As design rules shrink to conform with ultra‐large‐scale integration device dimensions, gate dielectrics for metal–oxide–semiconductor field effect transistor structures are required to be scaled to below ∼60 Å, where some properties of the device, such as interface roughness, that are negligible for thicker films become critical. Microroughness at the interface of ultrathin MOS capacitors has been shown to degrade these devices. The present study focuses on the interfacial region of ∼50 Å SiO2on Si, using the quantum oscillations in Fowler–Nordheim tunneling currents as a probe. The oscillations are sensitive to the electron potential and abruptness of the film and interfaces. In particular, inelastic scattering of the electrons will reduce the amplitude of the oscillations. The amplitude of the oscillations is used to examine the degree of microroughness at the interface that results from a preoxidation high temperature anneal in an inert ambient containing various amounts of H2O. Atomic force microscopy imaging has shown correlations supporting a microroughness induced change in the quantum oscillation amplitudes.