A New Low Temperature Thin Film Deposition Process: Energetic Cluster Impact (ECI)
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ABSTRACT A beam of metal cluster ions of variable size is deposited with variable kinetic energy on a substrate. Mirror-like and strongly adhering films are produced on unheated substrates for sufficiently high cluster impact energies. Numerical simulations provide the physical insight why this novel technique gives different, and sometimes superior results compared to conventional methods. Several examples are presented. I. INTRODUCTION Two parameters influence most thin film properties: the kinetic energy of the arriving atoms and the temperature of the substrate. Both parameters determine the surface mobility, if one is reduced, the other one has to be increased. We have developed a new physical vapor deposition method [1, 2] where a very high temperature (several thousand K) is generated at each impact, thus reducing the influence of the substrate temperature considerably. An intense beam of ionized large metal clusters (like AlCo+ 0 , Cu+ Ti+, or (TiN)o 0 0 is produced, electrically accelerated, and deposited with variable kinetic energy on a surface. A numerical simulation [3, 4] of a single impact is shown in Fig. 1. The top figure shows a Culoo0 cluster hovering over the surface. Its kinetic energy is 10 eV/atom, or 10 keV. Both cluster and surface have a temperature of 300 K. The time after the first contact (in units of 10-12 s) is indicated in each frame. At 0.2 ps, about half of the cluster has dived into the surface, which had little time to react. When the first atoms in the cluster get into contact with the surface they are scattered backwards. The next line of atoms in the cluster hit these recoiling atoms, etc. This produces a "nano" shock wave [1] which moves into the surface. Due to this strong compression, a pressure of about 1011 Pa and a temperature of 3.000 K is generated locally. At 0.5 ps, the nano shock wave has moved into the substrate, leaving behind a small temporary crater. At 1 and 5 ps, the compressed material expands, while about 50 ps later the area has cooled and a well annealed deposit is left. These extreme conditions lead to a solid anchoring of the deposited material. Because of the high local temperature caused by the impact of such a massive object, there is only little influence of the substrate temperature on the outcome of a single collision event. In the experiment, each spot is hit by a cluster about every second, which is about 11 orders of magnitude longer than a typical deposition event, so that diffusion processes between the cluster impacts can of course be influenced by the substrate temperature. Deposition by clusters was pioneered by Takagi and Yamada of the university of Kyoto, who called their process ICB, short for ionized cluster beam deposition [5]. The two methods are quite different as discussed below, so that we choose to call our method ECI, short for energetic cluster impact. 207 Mat. Res. Soc. Symp. Proc. Vol. 388 01995 Materials Research Society
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Fig. 1: Langevin Molecular Dynamics Simulation of an impact of a Culwoo cluster on a Cu surface-with
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