Chemical mechanical polishing of thin film diamond

TLDR

Nanocrystalline diamond films combine the high Young's modulus of single crystal diamond with low‑temperature growth, yet their competitive crystal growth leads to thickness‑dependent roughness that limits device performance. The study aims to reduce this roughness by applying chemical mechanical polishing to NCD films. Using a Logitech Tribo CMP system with a polyurethane/polyester cloth and alkaline colloidal silica slurry, the authors polished the films, characterized them with AFM, SEM, and XPS, and propose a wet‑oxidation–silica attachment–shearing mechanism. Roughness was lowered from 18.3 nm to 1.7 nm over 25 µm², reaching as low as 0.42 nm over 0.25 µm².

Abstract

The demonstration that Nanocrystalline Diamond (NCD) can retain the superior Young's modulus (1,100 GPa) of single crystal diamond twinned with its ability to be grown at low temperatures (<450 {\deg}C) has driven a revival into the growth and applications of NCD thin films. However, owing to the competitive growth of crystals the resulting film has a roughness that evolves with film thickness, preventing NCD films from reaching their full potential in devices where a smooth film is required. To reduce this roughness, films have been polished using Chemical Mechanical Polishing (CMP). A Logitech Tribo CMP tool equipped with a polyurethane/polyester polishing cloth and an alkaline colloidal silica polishing fluid has been used to polish NCD films. The resulting films have been characterised with Atomic Force Microscopy, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. Root mean square roughness values have been reduced from 18.3 nm to 1.7 nm over 25 {\mu}m$^2$, with roughness values as low as 0.42 nm over ~ 0.25 {\mu}m$^2$. A polishing mechanism of wet oxidation of the surface, attachment of silica particles and subsequent shearing away of carbon has also been proposed.