JP2008103634A - Solid-state imaging element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To maximally exploit the effect of improvement of light-receiving efficiency using an optical waveguide in a sold-state imaging element. <P>SOLUTION: A CCD image sensor 2 is provided with a plurality of light-receiving sections 10 arranged on a substrate 20 and receiving a light to execute photoelectric conversion, an optical waveguide 30 for guiding a light to the light-receiving sections 10, and a micro-lens 34 for collecting lights to an incident surface 30a of the optical waveguide 30. The optical waveguide 30 has a core layer 31 formed so that its width is the same from the light-receiving section 10 side to a light incident side and guiding a light toward the light-receiving sections 10, and a cladding layer 32 having a refractive index lower than that of the core layer 31 and surrounding the core layer 31. The optical waveguide 30 is formed so that (n<SB>1</SB><SP>2</SP>-n<SB>2</SB><SP>2</SP>)<SP>1/2</SP>≥NA is established, where NA is a numerical aperture of an optical system including the micro-lens 34 and the optical waveguide 30, and n<SB>1</SB>, n<SB>2</SB>are the refractive indexes of the core layer 31, and the cladding layer 32, respectively. Thus, an incident light guided by the optical waveguide 30 is guided without any leakage to an opening 28. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a solid-state imaging device such as a CCD used in a digital camera or a video camera.

  Solid-state imaging devices such as CCDs used for digital cameras and video cameras are required to have high resolution in order to improve image quality. In addition, along with miniaturization of digital cameras and video cameras, further miniaturization is required. Therefore, in order to increase the resolution and reduce the size, it is necessary to increase the number of pixels and increase the pixel arrangement density.

  In order to increase the number of pixels and increase the pixel arrangement density, the size of the pixels itself can be reduced. However, if the size is reduced, the amount of light incident on the pixels is reduced and the sensitivity is lowered. . For this reason, in the conventional solid-state image sensor, the microlens which condenses incident light was provided and the high sensitivity was aimed at. However, in the current solid-state imaging device in which the pixel size is miniaturized to about several hundred nm to several μm, a desired sensitivity cannot be obtained with only a microlens, and miniaturization has reached the limit.

Therefore, in order to achieve high sensitivity and miniaturization that could not be achieved only with a microlens, a solid-state imaging device in which a light guide for guiding incident light to the light receiving portion is formed on the light receiving portion constituting the pixel has been proposed. (See Patent Documents 1 and 2).
JP-A-5-235313 JP 2005-251804 A

  By the way, conventionally, when manufacturing a microlens, a heat-deformable resin as a raw material of the microlens is formed, patterned so that one microlens is assigned to one pixel, and then subjected to a reflow heat treatment. Since a desired convex lens shape is obtained, it is difficult to strictly control the size of the microlens. For this reason, in the invention described in Patent Documents 1 and 2, the size of the light guide path for efficiently guiding the light condensed by the microlens to the light receiving portion is not particularly defined. However, if the size of the microlens can be strictly controlled, it is necessary to optimize the size of the light guide path in order to increase the light receiving efficiency.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a solid-state imaging device capable of maximizing the effect of improving the light receiving efficiency by the light guide path.

In order to achieve the above object, the present invention provides a plurality of light receiving units arranged on a substrate that receive light and perform photoelectric conversion, and have substantially the same width from the light receiving unit side to the light incident side. A core layer that guides light toward the light receiving portion, a light guide path that has a lower refractive index than the core layer and that surrounds the core layer, and an incident surface of the light guide path condensing light to the solid-state imaging device and a microlens, the microlens and the numerical aperture of the optical system including the light guide NA, n ₁ the refractive index of the core layer and the cladding layer _{respectively,} n ₂ The following conditional expression is satisfied.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA

