The Sputter Deposition of Metal Multilayers
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THE SPUTTER DEPOSITION OF METAL MULTILAYERS R.E. SOMEKH, R.J. HIGHMORE, K. PAGE, R.J. HOME and Z.H. BARBER Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 SQZ, UK.
ABSTRACT We describe the strategy that we are using to make precision metal multilayers. Differential scanning calorimetry has been used with the Ni/Zr system to study the abruptness of the interface as a function of the sputtering pressure. For 1Onm period multilayers there is a monotonic increase in the width of the interface with increasing sputtering pressure. W/Si multilayers have been studied as a function of both the sputtering pressure and the relative thicknesses of tungsten and silicon. At reasonably low sputtering pressures a well textured (110) tungsten X-ray peak is seen which is compatible with the expected thickness of the tungsten layers. Finally, we report some preliminary work on sputtering from tungsten and silicon targets which are at different distances from the substrate so that the degrees of bombardment on the growing layers of the film can be independently varied. INTRODUCTION In the last few years the possibility of depositing a metal multilayer in which there is exact atomic registry from top to bottom of a thin film has come closer to reality. In the pursuit of such an objective we have been developing means of preparing precisely stacked multilayers. Various levels of ideality can be envisaged, the ultimate objective being to prepare a single crystal multilayer with atomically fiat interfaces. This would require a deposition rate stability and accuracy of 0.01% through the thickness of, for example, a 300 nm thick film in order to guarantee near-perfect atomic register. Polycrystalline or amorphous multilayers may be produced which would necessarily have less well defined interfaces but which could have equally good registry. Whether one can make atomically sharp interfaces with a single crystal system will probably ultimately depend upon the purity of the deposition system. Very high surface mobilities at modest temperatures minimise the surface/sub-surface diffusion process, as described by Schneider et al. [1]. Single crystal multilayers rely for their perfection on being prepared under near equilibrium thermodynamic conditions and minimal energy must be injected to ensure the best quality [2]. This is not true for polycrystalline and amorphous systems in which energetic bombardment during deposition aids mobility and promotes smoothness. In the case of single crystal multilayers smoothness and flatness arise from thermally induced surface mobility, and the distance between ledges in the layer by layer growth process is determined by thermal kinetics. This process is disrupted by any very energetic atoms which may create defects at the surface of the growing film. However, in order to deposit polycrystalline or amorphous multilayers mobility must be promoted to keep the film fiat as it grows. This can be achieved by bombarding the surface with energetic particles. I
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