Near-Field Defects Imaging in Thin DLC Coatings Using High-Frequency Scanning Acoustic Microscopy

Abstract

A high-frequency scanning acoustic microscope (SAM) operating at 1–1.3 GHz was used to investigate subsurface defects in diamond-like carbon (DLC) films that were 2–3 μm thick. Because the wavelength of the longitudinal wave in the film was comparable to the film thickness, the acoustical images obtained were near-field images. To interpret the features in the acoustical images, a multidisciplinary approach was utilized through a combination of SAM with atomic force microscopy (AFM), a focused ion beam (FIB) technique, and optical microscopy. Simulations based on the rigorous theoretical approach better predict the depth of the defect than the ray optical approach. It is shown that scattering of the shear waves by subsurface defects is responsible for the acoustical contrast of the cavity.