Front-illuminated image sensor and method of forming the same

Abstract

The present invention relates to a front-illuminated image sensor and a method for forming the same, comprising: providing a semiconductor substrate, the semiconductor substrate being formed with a photodiode region corresponding to a pixel unit; forming a plurality of metal interconnection layers on the semiconductor substrate; the method for forming at least one metal interconnection layer comprising: forming a light-dense medium region above the photodiode region; forming a metal layer around the light-dense medium region; forming a light-sparse medium region covering the metal layer and the light-dense medium region. The process method of the present invention is compatible with the current conventional CMOS semiconductor process, and the process is simple. At the same time, the refractive index, morphology, etc. of the light-dense medium region can be independently adjusted according to design requirements, and the design is flexible. In addition, by adding a light-dense medium material with a high refractive index to the photodiode region, it acts as a light guide, reducing the light signal loss caused by reflection and refraction, thereby improving the sensitivity of the image sensor and reducing crosstalk.

Description

Technical Field

[0001] The present invention relates to the technical field of image sensors, and in particular to a front-illuminated image sensor and a method for forming the same. Background Art

[0002] Image sensors are sensors that can sense optical image information and convert it into usable output signals. They are an important component of digital cameras. Depending on the components, they can be divided into two categories: CCD (Charge Coupled Device) and CMOS (Complementary Metal-Oxide Semiconductor). With the continuous development of CMOS integrated circuit manufacturing technology, especially CMOS image sensor design and manufacturing technology, CMOS image sensors have gradually replaced CCD image sensors and become the mainstream. CMOS image sensors have the advantages of higher industrial integration and lower power consumption.

[0003] CMOS image sensor products can be divided into FSI (FrontSide Illumination) and BSI (BackSide Illumination). In front-illuminated CMOS image sensor products, the BEOL (Back RndOfLine) process has at least two layers of metal wiring, that is, multiple layers of metal wires and dielectric layers. For the photosensitive area, the distance from the chip surface to the photosensitive surface is relatively large, which means that the incident light must travel a long distance before it can be absorbed by the photosensitive area. Not only is the light attenuation large, but the CRA (Chief Ray Angle, the angle between the main light and the normal direction of the imaging surface) of the chip cannot be too large, thus affecting the wide application of image sensors.

[0004] In addition, due to the existence of different dielectric layers, light will be reflected and refracted during propagation, resulting in light loss and energy reduction. In this way, the number of electrons reaching the bottom of the photodiode from the chip surface is reduced, reducing the light sensitivity and affecting the performance of the image sensor. Summary of the invention

[0005] The object of the present invention is to provide a front-illuminated image sensor and a method for forming the same, so as to solve the light loss in the light propagation process of the image sensor in the prior art and improve the sensitivity.

[0006] In order to solve the above technical problems, the present invention provides a method for forming a front-illuminated image sensor, comprising:

[0007] Providing a semiconductor substrate, wherein the semiconductor substrate is formed with a photodiode region corresponding to a pixel unit;

[0008] forming a multi-layer metal interconnection layer on the semiconductor substrate;

[0009] The method for forming at least one metal interconnection layer includes: forming a light-dense medium region above the photodiode region; forming a metal layer around the light-dense medium region; and forming a light-sparse medium region covering the metal layer and the light-dense medium region.

[0010] Optionally, the cross-sectional shape of the optically dense medium region is a regular trapezoid or a triangle.

[0011] Optionally, the refractive index of the optically denser medium is at least 15% greater than the refractive index of the optically rarer medium.

[0012] Optionally, the material of the optically dense medium is one of silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

[0013] Optionally, the material of the optically sparse medium is one of silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

[0014] Optionally, 2 to 4 metal interconnection layers are formed on the semiconductor substrate.

[0015] Optionally, the method for forming the metal interconnection layers from the first layer to the second top layer further includes: forming a through-hole structure connecting adjacent metal interconnection layers in the optically sparse dielectric region.

