TWI714295B - Fingerprint sensor embedded in a flat-panel display and a method of operating the same - Google Patents

Abstract

A fingerprint sensor embedded in a flat-panel display includes photo sensors; select transistors that are correspondingly connected in series with the photo sensors respectively; switch transistors that are correspondingly connected in series with the select transistors respectively, but are correspondingly connected in parallel with the photo sensors respectively; and a detection circuit that detects a signal passing one turned-on select transistor.

Description

Fingerprint sensor embedded in flat panel display and operation method thereof

The present invention relates to a flat panel display, in particular to a flat panel display embedded with a fingerprint sensor.

A mobile device (such as a smart phone) is a computer device that is small in size and can be held and operated by hand. Mobile devices usually have a touch screen, which occupies a large proportion (for example, 70%) of the front surface of the mobile device.

Modern mobile devices can perform many functions and are suitable for various purposes, such as social interaction, financial transactions, personal or business communications. In view of this, biometrics (such as fingerprints) technology is usually used to identify users and their identities to protect confidential data stored in mobile devices. Fingerprint recognition is not only a security method for identifying users, but also a quick way to access mobile devices.

Many mobile devices (such as smart phones) are equipped with fingerprint recognition, which usually includes physical buttons, located outside the touch screen on the front surface. The touch screen of mobile devices has a trend of increasing gradually, in order to respond to more and more powerful functions of mobile devices. However, placing the fingerprint recognition button on the front surface of the mobile device will hinder the trend of using large touch screens.

Therefore, some people propose to embed the fingerprint sensor in a flat panel display (such as a liquid crystal display). However, the signal-to-noise ratio (SNR) of the traditional fingerprint sensor is extremely low, which reduces the overall performance. Therefore, there is an urgent need to provide a novel fingerprint sensor with high performance.

In view of the foregoing, one of the objectives of the embodiments of the present invention is to provide a fingerprint sensor embedded in a flat panel display with enhanced signal-to-noise ratio and performance.

According to an embodiment of the present invention, the fingerprint sensor embedded in the flat panel display includes a light sensor, a selection transistor, a switching transistor, and a detection circuit. The selected transistors are respectively connected in series to the light sensor. The switching transistors are respectively connected in series to the selection transistors, and respectively connected in parallel to the optical sensors. The detection circuit detects a signal passing through a turned-on selective transistor.

The first figure shows a cross-sectional view of a flat panel display (such as a liquid crystal display 100) embedded with a fingerprint sensor, where the fingerprint sensor is integrated in the active area of the liquid crystal display 100. The liquid crystal display 100 may be a thin film transistor (TFT) liquid crystal display. Thin film transistor (TFT) liquid crystal displays can be formed using low temperature polysilicon (LTPS) technology, which is performed at a relatively low temperature (about 650 degrees Celsius or lower), while the traditional method is performed at 900 degrees Celsius or more. Low-temperature polysilicon (LTPS) can be used to manufacture large-size liquid crystal displays.

The liquid crystal display 100 may include a thin film transistor (TFT) substrate 11 on which a first dielectric layer 12 is formed. The first dielectric layer 12 may include silicon oxide or/and silicon nitride. A plurality of switching thin film transistors 13 are formed in the first dielectric layer 12 for display purposes. The switching thin film transistor 13 may include a polysilicon layer (as a channel) 131, a first metal layer M1 (as a gate) 132 is arranged on the polysilicon layer 131, and a second metal layer M2 (as a source and drain) 133 is arranged on the polysilicon. The surface of the layer 131 surrounds the first metal layer 132, wherein the first dielectric layer 12 isolates the first metal layer 132 and the second metal layer 133.

At least one light sensor (or light detector) 14 is formed in the first dielectric layer 12. The photo sensor 14 may include a polysilicon layer 131 and a second metal layer 133 on the surface thereof. The second metal layer 133 surrounds a channel for passing through the light beam representing the fingerprint, and then the light sensor 14 detects it. One end of the polysilicon layer 131 is doped with P-type dopants, and the other end is doped with N-type dopants, thereby forming a p-n interface as a photo sensor. In addition, the base metal layer (M0) 130 is disposed in the first dielectric layer 12 and located on the surface of the thin film transistor substrate 11 as a first light barrier to block or shield the backlight.

