Image sensor manufacturing method and image sensor
By forming a heavily doped P-type region below the floating diffusion section and restricting its diffusion, the transverse electric field strength is reduced, thus solving the problem of dark current in CMOS image sensors and improving the image quality of the image sensor.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANGHAI INTEGRATED CIRCUIT RESEARCH & DEVELOPMENT CENTER CO LTD
- Filing Date
- 2024-12-23
- Publication Date
- 2026-06-23
AI Technical Summary
In the miniaturization process of existing CMOS image sensors, the dark current problem caused by noise electrons in the floating diffuser cannot be eliminated by CDS, which affects image quality.
A P-type heavily doped region is formed below the floating diffusion section. By using etching and low-temperature epitaxial growth processes, the diffusion of the P-type heavily doped region is restricted, the lateral electric field strength is reduced, the recombination centers in the depletion region are reduced, and the image sensor structure is optimized.
It effectively reduces dark current, improves image quality of image sensors, and meets the requirements of high dynamic range.
Smart Images
Figure CN122269843A_ABST
Abstract
Description
Technical Field
[0001] This application relates to semiconductor technology, and in particular to a method for fabricating an image sensor and an image sensor. Background Technology
[0002] With the increasing popularity of new energy vehicles and the maturation of autonomous driving technology, CMOS (Complementary Metal Oxide Semiconductor) image sensors (CIS) have been widely used in the automotive industry, i.e., automotive image sensors. Automotive image sensors primarily capture optical signals through optical lenses (LENS), projecting these signals onto the image sensor. The image sensor converts the absorbed optical signals into electrical signals, which are then converted from analog to digital signals into digital image signals. These digital signals are then processed by an ISP (Image Signal Processor) chip, ultimately transforming the processed image into a specific format before being transmitted to the vehicle's autonomous driving system for recognition, and finally converted into the image displayed to the end user on the vehicle's infotainment system. Current automotive cameras primarily focus on surround-view and rear-view imaging, mainly used to assist drivers in scenarios such as automatic parking, traffic sign recognition, or triggering emergency braking. Automotive cameras need to collect as much environmental information as possible to help drivers make correct judgments. Therefore, automotive image sensors have higher requirements for image quality and dynamic range. High dynamic range image sensors have higher requirements for charge storage. However, as image sensors are miniaturized, the size of PDs (photodiodes) is gradually shrinking. The industry generally uses LOFIC (Lateral Overflow Integration Capacitor) technology to solve the problem of no space to collect the charge generated by PDs. This technology uses an external capacitor to collect the overflow charge.
[0003] Unlike conventional image sensors, LOFIC technology, when collecting excess charge, uses an external capacitor structure connected to the floating diffuser of the image sensor. During image sensor exposure, the floating diffuser generates noise electrons, primarily originating from the dark current in the shallow trench isolation structure of the image sensor. Conventional image sensors use CDS (Correlated Double Sampling) to eliminate this noise. Simply put, CDS samples two levels of the output electrical signal: one for the reset level and the other for the signal level. However, because the LOFIC circuit needs to read the external capacitor value, a reset operation cannot be performed during the readout process; otherwise, the external capacitor would be cleared to zero. Therefore, CDS cannot be used during this process, inevitably resulting in the reading of noise from the floating diffuser. This noise directly affects image quality. Summary of the Invention
[0004] The purpose of this application is to provide an image sensor fabrication method and an image sensor that can improve the dark current of shallow trench isolation structures, thereby optimizing image quality.
[0005] To achieve the above objectives, this application provides a method for fabricating an image sensor, comprising:
[0006] Two shallow trench isolation structures and a photosensitive pixel structure are formed in the region corresponding to each pixel unit in the substrate, and a pixel transmission gate is formed on the surface of the photosensitive pixel structure; the photosensitive pixel structure is located on the side of one of the shallow trench isolation structures opposite to the other shallow trench isolation structure;
[0007] An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away at least the substrate on both sides of the first ion implantation region;
[0008] A barrier layer is applied to the surface except the top surface of the first ion implantation region. Ion implantation is performed on the first ion implantation region. The concentration of implanted P-type ions is greater than the concentration of P-type ions in the substrate. After implantation, the barrier layer is removed to form a heavily doped P-type region.
[0009] After forming the P-type heavily doped region, a low-temperature epitaxial growth process is performed to epitaxially grow the substrate in the area etched away between the two shallow trench isolation structures.
[0010] An image sensor is obtained by forming an N-type lightly doped region and an N-type heavily doped region sequentially along the thickness direction above the P-type heavily doped region; the N-type heavily doped region is a floating diffusion section; the lateral area of the P-type heavily doped region is smaller than the lateral area of the N-type lightly doped region, and the distance from the P-type heavily doped region to the shallow trench isolation structure is greater than the distance from the P-type heavily doped region to the N-type lightly doped region.
