Back-illuminated image sensor and manufacturing method thereof

Abstract

This invention provides a back-illuminated image sensor and its manufacturing method, applicable to the field of semiconductor technology. Specifically, in the manufacturing method of the back-illuminated image sensor provided by this invention, damage to the silicon substrate surrounding the pixel region is reduced by adjusting the distance between the BSGND structure formed on the top surface of the back-illuminated image sensor stack and the pixel region within the pixel wafer of the back-illuminated image sensor stack. This reduces the generation of dark current in the device, thereby reducing leakage current and ultimately improving image quality in low light conditions.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to a back-illuminated image sensor and its manufacturing method. Background Technology

[0002] With the booming development of emerging fields such as artificial intelligence, the Internet of Things, and Industry 4.0, the market demand for CMOS image sensors is increasing day by day. Currently, in the field of semiconductor technology, back-illuminated CMOS image sensors (BSI) are made by bonding logic wafers and pixel wafers separately. Since logic wafers and pixel wafers are made separately, the manufacturing process is flexible and low-cost. In addition, it has the advantages of large wafer usable area and the ability to integrate multifunctional wafers, which makes it popular in the industry.

[0003] As market demand for back-illuminated CMOS image sensors (BSI) increases daily, the performance requirements are also rising. A key indicator of image sensing performance is dark current level. Dark current refers to the ability of a photodiode to generate charge and form a current signal that can be read out even in complete darkness. In image imaging, this manifests as "white" pixels, reducing image quality.

[0004] Currently, for back-illuminated CMOS image sensors (BSI), the generation of dark current should generally be minimized. The direct harm of dark current is that it can cause leakage current, resulting in white pixels. The main cause of dark current is silicon damage, so the dark current caused by silicon damage should be reduced in device applications.

[0005] In summary, how to provide a method to solve the problem of dark current caused by silicon defects in back-illuminated CMOS image sensors is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a back-illuminated image sensor and a method for manufacturing the same, so as to prevent dark current problems caused by defects in the silicon substrate of the back-illuminated CMOS image sensor and further improve the performance of the product image.

[0007] In a first aspect, to solve the above-mentioned technical problems, the present invention provides a method for manufacturing a back-illuminated image sensor, which may include at least the following steps:

[0008] A back-illuminated image sensor stack structure is provided, the back-illuminated image sensor stack structure includes mutually bonded logic wafers and pixel wafers with pixel regions formed therein, such that the bottom of the pixel wafers along the direction away from the bonding surface of the logic wafers and the pixel wafers serves as the top surface of the back-illuminated image sensor stack structure.

[0009] A BSGND structure is formed on the top surface of the back-illuminated image sensor stack structure, and the dark current of the back-illuminated image sensor is reduced by adjusting the distance between the BSGND structure and the pixel region.

[0010] Furthermore, the BSGND structure can be a hole-like structure used to electrically connect the device structure formed in the pixel wafer to an external circuit.

[0011] Furthermore, the step of adjusting the distance between the BSGND structure and the pixel region may include increasing the distance between the BSGND structure and the pixel region to a preset distance threshold.

[0012] Furthermore, the preset distance threshold can be 25μm to 250μm.

[0013] Furthermore, the step of providing a back-illuminated image sensor stack structure may include:

[0014] A pixel wafer having the pixel region is provided; the substrate of the pixel wafer having the pixel region is inverted and then bonded to the logic wafer; and...

[0015] At least the substrate of the pixel wafer is thinned.

[0016] Furthermore, the step of forming a BSGND structure on the top surface of the back-illuminated image sensor stack structure may include:

[0017] Using the trench pattern set on the mask, multiple trenches are formed on the substrate of the pixel wafer that is bonded to the logic wafer after the substrate is reversed, and the trenches are filled so that the top surface of the filled trench is flush with the top surface of the back-illuminated image sensor stack structure.

[0018] Furthermore, the step of increasing the distance between the BSGND structure and the pixel region to a preset distance threshold may include reducing the hole depth of the BSGND structure.

[0019] Furthermore, the pixel wafer also includes a dielectric layer and a metal interconnect layer embedded in the dielectric layer, the dielectric layer being formed between the pixel region and the BSGND structure.

