JP2006173487A - Imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging device which can be reduced in dark current. <P>SOLUTION: In the peripheral region of a photodiode PD, a guard band region 34 is so formed as to cover the side face of an N-type diffusion layer 33 which constitutes the photodiode PD. The guard band region 34 has the same conductivity type (N-type) as that of the N-type diffusion layer 33 which constitutes the photodiode PD, and has a dopant concentration lower than that of the N-type diffusion layer 33. With the guard band region 34 relaxing the concentration gradient in the lateral direction between the N-type diffusion layer 33 and a P-type epitaxial layer 32, a depletion layer formed in a PN junction portion expands in width due to the existence of the guard band region 34, and a substantial distance (width of the depletion layer) between the N-type diffusion layer 33 and the P-type epitaxial layer 32 becomes longer compared with the (conventional) case that there is no guard band region 34. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an imaging device.

  Conventionally, MOS-type imaging devices have been used to acquire various image data. This type of imaging device reads out the electric charge accumulated in the pn junction capacitance of the photo diode through a MOS transistor (for example, a field effect transistor (FET)).

  In general, it is said that an imaging device such as a MOS type has a narrow latitude, that is, a dynamic range, compared to a negative film used for photographing. The narrow latitude means that the dark part of the image is recorded as black pixel data, and the bright part of the image is recorded as white pixel data.

As a technique for expanding the dynamic range, there is a logarithmic conversion type imaging device (see, for example, Patent Document 1 and Non-Patent Document 1). For example, in the image cell disclosed in Patent Document 1, the light receiving element is connected between one terminal of a first MOS transistor and the gate terminal of a second MOS transistor, and the other terminal of the first MOS transistor. Is connected to one electrode of a voltage supply source. Then, logarithmic conversion is performed in the image cell by the first MOS transistor operating in the sub-threshold region, and the conversion result is output.
US Pat. No. 5,608,204 The Journal of the Video Media Society, Vol. 54, no. 2, pp. 224, 2000

Incidentally, a photodiode used as a light receiving element includes a P-type substrate or well and an N-type diffusion layer formed in the substrate or well, and a reverse bias is applied thereto. The electric field concentrates on the periphery of the PN junction, and a large leak current (dark current) flows from the N-type diffusion layer to the P-type substrate (or well) due to the strong electric field. For this reason, there is a problem in that high sensitivity cannot be realized because sensitivity under low illuminance with a small photocurrent cannot be obtained. The dark current in the PN junction is significantly larger at the peripheral portion than at the central portion of the joint surface. For example, dark current at the joint surface central portion whereas a 0.2~0.3pA / cm ^2, it becomes one order of magnitude higher 1~2pA / cm ² in the peripheral portion. For this reason, the sensitivity at low illuminance cannot be obtained due to the signal-to-noise (S / N) between the photocurrent and dark current generated in the photodiode.

  The present invention provides an imaging device capable of reducing dark current.

  The imaging device according to the present invention includes a photo diode having a PN junction, and a semiconductor layer around the PN junction of the photo diode is provided between a diffusion layer constituting the photo diode PD and the semiconductor layer. A guard band region is provided to moderate the concentration gradient. According to the present invention, the width of the depletion layer in the PN junction is increased, and the dark current is reduced by relaxing the electric field concentration.

  An imaging device according to the present invention includes a photodiode having a PN junction formed by a semiconductor layer of one conductivity type formed of a substrate or a well, and a diffusion layer of opposite conductivity type to the semiconductor layer, and the diffusion layer. A guard band region is provided in the periphery of the substrate to moderate the concentration gradient between the diffusion layer and the semiconductor layer. According to the present invention, the width of the depletion layer in the PN junction is increased, and the dark current is reduced by relaxing the electric field concentration.

  The guard band region has the same conductivity type as the diffusion layer, and is formed at a concentration lower than the concentration of the diffusion layer. This guard band region is obtained by forming a well of the same conductivity type as the diffusion layer so as to surround the periphery of the diffusion layer. Therefore, it is realized without complicating the manufacturing process.

  The guard band region has the same conductivity type as the semiconductor layer, and is formed at a concentration lower than the concentration of the semiconductor layer. This guard band region is obtained by forming a well having the same conductivity type as that of the semiconductor layer so as to surround the periphery of the diffusion layer. Therefore, it is realized without complicating the manufacturing process.

  The guard band region has a first layer having the same conductivity type as the diffusion layer and a concentration lower than the concentration of the diffusion layer, and a first layer having the same conductivity type as the semiconductor layer and a concentration lower than the concentration of the semiconductor layer. It consists of two layers. This guard band region is obtained by forming a well of the same conductivity type as the semiconductor layer so as to surround the periphery of the diffusion layer, and forming a well of the same conductivity type as the diffusion layer in the well. Therefore, it is realized without complicating the manufacturing process.