[0011] Optionally, forming two shallow trench isolation structures and a photosensitive pixel structure in the region corresponding to each pixel unit in the substrate, and forming a pixel transmission gate on the surface of the photosensitive pixel structure, includes:
[0012] After depositing a mask layer on the front side of the substrate, an etching process is performed to form two shallow trenches in the area corresponding to each pixel unit in the substrate.
[0013] After the shallow trenches are filled by the process, an annealing and grinding process is performed to form the shallow trench isolation structure;
[0014] After forming the shallow trench isolation structure, N-type heavily doped ion implantation, N-type lightly doped ion implantation, and P-type ion implantation are sequentially performed on the substrate corresponding to the pixel photosensitive area and the gate area to form an N-type heavily doped well region, an N-type lightly doped implantation region, and a clamped P-type implantation region arranged sequentially along the thickness direction, thereby obtaining the photosensitive pixel structure; the gate area is located on the side of one shallow trench isolation structure that is away from another shallow trench isolation structure; the pixel photosensitive area is located on the side of the gate area that is away from the shallow trench isolation structure.
[0015] After obtaining the photosensitive pixel structure, the pixel transmission gate is formed on the surface of the gate region by high-temperature thermal oxidation, photolithography and etching.
[0016] Optionally, the step of forming an N-type lightly doped region and an N-type heavily doped region sequentially arranged along the thickness direction above the P-type heavily doped region to obtain an image sensor includes:
[0017] During the epitaxial growth process, the N-type lightly doped region and the N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region through photolithography, ion implantation and cleaning processes, to obtain the image sensor.
[0018] Optionally, the step of forming an N-type lightly doped region and an N-type heavily doped region sequentially arranged along the thickness direction above the P-type heavily doped region to obtain an image sensor includes:
[0019] After epitaxial growth is completed, the N-type lightly doped region and the N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region through photolithography, ion implantation and cleaning processes to obtain the image sensor.
[0020] Optionally, the etching operation on the substrate between the two shallow trench isolation structures, at least etching away the substrate on both sides of the first ion implantation region, includes:
[0021] An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away a portion of the substrate width adjacent to the two shallow trench isolation structures, forming the first ion implantation region and the second ion implantation region arranged sequentially along the thickness direction.
[0022] Accordingly, an N-type lightly doped region and an N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region to obtain an image sensor, comprising:
[0023] The image sensor is obtained by forming the N-type lightly doped region and the N-type heavily doped region arranged sequentially along the thickness direction in the second ion implantation region.
[0024] Optionally, covering the surface other than the top surface of the first ion implantation region with a barrier layer includes:
[0025] Photoresist is coated on the front side of the substrate, except for the top surface of the second ion implantation region, and the photoresist is filled in the area etched between the two shallow trench isolation structures.
[0026] Optionally, the etching operation on the substrate between the two shallow trench isolation structures, at least etching away the substrate on both sides of the first ion implantation region, includes:
[0027] An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away the substrate on both sides and the surface of the first ion implantation region, thereby exposing the first ion implantation region.
[0028] Accordingly, an N-type lightly doped region and an N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region to obtain an image sensor, comprising:
[0029] The image sensor is obtained by forming the N-type lightly doped region and the N-type heavily doped region sequentially along the thickness direction in the substrate epitaxially grown above the P-type heavily doped region.
[0030] Optionally, covering the surface other than the top surface of the first ion implantation region with a barrier layer includes:
[0031] Photoresist is coated on the substrate surface and photolithography is performed to expose the first ion implantation region, while the photoresist is retained on the surface except for the top surface of the first ion implantation region.
[0032] To achieve the above objectives, this application also provides an image sensor, comprising: a plurality of pixel units; each pixel unit comprising: a substrate; the substrate having two shallow trench isolation structures and a photosensitive pixel structure disposed therein; the photosensitive pixel structure being located on a side of one of the shallow trench isolation structures opposite to the other shallow trench isolation structure; and a pixel transmission gate being disposed on the surface of the photosensitive pixel structure;
[0033] In the substrate between the two shallow trench isolation structures, a P-type heavily doped region, an N-type lightly doped region, and an N-type heavily doped region are sequentially disposed along the thickness direction; the N-type heavily doped region is a floating diffusion region; the concentration of P-type ions in the P-type heavily doped region is greater than the concentration of P-type ions in the substrate; the lateral area of the P-type heavily doped region is smaller than the lateral area of the N-type lightly doped region, and the distance from the P-type heavily doped region to the shallow trench isolation structure is greater than the distance from the P-type heavily doped region to the N-type lightly doped region.