[0016] Optionally, the method for forming at least one metal interconnect layer includes:

[0017] forming an optically dense medium on the semiconductor substrate, etching the optically dense medium, and retaining a portion of the optically dense medium above the photodiode region;

[0018] forming a metal layer covering the remaining semiconductor substrate and the optically dense medium, etching the metal layer, and retaining a portion of the metal layer around the optically dense medium;

[0019] A light-reducing medium covering the metal layer and the light-dense medium is formed, and the light-reducing medium is chemically and mechanically polished to expose the top of the light-dense medium.

[0020] Optionally, the method for forming the metal interconnection layer from the first layer to the second top layer further includes: etching the optically sparse medium to form a groove exposing a portion of the metal layer, and filling the groove with metal to form a through-hole structure.

[0021] Optionally, the width of the optically dense medium region is less than or equal to the width of the photodiode region.

[0022] Accordingly, another aspect of the present invention further provides a front-illuminated image sensor, comprising:

[0023] A semiconductor substrate having a photodiode region corresponding to a pixel unit formed therein;

[0024] A multi-layer metal interconnection layer formed on the semiconductor substrate;

[0025] At least one metal interconnection layer comprises: a light-dense medium region formed above the photodiode region; a metal layer located around the light-dense medium region; and a light-sparse medium region covering the metal layer and the light-dense medium region.

[0026] Optionally, the cross-sectional shape of the optically dense medium region is a regular trapezoid or a triangle.

[0027] Optionally, the refractive index of the optically denser medium is at least 15% greater than the refractive index of the optically rarer medium.

[0028] Optionally, the material of the optically dense medium is one of silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

[0029] Optionally, the material of the optically sparse medium is one of silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

[0030] Optionally, 2 to 4 metal interconnection layers are formed on the semiconductor substrate.

[0031] Optionally, the first to second top metal interconnection layers further have: a through hole structure formed in the optically sparse dielectric region and connecting adjacent metal interconnection layers.

[0032] Optionally, the width of the optically dense medium region is less than or equal to the width of the photodiode region.

[0033] Compared with the prior art, the front-illuminated image sensor and the method for forming the same in the present invention have the following beneficial effects:

[0034] In the present invention, the method for forming at least one metal interconnect layer includes: forming an optically dense medium region above the photodiode region; forming a metal layer around the optically dense medium region; and forming an optically sparse medium region covering the metal layer and the optically dense medium region. The process method of the present invention is compatible with the current conventional CMOS semiconductor process, has a simple process, and can independently adjust the refractive index, morphology, etc. of the optically dense medium region according to design requirements, and has flexible design.

[0035] In addition, by adding a light-dense dielectric material with a high refractive index to the photodiode area, it acts as a light guide, reducing the loss of light signals caused by reflection and refraction, thereby improving the sensitivity of the image sensor and reducing crosstalk. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flow chart of a method for forming a front-illuminated image sensor according to an embodiment of the present invention;

[0037] Figure 2 is a flow chart of a method for forming a metal interconnection layer in one embodiment of the present invention;

[0038] Figure 3 is a schematic cross-sectional view of a semiconductor substrate in one embodiment of the present invention;

[0039] Figure 4 is a cross-sectional schematic diagram of forming an optically dense medium in one embodiment of the present invention;

[0040] Figure 5 is a cross-sectional schematic diagram of the remaining optically dense medium in one embodiment of the present invention;

[0041] Figure 6 A cross-sectional schematic diagram of forming a metal layer in one embodiment of the present invention;

[0042] Figure 7 is a schematic cross-sectional view of the remaining metal layer in one embodiment of the present invention;

[0043] Figure 8 is a cross-sectional schematic diagram of forming an optically sparse medium in one embodiment of the present invention;

[0044] Fig. 9 is a cross-sectional schematic diagram of the remaining optically sparse medium in one embodiment of the present invention;

[0045] Fig.10 is a cross-sectional schematic diagram of forming a groove in one embodiment of the present invention;

[0046] Fig.11 A cross-sectional schematic diagram of forming a through-hole structure in one embodiment of the present invention;

[0047] Fig.12 FIG. 4 is a cross-sectional schematic diagram of forming a multi-layer metal interconnection line in one embodiment of the present invention. DETAILED DESCRIPTION

[0048] Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar generalizations without violating the connotation of the present invention, so the present invention is not limited to the specific implementation disclosed below.

