KR100761048B1 - High-sensitivity ccd image sensor and fabrication method thereof - Google Patents

Abstract

A high sensitivity CCD(Charge Coupled Device) image sensor and its manufacturing method are provided to obtain a high sensitivity image by increasing a signal charge storing space using an epitaxial layer between a photodiode and a P well. A high sensitivity CCD image sensor has an enhanced pixel structure. The pixel structure is composed of a first P well(2) for dividing an N type epitaxial wafer into an N type substrate(1) and an N type epitaxial layer(1-a), a photodiode(7) on a surface of the N type epitaxial layer, a heavily doped surface P+ layer(8) on the photodiode, a second P well(3) from an upper portion of the N type substrate to a predetermined portion of one side of the N type epitaxial layer, a vertical transfer channel(4) from an upper portion of the second P well to an epitaxial wafer surface of a side portion of the photodiode, a transfer gate(6) between the photodiode and the vertical transfer channel, and a channel stop layer. The channel stop layer(5) is deeply formed from the epitaxial wafer surface to the N type substrate to isolate completely the heavily doped surface P+ layer, the photodiode, the N type epitaxial layer, and one side of the first P well from an adjacent cell.

Description

High-sensitivity CCD image sensor and fabrication method

1 is a schematic diagram showing a pixel structure of a general CCD image sensor.

2A to 2H are cross-sectional views of a typical CCD image sensor.

Figure 3 is a schematic diagram showing the cell structure of the CCD image sensor as an embodiment of the present invention

4A to 4I are cross-sectional views of a CCD image sensor according to the embodiment of FIG. 3.

FIG. 5 is a schematic diagram illustrating a cell structure when only one channel stop layer is provided in the embodiment of FIG. 3.

FIG. 6 is a doping profile identified by section A-A 'of FIG.

FIG. 7 is a net doping profile identified by section B-B 'of FIG.

8 is a vertical potential profile compared with a conventional photodiode.

9 is an image captured by a conventional CCD image sensor.

10 is an image taken by the CCD image sensor according to the present invention.

<Explanation of symbols on main parts of the drawings>

1: N-Substrate 1-a: Epitaxy Layer

2: first P well 3: second P well

4: Vertical Transmission Channel (VCCD) 5: Channel Stop Layer (CS)

5-a: second channel stop layer (CS) 6: transfer gate

7: N-type photodiode (PDN) 8: high concentration surface P + layer

The present invention relates to an image sensor, and more specifically, to implant the channel stop layer (5) to the N-type substrate (1) to block the leakage current and noise flow, floating in the P-type well By forming a low concentration epitaxy layer (1-a) present in a state to increase the area of the photodiode and at the same time lowering the operating voltage of OFD (hereinafter referred to as OFD). The present invention relates to a CCD image sensor capable of capturing high sensitivity and high quality images.

In general, a CCD image sensor is a semiconductor device that converts an optical image into an electrical signal. A photodiode arranged in rows and columns converts light into signal charges, and the signal charges are sequentially moved in a vertical transmission channel and a horizontal transmission channel. Finally, the device senses and outputs the video signal charge transmitted through the sensing amplifier.

In manufacturing such a CCD image sensor, it is moving toward the integration of a larger number of pixels in the same area in accordance with the demand for high quality and high integration. However, the improvement in the degree of integration in the same area inevitably leads to a reduction in the area of the photodiode, resulting in a decrease in sensitivity due to a reduction in signal charge.

In addition, the dark current component generated by the diffusion of optical excitation carriers generated during the photoelectric conversion process of the photodiode and the presence of parasitic stray capacitance in the P well is increased with the increase of noise. It acts as a noise component to degrade the characteristics of the sensor to degrade the quality (quality), and the response speed of the incident image image is reduced due to the slow response of the photocurrent.

As a result, as described above, the reduction of the area of the photodiode and the inter-pixel interference in which the signal charges move to other adjacent pixels have emerged as important problems to be solved.

In order to solve the above problems, various efforts have been made to increase the optical sensitivity of an image sensor, and one of them is the ratio of the area of the photodiode to the total image sensor area in order to increase the optical sensitivity. In order to increase the fill factor and to reduce the inter-pixel interference, each pixel is completely isolated.