[0034] Optionally, the photosensitive pixel structure includes a pixel photosensitive region and a gate region; the gate region is located on the side of one of the shallow trench isolation structures facing away from another shallow trench isolation structure; the pixel photosensitive region is located on the side of the gate region facing away from the shallow trench isolation structure; both the pixel photosensitive region and the gate region include an N-type heavily doped well region, an N-type lightly doped implantation region, and a clamped P-type implantation region arranged sequentially along the thickness direction;
[0035] The pixel transmission gate is disposed on the surface of the gate region.
[0036] Obviously, the image sensor fabrication method provided in this application involves forming a heavily doped P-type region below a floating diffuser. This causes the depletion regions of the heavily doped P-type and lightly doped N-type regions below the floating diffuser to bend upwards from their original cubic trapezoidal shape into a crescent shape. This reduces the area of the depletion region falling within the shallow trench isolation structure, thereby reducing dark current. Simultaneously, etching and low-temperature epitaxial growth processes restrict the diffusion of the heavily doped P-type region, confining it to a region far from the shallow trench isolation structure. The lateral area of the heavily doped P-type region is smaller than that of the lightly doped N-type region. Since the electric field in the vertical direction of the heavily doped P-type region is much stronger than the electric field in the horizontal direction, the electric field strength near the shallow trench isolation structure in the horizontal direction is reduced. This results in fewer electrons or holes being absorbed by defects that can act as recombination centers compared to the original structure, thus reducing dark current. This application also provides an image sensor with the aforementioned beneficial effects. Attached Figure Description
[0037] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.
[0038] Figure 1 This is a schematic diagram of the structure of a traditional image sensor;
[0039] Figure 2 A flowchart illustrating an image sensor fabrication method provided in this application embodiment;
[0040] Figure 3 This is a schematic diagram of the structure of an image sensor provided in an embodiment of this application;
[0041] Figures 4 to 15 A schematic flowchart illustrating the first image sensor fabrication method provided in this application embodiment;
[0042] Figures 16 to 21 A schematic flowchart illustrating the second image sensor fabrication method provided in this application embodiment;
[0043] Figures 22 to 24 This is a schematic flowchart illustrating the third image sensor fabrication method provided in this application embodiment.
[0044] The annotations in the attached figures are explained as follows:
[0045] 1-Substrate; 2-Shallow trench isolation structure; 3-Pixel photosensitive structure; 31-Heavily doped N-type well region; 32-Lightly doped N-type implantation region; 33-Clamped P-type implantation region; 4-Pixel transfer gate; 5-Lightly doped N-type region; 6-Heavily doped N-type region; 7-Heavily doped P-type region; 8-Depletion region; 9-Mask layer; 10-Photoresist. Detailed Implementation
[0046] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0047] Traditional image sensors such as Figure 1As shown, this image sensor structure includes: a photodiode (i.e., pixel photosensitive structure 3), which is disposed on the substrate 1 to generate image charge in response to incident light; and a floating diffuser (i.e., N-type heavily doped region 6), which is close to the pixel photosensitive structure 3 and is isolated from the pixel photosensitive structure 3 using a shallow trench isolation structure 2. The shallow trench isolation structure 2 is generally formed using etching and ion implantation processes. The contribution of the shallow trench isolation process to dark current is mainly in the following two aspects:
[0048] (1) During the etching process, the shallow trench isolation structure 2 will inevitably produce defects. Under the action of the transverse electric field generated when the image sensor is working, it will act as a recombination center, absorbing electrons or holes to form dark current. This leakage current is the source of dark current in the shallow trench isolation structure 2.
[0049] (2) During ion implantation, a depletion region 8 is formed at the N-P junction of the shallow trench isolation structure 2. The depletion region 8 is a cubic trapezoid. This depletion region 8 is affected by defects in the shallow trench isolation structure 2 and mechanical stress during the film deposition process, resulting in dark current. The larger the junction between the depletion region 8 and the shallow trench isolation structure 2, the more dark current is generated. Therefore, this application provides an image sensor fabrication method that implants P-type ions with a higher doping concentration than the substrate 1 to form a heavily doped P-type region 7 below the floating diffusion section to bend the depletion region 8. At the same time, the region where the heavily doped P-type region 7 is located is moved away from the shallow trench isolation structure 2 to reduce the lateral electric field, thereby improving the dark current of the shallow trench isolation structure 2 and thus optimizing the image quality.
