JP2008078326A - Solid-state imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress shading of a solid-state imaging device in which a micro lens for condensing light on a light receiving element arranged in array on a light receiving surface is provided on each light receiving element. <P>SOLUTION: A single convex film 6, having a form of convex lens, that covers a light receiving surface 1a is provided on a micro lens 5. The refractive index of the convex film 6 is lower than that of the micro lens 5. The incident light 2c that enters the perimeter of the light receiving surface 1a is refracted into vertical direction by the convex film 6 to enter almost vertically in the micro lens 5. Due to the convex film 6, the micro lens 5 becomes a long focus one. For this reason, a wiring 4 formed above the perimeter of the light receiving element 3 shields less incident light, resulting in less shading. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid-state image sensor that collects light using a microlens on light-receiving elements arranged in an array on a light-receiving surface of a substrate, and more particularly to a low-shading solid-state image sensor.

  2. Description of the Related Art A solid-state imaging device in which light receiving elements are arranged in an array on a light receiving surface of a semiconductor substrate is widely used by being incorporated in a camera of a mobile terminal such as a digital still camera or a mobile phone. With the miniaturization of these portable terminals, the cameras have also been miniaturized, and at the same time, the solid-state imaging device has been miniaturized. Such downsizing of the solid-state imaging device results in a reduction in sensitivity due to the reduction of the light receiving element as a result of reducing the pixel area.

  In order to suppress the decrease in sensitivity due to the reduction of the light receiving element, a microlens is disposed on the light receiving element, and a solid-state imaging element that collects light incident on the pixel with the microlens and enters the light receiving element is used. . Hereinafter, the structure of a conventional solid-state imaging device using such a microlens will be described.

  FIG. 3 is a cross-sectional view of a conventional solid-state image sensor, and shows the structure of a solid-state image sensor using microlenses. FIG. 3A shows the relationship between the solid-state imaging device 13 and incident light. FIGS. 3B to 3D show first to third solid-state imaging devices having different structures, respectively. 3 (b) to 3 (d), the center (the center with respect to the left and right) is the element structure of the portion located at the center of the light receiving surface 1a shown in FIG. The element structure of the part located in the periphery of the light-receiving surface 1a shown to 3 (a) is represented.

  Referring to FIG. 3A, a light receiving surface 1a on which a light receiving element 3 is disposed is provided on a substrate 1 constituting a solid-state imaging device 13, and a periphery for driving the light receiving element 3 is provided around the light receiving surface 1a. A circuit (not shown) is provided. Light that has passed through a camera lens (not shown), which is a camera lens, becomes incident light 2a and 2c emitted from one point and enters the light receiving surface 1a. Of the incident light 2a and 2c, the incident light 2a incident near the center of the light receiving surface 1a enters the light receiving surface 1a substantially perpendicularly. However, since the incident light 2c incident near the periphery of the light receiving surface 1a is incident obliquely on the light receiving surface 1a, shading occurs as described below.

  Referring to FIG. 3B, in the first solid-state imaging device, the light receiving elements 3 are formed in an array shape, for example, a matrix shape, on the light receiving surface 1a, and an insulating interlayer film 7 is formed on each light receiving element 3. A microlens 5 is formed via the. The microlens 5 is arranged so that the central axis 5a (optical axis) of the microlens 5 passes through the center of the corresponding light receiving element 3. That is, the microlens 5 is disposed immediately above the light receiving element 3.

  In the interlayer film 7, wiring 4 formed in multiple layers around the light receiving element 3 is provided. The wiring 4 functions as a light shielding film, and incident light 2a and 2c are transmitted through a transparent window 4a surrounded by the wiring 4 and are incident on the light receiving element. At this time, since the incident light 2c incident on the light receiving element 3 near the periphery of the light receiving surface 1a is incident obliquely on the surface of the substrate 1, the incident light 2c is incident on a position shifted from the center of the light receiving element 3. . As a result, shading occurs in which a part of the incident light 2 c is shielded by the wiring 4. When the light receiving dimension of the light receiving element 3 (planar dimension of the light receiving region) is reduced, the height of the wiring (aspect ratio of the window 4a) with respect to the light receiving dimension is increased, and therefore shading is further increased. As a result, a large sensitivity distribution is generated in the light receiving surface 1a.

