JP2006024686A - Solid-state imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid-state imaging apparatus that has improved linearly in strength versus signal output and can suppress deterioration in the linearly of sensitivity when signal charges are small, and fixed pattern noise by the variations. <P>SOLUTION: The solid-state imaging apparatus includes an annular gate 11; a source region 12 formed inside the annular gate; a drain region 13 formed so that the outer periphery of the annular gate is covered; and a carrier pocket 14 formed annularly at the lower portion of the annular gate. In the solid-state imaging apparatus, the outer periphery section of the carrier pocket 14 formed annularly is rounded. Ideally, corners are preferably rounded so that the length in the direction of the gate length (channel length) in the annular gate and/or annular carrier pocket is the same at any part on the circumference. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a solid-state image pickup device having characteristics of high image quality and low power consumption, and more particularly to a solid-state image pickup device, and in particular, a ring shape capable of obtaining a signal output by modulating a threshold value (Vth) according to subject light. The present invention relates to image quality improvement of a solid-state imaging device having a gate shape.

  Solid-state imaging devices mounted on cellular phones and the like include CCD (charge coupled device) type image sensors and MOS type image sensors. The CCD type image sensor is excellent in image quality, and the MOS type image sensor has low power consumption and low process cost. In recent years, a threshold modulation type MOS solid-state imaging device having both high image quality and low power consumption has been proposed. A threshold modulation type MOS solid-state imaging device is disclosed in, for example, Patent Document 1.

  FIG. 7 is a schematic plan view of the modulation transistor of the threshold modulation type solid-state imaging device disclosed in Patent Document 1, and FIG. 8 is a sectional view taken along line AA of FIG.

  The threshold modulation type solid-state imaging device shown in FIGS. 7 and 8 includes a photodiode 20 (indicated by a two-dot chain line) and a transistor region called a modulation transistor 10, and the modulation transistor 10 is, for example, rectangular or The ring-shaped gate electrode 11 has an octagonal shape (the case of a quadrangular shape is shown in FIG. 1 of Patent Document 1 but an octagonal shape is shown in FIG. 7 of the present application). A source diffusion region 12 is formed in the center, and a drain diffusion region 13 is formed so as to surround the ring-shaped gate electrode 11. A high-concentration buried layer called a carrier pocket 14 in the well region 16 that contributes to threshold modulation under the channel region 15 in the channel width direction is provided outside the modulation transistor 10 with a light-receiving portion composed of a photodiode 20 And a portion having a low potential with respect to the signal charge is formed from the source diffusion region 12 to the drain diffusion region 13. The carrier pocket 14 has a function of accumulating photoelectrically converted signal charges.

  Photogenerated charges generated by light incident on the photodiode 20 are transferred to a P-type well region 16 provided below the ring-shaped gate electrode 11 and accumulated in a carrier pocket 14 formed in this region. . The threshold voltage (Vth) of the modulation transistor 10 is changed by the photo-generated charges accumulated in the carrier pocket 14. Thereby, a pixel signal corresponding to incident light (light intensity) can be obtained from a source contact (not shown) connected to the source region 12 of the modulation transistor 10. Since the ring-shaped gate electrode 11 has a quadrangular or octagonal ring, the carrier pocket 14 has the same shape.

In Patent Documents 2 to 4, there is no description of the term carrier pocket, but the same is described as the prior art, and a fixed pattern due to sensitivity linearity degradation and variations in light intensity versus signal output characteristics at low illuminance. In order to improve noise generation, a configuration in which a ring-shaped gate electrode is not used, a straight-shaped gate electrode, and an element isolation electrode is proposed.
Patent No. 2954992 Japanese Patent Laid-Open No. 10-65138 Japanese Patent Laid-Open No. 10-65137 Japanese Patent Laid-Open No. 10-144902

  However, although Patent Documents 2 to 4 have an electrode for element isolation by using a straight control electrode, there is a problem that it is difficult to form a separation layer located under the electrode. In addition, increasing the oxide film has a problem that process processing increases.

  On the other hand, in Patent Document 1, problems caused by using the ring-shaped gate 11 are as follows.