[0049] Secondly, the present invention is described in detail using schematic diagrams. When describing the embodiments of the present invention in detail, for the convenience of explanation, the schematic diagrams are only examples and should not limit the scope of protection of the present invention.

[0050] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and understandable, the front-illuminated image sensor and the method for forming the same of the present invention are described in detail below with reference to the accompanying drawings.

[0051] refer to Fig.12 As shown, a front-illuminated image sensor provided by the present invention includes: a semiconductor substrate 10, on which a photodiode region 11 corresponding to a pixel unit is formed; a plurality of metal interconnection layers 30, 40, 50 are formed on the semiconductor substrate 10; at least one metal interconnection layer 30, 40 or 50 has: a light-dense medium region 31', 41 or 51 formed above the photodiode region 11; a metal layer 32', 42 or 52 located around the light-dense medium region 31', 41 or 51; and a light-sparse medium region 33', 43 or 53 covering the metal layer and the light-dense medium region.

[0052] refer to Figure 1 As shown, the method for forming a front-illuminated image sensor of the present invention comprises the following steps:

[0053] Execute step S1, refer to Figure 3 As shown, a semiconductor substrate 10 is provided, and a plurality of photodiode (PD) regions 11 corresponding to pixel units (Pixel) are formed on the semiconductor substrate 10. The surface of the semiconductor substrate 10 has an insulating layer 20 and a through-hole structure 21 located in the insulating layer 20, the insulating layer 20 is used to protect the device structure in the semiconductor substrate 10, and the through-hole structure 21 is used to electrically lead out the device structure in the semiconductor substrate 10.

[0054] Execute step S2, refer to Fig.12 As shown, in the back-end process, multiple metal interconnection layers are formed on the semiconductor substrate 10, and 2 to 4 metal interconnection layers are formed on the semiconductor substrate 10. For example, in this embodiment, 3 metal interconnection layers are formed on the semiconductor substrate 10 as an example for explanation, including a first metal interconnection layer 30, a second metal interconnection layer 40, and a third metal interconnection layer (top metal interconnection layer) 50. The method for forming at least one metal interconnection layer includes: forming an optically dense dielectric region above the photodiode region; forming a metal layer around the optically dense dielectric region; and forming an optically sparse dielectric region covering the metal layer and the optically dense dielectric region.

[0055] In the present invention, the refractive index of the optically dense medium is at least 15% greater than that of the optically sparse medium. For example, the refractive index of the optically dense medium is greater than that of the optically sparse medium by 20%. By adding an optically dense medium material with a high refractive index to the photodiode region, it plays the role of a light guide, reduces the light signal loss caused by reflection and refraction in the medium layer of light, thereby improving the sensitivity of the image sensor and reducing crosstalk. Wherein, the material of the optically dense medium is one of silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), and hafnium dioxide (HfO2) or a combination thereof. The material of the optically sparse medium is one of silicon oxide (SiO2), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), and hafnium dioxide (HfO2) or a combination thereof.

[0056] In addition, in order to achieve electrical connection between each layer of metal interconnection layer, the method for forming the metal interconnection layer from the first layer to the next top layer (second layer) also includes: forming a through-hole structure connecting adjacent metal interconnection layers in the optically sparse medium area, and the through-hole structure is electrically connected to the metal layer in the adjacent metal interconnection layer.

[0057] refer to Figure 2 As shown, the method for forming at least one metal interconnect layer specifically includes the following steps:

[0058] Execute step S21, refer to Figure 4 As shown, a light-dense medium 31 is formed on the semiconductor substrate 10, and the light-dense medium 31 is etched to retain a portion of the light-dense medium region 31' above the photodiode region 11. Figure 5 As shown. In this embodiment, conventional photolithography technology can be used to etch the optically dense medium. For example, a photoresist (PH) is formed on the optically dense medium 31. After exposure, development, etching, removal of the photoresist and other steps, an optically dense medium region 31' is formed. The cross-sectional shape of the optically dense medium region 31' is a regular trapezoid or a triangle. For example, the angle of the bottom of the regular trapezoid is 80°, so that after the light enters the optically dense medium region 31', it is reflected back to the optically dense medium region 31' as much as possible on the side wall of the optically dense medium region 31', so that the light enters the photodiode region 11 as much as possible, thereby improving the optical performance of the image sensor. In addition, the width of the optically dense medium region 31' is less than or equal to the width of the photodiode region 11. The material of the optically dense medium is one of silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), hafnium dioxide (HfO2) or a combination thereof. In this embodiment, the optically dense medium 31 is silicon nitride as an example for explanation. The refractive index of silicon nitride is between 1.8 and 2.0.

