A backside illumination image sensor forming method and image sensor

Abstract

The application provides a back-illuminated image sensor forming method, comprising: before forming a metal interconnection layer of a first surface of the image sensor, performing at least twice temporary bonding processes, and forming a photodiode on the first surface, a functional transistor structure, and a deep trench isolation structure of a second surface opposite to the first surface, so as to relax a thermal budget limit of a process of the second surface, and improve an interface quality of the deep trench isolation structure of the second surface. Through the above scheme, the problem of insufficient thermal budget in the process of the back surface of the image sensor is solved, and the performance of the image sensor is improved. In addition, all devices are prepared in the same semiconductor substrate, and the temporary bonding carrier can be debonded and reused, thereby saving the process cost.

Description

Technical Field

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

[0002] With the continuous development of CMOS image sensor technology, back-illuminated image sensors have gradually become mainstream due to their superior performance. In a back-illuminated image sensor, light enters from the back of the sensor, eliminating the need for a front metal interconnect layer. Microlenses, color filters, and other structural layers can be placed close to the photodiode, significantly shortening the distance between the incident light and the photodiode. This reduces light loss and crosstalk, thereby significantly improving the sensitivity and other performance characteristics of the image sensor.

[0003] In current mainstream back-illuminated image sensor fabrication methods, the silicon wafer is typically flipped after the front-side processes (such as ion implantation, transistors, and metal interconnect layers) are completed, and then further thinned before the back-side processes are performed. Since the metal interconnect layer has already been fabricated at this point, the high-temperature steps in the back-side processes can easily cause the diffusion of metals (such as copper) within the interconnect layer, increasing leakage current and potentially leading to device failure. It can be seen that the metal interconnect layer formed during the front-side processes typically severely limits the thermal budget for the back-side processes; therefore, the temperature of conventional back-side processes generally does not exceed 500°C.

[0004] However, deep trench etching (BDTI) is often required in back-side fabrication for isolation and pinning. Thermal budget constraints also limit the repair process of the BDTI etching interface, hindering improvements in dark current and white point performance. Furthermore, epitaxy, boron diffusion, and polysilicon deposition are frequently used in image sensor fabrication to enhance device performance; however, these processes often exceed 500°C, making them difficult to use in conventional image sensor back-side fabrication due to thermal budget limitations of the metal interconnect layers.

[0005] Patent CN 115548037 A mentions that the problem of insufficient thermal budget during deep trench fabrication can be solved by placing the back-side deep trench process as the first step. While this approach can improve the thermal budget for interface repair after the deep trench process and reduce the impact of dark current, the wafer must undergo a front-side process at a temperature as high as 1100°C after bonding with the first carrier during fabrication. This requires extremely high bonding strength, making temporary bonding difficult. Subsequent debonding necessitates the physical or chemical removal of the first carrier, thus preventing its reuse and resulting in high costs. Summary of the Invention

[0006] The purpose of this invention is to provide a method for forming a back-illuminated image sensor, comprising: Before forming the metal interconnect layer on the first surface of the image sensor, at least two temporary bonding processes are performed to form a photodiode, a functional transistor structure on the first surface, and a deep trench isolation structure on the second surface opposite to the first surface, so as to relax the thermal budget constraints of the second surface process and improve the interface quality of the deep trench isolation structure on the second surface.

[0007] Preferably, the process of performing at least two temporary bonding operations includes: After the source and drain doping of the functional transistors on the first surface of the semiconductor substrate is activated, the first surface of the semiconductor substrate is temporarily bonded to the first carrier. The deep trench isolation structure is formed on the second surface of the semiconductor substrate; The second surface of the semiconductor substrate is temporarily bonded to the second carrier. The temporary bond between the first surface of the semiconductor substrate and the first carrier is released.

[0008] Preferably, after releasing the temporary bond between the first surface of the semiconductor substrate and the first carrier, the method further includes: A metal interconnect layer is formed on the first surface of the semiconductor substrate; The first surface of the semiconductor substrate is bonded to the third carrier. Release the temporary bond between the second surface of the semiconductor substrate and the second carrier. A microlens structure is formed on the second surface of the semiconductor substrate.

[0009] Preferably, after the source / drain doping activation of the functional transistor formed on the first surface, the method further includes: A silicide barrier layer is formed on the first surface; A first dielectric layer is deposited on the silicide barrier layer and then planarized to a preset height.

[0010] Preferably, the deep trench isolation structure forming the second surface includes: According to the preset photolithography pattern, the second surface is etched to form the first trench; A second dielectric layer is formed on the surface of the first trench.

[0011] Preferably, after forming the first trench, the method further includes: A first epitaxial layer is formed in the first trench using an epitaxial process.

[0012] Preferably, after forming the second dielectric layer, the method further includes: Boron-doped material is diffused into the first trench using a diffusion process.

[0013] Preferably, after forming the second dielectric layer, the method further includes: A polycrystalline semiconductor material is filled into the first trench to form a polycrystalline layer.

