The three-dimensional step structure at the buried Pb on Si͑111͒ 6ϫ6-Au interface is determined by utilizing the presence of quantum well states. We demonstrate that the spatial step positions as well as the step heights can be extracted nondestructively and with atomic layer precision by scanning tunneling microscopy and spectroscopy. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1506404͔ Thin metal films on semiconductor substrates are crucial components of many semiconductor devices. The physical properties of these thin films are to a large degree governed by the film morphology, which in turn is directly affected by the roughness of the buried metal-semiconductor interface. Despite large efforts in determining the interfaces' step structure with a variety of nondestructive tools, the real space three-dimensional step-structure determination of interfaces still remains a formidable task.Recently, there has been much progress in synthesizing ultrathin metal films on semiconductor substrates. In particular, it has been shown that atomically flat thin metallic films with nanometer scale thickness can be successfully grown. [1][2][3][4][5] In such an ultrathin metal film, electrons are subjected to strong confinement along the vertical direction, resulting in quantum well states ͑QWS͒. 2 Indeed, such QWS have been observed using photoemission 2,6,7 and scanning tunneling spectroscopy. [8][9][10][11] Since the energy levels of QWS depend sensitively upon the film thickness, it is therefore possible, from the measurement of local QWS, to determine the local thickness of the thin film from which the underlying step structure can be deduced. 12 Indeed, by studying individual Pb islands on Si͑111͒ substrates, Altfelder et al., found that the variation of QWS correlates with the substrate step structure visible at the edges of the metal island. 9 This correlation is, however, based on the ability to measure directly in scanning tunneling microscopy ͑STM͒ images the height of individual islands on the substrate ͑with wetting layer͒. Unfortunately such a simple correlation is no longer possible for a fully covering two-dimensional metal film, which is the ultimate goal for applications in semiconductor devices. In this letter we demonstrate the nondestructive methodology on how to determine the step structure at buried metal-semiconductor interfaces for fully covering twodimensional thin metal films using STM.As a model system we grew Pb films on Si͑111͒ 6ϫ6-Au surfaces at room temperature. On such a surface,