JP2005252391A - Imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus with a simple configuration for controlling the MTF without the need for provision of an apodization filter and an optical low pass filter on the optical system. <P>SOLUTION: The imaging apparatus consists of the optical system for forming an object image and of an imaging element provided with a plurality of light receiving elements 14 for applying photoelectric conversion to the object image formed by the optical system, and the imaging element is provided with an efficiency regulation section 19 for decreasing the utilizing efficiency of a formed luminous flux of the optical system from the center of the luminous flux toward the peripheral part as part of the structure of the light emitting elements. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an improvement in an imaging apparatus such as a digital still camera, a video camera, a surveillance camera, a mobile phone with an imaging function, or a focus detection apparatus for optical equipment that captures an image for focus adjustment.

  Conventionally, in the formation of a color image, monocular optics using a single light receiving element array in which light receiving elements having color filters such as R (red), G (green), and B (blue) are arranged in a mosaic pattern. An imaging technique that captures a single object image formed by elements and generates luminance information and color information corresponding to the number of light receiving elements by subsequent signal processing is widely used.

  On the other hand, the imaging apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-078214 previously filed and published by the applicant of the present application forms a plurality of object images using a compound eye optical element, and supports them. These object images are captured by a plurality of light receiving element arrays, and outputs from the light receiving element arrays are combined to form a single color image. An imaging apparatus using such a compound eye optical element is suitable for thinning.

  This imaging apparatus is configured to obtain a good MTF (Modulation Transfer Function) characteristic by using a filter whose transmittance continuously decreases from the center to the periphery of the stop, that is, a so-called apodization filter. When an apodization filter is used, the spatial frequency characteristics can be controlled so as to improve the response below the Nyquist frequency while suppressing the spatial frequency component above the Nyquist frequency determined by the light receiving element pitch of the image sensor.

  Such a compound-eye imaging apparatus can output image data exceeding 300,000 pixels per image with an apparatus thickness of only about 2.5 mm, and can be applied to various uses that require a reduction in thickness.

Further, in the technology related to the camera focus detection device, as disclosed in US Pat. No. 4,384,210, an optical low-pass filter is inserted in the imaging optical path, and a spatial frequency component exceeding the Nyquist frequency on the image sensor of the focus detection device. A technique for preventing projection of the image is known. If an MTF characteristic obtained by removing a spatial frequency component equal to or higher than the Nyquist frequency is obtained, so-called aliasing distortion is not superimposed on the output of the imaging device of the focus detection device, and accurate focus detection is possible.
JP 2001-078214 A USP 4384210

However, the conventional technology described above is
(1) If an apodization filter is disposed in front of or behind one lens, distortion (distortion aberration) increases, and a high-quality image cannot be obtained. Further, in order to form an apodization filter inside the lens, it is necessary to make two lenses, and the apparatus becomes complicated.
(2) The manufacturing process of the apodization filter such as vapor deposition and printing is complicated and expensive.
(3) Alternatively, the cost is increased due to the optical low-pass filter.
That was not enough.

Accordingly, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide an imaging apparatus having a simple configuration in which the MTF is controlled without providing an optical system with an apodization filter or an optical low-pass filter. That is.

  In order to solve the above-described problems and achieve the object, an imaging apparatus according to the present invention includes an optical system that forms an object image and a plurality of light-receiving elements that photoelectrically convert the object image formed by the optical system. An image sensor, and the image sensor includes, as part of the structure of the light receiving element, efficiency adjusting means for reducing the use efficiency of the imaging light beam of the optical system from the center to the periphery of the light beam. It is characterized by that.

  In the image pickup apparatus according to the present invention, the efficiency adjusting means includes a light diffusion layer formed in an optical path of light incident on the light receiving element.

In the imaging device according to the present invention, the light diffusion layer is configured by periodically arranging Si ₃ N ₄ and SiO ₂ .

  In the imaging apparatus according to the present invention, the efficiency adjusting unit includes a microlens formed in an optical path of light incident on the light receiving element.

  In the imaging apparatus according to the present invention, the microlens has a focal plane near the surface of the light receiving element.

  In the imaging apparatus according to the present invention, the microlens has a focal plane at an intermediate position between the macrolens and the surface of the light receiving element.

  In the imaging apparatus according to the present invention, the efficiency adjusting means includes an opening of 0.3 μm to 9.0 μm formed in the optical path of light incident on the light receiving element.

  In the imaging apparatus according to the present invention, the efficiency adjusting unit includes a microlens formed in an optical path of light incident on the light receiving element, and the opening is an intermediate between the microlens and the surface of the light receiving element. It is arranged at a position.

  According to the present invention, it is possible to provide an imaging apparatus having a simple configuration in which the MTF is controlled without including an apodization filter or an optical low-pass filter in the optical system.

(First embodiment)
1 to 4 are diagrams for explaining a first embodiment of an imaging apparatus according to the present invention.

  First, FIG. 2 is a side sectional view showing the imaging apparatus of the present embodiment.

