JP2004191630A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an excellent phase difference signal while obtaining a desirable image signal in the setting of microlens power. <P>SOLUTION: The imaging apparatus is equipped with a photographic lens and a solid-state image pickup element arranged on the scheduled image-formation surface of the photographic lens. The solid-state image pickup element has a plurality of photoelectric conversion cells respectively equipped with pixels 305 and 306 divided into two photoelectric conversion parts and a microlens 301 arranged in front of them, and is set to be expressed by ä(R/Z)×r0}×0.95<r<ä(R/Z)×r0}×1.05 assuming that the radius of curvature of the microlens is (r), the radius of curvature of the microlens obtained when the light receiving surface of the pixel and the exit pupil 312 of the photographic lens have conjugate relation to each other is r0, length on the surface of the exit pupil obtained when a side orthogonal to the divided boundary line of the pixel is projected to the surface of the exit pupil of the photographic lens through the microlens having the radius of curvature (r) is Z, and the radius of the surface of the exit pupil is R. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an imaging apparatus having a solid-state imaging device that performs imaging of a subject and focus detection using a phase difference method.
[0002]
[Prior art]
Japanese Patent Application Laid-Open No. 2000-156823 (Patent Document 1) discloses a configuration for performing imaging of a subject image and focus detection of the subject image by a phase difference method on the same solid-state imaging device. According to this configuration for focus detection, the size of the imaging device can be reduced, the cost can be reduced, error factors can be reduced, and the hill-climbing type focusing can be achieved, as compared with a structure in which a mechanism for a focus detection system is separately provided. Focus detection can be performed in a shorter time as compared with the method.
[0003]
Specifically, in a solid-state imaging device in which photoelectric conversion cells for converting an optical image formed by an optical system into electric signals are arranged two-dimensionally, at least a part of the photoelectric conversion cell group has focus detection. A photoelectric conversion cell that outputs a signal for focus detection, a microlens disposed on the photoelectric conversion unit, and a microlens and a photoelectric conversion unit. A light-shielding film layer having a specific opening disposed therebetween, wherein the photoelectric conversion cell has a first photoelectric conversion cell in which an opening of the light-shielding film layer is biased with respect to the optical center of the microlens. The first photoelectric conversion cell is classified into a first photoelectric conversion cell and a second photoelectric conversion cell in which the opening of the light-shielding film layer is biased in the opposite direction with respect to the optical axis center of the microlens. A first row of a regular array and the first row Having an array of a second row lined basic sequence comprising a second photoelectric conversion cells adjacent in the row in at least one region of the imaging region. With such a configuration, a subject image is formed on the image sensor based on a phase difference between a signal output by the connection of the first photoelectric conversion cells and a signal output by the connection of the second photoelectric conversion cells. The focus of the optical system can be adjusted.
[0004]
Separately, in the solid-state imaging device disclosed in Japanese Patent Application Laid-Open No. 3-28777 (Patent Document 2), the solid-state imaging device disclosed in Japanese Patent Application Laid-Open No. 2000-156823 includes an opening in a light-shielding film layer. In order to impart a bias to the portions, the adjacent photoelectric conversion cells are configured such that incident light is incident on relatively different portions of the respective photoelectric conversion portions due to the arrangement relationship between the respective photoelectric conversion portions and the microlenses. I have. Specifically, the central axis of the photoelectric conversion unit and the central axis of the microlens are biased in the opposite direction between the adjacent photoelectric conversion cells, or the optical axis of the microlens is set relative to the incident light. The light flux incident on each microlens is inclined at the same angle to the opposite side so that it reaches relatively different portions in the photoelectric conversion unit.
[0005]
On the other hand, a solid-state imaging device capable of performing addition of photoelectric conversion signals of a plurality of photoelectric conversion units in an image unit and further arbitrarily adding and non-addition of photoelectric conversion signals is disclosed in Japanese Patent Application Laid-Open No. 9-46596 (Patent Document 3). ing. When the subject luminance is bright, the pixel signal is output without addition to perform high-resolution imaging. When the subject luminance is dark, the pixel signal is added and output to perform high-sensitivity imaging.