Hereinafter, a pixel structure of a conventional CCD image sensor will be described with reference to FIG. 1.

1 is a schematic diagram showing a pixel structure of a general CCD image sensor.

Referring to FIG. 1, a CCD image sensor generally includes a first P well 2 and a second P well 3 formed on an N-type substrate 1 and an order above the first P well 2. Vertical transfer channel which is a photodiode region composed of N-type impurity layers 7 formed as described above, a high-concentration surface P + layer 8 for preventing surface depletion, and an N-type impurity layer formed at a predetermined distance from the photodiode. (4), a transfer gate 6 for interconnecting the photodiode and the vertical transmission channel 4, and a channel stop layer 5 for isolating adjacent cells on one surface of the photodiode.

In the CCD image sensor of FIG. 1 configured as described above, when light enters the photodiode region through the lens, the incident light is photoelectric conversion in the depletion layer of the N-type photodiode 7 region, resulting in an EHP (Electron-Hole Pair). Electrons, which are signal charges, are accumulated in the N-type photodiode 7 region, and holes are accumulated in the first P well 2 region. At this time, the signal charges accumulated in the N-type photodiode 7 are moved to the vertical transfer channel 3 through the transfer gate 6 by applying a bias to the transfer gate 6.

The CCD image sensor is a device that converts a light signal for an image into an electrical signal, and when strong light enters, excessively generated charges overflow and diffuse into the substrate, and bloom and invade adjacent photodiodes. Phenomenon occurs, and the captured image generates a strong false signal in a white band state and spreads to the surrounding image.

In order to suppress this blooming phenomenon, an OFD voltage is usually applied to the substrate to subtract excessively generated charges to the N-type substrate 1.

The OFD operation is performed by punch-thru between the N-type substrate 1 and the P well. That is, when a reverse bias (+ voltage) is applied to the N-type substrate 1 with the P well as the reference potential, the P-well is depleted, and the signal charge generated excessively in the N-type photodiode 7 is punch-through. It is discharged to the N-type substrate 1 through the P well of -Thru) state.

However, since the reverse bias for discharging excess charge of the photodiode in the OFD operation is applied to the entire N-type substrate 1, the signal charge of the vertical transfer channel 4 can be taken out together. Therefore, in order to prevent this at present, additional P-type impurities are implanted into the lower portion of the vertical transfer channel 4 to form the second P well 3, thereby preventing the depletion of the second P well 3 so that the vertical The loss of signal charges transmitted to the transmission channel 4 is prevented.

However, if the impurity concentration of the first P well 2 is high, depletion is difficult, so it is difficult to reduce the blooming phenomenon due to OFD operation, and the bias applied to the N-type substrate 1 becomes high, which adversely affects the overall operation of the CCD image sensor. do.

Therefore, in order to implement a high-sensitivity CCD image sensor, the N-type photodiode 7 should be made large in area and low in concentration so as to be fully depleted even at low voltage.

Herein, a manufacturing process of a conventional CCD image sensor will be described with reference to the accompanying drawings.

First, the P-type impurity is doped into the N-type wafer 1 as shown in FIG. 2A, thereby forming the first P well 2 as shown in FIG. 2B.

Subsequently, as shown in FIG. 2C, the P-type impurity is implanted into one surface of the N-type wafer to form the second P well 3.

Subsequently, as shown in FIG. 2D, the vertical transfer channel 4 is formed by doping N-type impurities on the second P well 3.

Subsequently, as shown in FIG. 2E, the P-type impurity is doped on the other surface of the N-type wafer 1 to form the channel stop layer 5.

Subsequently, as shown in FIG. 2F, a P-type impurity is doped on one surface of the vertical transfer channel 4 to form a transfer gate 6.

Finally, as shown in FIG. 2G, N-type impurities are sequentially doped on the surface of the N-type wafer 1 between the transfer gate 6 and the channel stop layer 5 to form an N-type photodiode 7 region. A high concentration surface P + layer 8 is formed to prevent surface depletion by doping a high concentration of P-type impurities to form a photodiode composed of P-type and N-type.

However, as described above, in the manufacturing process of the CCD image sensor, the P-type and N-type impurities are sequentially doped to form a photodiode, wherein the doping process forms a complete depletion of the photodiode for smooth OFD operation. Under appropriate doping conditions N-type impurities are doped.

