Patent Application
20070181923
This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-027201, filed Feb. 3, 2006, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to solid-state image sensor. In particular, the present invention relates to microlenses in solid-state image sensor which condense light incident on pixels.
2. Description of the Related Art
An effective method for reducing the sizes electronic cameras is to reduce the sizes of image areas and thus of optical systems. This requires the pixel size of CMOS sensors to be shrunk. To shrink the pixel size, attempts have been made to allow a plurality of photodiodes to share a transistor in each pixel to reduce the number of transistors per photodiode, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. H10-150182.
The conventional CMOS sensor has microlenses that condense incident light and photodiodes that convert the incident light condensed by the microlenses into charges. For example, as described in Published Japanese Patent No. S60-59752, each of the microlenses normally has a laterally asymmetrical sectional shape. Accordingly, the focus of the microlens is located immediately below its top, that is, almost immediately below the center of bottom surface of the microlens.
However, in the above configuration, the microlenses are spaced at equal intervals, so that the foci are spaced at equal intervals. As a result, a gate of a MOS transistor adjacent to each photodiode blocks incident light. This reduces the photosensitivity of the CMOS area sensor.
A solid-state image sensor according to an aspect of the present invention includes:
pixels on a semiconductor substrate, each of the pixels including a photodetecting section which photoelectrically converts incident light; and
lenses which condense the incident light on the photodetecting section, the lenses having a fixed curvature on an incident surface for the incident light, a top of the incident surface of each of the lenses being at a position different from that of a center of a bottom surface of each of the lenses in a direction horizontal to the bottom surface.
With reference to
As shown in the figures, a plurality of photodetecting sections, for example, photodiodes 2 are provided in a surface of a semiconductor substrate 1. The photodiodes 2 are formed by using, for example, ion implantation to doping impurities of a conductivity type opposite to that of the semiconductor substrate 1, in its surface. Gate electrodes 3 are each provided on the semiconductor substrate 1 between adjacent photodiodes 2 with a gate insulating film interposed therebetween. An insulating film 4 is provided on the semiconductor substrate 1 so as to cover the photodiodes 2 and gate electrodes 3. Microlenses 5 are provided on the insulating film 4 in association with the respective photodiodes 2. In this configuration, a plurality of pixels are formed each of which includes one photodiode.
The top P1 of the microlens 5 in accordance with the present embodiment is at a distance d1 from one end of the microlens 5 and at a distance d2 (≠d1) from the other end of the microlens 5. Accordingly, the position of the top P1 is different from that of center C1 of a bottom surface of the microlens 5 in a direction horizontal to the bottom surface. In other words, the top P1 of the microlens 5, that is, its focus F1, is present at a distance d3 from a perpendicular to the bottom surface of the microlens 5 which contains the center C1 (see
The top P1 of the microlens 5 lies opposite the gate electrode 3 across the center C1 of the microlens 5 in a direction in which the photodiode 2 and gate electrode 3 are arranged. That is, the top P1 of the microlens 5 is disposed at its end (second end) far from the gate electrode 3 in the direction in which the photodiode 2 and gate electrode 3 are arranged; the first end of the microlens 5 is near the gate electrode 3. In other words, in
In this configuration, upon reaching the microlens 5, the incident light Ll is refracted in accordance with Snell's law. The refracted incident light L1 forms an image on the photodiode 2 corresponding to the microlens 5. The photodiode 2 photoelectrically converts the incident light L1 into charges.
This configuration exerts the following effect.
(1) A decrease in the photosensitivity of the solid-state image sensor can be inhibited (1).
In the configuration in accordance with the present embodiment, the top of the microlens is away from the center of its bottom surface in the direction horizontal to the bottom surface of the microlens, which condenses incident light on the photodiode. This enables a decrease in the photosensitivity of the solid-state image sensor to be inhibited. This effect will be described below in detail.