  When the exit pupil distance (distance between the camera lens and the light receiving surface 1a) is shortened in order to reduce the size of the camera, the incident light 2c incident on the peripheral portion enters more obliquely, so that the shading is further increased. For this reason, downsizing of the camera and the solid-state imaging device is limited.

  In order to suppress the shading caused by the oblique incidence, a method has been devised in which the microlens 5 is displaced from the center of the light receiving element 3.

  Referring to FIG. 3C, in the second solid-state imaging device, the center of the microlens 5 and the light-receiving device 3 coincide with each other in the central portion of the light-receiving surface 1a as in the first solid-state imaging device. However, in the peripheral portion of the light receiving surface 1a, the micro lens 5 is disposed so that the central axis 5a (optical axis) of the micro lens 5 passes through the center of the light receiving surface 1a from the center of the corresponding light receiving element 3. That is, the microlens 5 is disposed at a position shifted from the position immediately above the light receiving element 3 toward the center side.

  In the second solid-state imaging device, incident light 2c incident obliquely on the peripheral portion is condensed by the microlens 5 at a position shifted to the center side from the first solid-state imaging device. Thus, since the incident light 2c is condensed near the center of the light receiving element 3, the periphery of the incident light 2c is less likely to be shaded by the wiring 4.

  However, in the second solid-state imaging device, since the microlens 5 is disposed obliquely above the light receiving element 3, the condensed incident light 2 c is incident on the surface of the light receiving element 3 obliquely. For this reason, when the wiring 4 is thick, a part of the incident light 2c is shielded by the wiring 4 and shading tends to occur.

  Further, when the wiring 4 is thick and the window 4a has a large aspect ratio, or when the interlayer film 7 is thick, it is necessary to use a long-focus microlens 5. The long-focus microlens 5 can be manufactured by a thin microlens having a small curvature or a microlens made of a material having a small refractive index.

However, since the microlens 5 uses resin reflow or gradation exposure (exposure that gradually changes the exposure amount from the center to the periphery of the microlens) in the manufacturing process, the shape formed by reflow or gradation exposure is limited. Therefore, it is difficult to precisely produce a thin microlens with a small curvature. Further, since the material of the microlens 5 is usually a transparent inorganic material such as SiO ₂ or a transparent resin, the material surface is restricted even if the refractive index is reduced. For this reason, it is difficult to produce the long-focus microlens 5.

  In order to solve the difficulty of shading incident light 2c incident obliquely and producing a long-focus microlens, a third solid-state imaging device in which a flat film 9 having a low refractive index is formed on the microlens 5 has been proposed. . (For example, refer to Patent Document 1).

  Referring to FIG. 3D, in the third solid-state imaging device, a flat film 9 having a flat upper surface made of a material having a lower refractive index than the microlens 5 is formed on the microlens 5 of the second solid-state imaging device. Provided. Incident light 2c obliquely incident on the light receiving element 3 in the periphery of the light receiving surface 1a is refracted on the surface of the flat film 9, and has a refraction angle smaller than an incident angle (an angle formed between the vertical direction of the incident surface and the incident light 2c). The light travels through the interlayer film 9 at an angle between the direction perpendicular to the incident surface and the refracted incident light 2 c, and is collected by the microlens 5.

  In this structure, since the incident light 2c is refracted by the flat film 9 in a direction approaching the vertical direction, the refracted incident light 2c has an incident angle smaller than the incident angle to the flat film 9 (that is, more with respect to the light receiving surface). Incidently on the surface of the microlens 5. For this reason, shading caused by oblique incidence can be reduced as compared with the case without the flat film 9.

