Douglas L. Medlin

Review of advances in cubic boron nitride film synthesis

Abstract Cubic boron nitride (cBN) has a number of highly desirable mechanical, thermal, electrical, and optical properties. Because of this, there has been an extensive worldwide effort to synthesize thin films of cBN. Film synthesis is difficult in that without significant levels of ion bombardment during growth, only sp2-bonded BN forms, not sp3-bonded cBN. Recently there has been considerable progress in improving the deposition techniques and cBN film quality. In addition, progress has been made in understanding how energetic deposition conditions can lead to cBN formation. However, unanswered questions remain and process improvements are still needed. In this paper we critically and comprehensively review recent developments in cBN film synthesis and characterization. First, the structures and stability of the BN phases and characterization techniques are described. Next, the key experimental parameters controlling cBN film formation and synthesis techniques are discussed. Following a review of microstructure, the proposed mechanisms of cBN formation and the observed mechanical and electrical properties of cBN films are analyzed. We conclude by highlighting the current impediments to the practical realization of cBN-film technology.

Thick stress-free amorphous-tetrahedral carbon films with hardness near that of diamond

We have developed a process for making thick, stress-free, amorphous-tetrahedrally bonded carbon (a-tC) films with hardness and stiffness near that of diamond. Using pulsed-laser deposition, thin a-tC films (0.1–0.2 μm) were deposited at room temperature. The intrinsic stress in these films (6–8 GPa) was relieved by a short (2 min) anneal at 600 °C. Raman and electron energy-loss spectra from single-layer annealed specimens show only subtle changes from as-grown films. Subsequent deposition and annealing steps were used to build up thick layers. Films up to 1.2 μm thick have been grown that are adherent to the substrate and have low residual compressive stress (<0.2 GPa). The values of hardness and modulus determined directly from an Oliver–Pharr analysis of nanoindentation experimental data were 80.2 and 552 GPa, respectively. We used finite-element modeling of the experimental nanoindentation curves to separate the “intrinsic” film response from the measured substrate/film response. We found a hardness ...

Ion‐assisted pulsed laser deposition of cubic boron nitride films

Ion‐assisted pulsed laser deposition has been used to produce films containing ≳85% sp3‐bonded cubic boron nitride (c‐BN). By ablating from a target of hexagonal boron nitride (h‐BN), BN films have been deposited on heated (50–800 °C) Si(100) surfaces. The growing films are irradiated with ions from a broad beam ion source operated with Ar and N2 source gasses. Successful c‐BN synthesis has been confirmed by Fourier transform infrared (FTIR) spectroscopy, high‐resolution transmission electron microscopy (TEM), selected‐area electron diffraction, electron energy‐loss spectroscopy, and x‐ray diffraction. The films are polycrystalline and show grain sizes up to 300 A. In addition, Rutherford backscattering, elastic recoil detection, and Auger electron spectroscopies have been used to further characterize the samples. The effects of varying ion current density, substrate growth temperature, growth time, and ion energy have been investigated. It is found that stoichiometric films with a high c‐BN percentage ca...

Microstructure of cubic boron nitride thin films grown by ion‐assisted pulsed laser deposition

A microstructural study of boron nitride films grown by ion‐assisted pulsed laser deposition is presented. Fourier transform infrared spectroscopy, electron‐energy‐loss spectroscopy, and electron‐diffraction measurements indicate that within the ion‐irradiated region on the substrate, the film consists of a high fraction of the cubic phase (cBN) with a small amount of the turbostratic phase; outside the irradiated region, only the turbostratic phase is detected. Conventional and high‐resolution electron microscopic observations show that the cBN is in the form of twinned crystallites, up to 40 nm in diameter. Particulates, formed by the laser ablation process, reduce the yield of cBN in the irradiated regions by shadowing local areas from the ion beam. The films exhibit a layered structure with an approximately 30‐nm‐thick layer of oriented turbostratic material forming initially at the silicon substrate followed by the cBN. The observations of oriented turbostratic material and twinned cBN crystallites a...

THE SYNTHESIS, CHARACTERIZATION, AND MECHANICAL PROPERTIES OF THICK, ULTRAHARD CUBIC BORON NITRIDE FILMS DEPOSITED BY ION-ASSISTED SPUTTERING

Significant ion irradiation is needed during growth to synthesize cubic boron nitride (cBN) films. This results in large film stresses, which have limited cBN film thicknesses to only a few hundred nm and represents a significant barrier in the development of cBN film technology. Using a new hybrid deposition technique, we have synthesized cubic BN films up to 700 nm (0.7 μm) thick. A compositional and structural analysis of the films using several standard characterization techniques confirms that relatively thick polycrystalline films with a high cBN content were synthesized. Thicker cBN films enable hardness measurements to be undertaken without major substrate effects. Nanoindentation measurements yield hardness values for the cubic BN films up to 60–70 GPa, which are greater than values measured for bulk cBN. The measured elastic modulus was observed to be lower than the bulk, and this can be accounted for by an elastic deformation of the silicon substrate. The mechanical properties of the cubic BN f...

