Solid-state imaging element

Abstract

A solid-state imaging element (1) is a solid-state imaging element comprising a microlens array (MLA) in which a plurality of regularly arranged microlenses (ML) are provided. In each microlens, a central part (CP) including an optical axis (AX) is formed with a first curvature radius (r1), and a peripheral part (PP) around the central part is formed with a second curvature radius (r2) smaller than the first curvature radius.

Description

solid-state imaging device

[0001] The present disclosure relates to a solid-state imaging device.

[0002] 2. Description of the Related Art A solid-state imaging device is known that includes a microlens array in which a plurality of microlenses are arranged periodically (see, for example, Japanese Patent Application Laid-Open No. 2003-129998).

[0003] When strong light is incident on this type of microlens array, a magenta diffraction pattern may appear due to the diffraction phenomenon, which is called petal flare.

[0004] JP 2013-77740 A

[0005] To suppress petal flare, it is conceivable to form an anti-reflection coating on the microlens, as described in Patent Document 1. However, in this case, an additional step of forming the anti-reflection coating is required. Also, for example, it is difficult to remove the anti-reflection coating once it has been formed (i.e., rework processing), so if a defect occurs in the coating formation, the product immediately becomes defective.

[0006] In view of the above circumstances, an embodiment of the present disclosure has an object to provide a solid-state imaging device that is suitably configured to suppress petal flare.

[0007] A solid-state imaging device according to an embodiment of the present disclosure includes a microlens array including a plurality of periodically arranged microlenses, each of which has a central portion including an optical axis formed with a first radius of curvature and a peripheral portion surrounding the central portion formed with a second radius of curvature smaller than the first radius of curvature.

[0008] According to an embodiment of the present disclosure, there is provided a solid-state imaging device that is suitably configured to suppress petal flare.

[0009] FIG. 1 is a cross-sectional schematic diagram of a solid-state imaging element according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing a pixel array of a solid-state imaging element according to an embodiment of the present disclosure. FIG. 3 is a diagram illustrating a configuration of a microlens according to an embodiment of the present disclosure. FIG. 4 is a diagram illustrating a microlens according to an embodiment of the present disclosure. FIG. 5 is a diagram illustrating a microlens according to Example 1 of the present disclosure. FIG. 6 is a diagram illustrating a microlens according to Example 2 of the present disclosure. FIG. 7 is a diagram illustrating a microlens according to Example 3 of the present disclosure. FIG. 8 is a diagram illustrating a microlens according to Example 4 of the present disclosure. FIG. 9 is a graph showing the intensity of reflected diffracted light in each example of the present disclosure. FIG. 10 is a schematic diagram illustrating, by way of example, a method for manufacturing a microlens array according to an embodiment of the present disclosure. FIG. 11 is a schematic diagram illustrating, by way of example, another method for manufacturing a microlens array according to an embodiment of the present disclosure.

[0010] The following description relates to a solid-state imaging device according to an embodiment of the present disclosure. Common or corresponding elements are denoted by the same or similar reference numerals, and redundant descriptions are appropriately simplified or omitted. Note that in this specification, the notation "to" includes the lower limit and upper limit. For example, values ​​N1 to N2 indicate a range in which the lower limit is value N1 and the upper limit is value N2.

[0011] 1 and 2 are diagrams showing the configuration of a solid-state imaging device 1 according to an embodiment of the present disclosure. Fig. 1 is a schematic cross-sectional view of the solid-state imaging device 1. Fig. 2 is a schematic diagram showing a pixel array of the solid-state imaging device 1. For convenience, only a portion of the solid-state imaging device 1 is shown in both Fig. 1 and Fig. 2.

[0012] In the following description, two mutually orthogonal directions are referred to as the X direction and the Y direction. The direction orthogonal to both the X direction and the Y direction is referred to as the Z direction. That is, the X, Y, and Z directions are orthogonal to each other. The three-axis coordinate system of XYZ is a left-handed system. For convenience, the axes extending in the X direction, Y direction, and Z direction may be referred to as the X axis, Y axis, and Z axis, respectively. The X direction may also be referred to as the horizontal direction or the left-right direction. The Y direction may also be referred to as the vertical direction or the up-down direction. The Z direction may also be referred to as the height direction or the front-back direction. Note that the names of directions are used for convenience to explain the relative positional relationships of components and do not indicate absolute directions. For example, depending on the orientation of the device, the X direction may not necessarily be the left-right direction but may be the up-down direction.

