Pixel structure to improve back-illuminated global shutter efficiency

Abstract

A structure for improving the efficiency of a back-illuminated global shutter, by forming at least one strong electric field at the position of a floating diffusion node region in a photosensitive pixel circuit, thereby generating an effect of shielding the floating diffusion node region, or by hollowing out the semiconductor material of the floating diffusion node facing the illumination direction through a process, so that a depletion region cannot be formed in this direction, or by forming a reflective layer or a photoresist layer that blocks light in the illumination direction, thereby reducing the charge generated by unexpected photosensitive noise and making it difficult for the charge generated by the photosensitive noise to reach the floating diffusion node region, thereby improving the shutter efficiency.

Description

[0001] This application is a divisional application of the Chinese invention patent application with application number 201910419018.8, application date May 20, 2019, and name “Pixel structure for improving back-illuminated global shutter efficiency”. Technical Field

[0002] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter, and more particularly to a structure for improving the shutter efficiency by forming an electric field to attract excess electrons near a floating diffusion node or by making a structure for shielding excess electrons from entering the floating diffusion node. Background Art

[0003] In known back-illuminated global shutter sensors, due to the characteristics of the global shutter, all photosensitive pixels of the sensor will be exposed at the same time, and then read out in batches after being temporarily stored. Since there will be time differences in the readout, and during the readout process, the photosensitive element may still be continuously stimulated by external light and produce unexpected photosensitive noise, the later the exposure data is read out, the more susceptible it is to such photosensitive noise. Since the components of photosensitive noise are the same as those of exposure data, both are charges, once the exposure data in the form of charges is contaminated by photosensitive noise, it cannot be separated, resulting in distortion of the final image signal, that is, the shutter efficiency will be reduced.

[0004] In order to solve such problems, the present invention provides an improvement in pixel structure, which improves shutter efficiency by forming an electric field to attract excess electrons near a floating diffusion node, or configuring a structure to shield excess electrons from entering the floating diffusion node. Summary of the invention

[0005] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter. By configuring the high-voltage terminal of an element in a photosensitive pixel circuit very close to a floating diffusion node region, at least one strong electric field is formed, thereby producing an effect of shielding the floating diffusion node region. This makes it difficult for charges generated by unexpected photosensitive noise to reach the floating diffusion node region, that is, they are absorbed by the strong electric field, thereby improving the shutter efficiency.

[0006] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter. By adding an additional n-doped region in a photosensitive pixel circuit to form a guard ring with the n+ doped region of a source follower element, at least one strong electric field is formed, thereby producing an effect of shielding a floating diffusion node region, making it difficult for charges generated by unexpected photosensitive noise to reach the floating diffusion node region, that is, being absorbed by the strong electric field, thereby improving the shutter efficiency.

[0007] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter. In order to make it difficult for charges generated by unexpected photosensitive noise to reach the floating diffusion node area, the semiconductor material of the floating diffusion node facing the illumination direction can be hollowed out through a process procedure so that a depletion region cannot be formed in this direction. This can prevent charges generated by unexpected photosensitive noise from having a path to reach the floating diffusion node, thereby improving the shutter efficiency.

[0008] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter. In order to make it difficult for charges generated by unexpected photosensitive noise to reach a floating diffusion node area, a structure equivalent to deep trench isolation can be formed on the floating diffusion node facing the illumination direction through a process procedure, so that a depletion region cannot be formed in this direction. This can prevent charges generated by unexpected photosensitive noise from having a path to reach the floating diffusion node, thereby improving the shutter efficiency.

[0009] The present invention relates to a structure for improving the efficiency of a back-illuminated global shutter. In order to make it difficult for charges generated by unexpected photosensitive noise to reach the floating diffusion node area, a reflective layer can be formed on the semiconductor material of the floating diffusion node facing the illumination direction through a process procedure, making it difficult for light to enter the semiconductor and generate charges. The charges generated by unexpected photosensitive noise can be reduced, thereby improving the shutter efficiency.

