WO2009066909A2

WO2009066909A2 - Unit pixel of image sensor including photodiode having stacking structure

Info

Publication number: WO2009066909A2
Application number: PCT/KR2008/006751
Authority: WO
Inventors: Byoung Su Lee
Original assignee: Siliconfile Technologies Inc
Current assignee: SK Hynix System IC Inc
Priority date: 2007-11-20
Filing date: 2008-11-17
Publication date: 2009-05-28
Anticipated expiration: 2010-05-20
Also published as: WO2009066909A3; KR20090051790A

Abstract

Provided is a unit pixel of an image sensor in which photodiodes are arranged in a stacking structure and transfer gates are provided for the respective photodiodes so that signals are sequentially transferred to extract information on a plurality of color components and, by which dark current generated at a substrate surface can be reduced by using a buried-type photodiode. Accordingly, since a buried-type photodiode is used in a unit pixel of an image sensor including a photodiode having a stacking structure, dark current generated by surface detect can be suppressed. In addition, since signals are sequentially transferred through transfer gates for respective photodiodes having a stacking structure, information on a plurality of color components can be extracted without having to use complex peripheral circuits.

Description

UNIT PIXEL OF IMAGE SENSOR INCLUDING PHOTODIODE HAVING STACKING STRUCTURE

Technical Field

[1] The present invention relates to a unit pixel of an image sensor, and more particularly, to a unit pixel of an image sensor in which photodiodes are arranged in a stacking structure and transfer gates are provided for the respective photodiodes so that signals are sequentially transferred to extract information on a plurality of color components, and by which dark current generated at a substrate surface can be reduced by using a buried-type photodiode. Background Art

[2] In general, a complementary metal oxide semiconductor (CMOS) image sensor is constructed of one unit pixel including one photodiode and three or four transistors (TRs) by performing a CMOS process. The CMOS image sensor includes a photodiode for sensing a light beam and a circuit part for processing the sensed light beam into an electrical signal. To increase a photosensitivity, it is important for the photodiode to occupy a large portion in a total area of a unit pixel.

[3] FIG. 1 is a circuit diagram illustrating a unit pixel of an image sensor having a conventional four-transistor (4TR) structure.

[4] The unit pixel of FIG. 1 is constructed of a photodiode PD that receives a light beam to generate photo-charges, a transfer transistor Tx that transfers the photo-charges accumulated in the photodiode to a floating diffusion region FD according to a turn-on operation, the floating diffusion region FD that receives the photo-charges accumulated in the photodiode, a reset transistor Rx that resets the floating diffusion region FD to a power voltage level VDD in response to a reset signal, a drive transistor Dx that has a variable turn-on level depending on an electrical signal corresponding to the photo-charges transferred from the floating diffusion region FD and that outputs the electrical signal in proportion to an amount of the photo-charges according to the turn-on level, and a select transistor Sx that is turned on in response to a select signal to output a unit pixel signal output through the drive transistor.

[5] That is, one unit pixel includes four (or three) transistors and one photodiode, and further includes a color filter that passes only a light beam in a specific wavelength range. As a result, a single color image is obtained.

[6] FIG. 2 is a plan view illustrating a layout of a unit pixel of an image sensor including three-color images with a conventional structure.

[7] Referring to FIG. 2, unit pixels for obtaining red (R), green (G), and blue (B) images are respectively required in order for the image sensor to include R, G, and B components (hereinafter, simply referred to as RGB). In general, the image sensor has a 2x2 structure constructed of one unit pixel U/Pl for obtaining the R image, two unit pixels U/P2 and U/P3 for obtaining the G image, and one unit pixel U/P4 for obtaining the B image.

[8] That is, only a specific color image is obtained from one unit pixel, and thereafter interpolation is performed using data of adjacent other color unit pixels. As a result, all color values can be estimated for corresponding unit pixels.

[9] Such a unit pixel layout has problems in that at least three unit pixels (i.e., RGB) are required to obtain all color images. Further, since respective unit pixels have different color values at different positions, an interpolation error occurring in an interpolation process may result in false color images. Furthermore, due to a highly complex interpolation algorithm, a portion to be subjected to digital image processing is increased, and a large number of memories are required.

