A semiconductor process and apparatus provide a planarized hybrid substrate (15) by thermally oxidizing SOI sidewalls (90) in a trench opening (93) to form SOI sidewall oxide spacers (94) which are trimmed while etching through a buried oxide layer (80) to expose the underlying bulk substrate (70) for subsequent epitaxial growth of an epitaxial semiconductor layer (96). In this way, SOI sidewall oxide spacers (94) are formed that prevent epitaxial SOI sidewalls from being formed in the trench opening (93) during the epitaxial growth step, and that can be readily removed during any subsequent STI etch process