Apple Reveals More Details of Touch ID for iPhone, iPad & beyond
On October 17, 2013, the US Patent & Trademark Office published a patent application from Apple that reveals more details behind Apple's new Touch ID fingerprint scanner. Apple reveals that they added a method to the sensor that would allow it to scan a fingerprint at a much higher resolution than other sensors on the market without electrically shocking the finger. Apple also notes that their Touch ID technology could eventually apply to an iPad, MacBook and beyond. Technically speaking, Apple's patent relates to circuits that may be used to support fingerprint sensing, including boost circuits, such as inductive boost circuits.
Apple's Patent Background
Fingerprint sensing technology has become widespread in use and is often used to provide secure access to sensitive electronic devices and/or data. Generally, capacitive fingerprint sensors may be used to determine an image of a fingerprint through measuring capacitance through each capacitive sensing element of a capacitive sensor. Thus, fingerprint ridges provide a higher capacitance in an underlying capacitive sensing element than do fingerprint valleys.
Capacitive fingerprint sensors come in at least two varieties, namely active and passive. Active capacitive sensors are often used in electronic devices to provide biometric security and identification of users.
The Active Capacitive Sensor
Active capacitive sensors initially excite the epidermis of the sensed finger. Capacitance to the epidermis is measured at each capacitive sensing element.
As one example, capacitance may be measured or determined by measuring a capacitive sensing element's voltage and/or charge during a low voltage phase and a high voltage phase of a modulation frequency for the capacitive sensing element array. The difference in voltages may be used to determine capacitance.
As shown in patent FIG. 1 below, the active fingerprint sensor may include both capacitive sensing element array (#102) on sensor chip 100 and drive ring 104. The voltage of capacitive sensing element array 102 is not directly driven or modulated, but instead drive ring 104 is modulated by drive amplifier 106. This, in turn, excites finger 108 and the voltage and/or charge at each capacitive sensing element of capacitive sensing element array 102 varies as drive ring 104 is modulated since finger's 108 voltage potential changes with the modulation of drive ring 104.
In such a sensor, the voltage that may be applied to the drive ring may be limited. Commonly, the drive ring voltage is no more than 4 volts peak-to-peak. Voltages above this may excite the finger to too high a voltage; this excessive excitation may be detected by a person as a "tingling" or uncomfortable feeling in their finger. Although the exact voltage at which one can sense the tingling varies from person to person, a 4 volt peak-to-peak voltage is generally considered as the threshold beyond which the feeling is noticeable.
Since the drive ring's voltage is restricted to avoid user perception, the thickness of any dielectric overlaying the sensor may also be limited. The thicker the dielectric between sensor pad and finger, the more attenuated the resulting capacitance and the blurrier the fingerprint image becomes. For dielectrics having a thickness or more than approximately 100 microns, the fingerprint image may become unreliable.
Another limitation arises when other parts of the user's finger or hand or body may capacitively couple through earth ground to the system, or directly to the system ground when touching other parts of the system. This capacitive coupling from the user to the system may be highly variable depending on how the user is touching the device. This parasitic coupling attenuates the voltage that the drive ring is able drive into the user's finger, and as such reduces the signal. The attenuation may be highly variable depending on how the user is touching the device.
Apple's Touch ID Solution
Apple's invention may take the form of an electronic device which includes a sensor pad comprising an array of individual capacitive sensing elements; a drive ring connected to the sensor pad; and a modulating circuit adapted to control a modulated signal received by the sensor pad while a drive signal received by the drive ring is maintained substantially at a boosted ground exceeding an electronic device ground level.
Generally, Apple's patent discusses a capacitive sensor, such as a fingerprint sensor. The sensor may be formed from a capacitive sensing element array; each capacitive sensing element of the array may register a voltage that varies with the capacitance of a capacitive coupling.
A finger may capacitively couple to the individual capacitive sensing elements of the sensor, such that the sensor may sense a capacitance between each capacitive sensing element and the flesh of the fingerprint. The capacitance signal may be detected by sensing the change in voltage on the capacitive sensing element as the relative voltage between the finger and the sensing chip is changed.
Alternately, the capacitance signal may be detected by sensing the change in charge received by the capacitive sensing elements as the relative voltage between the finger and the sensing chip is changed. Portions of the finger further away from the sensor may create a lower capacitance between the sensor and finger, and thus a lower signal on underlying capacitive sensing elements.
By contrast, portions of the finger closer to the sensor may create a higher capacitance between sensor and finger, and thus higher signals on underlying capacitive sensing elements. Thus, capacitive sensing elements underlying ridges of a fingerprint may register higher signals while capacitive sensing elements underlying valleys of the fingerprint may register a lower capacitance and lower signals.
The fingerprint sensor may include both a sensor pad and a drive ring. Both the sensor pad and drive ring may be placed beneath a dielectric, such that the finger does not directly touch either the sensor pad or drive ring but instead comes into contact with the interposed dielectric. In some examples, the finger may contact the drive ring.
The difference in signals at the sensor capacitive sensing elements may be used to map a fingerprint. The resolution of the sensor may vary with capacitive sensing element density, distance between the sensor pad's surface and the finger, and thickness of a dielectric covering the sensor pad.
