Advances in growth technologies have made it possible to grow nanocrystalline diamond (NCD) over large area on non-diamond substrates. NCD films are polycrystalline in nature, but retain all the superlative properties of single crystal diamond such as superhardness, superconductivity, high thermal conductivity and biocompatibility. Such properties make NCD an excellent material for fabricating ultrasensitive and quantum devices, e.g., MEMS and SQUIDs for a variety of applications. However, the downside of NCD is its inherent surface roughness, which can lead to reduced sensitivity. While the polishing of single crystal diamond or thick polycrystalline film is well developed, these techniques cannot be used to polish NCD. This is primarily due to wafer bow due to large differences in thermal expansion coefficients between NCD and the substrate (e.g., Si). To get around this problem we have developed a chemical mechanical polishing technique capable of generating smooth NCD surfaces. The process is based on a similar process used in the silicon industry for polishing metal interconnects. In this process NCD is polished using a silica based slurry in contact with a soft polyurethane pad. Using this process, we were able to reduce the roughness from 18.3 nm to 1.7 nm RMS over an area of 25 m2on a 360 nm thick NCD film. Furthermore, this technique can remove polishing tracks from both (100) and (111) single crystal diamond. Initial experiments were carried out using silica-based slurries and following experiments with other oxide (ceria and alumina) based slurries. Both slurries were able to polish thin NCD films. We observed that the pH of the slurry has minimal effect on polishing, while an inverse relationship exists between the abrasive particle diameter and polishing rate. A loss in superconductivity in boron doped diamond after mechanical polishing has previously been reported, however in this reported technique no loss in superconductivity was observed.
Finally, to study chemical aspects of the technique, we explored the effects of the addition of strong redox agents to the polishing fluid on the roughness reduction rate of NCD films. A series of NCD films were polished for 4 h and the roughness monitored by AFM after each hour. The base slurry used for polishing was colloidal silica SF1 from Logitech, which could reduce a nominal as-grown NCD RMS roughness of ~24 nm to ~2 nm RMS over 25 μm2 within 4 hours of polishing. Various redox agents such as hydrogen peroxide, ferric nitrate, potassium permanganate, oxalic acid and sodium thiosulfate were added to SF1 to study their effect on polishing rates. Oxalic acid produced the fastest polishing rate while hydrogen peroxide had very little effect on polishing possibly due to its volatile nature. XPS after polishing revealed little difference in the surface oxygen content, this is a possible indication that the addition of redox agents does not increase the density of oxygen containing species on the surface, but rather accelerates the process of attachment and removal of Si or O atoms on the diamond surface.