Furthermore, since the surface of the microlens 5 (the surface on which light is incident) is in contact with the flat film 9, the effective refractive index of the microlens 5 (the difference in refractive index between the microlens 5 and the external medium) is reduced. As a result, the focal length of the microlens 5 becomes longer. That is, it functions as a long focus microlens. Accordingly, even if the window 4a having a high aspect ratio or the thick correlation film 7 is used, high condensing efficiency is realized. Therefore, even when applied to a camera using a camera lens having a short exit pupil distance, a decrease in sensitivity is reduced. Can do.
JP-A-2005-57024

  As described above, by providing a flat film with a low refractive index on the microlens, shading caused by oblique incidence of incident light and shortening of the exit pupil distance of the camera is suppressed, and the camera and the solid-state imaging device can be made compact. The accompanying sensitivity reduction can be suppressed.

  However, it is difficult to make the incident light sufficiently perpendicularly incident on the microlens only by the change in the traveling direction of the incident light due to the flat film, and shading caused by the oblique incidence on the microlens cannot be sufficiently suppressed. Furthermore, in order to suppress shading caused by oblique incidence, the microlens must be shifted from the position directly above the light receiving element to the center of the light receiving surface in the periphery of the light receiving surface, and precise alignment is required. Manufacturing is difficult.

  In the present invention, even in a solid-state imaging device in which the exit pupil distance is short and incident light is incident obliquely on the light receiving surface, sufficient shading is achieved by allowing the incident light to the microlens to enter the light receiving surface sufficiently perpendicularly. An object of the present invention is to provide a solid-state imaging device that is suppressed by the above.

  In order to solve the above-described problems, a solid-state imaging device of the present invention includes a light receiving element arranged on a light receiving surface defined on a substrate, a microlens provided on each of the light receiving elements, and a microlens. And a convex film having a lower refractive index than the microlens that is provided and covers the light receiving surface in a convex lens shape.

  In the solid-state imaging device of the present invention, a convex film is provided on the microlens to cover the light receiving surface like a convex lens. Incident light incident obliquely on the light receiving surface is refracted in a direction approaching the light receiving surface by the convex lens-like convex film and is incident on the microlens. Since this convex film has the function of a convex lens, it refracts incident light more vertically than a flat film. Therefore, since incident light is incident on the microlens more perpendicularly than the flat film, shading caused by oblique incidence is more effectively suppressed.

  In addition, since the difference in refractive index with the convex film covering the microlens is small, the focal length of the microlens is increased, and shading is reduced even when a camera lens having a high aspect ratio window and a short exit pupil distance is used.

  The convex film described above should have a lower refractive index than the microlens so that the microlens does not lose its function as a condenser lens. From this point of view, the refractive index of the convex film is preferably 0.1 or more smaller than the refractive index of the microlens. On the other hand, it is desirable that the refractive index be large so that incident light is refracted greatly even by a convex film having a small curvature. From this viewpoint, the refractive index of the convex film is preferably within 0.2 from the refractive index of the microlens.

  The material of the microlens and the convex film may be an inorganic material or an organic material as long as it is transparent. The organic material is excellent in that it can be easily manufactured by reflow or gradation exposure. When an organic material is used, a phenolic resin having a high refractive index, for example, a refractive index of 1.5 to 1.7 is used for the microlens, and an acrylic resin having a refractive index of, for example, 1.4 to 1.5 is used for the convex film. Resin can be used.

  The convex film of the present invention described above has a convex lens-like top surface shape with the center of the light receiving surface as the optical axis so that incident light obliquely incident on the periphery of the light receiving surface approaches the vertical direction. Here, the convex lens-shaped top surface shape means that incident light radiated on the light receiving surface from one point on the vertical line (which coincides with the optical axis of the convex lens shape) passing through the center of the light receiving surface is parallel to the vertical line. It refers to the convex top shape that approaches. Such a convex lens-shaped upper surface shape is a rotational surface whose upper surface is convex upward with the optical axis of the microlens 5 as a rotation axis, and includes, for example, a spherical surface, an elliptical surface, a parabolic surface, or a combination of these surfaces. In addition, the case where a part is a flat surface is included.

