Manufacturing method of solid state imaging device
Patent Information
- Application Number
- JP2022072104
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2022-04-26
- Publication Date
- 2025-01-20
- Estimated Expiration
- 2042-04-26
AI Technical Summary
The existing methods for manufacturing solid-state image sensors with optical waveguides face issues such as increased number of steps, potential plasma damage to photodiodes, and yield loss due to unintentional metal film formation on the waveguide bottom, leading to reduced opening size and higher costs.
A manufacturing method involving etching an insulating film into a tapered shape, forming a metal film inside the waveguide, and using a hard mask to selectively remove the bottom portion and metal film, while employing chemical vapor deposition and plasma etching to minimize damage and ensure uniform coverage.
This method allows for a solid-state imaging device that functions effectively regardless of light beam incident angle, reducing plasma damage and maintaining aperture size, thus improving yield and reducing manufacturing complexity.
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Abstract
Description
[Technical Field]
[0001] The present invention relates to a method for manufacturing a solid-state imaging device having an optical waveguide. [Background technology]
[0002] In recent years, mirrorless cameras have become mainstream, replacing single-lens reflex cameras. These cameras are compact and lightweight, yet offer high image quality and performance. Mirrorless cameras are characterized by a shorter flange focal length than single-lens reflex cameras, and the angle of light incident on the solid-state image sensor tends to be larger than that of single-lens reflex cameras.
[0003] To address this issue, back-illuminated solid-state imaging devices can be used, which alleviate the limitations on the light angle by placing the photodiode on the silicon surface opposite the wiring layer. However, back-illuminated solid-state imaging devices are more difficult to manufacture and more expensive than front-illuminated solid-state imaging devices.
[0004] Patent Document 1 discloses a solid-state imaging device in which a metal reflective film is formed on the sidewall of an optical waveguide to form a reflective film with high reflectivity inside the optical waveguide. [Prior art documents] [Patent documents]
[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-268575 Summary of the Invention [Problem to be solved by the invention]
[0006] The solid-state imaging device disclosed in Patent Document 1 discloses a method for selectively depositing an aluminum (Al) film on the side surface of a waveguide. However, this method has the problem of increasing the number of steps, since it requires depositing a seed layer, an underlayer, and RIE (reactive ion etching) on the side surface of the waveguide.
[0007] Furthermore, there is a concern that multiple RIE processes may cause damage to the photodiode directly below the waveguide due to plasma.
[0008] Furthermore, if a metal film is unintentionally formed on the bottom surface of the waveguide, there is no means to remove it, and therefore such a solid-state imaging device must be discarded, which has the problem of affecting the yield.
[0009] An etching process is required to completely remove the metal film on the waveguide. During etching, a mask is required to protect the metal film on the side. Therefore, there is a problem that the opening is reduced by the thickness of the mask.
[0010] The present invention has been made in view of the above circumstances, and has as its object to provide a solid-state imaging device that can obtain the effect of a waveguide regardless of the incident angle of a light beam. [Means for solving the problem]
[0011] The means for solving the above problem is a method for manufacturing a solid-state imaging device, which has a plurality of sensor portions that photoelectrically convert incident light into image signals and in which an insulating film is formed on a surface of a semiconductor substrate, and which is characterized by comprising the steps of: etching the insulating film above the sensor portions into a tapered shape to form an optical waveguide; forming a metal film inside the optical waveguide; forming a hard mask on the metal film; selectively removing the bottom of the hard mask; and removing the metal film from openings in the hard mask. [Effects of the Invention]
[0012] According to the present invention, it is possible to provide a solid-state imaging device that can obtain the effect of a waveguide regardless of the incident angle of a light beam. [Brief explanation of the drawings]
[0013] [Figure 1] 1A and 1B are cross-sectional views of a solid-state imaging device showing a process for forming a metal film / hard mask according to an embodiment of the present invention; [Figure 2] 5A and 5B are cross-sectional views of a solid-state imaging device showing a process of forming an opening inside a waveguide according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION
[0014] The best mode for carrying out the present invention will be described below with reference to the drawings of the present application, but the present invention is not limited to this embodiment.
[0015] 1 and 2 are cross-sectional views of a solid-state imaging device in the manufacturing process according to an embodiment of the present invention.
[0016] In this embodiment, 1 is a semiconductor substrate, and the semiconductor substrate used in this embodiment is a silicon (Si) single crystal substrate (hereinafter referred to as Si substrate), 2 is a wiring layer, 3 is a metal film, 11 is a photodiode, and 31 is a hard mask.
[0017] FIG. 1 is a cross-sectional view of a solid-state imaging device showing the metal film / hard mask deposition process of this embodiment. Also, FIG. 1(a) is a cross-sectional view of a solid-state imaging device in which fabrication up to the wiring layer 2 has been completed on a Si substrate 1, and the wiring layer 2 above a photodiode 11 has been tapered and etched to form a waveguide. Furthermore, a plurality of photodiodes 11 are formed on the Si substrate 1, and the photodiodes 11 photoelectrically convert light incident on the Si substrate 1 into an image signal. The wiring layer 2 has a structure in which an insulating film is provided between layers, and therefore the wiring layer 2 corresponds to an insulating film.
