Method for forming backside illuminated image sensor

Abstract

The invention discloses a method for forming a back-illuminated image sensor. The method comprises the following steps of: bonding the front surface of an image sensor wafer with a bearing wafer; a trench isolation structure is formed on the front surface of the wafer; carrying out at least one of the following thinning processes on the back surface of the image sensor wafer so as to improve the flatness of the back surface of the image sensor wafer; the at least one thinning process comprises the following steps of: 1, thinning the back surface of the wafer through a first grinding fluid and / or a second grinding fluid, exposing the trench isolation structure, and removing a part of dielectric layer in the trench isolation structure through etching to form a shallow trench, continuously thinning the back surface of the wafer through the first grinding fluid and / or the second grinding fluid, so that the thinned thickness at least exceeds the depth of the shallow trench; or the process 2: thinning the back surface of the wafer for one time or multiple times through the first grinding fluid and / or the second grinding fluid; the etching selection ratios of the first grinding fluid and the second grinding fluid to the silicon and the dielectric layer are opposite.

Description

Technical Field

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

[0002] In image sensors, as the size of pixel units gradually decreases, the optical crosstalk between adjacent pixels becomes increasingly serious. Therefore, isolation technology between pixels is crucial. Common isolation structures are mainly achieved by first etching deep trenches on the silicon substrate, then filling them with dielectric layers and polysilicon, and thinning the back of the wafer after wafer bonding to completely expose the isolation structure, ultimately achieving isolation between pixels. However, during the back-side thinning process of the isolation structure, due to the difference in material between the dielectric layer and the single-crystal silicon substrate in the isolation structure, different grinding rates will be exhibited during the grinding process (commonly, the grinding rate of the dielectric layer is less than the grinding rate of single-crystal silicon or polycrystalline silicon), which leads to a height difference in the final grinding interface. The height of the silicon substrate in different areas is inconsistent, affecting the photosensitivity of the image sensor. Summary of the Invention

[0003] Based on the above problems, the present invention proposes a method for forming a back-illuminated image sensor, comprising: bonding the front side of an image sensor wafer to a carrier wafer; forming a trench isolation structure on the front side of the image sensor wafer; and subjecting the back side of the image sensor wafer to at least one of the following thinning processes to improve the flatness of the back side of the image sensor wafer; the at least one thinning process comprises: process 1: thinning the back side of the image sensor wafer using a first polishing liquid and / or a second polishing liquid to expose the trench isolation structure, and then removing part of the dielectric layer in the trench isolation structure by etching to form a shallow trench. , the back side of the image sensor wafer is further thinned by the first polishing liquid and / or the second polishing liquid, so that the thinned thickness is at least greater than the depth of the shallow groove, thereby achieving flattening of the back side of the image sensor wafer; or process two: the back side of the image sensor wafer is thinned once or multiple times by the first polishing liquid and the second polishing liquid, thereby achieving flattening of the back side of the image sensor wafer; wherein, for the first polishing liquid, the polishing rate of silicon is greater than the polishing rate of the dielectric layer in the trench isolation structure; for the second polishing liquid, the polishing rate of silicon is less than the polishing rate of the dielectric layer in the trench isolation structure.

[0004] In some embodiments, the process 1 further includes: continuing to thin the back side of the image sensor wafer one or more times using the first polishing liquid and the second polishing liquid to achieve planarization of the back side of the image sensor wafer.

[0005] In some embodiments, the method for forming the trench isolation structure on the front side of the image sensor wafer includes: etching a deep trench on the front side of the image sensor wafer; forming a dielectric layer on the sidewall and bottom of the deep trench; and filling the deep trench by depositing polysilicon to form the trench isolation structure.

[0006] In some embodiments, the dielectric layer includes at least one of silicon oxide, silicon nitride, or silicon oxynitride.

[0007] In some embodiments, the shallow trench is formed by removing a portion of the dielectric layer in the trench isolation structure using at least one of hydrofluoric acid or phosphoric acid.

[0008] In some embodiments, a metal interconnect structure is formed on the front side of the image sensor wafer.

[0009] In some embodiments, the method further includes forming a filter and a microlens on the back side of the image sensor wafer to form a back-illuminated image sensor.

[0010] The present invention also provides a back-illuminated image sensor, which is prepared by the above method.

[0011] Compared with the existing technology, the present invention proposes to remove part of the dielectric layer in the trench isolation structure by etching to form a shallow trench; perform a second thinning on the back side of the image sensor wafer so that the thinned thickness is at least greater than the depth of the shallow trench to improve the flatness of the back side of the image sensor wafer; and proposes to thin the back side of the image sensor wafer using two different polishing liquids to improve the flatness of the back side of the image sensor wafer. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 and Figure 2 The figure is a structural diagram of a back-illuminated image sensor during the formation process according to an embodiment of the present invention.

