Pixel Structure of Image Sensor, Preparation Method Thereof, and Image Sensor

Abstract

The present invention provides a pixel structure of an image sensor and a preparation method thereof, including: a semiconductor substrate, a photodiode and a floating diffusion region located in the semiconductor substrate; a pinned layer covering the upper surface of the photodiode; wherein, the thickness of the pinned layer gradually thins from the photodiode to the floating diffusion region on one side close to the floating diffusion region. By setting the pinned layer to have a gradually thinning structure on one side close to the floating diffusion, the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region, enhancing the switching performance of the transfer transistor; the pinned layer is formed by a selective epitaxy process, and the formed pinned layer has stable performance.

Description

Technical Field

[0001] The present invention relates to the technical field of image sensors, and in particular, to a pixel structure of an image sensor, a preparation method thereof, and an image sensor. Background Art

[0002] CMOS image sensors have the advantages of simple process, easy integration with other devices, small size, light weight, low power consumption, and low cost. Therefore, with the development of technology, CMOS image sensors are increasingly replacing CCD image sensors and being applied to various electronic products. Currently, CMOS image sensors have been widely used in static digital cameras, camera phones, digital video cameras, medical imaging devices (such as gastroscopes), vehicle-mounted imaging devices, etc. CMOS image sensor products can be divided into FSI (Front Side Illumination) and BSI (Back Side Illumination).

[0003] The pixel array, as the core module of an image sensor, is mainly composed of a photodiode, a transfer transistor, a reset transistor, a source follower transistor, etc. Among them, the photodiode is used to absorb photons and convert them into electrons, and the photodiode is composed of an N-type doped region and a semiconductor substrate; the transfer transistor is located between the photodiode and the floating diffusion region to achieve a switching function, and the role of the floating diffusion region is to collect the electrons converted by the photodiode and convert the electronic signal into a corresponding voltage signal according to its conversion gain ability.

[0004] In the prior art, an ion implantation technique is usually used to form a pinned layer on the surface of the n-type doped region. The depth of ion implantation is affected by the thickness of the dielectric layer (oxide) and the ion implantation machine, which may lead to uneven ion implantation depth. If the ion implantation is too deep, the photosensitivity will decrease and color cast will occur. If the ion implantation is too shallow, there will be no pinning effect. Moreover, the ion implantation technique will damage the surface of the photodiode and generate defects, and these defects act as dangling bonds on the substrate surface and generate noise, thereby affecting the performance of the image sensor.

[0005] In the prior art, the commonly used planar field effect transistor has no directional threshold gradient. When used as a transfer transistor, after being turned on, a large number of electrons will be adsorbed on the channel surface, and during the turn-off process, a part of the electrons will flow back into the photodiode, and this part of the backflow electrons is superimposed on the next frame of signal, generating drag noise. Summary of the Invention

[0006] The purpose of the present invention is to provide a pixel structure of an image sensor, a preparation method thereof, and an image sensor for enhancing the switching characteristics.

[0007] Based on the above considerations, the present invention provides a pixel structure of an image sensor, including:

[0008] A semiconductor substrate, a photodiode located within the semiconductor substrate, and a floating diffusion region;

[0009] A pinning layer covering the upper surface of the photodiode;

[0010] Wherein, the thickness of the pinning layer gradually thins from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

[0011] Optionally, the side of the pinning layer close to the floating diffusion region has a downward stepped structure from the photodiode to the floating diffusion region.

[0012] Optionally, the stepped structure is a slope-like structure.

[0013] Optionally, it further includes a transfer transistor, and the gate of the transfer transistor covers the side of the pinning layer close to the floating diffusion region, so that the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region.

[0014] Optionally, the pinning layer is composed of N pinning thin films stacked layer by layer on the upper surface of the photodiode; wherein, the upper pinning thin film covers the lower pinning thin film and extends to the upper surface of the photodiode on the side close to the floating diffusion region, and N≥2.

[0015] Optionally, the doping concentration of the pinning thin film gradually decreases from the bottom layer to the outer layer.

[0016] Optionally, the pinning layer is a single crystal structure.