[0020] Furthermore, the substrate of the pixel wafer can be a silicon substrate.

[0021] Secondly, based on the same inventive concept, the present invention also provides a back-illuminated image sensor, which can be manufactured using the back-illuminated image sensor manufacturing method described above.

[0022] Compared with the prior art, the technical solution of the present invention has at least one of the following beneficial effects:

[0023] In the manufacturing method of the back-illuminated image sensor provided by the present invention, the damage to the silicon substrate around the pixel is reduced by adjusting the distance between the BSGND structure formed on the top surface of the back-illuminated image sensor stack and the pixel region in the pixel wafer of the back-illuminated image sensor stack. This reduces the generation of dark current in the device, thereby reducing the lack of connection due to leakage current, and ultimately improving the image quality in low light. Attached Figure Description

[0024] Figure 1 This is a flowchart of a method for manufacturing a back-illuminated image sensor according to an embodiment of the present invention;

[0025] Figure 2 This is a top view showing that the distance between the BSGND structure and the pixel region in the prior art of this invention is 20μm.

[0026] Figure 3 This is a top view showing that the distance between the BSGND structure and the pixel area in one embodiment of the present invention is 200 μm. Detailed Implementation

[0027] As described in the background section, for back-illuminated CMOS image sensors (BSI), the generation of dark current should generally be minimized. The direct harm of dark current is leakage current, resulting in white pixels. The main cause of dark current is silicon damage; therefore, dark current caused by silicon damage should be reduced in device applications.

[0028] To this end, the present invention provides a back-illuminated image sensor and a method for manufacturing the same, in order to prevent dark current problems in back-illuminated CMOS image sensors caused by defects in their silicon substrates, and to further improve the performance of product images.

[0029] The back-illuminated image sensor and its manufacturing method proposed in this invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The advantages and features of this invention will become clearer from the following description. It should be noted that the drawings are all in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of this invention. Many specific details are set forth in the following description to provide a thorough understanding of this invention; however, this invention may be implemented in other ways different from those described herein, and therefore this invention is not limited to the specific embodiments disclosed below.

[0030] As shown in this application and claims, unless the context clearly indicates otherwise, the words "a," "an," "an," and / or "the" do not specifically refer to the singular and may also include the plural. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of explicitly identified steps and elements, which do not constitute an exclusive list, and the method or apparatus may also include other steps or elements. In detailing the embodiments of the present invention, for ease of explanation, the cross-sectional views showing the device structure may be partially enlarged without adhering to the general scale, and the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. Furthermore, in actual manufacturing, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0031] For example, refer to Figure 1 As shown, Figure 1 This is a schematic flowchart illustrating the manufacturing method of a back-illuminated image sensor provided in an embodiment of the present invention. Specifically, the manufacturing method of the back-illuminated image sensor provided by the present invention may include at least the following steps:

[0032] Step S100: A back-illuminated image sensor stack structure is provided. The back-illuminated image sensor stack structure includes mutually bonded logic wafers and pixel wafers with pixel regions formed inside them, such that the bottom of the pixel wafers along the direction away from the bonding surface of the logic wafers and the pixel wafers serves as the top surface of the back-illuminated image sensor stack structure.

[0033] In this embodiment, a pixel wafer can be provided first, and then a pixel region containing multiple pixel units and other structures can be formed on the wafer. Then, the bottom of the pixel wafer is flipped up so that its bottom substrate is facing up, and then the flipped pixel wafer with the pixel region is bonded to a pre-prepared logic wafer. The back-illuminated image sensor stack structure can be a semiconductor structure that has completed part of the back-illuminated image sensor process. Specifically, the logic wafer serves as a supporting substrate, which can be a semiconductor substrate, such as at least one of the following materials: silicon (Si), germanium (Ge), silicon germanium (SiGe), silicon carbide (SiC), silicon germanium carbide (SiGeC), indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), or other III / V compound semiconductors. It also includes multilayer structures composed of these semiconductors, or silicon-on-insulator (SOI), silicon-on-insulator stacked (SSOI), silicon-on-insulator stacked (S-SiGeOI), silicon-on-insulator (SiGeOI), and germanium-on-insulator (GeOI). Alternatively, it can be a double-sided polished wafer (DSP), or a ceramic substrate such as alumina, a quartz substrate, or a glass substrate. For example, in this embodiment, the logic wafer can be a silicon wafer. In other embodiments, the logic wafer can also be various composite substrates such as silicon-on-insulator (SOI) or silicon-on-insulator (SiGeOI). Those skilled in the art will understand that the substrate is not limited in any way, but can be selected according to the actual application. CMOS circuits and related structures may already be formed in the logic wafer.

