Metallisation and Silicidation of Porous Silicon
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METALLISATION AND SILICIDATION OF POROUS SILICON BERNARD J. AYLETT*, LYNDSAY G. EARWAKER**, AND JOHN M. KEEN*** *Dept. of Chemistry, Queen Mary and Westfield College, London El 4NS, U.K. **Dept. of Physics and Space Research, University of Birmingham, U.K. ***DRA-Royal Signals and Radar EstablishmenL Electronics Division, Malvern, U.K. ABSTRACT Thin films of cobalt, rhenium, or cobalt silicide have been deposited down the pores of porous silicon layers, using HCo(CO) 4 , HRe(CO) 5 , and SiH 3 Co(CO) 4 as precursors in a Chemical Vapour Infiltration and Decomposition (CVID) technique. INTRODUCTION The oxidation and nitridation of porous silicon (PS) to give regions with good insulating and/or dielectric properties has received much attention. By contrast, the conversion of porous silicon layers into conducting regions by metallisation or silicidation has been little studied. It is clear, however, that there are exciting possibilities for new VLSI structures if good conducting paths down into bulk silicon can be produced under mild conditions. Four methods have been reported to yield metal or metal silicide regions on or in porous silicon layers, namely (i) electrochemical deposition of metall; (ii) evaporation of metal on to the porous surface 2; (iii) metal or metal silicide deposition by MBE 3 ; and (iv) metal deposition by CVD, using WF6 as precursor4 . Only in the last case was good penetration of the pores achieved, but there is a risk that residual fluorides may lower the long-term stability of the resulting structure. In all cases, post-annealing of the metal deposit at elevated temperatures could lead to silicide formation, although this has not always been attempted. In this study, the potential of metal carbonyls and their derivatives to act as precursors for deposition down the pores was investigated. EXPERIMENTAL Apparatus. A conventional Pyrex high-vacuum system was used for the synthesis and handling of precursors. Solids were handled in a high-integrity glove box, purged with either argon or oxygen-free nitrogen. The techniques employed for analysis included Rutherford Back-Scattering (RBS), Nuclear Reaction Analysis (NRA), Scanning Electron Microscopy (SEM), and Extended X-ray Absorption Fine Structure analysis (EXAFS). The CVD apparatus was custom-built by Electro Gas Systems Ltd. of Cheadle, Cheshire. It comprised:(a) an inlet system for carrier gas, etc., metered by mass flow controller(s); (b) a precursor sample holder maintained at a constant temperature in the range -40 to +90 0 C (± 0.51C) by a thermostatted bath; (c) a horizontal quartz cylindrical reaction chamber; (d) a sample holder consisting of a block of single-crystal silicon; (e) an infrared lamp allowing rapid ramping of the substrate temperature up to about 800 0 C; (f) a pumping system giving an ultimate vacuum of better than lx l06 torr; and (g) a control system. Precursors. Cobalt carbonyl, Co2(CO)8, (Alfa Inorganics) was purified by sublimation in vacuo. Cobalt carbonyl hydride, HCo(CO) 4 , was first synthesised using the publis
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