[0013] The solid-state imaging device 1 is, for example, a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor, and is mounted in imaging devices such as video cameras, digital cameras, and smartphones.

[0014] 1, the solid-state imaging device 1 includes a semiconductor substrate 10. A plurality of photoelectric conversion elements 12 are arranged on the semiconductor substrate 10. The photoelectric conversion elements 12 are arranged in a matrix on the semiconductor substrate 10.

[0015] A partition wall 14 is formed on the semiconductor substrate 10. The partition wall 14 is formed in a wall shape and separates the photoelectric conversion elements 12. The partition wall 14 is formed so as to surround the outer periphery of each photoelectric conversion element 12 in a plan view.

[0016] The space defined by the partition wall 14 is filled with a sealing resin 16. The sealing resin 16 is a transparent resin that has a high transmittance at least in the visible light region. Examples of the material for the sealing resin 16 include acrylic resin, polyurethane resin, polyester resin, polyolefin resin, polycarbonate resin, polyethyleneimine resin, epoxy resin, and thioether resin.

[0017] The solid-state imaging device 1 includes a color filter 20. The color filter 20 is, for example, an on-chip color filter in a Bayer array in which red filters 20R, green filters 20G, and blue filters 20B are arranged in a mosaic pattern. The color filter 20 may be another filter, such as a complementary color filter. In FIG. 2 , reference numeral 20W denotes a partition wall between the red filters 20R, green filters 20G, and blue filters 20B.

[0018] The microlens array MLA is disposed on the color filter 20. The microlens array MLA is formed of, for example, an organic material such as acrylic resin, epoxy resin, or silicon resin, or an inorganic material such as SiN or SiO 2 .

[0019] A plurality of microlenses ML are formed in the microlens array MLA. The microlenses ML are arranged in a matrix. The microlenses ML are arranged for each pixel. That is, the solid-state imaging device 1 is configured such that one microlens ML is arranged for each pixel.

[0020] The solid-state imaging device 1 may have a QPD structure, in which one microlens ML and one filter (either a red filter 20R, a green filter 20G, or a blue filter 20B) are arranged for every four pixels arranged in two rows and two columns.

[0021] In the solid-state imaging device 1, not all pixels need to have a QPD structure. That is, a configuration including four pixels with a QPD structure and at least one microlens ML falls within the scope of the present invention. For example, the green filter 20G is configured as a QPD structure, and the red filter 20R and the green filter 20G are configured as a normal structure having one hemispherical microlens per pixel.

[0022] A plurality of flat optical elements, such as a seal glass 32, an optical low-pass filter 34, and an IR cut filter 36, are arranged on the microlens array MLA.

[0023] Light from an imaging lens (not shown) passes through the IR cut filter 36, the optical low-pass filter 34, and the seal glass 32 in this order, and is then incident on the microlens ML. The light incident on the microlens ML is collected by the microlens ML, passes through a filter (one of the red filter 20R, the green filter 20G, and the blue filter 20B), and is received by the photoelectric conversion element 12. The photoelectric conversion element 12 accumulates the received optical image as an electric charge according to the amount of light, and outputs an image signal.

[0024] Here, we will explain petal flare. Petal flare is a flare that appears in the shape of petals spaced apart around the optical axis of a microlens. When strong light is incident on a solid-state imaging device, the light is reflected at an angle to the optical axis by the microlens, and when it is reflected by a sealing glass or the like and then incidents on the pixel again, color mixing occurs and petal flare appears. The periodic, dense arrangement of microlenses causes a diffraction phenomenon, and petal flare, a petal-shaped diffraction pattern, appears around the strong light.

[0025] Petal flare can be suppressed by suppressing reflected light and reflected diffracted light in the microlenses. However, forming an anti-reflection coating on the microlenses as in the conventional method requires an additional process as described above, and has disadvantages such as poor reworkability. Note that "reflected diffracted light" is light diffracted by reflected light of perpendicular incident light that is perpendicularly incident on a microlens array in which microlenses are periodically arranged, and is light diffracted by reflected light that is reflected in a direction other than the perpendicular direction (a direction oblique to the optical axis) in the microlenses.