  FIG. 9 shows a current distribution during modulation in a low illuminance environment using the threshold-modulation MOS type solid-state imaging device disclosed in Patent Document 1. Here, only the B-B ′ line portion including the side portion and the corner portion of the modulation transistor 10 is shown, but current during modulation mainly flows through each side portion of the octagon. That is, the drain / source current IB flowing through the octagonal side is larger than the drain / source current IB 'flowing through the octagonal side. That is, IB> IB ′. This is because the current path (the distance between the drain and the source) is shorter at the side than at the corner, and the potential at the side is higher than that at the corner as shown in FIG. The difference in potential between the side and the corner is because the length of the carrier pocket in the gate length direction (that is, the pocket length in the direction across the gate) is longer at the corner than at the side. This is because a two-dimensional effect works and a corner portion having a longer pocket length causes a potential drop than a side portion.

  That is, in the pixel cell, the length L2 of the corner of the annular gate 11 is larger than the length L1 of the side (L2> L1), and the length l2 of the corner of the carrier pocket 14 is larger than the length of the side l1. Since (l2> l1), the corner portion has a potential drop more than the side portion.

  FIG. 10 shows a difference in potential of the carrier pocket 14 in the cross section along line B-B ′ of FIG. 9. Symbols Pa and Pb in FIG. 10 indicate potentials at points a and b in FIG.

  Point a corresponds to the position of the side portion in the carrier pocket 14, and point b corresponds to the position of the corner portion in the carrier pocket 14. Therefore, Pa represents the potential of the side portion in the carrier pocket 14, and Pb represents the potential of the corner portion in the carrier pocket 14. The angle including the point b forms a part of the ring-shaped gate electrode 11. If only a part of the angle has a low potential (deep), a signal charge (hole) is present in a low illumination environment. First collects from there, but the corner has a smaller area than the side (ie, it is a very small part of the carrier pocket region shown in satin), so its signal charge does not contribute much to the modulation of the signal output. . In other words, at low illuminance, in the process of accumulating the accumulated charge again from the cleared state, the charge accumulates from the point b of the corner having the lowest potential, but this is the only one that hits the corner of the carrier pocket region. Since the current is only partially and locally increased at corners where the potential is low and current does not easily flow, it is difficult to contribute to modulation of the signal output (channel current).

  As described above, when the potential of the carrier pocket 14 is partially or locally lowered like a corner portion, the source-drain current, that is, the channel current flows at the point a on the side where the potential of the carrier pocket 14 is high. On the other hand, the surface channel is also likely to conduct partially, but on the other hand, the signal charge is accumulated first at the point b at the corner having a low potential as described above, and is modulated with respect to the point a where the channel is partially conducted. (Since point a is different from point b), the slope of the signal output at low illuminance with low signal charge, that is, the slope of the light intensity vs. signal output characteristics is reduced. .

  As a result, the conventional light intensity versus signal output characteristic at low illuminance becomes the characteristic shown by the dotted line in FIG. In other words, when the light intensity is low, the signal output rises slowly and the linearity deteriorates (dotted line graph in FIG. 6), so-called black crushing occurs, or the linearity of individual pixels is uneven at low illumination. This causes non-uniformity of sensitivity, so-called fixed pattern noise.

  Accordingly, in view of the above problems, the present invention provides a solid-state imaging device capable of suppressing fixed pattern noise due to deterioration and variation in sensitivity linearity when light intensity versus signal output linearity is good and signal charge is small It is intended to do.

  The solid-state imaging device according to the present invention includes an annular gate, a source region formed inside the annular gate, a drain region formed so as to cover an outer periphery of the annular gate, and an annular shape below the annular gate. In the solid-state imaging device including the carrier pocket, the outer peripheral portion of the carrier pocket formed in an annular shape is rounded.

  According to the configuration of the present invention, if there are non-round portions such as corners on the outer periphery of the carrier pocket formed in an annular shape along the annular gate, the length of the carrier pocket in the gate length direction becomes longer. As a result, a potential drop occurs in the carrier pocket region of the portion. Therefore, by rounding the outer periphery of the annular carrier pocket so as to be rounded as a whole, the lowest part of the potential in the carrier pocket region is uniform over the entire periphery. Thus, the charge accumulated in the carrier pocket becomes uniform throughout the carrier pocket at low illuminance, and the accumulated charge can effectively contribute to the modulation of the channel current. The term “annular” means a closed loop formed by forming a polygonal line shape including an octagon and a quadrilateral as a band.

  In the present invention, it is preferable that the outer peripheral portion of the annular gate is rounded.

