S. Reinke

Recent results in cubic boron nitride deposition in light of the sputter model

Abstract In the field of c-BN deposition, ion-assisted processes are most common and best investigated. Understanding of the deposition mechanisms is the basic key to improve film quality and, therefore, to realize potential applications. Recently, Reinke et al. developed a sputter model for ion-assisted c-BN deposition in which material removal processes (sputtering and desorption) play major roles [1]. This model is used as a guideline in order to give an overview on c-BN deposition. The first part investigates the growth mechanisms of ion-assisted c-BN deposition. Therefore, a data collection which includes CVD and PVD deposition methods was performed. The resulting dependencies are discussed in light of the sputter model. By comparison with other models, it is found that most of the experimental facts can best be described in terms of the sputter model, whereas in the nucleation step compressive stress plays an additional role. In the second part, c-BN deposition is considered in a wider context. Current problems and some first approaches to overcome these problems are discussed. Furthermore boron nitride is compared with the sp 3 carbon modifications tetrahedral amorphous carbon (ta-C) and diamond. It is concluded that c-BN and ta-C rely on different growth mechanisms; the investigation of extended parameter ranges in the search for new ion-assisted carbon and BN modifications is proposed. Some approaches in c-BN deposition without ion assistance are discussed in accordance to diamond deposition. Nevertheless, the advances are small and no process yielding similar high-quality material as CVD diamond exists.

Nucleation of cubic boron nitride (c-BN) with ion-induced plasma-enhanced CVD

Abstract Nucleation and growth of c-BN films by means of the inductively coupled plasma technique was investigated. Two different sets of experiments were performed: deposition as a function of the bias voltage V B and experiments with varying deposition times. The films were characterized by Fourier transform IR spectroscopy, Auger electron spectroscopy and ellipsometry. Without substrate bias, crystalline h-BN was obtained, oriented with the c -axis normal to the surface. With sufficient bias voltage, a layer of vertically oriented h-BN was observed prior to the formation of c-BN, in agreement with transmission electron microscopy measurements published recently in the literature. We observed further a significant increase in the boron to nitrogen ratio prior to the formation of c-BN. Preferential sputtering of nitrogen with respect to boron with increasing ion bombardment seems to be the reason for this behaviour.

Mechanisms in ion induced c-BN growth

Abstract A literature survey on ion beam assisted deposition of boron nitride shows well defined ranges (domains) of microscopic parameters in which different phases of boron nitride occur. These are interpreted in terms of a growth model in which temperature dependent desorption of incoming material and selective sputtering of h-BN play dominating roles. This concept is transferred to chemical vapour deposition in which microscopic parameters cannot be quantified well. For this case a simple rule is developed which also shows the influence of sputtering during deposition. c-BN is compared with diamond-like carbon with respect to dominating deposition mechanisms.

Investigation of stress and adhesion of cubic boron nitride films

Abstract Cubic boron nitride (c-BN) films can be deposited by PVD and CVD methods. Massive ion bombardment of the growing film is required, leading to compressive stress and therefore poor adhesion. In a simple approach, stress is modelled by the balance of defect formation, due to ion bombardment, and thermal as well as ion-induced recombination processes. The model predicts stress to be reducible using high ion energies and high substrate temperatures, whereas the ion to atom flux ratio is of minor importance only. New results concerning c-BN nucleation show an orientational relationship between an initial textured h-BN layer and the first c-BN crystals. The influence of interlayers prior to c-BN deposition on the nucleation sequence and film adhesion is discussed.

Deposition of cubic boron nitride with an inductively coupled plasma

Cubic boron nitride (c-BN) films have been deposited by inductively coupled plasma chemical vapour deposition (CVD). With this technique films with a c-BN content of nearly 75% can be achieved. Investigations on the influence of the process parameters which are most important for depositing c-BN (ion flux, ion energy and substrate temperature) have been carried out. The results show good agreement with our theoretical sputter model. Furthermore, from these experiments it is possible to draw the conclusion that the physical vapour deposition and the ion-induced CVD of c-BN films rely on the same deposition mechanisms among which physical sputtering is most important.