At least one selective thin film transistor 14B is formed to match the photo sensor 14. The selective thin film transistor 14B may include a polysilicon layer (as a channel) 131, a first metal layer M1 (as a gate) 132 is arranged on the polysilicon layer 131, and a second metal layer M2 (as a source and drain) 133 is arranged on the polysilicon. The surface of the layer 131 surrounds the first metal layer 132, wherein the first dielectric layer 12 isolates the first metal layer 132 and the second metal layer 133. The selective thin film transistor 14B is connected to the corresponding photo sensor 14 through the second metal layer 133.

The liquid crystal display 100 may include a light source, such as a backlight module (not shown in the drawings), disposed under a thin film transistor (TFT) substrate 11. The light source of the liquid crystal display 100 may emit visible light or invisible light beams.

The liquid crystal display 100 may include a transparent planarization (PLN) layer 15 having a substantially flat top surface and formed on the first dielectric layer 12. The planarization layer 15 may include a transparent material, such as resin, to allow light to pass through. A third metal layer (M3) 134 is formed on the bottom of the planarization layer 15. The third metal layer 134 can be used as a second light barrier to block or shield (oblique) light outside the channel direction, so that it will not enter the light sensor 14.

The liquid crystal display 100 may include a second dielectric layer 16 formed on the surface of the planarization layer 15. The second dielectric layer 16 may include silicon oxide or/and silicon nitride. At least one conductive layer is formed in the second dielectric layer 16. As illustrated in the first figure, the at least one conductive layer may include a first indium tin oxide (ITO) layer 161 formed on the bottom of the second dielectric layer 16 (for example, formed on the surface of the planarization layer 15); and An indium tin oxide (ITO) layer 162 is formed on the top of the second dielectric layer 16 (for example, formed on the first indium tin oxide layer 161). The second dielectric layer 16 isolates the first indium tin oxide layer 161 and the second indium tin oxide layer 162. As shown in the first figure, the first indium tin oxide layer 161 can be connected to the second metal layer 133 of the switching thin film transistor 13.

The liquid crystal display 100 may include a liquid crystal (LC) layer 17 formed on the second dielectric layer 16. At least one transparent photo spacer 171 is provided in the liquid crystal layer 17 to isolate adjacent liquid crystal regions. The photo spacer 171 of the liquid crystal display 100 may include a transparent material, such as resin. The liquid crystal display 100 may further include a color filter (CF) layer 18 formed on the liquid crystal layer 17 and disposed on the bottom surface of the color filter (CF) substrate 19. The color filter layer 18 may include a plurality of color filters, such as red, green, and blue filters, for passing red light, green light, and blue light, respectively. The color filter layer 18 may also include at least one black filter to block light. The area not covered by the black filter is the display area. As shown in the first figure, the black filter is roughly aligned with the optical spacer 171 below. In this embodiment, the light sensor 14 is located in the active display area that is not covered by the black filter of the color filter layer 18.

The liquid crystal display 100 may include at least one lens area 20 disposed on the light sensor 14 and substantially aligned with the light sensor 14 in the vertical direction. In this embodiment, the lens area 20 is connected to the top surface of the planarization layer 15 and extends upward. The lens area 20 may include a transparent material, which may be the same as or different from the planarization layer 15. The lens area 20 is vertically elongated and passes through the second dielectric layer 16, the liquid crystal layer 17 and the color filter layer 18 (in order from bottom to top).

According to the above, the liquid crystal display 100 embedded with a fingerprint sensor includes a light source, a lens area 20 and a light sensor 14. The second figure shows a schematic diagram of the fingerprint sensor of this embodiment. The light source 21 emits a light beam to the finger 22. The lens area 20 serves as a rod lens 23 for focusing the light beam reflected from the fingerprint. The light sensor 14 is used as the light detector 24 to detect the light beam representing the fingerprint and convert it into an electronic signal.

The third diagram A shows the circuit diagram of the fingerprint sensor 300 embedded in the liquid crystal display 100 (first diagram). The fingerprint sensor 300 may include a plurality of light sensors 14, such as photodiodes. The fingerprint sensor 300 may include a plurality of selective transistors 14B (for example, selective thin film transistors TFT[N]~TFT[N+M]), which are respectively connected in series to the light sensor 14. Although an N-type thin film transistor is used as an example in the third diagram A, a P-type thin film transistor can also be used.

