasma pressure on the microstructure and on the mechanical and optical properties of amorphous carbon films deposited by dc-magnetron sputtering was investigated. A modified version of the computer code TRIM 98.01 was employed to simulate the thermalization of the sputtered carbon atoms. In this way, it provided the efficiency of transport and the amount of energy deposited at the growing surface per sputtered carbon atom as a function of the plasma pressure and according to Sigmund’s energy distribution of sputtered particles. Several techniques were employed to characterize the films properties: ion beam analysis provided the composition and areal density; profilometry provided the thickness and film/substrate bending, with which the internal stress could be calculated through Stoney’s equation, and nanoindentation provided the film hardness. In addition, the disorder of the amorphous network was characterized by Raman spectroscopy, while infrared spectroscopy provided information about chemical bonding. Transmission and reflection spectroscopies were carried out in the 400–2500 nm range, and the optical gap was obtained through the Tauc plot. The index of refraction was obtained at 1.3 μm according to the Abelès method. All these quantities were related to the growing conditions, providing a clear picture of the growth of polymer-like or diamond-like films. Finally, the present models for a-C films are reviewed in light of experimental results.

Deposition of hydrogenated amorphous CNx film in CH4/N2 RF discharge

Abstract Hydrogenated amorphous carbon nitride (a-CN x :H) films were prepared in a CH 4 /N 2 radio-frequency (RF) plasma with a wide pressure range of 0.02–1.0 torr. The deposition rates, infrared (IR) absorption spectra and photoluminescence (PL) spectra of the a-CN x :H films deposited were measured. The deposition rate depends strongly on both the source gas pressure and the CH 4 /N 2 composition ratio. When the N 2 content in the plasma was increased, no film deposition occurred at relatively low pressures. In the IR spectra of polymer-like soft films deposited at 1.0 torr, four absorption bands associated with NH (3200–3500 cm −1 ), CH (2800–3000 cm −1 ), CN and/or NN (2100–2250 cm −1 ), and NH and/or CN (1500–1800 cm −1 ) bonds were predominantly observed. For diamond-like hard films obtained at 0.1 torr the absorption intensity of the 2100–2250 cm −1 band was extremely low, but for those at 0.02 torr the absorption due to the CN bond became predominant. Intense PL with a broad band of 1.6–2.5 eV was observed for the polymer-like a-CN x :H films, and the PL intensity increased with increasing N 2 content in the plasma.

Optical Properties of Polymer-like Organic Thin Films Deposited by Plasma-Enhanced Chemical Vapor Deposition Using Toluene as the Precursor

Polymer-like organic thin films (POTFs) were deposited by plasma-enhanced chemical vapor deposition (PECVD) using toluene as the precursor. Refractive indices and extinction coefficients of the POTFs could be varied by changing the plasma power and the deposition temperature, which are easily and continuously controllable deposition parameters.

Polymer-like Organic Thin Films Deposited by Plasma Enhanced Chemical Vapor Deposition Using the Para-xylene Precursor as Low Dielectric Constant Interlayer Dielectrics for Multilevel Metallization

Polymer-like organic thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) using the para-xylene precursor. The effect of the plasma power on the dielectric constant and thermal stability was significant. As the plasma power was increased from 5 W to 60 W, the relative dielectric constant increased from 2.70 to 3.21. The film deposited at a higher plasma power showed higher thermal stability. The film deposited at 60 W was stable up to 450°C. All the deposited films were insulating under an applied field of ≤1 MV/cm.

Quantitative study of C—H bonding in polymerlike amorphous carbon films usingin situinfrared ellipsometry

Polymerlike hydrogenated amorphous carbon $(a\ensuremath{-}\mathrm{C}:\mathrm{H})$ films have been deposited by plasma CVD at low temperature and low pressure. Vibrational properties have been investigated by in situ infrared ellipsometry as a function of film thickness. Hydrogen distribution within the films has been changed by varying the ion energy impinging on the film surface. Vibrational properties of $\mathrm{C}\mathrm{---}\mathrm{H}$ stretching and bending modes have been analyzed as function of self-bias ${(V}_{\mathrm{bias}})$ in terms of frequency, bandwidth, and intensity. Absorption strengths are associated with effective charges that have been calculated for the different ${\mathrm{sp}}^{3}$ ${\mathrm{CH}}_{n}$ groups. In order to make a comparison with values of organic chemistry, a general review of infrared spectra including alkanes, alkenes, and aromatic hydrocarbons is presented. A dipole description taking into account the local environment of CH bonds is developed showing that methyl and methylene group effective charges are similar for polymeric $a\ensuremath{-}\mathrm{C}:\mathrm{H}$ and ${\mathrm{C}}_{x}{\mathrm{H}}_{y}$ organic compounds. Line broadening and frequency shift effects are found to depend on the type of CH groups and are explained through a model including strain and dipole-dipole interactions. The sensitivity of effective charges to the local environment and the determination of ${\mathrm{CH}}_{n}$ group densities are used to propose a description of the hydrogenated network structure.

Growth and composition of dual-plasma polymer-like amorphous carbon films

Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) and to interpret the observed changes in optical parameters in terms of a two-phase microscopic description of polymer-like a-C:H. In order to understand the kinetic results, a phenomenological growth model is proposed including the respective roles of ion bombardment and H atom flux in the activation and deactivation of surface sites available for CxHy radical incorporation (chemisorption) to the growth zone. The activation mechanism corresponds to a chemical modification at the growth zone sites. This adlayer model includes the physisorption of both H atoms and CxHy radicals and explains why the temperature dependence of the deposition rate is found to be opposite for the limiting cases of low and high H atom fluxes. Some consequences of the model on the film stoichiometry (H/C ratio) and microstructure (sp2 C/sp3 C ratio) have also been evaluated.

