Low-energy Ion-assisted Deposition Research Articles

Low-Energy Ion-Assisted Deposition of Boron Nitride Films in Surface-Wave Plasma

A surface-wave plasma operated at 2.45-GHz microwave (MW) is used to deposit hexagonal boron nitride (hBN) films by applying a negative or positive bias voltage to a substrate. Ion energy and flux onto the substrate are examined in terms of MW power, pressure, substrate location, and substrate bias. The mean ion energy is estimated from the sheath potential measured with a Langmuir probe and controlled between a few electronvolts and ~170 eV. hBN films are deposited on silicon in a gas mixture of He, N₂, H₂, and BFN₃ by varying either the ion energy or substrate temperature under a high ion flux condition (1017 cm−2s−1). The results show that the crystallinity of sp2-bonding network in the films depends upon ion energy more than substrate temperature.

Growth and Field Emission Properties of Boron Nitride Island Films by Low-energy Ion-assisted Deposition

ABSTRACTWe report the growth and field emission properties of boron nitride (BN) island films by chemical vapor deposition in inductively coupled plasma. Fine-grained island films with large surface roughness can be grown for initial sp2-bonded BN and subsequent cubic BN (cBN) phases by using low-energy (~20 eV) ion bombardment. Ultraviolet photoelectron spectroscopy indicates that the electron affinity is as low as 0.3 eV for both sp2-bonded BN and cBN phases. The evolution of cBN islands reduces the turn-on field down to around 9 V/μm and increases the current density up to 10-4 A/cm2. The surface potential barrier height is estimated to be about 3.4 eV for emission from the Fermi level.

Reflectivity and diffraction study of Co/Cu multilayer using synchrotron X-rays

Synchrotron powder diffraction measurements were obtained from a Co/Cu multilayer produced by low energy ion-assisted deposition. The experiment was carried out on the high resolution diffractometer on Station 2.3 at the SRS, Daresbury Laboratory. Exploiting the parallel beam optics, high photon flux and wavelength tunability of the instrument, the anomalous reflectivity method was used to examine the multilayer structure. With its inherent instrumental advantages, shifts in powder peak position have been measured using the sin 2 ψ diffraction technique, and the average in-plane stress present in the sample has been determined.

Microstructural modification in Co/Cu multilayers prepared by low energy ion-assisted deposition

The microstructure of Co/Cu multilayers deposited by low energy ion-assisted deposition was investigated. The samples studied were grown by unbalanced magnetron sputtering with and without an applied d.c. substrate bias of −50 V. Specular and off-specular X-ray reflectivity measurements were performed on the samples and revealed the presence of roughness at the interfaces that was partially correlated throughout the film. The effect of applying a −50 V bias was to suppress the correlation of the lower frequency roughness and to slightly reduce the bilayer period of the multilayer. The differences between the samples are discussed in terms of possible ion bombardment induced smoothing of the layers and densification of the microstructure.

Fundamental and applied aspects of the plasma-assisted nitriding process for aluminium and its alloys

The insufficient wear resistance of aluminium and its alloys is a well-known problem. Different surface treatment methods have been developed to increase the resistance. Compared to the electrochemical processes, the plasma-assisted nitriding of aluminium is a new method without ecological problems. In this paper different possibilities of plasma-activated nitriding are introduced. Due to the low electrical conductivity of the aluminium nitride coating formed, an rf plasma process has some advantages compared to a dc discharge. Furthermore, comparisons are made with high-energy ion implantation and a low-energy ion-assisted deposition. The morphology and composition of the resulting aluminium nitride layers are shown. A sputtering process prior to nitriding is of great importance. The influence of the gas phase composition and the plasma parameters were examined by plasma mass spectrometry. The ion energy distribution of the detected species indicates different possible charge transfer processes which are influenced by the gas phase composition.

The modification of alloys by low energy ion-assisted deposition

Thin film alloys of Al/Pt were produced by sputtering a binary target, comprising a piece of Al mounted adjacent to a piece of Pt, using a broad-beam Kaufman ion source. By suitable geometrical positioning of the target with respect to the substrate, different compositions of Al/Pt thin films were produced. The effect of irradiation by an additional 200 eV Ar ion beam during film deposition on the composition of the resulting alloy has been investigated. It was found that, for given deposition conditions, the films became richer in Pt as the ion-assisting current density increased. A simple model shows that this effect can be explained by preferential sputtering of Al from the alloy, although other irradiation-induced effects could contribute further to this Pt enrichment. TEM measurements indicate that the grain size of these films also increases with ion-assisting current density and AFM results show that the surface of the alloys becomes much smoother.