  In FIG. 2, 1 is a convex lens, which is an optical element, 2 is an optical axis of the convex lens 1, 3 is an imaging element that converts an object image formed by the convex lens 1 into an electrical signal, and 4 is a convex lens 1 and imaging element 3. It is a housing that holds and functions as a dark box. Although the convex lens 1 is illustrated as a single lens for the sake of simplicity of explanation, it is actually an optical element having a total positive power by combining a plurality of lenses, reflecting mirrors, or diffractive optical elements. Also good. A zoom function can also be applied. Furthermore, the optical element 1 may have an aperture for light shielding for cutting ghosts and flares.

  The imaging element 3 includes a plurality of light receiving elements, and is configured by regularly arranging millions of light receiving elements vertically and horizontally or obliquely in order to obtain image data of millions of pixels, for example.

  FIG. 1 is an enlarged cross-sectional view of a light receiving element array of the image sensor 3. In FIG. 1, 13 is a silicon substrate, 14 is a photoelectric conversion unit formed on the silicon substrate, 17 is an aluminum wiring for controlling charge accumulation operation and signal readout operation of each light receiving element, 11 is a microlens, 18 is The optical axis of the microlens 11, 15 is a color filter that transmits a predetermined color component, 16 is an insulating layer, and 19 is a light diffusion layer.

The material and refractive index of each component are as follows.
Microlens 11 Material: Resin, Refractive index: 1.58
-Color filter 15 Material: Resin, refractive index 1.5
Insulating layer 16 Material: SiO ₂ , refractive index: 1.46
Light diffusing layer 19 Material: periodic structure of Si ₃ N ₄ and SiO ₂ , refractive index: Si ₃ N ₄ = 2.0, SiO ₂ = 1.46
The light diffusion layer deflects incident light in various directions by the periodic structure of Si ₃ N ₄ and SiO ₂ . This structure is a diffraction grating or a SWS (sub-wave-length structure).

  The microlens 11 is for increasing the light collection efficiency to the photoelectric conversion unit 14, and the light receiving element having this structure has a focal point in the vicinity of the photoelectric conversion unit 14. Therefore, the light beam emitted from the convex lens 1 forms a pupil image of the convex lens 1 on the photoelectric conversion unit 14 by the action of the micro lens 11. However, since the light rays are dispersed and deflected as indicated by the arrow 20 in the light diffusion layer 19, the pupil image becomes blurred.

  As a result, the transmitted light utilization efficiency on the pupil of the convex lens 1 is as shown in FIG. In FIG. 3A, the horizontal axis represents the position on the pupil with the origin on the optical axis, and the vertical axis represents the transmitted light utilization efficiency. If the light diffusing layer 19 is not provided, the pupil image on the photoelectric conversion unit 14 is formed to be sharp to some extent, and is limited to either the aperture of the convex lens 1 or the size of the photoelectric conversion unit 14 to obtain a predetermined value. Efficiency drops sharply from 1 to zero at the pupil position. A characteristic 30 shown in the figure is an example of this, and the efficiency is zero at a position on the pupil exceeding a due to a limitation due to the size of the photoelectric conversion unit 14.

  On the other hand, the characteristic 30 represents the case where the light diffusion layer 19 is present, and the efficiency is smoothly lowered from the center to the periphery of the pupil. The change in transmitted light utilization efficiency means that the apparent light intensity has a distribution, and has the same meaning as an apodization filter. FIG. 3B is a schematic diagram of the effective light intensity distribution on the pupil plane based on the characteristic 30 in FIG. A pupil plane is arranged at the center on the XY coordinates, and the effective light intensity is expressed by density. The effective light intensity is the highest on the optical axis 33 and decreases with increasing distance from the optical axis.

  The effective light intensity distribution is strongly related to the MTF characteristic as in the case of the apodization filter. However, if the MTF characteristic depends not only on the distance from the optical axis but also on the direction, it is as if on the image an optical element. Since it appears that the (lens) has a manufacturing error such as ass and habits, it is preferable that the effective light intensity characteristic is an axial object.

  4 and 5 show the effective light intensity distribution in a cross section of the imaging apparatus.

  4 and 5, the background of the imaging device shown in FIG. 2 is blackened in order to express the effective light intensity distribution by density. FIG. 4 shows an image forming beam incident on the light receiving element 37 on the optical axis, and FIG. 5 shows an image forming beam incident on the light receiving element 38 around the photographing screen. Each of the light beam 35 shown in FIG. 4 and the light beam 36 shown in FIG. 5 has a high intensity portion at the center, and the intensity decreases toward the periphery of the light beam. In order to adjust the effective light intensity distribution on the pupil of the convex lens 1 of the light receiving element 38 located around the photographing screen, the microlens 11 shown in FIG.

  When the utilization efficiency of the imaged light beam is reduced from the center to the periphery of the light beam, the response of the optical MTF is low in the high frequency range exceeding the Nyquist frequency determined by the light receiving element pitch of the image sensor, and below the Nyquist frequency. Highly desirable characteristics such as high response in the frequency range. Therefore, a high-definition image with less moire and sharpness can be obtained.

