Formation of Si islands in the buried oxide layers of ultra-thin SIMOX structures implanted at 65 keV

The microstructures of separation by implanted oxygen (SIMOX) wafers, implanted at 65 keV with doses of 1.5-7.0 × 10¹⁷ O⁺/cm² at 500°C followed by a high temperature (1350°C) annealing with and without a protective cap, were studied using transmission electron microscopy to investigate the relationship between the formation of ultra-thin SIMOX structures and a variety of different preparation parameters. The study found that there is an optimum dose range corresponding to the implantation energy used. The samples synthesized at an oxygen dose of 2 × 10¹⁷ O⁺/cm² (annealed without a cap) or 2.5 × 10¹⁷ O⁺/cm² (annealed with a cap) consist of a thin silicon top layer with a low threading dislocation density, and a thin continuous buried oxide (BOX) layer free of Si islands. For samples implanted below the optimum dose, the BOX layer is discontinuous. Capping or non-capping the sample surface during annealing affects the formation of the BOX layer. For samples without a cap, internal thermal oxidation happens even in an ambient of low concentration of oxygen and makes the BOX layer grow continuously and free of Si islands.