ICMCTF2000 Session D1-3: Synthesis, Characterization and Applications of Boron Nitride, Carbon Nitride and Fullerene Like Structures

This paper describes a hot filament CVD process for carbon nitride thin films without hydrogen additions. The concentration of the nitrogen precursors NH₃ and N₂ relatively to methane has been systematically varied in order to analyze the influence of nitrogen incorporated into the CN_X films for the structure formation. Additionally total pressure and total gas flow rate were changed within a large scale. Various substrate temperatures between 1800 and 2300°C were applied with the aim to improve the deposition. But during one experiment the temperature of the carborized W filament was kept constant. It is a well-known fact that the substrate temperature and substrate bias voltage are further important parameter in chemical vapor deposition. Several materials such as (100) and (111) Si, SiO₂, quartzglas, and Ni, were used as substrates in order to study their influence on nucleation and growth of the CN_X films. As a result we succeed in screening of the possible range for CN_X film deposition.

The chemical composition and the structure of the deposited CN_X films were studied by RBS, TEM, and SEM. From the FTIR and Raman spectroscopy the vibrational properties were observed. From the results conclusions to the chemical bonding states are drawn. The structure of the films was analyzed by means of X-ray and electron diffraction using a transmission electron microscope for the latter. Additional STM investigations reveal cluster like microstructure of the film surface.

The mechanical properties of the CN_X thin films were investigated using nanoindentation measurements. From the load-displacement curve the hardness and the Young's modulus of the films were calculated. The results will be compared with reactively sputtered CN_X films containing different amount of nitrogen up to 42 at.%N.

A continuing interest in carbonitride has persisted since the prediction of its structure by Liu and Cohen due to the expected improvement of surface properties for a lot of applications. However, an Si-C-N ternary system may be preferred and could exhibit all useful characteristics of the binary compound. Very promising approaches to silicon carbonitride synthesis are RF magnetron sputtering and ion implantation techniques providing tailored stoichiometries at high purity. Si-C-N thin films were reactively sputtered using ¹⁵N enriched N₂/Ar sputtering gas and co-sputter targets with different Si/C areas resulting in defined and reproducible Si/C ratios at constant nitrogen concentrations. Alternatively, surface modification by sequential high fluence implantation of C and N ions into silicon allows for tuning the atomic fraction of all elements over a wide range. However, the implanted layers of interest are some ten nanometer below the surface. Both preparation techniques along with post-treatment procedures are discussed with respect to the attainability of stoichiometric phases, e.g. Si₂CN₄.

The chemical composition of the ternary systems were characterized by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and Fourier transform infrared spectroscopy (FTIR). In case of the buried implanted layers chemical binding states, however, only can be determined from XPS after Ar⁺ ion sputter etching. Therefore, it is necessary to reduce the Ar⁺ ion energy drastically (< 400 eV) in order to minimize the projected range for the etching ions to a negligible part of the XPS information depth. For quantification XPS and AES data were calibrated with absolute concentration values from n-RBS (non-Rutherford backscattering spectrometry). Furthermore, both preparation techniques have the advantage that ¹⁵N and ¹³C isotopes can be introduced into the layers enabling non-destructive nuclear reaction analysis (NRA) for depth profiling.

In plasma enhanced physical vapor deposition (PEPVD) of cubic boron nitride (c-BN) films the deposition parameter range for c-BN growth is strongly restricted. Therefore in large scale deposition devices systematically investigations are necessary to find out the parameter range for growth over a large area with nearly constant deposition rate and c-BN content.

Investigations are performed in a hollow cathode arc evaporation device (HCAED) in a power range of 2 - 5 kW. In the first step plasma parameters (electron density, mean energy of electrons, plasma potential) were measured spatially resolved in the substrate area of the deposition chamber. Gas species used are argon and nitrogen. In the second step the spatial resolved boron deposition rate was measured as function of HCAED-power. By evaluation of the data from plasma parameter and boron deposition rate measurements the ratio of ion to boron flux rate can be achieved. The ratio of ion to boron flux in dependence on ion energy is one crucial parameter in PEPVD of c-BN film nucleation and growth. Both flux rates to the substrate surface are strongly depending on the position of the substrate in the deposition chamber and on outer parameters such as pressure, power and gas mixture. Typical values of these outer parameters are: pressure of 0.6 Pa, power of 5 kW and ratio of argon to nitrogen of about 3:1. A simulation shows large area (>300mm) where c-BN film growth with nearly constant deposition rate is possible. The size of these area depends on the distance of the evaporator to the substrate surface. Experiments confirm the simulation. The c-BN content of the films was measured in situ and ex situ by IR reflection spectroscopy.

In the present work, attempts were made for deposition of c-BN films on unheated silicon substrates by IBAD technique and also Triode sputtering system. The films were characterized by infrared and Raman spectroscopy. High-resolution cross sectional TEM is used to confirm the optical results. Films with c-BN content up to 85% were obtained at temperature of 200°C and in a pure nitrogen discharge. It's well known that the development of intrinsic stress hinders the deposition of thick c-BN films. So, the intrinsic stress in the deposited films was studied and in order to better understand this stress behaviour in depth stress profiles have been performed by three different procedures. After the description of the procedures, the obtained results may be discussed in relation with the different process parameters.

Finally it has been found that on condition to change the ion flux or energy during growth it's possible to get less stressed films. This assumption was checked and cubic boron nitride films with stress values of 5 GPa and relatively good adhesion were obtained.