  In addition, since the high-frequency component is significantly reduced in the MTF of the defocused image, the effect of obtaining a soft and preferable blur is also great.

(Second Embodiment)
FIG. 6 to FIG. 11 are diagrams for explaining a second embodiment according to the present invention.

  In the second embodiment, imaging of an optical element is performed on an imaging apparatus using an imaging element having a refractive index distribution structure that reflects a light beam from the optical element and guides it to a photoelectric conversion unit and a compound eye optical element. An efficiency adjusting means for reducing the light beam utilization efficiency from the center to the periphery of the light beam is applied.

  In compound-eye imaging devices, the alignment of the optical axis of the optical system and the light receiving element is extremely important, and each compound eye is composed of a single lens because it dislikes that the optical axis shifts even with a small amount. It is desirable to do. In an optical system composed of a single lens, good optical aberration correction is usually not possible, but sufficient optical characteristics can be derived by using an efficiency adjusting means that adjusts the utilization efficiency of the imaging light beam. is there.

  First, FIG. 7 is a diagram illustrating the imaging apparatus of the present embodiment.

  FIG. 7A is a plan view of the imaging apparatus, FIG. 7B is a side view, and FIG. 7C is a plan view of an imaging element in which a semiconductor circuit as one element of the imaging apparatus is formed.

  An imaging apparatus 511 shown in FIG. 7 has a structure in which the compound eye optical element 512 and the imaging element 503 are integrated, and does not require a sensor package or a lens barrel. The object light incident on the optical element 512 from above in FIG. 7B forms a plurality of object images on the image sensor 503, and photoelectric conversion is performed by the light receiving element in the image sensor 503. The image sensor 503 is a CCD or CMOS sensor.

  The compound-eye optical element 512 is a plate-like transparent body made up of a single substrate and four convex lenses as an imaging action optical system. In the figure, reference numeral 501 denotes an optical element substrate for molding convex lenses 600a, 600b, 600c, and 600d that are image forming optical systems. The convex lenses 600b and 600d are not shown in the cross section shown in FIG.

  The optical element 512 has a structure in which an aspherical convex lens made of resin is added to the upper and lower surfaces of an optical element substrate 501 that is a flat glass substrate by a replica manufacturing method. In addition, a method in which the convex lens portion is formed integrally with the substrate by a method such as injection molding using a resin, compression molding, or the like, or glass molding using the entire glass or injection molding using the entire resin can be selected.

  A light shielding layer for cutting unnecessary light and an infrared cut filter are formed on the lower surface of the optical element substrate 501.

  Four object images are formed on the image sensor 503 by the optical element 512, and these are photoelectrically converted by the four light receiving element arrays 820a, 820b, 820c, and 820d provided on the image sensor and captured as electric signals. A light receiving element array 820a, 820b, 820c, and 820d shown in FIG. 7C is an array in which a large number of light receiving elements are arranged in a two-dimensional direction. In these four light receiving element arrays, a green transmission (G) filter, a red transmission (R) filter, a blue transmission (B) filter, and a green transmission (G) filter are formed, and four images are separated into three primary colors. Can be taken out.

  The distance between the image sensor 503 and the optical element 512 is adjusted by the thickness of the thermal ultraviolet curable epoxy resin 504 that bonds the transparent glass spacer 522 and the optical element 512. A TAB film 517 having an opening 517a is sandwiched between the spacer 522 and the image sensor 503, and the image sensor and the TAB film 517 are electrically connected through gold bumps. Further, the TAB film 517 is connected to an external electric circuit (not shown).

  The positional relationship of the light receiving element images when the light receiving element array is back projected onto the subject is such that the spatial phase of each light receiving element of the light receiving element arrays 820a, 820b, 820c, and 820d is shifted between the arrays. Then, sampling equivalent to that of an image pickup device having a Bayer array color filter is performed.

  In comparison with an imaging system that uses a single photographic lens, if the light receiving element pitch is fixed, it is compared with a Bayer array system in which a primary color filter is formed by combining two rows and two columns of light receiving elements on the image sensor. In this compound eye system, the size of the object image is 1 / √4. Accordingly, the focal length of the taking lens can be shortened to about 1 / √4 = 1/2. Therefore, it is extremely advantageous for making the camera thinner.

  Next, the structure of the light receiving element will be described with reference to FIGS.

  FIG. 8 is a detailed plan view of the image sensor shown in FIG.

  In FIG. 8, reference numerals 820a, 820b, 820c, and 820d denote light receiving element arrays. For the sake of explanation, each light receiving element array is a 35 light receiving element of 5 rows and 7 columns. In practice, an array of about 500 rows and 700 columns is used to increase the resolution and obtain a light-definition image. The broken lines surrounding the light receiving element arrays 820a, 820b, 820c, and 820d are for helping understanding of the positions of the arrays, and are not actually drawn on the image sensor. Reference numeral 101 denotes a microlens on the surface of the light receiving element, and reference numeral 102 denotes a high refractive index layer located in the back of the microlens 101. The microlens 101 is substantially square when viewed in a plan view in this direction, but its three-dimensional structure is a shape obtained by cutting an axisymmetric convex aspherical surface into a square.

