Hydrogen induced microstructural variation in diamond and diamond like carbon thin films

Hydrogen plays a crucial role in the growth of micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films grown by plasma assisted chemical vapour deposition (PACVD) processes. It selectively etches graphite phase and helps in stabilizing the diamond phase. The presence of various hydrocarbon species in the plasma and their reaction with atomic, excited or molecular hydrogen on the substrate surface decide the mechanism of diamond nucleation and growth. Several mechanisms have been proposed but the process is still not well understood. Control of hydrogen and other deposition parameters in the PACVD process leads to deposition of yet another class of materials called diamond like carbon. By varying the concentration of hydrogen it is possible to produce purely amorphous carbon films on the one hand and amorphous hydrogenated carbon films (with as high as 60% hydrogen) on the other. Very hard, optically transparent and electrically insulating films characterize the diamond like behaviour. The proportion of hydrogen and its bonding with carbon or hydrogen in the film can be varied to obtain very hard to very soft films which could be optically transparent or opaque. The microstructure of these films have been investigated by a large number of techniques. The results show interesting situations. This paper reviews the work on the role of hydrogen on the growth, structure and properties of MCD and DLC thin films.     

Stability of plasma-deposited amorphous hydrogenated boron films

An investigation of radio-frequency plasma-deposited amorphous hydrogenated boron films was conducted in order to determine the influence of the substrate temperature T_S and d.c. self-bias U_SB on the physical properties and film stability. We found three different regions of stability depending on T_S and U_SB. Films prepared at high d.c. self-bias and substrate temperature are mechanically unstable due to internal stress. They peel off during the deposition process or on first contact with the ambient atmosphere. Films deposited at low self-bias and substrate temperature were found to be chemically unstable. Exposed to the ambient atmosphere, they undergo chemical changes and incorporate large amounts of oxygen and carbon. These two different regions of instability are separated by chemically and mechanically stable films. The chemical composition and the structural properties of chemically stable and unstable films were determined by Fourier transform infrared spectroscopy, X-ray induced photoelectron spectroscopy and ion beam analysis. These measurements show that chemical stability correlates with the boron density. Chemically stable films reveal densities of more than 68% up to 99% of the density of crystalline boron. In general, elevated substrate temperature and ion energy cause densification of the films and increasing internal stress. Densification leads to chemical stability, while internal stress is the reason for mechanical instability.     

Synthesis and properties of plasma-deposited carbon condensates

Structural data, thermal characteristics, and theoretically calculated binding energies are reported for a graphite condensate obtained by carbon deposition from plasma. It is demonstrated that this condensate can be effectively used in self-propagating high-temperature synthesis processes.     

Field emission from diamond and diamond-like carbon films

Field emission from diamond and diamond-like carbon thin films deposited on silicon substrates has been studied. The diamond films were synthesized using hot filament chemical vapor deposition technique. The diamond-like carbon films were deposited using the radio frequency chemical vapor deposition method. Field emission studies were carried out using a sphere-to-plane electrode configuration. The results of field emission were analyzed using the Fowler-Nordheim model. It was found that the diamond nucleation density affected the field emission properties. The films were characterized using standard scanning electron microscopy, Raman spectroscopy, and electron spin resonance techniques. Raman spectra of both diamond and diamond-like films exhibit spectral features characteristic of these structures. Raman spectrum for diamond films exhibit a well-defined peak at 1333cm^?1. Asymmetric broad peak formed in diamond-like carbon films consists of D-band and G-band around 1550 cm^?1 showing the existence of both diamond (sp³ phase) and graphite (sp² phase) in diamond-like carbon films.     

Photoelectrochemical properties of plasma-deposited TiO2 thin films

Photoanodes were fabricated from TiO₂ films deposited onto titanium substrates by plasma-enhanced chemical vapor deposition. The photocurrent-wavelength and photocurrent-voltage properties of the anodes were determined and compared with those of thermally grown TiO₂ photoanodes. The plasma-deposited photoanodes displayed quantum efficiencies higher than those for the thermally grown films and comparable with those reported for single-crystal rutile. The microstructure of the plasma-deposited films appeared to be primarily responsible for the high quantum efficiencies.     

Wear resistance of nano-polycrystalline diamond with various hexagonal diamond contents

Wear resistance of nano-polycrystalline diamond (NPD) rods containing various amounts of hexagonal diamond has been tested with a new method for practical evaluation of the wear-resistance rate of superhard ceramics, in addition to the measurements of their Knoop hardness. The wear resistance of NPD has been found to increase with increasing synthesis temperature and accordingly decreasing proportion of hexagonal diamond. A slight increase in Knoop hardness with the synthesis temperature also has been observed for these samples, consistent with the results of the wear-resistance measurements. These results suggest that the presence of hexagonal diamond would not yield any observable increase in both hardness and wear resistance of NPD, contradictory to a recent prediction suggesting that hexagonal diamond is harder than cubic diamond. It is also demonstrated that NPD is superior to single crystal diamond in terms of relatively homogeneous wearing without any significant chipping/cracking.     