[0010] According to the above description, the technical means provided by the present invention can also be implemented in combination, such as designing the placement of the high-voltage terminal of the component, and hollowing out the semiconductor material of the floating diffusion node facing the light direction through the process procedure, so that no depletion area can be formed in this direction. Both can prevent the charges generated by the unexpected photosensitive noise from reaching the floating diffusion node, thereby improving the shutter efficiency.

[0011] The present invention provides a pixel structure, comprising a base layer and a floating diffusion node. The floating diffusion node is arranged in the base layer and close to the front surface of the base layer, wherein the base layer has a hollow area located directly above the floating diffusion node and a depletion area cannot be formed directly above the floating diffusion node.

[0012] In order to make the above and other purposes, features and advantages of the present invention more obvious, the following will be described in detail with reference to the accompanying drawings. In addition, in the description of the present invention, the same components are represented by the same symbols, which are hereby described together. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1A is a top view of a pixel structure according to an embodiment of the present invention;

[0014] Figure 1B It is along Figure 1A A cross-sectional view of the A-A' line;

[0015] Figure 2 is a schematic diagram of a pixel structure of another embodiment of the present invention, which shows a similar Figure 1A A cross-sectional view along line BB' in the pixel structure;

[0016] Figure 3 is a schematic diagram of a pixel structure of another embodiment of the present invention, which shows a similar Figure 1A A cross-sectional view along line BB' in the pixel structure;

[0017] Figure 4 is a schematic diagram of a pixel structure of another embodiment of the present invention, which shows a similar Figure 1A A cross-sectional view along line BB' in the pixel structure;

[0018] Figure 5A is a schematic diagram of a pixel array according to an embodiment of the present invention;

[0019] Figure 5B yes Figure 5A A cross-sectional view of a pixel structure;

[0020] Figure 6 is a schematic diagram of a pixel array according to another embodiment of the present invention.

[0021] Description of Reference Numerals

[0022] 101, 201, 301, 401, 501, 601 Photodiode

[0023] 102, 202, 302, 402 Charge transfer control transistor

[0024] 103 Source follower element

[0025] 104, 204, 304, 404 Reset transistor

[0026] 105 High voltage terminal

[0027] 106, 206, 306, 406, 506, 606 floating diffusion nodes

[0028] 307a, 307b, 407 Deep trench isolation area

[0029] 308 Reflection layer

[0030] 408 photoresist layer

[0031] 511, 611 microlens DETAILED DESCRIPTION

[0032] The purpose of the present invention is to improve shutter efficiency by utilizing improvements in pixel structure, by forming an electric field to attract excess electrons near a floating diffusion node, or by configuring a structure to shield excess electrons from entering a floating diffusion node. Since in a global shutter sensor circuit, charges generated during a normal exposure time but not yet read are temporarily stored in a floating diffusion node, once these charges are affected by photosensitive noise, the normal exposure value cannot be correctly analyzed. Therefore, how to prevent photosensitive noise from affecting these charges is one of the most important issues. The individual embodiments of the present invention below can be used in conjunction with each other in other embodiments, and all can work effectively.

[0033] Figure 1A A top view of a pixel structure of an embodiment of the present invention is shown, which utilizes a transistor as a source follower element 103 in a photosensitive pixel circuit, and the transistor has a high voltage terminal 105. The pixel structure of the photosensitive pixel circuit in this embodiment also includes a photodiode 101, a charge transfer control transistor (TG) 102, a reset transistor (Rst) 104, and a floating diffusion node (FD) 106; wherein the charge transfer control transistor 102 and the reset transistor 104 are arranged in a first direction (e.g., Figure 1A The two source follower elements 103 are arranged at positions very close to (eg, less than 0.3 microns in a 90 nanometer process) the floating diffusion node 106, for example Figure 1A The two source follower elements 103 are shown to be arranged in the second direction (eg, Figure 1A The second direction is, for example, perpendicular to the first direction. In the present invention, the two source follower elements 103 are connected to the same read line to output the exposure value stored in the floating diffusion node 106.

[0034] When the photosensitive pixel circuit operates normally, a high voltage is applied to the high voltage terminal 105, thereby forming a strong electric field to surround and shield the floating diffusion node 106 area (such as Figure 1A As shown in FIG. 1 ). Thus, when the charges generated by the unexpected photosensitive noise approach the floating diffusion node 106 region, they will be absorbed by the strong electric field and cannot reach the floating diffusion node 106 region, thereby improving the shutter efficiency.