  Such a convex film may be a convex film having a surface having a uniform curvature (spherical surface) from the center to the periphery of the light receiving surface or a surface in which the curvature gradually increases in the vicinity of the periphery. Further, the central portion of the light receiving surface can be a flat surface (curvature is 0) or a convex surface having a small curvature, and the peripheral portion of the light receiving surface can be a convex surface having a large curvature, that is, a curved surface that is convex toward the imaging side. The convex film having a small curvature at the central portion can increase the curvature of the peripheral edge when the convex film having the same thickness is used as compared with the case where the curvature is changed uniformly or gradually. For this reason, since the incident light is largely refracted in the vertical direction particularly at the peripheral portion where the incident angle becomes large, shading of the peripheral portion can be effectively suppressed.

  In the solid-state imaging device of the present invention, as described above, incident light is incident from a direction near to the microlens, so that the microlens can be disposed immediately above the light receiving device. For this reason, there is little shading resulting from oblique incidence. In the present invention, the present invention is not limited to this, and the microlens can be arranged at a peripheral portion of the light receiving surface while being shifted from directly above the light receiving element toward the center of the light receiving surface. In this configuration, since the amount of refraction of incident light required for the convex film may be small, the convex film can be easily manufactured.

  As described above, according to the present invention, incident light incident obliquely on the periphery of the light receiving surface is refracted in a direction near the vertical by the convex lens-like convex film and is incident on the microlens. A low shading camera can also be configured by combining a solid-state imaging device having a light receiving element window with a camera lens having a short exit pupil distance.

  The first embodiment of the present invention relates to a solid-state imaging device using a CMOS sensor.

  FIG. 1 is a cross-sectional view of a first embodiment of the present invention, showing a cross-sectional structure of a solid-state image sensor. FIG. 1A shows the relationship between the solid-state imaging device 13 and incident light. FIG. 1 (b) is an enlarged view of the vicinity of the light receiving element of the solid-state image sensor, in order from the right to the left of the paper surface, in the vicinity of the light receiving element located at the left end of the light receiving surface and in the center of the light receiving surface. The vicinity of the light receiving element located in the vicinity and the right end of the light receiving surface is shown.

  With reference to FIG. 1A, in the solid-state imaging device 10 according to the first embodiment, a rectangular light receiving surface of 2 mm × 1.5 mm is formed on the surface of a square silicon substrate 1 of 3.5 mm × 3.5 mm. 1a is defined, and 640 × 480 light receiving elements 3 are arranged in an array on the light receiving surface. In a camera incorporating the present solid-state imaging device 10, a camera lens (not shown) is disposed immediately above the light receiving surface 1a so that its optical axis coincides with a vertical line passing through the center of the light receiving surface 1a. Therefore, the incident light 2a incident on the center of the light receiving surface 1a through the camera lens is incident on the light receiving surface 1a perpendicularly. On the other hand, the incident light 2c incident on the periphery of the light receiving surface 1a through the camera lens enters the light receiving surface 1a obliquely with a large incident angle.

  Referring to FIG. 1B, the light receiving element 3 is composed of a photodiode formed on the surface of the light receiving surface 1a. In a region of the light receiving surface 1a where the light receiving element 3 is not formed, a transistor (not shown) constituting a CMOS circuit for driving the light receiving element 3 is formed.

  An interlayer film 7 having a thickness of 6 μm is formed on the substrate 1, and the wiring 4 of the CMOS circuit is embedded in the interlayer film 7. The interlayer film 7 is made of a transparent insulating film, and can include an insulating film forming a color filter in addition to an insulating film forming a CMOS circuit. The wiring 4 is composed of, for example, 3 to 5 layers of multilayer wiring, and is disposed so as to surround the periphery of the light receiving element 3. The wiring 4 forms a light shielding portion that shields incident light 2a and 2c. Therefore, a transparent window 4a for the incident light 2a and 2c, which is filled with the light-transmitting interlayer film 7 and shielded by the wiring 4 around the light-receiving layer 3, is formed.

