E. Bauer-Grosse

Diamond deposition by chemical vapor deposition process: Study of the bias enhanced nucleation step

In the case of diamond films synthesized by the microwave plasma assisted chemical vapor deposition technique (MPCVD), the bias enhanced nucleation (BEN) step has been developed to avoid the scratching pretreatment and to palliate the low nucleation density of diamond crystals on the most common substrate used: pristine single silicon substrates. This treatment that occurs before the diamond growth step often consists in the application of a negative bias voltage of the substrate/substrate holder system, which is electrically insulated from the reactor wall. In the case of the MPCVD process, this bias induces a complex superposition of two cold discharges: the microwave and the bias discharges. Unfortunately, this complex configuration leads to inhomogeneous deposits in terms of quality, nucleation rate, and thickness. Furthermore, the reproducibility of the BEN step is generally poor in terms of diamond deposits and electrical BEN parameters. In order to better understand and overcome this pretreatment s...

Three-dimensional simulation of CVD diamond film growth

Abstract As often reported in the case of CVD diamond synthesis, the elaboration conditions strongly affect the quality of the deposit interms of roughness, grain boundary quantity, structural defect density or non diamond phase insertion. It is recognised that this quality mainly depends of the texture evolution. To our knowledge, numerical simulations of this growth model that allow the diamond film quality to be predicted do not exist. In this way, we have developed a three-dimensional computer simulation of diamond growth. This numerical model is based on the homothetic growth of single diamond crystals in an Euclidean space and the selection of the enveloping surface obtained after the crystal interpenetration. By this way and in the limit of the exposed hypotheses, it is possible to build the topographies and to simulate its theoretical evolutions in the case of textured films synthesised on scratched silicon wafers according to the growth conditions such as the nucleation density, the growth rate ratio α = √3( V 100 / V 111 ) or the synthesis time. We have particularly studied the ratio R {hkl} = ( S {100} / S {100} + S {111} ) of the topography according to the synthesis time, in order to follow the evolution of surface morphology.

Observation of spectral and normal emissivity as a method of surface control during the growth of diamond films deposited by a microwave plasma-assisted CVD technique

During the growth of a diamond layer on a silicon substrate obtained by microwave plasma-assisted chemical vapour deposition, pyrometric observations show signal oscillations. We can prove that these variations are due to emissivity modifications linked with thickness increase. The purpose of the present study, in addition to its theoretical aspect concerning the conception of an emissivity model, is to show the possibility of using the emissivity observation (here a simple pyrometric measurement) as an in situ and real-time means of controlling the surface and therefore also the treatment process. The proposed model allows one at each moment, to calculate the thickness and the complex refractive index of the deposit, to measure its kinetic growth and therefore also to watch its possible evolution.

Thermal stability and crystallization studies of amorphous TM–C films

Abstract During the last years, several binary transition metals (TM)–Carbon systems have been explored with the aim of, first obtaining amorphous alloys in a wide range of composition, especially towards the carbon-rich concentrations and second, studying the thermal stability and the crystallization of these new materials. Sputtering has been chosen as means of elaboration to obtain the films and electron probe microanalysis was used to determine the composition. The as-sputtered amorphous state was detected by electron diffraction and/or X-ray. For each amorphous film, the thermal stability was studied by differential scanning calorimetry and the crystallization was followed by hot-stage transmission electron microscopy. The products of crystallization were identified by electron diffraction. In this paper, we present a comparison of the main results we have got on amorphous and then crystallized films belonging to the well-known Fe–C, Mn–C and Cr–C systems. The thermal stability increases from Fe–C to Cr–C systems. Depending both on the carbon content and the nature of the transition metal, various unknown carbides form from the amorphous films. We find that they are often isomorphous with interstitial compounds already existing among borides, nitrides, carbonitrides and other carbides. The emphasis is put in their structural description. It is thus demonstrated that the new structures can offer either prismatic, octahedral or both sites to the C atoms. This suggests that more than one type of local orders may exist in the amorphous state for these TM–C systems.

Morphometric analysis of diamond crystals elaborated by microwave plasma assisted chemical vapour deposition: application to textured films

Abstract Morphometric analysis was carried out on both isolated single-crystal and multiply twinned diamond particles (MTPs) elaborated by microwave plasma assisted chemical vapour deposition, in order to determine, in our reactor, the growth parameter α as a function of deposition parameters such as substrate temperature and methane concentration. The evolution of α allows the MTPs to be described, in particular the formation of grooves at the emergence of the twin plane. Free-standing diamond films, elaborated using the same conditions as for isolated particles, were characterized by X-ray texture measurements to deduce the growth parameter α associated with these films. We reveal that the orientation of the silicon substrate can strongly delay texture formation because of an unfavourable substrate effect. Finally, the growth parameter α associated with diamond films is higher than the α value associated with isolated particles obtained under the same experimental conditions.

