JP2008091660A - Imaging device - Google Patents

Abstract

An object of the present invention is to suppress attenuation of light when reflecting an optical waveguide and reduce a difference in received light amount generated between a central portion and a peripheral portion of an image sensor.
An optical waveguide 31 is provided on a light receiving element 20 in a CCD image sensor 12. The optical waveguide 31 is provided with a light reflecting layer 32 for reflecting incident light and guiding it to the light receiving element 20. The incident angle of the subject light with respect to the CCD image sensor 12 becomes an appropriate angle by the optical waveguide 31 and the imaging lens. The principal ray of incident light incident on the optical waveguide 31 reaches the light receiving element 20 without being reflected once or reflected by the optical waveguide 31.
[Selection] Figure 3

Description

  The present invention relates to an imaging apparatus provided with an optical waveguide that guides incident light to each light receiving element.

  CCD image sensors and CMOS image sensors are known as image sensors used in digital cameras. Each image sensor includes a plurality of light receiving elements that convert an optical image into signal charges for each pixel, and generates an image signal from the obtained signal charges. Image sensors have been increasing in number of pixels so that high-resolution images can be recorded. When the pixel density is increased to increase the number of pixels, the light receiving area where one light receiving element receives light is reduced. In view of this, an image sensor in which a microlens that collects incident light is provided in front of the light receiving element to improve the light use efficiency has been known.

In addition, an optical waveguide that guides incident light to the light receiving element is provided between the light receiving element and the microlens. Light that cannot reach the light receiving element directly is reflected by the optical waveguide, and the light reaching the light receiving element is bent by bending the light path. An image sensor with an increased amount is known (see Patent Documents 1 to 4). The optical waveguide is formed of a reflective layer having a high light reflectivity provided between the light receiving element and the microlens, for example, and the reflection layer suppresses the attenuation of incident light and improves the sensitivity of the image sensor.
JP-A-5-283661 JP 7-45805 A JP 2000-150845 A JP 2002-359363 A

  However, when a subject image is formed on the image sensor, the incident angle is small at the center of the image sensor and the light is incident almost vertically, whereas the incident angle is large at the peripheral part of the image sensor and the light is inclined. Incident from. That is, the incident angle of light is different between the central portion and the peripheral portion of the image sensor, and the number of times the incident light reflects on the optical waveguide increases at the peripheral portion of the image sensor. Even if an optical waveguide is provided, light attenuation due to reflection occurs not a little, but there is a disadvantage that the sensitivity of the light receiving element in the peripheral part of the image sensor having a large number of reflections is lower than that in the central part.

  An object of the present invention is to provide an imaging apparatus capable of reducing the sensitivity deviation of the light receiving elements between the central portion and the peripheral portion of the image sensor by paying attention to the above problems.

  In order to achieve the above object, the present invention provides an imaging lens for forming a subject image, a plurality of light receiving elements for converting the subject image into signal charges for each pixel, and reflecting incident light to each light receiving element. And a plurality of optical waveguides guided to the light receiving element, wherein the principal ray of incident light incident on the light receiving element from the imaging lens is reflected by the optical waveguide one time or less.

  The optical waveguide is formed in a cylindrical shape on the light receiving element, includes a light reflection layer through which incident light passes, and an incident angle of a chief ray of incident light incident on the optical waveguide is θ1. When the refractive index of the waveguide is N1, the length of the optical waveguide is L1, and the width is D1, tan (θ1 / N1) ≦ D1 / L1 is satisfied.

  The optical waveguide includes a high refractive index layer having a higher refractive index than a surrounding transparent body and formed in a columnar shape.

  A plurality of condensing lenses for condensing incident light from the imaging lens on each light receiving element are provided, and the position where the condensing lenses are provided is shifted to the center side of the light receiving surface on which the plurality of light receiving elements are arranged. It is characterized by that.

  The number of times that the light beam incident from the imaging lens to the light receiving element at the maximum incident angle is reflected by the optical waveguide is one or less.

  According to the present invention, the angle of incident light incident on the light receiving element from the imaging lens and the structure of the optical waveguide are optimized, and the number of times the principal ray of the incident light is reflected on the optical waveguide is less than one. Even for light incident on the periphery of the image sensor, the loss of incident light due to reflection on the optical waveguide can be minimized. That is, the amount of light that can be detected by the light receiving element located in the peripheral portion of the image sensor is increased, and an effect of reducing shading that reduces the light amount in the peripheral portion of the image can be obtained.

  In FIG. 1, an imaging apparatus 10 forms an image of incident subject light, forms an object image, and a CCD image sensor 12 that photoelectrically converts the object image formed by the image forming lens 11 as an image signal. It is comprised by. The imaging lens 11 is constituted by a plurality of lenses, for example, and has an optical axis 11a. The CCD image sensor 12 is provided on the image plane of the imaging lens 11. As is well known, when light from a specific object point on the object plane 13 forms an image, the chief ray from the object point away from the optical axis 11a has a large incident angle on the CCD image sensor 12, and the CCD image Incidently incident on the sensor 12. Further, the chief ray from an object point close to the optical axis 11a has a small incident angle on the CCD image sensor 12 and enters the CCD image sensor 12 almost perpendicularly. In addition, the upper limit light beam and the lower limit light beam from an object point not on the optical axis 11a are a light beam having a larger incident angle and a light beam having a smaller incident angle than the principal ray.