  Microlenses 5 are formed on the upper surface of the interlayer film 7. One microlens 5 is disposed immediately above each light receiving element 3 so that the optical axis 5 a of the microlens 5 passes through the center of the light receiving element 3. Therefore, the microlenses 5 are arranged in the same array as the light receiving elements 3.

  The microlens 5 is made of a phenol resin having a refractive index of 1.5 to 1.7, for example, 1.6. A convex film 6 is formed on the interlayer film 7 so as to cover the microlens 5. Of course, as the material of the microlens 5, other commonly used materials can be used.

  The convex film 6 is made of an acrylic resin having a refractive index of 1.4 to 1.5, for example, 1.5, and has a convex lens shape that covers the entire surface of the light receiving surface 1a. In the first embodiment, a convex film 6 formed by reflowing a rectangular resin pattern covering the light receiving surface 1a, or a convex shape formed by gradation exposure and development of the resin film covering the light receiving surface 1a. Membrane 6 was used. The convex film 6 formed by reflow or gradation exposure is excellent in that it is easy to manufacture.

  In the solid-state imaging device 10 of the first embodiment, the incident light 2a and 2c are first refracted by the convex film 6 in a direction approaching parallel to the central axis of the light receiving surface 1a (perpendicular to the light receiving surface 1a). Is done. At this time, since the upper surface of the convex film 6 is inclined closer to the periphery of the light receiving surface 1a than to the central portion, the incident light 2c is bent largely at the peripheral portion, and as a result, at an angle close to the perpendicular to the microlens 5. Incident. Thus, by using the convex film 6, the incident light 2c can be incident on the microlens 5 at an angle closer to the vertical than in the case of the flat film.

  Increasing the refractive index of the convex film 6 to refract the incident light 2a, 2c is preferable in order to make the incident light 2c incident on the microlens 5 at an angle close to perpendicular. However, if the refractive index of the convex film 6 is large and too close to the refractive index of the microlens 5, the focal point of the microlens 5 becomes too long, which is not preferable. In this embodiment, the refractive index of the convex film is 0.1 to 0.2 smaller than the refractive index of the microlens 5. By using the convex film 6 having a refractive index in such a range, the focal length of the microlens 5 can be easily controlled to an appropriate range.

  The solid-state imaging device 10 according to the first embodiment described above can be manufactured by the following steps.

  Referring to FIG. 1, first, light receiving elements 3, p-type transistors, and n-type transistors (not shown) are formed in an array on the upper surface of a silicon substrate 1. Next, the multilayer wiring 4 constituting the CMOS circuit is formed together with a part (lower layer) of the multilayer film 7. Next, a color filter (not shown) is formed as a part of the multilayer film 7 to form the multilayer film 7 having a flat upper surface.

  Next, the microlens 5 is formed on the upper surface of the multilayer film 7. The microlens 5 can be formed by forming a resin film pattern defining the microlens 5 by exposure and development, and reflowing the resin film pattern. Or you may form by exposing and developing the photosensitive resin film using gradation exposure. Furthermore, as is well known, it can also be formed by etching the lower transparent material layer using a convex resin film formed by reflow or gradation exposure as a mask. In this method, an appropriate transparent material can be used as the material for the convex film.

  The above manufacturing process is a process normally used for manufacturing a conventional solid-state imaging device.

  Next, the convex film 6 is formed. The convex film 6 can be formed by forming a rectangular resin film covering the light receiving surface 1a on the microlens 5 and reflowing the resin film into a convex lens shape. Alternatively, the convex film 6 having a convex lens shape may be formed by gradation exposure of the photosensitive resin film. Furthermore, by using the resin film formed by reflow or gradation exposure as a mask, the underlying transparent material film can be etched to form the convex film 6 made of the transparent material film.