  According to the configuration of the present invention, when there is a non-round portion such as a corner on the outer peripheral portion of the annular gate, the length of the channel current path, that is, the length of the drain-source distance increases. Since a potential drop occurs in the carrier pocket region of that portion, by rounding the outer periphery of the annular gate so as to be rounded as a whole, the lowest potential of the carrier pocket region is uniform over the entire circumference. The charge accumulated in the carrier pocket at the time of low illuminance becomes uniform throughout the carrier pocket, and the accumulated charge can effectively contribute to modulation with respect to the channel current.

  In the present invention, it is preferable that the carrier pocket or / and the inner peripheral portion of the annular gate are also rounded.

  According to the configuration of the present invention, the same effect as the rounding process of the outer peripheral part can be obtained by rounding the inner peripheral part of the carrier pocket and the inner peripheral part of the annular gate.

  A solid-state imaging device according to the present invention includes an annular gate formed in an annular shape including corners and sides, a source region formed inside the annular gate, and a drain formed so as to cover the outer periphery of the annular gate. In a solid-state imaging device including a region and a carrier pocket formed annularly below the annular gate, the corners of the annular gate and the carrier pocket have the same length as the side. As described above, the annular gate and the carrier pocket are rounded.

  According to the configuration of the present invention, the corners of the annular gate and the carrier pocket are cut and rounded, so that the lowest potential of the carrier pocket region has a uniform level over the entire circumference, and at low illuminance. The charge accumulated in the carrier pocket becomes uniform throughout the carrier pocket, and the accumulated charge can effectively contribute to the modulation of the channel current.

  Furthermore, in the present invention, the annular gate has an octagonal shape.

  According to the configuration of the present invention, when the annular gate has an octagonal shape, the carrier pocket along the annular gate also has an octagonal shape. The glass mask required for forming the annular gate can be formed at a relatively low cost in the shape of a straight line and a 45 degree or 90 degree line with respect to the straight line, and the square shape is relatively easy by the rounding process. Can be formed in a substantially circular shape.

  Furthermore, in the present invention, the rounding process is characterized in that a pattern is formed by a combination of a straight line and a 45-degree line or a 90-degree line using a lithography technique based on a glass mask pattern formation.

  The rounding process by the glass mask pattern formation is difficult because the circular processing is inherently difficult. However, according to the configuration of the present invention, a combination of straight lines and 45-degree lines, or only straight lines (including straight lines and 90-degree lines). The pattern formation processing of the glass mask is facilitated by performing the pattern formation by (). When a fine pattern such as a pixel cell is formed on a semiconductor substrate, the inner and outer circumferences of the annular gate and the carrier pocket formed annularly along the annular gate are patterned by a combination of a straight line and a 45 degree line or a 90 degree line. When exposure is performed with a projection exposure apparatus using a glass mask, the inner and outer peripheral patterns of the annular gate and carrier pocket formed after the exposure will be formed in a staircase pattern. When the staircase interval is narrowed to a certain extent, the inner and outer peripheral patterns after exposure are rounded due to the proximity effect of light, resulting in a round shape and a beautiful circular line. Note that the proximity effect of light means that when a fine pattern is formed by a projection exposure apparatus, if the surrounding pattern spacing is coarse or dense, the degree of diffraction and interference slightly changes, resulting in a pattern formed after exposure. A phenomenon in which a dimensional difference occurs.

  According to the solid-state imaging device of the present invention, the linearity of the light intensity versus the signal output is good, and it is possible to suppress the fixed pattern noise due to the deterioration of the linearity of the sensitivity when the signal charge is small and its variation.

  Embodiments of the invention will be described with reference to the drawings.

  A semiconductor circuit manufacturing technique is used to produce a solid-state imaging device. In the semiconductor manufacturing process, a lithography technique is used for forming a gate electrode that requires fine processing.

  The lithography technique is a technique for forming a semiconductor circuit on a wafer by a technique similar to photolithography, and is formed by first applying a photosensitive polymer (photoresist) on a semiconductor substrate. The thickness of the photoresist film is adjusted to a desired thickness by the viscosity and the rotation speed of the spinner, and the applied film is heated to remove the solvent. Next, ultraviolet rays are irradiated through a photomask on which a predetermined circuit pattern is drawn. The photochemical reaction occurs in the photoresist due to the ultraviolet light exposure, the photosensitive portion becomes soluble in the developer, and a photoresist pattern is formed on the semiconductor substrate. Next, the semiconductor substrate is etched using the photoresist as a mask to form a desired circuit pattern.