Material properties of ZrN film on silicon prepared by low-energy ion-assisted deposition

Abstract We investigated the crystalline quality and the resistivity of a ZrN film on an Si(100) substrate, prepared by an ion-assisted deposition method. With this method, metallic Zr is deposited by electron beam evaporation, while simultaneously bombarding the film with nitrogen ions of low ion energy E i (100–700 eV). The crystalline quality was improved and the resistivity was decreased by lowering the ion energy to 150 eV. However, the quality of the film prepared at E i = 100 eV became poorer than those at E i = 150 and 200 eV. The optimum arrival rate ratio N/Zr for the film quality depends slightly on the ion energy, and is about 1.2–1.5 for E i = 150 and 200 eV. Increasing the substrate temperature to 820 °C improves the crystalline quality of the ZrN film and decreases the resistivity. However, at 850 °C, a solid phase reaction between Si and Zr occured, and Zr silicide was sometimes formed. With the optimum deposition conditions, we can obtain a (100)-oriented ZrN film on Si with a resistivity as low as 11.4 μΩ cm.

A comparison between high- and low-energy ion mixing at different temperatures

High-energy ion mixing occurs when an ion beam of a few hundred keV bombards an interface under the surface. Low-energy ion mixing arises when an ion beam of a few keV bombards an interface near the surface during, e.g., sputter depth profiling and low-energy ion-assisted deposition. At low temperatures, the rate of both high- and low-energy ion mixing can be influenced by thermodynamic parameters, such as the heat of mixing and the cohesive energy of solids. These effects are demonstrated by ion mixing experiments using metallic bilayers consisting of high-atomic-number elements. A model of diffusion in thermal spikes is used to explain this similarity. Low-energy ion mixing can also be strongly affected by surface diffusion and the morphological stability of thin films. These effects are illustrated using results obtained from sputter depth profiling of Ag/Ni, Ag/Fe, and Ag/Ti bilayers at elevated temperatures. High-energy ion mixing at low temperatures can be influenced by the anisotropic momentum distribution in a collision cascade as seen from a set of marker experiments to determine the dominant moving species in high-energy ion mixing. An understanding of these similarities and differences between high- and low-energy ion mixing at different temperatures will provide useful guidelines for applications of ion mixing.

Optical and microstructural properties of hafnium dioxide thin films

We have applied a variety of analytical tools to educe the composition and morphology of thin films of hafnium dioxide deposited under a variety of conditions. Surface analytical techniques include scanning electron microscopy, transmission electron microscopy, electron diffraction, Rutherford backscattering spectrometry, and X-ray photoelectron spectroscopy. The results from these measurements are correlated with the observed optical properties. Electron diffraction reveals that films deposited below 300 °C are amorphous and spectrophotometry uncovers a small optical inhomogeneity. The refractive index and inhomogeneity are strongly influenced by the oxygen backfill present during film condensation. Ion-assisted deposition (IAD) removes the optical inhomogeneity, increases the refractive index, and eliminates the air-to-vacuum spectral shift observed in films deposited without IAD. In addition, low energy IAD is found to increase the refractive indices of the films without affecting the optical inhomogeneity. This is explained by the preferential sputtering of hydroxide from the growing film surface by the low energy ions.

Ion-Surface Interactions During Epitaxy

AbstractRecent work on low-energy ion-assisted deposition of epitaxial films is reviewed. Much of the interest in this area has centered on the use of very low ion energies (∼ 25 eV) and high fluxes (> 1 ion per deposited atom) obtained using novel ion-assisted deposition techniques. These methods have been applied in ultra-high vacuum, allowing the preparation of high-purity semiconductor materials. The following ion-surface interaction effects during epitaxy are discussed: improvements in crystalline perfection during low temperature epitaxy, ion damage effects, improved homogeneity and properties in III-V alloys grown within miscibility gaps, and changes in nucleation mechanism from Stranski-Krastanov to layer-by-layer.

High T c superconducting B1 phase MoN films prepared by low-energy ion-assisted deposition

Metastable B1 phase MoN was synthesized by low-energy ion-assisted thin-film growth techniques at deposition temperatures TD =400, 500, and 700 °C. The films were condensed under the action of mixed Ar+ and N2+ ion species at an ion to condensing atom arrival rate ratio of 1.0. For each deposition temperature, films were prepared at various ion energies in the range 2–200 eV. The presence of chemically reactive N2+ ions and the use of low ion energies allow both structural and chemical selectivity, facilitate increased incorporation of nitrogen, and promote ordering of the defect structure of cubic MoN. The films were characterized by x-ray diffraction, by measurements of superconducting temperature Tc and by temperature-dependent resistivity ρ(T). Collectively the film properties strongly depend on ion energy and deposition temperature. The composition, lattice parameter, crystalline preferred orientation, and grain size can be controlled by varying ion energy. The resistivity shows an anomalous temperature dependence that is typical of disordered metals, i.e., electron localization governs electrical conduction with the temperature coefficient of resistivity changing sign according to the Mooij correlation. At optimum deposition conditions the resistivity is metallike (phonon contribution ρthermal >0), Tc=6.2 K, the resistivity ratio RR=1.1, and ρ300 K =120 μΩ cm. Annealing of films at 700 °C causes an improvement in properties with Tc≂12 K, RR=1.7, and ρthermal =60–80 μΩ cm. In films with large thermal resistivities we observe a ρ∼T2 behavior at low temperatures and saturation of resistivity at high temperatures (ρsat =200–280 μΩ cm) which suggest that MoN with the B1 crystal structure is a strongly coupled and disordered high Tc superconductor.