[0013] Figures 3 to 10 The figure is a structural diagram of a back-illuminated image sensor during the formation process according to an embodiment of the present invention.

[0014] Figures 11 to 17 FIG. 4 is a schematic structural diagram of another back-illuminated image sensor during the formation process according to an embodiment of the present invention. DETAILED DESCRIPTION

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly describes the drawings required for describing the embodiments. Obviously, the drawings described below are merely examples or embodiments of the present invention. Those skilled in the art can apply the present invention to other similar scenarios based on these drawings without inventive effort. Unless otherwise apparent from the context or otherwise noted, the same reference numerals in the figures represent the same structure or operation.

[0016] refer to Figure 1 The image sensor wafer 10 includes a substrate 11. The substrate 11 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 11 can include a substrate with multiple epitaxial layers. In an embodiment of the present invention, the substrate 11 is single crystal silicon. A metal interconnect structure 13 is formed on the front side of the substrate 11. The front side of the image sensor wafer 10 is bonded to a carrier wafer 14. A trench isolation structure 12 is formed on the front side of the image sensor wafer 10. The trench isolation structure 12 includes a dielectric layer 121 and polysilicon 122.

[0017] refer to Figure 2 For back-illuminated image sensors, the backside of the image sensor wafer 10 must be thinned. Due to the material differences between the dielectric layer 121 in the trench isolation structure and the single-crystal silicon substrate 11, different polishing rates occur during polishing, resulting in a height difference h at the final polished interface. This inconsistent height of the silicon substrate in different areas affects the image sensor's photosensitivity.

[0018] An embodiment of the present invention provides a method for forming a back-illuminated image sensor to improve the flatness of the backside of an image sensor wafer after thinning. The method is described below with reference to the accompanying drawings.

[0019] refer to Figure 3 , providing a substrate 21. Substrate 21 can be a doped or undoped semiconductor material, such as silicon, germanium, silicon germanium, silicon germanium on insulator (SGOI), or a combination thereof. Substrate 21 can include a substrate with multiple epitaxial layers. In an embodiment of the present invention, substrate 21 is single crystal silicon.

[0020] refer to Figure 4 , etching the front surface of the substrate 21 to form a deep trench 211. Specifically, the deep trench 211 can be formed by dry etching. This step omits the processes of forming a hard mask on the front surface of the substrate 21, developing, photolithography, etc.

[0021] refer to Figure 5 A dielectric layer 221 and polysilicon 222 are deposited on the sidewalls and bottom of the deep trench 211 to form a trench isolation structure 22. The dielectric layer 221 includes at least one of silicon oxide, silicon nitride, or silicon oxynitride.

[0022] refer to Figure 6 A metal interconnect structure 23 is formed on the front surface of the substrate 21 to form the image sensor wafer 20. The front surface of the image sensor wafer 20 is bonded to the carrier wafer 24.

[0023] refer to Figure 7 , the back side of the image sensor wafer 20 is thinned for the first time, and the trench isolation structure 22 is exposed.

[0024] Specifically, the back side of the image sensor wafer is thinned by a first polishing liquid and / or a second polishing liquid, and the trench isolation structure 22 is exposed; wherein, for the first polishing liquid, the polishing rate of silicon is greater than the polishing rate of the dielectric layer 221 in the trench isolation structure 22; and for the second polishing liquid, the polishing rate of silicon is less than the polishing rate of the dielectric layer 221 in the trench isolation structure 22.

[0025] refer to Figure 8 The dielectric layer 221 in the trench isolation structure 22 is partially removed by etching to form a shallow trench 223. The shallow trench 223 is formed by removing the dielectric layer 221 in the trench isolation structure 22 using at least one of hydrofluoric acid or phosphoric acid.

[0026] refer to Figure 9 The back side of the image sensor wafer 20 is thinned for the second time so that the thinned thickness at least exceeds the depth of the shallow trench, so as to improve the flatness of the back side of the image sensor wafer 20.

[0027] Specifically, the back side of the image sensor wafer is further thinned by the first polishing liquid and / or the second polishing liquid, so that the thinned thickness is at least greater than the depth of the shallow groove 223, thereby achieving flattening of the back side of the image sensor wafer; wherein, for the first polishing liquid, the polishing rate of silicon is greater than the polishing rate of the dielectric layer 221 in the trench isolation structure 22; and for the second polishing liquid, the polishing rate of silicon is less than the polishing rate of the dielectric layer 221 in the trench isolation structure 22.

[0028] Furthermore, the back side of the image sensor wafer can be thinned once or multiple times using the first polishing liquid and / or the second polishing liquid to achieve flattening of the back side of the image sensor wafer; wherein, for the first polishing liquid, the polishing rate of silicon is greater than the polishing rate of the dielectric layer 221 in the trench isolation structure 22; and for the second polishing liquid, the polishing rate of silicon is less than the polishing rate of the dielectric layer 221 in the trench isolation structure 22.