  As described above, according to the present invention, an imaging device capable of reducing the dark current can be provided.

Hereinafter, an embodiment embodying the present invention will be described with reference to FIGS.
FIG. 3 is a schematic block circuit diagram of the solid-state imaging device.
The solid-state imaging device 10 includes an imaging unit 11, an internal clock generation circuit 12, a vertical scanning circuit 13, a horizontal scanning circuit 14, and an output circuit 15.

The imaging unit 11 includes a plurality of image cells Ca arranged in a matrix. FIG. 5 shows image cells Ca arranged in a matrix of m rows and n columns.
The internal clock generation circuit 12 receives the clock signal Φ0 and generates a vertical clock signal Φw and a horizontal clock signal Φt based on the clock signal Φ0.

  The vertical scanning circuit 13 is a vertical shift register, and is connected to row selection lines W1 to Wm and reset lines R1 to Rm that are paired with the row selection lines W1 to Wm. The horizontal scanning circuit 14 includes a plurality of (four in FIG. 3) amplifier circuits 16 and a shift register 17, to which column signal lines BL1 to BLn are connected. An image cell Ca is connected to intersections of the row selection lines W1 to Wm and the column signal lines BL1 to BLn. Each image cell Ca is connected to reset lines R1 to Rm that are paired with row selection lines W1 to Wm.

  The vertical scanning circuit 13 sequentially drives the row selection lines W1 to Wm based on the vertical clock signal Φw. The image cells Ca connected to the row selection lines W1 to Wm output photoelectric conversion signals to the column signal lines BL1 to BLn in response to drive signals supplied via the row selection lines W1 to Wm.

  Each of the column signal lines BL1 to BLn is connected to the amplifier circuit 16 constituting the horizontal scanning circuit 14. Each amplifying circuit 16 includes an amplifying unit that amplifies photoelectric conversion signals input via the column signal lines BL1 to BLn, and an analog-digital (A / D) converting unit that converts an output signal of the amplifying unit into a digital signal. including.

The shift register 17 constituting the horizontal scanning circuit 14 transfers the digital signal output from the amplifier circuit 16 to the output circuit 15 based on the horizontal clock signal Φt.
The output circuit 15 generates and outputs an output signal out obtained by extending the pulse width of the signal output from the horizontal scanning circuit 14.

FIG. 2 shows the image cell Ca connected to the intersection of the row selection line W1 and the column signal line BL1.
The image cell Ca includes a photodiode PD as a light receiving element and three transistors T1 to T3. The first to third transistors T1 to T3 are one-conductivity channel type transistors (N-channel type MOS transistors in this embodiment), and their back gates are connected to a low potential power source (in this embodiment, ground GND). Yes.

  The photodiode PD has an anode connected to a low potential power source (in this embodiment, the ground GND) as a first power source, and a cathode connected to the first transistor T1. The first transistor T1 has a first terminal (source terminal) connected to the photodiode PD and a second terminal (drain terminal) connected to a high potential power supply Vdd1 as a second power supply. The first transistor T1 has a gate terminal connected to the reset line R1, and a reset signal Φr1 described later is supplied to the gate terminal via the reset line R1. The first transistor T1 is a reset transistor, and is turned on / off in response to the reset signal Φr1, and the turned on first transistor T1 resets the potential of the cathode terminal of the photodiode PD to the high potential power supply Vdd1 level.

  A sense node N1, which is a connection point between the photodiode PD and the first transistor T1, is connected to the second transistor T2. The second transistor T2 has a gate connected to the sense node N1, a source connected to the third transistor T3, and a drain connected to the high potential power supply Vdd2. The second transistor T2 is an amplifying transistor, and outputs a signal (photoelectric conversion signal) obtained by amplifying the potential of the sense node N1. Therefore, the second transistor T2 constitutes an amplifier circuit that amplifies the output terminal (cathode) potential of the photodiode PD.

  The third transistor T3 has a first terminal (for example, source terminal) connected to the second transistor T2, and a second terminal (for example, drain terminal) connected to the column signal line BL1. The gate terminal of the third transistor T3 is connected to the row selection line W1, and a drive signal Φw1 described later is supplied to the gate terminal via the row selection line W1. The third transistor T3 is a row selection transistor, and is turned on / off in response to the drive signal Φw1 supplied via the row selection line W1, thereby connecting and separating the second transistor T2 and the column signal line BL1. Therefore, when the third transistor T3 is turned on, a signal (photoelectric conversion signal) output from the second transistor T2 is output to the column signal line BL1.