[0034] Furthermore, theoretically, the pixel wafer could be made of any of the substrate materials listed for the logic wafer. However, as mentioned in the background of this invention, since the device substrate is silicon, there is a problem of dark current caused by silicon defects, such as… Figure 2 As shown, the edges of the graphic are bright; the material of the pixel wafer proposed in this invention is also a silicon substrate.

[0035] It should be further explained that the step of providing a back-illuminated image sensor stack structure in the embodiments of the present invention may include: providing a pixel wafer having the pixel region, inverting the substrate of the pixel wafer having the pixel region and bonding it to the logic wafer; and at least thinning the substrate of the pixel wafer.

[0036] Furthermore, the pixel wafer in the back-illuminated image sensor stacking structure provided by the present invention may also include a dielectric layer and a metal interconnect layer embedded in the dielectric layer, wherein the dielectric layer is formed between the pixel region and the BSGND structure.

[0037] It is understood that, although not shown in the figures, other layers or components may be formed on the back-illuminated image sensor stack structure in the embodiments of the present invention, such as isolation structures, gate structures, contact holes and through holes, etc.

[0038] Step S200: A BSGND structure is formed on the top surface of the back-illuminated image sensor stack structure, and the dark current of the back-illuminated image sensor is reduced by adjusting the distance between the BSGND structure and the pixel area.

[0039] In this embodiment, according to the inventor's research, prior art typically requires forming a row of BSGND structures for electrical connection with external circuits on the front side of the stacked structure formed in step S100 above, i.e., on the back side of the substrate of the pixel wafer; wherein, the BSGND structure is a hole-like structure for electrically connecting the device structure formed in the pixel wafer to the external circuit. However, in the prior art, the distance between the BSGND structure formed on the back side of the pixel wafer substrate and the pixel region formed within the substrate of the pixel wafer is typically 20 μm, and back-illuminated image sensors formed with this distance will experience issues such as… Figure 2 The back-illuminated image sensor structure shown in the invention exhibits a problem of bright edges (an undesirable phenomenon in practical applications). Therefore, the inventors of this invention propose an inventive concept to improve the dark current of the device by changing this distance.

[0040] Specifically, in this embodiment of the invention, the step of adjusting the distance between the BSGND structure and the pixel region includes increasing the distance between the BSGND structure and the pixel region to a preset distance threshold. The range of the preset distance threshold to which the distance between the BSGND structure and the pixel region is increased can be 25μm to 250μm. For example, the preset distance threshold can be 25μm, 50μm, 100μm, 150μm, 200μm, and 250μm. Preferably, in this embodiment of the invention, the distance between the BSGND structure and the pixel region is increased to an optimal distance of 200μm to reduce damage to the silicon substrate around the pixel region, thereby reducing the generation of dark current in the device, and further reducing leakage current, ultimately improving image quality in low light.

[0041] Furthermore, the manufacturing method of the back-illuminated image sensor provided by the present invention also specifically provides a step of forming a BSGND structure on the top surface of the stacked structure of the back-illuminated image sensor, which may specifically include the following steps:

[0042] Using the trench pattern set on the mask, multiple trenches are formed on the substrate of the pixel wafer that is bonded to the logic wafer after the substrate is reversed, and the trenches are filled so that the top surface of the filled trench is flush with the top surface of the back-illuminated image sensor stack structure.

[0043] Furthermore, the step of increasing the distance between the BSGND structure and the pixel region to a preset distance threshold includes reducing the hole depth of the BSGND structure.

[0044] In this embodiment, the distance between the BSGND structure and the pixel region can be changed by altering the process parameters of the etching process that forms the trench, such as the etching amount of the etching gas (dry etching) or the concentration and etching time of the etching liquid, such as HF (wet etching).