  FIG. 1 shows a glass mask 1 used at the time of manufacture and an annular gate 11 formed on a semiconductor substrate using the mask 1 in the solid-state imaging device according to the first embodiment of the present invention.

  In FIG. 1A, reference numeral 1 denotes a glass mask as a photomask, and an octagonal annular gate pattern 2 is formed on the glass mask 1. The area | region of the cyclic | annular gate pattern 2 is a light impervious part, and the other area | regions 3 and 4 form the light transmissive part.

  1B is formed by using the glass mask 1 of FIG. 1A and a photolithography technique on a semiconductor substrate on which a photodiode, a source region, a drain region, and a carrier pocket (not shown) are formed in advance. An annular gate 11 is shown.

  FIG. 2 is an enlarged view of one side P or one side Q of the inner peripheral portion of the octagonal annular gate pattern 2 on the glass mask 1 of FIG. For one side P or Q, the dotted line portion indicates a case where the rounding process is not performed, and the solid line indicates a case where the rounding process is performed. In this example, when creating a glass mask, it is technically difficult to form a clean circular line as a mask pattern, but if a circle is created by a combination of a straight line and a 45-degree line, This is made by paying attention to the fact that the mask creation is technically relatively easy.

  3 is formed by performing a rounding process as shown in FIG. 2 so that the corners of the annular gate 11 and the carrier pocket 14 have the same length as the sides in the layout design of the pixel cell. A sensor cell (for one pixel) of the solid-state imaging device is shown. FIG. 4 shows the potential Pc of the pocket carrier pocket in the section taken along the line C-C 'of FIG.

  In FIG. 3, a MOS solid-state image sensor for one pixel constituting the solid-state image pickup device receives incident light and obtains photo-generated charges, accumulates photo-generated charges from the photodiodes 20, And a modulation transistor 10 that converts the signal voltage and outputs the signal voltage.

  The modulation transistor 10 includes an annular gate 11 formed in an annular shape, a source region 12 formed inside the annular gate 11, a drain region 13 formed so as to cover the outer periphery of the annular gate 11, and the annular transistor A carrier pocket 14 formed annularly along the annular gate 11 is provided below the gate 11.

  By performing the rounding process as shown in FIG. 3, the two-dimensional effect works uniformly by making the annular gate 11 and the carrier pocket 14 and the respective lengths L and l the same in each region of the entire circumference. The potential distribution in the carrier pocket 14 is not partially or locally nonuniform. The two-dimensional effect means that the influence from the drain / source region reaches the diffusion of the channel region.

  FIG. 4 shows the potential of the carrier pocket in the section taken along the line C-C ′ of FIG. 3. The same potential distribution is obtained in any cross section, not limited to the C-C 'line cross section.

  As a result, the accumulation of signal charges is uniformly accumulated in the entire carrier pocket, and the surface channel current easily flows. Such a sensor cell can contribute modulation to the entire channel even when the signal charge at low illuminance is small, and the slope of light intensity versus signal output can be greatly increased, so light intensity versus signal output at low illuminance. The deterioration of the linearity of characteristics can be prevented.

  FIG. 6 shows the difference in light intensity versus output characteristics for pixels with uniform and non-uniform potentials. According to the embodiment of the present invention, pixels having a uniform potential can be obtained, so that it is possible to obtain a characteristic with good linearity of light intensity versus signal output characteristic as shown by the solid line in FIG. Since the variation factor in each pixel is removed, it is possible to reduce sensitivity unevenness due to non-uniformity of linearity, so-called fixed pattern noise.

  In addition, making the annular gate 11 into an octagonal shape and rounding the angle can create the annular gate pattern on the glass mask at the time of manufacture with straight lines and lines of 45 degrees and 90 degrees. There are advantages you can do. However, it is meaningless to form the annular gate 11 in a quadrangular shape. This is because the number of work steps for creating a glass mask increases in order to obtain a substantially circular shape even when the rounding process is performed based on the quadrangular shape.

  FIG. 5 relates to the solid-state imaging device according to the second embodiment of the present invention, and illustrates another example of rounding processing. 1 shows an enlarged view of one side P or one side Q of the inner periphery of the octagonal annular gate pattern 2 on the glass mask 1 of FIG.

  In FIG. 2 of the first embodiment, a straight line and a 45-degree line for the straight line are used, but the present invention is not necessarily limited to this combination.

  Example 2 shows an example in which, when a glass mask is created, the corners are rounded with only a combination of a straight line and a 90-degree line as a mask pattern, that is, a straight line that does not use a 45-degree line.

