Diamond layers produced by chemical-vapor deposition (CVD) have been used in industry for more than two decades as a result of the outstanding properties of this material. Diamond excels in certain applications because of its high thermal conductivity, mechanical hardness, and optical properties. The conventional CVD process, however, has a slow processing speed. The process requires a vacuum in the millibar range, or at least a reaction chamber, so that layer growth rates amount to no more than a few microns per hour. In addition, the vacuum vessel and pump system are costly and the free space in the vessel is restricted.
The Bremer Institut für Angewandte Strahltechnik (BIAS; Bremen, Germany), the Federführung des Instituts für Strahlwerkzeuge (Stuttgart, Germany), and the General Physics Institute (Moscow, Russia) have collaborated to develop a technique based on laser-produced plasma that could greatly improve polycrystalline diamond deposition in industrial production.1 The process speeds up the deposition rate by nearly two orders of magnitude and allows processing at atmospheric pressure.
The researchers at BIAS have developed a so-called photon plasmatron, which operates in the open air and generates an optical discharge in a carrier gas mixed with a carbon-containing gas such as carbon dioxide (CO2) or methane at atmospheric pressure so that the workpiece can be easily moved below the processing zone (see figure).2 The optical discharge is a continuously burning plasma generated in the focus of the radiation of a continuous-wave CO2 laser with up to 12 kW optical power.
The plasma is generated and maintained by the absorption of laser radiation, essentially provided by the inverse Bremsstrahlung effect, in which free electrons colliding with molecules and atoms gain energy from radiation and cause increased ionization. A steady state is reached when the losses from the plasma cloud into the free atmosphere balance the energy input. "The plasma is a kind of continuous flash in which temperature rises to about 15,000 to 20,000 K, at which point you could easily melt bricks," says Simeon Metev, head of the research group at BIAS. To start the process, the plasma must be ignited by generating a few free electrons—for example, by thermal emission from a wire.
The researchers had to investigate the plasma and the reactant conditions to find the appropriate processing parameters that produce diamond layers and not soot. As in low-pressure CVD, optimum diamond deposition occurs at elevated surface temperature in the range 800°C to 1000°C, at which maximum growth rate and highest layer quality are obtained. Different carbon-containing gases as well as carrier gases such as argon, nitrogen, oxygen, hydrogen, and mixtures were investigated. Using molecules, photodissociation becomes a process competing with ionization, allowing the optimal percentage of admixture to be found.
Several analytical methods were applied to make sure that diamond layers had been generated. Raman spectra were taken across a layer of carbon deposited by a photon plasmatron. Traces reveal the Raman response of different carbon phases. From the similar relative heights of measured spectral peaks, it could be concluded that diamond amounts only to a fraction of all types of carbon produced; however, it is well known that Raman spectroscopy is about two orders of magnitude less sensitive for the diamond peaks than for other peaks, so that in fact diamond is by far the most prevalent phase of carbon in the central part of the layer. This conclusion is supported by Auger spectroscopic measurements, too, but most evidently by microscopic views of the deposited layers showing the diamond crystallite.
REFERENCES
- V. I. Konov, et al., Appl. Phys. A Mater. Sci. and Proc. 66, 575 (1998).
- S. Metev et al., "New technology for high rate synthesis of PC-diamond coatings in air with photon plasmatron," Diamond and Related Materials, to be published.