WO2023051231A1

WO2023051231A1 - Ultrasonic fingerprint apparatus and electronic device

Info

Publication number: WO2023051231A1
Application number: PCT/CN2022/118477
Authority: WO
Inventors: 杜灿鸿
Original assignee: 深圳市汇顶科技股份有限公司
Priority date: 2021-09-30
Filing date: 2022-09-13
Publication date: 2023-04-06

Abstract

An ultrasonic fingerprint apparatus (2) and an electronic device (3). The ultrasonic fingerprint apparatus (2) is arranged under a display screen (1) of the electronic device (3), so as to realize under-screen ultrasonic fingerprint recognition. The ultrasonic fingerprint apparatus (2) comprises an ultrasonic fingerprint chip (20) and a piezoelectric transducer (10), which is arranged above the ultrasonic fingerprint chip (20), wherein the piezoelectric transducer (10) comprises an upper electrode (120), a lower electrode (130) and a piezoelectric layer (110), which is located between the upper electrode (120) and the lower electrode (130); the lower electrode (130) is arranged on a surface of the ultrasonic fingerprint chip (20); the upper electrode (120) is connected to a signal transmission circuit (31) under the ultrasonic fingerprint chip (20); the signal transmission circuit (31) is used for generating an excitation signal and loading same onto the upper electrode (120), such that the piezoelectric layer (110) is excited to transmit an ultrasonic signal to a finger above the display screen (1); the ultrasonic fingerprint chip (20) is a CMOS chip, and comprises a signal receiving circuit (21); the signal receiving circuit (21) is connected to the lower electrode (130), so as to receive an ultrasonic detection signal, which is generated between the upper electrode (120) and the lower electrode (130) when an ultrasonic signal that is returned by the finger acts on the piezoelectric layer (110); and the ultrasonic detection signal is used for acquiring a fingerprint image of the finger.

Description

Ultrasonic Fingerprint Devices and Electronics

technical field

The embodiments of the present application relate to the field of fingerprint identification, and more specifically, relate to an ultrasonic fingerprint device and electronic equipment.

Background technique

With the advancement of society, mobile phones have become one of the indispensable electronic devices in modern life. Mobile phones currently on the market have one or more identity authentication methods, including digital passwords, gesture graphics, facial recognition, fingerprint recognition, and the like. Among them, fingerprint recognition has become the standard configuration of most mobile phones due to its convenient application, fast recognition speed, stability and reliability. Fingerprint recognition has also developed different technical routes, including capacitive fingerprint recognition, optical fingerprint recognition and ultrasonic fingerprint recognition.

Due to the strong penetrating ability of ultrasound, ultrasonic fingerprint recognition can not only identify the surface morphology of fingerprints, but also identify the signal of the dermis of the finger. Therefore, ultrasonic fingerprint recognition has gradually become a new fingerprint recognition method. Ultrasonic fingerprint devices usually include a piezoelectric transducer and an ultrasonic fingerprint chip. How to realize the integration between the piezoelectric transducer and the ultrasonic fingerprint chip and make the ultrasonic fingerprint chip have a higher circuit sensitivity has become a problem to be solved.

Contents of the invention

Embodiments of the present application provide an ultrasonic fingerprint device and electronic equipment, which can realize integration between a piezoelectric transducer and an ultrasonic fingerprint chip, and the ultrasonic fingerprint chip has high circuit sensitivity.

In the first aspect, an ultrasonic fingerprint device is provided. The ultrasonic fingerprint device is arranged under the display screen of an electronic device to realize ultrasonic fingerprint recognition under the screen. The ultrasonic fingerprint device includes an ultrasonic fingerprint chip and is arranged on the ultrasonic Piezoelectric transducer above the fingerprint chip;

The piezoelectric transducer includes an upper electrode, a lower electrode, and a piezoelectric layer between the upper electrode and the lower electrode, the lower electrode is arranged on the surface of the ultrasonic fingerprint chip, and the upper electrode Connected to the signal transmitting circuit below the ultrasonic fingerprint chip, the signal transmitting circuit is used to generate an excitation signal and load it to the upper electrode, so as to excite the piezoelectric layer to send an ultrasonic signal to the finger above the display screen;

The ultrasonic fingerprint chip is a CMOS chip, and the ultrasonic fingerprint chip includes a signal receiving circuit, and the signal receiving circuit is connected to the lower electrode to receive the ultrasonic signal returned by the finger when it acts on the piezoelectric layer. An ultrasonic detection signal generated between the upper electrode and the lower electrode, the ultrasonic detection signal is used to obtain the fingerprint image of the finger.

The embodiment of the present application adopts the method of integrating the piezoelectric layer and the CMOS chip to form the ultrasonic fingerprint device, which not only retains the advantage of low manufacturing cost of the piezoelectric layer, but also utilizes the CMOS integration process, and can manufacture complex integrated circuits on the CMOS, In order to improve the sensitivity of the circuit and improve the performance of ultrasonic fingerprint recognition.

In an implementation manner, the signal transmitting circuit includes a first switch and a second switch, the upper electrode is connected to an AC power supply through the first switch, and is connected to a DC power supply through the second switch; The signal receiving circuit includes an operational amplifier connected to the lower electrode, and a third switch connected between the lower electrode and a reference voltage or between the lower electrode and ground.