Structural and optical properties of plasma-deposited boron nitride films

The plasma-enhanced chemical vapor deposition of boron nitride films in a low pressure, parallel plate reactor incorporating an electromagnet was investigated. Films were deposited from gas mixtures of diborane, hydrogen and ammonia. The ratio of boron to nitrogen was approximately 1.7 when an ammonia-to-diborane ratio of 4 was used. The films had the following optical properties: a band gap in the range 5.6–5.8 eV, an absorption coefficient (at 6.0 eV) of about 1×10⁵ cm⁻¹ and an index of refraction of 1.7. In general the optical properties were identical, with or without the application of a low intensity magnetic field.     

Synthesis of carbon films containing diamond particles by electrolysis of methanol

Carbon films containing diamond particles were deposited onto a Si (100) substrate by electrolysis of methanol under a direct current potential of 1200 V, with a current density of about 52 mA/cm², at atmospheric pressure and in the temperature range of 50-55 °C. The surface morphology, microstructure and crystalline structure of the deposited films were characterized by scanning electron microscopy (SEM), Fourier transformation infrared (FTIR) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM) respectively. The SEM images show that the films are formed by particle clusters and a surrounding glassy phase. The Raman spectra of the films indicate that the particle clusters are composed of diamond and that the glassy phase is composed of amorphous carbon. The FTIR measurements suggest the existence of hydrogen which is mainly bonded to the sp³ carbon in the films. The transmission electron diffraction patterns further indicate that the particles in the films consist of single-crystalline diamond. Both TEM and Raman measurements have confirmed unambiguously the formation of diamond crystals in the deposit, although the particles are not uniformly distributed on the entire surface.     

Surface and bioproperties of nanocrystalline diamond/amorphous carbon nanocomposite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma chemical vapour deposition from CH₄/N₂ mixtures. In order to investigate their suitability as templates for the immobilization of biomolecules, e.g. for applications in biosensors, four differently prepared surfaces, namely as-grown, hydrogen plasma treated, oxygen plasma treated, and chemically treated with aqua regia, have been thoroughly characterized by methods such as XPS, TOF-SIMS, AFM, and contact angle measurements. In addition, in order to investigate the affinity of these surface to non-specific bonding of biomolecules, they have been exposed to bovine serum albumin (BSA). It turned out that already the as-grown surface is hydrogen terminated; the degree of the termination is even slightly improved by the hydrogen plasma treatment. Reaction with aqua regia, on the other hand, led to a partial destruction of the H-termination. The oxygen plasma treatment, finally, causes a termination by O and OH, rather than by carboxylic acid groups. In addition, an increase of sp² bonded carbon is observed. All surfaces were found to be susceptible to attachment of BSA proteins, but the coverage of the hydrogen terminated was lower than that of the O-terminated film. The highest BSA concentrations were found for the aqua regia sample where the H-termination has been removed partially. Finally, our results show that even minor surface contaminations have a great influence on the BSA coverage.     

Structure of diamond and diamond like carbon thin films grown by hot-filament chemical vapour deposition technique

Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH₄ and H₂ gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2μm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp ² andsp ³ form was obtained by a specially developed modelling technique. The typical values ofsp ³/sp ² ratio in our films are between 1·88–8·02. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Electrical properties of diamond films prepared from carbon disulfide and ethanol in hydrogen

Sulfur doping of diamond samples produced by the hot-filament chemical vapor deposition (CVD) process using carbon disulfide highly diluted in ethanol and hydrogen has been investigated. The polycrystalline morphology observed by Scanning Electron Microscopy (SEM) and the characteristic diamond Raman peak were practically not affected by addition of CS₂ in the range of 0.8-2.0% by volume. The electrical resistivity dependence due to CS₂ addition showed a “down-and-up” curve with minimum resistivity of about 3.6 × 10⁻⁴ Ωcm. Hall mobility as high as 325.9 cm²V⁻¹s⁻¹ have been achieved. PIXE and XRF measurements showed that sulfur (100-400 ppm) was incorporated during the deposition independent of the CS₂ added. Unintentional contaminations with Si, W, Cu and Cr have been detected. All the samples, measured by hot-point method and Hall effect, showed p-type characteristics.     

Effect of silver doping on optical property of diamond like carbon films

Optical properties of silver doped diamond like carbon films (Ag:DLC) deposited by the RF reactive sputtering technique were studied in detail. The chemical binding energy and the composition of the films were investigated by using an X-ray photoelectron spectroscopy. Optical transparency and optical band gap decreased with the silver incorporation to the DLC film. Optical band gap calculated from transmittance spectra decreased from 2.55 to 1.95 eV with a variation of Ag concentration from 0 to 12.5 at.%. Urbach parameter determined from the band tail of the transmittance spectra showed to increase with the doping concentration.     

