Diamond

Diamond

Welcome to the Advanced Materials Research Laboratory (AMRL) CVD diamond group webpage! Diamond has many outstanding physical and chemical properties such as extreme hardness, low coefficient of friction, corrosion resistance, ability to emit electrons and the ability to transmit sound faster than any other material. Nanocrystalline diamond can exhibit optimum properties for many applications in high technology electronics, heat technology or mechanical technology. Low wear and low friction are obtained for ultra smooth films. New technologies are to be developed for the preparation of thin films by CVD with very good lamination to substrates such as cutting tools, bearings, or packaging materials. The group pursues research in several related areas: the fundamental growth processes involved in the plasma deposition of these materials (diamond, nanocrystalline diamond, diamond like carbon and carbon nanotubes); processing of such films for MEMS applications, electro-optical, mechanical and tribological characterization of these materials and exploring the applications in different fields (hard and wear resistant coatings, MEMS, bio-compatiable substrates, flat panel displays, surface acoustic wave (SAW)devices, rotary shaft seals, electrochemical electrodes etc.).

Diamond Films Grown by MPCVD Technique

Though the initial work in the synthesis of diamond at low temperature and pressure was carried out in nineteen fifties, the activities in the area of chemical vapor deposited (CVD) diamond picked up really in the early eighties. A variety of CVD techniques such as microwave plasma CVD (MPCVD), hot filament CVD (HFCVD), oxy-acetylene torch CVD, dc plasmas, arc discharges, plasma jet CVD etc. have been used for the synthesis of diamond films. It is also noted that MPCVD and HFCVD, which have remained the leading techniques, have been used widely by a number of workers both in laboratories and in industries.

Diamond formed by sp³ hybridized carbon atoms is a unique structure in nature. The C atoms in diamond structure form strong covalent bonds resulting from the hybridization of the outer shell s and p electrons. Its unique properties make it suitable for a variety of commercial applications. Not only it is used prominently as a gemstone, but it is also a very useful industrial material. The diamond coated cutting tools, abrasive wheels, polycrystalline diamond inserts, diamond heat sinks, are a few products used routinely in industry.

Generally a carbon precursor gas (CH₄, C₂H₂, etc.) with large concentration of hydrogen (H₂). Hydrogen abstraction reactions produce highly reactive radicals such as -CH₃ which then deposit on the substrate, forming C-C bonds. At CVD conditions, the substrate is usually between 700 and 1000 ^oC. High atomic H concentration on the growing surface helps to etch graphite preferentially and stabilizes the sp³ phase of carbon as diamond. Excess hydrogen also introduces a large number of impurities and defects in the film. A good quality film is mostly diamond and contains very little graphite. This is determined by characterizing the amount of sp³ bonding (diamond) over sp² bonding (graphite). A good quality film also has a well-formed crystal structure and few impurities.

Perfection in diamond growth is achieved only on some particular substrates. In the CVD process the ratio of CH₄/H₂, substrate temperature and deposition pressure are the three most important parameters, which control the film quality. Other parameters vary depending on the system and technique used. Production of atomic hydrogen in the gas phase and atomic H concentration on the substrate surface are very important for obtaining good quality diamond films.

Possible Applications for Diamond Films

These films are expected to be used in a variety of applications from cutting tools to wear-resistant parts, and from electronic to optical applications. One advantage of CVD diamond technology over the high-pressure technology is low cost and its ability to coat on any shape. Because of diamond's several unique properties, different applications could be separated on the basis of their specific unique property. Based on their highest hardness, excellent wear resistant property and low friction diamond films are used for Cutting Tools, Protective Coatings, and Composite Additives. All three of these uses are currently in practice or development for CVD Diamond. Thermal conductivity in diamond is five times higher than that of copper and at the same time it is electrically insulator, which make diamond as an ideal candidate for thermal management applications. Diamond is chemically inert and transparent from UV to the far IR spectrum. This makes it well suited for use as protective optic coatings or even IR windows. Wide band gap and with suitable dopant, diamond films are ideal for high temperature and high power electronics devices. Negative electron affinity makes diamond as an ideal candidate for field emission display applications. Electronic properties of diamond are also suited for field effect transistors, piezoelectric effect devices, radiation detectors and UV photodetectors.

Nanocrystalline Diamond Films

At low partial pressures of methane, highly crystalline, primarily sp³ bonded diamond films are obtained. As the partial pressure increases, the crystalline morphology disappears, and a "lower quality," more disordered film is formed. This form is more amorphous in structure - disordered graphite containing small clusters of diamond nanocrytals. In recent years, it has been found that growing at specific conditions between these two extremes can yield high quality (mostly sp³) nanocrystalline diamond films, which possess a much smoother surface and enhanced electronic properties.
In MPCVD system using Methane or Fullerene Precursors - Nanocrystalline films can be grown from methane or fullerene precursors, with or without the addition of molecular hydrogen and with the presence of argon as a carrier gas. C₂ dimer is the active species during the growth. In MPCVD system Nanocrystalline films can also be made without the addition of atomic hydrogen by using N₂/CH₄ as the reactant gas. Again, the C₂ dimer is the active species. The films again are smooth with randomly oriented grains, ranging in size from 10 to 30 nm. Using the substrate biasing in MPCVD system during multistep deposition, smooth and stress free NCD films can be grown.

It is well known that the grain size of a film strongly affects its properties; this can be attributed to the grain boundary density. Nanocrystalline diamond films with high grain boundary density have been attracted enormous interest due to its fascinating mechanical, electrical, and optical properties. Due to the negative electron affinity diamond is an optimum candidate for field electron emission (FEE), which has potential applications in the areas such as flat panel display and microelectronic devices. Nanocrystalline diamond has superior mechanical, tribological, and thermal properties suitable for the rapidly expanding field MEMS technology. MEMS is a manufacturing strategy that integrates miniature mechanical devices and semiconductor microcircuitry on a silicon chip. Nanocrystalline diamond films have been examined for electron field emission, optical transparency, and resistance to wear. In these three areas, nanocrystalline film performance has exceeded that of conventional diamond films.

Diamond Films Grown at NNRC, USF

Figure. MPCVD chamber at USF. Inset shows the plasma ball.