Manufacturing method for solid-state image sensors

Abstract

To provide a manufacturing method of a solid state imaging device, which can obtain an effect of a waveguide independence of an incidence angle of a beam angle.SOLUTION: A manufacturing method of a solid state imaging device which has a plurality of sensor parts that performs a photo-electric conversion of an incident light into an image signal in which an insulation film is formed on a semiconductor substrate front surface, comprises: a step of forming a light conductive path by etching the insulation film of an upper part of each sensor part in a taper state; a step of forming a metal film into an inner part of the light conduct path; a step of forming a hard mask onto the metal film; a step of selectively removing a bottom part of the hard mask; and a step of removing the metal film by an open part of the hard mask.SELECTED DRAWING: Figure 1

Description

Technical Field

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[0001] The present invention relates to a method for manufacturing a solid-state imaging device having an optical waveguide.

Background Art

[0002] In recent years, in photographic cameras, mirrorless cameras that are small, lightweight, and have high performance in terms of image quality and performance are becoming the mainstream instead of single-lens reflex cameras. This mirrorless camera is characterized by a shorter flange back compared to a single-lens reflex camera, and the angle of the light beam incident on the solid-state imaging device tends to be larger than that of a single-lens reflex camera.

[0003] Therefore, there is a back-illuminated solid-state imaging device that relaxes the limitation of the light beam angle by arranging a photodiode on the silicon surface on the opposite side of the wiring layer. However, the back-illuminated solid-state imaging device has a higher manufacturing difficulty and higher cost compared to the surface-type solid-state imaging device.

[0004] Patent Document 1 discloses a solid-state imaging device in which a reflective film having high reflective characteristics is formed in an optical waveguide by forming a metal reflective film on the side wall of the optical waveguide.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The solid-state imaging device disclosed in Patent Document 1 discloses a method of selectively forming aluminum (Al) on the side surface of a waveguide. However, since a seed layer and an underlayer film need to be formed on the side surface of the waveguide and RIE (reactive ion etching) is required, there is a problem that the number of steps increases.

[0007] Furthermore, there is a concern that multiple RIE cycles may cause plasma damage to the photodiode directly beneath the waveguide.

[0008] Furthermore, if a metal film unintentionally forms on the bottom surface of the waveguide, there is no means to remove it, so such solid-state image sensors must be discarded, which presents a problem that affects the yield.

[0009] An etching process is necessary to reliably remove the metal film on the waveguide. During etching, a mask is required to protect the metal film on the sides. Therefore, a challenge arises: the opening is reduced by the thickness of the mask.

[0010] This invention has been made in view of the above circumstances, and aims to provide a solid-state image sensor that can obtain the waveguide effect regardless of the angle of incidence of the light ray. [Means for solving the problem]

[0011] A means for solving the above problem is a method for manufacturing a solid-state image sensor, the solid-state image sensor having a plurality of sensor units that photoelectrically convert incident light into an image signal, and having an insulating film formed on the surface of a semiconductor substrate, the method comprising the steps of: etching the insulating film on the upper part of the sensor unit in 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 the opening of the hard mask. Furthermore, the step of removing the metal film from the hard mask opening involves etching the metal film substantially perpendicular to the semiconductor substrate by anisotropic etching, and then etching the metal film horizontally by isotropic etching. It is characterized by doing so. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a solid-state image sensor that can obtain the waveguide effect regardless of the angle of incidence of the light ray. [Brief explanation of the drawing]

[0013] [Figure 1]Cross-sectional view of a solid-state image sensor showing the film formation process of a metal film / hard mask according to an embodiment of the present invention. [Figure 2] Cross-sectional view of a solid-state image sensor showing the process of forming an opening inside a waveguide according to an embodiment of the present invention. [Modes for carrying out the invention]

[0014] The best mode for carrying out the present invention will be described below with reference to the drawings of this application. However, the present invention is not limited to this embodiment.

[0015] This is an example of applying the manufacturing method for a solid-state image sensor of the present invention. Figures 1 and 2 are cross-sectional views of the solid-state image sensor during the manufacturing process.

[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 a Si substrate), 2 is a wiring layer, 3 is a metal film, 11 is a photodiode, and 31 is a hard mask.

[0017] Figure 1 is a cross-sectional view of a solid-state image sensor showing the deposition process of a metal film and hard mask in this embodiment. Figure 1(a) is a cross-sectional view of a solid-state image sensor in which the manufacturing process up to the wiring layer 2 on the Si substrate 1 has been completed, and the wiring layer 2 above the photodiode 11 has been etched in a tapered shape for waveguide formation. Furthermore, multiple photodiodes 11 are formed on the Si substrate 1, and the photodiodes 11 convert the incident light on the Si substrate 1 into an image signal using photoelectric conversion. Note that the wiring layer 2 has an insulating film between the layers, so the wiring layer 2 corresponds to the insulating film.