[0029] refer to Figure 10 , a filter 25 and a microlens 26 are formed on the back side 20 of the image sensor wafer to form a back-illuminated image sensor.

[0030] The present invention provides another method for forming a back-illuminated image sensor to improve the flatness of the backside of the image sensor wafer after thinning.

[0031] refer to Figure 11 , providing a substrate 31. Substrate 31 can be a doped or undoped semiconductor material, such as silicon, germanium, silicon germanium, silicon germanium on insulator (SGOI), or a combination thereof. Substrate 31 can include a substrate with multiple epitaxial layers. In an embodiment of the present invention, substrate 31 is single crystal silicon.

[0032] refer to Figure 12 , etching the front surface of the substrate 31 to form a deep trench 311. Specifically, the deep trench 311 can be formed by dry etching. This step omits the processes of forming a hard mask on the front surface of the substrate 31, developing, photolithography, etc.

[0033] refer to Figure 13 A dielectric layer 321 and polysilicon 322 are deposited on the sidewalls and bottom of the deep trench 311 to form a trench isolation structure 32. The dielectric layer 321 includes at least one of silicon oxide, silicon nitride, or silicon oxynitride.

[0034] refer to Figure 14 A metal interconnect structure 33 is formed on the front surface of the substrate 31 to form the image sensor wafer 30. The front surface of the image sensor wafer 30 is bonded to the carrier wafer 34.

[0035] refer to Figure 15 The back of the image sensor wafer 20 is thinned for the first time using the first grinding liquid. The back of the image sensor wafer 20 is not flat due to the first thinning.

[0036] refer to Figure 16 The back side of the image sensor wafer 20 is thinned a second time using a second grinding liquid.

[0037] For the first polishing slurry, the polishing rate of silicon is greater than the polishing rate of the dielectric layer 321 in the trench isolation structure 32 ; for the second polishing slurry, the polishing rate of silicon is less than the polishing rate of the dielectric layer 321 in the trench isolation structure 32 .

[0038] Before thinning the back side of the image sensor wafer 30 for the first time using the first grinding liquid, the process may further include: roughly grinding the back side of the image sensor wafer 30 to remove a portion of the silicon substrate 31 .

[0039] refer to Figure 17 , a filter 35 and a microlens 36 are formed on the back side 30 of the image sensor wafer to form a back-illuminated image sensor.

[0040] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be based on the scope defined by the claims.

Claims

1. A method for forming a back-illuminated image sensor, characterized in that: include: Bonding the front side of the image sensor wafer to the carrier wafer; A trench isolation structure is formed on the front side of the image sensor wafer; Performing at least one of the following thinning processes on the back side of the image sensor wafer to improve the flatness of the back side of the image sensor wafer; The at least one thinning process comprises: Process 1: Thinning the back side of the image sensor wafer using a first polishing slurry and / or a second polishing slurry to expose the trench isolation structure, then removing a portion of the dielectric layer in the trench isolation structure by etching to form a shallow trench, and further thinning the back side of the image sensor wafer using the first polishing slurry and / or the second polishing slurry to a thickness that at least exceeds the depth of the shallow trench, thereby achieving planarization of the back side of the image sensor wafer; or Process 2: thinning the back surface of the image sensor wafer once or multiple times using the first polishing liquid and / or the second polishing liquid to achieve planarization of the back surface of the image sensor wafer; Wherein, for the first polishing liquid, the polishing rate of silicon is greater than the polishing rate of the dielectric layer in the trench isolation structure; for the second polishing liquid, the polishing rate of silicon is less than the polishing rate of the dielectric layer in the trench isolation structure.

2. The method according to claim 1, wherein The process further comprises: The back side of the image sensor wafer is thinned one or more times by continuously using the first polishing liquid and / or the second polishing liquid to achieve planarization of the back side of the image sensor wafer.

3. The method according to claim 1, wherein The method for forming the trench isolation structure on the front side of the image sensor wafer includes: Etching a deep trench on the front side of the image sensor wafer; forming a dielectric layer on the sidewalls and bottom of the deep trench; The deep trench is filled by depositing polysilicon, thereby forming the trench isolation structure.

4. The method according to claim 3, wherein The dielectric layer includes at least one of silicon oxide, silicon nitride or silicon oxynitride.

5. The method according to claim 4, wherein The shallow trench is formed by removing a portion of the dielectric layer in the trench isolation structure using at least one of hydrofluoric acid or phosphoric acid.

6. The method according to claim 5, wherein A metal interconnection structure is formed on the front side of the image sensor wafer.

7. The method according to claim 6, wherein Also includes: Color filters and microlenses are formed on the back side of the image sensor wafer to form a back-illuminated image sensor.

8. A back-illuminated image sensor, characterized in that: Prepared by the method according to any one of claims 1 to 7.

Images