KR101360850B1 - Cmos image sensor - Google Patents

Abstract

A CMOS image sensor is disclosed. The CMOS image sensor includes a photodiode area generating and accumulating a charge corresponding to incident light by being formed in a P conductivity type semiconductor board and an OFD active area formed in the P conductivity type semiconductor board to be separated from the photodiode area to the side and discharging the excessive charges generated in the photodiode area to the outside. An NPN transistor can be formed in the active photodiode area, the P conductivity type semiconductor board, and the OFD active area.

Description

CMOS image sensor "

The present invention relates to a CMOS image sensor that can improve the blooming (blooming) phenomenon.

In general, an image sensor is an element that converts an optical image into an electrical signal. Typical examples of such image sensors are a charge coupled device (CCD) and a CMOS image sensor (CIS).

The charge coupled device includes a plurality of MOS capacitors for moving charges (or carriers) generated by light. In contrast, the CMOS image sensor includes a plurality of unit pixels and a CMOS logic circuit for controlling output signals of the unit pixels.

In the case of the CMOS image sensor, when the light intensity is strong, a signal charge more than the total capacitance of the photodiode is generated, and a blooming phenomenon in which charges overflow to adjacent unit pixels may occur.

In order to prevent this, a method of forming an overflow drain adjacent to the photodiode has been studied, but there is a problem that a separate transistor must be provided and a metal wiring for this is required.

The present invention is to provide a CMOS image sensor that can prevent the blooming phenomenon in a simpler way without having a separate transistor requiring a metal wiring.

The present invention aims to improve the dynamic range by preventing or improving the blooming phenomenon, and provides a CMOS image sensor that can prevent a decrease in the signal to noise ratio (SNR) in a bright environment. It is to.

Other objects of the present invention will become readily apparent from the following description.

For reference, the present application reveals that the research is supported by the Ministry of Knowledge Economy's technology innovation project (vehicle image sensor with improved function for safe driving, 10040047).

According to an aspect of the present invention, a CMOS image sensor comprising: a photodiode region formed in a P-conductive semiconductor substrate to generate and accumulate charges corresponding to incident light; And an overflow drain (OFD) active region formed in the P conductive semiconductor substrate so as to be laterally spaced apart from the photodiode region, and configured to discharge excess charge generated in the photodiode to the outside. An active region, the P-conducting semiconductor substrate, and the OFD active region form an NPN transistor.

The active region of the photodiode and the OFD active region may be spaced apart from each other within a distance of 0.5 μm to form a potential barrier of a predetermined potential barrier.

The OFD active region may be formed using a chain implant to control a distance from the active region of the photodiode.

A pinning layer of the photodiode may be extended to contact the OFD active region.

A P ion region may be formed between the photodiode and the overflow drain (OFD) active region.

The base of the NPN transistor may be grounded.

Other aspects, features, and advantages will become apparent from the following drawings, claims, and detailed description of the invention.

According to the embodiment of the present invention, there is an effect that the blooming phenomenon can be prevented by a simpler method without a separate transistor requiring metal wiring.

In addition, by improving or preventing the blooming phenomenon, the dynamic range is improved, and the signal to noise ratio (SNR) can be prevented from being reduced in a bright environment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing an equivalent circuit for a unit pixel of a CMOS image sensor for preventing blooming according to the related art. FIG.
2 is a diagram illustrating an equivalent circuit of a unit pixel of a CMOS image sensor for preventing blooming according to an embodiment of the present invention.
3 is a schematic plan view of a unit pixel illustrated in FIG. 2;
4 is a cross-sectional view taken along the line AA ′ of FIG. 3.
5A to 5C are cross-sectional views of unit pixels of a CMOS image sensor for preventing blooming according to other embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is capable of various modifications and various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

The terminology used in this application is used only to describe a specific embodiment and is not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "having" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Where an element such as a layer, region or substrate is described as being "on" or "onto" another element, the element may be directly on top of another element or may extend directly over it , Or an intervening element may exist. On the other hand, if one element is referred to as being "directly on" another element or "directly onto" another element, there are no other intermediate elements. Also, when an element is described as being "connected" or "coupled" to another element, the element may be directly connected to or directly coupled to another element, or an intermediate intervening element may be present have. On the other hand, if one element is described as being "directly connected" or "directly coupled" to another element, there are no other intermediate elements.

The terms "below" or "above" or "upper" or "lower" or "horizontal" or "lateral" Relative terms such as " vertical "may be used herein to describe a relationship to another element, layer or region of an element, layer or region, as shown in the figures. It should be understood that these terms are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

For the sake of convenience, the unit pixel including four transistors is described in the present specification, but the present invention is not limited thereto. For example, it is natural that the present invention can be applied without limitation to a unit pixel including three transistors or a unit pixel including five transistors.

