Optical fingerprint device

Abstract

The present invention provides an optical fingerprint device, in which at least one effective pixel unit corresponds to a microlens, and the size of the effective pixel unit is smaller than the size of the microlens, thereby reducing the area of ​​a single effective pixel unit and reducing the dark current effect. By arranging an overflow charge structure around the effective pixel unit, the photodiode is prevented from floating when too much charge is collected, which may migrate to adjacent effective pixel units, thereby improving image quality and fingerprint recognition accuracy. In addition, the design flexibility is increased and the overall performance of the optical fingerprint device is improved.

Description

Technical Field

[0001] The invention relates to an optical fingerprint device. Background Art

[0002] Current fingerprint recognition solutions include optical technology, silicon technology (capacitive / radio frequency), ultrasonic technology, etc. Among them, optical fingerprint recognition technology has been widely used in portable electronic devices.

[0003] Optical fingerprint recognition technology uses an optical imaging device based on the principle of total internal reflection (FTIR). Light strikes the surface of a transparent layer (such as organic or inorganic glass) bearing the fingerprint. The reflected light is captured by an image sensor. The amount of reflected light depends on the depth of the fingerprint's ridges and valleys, as well as the oil and moisture between the skin and the glass. After passing through the glass and reaching the center of a valley, light undergoes total internal reflection at the glass-air interface and is reflected to the image sensor. Light striking the ridges is not fully reflected but is absorbed by the ridge-glass interface or diffusely reflected elsewhere, forming an image of the fingerprint on the image sensor.

[0004] In optical fingerprint devices, larger microlenses are usually required to increase the energy of the incident light, so that the incident light with higher energy enters the pixel unit of the image sensor and is converted into an electrical signal, thereby obtaining higher fingerprint image quality.

[0005] Figure 1 、 Figure 2 A partial cross-sectional schematic diagram and a partial top view of a conventional optical fingerprint device are shown, wherein each pixel unit 110 corresponds to a microlens 120. Based on existing process conditions, the size of the microlens 120 (usually referring to the diameter D1) is generally 1μm-80μm; accordingly, the size of the pixel unit 110 (usually referring to the side length L1) is roughly equivalent to the size of the microlens 120.

[0006] In image sensors, dark current, often caused by carrier diffusion, surface and internal defects, and harmful impurities, can affect image quality. Dark current in a pixel is proportional to its area; therefore, larger pixel areas result in higher dark current. Although image sensors include dark current correction modules, dark current can be affected by process fluctuations, leading to poor chip-to-chip consistency. Furthermore, dark current increases with temperature, complicating chip performance inconsistencies across different environments. Therefore, in practical applications, it is often desirable to minimize or eliminate dark current.

[0007] In addition, when the photosensitive diode of a single pixel unit receives strong incident light and collects too much charge, a blooming process will occur, in which the charge migrates to the photosensitive diode of the adjacent pixel unit, affecting the image acquisition and processing of the adjacent pixel units, affecting the imaging quality, and thus affecting the accuracy of fingerprint recognition.

[0008] Furthermore, since pixel units correspond one-to-one with microlenses and are of comparable size, the area available for circuits or other structures in the image sensor chip is relatively limited, thus restricting the flexibility of the overall optical system design. Summary of the Invention

[0009] The purpose of the present invention is to provide an optical fingerprint device that reduces dark current and blooming effect, improves image quality and fingerprint recognition accuracy, increases design flexibility, and improves the overall performance of the optical fingerprint device.

[0010] Based on the above considerations, the present invention provides an optical fingerprint device, comprising a plurality of microlenses and pixel units arranged relatively to each other; at least one effective pixel unit corresponds to a microlens, and the size of the effective pixel unit is smaller than that of the microlens.

[0011] Preferably, an overflow charge structure is provided around the effective pixel unit.

[0012] Preferably, the overflow charge structure is an invalid pixel unit.

[0013] Preferably, the overflow charge structure is an overflow charge drain.

[0014] Preferably, the overflow charge drain includes an N-type doped region located outside the isolation region of the photodiode and an N-type heavily doped region located above the N-type doped region and close to the surface of the semiconductor substrate, and a voltage is applied to the N-type heavily doped region.

[0015] Preferably, each microlens corresponds to a plurality of pixel units arranged in an N*N array, where N is a natural number greater than or equal to 2.

