Method of forming a cmos image sensor

Abstract

The application provides a forming method of a CMOS image sensor, which comprises the following steps: providing a substrate; forming a photodiode region in the substrate; etching the photodiode region to form a groove in the photodiode region; forming a first oxide layer on the surface of the substrate, the first oxide layer covering the bottom wall of the groove and the sidewall of the groove; removing the first oxide layer; forming a second oxide layer on the surface of the substrate, the second oxide layer covering the bottom of the groove and the sidewall of the groove; forming a vertical gate in the groove; and filling the vertical gate with polycrystalline silicon doped with carbon and phosphorus. The first oxide layer compensates for the defects of the sidewall of the groove during etching the groove, the quality of the oxide layer is good during forming the second oxide layer, and the quality of the sidewall of the vertical gate formed in the groove subsequently is improved. Furthermore, the polycrystalline silicon doped with carbon and phosphorus is filled in the vertical gate, the failure rate of white pixel points is reduced, and the quality of the CMOS image sensor is improved.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a method for forming a CMOS image sensor. Background Art

[0002] In semiconductor process production, in order to increase the number of pixels per unit area of ​​the image sensor, the pixel size of the image sensor continues to shrink. The resulting problem is that the effective pixel area is greatly reduced, resulting in the generation of vertical gates. By etching, a groove is formed inside the substrate, and the groove extends to the depth of the photodiode. A vertical gate is formed in the groove. It is very easy to produce weak areas near the vertical gate, which in turn produces white pixels. Therefore, white pixels are a key parameter of semiconductor manufacturing CIS (CMOS image sensor) products. White pixels are an important indicator to characterize CIS image imaging. How to reduce the failure rate of white pixels through manufacturing processes has always been a difficult problem that needs to be overcome in process production.

[0003] In the prior art, a CMOS image sensor is formed by providing a substrate, forming a photodiode region within the substrate, forming a trench by dry etching the photodiode region, then forming an oxide layer to cover the sidewalls and bottom wall of the trench and the surface of the substrate, and finally forming a vertical gate by filling the oxide layer within the trench with polysilicon.

[0004] However, the inventors discovered that when using existing methods to form 55CIS products, the failure rate of white pixels was relatively high, which reduced image quality. Furthermore, during the trench etching process, the trench sidewalls were damaged, reducing the quality of the vertical gate sidewalls formed subsequently, further degrading the quality of the image sensor. Summary of the Invention

[0005] The present invention aims to provide a method for forming a CMOS image sensor, which can improve the quality of the oxide layer on the sidewall of the trench and reduce the failure rate of white pixels, thereby improving the image quality of the CMOS image sensor.

[0006] To achieve the above-mentioned object, the present invention provides a method for forming a CMOS image sensor, comprising:

[0007] providing a substrate;

[0008] forming a photodiode region in the substrate, and etching the photodiode region to form a trench in the photodiode region;

[0009] forming a first oxide layer on the surface of the substrate, wherein the first oxide layer covers the bottom wall and the sidewalls of the trench at the same time;

[0010] removing the first oxide layer to expose the surface of the substrate, the bottom of the trench, and the sidewalls of the trench;

[0011] forming a second oxide layer on the surface of the substrate, wherein the second oxide layer covers the bottom of the trench and the sidewalls of the trench;

[0012] forming a vertical gate in the trench; and

[0013] The vertical gate is filled with polysilicon doped with carbon and phosphorus.

[0014] Optionally, in the method for forming a CMOS image sensor, the thickness of the first oxide layer is 55 angstroms to 65 angstroms.

[0015] Optionally, in the method for forming a CMOS image sensor, the thickness of the second oxide layer is 85 angstroms to 95 angstroms.

[0016] Optionally, in the method for forming the CMOS image sensor, the method for forming the groove in the photodiode region includes: etching the photodiode region using a dry etching method to form the groove in the photodiode region.

[0017] Optionally, in the method for forming a CMOS image sensor, the first oxide layer includes silicon oxide.

[0018] Optionally, in the method for forming a CMOS image sensor, the first oxide layer is silicon dioxide.

[0019] Optionally, in the method for forming a CMOS image sensor, the second oxide layer includes silicon oxide.

[0020] Optionally, in the method for forming a CMOS image sensor, the second oxide layer is silicon dioxide.

[0021] Optionally, in the method for forming the CMOS image sensor, the method of filling the vertical gate with polysilicon doped with carbon and phosphorus includes:

[0022] The vertical gate is filled with polysilicon and doped with carbon gas and phosphorus gas.

