Method for forming backside illumination image sensor and backside illumination image sensor

Abstract

The application provides a forming method of a back-illuminated image sensor and an image sensor thereof, and the forming method comprises the following steps: providing a semiconductor substrate, wherein the semiconductor substrate has a front surface and a back surface corresponding to the front surface; forming a deep pinning region in a predetermined region of the semiconductor substrate; forming a side isolation region in the predetermined region of the semiconductor substrate; forming a photo-generated carrier collection region in the predetermined region of the semiconductor substrate; thinning the back surface of the semiconductor substrate, and forming a deep trench isolation region on the thinned back surface of the semiconductor substrate; wherein the side isolation region and the deep pinning region pin the bottom surface and / or the side surface of the deep trench isolation region. The application pins the bottom surface and / or the side surface of the deep trench isolation region by arranging the deep pinning region, so as to reduce the dark current of the image sensor and the image white point phenomenon.

Description

Technical Field

[0001] This invention relates to the field of image sensor technology, and more particularly to a method for forming a back-illuminated image sensor and the back-illuminated image sensor itself. Background Technology

[0002] Complementary metal-oxide-semiconductor (CMOS) technology is the dominant manufacturing process in the integrated circuit field due to its low cost, high integration density, and extremely low power consumption. In recent years, image sensors (CMOS Image Sensors, CIS) based on CMOS technology have developed rapidly. According to the different light-gathering surfaces, CIS can be divided into front-side illumination (FSI) CIS and back-side illumination (BSI) CIS. Compared to front-side illumination CIS, back-side illumination CIS has advantages such as better placement of back-end metal interconnects, higher photosensitivity, and a higher signal-to-noise ratio. BSI technology is gradually becoming the technology adopted by high-end, high-pixel CIS manufacturers.

[0003] Typically, back-illuminated CIS (CMOS Image Sensor) requires backside deep trench isolation (BDTI) to isolate adjacent pixels and address optical / electrical crosstalk. Obviously, forming the BDTI structure requires etching a single-crystal silicon crystal, which inevitably introduces defects, increasing interface defects. Therefore, it is necessary to passivate the BDTI interface and its defects. Depositing high-k materials such as hafnium oxide (HFO) and alumina (AlO) on the BDTI surface can partially passivate interface defects, but the passivation capability of high-k materials is insufficient.

[0004] Furthermore, high-pixel, small-size image sensors have certain requirements for full-well capacity (FWC). This is typically achieved by narrowing the width of the P-type isolation in the pixel region to prevent the P-type isolation area from offsetting too much of the N-type injection in the photodiode. At the same time, the N-type injection dose and injection depth of the photodiode are increased to meet the full-well capacity requirements of the pixel region. Generally, the P-type isolation in the pixel region is photoresist (PR) type, but the photoresist cannot be too high. If the photoresist is too high, it will collapse on the silicon wafer. The height of the photoresist limits the injection depth of boron (B) ions, resulting in insufficient depth of P-type isolation. In addition, a very narrow P-type isolation area may not cover the bottom of the BDTI, which will lead to insufficient pinning of the sides and bottom of the BDTI by the P-type isolation. Increasing the deep N-type injection dose and injection depth will result in insufficient pinning of the high-k material on the BDTI surface, thus exacerbating the white pixel phenomenon. Summary of the Invention

[0005] The purpose of this invention is to provide a method for forming a back-illuminated image sensor and a back-illuminated image sensor for reducing dark current and white spots in images.

[0006] Based on the above considerations, the present invention provides a method for forming a back-illuminated image sensor, comprising:

[0007] A semiconductor substrate is provided, the semiconductor substrate having a front surface and a back surface corresponding to the front surface;

[0008] A deep pinning region is formed in a predetermined area of ​​the semiconductor substrate;

[0009] A side isolation region is formed in a predetermined area of ​​the semiconductor substrate;

[0010] A photogenerated carrier collection region is formed in a predetermined area of ​​the semiconductor substrate;

[0011] Thinning is performed on the back surface of the semiconductor substrate, and a deep trench isolation region is formed on the thinned back surface of the semiconductor substrate.