[0016] Preferably, the size of each pixel unit is less than 8 μm.

[0017] Preferably, the optical fingerprint device further includes at least one of a light-blocking layer, a light-transmitting layer, and an infrared cutoff filter film located between the microlens and the pixel unit.

[0018] The optical fingerprint device of the present invention corresponds to a microlens through at least one effective pixel unit, and the size of the effective pixel unit is smaller than the size of the microlens, thereby reducing the area of ​​a single effective pixel unit and reducing the dark current effect. By arranging an overflow charge structure around the effective pixel unit, the photodiode avoids the floating process that may occur when too much charge is collected and migrates to the adjacent effective pixel unit, thereby improving image quality and fingerprint recognition accuracy. In addition, the design flexibility is increased and the overall performance of the optical fingerprint device is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Other features, objects and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments when read with reference to the accompanying drawings.

[0020] Figure 1 A partial cross-sectional schematic diagram of an optical fingerprint device in the prior art;

[0021] Figure 2 A partial top view schematic diagram of an optical fingerprint device in the prior art;

[0022] Figure 3 Schematic diagram of a partial cross-section of an optical fingerprint device according to embodiment 1 of the present invention;

[0023] Figure 4 Schematic diagram of a partial top view of the optical fingerprint device according to the first embodiment of the present invention;

[0024] Figure 5 A partial cross-sectional schematic diagram of an optical fingerprint device according to a second embodiment of the present invention;

[0025] Figure 6 Schematic diagram of a partial top view of an optical fingerprint device according to a second embodiment of the present invention;

[0026] Figure 7 Schematic cross-sectional view of the overflow charge drain of the optical fingerprint device according to the second embodiment of the present invention.

[0027] In the drawings, the same or similar reference numerals denote the same or similar devices (modules) or steps throughout different drawings. DETAILED DESCRIPTION

[0028] The present invention provides an optical fingerprint device, in which at least one effective pixel unit corresponds to a microlens, and the size of the effective pixel unit is smaller than the size of the microlens, thereby reducing the area of ​​a single effective pixel unit and reducing the dark current effect. By arranging an overflow charge structure around the effective pixel unit, the photodiode is prevented from floating when too much charge is collected, which may migrate to adjacent effective pixel units, thereby improving image quality and fingerprint recognition accuracy. In addition, the design flexibility is increased and the overall performance of the optical fingerprint device is improved.

[0029] In the following detailed description of preferred embodiments, reference will be made to the accompanying drawings, which form a part of the present invention. The accompanying drawings illustrate, by way of example, specific embodiments that can implement the present invention. The illustrative embodiments are not intended to be exhaustive of all embodiments according to the present invention. It will be understood that other embodiments may be utilized, and structural or logical modifications may be made, without departing from the scope of the present invention. Therefore, the following detailed description is not restrictive, and the scope of the present invention is defined by the appended claims.

[0030] The present invention is described in detail below with reference to specific embodiments.

[0031] Example 1

[0032] Figure 3 、 Figure 4 A partial cross-sectional schematic diagram and a partial top view schematic diagram of an optical fingerprint device according to a first embodiment of the present invention are shown. In the optical fingerprint device of this embodiment, one microlens 220 corresponds to nine pixel units arranged in a 3*3 array, at least one of which is an effective pixel unit. As an example, the middle pixel unit 210 is shown as an effective pixel unit, and the surrounding pixel units 230 are ineffective pixel units. Therefore, the size of the effective pixel unit 210 is smaller than the size of the microlens 220, that is, the side length L2 of the effective pixel unit 210 is smaller than the diameter D2 of the microlens 220. Compared with pixel units of the same size as the microlens in the prior art, the present invention reduces the area of ​​a single effective pixel unit and reduces the dark current effect.

[0033] Those skilled in the art will appreciate that, to reduce the effective pixel unit area and dark current, at least one effective pixel unit can be configured to correspond to a microlens. This can be achieved by ensuring that the effective pixel unit size is smaller than the microlens size. The size and number of areas surrounding the effective pixel units can be flexibly configured and can also be used to configure other circuit structures, increasing design flexibility.

[0034] Preferably, each microlens corresponds to a plurality of pixel units arranged in an N*N array, where N is a natural number greater than or equal to 2. Further preferably, the size of each pixel unit is less than 8 μm, thereby meeting the dark current suppression requirements in most application environments.

