Method for reducing dark current on surface of image sensor

Abstract

The invention provides a method for reducing dark current on the surface of an image sensor and the image sensor, and the method comprises the following steps: providing a substrate, and forming an epitaxial layer on the substrate; doping in the epitaxial layer to form a photodiode, wherein the photodiode is provided with a first region and a second region; the first region is close to the transfer transistor and the second region is away from the transfer transistor; and the doping depth of the second region is more than 10% greater than the doping depth of the first region so as to reduce dark current caused by surface defects of the image sensor substrate.

Description

Technical Field

[0001] The present invention relates to the field of image sensors, and particularly to a method for reducing the surface dark current of an image sensor. Background Art

[0002] As a device unit for converting optical signals into digital signals, CMOS image sensors are widely used in various emerging fields such as smart phones, tablet computers, automobiles, and medical applications. Generally, each pixel of a CMOS image sensor includes a photosensitive element and one or more transistors for reading out signals from the photosensitive element.

[0003] Traditional image sensors have a front-illuminated structure, in which a photodiode is formed under the substrate surface, and logic circuits are formed above the photodiode. Light reaches the photodiode after passing through the logic circuits. During this process, light passes through multiple layers, resulting in light loss or light crosstalk to adjacent image sensor unit chips, affecting the light response characteristics of the photodiodes of each image sensor unit chip. To overcome the above limitations, a backside illumination (BSI) image sensor has been proposed. In a backside-illuminated image sensor, light directly irradiates the photodiode from the back of the substrate without passing through the logic circuits. Therefore, the light response characteristics of the photodiode are improved.

[0004] However, a relatively large dark current usually exists in a backside-illuminated image sensor. When the image sensor is operating, the dark current is mixed into the signal current, causing signal interference and resulting in a decline in the performance of the image sensor. For more information on the dark current problem of image sensors, please refer to the Chinese patent application with the publication number 103137633A (publication date: June 5, 2013).

[0005] During the processes of forming the gate of the transfer transistor and the corresponding gate sidewalls, various processes such as etching are performed on the substrate surface. These processes will cause more defects on part of the substrate surface, and these defects will cause deep energy levels to be generated in the surface semiconductor substrate material, thereby causing the semiconductor substrate material to easily generate carriers even under lightless conditions. These carriers are easily introduced into the photodiode located below the defects, resulting in the generation of dark current. Therefore, a method for reducing the surface dark current of an image sensor is needed to solve the problem of relatively large surface dark current existing in existing image sensors. Summary of the Invention

[0006] Based on the above considerations, the present invention provides a method for reducing the dark current on the surface of an image sensor, comprising the following steps: providing a substrate, forming an epitaxial layer on the substrate; doping in the epitaxial layer to form a photodiode, the photodiode having a first region and a second region; the first region is close to the transfer transistor, and the second region is far from the transfer transistor; the doping depth of the second region is more than 10% greater than the doping depth of the first region to reduce the dark current on the surface of the image sensor.

[0007] Further, the doping depth of the second region varies in a gradient manner, wherein the gradient change is a gradual increase.

[0008] Further, the step of forming the photodiode comprises the following steps: using a first mask to implant a first dopant in the first region with a first bias power, and using a second mask to implant the first dopant in the second region with a second bias power, wherein the first bias power is less than the second bias power.

[0009] Further, the step of forming the photodiode may further comprise the following steps: using a third mask to form a pinning layer of the first conductivity type in the second region, the doping concentration of the pinning layer being higher than a preset value, and then using a fourth mask to dope and implant a doping substance of the second conductivity type in the first region and the second region to form the photodiode, and at the same time making the doping concentration of the pinning layer reach the preset value.

[0010] Further, at least a part of the first region overlaps with the transfer transistor in the direction perpendicular to the substrate.

[0011] Further, by controlling the bias power, the photodiode has a gradient-varying doping concentration along the direction away from the transfer transistor.

[0012] Further, the present application provides an image sensor prepared by the above method, specifically comprising: a substrate; an epitaxial layer of the first conductivity type on the substrate; a photodiode of the second conductivity type in the epitaxial layer; the photodiode having a first region and a second region; wherein the first region is close to the transfer transistor, and the second region is far from the transfer transistor; the doping depth of the second region is more than 10% greater than the doping depth of the first region to reduce the dark current on the surface of the image sensor.

[0013] Further, the gradient change is a gradual increase.

