Study and optimization of abrasive machining processes in crystal wafer manufacturing

Various important aspects of free abrasive machining (FAM) processes involved in crystal wafer manufacturing, as described in the following are studied in this dissertation to form the basis for practical improvements of the processes and further theoretical studies. The effect of crystal anisotropy on the process of slicing using wiresaw is studied and presented. A method is proposed to determine the direction of approach (DOA) which will give a better surface finish and reduce deviation from the desired surface normal by maintaining symmetry in material removal rates on the two sides of the wire. For non-centrosymmetric crystals, mechanical properties are likely to be different on opposite faces of the same wafer, which are often processed simultaneously in processes such as wiresawing, lapping and polishing. Hardness anisotropy of three common orientations of Lithium Niobate wafers is studied using nano-indentation technique. Hardness values on opposite faces for the same direction are found to be different. Furthermore, effect of multiple simultaneous indentations on material's response is studied in purview of wiresaw slicing using finite element method. For silicon, as the spacing between the abrasives decreases, the load required to achieve a prescribed depth' of indentation decreases. This study extends the previously proposed rolling-indenting model with single abrasive grit.; Impact of mixing abrasives on various parameters such as amount of material removed, material removal rate, surface roughness, particle size distribution and relative angular velocity is studied. It is shown that the material removal rate can be increased by properly mixing abrasive grits of different sizes, without significant changes of surface finish and quality. It is also shown that slurries undergo severe grain size transition during lapping. Last but not the least, thermal effects and heat transfer during wiresaw slicing are important factors in determining the surface finish of wafers and warp of sliced wafers. It is thus important to model theoretically the thermal aspects in manufacturing and to validate results with experiments. An in-house C++ code was developed to model the problem using finite element method. The simulation results for the temperature profile show an excellent match with experimental studies available in literature.