Hydrogen evolution and transport in semiconductors
Silicon-on-insulator structures are used for the fabrication of integrated electronic circuits, photonic devices and structures, and micro-electro-mechanical systems. The most common fabrication method for SOI is a hydrogen-induced cleavage technique in which ion-implanted hydrogen is employed to initiate and propagate cracks in a plane parallel to the silicon surface. Considerable research effort has been devoted to understanding this cleavage technique in (100) silicon but several fundamental issues remain unclear, including the role of stress on hydrogen platelet alignment. In addition, there is keen interest in extending the technique to other silicon orientations (i.e. (110) and (111)) and semiconductor materials (e.g. Ge). The intrinsic behaviour of hydrogen ion-implanted into Ge and Si was examined by ion beam analysis, optical profilometry and microscopy, to establish the influence of lattice damage and hydrogen evolution. In particular, hydrogen-induced blistering and crater formation under thermal annealing from T=300-650 degrees Celsius was studied to determine the activation energies in Ge and Si in several crystalline orientations. Similar techniques were employed as the influence of extrinsic applied stresses upon hydrogen's evolution within Si was studied, by mechanical stress application onto Si(100). XTEM was used to study the defect evolution related to the hydrogen and ion-implantation damage under anneals applied to samples under stress, in addition to samples produced in different stress conditions. Blistering rate and areal density was seen to follow logistic sigmoidal functions in all materials. Constant activation energies were measured for all Si samples under selected implantation conditions, but multiple activation energies were found in each Ge sample when the conditions were varied. Si(100) & Si(111) both blistered readily for all temperatures, Si(110) required higher H fluence and Ge showed inconsistent behaviour at different implantation conditions. Blister crater depth and roughness may be closer linked to local H concentration rather than total implantation fluence. High level doping of Si does not significantly change the dynamics of H blister formation, with potentially exploitable benefits for SOI production. Stress induced by ion implantation in Si and Ge is tensile, relaxes somewhat with thermal annealing, in the order of <1 MPa. Both 50 and 375um Si wafers behave similarly when implanted with H. Tensile stress applied to H-implanted thick Si(100) influenced hydrogen defect alignment within the lattice, shifting complexes to [110] and [100] planes following annealing. In ULTRATHIN Si, application of tensile stress may relatively diminish and compressive stress enhance diffusion of H, although any applied stress during implantation is seen to decrease H concentration. Applied stresses above 400 MPa cause the height of hydrogen surface blisters to decrease and density to increase. Blisters formed during annealing are not permanently decorated with nor contain hydrogen, whether under applied stress or not. Orientations of detectable defects are not strongly affected by application of stress, however concentrations are seen to decrease at high stress. The location of the ion-cut inducing defect does not appear to correspond to long term measurements of H or implantation damage, and may be even shallower, but this cannot be unambiguously confirmed.
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