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The depth between an oxide at the wafer surface and a poly silicon plug in a trench was determined with scatterometry using a polarized reflectometer. Poly recess depth is an important parameter in trench MOSFET technologies that needs to be monitored closely. Two cases from technologies with different trench width are presented. The first case has a narrow and deep poly recess, while the second case has a wide and shallow recess structure. The optical method is the only nondestructive way to control the etch process with respect to uniformity across the wafer, when the recess is too narrow for a measurement with atomic force microscopy. Three wafers were etched to different recess depths for each of the two technologies. There was very good agreement between the results from the optical technique, SEM cross sections and AFM.