[0017] The present invention also provides a method for manufacturing a pixel structure of an image sensor, characterized by including:

[0018] Providing a semiconductor substrate, and arranging a photodiode and a floating diffusion region within the semiconductor substrate;

[0019] Forming a pinning layer on the upper surface of the photodiode;

[0020] Wherein, the thickness of the pinning layer gradually thins from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

[0021] Optionally, it further includes the step of forming a transfer transistor, and the gate of the transfer transistor covers the side of the pinning layer close to the floating diffusion region, so that the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region.

[0022] Optionally, the step of forming a pinning layer on the upper surface of the photodiode includes:

[0023] S21: Form a patterned mask layer on the surface of the semiconductor substrate, where the patterned mask layer covers the surface of the semiconductor substrate and exposes part or all of the upper surface of the photodiode;

[0024] S22: On the exposed upper surface of the photodiode, form a first pinned film;

[0025] S23: Etch the outer edge portion of the mask layer to re-expose part of the upper surface of the photodiode near the floating diffusion region;

[0026] S24: Form a second pinned film to cover the first pinned film and the re-exposed part of the upper surface of the photodiode;

[0027] S25: Remove the patterned mask layer to form a pinned layer stacked by pinned films layer by layer, and the pinned layer has a downward stepped structure from the photosensitive region of the photodiode to the floating diffusion region near the floating diffusion region.

[0028] Optionally, the step of forming a pinned layer on the upper surface of the photodiode further includes: between step S24 and S25, repeating steps S23 - S24 one or more times to form a pinned layer stacked by multiple layers of pinned films layer by layer, and the pinned layer has a downward stepped structure from the photodiode to the floating diffusion region near the floating diffusion region.

[0029] Optionally, between step S24 and S25, repeating steps S23 - S24 multiple times to form a pinned layer stacked by multiple layers of pinned films layer by layer, and the pinned layer has a downward slope-like structure from the photodiode to the floating diffusion region near the floating diffusion region.

[0030] Optionally, the doping concentration of the pinned film gradually decreases from the bottom layer to the outer layer.

[0031] Optionally, the step of forming a pinned layer on the surface of the photodiode includes:

[0032] S31: Cover a pinned layer on the surface of the semiconductor substrate;

[0033] S32: On the surface of the pinned layer directly above the photodiode, form a patterned mask layer;

[0034] S33: Use the mask layer to etch the surface of the pinned layer near the floating diffusion region;

[0035] S34: Etch the mask layer near the floating diffusion region to re-expose part of the pinned layer;

[0036] S35: Etch the surface of the re-exposed pinned layer;

[0037] S36: Remove the patterned mask layer to finally form a pinned layer, and the thickness of the pinned layer gradually decreases from the photodiode to the floating diffusion region side near the floating diffusion region.

[0038] Optionally, the step of forming the pinned layer on the surface of the photodiode further includes: between steps S35 and S36, repeating steps S34 - S35 one or more times, and the pinned layer has a downward stepped structure from the photodiode to the floating diffusion region side near the floating diffusion region.

[0039] Optionally, between steps S35 and S36, repeat steps S34 - S35 multiple times, and the pinned layer has a downward slope-like structure from the photodiode to the floating diffusion region side near the floating diffusion region.

[0040] Optionally, the pinned layer is formed by selective epitaxy process.

[0041] The present invention also provides an image sensor, including the pixel structure described in any one of the above.

[0042] The pixel structure of the image sensor provided by the present invention, its manufacturing method, and the image sensor have the following beneficial effects:

[0043] By setting the pinned layer to have a gradually decreasing thickness near the floating diffusion side, the threshold voltage of the transfer transistor is distributed from high to low from the photodiode to the floating diffusion region, enhancing the switching performance of the transfer transistor;

[0044] The pinned layer is formed by selective epitaxy process, and the formed pinned layer has stable performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Other features, objects, and advantages of the present invention will become more apparent by reading the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0046] Figures 1 to 8 It is a schematic structural diagram showing the formation process of the pixel structure of Embodiment 1 of the present invention;

[0047] Figures 9 to 15 It is a schematic structural diagram showing the formation process of the pinned layer of Embodiment 2 of the present invention;

[0048] Figure 16 It is a schematic diagram of the pixel structure of Embodiment 2 of the present invention.