Thus, since the microlens 105 are spaced at equal intervals, the foci F101 are spaced at equal intervals. On the other hand, the photodiodes 102 are not spaced at equal intervals; a larger interval corresponds to the area between the photodiodes 102 in which the gate electrode 103 is sandwiched between the photodiodes 102, and a smaller interval corresponds to the area between the photodiodes 102 in which the gate electrode 103 is not present. As a result, the incident light L101 condensed by the microlens 105 is partly blocked by the gate electrode 103 (in
In contrast, in the configuration in accordance with the present embodiment, the top P1 of the microlens 5 is away from the center C1 of bottom surface of the microlens 5 in the direction horizontal to the bottom surface of the microlens 5 (that is, horizontal to the surface of the photodiode 2). Consequently, the microlenses 5 are spaced at equal intervals but the foci of the microlenses 5 are not. More specifically, as described with reference to
Now, with reference to
As shown in the figure, the solid-state image sensor in accordance with the present embodiment has the configuration described in the first embodiment with reference to
As shown in
The distances between the perpendicular V1 and the two metal wiring layers 6 located across the perpendicular V1 are defined as d7 and d8 (d7<d8). One of the metal wiring layers 6 which is near the perpendicular V1 is called a wire W1. The metal wiring layer 6 far from the perpendicular V1 is called a wire W2. Then, the top P2 of the microlens 5 lies opposite the wire W1 across the perpendicular V1. In other words, a straight line V2 joining the top P2 of the microlens 5 and the focus F2 together is located between the wire W2 and the perpendicular V1.
This configuration exerts the following effect.
(2) A decrease in the photosensitivity of the solid-state image sensor can be inhibited (2).
In the configuration in accordance with the present embodiment, the top P2 of the microlens 5 is away from the center C2 of bottom surface of the microlens 5 in the direction horizontal to the bottom surface of the microlens 5 as in the case of the first embodiment. Consequently, the microlenses 5 are spaced at equal intervals but the foci of the microlenses 5 are not. More specifically, as described with reference to
That is, the present embodiment exerts an effect similar to that of the first embodiment to prevent the incident light L2 from being blocked by the metal wiring layers 6. However, the metal wiring layers 6 are normally located at a higher level than the gate electrodes, described in the first embodiment, that is, located closer to the microlenses 5. Consequently, the metal wiring layers 6 are more likely to block the incident light L2 than the gate electrodes. Therefore, the use of the microlens 5 in accordance with the present embodiment is more effective than that in the first embodiment.
Now, a solid-state image sensor in accordance with a third embodiment of the present invention will be described. The present embodiment relates to a solid-state image sensor which includes the microlens 5 described in the first and second embodiments and in which an amplifying transistor is shared by two photodiodes.
As shown in the figure, the solid-state image sensor 10 comprises a clamp circuit 11, a sample-and-hold circuit 12, vertical selection circuit 13, a horizontal selection circuit 14, and a light receiving section 20.
The light receiving section 20 comprises a plurality of unit cells 21 which photoelectrically convert incident light.
The clamp circuit 11 is connected to one end of each of the vertical signal lines 22 to clamp a signal read onto the vertical signal line 22. The other end of the vertical signal line 22 is connected to a ground potential via a load transistor 23.
The sample-and-hold circuit 12 samples and holds a signal clamped by the clamp circuit 11. The signal held by the sample-and-hold circuit 12 is output to an output node OUT via a read transistor 24. A gate of the read transistor 24 is controlled by the horizontal selection circuit 14.
Now, the configuration of the unit cell 21 will be described with reference to
Each of the pixels 25 comprises a read transistor 28 and a photodiode 29. Gates of the two read transistors 28 included in the same unit cell 21 are connected to the read signal lines RD1 and RD2, respectively. A drain of each read transistor 28 is connected to an anode of the photodiode 29 in the corresponding pixel 25. A cathode of the photodiode 29 is grounded.
The signal output section 26 comprises an amplifying transistor 30, an address transistor 31, and a reset transistor 32. The amplifying transistor 30 has a gate connected to sources of the transistors 28 in both pixels 25, a source connected to the vertical signal line 22, and a drain connected to a source of the transistor 31. The address transistor 31 has a gate connected to the address signal line AD and a drain connected to a power supply potential VDD. The reset transistor 32 has a gate connected to the reset signal line RS, a source connected to the sources of the transistors 28 in both pixels 25, and a drain connected to the power supply potential VDD. That is, the one signal output section 26 is shared by the two pixels 25.
For the microlens 34 in the solid-state image sensor in accordance with the present embodiment, the relationship between its top P3 (focus F3) and the center C3 of its bottom surface is similar to that in the first embodiment. That is, the top P3 of the microlens 34, that is, its focus F3, is a distance d9 away from a perpendicular containing the center C3 of the bottom surface of the microlens so as to away from the gate electrode 33. In other words, as shown in
Now, operations of the solid-state image sensor configured as described above will be described. First, in the light receiving section 20, any of the unit cells 21 is selected. In this selecting operation, an address signal AD output by the vertical selection circuit 13 turns on the address transistor 31 in any of the unit cells 21. Further, the load transistor 23 connected to the corresponding vertical signal line 22 is turned on.