[0018] FIG. 1(b) is a cross-sectional view of FIG. 1(a) after a metal film 3 has been formed. Chemical vapor deposition (CVD) is a suitable method for forming the metal film 3, since it is required to form a uniform metal film 3 on the side surface of the waveguide. Specifically, when forming an Al film as the metal film 3, thermal CVD using trimethylaluminum (TMA) as a raw material can be used. In this case, the Al film thickness is preferably 90 nm to 120 nm.
[0019] Additionally, by depositing Al-Cu as the metal film 3 on the wiring layer 2 using physical vapor deposition (PVD), a lower temperature process can be used compared to thermal CVD, which is expected to reduce damage. Specifically, when using a thermal CVD process, the substrate is heated to temperatures approaching 300-500°C, which may cause thermal stress in the wiring layer 2, etc. Furthermore, the diffusion of impurities doped into the silicon may progress, causing the shape of the diffusion layer to change from the target. To reduce this damage, it is desirable to keep heating during semiconductor processing to a minimum. However, when using PVD, care must be taken to cover the inside of the waveguide, and a sputtering thickness of approximately 240 nm is required at the top end of the waveguide.
[0020] Furthermore, before forming the metal film 3 on the wiring layer 2, a barrier layer may be formed to prevent the diffusion of metal ions. By forming the barrier layer, it is possible to reduce the risk of wiring shorts and the formation of defect levels in silicon due to the diffusion of metal ions. Titanium nitride (TiN) can be used as the material for the barrier layer.
[0021] Furthermore, FIG. 1(c) shows a hard mask 31 formed after the metal film 3 of FIG. 1(b) has been formed. The hard mask 31 is formed, for example, from silicon oxide, silicon nitride, aluminum oxide, titanium nitride (TiN), or polysilicon (Poly-Si). It must be formed with a uniform thickness on the waveguide side as well. Therefore, CVD or atomic layer deposition (ALD) are suitable deposition methods. The hard mask 31 has a thickness of 50 nm to 200 nm. Note that a thin hard mask 31 is desirable because it will be removed by plasma etching in a subsequent process. However, a certain thickness is required to protect the waveguide metal film 3 from etching gas.
[0022] Next, FIG. 2 is a cross-sectional view of the solid-state imaging device showing the process of forming an opening inside the waveguide.
[0023] As shown in FIG. 2(a), an opening is formed at the bottom of the hard mask 31 formed in FIG. 1(c) to expose the metal film 3. Specifically, the hard mask 31 is removed by plasma etching, which is anisotropic etching. When the hard mask 31 is made of a silicon oxide film or a silicon nitride film, an etching gas containing a fluorine compound is used, and when the hard mask 31 is made of aluminum oxide, TiN, or Poly-Si, an etching gas containing a chlorine compound is used.
[0024] For plasma etching, a capacitance coupled plasma etcher (hereinafter referred to as a CCP plasma etcher) may be used, which applies RF power to a pair of parallel plates to generate plasma.
[0025] For plasma etching, an electron cyclotron resonance plasma etcher (hereinafter referred to as an ECR plasma etcher) may be used, which generates plasma by introducing microwaves into an etching chamber.
[0026] CCP plasma etchers and ECR plasma etchers can control the ion energy incident on the wafer by controlling the high frequency power (hereinafter referred to as bias high frequency power) applied to the electrode (hereinafter referred to as lower electrode) on which the wafer to be processed is mounted.
[0027] The etching gas may also contain a gas (hereinafter referred to as a sidewall protection material gas) that serves as a source of carbon (C) and hydrogen (H), such as C2H4, to generate a polymer that protects the sidewall of the waveguide during etching. Specifically, a gas such as a fluorine compound + C2H4 or a chlorine compound + C2H4 is used. After etching, as shown in Figure 2(a), a hard mask 31 remains only on the sidewall of the waveguide.
[0028] 2(a), the side surface of the waveguide has a tapered shape, and therefore when ions are incident approximately perpendicularly from top to bottom in plasma etching, which is anisotropic etching, the hard mask 31 on the side surface of the waveguide is also partially etched compared to a completely vertical surface. If a protective film is further formed on the hard mask, the protective film on the side surface of the waveguide is also partially etched in the same manner.
