JP2013016687A - Solid state image element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid state image element capable of providing a high quality output image by reducing a difference of a smear ratio in a pixel portion.SOLUTION: The solid state image element has a micro lens 16 and a color filter 17 that selects wavelengths of red, green and blue and includes an optical waveguide structure (optical waveguide 13 ...) that allows incident light after transmitting through the color filter 17 to propagate to a pixel opening part by using a material having a refractive index larger than that of a peripheral portion. The opening diameter of the optical waveguide 13 gets smaller from the pixel center toward the periphery portion. In particular, the opening diameter dat the central portion is larger than the opening diameter dat the outer periphery. In an R-pixel and a B-pixel also, the opening diameter of the optical waveguide is set with the same relationship.

Description

  The present invention relates to a solid-state imaging device.

With the recent increase in the number of pixels of solid-state imaging devices, the size (area) per pixel is in the direction of reduction. As the pixel size is reduced, it has become difficult to collect incident light on the solid-state imaging device. Therefore, means for guiding light to the photoelectric conversion unit using an optical waveguide has been proposed.
FIG. 11 is a diagram illustrating a configuration of a solid-state imaging device according to a conventional example using a waveguide structure.
As shown in FIG. 11, a photodiode portion 101 as a photoelectric conversion portion is formed on an upper layer portion of a silicon (Si) substrate 100, and a poly-oxide film is formed on the upper surface of the polysilicon portion through a gate insulating film 102 made of a silicon oxide film or the like. A transfer gate electrode 103 made of a silicon film is arranged to transfer the signal charge accumulated in the photodiode portion 101 to the floating diffusion (FD) 104.

A plurality of wiring films (including a light shielding film) 105 are formed on the silicon substrate 101 via an interlayer insulating film 106. Further, a color filter 109 is disposed on the upper surface of the insulating film 106 via a passivation film and a planarization film, and an on-chip microlens 110 is disposed thereon.
Corresponding to the positions of the microlens 110 and the photodiode portion 101, the transparent films 111A and 112A of the waveguides 111 and 112 are embedded in the insulating film 106, and the on-chip lens 110 and the photodiode portion 101 are optically connected. It is connected.

  As shown in the drawing, each of the waveguides 111 and 112 has an opening diameter corresponding to the position of the pixel, and the waveguide 111 disposed on the center side of the pixel portion is formed with a relatively small opening diameter. On the contrary, the waveguide 112 arranged on the peripheral side of the pixel portion is formed with a relatively large opening diameter, and suppresses a sensitivity difference (shading) between the center of the pixel portion and the pixel peripheral portion. In addition, the optical axes of the waveguides 111 and 112 of each pixel and the microlens 110 are also shifted in accordance with the position of the pixel. On the peripheral side of the pixel portion, the position of the microlens 110 is the center of the pixel portion. They are displaced in the direction.

Japanese Patent Laying-Open No. 2006-190766 (FIG. 1)

The above prior art has been described for the case where a CMOS solid-state image sensor is used. In a CCD solid-state image sensor, a reduction in smear ratio (noise / signal logarithm display) improves the quality of a captured image. In order to be very important.
In the above prior art, it has been disclosed that the aperture diameter of the optical waveguide is made larger in the peripheral portion than in the central portion in order to suppress the reduction in sensitivity in the peripheral portion as compared with the central portion of the pixel portion. In the configuration, the sensitivity difference (shading) can be suppressed, but the smear ratio in the peripheral portion is more markedly deteriorated than in the central portion.

  The sensitivity in the pixel portion can be easily corrected by signal processing using a DSP or the like. However, since it is very difficult to correct an optically generated smear ratio, it becomes a serious problem. This is because the smear ratio easily changes irregularly in the pixel array when the color component of incident light changes. When the correction does not function due to signal processing, problems such as non-uniform coloration of the output image occur.

  An object of the present invention is to provide a solid-state imaging device in which a difference in smear ratio in a pixel portion plane is suppressed and an image quality of an output image is high.