[0026] Therefore, in this embodiment, an anti-reflection coating is not formed on the microlenses ML, and the microlenses ML are formed in a shape suitable for suppressing reflected and diffracted light, thereby making it possible to suppress petal flare while avoiding an increase in the number of processes and a decrease in reworkability.

[0027] FIG. 3 is a diagram illustrating a microlens ML according to this embodiment. The microlens L1 shown in FIG. 3 is formed in an elliptical hemispherical shape with a curvature typical of a microlens for a solid-state imaging device. The microlens L2 shown in FIG. 3 is an elliptical spherical microlens with a curvature greater than that of the microlens L1. In FIG. 3, the entire area indicated by the solid line and the dashed dotted line is an elliptical hemisphere. That is, in the schematic diagram of FIG. 3, the microlenses L1 and L2 have a shape in which a portion of an elliptical hemisphere has been cut out.

[0028] As shown in Fig. 3, the microlens ML according to this embodiment has a shape that combines microlenses L1 and L2. By forming a two-stage shape by adding microlens L2 to microlens L1, more light with a large angle of incidence is allowed to pass through compared to a general microlens shape (for example, a shape of microlens L1 alone). Here, the microlens ML acts as a reflective diffraction grating. The condition under which light reinforces each other in a reflective diffraction grating is expressed by the following equation:

[0029] a [sin(θm) + sin(θi)] = mλ a: pitch λ: wavelength of light θm: angle of emergence of diffracted light θi: angle of incidence of light m: 0, 1, 2, ...

[0030] If the pitch is not changed from that of a microlens array with a general microlens shape, the angle of emergence of diffracted light also remains unchanged. Under these conditions, as the angle of incidence increases, the wavelength λ that satisfies the above formula also increases. In other words, by using a two-stage microlens, some microlenses on the short wavelength side do not satisfy the above formula. As a result, reflected diffracted light is reduced.

[0031] By adding the microlens L2 with a large curvature to the microlens L1 formed with a normal curvature, the diffraction efficiency of the reflected light at the microlens ML can be suppressed, thereby reducing the reflected diffracted light. Even when strong light is incident on the solid-state imaging device 1, the reflected diffracted light is reduced, thereby suppressing petal flare, which is a petal-shaped diffraction pattern.

[0032] It should be noted that the microlenses L1 and L2 (in other words, the central portion CP and peripheral portion PP) that form the basis of the microlens ML are not limited to a shape obtained by cutting out a portion of an elliptical hemisphere. For example, the microlens L1 may be formed as an elliptical hemisphere. The microlenses L1 and L2 may also have a shape obtained by cutting out a portion of a hemisphere. The microlenses L1 may also be formed as a hemisphere. The microlenses L1 and L2 may be designed so that a virtual curved surface obtained by virtually extending the central portion CP intersects with the edge of the second surface (exit surface) of the microlens ML. In other words, the microlenses L1 and L2 that form the basis of the microlens ML can have various shapes, allowing for design freedom.

[0033] 4 is a diagram showing a microlens ML according to this embodiment. The microlens ML has a central portion CP including the optical axis AX, and a peripheral portion PP surrounding the central portion CP. The central portion CP and the peripheral portion PP are formed rotationally symmetrically with respect to the optical axis AX. For convenience, the intersection between the entrance surface (first surface) of the microlens ML and the optical axis AX is referred to as the "first intersection point P1." The intersection between the exit surface (second surface) of the microlens ML and the optical axis AX is referred to as the "second intersection point P2."

[0034] The central portion CP and the peripheral portion PP have a shape obtained by cutting out a portion of an elliptical hemisphere. The central portion CP is formed with a radius of curvature r1 (an example of a first radius of curvature). The peripheral portion PP is formed with a radius of curvature r2 (an example of a second radius of curvature) that is smaller than the radius of curvature r1. The central portion CP and the peripheral portion PP may be formed in a shape different from the shape obtained by cutting out a portion of an elliptical hemisphere. For example, in the example of Equation 4 described below, the peripheral portion PP is formed in a shape different from the shape obtained by cutting out a portion of an elliptical hemisphere.

