We present a technique in which atomic force microscopy (AFM) at ultrasonic frequencies is used to measure the contact stiffness between an AFM tip and thin films on silicon substrates. In this method, the resonance frequencies of the cantilever flexural modes are used to determine the tip-sample contact stiffness. We present experimental results, showing that the contact stiffness is highly sensitive to the thickness of thin metal and polymer films. These results are compared with those from out theoretical model, which we call the Contact Stiffness Algorithm (CSA), that may be used to calculate the contact stiffness between an AFM tip and an arbitrarily layered sample. Unlike transmission electron microscopy (TEM) or scanning electron microscopy (SEM) on a cross-section of the sample, this film thickness measurement technique is non- destructive. It is also capable of high lateral spatial resolution, provided that a sharp AFM tip is used. We present images of a photoresist film on silicon with contrast resulting from the elastic properties of the sample.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.