  In order to solve the above problems, a solid-state imaging device of the present invention includes a semiconductor substrate having a plurality of light receiving portions formed in an imaging region, a first transparent film formed on the semiconductor substrate, and a top surface of the light receiving portion. A second transparent film formed inside the first transparent film, wherein the refractive index of the second transparent film is greater than the refractive index of the first transparent film, The diameter of the upper surface of the second transparent film located in the center of the imaging region is larger than the diameter of the second transparent film located in the peripheral part of the imaging region.

  The solid-state imaging device according to the present invention further includes a color filter above the light receiving unit and on the second transparent film in the configuration described above, and the red color of the color filter in the center of the imaging region. The diameter of the upper surface of the second transparent film having the color filter is larger than the diameter of the upper surface of the second transparent film having the blue color filter among the color filters.

  In the above configuration, the solid-state imaging device according to the present invention has a diameter of an upper surface of the second transparent film including a green color filter of the color filter in a central portion of the imaging region. The diameter is larger than the diameter of the upper surface of the second transparent film having the blue color filter and smaller than the diameter of the upper surface of the second transparent film having the red color filter of the color filter. .

The solid-state imaging device of the present invention is characterized in that, in the above configuration, the second transparent film is formed independently for each of the plurality of light receiving portions.
In the above-described configuration, the solid-state imaging device according to the present invention is configured such that the second transparent film is formed across the plurality of light receiving portions and protrudes in a convex shape above each of the plurality of light receiving portions. Features.

  In the above-described configuration, the solid-state imaging device of the present invention is configured such that the convex portion of the second transparent film is offset toward the center of the imaging region in the periphery of the imaging region with respect to the light receiving unit. It is characterized by.

  According to the present invention, the in-plane distribution of smear is reduced, and the smear ratio at the outer periphery of the pixel portion is improved, so that product quality deterioration can be prevented. Further, since light confinement with respect to oblique light is strengthened, color mixture to adjacent pixels is also reduced.

1 is a schematic block diagram showing a schematic configuration of a solid-state imaging device 1 according to a first embodiment of the present invention. 1 is a schematic cross-sectional view showing a partial configuration of a solid-state image sensor 1. (A) is a schematic top view of the solid-state imaging device 1, and (b) is a schematic cross-sectional view showing the structure of an optical waveguide in the pixel unit plane. It is a characteristic view which shows a smear ratio. (A) is a schematic cross-sectional view for explaining the mechanism of smear ratio improvement when the diameter of the optical waveguide is thin, and (b) shows the mechanism of smear ratio improvement when the diameter of the optical waveguide is large. It is a schematic cross section for. (A) is a characteristic diagram showing the relationship between the aperture diameter, the incident angle and the smear ratio when blue light is incident, and (b) is the relationship between the aperture diameter, the incident angle and the smear ratio when red light is incident. FIG. It is a schematic cross section which shows the structure of the solid-state image sensor in the 2nd Embodiment of this invention. (A) is a schematic top view of the solid-state image sensor in 2nd Embodiment, (b) is a schematic cross section which shows the structure of the optical waveguide in the pixel part surface in 2nd Embodiment. is there. It is a characteristic view which shows the relationship between a convex part offset amount and incident light intensity. It is a schematic process drawing which shows the process flow of the solid-state image sensor concerning 2nd Embodiment. It is a figure which shows schematic structure of the solid-state image sensor of a prior art.

Hereinafter, a solid-state imaging device according to an embodiment of the present invention will be described with reference to the drawings. Unless otherwise specified, the same symbols are used to indicate the same items. The point of the present invention is to improve the smear ratio in the periphery of the imaging region (pixel portion).
[First Embodiment]
A first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a solid-state imaging device according to the first embodiment of the present invention. Note that the pixel pitch of the solid-state imaging device described in this embodiment is 1.34 μm.

In the solid-state imaging device 1 according to the present embodiment, as shown in FIG. 1, vertical CCD units 5 for transferring charges in the vertical direction (Y direction in the figure) are periodically arranged in the X direction. Between the adjacent vertical CCD units 5, an R pixel 2 for red light (R), a G pixel 3 for green light (G), and a B pixel 4 for blue light (B) are formed. (Bayer) array.
Each pixel is provided with a readout gate (not shown), and charges generated by light input at each pixel are transferred to the vertical CCD unit 5. Electrodes (not shown) are formed in the vertical CCD unit 5, and the electric charges transferred to the vertical CCD unit are transferred to the horizontal CCD unit 6 by sequentially applying a potential to the electrodes.

