Diffusion Instability and Tapering of Nickel Silicide Intrusions in Silicon Nanowires
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Diffusion Instability and Tapering of Nickel Silicide Intrusions in Silicon Nanowires Alex Katsman1, Michael Beregovsky2, and Yuval E. Yaish2 1 2
Department of Materials Engineering, Technion, Haifa 32000, Israel Department of Electrical Engineering, Technion, Haifa 32000, Israel
ABSTRACT Thermally activated axial intrusion of nickel silicides into the silicon nanowire (NW) from prepatterned Ni reservoirs is used in formation of nickel silicide/silicon contacts in SiNW field effect transistors. This intrusion consists usually of different nickel silicide phases which grow simultaneously during thermal annealing (TA). The growth is often accompanied by local thickening and tapering of the NW, up to full disintegration of segments adjacent to the silicon. In the present work this process was investigated in SiNWs of 30-60 nm in diameters with prepatterned Ni electrodes after a TA at 420-440qC and times up to 15 s. The process was analyzed in the framework of a model taking into account simultaneous formation of two silicide phases in the NW. Additional flux of atoms caused by the NW curvature gradients due to different radii of different silicides was taken into account as well. For a certain set of parameters thickening of the nickel-rich silicide intrusion and tapering of the monosilicide part of intrusion were obtained. INTRODUCTION Field-effect transistors (FETs) can be fabricated based on NiSi/Si nanowire (NW) heterostructures in which the source-drain contacts are defined by the metallic NiSi NW regions [1-10]. Thermally activated axial intrusion of nickel silicides into the SiNW from pre-patterned Ni reservoirs is used in formation of nickel silicide/silicon contacts in SiNW FETs. However, the formation of a precisely controlled silicon/silicide nanostructure remains a challenging problem in nanotechnology today. The axial intrusion consists usually of different nickel silicides which grow simultaneously during thermal annealing (TA) [8, 9]. The growth is often accompanied by local thickening and tapering of the silicide NW up to full disintegration of the NW segment adjacent to Si. Up to date, comprehensive understanding of these processes is still missing. In the present work diffusion growth and possible instability of two-phase nickel silicide intrusions in SiNWs were investigated at temperatures of 420-440qC and times up to 15 s. EXPERIMENTAL DETAILS The SiNWs were synthesized by the vapor-liquid-solid growth technique in an ultrahigh vacuum chemical vapor deposition chamber, with silane as the silicon precursor and gold as the catalyst [11, 12]. Typical diameters of NWs grown in the and directions were 35-60 nm and 25-40 nm, respectively. Samples were fabricated by a method described previously [8], with the lone exception being that the SiNWs were randomly dispensed onto a sacrificial 150 nmthick resist layer (deposited on the Si3N4/Si substrate) which allowed to obtain a suspended SiNW structure. In order to form nickel silicide/silicon contacts, a series of anneals of the Ni/SiNW sample were perform
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