Nano-crystalline diamond films synthesized at low temperature and low pressure by hot filament chemical vapor deposition

Nano-crystalline diamond (NCD) films on silicon were synthesized at a low temperature of 450–550 °C and a low pressure of ∼7 Torr by a hot filament chemical vapor deposition (HFCVD) in a mixture of CH4 and H2. The synthesis process included pretreatment, nucleation, growth and in situ annealing steps. The Si wafers were ultrasonically pretreated in a suspension of acetone and diamond powder (20–40 μm in diameter). It was found that the key parameter to obtain smooth NCD films by conventional HFCVD without Ar was the high nucleation density (ND). Optimal ultrasonic pretreatment with coarser diamond powder and nucleation at 2.5% CH4 greatly increased the ND to about 1.5 × 1011 cm− 2. The NCD films with average grain sizes in the range of 40–70 nm, thickness less than 500 nm and surface roughness of ∼19 nm were grown at 1.5% CH4, 550 °C and 7 Torr, their optical transmittance in the near infrared (NIR) range was as high as 85%.