Elastic-property measurements of ultrathin films using atomic force acoustic microscopy

Atomic force acoustic microscopy (AFAM), an emerging technique that affords nanoscale lateral and depth resolution, was employed to measure the elastic properties of ultrathin films. We measured the indentation modulus M of three nickel films approximately 50, 200, and 800 nm thick. In contrast to conventional methods such as nano indentation, the AFAM results remained free of any influence of the silicon substrate, even for the 50 nm film. X-ray diffraction and scanning electron microscopy results indicated that the films were nanocrystalline with a strong preferred (111) orientation. Values of M ranged from 210 to 223 GPa, lower than expected from values for bulk nickel. The reduced values of the elastic modulus may be attributed to grain-size effects in the nanocrystalline films.