Some observations on the wear of diamond tools in ultra-precision cutting of single-crystal silicon

Single-crystal silicon is not only a dominant substrate material for the fabrication of micro-electro and micro-mechanical components but also an important infrared optical material. Since silicon is a nominally brittle material, currently it is finished by grinding, lapping and chemo-mechanical polishing (CMP). However, silicon can be deformed plastically in machining, yielding ductile chips under the influence of high hydrostatic pressure. Therefore, an alternate approach would be to machine silicon with a single point tool in the ductile mode without the need for subsequent polishing. This way damage due to brittle fracture can be minimized and the productivity of complex-shaped components can be significantly improved. This technology involves the use of an extremely rigid, ultra-precision machine tool and a single-crystal diamond tool with a high negative rake angle. However, one of the problems existing in the industrial application of the ductile machining technology is the wear of diamond tools. Tool wear not only raises the machining cost but also degrades the product quality. The tool wear problem becomes particularly serious when machining large radius components. This paper deals with the performance of diamond cutting tools during single point diamond turning of single-crystal silicon substrates at a machining scale smaller than 1 μm. The cutting edge, the finished surface and the cutting chips were examined by scanning electron microscope (SEM) and the micro-cutting forces were measured. It was found that the tool wear could be generally classified into two types: Micro-chippings and gradual wear, the predominant wear mechanism depending on undeformed chip thickness. In ductile mode cutting, flank wear was predominant and the flank wear land was characterized by trailing micro-grooves and step structures. The tool wear causes micro-fracturing on machined surface, yields discontinuous chips and raises cutting forces and force ratio. Experimental results also indicate that it is possible to prolong the ductile cutting distance by using an appropriate coolant.