Effect of crystalline orientation in the ductile-regime machining of silicon

This paper investigates the effect of crystallographic orientation in ductile-regime (DR) machining of (100) silicon wafers. Single crystalline diamond tools with 10-40 nm edge sharpness were used to machine the wafers at either constant depths of cut, or a taper mode to vary the depths of cut up to 1 mum. The feedrates were normalised as percentages of tool nose radii, and the machining process was performed using an ultraprecision machining system. The surface and subsurface integrity were then characterised with an atomic force microscope, a phase shift interferometer, and an ion beam system. The measured surface roughness of silicon was compared with those of copper alloys, and the calculated values. A ductile-regime was achieved when machining along the <110> directions when the maximum chip thickness of less than 0.5 mum. Machining conditions that formed thicker chips led to pitting, microcracks and slip lines. Such defects, which could be more than 1 mum deep, were found along the <110> directions and occasionally along the <100> directions. Surface roughness below 10 nm was measured in a DR area, but was as high as 170 nm in pitted areas. When the depth of cut was of the magnitude of the tool edge sharpness, the surface finish was degraded by radial cracks in the lateral plane owing to rubbing between the tool and the workpiece. The surface finish of the silicon, therefore, was rougher than that of copper alloys that were machined using similar parameters.