In the horizontal CCD unit, the electric charge is transferred in the X direction in the figure, output to the amplifier unit 7, and processed as an image signal.
FIG. 2 is a schematic cross-sectional view of the solid-state imaging device 1 according to the first embodiment of the present invention.
In FIG. 2, a light receiving portion (photodiode: PD) 9 formed by implanting an n-type impurity or the like is disposed in the surface layer portion of the silicon substrate 8 so as to be spaced apart. In the light receiving unit 9, charges are accumulated by photoelectric conversion. Light captured by the microlens 16 enters the light receiving unit 9. When incident light taken in by the microlens 16 passes through the green (G) filter 17, components other than green light (G light) are absorbed and transmitted through the intermediate layer 15 and then guided to the G light optical waveguide 13. .

The intermediate layer 15 is used for planarization before forming the color filter. The optical waveguide 13 is made of a SiN-based material having a large refractive index (n≈2). Since it is larger than the refractive index (n≈1.5) of the intermediate layer 15, it has the ability to confine light. The height of the optical waveguide was L≈1.5 μm.
The G light incident on the G light optical waveguide 13 enters the G pixel 3. When passing through the red (R) filter 18, red light (R light) propagates through the R light optical waveguide 14 and reaches the R pixel 2.

Similarly, blue (B light) not shown in FIG. 2 passes through the blue filter and the B light optical waveguide and enters the B pixel.
The green light incident on the G pixel 3 is photoelectrically converted by the light receiving unit (PD) 9 in the G pixel 3 and charges are accumulated. The accumulated charge moves to the vertical CCD unit 5 by applying a potential to the read gate electrode 11. The vertical CCD unit 5 and the read gate electrode 11 are separated by a gate oxide film 10.

The read gate electrode 11 is covered with a light shielding film 12 and prevents unwanted light from entering the vertical CCD section 5 below the read gate electrode 11 and suppresses the occurrence of smear.
FIG. 3A is a schematic top view of the optical waveguide structure of the solid-state imaging device 1 according to the first embodiment of the present invention as viewed from above. Here, FIG. 3A is a schematic top view of the AA ′ cross section in FIG. 2 as viewed from above, and the pixel (imaging) part of the solid-state imaging device 1 is an area of the pixel part 26. The symbols R, G, and B mean the R light optical waveguide 14, the G light optical waveguide 13, and the B light optical waveguide 19, respectively.

As shown in FIG. 3A, in the present embodiment, the opening diameter of the G light optical waveguide 13 at the center of the pixel portion is d _G1 , and the opening diameter of the G light optical waveguide 13 at the outermost horizontal end of the pixel portion is the same. d _G2 , where the opening diameter of the G light optical waveguide 13 farthest from the center of the pixel is d _G3 , the opening diameter of the optical waveguide is changed so as to satisfy the following relationship.
[Expression 1] d _G1 > d _G2 > d _G3
FIG. 3B is a schematic cross-sectional view showing the CC ′ cross section of FIG. 3A, and shows the center of the pixel portion and the horizontal outermost edge of the pixel portion.

  As shown in FIG. 3B, at the center of the pixel portion, light is incident substantially perpendicular to the optical waveguide, but oblique light is incident on the optical waveguide as it becomes the outer peripheral portion. With the recent thinning of digital cameras, the incident angle at the outermost periphery of the pixel portion tends to increase. Since the incident angle θ during zooming may be 15 ° at the maximum, it is necessary to improve the characteristics up to θ = 15 °.

FIG. 4 shows the incident angle characteristic of the smear ratio when the aperture diameter of the optical waveguide is changed in the pixel portion.
In FIG. 4, the smear ratio of the pixel at the center is better as the aperture diameter of the optical waveguide is larger. The opening diameter is desirably 730 nm or more. In the present embodiment, d _G1 = 930 nm.
On the other hand, the smear ratio at the horizontal end (H end) is improved when the aperture diameter of the optical waveguide is reduced as the incident angle increases. Assuming that the incident angle is as good as 15 °, it is desirable to set the incident angle to 530 nm to 730 nm. In the present embodiment, d _G2 = 630 nm. In addition, the opening diameter of the optical waveguide 13 for G light that is farthest from the center of the pixel is d _G3 = 600 nm.

