Diamond thin film preparation by microwave plasma chemical vapor deposition under the open-air condition

Summary form only given: A diamond thin film has many attractive properties such as high hardness, large heat conductivity etc. Microwave plasma chemical vapor deposition (MWPCVD) is an effective method for producing a diamond thin film. However, the deposition rate of the conventional low-pressure MWPCVD is very low. Recently, the authors have succeeded to produce a diamond thin film on a Si substrate with high deposition rate (~100 m/h) under the open-air condition, i.e. under the atmospheric pressure and without chamber, using microwave plasma system with a torch (electrode) of coaxial structure. Since the system has open-air structure, vacuum system is not required, which results low cost. However, the stable deposition condition for both high deposition rate and high film quality has not established. In this study, the effect of the gap distance between the electrode and the substrate is studied experimentally to find the best condition for high quality diamond thin film preparation. The electrode has coaxial structure and the inside diameter of the outside copper electrode is 12 mm and the outside diameter of the inside tungsten electrode is 4 mm. It is attached to the waveguide vertically. Typically 100 W microwave power (2.45 GHz) is fed to the electrode and the plasma is initiated at the bottom tip of the electrode. The gap length between the electrode and the substrate was set at three different lengths: 0.4, 0.6 and 0.8 mm. The substrate was silicon of about 1 cm², which was scratched with diamond powder before plasma processing. Source gas flow rate was kept at 4 sccm (CH₄) and 100 sccm (H₂). Growth time was set at 20 min. Films deposited as circular shape with 2 mm in diameter and they consisted of many grains with a few to 40 m in diameter. The grain size tended to become larger with longer gap length. This would be attributed to the substrate temperature; although it was not measured, from the substrate color during p- asma duration, the substrate temperature was obviously higher for longer gap length. Some of the deposited film had dense crystal grain, whereas some of them had pinhole, i.e. the position where the crystal grain was not deposited. The crystal properties were analyzed by Raman spectroscopy. In each film, although some local crystallization of diamond bonds was observed, most part of the films was amorphous. Further study for optimization of conditions including CH₄/H₂ ratio is under progress.