Generally, as the dielectric increases in thickness, the capacitance between the finger and sensor grows increasingly attenuated, and so the signal differences may attenuate, as well. Certain embodiments may address this attenuation by applying a higher, less variable, relative voltage change between the finger and the sensor pad, leading to a higher detectable signal at the sensor, as described in more detail herein.
By driving a higher voltage to the sensor, the capacitive coupling between the sensor and finger may compensate for the thickness of the dielectric. By increasing the voltage in this fashion, the capacitance between finger and capacitive sensing elements of the sensor pad may be increased, thereby obtaining better resolution and imaging of the fingerprint.
The sensor may be driven at a higher voltage, such as 12 volts peak-to-peak, 16 volts peak-to-peak, or even more, without inducing any physical sensation in the finger or hand of a user. This may be accomplished by maintaining the drive ring's voltage at a system ground voltage, while the sensor array is modulated with the higher peak-to-peak voltage signal.
By driving a voltage to the sensor relative to system ground, while maintaining the drive ring's voltage at system ground, minimizes the issue of signal attenuation due to the highly variable capacitive coupling between the user and the system ground as a result of other fingers, hands, or body parts coming in contact with other parts of the device.
In Apple's patent FIG. 2 noted below, we see a depiction of one sample embodiment of a fingerprint sensor. The sensor may include sensor pad (#202) formed by an array of individual capacitive sensing elements. Although sensor the pad is shown, the individual capacitive sensing elements are omitted for purposes of clarity.
As previously mentioned, each capacitive sensing element may be capacitively coupled to a portion of finger overlying that capacitive sensing element; the distance between the overlying finger portion and capacitive sensing element determines the capacitance between the two and thus the signal registered at the capacitive sensing element. As distance decreases, signal increases. Each capacitive sensing element is also capacitively coupled to other adjacent overlying finger portions, and as the distance increases, this leads to a blurring effect, which reduces the total signal difference between capacitive sensing elements directly under finger ridges and finger valleys.
The fingerprint sensor may also include first and second drive amplifier 206 and 208, as depicted in FIG. 2. The first drive amplifier may drive a sensor integrated circuit that includes the sensor pad such as an application-specific integrated circuit (ASIC) with a modulating voltage, to induce relative voltage between the sensor pad and the user's finger.
The second drive amplifier may, provide voltage and current to the sensor integrated circuit, so that the sensor integrated circuit can operate from the relative voltage difference between first and second drive amplifiers. The second drive amplifier may supply a modulating voltage in sync with first drive amplifier such that the relative voltage between the outputs of the first and second amplifiers is approximately constant. The first and second drive amplifiers may be connected by a capacitor (#210), which can help both the amplifiers keep the relative voltage between the first and second drive amplifier outputs constant.
The Home Button Fingerprint Sensor for iDevices & beyond
Apple's patent FIG. 3 depicts an iPhone incorporating a fingerprint sensor, such as that shown in patent FIG. 2. Beyond the iPhone, the electronic device may be a tablet computing device, a notebook computer, a personal digital assistant, a desktop computer, a portable media player, and the like.
According to Apple, the sensor pad may be placed below an input mechanism, such as the home button #302 as noted in patent FIG. 3 for the iPhone.
In some embodiments, a flex circuit may extend from the input mechanism stack-up and connect the sensor chip to a signal provider chip that may facilitate modulating the voltage and/or operating frequency of the fingerprint sensor chip. We've seen this in the iFixit teardown.
Future Devices may include the Fingerprint Scanner beneath the Display Screen
Apple further notes that the sensor pad may alternatively be placed beneath a portion of the display screen noted as #304 of a device, beneath a sidewall noted as #306 or another portion of the device's enclosure, and the like. Essentially, any portion of the electronic device's enclosure may house the fingerprint sensor.
In the home button, an ink layer and/or adhesive may be placed between the button's bottom surface and the sensor chip's top surface. The adhesive may bond the chip to the button, for example. One or more solder balls may affix the fingerprint sensor chip to a flex conductor. The solder balls may generally be placed near the center of the fingerprint sensor chip to reduce the likelihood of cracking due to stress.
In still other embodiments, a number of fingerprint sensors as described herein may be employed in a single electronic device. For example, the fingerprint sensors may be arranged beneath a cover glass or outer surface of an electronic device. In some embodiments, the sensors may be arranged in an array or other ordered pattern. In other embodiments, the sensors may be placed randomly or semi-randomly beneath the surface.
In still other embodiments, the sensors may be placed within or beneath a display stack of an electronic device incorporating a display, such as a mobile phone, tablet computing device, computer display screen, and the like. In such embodiments, the sensor(s) may capacitively sense a fingerprint through the display and/or cover glass of the display.
Apple's patent FIG. 6 is a schematic illustration of a Fingerprint sensing system.
Apple credits Jean-Marie Bussat, Steven Hotelling, and Benjamin Lyon as the inventors of this patent application which was originally filed under serial number 842635 in Q2 2013 or 6 months prior to the debut of the Touch ID feature for the iPhone 5S.
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