Using lasers to induce locally confined subsurface modifications in materials is a well-established technique. Indeed, subsurface modification of dielectric materials is of great importance for optical applications such as the production of waveguides, gratings, lenses and three-dimensional data storage. Furthermore the technique is of growing interest for engineering subsurface modifications in semiconductors. Of particular interest for the semiconductor industry is the process of subsurface laser-dicing that enables the accurate sectioning of wafers and devices. Subsurface laser-dicing involves inducing closely spaced subsurface modifications that act to guide subsequent crack propagation in a highly controlled manner during cleaving. However, neither the morphology of the induced modifications, nor the mode by which they act to guide crack propagation, has previously been investigated in any detail. As such this work examines these factors with the aims of understanding subsurface laser-induced damage, improving wafer-dicing performance and reducing surface debris.
In this work columnar laser modifications have been produced subsurface in two laser regimes; induced either by two photon absorption with a 1550 nm laser or by weak linear absorption with a 1064 nm laser. Furthermore, modifications have been produced both in isolation to examine the fundamental morphology, and closely spaced (2 µm lateral separation) as used for dicing to investigate modification interactions. The extent of the modified zone is between 30 and 50 µm long and the modified zone is 50-150 µm subsurface.
For analysis, the plane of modifications was exposed either by cleaving the sample or by tripod polishing; the latter requiring extreme care to ensure the polished surface was coplanar and did not induce undesired cleavage. Focussed ion beam (FIB) milling was used to prepare cross-sectional transmission electron microscopy (TEM) samples from the exposed surface. Locating single laser modified zones many microns below the surface and obtaining cross sections of them is not trivial and requires careful control over zone location and sample preparation methods. The analysis was conducted using a combination of scanning electron microscopy (SEM), TEM and Raman micro-spectroscopy, including mapping.
Thus far, the modifications have been found to contain a range of features including (1) lattice defects related to epitaxial solidification of molten zones created by laser heating, (2) voids induced by volume contraction of molten Si due to its higher density than the solid, (3) melt quenched amorphous Si and (4) pressure-induced high density Si phases. Furthermore, in undiced samples discontinuous cracks between modifications have been noted once the sample is thinned for TEM, providing insight into the cracking process.
The origin of these features will be discussed based on two key processes; the redistribution of mass within the melt volume, and the radial solidification of Si from the melt. Additionally the interaction between neighbouring modifications is considered.