Effect of resputtering on composition of WSix films deposited by multilayer sputtering

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Abstract
WSix films for use as gates in a GaAs‐based self‐aligned gate field‐effect transistor (FET) process were formed by sequential sputter deposition of multiple layers of W and Si. The power applied to the W‐ and Si‐opposed planar magnetron targets could be individually controlled and was fixed at 530 and 500 W, respectively. The thickness of the W and Si layers decreased from ∼6 to 0.3 nm as the rotation speed of the substrate carousel was increased from 2 to 30 rpm. However, the W and Si thicknesses decreased at different rates. From Rutherford backscattering spectrometry measurements of films deposited on carbon‐coated substrates, the films became depleted in Si as the rotation rate increased; the composition varied from WSi0.58 at 2 rpm to WSi0.16 at 30 rpm. This large change in composition for constant sputtering conditions can be explained by the resputtering of the thin Si layers by energetic neutral Ar atoms reflected from the W target. Films with a Si/W ratio of 0.16 showed good thermal stability and were excellent Schottky barriers after rapid thermal annealing at 900 °C for 6 s. FET’s with 0.6 μm‐long W0.16Si gates gave fT≥20 GHz.

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