ABSTRACTINVESTIGATION OF DIAMOND ETCHING BY A MICROWAVE PLASMA-ASSISTED SYSTEMByDzung Tri TranDiamond deposition technology advances have opened several potential applications for diamond-based devices and components. Many diamond applications, such as micro-electro-mechanical systems (MEMS) fabrication and electronic devices, require the micro-structuring of the diamond and other applications, such as optical and thermal management components, require smoothing the diamond surface. Because of the high chemical inertness property of diamond, a key technique for micro-structuring and surface modification of diamond is plasma-assisted etching. The objective of this study is to investigate and develop processes and the associated understanding of plasma-assisted etching of diamond for micro-structuring and smoothing of diamond substrates. The etching of three types of chemical vapor deposition (CVD) diamond including nanocrystalline diamond (NCD), microcrystalline diamond (MCD) and single crystal diamond (SCD) is investigated using a 2.45 GHz microwave plasma-assisted etching reactor system. The plasma reactor has a 25 cm diameter discharge located inside a 30 cm diameter cavity applicator and it has an independent rf bias capability for the substrate holder that facilitates ion energy controlled reactive ion etching at low pressures. The feed gases for the etching process include mixtures of oxygen (O2), sulphur hexafluoride (SF6), and argon (Ar). The etching reactor operation is investigated for both magnetized electron cyclotron resonance (ECR) and non-magnetized plasma operation for the pressure ranges of 1-40 mTorr and 4-100 mTorr, respectively. The plasma characteristics are investigated using visual plasma discharge observations and single Langmuir probe measurements. For both ECR and non-magnetized plasma reactor operation, a high density plasma with charge particle densities of is obtained. The etch rate, anisotropic etching profile, and surface roughness are measured versus input etching reactor parameters including pressure, substrate bias, microwave power and gas mixtures. Anisotropic etching is demonstrated and the measured etching rates range from 4 - 15 μm/h. A highly anisotropic etching profile is obtained at a pressure of 4 mTorr. The selectivity of the plasma-assisted diamond etching process is measured for various mask materials including aluminum, gold, titanium, silicon dioxide and silicon nitride. Aluminum gave the highest selectivity with a value of 56. The use of the plasma-assisted diamond etching process is also investigated for the smoothing or polishing of rough microcrystalline diamond (MCD) surfaces. Three plasma-assisted polishing methods investigated include the use of (1) plasma-assisted etching of MCD films coated with a sacrificial layer and etched with a selectivity of one, (2) photo-resist reflow on the rough MCD surface to expose the high portions of the MCD sample, and (3) micro-roughing of the surface by plasma-assisted etching prior to mechanical polishing. The surface roughness reduction rate and the final surface roughness obtained by the three techniques are studied and comparisons are made. The plasma-assisted smoothing of MCD samples from a surface roughness of 3800 nm down to 50 nm is demonstrated.
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