In an implementation manner, the ultrasonic fingerprint chip further includes a control circuit, and each pulse period of the excitation signal includes a first period and a second period, and the control circuit is used to: in the first period, control The first switch and the third switch are closed, and the second switch is opened, so as to apply the excitation signal to the upper electrode through the AC power supply; during the second period, control the first The switch and the third switch are opened, and the second switch is closed to pass the ultrasonic detection signal received from the lower electrode through the operational amplifier.

In an implementation manner, the lower electrode is an electrode array composed of a plurality of electrodes, and the plurality of electrodes are respectively connected to different operational amplifiers.

In one implementation, the lower electrode is an electrode array composed of a plurality of electrodes, and the plurality of electrodes are M groups, wherein each group includes N electrodes, and the N electrodes are respectively passed through N fourth switches connected to the same operational amplifier, and the control circuit is further configured to: in the second period, time-divisionally control the N fourth switches, so as to time-divisionally control the N fourth switches through the same operational amplifier The electrodes receive the ultrasonic detection signal.

In one implementation, the operational amplifier includes a positive input terminal, a negative input terminal, and an output terminal, and the signal receiving circuit further includes a capacitor unit and a reset switch, and the capacitor unit is connected across the negative input terminal and the Between the output terminals, the reset switch is connected in parallel with the capacitor unit, the negative input terminal is used for connecting the lower electrode, and the positive input terminal is used for connecting a reference voltage.

In an implementation manner, the ultrasonic fingerprint chip further includes a shielding electrode disposed on its surface, the shielding electrode is disposed along the periphery of the lower electrode, and the shielding electrode is grounded or connected to a fixed voltage. The shielding electrode can make the fringe electric field of the lower electrode more uniform, and reduce the sensitivity of transmitting and receiving signals in the edge region of the lower electrode caused by the inhomogeneity of the fringe electric field; and, the shielding electrode can prevent external interference signals from passing through the fringe region of the lower electrode Coupled to the lower electrode to avoid interference signals from affecting the receiving sensitivity.

In an implementation manner, the ultrasonic fingerprint chip further includes driving wires arranged on its surface, the upper electrode is drawn from the upper surface of the piezoelectric layer to the surface of the ultrasonic fingerprint chip, and passes through the The driving wire is connected to one end of the lead, and the other end of the lead is connected to the signal transmitting circuit.

On the one hand, the density of the upper electrode material is very low and cannot support the bonding process. By setting the driving wires on the ultrasonic fingerprint chip, the upper electrode can be led out to the surface of the ultrasonic fingerprint chip through the driving wires. On the other hand, since a CMOS chip is used, a bonding process can be implemented on the surface of the chip, so as to connect the driving line and the signal transmitting circuit through the wire. In this way, the interconnection between the upper electrode and the signal transmitting circuit is realized.

In an implementation manner, the ultrasonic fingerprint chip further includes a passivation layer covering the driving wiring, the passivation layer is provided with an opening, and the upper electrode is connected from the piezoelectric layer. The surface extends into the window, so as to connect the part of the driving trace located in the window. In this way, the electrical connection between the upper electrode and the driving wiring is realized.

In an implementation manner, the material of the piezoelectric layer is polyvinylidene fluoride PVDF, or the material of the piezoelectric layer is polyvinylidene fluoride-trifluoroethylene copolymer PVDF-TrFE.

In an implementation manner, the frequency of the ultrasonic signal is between 9 MHz and 14 MHz.

In one implementation, the upper electrode is a silver paste coating formed on the upper surface of the piezoelectric layer, the silver paste coating is a mixture of silver powder and glue, and the acoustic impedance of the upper electrode is 5MRayl to 10MRayl.

In an implementation manner, the ratio D1/λ between the thickness D1 of the upper electrode and the wavelength λ of the ultrasonic signal is negatively correlated with the transmittance of the ultrasonic signal in the upper electrode, D1= λ×P=(v/f)×P, wherein, v is the transmission velocity of the ultrasonic signal in the upper electrode, f is the frequency of the ultrasonic signal, and P is D1/ The value of lambda. In this way, the optimal thickness D1 of the upper electrode can be effectively determined according to the frequency and sound velocity of ultrasonic waves used in ultrasonic fingerprint detection, and the signal transmission performance of the ultrasonic fingerprint device can be improved.

For example, the thickness D1 of the upper electrode is between 4um and 8um.

In an implementation manner, the material of the ultrasonic fingerprint chip is silicon, and the acoustic impedance of the ultrasonic fingerprint chip is 14 MRayl to 20 MRayl.

In an implementation manner, the thickness of the ultrasonic fingerprint chip D2=λ/4+(λ/2)×k, where λ is the wavelength of the ultrasonic signal, and k is 0 or a positive integer. Since the round-trip time t=2×D2/u of the ultrasonic signal transmitted in the ultrasonic fingerprint chip, u is the transmission speed of the ultrasonic signal in the ultrasonic fingerprint chip, when t=T/2+T× When k, T is the cycle of the ultrasonic signal. When the back-facing ultrasonic wave returns to the surface of the ultrasonic fingerprint chip, the difference between the initial phase of its vibration and the initial phase of the sinusoidal ultrasonic wave is 180°, which is exactly 360° after phase superposition, which is quite Because the reflected backward ultrasonic wave is just in phase with the sinusoidal ultrasonic wave, the amplitude of the ultrasonic wave is enhanced. Therefore, the thickness D2=λ/4+(λ/2)×k of the ultrasonic fingerprint chip obtained by 2×D2/u=T/2+T×k can improve the signal transmission performance of the ultrasonic fingerprint device.