When the incident angle of light to the microlens is θ and the angle between the light incident on the core layer and a line perpendicular to the light receiving portion is γ ′, the following conditional expression is preferably satisfied.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′, where NA ′ = n ₁ sin γ ′ = [NA {(n ₁ / sin α) ² −1} ^1/2 + n ₁ − (n ₁ ² − NA ² ) ^1/2 ] sin α / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 , sin α = NA cos θ + sin θ {1− (n ₁ ² −NA ² ) ^1/2 } / {N ₁ ² -2 (n ₁ ² -NA ² ) ^1/2 +1} ^1/2

Further, the present invention provides a plurality of light receiving portions arranged on a substrate, receiving light and performing photoelectric conversion, and tapered so that the width gradually increases from the light receiving portion side toward the light incident side. A light guide path formed of a core layer that guides light toward the light receiving portion, a light guide path that has a lower refractive index than the core layer and that surrounds the core layer, and light incident on an incident surface of the light guide path In a solid-state imaging device including a microlens that collects light, an angle formed by a line perpendicular to the light receiving portion in the core layer of light incident perpendicularly to the light receiving portion is γ, the microlens, and the light guide When the numerical aperture of the optical system including the optical path is NA, the taper angle of the core layer is φ, and the refractive indices of the core layer and the cladding layer are n ₁ and n ₂ , respectively, the following conditional expressions are satisfied: And
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ″, where NA ″ = n ₁ sin (γ + φ) = NA cos φ + sin φ (n ₁ ² −NA ² ) ^1/2

When the incident angle of light to the microlens is θ and the angle between the light incident on the core layer and a line perpendicular to the light receiving portion is γ ′, the following conditional expression is preferably satisfied.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′ ″, where NA ′ ″ = n ₁ sin (γ ′ + φ) = [[NA {(n ₁ / sin α) ² −1} ^{1 / 2} + n ₁ − (n ₁ ² −NA ² ) ^1/2 ] cos φ + [{(n ₁ ² −NA ² ) ^1/2 −1} {(n ₁ / sin α) ² −1} ^1/2 + NA] sin φ ] Sin α / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 , sin α = NA cos θ + sin θ {1− (n ₁ ² −NA ² ) ^1/2 } / {n ₁ ² − 2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2

According to the solid-state imaging device of the present invention, when the core layer is formed so that the width is substantially the same from the light receiving unit side to the light incident side, the optical system including the microlens and the light guide path When the numerical aperture is NA and the refractive indexes of the core layer and the cladding layer are n ₁ and n ₂ , respectively, (n ₁ ² −n ₂ ² ) ^1/2 ≧ NA is satisfied and light enters from the light receiving unit side. When the core layer is formed in a tapered shape so that the width gradually increases toward the side, when the incident angle of light to the microlens is θ and the taper angle of the core layer is φ, (n ₁ ² −n ₂ ² ) ^1/2 ≧ NA ″, NA ″ = n ₁ sin (θ + φ) = NAcosφ + sinφ (n ₁ ² −NA ² ) ^1/2 is satisfied, so that incident light guided to the light guide Can be guided to the light receiving section without leakage. Therefore, it is possible to maximize the effect of improving the light receiving efficiency by the light guide path.

  In FIG. 1, a CCD image sensor 2 includes a light receiving unit 10, a read transfer gate (hereinafter abbreviated as TG) 11, a vertical CCD (hereinafter abbreviated as VCCD) 12, and a horizontal CCD (hereinafter abbreviated as HCCD) 13. And an output amplifier 14.

  A plurality of light receiving units 10 are arranged in a matrix at a predetermined pitch in the vertical direction (arrow A direction) and the horizontal direction (arrow B direction). The light receiving unit 10 receives light incident through the microlens 34 and the light guide path 30 (both see FIG. 2), performs photoelectric conversion, and generates and accumulates signal charges corresponding to the amount of incident light. The TG 11 is provided in each light receiving unit 10 and transfers the signal charge accumulated in the light receiving unit 10 to the VCCD 12.

