Chemical mechanical polishing of thin film diamond

TL;DR

This study demonstrates effective chemical mechanical polishing of nanocrystalline diamond films, significantly reducing surface roughness and proposing a polishing mechanism to enhance film smoothness for device applications.

Contribution

It introduces a CMP process for NCD films using specific materials and characterizes the resulting surface improvements and polishing mechanism.

Findings

Roughness reduced from 18.3 nm to 1.7 nm over 25 μm²

Surface roughness as low as 0.42 nm over 0.25 μm² achieved

Proposed polishing mechanism involves wet oxidation and silica particle shearing

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.