[0006]
Further, in a solid-state imaging device including a photoelectric conversion cell group having one microlens, a color filter, and one pixel divided into two photoelectric conversion units in one photoelectric conversion cell, the output of each photoelectric conversion unit is respectively A first read mode (non-addition mode) in which reading is performed independently; and a second reading mode (addition mode) in which outputs of two photoelectric conversion units in the same pixel are added and read. Japanese Patent Laid-Open No. 2001-124984 discloses an invention in which a phase difference signal for focus detection is read in a first read mode and an image signal is read in a second read mode. ing. In the image sensor configured as described above, one photoelectric conversion cell can output both a phase difference signal for focus detection and an image signal for forming an image, which is effective for both focus detection and image generation.
[0007]
[Patent Document 1]
JP 2000-156823 A [Patent Document 2]
JP-A-3-28777 [Patent Document 3]
JP-A-9-46596 [Patent Document 4]
Japanese Patent Application Laid-Open No. 2001-124984 [Problems to be Solved by the Invention]
Thus, from a photoelectric conversion cell having one microlens, a color filter and one pixel divided into two photoelectric conversion units in one photoelectric conversion cell, an image signal for forming an image is also used for focus detection. If the phase difference signal can also be output, there are two conflicting requirements in the shape of the microlens.
[0008]
First, with respect to an image signal output for forming an image, reading is performed by adding both of the two photoelectric conversion units (addition mode).
[0009]
Regarding the subject image formed by the output of the image signal, it is desirable that the relationship between the aperture value of the photographing lens and the amount of incident light reaching the image sensor complies with the APEX (Additive System of Photographic Exposure). In other words, if the area of the exit pupil of the photographing lens, which changes according to the set aperture value, and the amount of light incident on the image sensor light-receiving unit (both divided into two photoelectric conversion units) are in a proportional relationship, the silver halide camera and the silver halide camera can be used. Similarly, the parameters of the object brightness, the sensitivity of the image sensor, the shutter speed, and the aperture value can be uniquely set. In order to satisfy this relationship, ideally, the light receiving section of the image sensor and the exit pupil of the imaging lens have a conjugate relationship with each other, that is, the light beam emitted from the exit pupil forms an image on the light receiving section surface of the image sensor. Even if the power of the microlens is set such that the aperture value (open aperture value) at which the exit pupil of the photographing lens is maximized, the projected image obtained by projecting the light receiving unit onto the exit pupil via the microlens is smaller than the exit pupil. It is sufficient that the area of the light receiving unit and the projection magnification are set so that the distance becomes larger.
[0010]
However, in practice, each cell is provided with an amplifier circuit, a transfer unit, and the like in addition to the light receiving unit. Therefore, with the recent increase in the number of pixels, the light receiving unit area ratio per cell tends to decrease. . Therefore, the light receiving unit cannot actually occupy a sufficient area, and the projected image projected on the exit pupil is smaller than the exit pupil, that is, the relationship that the projected image includes the exit pupil at the open aperture value cannot be realized. ing. In other words, the light beam emitted from the exit pupil does not reach the light receiving unit and is not photoelectrically converted depending on the emitted position.
[0011]
Therefore, the state of shifting the power of the microlens from the power for realizing the imaging state to the power for forming the virtual image state, that is, the micro state, so that the light beam as wide as possible among the light beams emitted from the exit pupil reaches the light receiving unit. A technique of setting power so that an image plane formed by a lens is located behind an actual light receiving unit surface has been adopted. In the imaging device in which the power of the microlens is set as described above, even when the area of the light receiving portion is not sufficient, the area of the exit pupil and the area of the exit pupil are smaller than those of the imaging device having the microlens power for realizing the imaging state. The proportional relationship between the amounts of light incident on the light receiving unit is improved. Therefore, in an image sensor that outputs only an image signal, the power of the microlens may be set by paying attention only to this direction.
[0012]
On the other hand, as for the phase difference signal output for performing focus detection, reading is performed independently from each of the two divided photoelectric conversion units (non-addition mode).
[0013]
Regarding focus detection by a general phase difference method having a separate conventional AF unit, a defocus amount D is calculated by using an image shift amount P and a proportionality constant K.