  The VCCD 12 is provided between the vertical columns of the light receiving unit 10. The VCCD 12 transfers the signal charge transferred from the light receiving unit 10 via the TG 11 in the vertical direction line by line toward the HCCD 13. The last end of each VCCD 12 is connected to the HCCD 13. The HCCD 13 receives the signal charges output from the last end of the VCCD 12 line by line, and transfers the signal charges to the output amplifier 14 each time the signal charge for one line is received. The output amplifier 14 converts the signal charge for one row from the HCCD 13 into a signal voltage corresponding to the charge amount, and outputs the signal voltage to the outside of the CCD image sensor 2 as an imaging signal.

In FIG. 2 showing a cross section taken along the line aa ′ in FIG. 1, for example, a p-type well layer 21 is formed on an n-type semiconductor substrate 20 made of silicon. In the p-type well layer 21, a first n-type layer 22 for accumulating signal charges is formed. A p ⁺ type layer 23 that accumulates holes is formed on the first n-type layer 22. A second n-type layer 24 is formed on the surface layer of the p-type well layer 21. The second n-type layer 24 is separated from the first n-type layer 22 by the p-type well layer 21 or the p ⁺ -type layer 23.

  The light receiving unit 10 is a so-called embedded photodiode configured by a pnpn junction of each layer including the substrate 20. The TG 11 is constituted by a p-type well layer 21 between the first and second n-type layers 22 and 24, and the VCCD 12 is constituted by a second n-type layer 24.

  A transfer electrode (for example, made of polycrystalline silicon) 26 is formed above the second n-type layer 24 via a first insulating layer (for example, made of silicon oxide) 25. The transfer electrode 26 also extends on the p-type well layer 21 between the first and second n-type layers 22 and 24. A drive voltage for controlling the vertical transfer of signal charges by the VCCD 12 and the read transfer of signal charges from the first n-type layer 22 to the second n-type layer 24 by the TG 11 is applied to the transfer electrode 26.

  On the second insulating layer (for example, made of silicon oxide) 27 covering the periphery of the transfer electrode 26, a light shielding layer (for example, made of tungsten) 29 that shields light from a region other than the opening 28 of each light receiving unit 10. Is formed. A light guide path 30 is formed on the opening 28.

  The light guide 30 guides incident light toward the opening 28, and has a core layer 31 made of a material having a high refractive index, for example, silicon nitride (refractive index 1.9 to 2.0), and an insulating property. And a clad layer 32 made of a material having a low refractive index, for example, BPSG (Boron phosphorus silicate glass, refractive index 1.4 to 1.5).

  The core layer 31 is formed in a substantially cylindrical shape, and is arranged so that the center thereof coincides with the center of the opening 28. The surface of the cladding layer 32 is flattened by a CMP (Chemical Mechanical Polishing) method or the like.

  On the cladding layer 32, a color filter 33 and a microlens 34 are provided. The color filter 33 is made of a resist material containing a dye that transmits light of a specific color, for example, red, green, blue, or cyan, magenta, and yellow. The microlens 34 is arranged so that the optical axis L passing through the center thereof passes through the center of the opening 28 and is perpendicular to the surface of the opening 28, and incident light parallel to the optical axis L is efficiently incident on the light guide 30. The curvature is such that the light is condensed toward the surface 30a. In order to avoid complications, only the color filter 33 and the microlens 34 are hatched, and other portions are omitted.

  Here, in order to guide the incident light guided to the light guide path 30 toward the opening 28 without omission, a condition in which the incident light does not transmit from the core layer 31 to the cladding layer 32, that is, the core layer 31 and the cladding layer 32, What is necessary is just to obtain | require the conditions from which incident light totally reflects in the interface of this.

