Chemical and Physical Modification of Diamond Surfaces

Kealey, Christopher P (1999) Chemical and Physical Modification of Diamond Surfaces. PhD thesis, University of Glasgow.

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In recent years it has become possible to grow a film of diamond, on substrates such as silicon, at low temperature and pressure by a process called chemical vapour deposition or CVD. These films have been found to exhibit properties comparable to those of natural diamond. As a result there has been a surge of interest in the use of the material as an alternative to natural, or HPHT (High Pressure High Temperature) synthetic diamond, in optical and microelectronic applications. Unlike natural or HPHT diamond, which are monocrystalline, these films are polycrystalline and have a rough surface. For many of the potential uses, such as a heat sink in microelectronic devices, smooth surfaces on the nanometre scale are required. Polishing is seen to be one way of smoothing the rough surface of a CVD diamond film, however traditional methods, using harsh mechanical conditions have proved unsatisfactory often resulting in a damaged film. The development of a chemomechanical polishing process, involving chemical reaction at the diamond surface and subsequent removal of of surface species formed, is one possible alternative to the traditional mechanical methods. The first step in the development of a chemomechanical polishing process involves a detailed study of the chemistry possible on diamond and in particular those reactions, which may involve etching of the diamond surface. In this work the behaviour of various fluorine compounds towards diamond films and powders has been determined. Reactions of F2 or CIF3 with hydrogen-pretreated diamond at ambient temperature led to the removal of hydrogen as HF or HCl. DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) of powder samples has shown the loss of C-H and its replacement with C-F. When the reaction temperature is increased to 673 K, analysis of the gas phase using FTIR spectroscopy showed the presence of a fluorocarbon mixture and DRIFTS analysis of the diamond surface showed bands corresponding to CF, CF2 and CF3 moieties. Etching of :CF2 from the diamond surface and the subsequent reactions of the former is believed to be responsible for the fluorocarbon mixture. However, SEM (Scanning Electron Microscopy) analysis of film samples following reaction at 673 K revealed that a massive etching reaction had not occurred, since the gross morphology of the film surface was maintained. EFTEM (Energy-Filtered Transmission Electron Microscopy) analysis on the nanometre scale was more informative, this technique allowed the density of sp2/sp3 bonding character in the sample to be determined. The carbon environment at grain boundaries and grain edges was shown to be predominately sp2 prior to reaction with CIF3 and sp3 after reaction with CIF3. This indicated a propensity for reaction at grain boundaries or crystal edges and these are expected to be the likely sites for etching of carbon. XPS (X-ray Photoelectron Spectroscopy) allowed quantitative analysis of the fluorinated diamond surface; carbon to fluorine ratios could be calculated. The key feature of this part of the work was evidence for a subsurface reaction at 523 K, resulting in the formation of a fluorinated layer, with a greater than monolayer thickness. A model for the structure of this layer was provided by comparison with the related compound, graphite fluoride (CF)n. Fluorinated diamond surfaces have been shown to be stable to further reaction with F2 or CIF3, a feature in common with many carbon-fluorine compounds. This indicates the presence of a kinetic barrier t extensive reaction, which accounts for the absence of a gross change in the morphology of fluorine-treated film surfaces. The interaction of [18F]-HF ([18F] t1/2 110 min, beta+-emitter) has been used to probe the extent and type of interactions that are possible between HF and a variety of diamond surfaces. Standard pretreatments were hydrogenation (99% H2, 1173 K, 1 h) or oxygenation (hydrogenation + 20% O2/Ar, 673 K, 1 h). Fluorinations were performed with F2, CIF3 or HF at ambient temperature or at 673 K with F2 or CIF3 and 573 K with HF. The extent of [18F] uptake on the surface was readily detected in all cases, even when H18F treatment was at ambient temperature. It has been established that the fluorine laid down on the surface is labile and is subject to hydrolysis upon exposure to moist air. Fluorine exchange reactions and DRIFTS analysis were used. These facts are not consistent with C-F termination of the surface in the reaction with HF, and this suggests a very different behaviour when compared with the reaction of F2 with diamond. Studies in the polishing behaviour of fluorine-treated diamond surfaces by traditional methods and the use of fluorine containing reagents in solution, such as [HF2]-, had limited success. The outcome of the experiments could be rationalised in terms of the limited fluorination using F2 or CIF3 and the lack of interaction between HF and diamond surfaces in an aqueous environment. The knowledge gained in the study of diamond surface chemistry had an important role in the development of a promising process for the polishing of diamond film.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: John M Winfield
Keywords: Physical chemistry
Date of Award: 1999
Depositing User: Enlighten Team
Unique ID: glathesis:1999-76345
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 15:23
Last Modified: 19 Nov 2019 15:23

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