For example, the thickness D2 of the ultrasonic fingerprint chip is between 100um and 140um.

In one implementation, the ultrasonic fingerprint device further includes a reinforcing board 30, the ultrasonic fingerprint chip and the signal transmitting circuit are arranged on the reinforcing board, and the ultrasonic fingerprint chip and the reinforcing board They are connected by a glue layer, the acoustic impedance of the glue layer is less than or equal to 5MRayl, so as to ensure that the reflectivity of the interface between the ultrasonic fingerprint chip and the glue layer on the lower surface is large enough to ensure the signal volume of the ultrasonic signal.

In an implementation manner, the piezoelectric layer is a piezoelectric film, and the thickness of the piezoelectric film is 9um.

In a second aspect, an electronic device is provided, including: a display screen; and, according to the first aspect or the ultrasonic fingerprint device described in any implementation manner of the first aspect, the ultrasonic fingerprint device is arranged on the display screen Below, to realize the ultrasonic fingerprint recognition under the screen.

Description of drawings

FIG. 1 is a schematic perspective view of an ultrasonic fingerprint device according to an embodiment of the present application.

FIG. 2 is a schematic cross-sectional view of an ultrasonic fingerprint device according to an embodiment of the present application.

Fig. 3 is a schematic diagram of electric field distribution at the edge of the lower electrode when there is no shielding electrode.

Fig. 4 is a schematic diagram of electric field distribution at the edge of the lower electrode when there is a shielding electrode.

Fig. 5 is a schematic diagram of a possible implementation manner of the signal receiving circuit of the embodiment of the present application.

Fig. 6 is a schematic diagram of another possible implementation manner of the signal receiving circuit of the embodiment of the present application.

Fig. 7 is a schematic diagram of a possible structure of the receiving circuit.

Fig. 8 is a schematic diagram of the thickness of the upper electrode, the frequency of the ultrasonic signal and its transmittance on the upper electrode.

Fig. 9 is a schematic diagram of the corresponding relationship between the ratio of the thickness of the upper electrode to the wavelength of the ultrasonic signal and the transmittance.

Fig. 10 is a schematic diagram of the transmission of ultrasonic signals in the ultrasonic fingerprint chip.

Fig. 11 is a schematic block diagram of an electronic device according to an embodiment of the present application.

Detailed ways

The technical solution in this application will be described below with reference to the accompanying drawings.

Ultrasonic fingerprint detection is implemented based on piezoelectric transducers, which are usually sandwich structures, which include an upper electrode, a lower electrode, and a piezoelectric layer between the upper electrode and the lower electrode. Piezoelectric transducers need to be connected with relatively complex transmitting circuits and receiving circuits to work, and the transmitting and receiving circuits are usually made into application-specific integrated circuits to reduce the size and complexity of the system. Integrating the piezoelectric transducer with the integrated circuit can further reduce the size of the system while minimizing the noise caused by the interconnection lines between the piezoelectric transducer and the circuit.

The ultrasonic fingerprint device can adopt the "MEMS+CMOS" structure, wherein, CMOS is a conventional integrated circuit technology, and the MEMS piezoelectric transducer is made on the surface of the CMOS chip. This configuration process is complicated and the cost is high; the ultrasonic fingerprint device also The structure of "piezoelectric film + TFT" can be adopted, and the piezoelectric film can be made on the surface of TFT by smearing. Due to the low cost of the TFT circuit, the ultrasonic fingerprint device with this structure is simple and low-cost. It is impossible to make complex circuits, resulting in low circuit sensitivity and affecting the performance of the ultrasonic fingerprint device.

For this reason, the embodiment of the present application adopts the structure of "piezoelectric film + CMOS" and combines a certain integration process to form an ultrasonic fingerprint device. While reducing the cost, the circuit sensitivity of the ultrasonic fingerprint device is also ensured.

FIG. 1 and FIG. 2 show schematic diagrams of an ultrasonic fingerprint device according to an embodiment of the present application. FIG. 1 is a perspective view of an ultrasonic fingerprint device 2 , and FIG. 2 is a cross-sectional view of the ultrasonic fingerprint device 2 . As shown in Figures 1 and 2, the ultrasonic fingerprint device 2 is arranged under the display screen 1 of the electronic device to realize ultrasonic fingerprint recognition under the screen. The ultrasonic fingerprint device 2 includes an ultrasonic fingerprint chip 20 and an Piezoelectric transducer 10.

Wherein, the piezoelectric transducer 10 includes an upper electrode 120, a lower electrode 130, and a piezoelectric layer 110 between the upper electrode 120 and the lower electrode 130, the lower electrode 130 is arranged on the surface of the ultrasonic fingerprint chip 20, and the upper electrode 120 and The signal transmitting circuit 31 under the ultrasonic fingerprint chip 20 is connected, and the signal transmitting circuit 31 is used to generate an excitation signal and load it to the upper electrode 120 to excite the piezoelectric layer 110 to send an ultrasonic signal to the finger above the display screen 1 .

Ultrasonic fingerprint chip 20 is a complementary metal oxide semiconductor (Complementary Metal-Oxide-Semiconductor Transistor, CMOS) chip, and ultrasonic fingerprint chip 20 includes signal receiving circuit 21, and signal receiving circuit 21 is connected with lower electrode 130, to receive the ultrasonic signal that finger returns The ultrasonic detection signal generated between the upper electrode 120 and the lower electrode 130 when acting on the piezoelectric layer 110 is used to obtain the fingerprint image of the finger.

