Final PhD Seminar
The morphology of deep subsurface modifications induced by pulsed laser irradiation for Si wafer dicing
A key step in the production of IC devices, which are batch produced, is the dicing of wafers into individual components. A promising new method for dicing aims to achieve this by controlled wafer cleaving, allowing for significantly higher packing densities on a wafer, and thus higher yields. This method makes use of arrays of laser-induced subsurface modifications and has been successfully demonstrated. However, the morphology had not previously been investigated in any depth. My work, and this talk, investigates the morphology in part for the sake of optimising the dicing process.
The system is also scientifically interesting due to the unusual subsurface laser absorption process, rapid melting and solidification that occurs as a consequence, and the coupling of density and pressure for subsurface modifications.
Examination, primarily by transmission electron microscopy and Raman spectroscopy, reveals extreme solidification behaviour that extends upon the knowledge from more conventional approaches. The interaction between multiple solidification interfaces results in an interesting interplay between energetically dominated and kinetically dominated behaviour. Furthermore, rapid solidification results in a series of transitions in the nature of the solidification process.
Being subsurface, variations in density that occur during heating and melting also cause a variation in pressure within the modified region. This results in the formation of voids and pressure induced high density crystalline phases under transient conditions that differ significantly to more common indentation and diamond anvil cell approaches to applying pressure.
This work brings clarity to the nature of the modifications, suggests an approach to improve dicing using the modifications and provides useful insight into high speed solidification behaviour and pressure induced phase transformations under transient conditions.