Hot filament CVD is one of the first techniques to prepare diamond films successfully. Compared with other methods, hot filament CVD requires simple equipment, keeps film formation under control and generates large-area diamond films. The main shortcoming is the low heat efficiency of using a hot filament as the heat source. The growth rate is 0.3 – 2 μm/h.
In the growth process, the distance between the hot filament and the substrate is crucial. Without any auxiliary heat source, it should be less than 10 mm. In addition, the film is difficult to make uniform and contains non-diamond phase. To improve diamond quality and growth rate, a direct voltage is applied between the hot filament and the substrate to generate plasma and raise the filament temperature. The hot filament distorts after several working hours at high temperature: the longer the W or Ta filament, the greater the distortion, and the higher the carbon concentration, the greater the distortion. This makes the distance change difficult to control. Meanwhile, the surface temperature and the uniformity of the DC plasma distribution between the filament and the substrate are also affected. The figure below is the equipment sketch of a so-called advanced hot filament CVD (AHFCVD). Symmetrical diamond (thick) films of Ф100 mm can be prepared using this equipment.