The ultrasonic fingerprint device 2 is formed by integrating the piezoelectric layer 110 and the CMOS chip 20, which not only retains the low manufacturing cost of the piezoelectric layer 110, but also utilizes the CMOS integration process to make complex integrated circuits on CMOS. In order to improve the sensitivity of the circuit and improve the performance of ultrasonic fingerprint recognition.

The piezoelectric layer 110 is a piezoelectric film formed of a material having a piezoelectric effect, such as polyvinylidene fluoride (PVDF) or polyvinylidene fluoride-trifluoroethylene copolymer (PVDF-TrFE).

Further, as shown in FIG. 1 and FIG. 2 , the ultrasonic fingerprint device 2 also includes a reinforcing board 30, on which the ultrasonic fingerprint chip 20 and the signal transmitting circuit 31 are arranged, and the ultrasonic fingerprint chip 20 and the reinforcing board 30 are arranged on the reinforcing board 30. They are connected by an adhesive layer 23, such as glue, die attach film (Die Attach Film, DAF) or epoxy resin. The reinforcement board 30 may be a circuit board 30 , and the signal transmitting circuit 31 may be fabricated on the surface of the circuit board 30 .

The waveguide layer 40 includes a protective layer for the upper electrode 120 , a coupling layer between the piezoelectric transducer 10 and the display screen 1 , an adhesive layer between the coupling layer and the upper electrode 120 and other stacked layers. When the piezoelectric layer 110 is in the transmitting state, the ultrasonic signal is coupled to the application layer 1 through the waveguide layer 40. The application layer 1 is the object of the ultrasonic wave, including the cover plate, the display screen, and other objects on which the ultrasonic signal acts. Here, the display screen 1 as an example; when the piezoelectric layer 110 is in a receiving state, the waveguide layer 40 couples ultrasonic waves from the application layer 1 to the piezoelectric layer 110. The waveguide layer 40 also serves as a protective layer for the upper electrode 120 to prevent the upper electrode 120 from being damaged due to exposure to the environment.

The ultrasonic fingerprint chip 20 also includes a shielding electrode 25, which is arranged along the periphery of the lower electrode 130, and the shielding electrode 25 is grounded to GND or connected to a fixed voltage. Hereinafter, the shield electrode 25 is grounded as an example.

The ultrasonic fingerprint chip 20 includes multiple metal layers, and the top one is the top metal layer TM. The lower electrode 130 and the shielding electrode 25 are fabricated on the top metal layer TM, and the lower electrode 130 is connected to the receiving circuit 21 in other metal layers of the ultrasonic fingerprint chip 20 . As shown in FIG. 3 , when the shielding electrode 25 is not provided on the side of the lower electrode 120, the electric field in the edge region of the lower electrode 130 is not uniform, and the electric field in the edge region of the lower electrode 130 is weakened, which will lead to signal fluctuations in the edge region of the lower electrode 130. Transmit and receive sensitivity is reduced.

As shown in Figure 4, when the shielding electrode 25 is arranged on the side of the lower electrode 120, the shielding electrode 25 bears the uneven electric field at the edge of the lower electrode 130, so that the electric field at the edge region of the lower electrode 130 is more uniform, reducing the Sensitivity of signal transmission and reception in the edge region of the lower electrode 130 is reduced due to the non-uniform electric field. Moreover, the shielding electrode 25 can also prevent external interference signals from being coupled to the lower electrode 130 through the edge region of the lower electrode 130 , so as to prevent the interference signals from affecting the receiving sensitivity.

FIG. 4 is a cross-sectional view of the ultrasonic fingerprint device 2 , the shielding electrode 130 is disposed on the right side of the lower electrode 130 . In practical applications, the shielding electrode 130 can also be arranged on other sides of the lower electrode 130 , or be arranged around the lower electrode 130 on the top metal layer TM.

Continuing to refer to FIG. 2 to FIG. 4 , in an implementation manner, the ultrasonic fingerprint chip 20 further includes a driving wire 22 disposed on its top metal layer TM, and the upper electrode 120 is drawn from the upper surface of the piezoelectric layer 110 to the ultrasonic fingerprint chip. The surface of the chip 20 is connected to one end of the lead 50 through the driving wire 22 , and the other end of the lead 50 is connected to the signal transmitting circuit 31 .

On the one hand, since the density of the material of the upper electrode 120 is very low, it cannot support the bonding process. By setting the driving wire 22 on the ultrasonic fingerprint chip 20, the upper electrode 120 can be led out to the ultrasonic fingerprint chip 20 through the driving wire 22. surface. On the other hand, since the ultrasonic fingerprint chip 20 adopts a CMOS chip, a bonding process can be implemented on the chip surface to connect the driving wire 22 and the signal transmitting circuit 31 through the lead wire 50 . In this way, the interconnection between the upper electrode 120 and the signal transmitting circuit 31 is realized.

The passivation layer 24 is a protective layer on the top metal layer TM on the surface of the ultrasonic fingerprint chip 20. The lower electrode 130, the shielding electrode 25, and the driving wiring 22 are all covered with the passivation layer 24, but the passivation layer is located on the lower electrode. The region above the shield electrode 130 and the shield electrode 25 needs to open a window so as to make contact between the bottom electrode 130 and the piezoelectric layer 110 .

