Infrared spectroscopy and secondary ion mass spectrometry are used to elucidate the mechanism by which co-implantation of He with H facilitates the shearing of crystalline Si. By studying different implant conditions, we can separate the relative contributions of damage, internal pressure generation, and chemical passivation to the enhanced exfoliation process. We find that the He acts physically as a source of internal pressure but also in an indirect chemical sense, leading to the reconversion of molecular H2 to bound Si–H in “VH2-like” defects. We postulate that it is the formation of these hydrogenated defects at the advancing front of the expanding microcavities that enhances the exfoliation process.

Since the first report of the Unibond process,l there has been much interest in reproducing Si exfoliation by H implantation and in understanding the mechanism leading to such a remarkably uniform shearing. We have previously demonstrated that, contrary to the initial speculation? there are in fact three distinct aspects to the process3 i) The generation of damage to the crystalline material by the implantation; ii) The unique surface chemistry of hydrogen and silicon that drives the thermal evolution of this damage region and; iii) The creation of internal pressure that ultimately causes exfoliation of the overlying Si layer. Therefore, a detailed understanding of the exfoliation mechanism involves the study of initial damage, of H-passivation of various internal structures and of the mechanical forces exerted by trapped gases as a function of hydrogen implantation doseldepth and annealing temperature. In this work, we have used different hydrogen implantation conditions (ion energies ranging from 1 V to 1 MeV and substrate crystallographic orientations) as well as co-implantation of a variety of other elemental species, in combination with novel spectroscopic configurations, to further explore these different mechanistic aspects.Infrared spectroscopy has played a key role in elucidating the microscopic details of the process, due to its high sensitivity and selectivity and inherent non-destructiveness. However, the frequency range accessible was limited to above 1500 cm-1 so that only the Si-H stretching vibrations could be observed. Recently, we have developed novel optical configurations that allow probing of the Si-H bending modes (at -600-650 cm-1) and scissor modes (850-910 cm-I), allowing definitive identification of the different defect modes. Using this approach, in combination with a variety of other techniques, we have been able to definitively show that exfoliation consists of the following distinct mechanistic steps: Above the critical dose of 6 x 1016/cm2, the IR spectrum shows evidence for monohydride-terminated, multi-vacancy defects that are typically found in hydrogenated amorphous silicon. The formation of such a "multivancy" defect region is critical to exfoliation, because it allows both formation of agglomerated defects and the evolution of molecular H2. These defects, in turn, develop into (100) and (1 11) internal cracks which act as traps for the H2 leading to the build-up of internal pressure and subsequent shearing. It is the synergetic combination of H-passivation of internal surfaces and H2 pressure within these intemal cracks that leads to the shearing in the presence of the joined wafer, that acts as a mechanical stiffener. Importantly, in the absence of the stiffener, the surface 'blisters'; in the absence of sufficient damage (below the critical dose), the hydrogen diffuses away from the implanted region, preventing exfoliation.Recent experiments4 have isolated the physical and chemical contributions to exfoliation by co-implanting He, Li and Si along with H and demonstrated that...