Black level calibration method of image sensor

Abstract

The application provides a black level calibration method of an image sensor, the image sensor comprising a photosensitive pixel array and a light shielding pixel array, the calibration method comprising: acquiring black levels of the photosensitive pixel array and the light shielding pixel array respectively, and obtaining a black level difference value of the photosensitive pixel unit and the light shielding pixel unit; adjusting the light shielding pixel array structure through the black level difference value, so that the black levels of the photosensitive pixel array and the light shielding pixel array are equal or close. The black level of the light shielding pixel is adjusted by changing the design of the main dark current source of the light shielding pixel, the average values of the black levels of the light shielding pixel and the photosensitive pixel are equal or approximately equal, and finally the black level calibration is realized; without increasing the circuit complexity or the number of mask plates, the accurate black level calibration of the image sensor is realized.

Description

Technical Field

[0001] This invention relates to the field of image sensor technology, and in particular to a black level calibration method for image sensors. Background Technology

[0002] In the field of image sensors, black level refers to the value of an image sensor under dark conditions, which is not a valid signal of the image. Due to the presence of non-ideal factors such as intrinsic thermal noise, PN junction dark current, interface dark current, and defect dark current, the black level of an image sensor under dark conditions is not 0. Especially under high temperature, high gain, and long exposure conditions, the black level value will be much greater than 0, seriously affecting the signal-to-noise ratio of the image under high temperature and low light conditions. Therefore, image sensor designs generally include photosensitive pixel arrays and light-shielding pixel arrays. The light-shielding pixel array is not photosensitive, and the black level value can be statistically calculated using the light-shielding pixel array. Subtracting the black level value from the image data of the photosensitive pixel array, this black level calibration design (ideally, the average signal of the photosensitive pixels in no light minus the average signal of the light-shielding pixels equals 0) can eliminate the interference of black level and significantly improve the signal-to-noise ratio under low light conditions.

[0003] In practical applications, on the one hand, the light-shielding pixel array is generally located at the edge of the entire pixel array; on the other hand, the light-shielding pixel array requires an additional black-shielding layer. These two factors lead to differences in the manufacturing performance between the pixels in the light-shielding array and the pixels in the photosensitive array, mainly manifested in differences in stress, irradiation, and plasma damage during the manufacturing process. These differences ultimately result in the black level (dark current) of the light-shielding black pixel array not being equal to that of the photosensitive pixel array in the absence of light. Moreover, this difference increases significantly with rising temperature, making black level calibration unable to effectively eliminate black level noise and affecting the image signal-to-noise ratio.

[0004] To address this issue, the main current solutions are: 1) Improving the manufacturing process to achieve a low dark current solution. However, pixel processes are sensitive, making it difficult to reduce dark current, and achieving performance consistency across different process lines is also very challenging; 2) Using an additional black level calibration circuit. This solution increases design complexity and area.

[0005] Therefore, how to achieve accurate black level calibration of image sensors while controlling design, process complexity, and cost is an urgent problem that needs to be solved. Summary of the Invention

[0006] The purpose of this invention is to provide a black level calibration method for an image sensor, which enables accurate black level calibration of the image sensor.

[0007] Based on the above considerations, the present invention provides a black level calibration method, wherein the image sensor comprises a photosensitive pixel array and a light-blocking pixel array, characterized in that the calibration method includes:

[0008] The black level of the photosensitive pixel array and the light-blocking pixel array are collected respectively to obtain the black level difference between the photosensitive pixel unit and the light-blocking pixel unit;

[0009] The black level difference is used to adjust the light-blocking pixel array structure so that the black level of the photosensitive pixel array is equal to or close to that of the light-blocking pixel array.

[0010] Optionally, the method for adjusting the light-shielding pixel array structure includes: adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array.

[0011] Optionally, the method for adjusting the light-shielding pixel array structure includes: adjusting the doping concentration or the size of the trench isolation region in the light-shielding pixel array.

[0012] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or size of the N-type doped region.

[0013] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or size of the P-type isolation doped region of the photodiode in the light-shielding pixel array.