In addition, in order to realize the electrical connection between the upper electrode 120 and the driving wire 22, the passivation layer 24 is also provided with an opening 241 corresponding to the driving wire 22, and the upper electrode 120 extends from the upper surface of the piezoelectric layer 110. to the inside of the window 241 to connect the part of the driving wire 22 inside the window 241 .

Hereinafter, the signal receiving circuit 21 and the signal transmitting circuit 31 of the embodiment of the present application will be described in detail.

In one implementation, referring to FIG. 2 , the signal transmitting circuit 31 includes a first switch K1 and a second switch K2, the upper electrode 120 is connected to an AC power supply AC through the first switch K1, and connected to a DC power supply through the second switch K2. Power supply DC; the signal receiving circuit 21 includes an operational amplifier 211 connected to the lower electrode 130 and a third switch K3 connected between the lower electrode 130 and the reference voltage Vx or between the lower electrode 130 and the ground GND.

The ultrasonic fingerprint chip 20 also includes a control circuit (not shown in FIG. 2 ), the excitation signal generated by the signal transmitting circuit 31 has a pulse cycle, wherein each pulse cycle includes a first period T1 and a second period T2, and the control circuit is used for:

In the first period T1, the first switch K1 and the third switch K3 are controlled to be closed, and the second switch K2 is turned off, so as to apply an excitation signal with a frequency f to the upper electrode 120 through the alternating current power supply AC, so as to excite the piezoelectric layer through the excitation signal 110 generating an ultrasonic signal;

In the second period T2, the first switch K1 and the third switch K3 are controlled to be turned off, and the second switch K2 is turned on. Since the direct current power supply DC is connected to the upper electrode 120 at this time, when the ultrasonic signal returned by the finger acts on the piezoelectric layer 110, An AC reception signal is generated between the upper electrode 120 and the lower electrode 130, that is, the ultrasonic detection signal of the finger. Since the upper electrode 120 is connected to a fixed direct current power supply DC, all the received signals will be input to the operational amplifier 211 and completed through the operational amplifier 211. The capture of the electrical signal converted from the ultrasonic signal.

2 to 4 above, the lower electrode 130 is a single electrode as an example. When imaging a fingerprint, an electrode array composed of multiple electrodes is usually used as the lower electrode 130, that is to say, the lower electrode 130 is divided into transmission Multiple channels of ultrasonic detection signals, where each electrode corresponds to a pixel in the fingerprint image.

For example, as shown in FIG. 5 , multiple electrodes in the electrode array of the lower electrode 130 are respectively connected to different operational amplifiers 211 , each electrode can work independently, and multiple electrodes simultaneously output ultrasonic detection signals of fingers.

For another example, as shown in FIG. 6, the plurality of electrodes in the electrode array of the lower electrode 130 can be divided into M groups, wherein each group includes N electrodes, and the N electrodes of each group pass through N fourth switches K41 to K4n is connected to the same operational amplifier 211 . That is, every N electrodes share one operational amplifier 211 . At this time, the control circuit is also used to: in the second period T2, control the N fourth switches K41 to K4n in time division, so as to receive the ultrasonic detection signals of the finger from the N electrodes through the same operational amplifier 211 in time division.

In FIG. 6 , N=4 is taken as an example, that is, every 4 electrodes share one operational amplifier 211 . Switch K41, switch K42, switch K43 and switch K44 are time-division multiplexed. In the second period T2, the first switch K1 and the third switch K3 are turned off, the second switch K2 is turned on, and the switch K41, the switch K42, the switch K43 and the switch K44 are turned on at different times, so as to respectively receive corresponding electrodes at different times. The output ultrasonic fingerprint signal.

FIG. 7 shows a possible structure of the signal receiving circuit 21 . Each of the above-mentioned switches can be realized by, for example, transistors. As shown in FIG. 7, the operational amplifier 211 includes a positive input terminal (+), a negative input terminal (-) and an output terminal V _OUT , and the signal receiving circuit 21 also includes a capacitor unit C _INT and a reset switch RES, and the capacitor unit C _INT spans Connected between the negative input terminal (-) and the output terminal (V _OUT ), the reset switch RES is connected in parallel with the capacitor unit C _INT , the negative input terminal (-) is used for connecting the lower electrode 130, and the positive input terminal (+) is used for Connect the reference voltage Vx.

In order to further improve the signal transmission performance of the ultrasonic fingerprint device 2, its acoustic performance needs to be optimized, for example, the thickness D1 of the upper electrode 120, the thickness D3 of the piezoelectric layer, the thickness D2 of the ultrasonic fingerprint chip 20, and the adhesive layer 23 in the stack The thickness etc. are optimized to reduce the loss caused by each stack to the ultrasonic signal transmission process. For different operating frequencies f of ultrasonic signals, the optimal thickness of each laminate is different.

The ultrasonic signal is generated by the signal transmitting circuit 31 , and the application does not limit the specific structure of the signal transmitting circuit 31 , as long as it can generate an ultrasonic signal of frequency f. The operating frequency f of the ultrasonic signal used for ultrasonic fingerprint identification is usually between 9 MHz and 14 MHz; preferably, the frequency f is between 11 MHz and 15 MHz.

First, the design of the thickness D1 of the upper electrode 120 will be described.

Usually, the upper electrode 120 is a silver paste coating formed on the upper surface of the piezoelectric layer 110, the silver paste coating is a mixture of silver powder and glue, the transmission speed of the ultrasonic signal in the upper electrode 120 is the speed of sound, and the speed of the upper electrode 120 The acoustic impedance is between silver powder and glue. For example, the sound velocity is between 1.5um/ns and 2.5um/ns, and the acoustic impedance is between 5MRayl and 10MRayl.

