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This work describes results obtained from detailed TEM, TED, HREM and SIMS analysis of the as-implanted and annealed microstructures of high dose (O.lxl0 17/cm2 to 1.7xl0¹⁸/cm²) oxygen implanted silicon (Si). Molecular oxygen (O₂⁺) has been implanted into Si wafers at equivalent energies of 200keV/O + , 90keV/O +, 70keV/O+ and 50keV/O+ to form, after annealing in flowing N₂ or Ar + ½ %O₂, buried SiO₂ layers below single crystal surface Si layers. This process is called Separation by the IMplantation of OXygen (SIMOX). The energies and doses investigated are potentially suitable for the fabrication of two types of "thin-film" SIMOX substrates, which have major potential benefits for high-performance CMOS devices. This work is concerned with investigating the as-implanted and annealed microstructures, understanding the basic processes and mechanisms taking place during implantation and annealing, and establishing optimum fabrication parameters. Similar microstructures and changes in microstructure as a function of the dose are observed for the different implant energies investigated. For all the different energies and doses investigated, SiO₂ precipitates are present after implantation. Five different precipitate morphologies are observed. The precipitate morphology depends on the oxygen concentration and the depth below the surface. The local and long range strain also play a role in determining the precipitate morphology. For doses of 0.5xl0¹⁸O/cm² to 0.7xl0¹⁸O/cm² at 200keV defects at the wafer surface are non-uniformly distributed across the implanted area. The regions of defects are in plan-view rectangular in shape with edges parallel to <100> directions. The percentage of the implanted surface that is covered by these rectangular regions depends on both the dose and the time-averaged beam current density. This is the first known report of such non-uniform distribution across the implanted area of defects at the wafer surface and their occurrence in regions with precise rectangular shapes. Previously unreported "line" defects below the peak of the as-implanted oxygen distribution for these energies are investigated. They are considered to be platelets on {100} planes and edge dislocation loops on {110} planes. After annealing, two major types of defects are present, threading dislocations in the surface Si layer and Si islands within the buried SiO₂ layer. Correlation of as-implanted and annealed microstructure suggests that the threading dislocations originate in the defects present at the wafer surface after implantation and grow down during annealing. The Si islands originate from Si isolated from the surface Si layer and the substrate during implantation or annealing. The optimum dose for forming a SIMOX structure at a particular energy with both a low threading dislocation density and a low Si island density is just greater than the minimum dose for forming a continuous buried SiO₂ layer after annealing. In order to try and reduce the density of Si islands within the buried SiO₂ layer, graded low energy implants and interim rapid thermal anneals are investigated. Their influence on the microstructure is reported. The experimental results enable, for the implantation and anneal conditions used, the likely threading dislocation and Si island density after annealing to be estimated for a particular dose and energy. Simple models have been proposed for calculating typical oxygen diffusion lengths during implantation, the thicknesses of the buried SiO₂ and surface Si layers after annealing and conversely the implant dose and energy required to fabricate a SIMOX substrate with a certain thickness of buried SiO₂ and a certain thickness of surface Si layer after annealing. Thin-film SIMOX substrates consisting of a thin surface Si layer above a thin buried oxide layer suitable for high-performance "fully depleted" CMOS devices have been successfully fabricated by implanting doses of ≥0.35 and ≥0.33xl0¹⁸O/cm² at energies of 90keV and 70keV, repectively. Thin-film SIMOX substrates with a thin buried oxide layer below a standard thickness surface Si layer suitable for radiation hard circuits with reduced circuit self-heating have been successfully fabricated by implanting doses of ≥0.56xl0¹⁸ O/cm² at an energy of 200keV.