Laser Assisted Deposition of Amorphous Hydrogen Containing Carbon at Room Temperature
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ABSTRACT Hydrogen-containing amorphous carbon films were produced from CH,I, by photolytical laser assisted chemical vapour deposition at room temperture. An Ar* laser operated in the ultraviolet spectral region (351 and 364 nm) was used as the excitation source. The beam was focused in parallel to the Si(100) substrate. The deposition process was studied for different partial pressures of CHI 2 and for different heights of the laser beam above the substate surface. The mass thickness of the film was determined by energy dispersive X-ray spectrometry. The hydrogen content of the carbon films was determined by nuclear reaction analysis and the iodine content in the films was obtained by Auger electron spectroscopy and energy dispersive Xray spectrometry. Carbon hybridization was investigated by Raman and infrared spectroscopy while morphology was examined by atomic force microscopy. INTRODUCTION Amorphous carbon films often contain hydrogen if the precursor is a hydrocarbon. A low hydrogen content often results in dense, hard and optically opaque films [I ).An increased amount of hydrogen will, in general, decrease the density and produce a softer film which still may be optically opaque. The most common technique for production of amorphous hydrogen containing carbon (a-C:H) films is plasma assisted chemical vapour deposition [2] where volatile hydrocarbons are used. Aromatic hydrocarbons, such as benzene and toluene, have been used to produce a-C:H films by excimer laser assisted chemical vapour deposition [3]. These species generally have a large cross-section for ultraviolet light. In this paper a-C:H films were produced by photolytic laser assisted chemical vapour deposition of CH,I, which absorbs photons having a wavelength of 350 nm. Absorption of a photon results in dissociation of CH2 12 with a subsequent release of an iodine atom [4]. This investigation was performed at room temperature where the influence of experimental parameters such as precursor partial pressure and beam/substrate distances on the hydrogen and iodine content of the film was investigated. EXPERIMENTAL In these experiments the near UV lines (351 and 364 nm) of an Ar÷ laser were used. The laser beam was focused by three lenses, as shown in Fig 1.The first lens,with a focal length of 80 mm, was used to expand the beam to achieve a lower power density on the entrance window. The second lens with a focal length of 400 mm was used to achieve a parallel beam. Finally the last lens with a focal length of 250 mm was used to focus the beam in front of the sample and in parallel to the substrate. The reactor was made of stainless steel and the gas flows were controlled with mass flow contollers (MFC). Argon was used as purge gas in order to prevent deposition on the laser entrance window during the experiments. The partial pressure of CH,I was set by the temperature in a refluxer using argon as the carrier gas. The sample was placed in the reactor on a holder with a parallel configuration in order to irradiate only the gas phase and not 613 Mat.
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