[10] To address these problems, there are ongoing researches on a unit pixel of an image sensor including a photodiode having a stacking structure.

[11] FIG. 3 is a cross-sectional view illustrating a unit pixel of an image sensor including a photodiode having a conventional stacking structure. FIG. 4 is a graph illustrating an energy band of the unit pixel of FIG. 3, viewed along line B-B' of FIG. 3. FIG. 5 is a graph illustrating a sensitivity spectrum of photodiodes having a conventional stacking structure.

[12] In a case where multi-layered photodiodes are arranged as shown in FIG. 3, an electron-hole generation rate differs depending on a wavelength of a light beam when the light beam is incident into a semiconductor. An electron-hole pair generation rate for RGB is as shown in FIG. 4.

[13] When the light beam is incident, electrons generated by the light beam are gathered in wells of a first photodiode PDl, a second photodiode PD2, and a third photodiode PD3 according to regions where the electrons are generated. More specifically, a blue (B) light beam having a short wavelength is strongly absorbed in a portion near a substrate surface and thus is gathered in the well of the third photodiode PD3. A green (G) light beam is gathered in the well of the second photodiode PD2. A red (R) light beam having a long wavelength is almost uniformly absorbed according to a depth of a unit pixel and is mostly gathered in the well of the first photodiode PDl.

[14] That is, a signal spectrum depending on a wavelength of a light beam having a constant intensity in a visible band has a pattern of FIG. 5. Such a spectrum can be used to combine signals of the first to third photodiodes PDl, PD2, and PD3. Accordingly, RGB values can be obtained using one unit pixel.

[15] The unit pixel of FIG. 3 has a structure in which the third photodiode PD3 is exposed to the substrate surface. Many defect levels exist in the substrate surface, and thermally excited electrons are generated and gathered in the third photodiode PD3. As a result, a lot of electrons are generated even if a light beam is not incident, which may cause generation of dark noise.

[16] As shown in FIG. 5, the third photodiode PD3 is strongly reactive to a light beam having a wavelength short than 400 nanometers (nm). Thus, it is difficult to extract a signal in a blue band (i.e., a center wavelength of 448 nm and a bandwidth of 60 nm). Accordingly, a band-pass filter having a pass band of 400 nm to 700 nm is required to reduce reactions in an infrared (IR) band and an ultraviolet (UV) band.

[17] In addition, the conventional structure has a problem in that a unit pixel size is increased since additional circuits are required to obtain data by processing an electrical signal from a light beam sensed by a photodiode of the unit pixel. Disclosure of Invention Technical Problem

[18] In order to solve the aforementioned problems, an object of the present invention is to provide a unit pixel of an image sensor in which transfer gates are provided for respective photodiodes arranged in a stacking structure so that signals are sequentially transferred to extract information on a plurality of color components, and by which dark current generated at a substrate surface can be reduced by using a buried-type photodiode. Technical Solution

[19] According to an aspect of the present invention, there is provided a unit pixel of an image sensor including a photodiode having a stacking structure, comprising: at least two photodiodes formed in the stacking structure on a semiconductor substrate, wherein an uppermost-layered photodiode of the at least two photodiode is buried in the semiconductor substrate, and photo-charges accumulated in the at least two photodiodes are transferred to a floating diffusion region through at least two transfer gates.

[20] According to another aspect of the present invention, there is provided a unit pixel of an image sensor including a photodiode having a stacking structure, comprising at least two photodiodes formed in the stacking structure on a semiconductor substrate, wherein the at least two photodiode are buried in the semiconductor substrate, and photo-charges accumulated in the at least two photodiodes are transferred to a floating diffusion region through at least two transfer gates.