[0049] In the figures, throughout different views, the same or similar reference numerals represent the same or similar devices (modules) or steps. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention can be implemented 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 present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0051] Secondly, the present invention is described in detail with reference to schematic diagrams. When describing the embodiments of the present invention, for the sake of clarity, the schematic diagrams are only examples and should not limit the scope of protection of the present invention.

[0052] The present invention provides a pixel structure of an image sensor, as Figure 8 shown, comprising:

[0053] a semiconductor substrate 11, a photodiode 12 and a floating diffusion region 15 located within the semiconductor substrate 11;

[0054] a pinning layer 13 covering the upper surface of the photodiode 12;

[0055] wherein, the thickness of the pinning layer 13 gradually thins from the photodiode to the floating diffusion region side near the floating diffusion region 15.

[0056] The pinning layer is generally disposed on the surface of the photodiode, so that when the image sensor is irradiated with light, the photoelectrons formed in the photodiode are isolated, preventing the photoelectrons from escaping from the surface of the photodiode and improving the performance of the image sensor.

[0057] The thickness of the pinning layer gradually thins from the photodiode to the floating diffusion region side near the floating diffusion region, and can be a downward stepped structure or a downward slope-like structure formed by multiple steps. In this embodiment, the pinning layer 13 forms a three-layer stepped structure on the floating diffusion region side.

[0058] The material of the semiconductor substrate 11 includes semiconductor materials such as silicon, germanium, silicon germanide, and gallium arsenide. Those skilled in the art can select the type of semiconductor substrate according to the semiconductor devices formed on the semiconductor substrate. Therefore, the type of semiconductor should not limit the scope of protection of the present invention.

[0059] As an example, as Figure 8 shown, the transfer transistor gate 16 covers the downward stepped structure 14 on the side of the pinning layer 13 near the floating diffusion region 15. The transfer transistor gate covers the pinning layer, and can form a lateral parasitic electric field. Moreover, due to the change in the thickness of the pinning layer, the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region, thereby enhancing the switching characteristics of the transfer transistor. Of course, the pixel structure provided by the present invention also includes a reset transistor (not shown), a source follower transistor (not shown), etc., and their technical solutions are the same as those in the prior art and will not be elaborated herein.

[0060] As an example, in this embodiment, the pinning layer 13 is composed of three pinning thin films stacked layer by layer on the upper surface of the photodiode; among them, the upper pinning thin film covers the lower pinning thin film and extends to the upper surface of the photodiode near the floating diffusion region, thereby forming a stepped structure. That is, the first thin film 131 covers the upper surface of the photodiode; the second thin film 132 covers the first thin film 131 and extends to the upper surface of the photodiode; the third thin film 133 covers the second thin film 132 and extends to the upper surface of the photodiode. Of course, the number of pinning thin films in the pinning layer is not limited to the three pinning thin films in this embodiment as an example, and two or more pinning thin films can be set as needed. The multi-layer pinning thin films can also form a stepped structure similar to a slope.

[0061] At the same time, the doping concentration of each pinning thin film can be set. The doping concentrations of each pinning thin film can be the same or different. As an example, in this embodiment, the doping concentration of the pinning thin film gradually decreases from the bottom layer to the outer layer, that is, the doping concentration of the first pinning thin film is greater than that of the second pinning thin film, and the doping concentration of the second pinning thin film is greater than that of the third pinning thin film. Of course, it can also be adjusted according to actual needs, and no limitation is made here.

[0062] It should be noted that the pinning layer can not only be formed by stacking two or more pinning thin films, but also can be formed by etching a single pinning thin film multiple times.

[0063] The pinning layer is preferably a single crystal structure. The pinning layer with a single crystal structure has good high-temperature stability and can be not damaged in subsequent processes.

[0064] The present invention also provides a method for manufacturing an image sensor pixel structure, including the steps of:

[0065] Providing a semiconductor substrate, and arranging a photodiode and a floating diffusion region in the semiconductor substrate;

[0066] Forming a pinning layer on the upper surface of the photodiode;

[0067] Among them, the thickness of the pinning layer gradually thins from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

[0068] The following is a detailed description of the method for manufacturing the pixel structure through Figures 1 to 8 the following.