As a result, the smear ratio at the pixel periphery is improved by 2.5 dB compared to the case where the size of the optical waveguide is uniform in the plane.
In order to improve the smear ratio in the plane of the pixel portion, as shown in FIG. 3A, it is possible to obtain an effect by dividing the pixel portion into nine at least and optimizing the aperture diameter of the optical waveguide in each region. . Ideally, it is best to continuously change (reduce) the aperture diameter from the center of the pixel to the periphery of the pixel, but it is realistic to change it stepwise due to mask resolution and mask design constraints. It is.

The mechanism for improving the smear ratio will be described with reference to FIG. FIG. 5A is a schematic cross-sectional view in the vicinity of the outer peripheral portion when the diameter of the optical waveguide is thin, and FIG. 5B is a schematic cross-sectional view in the vicinity of the outer peripheral portion when the diameter of the optical waveguide is large.
When the incident angle increases and the oblique light component increases, when the diameter of the optical waveguide shown in FIG. 5A is small, the incident light is reflected at the boundary between the optical waveguide 13 and the intermediate layer 15 and is directly reflected on the light receiving unit 9. Easy to enter.

Further, if the optical waveguide is thin, incident light is confined in a thin region, so that the light diffracted by the light shielding film 12 is reduced, so that the light directly incident on the vertical CCD unit 5 is small. In other words, the ability to bend light that is incident obliquely and guide it to the light receiving unit 9 is high.
On the other hand, when the diameter of the optical waveguide is large as shown in FIG. 5B, the location of reflection of the incident light changes and hits the light shielding film 12 so that diffracted light is likely to be generated. Diffracted light is incident on the vertical CCD unit 12 and is a main factor that causes smear, so that the diameter of the optical waveguide is large is a disadvantage.

Although it is conceivable to increase the height of the optical waveguide in order to change the light reflection position, the distance between the light receiving unit 9 and the microlens 16 is increased. It is not realistic to be strict.
In the present embodiment, the distance between the light receiving unit 9 and the bottom surface of the microlens 16 is about 3 μm.

Further, in the present embodiment, as shown in FIG. 3A, the opening diameters of the waveguides for R light, G light, and B light are changed. When the aperture diameters of the optical waveguides corresponding to the respective colors are d _R , d _G , and d _B , the following relationship is satisfied.
[Equation 2] d _R ≧ d _G ≧ d _B
Since the B light has a short wavelength, it can be confined in the optical waveguide even if the aperture diameter is small, and the smear ratio can be improved because the light is confined in a narrow range and the scattered light can be reduced.

On the other hand, since the wavelength of R light is long, the confinement ability is improved by increasing the aperture diameter. When the aperture diameter with respect to the R light is small, there is a risk that the light oozes out during the waveguide and the smear ratio is deteriorated.
In addition, if there is a large amount of oozing light, there is a danger of color mixing to adjacent pixels.
FIG. 6 shows the smear ratio when the aperture diameter is changed for each color. For easy comparison, characteristics of B light and R light having a long wavelength are shown. FIG. 6A shows the characteristics of the B light optical waveguide, and FIG. 6B shows the characteristics of the R light optical waveguide.

  As shown in FIGS. 6A and 6B, comparing the characteristics of the B light and the R light, it can be seen that the B light has a smaller opening diameter with a better smear ratio than the R light. Therefore, in the present embodiment, the solid-state imaging device is manufactured by changing the waveguide opening diameter at the center of the pixel and at the periphery of the pixel and at the same time changing the waveguide opening diameter for each color. As described above, it is very important to change the aperture diameter for each color and improve the smear ratio for all colors. When shooting an image close to a single color, if the smear ratio for a specific color is poor, the output image is likely to be colored. For example, when the smear ratio with respect to the R light is poor, there is a problem that the output image is colored red when a blue sky is photographed.