When light is incident in parallel to the optical axis L (incidence angle 0 °), as shown in FIG. 3, the light a incident from the outermost peripheral portion of the microlens 34 is incident from the center side of the microlens 34. The light b has a larger incident angle to the interface between the core layer 31 and the clad layer 32. Therefore, if the light a is totally reflected, all incident light guided to the light guide path 30 is totally reflected. Therefore, finally, a condition that the light a is totally reflected at the interface between the core layer 31 and the cladding layer 32 may be obtained. That is, as shown in FIG. 4, the angle formed by the optical axis L in the core layer 31 of light incident parallel to the optical axis L is γ, and the refractive indices of the core layer 31 and the cladding layer 32 are n ₁ , respectively. When n ₂ , Snell's law
sin (π / 2−γ) ≧ n ₂ / n ₁ (1)
It becomes. When this equation (1) is transformed,
(N ₁ ² −n ₂ ² ) ^1/2 ≧ n ₁ sin γ
Here, since n ₁ sin γ = NA, the above equation is
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA (2)
It becomes. The light guide 30 is formed so that the refractive index of the core layer 31 and the clad layer 32 satisfies the formula (2).

  Next, the operation of the CCD image sensor 2 having the above configuration will be described. The light incident on the CCD image sensor 2 is condensed on the incident surface 30 a by the microlens 34. The light condensed on the incident surface 30 a is guided to the opening 28 by the light guide path 30 because the refractive index of the core layer 31 is higher than that of the cladding layer 32. At this time, since the refractive index of the core layer 31 and the cladding layer 32 is formed so as to satisfy the formula (2), all the incident light guided to the light guide path 30 is at the interface between the core layer 31 and the cladding layer 32. It is reflected and led to the opening 28 without leakage.

  When light is received by the light receiving unit 10, photoelectric conversion is performed by the light receiving unit 10, and signal charges corresponding to the amount of incident light are generated and accumulated. When a drive voltage is applied to the transfer electrode 26, the signal charge accumulated in the light receiving unit 10 is transferred to the VCCD 12 via the TG 11. The signal charges transferred to the VCCD 12 are transferred vertically to the VCCD 12 and transferred to the HCCD 13. The signal charge transferred to the HCCD 13 is horizontally transferred to the HCCD 13, converted into a signal voltage by the output amplifier 14, and output to the outside as an imaging signal.

  As described above, since the light guide path 30 is formed so as to satisfy the expression (2), the incident light guided to the light guide path 30 can be efficiently incident on the light receiving unit 10, and the CCD image sensor 2 has a high height. Sensitivity can be promoted. As a result, the CCD image sensor 2 can be reduced in size and resolution.

In the above embodiment, only incident light parallel to the optical axis L is described. However, not only incident light parallel to the optical axis L but also light is incident from an oblique direction not parallel to the optical axis L. In this case, in order to guide the incident light guided to the light guide path 30 toward the opening 28 without omission, the outermost peripheral edge of the microlens 34 shown in FIG. What is necessary is just to obtain conditions under which the light a ′ incident from the part is totally reflected at the interface between the core layer 31 and the cladding layer 32. That is, as shown in FIG. 6, when the angle formed by the optical axis L in the core layer 31 of the light incident obliquely with respect to the optical axis L is γ ′,
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′
However, NA ′ = n ₁ sin γ ′ = [NA {(n ₁ / sin α) ² −1} ^1/2 + n ₁ − (n ₁ ² −NA ² ) ^1/2 ] sin α / {n ₁ ² −2 ( n ₁ ² −NA ² ) ^1/2 +1} ^1/2 ,
sin α = NA cos θ + sin θ {1- (n ₁ ² −NA ² ) ^1/2 } / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 (3)
It is preferable to satisfy.

  In the said embodiment, although the core layer 31 is made into the substantially cylindrical shape, square pillar shape may be sufficient. Further, as shown in FIG. 7, a light guide path 42 including a core layer 40 and a cladding layer 41 that are tapered so that the width gradually increases from the light receiving unit 10 side toward the light incident side is provided. It may be used.