1 is a diagram illustrating an equivalent circuit of a unit pixel of a CMOS image sensor for preventing blooming according to the related art. For reference, FIG. 1 relates to an equivalent circuit of a unit pixel disclosed in Patent Application No. 2006-0076840 (CMOS image sensor).

Referring to FIG. 1, a unit pixel of a CMOS image sensor includes one photo diode (PD) and four NMOS transistors (TG, RG, SF, and SEL).

The transfer transistor (TG) transfers the optical charges collected in the photodiode PD to the floating diffusion region (FD), and the reset transistor (RG) floats to a desired value. A function of setting the potential of the diffusion region FD and discharging electric charges to reset the floating diffusion region.

The source follower transistor (SF) acts as a source follower buffer amplifier, and the select transistor (SG) allows addressing as a switching role. .

In addition, an overflow transistor (OG), which is used as a blooming path for removing an optical charge exceeding the capacity of the photodiode PD, is further included.

The overflow transistor OG serves to prevent a blooming phenomenon in which the optical charges exceeding the capacity of the photodiode PD flows into the neighboring unit pixels so that the neighboring unit pixels become bright and blurry on the output screen.

The operation of the unit pixel of the CMOS image sensor will be briefly described below.

First, when the reset transistor RG is turned on, the potential of the floating diffusion region FD becomes the applied voltage VDD. When light is incident on the photodiode PD from the outside, an electron-hole pair (EHP) is generated, and signal charges are accumulated in the source region of the transfer transistor TG.

When the transfer transistor TG is turned on, the accumulated signal charge is transferred to the floating diffusion region FD to change the potential of the floating diffusion region FD and at the same time the gate potential of the source follower transistor SF. At this time, when the select transistor SEL is turned on by the selection signal, data is output to the output terminal Out.

When the reset transistor RG is turned on again, the potential of the floating diffusion region FD becomes the applied voltage VDD, and this process is repeated to output an image signal. When the overflow transistor OG is turned on, the optical charge exceeding the capacity of the photodiode PD generated by the light incident on the photodiode PD while the image signal is output is removed.

However, the unit pixel of the CMOS image sensor for preventing the blooming phenomenon shown in FIG. 1 should additionally include an overflow transistor in the structure of the unit pixel of the conventional CMOS image sensor, and a metal wiring for connecting the same is required. In addition, there is a problem that a charge pump or the like is required to set an appropriate voltage level to turn on the overflow transistor.

Therefore, there is a need for a CMOS image sensor that can be prevented blooming by a simpler structure.

FIG. 2 is a diagram illustrating an equivalent circuit of a unit pixel of a CMOS image sensor for preventing blooming, according to an embodiment of the present invention, FIG. 3 is a schematic plan view of the unit pixel illustrated in FIG. 2, and FIG. 4. Is a cross-sectional view taken along the line AA ′ of FIG. 3. 5A to 5C are cross-sectional views of unit pixels of a CMOS image sensor for preventing blooming according to other embodiments of the present invention.

Referring to FIG. 2, the unit pixel of the CMOS image sensor according to the present exemplary embodiment includes one photo diode (PD) and four NMOS transistors (Tx, Rx, Dx, and Sx). In addition, an NPN transistor 210 is further provided for removing the blooming phenomenon. That is, the NPN transistor 210 is provided in place of the overflow transistor OG provided in the unit pixel of the CMOS image sensor according to the related art.

3 is a plan view illustrating the region 220 of the unit pixel illustrated in FIG. 2, and FIG. 4 is a cross-sectional view taken along line AA ′ of FIG. 3.

Referring to FIG. 4, the unit pixel of the CMOS image sensor may include a P conductive semiconductor substrate 410, a photodiode 420, a floating diffusion region 430, a source / drain region 440, and an overflow drain (OFD). The active region 450 includes a transfer transistor Tx and a reset transistor Rx.

The photodiode 420, which absorbs light energy and accumulates charges, may include an N-type active region 422 and a P-conductive pinning layer 425.

The floating diffusion region 430 receives charges accumulated in the photodiode 420 through the transfer transistor and may be formed by implanting ions of N conductivity type.

The active region 450 of the OFD region is formed of an N conductive ion region, and in the relation between the active region 450 of the OFD region, the P conductive semiconductor substrate 410, and the active region 422 of the photodiode 420. NPN transistor 210 is formed.

* In the case of the overflow transistor described above with reference to FIG. The active region 450 and the active region 422 of the photodiode 420 are formed at appropriate positions so that the potential barrier between them is adjusted to an appropriate level.

The method of adjusting the potential barrier between the active region 450 of the OFD region and the active region 422 of the photodiode 420 to an appropriate level is, for example, the active region 450 and photodiode 420 of the OFD region. The active region 422 may be spaced apart from each other by an appropriate distance, or the active region 450 of the OFD region may be formed using a chain implant using N-conductive ions. Hereinafter, each of these will be described with reference to related drawings.