D @ KP ... (1)
Where the proportionality constant K is proportional to the reciprocal of the base line length L, which is the distance between the apertures of the AF diaphragm placed in front of the secondary imaging lens which is one of the AF unit components.
[0014]
In this method in which focus detection is performed by obtaining a focus detection signal in the non-addition mode, the expression (1) is also satisfied. However, in this case, the base line length L according to the proportionality constant K is such that when the light receiving surfaces of the photoelectric conversion unit divided into two are an α plane and a β plane, respectively, the α plane and the β plane are emitted through a microlens. In the projection image projected on the pupil, the portion surrounded by the exit pupil corresponds to the distance between the centers of gravity of the image formed.
[0015]
When the power of the microlens is set to form a virtual image state, the distance between the centers of gravity indicates that the projected image on the exit pupil shows an intensity distribution. (Hereinafter, referred to as “pupil intensity distribution”), so that the intensity is shorter than that of a microlens power that realizes an imaging state in which the distribution is uniform.
[0016]
When the base line length becomes shorter, the sensitivity of the image shift amount P becomes higher than the equation (1), that is, even a small error of P has a large effect on the defocus amount D which is a calculated value. Accordingly, if the base line length is short, the focus detection error increases, so that it is desirable that the base line length be long. This leads to the fact that it is desirable to set the microlens to a power that realizes an imaging state that does not generate an intensity distribution.
[0017]
As described above, in setting the power of the microlenses, the virtual image state is formed to obtain a preferable image signal, and the image forming state is realized to obtain a good phase difference signal. There will be conflicting demands.
[0018]
Therefore, the present invention has been made in view of the above-described problem, and an object of the present invention is to make it possible to obtain a favorable phase difference signal while obtaining a preferable image signal in setting a microlens power. .
[0019]
[Means for Solving the Problems]
In order to solve the above-described problems and achieve the object, an imaging apparatus according to the present invention includes a photographic lens and a solid-state imaging device arranged on a predetermined imaging plane of the photographic lens. A plurality of photoelectric conversion cells each including a pixel divided into two photoelectric conversion units and a microlens disposed in front of the pixel, wherein the curvature radius r of the microlens is determined by comparing the light receiving surface of the pixel with the light receiving surface of the pixel. The microlens having a radius of curvature r0 when the exit pupils of the lens are in a conjugate relationship with each other, and having a radius of curvature r on the exit pupil plane of the photographing lens whose side orthogonal to the divided boundary line of the pixel. When the length on the exit pupil plane when projected through is Z and the radius of the exit pupil plane is R,
{(R / Z) × r0} × 0.95 <r <{(R / Z) × r0} × 1.05
It is characterized by having been set as represented by.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment in which the present invention is applied to a digital still camera will be described in detail with reference to the accompanying drawings.
[0021]
FIG. 1 is a sectional view schematically showing a main part of a digital still camera according to the present embodiment.
[0022]
In FIG. 1, reference numeral 3 denotes a solid-state image sensor, which is arranged on a predetermined image-forming surface of a photographing lens 2 of a digital still camera 1 (for convenience, two lenses are shown, but actually constituted by a large number of lenses). ing. The digital still camera 1 includes a CPU 10 that controls the entire camera, an image sensor control circuit 11 that drives and controls the solid-state image sensor 3, an image processing circuit 14 that processes an image signal captured by the solid-state image sensor 3, , A liquid crystal display element driving circuit 15 that drives the liquid crystal display element, an eyepiece 4 for observing a subject image displayed on the liquid crystal display element 5, and an image captured by the solid-state imaging element 3. , And an interface circuit 13 for outputting an image processed by the image processing circuit 14 to the outside. The memory circuit 12 also records shooting lens specific information.
[0023]
The taking lens 2 is adjusted to a focused state by the taking lens driving mechanism 16 based on the focus adjustment information sent from the CPU 10. The CPU 10 also serves as a focus detection unit. Reference numeral 20 denotes an aperture device which is stopped down to a predetermined aperture value by an aperture drive mechanism 17.
[0024]
Next, the solid-state imaging device 3 used in the present embodiment will be described.
