Extremely smooth (6 nm RMS roughness over 4 mu m(2)), thin (100 nm), and continuous ultrananocrystalline diamond (UNCD) films were synthesized by microwave plasma chemical vapor deposition using a 10 nm tungsten (W) interlayer between the silicon substrate and the diamond film. These UNCD films possess a high content of sp(3)-bonded carbon. The W interlayer significantly increased the initial diamond nucleation density, thereby lowering the surface roughness, eliminating interfacial voids, and allowing thinner UNCD films to be grown. This structural optimization enhances the films' properties and enables its integration with a wide variety of substrate materials. (c) 2006 Elsevier B.V. All rights reserved.