Ultrasonic authenticating button

A method of detecting a finger using an ultrasonic button, wherein the ultrasonic button comprises an ultrasonic transmitter, a piezoelectric receiver layer, and an ultrasonic sensor array, comprising:

• transmitting an ultrasonic wave from the ultrasonic transmitter, wherein the ultrasonic wave passes through a platen layer and a first matching layer;
• matching an acoustic impedance of the platen layer with an acoustic impedance of ridges of the finger at the first matching layer, wherein the first matching layer has an acoustic impedance approximately equal to a geometric mean of the acoustic impedance of the ridges of the finger and the acoustic impedance of the platen layer;
• receiving a reflected wave of the ultrasonic wave at the piezoelectric receiver layer, wherein the reflected wave passes through the platen layer and the first matching layer;
• detecting the finger based on the reflected wave using the ultrasonic sensor array;
• and
• providing a haptic feedback to the finger with the ultrasonic sensor array and a haptic device, wherein the ultrasonic sensor array and the haptic device are mechanically coupled to each other.

An ultrasonic button, comprising:

• means for transmitting an ultrasonic wave;
• wherein the ultrasonic wave passes through a platen layer and a first matching layer;
• means for matching an acoustic impedance of the platen layer with an acoustic impedance of ridges of a finger;
• means for receiving a reflected wave of the ultrasonic wave, wherein the reflected wave passes through the platen layer and the first matching layer, wherein the first matching layer has an acoustic impedance approximately equal to a geometric mean of the acoustic impedance of the ridges of the finger and the acoustic impedance of the platen layer;
• means for detecting the finger based on the reflected wave;
• and
• means for providing a haptic feedback to the finger, wherein the means for receiving a reflected wave of the ultrasonic wave and the means for providing a haptic feedback to the finger are mechanically coupled to each other.