ICMCTF2001 Session D2-2: Synthesis and Characterization of Diamond and Carbon-based Materials

We have succeeded in synthesizing cBN films on Si substrates by DC plasma jet CVD with substrate bias in an Ar-N₂-BF₃-H₂ gas system¹. The gas pressure during the deposition was 50 Torr and the most appropriate substrate bias was -85 V. The phase purity of the cBN films was above 90 % by IR spectroscopy and sharp X-ray diffraction peaks of cBN was observed. Clear Raman peaks of TO and LO phonon modes of cBN appeared for the first time for cBN films. The crystal size was found to be 0.2 x 3 µm by TEM but the crystals contain a large amount of defects such as stacking faults and microtwins. TEM also revealed that presence of interface layers of amorphous and h- or t-BN between the cBN and the substrate. cBN films of above 20 µm in thickness were obtained and the deposition rate at the initial growth stage was 0.3 µm/min. This is the highest growth rate reported so far for cBN films. Positive bias to the substrate or the use of B₂ H₆ or BC_l3 gas, instead of BF₃, didn't give cBN under similar deposition conditions.

Our diamond deposition by DC plasma jet or by RF induction thermal plasma from a Ar- H₂- CH₄ gas system at 140 Torr or at 1 atm gave high quality diamond films with a growth rate of 10 - 2 µm/min. Positive bias to the substrates enhanced the deposition rate, from 6 to 15 µm/min, for example, but negative bias decreased the deposition rate and the crystal quality, inducing the secondary nucleation and the formation of small diamond crystals with {100} faces².

Thermal plasma CVD is known to give the highest deposition rate of diamond among all the CVD methods so far known. It also gave cBN with high growth rate, although the growth rate was still below one tenth of diamond growth. The deposition of cBN at a pressure as high as 50 Torr suggests the possibility of real CVD of this process, as different from the low pressure (several tens mTorr) CVD reported so far. On the other hand, fluorine and negative bias were necessary for cBN growth. These facts suggest that fluorine is effective for the preferential growth of sp³ bonds as atomic hydrogen is in diamond CVD, but the surface chemistry of the growth is rather different from that of diamond.

¹S. Matsumoto and W.J. Zhang, Jpn. J. Appl. Phys. 39 (2000) L442.

²S. Matsumoto et al, Pure & Appl. Chem. 64 (1992) 751.

Diamond-like carbon (DLC) is an amorphous carbon containing a certain fraction of hydrogen. DLC exhibits superior mechanical as well as tribological behaviour and promising biocompatibility. Additional elements like nitrogen, silicon and titanium may effect a further adaptation of the DLC properties according very specific demands. In order to enlarge the applicability of DLC for biomedical purposes, the addition of calcium which is able to influence bone formation may be of considerable relevance. The intention of the study is to gain first information about the influence of Ca-O-incorporation on the mechanical and structural properties of DLC.

The generation of the conventional DLC and the Ca-O-DLC of about 5 µm thickness was carried out by exploiting a direct current discharge for plasma formation. The CaO was dissolved in water. For the generation of Ca-O-DLC, the gaseous precursor (benzene) together with the CaO-H2O vapor is decomposed due to the direct current discharge under a pressure of between 0.005 and 0.05 Pa. For processing conventional DLC, only benzene is supplied into the plasma chamber. The negative bias of the titanium-substrate attracts ions, loaded particles other decomposed fragments and leads to the coating formation.

The characterization of the DLC micro-structure was carried out by Raman spectroscopy and XPS measurements. Hardness determination, scratch and abrasive wear tests give information about the mechanical and tribological performance of the modified coatings. TEM investigations and cell culture tests are planed for gaining further inside about the micro-structure and the biocompatibility of the Ca-O-DLC.