JP2005259824A - Solid state imaging device and method of forming asymmetric optical waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently condense lights incident on an optical waveguide, irrespective of the position of pixels in a solid state imaging device. <P>SOLUTION: The solid state imaging device (30) is composed of a plurality of pixels disposed on an expected image forming plane of a photographing lens. Each pixel has a microlens (31), a light receiver (33) for converting an incident light to an electric signal, and an optical waveguide (37) made of a transparent high-refractive index material disposed between the microlens and the receiver to guide the light from the microlens to the receiver. The optical waveguide has a different shape (37a-37d) suited to the position of each pixel in the imaging device, and the shape meets the condition that the incident light from the microlens makes the total reflection in the optical guide means. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solid-state imaging device used in an imaging apparatus such as a digital still camera and a method for forming an asymmetric optical waveguide used in the solid-state imaging device.

  In recent years, solid-state imaging devices used in imaging devices such as digital still cameras have improved the image quality by increasing the number of pixels, while reducing the cost by reducing the chip size. For this reason, the size of one pixel constituting the solid-state imaging device is decreasing year by year, and the area of the light receiving unit is also decreasing accordingly.

  When the area of the light receiving portion is reduced, the light receiving sensitivity is lowered. Therefore, a light receiving element in which an optical waveguide is provided between the light incident surface and the light receiving portion to improve the light collecting characteristic has been proposed (for example, Patent Document 1). reference). In this light receiving element, a symmetrical optical waveguide composed of a high refractive index material is provided on the light incident side of the light receiving portion, and a low refractive index material is provided around the optical waveguide to totally reflect incident light at the boundary surface. This improves the light condensing characteristics.

  In addition, by shifting the optical axis of the microlens (on-chip lens) formed on each light-receiving unit toward the center from the optical axis of the light-receiving unit at the periphery of the solid-state imaging device, light incident from an oblique direction can be efficiently The structure which concentrates on a light-receiving part is proposed (for example, refer patent document 2).

  FIG. 8A is a cross-sectional view of pixels arranged in the peripheral portion of the solid-state imaging device 30 that can be considered when the above-described conventional example is combined, and shows a state of light rays incident from a photographing lens (not shown). In the figure, reference numeral 31 denotes a microlens for condensing incident light on the light receiving unit 33, and is arranged at a position eccentric to the optical axis side of a photographing lens (not shown) with respect to the light receiving unit 33. An optical waveguide 36 made of a material having a high refractive index is formed on the light incident side of the light receiving portion 33, and the incident light refracted by the microlens 31 is the optical waveguide 36 and an interlayer insulating film that is a low refractive index material. The light is totally reflected at the boundary surface with the light 35 and guided to the light receiving unit 33.

JP-A-5-235313 Japanese Patent No. 2600250

  However, if the photographic lens is made smaller in order to further reduce the size of the camera, the distance between the solid-state image sensor and the exit pupil of the photographic lens is shortened, and the angle of light incident on the solid-state image sensor is further increased. End up. For this reason, as shown in the ray trace diagram of FIG. 8B, in the conventional solid-state imaging device having a symmetrical optical waveguide, a light flux that does not satisfy the total reflection condition is generated at the refractive index interface, and the incident light is efficiently generated. There was a disadvantage that the light could not be condensed.

  As described above, since the angle of light incident on the solid-state image sensor differs depending on the position of each pixel constituting the solid-state image sensor, there is a disadvantage that the sensitivity varies depending on the pixels when the optical waveguide having the same shape is formed.

  The present invention has been made in view of the above problems, and an object thereof is to efficiently collect light incident on an optical waveguide regardless of the position of a pixel in a solid-state imaging device.