4 illustrates a structure in which a potential barrier at an appropriate level is formed by using a distance between the active region 450 of the OFD region and the active region 422 of the photodiode 420.

For reference, when the active region 450 of the OFD region and the active region 422 of the photodiode 420 contact each other, the active region 450 of the OFD region and the active region 422 of the photodiode 420 are applied. It becomes equal to the voltage VDD, and thus the electric charge generated in the photodiode 420 will all escape to the overflow drain (OFD).

In contrast, as the distance between them increases, the voltage level between the active region 450 of the OFD region and the active region 422 of the photodiode 420 (ie, the P-conducting region therebetween) gradually increases to 0V. Getting closer.

Therefore, when the distance between the two becomes far enough, when the voltage level of the P-conducting type region described above becomes 0V, the function for preventing the blooming phenomenon cannot be performed. Thus, the voltage level between the two is a predetermined voltage level (for example, 0.2V to 0.3V). This means that if more charge is accumulated than the capacity of the photodiode 420, the charge overflows in all directions. If there is a low voltage level leading to the OFD, the charge overflowed from the photodiode 420 becomes active in the OFD region. This is because the region 450 overflows.

It was confirmed that the distance between the active region 450 of the OFD region and the active region 422 of the photodiode 420 having such a voltage level and having an appropriate potential barrier was within 0.5 μm.

In FIG. 5, N-type ions are used as the active region 450 of the OFD region to form an appropriate level potential barrier between the active region 450 of the OFD region and the active region 422 of the photodiode 420. The structure to which the method of forming using a chain implant is applied is illustrated.

The active region 510 of the OFD region formed using the chain implant is composed of a high concentration first region 512 and a low concentration second region 514.

This increases the depth of the active region 510 in the OFD region, and also the active region of the photodiode 420 using the second region 514 having a relatively low concentration and being less affected by overflow drain (OFD). This is to precisely adjust the distance to the (422).

In FIG. 5B, the pinning layer 425 of the photodiode 420 contacts the active region 510 of the OFD region to remove surface noise generated at the boundary between the OFD region and the photodiode 420. Illustrated is a structure configured to expand until it is.

FIG. 5C illustrates a structure in which the P-conductive ion region 550 is independently formed between the active region 450 of the OFD region and the active region 422 of the photodiode 420.

This is because both sides of the N conductive ions implanted into the P conductive semiconductor substrate 410 to form the active region 450 of the OFD region and the active region 422 of the photodiode 420, respectively. The reason is that the N-conducting ion regions are in contact with each other, thereby forcibly separating them and forming the NPN transistor 210. Also. In the case where the active region 422 of the photodiode 420 is extended to the OFD region, the P-conductive ion region 550 may be formed in the boundary region to maintain the characteristics between pixels.

In addition, the present invention ensures efficient OFD characteristics, and prevents charge generated during integration time in the strong light environment from overflowing to the floating diffusion region (FD) through the transfer transistor (Tx). Can be. This appears as an effect of suppressing and eliminating the black sun phenomenon.

In general, a black line improvement circuit exists for each column of a pixel array in order to remove the black line phenomenon. If OFD operates well in this embodiment, the above-described black line improvement circuit is unnecessary. In addition, there is an advantage that can be improved by the chip area.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the following claims And changes may be made without departing from the spirit and scope of the invention.

210: NPN transistor 410: P conductive substrate
420: photodiode 430: floating diffusion region
440: source / drain area 450, 510: OFD active area

Claims (5)

In the CMOS image sensor,
A photodiode region formed in the P conductive semiconductor substrate to generate and accumulate charges corresponding to incident light; And
And an overflow drain (OFD) active region formed in the P conductive semiconductor substrate so as to be laterally spaced apart from the photodiode region, and configured to discharge excess charge generated in the photodiode to the outside.
The active region of the photodiode and the OFD active region are respectively formed such that the P-conductive semiconductor substrate is interposed so that the active region of the photodiode, the interposed P-conductive semiconductor substrate and the OFD active region are NPN. Forming transistors,
The OFD active region is a CMOS image sensor, characterized in that formed using a chain implant (chain implant) to adjust the distance from the active region of the photodiode.
The method of claim 1,
And the active area of the photodiode and the OFD active area are spaced apart from each other within a distance of 0.5um to form a potential barrier of a predetermined potential barrier.
3. The method according to claim 1 or 2,
And a pinning layer of the photodiode is extended to contact the OFD active region.
3. The method according to claim 1 or 2,
And a P ion region is formed between the photodiode and the overflow drain (OFD) active region.
The method of claim 1,
And the base of the NPN transistor is grounded.

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