[0012] The side isolation zone and the deep pinning zone pin the bottom surface and / or side surface of the deep trench isolation zone.

[0013] Optionally, the steps for forming a deep pinning zone include:

[0014] At least one full-surface ion implantation process is performed on the front surface of the semiconductor substrate, with the depth of the full-surface ion implantation being greater than or equal to the depth of the side isolation region, so as to form a deep pinning region below the side isolation region.

[0015] Optionally, two or more full-body ion implantation processes may be performed, with the ion implantation depth increasing or decreasing successively, to pin the entire side of the deep trench isolation region.

[0016] Optionally, the full-scale ion implantation reaches the back surface of the thinned semiconductor substrate to pin the back surface of the thinned semiconductor substrate.

[0017] Optionally, the dose and energy of the full-area ion implantation decrease or increase with each implantation to form a doping concentration gradient from deep to shallow in the direction from the back surface of the semiconductor substrate to the bottom of the side isolation region, preventing charges generated at the bottom and / or side surfaces and / or the back surface of the deep trench isolation region from entering the photogenerated carrier collection region.

[0018] Optionally, the step of forming the deep pinning zone includes:

[0019] A diffusion process is performed on the back surface of the semiconductor substrate to form a doping concentration gradient from deep to shallow in the direction from the back surface of the semiconductor substrate to the side isolation region, so as to pin the back surface of the semiconductor substrate and / or the bottom and / or side surfaces of the deep trench isolation region.

[0020] Optionally, the step of forming the side isolation zone includes:

[0021] A patterned photoresist layer is formed on the positive surface of the semiconductor substrate;

[0022] Using the patterned photoresist layer as a mask, a side isolation region is formed through an ion implantation process.

[0023] Optionally, the step of forming the deep trench isolation zone includes:

[0024] A deep trench is formed on the back surface of the thinned semiconductor substrate by etching, which is connected to the side isolation region.

[0025] A passivation layer is formed on the bottom surface, side surface, and back surface of the semiconductor substrate of the deep trench;

[0026] The deep trench is filled with oxides or nitrides to form a deep trench isolation zone.

[0027] The present invention also provides a back-illuminated image sensor, comprising:

[0028] A semiconductor substrate, wherein a photogenerated carrier collection region is provided within the semiconductor substrate;

[0029] The photogenerated carrier collection region has a side isolation region and a deep trench isolation region with an opening located on the back surface of the semiconductor substrate.

[0030] Among them, a deep pinning region is also provided between the bottom of the side isolation region and the back surface of the semiconductor substrate to pin the bottom and / or side of the deep trench isolation region.

[0031] Optionally, the deep pinning region extends from the bottom of the side isolation region to the back surface of the semiconductor substrate to pin the entire side of the deep trench isolation region.

[0032] Optionally, the deep pinning region completely covers the back surface of the semiconductor substrate to pin the back surface of the semiconductor substrate.

[0033] Optionally, the doping concentration of the deep pinning region gradually increases from the bottom of the side isolation region to the back surface of the semiconductor substrate to prevent electrons from entering the photodiode region from defects on the bottom and / or side and / or back surface of the deep trench isolation region.

[0034] Optionally, the width of the side isolation region is 10-100 nm wider than the width of the deep trench isolation region.

[0035] The back-illuminated image sensor formation method and the back-illuminated image sensor provided by the present invention have the following beneficial effects:

[0036] By performing at least one comprehensive ion implantation process, the bottom and sides of the deep trench isolation structure are fully pinned, reducing dark current in the image sensor and white spot phenomenon in the image.

