Tilted sample acoustic microscopy for anisotropy measurement

The conventional acoustic microscope with the spherical lens insonifies the samples with circularly symmetric fields. In case of anisotropic samples, this field distributions result in an anisotropy insensitive V(Z) curve, since the material properties are averaged overall azimuthal directions. The circularly symmetric excitation and detection should be avoided to improve the sensitivity to anisotropy of the sample. We present a method in which the sample is simply tilted to improve anisotropy sensitivity of the acoustic microscope with high spatial resolution. No change in the conventional lens and transducer structure is required. A theoretical model based on angular spectrum approach is developed to calculate the V(Z) curves for the tilted sample acoustic microscope. The directivity and resolution of the tilted sample acoustic microscope is evaluated as a function of lens and material parameters and the tilt angle using the developed approach. Validity of calculations is tested by comparing measured and calculated V(Z) curves for (100) silicon and excellent agreement is observed. The anisotropy of surface waves in (100) gallium arsenide is also observed by a tilted sample acoustic microscope in agreement with calculations