Effect of dopant implantation on the properties of TaSi2/poly-Si composites

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Abstract
Refractory metal silicide/poly‐Si composites are replacing poly‐Si in VLSI circuits in order to reduce runner resistance without sacrificing poly‐Si MOS compatibility. In a typical integrated circuit (IC) fabrication sequence, the gate serves as a mask for the formation of self‐aligned sources and drains. It is, therefore, exposed to various heavy implants of B, P, or As. These implants subsequently redistribute during further processing. This paper investigates the effect of dopant implantation and redistribution on the mechanical, structural and electrical properties of silicided poly‐Si. Implantation increases the sheet resistance of 2500 Å of sintered TaSi2 by a factor of 2–3. This coincides with a decrease in the silicide stress, proportional to the implant range in TaSi2, and to a lesser degree, the implant dose. These stress changes, however, recover by 500 °C, while the TaSi2 resistivity returns to its nominal value (2–2.5Ω/⧠) following a 900 °C heat cycle. Doses as high as 1E16As/cm2 do not render TaSi2 amorphous. If the underlying LPCVD poly‐Si is initially undoped, the nature of the impurity appears to influence subsequent poly‐Si recrystallization. With P/As, the poly‐Si grains grow during a high temperature anneal. However, with B, the grains remain needle‐like. This recrystallized poly‐Si grain structure is nevertheless significantly different from the large, equiaxed grains developed when the poly‐Si is doped in a PBr3 gas flow, prior to TaSi2 deposition. N‐type impurities readily redistribute from the silicide into poly‐Si by 950 °C. Thus, a single n+ source/drain implant can effectively dope the gate, buried gate‐substrate contacts and junctions in NMOS circuits. The corresponding gate metal work function φm is comparable to the reported values for n+ poly‐Si. B, on the other hand, is retained in the silicide. This results in a somewhat lower φm value than expected for p+ poly‐Si.

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