Line trenches are formed in a stack of a bulk semiconductor substrate and an oxygen-impermeable layer such that the depth of the trenches in the bulk semiconductor substrate is greater than the lateral spacing between a pair of adjacently located line trenches. Oxygen-impermeable spacers are formed on sidewalls of the line trenches. An isotropic etch, either alone or in combination with oxidation, removes a semiconductor material from below the oxygen-impermeable spacers to expand the lateral extent of expanded-bottom portions of the line trenches, and to reduce the lateral spacing between adjacent expanded-bottom portions. The semiconductor material around the bottom portions is oxidized to form a semiconductor oxide portion that underlies multiple oxygen-impermeable spacers. Semiconductor-on-insulator (SOI) portions are formed above the semiconductor oxide portion and within the bulk semiconductor substrate.