  In order to achieve the above object, the solid-state imaging device of the present invention consisting of a plurality of pixels arranged on the planned imaging plane of the photographic lens is composed of a microlens and a photoelectric conversion that converts incident light into an electrical signal. And a light guide means that is made of a transparent high refractive index material and is disposed between the microlens and the photoelectric conversion means and guides light from the microlens to the photoelectric conversion means. The light guide means has a different shape according to the position of each pixel in the solid-state imaging device, and the shape satisfies the condition that light incident from the microlens is totally reflected in the light guide means. Fulfill.

  According to another configuration, the solid-state imaging device according to the present invention, which is arranged on the planned imaging plane of the photographing lens and includes a plurality of pixels, each pixel includes a microlens and a photoelectric that converts incident light into an electrical signal. A conversion unit, and a condensing unit that is made of a transparent high refractive index material and is disposed between the microlens and the photoelectric conversion unit, and condenses the light from the microlens on the photoelectric conversion unit. And the condensing means has different shapes depending on the position of each pixel in the solid-state imaging device, and the shape allows light incident from the microlens to be substantially within the region of the photoelectric conversion means. Thus, it has a shape to collect light and a light collecting power.

  According to the above configuration, it is possible to efficiently collect light incident on the optical waveguide regardless of the position of the pixel in the solid-state imaging device.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components exemplified in this embodiment should be changed as appropriate according to the configuration of the apparatus to which the present invention is applied and various conditions. However, the present invention is not limited to these examples.

<First Embodiment>
FIG. 1 is a plan view of a solid-state imaging device according to the first embodiment of the present invention.

  In FIG. 1, reference numeral 30 denotes a solid-state image sensor. Each pixel of the solid-state image sensor 30 includes a microlens 31 for condensing light from a photographing lens (not shown), and an electrical signal corresponding to the amount of incident light. And a light guide 36 that guides the light from the microlens 31 to the light receiver 33. In FIG. 1, only 8 × 6 pixels are shown for simplification of explanation, but actually hundreds of thousands to millions of pixels are arranged. Although the light receiving portions 33 of the respective pixels are arranged at equal intervals, the microlens 31 is arranged eccentrically with respect to the light receiving portion 33 toward the center side of the solid-state imaging device 30 (the optical axis side of a photographing lens (not shown)). Yes. The optical waveguide 36 has a quadrangular pyramid shape, and has a different shape depending on the position of each pixel.

  2A is a pixel located at a, b, and c in FIG. 1 (hereinafter referred to as pixels a, b, and c), and FIG. 2B is a pixel that is located at d (hereinafter referred to as a pixel). It is a figure which each shows the cross section of d.). Note that “a” to “d” after the reference number indicating the configuration of each pixel are for specifying the corresponding pixels “a” to “d”, respectively. As can be seen from FIG. 1, the pixels a and b are located substantially at the center of the solid-state image sensor 30, the pixel c is adjacent to the center pixel b, and the pixel d is located at the end of the solid-state image sensor 30. .

In FIG. 2, 32 is a Si substrate, and light receiving portions 33a to 33d are formed. Reference numeral 38 denotes a wiring electrode, which is formed between interlayer insulating films 35 made of SiO ₂ or the like having a low refractive index. Reference numeral 34 denotes a transfer electrode for transferring photoelectric charges generated in the light receiving portions 33a to 33d to a floating diffusion portion (FD portion) (not shown). Reference numerals 36a to 36d denote optical waveguides made of SiN or the like having a high refractive index, and the central axis thereof substantially coincides with the central axes of the light receiving portions 33a to 33d. The light incident sides of the optical waveguides 36a to 36d are formed so that the openings are widened so that more light can enter. Reference numerals 37a to 37d denote color filters, and microlenses 31a to 31d are formed through a planarization layer. The lens shapes of the microlenses 31a to 31d are determined so as to condense light beams incident from a photographing lens (not shown) onto the light receiving portions 33a to 33d.

  In the pixels a and b located at the center of the solid-state image pickup device 30, the principal ray of light incident from a photographing lens (not shown) is incident substantially perpendicularly, so that the optical axes 41a and 41b of the microlenses 31a and 31b and the light beams are guided. The central axes 40a and 40b of the waveguides 36a and 36b are configured to substantially coincide with each other. The optical waveguides 36a and 36b are in the shape of a regular quadrangular pyramid that is symmetric with respect to the central axes 40a and 40b.