The fingerprint sensor 300 may include a detection circuit 31 for detecting a signal (such as current) of the light sensor 14 of one of the branches to be detected. The drain of the selection transistor 14B is electrically connected to the cathode of the corresponding photo sensor 14 (the anode is electrically connected to the common voltage VCM), and the source of the selection transistor 14B is electrically connected to the input of the detection circuit 31 end.

The third diagram B respectively shows the timing diagram of the control signals TFT[N]~TFT[N+M] for selecting the gate of the transistor 14B. As shown in the third diagram B, only one selection transistor 14B is turned on at a time, and the selection transistors 14B are turned on in a predetermined order, as illustrated in the third diagram B.

The impedance of the selective transistor 14B that is turned on usually has several thousand ohms, and the impedance of the selective transistor 14B that is turned off usually has several mega ohms. Since the signal current of the light sensor 14 is generally only a few pico amperes, and the impedance of the selected transistor 14B that is turned off is also only several million ohms, the detection circuit 31 not only receives the selected transistor 14B that is turned on. At the same time, it also receives the signal of the selected transistor 14B that is turned off, which affects the signal-to-noise ratio (SNR) and performance of the fingerprint sensor 300.

Fig. 4A shows a circuit diagram of a fingerprint sensor 400 embedded in a flat panel display (such as the liquid crystal display 100 in Fig. 1) according to an embodiment of the present invention. The fingerprint sensor 400 of this embodiment is similar to the fingerprint sensor 300 of FIG. 3A, and may include the light sensor 14, the selective transistor 14B, and the detection circuit 31, the details of which will not be repeated.

According to one of the features of this embodiment, the fingerprint sensor 400 may further include a plurality of switching transistors 41 (for example, switching thin film transistors), which are respectively connected in series to the selection transistors 14B, but are connected in parallel to the photo sensor 14 respectively. Although the P-type thin film transistor is used as an example in this embodiment, an N-type thin film transistor can also be used. In a preferred embodiment, the selection transistor 14B includes an N-type thin film transistor, and the switching transistor 41 includes a P-type thin film transistor.

The source of the switching transistor 41 is electrically connected to the drain of the corresponding selection transistor 14B (and the cathode of the photo sensor 14), and the drain of the switching transistor 41 is electrically connected to the bias voltage VBIAS.

The fourth diagram B respectively shows a timing diagram of the control signals TFT[N]~TFT[N+M] for selecting the gate of the transistor 14B and switching the transistor 41. It is worth noting that if the switching transistor 41 uses an N-type thin film transistor, the polarity of the control signals TFT[N]~TFT[N+M] needs to be reversed.

According to another feature of this embodiment, when the selection transistor 41 is turned on, the corresponding switching transistor 41 must be turned off; when the selection transistor 41 is turned off, the corresponding switching transistor 41 must be turned on. In other words, the switching state (ie, on or off) of the switching transistor 41 is opposite to the switching state of the corresponding selection transistor 14B.

Although the signal current of the photo sensor 14 is only a few pico amperes and the impedance of the selected transistor 14B that is turned off is only several million (mega) ohms, since the corresponding switching transistor 41 is turned on, the detection circuit 31 Only the signal through the selected transistor 14B that is turned on is received, but the signal through the selected transistor 14B that is turned off is not received, thereby enhancing the signal-to-noise ratio (SNR) of the fingerprint sensor 400 and improving the performance of the fingerprint sensor 400.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention; all other equivalent changes or modifications made without departing from the spirit of the invention should be included in the following Within the scope of patent application.