[0059] Execute step S22, refer to Figure 6As shown, a metal layer 32 covering the remaining semiconductor substrate 10 and the optically dense medium 31' is formed, the metal layer 32 is etched, and a portion of the metal layer 32' around the optically dense medium 31' is retained. The metal layer 32' is located above the through-hole structure 21 and is electrically connected to the through-hole structure 21 to achieve electrical connection with the device structure in the semiconductor substrate 10. Figure 7 shown.

[0060] Execute step S23, refer to Figure 8 As shown, a light-sparse medium 33 is formed to cover the metal layer 32', the light-dense medium 31' and the remaining semiconductor substrate 10. Next, the light-sparse medium 33 is chemically mechanically polished (CMP), a portion of the light-sparse medium 33' is retained, and the light-dense medium 31' is etched to stop and expose the top of the light-dense medium 31'. Fig. 9 The material of the optically sparse medium is one of silicon oxide (SiO2), silicon nitride (SiN), silicon oxynitride (SiON), silicon carbide (SiC), hafnium dioxide (HfO2) or a combination thereof. In this embodiment, the optically sparse medium 33 is silicon oxide as an example for description, and the refractive index of silicon oxide is between 1.4 and 1.6.

[0061] Further, the second metal interconnection layer 40 and the third metal interconnection layer (top metal interconnection layer) 50 are formed on the first metal interconnection layer 30 by the above-mentioned formation method, and a protective layer 60 is formed on the top metal interconnection layer 50. The process method of the present invention is compatible with the current conventional CMOS semiconductor process, and the process is simple. At the same time, the refractive index, morphology, etc. of the optically dense medium region 31', 41, 51 in each metal interconnection layer 30, 40, 50 can be independently adjusted according to the design requirements, that is, the optically dense medium of each layer can be made of materials with different refractive indices, and its morphology can also be different, and the design is flexible, thereby meeting different design requirements. In the present invention, an optically dense medium 31', 41, 51 with a high refractive index is formed on the photodiode region 11, and the refractive index of the optically dense medium 31', 41, 51 is greater than that of the surrounding optically sparse medium 33', 43, 53, and plays the role of a light guide, so that the light forms a total reflection during the propagation process in the dielectric layer of the back-end process, thereby reducing the light signal loss caused by reflection and refraction, improving the sensitivity of the image sensor, and reducing crosstalk.

[0062] In addition, in order to realize the electrical connection of each metal interconnection layer 30, 40, 50, the method for forming the metal interconnection layers 30, 40 from the first layer to the second top layer (the second layer) further includes: etching the optically sparse medium to form a groove exposing a portion of the metal layer, and filling the groove with metal to form a through-hole structure. Taking the first metal interconnection layer 30 as an example, it is also necessary to form a through-hole structure in the optically sparse medium 33'. Specifically, first, refer to Fig.10As shown, the optical thinning dielectric 33' is etched to form a groove 34 exposing a portion of the metal layer 32'. Fig.11 As shown, metal is filled in the groove 34 to form a through-hole structure 35. The optically dense medium 31', the metal layer 32', the optically sparse medium 33' and the through-hole structure 35 together form the first metal interconnection layer 30. The second metal interconnection layer 40 can be formed into a through-hole structure 45 by the same process. Fig.12 The structure of the metal interconnection layer in the present invention will not be described in detail here.

[0063] In summary, the present invention provides a front-illuminated image sensor and a method for forming the same, including: providing a semiconductor substrate, the semiconductor substrate being formed with a photodiode region corresponding to a pixel unit; forming a plurality of metal interconnect layers on the semiconductor substrate; the method for forming at least one metal interconnect layer includes: forming a light-dense medium region above the photodiode region; forming a metal layer around the light-dense medium region; forming a light-sparse medium region covering the metal layer and the light-dense medium region. By adding a light-dense medium material with a high refractive index to the photodiode region, it acts as a light guide, reducing the light signal loss caused by reflection and refraction, thereby improving the sensitivity of the image sensor and reducing crosstalk.

