KR20060093385A - Image sensor - Google Patents

Abstract

Disclosed are an image sensor capable of preventing dark current caused by crystal defects. The sensor includes a deep p-well formed in a semiconductor substrate and a device isolation barrier doped with impurities at a higher concentration than the deep p-well, while contacting one side of the photodiode and photodiode formed on the deep p-well.

p-well, isolation barrier, photodiode, reset gate

Description

Image sensor

1 is a cross-sectional view of an image sensor including a conventional reset gate.

2A is a circuit diagram illustrating a unit pixel of an image sensor according to example embodiments. FIG. 2B is a layout illustrating the unit pixel of FIG. 2A.

3 to 5 are process cross-sectional views illustrating a method of manufacturing the image sensor according to the first embodiment of the present invention.

6 is a cross-sectional view illustrating a method of manufacturing an image sensor according to a second embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

100; Semiconductor substrate 102; Deep p-well

104; First p-well 106; Second p-well

108; Device isolation cabinet 110; Photodiode

120; Reset gate 130; Transfer gate

 The present invention relates to an image sensor, and more particularly to an image sensor having a photodiode separated by a PN junction.

A photodiode is a light receiving element that converts an optical signal into an electrical signal. Photodiodes are widely used for optical pick-up devices such as CD-ROMs, DVDs, and the like.

On the other hand, when light enters the photodiode, electron-hole pairs are generated in the depletion region. The generated electron-hole pairs travel through transistors or wiring lines connected to the outside. That is, the current of the photodiode changes essentially with the optical generation rate of the carrier, which provides a useful way of converting a time-varying optical signal into an electrical signal. On the other hand, the image sensor is composed of a photodiode portion for sensing the light to generate a current and a logic portion for processing the sensed light as an electrical signal and data.

1 is a cross-sectional view of an image sensor including a conventional reset gate.

Referring to FIG. 1, a conventional image sensor includes a photodiode 20 separated by an isolation layer 18 and formed on a semiconductor substrate 10. The deep p-well 12 and the first p-well 14 are sequentially formed in the semiconductor substrate 10. The second p-well 16 is formed from an upper end of the semiconductor substrate 10 to a portion of the upper portion of the deep p-well 12 to include the device isolation layer 18 defining the photodiode 20. The photodiode 20 generally consists of a p-type photodiode 21 and an n-type photodiode 22. The reset gate 30 is for resetting the floating diffusion node, and has a structure in which a doped polysilicon 31 and a gate oxide layer 32 are stacked.

The device isolation layer 18 is formed of an insulating film, for example, a silicon oxide film, and is formed in an STI or LOCOS method in order to prevent signal interference or overflow between adjacent photodiodes 20. However, crystal defects occur at the interface between the device isolation film 18 and the second p-well 16 generated in the process of forming the device isolation film 18. Crystal defects at the interface are important factors that increase the dark current by acting as pixel defects or noise components. The dark current may deteriorate the screen characteristics when the illumination is low.

Accordingly, an aspect of the present invention is to provide an image sensor capable of preventing dark current caused by crystal defects at an interface of an isolation layer.

The image sensor according to the present invention for achieving the technical problem is in contact with a semiconductor substrate, a deep p-well formed on the semiconductor substrate, a photodiode formed on the deep p-well and one side of the photodiode, Device isolation barriers doped with impurities at a higher concentration than deep p-wells.

The device isolation barrier may be made of an impurity of the same conductivity type as the deep p-well or an impurity of a conductivity type opposite to the deep p-well.

A second p-well formed from an upper end of the semiconductor substrate over a portion of the deep p-well and defining the photodiode, wherein the device isolation barrier is formed in the second p-well; Can be embedded.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments described below may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

The unit pixel of a typical CMOS image sensor is composed of one photodiode (PD) and four NMOS transistors. Four NMOS transfer transistors for transporting the concentrated photocharges from the photodiode (PD) to the floating diffusion node (FD), set the potential of the floating diffusion node (FD) to the desired value, and charge (Cpd) A reset transistor for resetting the floating diffusion node (FD) by discharging it, a drive transistor (Dx) serving as a source follower-buffer amplifier, and a select transistor for addressing by switching. do.

