FIG. 5C illustrates a cross section view of an ultrasonic fingerprint sensor 580, according to another embodiment. Ultrasonic fingerprint sensor 580 operates in a similar manner to, and includes the components of, ultrasonic fingerprint sensor 500 of FIG. 5A, with the exception of the shape of contact layer 590. As illustrated, contact layer 590 has a concave profile, such that contact surface 595 is not flat. While one point or small region of contact surface 595 may be parallel to sensing layer 520 (e.g., the mid-point or valley of the concave shape), contact surface 595 is substantially non-flat and non-parallel to substantially flat sensing layer 520.
Due to the varying thickness of contact layer 590, ultrasonic beams sent from different locations at sensing layer 520 would result in ToF variations, as the path traversed by the ultrasonic beams would vary across the contact layer. In some embodiments, in order to select the signal of interest, e.g., the signal from waves directly reflected from the object on the surface, a measurement window is used.