[0050] Please refer to Figure 2 , Figure 2 A flowchart of an image sensor fabrication method provided in this application embodiment, the method may include:
[0051] S101: Two shallow trench isolation structures and a photosensitive pixel structure are formed in the region corresponding to each pixel unit in the substrate, and a pixel transmission gate is formed on the surface of the photosensitive pixel structure; the photosensitive pixel structure is located on the side of one shallow trench isolation structure away from the other shallow trench isolation structure.
[0052] It should be noted that the substrate 1 in this embodiment is a lightly doped P-type substrate. The concentration of P-type ions in the substrate 1 is less than the concentration of P-type ions in the heavily doped P-type region 7. The type of substrate 1 may be, but is not limited to, a P-type EPI (Epitaxy) silicon wafer.
[0053] This embodiment does not limit the specific method of forming the shallow trench isolation structure 2, and the following methods may be used, but are not limited to:
[0054] After depositing a mask layer 9 on the front side of substrate 1, an etching process is performed to form two shallow trenches in the area corresponding to each pixel unit in substrate 1.
[0055] After filling the shallow trenches, an annealing and grinding process is performed to form a shallow trench isolation structure 2.
[0056] This embodiment does not limit the specific material to be filled; for example, the material to be filled can be SiN or SiO2.
[0057] This embodiment does not limit the specific method of forming the photosensitive pixel structure. It can be, but is not limited to, the following method: After forming the shallow trench isolation structure 2, the substrate 1 corresponding to the pixel photosensitive area and the gate area is sequentially implanted with N-type heavily doped ions, N-type lightly doped ions, and P-type ions to form an N-type heavily doped well region 31, an N-type lightly doped implantation region 32, and a clamped P-type implantation region 33 arranged sequentially along the thickness direction to obtain the photosensitive pixel structure; the gate area is located on the side of one shallow trench isolation structure 2 away from the other shallow trench isolation structure 2; the pixel photosensitive area is located on the side of the gate area away from the shallow trench isolation structure 2.
[0058] This embodiment does not limit the specific method of forming the pixel transfer gate 4. It can be, but is not limited to, the following method: after obtaining the photosensitive pixel structure, the pixel transfer gate 4 is formed on the surface of the gate region through high-temperature thermal oxidation, photolithography, and etching. This embodiment does not limit the specific material of the pixel transfer gate 4; the material of the pixel transfer gate 4 can be, but is not limited to, polysilicon.
[0059] S102: Perform an etching operation on the substrate between the two shallow trench isolation structures, at least etching away the substrate on both sides of the first ion implantation region.
[0060] This embodiment does not limit the specific method of etching the substrate 1, as long as it can ensure that the substrate 1 on both sides of the first ion implantation region can be etched away. For example, the following two methods can be used:
[0061] (1) Etching operation is performed on the substrate 1 between the two shallow trench isolation structures 2 to etch away the portion of the substrate 1 that is adjacent to the two shallow trench isolation structures 2 respectively, forming the first ion implantation area and the second ion implantation area arranged sequentially along the thickness direction.
[0062] Accordingly, step S105 may include:
[0063] An image sensor is obtained by forming an N-type lightly doped region 5 and an N-type heavily doped region 6 arranged sequentially along the thickness direction in the second ion implantation region.
[0064] This embodiment does not limit the specific etching width. For example, the etching width can be 0.08μm to 2μm, including the values at both ends.
[0065] (2) Etching operation is performed on the substrate 1 between the two shallow trench isolation structures 2 to etch away the substrate 1 on both sides and the surface of the first ion implantation area, exposing the first ion implantation area.
[0066] Accordingly, step S105 may include:
[0067] An image sensor is obtained by forming an N-type lightly doped region 5 and an N-type heavily doped region 6 sequentially arranged along the thickness direction in a substrate 1 epitaxially grown above a P-type heavily doped region 7.
[0068] S103: Cover the surface except the top surface of the first ion implantation region with a barrier layer, perform ion implantation on the first ion implantation region, and the concentration of implanted P-type ions is greater than the concentration of P-type ions in the substrate. After implantation, remove the barrier layer to form a heavily doped P-type region.
[0069] It should be noted that, in this embodiment, the top surface of the first ion implantation region refers to the surface of the first ion implantation region facing the floating diffusion section.
[0070] This embodiment does not limit the specific type of the barrier layer. The barrier layer may be, but is not limited to, photoresist 10.
[0071] This embodiment does not limit the specific method of covering the barrier layer; for example, the following two methods can be used:
[0072] (1) Photoresist 10 is coated on the area on the front side of substrate 1 except for the top surface of the second ion implantation area, and photoresist 10 is filled in the area etched away between the two shallow trench isolation structures. It should be noted that this method can be applied to the above etching operation (1).