[0029] Since the hard mask 31 has already been etched from the bottom of the waveguide in FIG. 1(c), the metal film 3 is etched by the plasma etching in FIG. 2(a). If the etching gas contains a sidewall protection material gas, a protective film is also deposited on the bottom. However, under conditions where the etching rate exceeds the deposition rate of the protective film, the metal film 3 is etched. At the same time, a protective film is deposited on the surface of the metal film 3 on the side of the waveguide, which is an extension of the hard mask 31. If the metal film 3 on the side of the waveguide is not protected by a protective film, isotropic etching of the metal film 3 located between the bottom of the waveguide and the hard mask 31 begins immediately after etching of the hard mask bottom, as shown in FIG. 2(b). As a result, it becomes difficult to control the start timing of horizontal etching of the metal film 3 located between the bottom of the waveguide and the hard mask 31 in the next process, FIG. 2(c). Example 1
[0030] Next, FIG. 2(b) shows the metal film 3 exposed in the opening after the hard mask opening is formed and then removed by anisotropic etching. When the metal film 3 is made of aluminum, an etching gas containing a chlorine compound is used. The etching gas may also contain a sidewall protection material gas. Furthermore, etching of the hard mask 31 and etching of the metal film 3 may be performed consecutively in a single etching process. In this case, the number of processes can be reduced.
[0031] After the metal film 3 has been anisotropically etched, the horizontal surfaces of the metal film 3 remaining under the hard mask 31 in FIG. 2(b) are removed by isotropic etching, as shown in FIG. 2(c).
[0032] The isotropic etching may be performed by stopping or reducing the bias high frequency power on the lower electrode side during the plasma etching. In this case, the isotropic etching can be performed consecutively with the plasma etching, thereby reducing the number of steps.
[0033] Isotropic etching may be performed by stopping the supply of the sidewall protection material gas. As described above, since the waveguide sidewalls have a tapered shape, the protective film on the sidewalls is etched even when ions are incident at a substantially normal angle. Therefore, when the supply of the sidewall protection material gas is stopped, the protective film on the sidewalls disappears, and isotropic etching is achieved. In this case, the number of steps can be reduced because it can be performed consecutively with plasma etching.
[0034] Thereafter, a transparent material is filled into the waveguide hole, which may be the same material as the hard mask 31 or a transparent organic resin with a high refractive index. Example 2
[0035] 2(b), after forming the opening in the hard mask 31, isotropic etching is performed to remove the horizontal surfaces of the metal film 3 in the opening and the metal film 3 below the hard mask 31. If plasma etching is used to etch the metal film 3, plasma damage may occur to the semiconductor layer below the waveguide, but wet etching can be used to suppress the plasma damage caused by plasma etching.
[0036] The waveguide holes are then filled with a transparent material, which can be the same material as the hard mask or a transparent organic resin with a high refractive index.
[0037] In either etching method, the hard mask may be removed by wet etching or the like before embedding the transparent material inside the waveguide. If the hard mask is made of a transparent material, it can be left unremoved and the subsequent steps can proceed, allowing it to function as a protective film for the metal film 3. If the material is not transparent, it is preferable to remove it.
[0038] From the above, in the solid-state imaging device of the present application, obstacles on the optical waveguide are reliably removed by etching the metal film 3 after forming the hard mask 31, and further, by suppressing the reduction of the opening, the effect of the waveguide can be obtained regardless of the incident angle of the light beam.
[0039] Although the solid-state imaging device of the present embodiment has been described based on a front-illuminated solid-state imaging device, it can also be applied to a back-illuminated solid-state imaging device. [Explanation of symbols]
[0040] 1. Semiconductor substrate (Si substrate) 2 wiring layers 3 Metal Film 11 Photodiode 31 Hard Mask
Claims
1. A method for manufacturing a solid-state imaging device having a plurality of sensor units that photoelectrically convert incident light into an image signal, the method comprising forming an insulating film on a surface of a semiconductor substrate, the method comprising the steps of: a step of forming an optical waveguide by etching the insulating film above the sensor portion into a tapered shape; forming a metal film inside the optical waveguide; forming a hard mask on the metal film; selectively removing a bottom portion of the hard mask; removing the metal film from the hard mask opening; 2. A method for manufacturing a solid-state imaging device comprising the steps of:
2. The step of removing the metal film from the hard mask opening includes:
2. The method for manufacturing a solid-state image pickup device according to claim 1, wherein the metal film is etched substantially perpendicularly to the semiconductor substrate by anisotropic etching, and then the metal film is etched horizontally by isotropic etching.
3. 3. The method for manufacturing a solid-state image pickup device according to claim 2, wherein the anisotropic etching is performed using a gas containing a fluorine compound or a chlorine compound.
4. The step of removing the metal film from the hard mask opening includes:
2. The method for manufacturing a solid-state image pickup device according to claim 1, wherein the etching is isotropic etching.
5. 5. The method for manufacturing a solid-state image sensor according to claim 4, wherein the isotropic etching is wet etching.
6. 5. The method for manufacturing a solid-state image pickup device according to claim 4, wherein the isotropic etching is performed by stopping or reducing bias high frequency power in plasma etching.
7. 4. The method for manufacturing a solid-state image pickup device according to claim 3, wherein the anisotropic etching uses an etching gas and a protective material gas containing carbon (C) and hydrogen (H) elements.
8. 8. The method for manufacturing a solid-state image pickup device according to claim 7, wherein the supply of the protective material gas is stopped during the isotropic etching.