Imaging distribution of local stiffness over surfaces using atomic force acoustic microscopy

We report a systematic study to determine local elastic properties of surfaces using atomic force acoustic microscopy(AFAM). AFAM is a combination of atomic force microscopy (AFM) and acoustic waves. We describe the technique and principle of AFAM in detail and interpret the obtained images using simple arguments. We have used (1) polished commercial piezoelectric PZT, Pb(Zr,Ti)O-3 ceramic, (2) thin film of PZT deposited by the sol-gel technique and (3) thin film of Au deposited on a Si(001) substrate to elucidate the capability of the AFAM technique to image the distribution of local stiffness over the sample surface. We have also used a complementary technique such as force-distance measurements using the AFM mode to support the interpretation of the AFAM images. We have determined semi-quantitatively the change in the local stiffness over the sample surface using both the force-distance measurement and the change in the contact resonance peak frequency at various regions. We have also shown that the AFAM technique can be used to get a better surface image contrast where contact mode AFM shows poor contrast.