In this embodiment, since the aperture diameter of the optical waveguide is small around the pixel portion, the sensitivity around the pixel portion is likely to decrease, but the sensitivity at the outer peripheral portion of the pixel portion can be corrected using a DSP (digital signal processor) or the like. The difference in sensitivity (shading) of the output image can be prevented.
According to the first embodiment of the present invention, smear is reduced and smear in the entire pixel portion is improved, so that product quality deterioration can be prevented. In addition, since light confinement with respect to oblique light around the pixel portion is enhanced, scattered light to adjacent pixels is attenuated, and color mixing is also reduced.

Although this embodiment has been described by taking a CCD device as an example, it is clear that a solid-state imaging device (for example, a CMOS sensor) employing an optical system using an optical waveguide has a noise reduction effect and a color mixing prevention effect. It is.
[Second Embodiment]
The smear reduction method will be described also in the present embodiment.

The basic configuration of the solid-state imaging device of the present embodiment is the same as that of the first embodiment, but the structure of the optical waveguide is different. FIG. 7 shows a partial cross-sectional structure of the solid-state imaging device according to the present embodiment.
In the first embodiment, the optical waveguide is formed in a straight line. On the other hand, as shown in FIG. 7, the optical waveguide of the present embodiment is formed by a convex portion 19 and a buried portion 20. Since the convex portion and the embedded portion are formed of a SiN-based material having a large refractive index (n≈2), it is easy to confine light. The height h of the convex part is preferably about 150 nm to 500 nm. If the height h is 150 nm or more, there is an ability to bend incident light.

On the other hand, when the height h is 600 nm or more, the waveguide loss generated during the wave guide of the convex portion is increased, and the sensitivity is likely to be lowered. Moreover, since the light oozing out from the convex portion tends to be scattered light, the risk of increasing smear increases.
FIG. 8A is a schematic top view of the BB ′ cross section of FIG. 7 viewed from above. FIG. 8B is a schematic cross-sectional view showing a part of FIG. 8A and shows the structure of the center of the pixel portion and the outermost horizontal edge of the pixel portion.

The structure of the present embodiment is characterized in that the convex portion having the effect of the optical waveguide and the embedded portion are independent.
As shown in FIGS. 8A and 8B, since light enters the outer periphery of the pixel portion from an oblique direction, shrink (corresponding to offset the optical structure near the center of the pixel) is necessary. As described above, when the convex portions are formed independently, only the convex portions are shifted in the pixel center direction, and the incident light quantity to the pixel portion can be easily adjusted.

By optimizing the position of the convex portion, it is possible to prevent the decrease in sensitivity as much as possible, so that it is difficult for the sensitivity to decrease.
FIG. 9 shows a change in sensitivity when only the convex portion is shifted. FIG. 9 shows the sensitivity of the pixel at the horizontal end in FIG. 8A (point P in FIG. 8A). In the drawing, the horizontal axis indicates the shift amount of the convex portion 19 in the pixel center direction, and the vertical axis indicates the incident light intensity of the light to the pixel portion. FIG. 9 shows the result when the light is incident on the point P at an angle of 8 °.

As can be seen from FIG. 9, when the convex portion is shifted by about 100 nm toward the center of the pixel, the incident light intensity is maximized, and the incident light intensity is increased by 3.5% compared to the case where the protrusion is not shifted. Therefore, sensitivity at the horizontal end of the pixel portion is improved, and sensitivity shading is reduced.
Also in this embodiment, as shown in FIG. 8A, the opening diameter of the optical waveguide is changed in the pixel portion. As in the first embodiment, the smear of the pixel in the central portion employs a structure in which the opening diameter of the optical waveguide is larger than the opening diameter of the horizontal end (H end), and the smear ratio in the pixel portion The smear ratio at the horizontal end is improved.

FIG. 10 shows a process flow of the optical structure of the solid-state imaging device of the present embodiment.
As shown in FIG. 10A, after the read gate electrode 11 and the light shielding film 12 are formed, the periphery of the light shielding film 12 is covered with an oxide film 22 having a low refractive index. This is because light is confined using a difference in refractive index with the buried portion 20 formed of a SiN-based material having a high refractive index.
Next, an intermediate layer 23 is formed on the oxide film 22. The intermediate layer 23 is formed for the purpose of adjusting the reflectance and reducing defects in the Si substrate using hydrogen contained in the intermediate film 23.

