Image sensor and forming method thereof

Abstract

The invention discloses a method for forming an image sensor. The method at least comprises the following steps: providing a silicon substrate; etching the front surface of the silicon substrate to form a groove; forming a composite dielectric layer on the surface of the groove, and filling polycrystalline silicon doped with boron element to form a groove filling structure; the composite dielectric layer at least comprises a nitrogen-containing layer so as to reduce the diffusion of boron elements in the polycrystalline silicon to the side wall of the groove. The nitrogen-containing layer is introduced into the composite dielectric layer, so that boron in heavily doped p-type polycrystalline silicon can be prevented from diffusing to a Si / SiO2 interface, and the leakage current between Si and Poly is prevented from being increased.

Description

Technical Field

[0001] The present invention relates to the field of integrated circuits, and particularly to an image sensor and a method for forming the same. Background Art

[0002] In recent years, with the continuous iteration of image sensor technology, deep trench structures have been widely used in image sensors, especially in small pixel image sensors. As the pixel size shrinks, the method of achieving p-type isolation through boron implantation will cause lateral diffusion of boron during implantation and subsequent thermal processes, resulting in a reduction in the effective volume of the photodiode, severely limiting the full well capacity (FWC) of the photodiode. The deep trench structure only requires a small physical space to achieve physical isolation between different pixels, reducing the physical space required for the isolation region, which is beneficial to the improvement of the full well capacity of small pixel image sensors. At the same time, the deep trench structure can also be used for isolation between different modules of the peripheral circuit of the image sensor. On the other hand, the deep trench structure can be used to fabricate deep trench MOS capacitors.

[0003] The existing deep trench structure of an image sensor is composed of silicon (Si) / silicon oxide (SiO 2 ) / polycrystalline (poly), where Poly is usually heavily doped polysilicon. For the pixel region, using heavily doped p-type polysilicon in the isolation structure can form a denser hole layer near the deep trench, making it easier for the photodiode to deplete, and at the same time being beneficial to reducing the dark current. However, during the process of fabricating the image sensor, boron in the heavily doped p-type polysilicon will diffuse into SiO 2 / Si under the action of heat, changing the dielectric properties of SiO 2 , resulting in an increase in the leakage current between the isolation structure Si and Poly or even causing punch-through. If a thick SiO 2 is used as the dielectric layer, although it can inhibit the diffusion of boron into Si, the process cost is relatively high. Summary of the Invention

[0004] Based on the problems of the prior art, the present invention provides a deep trench structure with a composite dielectric layer for an image sensor. Specifically, the present invention provides a method for forming an image sensor, at least including: providing a silicon substrate; etching the front surface of the silicon substrate to form a trench; forming a composite dielectric layer on the surface of the trench and filling it with polysilicon doped with boron element to form a trench filling structure; the composite dielectric layer at least includes a nitrogen-containing layer to reduce the diffusion of boron element in the polysilicon to the sidewall of the trench.

[0005] In some embodiments, the method further includes: thinning the back surface of the silicon substrate to expose the polysilicon; the trench filling structure is used to form at least one of the following structures: a trench isolation structure between different pixels of an image sensor; a trench capacitor of an image sensor; a trench isolation structure between different modules of the peripheral circuit of an image sensor.

[0006] In some embodiments, the composite dielectric layer includes a first silicon oxide layer and a nitrogen-containing layer formed in sequence on the surface of the trench; the nitrogen-containing layer includes: silicon nitride, silicon oxynitride, or a combination of the two.

[0007] In some embodiments, the composite dielectric layer includes a first silicon oxide layer, a nitrogen-containing layer, and a second silicon oxide layer formed in sequence on the surface of the trench; the nitrogen-containing layer includes: silicon nitride, silicon oxynitride, or a combination of the two.

[0008] In some embodiments, the formation process of the first silicon oxide layer includes: one or a combination of oxidation, atomic layer deposition, and chemical vapor deposition; the formation process of the nitrogen-containing layer includes: one or a combination of chemical vapor deposition, atomic layer deposition, annealing, and implantation; the formation process of the second silicon oxide layer includes: one or a combination of atomic layer deposition, chemical vapor deposition, and oxidation.

[0009] In some embodiments, the thickness of the first silicon oxide layer is 0 to 20 nanometers; the thickness of the nitrogen-containing layer is 0 to 20 nanometers; the thickness of the second silicon oxide layer is 0 to 10 nanometers.

[0010] In some embodiments, the polysilicon is connected to a voltage.

[0011] The present invention also provides an image sensor, at least including: a silicon substrate; a trench filling structure formed in the silicon substrate; the trench filling structure includes a composite dielectric layer and polysilicon doped with boron element; the composite dielectric layer includes at least one nitrogen-containing layer to reduce the diffusion of boron element in the polysilicon to one side of the nitrogen-containing layer.