[0069] As Figure 1 shown, a semiconductor substrate 11 is provided, and a photodiode 12 and a floating diffusion region 15 are arranged in the semiconductor substrate 11.

[0070] As Figure 7As shown, a pinning layer 13 is formed on the upper surface of the photodiode 12. The pinning layer 13 has a downward stepped structure 14 in the direction from the photodiode 12 to the floating diffusion region 15 on one side close to the floating diffusion region 15.

[0071] In this embodiment, the steps of forming the pinning layer 13 are specifically as follows:

[0072] As Figure 2 shown, step S21 is performed to form a patterned mask layer 17 on the surface of the semiconductor substrate 11. The patterned mask layer 17 covers the surface of the semiconductor substrate 11 and exposes part or all of the upper surface of the photodiode 12.

[0073] As Figure 3 shown, step S22 is performed to form a first pinning thin film 131 on the exposed upper surface of the photodiode 12.

[0074] As Figure 4 shown, step S23 is performed to etch the outer edge portion of the mask layer 17 to re-expose part of the upper surface of the photodiode 12 on the side close to the floating diffusion region.

[0075] As Figure 5 shown, step S24 is performed to form a second pinning thin film 132. The second pinning thin film 132 covers the first pinning thin film 131 and covers the re-exposed upper surface of the photodiode 12.

[0076] In this embodiment, steps S23 - S24 are also repeated once to form a third pinning thin film 133. The number of repetitions of steps S23 - S24 can be set according to the number of layers of the pinning thin film. In other embodiments, steps S23 - S24 can be repeated multiple times to form a pinning layer with multiple layers of pinning thin films stacked. In another embodiment, the multiple layers of pinning thin films can also form a stepped structure similar to a slope. In another embodiment, steps S23 - S24 may not be repeated, and a pinning layer composed of two layers of pinning thin films stacked can be obtained.

[0077] As Figure 7 shown, step S25 is performed to remove the patterned mask layer 17, and finally a pinning layer 13 composed of three layers of pinning thin films is formed, and it has a downward stepped structure 14 in the direction from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

[0078] Preferably, each pinning film is formed by selective epitaxy. Compared with the prior art in which the pinning layer is formed by ion implantation, the epitaxial method for forming the pinning layer in the present invention can eliminate the inconsistency in ion implantation depth caused by the uneven thickness of the dielectric layer during ion implantation, improve the optical performance uniformity in the preparation of wafer-level image sensors, and reduce noise. Moreover, the ion implantation process is omitted, defects caused by the damage to the surface of the photodiode during the ion implantation process are reduced, and the process steps are also saved.

[0079] Moreover, the doping concentration of each pinning film can be set. The doping concentrations of the pinning films can be the same or different. As an example, in this embodiment, the doping concentration of the pinning film gradually decreases from the first layer to the third layer.

[0080] On the structure formed above, a transfer transistor gate 16 is formed, as Figure 8 shown. The transfer transistor gate 16 covers the downward stepped structure 14 of the pinning layer 13, so that the threshold voltage of the transfer transistor shows a gradient distribution from high to low in the direction from the photodiode to the floating diffusion region, enhancing the switching characteristics of the transfer transistor.

[0081] The present invention also provides an image sensor (not shown), which includes the pixel structure provided above, and of course also includes the peripheral circuits around the pixel structure, etc.

[0082] Embodiment 2

[0083] This embodiment provides a pixel structure and a method for preparing the pixel structure. Its technical solution is similar to that of Embodiment 1. The difference from Embodiment 1 is that the pinning layer of its pixel structure and the method for forming the pinning layer are different.