  On the other hand, in the pixels c and d that are not located at the center of the solid-state imaging device 30, the principal ray of light incident from a photographing lens (not shown) is incident at a predetermined angle determined by the exit pupil of the photographing lens and the position of the pixel. To do. Therefore, the optical axes 41c and 41d of the microlenses 31c and 31d are set so as to be eccentric to the center of the pixel with respect to the central axes 40c and 40d of the optical waveguides 36c and 36d. The optical waveguides 36c and 36d are in the form of a truncated pyramid that is asymmetric with respect to the central axes 40c and 40d. That is, the angle of the inclined surface on the central side of the solid-state imaging device 30 with respect to the central axes 40c and 40d of the optical waveguides 36c and 36d is constant, but the peripheral side of the solid-state imaging device 30 with respect to the central axes 40c and 40d. The angle of the inclined surface is formed to be different depending on the position of the pixel. That is, since the incident angle of the principal ray from the photographing lens becomes deeper as it goes to the peripheral portion of the solid-state imaging device 30, the inclined surface positioned on the peripheral side so that the incident angle to the inclined surface of the optical waveguide 36 becomes shallow. These angles are formed so as to approach perpendicular to the Si substrate 32.

  As described above, the opening areas of the optical waveguides 36c and 36d are reduced by bringing the inclined surfaces on the peripheral side of the optical waveguides 36c and 36d close to vertical, but the positions of the microlenses 31c and 31d are solid as described above. By decentering to the center of the image sensor 30, the light collected by the microlenses 31c and 31d is guided to the same extent as the light collected by the microlenses 31a and 31b is incident on the optical waveguides 36a and 36b. The light can enter the waveguides 36c and 36d, respectively. As described above, the installation position of the microlens of each pixel is appropriately changed based on conditions such as the distance from the exit pupil of the photographing lens, the focusing power of the microlens, and the distance to the opening of the optical waveguide. It is.

  FIG. 3 shows the state of the light beam incident on the pixel d at the end of the solid-state image sensor 30 when the distance between the exit pupil and the solid-state image sensor 30 is shortened by further downsizing a photographing lens (not shown). FIG. The light beam incident on the solid-state imaging device 30 at a deep angle is refracted by the micro lens 31d and enters the optical waveguide 36d made of a high refractive index material. Here, since the inclined surface on the peripheral portion side of the optical waveguide 36d is formed so as to be substantially perpendicular to the Si substrate 32, the angle of the light beam incident on the inclined surface becomes shallow, and the total reflection condition is satisfied. As a result, the light beam incident on the solid-state imaging device 30 at a deep angle is also totally reflected by the inclined surface of the optical waveguide 36d and efficiently guided to the light receiving unit 33d.

  A process for forming the optical waveguide 36 having a different asymmetric shape depending on the position of the solid-state imaging device 30 will be described with reference to the photomask explanatory diagram of FIG. 4 and the optical waveguide formation process explanatory diagram of FIG.

  FIG. 4 is an example of a photomask used for forming an optical waveguide of one pixel of the solid-state imaging device 30 of the present invention. In the first embodiment, a dot pattern selectively drawn on a light-transmitting support 11 such as a quartz substrate, that is, a light-shielding film pattern by a desired dot pattern in which the density of dots 13 is changed stepwise. 12 is formed to constitute the photomask 10. The dots 13 have a fixed shape and are formed in the same square in this example. In the first embodiment, when viewed in the direction of AA ′ in the drawing, the dot density is the highest between A and C and between D and A ′ (shaded portion in FIG. 4), and conversely D There is no dot between -E. Further, between the lines CE, the dots 13 are distributed and configured so that the dot density gradually decreases. In FIG. 4, the dot density distribution as described above is provided, but the dot density distribution of the light shielding pattern is designed to be different depending on the position of the pixel on the solid-state imaging device 30 with respect to the optical waveguide to be formed.

