Among structures, methods, devices, and systems for isolation trenches, a semiconductor device is provided that includes a substrate and an isolation trench structure. One such isolation trench structure includes a first isolation trench portion associated with a surface of the substrate and having a first pair of opposing sidewalls that are each substantially perpendicular to the surface of the substrate. A second isolation trench portion includes a second pair of sidewalls within the substrate that are each angled obliquely with respect to the surface of the substrate, where the second isolation trench portion has a separation between the second pair of sidewalls that decreases as a distance from the first isolation trench portion increases. A third isolation trench portion includes a third pair of sidewalls within the substrate that are each substantially perpendicular to the surface of the substrate.