JP3618842B2 - Photoelectric conversion device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photoelectric conversion device, and more particularly to a photoelectric conversion device in which a photogate electrode is formed on a photoelectric conversion region.
[0002]
[Prior art]
Conventionally, as a photoelectric conversion element, there is a metal-oxide-semiconductor MOS structure that enables photoelectric conversion, and there are an FET type and a CCD type depending on the optical carrier movement method. Further, depending on the material component of the semiconductor, the light wavelength sensitivity differs corresponding to the band gap and impurity level of the material, and Si has light sensitivity in the visible region. The photoelectric conversion elements have been employed in various fields such as line area image sensors, copiers, facsimiles, solar cells, etc., as photoelectric conversion efficiency is improved and density is increased. Photoelectric conversion devices using the MOS structure FET type photoelectric conversion elements are expected to be widely used because the manufacturing process can be shortened and the transfer of photocharges can be performed at high speed.
[0003]
One of the photoelectric conversion solid-state imaging devices using a photogate that accumulates photocharges in the MOS structure FET type gate electrode is a CMOS process compatible sensor (hereinafter abbreviated as a CMOS sensor). This type of sensor has been published in documents such as IEEE TRANSACTIONS ON ELECTRON DEVICE VOL41, PP452-453 1994.
[0004]
FIG. 7A shows a circuit configuration diagram, a cross-sectional view of the pixel portion of the CMOS sensor, and FIG. 7B shows a plan view thereof.
[0005]
In FIG. 7A, 1 is a Si substrate, 2 is a P-type well formed on the Si substrate, 3 is a thick SiO ₂ film which is an element isolation region, 4 is a SiO ₂ film which is a gate insulating film, and 5 is a photo A poly-Si electrode, which is a gate electrode, 6 is a transfer gate for transferring photocharge accumulated under the photogate to a floating diffusion portion (hereinafter referred to as FD portion) 7, and 8 is a photocharge of the FD portion 7. A source follower amplifier MOS transistor for transmitting in a source follower format, 9 is a selection switch MOS transistor for selecting a pixel in which photoelectric charges are accumulated, and 10 is a vertical output line for selectively reading out the photoelectric charges of pixels arranged vertically and horizontally in the vertical direction. , 11 are load MOS transistors having a load function of the source follower amplifier MOS transistor 8.
[0006]
In FIG. 7B, the control pulse φPG terminal is connected with a large area of the photogate 5 that greatly contributes to photoelectric conversion, the transfer gate electrode 6 connected to the control pulse φTX terminal, and the control pulse φR terminal are connected. A gate electrode of the reset MOS transistor, an n ⁺ portion connected to the power supply VDD, and an FD portion 7 connected to the gate electrode of the source follower MOS transistor 8 are formed.
[0007]
Hereinafter, the operation of the photoelectric conversion device will be described. By supplying a positive pulse to the control pulse φPG and applying a positive voltage to the photogate 5, a depletion layer is formed under the photogate 5 below the poly-Si electrode 5 in the P-type well 2 in FIG. A photoelectric conversion unit 12 is generated in a portion surrounded by a dotted line. The area other than the photogate 5 is shielded from light.
[0008]
Here, an electron-hole pair is generated in the P-type well 2 by the light transmitted through the photogate 5 and incident on the depletion layer. The positive pulse electric field of the control pulse φPG causes the holes to move toward the P-type well. The electrons move below the photogate 5. The photoelectric charge of the amount of electrons corresponding to the amount of light is accumulated under the photogate 5, and when the transfer period starts, the control pulse φPG is lowered and the control pulse φTX is set high, thereby setting the transfer gate 6 high and setting the transfer MOS transistor Through the FD unit 7. Electrons are accumulated in a portion indicated by a dotted line below the FD portion 7 in FIG. 7A, and the potential change of the FD portion 7 is made high by the source follower operation by the source follower amplifier MOS transistor 8 and the gate is changed to the gate. The voltage VG is set high, the load MOS transistor 11 is turned on, the selection switch MOS transistor 9 is turned on, and the output OUT is made to the outside through the vertical output line 10. In this way, the photoelectric charge reading operation under the photogate 5 having the photoelectric conversion portion is performed.
