Laser induced subsurface modification of materials provides a route for rapid energy deposition while simultaneously constraining the material’s response due to the surrounding unaffected material. This method is of interest for creating functional subsurface modifications, for example as part of a Si wafer dicing method where the modifications guide crack propagation during cleaving, allowing for accurate sectioning of wafers. Furthermore this method has many parallels with pulse laser melting, typically used to attain highly non-equilibrium semiconductor doping, while extending the material science into a subsurface regime where new behaviours are manifested.
One such behaviour is the formation of voids within the melt volume due to the combination of higher melt density and constrained melt volume. However investigations of these voids are nontrivial as they constitute only a small portion of the modified volume, while simultaneously existing deep subsurface.
Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was used to investigate the voids that persisted after solidification to examine their morphology and relate this to the morphology without a void.
SEM of modifications exposed by cleaving shows that most modifications contain one persistent void with a roughly cylindrical profile, as confirmed by TEM. By conservation of mass, a subsurface void must indicate that excess material is distributed elsewhere in the modified volume. During solidification this implies that the void should refill and indeed this appears to partially occur. Cones of material with crystallography that does not match the bulk appear to result from melt injected into the void during radial solidification. Rapid cooling of these jets often results in solidification to an amorphous state. Nevertheless, even partially refilled, the persistence of voids indicates that excess material remains in a high density state elsewhere in the modification.