Demand for high quality silicon chips in the electronics industry necessitates stringent manufacturing requirements. It is known that nanogrinding of silicon wafers results in subsurface damage. For optimal performance of chips, it is integral that damaged layers are removed.
This thesis is a study of the effect of subsurface damage on wafer mechanical properties; namely elastic modulus and hardness. It is known that subsurface damage has a correlation with grinding conditions. This information was used to link grinding conditions with wafer mechanical properties.
It was found that grinding wheels with relatively large grit sizes produce wafers with low subsurface stiffness. Lower grit sizes generally resulted in higher stiffness. A significant increase in wafer subsurface stiffness was found when transitioning from a #800 to #2000 mesh grinding wheel. From the results, a mixture of heat and pressure from grinding conditions were attributed to the differing levels of subsurface damage. There was a general trend for elastic modulus to increase with grinding wheel mesh size, and this was explained through the degree of subsurface damage in samples. No trend between hardness and subsurface damage was evident. This was likely testing-related and due to the effects of sample surface roughness.
The studies were conducted using nanoindentation; a method which extracts hardness and elastic modulus through load-displacement information, gathered from indenting a surface with a diamond tip, at the nanoscale. Nanoindentation is used due to the small size of wafers (approx. 250 ìm thick), where conventional materials testing methods are not practical. Silicon substrates were further characterised using atomic force microscopy, an imaging technology used to analyse surfaces.
In order to investigate the influence of grinding conditions on subsurface damage, various ground monocrystalline (1 0 0) silicon samples were tested under #400, #800, #2000, #3000 and #5000 mesh grinding wheels.
The surface roughness of samples was found to affect results quite significantly. This issue was alleviated somewhat through performing a systematic series of tests, however, large deviations in some data were still present.
Through performing a series of indentations, mechanical properties of ground silicon substrates can be analysed, to give an indication as to the level of incurred subsurface damage. With further development of the results presented, nanoindentation could be used as a subsurface quality assessor for ground silicon wafers.