  In addition, the optical axis of the microlens is at the center of the square, and the high refractive index layer 102 is decentered with increasing distance from the center of each light receiving element array. This is for efficiently guiding the light beam from the optical element to the photoelectric conversion unit. As will be described later, since this light receiving element has a refractive index distribution structure that reflects the light beam from the optical element and guides it to the photoelectric conversion unit, it is possible to increase the tolerance of the manufacturing error related to the eccentricity of the microlens. It is like that.

  Further, reference numeral 514 denotes an A / D conversion circuit that converts output signals from the light receiving element arrays 820a, 820b, 820c, and 820d into digital signals, and 515 indicates timing signals for photoelectric conversion operations of the light receiving element arrays 820a, 820b, 820c, and 820d. A timing generator 516 is an image processing circuit. If the light receiving element is a CMOS sensor, it is easy to mount these circuits on the semiconductor chip 503.

  FIG. 6 is an enlarged cross-sectional view of the light receiving element array 111 shown in FIG. In the light receiving element array 111, four continuous light receiving elements in the center are extracted from the light receiving element array 820b.

  In FIG. 6, 103 is a silicon substrate, 104 is a photoelectric conversion unit formed on the silicon substrate, 107 is an aluminum wiring for controlling charge accumulation operation and signal readout operation of each light receiving element, 101 is the above-described microlens, Reference numeral 108 denotes an optical axis of the microlens 101, 105 denotes a color filter that transmits a predetermined color component, 106 denotes a low refractive index layer, 102 denotes a high refractive index layer that forms a refractive index distribution structure together with the low refractive index layer 106, 109 denotes It is a light diffusion layer.

The material and refractive index of each component are as follows.
Microlens 101 Material: Resin, Refractive index: 1.58
Color filter 105 Material: Resin, Refractive index: 1.5
Low refractive index layer 106 Material: SiO ₂ , Refractive index: 1.46
High refractive index layer 102 Material: Si ₃ N ₄ , Refractive index: 2.0
Light diffusion layer 109 Material: Periodic structure of Si ₃ N ₄ and SiO ₂ Refractive index: Si ₃ N ₄ = 2.0, SiO ₂ = 1.46
The light diffusion layer deflects incident light in various directions by the periodic structure of Si ₃ N ₄ and SiO ₂ . This structure is a diffraction grating or a SWS (sub-wave-length structure).

  The microlens 101 is for increasing the efficiency of condensing light onto the photoelectric conversion unit 104. In the light receiving element having this structure, the microlens 101 has a focal point near the center of the high refractive index layer 102. Therefore, the light beam emitted from the convex lens 600 b of the optical element 512 forms a pupil image of the convex lens 600 b inside the high refractive index layer 102 by the action of the microlens 101. However, since light rays are dispersed and deflected in the light diffusion layer 109, the pupil image is blurred.

  Further, the high refractive index layer 102 is covered with a low refractive index layer 106, and light incident from the high refractive index layer 102 to the interface with the low refractive index layer 106 beyond the critical angle is totally reflected. This structure is for guiding the light beam obliquely incident on the light receiving element in the peripheral portion of the screen to the photoelectric conversion unit.

The refractive index structure in which the high-refractive index layer 102 is surrounded by the low-refractive index layer 106 is obtained by removing SiO ₂ once formed flat enough to sufficiently cover the aluminum wiring 107 by etching into a quadrangular column and then adding Si 2 ₃ N ₄ is embedded, smoothed, and a SiO ₂ layer is placed thereon.

  9 and 10 are diagrams for explaining the behavior of light incident on the light receiving element. FIG. 9 is a diagram showing one of the light receiving elements in the center of the screen, and FIG. It is the figure which extracted one.

  In FIG. 9, reference numerals 119 and 120 denote light rays that are emitted from the periphery of the pupil of the convex lens 600 b and enter the light receiving element 117. Since the light receiving element 117 is located at the center of the light receiving element array 820b and is the center of the image circle formed by the convex lens 600b, the light beam 119 and the light beam 120 are inclined in the opposite direction by an equal angle with respect to the optical axis 108. In addition, the light beam 119 and the light beam 120 form an image of the pupil end portion in the region 123 and the region 124 in the high refractive index layer 102, and then enter the photoelectric conversion unit 104 while being spread and converted into an electric signal. However, since light rays are dispersed and deflected as indicated by an arrow 130 in the light diffusion layer 109, the pupil image is in a blurred state, and some light rays cannot enter the high refractive index layer 120.