[0014] Further, the photodiode has a gradient-varying doping concentration along the direction away from the transfer transistor.

[0015] Further, at least a part of the first region overlaps with the transfer transistor in the direction perpendicular to the substrate.

[0016] The method for reducing the surface dark current of the image sensor provided by the present invention has the following beneficial effects: By moving at least a part of the photodiode away from the substrate surface, the dark current generated by the carriers conducted to the photodiode region below the defect under lightless conditions due to the deep energy levels generated on the substrate surface caused by more defects on the substrate surface is reduced; further, by setting the doping depth of the second region to be at least 10% greater than that of the first region, the effect of reducing the surface dark current can be achieved while ensuring the full well capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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.

[0018] Figure 1 It shows a schematic structural diagram of the photodiode region in the prior art; Figure 2 It shows a schematic structural diagram of the photodiode region of the present invention; REFERENCE SIGNS

[0019] 1 Floating diffusion region 2 Transfer transistor 3 Shallow trench isolation 4 Epitaxial layer 5 Photodiode EMBODIMENTS

[0020] In order to make the objectives, technical solutions, and advantages of the present application more apparent, exemplary embodiments according to the present application will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. It should be understood that the present application is not limited by the exemplary embodiments described herein. Based on the embodiments of the present application described herein, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the protection scope of the present application.

[0021] In the following description, numerous specific details are given to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without one or more of these details. In other examples, some technical features well known to those skilled in the art are not described to avoid confusion with the present application.

[0022] It should be understood that the present application can be implemented in different forms and should not be construed as limited to the embodiments presented herein. On the contrary, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the present application to those skilled in the art.

[0023] The purpose of the terms used herein is only to describe specific embodiments and is not a limitation of the present application. When used herein, the singular forms "a", "an" and "the" are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms "comprising" and / or "including", when used in this specification, determine the presence of the stated features, integers, steps, operations, elements and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups. When used herein, the term "and / or" includes any and all combinations of the related listed items.

[0024] To thoroughly understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solutions proposed by the present application. The preferred embodiments of the present application are described in detail below. However, in addition to these detailed descriptions, the present application may have other implementation manners.

[0025] Figure 1 is a schematic diagram of a photodiode in the prior art, as Figure 1 shown, in a back-illuminated image sensor, the photodiode is formed under the surface of the substrate, and light directly irradiates the photodiode from the back of the substrate without passing through the logic circuit. However, during the process of forming the transfer transistor, various etching, polishing and other processes will have a certain impact on the substrate directly above the photodiode, which will lead to various defects in the substrate material. These defects will cause deep energy levels in the surface semiconductor substrate material, thereby causing the semiconductor substrate material to easily generate carriers even under lightless conditions. These carriers are easily introduced into the photodiode located below the defect, resulting in the generation of dark current. To solve the above problems, the present application provides a method that can effectively reduce the surface dark current of an image sensor.

[0026] Mainly by forming an epitaxial layer on the substrate, and then doping in the epitaxial layer to form a photodiode, by setting the photodiode as regions with different doping depths; the first region is close to the transfer transistor, and the second region is far from the transfer transistor; and the doping depth of the second region is more than 10% greater than that of the first region, the surface dark current of the image sensor can be reduced.

[0027] As Figure 2As shown, an epitaxial layer 4 is prepared on a substrate doped with a first conductive type (not shown in the figure). In the epitaxial layer, a photodiode 5 with different depths in each part is formed by ion implantation. According to the different implantation depths, it can be divided into two regions, 5a and 5b. The ion implantation depth of the 5b region is more than 10% greater than that of the 5a region, preferably 50%.

[0028] Furthermore, the ion implantation region of the photodiode can be set to vary in a gradient manner. For example, it gradually becomes deeper along the direction away from the transfer transistor, which can make the photodiode region away from the surface, thereby achieving the technical effect of effectively reducing the surface dark current.

[0029] The preparation device of the photodiode in this embodiment can be a device integrating multiple functions such as coating, lithography, etching, and ion implantation, which can realize the formation of the substrate shielding layer and the coating process of the photoresist layer. It can also be a device integrating multiple modules that realize multiple functions such as coating, lithography, etching, and ion implantation.

[0030] In this embodiment, to prepare the photodiode 5 with different ion implantation depths, two mask plates with different patterns can be used respectively. When using the mask plate with the 5a cross-section pattern, the power of ion implantation is controlled to be less than that when using the mask plate with the 5b cross-section pattern, so that the ion implantation depth of the 5b region is more than 10% greater than that of the 5a region.