[0035] It should be noted that any reference to an element using a designation such as "first," "second," etc., used in this disclosure does not generally limit the quantity or order of those elements. These designations are used for convenience to distinguish between two or more elements. Thus, reference to a first and a second element does not imply, for example, that only two elements are employed, that the first element must precede the second element, etc.

[0036] The radius of curvature r1 is the radius of curvature of the central portion CP on the optical axis AX. The radius of curvature r2 is the radius of curvature of the peripheral portion PP. More specifically, the radius of curvature r2 is the radius of curvature of a circle C2 (see FIG. 4) that approximates the curved shape of the peripheral portion PP.

[0037] 4, by forming the microlens ML in a two-stage shape with a central portion CP and a peripheral portion PP, reflected and diffracted light is reduced, thereby suppressing petal flare.

[0038] The radius of curvature r1 is larger than the radius of curvature r2, and is, for example, 1.92 μm or less. By setting the radius of curvature r1 to 1.92 μm or less, the reflected and diffracted light is further reduced, thereby further suppressing petal flare.

[0039] More preferably, the radius of curvature r1 may be set to 1.86 μm or less. By setting the radius of curvature r1 to 1.86 μm or less, reflected and diffracted light is further reduced, and petal flare is suppressed.

[0040] The radius of curvature r2 is, for example, 0.3 μm to 0.9 μm. By setting the radius of curvature r2 to a value within this range, reflected and diffracted light is further reduced, and petal flare is further suppressed.

[0041] To further suppress petal flare, the radius of curvature r2 may be set to 0.31 μm to 0.87 μm.

[0042] The distance between the first intersection point P1 and the second intersection point P2 is referred to as a "first distance D1." An optical axis-orthogonal plane including the boundary line between the central portion CP and the peripheral portion PP is referred to as an "optical axis-orthogonal plane PL." The distance between the optical axis-orthogonal plane PL and the second intersection point P2 is referred to as a "second distance D2." The microlens ML is formed, for example, so that the ratio RT1 (= D2 / D1) of the second distance D2 to the first distance D1 is 0.2 to 0.7.

[0043] By setting the ratio RT1 to a value within this range, reflected and diffracted light is further reduced, and petal flare is further suppressed.

[0044] More preferably, the microlenses ML may be formed so that the ratio RT1 is 0.20 to 0.66. By setting the ratio RT1 to a value within this range, reflected and diffracted light is further reduced, and petal flare is further suppressed.

[0045] The radius of the central portion CP is referred to as a "first radius Rd1." The radius of the microlens ML consisting of the central portion CP and the peripheral portion PP is referred to as a "second radius Rd2." The microlens ML is formed, for example, so that the ratio RT2 (=Rd1 / Rd2) of the first radius Rd1 to the second radius Rd2 is 0.5 to 0.9.

[0046] By setting the ratio RT1 to a value within this range, reflected and diffracted light is further reduced, and petal flare is further suppressed.

[0047] More preferably, the microlenses ML may be formed so that the ratio RT2 is 0.6 to 0.9. By setting the ratio RT2 to a value within this range, reflected and diffracted light is further reduced, and petal flare is further suppressed.

[0048] Next, the microlenses of each example (Comparative Examples 1 and 2 and Examples 1 to 5) will be described. Comparative Examples 1 and 2 are simple elliptical hemispherical microlenses. Examples 1 to 5 are microlenses ML having a central portion CP and a peripheral portion PP. As an example, FIGS. 5 to 8 show the microlenses ML of Examples 1 to 4. FIGS. 5 to 8 are cross-sectional views of the microlenses ML in the diagonal direction (the direction of line A1-A2 in FIG. 2). That is, in each of FIGS. 5 to 8, the horizontal axis indicates the diagonal direction parallel to the XY plane. The vertical axis indicates the height direction (Z direction).

[0049] Petal flare occurs in various types of solid-state image sensors. Here, a solid-state image sensor with a Bayer array QPD (Quad Phase Detection) structure will be taken as an example to explain the microlenses of each example.