[0045] In summary, in the manufacturing method of the back-illuminated image sensor provided by the present invention, the damage to the silicon substrate around the pixel region is reduced by adjusting the distance between the BSGND structure formed on the top surface of the back-illuminated image sensor stack and the pixel region within the pixel wafer in the back-illuminated image sensor stack. This reduces the generation of dark current in the device, thereby reducing the lack of connection due to leakage current, and ultimately achieving the goal of improving image quality in low light.

[0046] It should be noted that although the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the present invention. For any person skilled in the art, many possible variations and modifications can be made to the technical solutions of the present invention based on the disclosed technical content, or equivalent embodiments can be modified accordingly, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention shall still fall within the scope of protection of the present invention.

[0047] It should also be understood that, unless otherwise specified or indicated, the terms “first,” “second,” “third,” etc., in the specification are used only to distinguish the various components, elements, and steps in the specification, and not to indicate the logical or sequential relationships between the various components, elements, and steps.

[0048] Furthermore, it should be recognized that the terminology described herein is used only to describe particular embodiments and not to limit the scope of the invention. It must be noted that the singular forms “a” and “an” used herein and in the appended claims include plural bases unless the context clearly indicates otherwise. For example, a reference to “a step” or “an apparatus” means a reference to one or more steps or apparatuses, and may include secondary steps and secondary apparatuses. All conjunctions used should be understood in the broadest sense. Also, the word “or” should be understood to have the definition of logical “or” rather than logical “exclusive OR”, unless the context clearly indicates otherwise. Furthermore, implementation of the methods and / or devices in embodiments of the invention may include performing selected tasks manually, automatically, or in combination.

Claims

1. A method for manufacturing a back-illuminated image sensor, characterized in that, It should include at least the following steps: A back-illuminated image sensor stack structure is provided, the back-illuminated image sensor stack structure includes mutually bonded logic wafers and pixel wafers with pixel regions formed therein, such that the bottom of the pixel wafers along the direction away from the bonding surface of the logic wafers and the pixel wafers serves as the top surface of the back-illuminated image sensor stack structure. A BSGND structure is formed on the top surface of the back-illuminated image sensor stack structure, and the dark current of the back-illuminated image sensor is reduced by adjusting the distance between the BSGND structure and the pixel area. The step of adjusting the distance between the BSGND structure and the pixel region includes increasing the distance between the BSGND structure and the pixel region to a preset distance threshold, wherein the preset distance threshold is 25μm~250μm.

2. The method for manufacturing a back-illuminated image sensor as described in claim 1, characterized in that, The BSGND structure is a hole-like structure used to electrically connect the device structure formed in the pixel wafer to an external circuit.

3. The method for manufacturing a back-illuminated image sensor as described in claim 1, characterized in that, The steps of providing a back-illuminated image sensor stack structure include: A pixel wafer having the pixel region is provided; the substrate of the pixel wafer having the pixel region is inverted and then bonded to the logic wafer; and... At least the substrate of the pixel wafer is thinned.

4. The method for manufacturing a back-illuminated image sensor as described in claim 3, characterized in that, The step of forming a BSGND structure on the top surface of the back-illuminated image sensor stack structure includes: Using the trench pattern set on the mask, multiple trenches are formed on the substrate of the pixel wafer that is bonded to the logic wafer after the substrate is reversed, and the trenches are filled so that the top surface of the filled trench is flush with the top surface of the back-illuminated image sensor stack structure.

5. The method for manufacturing a back-illuminated image sensor as described in claim 1, characterized in that, The step of increasing the distance between the BSGND structure and the pixel region to a preset distance threshold includes reducing the hole depth of the BSGND structure.

6. The method for manufacturing a back-illuminated image sensor as described in claim 1, characterized in that, The pixel wafer also includes a dielectric layer and a metal interconnect layer embedded in the dielectric layer, wherein the dielectric layer is formed between the pixel region and the BSGND structure.

7. The method for manufacturing a back-illuminated image sensor as described in claim 1, characterized in that, The substrate of the pixel wafer is a silicon substrate.

8. A back-illuminated image sensor, characterized in that, It is prepared using the manufacturing method of the back-illuminated image sensor according to any one of claims 1 to 7.

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