[0014] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or the size of the doped region of the P-type pinned region of the photodiode in the light-shielding pixel array.

[0015] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance between the P-type pinned region and the heavily doped N-type region of the photodiode in the light-shielding pixel array.

[0016] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the size of the overlap between the P-type pinned region and the N-type doped region of the photodiode in the light-shielding pixel array.

[0017] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance between the heavily doped P-type ground region and the heavily doped N-type region of the photodiode in the light-shielding pixel array.

[0018] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the position of the heavily doped P-type ground region or the position of the heavily doped N-type region of the photodiode in the light-shielding pixel array.

[0019] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance from the N-type doped region of the photodiode to the heavily doped P-type ground region in the light-shielding pixel array.

[0020] Optionally, adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance between the heavily doped N-type region and the P-type isolation doped region of the photodiode in the light-shielding pixel array.

[0021] Optionally, adjusting the doping concentration or the size of the trench isolation region in the light-shielding pixel array can be achieved by adjusting the distance from the N-type doped region of the photodiode in the light-shielding pixel array to the interface of the trench isolation region.

[0022] Optionally, adjusting the doping concentration or size of the trench isolation region in the light-shielding pixel array can be achieved by adjusting the distance from the N-type doped region of the photodiode in the light-shielding pixel array to the shallow trench isolation or deep trench isolation interface.

[0023] Optionally, adjusting the doping concentration or the size of the trench isolation region in the light-shielding pixel array includes adjusting the distance from the boundary of the P-type isolation doped region of the photodiode in the light-shielding pixel array to the boundary of the trench isolation structure, wherein the P-type isolation doped region surrounds the trench isolation structure.

[0024] The black level calibration method of the present invention has the following beneficial effects:

[0025] 1) By changing the design of the main dark current source of the light-shielding pixel, the black level of the light-shielding pixel is adjusted, so that the average black current of the light-shielding pixel and the photosensitive pixel are equal or approximately equal, and finally the black level calibration is achieved.

[0026] 2) Achieve accurate black level calibration of image sensors without increasing circuit complexity or the number of photomasks. Attached Figure Description

[0027] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings.

[0028] Figure 1 The diagram shown is a flowchart of the black level calibration method provided by the present invention.

[0029] Figure 2 The diagram shows a schematic of the light-shielding pixel structure of an existing image sensor.

[0030] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0031] Numerous specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0032] Secondly, the present invention is described in detail using schematic diagrams. When describing the embodiments of the present invention in detail, the schematic diagrams are merely examples for ease of explanation and should not limit the scope of protection of the present invention.

[0033] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, a black level calibration method for an image sensor according to the present invention will be described in detail below with reference to the accompanying drawings.

[0034] This invention provides a black level calibration method for an image sensor, the image sensor comprising a photosensitive pixel array and a light-blocking pixel array, the calibration method comprising:

[0035] The black level of the photosensitive pixel array and the light-blocking pixel array are collected respectively to obtain the black level difference between the photosensitive pixel unit and the light-blocking pixel unit;

[0036] The black level difference is used to adjust the light-shielding pixel array structure so that the black level of the photosensitive pixel array and the light-shielding pixel array are equal or close. In the implementation example, considering that in most cases the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the example is only presented with a negative black level difference between the photosensitive pixel unit and the light-shielding pixel unit.

[0037] like Figure 2 As shown, the typical image sensor's light-shielding pixel structure includes: 11 is an N-type doped region, 12 is the transmission transistor gate, 13 is a P-type isolation doped region, 14 is a heavily doped N-type region, 15 is a P-type pinned region, 16 is a P-type ground region, 17 is a trench isolation structure, 18 is a row select transistor gate, 19 is a source follower transistor gate, 20 is a reset transistor gate, 21 is a floating diffusion region, and the black-shielding layer and related contact holes, vias, and metal interconnects are not shown.

[0038] Methods for adjusting the structure of a light-shielding pixel array by adjusting the black level difference include: adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array.