[0037] A doping concentration gradient from shallow to deep is formed from the bottom of the side isolation region to the back surface of the semiconductor substrate, which can help reduce the impact of the increased full-coverage ion implantation of the present invention on the full-well capacity; it can also more effectively prevent surface defect charges from entering the photogenerated carrier region from the surface of the deep trench isolation region.

[0038] The white spot phenomenon in image sensors can be improved without adding an additional photomask layer. Attached Figure Description

[0039] Other features, objects, and advantages of the invention will become more apparent from the following detailed description of non-limiting embodiments, taken in conjunction with the accompanying drawings.

[0040] Figure 1 The flowchart shown is a method for forming a back-illuminated image sensor provided by the present invention;

[0041] Figure 2 The diagram shown is a schematic diagram of a structure in which a deep pinning region is formed in a predetermined area of ​​a semiconductor substrate, as provided by the present invention.

[0042] Figure 3 The diagram shows a schematic of the structure provided by the present invention, in which a side isolation region and a photogenerated carrier collection region are formed in a predetermined region of a semiconductor substrate;

[0043] Figure 4 The diagram shown is a schematic representation of the structure for forming deep trenches provided by the present invention.

[0044] Figure 5 The diagram shown is a structural schematic of the back-illuminated image sensor provided by the present invention.

[0045] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0046] Numerous specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0047] Secondly, the present invention is described in detail using schematic diagrams. When describing the embodiments of the present invention in detail, the schematic diagrams are merely examples for ease of explanation and should not limit the scope of protection of the present invention.

[0048] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0049] This invention provides a method for forming a back-illuminated image sensor, such as... Figure 1 As shown, the method includes:

[0050] S1: Provide a semiconductor substrate having a front surface and a back surface corresponding to the front surface;

[0051] S2: A deep pinning region is formed in a predetermined area of ​​the semiconductor substrate;

[0052] S3: A side isolation region is formed in a predetermined area of ​​the semiconductor substrate;

[0053] S4: A photogenerated carrier collection region is formed in a predetermined area of ​​the semiconductor substrate;

[0054] S5: Thinning is performed on the back surface of the semiconductor substrate, and a deep trench isolation region is formed on the thinned back surface of the semiconductor substrate; wherein the isolation region and the deep pinning region pin the bottom surface and / or side surface of the deep trench isolation region. By setting the deep pinning region, the generation of dark current and image white spots is reduced.

[0055] The method for forming a back-illuminated image sensor provided by the present invention will be described in detail below with reference to the accompanying drawings.

[0056] Please refer to Figure 2 Steps S1 and S2 are performed to provide a semiconductor substrate 11, which has a front surface 111 and a back surface 112 corresponding to the front surface; a deep pinning region 12 is formed in a predetermined area of ​​the semiconductor substrate 11.

[0057] The semiconductor substrate can be a silicon substrate, or the material of the semiconductor substrate can also be germanium, silicon germanide, silicon carbide, gallium arsenide, or indium gallium dihydrogen phosphate. The semiconductor substrate can also be a silicon substrate on an insulator surface or a germanium substrate on an insulator surface, or a substrate with an epitaxy layer (Epi layer) grown on it. In this embodiment, the semiconductor substrate 11 is selected as a silicon substrate.

[0058] Please continue to refer to Figure 2 And please combine Figure 4 The step of forming the deep pinned region 12 includes: performing at least one blanket implantation process on the front surface 111 of the semiconductor substrate 11, wherein the depth of the blanket implantation is greater than or equal to the depth of the side isolation region 13, so as to form the deep pinned region 12 below the side isolation region 13.

[0059] The doping type of the deep pinning region 12 is opposite to that of the photogenerated carrier collection region 14. If the dopant ions in the photogenerated carrier collection region 14 are N-type, then the dopant ions in the deep pinning region 12 are P-type, such as B, Ga, or In; conversely, if the dopant ions in the photogenerated carrier collection region 14 are P-type, then the dopant ions in the deep pinning region 12 are N-type, such as P, As, or Sb.

