Subsurface modification in Si induced by nanosecond laser irradiation
Semiconductor devices are pivotal to modern technology. These devices are batch produced on wafers which must be diced. Conventionally, dicing involves destroying the material between each chip, dictating a minimum spacing between modifications. It follows that eliminating this material loss will allow more chips to be created per wafer. This thesis examines subsurface modifications that form part of a dicing process with no material loss. Subsurface modifications can be induced by focussed pulsed laser irradiation when the photon energy is near or below the material's bandgap. Intensity dependent absorption is used to selectively modify the region near focus. This technique is useful both for technological applications and scientific investigation of material properties and laser-material interaction. This thesis examines columnar modifications created subsurface in Si wafers due to melting induced by nanosecond pulsed laser irradiation. The wafer dicing technique uses rows of closely spaced modifications that can preferentially guide crack propagation over the natural cleavage planes when the sample is cleaved. The modifications themselves are poorly characterised and it is this deficiency that this thesis seeks to address. The work examines both isolated modifications, where the laser-matter interaction can be probed, and closely spaced modifications, which can provide insight into the dicing process. Additionally, modifications created under several different laser conditions are investigated. Transmission electron microscopy is used to examine both the short and long axis of the modifications. Scanning electron microscopy and Raman microspectroscopy are also used. Detailed examination of the short axis of the modifications reveal a range of morphological features. These features can be explained by extending upon the literature of one-dimensional pulsed laser melting experiments into two-dimensions and conditions of higher cooling rates. Rapid, accelerating solidification occurs along multiple crystal orientations and propagates inwards from the melt periphery while simultaneously transitioning through several rate-dependent regimes of solidification behaviour. Examining the long axis allows the solidification process to be understood in three-dimensions. Here, variations observed in the short axis morphology are related to the dependence on the position along the long axis of the modifications. Solidification is also subject to perturbations, often density related. These originate in the densification of Si as it melts, which drives the formation of voids. During solidification the voids are partially, but incompletely refilled. The material displaced by the remanent voids is largely concentrated in the final regions of the modified volume to solidify, creating compressive strain. However, there are also crystallites of high density Si allotropes. These allotropes form if pockets of high density melt lack room to expand during solidification. Interestingly, although these allotropes have been observed in near-static near-equilibrium pressure loading techniques (diamond anvil cell or nanoindentation loading), the pathway by which the Si allotropes form must clearly differ in this work. It is also noteworthy that the manifestation of the density dependant morphological features is highly dependent on the size of the modifications, and duration of the laser pulse used to create them. The modifications were also found to contain many microcracks after solidification. During cleaving this removes the need to nucleate cracks. Instead, many cracks propagate simultaneously before joining together, completing the dicing process. Thus, this thesis has rectified the deficiency in knowledge of the modification morphology. In doing so, new solidification and high pressure related behaviour has been observed. Collectively, this information should serve as the basis to improve the dicing technique in the future.
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