Ion acoustic microscopy for imaging of buried structures based on a focused ion beam system

An intensity modulated focused ion beam (FIB) which is hitting the surface of a solid, leads to a small temperature variation in the near subsurface region which is sufficient for thermal elastic wave generation. The measurement of these waves can be used for analysis and imaging of surface as well as subsurface structures in the material. The modified FIB equipment IMSA-100 working with 35 keV Ga+ and Au+ ions and a current of about 3 nA was employed to obtain acoustic images from structures on silicon and glass targets. The acoustic signals were detected using a PZT transducer delivering an output voltage of 50 - 100 nV. The modulation frequency was varied in the range of 60kHz - 170kHz. The obtained lateral resolution of the ion acoustic images at these frequencies was about 15 µm on silicon and about 7µm on glass. We are going to increase the modulation frequency up to MHz range in order to reach sub-micron resolution.