[0060] In this embodiment, the photogenerated carrier collection region 14 is doped with N-type and the deep pinning region 12 is doped with P-type, that is, P-type full-scale ion implantation is performed to form a P-type doped deep pinning region 12.

[0061] The number of implantation cycles in the full-coverage ion implantation process can be specifically determined based on the distance from the side isolation region 13 to the back surface 113 of the thinned semiconductor substrate. Multiple full-coverage ion implantations can be performed to achieve pinning of the entire side of the deep trench isolation structure; the implantation depth of the full-coverage ion implantation can reach the back surface of the thinned semiconductor substrate to pin the back surface of the semiconductor substrate. In this embodiment, three P-type full-coverage ion implantations are performed sequentially on the front surface of the semiconductor substrate to form pinning sub-layers 121, 122, and 123, respectively. This forms a deep pinning region 12 composed of pinning sub-layers 121, 122, and 123 between the bottom of the side isolation region 13 and the back surface 113. This allows pinning of the entire side 151 and bottom 152 of the deep trench isolation region, as well as the back surface 113 of the thinned semiconductor substrate, thereby reducing dark current and image white spots caused by surface defects in the deep trench isolation region and the semiconductor substrate.

[0062] Furthermore, the dosage of full-area ion implantation can be gradually decreased or increased with each implantation, forming a doping concentration gradient from deep to shallow along the direction from the back surface of the semiconductor substrate to the bottom of the side isolation region. This prevents electrons or holes generated at the bottom and / or side surfaces and / or the back surface of the semiconductor substrate from entering the photogenerated carrier collection region. Simultaneously, it also avoids adverse effects of full-area ion implantation on the full-well capacity of the photodiode. In this embodiment, the concentration of P-type doped ions in the formed pinned layers 121, 122, and 123 decreases sequentially to prevent electrons at surface defects from entering the photogenerated carrier collection region.

[0063] like Figure 3 As shown, S3 and S4 are performed to form a side isolation region 13 and a photogenerated carrier collection region 14 of a photodiode in a predetermined region of the semiconductor substrate 11.

[0064] The doping type of the side isolation region 13 is opposite to that of the photogenerated carrier collection region. In this embodiment, the doping type of the side isolation region 13 is P-type.

[0065] In this embodiment, the step of forming the side isolation area 13 specifically includes:

[0066] A patterned photoresist layer (not shown) is formed on the front surface 111 of the semiconductor substrate.

[0067] Using the patterned photoresist layer (not shown) as a mask, a side isolation region 13 is formed by a P-type ion implantation process.

[0068] Of course, after forming the side isolation region and the photogenerated carrier collection region, there are also steps to form other transistors and metal interconnect layers. The formation method is the same as the front-side process formation method of the existing back-illuminated image sensor, and will not be described in detail here.

[0069] like Figures 4 to 5 As shown, step S5 is performed, thinning is performed on the back surface 112 of the semiconductor substrate, and a deep trench isolation region 15 is formed on the thinned back surface 113 of the semiconductor substrate. After thinning, a deep pinning region 12 is exposed to achieve pinning of the back surface of the semiconductor substrate. The bottom surface 152 and the side surface 151 of the deep trench isolation region 15 are pinned by the side isolation region 13 and the deep pinning region 12.

[0070] Specifically, please refer to Figures 4 to 5 In this embodiment, the step of forming the deep trench isolation area includes:

[0071] The back surface 113 of the thinned semiconductor substrate is etched to form a deep trench 16 connected to the isolation region;

[0072] A passivation layer 17 is formed on the bottom surface, side surface and back surface of the semiconductor substrate of the deep trench 16;

[0073] The deep trench is filled with oxides or nitrides to form a deep trench isolation zone 15.

[0074] The passivation layer 17 can be a high-k dielectric layer, such as hafnium oxide or aluminum oxide, which can partially passivate surface defects. In other embodiments, the passivation layer may not be formed during the step of forming the deep trench isolation region.