Advantageous Effects

[21] According to the present invention, a buried-type photodiode is used in a unit pixel of an image sensor including a photodiode having a stacking structure so as to protect the photodiode against introduction of thermal charges generated at a surface of a semiconductor substrate and electrons generated by incidence of short-wavelength radiation. Therefore, a sensitivity is decreased in a short-wavelength band, and thus a signal for a blue light beam can be obtained without having to use an extra band-pass filter for blocking an ultra violet (UV) light beam. Further, dark current generated by surface detect can be suppressed.

[22] In addition, since signals are sequentially transferred through transfer gates for respective photodiodes having a stacking structure, information on a plurality of color components can be extracted without having to use complex peripheral circuits. Brief Description of the Drawings

[23] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

[24] FIG. 1 is a circuit diagram illustrating a unit pixel of an image sensor having a conventional four-transistor (4TR) structure;

[25] FIG. 2 is a plan view illustrating a layout of a unit pixel of an image sensor including three-color images with a conventional structure;

[26] FIG. 3 is a cross-sectional view illustrating a unit pixel of an image sensor including a photodiode having a conventional stacking structure;

[27] FIG. 4 is a graph illustrating an energy band of the unit pixel of FIG. 3, viewed along line B-BOf FIG. 3;

[28] FIG. 5 is a graph illustrating a sensitivity spectrum of photodiodes having a conventional stacking structure;

[29] FIG. 6 is a plan view illustrating a unit pixel of an image sensor including a photodiode having a stacking structure according to an embodiment of the present invention;

[30] FIG. 7 is a cross-sectional view illustrating the unit pixel of FIG. 6;

[31] FIG. 8 is a graph illustrating an energy band of the unit pixel of FIG. 7, viewed along line A-AOf FIG. 7;

[32] FIG. 9 is a graph illustrating a sensitivity spectrum of photodiodes having a stacking structure according to an embodiment of the present invention;

[33] FIG. 10 is a waveform diagram illustrating control signals of a unit pixel of an image sensor including a photodiode having a stacking structure according to an embodiment of the present invention; and

[34] FIG. 11 is a cross-sectional view of a unit pixel of an image sensor including a photodiode having a stacking structure according to another embodiment of the present invention. Best Mode for Carrying Out the Invention

[35] Hereinafter, the present will be described in detail with reference to accompanying drawings.

[36] FIG. 6 is a plan view illustrating a unit pixel of an image sensor including a photodiode having a stacking structure according to an embodiment of the present invention. FIG. 7 is a cross-sectional view illustrating the unit pixel of FIG. 6.

[37] Referring to FIG. 7, the unit pixel includes at least two photodiodes formed in a stacking structure on a semiconductor substrate.

[38] More specifically, the unit pixel includes a first photodiode PDl formed in a semiconductor substrate 700, a second photodiode PD2 formed above the first photodiode PDl, and a third photodiode PD3 formed above the second photodiode PD2.

[39] The first and second photodiodes PDl and PD2 are doped with a first type of impurities. A region doped with a second type of impurities is located between the first photodiode PDl and the second photodiode PD2 to separate the two photodiodes. In addition, a region doped with the second type of impurities exists between the second photodiode PD2 and the third photodiode PD3 to separate the two photodiodes PD2 and PD3.

[40] The third photodiode PD3, i.e., an uppermost-layered photodiode, is not exposed to a surface of the semiconductor substrate but buried in the semiconductor substrate. Therefore, introduction of thermal charges can be prevented. The thermal charges are generated by surface defection existing on the surface of the semiconductor substrate 700.

[41] A light beam having a short wavelength is mostly absorbed in a surface portion of the semiconductor substrate 700. Thus, in a structure in which the third photodiode PD3 is buried in the semiconductor substrate, charges of the surface portion are generated by the light beam having the short wavelength and are not introduced to the third photodiode PD3. As a result, noise generation is not significant. In addition, as shown in FIG. 9, a sensitivity of the third photodiode PD3 is decreased near 350 nm. Thus, an extra optical device for eliminating an ultraviolet (UV) light beam is not required.

[42] In an image sensor including a photodiode having the conventional staking structure, additional circuits have been required to obtain data by performing an operation in which photo-charges accumulated in each photodiode are respectively transferred to floating diffusion regions and the transferred photo-charges are processed into electrical signals.