  The image cell Ca configured in this manner operates according to the drive signal Φw1 and the reset signal Φr1 supplied via the row selection line W1 and the reset line R1. The drive signal Φw1 and the reset signal Φr1 have waveforms shown in FIG. The photo diode PD accumulates signal charges by photoelectric conversion after being reset to the high potential power source Vdd1 level by the first transistor T1. Since the photodiode PD is in a reverse bias state by reset and functions as a capacitor equivalently, the voltage of the sense node N1 changes due to accumulation of signal charges. The second transistor T2 amplifies the voltage, and the amplified voltage is output to the column signal line BL1 via the third transistor T3 that is turned on in response to the drive signal Φw1 at the H level. Thereafter, the first transistor T1 is turned on in response to the H level reset signal Φr1 to reset the photodiode PD.

  The amplifying circuit 16 shown in FIG. 3 amplifies the signal read out to the column signal line BL1, samples and performs A / D conversion based on the horizontal clock signal Φt. As shown in FIG. 4, the timing of the horizontal clock signal Φt is set so as to be sampled when a sufficient photocurrent is generated in the photodiode PD. The shift register 17 transfers the output signal of the amplifier circuit 16 to the output circuit 15, and the output circuit 15 generates an output signal out obtained by expanding the pulse width of the input signal to a predetermined pulse width (width tk in this embodiment). And output it.

  FIG. 1 is a partial cross-sectional view of a chip of a solid-state imaging device 10 as an imaging device. This chip includes a P-type silicon substrate 31 and a P-type epitaxial layer 32 as a semiconductor layer formed thereabove. An N-type diffusion layer 33 is formed in the P-type epitaxial layer 32 from the upper surface to a predetermined depth. The N-type diffusion layer 33 and the P-type epitaxial layer 32 constitute a PN junction photodiode PD.

  A guard band region 34 is formed in the periphery of the photo diode PD. The guard band region 34 is formed so as to cover the side surface of the N-type diffusion layer 33. Further, the guard band region 34 of the present embodiment is formed so as to cover the periphery of the bottom surface of the N-type diffusion layer 33 (a bonding surface where the N-type diffusion layer 33 and the P-type epitaxial layer 32 are in contact in the vertical direction). .

  The guard band region 34 is provided in order to moderate the concentration gradient between the N type diffusion layer 33 and the P type epitaxial layer 32 in the lateral direction. For example, the guard band region 34 has the same conductivity type (N-type) as the N-type diffusion layer 33 constituting the photodiode PD, and the impurity concentration is lower than the impurity concentration of the N-type diffusion layer 33. . Accordingly, the concentration difference between the N-type diffusion layer 33 and the guard band region 34 and the concentration difference between the P-type epitaxial layer 32 and the guard band region 34 are compared with the concentration difference between the N-type diffusion layer 33 and the P-type epitaxial layer 32. Become smaller. Further, the depletion layer formed in the PN junction is widened by forming the guard band region 34, and a substantial distance between the N-type diffusion layer 33 and the P-type epitaxial layer 32 (depletion layer width). ) Is longer than when the guard band region 34 is not provided (conventional example). As a result, the substantial resistance value between the N-type diffusion layer 33 and the P-type epitaxial layer 32 becomes larger than that of the conventional example, and therefore, a leak current (flowing from the N-type diffusion layer 33 toward the P-type epitaxial layer 32 ( (Dark current) is smaller than in the conventional example.

  In the guard band region 34, for example, an N-type well is formed in an annular shape in a plan view, and impurities are implanted to a predetermined depth inside and inside the annular well to form an N-type diffusion layer 33. It is obtained by forming.

  The concentration gradient due to the guard band region 34 is desirably about 2 to 10 times as gentle as the junction depth compared to the concentration gradient in the vertical direction of the center of the photodiode PD. For example, when the impurity concentration of the P-type epitaxial layer 32 in which the impurities are diffused substantially uniformly is set to the order of 10 15, the maximum value of the phosphorus concentration in the center is on the order of the 19th power of the maximum concentration 10 PN junction depth. For the N-type diffusion layer 33 having a thickness of about 1.0 μm, a guard band region 34 of a low-concentration N-type region having a depth of 3 μm is formed on the order of 10 16. It has been confirmed that the photodiode PD configured in this way has a dark current reduced to 1/5 or less as compared with a photodiode in which the guard band region 34 is not formed.

  In the CMOS device, the junction depth of the N well or P well is several μm and the concentration is low. Therefore, the guard band region 34 is realized without complicating the manufacturing process by surrounding the end portion of the PN junction of the photodiode PD with the N well or the P well.

