Synthesis Induced by Laser Irradiation at Liquid/Solid Interfaces
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SYNTHESIS INDUCED BY LASER IRRADIATION AT LIQUID/SOLID INTERFACES SIU-WAI CHAN,
D. DIJKKAMP, X.D. WU, T. VENKATESAN AND C.C. CHANG
&ell Communications Research, Inc., Red Bank, NJ 07701 Physics Department, Rutgers University, Piscataway, NJ 08854 ABSTRACT Synthesis of transition metal nitrides induced by excimer laser irradiation of metal foils immersed in liquid nitrogen was studied. Nitrogen incorporation was found by sputtering Auger Electron Spectroscopy in all six investigated metals (Ti, Zr, Hf, V, Nb and Ta). X-ray diffraction was used to determine the compounds formed. The morphologies of the irradiated surfaces often indicated that the surfaces had gone through a molten state. Micronthick mononitrides were grown on Ti, Zr and Hf, whereas Ta and Nb formed Ta2 N and Nb N respectively. The nitrides on V were not satisfactorily identified. The extent of nitridation correlates with the thermodynamic driving force, i.e. the Gibbs free energy for nitride formation. INTRODUCTION When a molten metal surface is in contact with a large supply of reactants a significant amount of reaction may occur at the interface. This idea has been utilized for the formation of niobium nitrides by pulsed ruby laser irradiation of Nb immersed in liquid nitrogen [1] and very recently for the production of titanium and zirconium nitrides using a pulsed CO 2 laser [2]. Transition metal nitrides have been reported to be high temperature superconducting materials [3]. Besides, TiN and ZrN are excellent diffusion barriers for use in VLSI-technology [4,5]. The high heating and cooling rates which can be obtained in pulsed laser irradiation may result in the formation of metastable phases with desirable properties. In addition, laser synthesis offers the possibility of direct beam writing of nitride patterns. In this paper we report the results of a systematic investigation into the
feasibility of pulsed laser fabrication of nitrides on transition metals immersed in liquid nitrogen. EXPERIMENT A Lambda Physik EMG-200 KrF excimer laser (248 nm, 30 ns) with a maximum output of 1 J/shot was used. The beam was collimated to assure a homogeneous energy density and focussed by a quartz lens with a 25 cm focal length. The energy density as a function of distance to the quartz lens was calibrated by positioning an aperture with known area in the center of the beam and measuring the transmitted energy with a GenTec model 500-ED Joulemeter. An aluminum coated mirror with calibrated reflectivity was used to direct the laser beam onto the samples which were secured in a styrofoam container filled with )iquid nitrogen. The energy density was varied between 0.5 and 9 J/cm by translation of the styrofoam container. This method was preferred above the admittedly more accurate use of attenuators, in order to
obtain sufficiently large beam spots. The quoted energy densities are accurate to within 20 %. Metal foils of 0.25 mm thickness and more than 99.5 % purity supplied by Johnson Matthey, Inc., were used. A liquid nitrogen layer of about 2 mm was
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