Ultra-precise processing of silicon wafers using machining, grinding, and polishing processes is critical for many applications in IR optics, solar cells, and MEMS devices, all of which require a smooth undamaged silicon surface. The basic action involved in these processes is a series of overlapping scratches on the silicon wafer by a hard material (defined geometry cutting tool or arbitrary shaped abrasive particle) subjecting the silicon to rapid mechanical loading (as the tool engages) and unloading (as the tool passes by). This results in surface and sub-surface damages (e.g. phase transformation, residual stress, and fracture) because of silicon’s inherent material properties. Under some conditions such as shallow depths of scratching and when silicon is subjected to high hydrostatic pressures (~ 12 GPa) plastic deformation becomes more favorable than fracture. However, these conditions also result in tensile surface residual stresses. Hence, a study on loading and unloading affects, determine conditions that minimize or eliminate the surface and sub-surface damages, during scratching of silicon is needed.

This research studies experimentally, via a unique rotational double-taper scratching setup, ductile-brittle transition in p-type (100) single crystal silicon using a conical diamond tool at room temperature and scratching speeds ranging between 0.1 m/s and 0.3 m/s. A well-defined quantitative way to determine critical depth of cut via linear crack density per unit crack length is proposed. Using this data, the critical depth of cut has been comprehensively studied at all speed ranges. The thesis reports residual stress and their evolution during ductile-brittle transition. Data also shows that with an increase in scratch depth, residual stresses transition from compressive to tensile at the bottom of the scratched groove and vice-versa on the top surface. Interestingly this transitions in residual stresses coincide with the transition from ductile to brittle mode of material removal. Hence, we postulate that conditions close to the critical depth value should result in the least stress surface in silicon.

The thesis further studies the fundamental difference between increasing depth and decreasing depth scratching. Using a force-depth plot increasing depth scratch and decreasing depth scratch is compared. It is observed that after the ductile-brittle transition occurs, higher forces are required to deform the material during increasing depth scratching. This is because large surface and sub-surface damages with the presence of radial, median and lateral cracks make the material weaker in decreasing depth case. Using such force-depth plots, for the first time, indentation and scratching operations are compared.

The thesis also investigates the effect of temperatures (25-500 °C) on the phase transformations in rotational scratching, along various crystal orientations, at speeds comparable to diamond turning process (1 m/s). Analytical pressure calculations show that at higher temperatures, phase transformations can happen in silicon at significantly lower pressures. This study is expected to help tune heat-assisted diamond turning conditions and improve surface formation.