KR100790214B1 - CMOS image sensor and its manufacturing method - Google Patents

Abstract

The present invention provides a CMOS image sensor capable of suppressing reset noise due to a potential change in a sensing region generated when the reset transistor is turned off, and a method of manufacturing the same. The CMOS image sensor according to the present invention includes a reset transistor connected to a sensing area and a power supply voltage, respectively, and configured to discharge electrons stored in the sensing area to a power supply voltage when the CMOS image sensor is turned on. It is characterized by having a thicker thickness in the portion adjacent to the sensing area.

Image sensor, reset transistor, sensing area, potential, reset noise, gate oxide

Description

CMOS image sensor and its manufacturing method {CMOS IMAGE SENSOR AND METHOD OF MANUFACTURING THE SAME}             

1 is a unit pixel circuit diagram of a general CMOS image sensor.

2 is a cross-sectional view of a reset transistor of a conventional CMOS image sensor.

FIG. 3 is a diagram illustrating potentials of on / off operations of the reset transistor of FIG. 2. FIG.

4A to 4C are cross-sectional views illustrating a method of manufacturing a reset transistor of a CMOS image sensor according to an exemplary embodiment of the present invention.

FIG. 5 is a view illustrating a potential according to an on operation of the reset transistor of FIG. 4. FIG.

FIG. 6 is a view illustrating a potential according to an off operation of the reset transistor of FIG. 4. FIG.

※ Explanation of code for main part of drawing

41 semiconductor substrate 42 field oxide film

43: gate oxide film 44: gate

45 photoresist pattern 46 N-type impurity ion

47: spacer 48: drain                 

49: source 50: channel area

SR: Sensing area VDD: Power supply voltage

Rx: Reset Transistor

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a complementary metal oxide semiconductor (CMOS) image sensor technology, and more particularly, to a CMOS image sensor capable of suppressing reset noise and a method of manufacturing the same.

CMOS image sensor is not only the growth in the existing image sensor market centered on TV or video, but also digital still camera, PC camera, digital Demand is expected to grow in proportion to the growth of the camcorder and PCS market, and also in the fields of video game devices, security cameras, medical micro cameras and HDTVs. Demand is expected to increase.

FIG. 1 is a pixel circuit diagram of the CMOS image sensor. As shown in FIG. 1, a unit pixel includes one photodiode PD and four MOS transistors. The four transistors are composed of a transfer transistor (Tx), a reset transistor (Rx), a drive transistor (MD), and a select transistor (Sx), and a capacitance (Cfd) is provided in a floating diffusion region, which is a sensing region (SR). The load transistor is formed outside the unit pixel to read an output signal.

Here, the reset transistor Rx is connected to the sensing region SR and the power supply voltage VDD, respectively, and when turned on, discharges electrons stored in the sensing region SR to the power supply voltage VDD. When the signal is turned off, electrons in the channel region under the gate of the reset transistor Rx move to one of the sensing region SR and the power supply voltage VDD.

2 and 3 are diagrams for explaining the operation of the reset transistor (Rx), Figure 2 is a cross-sectional view of the reset transistor (Rx), Figure 3 is on / off (ON / OFF) of the reset transistor (Rx). It is a figure which shows the potential according to operation.

As shown in FIG. 2, the reset transistor Rx includes a semiconductor substrate 1 in which a reset transistor region is defined by a field oxide film 2, a gate 4 formed on the substrate 1, and a gate 4. It is formed in the substrate 1 on both sides, and is comprised of the sensing area SR and the drain and source 6 and 7 as a power supply voltage VDD. In addition, a gate oxide film 3 is interposed between the gate 4 and the substrate 1, and spacers 5 are formed on both sides of the gate 4.

At this time, when the reset transistor Rx is turned off, as shown in FIGS. 2 and 3, the electrons of the channel region 8 under the gate 4 are either one of the sensing region SR and the power supply voltage VDD. For example, when the electron moves to the power supply voltage VDD, the potential of the power supply voltage VDD remains unchanged, whereas when the electron moves to the sensing area SR, the potential of the sensing area SR is changed. Will be lowered.                         

However, since the electrons under the gate 4 of the reset transistor Rx do not always move to the sensing area SR by a predetermined amount, the potential of the sensing area is changed every time the sensing area SR is reset. There is a problem that changes, and this potential fluctuation causes a reset noise.

In addition, since the number of the image sensor is composed of one or more unit pixels according to the use, the number of reset noises generated in each unit pixel causes a problem of deteriorating the characteristics of the image sensor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a CMOS image sensor capable of suppressing reset noise due to a potential change in a sensing region generated when the reset transistor is turned off.

In addition, another object of the present invention is to provide a method of manufacturing the CMOS image sensor.

In order to achieve the object of the present invention, the CMOS image sensor according to the present invention includes a reset transistor connected to the sensing area and the power supply voltage, respectively, and discharges the electrons stored in the sensing area to the power supply voltage when turned on, the reset transistor The gate oxide film has a thicker thickness in a portion adjacent to the sensing region than in a portion adjacent to the power supply voltage.