  The solid-state imaging device 10 according to the first embodiment is manufactured through the above steps.

  FIG. 2 is a cross-sectional view of a second embodiment of the present invention, showing a cross-sectional structure of a solid-state image sensor. FIG. 2A shows the relationship between the solid-state imaging device 11 and incident light. FIG. 2 (b) is an enlarged view of the vicinity of the light receiving element of the solid-state image sensor, in order from the right to the left of the paper surface, in the vicinity of the light receiving element located at the left end of the light receiving surface and the center of the light receiving element. This represents the vicinity of the light receiving element located near the right end of the light receiving surface.

  2nd Embodiment of this invention is related with the solid-state image sensor 11 from which the shape of the convex film | membrane 6 of the solid-state image sensor 10 which concerns on 1st Embodiment, and the arrangement position of the micro lens 5 differ.

  In the solid-state imaging device 11 according to the second embodiment, referring to FIG. 2, the central surface of the convex film 6 has a substantially flat surface or a flat surface close to a flat surface, and a peripheral portion of the light receiving surface 1 a. The microlens 5 to be arranged is different from the first embodiment in that the microlens 5 is arranged so as to be shifted from the center of the light receiving element 3 to the center. Other configurations are the same as those of the first embodiment.

  In the second embodiment, since the convex film 6 is flat at the center portion, even if the convex film has the same thickness as that of the first embodiment, a large curvature surface 6b having a large curvature is formed in the peripheral portion. Accordingly, the incident light 2c incident obliquely on the periphery can be refracted in the vertical direction to a greater extent than in the first embodiment. For this reason, shading is effectively prevented.

  Furthermore, since the microlens 5 is arranged on the center side from directly above the light receiving element 3, the incident light 2c refracted by the convex film 6 does not enter the vertical direction sufficiently, and even if it is incident at a slight angle, the incident light 2c is obliquely incident. The resulting shading is suppressed.

  In addition, when the incident light 2c can be incident on the microlens 5 sufficiently vertically by the convex film 6, the microlens 5 may be disposed immediately above the light receiving element 3 also in the second embodiment. In the second embodiment, since the large curvature surface 6b of the convex film 6 is disposed on the peripheral portion of the light receiving surface 1a, the incident light 2c incident on the peripheral portion can be easily incident on the microlens 5 vertically. . For this reason, even if the microlens 5 is disposed immediately above the light receiving element 3, shading of the peripheral portion is effectively suppressed.

  By applying the present invention to a solid-state image sensor of a small camera incorporated in a portable device, a small and low shading camera can be realized.

Sectional drawing of 1st Embodiment of this invention Sectional drawing of 2nd Embodiment of this invention Cross-sectional view of a conventional solid-state image sensor

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 1a Light-receiving surface 2a, 2c Incident light 3 Light-receiving element 4 Wiring 5 Micro lens 5a Central axis 6 Convex film 6a Flat surface 6b Large curvature surface 7 Interlayer film 9 Flat film 10, 11, 13 Solid-state image sensor

Claims (5)

In a solid-state imaging device having a light receiving element arranged on a light receiving surface defined on a substrate, and a microlens provided on each of the light receiving elements,
A solid-state imaging device comprising a convex film provided on the microlens and covering the light receiving surface in a convex lens shape and having a refractive index lower than that of the microlens.   2. The solid-state imaging device according to claim 1, wherein the convex film has a curved surface that is flat at a central portion of the light receiving surface and is convex toward an imaging side at a peripheral portion of the light receiving surface.   The solid-state imaging device according to claim 1, wherein the microlens is arranged so that a central axis of the microlens passes through a center of the light receiving element.   4. The solid-state imaging device according to claim 1, wherein a difference in refractive index between the microlens and the convex film is 0.1 or more and 0.2 or less. The refractive index of the microlens is 1.5 or more and 1.7 or less,
5. The solid-state imaging device according to claim 1, wherein a refractive index of the convex film is 1.4 or more and 1.5 or less.

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