[0018] Figure 1(b) is a cross-sectional view of Figure 1(a) after the metal film 3 has been deposited. Since a uniform metal film 3 is required on the waveguide side surface, chemical vapor deposition (CVD) is suitable for depositing the metal film 3. Specifically, when depositing an Al film as the metal film 3, thermal CVD using trimethylaluminum (TMA) as the raw material can be used. The desired Al film thickness is 90 nm to 120 nm.

[0019] Also, by forming a metal film 3 as a wiring layer 2 by physical vapor deposition (PVD: Physical Vapor Deposition) of Al-Cu, it is possible to reduce damage by using a process at a lower temperature compared to thermal CVD. Specifically, when using a thermal CVD process, since the substrate is heated up to nearly 300 to 500 °C, it is considered that thermal stress may occur in the wiring layer 2 or the like. Furthermore, it is also considered that the diffusion of impurities doped in silicon progresses and the shape of the diffusion layer changes from the target. In order to reduce these damages as well, it is desirable to minimize heating during the semiconductor process. However, when using PVD, attention must be paid to the coverage inside the waveguide, and a sputter thickness of about 240 nm is required at the upper end of the waveguide.

[0020] Also, before forming the metal film 3 on the wiring layer 2, a barrier layer for preventing the diffusion of metal ions may be formed. By forming the barrier layer, it is possible to reduce wiring shorts due to the diffusion of metal ions and the formation of defect levels in silicon. Examples of the material of the barrier layer include titanium nitride (TiN). [[ID=*]]

[0021] Furthermore, FIG. 1(c) shows a hard mask 31 formed after the formation of the metal film 3 in FIG. 1(b) described above. The hard mask 31 is formed of, for example, any of a silicon oxide film, a silicon nitride film, aluminum oxide, titanium nitride (TiN), and polysilicon (Poly-Si). It is also necessary to form a film with a uniform thickness on the side surface of the waveguide. Therefore, a CVD method or an atomic layer deposition method (ALD: Atomic Layer Deposition) is suitable as the film formation method. The film thickness of the hard mask 31 is 50 nm to 200 nm. Note that since the hard mask 31 is removed by plasma etching in a subsequent process, it is desirable that the film thickness be thin, but it is necessary to ensure a certain film thickness because it is necessary to protect the metal film 3 of the waveguide from the etching gas.

[0022] Next, FIG. 2 is a cross-sectional view of a 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 performing plasma etching, which is anisotropic etching. When the hard mask 31 is composed of a silicon oxide film or a silicon nitride film, an etching gas containing a fluorine compound is used. When the hard mask 31 is composed of aluminum oxide, TiN, or Poly-Si, an etching gas containing a chlorine compound is used.

[0024] For plasma etching, a capacitively coupled plasma (CCP) etcher that generates plasma by applying RF power to a pair of parallel plates (hereinafter referred to as a CCP plasma etcher) may be used.

[0025] For plasma etching, an electron cyclotron resonance (ECR) plasma etcher that generates plasma by introducing microwaves into an etching chamber (hereinafter referred to as an ECR plasma etcher) may be used.

[0026] The CCP plasma etcher and the ECR plasma etcher 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 an electrode (hereinafter referred to as the lower electrode) on which the wafer to be processed is mounted.

[0027] In addition, the etching gas may contain a gas that serves as a source of carbon (C) and hydrogen (H), such as C2H4 (hereinafter referred to as a sidewall protection material gas), for generating a polymer that protects the side surface of the waveguide during etching. Specifically, gases such as fluorine compound + C2H4 or chlorine compound + C2H4 are used. After etching, as shown in Fig. 2(a), only the hard mask 31 remains on the side surface of the waveguide.

[0028] Here, as shown in Figure 2(a), the waveguide side surface is tapered. Therefore, when ions are incident approximately perpendicularly from top to bottom in Figure 2(a) as anisotropic plasma etching, the hard mask 31 on the waveguide side surface is also partially etched compared to a perfectly perpendicular surface. If a protective film is formed on top of the hard mask, the protective film on the waveguide side surface is also partially etched in the same way.

