Photon stimulated CVD of amorphous carbon thin films : By Mikael Lindstram

Abstract: Amorphous carbon has attracted much interest for several decades now because of its complex crystal structure and microstructure. Depending on the carbon sp2/sp3 bonding ratio, these materials yield a broad spectrum of useful properties; which can be tailored for specific applications. Room- and low-temperature deposition of amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) films have here been investigated using photon stimulation from UV -lasers and a broadband halogen lamp.Laser-assisted photolytic dissociation of CH2I2 at room temperature resulted in a-C:H films. Depending on the laser wavelength, these films contained up to 50 % hydrogen and up to 18 % iodine. Optimising the 275-305 nm and 350 nm laser light deposition processes reduced the iodine content of the films to less than 1 %. The mechanism for film formation differs for 248 nm laser light, where a subsequent two -photon process is believed to remove both iodine atoms homogeneously. The resulting films contain larger amounts of iodine and smaller amounts of hydrogen than films deposited using275-305 nm and 350 nm laser light. Model calculations could describe well thekinetics and the iodine incorporation into the films as a function of the process parameters.Use of a broadband halogen lamp as the activating source resulted in a-C films from the molecule CH2I2. Irradiation was made perpendicular to the substrate surface. Depending on the substrate and deposition temperature, thermal deposition or photolytic deposition dominated the growth process initially. When an intact a-C layer had been formed, the process became primarily thermal. Less than 1 % hydrogen incorporation into the a-C films resulted from the lamp process. The low hydrogen content is explained by the thermal process leading to dehydration of the growing a-Cfilm. The films were nano-crystalline, mirror like, hard, and had a low friction coefficient combined with a high elasticity. These unique properties are attractive in many areas of application: tribology, corrosion protection, medical implants, etc. Finally, modelling was able to describe well the kinetics of the process.