The microstructures of SIMOX（Separation by IMplanted OXygen） structures implanted at 170keV with doses between 0.6x1018O+/cm2and 1.8x1018O+/cm2

as-implanted and annealed at 1300℃ in Ar+0.5%O2

have been investigated by SIMS

RBS and TEM.The results show that implantation with dose as low as 0.6x1018O+/cm2 can form high qualitative surface silicon layer with atom-abrupt interfaces between Si/SiO2 after annealing

but there exist silicon islands with size as up to 60% the SiO2 width in the continuous oxide layer. Conventional 1.8x1018O+/cm2 implantation can cause big dislocation density which will affect the microstructure at the wafer surface after annealing.The experimentaly determined values of layer thickness are compared with values calculated from POISS（Program of Oxygen Implantation in Silicon Substrates） we developed earlier.It is found that the optimum dose for both surface Si and buried oxide is around the critical dose that forms an as-implanted continuous buried layer.