[0073] (2) Photoresist 10 is coated on the surface of substrate 1 and photolithography is performed to expose the first ion implantation region, while the photoresist 10 is retained on the surface except for the top surface of the first ion implantation region. It should be noted that this method can be applied to the above etching operation (2).
[0074] This embodiment does not limit the specific type of P-type ion. For example, the type of P-type ion can be boron or other types of P-type ions.
[0075] This embodiment does not limit the specific concentration of P-type ions; for example, the concentration of P-type ions can be 900E. 12 ~600E 13 And includes the values at both ends.
[0076] This embodiment does not limit the implantation energy during ion implantation. For example, the implantation energy can be 30 KeV~90 KeV, including both values.
[0077] S104: After forming the P-type heavily doped region, a low-temperature epitaxial growth process is performed to epitaxially grow the substrate in the area etched between the two shallow trench isolation structures.
[0078] It should be noted that the material used in this embodiment for the low-temperature epitaxial growth process is the same as that used in substrate 1.
[0079] This embodiment does not limit the specific temperature of the low-temperature epitaxial growth process. The temperature of the low-temperature epitaxial growth process can be, but is not limited to, room temperature.
[0080] S105: An image sensor is obtained by forming an N-type lightly doped region and an N-type heavily doped region sequentially arranged along the thickness direction above the P-type heavily doped region; the N-type heavily doped region is a floating diffusion part; the lateral area of the P-type heavily doped region is smaller than the lateral area of the N-type lightly doped region, and the distance from the P-type heavily doped region to the shallow trench isolation structure is greater than the distance from the P-type heavily doped region to the N-type lightly doped region.
[0081] It should be noted that the lateral area in this embodiment refers to the area on a plane perpendicular to the thickness direction. In this embodiment, the N-type lightly doped region 5 is an NMOS (N-Metal-Oxide-Semiconductor) lateral lightly doped region, abbreviated as NLDD region. The width of the N-type lightly doped region 5 (the width perpendicular to the thickness direction) may be, but is not limited to, equal to the spacing between the two shallow trench isolation structures 2; the N-type lightly doped region 5 may, but is not limited to, surround the side surface (the surface of the N-type heavy doped region 6 near the shallow trench isolation structure 2) and the bottom surface (the surface of the N-type heavy doped region 6 near the P-type heavy doped region 7).
[0082] This embodiment does not limit the specific method of forming the N-type lightly doped region 5 and the N-type heavily doped region 6. For example, the following two methods can be used:
[0083] (1) During the epitaxial growth process, an image sensor is obtained by forming an N-type lightly doped region 5 and an N-type heavily doped region 6 sequentially arranged along the thickness direction above the P-type heavily doped region 7 through photolithography, ion implantation and cleaning processes.
[0084] (2) After the epitaxial growth is completed, an N-type lightly doped region 5 and an N-type heavily doped region 6 are formed on the P-type heavily doped region 7 along the thickness direction by photolithography, ion implantation and cleaning processes to obtain an image sensor.
[0085] Based on the above embodiments, this application employs a P-type heavily doped region formed below the floating diffusion section. This causes the depletion regions of the P-type heavily doped region and the N-type lightly doped region below the floating diffusion section to bend upwards from their original cubic trapezoidal shape into a crescent shape. This reduces the area of the depletion region falling within the shallow trench isolation structure, thereby reducing dark current. Simultaneously, through etching and low-temperature epitaxial growth processes, the diffusion of the P-type heavily doped region is restricted, confining it to a region far from the shallow trench isolation structure. The lateral area of the P-type heavily doped region is smaller than that of the N-type lightly doped region. Since the electric field in the vertical direction of the P-type heavily doped region is much stronger than the electric field in the horizontal direction, the electric field strength near the shallow trench isolation structure in the horizontal direction is reduced. This results in fewer electrons or holes being absorbed by defects that can act as recombination centers compared to the original structure, thereby reducing dark current.
[0086] Please refer to Figure 3 , Figure 3 This is a schematic diagram of the structure of an image sensor provided in an embodiment of this application. The structure may include: multiple pixel units; each pixel unit includes: a substrate 1; two shallow trench isolation structures 2 and a photosensitive pixel structure are disposed in the substrate 1; the photosensitive pixel structure is located on the side of one shallow trench isolation structure 2 away from the other shallow trench isolation structure 2; a pixel transmission gate 4 is disposed on the surface of the photosensitive pixel structure;
[0087] In the substrate 1 between the two shallow trench isolation structures 2, a P-type heavily doped region 7, an N-type lightly doped region 5, and an N-type heavily doped region 6 are sequentially arranged along the thickness direction; the N-type heavily doped region 6 is a floating diffusion part; the concentration of P-type ions in the P-type heavily doped region 7 is greater than the concentration of P-type ions in the substrate 1; the lateral area of the P-type heavily doped region 7 is smaller than the lateral area of the N-type lightly doped region 5, and the distance from the P-type heavily doped region 7 to the shallow trench isolation structure 2 is greater than the distance from the P-type heavily doped region 7 to the N-type lightly doped region 5.