The ultrasonic signal generated by the piezoelectric layer 110 must pass through the silver paste to reach the waveguide layer 40, and the transmittance of the silver paste to the ultrasonic wave is related to the following factors: operating frequency f, thickness D1 of the silver paste, and acoustic parameters of the silver paste , the acoustic parameters of the piezoelectric layer 110 and the waveguide layer 40 .

In Fig. 8, the sound velocity in the above electrode 120 is 2um/ns, the acoustic impedance is 8MRayl, the acoustic impedance of the piezoelectric layer 110 and the waveguide layer 40 are both 2MRayl as an example, different thicknesses D1 and different frequencies f are obtained based on theoretical calculations The change of the transmittance of the corresponding ultrasonic signal.

It can be seen that the smaller the thickness D1 is, the lower the frequency f of the ultrasonic signal is, and the higher the transmittance of the ultrasonic signal in the upper electrode 120 is. If D1/λ is used to measure the transmittance, a rule independent of frequency f can be obtained. As shown in FIG. 9, the ratio D1/λ between the thickness D1 of the upper electrode 120 and the wavelength λ of the ultrasonic signal is related to The transmittance in the upper electrode 120 is negatively correlated. Wherein, the smaller D1/λ is, the larger the transmittance of the ultrasonic signal is.

In one implementation, D1=λ×P=(v/f)×P, wherein, v is the transmission speed of the ultrasonic signal in the upper electrode 120, f is the frequency of the ultrasonic signal, and P is the transmittance relative to the target The corresponding value of D1/λ. The target transmittance is, for example, the desired transmittance. In this way, the optimal thickness D1 of the upper electrode 120 can be effectively determined according to the ultrasonic frequency f and sound velocity v used in ultrasonic fingerprint detection, and the signal transmission performance of the ultrasonic fingerprint device 2 can be improved.

Taking the acceptance criterion of a transmittance of 80% as an example, as shown in FIG. 9 , it can be obtained that D1/λ corresponding to a transmittance of 80% should be less than 0.06. Assuming that the frequency f is between 10 MHz and 20 MHz, the transmission speed v of the ultrasonic signal in the upper electrode 120 is 2 um/ns. Based on D1=λ×P=(v/f)×P, when f=20MHz, the thickness of the corresponding upper electrode 120 D1=(2um/ns/20MHz)×0.06=6um; when f=10MHz, the corresponding The thickness D1 of the upper electrode 120 = (2um/ns/10MHz) × 0.06 = 12um; when f is other values, similarly, the optimal thickness D1 can be obtained. The lower the frequency f, the greater the allowable thickness D1, see Table 1 for details.

Table I

Based on the above method, the thickness D1 of the upper electrode 120 in the embodiment of the present application may be between 4um and 8um.

The design of the thickness D3 of the piezoelectric layer 110 depends on the operating frequency f, the material properties of the piezoelectric layer 110 and the like. Generally speaking, the higher the operating frequency f, the thinner the optimum thickness D3 of the piezoelectric layer 110 is. The present application does not limit the thickness D3 of the piezoelectric layer 110 . For example, the thickness D3 of the piezoelectric layer 110 may be 9 um, for example.

Next, the design of the thickness D2 of the ultrasonic fingerprint chip 20 will be described.

As shown in FIG. 10 , when the piezoelectric layer 110 vibrates, in addition to the ultrasonic signal transmitted through the piezoelectric layer 110 to the waveguide layer 40 , that is, the forward ultrasonic wave 101 , there is also a part of the ultrasonic signal transmitted to the inside of the ultrasonic fingerprint chip 20 . That is, the back-facing ultrasonic wave 102a, the back-facing ultrasonic wave 102a is transmitted to the interface of the "chip-adhesive layer" at the bottom of the ultrasonic fingerprint chip 20, the back-facing ultrasonic wave 102a will be reflected back to the ultrasonic fingerprint chip 20, and this part of the reflected back-facing ultrasonic wave 102b is transmitted to the guide When the wave layer 40 is superimposed with the forward ultrasonic wave 101, the thickness D2 of the ultrasonic fingerprint chip 20 will affect the transmission delay of the reflected back-facing ultrasonic wave 102b, thereby affecting its phase, resulting in the reflected back-facing ultrasonic wave 102b being different from the forward-facing ultrasonic wave 102b. There are differences in the superimposition effect between ultrasonic 101.

The material of the ultrasonic fingerprint chip 20 is silicon, and the single crystal silicon is processed into the ultrasonic fingerprint chip 20. The processing technology of the ultrasonic fingerprint chip 20 will affect the acoustic parameters of the single crystal silicon. Generally speaking, the acoustic parameters of the ultrasonic fingerprint chip 20 The impedance is 14Mrayl to 20Mrayl, the typical value is 15Mrayl.

In order to ensure that the reflectivity of the interface between the ultrasonic fingerprint chip 20 and the adhesive layer 23 on its lower surface is large enough to ensure the signal volume of the ultrasonic signal, in some implementations, the acoustic impedance of the adhesive layer 23 is less than or equal to 5 MRayl, for example 2MRayl to 4Mrayl, the typical value is 3Mrayl. Compared with silicon, the acoustic impedance of the adhesive layer 23 is much smaller. The acoustic impedance of the acoustic fingerprint chip 20 is 15Mrayl, and the acoustic impedance of the adhesive layer 23 is 3Mrayl. Rate R=(Z _Si -Z _gl )/(Z _Si +Z _gl )=(15-3)/(15+3)=66.7%, such a reflectivity is still very large, which can meet the needs of ultrasonic fingerprint detection .