100: LCD display

11: Thin film transistor substrate

12: The first dielectric layer

13: Switching thin film transistors

130: base metal layer

131: Polysilicon layer

132: The first metal layer

133: second metal layer

134: third metal layer

14: light sensor

14B: Choose thin film transistors

15: Planarization layer

16: second dielectric layer

161: first indium tin oxide layer

162: second indium tin oxide layer

17: Liquid crystal layer

171: photo spacer

18: Color filter layer

19: Color filter substrate

20: lens area

21: light source

22: finger

23: Rod lens

24: Light detector

300: Fingerprint sensor

31: Detection circuit

400: Fingerprint sensor

41: Switching transistor

M0: base metal layer

M1: The first metal layer

M2: second metal layer

M3: third metal layer

TFT: thin film transistor

PLN: Flattening

ITO: indium tin oxide

LC: liquid crystal

CF: Color filter

TFT[N]~TFT[N+M]: Thin film transistor/control signal

VCM: Common voltage

VBIAS: Bias voltage

The first figure shows a cross-sectional view of a flat panel display embedded with a fingerprint sensor. The second figure shows a schematic diagram of the fingerprint sensor of this embodiment. The third figure A shows the circuit diagram of the fingerprint sensor embedded in the liquid crystal display (the first figure). The third diagram B respectively shows the timing diagram of the control signal for selecting the gate of the transistor. Figure 4A shows a circuit diagram of the fingerprint sensor embedded in the flat panel display (Figure 1) according to an embodiment of the present invention. The fourth diagram B respectively shows the timing diagrams of the control signals for selecting the transistor and switching the gate of the transistor.

400: Fingerprint sensor

14: light sensor

14B: Choose thin film transistors

31: Detection circuit

41: Switching transistor

TFT[N]~TFT[N+M]: Thin film transistor/control signal

VCM: Common voltage

VBIAS: Bias voltage

Claims (13)

A fingerprint sensor embedded in a flat panel display, comprising: a plurality of light sensors; a plurality of selection transistors, respectively connected in series to the light sensors; a plurality of switching transistors, respectively connected in series to the selection transistors , And are respectively connected in parallel to the light sensors; and a detection circuit, which detects a signal passing through a turned-on selective transistor; wherein the drain of the selective transistor is electrically connected to the corresponding light sensor The cathode of the selection transistor is electrically connected to the input terminal of the detection circuit. For example, the fingerprint sensor embedded in the flat panel display of claim 1, wherein the switching state of the switching transistor is opposite to the switching state of the corresponding selection transistor. For example, the fingerprint sensor embedded in the flat panel display of claim 2, wherein when the selection transistor is turned on, the corresponding switching transistor is turned off; when the selection transistor is turned off, the corresponding switching transistor is turned on . For example, the fingerprint sensor embedded in the flat panel display of claim 1, wherein the flat panel display includes a liquid crystal display. For example, the fingerprint sensor embedded in the flat panel display of claim 1, wherein the light sensor includes a photodiode. For example, the fingerprint sensor embedded in the flat panel display of claim 1, wherein the selection transistor includes an N-type thin film transistor, and the switching transistor includes a P-type thin film transistor. For example, in the fingerprint sensor embedded in the flat panel display of claim 1, only one of the selective transistors is turned on at a time. For example, the fingerprint sensor embedded in the flat panel display of claim 1, wherein the source of the switching transistor is electrically connected to the drain of the corresponding selection transistor and the cathode of the corresponding photo sensor, and the switching transistor The drain of the crystal is electrically connected to the bias voltage. An operating method for a fingerprint sensor embedded in a flat panel display includes: providing a plurality of branches, each branch includes a light sensor, a selection transistor and a switching transistor, the switching transistor is connected in series to the selection correspondingly The transistor is correspondingly connected in parallel to the photo sensor; the drain of the selected transistor is electrically connected to the cathode of the corresponding photo sensor; the selected transistor of a branch to be detected is turned on and the branch to be detected is turned off The switching transistor, but turning off the selection transistors of the other non-test branches and conducting the switching transistors of the non-test branches; and detecting the signal of the conduction selection transistor passing through the branch to be tested. For example, the operation method of a fingerprint sensor embedded in a flat panel display according to claim 9, wherein the flat panel display includes a liquid crystal display. For example, the operation method of a fingerprint sensor embedded in a flat panel display of claim 9, wherein the light sensor includes a photodiode. Such as claim 9 of the operating method of a fingerprint sensor embedded in a flat panel display, wherein the selection transistor includes an N-type thin film transistor, and the switching transistor includes a P-type thin film transistor. For example, the operation method of the fingerprint sensor embedded in the flat panel display of claim 9 further includes electrically connecting the source of the switching transistor to the drain of the corresponding selection transistor and the cathode of the corresponding photo sensor, And the drain of the switching transistor is electrically connected to the bias voltage.

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