Today the U.S. Patent and Trademark Office officially granted Apple a patent that relates to improving the accuracy of ultrasonic touch sensing and fingerprint imaging on buttons like the iPad Air's Touch ID on the top button as shown in our cover graphic. While the execution of ultrasonic fingerprint imaging on the iPad Air's top button looks simple enough to consumers, the technicalities behind this feature is quite steep.

Apple notes that many types of electronic devices are presently available that are capable of receiving touch input to initiate operations. Examples of such devices include desktop, laptop and tablet computing devices, smartphones, media players, wearables such as watches and health monitoring devices, smart home control and entertainment devices, headphones and earbuds, and devices for computer-generated environments such as augmented reality, mixed reality, or virtual reality environments.

The potential for easy access to device functions and/or sensitive information creates a need for secure authentication and access.

Ultrasonic touch sensing and fingerprint imaging systems can be adversely affected by acoustic impedance mismatches as ultrasonic waves travel to a touch surface such as a sensing plate. These mismatches can cause undesired reflections of the ultrasonic waves, in addition to a reduction in the energy of the ultrasonic waves that eventually reach the sensing plate and are reflected back with different amplitudes depending on whether an object (e.g., a fingerprint ridge) is touching the sensing plate.

Geometric Structures For Acoustic Impedance Matching And Improved Touch Sensing And Fingerprint Imaging

Apple's granted patent focuses on improving the accuracy of ultrasonic touch sensing and fingerprint imaging using acoustic impedance matching.

Acoustic impedance mismatches between an ultrasonic transducer array and a sensing plate can be reduced to maximize energy transfer and minimize parasitic reflections. A reduction in acoustic impedance mismatches can be accomplished using (i) a composite epoxy having a higher acoustic impedance than epoxy alone, (ii) one or more matching layers having an acoustic impedance that is approximately the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array, (iii) pores or perforations in the sensing plate, or (iv) geometric structures formed in the sensing plate. In addition, parasitic reflections can be suppressed using an absorbent layer.

Apple's patent FIG. 2 below illustrates a block diagram of an electronic device including ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching; FIG. 4A illustrates a touch sensing stackup using an array of piezoelectric micro-machined ultrasonic transducers (PMUTs); FIG. 4B illustrates parasitic reflections from acoustic impedance mismatches and a finger signature reflected back from the touch surface of a sensing plate when an epoxy with low acoustic impedance is used. 

Apple's patent FIG. 6A above illustrates a matching layer affixed to a sensing plate while separated from an epoxy and a PMUT array; FIG. 6B illustrates a matching layer sandwiched between a sensing plate and a PMUT array; FIG. 6C illustrates the transmittance of energy of an ultrasonic signal through a single matching layer of a particular material over a range of frequencies and layer thicknesses.

For richer details, engineers will appreciate viewing Apple's granted patent  11715321.

Some of the Team Members on this Apple Project

• Ehsan Khajeh: Sensor & Hardware Lead a
• Camille L.M. Everhart, PhD: Product Design Engineer
• Dan Hiemstra: Senior Product Design Engineer
• George Mak: Product Design Engineer