In addition, in order to achieve the object of the present invention, a CMOS image sensor according to the present invention comprises a semiconductor substrate of a first conductivity type in which a reset transistor region is defined by a field oxide film; A gate oxide film formed on the reset transistor region; A gate formed on the gate oxide film; A spacer formed on the gate sidewall; A sensing region of the second conductivity type formed on the substrate on both sides of the gate, and a drain and a source as a power supply voltage; And a channel region formed under the gate, wherein the gate oxide film has a thicker thickness at a portion adjacent to the sensing region than at a portion adjacent to the power supply voltage.

In addition, in order to achieve the above object of the present invention, a method of manufacturing a CMOS image sensor according to the present invention comprises the steps of defining a reset transistor region by forming a field oxide film on the first conductivity type semiconductor substrate; Sequentially forming a gate oxide film and a gate of the reset transistor on the region; Implanting second conductivity type impurity ions into a portion of the gate; Heat treating the substrate to grow a gate oxide thickness below a portion of the gate; Forming a second conductivity type LDD region on the substrate on both sides of the gate; Forming a spacer on the gate sidewall; And implanting a second conductivity type high concentration impurity into the LDD regions on both sides of the spacer to form a drain and a source as a sensing region and a power supply voltage, wherein a portion of the gate is adjacent to the sensing region.

Here, the first conductivity type is P type and the second conductivity type is N type.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.                     

4A to 4C are cross-sectional views illustrating a method of manufacturing a reset transistor of a CMOS image sensor according to an exemplary embodiment of the present invention, and FIGS. 5 and 6 illustrate a reset transistor of a CMOS image sensor turned on / off. Potential of each is shown.

First, a manufacturing method of a CMOS image sensor according to an exemplary embodiment of the present invention will be described with reference to FIGS. 4A to 4C.

Referring to FIG. 4A, a field oxide film 42 is formed on a P-type semiconductor substrate 41 to define a reset transistor region, and a gate oxide film 43 of a reset transistor Rx (see FIG. 1) is formed on the region. The gate 44 is formed sequentially. Next, the photoresist pattern 45 is formed on the substrate 41 to expose a portion of the gate 44 of the reset transistor Rx adjacent to the sensing region SR (see FIG. 1), and then the exposed gate 44 is exposed. N-type impurity ions 46 are implanted into the portion of.

Then, the photoresist pattern 45 is removed by a known method, and a predetermined heat treatment process is performed to remove surface defects of the substrate 41. At this time, the gate oxide film 43 under the portion of the gate 44 is grown by the N-type impurity ions implanted in the portion of the gate 44, and as shown in FIG. 4B, adjacent to the sensing region SR. The thickness of the gate oxide film 43 in the portion becomes thick.

Referring to FIG. 4C, an N-type LDD region is formed on a substrate on both sides of the gate 44 by performing a known LDD process, and a spacer 47 formed of an insulating layer is formed on the sidewall of the gate 44 by a spacer process. Thereafter, high concentration N-type impurities are implanted into the LDD regions on both sides of the spacer 47 to form N-type drains and sources 48 and 49 as the sensing region SR and the power supply voltage VDD.

Next, the operation of the reset transistor of the CMOS image sensor described above will be described with reference to FIGS. 4C, 5, and 6.

As shown in FIG. 4C, the gate oxide layer 43 of the reset transistor Rx according to the present invention has a thickness thicker in the region adjacent to the sensing region SR than in the region adjacent to the power supply voltage VDD. That is, according to the present invention, when the voltage applied to the gate of the transistor is the same, the channel potential becomes lower as the thickness of the gate oxide film becomes thicker, and the thickness of the gate oxide film 43 in the portion adjacent to the sensing region SR is reduced. For example, when the reset transistor is turned on, as shown in FIG. 5, the potential of the channel region 50 becomes larger toward the power supply voltage VDD, and when the reset transistor is turned off, as shown in FIG. 6. Likewise, the electrons are selectively moved to the power supply voltage VDD without being moved to the sensing region SR and are discharged.

Therefore, the potential change of the sensing area is prevented, and the reset noise caused by the potential change is suppressed, thereby preventing deterioration of characteristics of the image sensor.

That is, according to the present invention, when the reset transistor is turned on, the gate oxide film thickness of the reset transistor is made thicker than the portion adjacent to the power supply voltage, and the channel potential of the sensing region is lowered when the reset transistor is turned on, so that the reset transistor is turned off. It prevents electrons from moving to the sensing area and can selectively discharge electrons into the power supply voltage area.

The present invention is not limited to the above embodiments, and various modifications can be made without departing from the technical spirit of the present invention.

According to the present invention described above, the potential change of the sensing area is prevented, and the reset noise caused by the potential change is suppressed, thereby resulting in an effect of preventing the deterioration of characteristics of the image sensor.

Claims (5)

delete delete delete Defining a reset transistor region by forming a field oxide film on the first conductivity type semiconductor substrate; Sequentially forming a gate oxide film and a gate of a reset transistor on the region; Implanting second conductivity type impurity ions into a portion of the gate; Heat treating the substrate to grow a gate oxide thickness below the portion of the gate; Forming a second conductivity type LDD region on the substrate on both sides of the gate; Forming a spacer on the gate sidewall; And Implanting a second conductivity type high concentration impurity into the LDD regions on both sides of the spacer to form a drain and a source as a sensing region and a power supply voltage; And a portion of the gate is a portion adjacent to the sensing region. The method of claim 4, wherein And the first conductivity type is P type and the second conductivity type is N type.

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