[0012] In some embodiments, the trench filling structure is used to form at least one of the following structures: a trench isolation structure between different pixels of an image sensor; a trench capacitor of an image sensor; a trench isolation structure between different modules of the peripheral circuit of an image sensor Compared with the prior art, the deep trench structure provided by the present invention is a composite dielectric layer. By introducing a nitrogen-containing layer into the composite dielectric layer, the diffusion of boron in the heavily doped p-type polysilicon to the Si / SiO 2 interface can be blocked, avoiding an increase in the leakage current between Si and Poly. There is an optional SiO between the nitrogen-containing layer and Poly 2The layer serves as a stress buffer layer to reduce stress. This deep trench structure can be used in the trench isolation structure and MOS capacitor structure in an image sensor. Since the longitudinal space is utilized, the MOS capacitor can increase the capacitance per unit area. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not restrictive, and in these embodiments, the same reference numerals represent the same structures.

[0014] Figure 1 It is a schematic structural diagram of an image sensor according to an embodiment of the present invention.

[0015] Figure 2 It is a flowchart of a method for forming an image sensor according to an embodiment of the present invention.

[0016] Figures 3 to 10 is Figure 2 a schematic structural diagram during the formation process of the image sensor. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some examples or embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, the present invention can also be applied to other similar scenarios based on these drawings. Unless obvious from the language context or otherwise stated, the same reference numerals in the figures represent the same structures or operations.

[0018] As Figure 1 shown, the image sensor 100 includes a pixel array 110, a logic circuit 120, and a storage array 130. The pixel array 110 includes a plurality of pixel units 111, and semiconductor devices ( Figure 1 not shown in

[0019] Figure 2 It is a flowchart of a method for forming an image sensor according to an embodiment of the present invention.

[0020] The method includes the following steps.

[0021] Step S11: Provide a silicon substrate.

[0022] For step S11, refer to Figure 3 , the silicon substrate 200 can be a doped or undoped semiconductor material, such as silicon, germanium, silicon germanium, silicon germanium on insulator (SGOI), or a combination thereof. The substrate 101 can include a substrate with multiple epitaxial layers. The substrate 101 includes a pixel region, a logic region, and a storage region of the image sensor. The pixel region of the image sensor is used to receive external optical signals and convert them into electrical signals for imaging. The specific structures of the pixel region and the logic region of the image sensor will not be elaborated here.

[0023] Step S12: Etch the front surface of the silicon substrate to form trenches; For step S12, refer to Figure 4 , by dry etching the front surface of the silicon substrate 200, trenches 210 are formed. This step omits processes such as forming a hard mask layer, photoresist, and exposure and development on the front surface of the silicon substrate.

[0024] Step S13: Form a composite dielectric layer on the surface of the trenches and fill polysilicon doped with boron element to form a trench filling structure; the composite dielectric layer includes at least one nitrogen-containing layer to reduce the diffusion of boron element in the polysilicon to the trench sidewalls.

[0025] For step S13, refer to Figure 5 , a composite dielectric layer 220 is formed on the surface of the trenches 210. The composite dielectric layer 210 includes at least one nitrogen-containing layer.

[0026] In one embodiment, refer to Figure 6 , the composite dielectric layer 220 includes a first silicon oxide layer 221 and a nitrogen-containing layer 222 formed in sequence on the surface of the trenches 210. The nitrogen-containing layer 222 includes: silicon nitride, silicon oxynitride, or a combination of both. The formation process of the first silicon oxide layer 221 includes: one or more combinations of oxidation, atomic layer deposition, and chemical vapor deposition; the formation process of the nitrogen-containing layer 222 includes: one or more combinations of chemical vapor deposition, atomic layer deposition, annealing, and implantation. The thickness of the first silicon oxide layer 221 is 0 to 20 nanometers; the thickness of the nitrogen-containing layer 222 is 0 to 20 nanometers.

[0027] In yet another embodiment, refer to Figure 7, the composite dielectric layer 220 includes a first silicon oxide layer 221, a nitrogen-containing layer 222, and a second silicon oxide layer 223 that are sequentially formed on the surface of the trench 210. The nitrogen-containing layer 222 includes: silicon nitride, silicon oxynitride, or a combination of both. The formation process of the first silicon oxide layer 221 includes: one or more combinations of oxidation, atomic layer deposition, and chemical vapor deposition; the formation process of the nitrogen-containing layer 222 includes: one or more combinations of chemical vapor deposition, atomic layer deposition, annealing, and implantation; the formation process of the second silicon oxide layer 223 includes: one or more combinations of atomic layer deposition, chemical vapor deposition, and oxidation. The thickness of the first silicon oxide layer 221 is from 0 to 20 nanometers; the thickness of the nitrogen-containing layer 222 is from 0 to 20 nanometers; the thickness of the second silicon oxide layer 223 is from 0 to 10 nanometers.