[0088] This embodiment does not limit the specific structure of the photosensitive pixel structure. The photosensitive pixel structure may include, but is not limited to, a pixel photosensitive area and a gate area. The gate area is located on the side of one shallow trench isolation structure 2 away from another shallow trench isolation structure 2. The pixel photosensitive area is located on the side of the gate area away from the shallow trench isolation structure 2. Both the pixel photosensitive area and the gate area include an N-type heavily doped well region 31, an N-type lightly doped implantation region 32 and a clamped P-type implantation region 33 arranged sequentially along the thickness direction. A pixel transmission gate 4 is provided on the surface of the gate area.
[0089] Based on the above embodiments, the image sensor of this application is obtained by the above image sensor preparation method and also has the above-mentioned beneficial effects.
[0090] The following examples illustrate the image sensor fabrication process. Please refer to them. Figures 4 to 15 , Figures 4 to 15The flowchart of the first image sensor fabrication method provided in this application embodiment is as follows:
[0091] 1. For example Figure 4 As shown, a P-type EPI silicon wafer is prepared as substrate 1;
[0092] 2. For example Figure 5 and Figure 6 As shown, after depositing the corresponding mask layer 9 on the front side of the substrate 1, an etching process is performed to form shallow trenches.
[0093] 3. For example Figure 7 As shown, the shallow trenches are filled with SiN or SiO2, and then annealing and grinding processes are performed to form the shallow trench isolation structure 2.
[0094] 4. For example Figure 8 As shown, following the steps, N-type heavily doped ion implantation (corresponding to the formation of N-type heavily doped well region 31), N-type lightly doped ion implantation (corresponding to the formation of N-type lightly doped implantation region 32), and P-type ion implantation (corresponding to the formation of clamped P-type implantation region 33) are performed on the pixel photosensitive region and gate region to form the corresponding photosensitive pixel structure in the substrate 1. The structure of the gate region is not shown in the figure. In fact, the gate region is located between the shallow trench isolation structure 2 and the pixel photosensitive region, and includes N-type heavily doped well region 31, N-type lightly doped implantation region 32 and clamped P-type implantation region 33 arranged sequentially along the thickness direction.
[0095] 5. For example Figure 9 As shown, a pixel transmission gate 4 is formed on the surface of the gate region through high-temperature thermal oxidation, photolithography, and etching.
[0096] 6. For example Figure 10 As shown, etching is performed on both sides of the first ion implantation region to remove the substrate 1 adjacent to the shallow trench isolation structure 2. The etching width can be 0.08 μm to 2 μm (including the values at both ends);
[0097] 7. For example Figure 11 , Figure 12 and Figure 13 As shown, except for the first ion implantation region, the remaining areas are coated with photoresist 10 to block P-type ion implantation. Ion implantation is performed on the first ion implantation region, and the concentration of implanted P-type ions is greater than the concentration of P-type ions in the substrate 1. The type of P-type ions can be boron or other types of P-type ions, and the concentration of P-type ions can be 900E. 12 ~ 600E 13(Including the values at both ends), the implantation energy can be 30Kev~90Kev (including the values at both ends); in particular, the etched holes also need to be filled with photoresist 10 to prevent P-type ions from being implanted into this region. Due to the presence of the etched area, the heavily doped P-type region 7 is confined to a range (i.e., confined between the two etched areas). After implantation, the photoresist 10 is removed;
[0098] 8. For example Figure 14 As shown, a low-temperature epitaxial growth process is then performed (the temperature in the low-temperature epitaxial growth process in this embodiment can be room temperature) to regenerate the etched substrate 1. The material of the regenerated substrate 1 is grown using P-type epitaxial growth, which is consistent with the original substrate 1.
[0099] 9. For example Figure 15 As shown, N-type lightly doped region 5 and N-type heavily doped region 6 are formed through processes such as photolithography, ion implantation, and cleaning. This step can be performed during epitaxial growth or after epitaxial growth is completed. Figure 3 As shown, due to the etching and low-temperature epitaxial growth process used on both sides of the P-type heavily doped region 7, its diffusion is extremely limited, resulting in a smaller lateral area for the P-type heavily doped region 7 compared to the N-type lightly doped region 5. Simultaneously, because the P-type heavily doped region 7 is far from the shallow trench isolation structure 2 in the lateral direction (perpendicular to the thickness direction) and close to the N-type lightly doped region 5 in the vertical direction (thickness direction), the electric field generated in the vertical direction is much greater than that in the lateral direction, thus reducing the electric field strength of the shallow trench isolation structure. Furthermore, because the P-type heavily doped region 7 is formed below the floating diffusion section, the depletion region 8 of the P-type heavily doped region 7 and the N-type lightly doped region 5 below the floating diffusion section bends upward from its original cubic trapezoidal shape into a crescent shape, thereby reducing the area of the depletion region 8 falling on the shallow trench isolation structure 2.