[0023] In the method for forming a CMOS image sensor provided by the present invention, a first oxide layer is first formed on the sidewalls of the trench to fill the trench sidewalls, compensating for defects in the trench sidewalls during trench etching. A second oxide layer is then formed, resulting in a higher quality oxide layer, thereby improving the quality of the sidewalls of the vertical gate subsequently formed within the trench. Furthermore, the vertical gate is filled with polysilicon doped with carbon and phosphorus. When electrons generated in the photodiode region are transmitted, the carbon and phosphorus-doped polysilicon can absorb impurity ions in the transmission trench, thereby reducing the failure rate of white pixels and further improving the quality of the CMOS image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a flow chart of a method for forming a CMOS image sensor according to an embodiment of the present invention;

[0025] Figure 2 is a schematic diagram of a CMOS image sensor after forming a photodiode according to an embodiment of the present invention;

[0026] Figure 3 is a schematic diagram of a CMOS image sensor after grooves are formed according to an embodiment of the present invention;

[0027] Figure 4 is a schematic diagram of a CMOS image sensor after forming a first oxide layer according to an embodiment of the present invention;

[0028] Figure 5 is a schematic diagram of a CMOS image sensor after removing the first oxide layer according to an embodiment of the present invention;

[0029] Figure 6 is a schematic diagram of a CMOS image sensor after forming a second oxide layer according to an embodiment of the present invention;

[0030] Figure 7 is a schematic diagram of a CMOS image sensor after forming a vertical gate according to an embodiment of the present invention;

[0031] In the figure: 110 - substrate, 120 - photodiode region, 130 - trench, 140 - first oxide layer, 150 - second oxide layer, 160 - vertical gate. DETAILED DESCRIPTION

[0032] The following is a more detailed description of the specific embodiments of the present invention with reference to schematic diagrams. The advantages and features of the present invention will become more apparent from the following description. It should be noted that the drawings are greatly simplified and not to exact scale, and are only used for the purpose of conveniently and clearly illustrating the embodiments of the present invention.

[0033] Hereinafter, the terms "first," "second," and the like are used to distinguish between similar elements and are not necessarily used to describe a particular order or chronological sequence. It is to be understood that these terms used in this manner are interchangeable where appropriate. Similarly, if a method described herein comprises a series of steps, the order in which the steps are presented herein is not necessarily the only order in which the steps may be performed, and some of the steps described may be omitted and / or other steps not described herein may be added to the method.

[0034] Please refer to Figure 1 The present invention provides a method for forming a CMOS image sensor, comprising:

[0035] S11: providing a substrate;

[0036] S12: forming a photodiode region in the substrate, and etching the photodiode region to form a trench in the photodiode region;

[0037] S13: forming a first oxide layer on the surface of the substrate, wherein the first oxide layer covers the bottom wall and the sidewalls of the trench at the same time;

[0038] S14: removing the first oxide layer to expose the surface of the substrate, the bottom of the trench, and the sidewalls of the trench;

[0039] S15: forming a second oxide layer on the surface of the substrate, wherein the second oxide layer covers the bottom of the trench and the sidewalls of the trench;

[0040] S16: forming a vertical gate in the trench; and

[0041] S17: Filling the vertical gate with polysilicon doped with carbon and phosphorus.

[0042] First, execute step S11 and step S12, please refer to Figure 2 , providing a substrate 110, which can be a wafer, and then forming a photodiode region 120 in the substrate 110. The specific formation method is to perform ion implantation on the substrate 110 on one side of the trench 120 to form a P-type ion region and an N-type ion region, respectively. The P-type ion region is located above the surface of the N-type ion region or the N-type ion region is located above the surface of the P-type ion region, and a PN junction is formed between the P-type ion region and the N-type ion region, thereby forming the photodiode region 120. Next, please refer to Figure 3 , etching the photodiode region 120 to form a groove 130. In the embodiment of the present invention, the photodiode region 120 may be etched using a dry etching method. In other embodiments of the present invention, other etching methods may also be used.

[0043] The inventors discovered that in step S11, since dry etching is used to etch the photodiode region 120 to form the groove 130, damage will occur to the sidewalls of the groove 130. If the damage is not treated at this time, an oxide layer will be formed on the sidewalls of the groove 130. When a vertical gate is formed in the oxide layer, the stress at the angle of the vertical gate is large because the angle of the vertical gate is a right angle, which will damage the oxide layer and indirectly reduce the quality of the CMOS image sensor. Therefore, the embodiment of the present invention adopts a series of measures to reduce the damage to the oxide layer to improve the quality of the CMOS image sensor. These measures are "step S13: forming a first oxide layer on the surface of the substrate, and the first oxide layer also covers the sidewalls of the groove; step S14: removing the first oxide layer to expose the surface of the substrate, the bottom of the groove and the sidewalls of the groove; and step S15: forming a second oxide layer on the surface of the substrate, and the second oxide layer also covers the bottom of the groove and the sidewalls of the groove". Please refer to Figure 4 In step S13, a furnace tube growth process is used to form a first oxide layer 150 on the substrate 110. The thickness of the first oxide layer 150 is 55 angstroms to 65 angstroms, for example, 60 angstroms. In other embodiments of the present invention, it can also be other thicknesses, such as 61 angstroms. The growth temperature of the furnace tube growth is 700°C to 900°C, for example, 800°C. In other embodiments of the present invention, it can also be other temperatures, such as 850°C. The formed first oxide layer 140 covers the bottom wall and sidewalls of the trench 130 and extends to cover the surface of the substrate 110. The first oxide layer 140 can fill the defects generated by etching. The first oxide layer 140 includes silicon oxide. In other embodiments of the present invention, in addition to silicon oxide, it can also be silicon nitride or aluminum oxide, or a combination of silicon oxide, silicon nitride and aluminum oxide. More specifically, in the embodiment of the present invention, the silicon oxide selected for the first oxide layer 140 is silicon dioxide.