Since the round-trip time t=2×D2/u of the ultrasonic signal transmitted in the ultrasonic fingerprint chip 20, u is the transmission speed u of the ultrasonic signal in the ultrasonic fingerprint chip 20, when t=T/2+T×k, T is For the period of the ultrasonic signal, k is 0 or a positive integer. When the back-facing ultrasonic wave returns to the surface of the ultrasonic fingerprint chip 20, the difference between the initial phase of its vibration and that of the sinusoidal ultrasonic wave is 180°, and the phase superposition is exactly 360°. It is equivalent to that the reflected back-to-back ultrasonic wave is just in phase with the sinusoidal ultrasonic wave, so that the amplitude of the ultrasonic wave is enhanced. Therefore, according to 2×D2/u=T/2+T×k, the thickness D2=λ/4+(λ/2)×k of the ultrasonic fingerprint chip 20 that can be obtained, thereby improve the signal transmission performance of the ultrasonic fingerprint device 2 . When k=0, that is, when the thickness D2 of the ultrasonic fingerprint chip 20 is 1/4 of the wavelength λ of the ultrasonic signal, the ultrasonic fingerprint chip 20 has the thinnest thickness with the best transmission performance.

As shown in FIG. 11 , the present application also provides an electronic device 3 , which includes a display screen 1 ; and the above-mentioned ultrasonic fingerprint device 2 . The ultrasonic fingerprint device 2 is located under the display screen 1, so as to realize the ultrasonic fingerprint recognition under the screen.

As an example and not a limitation, the electronic device in the embodiment of the present application may be a portable or mobile computing device such as a terminal device, a mobile phone, a tablet computer, a notebook computer, a desktop computer, a game device, a vehicle electronic device, or a wearable smart device, and Electronic databases, automobiles, bank ATMs (Automated Teller Machines, ATMs) and other electronic equipment. The wearable smart devices include full-featured, large-sized devices that can achieve complete or partial functions without relying on smart phones, such as smart watches or smart glasses, and devices that only focus on a certain type of application functions and need to be integrated with other devices such as smart phones. Cooperating equipment, such as various smart bracelets, smart jewelry and other equipment for physical sign monitoring.

It should be noted that, on the premise of no conflict, each embodiment described in this application and/or the technical features in each embodiment can be combined with each other arbitrarily, and the technical solution obtained after the combination should also fall within the protection scope of this application .

The systems, devices and methods disclosed in the embodiments of the present application may be implemented in other ways. For example, some features of the method embodiments described above may be omitted or not implemented. The device embodiments described above are only illustrative, and the division of units is only a logical function division. In actual implementation, there may be other division methods, and multiple units or components may be combined or integrated into another system. In addition, the coupling between the various units or the coupling between the various components may be direct coupling or indirect coupling, and the above coupling includes electrical, mechanical or other forms of connection.

Those skilled in the art can clearly understand that for the convenience and brevity of description, the specific working process and technical effects of the devices and equipment described above can refer to the corresponding processes and technical effects in the foregoing method embodiments, here No longer.

It should be understood that the specific examples in the embodiments of the present application are only to help those skilled in the art better understand the embodiments of the present application, rather than limit the scope of the embodiments of the present application. Those skilled in the art can Various improvements and modifications are made, and these improvements or modifications all fall within the protection scope of the present application.

The above is only a specific implementation of the application, but the scope of protection of the application is not limited thereto. Anyone familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the application. Should be covered within the protection scope of this application. Therefore, the protection scope of the present application should be determined by the protection scope of the claims.

Claims

An ultrasonic fingerprint device, characterized in that, the ultrasonic fingerprint device is arranged under the display screen of an electronic device to realize ultrasonic fingerprint identification under the screen, the ultrasonic fingerprint device includes an ultrasonic fingerprint chip, and an ultrasonic fingerprint device arranged on the ultrasonic fingerprint chip The upper piezoelectric transducer;

The piezoelectric transducer includes an upper electrode, a lower electrode, and a piezoelectric layer between the upper electrode and the lower electrode, the lower electrode is arranged on the surface of the ultrasonic fingerprint chip, and the upper electrode Connected to the signal transmitting circuit below the ultrasonic fingerprint chip, the signal transmitting circuit is used to generate an excitation signal and load it to the upper electrode, so as to excite the piezoelectric layer to send an ultrasonic signal to the finger above the display screen;

The ultrasonic fingerprint chip is a complementary metal oxide semiconductor CMOS chip, and the ultrasonic fingerprint chip includes a signal receiving circuit, and the signal receiving circuit is connected to the lower electrode to receive the ultrasonic signal returned by the finger to act on the pressure sensor. The electrical layer is an ultrasonic detection signal generated between the upper electrode and the lower electrode, and the ultrasonic detection signal is used to obtain the fingerprint image of the finger.
The ultrasonic fingerprint device according to claim 1, characterized in that,

The signal transmitting circuit includes a first switch and a second switch, the upper electrode is connected to an AC power supply through the first switch, and connected to a DC power supply through the second switch;