2A is a circuit diagram illustrating a unit pixel of an image sensor according to example embodiments. FIG. 2B is a layout illustrating the unit pixel of FIG. 2A.

2A and 2B, a transfer transistor in which a drain-source path is formed between the photodiode PD, the photodiode PD, and the floating diffusion node FD, and a control signal Tx is input to the gate, and a floating diffusion A reset transistor in which a source-drain path is formed between the node FD and the power supply voltage and a control signal Rx is input to the gate, a gate is connected to the floating diffusion node FD, and a drain is connected to the power supply voltage. The drive transistor may include an input drive transistor, a select transistor having a drain connected to a source of the drive transistor, a control signal Sx input to a gate, and a source connected to a unit pixel output. In addition, a load transistor is formed between the source of the select transistor and the ground power source, and a bias voltage is input to the gate.

One side of the gate of the transfer transistor (hereinafter referred to as the transfer gate Tx) is formed while overlapping an active region in which the photodiode PD is to be formed, and a floating diffusion region FD is formed in the active region below the other side of the transfer gate Tx. do. Here, the photodiode PD has a relatively large area, and the area is narrowed while giving a bottle neck effect to the path of the floating diffusion node FD from the photodiode PD.

An active region in which the reset transistor, the drive transistor, and the select transistor are to be formed extends counterclockwise around the floating diffusion node FD. Here, the gate of the reset transistor (hereinafter referred to as the reset gate Rx), the gate Dx of the drive transistor, and the gate Sx of the select transistor are arranged while crossing the upper portion of the active region at predetermined intervals.

The present invention will be described based on the first embodiment of the image sensor including the reset gate (Rx) and the second embodiment of the image sensor including the transfer gate (Tx).

First embodiment

3 to 5 are process cross-sectional views illustrating a method of manufacturing the image sensor according to the first embodiment of the present invention.

Referring to FIG. 3, a deep p-well 102 is formed by ion implanting a high concentration of p-type impurities into a semiconductor substrate 100, for example, an n-type silicon substrate. The deep p-well 102 is to reduce the resistance to carrier movement. Thereafter, the first p-well 104 is formed to a predetermined thickness on the deep p-well 102. The thickness and specific resistance of the first p-well 104 have an important effect on the energy conversion efficiency and signal processing speed of the photodiode. The thickness of the P-well 104 is preferably 1 to 8 µm and the specific resistance is 100 to 200 Ωcm.

As shown in FIG. 4, a second p-well 106 defining a region in which the photodiode 110 is to be formed and an active region is formed. The second p-well 106 is formed over a portion of the top of the deep p-well 102 from the top of the first p-well 104. Thereafter, a reset gate (Rx) 120 including a first gate oxide layer 122 and a doped polysilicon layer 121 is formed on the active region.

As shown in FIG. 5, an impurity of higher concentration than the deep p-well 102 is ion-implanted into the second p-well 106 to form the device isolation barrier 108. In this case, the device isolation barrier 108 may be a p-type impurity or an n-type impurity. The device isolation barrier 108 forms a potential barrier on the sidewall of the photodiode 110 to suppress the movement of carriers, especially electrons.

Thereafter, a photodiode 110 defined by the device isolation barrier 108 and composed of a p-type photodiode 111 and an n-type photodiode 112 is formed. Although not shown, a conventional photolithography process may be used to form the device isolation barrier 108 and the photodiode 110. The N-type photodiode 112 may be formed by ion implanting arsenic (As) and then activating arsenic ions. One side of the N-type photodiode 112 is aligned with one side of the reset gate 120. Then, the p-type photodiode 111 is formed on the n-type photodiode 112 aligned with one side of the reset gate 120 (not shown but may be one side of the spacer). The P-type photodiode 111 may be formed by ion implantation of p-type impurities, for example, boron (B).