[0035] Figure 1B for Figure 1AA cross-sectional view along line AA' in a pixel structure of FIG. The pixel structure includes a substrate layer 100, a floating diffusion node 106, and two source follower elements 103. The substrate layer 100 is, for example, a semiconductor material used for the substrate layer of a CMOS image sensor. The floating diffusion node 106 is disposed in the substrate layer 100 and is located near the front side 100f of the substrate layer 100. The two source follower elements 103 are disposed on two opposite sides of the floating diffusion node 106. Each source follower element 106 has an n+ doped region (i.e., the above-mentioned high-voltage terminal 105) located in the substrate layer 100. The n+ doped region is used to form an electric field around the floating diffusion node 106 in the substrate layer 100 when the pixel structure is in operation to shield the floating diffusion node 106, so as to prevent noise-carrying carriers from entering the floating diffusion node 106.

[0036] In other embodiments, a guard ring may be formed in the vicinity of the floating diffusion node 106 region. In this embodiment, the base layer 100 also includes an n-doped region 107 located directly above the floating diffusion node 106. When a voltage is applied to the n+ doped region 105 of the two source follower elements 103, a strong electric field may be formed to surround and shield the floating diffusion node 106 region. The n+ doped region 105 and the n- doped region 107 are connected due to a punch-through phenomenon, and the guard ring is formed around the floating diffusion node 106 in the base layer 100 to shield the floating diffusion node 106 from receiving noise charges. In this way, when the charges generated by unexpected photosensitive noise approach the floating diffusion node 106 region, they will be absorbed by the strong electric field formed by the guard ring and cannot reach the floating diffusion node 106 region, thereby improving the shutter efficiency.

[0037] Figure 2 Another embodiment of the present invention is shown. Figure 1A A cross-sectional view along line segment BB' in the pixel structure. Figure 2Only the most relevant parts of the pixel structure of the photosensitive pixel circuit are shown, namely, the photodiode 201, the charge transfer control transistor (TG) 202, the reset transistor (Rst) 204 and the floating diffusion node (FD) 206; wherein the floating diffusion node 206 is arranged on the side close to the front surface 200f in the base layer 200. In this embodiment, in order to make it difficult for the charges generated by the unexpected photosensitive noise to reach the floating diffusion node 206 area, the semiconductor material of the floating diffusion node 206 facing the light direction can be hollowed out through the process procedure so that no depletion region can be formed in this direction, that is, the base layer 200 has a hollow region 200H located directly above the floating diffusion node 206 and no depletion region can be formed directly above it during operation. Since the main function of the floating diffusion node 206 region is to temporarily store the charge in the state originally from the photodiode 201, waiting for the subsequent circuit to read, the floating diffusion node 206 region only needs a very thin half substrate to have complete functions, so the semiconductor material of the floating diffusion node 206 facing the light direction can be hollowed out to a degree very close to the floating diffusion node 206 without affecting the circuit function. For example, the distance D between the bottom of the hollow area 200H and the top surface of the floating diffusion node 206 is 0.5 microns to 1 micron and the bottom area of ​​the hollow area 200H is greater than or equal to the cross-sectional area of ​​the floating diffusion node 206. Such a structure can prevent the charge generated by the unexpected photosensitive noise from reaching the floating diffusion node 206 by a path, thereby improving the shutter efficiency. The dug opening can be closed with other materials to enhance the mechanical stability of the semiconductor structure. For example, the inner surface of the hollow area 200H is first covered with a dielectric layer, and then an insulating layer is filled in the hollow area 200H on the dielectric layer.