  On the other hand, 121 and 122 in FIG. 10 are light rays that are emitted from the periphery of the pupil of the convex lens 600 b and enter the light receiving element 118. Since the refractive index distribution structure is equivalent to the enlargement of the area of the photoelectric conversion unit, the utilization efficiency of light incident obliquely on the light receiving element array is high. By utilizing the property of effectively using light incident obliquely on the light receiving element array, it is possible to realize a thinner compound eye imaging device by adopting an optical system with a short focal length.

  Since the light receiving element 118 is located in the periphery of the light receiving element array 820b and is around the image circle formed by the convex lens 600b, the light beam 121 and the light beam 122 are inclined by different angles with respect to the optical axis 108. Further, the light beam 121 and the light beam 122 form an image of the pupil end portion in the region 125 and the region 126 in the high refractive index layer 102. Then, while spreading, a part of the light is totally reflected by the region 127 at the interface between the high refractive index layer 102 and the low refractive index layer 106, and efficiently enters the photoelectric conversion unit 104 without passing through the low refractive index layer 106. Converted to a signal. However, since light rays are dispersed and deflected as indicated by an arrow 131 in the light diffusion layer 109, the pupil image is in a blurred state, and some light rays cannot enter the high refractive index layer 120.

  Due to the light diffusing action of the light diffusing layer 109, the transmitted light utilization efficiency similar to that shown in the first embodiment can be obtained.

  FIG. 11 is a schematic diagram showing the effective light intensity distribution in a cross section of the imaging apparatus.

  Like FIG. 4 and FIG. 5, in FIG. 11, the effective light intensity distribution is expressed by density.

  In FIG. 11, 146 is a compound eye optical element, 140a and 140c are two of four convex lenses, 141a is an optical axis of the convex lens 140a, 141c is an optical axis of the convex lens 140c, and 142 is an object formed by the compound eye optical element 146. An image sensor 143 that converts an image into an electrical signal is a housing that holds the compound-eye optical element 146 and the image sensor 142 and functions as a dark box. Reference numeral 144 denotes a light beam that passes through the convex lens 140a and forms an object image on the light receiving element on the optical axis 141a. Reference numeral 145 denotes a light beam that passes through the convex lens 140c and forms an object image on the light receiving element on the optical axis 141c.

  Both the light beam 144 and the light beam 145 have a high intensity portion at the center, and the intensity decreases toward the periphery of the light beam.

  When the utilization efficiency of the imaged light beam is reduced from the center to the periphery of the light beam, the response of the optical MTF is low in the high frequency range exceeding the Nyquist frequency determined by the light receiving element pitch of the image sensor, and below the Nyquist frequency. Highly desirable characteristics such as high response in the frequency range. Therefore, a high-definition image with less moire and sharpness can be obtained.

  Further, as described above, since a very strict manufacturing error is required, each compound eye is preferably composed of one lens. In an optical system composed of a single lens, a considerably large distortion occurs when the stop is arranged on the front side or the rear side of the lens. However, when the light receiving element structure of the present embodiment is used, it is substantially inside the biconvex lens. Since this is equivalent to forming a diaphragm, distortion can be suppressed to a small level.

(Third embodiment)
FIG. 12 is a diagram for explaining a third embodiment of the imaging apparatus according to the present invention, and is an enlarged sectional view of a light receiving element array of the imaging element.

  In FIG. 12, 313 is a silicon substrate, 314 is a photoelectric conversion unit formed on the silicon substrate, 317 is an aluminum wiring for controlling charge accumulation operation and signal readout operation of each light receiving element, 311 is a micro lens, 318 is An optical axis of the micro lens 311, 315 is a color filter that transmits a predetermined color component, and 316 is an insulating layer.

The material and refractive index of each component are as follows.
Microlens 311 Material: Resin, Refractive index: 1.58
Color filter 315 Material: Resin, Refractive index: 1.5
Insulating layer 316 Material: SiO ₂ , refractive index: 1.46
The microlens 311 is for increasing the efficiency of condensing light onto the photoelectric conversion unit 314. In the light receiving element having this structure, the microlens 311 has a focus at a position indicated by 320. Therefore, the light beam emitted from the optical element forms a pupil image of the optical element near the point 320 by the action of the micro lens 311.

  The focal position 320 is in the middle of the aluminum wiring 317 and the color filter 315, and is usually about several μm away from the photoelectric conversion unit 314.

  FIG. 13 is a trace diagram of light rays incident on the light receiving element of the image sensor.

  The microlens 311 melts a resin etched in a disk shape at a high temperature and forms it into a spherical shape using the surface tension of the resin. Therefore, spherical aberration can be easily left, and the light beam that has passed through the peripheral portion can be bent more strongly than the declination angle that converges at one point compared to the light beam that has passed through the central portion of the microlens 311. As a result, as shown in FIG. 13, the degree of blurring of the pupil image before and after the focal position 320 is greatly different, and the edge of the pupil image is clear on the microlens 311 side from the focal position 320, and the edge of the pupil image is on the opposite side. Becomes unclear. Utilizing this property, focusing is performed in front of the photoelectric conversion unit 314, and a pupil image with an unclear edge is projected onto the photoelectric conversion unit 314, so that the transmission light utilization efficiency on the pupil is smooth. A changing characteristic can be obtained.

