A simple method to precisely measure the trench depth of VLSI DRAM's capacitor cells is presented. The measurement system uses a Michelson interferometer and a white light source. The trench depth is transformed into the optical path difference between the central peak and one of sideband peaks produced during one-way scan of a mirror in a Michelson interferometer. The measurement error was withinpm0.2 mum for trench depths of2-5 mum. The multimode fiber used in the system facilitates wafer setup so that the system can be introduced into an in-process measurement system.