[0075] In other embodiments, a diffusion process can also be used to form the deep pinning region. As an example, the steps of forming the deep pinning region using a diffusion process include:

[0076] Doping ions are diffused onto the back surface of the semiconductor substrate to form a doping concentration gradient from deep to shallow in the direction from the back surface of the semiconductor substrate to the side isolation region, so as to pin the back surface of the semiconductor substrate and / or the bottom and / or side surfaces of the deep trench isolation region.

[0077] It should be noted that the method for forming a back-illuminated image sensor provided by the present invention does not restrict the order in which the side isolation region, the photogenerated carrier collection region, and the deep pinning region are formed. In other embodiments, the deep pinning region may be formed first, followed by the side isolation region and the photogenerated carrier collection region.

[0078] The present invention also provides a back-illuminated image sensor, such as... Figure 5 The back-illuminated image sensor includes:

[0079] Semiconductor substrate 11, wherein a photogenerated carrier collection region 14 is provided in the semiconductor substrate 11;

[0080] The photogenerated carrier collection region 14 has a side isolation region 13 and a deep trench isolation region 15 with an opening located on the back surface of the semiconductor substrate.

[0081] Among them, a deep pinning region 12 is provided between the bottom of the side isolation region 13 and the back surface 113 of the semiconductor substrate to pin the bottom and side surfaces of the deep trench isolation region 15, so as to reduce the generation of dark current and image white spots.

[0082] The deep pinning region extends from the bottom of the side isolation region to the back surface of the semiconductor substrate to pin the entire side of the deep trench isolation region. The deep pinning region may also completely cover the back surface of the semiconductor substrate to pin it. The doping concentration of the deep pinning region gradually increases from the bottom of the side isolation region to the back surface of the semiconductor substrate to prevent electrons or holes from entering the photodiode region from defects on the bottom or side surfaces of the deep trench isolation region or the back surface of the semiconductor substrate.

[0083] In this embodiment, the deep pinning region consists of three pinning sub-layers, namely 121, 122, and 123. The concentration of P-type doped ions in the three pinning sub-layers gradually increases. Optionally, the width w1 of the side isolation region is 10-100 nm wider than the width w2 of the deep trench isolation region. In this invention, by pinning the sides of the deep trench isolation region 15 through the deep pinning region 12, the width of the side isolation region 13 can be reduced. In this embodiment, the width of the side isolation region 13 is 10-100 nm wider than the width of the deep trench isolation region 15.

[0084] In summary, the present invention provides a method for forming a back-illuminated image sensor and the image sensor thereof, which achieves full pinning of the bottom and sides of the deep trench isolation structure through at least one full-area ion implantation, thereby reducing dark current and white spot phenomenon in the image sensor.

[0085] By forming a doping concentration gradient from shallow to deep from the bottom of the side isolation region to the back surface of the semiconductor substrate, it not only helps to reduce the impact of the increased full-well ion implantation of the present invention on the full-well capacity, but also more effectively prevents surface defect charges from entering the photogenerated carrier region from the surface of the deep trench isolation region.

[0086] This invention improves the white spot phenomenon of image sensors without adding an additional photomask layer.

[0087] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and not restrictive in any way. Furthermore, it is clear that the word "comprising" does not exclude other elements and steps, and the word "a" does not exclude a plurality. Multiple elements recited in the apparatus claims may also be implemented by a single element. The terms "first," "second," etc., are used to denote names and do not indicate any particular order.

Claims

1. A method for forming a back-illuminated image sensor, characterized in that, include: A semiconductor substrate is provided, the semiconductor substrate having a front surface and a back surface corresponding to the front surface; A deep pinning region is formed in a predetermined area of ​​the semiconductor substrate; A side isolation region is formed in a predetermined area of ​​the semiconductor substrate; A photogenerated carrier collection region is formed in a predetermined area of ​​the semiconductor substrate; Thinning is performed on the back surface of the semiconductor substrate, and a deep trench isolation region is formed on the thinned back surface of the semiconductor substrate. The deep pinning region is close to the back surface of the semiconductor substrate, the side isolation region is a doped region, and the side isolation region and the deep pinning region pin the bottom surface and / or side surface of the deep trench isolation region.