[0038] Figure 3 Another embodiment of the present invention is shown. Figure 1A A cross-sectional view along line segment BB' in the pixel structure. Figure 3Only the most relevant parts of the pixel structure of the photosensitive pixel circuit are shown, namely, the photodiode 301, the charge transfer control transistor (TG) 302, the reset transistor (Rst) 304, the floating diffusion node (FD) 306, the deep trench isolation (DTI) regions 307a, 307b and the reflective layer 308 (optional implementation), wherein the floating diffusion node 306 is arranged on the side of the base layer 300 close to the front surface 300f. In this embodiment, in order to make it difficult for the charges generated by the unexpected photosensitive noise to reach the floating diffusion node 306 region, the deep trench isolation regions 307a, 307b can be formed through the process procedure, and the depth of the deep trench isolation regions 307a, 307b is preferably close to the floating diffusion node 306 region, for example, extending from the back surface 300b of the base layer 300 toward the front surface 300f to the top surface of the floating diffusion node 306, but not located directly above the floating diffusion node 306, so as to block the charges generated by the photosensitive noise and prevent the charges from entering the floating diffusion node 306 region. In this embodiment, the deep trench isolation regions 307a and 307b surround the floating diffusion node 306 so that the charged carriers outside the deep trench isolation regions 307a and 307b cannot enter the floating diffusion node 306. Figure 3 Two deep trench isolation areas are shown. This is due to Figure 3 In cross-sectional view, the deep trench isolation region is preferably formed into a tube shape extending from the back surface 300b toward the front surface 300f.

[0039] In addition, a reflective layer 308 may be formed on the semiconductor substrate (e.g., the back side 300b of the base layer 300) in the direction of the floating diffusion node 306 region, for example, directly above the floating diffusion node 306, which can further reduce the light entering the substrate and the charge of the photosensitive noise. Since the position of the photodiode 301 is not blocked by the reflective layer 308, the photosensitivity is not reduced. The reflective layer 308 can be formed using a metal material, such as copper or aluminum, but is not limited thereto. Similarly, the inner surface of the deep trench isolation region can be covered with a dielectric layer and the deep trench isolation region can be filled with an insulating layer on the dielectric layer to increase the structural strength.

[0040] Figure 4 Another embodiment of the present invention is shown. Figure 1A A cross-sectional view along line segment BB' in the pixel structure. Figure 4Only the most relevant parts of the pixel structure of the photosensitive pixel circuit are shown, namely, the photodiode 401, the charge transfer control transistor (TG) 402, the reset transistor (Rst) 404, the floating diffusion node (FD) 406, the deep trench isolation region 407 and the photoresist layer 408. In this embodiment, in order to make it difficult for the charges generated by the unexpected photosensitive noise to reach the floating diffusion node 306 region, the deep trench isolation region 407 can be formed through the process procedure. The difference from the deep trench isolation regions 307a and 307b of FIG. 3 is that in this embodiment, the back side 400b of the base layer 400 is located directly above the floating diffusion node 406 and is also formed by etching a lateral hollow region connected to the deep trench isolation region extending downward (such as Figure 3 The deep trench isolation region 407 has an arched structure or a cup-shaped structure, covering the floating diffusion node 406 region, and can more completely block the charges generated by the photosensitive noise. In this embodiment, a photoresist layer 408 can also be retained (after the etching process) on the semiconductor substrate (e.g., the back side 400b of the base layer 400) in the direction of the floating diffusion node 406 region to surround the deep trench isolation region 407, so as to exert an effect similar to that of the reflective layer 308 in the previous embodiment, and further reduce the light entering the substrate and generating the charges of the photosensitive noise.

[0041] It is worth noting that when using the deep trench isolation structure, the deep trench isolation structure can be formed to the maximum extent, that is, in addition to the photodiode area, the other parts of the light-receiving surface can be formed with the deep trench isolation structure to reduce the charge generated by the photosensitive noise. Similarly, the deep trench isolation area 407 can be filled with other materials to increase the structural strength.

[0042] Figure 5A An embodiment of the pixel array of the present invention is shown. In this embodiment, a microlens 511 is formed on the light-receiving surface of the photosensitive pixel circuit to change the light distribution condition, thereby reducing the amount of light that may enter the floating diffusion node 506 area. For example, a convex lens is placed above the photodiode area 501. Since the convex lens has a light-collecting effect, a larger proportion of the light entering the light-receiving surface of the photosensitive pixel will be concentrated in the photodiode area 501, thereby reducing the light entering the floating diffusion node 506 area and generating the charge of the photosensitive noise.

