On the thermal aspects of ductile regime micro-scratching of single crystal silicon for NEMS/MEMS applications

Ductile regime machining, DRM, of silicon offers an advantageous alternative to produce a material base that is suitable for MEMS/NEMS fabrication. DRM of brittle materials is due to the joined influence of a thermal and a mechanical effect. Due to the relative facility by which the mechanical behavior of Si in the ductile regime can be characterized, mechanical influences in DRM have received considerable attention. Thermal influences, however, were not the subject of many investigations due to the difficulties encountered in monitoring the thermal behavior of the ductile silicon phase. This paper reports on a series of micro-scratching experiments that were designed to remedy the current gap in literature. The experiments mimic DRM by scratching the surface of single crystal silicon wafers using a stylus with spherically capped diamond tip. In situ electrical resistivity measurements of the wafer surface allowed monitoring the behavior of Silicon in the ductile regime, electrical resistivity measurements were teamed to temperature computations. Applying the Wiedemann-Franz-Lorentz law, thermal conduction fields in the wafers characterized. As such, the thermal environment in the workpiece-stylus contact zone during the brittle-ductile transformation was characterized. The results indicate that: while covalent silicon is an average thermal conductor (K = 120-124 W/m K), the ductile phase is rather insulative (K ≈ 1-2.5 W/m K). The impact of low thermal conductivity on silicon machining and its' effects on process parameters is detailed and the implications for tool wear and geometry are discussed.