[43] On the other hand, in an image sensor including a photodiode having a stacking structure according to an embodiment of the present invention, transfer gates are provided for the respective first to third photodiodes PDl to PD3 and thus photo- charges accumulated in the respective photodiodes are sequentially transferred to the floating diffusion regions. As a result, additional circuits for signal processing are not required.

[44] More specifically, the third photodiode PD3 separated from the substrate surface is electrically connected to the second photodiode PD2 by a third transfer gate T3. The second photodiode PD2 is connected to the first photodiode PDl by a second transfer gate T2. The first photodiode PDl is connected to a floating diffusion region FD by a first transfer gate Tl. Further, the floating diffusion region FD is connected to the first transfer gate Tl and a power voltage VDD.

[45] The aforementioned connections are used to sequentially transfer photo-charges accumulated in the respective photodiodes and to process data. As a result, additional circuits for data processing are not required.

[46] FIG. 10 is a waveform diagram illustrating control signals of a unit pixel of an image sensor including a photodiode having a stacking structure according to an embodiment of the present invention.

[47] An operation of the unit pixel of FIG. 10 will be described below with reference not only to FIG. 10 but also to FIGs. 6 to 9.

[48] First, in a reset operation, a power voltage VDD is applied to a reset transistor Rx, a first transfer gate Tl, a second transfer gate T2, and a third transfer gate T3. In this state, a floating diffusion region FD has a highest voltage level, followed by the first photodiode PDl, the second photodiode PD2, and the third photodiode PD3, in that order.

[49] Next, a ground voltage GND is applied to the reset transistor Rx, the first transfer gate Tl, the second transfer gate T2, and the third transfer gate T3. As a result, the respective photodiodes have energy bands of FIG. 8.

[50] When a light beam is incident in this state, electrons generated by the light beam are distributed according to regions where the electrons are generated. More specifically, electrons generated by a UV light beam are gathered in a surface of a semiconductor substrate, and electrons generated by a blue light beam are mostly accumulated in the third photodiode PD3. Further, electrons generated by a green light beam are mostly accumulated in the second photodiode PD2, and electrons generated by a red light beam are mostly accumulated in the first photodiode PDl.

[51] An integration time depending on light incidence is as shown in FIG. 10. In a state where exposure is finished, data can be obtained as follows.

[52] After the integration time elapses, a voltage level of the floating diffusion region FD is measured by turning on/off the reset transistor Rx. The measured voltage level is used as a reset level. Next, when the first transfer gate Tl is turned on/off, electrons ac- cumulated in the first photodiode PDl are introduced to the floating diffusion region FD. Next, a voltage level of the first photodiode PDl (i.e., PDl level) is obtained by measuring the voltage level of the floating diffusion region FD.

[53] Next, electrons remaining in the floating diffusion region FD are eliminated by turning on/off the reset transistor Rx again, and the first and second transfer gates Tl and T2 are turned on/off. Then, electrons of the second photodiode PD2 are introduced to the floating diffusion region FD. Next, a voltage level of the second photodiode PD2 (i.e., PD2 level) is obtained by measuring the voltage level of the floating diffusion region FD.

[54] Next, electrons remaining in the floating diffusion region FD are eliminated by turning on/off the reset transistor Rx again, and the first to third transfer gates Tl to T3 are turned on/off. Then, electrons of the third photodiode PD3 are introduced to the floating diffusion region FD. Next, a voltage level of the third photodiode PD3 (i.e., PD3 level) is obtained by measuring the voltage level of the floating diffusion region FD.

[55] Real voltage levels of the respective photodiodes are obtained according to the aforementioned process and can be expressed as follows.

[56] Real voltage level of first photodiode (PDl level_real) = PDl level - Reset level

[57] Real voltage level of second photodiode (PD2 level_real) = PD2 level - Reset level

[58] Real voltage level of third photodiode (PD3 level_real) = PD3 level - Reset level

[59] Accordingly, real RGB values can be obtained by linearly combining the real voltage level of the first photodiode (i.e., PDl level_real), the real voltage level of the second photodiode (i.e., PD2 level_real), and the real voltage level of the third photodiode (i.e., PD3 level_real).

