An isolation region is formed on a substrate by forming spaced apart mesas on the substrate. A first insulation region is then formed on the substrate and second insulation regions are formed on the mesas, the first insulation region being disposed between and spaced apart from a respective one of the mesas, a respective one of the second insulation regions capping a respective one of the mesas. Preferably, the first and second insulation regions are formed by forming sidewall spacers adjacent sidewall portions of the mesas and oxidizing portions of the mesas opposite the substrate and a portion of the substrate disposed between the sidewall spacers. Spaced apart trenches are formed in the substrate on opposite sides of the first insulation region, a respective one of the trenches being disposed between the first insulation region and a respective one of the mesas, preferably by removing the sidewall spacers and underlying portions of the substrate. An insulating layer is formed on the substrate, filling the trenches and covering the first insulation region, and the substrate is planarized to remove portions of the insulating layer and the second insulation regions and thereby expose underlying portions of the mesas and leave a third insulation region spanning the trenches.