[0035] In addition, the invalid pixel unit 230 arranged around the effective pixel unit 210 can serve as an overflow charge structure. That is, when the photodiode of the effective pixel unit 210 collects too much charge and overflows, due to the existence of the invalid pixel unit 230, the overflowed charge in the effective pixel unit 210 is prevented from entering the adjacent effective pixel unit and affecting the image acquisition and processing of other effective pixel units, thereby reducing the floating effect and improving the image quality and fingerprint recognition accuracy.

[0036] Preferably, a light-blocking layer can be provided between the microlens 220 and the pixel units 210 and 230 to address the problem of incident light entering adjacent pixel units of the image sensor, thereby causing signal crosstalk. The provision of a light-blocking layer and a light-transmitting layer facilitates smooth light entry, and an infrared cutoff filter is provided to reduce infrared light in the incident light from entering the image sensor, causing noise crosstalk and image distortion, thereby improving the optical performance of the optical fingerprint device. Therefore, the optical fingerprint device of the present invention further includes at least one of a light-blocking layer, a light-transmitting layer, and an infrared cutoff filter located between the microlens and the pixel unit.

[0037] Example 2

[0038] Figure 5 、 Figure 6 A partial cross-sectional schematic diagram and a partial top view schematic diagram of an optical fingerprint device according to a second embodiment of the present invention are shown. In the optical fingerprint device of this embodiment, one microlens 320 corresponds to one effective pixel unit 310, and the size of the effective pixel unit 310 is smaller than the size of the microlens 320, that is, the side length L3 of the effective pixel unit 310 is smaller than the diameter D3 of the microlens 320. Compared with pixel units of the same size as the microlens in the prior art, the present invention reduces the area of ​​a single effective pixel unit and reduces the dark current effect.

[0039] Different from the first embodiment, the optical fingerprint device of the second embodiment does not use the invalid pixel unit as the overflow charge structure, but sets an overflow charge drain in the area 330 around the effective pixel unit 310 as the overflow charge structure. The specific structure is as follows: Figure 7As shown, the overflow charge drain 334 includes an N-type doped region 332 located outside the isolation region 331 of the photodiode 311 and an N-type heavily doped region 333 located above the N-type doped region 332 and close to the surface of the semiconductor substrate. The N-type heavily doped region 333 is applied with a voltage VDD to extract excess charge from the photodiode 311, thereby preventing the overflow charge in the effective pixel unit 310 from entering the adjacent effective pixel units and affecting the image acquisition and processing of other effective pixel units, thereby reducing the floating effect and improving the image quality and fingerprint recognition accuracy.

[0040] Preferably, the optical fingerprint device further includes at least one of a light-blocking layer, a light-transmitting layer, and an infrared cutoff filter film located between the microlens and the pixel unit.

[0041] In summary, the optical fingerprint device of the present invention corresponds to a microlens through at least one effective pixel unit, and the size of the effective pixel unit is smaller than the size of the microlens, thereby reducing the area of ​​a single effective pixel unit and reducing the dark current effect. By arranging an overflow charge structure around the effective pixel unit, the photodiode avoids the floating process of migrating to the adjacent effective pixel unit when too much charge is collected, thereby improving image quality and fingerprint recognition accuracy. In addition, the design flexibility is increased and the overall performance of the optical fingerprint device is improved.

[0042] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-restrictive in any manner. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude pluralities. Multiple elements stated in a device claim may also be implemented by a single element. Terms such as first and second are used to indicate names and do not imply any particular order.

Claims

1. An optical fingerprint device, characterized in that: It includes a plurality of micro lenses and pixel units arranged relatively; At least one effective pixel unit corresponds to a microlens, and a size of the effective pixel unit is smaller than a size of the microlens; An overflow charge structure is provided around the effective pixel unit; The overflow charge structure is an invalid pixel unit.

2. The optical fingerprint device according to claim 1, wherein: Each microlens corresponds to a plurality of pixel units arranged in an N*N array, where N is a natural number greater than or equal to 2.

3. The optical fingerprint device according to claim 2, characterized in that: The size of each pixel unit is less than 8μm.

4. The optical fingerprint device according to claim 1, wherein: It also includes at least one of a light blocking layer, a light transmitting layer, and an infrared cut-off filter film located between the microlens and the pixel unit.

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