Noble Gas Incorporation in Sputtered and Ion Beam Assisted Grown Silicon Films
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NOBLE GAS INCORPORATION IN SPUTTERED AND ION BEAM ASSISTED GROWN SILICON FILMS * A. van Veen, Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands. ** M.J.W. Greuter, L. Niesen, Department of Physics, University of Groningen, Nijenborgh 4, NL-9747 AG Groningen, The Netherlands. *** B. Nielsen, K.G. Lynn, Brookhaven National Laboratory, Physics Department, Upton NY, USA.
ABSTRACT Gas desorption measurements have been performed on sputter deposited silicon films. The sputter gas was argon or krypton. Parameters influencing the incorporation process e.g. bias voltage, substrate temperature and arrival rate ratio of silicon and noble gas atoms have been systematically varied. The films, a-Si and c-Si, have been characterised by various techniques for composition and defect analysis. A model has been applied to describe the composition of the growing silicon layer. Underlying mechanisms like gas-gas sputtering have been studied in separate ion implantation experiments. For a-Si concentrations as high as 6% Ar and Kr have been found. An important effect is the injection of self-interstitial atoms caused by the low energy heavy ion bombardment. It causes the layer to grow without large open volume defects.
INTRODUCTION Low energy ion bombardment is employed to assist and control silicon film growth (1]. In sputter deposition a negative bias at the substrate causes a considerable flux of low energy (Ar)-ions hitting the growing film. In ion assisted MBE an extra ion gun can provide the required ions. Beneficial effects on the growth process have been reported e.g. Ohmi et al [2] found growth of homoepitaxial Si-films at lower temperature compared to conventional thermal molecular epitaxy (MBE). Chapman has reviewed the early work on thin film production by noble gas sputtering [3]. Gas release from sputtered silicon films has been reported by Taoufik et al [5] and van Veen et al [6]. Results on the basic incorporation processes of noble gas atoms in metals have been reported by Kornelsen [6], Edwards [7] and van Veen [8]. In this article experimental results are presented for gas incorporation in sputtered silicon films. It is tried to model the incorporotion process quantitatively.
EXPERIMENTAL Thin films were produced by sputtering of silicon in a low pressure (0.01-0.1 Pa) argon or krypton plasma similarly as described in [8]. The standard energy of the noble gas ions striking the target was 1 keV and 6 2 the standard flux amounted to 6x101 cm" s-1. The bias voltage of the substrate could be varied from 0-300 V, and the temperature could be controlled from 350 - 900 K. The ion flux at the substrate varied from 02 1016 cm- s-1 depending on the bias voltage. The ion flux could be reduced without changing the plasma parameters by applying a pulsed bias voltage to the substrate (duty cycle from 10-100%). The flux of sputtered silicon atoms arriving at the substrate was calibrated by means of a quartz crystal microbalance and by weighing of the deposited film. Film
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