Further, a reset operation is performed to set the vertical signal line 22 to a given reference potential. In the reset operation, the vertical selection circuit 13 asserts a reset signal RS to turn on the reset transistor 32 in the selected unit pixel. Turning on the reset transistor 32 provides VDD to the gate of the amplifying transistor 30 via a current path in the transistor 32 to turn on the transistor 30. Then, since the address transistor 31 is on, the vertical signal line 22 is set to the given reference potential through a path extending from the power supply potential VDD to the vertical signal line 22 via the current path in the transistors 31 and 30.
The vertical selection circuit 13 then selects one of the read signal lines RD1 and RD2. The read transistor 28 connected to the selected read signal line RD1 or RD2 is then turned on. Consequently, in the pixel 25 with the transistor 28 turned on, charges generated in the photodiode 29 in response to incident light reach the gate of the amplifying transistor 30 via the current path in the transistor 28. This varies the potential of the vertical signal line 22 depending on the result of photoelectric conversion in the photodiode 29. Specifically, an image signal is provided to the vertical signal line 22 on the basis of the charges from the photodiode 29. The image signal is read onto an output node OUT via the clamp circuit 11, sample-and-hold circuit 12, and read transistor 24.
As described above, the solid-state image sensor in accordance with the present embodiment exerts the effect (1), described in the first embodiment. The effect (1) is particularly significant when the signal output section 26 is shared by a plurality of pixels as in the present embodiment. As shown in
Consequently, the shape of each pixel 25 is laterally asymmetrical in the direction shown in
However, in the present embodiment, the top P3 of the microlens 34, which condenses incident light on the photodiode 29, is away from the center C3 of the bottom surface of the microlens 34 in the direction horizontal to the bottom surface of the microlens 34. This prevents the incident light from being blocked by the gate electrode. Thus, a decrease in the photosensitivity of the solid-state image sensor can be inhibited as described in the first embodiment.
Now, a solid-state image sensor in accordance with a fourth embodiment of the present invention will be described. The present embodiment is the same as the third embodiment except that each unit cell 21 includes four pixels 25 and that the light receiving section 20 includes read signal lines RD3 and RD4 in addition to the read signal lines RD1 and RD2. The vertical selection circuit selects any of the read signal lines RD1 to RD4.
As shown in
In the light receiving section 20 of the solid-state image sensor configured as described above, the photodiodes 29 in the pixels 25-1 and 25-3 in the odd columns are isolated from the corresponding signal output section 26 in the respective pixels 25-1 and 25-3, and the photodiodes 29 in the pixels 25-2 and 25-4 in the even columns are isolated from the corresponding signal output section 26 in the respective pixels 25-2 and 25-4. The top P3 of the microlens 34 corresponding to each pixel is placed opposite the adjacent pixel across the center C3 along the first direction. Accordingly, in the example in
The above solid-state image sensor also exerts the effect (1), described in the first and third embodiments.
Now, a solid-state image sensor in accordance with a fifth embodiment of the present invention will be described. In the present embodiment, the microlens 5, described in the first and second embodiments, is applied to Jpn. Pat. Appln. KOKAI Publication No. 2006-302970.
The solid-state image sensor in accordance with the present embodiment is the same as the configuration described in the third embodiment with reference to
As shown in the figure, the unit cell 21 has a configuration similar to that described in the third embodiment with reference to
That is, in
In other words, the unit cells 21 each include two pixels 25 adjacent to each other in the first direction (vertical direction) and are arranged in a checkered pattern within the light receiving section 20. The signal output section 26 extends from between the two pixels 25 in the same unit cell 21 over the adjacent unit cell 21 in the second direction (horizontal direction). Moreover, the photodiodes 29 included in one of the pixels 25 in one unit cell 21 and in the diagonally adjacent unit cell 21 are arranged along the same horizontal line.
This configuration can also use the microlens 34, described in the third embodiment. The gates 33 of the pixels 25 adjacent to each other in the second direction are arranged opposite each other in the first direction across the center of the pixels 25. Consequently, between the two pixels 25 adjacent to each other in the second direction, the position of top P3 of the microlens 34, that is, its focus F3, in one of the pixels 25 is opposite to that in the adjacent pixel in the first direction. The configuration in accordance with the present embodiment also exerts the effect (1), described in the first and third embodiments.
Now, a solid-state image sensor in accordance with a sixth embodiment of the present invention will be described. The present embodiment varies the curvature of the microlens depending on its position in the light receiving section to improve the photosensitivity of the solid-state image sensor.