[0100] Optionally, from step 5 of the first image sensor fabrication method embodiment described above (i.e. Figure 9 (The steps shown) can be followed by an alternative approach; please refer to [the provided text]. Figures 16 to 21 , Figures 16 to 21 This is a schematic flowchart of a second image sensor fabrication method provided in an embodiment of this application. The specific process is as follows:
[0101] 1. For example Figure 16 As shown, after the pixel transmission gate 4 is formed, the substrate 1 on both sides and the surface of the first ion implantation region is etched away.
[0102] 2. For example Figure 17 As shown, photoresist 10 is coated on the surface of substrate 1 and photolithography is performed to expose the area that needs to be P-type ion implanted, namely the first ion implantation area, while the other parts are completely covered by photoresist 10.
[0103] 3. For example Figure 18 As shown, P-type ion implantation is performed, and P-type ions are formed deep below the floating diffuser and far away from the shallow trench isolation structure 2;
[0104] 4. Optional, such as Figure 19 , Figure 20 and Figure 21 As shown: After removing the photoresist 10, a low-temperature epitaxial growth process is performed. This process requires low-temperature growth and uses the same type of P-type epitaxial growth as substrate 1. Finally, photolithography, ion implantation, and cleaning processes are used to complete the formation of the lightly doped N-type region 5 and the heavily doped N-type region 6.
[0105] Optional, such as Figure 22 , Figure 23 and Figure 24 As shown: N-type lightly doped region 5 and N-type heavily doped region 6 can be Figure 20 After these steps, photolithography, ion implantation, and cleaning processes are performed to form the final product, or it can be like... Figure 23 As shown, this is completed during the epitaxial growth process.
[0106] This document uses specific examples to illustrate the principles and implementation methods of this application. The various embodiments are progressive, with each embodiment focusing on its differences from others. Similar or identical parts between embodiments can be referred to interchangeably. The descriptions of the embodiments above are merely illustrative of the method and core ideas of this application. For those skilled in the art, various improvements and modifications can be made to this application without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims of this application.
[0107] It should also be noted that, in this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes the element.
Claims
1. A method for fabricating an image sensor, characterized in that, include: Two shallow trench isolation structures and a photosensitive pixel structure are formed in the region corresponding to each pixel unit in the substrate, and a pixel transmission gate is formed on the surface of the photosensitive pixel structure. The photosensitive pixel structure is located on one side of one of the shallow trench isolation structures, away from the other shallow trench isolation structure; An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away at least the substrate on both sides of the first ion implantation region; A barrier layer is applied to the surface except the top surface of the first ion implantation region. Ion implantation is performed on the first ion implantation region. The concentration of implanted P-type ions is greater than the concentration of P-type ions in the substrate. After implantation, the barrier layer is removed to form a heavily doped P-type region. After forming the P-type heavily doped region, a low-temperature epitaxial growth process is performed to epitaxially grow the substrate in the area etched away between the two shallow trench isolation structures. An image sensor is obtained by forming an N-type lightly doped region and an N-type heavily doped region sequentially along the thickness direction above the P-type heavily doped region; the N-type heavily doped region is a floating diffusion section; the lateral area of the P-type heavily doped region is smaller than the lateral area of the N-type lightly doped region, and the distance from the P-type heavily doped region to the shallow trench isolation structure is greater than the distance from the P-type heavily doped region to the N-type lightly doped region.
2. The image sensor fabrication method according to claim 1, characterized in that, The process of forming two shallow trench isolation structures and a photosensitive pixel structure in the region corresponding to each pixel unit in the substrate, and forming a pixel transmission gate on the surface of the photosensitive pixel structure, includes: After depositing a mask layer on the front side of the substrate, an etching process is performed to form two shallow trenches in the area corresponding to each pixel unit in the substrate. After the shallow trenches are filled by the process, an annealing and grinding process is performed to form the shallow trench isolation structure; After forming the shallow trench isolation structure, N-type heavily doped ion implantation, N-type lightly doped ion implantation, and P-type ion implantation are sequentially performed on the substrate corresponding to the pixel photosensitive area and the gate area to form an N-type heavily doped well region, an N-type lightly doped implantation region, and a clamped P-type implantation region arranged sequentially along the thickness direction, thereby obtaining the photosensitive pixel structure; the gate area is located on the side of one shallow trench isolation structure that is away from another shallow trench isolation structure; the pixel photosensitive area is located on the side of the gate area that is away from the shallow trench isolation structure. After obtaining the photosensitive pixel structure, the pixel transmission gate is formed on the surface of the gate region by high-temperature thermal oxidation, photolithography and etching.