  Next, after forming the intermediate layer 23, a SiN-based material is deposited as the buried portion 20. In this embodiment mode, it is formed by a CVD method. When the embedded portion and the convex portion are formed separately as in the present embodiment, the thickness of forming the SiN material per time is reduced. When the amount of film formation increases, voids are likely to be generated in the film, but when only the buried portion is formed, cavities are less likely to be generated, so that the yield does not deteriorate.

When the cavity is generated, the light is scattered, so that the sensitivity is lowered and the smear ratio is easily deteriorated.
Next, after forming the buried portion 20 of SiN type material, the surface is planarized by CMP or etching.
As shown in FIG. 10B, after the surface is flattened, a convex SiN film 24 is formed.
Next, as shown in FIG. 10C, a resist 25 is applied, exposed with a mask, and patterned.

As shown in FIG. 10D, dry etching is performed after patterning to form the convex portion 26.
As can be seen from the process flow, the design of the convex portion 26 is easy because it is formed in a separate process from the embedded portion. Specifically, it is easy to deal with such as changing the opening diameter of the convex portion for each color.

In the present embodiment, the case where the upper surface of the convex portion is a plane is described as an example, but the same effect can be expected even when the upper surface of the convex portion is curved (convex lens shape).
According to the embodiment of the present invention, smear is reduced and smear in the entire pixel portion is improved, so that product quality deterioration can be prevented. In addition, a decrease in sensitivity around the pixel portion is also suppressed.
In addition, since light confinement with respect to oblique light around the pixel portion is strengthened, color mixture in adjacent pixels is also reduced.

  Although this embodiment has been described by taking a CCD device as an example, it is clear that a solid-state imaging device (for example, a CMOS sensor) employing an optical system using an optical waveguide has a noise reduction effect and a color mixing prevention effect. It is.

  The present invention is useful for realizing a solid-state imaging device that uses input light captured by a light receiving unit as a video signal.

1. 1. Solid-state image sensor R pixel G pixel 4. B pixel 5. Vertical CCD unit Horizontal CCD unit 7. Amplifier section 8. Silicon substrate 9. Light receiving unit 10. 10. Gate oxide film Read gate electrode 12. Light shielding film 13. G optical waveguide 14. 14. Optical waveguide for R light Intermediate layer 16. Microlens 17. Green filter 18. Red filter 19. Convex part 20. Embedded unit 21. B optical waveguide 22. Oxide film 23. Intermediate layer 24. Convex SiN film 25. Resist 26. Pixel part

Claims (6)

A semiconductor substrate having a plurality of light receiving portions formed in an imaging region;
A first transparent film formed on the semiconductor substrate;
A second transparent film formed on the light receiving portion and formed inside the first transparent film;
The refractive index of the second transparent film is larger than the refractive index of the first transparent film,
Solid-state imaging characterized in that the diameter of the upper surface of the second transparent film located in the central part of the imaging region is larger than the diameter of the second transparent film located in the peripheral part of the imaging region apparatus. Above the light receiving unit, further comprising a color filter on the second transparent film,
In the central part of the imaging region, the diameter of the upper surface of the second transparent film provided with the red color filter of the color filter is the second transparent film of the previous period provided with the blue color filter of the color filters. The solid-state imaging device according to claim 1, wherein the solid-state imaging device is larger than a diameter of an upper surface of the solid-state imaging device. In the center of the imaging area,
The diameter of the upper surface of the second transparent film having the green color filter of the color filters is larger than the diameter of the upper surface of the second transparent film having the blue color filter of the color filters, 3. The solid-state imaging device according to claim 2, wherein the color filter is smaller than a diameter of an upper surface of the second transparent film provided with a red color filter of the color filters.   The solid-state imaging device according to claim 1, wherein the second transparent film is formed independently for each of the plurality of light receiving units.   The said 2nd transparent film is formed ranging over these several light-receiving parts, and protrudes in convex shape upwards for every said several light-receiving parts, The any one of Claims 1-3 characterized by the above-mentioned. The solid-state imaging device described. In the periphery of the imaging area,
The solid-state imaging device according to claim 5, wherein the convex portion of the second transparent film is offset toward the center of the imaging region with respect to the light receiving unit.

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