[0028] Further, referring to Figure 8 , polysilicon 230 doped with boron element is filled to form a trench filling structure formed by the polysilicon 230 and the composite dielectric layer 220. The nitrogen-containing layer 222 can reduce the diffusion of the boron element in the polysilicon 230 to the sidewalls of the trench 210.

[0029] Further, referring to Figure 9 , a metal interconnect layer 240 is formed on the front surface of the silicon substrate 200. The carrier wafer 250 is bonded to the metal interconnect layer 240.

[0030] Further, referring to Figure 10 , the back surface of the silicon substrate 200 is thinned to expose the polysilicon 230. A voltage can be applied to the polysilicon 230.

[0031] The trench filling structure can be used to form at least one of the following structures: a trench isolation structure between different pixels of an image sensor; a trench capacitor of an image sensor; a trench isolation structure between different modules of the peripheral circuit of an image sensor.

[0032] Further, in order to form a complete image sensor structure, structures such as a metal grid, a filter, and a microlens can also be formed on the back surface of the silicon substrate 200, which will not be elaborated here.

[0033] The basic concepts have been described above. Obviously, for those skilled in the art, the above detailed disclosure is only an example and does not constitute a limitation to the present invention. Although not explicitly stated here, those skilled in the art may make various modifications, improvements, and corrections to the present invention. Such modifications, improvements, and corrections are proposed in the present invention, so such modifications, improvements, and corrections still fall within the spirit and scope of the exemplary embodiments of the present invention.

[0034] It should be understood that the embodiments described in the present invention are only used to illustrate the principles of the embodiments of the present invention. Other variations may also fall within the scope of the present invention. Therefore, by way of example and not limitation, alternative configurations of the embodiments of the present invention may be regarded as consistent with the teachings of the present invention. Accordingly, the embodiments of the present invention are not limited to the embodiments explicitly presented and described in the present invention.

Claims

1. A method for forming an image sensor, characterized in that, at least comprising: providing a silicon substrate; etching the front surface of the silicon substrate to form trenches; forming a composite dielectric layer on the surface of the trenches and filling polysilicon doped with boron element to form a trench filling structure; the composite dielectric layer at least comprises a nitrogen-containing layer to reduce the diffusion of boron element in the polysilicon to the side walls of the trenches.

2. The method according to claim 1, characterized in that, further comprising: thinning the back surface of the silicon substrate to expose the polysilicon; the trench filling structure is used to form at least one of the following structures: a trench isolation structure between different pixels of the image sensor; a trench capacitor of the image sensor; a trench isolation structure between different modules of the peripheral circuit of the image sensor.

3. The method according to claim 2, characterized in that, the composite dielectric layer comprises a first silicon oxide layer and a nitrogen-containing layer formed sequentially on the surface of the trenches; the nitrogen-containing layer comprises: silicon nitride, silicon oxynitride or a combination of both.

4. The method according to claim 2, characterized in that, the composite dielectric layer comprises a first silicon oxide layer, a nitrogen-containing layer and a second silicon oxide layer formed sequentially on the surface of the trenches; the nitrogen-containing layer comprises: silicon nitride, silicon oxynitride or a combination of both.

5. The method according to claim 3 or 4, characterized in that, the formation process of the first silicon oxide layer comprises: one or more combinations of oxidation, atomic layer deposition, chemical vapor deposition; the formation process of the nitrogen-containing layer comprises: one or more combinations of chemical vapor deposition, atomic layer deposition, annealing, implantation; the formation process of the second silicon oxide layer comprises: one or more combinations of atomic layer deposition, chemical vapor deposition, oxidation.

6. The method according to claim 3 or 4, characterized in that, the thickness of the first silicon oxide layer is 0 to 20 nanometers; the thickness of the nitrogen-containing layer is 0 to 20 nanometers; the thickness of the second silicon oxide layer is 0 to 10 nanometers.

7. The method according to claim 1, characterized in that, the polysilicon is connected to a voltage.

8. An image sensor, characterized in that, at least comprising: a silicon substrate; a trench filling structure formed in the silicon substrate; the trench filling structure comprises a composite dielectric layer and polysilicon doped with boron element; the composite dielectric layer at least comprises a nitrogen-containing layer to reduce the diffusion of boron element in the polysilicon to one side of the nitrogen-containing layer.

9. The image sensor according to claim 8, characterized in that, the trench filling structure is used to form at least one of the following structures: a trench isolation structure between different pixels of the image sensor; a trench capacitor of the image sensor; a trench isolation structure between different modules of the peripheral circuit of the image sensor.

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