[0039] Specifically, refer to Figure 2 The area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array can be adjusted in the following ways:

[0040] ① Adjust the doping concentration or size of the N-type doped region 11 in the light-shielding pixel array. If the black level of the photosensitive pixel unit is lower than the black level of the light-shielding pixel unit, the doping concentration or size of the N-type doped region 11 of the photodiode in the light-shielding pixel array can be reduced; if the black level of the photosensitive pixel unit is higher than the black level of the light-shielding pixel unit, the doping concentration or size of the N-type doped region 11 of the photodiode in the light-shielding pixel array can be increased.

[0041] ② Adjust the doping concentration or size of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the doping concentration or size of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the doping concentration or size of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array can be decreased.

[0042] ③ Adjust the doping concentration or size of the P-type pinned region 15 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is lower than the black level of the light-shielding pixel unit, the doping concentration or size of the P-type pinned region 15 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is higher than the black level of the light-shielding pixel unit, the doping concentration or size of the P-type pinned region 15 of the photodiode in the light-shielding pixel array can be decreased.

[0043] ④ Adjust the distance between the P-type pinned region 15 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the P-type pinned region 15 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the P-type pinned region 15 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be decreased.

[0044] ⑤ Adjust the size of the overlap between the P-type pinned region 15 and the N-type doped region 11 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the size of the overlap between the P-type pinned region 15 and the N-type doped region 11 of the photodiode in the light-shielding pixel array can be reduced; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the size of the overlap between the P-type pinned region 15 and the N-type doped region 11 of the photodiode in the light-shielding pixel array can be increased.

[0045] ⑥ Adjust the distance between the heavily doped P-type ground region 16 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the heavily doped P-type ground region 16 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the heavily doped P-type ground region 16 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be decreased.

[0046] ⑦ Adjust the position of the heavily doped P-type ground region 16 or the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the heavily doped P-type ground region 16 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the heavily doped P-type ground region 16 and the heavily doped N-type region 14 of the photodiode in the light-shielding pixel array can be decreased.

[0047] ⑧ Adjust the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the heavily doped P-type ground region 16. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the heavily doped P-type ground region 16 can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the heavily doped P-type ground region 16 can be decreased.

[0048] ⑨ Adjust the distance between the heavily doped N-type region 14 and the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the heavily doped N-type region 14 and the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the heavily doped N-type region 14 and the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array can be decreased.

[0049] The method for adjusting the light-shielding pixel array structure can also be achieved by adjusting the doping concentration or the size of the trench isolation region in the light-shielding pixel array.

[0050] Specifically, refer to Figure 2 The doping concentration or size of the trench isolation region in the light-shielding pixel array can be adjusted in the following ways:

[0051] ① Adjust the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the interface of the trench isolation region 17. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the interface of the trench isolation region 17 can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance between the N-type doped region 11 of the photodiode in the light-shielding pixel array and the interface of the trench isolation region 17 can be decreased.

[0052] ② Adjust the distance from the N-type doped region 11 of the photodiode in the light-shielding pixel array to the shallow trench isolation or deep trench isolation interface. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance from the N-type doped region 11 of the photodiode in the light-shielding pixel array to the shallow trench isolation or deep trench isolation interface can be increased; if the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance from the N-type doped region 11 of the photodiode in the light-shielding pixel array to the shallow trench isolation or deep trench isolation interface can be decreased.

[0053] ③ Adjust the distance from the boundary of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array to the boundary 17 of the trench isolation structure, wherein the P-type isolation doped region surrounds the trench isolation structure. If the black level of the photosensitive pixel unit is less than the black level of the light-shielding pixel unit, the distance from the boundary of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array to the boundary 17 of the trench isolation structure can be increased. If the black level of the photosensitive pixel unit is greater than the black level of the light-shielding pixel unit, the distance from the boundary of the P-type isolation doped region 13 of the photodiode in the light-shielding pixel array to the boundary 17 of the trench isolation structure can be decreased.

