Experimental study on reduction of hydrogen content in low z thin films by controlling DC glow discharge conditions

Boronization as wall conditioning has been performed in many plasma devices, such as, TEXTOR at 150-350°C, DIII-D at 300°C, and JT-60U at 300°C, resulting in well reduced oxygen impurities and hydrogen recycling. In Large Helical Device (LHD), the boronization with glow discharge is also considered as the main technique to regulate wall surfaces. However, the temperature of the vacuum vessel is limited below 100°C. In this low temperature operation, the hydrogen content in the boron film is not sufficiently reduced. Therefore, a new technique to reduce the hydrogen content in boron film is desired by controlling glow discharge conditions during the boronization. To make clear the dependence of H content on discharge conditions and to establish techniques to evaluate the H content are main aims of this thesis.

The evaluation methods of hydrogen content were newly developed or modified. These are flash filament method (FF), residual gas analysis method (RGA) and ion beam analysis method (IBA). The principle and procedure of these methods are presented. In particular, the newly developed FF method gives us three advantages, that are, simple and quick operation, in-situ measurements, and absolute measurement.

In the newly constructed experimental facility named plasma processing teststand (PPT), in-situ analyses under films coating were carried out by using an oscillating quartz crystal for monitoring the thin film thickness, a movable Langmuir probe to measure the electron temperature and the electron density in the discharge, a quadrupole mass analyzer for monitoring gas evolution during the discharge and FF measurements, and a thermocouple and an infrared TV camera to monitor temperatures of films surface on the FF or on the cathode liner.

Hydrogenated amorphous boron films using decaborane or diborane gas and carbon films using methane gas were produced with well regulated conditions in plasma-assisted chemical vapor deposition P- CVD in DC glow discharges. Extensive and systematic researches to reduce the hydrogen content in low Z thin films were carried out at the room temperature condition in PPT by measuring 5 dependences on controllable parameters, which are discharge power, total gas pressure, pumping speed, gas flow rate and gases mixing ratio.

For the boron films from decaborane, it was found that the hydrogen content was successfully reduced with increasing the film growth rate. In cases of diborane and methane, although the discharge power dependence was in good agreement with decaborane, the pressure dependence did not agree with decaborane. For decaborane lower pressure results in the lower H content. However, for diborane and methane the higher growth rate of films leads lower H content as well as decaborane.

In order to make clear the influence of electrical potential of FF., boron films were produced on AI samples by floating or connecting to the cathode potential, and then, depth profiles of H were analyzed with the ERD of IBA technique. For this purpose, a new modified method using RBS was established to monitor the probing ion flux. Results showed that a high growth rate of film formation and the floating surface were effective in reducing hydrogen content in B films. From this results it is conjectured that energetic ions flux should be reduced but the radical one should be increased.

The results measured by using RGA and ion beam analysis ERD. showed a good agreement with FF.

Hydrogen reduction mechanisms to explain the experimental findings were discussed.