  As shown in FIG. 2, the CCD image sensor 12 includes a plurality of light receiving elements 20 made of photodiodes arranged in a plane. The light detected by the light receiving element 20 is accumulated as a signal charge corresponding to the amount of light received. The CCD image sensor 12 is an interline transfer type CCD image sensor, and is provided with a vertical transfer CCD 21 and a horizontal transfer CCD 22. The vertical transfer CCD 21 transfers signal charges generated by the plurality of light receiving elements 20 arranged in the vertical direction. The horizontal transfer CCD 22 is connected in common to the last stage of the plurality of vertical transfer CCDs 21, and horizontally transfers the signal charges transferred by the vertical transfer CCD 21.

  The accumulated signal charges are read to the vertical transfer CCD 21 at predetermined read cycles via a read transfer gate connecting the light receiving element 20 and the vertical transfer CCD 21. The vertical transfer CCD 21 performs vertical transfer in which the read signal charges are transferred to the horizontal transfer CCD 22 step by step. The horizontal transfer CCD 22 receives the signal charge transferred from each final stage of the vertical transfer CCD 21, and performs horizontal transfer of the signal charge for one stage toward the output amplifier 23. The output amplifier 23 outputs a voltage signal corresponding to the amount of signal charge output from the horizontal transfer CCD 22 as an image signal.

  In FIG. 3, a transfer electrode 26, an insulating layer 27 made of silicon oxide covering the top and side portions of the transfer electrode 26, and a light shielding layer 29 covering the insulating layer 27 are formed above the vertical transfer CCD 21. Yes. In the light shielding layer 29, an opening 30 is formed on the light receiving element 20, and the light is incident on the light receiving element 20 to prevent the light from entering the vertical transfer CCD 21. The light shielding layer 29 is made of a light shielding material such as tungsten or aluminum.

  An optical waveguide 31 is provided in the opening 30. The optical waveguide 31 is filled with a transparent insulating material such as silicon dioxide, and includes a light reflecting layer 32 made of a metal having a high reflectivity. Examples of the highly reflective material used for the light reflecting layer 32 include aluminum, silver, and gold. The light reflection layer 32 prevents incident light from being reflected or absorbed by the light shielding layer 29 before reaching the light receiving element 20, and suppresses light loss by guiding the incident light to reach the light receiving element 20.

  A planarizing layer 33 is formed on the light shielding layer 29 and the optical waveguide 31. The planarizing layer 33 is made of BPSG (Boron Phosphorous Silicate Glass). A color filter 34 that transmits light of a specific color such as red, green, or blue is provided on the opening 30. On the color filter 34, a convex microlens 35 for condensing incident light is provided. The center of the light receiving surface of the light receiving element 20, the center of the optical waveguide 31, and the center of the microlens 35 are located on the same axis.

  In FIG. 4, when incident light reflects the optical waveguide 31, a loss of incident light due to reflection occurs. For this reason, the imaging lens 11 is suitable for the structure of the optical waveguide 31, and the number of times the incident light is reflected by the optical waveguide 31 is limited to suppress light loss. In order for the imaging lens 11 to be suitable for the CCD image sensor 12, when the refractive index inside the optical waveguide 31 is N1, the length of the optical waveguide 31 is L1, and the width of the optical waveguide 31 is D1, The condition is that incident light is incident on the optical waveguide 31 at an incident angle θ1 expressed as tan (θ1 / N1) ≦ D1 / L1.

  The length L1 of the optical waveguide 31 is substantially equal to the total thickness of the transfer electrode 26, the insulating layer 27, and the light shielding layer 29, and the width D1 of the optical waveguide 31 is the light receiving element 20 corresponding to the pixel density of the CCD image sensor 12. Is approximately equal to the width of the light receiving surface. When the structure of the optical waveguide 31 is determined, the upper limit value of the incident angle of the principal ray when entering the optical waveguide 31 is determined. When the upper limit value is determined, the maximum incident angle allowed when the principal ray is incident on the CCD image sensor 12 from the imaging lens 11 is obtained, and the imaging lens 11 that is optimally combined with the CCD image sensor 12 is obtained. The angle of view is determined.

  By using the imaging lens 11 determined in this manner, the principal ray of incident light incident on each light receiving element is reflected once on the optical waveguide 31 and incident on the light receiving element 20 or reflected on the optical waveguide 31. Without being incident on the light receiving element 20. Since the principal ray from a predetermined object point is not reflected a plurality of times in the optical waveguide 31, the attenuation of incident light due to the reflection in the optical waveguide 31 is effectively suppressed. In particular, the light incident on the periphery of the CCD image sensor 12 tends to increase in the number of reflections in the optical waveguide 31 due to the large incident angle. The difference in the amount of received light that occurs between the central portion and the peripheral portion of the image sensor 12 can be reduced.

