Nanostructuring of diamond via two photon UV etching at sub-ablation fluences

Full text: Two photon UV etching of diamond is a relatively newly discovered process, wherein exposure of a diamond surface in air to UV light with fluence below the ablation threshold results in slow, controllable removal of material with average rates typically much less than one atomic layer per pulse. Numerous experiments have been performed to investigate the properties if this process, the results of which will be presented. These include measurements of etch rate under a variety of conditions, imaging of nanostructures arising from etching, analysis of surface chemical states via x-ray spectroscopy, post-process modification of complex nanostructures by UV etching, and observation of the behaviour of etching as a function of oxygen partial pressure. It has been found that the rate of material removal has a quadratic dependence on the laser intensity, and the profile of etched pits are proportional to the square of the etching beam's intensity profile; both of which are indicative of a two photon process. It also appears to be thresholdless – average etch rates have been observed from as low as 10^-9 nm/pulse to as high as 10^-2 nm/pulse with consistent behaviour. The presence of atmospheric oxygen is necessary for etching to occur, indicating that the process is photo-chemical in nature and sustained by surface oxidation. X-ray spectroscopy has found the etched diamond surface to be free of graphite (which is normally associated with the laser ablation of diamond), and instead shows a clean, oxygen terminated surface. The lack of redeposited detritus indicates that material leaves as highly volatile species; likely CO or CO₂. Etching also causes roughening of the surface, as the diamond tends to form microfacets along (111) planes. Interestingly, the morphologies of these structures are strongly dependent on the angle between the polarisation of the etching laser and the underlying crystal lattice. Figure 1 shows an example of a (001) oriented surface and the differences in nanostructure which arise from rotation of the beam polarisation. This seems to indicate an interaction with highly directional components of the surface – likely surface bonds. These characteristics of material removal, which to the authors' knowledge are distinctive to diamond, have led to a broad description of a 'cold' etch mechanism where two photon absorption occurs at the diamond surface, which results in the ejection of carbon and oxygen atoms followed by readsorption of oxygen from the atmosphere to regenerate the oxygen termination layer. However, many aspects of the mechanism are poorly understood. Important characteristics that determine minimum feature size, as well as the smoothness and chemical characteristics of etched surfaces are yet to be elucidated. Crucially, little is known about the two photon excitation and how it results in the desorption of material. It is the goal of our ongoing work to learn more about the etching mechanism, both to identify physical processes not observed elsewhere, as well as to assess the viability of UV etching for technological applications. (author)