2. The method for forming a back-illuminated image sensor according to claim 1, characterized in that, The steps to form a deep pinning zone include: At least one full-surface ion implantation process is performed on the positive surface of the semiconductor substrate, with the depth of the full-surface ion implantation being greater than or equal to the depth of the side isolation region, so as to form a deep pinning region below the side isolation region.

3. The method for forming a back-illuminated image sensor according to claim 2, characterized in that, Perform two or more full-scale ion implantation processes, with the ion implantation depth increasing or decreasing successively, to pin the entire side of the deep trench isolation region.

4. The method for forming a back-illuminated image sensor according to claim 2, characterized in that, The implantation depth of the full-scale ion implantation reaches the back surface of the thinned semiconductor substrate to pin the back surface of the thinned semiconductor substrate.

5. The method for forming a back-illuminated image sensor according to claim 2, characterized in that, The dose and energy of the full-area ion implantation decrease or increase with each implantation to form a doping concentration gradient from deep to shallow in the direction from the back surface of the semiconductor substrate to the bottom of the side isolation region, preventing charges generated at the bottom and / or side surfaces and / or the back surface of the deep trench isolation region from entering the photogenerated carrier collection region.

6. The method for forming a back-illuminated image sensor according to claim 1, characterized in that, The step of forming the deep pinning zone includes: A diffusion process is performed on the back surface of the semiconductor substrate to form a doping concentration gradient from deep to shallow in the direction from the back surface of the semiconductor substrate to the side isolation region, so as to pin the back surface of the semiconductor substrate and / or the bottom and / or side surfaces of the deep trench isolation region.

7. The method for forming a back-illuminated image sensor according to claim 1, characterized in that, The steps for forming the side isolation zone include: A patterned photoresist layer is formed on the positive surface of the semiconductor substrate; Using the patterned photoresist layer as a mask, a side isolation region is formed through an ion implantation process.

8. The method for forming a back-illuminated image sensor according to claim 1, characterized in that, The steps for forming the deep trench isolation zone include: A deep trench is formed on the back surface of the thinned semiconductor substrate by etching, which is connected to the side isolation region. A passivation layer is formed on the bottom surface, side surface, and back surface of the semiconductor substrate of the deep trench; The deep trench is filled with oxides or nitrides to form a deep trench isolation zone.

9. A back-illuminated image sensor, characterized in that, include: A semiconductor substrate, wherein a photogenerated carrier collection region is provided within the semiconductor substrate; The photogenerated carrier collection region has a side isolation region and a deep trench isolation region with an opening located on the back surface of the semiconductor substrate. The side isolation region is a doped region, and a deep pinning region is also provided between the bottom of the side isolation region and the back surface of the semiconductor substrate to pin the bottom and / or side of the deep trench isolation region.

10. The back-illuminated image sensor according to claim 9, characterized in that, The deep pinning region extends from the bottom of the side isolation region to the back surface of the semiconductor substrate to pin the entire side of the deep trench isolation region.

11. The back-illuminated image sensor according to claim 9, characterized in that, The deep pinning region completely covers the back surface of the semiconductor substrate to pin the back surface of the semiconductor substrate.

12. The back-illuminated image sensor according to claim 9, characterized in that, The doping concentration of the deep pinning region gradually increases from the bottom of the side isolation region to the back surface of the semiconductor substrate to prevent charges at defects on the bottom and / or side and / or back surface of the deep trench isolation region from entering the photogenerated carrier collection region.

13. The back-illuminated image sensor according to claim 9, characterized in that, The width of the side isolation zone is 10-100 nm wider than the width of the deep trench isolation zone.

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