Synthesis of highly oriented CVD diamond films by ultra short bias enhanced nucleation step

Abstract Highly oriented diamond films have been deposited on silicon substrate by the MPCVD technique (microwave plasma assisted chemical vapour deposition) using an ultra short bias enhanced nucleation process (so called USBEN). We focus our attention on two points: the homogeneity of the deposit in order to perform a precise characterisation whatever surface location (on 1×1 cm2 of single silicon substrate); and the simplification of the successive steps usually performed in the BEN process. This is carried out by optimising the microwave cavity and the d.c. discharge extension and by keeping the pretreatments just necessary to obtain high nucleation density with an acceptable epitaxial ratio and a good homogeneity. This leads to a drastic reduction of the bias time of only 30 s for low bias voltage. As we obtain a highly oriented diamond film with a short bias pretreatment without preliminary carburation step, we discuss the substrate transformation under a weak bombardment duration of ions having a quite low energy. We think that the bias step probably consists to a slight modification of the substrate surface.

Initial growth phase of diamond thin films observed by thermal emission spectrometry

Abstract Pyrometric interferometry has been performed in the formation of diamond crystals and films synthesised on pretreated single silicon substrates by a microwave plasma assisted chemical vapour deposition (MPCVD) technique. The growth mode of this material induces particular variations of the emissivity ratio e / e 0 that we are able to simulate theoretically, taking into account the nucleation mode (Volmer-Weber), the nucleation density N and the growth rate v of the diamond crystallites. The comparison between the experimental and the theoretical curves of the e / e 0 variations allows us to understand better the first stage of the CVD diamond formation in terms of kinetic and nucleation density. Finally, the real-time monitoring of these parameters is a very attractive objective for the precise control of the deposition process, for which the first stages are crucial for the subsequent diamond film quality.

Prediction of the feasibility of oriented diamond films by microwave plasma-assisted CVD

After the nucleation step, the fabrication of textured or heteroepitaxial diamond film depends strongly on the direction of the fastest growth (vector denoted dv max) associated with every diamond particle. From quantification of this vector for single-crystal and multiply twinned particles (MTPs), we can determine the relative feasibility of textured or heteroepitaxial films in relation to preparation conditions such as the hydrocarbon concentration or substrate temperature. This theoretical study, that takes account of the results of morphological and structural characterizations of textured diamond films, reveals the difficulty of producing 〈111〉 textured films that lead to 〈011〉 textured films, and the ease of obtaining 〈001〉 textured films.

Quantitative characterization of the true epitaxial ratio in the first stage of the MPCVD diamond synthesis

Isolated diamond crystals have been deposited on silicon by the microwave plasma assisted chemical vapor deposition technique using the bias-enhanced nucleation step. The true epitaxial ratio, defined as the number of epitaxied crystals to the total crystal number before coalescence, has been calculated by high-resolution scanning electron microscopy coupled to image analysis software. This ratio is correlated to the conditions of the etching step which precedes the bias step. This investigation allows us to determine the better conditions to obtain homogeneous epitaxied deposit with an optimum epitaxial ratio in the absence of a preceding carburization step, and to propose a qualitative explanation about the effect of the etching step on the silicon substrate.

Silicon substrate preparation for epitaxial diamond crystals

Using the microwave plasma-assisted chemical-vapour deposition technique (MPCVD), we study the role of the (100) silicon substrate preparation prior to the ultra short bias enhanced nucleation (USBEN) step via various pre-treatments. We study the effect of the silicon HF cleaning coupled with the hydrogen plasma exposure in order to determine the efficiency of these pre-treatments to eliminate the native oxide layer on the silicon surface prior to the bias step. We show that the residual oxygen content in the gas phase during the hydrogen plasma exposure can strongly affect the nucleation density because of the formation of an oxide layer which is detrimental for the synthesis of highly oriented diamond (HOD) films. Moreover, we study the effect of the carburation step, often used for the synthesis of HOD films and show that it raises the percentage of epitaxial crystals and the crystal density. Thus, we show that the achievement of high epitaxial crystal density is not only due to the BEN parameters but also to the silicon substrate preparation. As a conclusion, it appears that the silicon substrate preparation prior to BEN is fundamental for controlling the quality of epitaxial diamond films.