[0009]
Since the source follower amplifier MOS transistor 8 is provided for each pixel, non-destructive due to excessive photocharge and high-sensitivity output OUT without transfer loss can be performed. In addition, since the amplifier MOS transistor 8 is operated by the source follower, the FD unit 7 is read to make a dark output when the control pulse φTX is low, and the photocharge is transferred to the FD unit 7 to read by making the control pulse φTX high. To light output. According to the read timing of each control pulse, the dark output and the bright output are sampled and canceled to cancel kTC noise, low frequency 1 / f noise, and FPN. Furthermore, since the MOS structure of the pixel portion and the MOS structure of the peripheral circuit such as the scanning circuit can be formed by the same CMOS process, the yield and cost are more advantageous than other CCD sensors.
[0010]
[Problems to be solved by the invention]
However, in the above conventional example, since the photoelectric conversion element is completely covered with a poly-Si electrode that is a photogate electrode, the light absorption of the poly-Si electrode has a drawback that sensitivity, in particular, blue sensitivity is lowered. It was. FIG. 8 shows an example of the transmission characteristics of poly-Si. According to the figure, it can be seen that there is much absorption of visible light, particularly light having a wavelength of blue (near 500 nm) or less.
[0011]
The object of the present invention is to increase the sensitivity of a solid-state imaging device having a photogate formed of a poly-Si electrode.
[0012]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, a photogate is provided only in a part on the photoelectric conversion region. If the concentration of the semiconductor layer of the photoelectric conversion portion is low to some extent (1 × 10 ¹⁶ cm ⁻³ or less) ) Since a depletion layer of several μm spreads in the lateral direction under the photogate, it has a sufficient ability to collect photoelectric conversion carriers. In particular, if light is condensed on a photoelectric conversion region other than a photogate by a microlens in an area sensor, an ideal spectral sensitivity characteristic can be obtained without causing a decrease in blue sensitivity.
[0013]
Specifically, in a photoelectric conversion device having a photogate electrode for forming a depletion layer and a pixel having the depletion layer as a photoelectric conversion region, the photogate electrode is formed only in a part of the pixel region. And A photogate electrode for forming a depletion layer; a transfer gate for transferring charges generated in the depletion layer to the floating diffusion layer; and a MOS amplifier for reading a potential change of the floating diffusion layer by a source follower operation. In the above photoelectric conversion device, the photogate electrode is formed only in a part of the pixel region including the depletion layer. Further, a microlens is formed on each of the pixels, and the photogate electrode is not formed in a condensing region of the microlens. Furthermore, the well concentration of the pixel region where the photogate electrode is not formed is 1 × 10 ¹⁴ cm ⁻³ or more and 2 × 10 ¹⁵ cm ⁻³ or less, and the opening of the pixel region is 4 × 4 μm ² or less. The sensitivity can be improved and the influence of crosstalk can be improved.
[0014]
Further, a P-type well formed on the Si substrate, a gate insulating film formed on a part of the P-type well, a photogate electrode formed on the gate insulating film, and a photocharge accumulated under the photogate A photoelectric conversion device comprising a transfer gate for transferring to a floating diffusion portion and a source follower amplifier MOS transistor for transmitting photocharges of the floating diffusion portion in a source follower format, wherein the photogate electrode shape is formed on the gate insulating film It is characterized by having a ring shape, an antenna shape or a comb shape.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings together with each example.
[0016]
(First embodiment)
1 and 2 are a conceptual sectional view and a plan view of an embodiment according to the present invention. In Figure 1, 1 is a Si substrate, 2 denotes a P-type well, the thick SiO ₂ film is an element isolation region 3, SiO ₂ film which is the gate insulating film of the MOS 4, 5 is a photo gate electrode poly Si electrode , 6 is a transfer gate for transferring photocharge accumulated under the photogate to 7 floating diffusion parts (hereinafter referred to as FD part), 8 is a source follower amplifier MOS transistor, 9 is a selection switch MOS transistor, 10 Is a vertical output line for selectively reading out photocharges of pixels arranged vertically and horizontally in the vertical direction, 11 is a load MOS transistor having a load function of the source follower amplifier MOS transistor 8, and 12 is formed by the potential of the photogate 5. The depletion layer becomes a photoelectric conversion region.
[0017]
In FIG. 2, the control gate φPG terminal is connected with the photogate 5 that greatly contributes to photoelectric conversion as a ring-shaped large area, the transfer gate electrode 6 to which the control pulse φTX terminal is connected, and the control pulse φR terminal is connected. A gate electrode of the reset MOS transistor, an n ⁺ portion to which the power supply VDD is connected, and an FD portion 7 to which the gate electrode of the source follower MOS transistor 8 is connected are formed.