[0054] This invention adjusts the black level (dark current) of the light-shielding pixel by changing the design of the main dark current source of the light-shielding pixel, so that the average black current of the light-shielding pixel and the photosensitive pixel are equal or approximately equal. In particular, the black level calibration value at high temperature (the photosensitive pixel signal value in the absence of light - the light-shielding pixel signal value) is equal to 0 or approximately equal to 0. This achieves accurate black level calibration of the image sensor without increasing the circuit complexity or the number of photomasks.

[0055] Of course, the methods for adjusting the structure of the light-shielding pixel unit are not limited to those mentioned above. Other methods can also be used to adjust the structure of the light-shielding pixel array, which are not limited here.

[0056] In summary, the present invention provides a black level calibration method for image sensors, which obtains the black level difference between the photosensitive pixel unit and the light-blocking pixel unit by separately acquiring the black level of the photosensitive pixel array and the light-blocking pixel array; and adjusts the structure of the light-blocking pixel array by the black level difference so that the black level of the photosensitive pixel array and the light-blocking pixel array are equal or close.

[0057] 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 characteristics of the invention. Therefore, the embodiments should be considered exemplary and not restrictive in any way. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude a plurality. Multiple elements recited in the apparatus claims may also be implemented by a single element. The terms "first," "second," etc., are used to denote names and do not indicate any particular order.

Claims

1. A method for black level calibration of an image sensor, the image sensor comprising a photosensitive pixel array and a light-blocking pixel array, characterized in that, The calibration method includes: The black level of the photosensitive pixel array and the light-blocking pixel array are collected respectively to obtain the black level difference between the photosensitive pixel unit and the light-blocking pixel unit; The black level difference is used to adjust the light-blocking pixel array structure so that the black level of the photosensitive pixel array is equal to or close to that of the light-blocking pixel array.

2. The black level calibration method for an image sensor according to claim 1, characterized in that, The method for adjusting the structure of the light-shielding pixel array includes: adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array.

3. The black level calibration method for an image sensor according to claim 1, characterized in that, The method for adjusting the structure of the light-shielding pixel array includes: adjusting the doping concentration or the size of the trench isolation region in the light-shielding pixel array.

4. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or size of the N-type doped region.

5. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or size of the P-type isolation doped region of the photodiode in the light-shielding pixel array.

6. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the doping concentration or the size of the doped region of the P-type pinned region of the photodiode in the light-shielding pixel array.

7. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance from the P-type pinned region to the heavily doped N-type region of the photodiode in the light-shielding pixel array.

8. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the size of the overlap between the P-type pinned region and the N-type doped region of the photodiode in the light-shielding pixel array.

9. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance between the heavily doped P-type ground region and the heavily doped N-type region of the photodiode in the light-shielding pixel array.

10. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the position of the heavily doped P-type ground region or the position of the heavily doped N-type region of the photodiode in the light-shielding pixel array.

11. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance from the N-type doped region to the heavily doped P-type ground region of the photodiode in the light-shielding pixel array.

12. The black level calibration method for an image sensor according to claim 2, characterized in that, Adjusting the area and / or electric field of the PN junction of the photodiode in the light-shielding pixel array includes adjusting the distance between the heavily doped N-type region and the P-type isolated doped region of the photodiode in the light-shielding pixel array.

13. The black level calibration method for an image sensor according to claim 3, characterized in that, Adjusting the doping concentration or size of the trench isolation region in the light-shielding pixel array can be achieved by adjusting the distance from the N-type doped region of the photodiode in the light-shielding pixel array to the interface of the trench isolation region.

14. The black level calibration method for an image sensor according to claim 3, characterized in that, Adjusting the doping concentration or size of the trench isolation region in the light-shielding pixel array can be achieved by adjusting the distance from the N-type doped region of the photodiode in the light-shielding pixel array to the shallow trench isolation or deep trench isolation interface.

15. The black level calibration method for an image sensor according to claim 3, characterized in that, Adjusting the doping concentration or size of the trench isolation region in the light-shielding pixel array includes adjusting the distance from the boundary of the P-type isolation doped region of the photodiode in the light-shielding pixel array to the boundary of the trench isolation structure, wherein the P-type isolation doped region surrounds the trench isolation structure.

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