3. The image sensor fabrication method according to claim 1, characterized in that, The image sensor is obtained by forming an N-type lightly doped region and an N-type heavily doped region sequentially arranged along the thickness direction above the P-type heavily doped region, comprising: During the epitaxial growth process, the N-type lightly doped region and the N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region through photolithography, ion implantation and cleaning processes, to obtain the image sensor.
4. The image sensor fabrication method according to claim 1, characterized in that, The image sensor is obtained by forming an N-type lightly doped region and an N-type heavily doped region sequentially arranged along the thickness direction above the P-type heavily doped region, comprising: After epitaxial growth is completed, the N-type lightly doped region and the N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region through photolithography, ion implantation and cleaning processes to obtain the image sensor.
5. The method for fabricating an image sensor according to any one of claims 1 to 4, characterized in that, The etching operation on the substrate between the two shallow trench isolation structures, at least etching away the substrate on both sides of the first ion implantation region, includes: An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away a portion of the substrate width adjacent to the two shallow trench isolation structures, forming the first ion implantation region and the second ion implantation region arranged sequentially along the thickness direction. Accordingly, an N-type lightly doped region and an N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region to obtain an image sensor, comprising: The image sensor is obtained by forming the N-type lightly doped region and the N-type heavily doped region arranged sequentially along the thickness direction in the second ion implantation region.
6. The image sensor fabrication method according to claim 5, characterized in that, The step of covering the surface other than the top surface of the first ion implantation region with a barrier layer includes: Photoresist is coated on the front side of the substrate, except for the top surface of the second ion implantation region, and the photoresist is filled in the area etched between the two shallow trench isolation structures.
7. The method for fabricating an image sensor according to any one of claims 1 to 4, characterized in that, The etching operation on the substrate between the two shallow trench isolation structures, at least etching away the substrate on both sides of the first ion implantation region, includes: An etching operation is performed on the substrate between the two shallow trench isolation structures to etch away the substrate on both sides and the surface of the first ion implantation region, thereby exposing the first ion implantation region. Accordingly, an N-type lightly doped region and an N-type heavily doped region are formed sequentially along the thickness direction above the P-type heavily doped region to obtain an image sensor, comprising: The image sensor is obtained by forming the N-type lightly doped region and the N-type heavily doped region sequentially along the thickness direction in the substrate epitaxially grown above the P-type heavily doped region.
8. The image sensor fabrication method according to claim 7, characterized in that, The step of covering the surface other than the top surface of the first ion implantation region with a barrier layer includes: Photoresist is coated on the substrate surface and photolithography is performed to expose the first ion implantation region, while the photoresist is retained on the surface except for the top surface of the first ion implantation region.
9. An image sensor, characterized in that, include: Multiple pixel units; The pixel unit includes: a substrate; two shallow trench isolation structures and a photosensitive pixel structure are disposed in the substrate; the photosensitive pixel structure is located on the side of one of the shallow trench isolation structures opposite to the other shallow trench isolation structure; a pixel transmission gate is disposed on the surface of the photosensitive pixel structure; In the substrate between the two shallow trench isolation structures, a P-type heavily doped region, an N-type lightly doped region, and an N-type heavily doped region are sequentially disposed along the thickness direction; the N-type heavily doped region is a floating diffusion region; the concentration of P-type ions in the P-type heavily doped region is greater than the concentration of P-type ions in the substrate; the lateral area of the P-type heavily doped region is smaller than the lateral area of the N-type lightly doped region, and the distance from the P-type heavily doped region to the shallow trench isolation structure is greater than the distance from the P-type heavily doped region to the N-type lightly doped region.
10. The image sensor according to claim 9, characterized in that, The photosensitive pixel structure includes a pixel photosensitive region and a gate region; the gate region is located on one side of one of the shallow trench isolation structures away from another shallow trench isolation structure; the pixel photosensitive region is located on the side of the gate region away from the shallow trench isolation structure; both the pixel photosensitive region and the gate region include an N-type heavily doped well region, an N-type lightly doped implantation region and a clamped P-type implantation region arranged sequentially along the thickness direction. The pixel transmission gate is disposed on the surface of the gate region.