[0043] Figure 5A The implementation method can be implemented in conjunction with the above Figure 1A-1B , Figure 2 , Figure 3 and Figure 4 Implementation. Figure 5AAs shown, each pixel structure of the pixel array 50 of the embodiment of the present invention includes a photodiode 501 located in the substrate layer 100 to 400 and located in the lateral direction ( Figure 5A The microlens 511 is disposed on the back side of the substrate layers 100 to 400 and is aligned with the photodiode 501 , and may or may not cover the floating diffusion node 506 .

[0044] Please refer to Figure 5B As shown, it is Figure 5A In order to further concentrate the incident light to the photodiode (PD) 501 to reduce the noise charge generated by the incident light to other areas of the semiconductor material, the pixel structure of the embodiment of the present invention further includes a light guide 521 located between the microlens 511 and the photodiode 501. In addition, a light blocking layer or a light reflecting layer 508 (such as copper or aluminum) may be formed on the floating diffusion node 506 to reduce the incident light transmitted to the semiconductor material above the floating diffusion node 506.

[0045] Figure 6 Another embodiment of the pixel array of the present invention is shown. In this embodiment, the adjacent photosensitive pixel circuits of the pixel array 60 can be arranged and replaced so that the photodiode regions 601 of the adjacent photosensitive pixel circuits are close to each other, so that a larger microlens 611 can be shared to concentrate the light to multiple photodiode regions 601. The floating diffusion node 606 regions distributed around can reduce the photosensitive noise charge generated by the light entering. Compared with FIG. 5, the advantage of this embodiment is that the number of microlenses can be reduced, and the larger microlenses are easier to manufacture, which can further improve the manufacturing yield.

[0046] Figure 6 In the embodiment, the micro lens 611 may not cover or partially cover the floating diffusion node 606. Similarly, a light guide may be formed between the micro lens 611 and the photodiode 601 (see Figure 5B ) and other optical structures to improve photosensitivity and reduce noise charge.

[0047] According to the above description, the technical means provided by the present invention can also be implemented in combination, such as designing the high voltage terminal configuration position of the source follower element, and hollowing out the semiconductor material of the floating diffusion node facing the light direction through the process procedure, so that no depletion region can be formed in this direction. Both of them can prevent the charges generated by the unexpected photosensitive noise from reaching the floating diffusion node, thereby improving the shutter efficiency.

[0048] Although the present invention has been disclosed through the above examples, they are not intended to limit the present invention. Any person skilled in the art with ordinary knowledge in the art to which the present invention belongs can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the appended claims.

Claims

1. A pixel structure, comprising: basal layer; and a floating diffusion node, the floating diffusion node being arranged in the base layer and close to the front surface of the base layer, in, The base layer has a hollow region located directly above the floating diffusion node, and a depletion region cannot be formed directly above the floating diffusion node; and A cross-sectional area of ​​a side of the hollow region close to the floating diffusion node is larger than a cross-sectional area of ​​another side away from the floating diffusion node. 2 . The pixel structure according to claim 1 , wherein an inner surface of the hollow area is covered with a dielectric layer. 3 . The pixel structure according to claim 2 , wherein the hollow area is filled with an insulating layer on the dielectric layer to enhance the mechanical stability of the structure. 4 . The pixel structure according to claim 1 , wherein a distance between a bottom of the hollow region and a top surface of the floating diffusion node is 0.5 μm to 1 μm. 5 . The pixel structure according to claim 1 , wherein a bottom area of ​​the hollow region is greater than or equal to a cross-sectional area of ​​the floating diffusion node. 6 . The pixel structure according to claim 1 , further comprising a photodiode located in the base layer and in a lateral direction of the floating diffusion node. 7 . The pixel structure according to claim 6 , further comprising a microlens disposed on the back surface of the base layer and aligned with the photodiode. 8 . The pixel structure according to claim 7 , further comprising a light guide element located between the microlens and the photodiode. 9 . The pixel structure according to claim 6 , further comprising a microlens disposed on the back surface of the base layer and the photodiode is not located in a central area of ​​the microlens.

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