[0014] Preferably, forming a microlens structure on the second surface of the semiconductor substrate includes: A high dielectric constant layer is formed on the second surface; A metal grid is formed on the second surface according to the pixel unit position; A color filter is formed by filling the metal grid with material. A microlens structure is formed on the color filter.

[0015] Preferably, the at least two temporary bonding processes include: A matte layer and / or a reflective layer are formed on the surfaces to be bonded; Set a light-emitting layer; When it is necessary to unbond, the temporary bond is released by irradiating the light-releasing layer with a laser to avoid damaging the substrate.

[0016] The present invention also provides an image sensor formed using the back-illuminated image sensor forming method described above.

[0017] This invention addresses the problem of insufficient thermal budget in the back-side fabrication process of image sensors through the aforementioned solution. Before fabricating the metal interconnect layer, the front side of the semiconductor substrate is temporarily bonded to a first carrier wafer. Afterward, the silicon wafer is flipped and thinned, and partial back-side processes are performed, such as deep trench etching, epitaxy, boron diffusion, polycrystalline semiconductor deposition, oxidation, and annealing. The back side of the semiconductor substrate is then temporarily bonded to a second carrier wafer, and the bond to the first carrier wafer is released before fabricating the metal interconnect layer. Subsequently, the front side is temporarily bonded to a third carrier wafer and debonded from the second carrier wafer before proceeding with subsequent back-side fabrication of the image sensor. Since the temperature tolerance of the temporary bond exceeds that of the metal interconnect layer, the thermal budget for the back-side fabrication of the image sensor is increased, providing room for improving image sensor performance through processes such as epitaxy, boron diffusion, annealing, and oxidation. Furthermore, all devices are fabricated on the same semiconductor substrate, and the temporarily bonded carrier wafers can be debonded and reused, significantly reducing process costs. Attached Figure Description

[0018] 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.

[0019] Figure 1 This is a schematic flowchart illustrating the formation process of a back-illuminated image sensor in one embodiment of the present invention. Figures 2-12This is a schematic diagram of the structure of the back-illuminated image sensor in different embodiments of the present invention.

[0020] Throughout the figures, the same or similar reference numerals denote the same or similar devices (modules) or steps. Detailed Implementation

[0021] The purpose of this invention is to provide a method for forming a back-illuminated image sensor to solve the problem of insufficient thermal budget in the back-side processing of image sensors in existing processes. Specifically, before forming the metal interconnect layer M1 on the first surface A of the image sensor, at least two temporary bonding processes are performed to form a photodiode, a functional transistor structure, and a deep trench isolation structure on the second surface B opposite to the first surface A. This relaxes the thermal budget constraints of the second surface B process and improves the interface quality of the deep trench isolation structure on the second surface B.

[0022] By employing temporary bonding, this invention advances some of the back-side processing of the image sensor to before the formation of the metal interconnect layer, thus solving the problem of insufficient thermal budget in the back-side processing. Furthermore, with the thermal budget increased, processes such as epitaxy, diffusion, oxidation, and annealing can be performed on the back-side of the image sensor to improve its performance.

[0023] In a preferred embodiment, during the temporary bonding process, a matte layer and / or a reflective layer are first formed on the surface to be bonded; then a light-emitting layer is provided. Preferably, the light-emitting layer can be one or more of organic materials, organometallic materials, inorganic materials, etc. When it is necessary to unbond, the temporary bonding can be released by irradiating the light-emitting layer with a laser to avoid damaging the substrate.

[0024] In one alternative implementation, such as Figure 1 As shown, the step of performing at least two temporary bonding processes in this invention includes: Step S100: After the source and drain doping activation of the functional transistor on the first surface A of the semiconductor substrate 100, the first surface A of the semiconductor substrate 100 is temporarily bonded to the first carrier 200, such as... Figure 2 As shown; Step S200: The deep trench isolation structure 110 is formed on the second surface B of the semiconductor substrate 100, such as... Figure 6 As shown; Step S300: Temporarily bond the second surface B of the semiconductor substrate 100 to the second carrier 300, such as... Figure 7 As shown; Step S400: Release the temporary bond between the first surface A of the semiconductor substrate 100 and the first carrier 200, such as... Figure 8As shown.

[0025] Preferably, after the source and drain doping activation of the functional transistor located on the first surface A is formed in step S100, the following steps can be performed first: Step S110: Form a silicide barrier layer 131 on the first surface A; Step S120: Deposit a first dielectric layer 132 on the silicide barrier layer 131 and perform a planarization process to a preset height.

[0026] Similarly, in a preferred embodiment, the following steps can be used when forming the deep trench isolation structure 110 of the second surface B in step S200: Step S210: According to the preset photolithography pattern, etch the second surface B to form the first trench 111, as shown below. Figure 3 As shown; Step S220: Form a second dielectric layer 112 on the surface of the first trench 111, such as Figure 5 As shown.

