Study of Wsi 2 and Cosi 2 Thin Films Deposited by Laser Ablation
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and films deposition. Laser ablation has a number of advantages compared with common films preparation techniques [2]. It is compatible with high vacuum apparatures and does not require a working gas. With the laser radiation flux density exceeding 10i" W/m 2 in the operative zone, the target material sputtered occurs without liquid phase formation, and laser ablation occurs, as a result of which the film stoichiometry conforms to the target chemical composition. This opens up reasonable possibilities of using lasers to prepare films of materials with complex stoichiometry. The high effective rate (105 A/s) enables one to reduce contamination of the films by residual gases of vacuum system. Owing to the high energy of the condensate atoms (T z 105 K), it becomes possible to obtain epitaxial films at lower substrate temperatures than with other techniques. The possibility of obtaining cobalt and tungsten disilicide films by laser ablation of high purity cast targets, prepared by vacuum-melting techniques, as well as results of experimental studies of the films CoSi2 and WSi 2 , it was reported. EXPERIMENTAL DETAILS The thin films of WSi 2 and CoSi 2 were deposited by laser ablation in vacuum, using set up shown in Fig. 1. A solid neodymium glass laser was used; the wavelength was 1.06 pm, the pulse frequency 10 Hz and the pulse energy 0.8 J. The vacuum chamber was evacuated 659 Mat. Res. Soc. Symp. Proc. Vol. 354 a 1995 Materials Research Society
to a pressure of - 1 10-' Pa by 500 L/s a turbomolecular pump The laser beam was focused by a lens to a spot size of 2-3 mm. The distance from the target to the substrate was 70 mm. The substrates were heated by an IR heater with halogen lamps. The substrates were Si(100) and (111) wafers. Phosphorus - doped silicon wafers (4-7 fl/cm) were chemically cleaned usimg standard processes. The samples were then dipped into dilute HF solution for 2.5 min before they were loaded in the laser deposition chamber. Immediately before the evaporation the substrates were heated in vacuum at 8500 C for 10min. The substrate temperature was varied within 100-8000 C and the deposition time was 5-10 min. The film thickness measured by profilometry.
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Fig. 1. Experimental set-up for laser ablation and deposition thin films.
The electrical resistivity of both the cast targets and deposited thin films was measured by a four-probe method. X-ray phase analysis of the cast specimens was performed using a Siemems D-500 diffractometr; local X-ray spectral analysis was performed with a JXA5 apparatus. The phase composition of the films was studied using a complex D/max Rc Rigaku diffractometer in the asymetric grazing incidence Bragg geometry. The X-ray incidence angle (CuK 0 ) was 0.1-5'. The materials used to prepare cobalt and tungsten disilicides targets were commercial 5N purity silicon and vacuum -zone refining in a crucibleless electron beam zone furnace, tungsten and cobalt roads. Typical contents of impurities in the cast zone - refined
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