Ion-assisted Methods Research Articles

Deposition of boron–carbon multilayer coatings by RF plasma sputtering

Boron–carbon coatings are typically accomplished by ion-assisted methods that generate energetic species of tens to hundreds of electron volt to facilitate the nucleation and growth processes. These energetic particles simultaneously induce a significant intrinsic stress leading to the film delamination, limiting the achievable thickness of the films to approximately a few hundred nanometers and creating difficulties for their technical applications. In this paper we propose a plasma method using an RF (13.56MHz) capacitive system as a deposition technique to realize B–C coatings with a low residual intrinsic stress. A stable coating has been obtained by deposition of a multilayered structure in conditions of very low deposition rate. SEM micrographs show that the layers making up the coating are dense, without pores or cracks, and with a good adhesion to the substrates.

Tailoring carbon nanotips in the plasma-assisted chemical vapor deposition: Effect of the process parameters

Carbon nanotips have been synthesized from a thin carbon film deposited on silicon by bias-enhanced hot filament chemical vapor deposition under different process parameters. The results of scanning electron microscopy indicate that high-quality carbon nanotips can only be obtained under conditions when the ion flux is effectively drawn from the plasma sustained in a CH4+NH3+H2 gas mixture. It is shown that the morphology of the carbon nanotips can be controlled by varying the process parameters such as the applied bias, gas pressure, and the NH3/H2 mass flow ratios. The nanotip formation process is examined through a model that accounts for surface diffusion, in addition to sputtering and deposition processes included in the existing models. This model makes it possible to explain the major difference in the morphologies of the carbon nanotips formed without and with the aid of the plasma as well as to interpret the changes of their aspect ratio caused by the variation in the ion/gas fluxes. Viable ways to optimize the plasma-based process parameters to synthesize high-quality carbon nanotips are suggested. The results are relevant to the development of advanced plasma-/ion-assisted methods of nanoscale synthesis and processing.

Nanodiamond growth on diamond by energetic plasma bombardment

The present work studies the growth of nanodiamond under prolonged DC glow discharge plasma bombardment of 1 μm thick polycrystalline CVD diamond. Using Raman spectroscopy, near edge X-ray absorption fine structure (NEXAFS) and secondary ion mass spectroscopy (SIMS) it is shown that nanodiamond formation on diamond starts directly, skipping the nucleation stage required to form the precursor material with the appropriate density and hydrogen content. On the other hand, the amorphous carbon/nanodiamond composite structure is substantial to the nanodiamond nucleation under energetic plasma bombardment and cannot be eliminated by starting with a micro-crystalline diamond substrate. The results give additional insight to the phenomena of nanocrystalline nucleation and growth under energetic particle bombardment relevant to a variety of systems including bias enhanced nucleation of diamond and cBN deposition by ion assisted methods.

Growth dynamics of ultrasmooth hydrogenated amorphous carbon films

We have studied the growth dynamics of ultrasmooth hydrogenated amorphous carbon films deposited on silicon substrates by electron cyclotron resonance chemical vapor deposition from argon/methane gas mixtures applying a high negative external bias. The surface morphology of films deposited for different growth times under the same experimental conditions was analyzed by atomic force microscopy. Our analysis leads to values of the growth, roughness, and coarsening exponents of 0, 0.1, and 0.5, respectively. As it has been recently proposed that the growth dynamics of amorphous films by ion-assisted methods should obey the Edwards-Wilkinson (EW) growth mode, we have analyzed the compatibility of our data with this model. Our analysis indicates that, although the scaling data could be interpreted in terms of the EW model, the relative large data error bars and the film ultrasmoothness preclude the unambiguous assessment of the EW growth mode for our film growth evolution. In our system, the interplay of shadowing, physical sputtering and enhanced surface mobility ion-induced effects contribute likely to the leveling and final ultrasmoothness of the film surface.

