High-resolution picosecond acoustic microscopy for non-invasive characterization of buried interfaces

Non-destructive investigation of buried interfaces at high-resolution is critical for integrated circuit and advanced packaging research and development. In this letter, we present a novel non-contact microscopy technique using ultrahigh frequency (GHZ range) longitudinal acoustic pulses to form images of interfaces and layers buried deep inside a silicon device. This method overcomes fundamental limitations of conventional scanning acoustic microscopy by directly generating and detecting the acoustic waves on the surface of the sample using an ultrafast pump-probe optical technique. We demonstrate our method by imaging copper lines buried beneath a 6-μm silicon wafer; the lateral spatial resolution of 3 μm is limited by the laser spot size. In addition to the high lateral spatial resolution, the technique has picosecond (ps) time resolution and therefore will enable imaging individual interconnect layers in multi-layer stacked devices.