The development of technologies and technologies capable of depositing continuous thin diamonds, semiconductors and advances in thin film deposition have inspired the diamond-like carbon film industry. These new technologies enable the deposition of diamond thin films on the surface of both semiconducting and non-semiconducting materials, which can be used in a variety of applications, both electronic and non-electronic. Two different coating methods have been developed, one relying on a sustained excited plasma discharge from a low-pressure atmosphere of hydrocarbon gas (plasma-assisted CVD), and one relying on the direct deposition of carbon films, With or without simultaneous bombardment by a high flux of energetic ions (ion beam enhanced deposition).
Plasma Assisted Chemical Vapor Deposition (PACVD) Technology
Diamond-like carbon films are deposited using plasma-assisted CVD techniques by exciting a hydrogen-hydrocarbon-argon mixture in glow discharge9-15 or microwave irradiation16-18 (Fig. 34.1). In both cases, a plasma is generated and free carbon atoms are generated through thermal decomposition of hydrocarbon gas components. The carbon atoms released in the plasma have sufficient energy to form tetragonal carbon-carbon (diamond) bonds, enabling the condensation of diamond and diamond-like carbon films. The films produced are actually a mixture of triangularly bonded carbon (graphite), tetragonally bonded carbon (diamond), and other allotropic crystal forms.
To dissociate the hydrocarbon starting gas and provide sufficient thermal energy to form trigonal and/or tetragonal carbon bonds, temperatures in the plasma discharge need to exceed 2000°F. Deposition rates of approximately 1 μm/h can be achieved. The presence of free hydrogen in the process gas helps to promote film growth with a higher concentration of tetragonally bonded diamond than trigonally bonded graphite. This is because graphite bonds are more chemically reactive than diamond bonds, leading to selective etching of the graphite component of the film by free hydrogen gas in the plasma. However, the presence of hydrogen in the deposited diamond films can generate high levels of tensile stress, which can lead to embrittlement and buckling.
Figure 34.1 Synthesis of diamond films using the plasma-assisted chemical vapor deposition process (PACVD). A range of hydrogenated diamond and diamond-like carbon thin film structures can be produced at low pressures and temperatures in the 2000°F range.
Applications for diamond films formed by PACVD are limited to those where substrates can be heated to temperatures in excess of 2000°F. Furthermore, the diamond films deposited by this technique are grown epitaxially and are therefore recommended for condensation and adhesion on crystalline substrates such as silicon and germanium. Therefore, diamond thin films deposited by PACVD technique are most suitable for applications in semiconductor structures and devices. Field Effect Transistors are fabricated by forming Ohmic and Schottky contacts on a semiconducting diamond substrate deposited by a CVD process. These films can also be used as passivation layers on integrated circuits because they can be made very hard and resistant to acids, bases and organic solvents. The formation of diamond-like carbon films on metal, ceramic, and of course plastic substrates requires techniques that do not require high temperatures and are insensitive to the metallurgy of the substrate to be coated.
Ion Beam Enhanced Deposition (DIAMOND)
By bombarding thin films of carbon atoms with energetic ions, the high temperatures and pressures required to form tetragonally bonded diamond structures can be provided at the microscopic level. Ions bombarded with energies as low as 100 eV produce picosecond temperature and pressure spikes at target surfaces exceeding 7300°F and 120,000 atm. Thermal agitation and shock waves accompanied by ion impact at 100 eV can form single crystal diamond nuclei with a diameter of approximately 1 nm. Thus, under ion bombardment, carbon atoms in a continuous film deposited on a substrate surface can combine at the surface to form all possible carbon-carbon bond combinations, including the uniform tetragonal bond exhibited by natural diamond.
Ion beam-based deposition processes can be achieved by directly accelerating and implanting carbon atoms20-23 or by depositing thin carbon films while bombarding the films with an additional high-energy ion beam24-27 or by depositing thin carbon films while bombarding the films with additional high-energy ion beams. Ion beam,24-27 is called ion beam enhanced deposition. The latter technique is more flexible because a wider range of film morphologies and properties can be produced, higher deposition rates can be achieved, and a wider variety of materials can be coated. In the ion beam enhanced deposition process, a carbon film is deposited on the surface to be diamond-coated while irradiating the surface with a secondary high-energy ion beam (Fig. 34.2). The energy of the carbon atoms can be changed during the deposition process, as can the energy of the secondary ion beam. No additional energy in the form of heat is required, so coating temperatures can be maintained below 150°F. Additionally, these films are initially mixed into the surface being coated, allowing them to be easily formed on virtually any substrate and optimizing film adhesion.