The solid-state image sensor is configured as described in the first embodiment with reference to
The present configuration exerts the following effect.
(3) A decrease in the photosensitivity of the solid-state image sensor can be inhibited (3).
With the configuration in accordance with the present embodiment, incident light efficiently enters the photodiode 29 even at the ends of the surface of the light receiving section 20. This enables the inhibition of a decrease in the photosensitivity of the solid-state image sensor. This effect will be described in connection with the case where the microlens 43 has a fixed curvature in the center and at the ends of the light receiving section.
As shown in
In contrast, in the present embodiment, the curvature of the microlens 43 decreases consistently with increasing distance from the center of the light receiving section 20 as shown in
Now, a solid-state image sensor in accordance with a seventh embodiment of the present invention will be described. The present embodiment corresponds to the combination of the first to fifth embodiments with the sixth embodiment.
As shown in
The configuration in accordance with the present embodiment exerts the effects (1) and (2), described in the first to third embodiments, and the effect (3), described in the sixth embodiment.
As described above, in the solid-state image sensor in accordance with the first to fifth embodiments of the present invention, the incident surfaces of the microlenses, which condense incident light on the photodiodes, have a fixed e curvature. Further, the top of each microlens is away from the center of its bottom surface in the horizontal direction. Consequently, the microlens has a focus at a position away from the center of the corresponding pixel. This makes it possible to prevent incident light from being blocked by the gate electrode or the like. A decrease in the photosensitivity of the solid-state image sensor can thus be inhibited.
Further, the solid-state image sensor in accordance with the sixth and seventh embodiments, the curvature of the microlens is larger in the center of the light receiving section and is smaller at the ends. This allows light to efficiently enter the photodiode even at the ends of the light receiving section, where light is obliquely incident on the microlens.
In the first to seventh embodiments, the term “fixed” curvature accepts such errors as described below.
However, it is assumed that the microlens is laterally asymmetrical with respect to the optical axis OP1 and that the curvature and focal distance on the left side of the optical axis OP1 are R and f, respectively, whereas the curvature and focal distance on the right side of the optical axis OP1 are R′ and f′, respectively (CASE2). Then, the beams condensed by the microlens 52 do not interest at one point. Thus, in an area where light condensed at the rightmost position of the microlens 52 and light condensed at the leftmost position of the microlens 52 cross, the beam has a width x expressed by:
x=a·|f−f′|/(f+f′)
where a is the radius of the microlens 52. Since light is an electromagnetic wave, it originally has a spread equal to its wavelength. Accordingly, no practical problem occurs provided that the width x is equal to the wavelength λ. In particular, with a visual light sensor, x shorter than 555 nm reduces the adverse effect of the difference in the curvature of the microlens 50 between its right and left sides; at a wavelength of 555 nm, human beings have the highest visibility. That is, the following equation is desirably satisfied.
x=a·|f−f′|/(f+f′)<λ(=555 nm)
Of course, λ can be appropriately selected by the solid-state image sensor. The relation between the radius of curvature and the focus of the microlens 52 is expressed by:
(1/f)=(nL−1)/R
where nL is the refractive index of the microlens 52. Consequently, the following equation is derived.
x=a·|R−R′|/(R+R′)<λ(=555 nm)
The above range corresponds to the “fixed curvature” in the above embodiments.
The microlens may be a cylindrical lens 5 such as the one shown in
First, as shown in
Further, in the description of the above embodiments, the microlens 5 is rectangular, and the gate electrode 3 is placed parallel to one side of the microlens 5. However, the gate electrode 3 may be placed obliquely to one side of the microlens 5. This case will be described with reference to
As shown in the figures, the microlens 5 is rectangular and its sides are formed along a first direction or a second direction perpendicular to the first direction. The gate electrode 3 and the photodiode 2 are arranged at 45° to the first and second directions. In this configuration, the position of top P1 of the microlens 5, that is, the position of the focus Fl, is different from that of the center C1 of bottom surface of the microlens 5 in a horizontal plane. More specifically, as shown in the figures, the top P1 (focus F1) is located opposite the gate electrode 3 across the center C1 in the direction in which the photodiode 2 and the gate electrode 3 are arranged.
The third to seventh embodiments can also be applied to a configuration having metal wiring layers and gates as is the case with the second embodiment or having metal wiring layers but not gates. If the configuration has metal wiring layers and gates, incident light is more likely to be blocked because the metal wiring layers are normally provided above the gates. Therefore, the curvature of the microlens is desirably designed giving more considerations to the metal wiring layers than to the gates.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-027201 | Feb 2006 | JP | national |