In the case of the system shown in FIG. 7, the condition that the incident light is totally reflected at the interface between the core layer 40 and the clad layer 41 when the light enters parallel to the optical axis L is that the core layer 31 is substantially cylindrical. Similarly to the case, the light a shown in FIG. 8 may be required to be totally reflected at the interface between the core layer 40 and the cladding layer 41. That is, as shown in FIG. 9, the taper angle of the core layer 40 (the inclination angle of the cross section of the core layer 40) is φ and the optical axis L in the core layer 31 of the light incident in parallel to the optical axis L is formed. From Snell's law, when the angle is γ,
sin {π / 2− (φ + γ)} ≧ n ₂ / n ₁ (4)
It becomes. When this equation (4) is transformed into an equation including the numerical aperture NA,
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ″,
However, NA ″ = n ₁ sin (θ + φ) = NA cos φ + sin φ (n ₁ ² −NA ² ) ^1/2 (5)
It becomes. The light guide path 42 is formed so that the refractive index of the core layer 40 and the clad layer 41 and the taper angle of the core layer 40 satisfy the formula (5).

Further, in the case where the core layer 31 is substantially cylindrical in order to guide the incident light guided to the light guide path 42 toward the opening 28 without leakage when light is incident from an oblique direction not parallel to the optical axis L. Similarly to the above, it is only necessary to obtain a condition that the light a ′ shown in FIG. 10 is totally reflected at the interface between the core layer 40 and the clad layer 41. That is, as shown in FIG. 11, when the angle formed by the optical axis L in the core layer 40 of light incident obliquely with respect to the optical axis L is γ ′,
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′ ″,
However, NA ′ ″ = n ₁ sin (θ + φ) = [[NA {(n ₁ / sin α) ² −1} ^1/2 + n ₁ − (n ₁ ² −NA ² ) ^1/2 ] cos φ + [{( n ₁ ² −NA ² ) ^1/2 −1} {(n ₁ / sin α) ² −1} ^1/2 + NA] sin φ] sin α / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 ,
sin α = NA cos θ + sin θ {1- (n ₁ ² −NA ² ) ^1/2 } / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 (6)
It is preferable to satisfy.

  In the CCD image sensor having the light guide path 30 shown in FIG. 2, the size of the pixel formed by the light receiving portion (effective diameter D of the microlens) and the amount of incident light when the light guide path is not provided are 100%. There is a relationship as shown in FIG. That is, the increase rate becomes maximum when the pixel size is 2.05 μm, and thereafter, the increase rate decreases as the pixel size increases.

  Further, in the CCD image sensor as described above, between the pixel arrangement pitch and the incident light amount at an incident angle of 15 ° when the light amount of parallel incident light is 1, as shown in FIG. There is a relationship. That is, the amount of incident light increases as the pitch becomes wider, and the amount of incident light is larger overall when the light guide is present than when the light guide is not present. In the above embodiment, the pixel size and pitch are set with reference to the characteristics shown in FIGS. The numerical values in the graph indicate the pixel size on the left side and the opening width in the parenthesis on the right side.

  In the above embodiment, the interline transfer type CCD image sensor 2 is described as an example. However, the present invention is not limited to this, and other solid-state devices such as a frame transfer type CCD image sensor and a CMOS image sensor. It is also possible to apply to an image sensor.

It is a schematic plan view which shows the structure of a CCD image sensor. It is sectional drawing which follows the a-a 'line of FIG. It is a figure for demonstrating the conditional expression for the light which injected in parallel with the optical axis at the interface of a core layer and a clad layer to totally reflect. It is a figure for demonstrating the conditional expression for the light which injected in parallel with the optical axis at the interface of a core layer and a clad layer to totally reflect. It is a figure for demonstrating the conditional expression for the light which slanted with respect to the optical axis to totally reflect in the interface of a core layer and a clad layer. It is a figure for demonstrating the conditional expression for the light which slanted with respect to the optical axis to totally reflect in the interface of a core layer and a clad layer. It is sectional drawing which shows another embodiment. FIG. 8 is a diagram for explaining a conditional expression for total reflection of light incident parallel to the optical axis at the interface between the core layer and the clad layer in the system shown in FIG. 7. FIG. 8 is a diagram for explaining a conditional expression for total reflection of light incident parallel to the optical axis at the interface between the core layer and the clad layer in the system shown in FIG. 7. FIG. 8 is a diagram for explaining a conditional expression for totally reflecting light incident obliquely with respect to the optical axis at the interface between the core layer and the clad layer in the system shown in FIG. 7. FIG. 8 is a diagram for explaining a conditional expression for totally reflecting light incident obliquely with respect to the optical axis at the interface between the core layer and the clad layer in the system shown in FIG. 7. It is a graph which shows the relationship between pixel size and the increase rate of incident light quantity. It is a graph which shows the relationship between the arrangement pitch of a pixel, and the incident light quantity in the case of an incident angle of 15 degrees.