[0025]
2. Description of the Related Art Conventionally, a solid-state imaging device has a MOS structure including a metal, an oxide, and a semiconductor capable of performing photoelectric conversion, and is classified into an FET type and a CCD type according to a method of moving an optical carrier. In the present embodiment, a CMOS compatible sensor (hereinafter referred to as a “CMOS sensor”), which is one of such amplification type solid-state imaging devices, is used.
[0026]
In the CMOS sensor according to the present embodiment, two photoelectric conversion units are configured for one pixel, and a floating diffusion region (hereinafter, referred to as an FD region) and a source follower amplifier, which are conventionally provided for each photoelectric conversion unit, are replaced by two photoelectric conversion units. And only one photoelectric conversion region is connected to the FD region via a transfer MOS transistor switch. Therefore, the charges of the two photoelectric conversion units can be transferred to the floating diffusion unit simultaneously or separately, and the addition and non-addition of the signal charges of the two photoelectric conversion units can be performed only by the timing of the transfer MOS transistor connected to the FD region. Easy to do. Utilizing this structure, a first output mode for performing photoelectric conversion output by a light beam from the entire exit pupil of the photographing lens, and a second output mode for performing photoelectric conversion output by a light beam from a part of the exit pupil of the imaging lens The mode can be switched. In the first output mode in which signals are added at a pixel level, a signal with less noise can be obtained as compared with a method in which signals are read and then added.
[0027]
FIG. 2 is a circuit configuration diagram of an area sensor unit in the image sensor 3. Although FIG. 1 shows a two-dimensional area sensor having 2 columns × 2 rows of pixels, in practice, the number of pixels is increased in both the column direction and the row direction to obtain a practical resolution.
[0028]
In FIG. 2, reference numerals 101 and 201 denote a photodiode-like first photoelectric conversion unit (hereinafter, referred to as “α unit”) and a second photoelectric conversion unit (hereinafter, referred to as “β unit”) each including a MOS transistor gate and a depletion layer below the gate. , 102 and 202 are photogates with the symbol of a capacitor on the figure, 103 and 203 are transfer switch MOS transistors for transferring charges by photoelectric conversion of the α section 101 and β section 201, and 104 are reset charges of the floating diffusion section FD. A reset follower MOS transistor 105, a source follower amplifier MOS transistor for converting a charge of the floating diffusion portion FD into a voltage by a source follower type and amplifying the voltage, and a vertical select switch MOS transistor 106 for selecting a pixel by a pulse φS0 from a horizontal scanning portion 116. , Reference numeral 107 denotes a load MOS transistor serving as a load of the source follower amplifier MOS transistor 105 that amplifies in a source follower type, 108 denotes a dark output transfer MOS transistor that transfers dark charge of the floating diffusion FD, and 109 denotes an image of the floating diffusion FD when imaging. A bright output transfer MOS transistor 110 outputs a bright output by turning on the dark output transfer MOS transistor 108, and a dark output storage capacitor CTN 111 stores a dark output when the dark output transfer MOS transistor 108 turns on. The bright output storage capacitors CTS, 112 and 204 to be stored are vertical transfer MOS transistors that are turned on / off by a control pulse from the vertical scanning unit 115, and 113 and 205 are vertical output line reset MOs that reset the vertical output lines. Transistor, 114 is a differential output amplifier that outputs a difference between a bright output and a dark output, 115 is a vertical scanning unit that outputs a pulse for controlling the vertical transfer MOS transistors 112 and 204, and 116 is an α unit 101 and a β unit 201. The horizontal scanning unit outputs a transfer pulse for reading out electric charges, a reset pulse, a trigger pulse, and a selection pulse.
[0029]
FIG. 3 is a cross-sectional view illustrating a circuit configuration of a light receiving unit for one pixel.
[0030]
In FIG. 3, 117 is a P-type well, 118 and 208 are gate oxide films, 119 and 209 are first-layer poly-Si, 120 and 220 are second-layer poly-Si, and 121 is an n + floating diffusion region. The same parts as those in FIG. 2 are denoted by the same reference numerals. For example, there are an α section 101, a β section 201, photogates 102 and 202, a reset MOS transistor 104, a source follower amplifier MOS transistor 105, a vertical selection switch MOS transistor 106, and a load MOS transistor 107.
