Influence of substrate bias pretreatment on growth of diamond films by HFCVD

In the hot filament CVD (HFCVD) process, the nucleation process is enhanced by ion bombardment of the negatively biased substrate. To achieve ion bombardment at voltages as low as possible, a hybrid HFCVD technique was developed utilising a dual plasma arrangement. The I-V measurements of the plasma formed between the substrate and the filaments showed that the saturation of the total current depends on the process duration and substrate bias. A low nucleation bias voltage, i.e. less than or equal to100 V, results in poor nucleation and non-continuous film formation. An increase in substrate bias voltage up to 120 V results in homogeneous nucleation and subsequent deposition of diamond films consisting mainly of large rectangle-like crystals. Further increase in nucleation voltage to greater than or equal to 140 V leads to the formation of films with increased density of grains, which show edges with streak and cracking features indicating a twinning growth. During nucleation, the total current on the substrate has a tendency to increase with time, except for the sample nucleated at 100 V The time constant, defined as the point when the total current starts to increase, is reduced with increase in substrate bias. It is proposed that this effect is related to the increased density of diamond nucleation sites.