  Next, an optical waveguide forming method using a lithography process for patterning a resist film using the photomask 10 will be described with reference to a process explanatory diagram of FIG.

  As shown in FIG. 5A, an interlayer insulating film 35 including a wiring electrode 38 is formed on a Si substrate 32 on which a light receiving portion 33 is formed, and a positive resist film (positive type) is formed on the interlayer insulating film 35. The photosensitive resin film) 20 is formed. Then, a photomask 10 having a predetermined dot density distribution is arranged in a corresponding region of each pixel, and exposure is performed through the photomask 10. In this case, in the direction of A-A ′ of the photomask 10, the density of the dots 13 changes depending on the position as described above, and the exposure density is controlled.

  By further developing the resist film 20, an asymmetric pattern corresponding to the exposure density of the photomask 10 is formed as shown in FIG. 5B.

  Next, by performing reactive ion etching, the pattern of the resist film 20 is transferred to the interlayer insulating film 35 as shown in FIG. 5C, and a concave portion that becomes an asymmetric optical waveguide is formed immediately above the light receiving portion 33. It is formed.

  Further, as shown in FIG. 5D, an optical waveguide 36 is formed by embedding SiN which is a high refractive index material in an asymmetric recess formed in the interlayer insulating film 35.

  As described above, according to the first embodiment, the shape of the optical waveguide is changed according to each position of the pixel so that the light incident on the optical waveguide is totally reflected in the optical waveguide. Regardless of the position of the pixel in the image sensor, the light incident on the optical waveguide can be efficiently guided to the light receiving unit.

<Modification>
In the first embodiment, the optical waveguide 36 of each pixel constituting the solid-state image sensor 30 is an inclined surface located on the peripheral side with respect to the center of the solid-state image sensor 30 as it goes to the periphery of the solid-state image sensor 30. Although an example is shown in which the angle is changed so as to approach the vertical, the same effect as in the first embodiment can be obtained even if the shape is as shown in the schematic cross-sectional view of the solid-state imaging device in FIG. It is possible to achieve.

  That is, at least a part of the inclined surface located on the peripheral side with respect to the center of the solid-state imaging device includes a surface perpendicular to the Si substrate 32, and the ratio of the inclined surface with a constant inclination angle to the surface perpendicular to the Si substrate 32 is set. By being configured so as to increase toward the periphery of the solid-state image sensor 30, the total reflection condition is satisfied even for a light beam incident on the solid-state image sensor 30 at a deep angle, and the light is efficiently guided to the light receiving unit 33. Making it possible.

  Furthermore, in addition to the configuration described in the first embodiment in which the inclined surface on the peripheral portion side of the optical waveguide is brought closer to the vertical as it goes to the periphery of the solid-state imaging device, the inclined surface on the center side is inclined to widen the opening area. You may do it. However, it is necessary to set the angle to maintain the total reflection condition even on the inclined surface on the center side.

  Moreover, in the said 1st Embodiment, although the shape of the optical waveguide was made into the square frustum, it is not restricted to this, It is also possible to comprise by the shape of a polygonal frustum or a truncated cone.

<Second Embodiment>
In the first embodiment, the example in which light is guided to the light receiving unit using total reflection at the refractive index interface of the optical waveguide is shown. However, the configuration shown in the schematic cross-sectional view of the solid-state imaging device in FIG. It is also possible to achieve the same effect as in the first embodiment.

  That is, an optical waveguide 39 having a lens action is provided between the wiring electrode 38 and the color filter 37, and the radius of curvature on the peripheral portion side of the lens body that is the optical waveguide 39 decreases as it goes to the peripheral portion of the solid-state imaging device 30. With this configuration, incident light can be efficiently guided to the light receiving unit 33 by providing a stronger refracting action on a light beam incident on the solid-state imaging device 30 at a deep angle.