[0084] The pinning layer in this embodiment is formed by multiple etching of a single pinning film. Figures 9 to 15 The method for forming the pinning layer in this embodiment is introduced in detail, and the specific steps are as follows:

[0085] As Figure 9 shown, step S31 is performed to cover the pinning layer 23 on the surface of the semiconductor substrate 21. A photodiode 22 and a floating diffusion region 25 are provided in the semiconductor substrate 21, and the pinning layer 23 is formed by selective epitaxy;

[0086] As Figure 10 shown, step S32 is performed to form a patterned mask layer 27 on the surface of the pinning layer 23 directly above the photodiode 22;

[0087] As Figure 11 shown, step S33 is performed to etch the surface of the pinning layer 23 using the mask layer 27;

[0088] AsFigure 12 As shown, step S34 is performed to etch the mask layer 27 on one side close to the floating diffusion region 25, and part of the surface of the pinned layer 23 is exposed again.

[0089] As Figure 13 shown, step S35 is performed to etch the surface of the pinned layer 23 using the mask layer 27.

[0090] As Figure 14 shown, in this embodiment, steps S34 - S35 are repeated once to form a three - layer stepped pinned layer 23. The number of repetitions of steps S34 - S35 can be set according to the number of steps of the stepped structure of the pinned layer. In other embodiments, steps S34 - S35 can be repeated multiple times to form a multi - layer stepped pinned layer. In another embodiment, steps S34 - S35 can also be repeated multiple times to form a stepped structure similar to a slope. In another embodiment, steps S34 - S35 may not be repeated to obtain a two - layer stepped pinned layer.

[0091] As Figure 15 shown, step S36 is performed to remove the patterned mask layer 27, and finally the pinned layer 23 is formed. The thickness of the formed pinned layer 23 gradually decreases from the photodiode to the floating diffusion region direction, and the pinned layer has a downward stepped structure 24 from the photodiode to the floating diffusion region direction on the side close to the floating diffusion region.

[0092] On the structure formed above, a transfer transistor gate 26 is formed. As Figure 16 shown, the transfer transistor gate 26 covers the downward stepped structure 24 of the pinned layer 23, so that the threshold voltage of the transfer transistor is distributed from high to low from the photodiode to the floating diffusion region direction, enhancing the switching characteristics of the transfer transistor.

[0093] Other technical solutions are the same as those in Embodiment 1 and will not be elaborated here.

[0094] In summary, the present invention provides a pixel structure of an image sensor and a method for preparing the same. The pixel structure includes: a semiconductor substrate, a photodiode and a floating diffusion region located in the semiconductor substrate; a pinned layer covering the upper surface of the photodiode; wherein, the thickness of the pinned layer gradually decreases from the photodiode to the floating diffusion region direction on the side close to the floating diffusion region. By setting the pinned layer to have a gradually decreasing thickness on the side close to the floating diffusion region, the threshold voltage of the transfer transistor is distributed from high to low from the photodiode to the floating diffusion region direction, enhancing the switching performance of the transfer transistor; the pinned layer is formed by a selective epitaxy process, and the formed pinned layer has stable performance.

[0095] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above-described exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, in any case, the embodiments should be regarded as exemplary and non-limiting. In addition, obviously, the term "including" does not exclude other elements and steps, and the word "a" does not exclude a plurality. A plurality of elements recited in the apparatus claims can also be implemented by one element. The terms first, second, etc. are used to denote names and do not denote any particular order.

Claims

1. A pixel structure of an image sensor, characterized in that, Comprising: A semiconductor substrate, a photodiode located within the semiconductor substrate, and a floating diffusion region; A pinning layer covering the upper surface of the photodiode and at least partially located above the surface of the semiconductor substrate; Wherein, on the side of the pinning layer close to the floating diffusion region, the thickness gradually thins from the photodiode to the floating diffusion region.

2. The pixel structure of the image sensor according to claim 1, wherein On the side of the pinning layer close to the floating diffusion region, it has a downward stepped structure from the photodiode to the floating diffusion region.

3. The pixel structure of the image sensor according to claim 2, characterized in that, The stepped structure is a slope-like structure.

4. The pixel structure according to claim 1, characterized in that It further includes a transfer transistor, and the gate of the transfer transistor covers the side of the pinning layer close to the floating diffusion region, so that the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region.

5. The pixel structure according to claim 1, wherein The pinning layer is composed of N pinning thin films stacked layer by layer from the upper surface of the photodiode; wherein, the upper pinning thin film covers the lower pinning thin film and extends to the upper surface of the photodiode on the side close to the floating diffusion region, and N≥2.

