In mobile devices, such as mobile phones, tablet computers and wearable devices, it is often desirable to protect the display and touch interfaces of the mobile devices with hard materials that can improve the mechanical robustness and reliability. However, adding a layer of hard materials between the sensor and the finger, such as a platen made of glass or plastic, can cause diffraction effects to the display and touch interfaces of the mobile devices.
FIG. 1A illustrates adverse effects of diffraction in an ultrasonic sensor. As shown in FIG. 1A, the diffraction effects can cause areas of the image to be severely attenuated, such as areas referenced by arrows 104. In addition, the diffraction effects can cause areas of the image be inverted with respect to other areas, such as areas references by arrows 106. Moreover, the signal to noise ratio (SNR) may be lower compared to a bare sensor. These adverse effects can lead to poor fingerprint match performance. Thus, it is desirable to correct the effects of diffraction in an ultrasonic sensor.