[0050] Equations 1 to 4 are functions that represent the shape of the microlenses ML in Examples 1 to 4, respectively. These functions are modified functions of an elliptical hemisphere with a circular base and an elliptical side cross section. In each of Equations 1 to 4, the upper equation represents the shape of the central portion CP. The lower equation represents the shape of the peripheral portion PP. In the upper equations, the value of the denominator of the fraction with x and y as numerators represents the radius of the base when the central portion CP is an elliptical hemisphere. In the lower equations, the value of the denominator of the fraction with x and y as numerators represents the radius of the base when the peripheral portion PP is an elliptical hemisphere. However, in the example of Equation 4, the peripheral portion PP has a shape that cannot be expressed by a simple elliptical hemisphere. Therefore, in the lower equation of Equation 4, the value of the denominator of the fraction with x and y as numerators does not represent the radius of the base.

[0051] (Formula 1)

[0052] (Formula 2)

[0053] (Formula 3)

[0054] (Formula 4)

[0055] 9 is a graph showing the intensity of reflected diffracted light in each example. In FIG. 9, the vertical axis represents the ratio RT0 (unit: %), and the horizontal axis represents the fill factor (unit: %). The ratio RT0 is the ratio of the intensity of reflected diffracted light (an example of second light intensity) to the intensity of normal incident light (an example of first light intensity) that is perpendicularly incident on the microlens ML. The fill factor is the occupancy rate of the microlens ML in the pixel. The fill factor is 99% in all examples.

[0056] The graph in Figure 9 was created under the following conditions. The "pixel set" below indicates a set of pixels arranged in two rows and two columns in the QPD. In the optical simulation performed to create the graph, Rigorous Coupled-Wave Analysis (RCWA) was used as the wave analysis method. Synopsys' DiffractMOD was used as the optical simulator.

[0057] The wavelength of the incident light was changed in 10 nm increments within the range of 600 nm to 700 nm. The ratio RT0 shown in the graph is the average value of the ratio RT0 at each wavelength.

[0058] <<Block between pixels>> 0.2 μm <<Block between RGB filters>> 0.2 μm <<Pitch between pixel sets>> 2.394 μm <<Wavelength of incident light>> 600 nm to 700 nm <<Angle of incidence>> 0 degrees (vertical incident light)

[0059] Table 1 shows various values ​​of the microlenses in each example. Specifically, Table 1 shows the ratio RT0, ratio RT1, ratio RT2, radius of curvature r1, radius of curvature r2, and device characteristics. The ratio RT0 has the same value as in Table 1. The device characteristics indicate whether reflected and diffracted light is reduced using two levels: "x" and "o."

[0060] (Table 1)

[0061] As shown in FIG. 9 and Table 1, in Comparative Examples 1 and 2, the ratio RT0 exceeds 1.4%. That is, in Comparative Examples 1 and 2, the reflected diffracted light cannot be sufficiently reduced. In contrast, in Examples 1 to 5, the reflected diffracted light is sufficiently reduced. Specifically, the ratio RT0 is 1.4% or less.

[0062] FIG. 10 is a schematic view illustrating an example of a method for manufacturing a microlens array MLA according to an embodiment of the present disclosure.

[0063] In the example of FIG. 10 , a base layer 40 that forms the basis of the microlens array MLA is formed on the color filter 20. Furthermore, a sacrificial layer (not shown) made of a photosensitive resin is formed on the base layer 40. When the sacrificial layer is exposed and developed with a pattern (referred to as a "first pattern" for convenience) that corresponds to the position of each pixel set (a set of pixels PX1 to PX4), sacrificial patterns 50A are formed on the base layer 40, as shown in step S101. Next, heat flow is used to flow each of the sacrificial patterns 50A corresponding to each pixel set, and as shown in step S102, the sacrificial patterns 50A are deformed into a rounded shape. The deformed sacrificial patterns 50A are referred to as sacrificial patterns 50a.

[0064] A sacrificial layer (not shown) is formed on the sacrificial pattern 50a. When the sacrificial layer is exposed and developed with a pattern (referred to as a "second pattern" for convenience) corresponding to the position of the center portion CP of each microlens ML, a sacrificial pattern 50B is formed on the sacrificial pattern 50a, as shown in step S103. Next, the sacrificial patterns 50a and 50B corresponding to each pixel set are individually flowed by heat flow. By heat flowing under appropriate conditions, the sacrificial patterns 50a and 50B can be deformed into a shape similar to the microlens array MLA, as shown in step S104. The deformed pattern is referred to as a sacrificial pattern 50C.