6. The pixel structure according to claim 5, wherein The doping concentration of the pinning thin film gradually decreases from the bottom layer to the outer layer.

7. The pixel structure according to claim 1, wherein The pinning layer is a single crystal structure.

8. A method for preparing an image sensor pixel structure, characterized in that, Comprising: Providing a semiconductor substrate, and arranging a photodiode and a floating diffusion region within the semiconductor substrate; Forming a pinning layer on the upper surface of the photodiode; Wherein, on the side of the pinning layer close to the floating diffusion region, the thickness gradually thins from the photodiode to the floating diffusion region.

9. The method for preparing the pixel structure according to claim 8, wherein, It further includes the step of forming a transfer transistor, and the gate of the transfer transistor covers the side of the pinning layer close to the floating diffusion region, so that the threshold voltage of the transfer transistor is distributed from high to low in the direction from the photodiode to the floating diffusion region.

10. The manufacturing method of the pixel structure according to claim 8, wherein The step of forming a pinning layer on the upper surface of the photodiode includes: S21: Forming a patterned mask layer on the surface of the semiconductor substrate, the patterned mask layer covering the surface of the semiconductor substrate and exposing part or all of the upper surface of the photodiode; S22: Forming a first pinning thin film on the exposed upper surface of the photodiode; S23: Etching the outer edge part of the mask layer to re-expose part of the upper surface of the photodiode on the side close to the floating diffusion region; S24: Forming a second pinning thin film, covering the first pinning thin film and covering the re-exposed part of the upper surface of the photodiode; S25: Removing the patterned mask layer to form a pinning layer composed of stacked pinning thin films layer by layer, and the pinning layer has a downward stepped structure from the photosensitive region of the photodiode to the floating diffusion region on the side close to the floating diffusion region.

11. The method for preparing the pixel structure according to claim 10, wherein The step of forming a pinning layer on the upper surface of the photodiode further includes: between step S24 and S25, repeating steps S23~S24 once or more times to form a pinning layer composed of multiple stacked pinning thin films layer by layer, and the pinning layer has a downward stepped structure from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

12. The method for preparing the pixel structure according to claim 11, wherein Between step S24 and S25, repeating steps S23~S24 multiple times to form a pinning layer composed of multiple stacked pinning thin films layer by layer, and the pinning layer has a downward slope-like structure from the photodiode to the floating diffusion region on the side close to the floating diffusion region.

13. The method for preparing the pixel structure according to claim 10, wherein The doping concentration of the pinning thin film gradually decreases from the bottom layer to the outer layer.

14. The manufacturing method of the pixel structure according to claim 8, characterized in that, The steps of forming a pinned layer on the surface of the photodiode include: S31: Cover the pinned layer on the surface of the semiconductor substrate; S32: Form a patterned mask layer on the surface of the pinned layer directly above the photodiode; S33: Etch the surface of the pinned layer near the floating diffusion region using the mask layer; S34: Etch the mask layer near the floating diffusion region to re-expose part of the pinned layer; S35: Etch the surface of the pinned layer that is re-exposed; S36: Remove the patterned mask layer to finally form the pinned layer, and the thickness of the pinned layer gradually decreases from the photodiode to the floating diffusion region near the floating diffusion region.

15. The manufacturing method of the pixel structure according to claim 14, characterized in that, The steps of forming a pinned layer on the surface of the photodiode further include: between step S35 and S36, repeating steps S34 to S35 one or more times, and the pinned layer has a downward stepped structure from the photodiode to the floating diffusion region near the floating diffusion region.

16. The method for preparing the pixel structure according to claim 15, wherein, Between step S35 and S36, repeating steps S34 to S35 multiple times, and the pinned layer has a downward slope-like structure from the photodiode to the floating diffusion region near the floating diffusion region.

17. The method for preparing the pixel structure according to claim 10 or 14, characterized in that, The pinned layer is formed by selective epitaxy process.

18. An image sensor, characterized in that, It includes the pixel structure according to any one of claims 1 to 7.

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