A silicon micromachining method that is able to create deep silicon dioxide blocks at selected locations in a silicon substrate is presented. The process combines deep-reactive-ion etching (DRIE), thermal oxidation, deposition of silicon dioxide and optional planarization. Design issues and parameters for the creation of such blocks are discussed. The selectively defined silicon dioxide blocks allow the integration of silicon surface and bulk micromachining and thick large-area isolation regions for integrated circuits. The performance enhancement that this approach enables is exemplified in the fabrication of an on-chip tunable capacitor and a monolithic transformer on 20-µm-deep silicon dioxide blocks. (Cover story)