The signal receiving circuit includes an operational amplifier connected to the lower electrode, and a third switch connected between the lower electrode and a reference voltage or between the lower electrode and ground.
The ultrasonic fingerprint device according to claim 2, wherein the ultrasonic fingerprint chip further comprises a control circuit, each pulse period of the excitation signal includes a first period and a second period, and the control circuit is used for:

During the first period, controlling the first switch and the third switch to be closed, and the second switch to be opened, so as to apply the excitation signal to the upper electrode through the AC power supply;

During the second period, the first switch and the third switch are controlled to be turned off, and the second switch is turned on, so as to pass the ultrasonic detection signal received from the lower electrode by the operational amplifier.
The ultrasonic fingerprint device according to claim 3, wherein the lower electrode is an electrode array composed of a plurality of electrodes, and the plurality of electrodes are respectively connected to different operational amplifiers.
The ultrasonic fingerprint device according to claim 3, wherein the lower electrode is an electrode array composed of a plurality of electrodes, and the plurality of electrodes are M groups, wherein each group includes N electrodes, and the N electrodes are The electrodes are respectively connected to the same operational amplifier through N fourth switches, and the control circuit is also used for:

In the second period, the N fourth switches are time-divisionally controlled to receive the ultrasonic detection signals from the N electrodes through the same operational amplifier in time-division.
The ultrasonic fingerprint device according to any one of claims 2 to 5, wherein the operational amplifier includes a positive input terminal, a negative input terminal and an output terminal, and the signal receiving circuit also includes a capacitor unit and a reset switch , the capacitor unit is connected between the negative input terminal and the output terminal, the reset switch is connected in parallel with the capacitor unit, the negative input terminal is used to connect the lower electrode, and the positive input terminal terminal is used to connect the reference voltage.
The ultrasonic fingerprint device according to any one of claims 1 to 5, wherein the ultrasonic fingerprint chip further includes a shielding electrode arranged on its surface, and the shielding electrode is arranged along the periphery of the lower electrode, so The shielding electrode is grounded or connected to a fixed voltage.
The ultrasonic fingerprint device according to any one of claims 1 to 7, characterized in that, the ultrasonic fingerprint chip also includes a driving wire arranged on its surface, and the upper electrode is connected from the upper surface of the piezoelectric layer Lead out to the surface of the ultrasonic fingerprint chip, and connect to one end of the lead wire through the driving wire, and the other end of the lead wire is connected to the signal transmitting circuit.
The ultrasonic fingerprint device according to claim 8, characterized in that, the ultrasonic fingerprint chip further comprises a passivation layer covering the driving wiring, the passivation layer is provided with windows, and the upper electrode extending from the upper surface of the piezoelectric layer into the opening, so as to connect the part of the driving wiring located in the opening.
The ultrasonic fingerprint device according to any one of claims 1 to 9, characterized in that, the material of the piezoelectric layer is polyvinylidene fluoride PVDF, or the material of the piezoelectric layer is polyvinylidene fluoride - Trifluoroethylene copolymer PVDF-TrFE.
The ultrasonic fingerprint device according to any one of claims 1 to 10, characterized in that the frequency of the ultrasonic signal is between 9 MHz and 14 MHz.
The ultrasonic fingerprint device according to any one of claims 1 to 11, wherein the upper electrode is a silver paste coating formed on the upper surface of the piezoelectric layer, and the silver paste coating is silver powder and glue, the acoustic impedance of the upper electrode is 5MRayl to 10MRayl.
The ultrasonic fingerprint device according to any one of claims 1 to 12, wherein the ratio D1/λ between the thickness D1 of the upper electrode and the wavelength λ of the ultrasonic signal is the same as that of the ultrasonic signal at The transmittance in the upper electrode is negatively correlated, D1=λ×P=(v/f)×P, where v is the transmission speed of the ultrasonic signal in the upper electrode, and f is the ultrasonic signal frequency, P is the value of D1/λ corresponding to the target transmittance.
The ultrasonic fingerprint device according to claim 13, wherein the thickness D1 of the upper electrode is between 4um and 8um.
The ultrasonic fingerprint device according to any one of claims 1 to 14, characterized in that the material of the ultrasonic fingerprint chip is silicon, and the acoustic impedance of the ultrasonic fingerprint chip is 14 MRayl to 20 MRayl.
The ultrasonic fingerprint device according to any one of claims 1 to 15, wherein the thickness of the ultrasonic fingerprint chip D2=λ/4+(λ/2)×k, where λ is the wavelength of the ultrasonic signal , k is 0 or a positive integer.
The ultrasonic fingerprint device according to claim 16, wherein the thickness D2 of the ultrasonic fingerprint chip is between 100 um and 140 um.
The ultrasonic fingerprint device according to any one of claims 1 to 17, characterized in that, the ultrasonic fingerprint device further comprises a reinforcing plate 30, and the ultrasonic fingerprint chip and the signal transmitting circuit are arranged on the reinforcing plate 30. On the board, the ultrasonic fingerprint chip is connected to the reinforcing board through an adhesive layer, and the acoustic impedance of the adhesive layer is less than or equal to 5 MRayl.
The ultrasonic fingerprint device according to any one of claims 1 to 18, wherein the piezoelectric layer is a piezoelectric film, and the thickness of the piezoelectric film is 9um.
An electronic device, characterized in that it comprises:

display screen; and,

According to the ultrasonic fingerprint device according to any one of claims 1 to 19, the ultrasonic fingerprint device is arranged under the display screen to realize ultrasonic fingerprint recognition under the screen.