[0065] Dry etching is performed on the base material layer 40 and the sacrificial pattern 50C (see step S105). As a result, as shown in step S106, the sacrificial pattern 50C disappears and is transferred to the base material layer 40, thereby forming a microlens array MLA on the color filter 20.

[0066] FIG. 11 is a schematic view illustrating another method for manufacturing a microlens array MLA according to an embodiment of the present disclosure.

[0067] In the example of FIG. 11 , a base layer 40 is formed on the color filter 20. Furthermore, a sacrificial layer is formed on the base layer 40. The sacrificial layer is exposed and developed with a first pattern. As a result, a sacrificial pattern 50A is formed on the base layer 40, as shown in step S201. Next, a sacrificial layer is formed on the sacrificial pattern 50A. This sacrificial layer is exposed and developed with a second pattern. As a result, a sacrificial pattern 50B is formed on the sacrificial pattern 50A, as shown in step S202. When the two-layer sacrificial patterns 50A and 50B are thermally flowed, a sacrificial pattern 50C similar to the microlens array MLA is formed on the base layer 40, as shown in step S203. Dry etching is performed on the base layer 40 and the sacrificial pattern 50C (see step S204). As a result, the sacrificial pattern 50C disappears and is transferred to the base layer 40, forming the microlens array MLA on the color filter 20, as shown in step S205.

[0068] The microlens array MLA may be manufactured using a method using a gray-tone mask. The gray-tone mask is a quartz substrate on which a light-shielding film with variable light transmittance is formed to correspond to the lens elements to be manufactured. The gray-tone mask is configured, for example, so that the light transmittance is lower in the region corresponding to the center portion CP of each microlens ML than in the region corresponding to the peripheral portion PP of each microlens ML. The microlens array MLA can also be manufactured using such a gray-tone mask.

[0069] The above is a description of exemplary embodiments of the present disclosure. The embodiments of the present disclosure are not limited to those described above, and various modifications are possible within the scope of the technical idea of ​​the present disclosure. For example, the embodiments of the present application also include appropriate combinations of embodiments explicitly shown in the specification or obvious embodiments.

[0070] This application is based on Japanese Patent Application No. 2024-107826, filed on July 4, 2024, the contents of which are incorporated herein in their entirety.

Claims

1. A solid-state imaging device comprising a microlens array having a plurality of periodically arranged microlenses, wherein the microlenses have a central portion including an optical axis formed with a first radius of curvature, and a peripheral portion around the central portion formed with a second radius of curvature smaller than the first radius of curvature.

2. The solid-state imaging device according to claim 1, wherein one microlens is arranged for each pixel.

3. The solid-state imaging element of claim 1, wherein when a first distance is defined as a distance between a first intersection point of a first surface of the microlens and the optical axis and a second intersection point of a second surface of the microlens and the optical axis, and a second distance is defined as a distance between a plane perpendicular to the optical axis that includes the boundary line between the central portion and the peripheral portion and the second intersection point, the microlens is formed so that the ratio of the second distance to the first distance is 0.2 or more and 0.7 or less.

4. The solid-state imaging device according to claim 3, wherein the ratio of the second distance to the first distance is 0.20 or more and 0.66 or less.

5. The solid-state imaging device according to claim 1, wherein when the radius of the central portion is a first radius and the radius of the microlens consisting of the central portion and the peripheral portion is a second radius, the microlens is formed so that the ratio of the first radius to the second radius is 0.5 or more and 0.9 or less.

6. The solid-state imaging device according to claim 5, wherein the ratio of the first radius to the second radius is 0.6 or more and 0.9 or less.

7. The solid-state imaging device according to claim 1, wherein the first radius of curvature is 1.92 μm or less.

8. The solid-state imaging device according to claim 1, wherein the first radius of curvature is 1.86 μm or less.

9. The solid-state imaging device according to claim 1, wherein the second radius of curvature is 0.3 μm or more and 0.9 μm or less.

10. The solid-state imaging device according to claim 1, wherein the second radius of curvature is 0.31 μm or more and 0.87 μm or less.

11. The solid-state imaging device according to claim 1, wherein when the intensity of normal incident light that is normal to the microlens is defined as a first light intensity and the intensity of light diffracted by the normal incident light that is reflected by the microlens is defined as a second light intensity, the second light intensity is 1.4% or less of the first light intensity.

Images