  According to the second embodiment, similarly to the first embodiment, light incident on the optical waveguide can be efficiently guided to the light receiving unit regardless of the position of the pixel in the solid-state imaging device.

It is a top view of the solid-state image sensing device in a 1st embodiment of the present invention. It is a schematic sectional drawing which shows the structure of the one part pixel of the solid-state image sensor shown in FIG. It is a ray trace figure of a peripheral pixel in a 1st embodiment of the present invention. It is a figure which shows an example of the photomask used in order to form the optical waveguide in the 1st Embodiment of this invention. It is the schematic explaining the process for forming the optical waveguide in the 1st Embodiment of this invention. It is a schematic sectional drawing which shows another structure of the one part pixel of the solid-state image sensor shown in FIG. It is a schematic sectional drawing which shows the structure of the one part pixel of the solid-state image sensor in the 2nd Embodiment of this invention. It is a ray trace figure of one pixel of the conventional solid-state image sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Photomask 20 Resist film 30 Solid-state image sensor 31 Micro lens 32 Si substrate 33 Light-receiving part 34 Transfer electrode 35 Interlayer insulating film 36, 39 Optical waveguide 37 Color filter 38 Wiring electrode

Claims (8)

A solid-state image sensor composed of a plurality of pixels disposed on a planned imaging plane of a photographing lens, each pixel being
A microlens,
Photoelectric conversion means for converting incident light into an electrical signal;
A light guide means that is made of a transparent high refractive index material and is arranged between the microlens and the photoelectric conversion means, and guides light from the microlens to the photoelectric conversion means;
The light guide means has a different shape depending on the position of each pixel in the solid-state imaging device, and the shape satisfies a condition that light incident from the microlens is totally reflected in the light guide means. A solid-state imaging device characterized by the above.   The microlens is arranged eccentric to the center side of the solid-state image sensor with respect to the optical axis of the photoelectric conversion unit according to the position of the corresponding pixel from the center of the solid-state image sensor, and the light guide unit is The solid-state imaging device according to claim 1, further comprising an opening through which light from the microlens is substantially entirely incident.   3. The solid-state imaging device according to claim 1, wherein the light guide means has a polygonal frustum shape or a truncated cone shape, and has an upper surface wider than a bottom surface.   The light guide means has a shape in which an angle of an inclined surface located on the peripheral side of the solid-state image sensor approaches more vertically as the position of the corresponding pixel is away from the center of the solid-state image sensor. Item 6. The solid-state imaging device according to Item 3. A solid-state image sensor composed of a plurality of pixels disposed on a planned imaging plane of a photographing lens, each pixel being
A microlens,
Photoelectric conversion means for converting incident light into an electrical signal;
Condensing means for condensing the light from the microlens to the photoelectric conversion means, which is composed of a transparent high refractive index material and is disposed between the microlens and the photoelectric conversion means,
The condensing means has a different shape depending on the position of each pixel in the solid-state imaging device, and the shape is collected so that light incident from the microlens is substantially within the region of the photoelectric conversion means. A solid-state imaging device having a shape to shine and a light collecting power.   The microlens is arranged eccentric to the center side of the solid-state image sensor with respect to the optical axis of the photoelectric conversion unit according to the position of the corresponding pixel from the center of the solid-state image sensor, The solid-state image pickup device according to claim 5, further comprising an opening through which almost all light from the microlens enters.   The condensing means has a shape in which the radius of curvature on the peripheral side of the solid-state image sensor becomes smaller as the position of the corresponding pixel moves away from the center of the solid-state image sensor. The solid-state imaging device described. Applying a resist on an interlayer insulating film including a wiring layer;
Exposing and developing the resist using a photomask having a light-shielding pattern in which the dot density is changed stepwise on a light-transmitting support; and
And a step of patterning the resist into an asymmetric shape and then transferring the asymmetric shape to the interlayer insulating film by an etching process.

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