  By such an operation, the transmitted light utilization efficiency similar to that shown in the first embodiment can be obtained.

  When the utilization efficiency of the imaged light beam is reduced from the center to the periphery of the light beam, the response of the optical MTF is low in the high frequency range exceeding the Nyquist frequency determined by the light receiving element pitch of the image sensor, and below the Nyquist frequency. Highly desirable characteristics such as high response in the frequency range. Therefore, a high-definition image with less moire and sharpness can be obtained.

  Although the microlens is formed on the surface of the image sensor here, it may be formed as an inner layer lens at an intermediate position between the surface and the photoelectric conversion unit.

(Fourth embodiment)
FIG. 14 is a diagram for explaining a fourth embodiment of the imaging apparatus according to the present invention, and is a perspective view of a single-lens reflex camera.

  In FIG. 14, 401 is an imaging lens for forming an object image, and 402 is an optical axis of the imaging lens 401. The imaging lens 401 can adjust the imaging position in the direction of the optical axis 402 by an energy source (not shown) and a driving mechanism (not shown). The imaging lens 401 may be a zoom lens or a shift lens in addition to a single focus lens. Further, it may be exchangeable for an imaging lens having various characteristics (F number, focal length, etc.).

  The light beam emitted from the imaging lens 401 is divided into transmitted light and reflected light by a half mirror 403 provided obliquely. Reference numeral 404 denotes a penta roof prism for guiding the reflected light to the eyepiece lens 409, and reference numeral 405 denotes an elliptical surface mirror that reflects and converges the transmitted light. A focal plane shutter (not shown) and an area sensor serving as an imaging surface are arranged behind the elliptical surface mirror 405. The light beam reflected by the half mirror 403 is emitted from the eyepiece lens 409 through the penta roof prism 404, and the user of the camera can observe the object image as an erect image through the eyepiece lens 409. In the imaging state, the half mirror 403 and the elliptical surface mirror 405 are retracted from the photographing optical path, the focal plane shutter is opened, and an appropriate amount of light is exposed to the area sensor.

  406 is a curved mirror, 407 is a re-imaging lens having four elliptical convex surfaces 407a, 407b, 407c, and 407d on the exit surface, 408 is an image sensor for focus detection, including an elliptical surface mirror 405, It is an element constituting a phase difference detection type focus detection device. The re-imaging lens 407 is an optical element referred to in the present invention.

  The light beam reflected by the elliptical surface mirror 405 is further reflected by the curved mirror 406 and enters the re-imaging lens 407. Since the elliptical surface mirror 405 is disposed in front of the imaging surface, a primary image of the object is formed between the elliptical surface mirror 405 and the curved mirror 406. Further, since the re-imaging lens has the four exit surfaces 407a, 407b, 407c, and 407d as described above, four secondary object images are formed on the image sensor 408 by these actions.

  The elliptical surface mirror 405 has a function of placing the exit pupil of the imaging lens 401 and the entrance pupil of the re-imaging lens 407 in a conjugate relationship, and a light beam used for focus detection through the re-imaging lens 407 forms an image. A region passing through the exit pupil of the lens 401 is restricted. In general, when the focus detection light beam is shifted by the imaging lens 401 in the focus detection of the phase difference detection method, it is necessary to set the focus detection light beam so that the focus detection accuracy is not lowered. In particular, in an imaging system in which the imaging lens can be exchanged, the assumed exit pupil is used as a representative position of the exchangeable imaging lens group to guarantee that no focus detection light beam can be produced regardless of which imaging lens is mounted. .

410 is a porous mask for preventing the occurrence of ghosts. The porous mask 410 has four openings, and these openings are sized so that the effective light beam of the re-imaging lens 407 can pass with a margin. The curved mirror 406 is made of a resin filled with a filler such as silica in order to suppress changes in shape depending on temperature and humidity, and is molded so that the filler does not appear on the mirror surface portion.
The re-imaging lens 407 is formed of a composite material in which niobium oxide nanoscale particles are uniformly dispersed in acrylic, and a dielectric multilayer film for reflecting infrared light is formed on the incident surface. Therefore, the secondary image of the object formed on the image sensor 408 is obtained by removing the infrared light component, and by making the image sensor 408 sensitive to a wavelength of light longer than 400 nm, Sensor output can be obtained. When capturing an object image of visible light formed by the imaging lens 401, it is desirable to obtain a sensor output for visible light and perform focus detection based on visible light.

  FIG. 15 is a cross-sectional view of the light receiving element portion of the image sensor 408.

  In FIG. 15, light enters the image sensor from above. The image sensor 408 is a CMOS sensor having an on-chip microlens, and the F number of the focus detection light beam can be defined by the action of the microlens.

