Film Thickness Dependence of Silicon Reduced LPCVD Tungsten on Native Oxide Thickness

Abstract

Tungsten was deposited on silicon wafers via the silicon reduction of in a low pressure chemical vapor deposition apparatus using and argon gases. For depositions performed at 400°C, 0.3 torr total pressure, a strong dependence of film formation on surface oxide thickness was observed. For oxides ranging from 0 to 15Å, a monotonic increase in tungsten film thickness occurs, reaching a maximum of approximately 1000Å at 20Å. Above this oxide thickness, the tungsten film thickness decreases sharply, reaching less than 20Å at 40Å of oxide, thus indicating the onset of "selectivity," i.e., no tungsten deposition. The tungsten formed for oxides ranging from 0–15Å is α‐W with a resistivity of approximately 18 μΩ‐cm. For a 20Å oxide thickness, β‐W is formed with a resistivity of approximately 130 μΩ‐cm. Rapid thermal annealing in forming gas at 950°C for 10s does not affect the α‐W films, but converts the β‐W films into α‐W. For VLSI applications such as barrier metal formation or contact hole planarization, extreme care has to be taken to limit the native oxide thickness below 15Å to ensure reproducible, in terms of thickness and uniformity, film depositions.