An adequate measurement metrology to nondestructively verify the integrity of dielectric gap-fill in a deep trench (DT) capacitor of deep-submicron DRAM product was proposed. Because of the geometric structure in the DT capacitor, the vertical cylindrical electrode isolator approximately provides a parasitic NMOSFET. Through the electrical measurement, people can analyze these drain-to-source electrical characteristics. Some of most valuable device parameters, threshold voltage (Vt) and mobility (un), correlate to the interface integrity and the surface roughness between silicon substrate and gap-fill oxide (or liner oxide). In other words, as these values are obtained, the degradation level of this interface or gap-fill quality can be clarified. Indirectly, the charge storage quality of this capacitor, avoiding the leakage path, is able to be improved with the process modification.