CO2 laser annealing of plasma-deposited high-T c superconducting thick films

Recent advances in the fabrication of high-T _c superconducting thick films demand processing techniques which can eliminate film/substrate interdifiusion that occurs during subsequent post-annealing heat treatment after the film is deposited, thereby limiting the application of the thick films for devices. The present study evaluates laser annealing techniques for plasma-deposited Y-Ba-Cu-O thick films using a high-energy CO₂ laser (10.6Μm) in a continuous wave mode. The results are compared with those obtained by conventional furnace annealing techniques necessary for post-heat treatment of as-deposited superconducting thick films. The high-T _c superconducting phase is recovered by cationic diffusion during subsequent post-annealing heat treatment. Crystallographic phases and microstructural characterization have been performed using XRD, SEM, and EPMA analytical techniques. The significance of the technology lies in the elimination of film/substrate interdiffusion problems, thereby resulting in high-quality superconducting thick films. The technology will significantly reduce the post-annealing times usually required by conventional furnace annealing techniques.     

Tribological behaviour of diamond and diamondlike carbon films: status and prospects

Abstract

Diamond and diamondlike carbon films possess several unique characteristics that govern their tribological properties, in particular their high hardness and lubricity nature. In this paper, the influence of the deposition methods on the adhesion of the films and on their wear resistance and frictional properties is summarised. It is observed that the deposition technique contributes greatly to the variation in microstructure of the diamond or diamondlike carbon film. The multitude of tribotesters (pin-on-disc, reciprocating ball-on-plate, ball-on-three flat, etc.) used to evaluate these films also complicate the interpretation of the tribological performance and confuse direct comparisons of the deposition techniques. Also, the effect of non-diamond carbons and hydrogen in the film is elucidated in relation to the friction coefficient. The application of these films to rolling and sliding components in various industries is highlighted, together with their limitations. The hard brittle nature of the film makes it an excellent candidate in abrasive and erosive applications, but less desirable in rolling contact. Choice of substrate material is critical in order to support the film under high contact stresses.

MST/1692     

Properties of plasma-deposited films using ethylene and fluoroethylenes as starting monomers

Some properties of plasma-deposited films using each of the members of the ethylene and fluoroethylene series of the general formula C₂H_NF_4?N (N = 0–4) as starting monomers, under otherwise identical deposition conditions, were determin as a function of the fluorine/carbon ratio of the starting monomer. These included film stoichiometry, deposition rate, film stress, hydrophobicity, dielectric constant (? ≈ 2.6?3.7), dielectric strength (> 200 V/μm), and thermal stability.     

Characterization of hypervelocity impact craters on chemical vapour-deposited diamond and diamond-like carbon films

Microwave plasma chemical vapour-deposited (CVD) process has been used to grow polycrystalline diamond films over silicon substrates. Diamond-like carbon (DLC) thin films were grown over silicon substrates using a microwave plasma disc reactor. Reactant gases of CH4 and H2 were used in both CVD processes. Some preliminary feasibility tests were performed on the possible applicability of diamond and diamond-like carbon thin films for space-protective applications against artificially simulated electrically actuated plasma drag hypervelocity impact of olivine particles. As-deposited films were analysed by Raman for their chemical nature. The morphology and dimensions of hypervelocity impact craters in diamond and DLC films was also studied by scanning electron microscopy (SEM) and optical microscopy. The velocity of debris particles was determined by high-speed photography using a streak camera. The size of the impact particles was determined by measuring the size of the holes formed in the mylar sheet mounted just above the target diamond and DLC film/silicon and coordinates of the impact sites were determined using the same apparatus. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electron emission characterization of diamond thin films grown from a solid carbon source

Diamond films of various morphologies and compositions have been deposited on silicon substrates by a plasma-enhanced chemical transport (PECT) process from a solid carbon source. Electron emission efficiency of these diamond films is related to their morphology and composition. The electric field required to excite emission in a boron-doped polycrystalline diamond film ranged between 20 to 50 MV m⁻¹. In an undoped conducting nanocrystalline diamond composite film, the field was as low as 5–11 MV m⁻¹. The cold field electron emission of these films is confirmed from the Fowler-Nordhelm plots of the data. Enhancement of electron emission by band-bending and by the nanocrystalline microstructure are discussed. New diamond emitters made of nanocrystalline boron-doped diamond composite are proposed.     

Ion beam synthesis of the diamond like carbon films for nanoimprint lithography applications

Ion beam deposited hydrogenated undoped as well as SiO_x (SiO_x + N₂, SiO_x + Ar) doped DLC thin films were deposited and evaluated as possible anti-adhesive layers for nanoimprint lithography. Film surface contact angle with water was investigated as a measure of the surface free energy and anti-sticking properties. Contact angle of the DLC films was independent of SiO_x doping and ion beam energy. Air-annealing resistance in terms of the contact angle with water of the synthesized diamond like carbon films was investigated. Optical transmittance spectra of the DLC films in UV-VIS range were measured to investigate it as possible anti-sticking layers for UV imprint lithography applications. DLC films with the most promising combination of the UV absorption and anti-sticking properties were revealed. Preliminary imprint tests with uncoated and thin DLC film coated hot imprint stamps were performed.