A method of forming a semiconductor device starts with a substrate of silicon, a first insulation layer on the silicon, and a silicon layer on the first insulation layer. The silicon layer and the insulation layer are removed just from a second substrate area. A second insulation layer is formed over the silicon layer in the substrate first area and over the silicon in the second substrate area. A first plurality of trenches is formed in the first substrate area that each extends through all the layers and into the silicon. A second plurality of trenches is formed in the second substrate area that each extends through the second insulation layer and into the silicon. An insulation material is formed in the first and second trenches. Logic devices are formed in the first substrate area, and memory cells are formed in the second substrate area.