[0064] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and 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 technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for forming a front-illuminated image sensor, characterized in that: include: Providing a semiconductor substrate, wherein the semiconductor substrate is formed with a photodiode region corresponding to a pixel unit; forming a multi-layer metal interconnection layer on the semiconductor substrate; The method for forming at least one metal interconnect layer comprises: forming an optically dense medium region above the photodiode region; forming a metal layer around the optically dense medium region; forming an optically sparse medium region covering the metal layer and the optically dense medium region; The method for forming at least one metal interconnect layer comprises: forming an optically dense medium on the semiconductor substrate, etching the optically dense medium, and retaining a portion of the optically dense medium above the photodiode region; forming a metal layer covering the remaining semiconductor substrate and the optically dense medium, etching the metal layer, and retaining a portion of the metal layer around the optically dense medium; forming an optically sparse medium covering the metal layer and the optically dense medium, and chemically and mechanically grinding the optically sparse medium to expose the top of the optically dense medium; The method for forming the metal interconnection layer from the first layer to the second top layer also includes: etching the optically sparse medium to form a groove exposing a portion of the metal layer, and filling the groove with metal to form a through-hole structure.

2. The method for forming a front-illuminated image sensor according to claim 1, wherein: The cross-sectional shape of the optically dense medium region is a regular trapezoid or a triangle.

3. The method for forming a front-illuminated image sensor according to claim 1, wherein: The refractive index of the optically denser medium is at least 15% greater than the refractive index of the optically rarer medium.

4. The method for forming a front-illuminated image sensor according to claim 1, wherein: The material of the optically dense medium is one of silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

5. The method for forming a front-illuminated image sensor according to claim 1, wherein: The material of the optically sparse medium is one of silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

6. The method for forming a front-illuminated image sensor according to claim 1, wherein: Two to four metal interconnection layers are formed on the semiconductor substrate.

7. The method for forming a front-illuminated image sensor according to claim 1, wherein: The method for forming the metal interconnection layers from the first layer to the second top layer further includes: forming a through-hole structure connecting adjacent metal interconnection layers in the optically sparse dielectric region.

8. The method for forming a front-illuminated image sensor according to claim 1, wherein: The width of the optically dense medium region is less than or equal to the width of the photodiode region.

9. A front-illuminated image sensor, characterized in that: include: A semiconductor substrate having a photodiode region corresponding to a pixel unit formed therein; A multi-layer metal interconnection layer formed on the semiconductor substrate; At least one metal interconnect layer comprises: a light-dense medium region formed above the photodiode region; a metal layer located around the light-dense medium region; and a light-sparse medium region covering the metal layer and the light-dense medium region. An optically dense medium is formed on the semiconductor substrate, and a portion of the optically dense medium above the photodiode region is retained by etching; A metal layer covering the remaining semiconductor substrate and the optically dense medium, and retaining a portion of the metal layer around the optically dense medium by etching; An optically sparse medium covering the metal layer and the optically dense medium, wherein the optically dense medium is exposed at the top by chemical mechanical polishing; The first to second top metal interconnection layers also have: a through hole structure formed in the optically sparse dielectric region to connect adjacent metal interconnection layers.

10. The front-illuminated image sensor according to claim 9, characterized in that: The cross-sectional shape of the optically dense medium region is a regular trapezoid or a triangle.

11. The front-illuminated image sensor according to claim 9, characterized in that: The refractive index of the optically denser medium is at least 15% greater than the refractive index of the optically rarer medium.

12. The front-illuminated image sensor according to claim 9, wherein: The material of the optically dense medium is one of silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

13. The front-illuminated image sensor according to claim 9, characterized in that: The material of the optically sparse medium is one of silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, and hafnium dioxide, or a combination thereof.

14. The front-illuminated image sensor according to claim 9, wherein: Two to four metal interconnection layers are formed on the semiconductor substrate.

15. The front-illuminated image sensor according to claim 9, wherein: The width of the optically dense medium region is less than or equal to the width of the photodiode region.

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