[0018]
Here, the operation of the photoelectric conversion device will be described. Electron-hole pairs are generated in the depletion layer shown by the dotted line in FIG. 1 in the P-type well 2 directly into the ring shape of the photogate 5 or by light incident through the photogate 5 and entering the depletion layer. Due to the positive pulse electric field of the control pulse φPG, holes move toward the P-type well and electrons move below the photogate 5. Photoelectric charges of the amount of electrons corresponding to the amount of light from the target image are accumulated under the photogate 5, and when the transfer period starts, the control pulse φPG is lowered and the control pulse φTX is set high to transfer the transfer gate 6 high. It is completely transferred to the FD section 7 through the MOS transistor. Electrons are accumulated in the portion indicated by the dotted line below the FD portion 7 in FIG. 1, and the change in potential of the FD portion 7 is made high by the source follower operation by the source follower amplifier MOS transistor 8 and the gate voltage VG is set to high. As high, the load MOS transistor 11 is turned on, the selection switch MOS transistor 9 is turned on, and the output is output OUT to the outside through the vertical output line 10. In this way, the readout operation of the photogate 5 having the photoelectric conversion portion and the photocharge in the ring is performed.
[0019]
What is characteristic in this embodiment is that the photogate 5 is formed in a ring shape. Therefore, since there is no poly-Si electrode 5 in the central portion of the photoelectric conversion region, light absorption by the poly-Si electrode does not occur, and photons hν are directly input to the depletion layer, thereby improving the photosensitivity. Further, since the optical sensitivity can be increased, the poly-Si film can be made thicker than the conventional example, so that high-speed driving can be performed. The photogate electrode of the source follower MOS transistor 8 is not limited to poly-Si, and materials such as polycide, silicide, and salicide can be used. Further, since the resistance of poly-Si does not increase so much by making the photogate 5 into a ring shape, high-speed driving is possible. Further, the manufacturing process may be the same as the conventional one.
[0020]
According to this embodiment, a highly sensitive CMOS sensor can be realized. It is obvious that the present invention is not limited to a CMOS sensor, but can be applied to a sensor such as a CMD using a photogate.
[0021]
Further, in this embodiment, a depletion layer is formed in the entire region of the opening by setting, for example, a well concentration of 2 × 10 ¹⁵ cm ⁻³ and an opening area of 4 × 4 μm ^{2 in the} pixel region where the photogate electrode is not formed. The photoelectric conversion characteristics are improved. This is because a depletion layer of about 2 μm is required to obtain the sensitivity of visible light (wavelength: 400 to 800 μm), so the well concentration is a concentration that forms a p-type well, for example, 1 × 10 ¹⁴ cm ⁻³ or more. It is desirable to be 2 × 10 ¹⁵ cm ⁻³ or less. In addition, since light absorption in a neutral region other than the depletion layer causes crosstalk, it is preferable to eliminate this region. Therefore, from the balance between improvement in photosensitivity and reduction in crosstalk, the opening area is reduced to 0. 4 × 4 μm ² not included.
[0022]
(Second embodiment)
FIG. 3 is a plan view of the pixel portion of the second embodiment according to the present invention. Reference numeral 13 denotes a depletion layer having a MOS structure formed in a thick SiO ₂ film which is an element isolation region. What is characteristic in this embodiment is that the photogate 5 is formed not in a ring shape but in an antenna shape. This embodiment is particularly effective for a line sensor having rectangular pixels. In the case of a line sensor, in FIG. 3, the pixel portions are arranged in a line in the vertical direction, and the photocharges of the pixel portions are transferred to the FD portion 7 connected to the gate of the source follower MOS transistor 8 at the timing of the control pulses φPG and φTX. Can be transferred and read. Therefore, the number of pixels of the line sensor can be arranged with high density, and the resolution on the line can be improved. However, FIG. 3 does not show the reset MOS transistor and the control pulse φR terminal.
[0023]
In this embodiment, since the poly gate area can be further reduced, a more sensitive CMOS sensor can be realized.
[0024]
(Third embodiment)
FIG. 4 is a plan view of the pixel portion of the third embodiment according to the present invention. Reference numeral 13 denotes a depletion layer having a MOS structure formed in a thick SiO ₂ film which is an element isolation region. The poly Si layer electrode 5 is comb-shaped and surrounds the outside of the depletion layer 13 so that photons hν can be directly input to the inside. Is formed. What is characteristic in this embodiment is that the photogates 5 are formed on both sides of the pixel depletion layer 13. When the photogate 5 is provided at the center of the pixel as in the second embodiment, so-called crosstalk is increased in which carriers generated at the end of the pixel leak into the adjacent pixel. Therefore, when the crosstalk is severe, a method of reducing the crosstalk by providing photogates 5 on both sides of the pixel as in this embodiment is effective.