Explanation of symbols

2 CCD image sensor 10 Light-receiving part 20 Substrate 30, 42 Light guide 30a, 42a Incident surface 31, 40 Core layer 32, 41 Clad layer 34 Micro lens

Claims (4)

A plurality of light receiving units arranged on the substrate and receiving light to perform photoelectric conversion are formed to have substantially the same width from the light receiving unit side to the light incident side, toward the light receiving unit. Solid comprising: a core layer that guides light; a light guide that has a lower refractive index than the core layer, and that includes a clad layer that surrounds the core layer; and a microlens that collects light on an incident surface of the light guide In the image sensor,
Solids satisfying the following conditional expression, where NA is the numerical aperture of the optical system including the microlens and the light guide path, and n ₁ and n ₂ are the refractive indexes of the core layer and the cladding layer, respectively. Image sensor.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA The following conditional expression is satisfied, where θ is an incident angle of light to the microlens and γ ′ is an angle between a light incident on the core layer and a line perpendicular to the light receiving portion. Item 2. The solid-state imaging device according to Item 1.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′,
However, NA ′ = n ₁ sin γ ′ = [NA {(n ₁ / sin α) ² −1} ^1/2 + n ₁ − (n ₁ ² −NA ² ) ^1/2 ] sin α / {n ₁ ² −2 ( n ₁ ² −NA ² ) ^1/2 +1} ^1/2 ,
sin α = NA cos θ + sin θ {1- (n ₁ ² −NA ² ) ^1/2 } / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2 A plurality of light receiving units arranged on a substrate, receiving light and performing photoelectric conversion, and tapered so that a width is gradually increased from the light receiving unit side toward a light incident side. A core layer that guides light toward a portion, a light guide path that has a lower refractive index than the core layer and that includes a cladding layer that surrounds the core layer, and a microlens that collects light on an incident surface of the light guide path In a solid-state imaging device comprising:
The angle between the light incident perpendicularly to the light receiving portion and the line perpendicular to the light receiving portion in the core layer is γ, the numerical aperture of the optical system including the microlens and the light guide is NA, and the core layer A solid-state imaging device, wherein the following conditional expression is satisfied, where φ is a taper angle, and n ₁ and n ₂ are refractive indexes of the core layer and the cladding layer, respectively.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ″,
However, NA ″ = n ₁ sin (γ + φ) = NA cos φ + sin φ (n ₁ ² −NA ² ) ^1/2 The following conditional expression is satisfied, where θ is an incident angle of light to the microlens and γ ′ is an angle between a light incident on the core layer and a line perpendicular to the light receiving portion. Item 6. The solid-state imaging device according to Item 3.
(N ₁ ² −n ₂ ² ) ^1/2 ≧ NA ′ ″,
However, NA ′ ″ = n ₁ sin (γ ′ + φ) = [[NA {(n ₁ / sin α) ² −1} ^1/2 + n ₁ − (n ₁ ² −NA ² ) ^1/2 ] cos φ + [ {(N ₁ ² −NA ² ) ^1/2 −1} {(n ₁ / sin α) ² −1} ^1/2 + NA] sinφ] sin α / {n ₁ ² −2 (n ₁ ² −NA ² ) ^{1 / 2} +1} ^1/2 ,
sin α = NA cos θ + sin θ {1- (n ₁ ² −NA ² ) ^1/2 } / {n ₁ ² −2 (n ₁ ² −NA ² ) ^1/2 +1} ^1/2

Images