ICMCTF2001 Session D1-1: Synthesis, Characterization and Applications of Boron Nitride, Carbon Nitride and Fullerene Structures

Carbonitride as well as Silicon Carbonitride thin films have been the subject of great interest in recent years due to the expected improvement of surface properties for a lot of applications. Various precursor based techniques have been employed to synthezise the pure materials. However, most of these efforts result in amorphous films or tiny crystals embedded in amorphous matrices of deficient nitrogen content and considerable hydrogen and oxygen content, respectively. Very promising approaches to Si-C-N synthesis are R.F. magnetron sputtering and ion implantation providing tailored stoichiometries at high purity. Silicon carbonitrides were reactively sputtered using ¹⁵N enriched N₂/Ar sputter 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. Both techniques enable us to synthezise ternary systems of more than 52 at.% nitrogen content, which are stable up to 1000 °C.

The chemical composition of the Si-C-N films was characterized by means of X-ray photoelectron spectroscopy (XPS). In case of the buried implanted layers chemical binding states were attainable after sputter etching using 300 eV Ar ions of a projected range minimized to a negligible part of the XPS information depth. In addition, Auger electron spectroscopy (AES), FTIR spectroscopy, and Raman spectroscopy were used to achieve a comprehensive characterization. For quantification XPS and AES data were calibrated with absolute concentration values from 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 for depth profiling.

The conditions for the growth of sp³-coordinated (cubic) boron nitride by r.f. magnetron sputter deposition from hBN - targets have been studied in dependence of the negative r.f. self - biasing voltage at the Si (100) substrates and the substrate temperature. When tracing the growth rate under constant deposition parameters (substrate bias and temperature 200 V and 600°C, respectively,) quite surprisingly a negative growth rate was observed which started at film thicknesses of about 100 nm and caused a thickness shrinking by about 30%. Subsequently the films continued to grow with an again positive growth rate being slightly different from its initial value. The shrinking effect is connected with an onset of the sp³-specific IR-absorbance and the development of a transiently increased surface roughness of the films. Hence, we conclude that the cBN-nucleation starts as an sp ² - sp ³ - phase transition within the highly ordered"turbostratic" BN-base layer which is formed during the initial stage of the deposition process and becomes clearly visible in our own high resolution TEM images of the respective films.

As soon as the nucleation into cBN has been completed across the entire film surface, the cBN-growth was found to occur at substrate bias voltages down to about 60 V for nitrogen as working gas and down to substrate temperatures around room temperature, even for Ar as working gas. Thus, we propose a two- step deposition process under which at first the required specificly ordered hBN-base layer is formed at elevated bias voltage and temperature. Both quantities can then be reduced to the lower values for which the cBN growth continues after completion of the initial nucleation process.