The device isolation barrier 108 according to the first embodiment of the present invention is formed by ion implantation of a high concentration of impurities, thereby forming a potential barrier between the photodiode 110 or between the photodiode 110 and a transistor adjacent to the photodiode. The interference between them can be suppressed. In addition, when the illuminance is high, electrons overflowed from the photodiode 110 flow through the deep p-well 102 formed under the photodiode 110 to the semiconductor substrate 100. The deep p-well 102 has a lower concentration of impurities than the device isolation barrier 108 and has a relatively low potential barrier for electrons. Accordingly, overflow between the photodiodes 110 can be prevented. Furthermore, since the photodiode 110 and the gate insulating film 122 of the reset transistor can be uniformly aligned, the characteristics of the photodiode can be made uniform.

Second embodiment

6 is a cross-sectional view illustrating an image sensor according to a second exemplary embodiment of the present invention. In the second embodiment of the present invention, the process of forming the semiconductor substrate 100, the deep p-well 102, the first p-well 104, and the photodiode 110 is described with reference to FIGS. 3 to 5. Same as the first embodiment. Accordingly, a description will be given focusing on the differences compared with the first embodiment.

Referring to FIG. 6, in the transfer gate 130, the second gate insulating layer 132 and the doped polysilicon layer 131 are stacked so that one side of the transfer gate 130 is aligned with the photodiode 110. The other side of the transfer gate 130 is implanted with n-type impurities to form a floating diffusion node 134. An active region of the floating diffusion node 134 and the transfer gate 130 may be formed between the device isolation region 108 and the photodiode 110.

 The device isolation barrier 108 according to the second embodiment of the present invention is formed by ion implantation of a high concentration of impurities, thereby forming a potential barrier between the photodiode 110 or between the photodiode 110 and a transistor adjacent to the photodiode. The interference between them can be suppressed. In addition, when the illuminance is high, electrons overflowed from the photodiode 110 flow through the deep p-well 102 formed under the photodiode 110 to the semiconductor substrate 100. The deep p-well 102 has a lower concentration of impurities than the device isolation barrier 108 and has a relatively low potential barrier for electrons. Accordingly, overflow between the photodiodes 110 can be prevented. Further, since the photodiode 110 and the gate insulating film 132 of the transfer transistor can be uniformly aligned, the characteristics of the photodiode can be made uniform.

As mentioned above, although the present invention has been described in detail with reference to preferred embodiments, the present invention is not limited to the above embodiments, and various modifications may be made by those skilled in the art within the scope of the technical idea of the present invention. It is possible.

According to the above-described image sensor according to the present invention, the device isolation barrier is formed by ion implantation of a high concentration of impurities, thereby forming a potential barrier between the photodiode or between the photodiode and the adjacent transistors to reduce the dark current generated in the device isolation barrier. Can be greatly reduced.

In addition, when the illuminance is high, electrons overflowed from the photodiode may flow through the deep p-well formed under the photodiode to the semiconductor substrate to prevent the overflow between the photodiodes.

Furthermore, since the photodiode and the gate insulating film of the transistor can be uniformly aligned, the characteristics of the photodiode can be made uniform.

Claims (5)

Semiconductor substrates; A deep p-well formed in said semiconductor substrate; A photodiode formed on the deep p-well; And And a device isolation barrier doped with impurities at a concentration higher than said deep p-well between said photodiode and photodiode and between a photodiode and an adjacent transistor. The image sensor of claim 1, wherein the device isolation barrier is made of the same conductivity type as the deep p-well. The image sensor of claim 1, wherein the device isolation barrier is made of a conductive impurity opposite to the deep p-well. The image sensor of claim 1, further comprising a second p-well formed from an upper end of the semiconductor substrate to a portion of an upper portion of the deep p-well, and defining the photodiode. The image sensor of claim 4, wherein the device isolation barrier is embedded in the second p-well.

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