Atomic force acoustic microscopy methods to determine thin-film elastic properties

We discuss atomic force acoustic microscopy (AFAM) methods to determine quantitative values for the elastic properties of thin films. The AFAM approach measures the frequencies of an AFM cantilever's first two flexural resonances while in contact with a material. The indentation modulus M of an unknown or test material can be obtained by comparing the resonant spectrum of the test material to that of a reference material. We examined a niobium film (d=280+/-30 nm) with AFAM using two separate reference materials and two different cantilever geometries. Data were analyzed by two methods: an analytical model based on conventional beam dynamics, and a finite element method that accommodated variable cantilever cross section and viscous damping. AFAM values of M varied significantly depending on the specific experimental configuration and analysis technique. By averaging values obtained with both reference materials, very good agreement (5-10 % difference) with values determined by other methods was achieved. These results provide insight into using AFAM methods to attain reliable, accurate measurements of elastic properties on the nanoscale. (C) 2003 American Institute of Physics.