Photoluminescence of polymer-like amorphous carbon films grown in different plasma reactors

The visible photoluminescence (PL) properties (emission and excitation spectra) of hydrogenated amorphous carbon (a-C:H) have been investigated for polymer-like thin films grown at low substrate temperature and small ion energies. For a large number of a-C:H films grown in three reactors (dual-mode plasma, radio frequency plasma and electron cyclotron resonance plasma) the emission spectra appear as a sum of three peaks with energies (a) 2.28 eV, (b) 2.65 eV and (c) 2.95 eV, being independent of the excitation energy (3.54 eV or 4.13 eV). Their relative intensities depend on the growth parameters while their positions are almost independent. Preliminary picosecond time-resolved PL data indicate a decrease of the decay time for increasing emission energies. A two-phase model of a-C:H is proposed to explain the PL excitation spectra.

Laser-induced chemical vapor deposition of nanostructured silicon carbonitride thin films

Laser-induced chemical vapor deposition of silicon carbonitride thin films has been investigated using a continuous wave CO2 laser in parallel configuration with the substrate. The reactant gases in this process, hexamethyl disilazane and ammonia, are rapidly heated by CO2 laser radiation due to their absorption of the laser energy. Polymerlike silicon carbonitride films or agglomerated nanosized particles are formed depending on process conditions. Dense, smooth films or nanostructured deposits have been synthesized at low substrate temperatures (Ts<300 °C) on quartz, copper, and silicon and can be obtained with controlled microstructures. Surface morphology, composition, and type of chemical bonding have been studied with electron microscopy and spectroscopic analysis and are correlated to the most important laser process parameters. X-ray photoelectron spectroscopy and reflectance Fourier transform infrared spectroscopy show that the deposits consist of Si–N, Si–C, and Si–O bonds, linked together in a x-ray amorphous, polymerlike structure. The nitrogen content is about 40% and can be varied by adding ammonia to the reactant gas flow. The layers are readily contaminated with oxygen after exposure to air, caused by hydrolysis and/or oxidation.

Investigations of the surface chemistry of silicon substrates etched in a rf-biased inductively coupled fluorocarbon plasma using Fourier-transform infrared ellipsometry

In situ Fourier-transform infrared (FTIR) ellipsometry has been performed on silicon substrates processed in a rf-biased transformer coupled plasma reactor. Plasmas in CHF3, CF4, C2F6, and C4F8 have been used. The reaction layer, which is present on the surface of the silicon wafer during the plasma process, has been analyzed in detail, addressing both chemical composition and thickness. The absolute reliability (expressed in terms of thickness) of the results is of the order of 0.01 nm, which corresponds to 3% of a monolayer. The instabilities of a silicon surface, which have been observed under specific conditions, can be of the order of tens of percents of a monolayer, which clearly illustrates the advantage of using a real in situ technique like FTIR ellipsometry over quasi in situ techniques like x-ray photoemission spectroscopy and Auger electron spectroscopy. For CHF3 plasmas it has been found that, if the bias increases to moderate levels (30–50 V), the fluorocarbon film deposition rate decreases and the silicon etching reaction rate increases. The reaction layer changes from a thick, predominantly CFx polymerlike film to a thin, carbon dominated layer of plasma and etching products showing vibrational absorptions of SiFx, C–C, and CF2. Increasing the bias voltage in a CHF3 plasma has a similar effect as increasing the F/C ratio of the feed gas.

Photoluminescence and Raman studies of hydrogenated amorphous carbon films deposited by the rf plasma-enhanced CVD method

Hydrogenated amorphous carbon films (a-C:H) were prepared by rf plasma-enhanced chemical vapour deposition. The film properties were examined by transmission electron microscopy, Raman and photoluminescence (PL) spectrometries. The results show clearly that the observed phenomena depend on the negative self-bias voltage (−Vb). The transitional bias for diamond-like carbon films and polymer-like carbon films is above and below 80 V, respectively. The PL of polymer-like carbon films can be easily observed by the eyes. When the irradiation laser power density is greater than 1 kW/cm2, the PL peak position and intensity change with the exposure time. Initially, the PL intensity increases by twice its original value, and the spectrum begins to generate a new peak at 534 nm with a shoulder at 570 nm. Continuous irradiation gradually reduced the PL intensity to zero. The light-induced PL change of a-C:H films is enhanced by an increase of laser infrared radiation density.

Thermo-oxidative erosion of amorphous hydrogenated carbon films

Amorphous C:D (or C:H) films were deposited on the inner surface of a spherical ultrahigh vacuum chamber by means of a direct current glow discharge in a ∼75% D2 (or H2) and ∼25% CD4 (CH4) gas mixture. Laser Raman spectroscopy analysis of the film exhibited the absence of a diamond peak at 1334.3 cm−1. The general appearance of the spectra was indicative of a polymerlike film. Upon exposure of the C:D film to 18O2, almost all of the released deuterium was seen to be in the D218O chemical form, with less than 1% released as D2. C18O2 was observed to be the main C-containing reaction product of the 18O2 exposure. No methane release was detected. x-ray photoelectron spectroscopy and secondary ion mass spectroscopy analyses of the films indicated the appearance of carbonyl groups (≳C=O) and an increase in the concentration of hydroxyl groups (—COD) due to surface oxidation. The generally accepted scheme for simple thermal oxidation of hydrocarbon polymers was used to provide a plausible reaction mechanism leading to emissions of the reaction products D2O, CO2, and CO.