  In FIG. 15, 151 is a silicon substrate, 152 is a photoelectric conversion portion of a buried photodiode, 153 is a polysilicon wiring layer, 154 is a first wiring layer made of aluminum or copper, and 155 is an opaque second wiring using aluminum or copper. 156 is an interlayer insulating film and passivation film made of silicon oxide film, hydrophobic porous silica, silicon oxynitride film, silicon nitride film, etc., 158 is a microlens, 157 is a distance from the second wiring layer to the microlens. It is a planarization layer for setting with high accuracy. The second wiring layer 155 is a metal film having openings provided discretely, and does not transmit visible light except for the openings. Therefore, the second wiring layer 155 has both an electrical function for operating the image sensor 108 and an optical function for controlling the angle characteristics of the received light beam. The flattening layer 157 is formed by a method of curing after spin coating a thermosetting resin or an ultraviolet curable resin, or bonding a resin film.

  A circuit unit (not shown) is connected to the end of each photoelectric conversion unit 152. In the circuit section, there are a transfer MOS transistor that functions as a transfer switch, a reset MOS transistor that supplies a reset potential, a source follower amplifier MOS transistor, a select MOS transistor for selectively outputting a signal from the source follower amplifier MOS transistor, and the like. Have.

Here, the microlens 158 is formed of resin, SiO ₂ , TiO ₂ , Si ₃ N ₄ , and the like, and is used not only for focusing but also for imaging. It is a symmetric aspherical lens. Since it is a shape having a symmetry axis 160 and is circular when viewed in plan, a plurality of microlenses can be provided in one pixel, and the light receiving area of one pixel can be maintained large. In this way, a sufficient image sensor output can be obtained even for a low-luminance object. Furthermore, in order to suppress surface reflection of light, a thin film having a low refractive index or a fine structure (so-called sub-wavelength structure) having a wavelength equal to or less than the wavelength of visible light may be formed on the surface of the microlens.

  The light beam emitted from the re-imaging lens 407 first enters the micro lens 158 of the image sensor 408, and then the component that has passed through the opening 155a provided in the second wiring layer enters the photoelectric conversion unit 152, and the electric signal Is converted to Since the light shielding layer for forming the opening is also used as the second wiring layer, it is not necessary to provide a light shielding layer for the opening, and the configuration of the image sensor can be simplified.

  Here, the utilization efficiency of the imaging light flux of the re-imaging lens 407 will be considered. Even if the microlens 158 is an ideal lens having no aberration, if the size of the aperture 155a is small, the back projection image of the aperture 155a does not become a sharp image due to the diffraction limit determined by the F number. In general, the response of the MTF characteristic monotonously decreases with increasing spatial frequency due to the diffraction limit, and becomes zero at a specific frequency. This frequency is called a cut-off frequency, and the cut-off frequency N is expressed by the following equation.

N = 1 / (Fλ)
Here, F is the F number of the ideal lens, and λ is the wavelength of light.

  Assuming an imaging device that captures visible light with a light receiving element pitch of about 2 μm to 20 μm. For example, if F = 5 and λ = 0.0005 mm, the cutoff frequency is 400 lines / mm. This is converted to a wavelength of 0.0025 mm, and it can be seen that an object having a size of several μm cannot be sharply formed because the contrast of the image is lowered. Thus, in the imaging region where the image degradation due to the diffraction limit is large, no matter how much the geometric optical aberration of the microlens 158 is improved, it hardly contributes to the improvement of the image contrast.

  On the other hand, if such characteristics are used, it can be used as an efficiency adjusting means for reducing the utilization efficiency of the imaged light flux of the optical element from the central portion to the peripheral portion of the light flux.

  When the size of the opening 155a of the second wiring layer 155 shown in FIG. 15 is set to a small value of about 0.3 μm to 9.0 μm, the outline of the backprojected image by the microlens 158 is blurred. FIG. 17 shows a back-projected image of the aperture 155a on the exit pupil of the re-imaging lens 407 by the micro lens 158, and 143a, 143b, 143c, and 143d re-image the aperture 155a of each light receiving element by the micro lens 158. It is an image that is back projected onto the exit pupil of the lens 407. The eccentric amount of the microlens is set so that the images of the openings 155a of the respective light receiving elements overlap on the exit pupils of the four convex lenses.

  Thus, the fact that the contour of the backprojection image is blurred means that the amount of light that can pass through the opening 155a in the vicinity of the contour of the backprojection image is gradually changing.

  FIG. 16 is a schematic diagram showing the effective light intensity distribution in a cross section of the imaging apparatus.

  Like FIG. 4, FIG. 5, and FIG. 11, in FIG. 16, the effective light intensity distribution is expressed by density. In FIG. 16, 407 is a re-imaging lens, 407a and 407b are two of the four convex lenses, 441a is the optical axis of the convex lens 407a, 441b is the optical axis of the convex lens 407b, and 408 is formed by the re-imaging lens 407. An image sensor 443 that converts the object image into an electrical signal is a housing that holds the re-imaging lens 407 and the image sensor 408 and functions as a dark box. Reference numeral 444 denotes a light beam that passes through the convex lens 407a and forms an object image on the light receiving element on the optical axis 441a. Reference numeral 445 denotes a light beam that passes through the convex lens 407b and forms an object image on the light receiving element on the optical axis 441b.