  Next, a second embodiment of the present invention will be described. The CCD image sensor 40 is provided with an optical waveguide 42 having a transparent high refractive index layer 41 on the light receiving element 20. The optical waveguide 42 is filled with a low refractive index material constituting the planarizing layer 43 between the high refractive index layer 41 and the light shielding layer 29, that is, around the high refractive index layer 41. In the optical waveguide 42, the high refractive index layer 41 functions as a core and the planarization layer 43 functions as a cladding, and totally reflects light incident on the high refractive index layer 41 and guides it to the light receiving element 20.

  In order to determine the combination of the CCD image sensor 40 including the optical waveguide 42 having such a structure and the imaging lens, the length of the optical waveguide 42 (the thickness of the high refractive index layer 41) is set to L1, and the optical waveguide 42 When the width (the width of the high refractive index layer 41) is D1 and the refractive index of the high refractive index layer 41 is N1, the high refractive index layer 41 has an incident angle θ1 that satisfies tan (θ1 / N1) ≦ D1 / L1. The condition is that the chief ray is incident. By using an imaging lens that satisfies this condition, the principal ray incident on the high refractive index layer 41 is reflected once by the optical waveguide 42 and incident on the light receiving element 20 or received without being reflected by the optical waveguide 42. The light can be directly incident on the element 20. That is, since the chief ray that is attenuated by a plurality of reflections does not occur in the optical waveguide 42, the difference in the amount of received light due to the reflection of the optical waveguide 42 between the central portion and the peripheral portion of the CCD image sensor 40 is reduced.

  In the embodiment described above, the center of the light receiving element, the center of the optical waveguide, and the center of the microlens are located on the same axis. Here, when the microlens is shifted toward the center of the image sensor, the incident angle of the principal ray incident on the optical waveguide can be reduced. As a result, even when the incident angle of the subject light incident on the image sensor from the imaging lens increases, the number of reflections of the principal ray in the optical waveguide can be reduced to one or less, and the imaging lens can be easily designed.

  In order to further reduce the difference in the amount of light received due to reflection in the optical waveguide between the central part and the peripheral part of the image sensor, not only the principal ray incident on the optical waveguide but also the upper limit where the incident angle is larger than the principal ray It is preferable that the number of reflections of all the light beams including the light beam or the lower limit light beam is 1 or less. When a material having a high light reflectance is used for the light shielding layer that shields the signal charge transfer path, the opening formed in the light shielding layer can also be used as an optical waveguide. The image sensor used in the present invention is not limited to a CCD image sensor, and can be applied to a CMOS image sensor.

It is a block diagram of an imaging device. It is a conceptual diagram of a CCD image sensor. It is sectional drawing of a CCD image sensor. It is a conceptual diagram explaining the optical path of the chief ray which injects into an optical waveguide. It is sectional drawing of the CCD image sensor of 2nd Embodiment. It is explanatory drawing for demonstrating the positional relationship of a micro lens and a light receiving element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Image pick-up device 11 Imaging lens 11a Optical axis 12 CCD image sensor 20 Light receiving element 29 Light shielding layer 31 Optical waveguide 32 Light reflection layer 33 Flattening layer 35 Microlens 40 CCD image sensor 41 High refractive index layer 42 Optical waveguide 43 Flattening layer

Claims (5)

An imaging lens that forms a subject image, a plurality of light receiving elements that convert the subject image into signal charges for each pixel, and a plurality of optical waveguides that reflect incident light and guide it to each light receiving element,
An imaging apparatus characterized in that the principal ray of incident light incident on the light receiving element from the imaging lens is reflected by the optical waveguide one time or less. The optical waveguide is formed in a cylindrical shape on the light receiving element, includes a light reflection layer through which incident light passes, and an incident angle of a chief ray of incident light incident on the optical waveguide is θ1. When the refractive index of the light transmission part of the waveguide is N1, the length of the optical waveguide is L1, and the width is D1,
tan (θ1 / N1) ≦ D1 / L1
The imaging device according to claim 1, wherein:   The imaging apparatus according to claim 2, wherein the optical waveguide includes a high-refractive index layer having a higher refractive index than a surrounding transparent body and formed in a columnar shape.   A plurality of condensing lenses for condensing incident light from the imaging lens on each light receiving element are provided, and the position where the condensing lenses are provided is shifted to the center side of the light receiving surface on which the plurality of light receiving elements are arranged. The image pickup apparatus according to claim 1, wherein the image pickup apparatus is an image pickup apparatus.   The imaging apparatus according to claim 1, wherein a light beam incident at a maximum incident angle from the imaging lens to the light receiving element is reflected by the optical waveguide one time or less.

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