[0031]
FD region 121 is connected to α section 101 and β section 201 via transfer MOS transistors 103 and 203. In FIG. 3, the α portion 101 and the β portion 201 are drawn apart from each other. However, in reality, the boundary portion is extremely small, and in practical use, the α portion 101 and the β portion 201 may be regarded as being in contact with each other. Further, the above-described light receiving surface is an effective surface which is a boundary surface between the gate oxide films 118 and 208 and the light receiving portions α section 101 and β section 201 and can actually receive photons. Hereinafter, of the effective surfaces, the light receiving surface corresponding to α portion 101 is referred to as α surface, and the light receiving surface corresponding to β portion 201 is referred to as β surface.
[0032]
FIG. 4 is a perspective view schematically showing a physical structure for one pixel on the microlens side from the light receiving surface of the image sensor 3.
[0033]
In FIG. 4, reference numeral 300 denotes a central axis of the pixel, reference numeral 301 denotes a microlens portion, and the height is A, and the radius of curvature is r. Reference numerals 302 to 304 denote layers having different refractive indexes, and the heights of the layers are B, C, and D, respectively. These layers are layers for including a filter layer and a wiring layer. Reference numerals 305 and 306 denote α planes and β planes, respectively, which are adjacent to each other around the central axis as shown in the figure, and both of which are represented by k1 × k2. The sum of the microlens height A and the heights B, C, and D of the respective layers is defined as h.
[0034]
FIG. 5 is a schematic diagram showing the relationship between one pixel in FIG. 4 and the exit pupil of the photographing lens. That is, in FIG. 4, the photographing lens optical axis 300 is set so as to face a side (side of the α plane and the β plane having a length k2) orthogonal to a boundary line (length k1) dividing the α plane 305 and the β plane 306. It is the figure which caught from the side. In the figure, the relation between the relative size and the distance is ignored for the sake of explanation. Reference numerals 300 to 306 denote the same parts and the same parts in FIG.
[0035]
In FIG. 5, reference numeral 310 denotes a microlens vertex plane, which is a plane that includes the microlens vertex 331 and is orthogonal to the photographing lens optical axis 300. Reference numeral 311 denotes a converted light receiving surface with respect to the light receiving surface (a plane including the α plane 305 and the β plane 306). The distance S from the microlens vertex 331 is expressed by Abbe's zero invariant by the following equation (2).
[0036]
1 / S = (1−νA) / r + 1 / (A / νA + B / νB + C / νC + D / νD) (2)
Here, νA, νB, νC, and νD represent the respective refractive indexes of the microlens 301 and the layers 302 to 304, and r represents the radius of curvature of the microlens.
[0037]
Reference numeral 312 denotes an exit pupil plane of the photographing lens, which is arranged at a distance of H from the microlens vertex 331. At this time, the radius of the exit pupil is R.
[0038]
Next, of the rays emitted from the end point 330 (image height k2) of the α plane 305 toward the exit pupil plane 312, the point at which the ray passing through the microlens vertex 331 reaches the exit pupil plane is defined as the exit pupil upper end point 332. Is defined. In other words, of the light rays incident from the subject side, the light rays passing through the exit pupil upper end point 332 and the microlens vertex 331 are refracted at the microlens vertex and the boundary between the layers, and reach the end point 330 of the α plane 305. Means that.
[0039]
At this time, the distance Z (322) of the exit pupil upper end point 332 from the photographing lens optical axis 300 is obtained as follows. First, the end point 330 (image height k2) of the α plane is
y = k2 · S / (A / νA + B / νB + C / νC + D / νD) (3)
Corresponds to the point 333 of the image height y. Next, a straight line is drawn from this point to the exit pupil plane 312 via the microlens vertex 331. Since the intersection of this straight line and the exit pupil plane 312 is the exit pupil upper end point 332, the distance Z from the photographing lens optical axis 300 is Z = y (H / S) (4)
It will be represented by
[0040]
Now, if the light beam refracted by the microlens forms an image on the light receiving surface, the radius of curvature of the microlens at that time is defined as r0.
1 / H = (1−νA) / r0 + 1 / (A / νA + B / νB + C / νC + D / νD) (5)
It is possible to obtain r0 from the relational expression.