  In the case of the second embodiment, the same effect as that of the first embodiment shown in FIG. 2 can be obtained. That is, the rounding process of the second embodiment makes it possible to suppress the fixed pattern noise due to the deterioration of the linearity of the sensitivity when the signal intensity is small and the linearity of the light intensity versus the signal output is small and the variation thereof.

  In the pixel cell layout design, the corners of the ring-shaped gates and carrier pockets are rounded so that they are the same as the lengths of the sides. However, the effect will be greater if they are close to the same length.

  In the layout design of the pixel cell in the solid-state imaging device, computer processing is introduced, and lines or points (called grids) with a certain vertical and horizontal interval are assumed so as to facilitate the computer processing. The pattern and wiring of each part including the pixel cell are designed to follow the grid.

  Therefore, due to the grid limitation of the layout design, it is not possible to carry out finer rounding processing than that, but on glass masks and semiconductor substrates, it is equivalent to rounding processing due to insufficient resolution in the photo process and edging process in the semiconductor process. Therefore, it is unnecessary to refine the grid and increase the manufacturing cost of the glass mask.

  As for the rounding process of the carrier pocket 14 formed in an annular shape, a p-type (or n-type) high-concentration buried layer is formed in the p-type (or n-type) well region below the channel region as the carrier pocket 14. 2 can be formed in a circular shape with no corners by using a mask similar to that shown in FIG. 2 or FIG.

  INDUSTRIAL APPLICABILITY The present invention is effective in improving the linearity of light output versus signal output characteristics in a solid-state imaging device having an annular gate shape, and particularly improving the linearity of signal output with respect to incident light at low illuminance.

FIG. 3 is a diagram illustrating a glass mask used at the time of manufacture and an annular gate formed on a semiconductor substrate using the mask in the solid-state imaging device according to the first embodiment of the present invention. The figure which expands and shows the one side P of the outer peripheral part of the cyclic | annular gate pattern on the glass mask of FIG. 1, or the one side Q of the inner peripheral part. FIG. 5 is a schematic plan view showing a sensor cell of a solid-state imaging device formed by performing rounding so that the corners of the annular gate and the carrier pocket have the same length as the side in the layout design of the pixel cell. The figure which shows the potential of the carrier pocket in the C-C 'line cross section of FIG. FIG. 10 is an enlarged view showing another example of rounding processing in the solid-state imaging device according to the second embodiment of the present invention. The figure which shows the difference of the light intensity versus output characteristic in a pixel with a uniform potential, and a pixel with a non-uniform potential. The typical top view which looked at the modulation transistor of the solid-state imaging device of the threshold modulation system disclosed by patent document 1 from the top. AA line sectional view of Drawing 7. The figure which shows the electric current distribution at the time of the modulation | alteration in a low illumination environment using the MOS type solid-state imaging device of the threshold value modulation system of patent document 1. FIG. The figure which shows the potential of the carrier pocket in the B-B 'line cross section of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Glass mask, 2 ... Ring gate pattern, 10 ... Modulation transistor, 11 ... Ring gate, 12 ... Source region, 13 ... Drain region, 14 ... Carrier pocket.

Claims (6)

An annular gate; a source region formed inside the annular gate; a drain region formed so as to cover an outer periphery of the annular gate; and a carrier pocket formed annularly below the annular gate. In a solid-state imaging device,
A solid-state imaging device, wherein an outer peripheral portion of the carrier pocket formed in an annular shape is rounded.   The solid-state imaging device according to claim 1, wherein an outer peripheral portion of the annular gate is rounded.   The solid-state imaging device according to claim 2, wherein the carrier pocket or / and the inner peripheral portion of the annular gate are also rounded. An annular gate formed in an annular shape including corners and sides, a source region formed inside the annular gate, a drain region formed so as to cover the outer periphery of the annular gate, and along the annular gate A solid-state imaging device including a carrier pocket formed annularly below the
A solid-state imaging device, wherein the annular gate and the carrier pocket are rounded so that the corners of the annular gate and the carrier pocket have the same length as the side. .   The solid-state imaging device according to claim 1, wherein the annular gate has an octagonal shape. The rounding process is performed by using a lithography technique based on pattern formation of a glass mask, and pattern formation is performed using a combination of a straight line and a 45-degree line or a 90-degree line. The solid-state imaging device described in 1.

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