[0025]
In this embodiment, a CMOS sensor with little crosstalk can be realized. In particular, if it is configured as a line sensor and it is necessary to obtain an accurate image signal for each pixel, for example, if the image signal is digitally processed and reproduced for each pixel, the influence of crosstalk This structure is advantageous because it is difficult to remove at a later stage. However, when it is necessary to maintain continuity with adjacent pixels in the case of the structure as a line sensor, a certain amount of crosstalk may be effective. Therefore, the configuration of FIG. 3 is better depending on the situation.
[0026]
(Fourth embodiment)
FIG. 5 is a plan view of a pixel according to the fourth embodiment of the present invention. What is characteristic in the present embodiment is that the photogate 5 is formed in a mesh shape. When the pixel size is large, a region where the electric field of the photogate 5 does not reach is generated. Therefore, the depletion layer is generated in the entire photoelectric conversion region by forming the photogate 5 in a mesh shape.
[0027]
This embodiment can realize a CMOS sensor having a large pixel area.
(5th Example)
FIG. 6 is a cross-sectional view of a pixel portion of a photoelectric conversion device including the ring-shaped and comb-shaped photogate 5 shown in the first and third embodiments of the present invention. In the figure, 13 is a light shielding film that shields the FD portion and reset MOS transistor portion other than the depletion layer portion, 14 is a color filter layer for reading out only a predetermined wavelength component from each pixel, 15 is a flattening layer, Reference numeral 16 denotes a microlens that collects reflected light from the target image.
[0028]
This embodiment is characterized in that light is condensed on a photoelectric conversion region without a photogate using an on-chip microlens. In this embodiment, since the incident light is completely absorbed by the photogate, the sensitivity is most improved. If the microlens can be formed in a subdivided shape, it can be focused on a portion without the photogate 5 of the second and fourth embodiments. In addition, regardless of the presence or absence of the photogate 5 portion, even if the microlens 16 collects light on the entire depletion layer, the photosensitivity is improved as compared with the case without the microlens 16 and a high level image signal is obtained. Can do.
[0029]
However, a high-sensitivity CMOS sensor can be realized by forming the photogate 5 in a part of the photoelectric conversion region and concentrating light on the photoelectric conversion region where the photogate 5 is not formed intensively.
[0030]
【The invention's effect】
As described above, according to the present invention, since the poly Si electrode that absorbs a large amount of blue region is reduced in size in the pixel region, a CMOS process compatible sensor with good blue sensitivity can be realized. A good sensor can be created at low cost.
[Brief description of the drawings]
FIG. 1 is a pixel configuration diagram of a first embodiment according to the present invention.
FIG. 2 is a plan view of a pixel according to a first embodiment of the present invention.
FIG. 3 is a plan view of a pixel according to a second embodiment of the present invention.
FIG. 4 is a plan view of a pixel according to a third embodiment of the present invention.
FIG. 5 is a plan view of a pixel according to a fourth embodiment of the present invention.
FIG. 6 is a plan view of a pixel according to a fifth embodiment of the present invention.
7A and 7B are a conceptual cross-sectional view and a plan view of a conventional photoelectric conversion device.
FIG. 8 is a graph showing the relationship of light transmittance with respect to the wavelength of poly-Si.
[Explanation of symbols]
1 n-type Si substrate 2 P-type well 3 element isolation oxide film 4 gate oxide film 5 poly-Si photogate 6 transfer gate 7 floating diffusion portion 8 amplification MOS transistor 9 selection switch MOS
10 Vertical output line 11 Load MOS of source follower
12 Depletion layer 13 Photoelectric conversion region

Claims (1)

P-type well formed on a Si substrate, a gate insulating film formed on a part of the P-type well, a photogate electrode formed on the gate insulating film, and a photo-charge accumulated under the photogate for floating diffusion In a photoelectric conversion device in which a plurality of pixels each including a transfer gate for transferring to a portion, a source follower amplifier MOS transistor that transmits photocharges of the floating diffusion portion in a source follower format, and an element isolation region are arranged,
The photoelectric conversion device is characterized in that the shape of the photogate electrode is an antenna shape on the gate insulating film and does not cover the element isolation region.

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