[0029] Furthermore, since the hard mask 31 has already been etched on the waveguide bottom surface as shown in Figure 1(c), the metal film 3 is etched by plasma etching as shown in Figure 2(a). At this time, if the etching gas contains a sidewall protective material gas, a protective film will also be deposited on the bottom. However, if the etching rate exceeds the deposition rate of the protective film, the metal film 3 will be etched. Simultaneously, a protective film is deposited on the surface of the metal film 3 along the extension of the hard mask 31 located on the waveguide side. If the metal film 3 located on the waveguide side is not protected by a protective film, as shown in Figure 2(b), isotropic etching of the metal film 3 located between the waveguide bottom and the hard mask 31 begins immediately after the etching of the hard mask bottom is completed. As a result, it becomes difficult to control the timing of the start of horizontal etching of the metal film 3 located between the waveguide bottom and the hard mask 31 in the next step, Figure 2(c). (Example 1)

[0030] Next, Figure 2(b) shows the metal film 3 exposed in the hard mask opening removed by anisotropic etching after the opening has been formed. When the metal film 3 is made of aluminum, an etching gas containing a chlorine compound is used. The etching gas may contain a sidewall protective material gas. Furthermore, the etching of the hard mask 31 and the 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 anisotropic etching of the metal film 3, the horizontal planes of the metal film 3 remaining under the hard mask 31 are removed by isotropic etching as shown in Figure 2(b), as shown in Figure 2(c).

[0032] Isotropic etching may also be performed by stopping or reducing the bias high-frequency power on the lower electrode side during plasma etching. In this case, it can be performed continuously with plasma etching, thus reducing the number of steps.

[0033] Isotropic etching can also be performed by stopping the supply of the sidewall protective material gas. As mentioned above, since the waveguide sidewalls are tapered, the protective film on the sidewalls is etched even when ions are incident approximately perpendicularly. Therefore, stopping the supply of the sidewall protective material gas causes the protective film on the sidewalls to disappear, and isotropic etching is achieved. In this case as well, it can be performed continuously with plasma etching, thus reducing the number of steps.

[0034] Subsequently, a transparent material is embedded inside the waveguide hole. The transparent material can be the same material as the hard mask 31 or a transparent organic resin with a high refractive index. (Example 2)

[0035] Furthermore, as shown in Figure 2(b), after forming the opening in the hard mask 31, the metal film 3 in the opening and the horizontal plane of the metal film 3 under the hard mask 31 are removed by isotropic etching. Using plasma etching to etch the metal film 3 may cause plasma damage to the semiconductor layer under the waveguide, but using wet etching can suppress the plasma damage caused by using plasma etching.

[0036] Next, a transparent material is embedded inside the waveguide hole. The transparent material can be the same material as the hard mask or a transparent organic resin with a high refractive index.

[0037] Furthermore, in any 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 will function as a protective film for the metal film 3 if left unremoved and the subsequent processes proceed. If the material is not transparent, removal is desirable.

[0038] From the above, the solid-state image sensor of the present invention reliably removes obstacles on the optical waveguide by etching the metal film 3 after the formation of the hard mask 31, and further suppresses the reduction of the aperture, thereby obtaining the waveguide effect regardless of the incident angle of the light ray.

[0039] Although the solid-state image sensor in this embodiment has been described based on a front-illuminated solid-state image sensor, it can also be applied to a back-illuminated solid-state image sensor. [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 image sensor having multiple sensor units that photoelectrically convert incident light into an image signal, and having an insulating film formed on the surface of a semiconductor substrate, The process of forming an optical waveguide by tapering the insulating film on the upper part of the sensor portion, The process of forming a metal film inside the optical waveguide, A step of forming a hard mask on the metal film, A step of selectively removing the bottom portion of the hard mask, A step of removing the metal film from the hard mask opening. It has, The step of removing the metal film from the hard mask opening is: A method for manufacturing a solid-state image sensor, characterized by etching the metal film substantially perpendicular to the semiconductor substrate by anisotropic etching, and then etching the metal film horizontally by isotropic etching.

2. The method for manufacturing a solid-state image sensor according to claim 1, characterized in that the anisotropic etching is etching using a gas containing a fluorine compound or a chlorine compound.

3. The method for manufacturing a solid-state image sensor according to claim 2, characterized in that the anisotropic etching is performed using an etching gas and a protective material gas containing carbon (C) and hydrogen (H) elements.

4. The method for manufacturing a solid-state image sensor according to claim 3, characterized in that the isotropic etching is performed by stopping the supply of the protective material gas.

5. A method for manufacturing a solid-state image sensor having a plurality of sensor units that photoelectrically convert incident light into an image signal, and having an insulating film formed on the surface of a semiconductor substrate, The process of forming an optical waveguide by tapering the insulating film on the upper part of the sensor portion, The process of forming a metal film inside the optical waveguide, A step of forming a hard mask on the metal film, A step of selectively removing the bottom portion of the hard mask, A step of removing the metal film from the hard mask opening. It has, The step of removing the metal film from the hard mask opening is: A method for manufacturing a solid-state image sensor, characterized by isotropic etching.

6. The method for manufacturing a solid-state image sensor according to claim 5, characterized in that the isotropic etching is wet etching.

7. The method for manufacturing a solid-state image sensor according to claim 5, characterized in that the isotropic etching involves stopping or reducing the bias high-frequency power in plasma etching.

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