R. Freudenstein

Stress of c-BN thin films: a parameter investigation

Abstract Boron nitride thin films were ion-beam-assisted deposited on bare silicon substrates or silicon cantilever structures. The resulting film stress was studied by determination of the resulting bending of the beam using either a profilometer or optical microscopy, respectively. Employing the latter method, the reliability of the stress measurement on the BN-coated cantilever structure could be increased. The theoretically predicted E I O N - 2 / 3 dependence of the resulting film stress on the ion energy was verified. Generally, the stress of the coatings increased with increasing content of cubic boron nitride (c-BN) in the sample. However, no correlation between the c-BN infrared peak position and the film stress could be observed. In addition, the development of stress — i.e., the distribution of the total film stress — was studied. Therefore, an as-deposited coating containing c-BN was subsequently resputtered and examined after each sputter step. The total stress of the non-cubic base layer increased gradually with increasing thickness. After the onset of c-BN growth, the film stress exhibited only a marginal increase. So far, any reduction of film stress after c-BN nucleation has not been observed.

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.

A concept for the deposition of adherent cubic boron nitride films

Abstract Measures to reduce the high compressive stress of c-BN films and to improve their adhesion are discussed. A simple model of stress formation in c-BN films reveals that high ion energies and low Ar/N2 ratios should be used for stress reduction. These predictions are experimentally confirmed by means of depth resolved stress measurements using back-etching techniques with films deposited on cantilever substrates for accurate stress measurements. By suitable choice of ion energy and Ar/N2 ratio, the stress can be reduced by a factor of four to values below 5 GPa. The adhesion of c-BN films, on the other hand, can be improved by engineering the interface; controlled surface roughening and the deposition of gradient interlayer B→BN both lead to better adhesion. Based on these results, a multi-step process is proposed for the deposition of thick, well-adherent c-BN films.

Dependence of the stress of c-BN films on the major deposition parameters: theoretical and experimental studies

Abstract High compressive stress is one of the major problems preventing any application of cubic boron nitride thin films at the present time. In order to solve this problem, a detailed understanding of the origins of stress formation in c-BN films is required. According to Davis’ model, the stress is determined by a balance of defect (interstitial) generation and recombination. By means of TRIM calculations, we investigated the interstitial generation as a function of important process parameters such as the ion energy and ion mass. Based on these calculations, the Davis model predicted, for example, a stress reduction with increasing ion energy. In our experimental work, we investigated the stress of depth-resolved c-BN films by means of the bending of cantilevers on specially designed substrates. The dependence of the stress on the major deposition parameters (ion energy, ion mass (by varying the Ar/N2 ratio) was established, e.g. a decrease in stress was observed with increasing energy, as predicted by the model. Furthermore, the incorporation of Ar ions within the films was studied by means of ERD measurements. Although the argon content increases with increasing Ar fraction in the ion beam to a maximum value of approximately 2%, it can be concluded that Ar yields a major stress contribution. Rather, the incorporated Ar is situated at the grain boundaries of the nanocrystalline films which also play an important role in stress release.

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.

Improvement of the adhesion of c-BN films by bias-graded h-BN interlayers

The adhesion of cubic boron nitride films can be improved considerably by inserting bias-graded interlayers, consisting of layers deposited with step-wise increased bias voltage. A study of the nucleation of the first h-BN layers on the silicon substrate by atomic force microscopy shows that in the case of our inductively coupled plasma CVD process without interlayer the standard nucleation layer nucleates itself in the island mode, which is not the case for the bias-graded adhesion interlayers. However, even if bias-graded interlayers are used, the standard nucleation layer with the h-BN basal planes standing upright is observed on top of the interlayer prior to c-BN formation. A study of the interlayer system by Fourier transform infrared spectroscopy, elastic recoil detection and high resolution transmission electron microscopy shows that the crystalline fraction, the orientation of the nanocrystals, and the density of the interlayer increase with increasing bias voltage. Possible reasons for the improved adhesion are discussed on the basis of these findings.

Investigation of the nucleation of c-BN by AFM measurements

The poor adhesion commonly observed for c-BN thin films is caused by the high compressive stress of the films and the low adhesion strength at the interface. In general, the adhesion strength of a coating is determined by the nature of its interface to the substrate and, therefore, influenced by the nucleation process. In order to evaluate the mechanisms of c-BN nucleation a nucleation sequence was deposited by inductively coupled plasma CVD (ICP-CVD) and investigated by AFM measurements. The surface roughness of all films is rather high and increases with deposition time. The height differences within the films are also rather large. The AFM images show small rounded features at the surface, which become more and more prominent with deposition time. It is speculated that they are the first c-BN nanocrystals formed on a h-BN nucleation layer. Finally, our results will be discussed in view of the growth mode of the nucleation layer, which can be correlated with the delamination behavior of the c-BN films.

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.

Correlation between stress values of cubic boron nitride thin films and intrinsic film properties or the infrared peak position

Boron nitride thin films were ion beam assisted deposited on silicon substrates. The c-BN content was investigated with IR spectroscopy. The intrinsic film stress, studied by measuring the resulting bending of the substrate using a profilometer, increases generally with increasing c-BN content. In addition, the IR peak position of the c-BN reststrahlen band was determined for adherent coatings and after the films completely peeled off the substrate. This stress relief due to the peeling-off induces a shift of the IR peak position to smaller wavenumbers. However, no correlation between the total stress obtained by the bending experiments and the IR peak position for adherent films could be found. In contrast, the value of the shift of the c-BN peak position to smaller wavenumbers after peeling-off shows a good correlation with the stress values from the bending method. Several mechanisms which may influence the peak position, such as stress, coalescence and film thickness, will be discussed. Finally, we conclude that an estimate of c-BN film stress from IR data is only possible on the basis of the peak shift after peeling-off, which on the other hand means that stress measurement using the IR peak position has to be a destroying technique.

Depth resolved comparative investigation of phase formation and stress build-up in cubic boron nitride films

Abstract Cubic boron nitride films have been deposited by means of ion beam assisted deposition (IBAD) and electron cyclotron resonance plasma CVD. Specially designed silicon cantilever substrates were used which allow precise stress measurements. After deposition, the films were etched back in steps of approximately 5–20 nm by Ar ions. After each step, the stress, the thickness and also FTIR spectra were measured. By this means, the stress distribution, the development of the c-BN content and also the thicknesses of the h-BN nucleation layer and the transition layer h-BN → c-BN could be determined. Major differences were found between the two types of layers: IBAD films possess a nucleation layer with a thickness of 15–20 nm and a stress of approximately 3 GPa. After c-BN nucleation, a transition region of approximately 20 nm is observed in which the c-BN content rapidly increases to the final value, which can exceed 90%. However, the c-BN top layer possesses a high compressive stress with values up to 20 GPa. In contrast, for ECR films the transition from the h-BN nucleation layer to the c-BN top layer is much more gradual (nucleation layer thickness approx. 50 nm). Simultaneously, the c-BN content of the top layer is limited to approximately 70% while its stress is much lower (2–3 GPa). The reasons for these differences in phase formation and stress build-up are discussed in terms of the differences between the two techniques.