[0044] Then, execute step S14 and step S15, please refer to Figure 5 , the first oxide layer 140 is etched away to expose the surface of the substrate 110, the bottom wall of the trench 130 and the sidewall of the trench 130. Figure 6A second oxide layer 150 is formed on the surface of the substrate 110. The second oxide layer 150 can also be grown using a furnace growth process, and the process conditions can refer to the conditions for forming the first oxide layer 140. The second oxide layer 150 covers the surface of the substrate 110, the bottom of the trench 130, and the sidewalls of the trench 130. The thickness of the second oxide layer 150 is 85 to 95 angstroms, for example, 90 angstroms. In other embodiments of the present invention, other thicknesses can also be used, such as 91 angstroms. The formation of the first oxide layer 150 compensates for damage to the sidewalls of the trench 120. The second oxide layer 150 improves the quality of the oxide layer. Even if the corners of the vertical gate formed subsequently are subjected to high stress, the second oxide layer will not be damaged, thereby improving the quality of the image sensor.

[0045] Next, execute step S16, please refer to Figure 7 Polysilicon is formed on the second oxide layer 150 using a furnace tube growth process, and its preferred growth temperature is 450°C to 700°C. For example, the growth temperature in the embodiment of the present invention can be 550°C. In other embodiments of the present invention, it can also be other temperatures, such as 500°C. In this embodiment, polysilicon fills the trench 130 to form a vertical gate 160 in the trench 130. The thickness of the polysilicon 170 is not limited by the present invention and can be adjusted according to actual conditions.

[0046] Next, step S7 is performed to continue filling vertical gate 160 with polysilicon. During the filling process, carbon-containing gas and phosphorus-containing gas are doped. The carbon-containing gas flow rate is 40 sccm to 150 sccm, and the phosphorus-containing gas concentration is 1E25 atom / cm2 to 7E20 atom / cm2. When the carbon- and phosphorus-doped polysilicon is filled into vertical gate 160, electrons generated by photodiode region 140 are transferred to the floating diffusion region through the transfer channel. The carbon- and phosphorus-doped polysilicon can absorb impurity ions in the transfer channel, thereby reducing the failure rate of white pixels and improving the quality of the CMOS image sensor.

[0047] In summary, a first oxide layer is first formed on the sidewalls of the trench to fill the trench sidewalls, compensating for defects in the trench sidewalls during trench etching. A second oxide layer is then formed, resulting in a higher quality oxide layer, thereby improving the quality of the sidewalls of the vertical gate subsequently formed within the trench. Furthermore, by filling the vertical gate with polysilicon doped with carbon and phosphorus, the carbon- and phosphorus-doped polysilicon can absorb impurity ions from the transmission trench during electron transmission in the photodiode region, thereby reducing the failure rate of white pixels and improving the quality of the CMOS image sensor.

[0048] The above description is merely a preferred embodiment of the present invention and does not limit the present invention in any way. Any person skilled in the art who, without departing from the scope of the present invention, makes any equivalent substitution, modification, or other changes to the technical solution and technical content disclosed in the present invention shall be deemed to be within the scope of the present invention and still fall within the scope of protection of the present invention.

Claims

1. A method for forming a CMOS image sensor, characterized in that: include: providing a substrate; forming a photodiode region in the substrate, and etching the photodiode region to form a trench in the photodiode region; forming a first oxide layer on the surface of the substrate, wherein the first oxide layer covers the bottom wall and the sidewalls of the trench at the same time; removing the first oxide layer to expose the surface of the substrate, the bottom of the trench, and the sidewalls of the trench; forming a second oxide layer on the surface of the substrate, wherein the second oxide layer covers the bottom of the trench and the sidewalls of the trench; forming a vertical gate in the trench; as well as The vertical gate is filled with polysilicon doped with carbon and phosphorus.

2. The method for forming a CMOS image sensor according to claim 1, wherein: The thickness of the first oxide layer is 55 angstroms to 65 angstroms.

3. The method for forming a CMOS image sensor according to claim 1, wherein: The thickness of the second oxide layer is 85 angstroms to 95 angstroms.

4. The method for forming a CMOS image sensor according to claim 1, wherein: The method for forming a trench in the photodiode region includes: etching the photodiode region using a dry etching method to form a trench in the photodiode region.

5. The method for forming a CMOS image sensor according to claim 1, wherein: The first oxide layer includes silicon oxide.

6. The method for forming a CMOS image sensor according to claim 5, wherein: The first oxide layer is silicon dioxide.

7. The method for forming a CMOS image sensor according to claim 1, wherein: The second oxide layer includes silicon oxide.

8. The method for forming a CMOS image sensor according to claim 7, wherein: The second oxide layer is silicon dioxide.

9. The method for forming a CMOS image sensor according to claim 1, wherein: The method of filling the vertical gate with polysilicon doped with carbon and phosphorus includes: The vertical gate is filled with polysilicon and doped with carbon gas and phosphorus gas.

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