Growth of cubic BN films on β‐SiC by ion‐assisted pulsed laser deposition

Cubic BN(c‐BN) films were deposited on cubic SiC (β‐SiC) films on Si(100) by ion‐assisted pulsed laser deposition. The films were nearly phase pure, with c‐BN fractions of up to ∼90% as determined by infrared spectroscopy. Cross‐sectional transmission electron microscopy showed that much of the film/substrate interface had a thin amorphous layer next to the β‐SiC, followed by hexagonal/turbostratic BN (h‐BN/t‐BN), and then polycrystalline c‐BN, as commonly observed on Si substrates. However, there are also c‐BN crystals that extend to within 10 A of the SiC interface, with no intervening h‐BN/t‐BN layer. A sharp falloff in c‐BN content was observed for substrate temperatures <150 °C, and below 100 °C c‐BN did not form for any ratio of the ion current flux to the deposition flux. At a different ion‐to‐substrate angle (20° closer to glancing incidence) the falloff in c‐BN content for T<150 °C was less sharp. The existence of a critical temperature for c‐BN formation does not result from a nitrogen deficienc...

Analysis of residual stress in cubic boron nitride thin films using micromachined cantilever beams

Abstract We have investigated the residual stress of cubic boron nitride (cBN) thin films using a cantilever-beam deflection method. The residual stresses were determined by measuring the deflection of cantilever beams micromachined from (100)-oriented silicon, and the hardness and elastic modulus of sp 2 -bonded BN (sp 2 -BN) were determined from nanoindentation experiments. Residual stresses were compressive, and varied from approximately 0.15 GPa to 1.2 GPa for sp 2 -BN films and from 0.5 to 5.5 GPa for cBN-containing films; the lower stresses are below a threshold stress level for cBN formation previously suggested by McKenzie et al. A correlation between the relative cBN content and the compressive film stress was also established: larger residual stresses occurred in the films containing a greater percentage of the cubic phase. We find that both sp 2 -BN and predominantly cBN films have approximately the same elastic strain. Thus, assuming that the strain is independent of composition, we develop a quantitative model that well describes the observed correlation of compressive film stress with cBN content. Within the model, the film stress increases with increasing cBN content simply because the elastic modulus of cBN is substantially larger than that of sp 2 -BN. The relevance of this analysis to the mechanism of cBN formation is discussed.

Effects of ambient conditions on the adhesion of cubic boron nitride films on silicon substrates

Abstract The influence of environmental conditions on cubic boron nitride (cBN) film adhesion to silicon substrated was investigated. cBN films were deposited on (100)-oriented silicon substrates by ion-assisted pulsed laser deposition. The irradiating ions were mixtures of nitrogen with (i) argon, (ii) krypton and (iii) xenon. Under room-ambient conditions, the films delaminated in the following time order: N/Xe, N/Kr and N/Ar. cBN films deposited using N/Xe ion-assisted deposition were exposed to four environmental conditions for several weeks: a 1 mTorr vacuum, high humidity, dry oxygen and dry nitrogen. Films exposed to the humid environment delaminated whereas those stored under vacuum or in dry gases did not. Films stored in dry nitrogen were removed after nearly 2 weeks and placed in the high-humidity chamber; these films subsequently delaminated with 14 h. It is concluded that the presence of a humid environment facilitates the delamination of BN thin films from silicon substrates. A model of the interaction between water and the interfacial material is also presented.

Orientation relationships in heteroepitaxial aluminum films on sapphire

Abstract We investigate the microstructure and orientation relationships observed in aluminum thin films deposited epitaxially on sapphire (0001). The films consist of grains with three distinct types of orientation relative to the substrate. The primary orientation is such that (0001)Al2O3‖(111)Al and [10 1 0] Al2O3‖ [ 1 10] Al. This configuration, which matches the close-packed planes and directions of the metal film with those of the oxygen ion sublattice in the sapphire substrate, allows for growth of two symmetrically equivalent orientation variants resulting in a film composed of interlocking regions of these two domains. Unexpectedly, two additional orientation types are identified in the films. As in the primary variant, the close-packed aluminum {111} planes remain parallel with the sapphire basal planes. However, these orientations are rotated about the aluminum [111] axis such that [10 1 0] Al2O3 is parallel to directions near either [ 1 2 1 ] Al (30° rotation) or [5 4 1 ] Al (∼11° rotation).

Crystallographic texture in cubic boron nitride thin films

We examine the crystallographic texture exhibited by cubic boron nitride (cBN) in thin films grown by ion‐assisted deposition. Our analysis indicates that the cBN is preferentially oriented such that individual crystallites have at least one [111] direction lying in the plane of the film but are otherwise randomly oriented about (1) the substrate normal and (2) the in‐plane cBN [111] axis. This preferential orientation is consistent with an alignment between the cBN {111} planes and the basal planes of the layer of highly oriented graphitic boron nitride that forms in the initial stages of film growth.