[60] FIG. 11 is a cross-sectional view of a unit pixel of an image sensor including a photodiode having a stacking structure according to another embodiment of the present invention.

[61] In the unit pixel of FIG. 11, first and second photodiodes PDl and PD2 are buried in a semiconductor substrate instead of being in contact with a surface of the semiconductor substrate. Since an area doped with high concentration has a buried structure instead of being exposed to the substrate surface, there is an advantage in that dark current generated at the substrate surface can be prevented.

[62] Other structures and operations of the unit pixel of FIG. 11 are the same as described above except that not only the third photodiode PD3 but also the first and second photodiodes PDl and PD2 are not in contact with the surface of the semiconductor substrate but buried in the semiconductor substrate. Therefore, detailed descriptions thereof will be omitted.

[63] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A unit pixel of an image sensor including a photodiode having a stacking structure, comprising at least two photodiodes formed in the stacking structure on a semiconductor substrate, wherein an uppermost-layered photodiode of the at least two photodiode is buried in the semiconductor substrate, and photo-charges accumulated in the at least two photodiodes are transferred to a floating diffusion region through at least two transfer gates.

[2] The unit pixel of claim 1, wherein the at least two photodiodes comprise: a first photodiode formed in the semiconductor substrate; a second photodiode formed above the first photodiode; and a third photodiode formed above the second photodiode.

[3] The unit pixel of claim 2, wherein the first and second photodiodes are separated from each other by a region doped with impurities whose types are different from those of the first and second photodiodes, and wherein the second and third photodiodes are separated from each other by a region doped with impurities whose types are different from those of the second and third photodiodes.

[4] The unit pixel of any one of claims 2 and 3, wherein the at least two transfer gates comprise: a first transfer gate that transfers photo-charges accumulated in the first to third photodiodes to the floating diffusion region; a second transfer gate that electrically connects the first photodiode and the second photodiode; and a third transfer gate that electrically connects the second photodiode and the third photodiode.

[5] The unit pixel of claim 4, wherein the first photodiode accumulates photo- charges generated mostly by a light beam having a red wavelength.

[6] The unit pixel of claim 5, wherein the second photodiode accumulates photo- charges generated mostly by a light beam having a green wavelength.

[7] The unit pixel of claim 6, wherein the third photodiode accumulates photo- charges generated mostly by a light beam having a blue wavelength.

[8] A unit pixel of an image sensor including a photodiode having a stacking structure, comprising at least two photodiodes formed in the stacking structure on a semiconductor substrate, wherein the at least two photodiode are buried in the semiconductor substrate, and photo-charges accumulated in the at least two photodiodes are transferred to a floating diffusion region through at least two transfer gates.

[9] The unit pixel of claim 8, wherein the at least two photodiodes comprise: a first photodiode formed in the semiconductor substrate; a second photodiode formed above the first photodiode; and a third photodiode formed above the second photodiode.

[10] The unit pixel of claim 9, wherein the first and second photodiodes are separated from each other by a region doped with impurities whose types are different from those of the first and second photodiodes, and wherein the second and third photodiodes are separated from each other by a region doped with impurities whose types are different from those of the second and third photodiodes.

[11] The unit pixel of any one of claims 9 and 10, wherein the at least two transfer gates comprise: a first transfer gate that transfers photo-charges accumulated in the first to third photodiodes to the floating diffusion region; a second transfer gate that electrically connects the first photodiode and the second photodiode; and a third transfer gate that electrically connects the second photodiode and the third photodiode.

[12] The unit pixel of claim 11, wherein the first photodiode accumulates photo- charges generated mostly by a light beam having a red wavelength.

[13] The unit pixel of claim 12, wherein the second photodiode accumulates photo- charges generated mostly by a light beam having a green wavelength.

[14] The unit pixel of claim 13, wherein the third photodiode accumulates photo- charges generated mostly by a light beam having a blue wavelength.