[0031] In another embodiment, the method for preparing the photodiode with different injection depths in each region can also be to perform a first conductive type ion implantation with a relatively high power using a mask plate with a 5a + 5b cross-section pattern to obtain a pinned layer with an ion doping concentration reaching a preset value, and then perform a second conductive type ion implantation with a relatively low power using a mask plate with a 5b cross-section pattern to reduce the ion doping concentration on the surface of the pinned layer in the 5b region, so that the overall ion implantation region depth of the 5b region is more than 10% greater than that of the 5a region. In this embodiment, it is preferably that the ion implantation depth of the 5b region is 20% greater than that of the 5a region. The surface dark current of the device obtained through simulation is Figure 1 90.8% smaller than that of the photodiode in

[0032] In another embodiment, the method for preparing a photodiode with different implantation depths in each region may also be to perform a first-conductivity-type ion implantation with a low power using a mask plate having a 5b cross-sectional pattern to obtain a pinned layer with an ion doping concentration less than a preset value, and then perform a second-conductivity-type ion implantation with a high power using a mask plate having a 5a + 5b cross-sectional pattern to reduce the ion doping concentration on the surface of the 5b region. Finally, the overall ion implantation depth of the 5b region is more than 10% greater than that of the 5a region. In this embodiment, it is preferred that the ion implantation depth of the 5b region is 40% greater than that of the 5a region. The dark current on the device surface obtained through simulation is Figure 1 93.4% smaller than that of the photodiode in

[0033] In other embodiments, to make the ion implantation depth of the entire photodiode show a gradient decrease, it can be achieved by using multiple mask plates or changing the distribution of the bias electric field during ion implantation, or by only setting the ion implantation depth of the 5b region or the 5a region to show a gradient decrease distribution.

[0034] 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 without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, in any case, the embodiments should be regarded as exemplary and non-limiting. In addition, obviously, the term "comprising" does not exclude other elements and steps, and the term "a" does not exclude a plurality. A plurality of elements stated 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 method for reducing the dark current on the surface of an image sensor, characterized in that: A substrate is provided, and an epitaxial layer is formed on the substrate; A photodiode is formed by doping in the epitaxial layer. The photodiode has a first region and a second region. The first region is close to the transfer transistor, and the second region is far from the transfer transistor; The doping depth of the second region is at least 10% greater than that of the first region to reduce the dark current on the surface of the image sensor.

2. The method according to claim 1, characterized in that: The doping depth of the second region changes in a gradient manner, and the gradient change is a gradual increase.

3. The method according to claim 1, characterized in that, The step of forming the photodiode includes the following steps: injecting a first dopant into the first region with a first bias power using a first mask, and injecting the first dopant into the second region with a second bias power using a second mask, where the first bias power is less than the second bias power.

4. The method according to claim 1, characterized in that, The step of forming the photodiode includes the following steps: forming a pinned layer of the first conduction type in the second region using a third mask, where the doping concentration of the pinned layer is higher than a preset value, and then doping and injecting a doping substance of the second conduction type into the first region and the second region using a fourth mask to form the photodiode, and simultaneously making the doping concentration of the pinned layer reach the preset value.

5. The method according to claim 1, wherein the first region at least partially overlaps with the transfer transistor in the direction perpendicular to the substrate.

6. The method according to claim 1, by controlling the bias power so that the doping concentration of the photodiode has a gradient change along the direction away from the transfer transistor.

7. An image sensor prepared by the method according to any one of claims 1 - 6, characterized in that, Comprising: A substrate; An epitaxial layer of the first conduction type is provided on the substrate; A photodiode of the second conduction type is provided in the epitaxial layer; The photodiode has a first region and a second region; Wherein the first region is close to the transfer transistor, and the second region is far from the transfer transistor; The doping depth of the second region is more than 10% greater than that of the first region to reduce the dark current on the surface of the image sensor.

8. The image sensor according to claim 7, characterized in that: The gradient change is a gradual increase.

9. The image sensor according to claim 7, characterized in that: The photodiode has a doping concentration that changes in a gradient manner along the direction away from the transfer transistor.

10. The image sensor according to claim 7, characterized in that: At least a part of the first region overlaps with the transfer transistor in the direction perpendicular to the substrate.

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