  In FIG. 1, a P-type well 36 is formed below an oxide film 35 (field oxide film: LOCOS) for element isolation formed between adjacent photodiodes PD. The P-type well 36 is formed with a higher impurity concentration than the impurity concentration of the P-type epitaxial layer 32. The P-type well 36 prevents a channel from being formed between adjacent photodiodes PD, and prevents mutual interference between the photodiodes PD.

As described above, according to the present embodiment, the following effects can be obtained.
(1) A guard band region 34 is formed around the photodiode PD so as to cover the side surface of the N-type diffusion layer 33 constituting the photodiode PD. The guard band region 34 has the same conductivity type (N-type) as the N-type diffusion layer 33 constituting the photodiode PD, and has an impurity concentration lower than that of the N-type diffusion layer 33. Therefore, since the guard band region 34 makes the concentration gradient between the N-type diffusion layer 33 and the P-type epitaxial layer 32 gentle in the lateral direction, the depletion layer formed at the PN junction forms the guard band region 34. As a result, the width is widened, and the substantial distance between the N-type diffusion layer 33 and the P-type epitaxial layer 32 (the width of the depletion layer) is longer than when the guard band region 34 is not provided (conventional example). Become. As a result, the substantial resistance value between the N-type diffusion layer 33 and the P-type epitaxial layer 32 becomes larger than that in the conventional example, and therefore, a leakage current flowing from the N-type diffusion layer 33 toward the P-type epitaxial layer 32 ( Dark current) can be reduced compared to the conventional example, and as a result, the sensitivity of the image cell Ca, that is, the sensitivity of the solid-state imaging device 10 can be increased.

In addition, you may implement each said embodiment in the following aspects.
In the above embodiment, the guard band region 34 is formed so as to cover the side surface and bottom surface periphery of the N-type diffusion layer 33, but the shape and formation method may be changed as appropriate, for example, as shown in FIG. Alternatively, it may be formed on the entire bottom surface of the N-type diffusion layer 33.

  In the above embodiment, as shown in FIG. 6, the P-type layer 41 formed by channel doping in order to change the threshold voltage of the first transistor T <b> 1 may also be formed in the photodiode PD. The P-type layer 41 prevents the N-type diffusion layer 33 constituting the photodiode PD from coming into direct contact with the oxide film 42 formed on the upper surface of the P-type epitaxial layer 32.

  In the above embodiment, the guard band region 34 has the same conductivity type as the N-type diffusion layer 33. However, if the concentration gradient between the N-type diffusion layer 33 and the P-type epitaxial layer 32 can be moderated, the guard band region 34 You may change a structure and a structure suitably. For example, the guard band region 34 may be formed of a P-type semiconductor having a conductivity type opposite to that of the N-type diffusion layer 33. Further, the guard band region 34 may be constituted by the N-type first layer and the P-type second layer.

  In the above embodiment, the configuration of the image cell Ca may be changed as appropriate. For example, the first transistor T1 connected in series to the photodiode PD may be embodied in an image cell that performs logarithmic conversion by operating in the sub-threshold region.

It is a partial schematic sectional drawing of the imaging device of one Embodiment. It is a circuit diagram which shows an image cell. It is a block circuit diagram of a solid-state imaging device. It is a wave form diagram which shows operation | movement of a solid-state imaging device. It is a partial schematic sectional drawing of another imaging device. It is a partial schematic sectional drawing of another imaging device.

Explanation of symbols

  32 ... P-type epitaxial layer, 33 ... Diffusion layer, 34 ... Guard band region, PD ... Photo diode.

Claims (5)

In an imaging device comprising a photodiode having a PN junction,
An imaging device characterized in that a guard band region is provided in a semiconductor layer around the PN junction of the photo diode to moderate a concentration gradient between the diffusion layer constituting the photo diode PD and the semiconductor layer. . In an imaging device including a photodiode having a PN junction formed by a semiconductor layer of one conductivity type formed of a substrate or a well and a diffusion layer of opposite conductivity type to the semiconductor layer,
An imaging device comprising: a guard band region that moderates a concentration gradient between the diffusion layer and the semiconductor layer around the diffusion layer.   The imaging device according to claim 1, wherein the guard band region has the same conductivity type as the diffusion layer and is formed at a concentration lower than the concentration of the diffusion layer.   The imaging device according to claim 1, wherein the guard band region has the same conductivity type as the semiconductor layer and is formed at a concentration lower than the concentration of the semiconductor layer.   The guard band region has a first layer having the same conductivity type as the diffusion layer and a concentration lower than the concentration of the diffusion layer, and a first layer having the same conductivity type as the semiconductor layer and a concentration lower than the concentration of the semiconductor layer. The imaging device according to claim 1, comprising two layers.

Images