Barrier properties of carbon films deposited on polymer-based devices in aggressive environments

Abstract The physical and chemical properties of diamond-like carbon (DLC) and carbyne-containing films on different substances, in particular on plastic materials, are advantageous for a great number of novel applications. These coatings, which possess the necessary mechanical characteristics of high adhesion to the plastic substrate material, chemical inertness and biocompatibility, have been proved to be hypoallergenic. In present paper, we report on the use of DLC and carbyne-containing films, formed by different ion-assisted methods, for strengthening and protecting of the surface of polymer-based medical products and for providing new functional properties. The medical products examined are: probes of different type, catheters, drainage tubes, polymer contact lenses and contact lens cases. It is shown that the DLC and carbyne-containing films are biocompatible, possess high chemical inertness to biological media of the human body, and provide the necessary rigidity to products. Antibacterial activity, a wide range of contact wetting angle changes, and a wide range of optical and colour characteristics open unlimited possibilities for the improvement of medical wares. Results on the mechanical properties of probes and contact lenses coated by carbon films, both from in vitro and clinical testing, are also presented.

A barometer of plasma and ion-assisted methods of surface modification

Plasma and ion-assisted methods of surface modification have been studied for over three decades in the field of metals, semiconductors, ceramics, polymers, etc. A large number of different methods and approaches exist. A typical factor is the exploitation of the beneficial effects of ion bombardment on material surfaces. The methods have been developed into industrial processes for the manufacture of electronic materials and of protective coatings and surface treatments for metals. New methods, such as plasma-based ion implantation, have recently been developed, indicating the speed at which techniques are currently advancing. A survey has been carried out of plasma and ion-assisted methods of surface modification. A total of 44 people working in industry or research answered a series of questions on the present situation and future prospects in the field. For example, comments were requested on the following aspects relating to the particular methods used. (1) An evaluation of the current state of the art and the future of the method. (2) The major force driving the development process. (3) The major difficulties in commercialization. (4) The major theoretical challenges associated with the method. The answers received and comments given were analysed to form a picture of the current situation in the field of plasma and ionassisted methods of surface modification, and a view of the future for the community. This paper provides a summary of the results.

Critical test of the c-BN sputter model

Cubic BN (c-BN) films are commonly deposited with ion-assisted methods (chemical vapour deposition and physical vapour deposition). It will be shown that all ion-assisted techniques work within the same range of deposition parameters which implies a common growth mechanism. Recently, we proposed a model for ion-induced c-BN deposition in which material removal processes due to sputtering and desorption play the major roles. Measurements of the sputter yields of BN films confirm the basic assumptions of the model with respect to sputtering. Nevertheless, under special conditions, additional influences of compressive stress and film stoichiometry have to be taken into account which are, however, not contradictory to the model.

Preparation of thin hard boron nitride films by r.f. magnetron sputtering

Abstract Films (0.1–1 μm) of boron nitride were grown on silicon (111), KCl and quartz by r.f. magnetron sputtering of a BN target in an Ar or Ar-N2 atmosphere. The deposition was carried out at two pressures: 2 × 10−3 and 2 × 10−2 Torr. The compositions and structures of the BN films were determined by electron probe microanalysis, electron diffraction and IR analysis. Films obtained at an argon pressure of 2 × 10−3 Torr had a stoichiometric composition. They were dense and hard and had low carbon and oxygen impurity levels. Films exhibited a high microhardness similar to that for films deposited by ion-assisted methods. Films obtained at an argon pressure of 2 × 10−2 Torr had a small excess of boron and increased impurity levels. They were soft and porous. All films were either amorphous or microcrystalline with a hexagonal structure. Application of a negative bias voltage (60–100 V) to the substrate stabilized the cubic phase that was confirmed by three methods. The deposition conditions for obtaining stoichiometric hard BN films and the applicability of the above-mentioned analysis methods are discussed.

