Improved sub-surface AFM using photothermal actuation

In this paper, we present an AFM based subsurface measurement technique that can be used for overlay and critical dimensions (CD) measurements through optically opaque layers. The proposed method uses the surface elasticity map to resolve the presence and geometry of subsurface structures. To improve the imaging performance of the AFM based subsurface measurements, we made use of photothermal excitation of the AFM cantilever together with a frequency modulation scheme. The experimental results show a significant improvement in the quality of the image, which leads to a more accurate and reliable CD and overlay measurement.