  Both the light beam 444 and the light beam 445 have a high intensity portion at the center, and the intensity decreases toward the periphery of the light beam.

  If the utilization efficiency of the imaging light beam is decreased from the center to the periphery of the light beam, the response of the MTF of the re-imaging lens 407 is low in a high frequency region exceeding the Nyquist frequency determined by the light receiving element pitch of the image sensor 408. Therefore, it is possible to obtain extremely desirable characteristics such as high response in a frequency range below the Nyquist frequency. Therefore, aliasing distortion does not occur in the output of the image sensor, and by using this output, highly accurate focus detection can be performed.

  As described above, according to the above-described embodiment, without using an apodization filter for the optical element of the imaging device, the substantial MTF of the optical element is set to the Nyquist frequency determined by the light receiving element pitch of the imaging element. It was possible to achieve extremely desirable characteristics such as a low response in the high frequency range exceeding and a high response in the frequency range below the Nyquist frequency.

  Since it is not necessary to make two lenses in order to form an apodization filter inside the lens, the configuration of the apparatus is not complicated. This effect is particularly effective when a compound eye optical element is used.

  Further, when the MTF is controlled, the high-frequency component is remarkably reduced in the MTF of the defocused image, so that a soft and preferable blur can be obtained.

  In an optical system composed of a single lens, a considerably large distortion occurs when the stop is arranged on the front side or the rear side of the lens. However, when the light receiving element structure of the present embodiment is used, it is substantially inside the biconvex lens. Since this is equivalent to forming a diaphragm, the distortion can be kept small.

  Since an apodization filter is not required, the cost of the imaging device can be made extremely low.

  Since an optical low-pass filter is not required, the cost of the image pickup apparatus can be extremely reduced.

  When applied to a focus detection device, high focus detection accuracy could be obtained.

It is an expanded sectional view of the photo acceptance unit row of an image sensor. It is sectional drawing which shows an imaging device. FIG. 3A is a diagram showing the transmitted light utilization efficiency on the pupil, and FIG. 3B is a schematic diagram of the effective light intensity distribution on the pupil surface. It is a figure showing the imaging light beam which injects into the light receiving element 37 on an optical axis. It is a figure showing the imaging light beam which injects into the light receiving element 38 of an imaging | photography screen periphery. 3 is an enlarged cross-sectional view of a light receiving element array 111. FIG. FIG. 7A is a plan view of the imaging apparatus, FIG. 7B is a side view, and FIG. 7C is a plan view of an imaging element in which a semiconductor circuit as one element of the imaging apparatus is formed. It is a detailed top view of an image sensor. It is the figure which extracted one of the light receiving elements of the screen center part. It is the figure which extracted one of the light receiving elements of a screen peripheral part. It is the schematic diagram which showed the effective light intensity distribution in the cross section of the imaging device. It is an expanded sectional view of the photo acceptance unit row of an image sensor. It is a trace figure of the light ray which injects into the light receiving element of an image pick-up element. It is a perspective view of the optical system of a single-lens reflex camera. 2 is a cross-sectional view of a light receiving element portion of an image sensor 108. FIG. It is the schematic diagram which showed the effective light intensity distribution in the cross section of the imaging device. It is the figure which showed the back projection image by the micro lens 158 of the opening 155a on the exit pupil of the re-imaging lens 407.

Explanation of symbols

19 Light diffusion layer 109 Light diffusion layer 102 High refractive index layer 311 158 Micro lens 155 Second wiring layer

Claims (8)

An optical system for forming an object image;
An image sensor including a plurality of light receiving elements that photoelectrically convert an object image formed by the optical system;
The imaging device includes, as a part of the structure of the light receiving element, efficiency adjusting means for reducing the use efficiency of the imaging light flux of the optical system from the central portion to the peripheral portion of the light flux. .   The imaging apparatus according to claim 1, wherein the efficiency adjusting unit includes a light diffusion layer formed in an optical path of light incident on the light receiving element. The imaging device according to claim 2, wherein the light diffusion layer is configured by periodically arranging Si ₃ N ₄ and SiO ₂ .   The imaging apparatus according to claim 1, wherein the efficiency adjusting unit includes a microlens formed in an optical path of light incident on the light receiving element.   The imaging apparatus according to claim 4, wherein the microlens has a focal plane in the vicinity of a surface of the light receiving element.   The imaging apparatus according to claim 4, wherein the micro lens has a focal plane at an intermediate position between the macro lens and the surface of the light receiving element.   The imaging apparatus according to claim 1, wherein the efficiency adjusting unit includes an opening of 0.3 μm to 9.0 μm formed in an optical path of light incident on the light receiving element.   The efficiency adjusting unit includes a microlens formed in an optical path of light incident on the light receiving element, and the opening is disposed at an intermediate position between the microlens and the surface of the light receiving element. The imaging device according to claim 7.

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