[0041]
Therefore, the radius of curvature r of the microlens is calculated by using the above R, Z and r0 as {(R / Z) × r0} × 0.95 <r <{(R / Z) × r0} × 1.05. 6)
Is set so as to satisfy.
[0042]
For example, various values are set as shown in FIG. All units are expressed in μm. At this time, assuming that the taking lens has an open aperture of F1.0, R = 40000, and Z = 26910, r0 = 3.50 is obtained by using the above equations (2) to (5). At this time, it can be confirmed that r satisfies the expression (6).
[0043]
The transition of the base line length with respect to the F value and the transition of the light quantity proportionality with respect to the F value when this application example is implemented are compared with the case where the radius of curvature is outside the applicable range that does not satisfy the above equation (6). The results of the simulation are shown in FIGS. 7 and 8 (the simulation was performed in the range from F1.0 to F8.0).
[0044]
In FIG. 7, the horizontal axis represents the transition of the F value in logarithmic representation, and the vertical axis represents the base line length. It can be seen that the above application example secures a longer base length at each F value. That is, even if the aperture of the taking lens is reduced, it is possible to perform phase difference AF with higher accuracy than when a micro lens outside the applicable range is provided.
[0045]
In FIG. 8, the horizontal axis represents the transition of the F value in logarithmic representation, and the vertical axis represents the difference in the number of steps of each F value based on F1.8. In the case of ideally following the APEX, all F-numbers should show 0. However, in reality, the relationship between the aperture and the received light amount does not exactly follow the proportional relationship. If the light amount is reduced only by the amount, it means +0.3 stops with respect to the reference aperture value. In this application example, it can be seen that the F value changes on the side closer to 0 as compared to the outside of the application range. Therefore, it can be said that a more favorable result is obtained also in the light quantity proportionality.
[0046]
As described above, according to the above-described embodiment, it is possible to provide an imaging element in which power is set in an equilibrium region where a favorable image signal can be obtained while obtaining a favorable image signal in setting the microlens power. Can be. That is, a preferable image signal is to maintain a light quantity proportional relationship closer to APEX, and a good phase difference signal is a signal having a small proportional constant sensitivity.
[0047]
【The invention's effect】
As described above, according to the present invention, in setting the microlens power, a favorable phase difference signal can be obtained while obtaining a preferable image signal.
[Brief description of the drawings]
FIG. 1 is a sectional view schematically showing a main part of a digital still camera according to an embodiment of the present invention.
FIG. 2 is a circuit configuration diagram of an image sensor.
FIG. 3 is a cross-sectional view illustrating a circuit configuration of a light receiving unit for one pixel.
FIG. 4 is a perspective view schematically showing a physical structure of one pixel.
FIG. 5 is a schematic diagram illustrating a relationship between a pixel and an exit pupil of a photographing lens.
FIG. 6 is a diagram showing various setting values in one embodiment of the present invention.
FIG. 7 is a diagram illustrating a transition of a base length with respect to an F value according to an embodiment.
FIG. 8 is a diagram showing a change in light quantity proportionality to an F value in one embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Digital still camera 2 Photographing lens 3 Solid-state image sensor 4 Eyepiece 5 Liquid crystal display device 10 CPU
DESCRIPTION OF SYMBOLS 11 Image sensor control circuit 12 Memory circuit 13 Interface circuit 14 Image processing circuit 15 Liquid crystal display element drive circuit 16 Photographing lens drive mechanism 16
17 Aperture drive mechanism 20 Aperture device

Claims (1)

A shooting lens,
A solid-state imaging device arranged on a predetermined imaging plane of the taking lens,
The solid-state imaging device has a plurality of photoelectric conversion cells each including a pixel divided into two photoelectric conversion units and a microlens disposed in front of the pixel, and sets a radius of curvature r of the microlens to the pixel. When the light receiving surface and the exit pupil of the photographing lens have a conjugate relationship with each other, the radius of curvature of the microlens is r0, and the side orthogonal to the divided boundary line of the pixel is the radius of curvature r on the exit pupil surface of the photographing lens. When the length on the exit pupil plane when projected through the microlens having
{(R / Z) × r0} × 0.95 <r <{(R / Z) × r0} × 1.05
An imaging apparatus characterized by being set as represented by:

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