[0027] Furthermore, after forming the first trench 111 in step S210, a first epitaxial layer 113 can be formed within the first trench 111 using an epitaxial process, such as... Figure 4 As shown.

[0028] Since the metal interconnect layer M1 on the first surface A has not yet been formed during the above steps, the temperature during the process can exceed 500°C, up to 700°C, which the temporary bonding can withstand, greatly improving the thermal budget. Therefore, after step S220, optionally, some high-temperature processes can be performed to supplement and repair the structure within the deep trench, for example, boron-doped material can be diffused into the first trench 111 using a diffusion process; or polycrystalline semiconductor material can be filled into the first trench 111 to form a polycrystalline layer 114, such as... Figure 6 As shown. Further optimization of image sensor performance.

[0029] Optionally, based on this, after unbonding in step S400, the following steps can be performed: Step S500: A metal interconnect layer M1 is formed on the first surface A of the semiconductor substrate 100, such as... Figure 9 As shown; Step S600: Bond the first surface A of the semiconductor substrate 100 to the third carrier 400, such as... Figure 10 As shown; Step S700: Release the temporary bond between the second surface B of the semiconductor substrate 100 and the second carrier 300, such as... Figure 11 As shown; Step S800: A microlens structure 120 is formed on the second surface B of the semiconductor substrate 100, such as... Figure 12 As shown.

[0030] Preferably, the formation of the microlens structure 120 in step S800 can specifically take the following steps: Step S810: Form a high dielectric constant layer 121 on the second surface B; Step S820: Form a metal grid 122 on the second surface B according to the pixel unit position; Step S830: Fill the metal grid 122 with material to form a color filter 123; Step S840: Form a microlens structure 120 on the color filter 123.

[0031] The present invention also provides an image sensor formed using the back-illuminated image sensor forming method described above.

[0032] 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 plural. 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: Before forming the metal interconnect layer on the first surface of the image sensor, at least two temporary bonding processes are performed to form a photodiode, a functional transistor structure on the first surface, and a deep trench isolation structure on the second surface opposite to the first surface, so as to relax the thermal budget constraints of the second surface process and improve the interface quality of the deep trench isolation structure on the second surface.

2. The method for forming a back-illuminated image sensor as described in claim 1, characterized in that, The process of performing at least two temporary bonding processes includes: After the source and drain doping of the functional transistors on the first surface of the semiconductor substrate is activated, the first surface of the semiconductor substrate is temporarily bonded to the first carrier. The deep trench isolation structure is formed on the second surface of the semiconductor substrate; The second surface of the semiconductor substrate is temporarily bonded to the second carrier. The temporary bond between the first surface of the semiconductor substrate and the first carrier is released.

3. The method for forming a back-illuminated image sensor as described in claim 2, characterized in that, After releasing the temporary bond between the first surface of the semiconductor substrate and the first carrier, the method further includes: A metal interconnect layer is formed on the first surface of the semiconductor substrate; The first surface of the semiconductor substrate is bonded to the third carrier. Release the temporary bond between the second surface of the semiconductor substrate and the second carrier. A microlens structure is formed on the second surface of the semiconductor substrate.

4. The method for forming a back-illuminated image sensor as described in claim 1, characterized in that, After the source and drain doping activation of the functional transistor formed on the first surface, the method further includes: A silicide barrier layer is formed on the first surface; A first dielectric layer is deposited on the silicide barrier layer and then planarized to a preset height.

5. The method for forming a back-illuminated image sensor as described in claim 1, characterized in that, The deep trench isolation structure forming the second surface includes: According to the preset photolithography pattern, the second surface is etched to form the first trench; A second dielectric layer is formed on the surface of the first trench.

6. The method for forming a back-illuminated image sensor as described in claim 5, characterized in that, After the formation of the first trench, the method further includes: A first epitaxial layer is formed in the first trench using an epitaxial process.

7. The method for forming a back-illuminated image sensor as described in claim 5, characterized in that, After forming the second dielectric layer, the method further includes: Boron-doped material is diffused into the first trench using a diffusion process.

8. The method for forming a back-illuminated image sensor as described in claim 5, characterized in that, After forming the second dielectric layer, the method further includes: A polycrystalline semiconductor material is filled into the first trench to form a polycrystalline layer.

9. The method for forming a back-illuminated image sensor as described in claim 1, characterized in that, The formation of the microlens structure on the second surface of the semiconductor substrate includes: A high dielectric constant layer is formed on the second surface; A metal grid is formed on the second surface according to the pixel unit position; A color filter is formed by filling the metal grid with material. A microlens structure is formed on the color filter.

10. The method for forming a back-illuminated image sensor as described in claim 1, characterized in that, The at least two temporary bonding processes include: A matte layer and / or a reflective layer are formed on the surfaces to be bonded; Set a light-emitting layer; When it is necessary to unbond, the temporary bond is released by irradiating the light-releasing layer with a laser to avoid damaging the substrate.

11. An image sensor, characterized in that, It is formed using the back-illuminated image sensor formation method as described in claims 1 to 10.

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