Investigation of the nucleation layer in c-BN film growth

Abstract The nucleation of c-BN during an inductively coupled plasma chemical vapour deposition (CVD) process is investigated. The transition from the textured h-BN nucleation layer to the c-BN layer turned out to be not abrupt but to take place rather gradually in several steps. Increasing the substrate temperature sharpens the transition and increases the maximum c-BN content. It is speculated that this is correlated with a decreasing hydrogen content of the films at higher temperatures. Experiments with different bias voltages during the nucleation step revealed that c-BN nucleation is possible only within the well-known c-BN region of the parameter space. Thus, this region might be determined by the nucleation of c-BN rather than by the growth step. Experiments with boron-rich interlayers finally results in extremely high c-BN contents of the subsequent c-BN layers.

Investigation of gas phase mechanisms during deposition of diamond films

Abstract The deposition of diamond films is investigated by optical emission spectroscopy measurements. Using the microwave chemical vapour deposition technique, the system C:H:O is surveyed starting from hydrogen-rich oxygen-free mixtures by gradually adding oxygen and reducing the hydrogen content until finally reaching C:H:O = 1:4:1, i.e. CH4 + 1 2 O2. CH and C2 are shown to give good measures for different active carbon species, CH being correlated with the growth rate while C2 correlates inversely to the film quality. The main role of O2 is the transformation of active carbon into the more stable carbon monoxide.

The influence of hydrogen on nucleation and growth of cubic boron nitride films

The presence of hydrogen in the gas phase of (ion-assisted) CVD processes is detrimental to the formation of cubic boron nitride (c-BN). By elastic recoil detection measurements it is shown that this is caused by the incorporation of hydrogen, especially into the h-BN nucleation layer, thus causing a degradation of this nucleation layer as well as the subsequent c-BN layer. Experiments carried out to solve this hydrogen problem revealed that the hydrogen content of CVD c-BN films can be reduced by either the use of precursors in which hydrogen is (partially) replaced by halogenes, or the use of high substrate temperatures. The reduced hydrogen content reflects in the nucleation behaviour of c-BN films as well as in their properties.

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.

On the role of ion bombardment in cubic boron nitride deposition

Abstract Thin film deposition of cubic boron nitride (c-BN) is nowadays investigated in two different ways; the first is ion assisted deposition, the second a chemical approach without significant ion assistance. The latter is less well investigated, and up to now its suitability for c-BN deposition has not been proved. On the other hand, for ion assisted deposition universal parameter ranges have been found, and first models focusing on special aspects of the ion bombardment (subplantation, stress, sputtering) exist. The sputter model which was developed by our group is slightly modified, assuming the modification with the highest growth velocity to be favoured. With this formulation, all macroscopic parameter dependencies (ion to atom ratio, ion energy, ion mass, angle of ion incidence and substrate temperature) are well described. In this contribution, the microscopic processes during deposition will be discussed in light of recent experiments. The nucleation of c-BN yielding different structures (an amorphous layer followed by textured h-BN and finally nanocrystalline c-BN) and parameter dependencies is also included. The role of ion bombardment in general is twofold: on the one hand, it is up to now inevitably necessary to grow the c-BN phase at all temperatures; on the other hand, it causes problems of which high stress and, as a consequence, poor adhesion are the most significant. Adhesion failure occurs, if the forces at the interface exceed the atomic adhesion forces. Therefore, besides the reduction of film stress, improvement of the interface is worthy of investigation. Modelling of the latter is complicated by the complex nucleation sequence mentioned above. Some conventional means to improve adhesion (adhesion layers, stress reduction by annealing or high energy ion implantation) exist. Nevertheless, in view of applications, improvement of adhesion by a suitable choice of deposition parameters is highly desirable. In a simple model, stress is related to the defect concentration within the material which is governed by a balance between defect production due to ion bombardment, and thermal as well as ion assisted recombination processes. The model predicts the decrease of the stress with increasing ion energy in agreement with experimental data. The ion to atom ratio is of minor importance, whereas increasing the angle of ion incidence and the substrate temperature are predicted to decrease stress. Furthermore, approaches to change the nucleation behaviour during deposition are outlined. These are expected to improve the interface characteristics in view of the adhesion problem.