Process and property relationships in hard coatings made by plasma- and ion-assisted methods

The relationship between the process parameters and the properties of the resultant hard coatings is discussed. Two approaches are considered: plasma-assisted chemical vapor deposition (PACVD) and ion-assisted physical vapor deposition (IAPVD). In PACVD the plasma is used to activate the gaseous precursors and the film is formed during ion-molecule and radical-molecule reactions in the plasma bulk and at the plasma-surface boundary. In IAPVD, partially ionized metal vapor is injected into an argon-supported plasma together with a reactive gas; these precursors react at the substrate surface to form the film under intense ion bombardment and at a reduced pressure. The correlation between experimental variables and properties of the deposits is illustrated with hard nitride films, namely Si 3N 4 made by PACVD and TiN made by IAPVD. Two picostructural examples are illustrated: argon entrapment in TiN made by IAPVD, and lattice strain effects on the Raman spectrum of diamond films made by (microwave) PACVD.

Process and property relationships in hard coatings made by plasma and ion-assisted methods

Abstract

Properties of diamond-like films formed by ion-assisted methods for multilayer structures

The electrical and optical properties of α-C:H films with different thickness prepared by plasma-ion beam deposition formed from cyclohexen (C 6H 12) vapours and by sputtering of graphite target in Ar,C 6H 12 and their mixtures were investigated. The values of the optical band gap and the resistivity of the films were determined and found to range from 0.6 to 2.0 eV and from 1.0 to 10 8 ω cm respectively. The values strongly depended on the method and deposition conditions as well as on the thickness of the film. The resistivity measurements of the films in the temperature range of 77–300 K show that the jump mechanism of conductivity with variable length of jump along the localized states at the Fermi level with decreasing thickness transforms from three-dimensional to two-dimensional one. It is shown that the multilayer structures formed on the base of α-C:H films have two values of optical band gap corresponding to the values of consisting layers. It might be related to effect of optical absorption quantization in the structures.

Plasma and particle flux characterization of the a-C:H deposition process by ion-assisted methods

In the present study mass spectrometric measurements were carried out on the ion flux to the substrate during the a-C:H deposition process by ion plating and r.f. glow discharge. For this purpose a portion of ion flux striking the substrate was passed through an aperture into a differentially pumped chamber, in which the energy distribution of the ions is measured by an electrostatic analyser and the e/m value by a quadrupole mass spectrometer. The discharge conditions were measured with a Langmuir probe. Depending on the discharge conditions and the hydrocarbon gas used, typical ion groups are observed. These ion groups are generated by electron impact or ion-molecule reactions.

Preparation of amorphous i-C films by ion-assisted methods

The deposition of hydrogenated amorphous carbon films by an electron cyclotron resonance (ECR) microwave plasma and an ECR microwave ion beam source is described. Optical, electrical and mechanical measurements and different analytical methods are used for film characterization. The films deposited at a rate of about 200 nm min -1 develop a high electrical resistivity ( 10 9-10 14 Ω cm ), good IR transparency, an optical gap of about 1.7 eV and a hardness up to 30 GPa (measured on silicon substrates). Sputtering deposition experiments were performed with the aim of investigating the influence of the hydrogen on the layer properties as well as the possibilities of deposition on highly insulated substrates. Layers made by means of the sputtering deposition method indicated that hydrogen might be involved in the set-up of a metastable ion-assisted hydrogenated amorphous carbon.

Ion beam enhanced diffusion of B during Si molecular beam epitaxy

Enhanced diffusion of B is observed during the growth of ion bombarded epitaxial layers by Si molecular beam epitaxy. Ion-assisted methods are generally required for high levels of n-type doping, and we find that the damage caused by the low-level ion bombardment is responsible for the enhanced diffusion of B. Furthermore, the concentration profiles of as-grown and post-growth annealed samples show that the diffusion is a transient effect that occurs at the growth temperature of 600–700 °C. Simulation of the diffusion process demonstrates that nearly all of the B is participating in the diffusion and that the built-in electric field at the p-n junction leads to a further smearing of the B profile.