That is, the design rule of the CCD image sensor is reduced and the area of the photodiode is also reduced according to the demand for high integration of pixels. Therefore, doping of a high concentration of impurities is required to compensate for the reduced area of the photodiode. In order to realize a high-sensitivity CCD image sensor, there is a difficulty in forming a photodiode with a large concentration of the photodiode and at the same time.

In addition, as mentioned in the introduction of the present application, since the channel stop layer 5 is partially isolated from the adjacent cells in the conventional CCD image sensor, a few carriers existing in the first P well 2 are diffused. As a result, the N-type photodiode 7 penetrates into the adjacent N-type photodiode 7, thereby generating a dark current that induces a current component even when there is no photocharge due to light. The dark current generated in such a form causes a signal to fall on the screen even when the light is low, and a particularly uneven dark signal causes a defect in the screen.

The present invention has been made to solve the above problems, by forming a CCD image sensor on a low concentration epitaxy wafer, it is completely depleted even at low OFD voltage to discharge excess electrons of the photodiode Blooming phenomenon It is an object of the present invention to provide a high-quality CCD image sensor that can prevent.

In addition, the present invention provides a CCD image sensor capable of blocking the leakage current of the photoelectrically converted signal charge and preventing the influx of noise by deeply forming the channel stop layer 5 to isolate the adjacent pixels to the N-sub layer. For other purposes.

In addition, the present invention is formed by floating the epitaxy layer on the lower portion of the N-type epitaxy wafer in contact with the photodiode, so that a large amount of photoelectric conversion signal charge to the photodiode and low concentration epitaxy layer. It is another object of the present invention to provide a CCD image sensor having a high sensitivity and a high degree of integration capable of capturing an image with an image.

In addition, the present invention aims to form a photodiode having a large area by forming a low concentration epitaxy layer by performing a step of implanting P-type impurities with high energy to deeply form an epitaxy wafer.

In order to achieve the above technical problem, a high-sensitivity, high-definition CCD image sensor according to the present invention comprises a P-well formed on an N-type epitaxy wafer (Epitaxyal wafer) in which low concentration epitaxy is grown, and a surface of the epitaxy wafer. A photodiode composed of an N-type impurity layer 7, a high concentration surface P + layer 8 formed on the photodiode, And an epitaxy layer (1-a) formed between the P well and the photodiode, and a channel stop layer (5) that isolates the adjacent cells from the epitaxy wafer surface to the P-type well.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. 3 illustrates a cell structure of a CCD image sensor as an embodiment of the present invention.

First, the first P well 2 formed on the N type substrate 1, the N type epitaxy layer 1-a formed on the first P well 2, and the N type epitaxy layer. (1-a) formed on the photodiode region composed of an N-type impurity layer 7 formed on the upper side, a high concentration surface P + layer 8 formed on the photodiode, and on the N-type substrate 1 A second P well 3 to be formed, a vertical transfer channel 4 as an N-type impurity layer formed thereon, a transfer gate 6 formed between the photodiode region and the vertical transfer channel 4, and an adjacent cell. And a channel stop layer (5) formed from the epitaxy wafer surface to the P-type well, which is completely isolated.

Preferably, the N-type epitaxy layer 1-a, which is the same material as the epitaxy wafer, is formed in contact with the photodiode in a floating state by being surrounded by P-type impurities on the left and right sides.

The N-type epitaxy layer 1-a is vertically formed between the photodiode and the first P well 2 so that the area of the photodiode is extended to the epitaxial layer 1-a, so that the amount of charge photoelectrically converted It is possible to obtain a high sensitivity CCD image sensor.

In addition, since the channel stop layer 5 is deeply formed in the vertical direction to the N-type substrate 1, noise and leakage current between adjacent cells can be prevented.

In addition, the area of the first P well 2 region is reduced by the width of the N-type epitaxy layer 1-a, thereby reducing the minority carriers present in the P well, thereby reducing the dark current generated by dark current can be reduced.

As described above, the manufacturing method for forming a CCD image sensor according to the present invention includes implanting P-type impurities into an N-type epitaxy wafer to form a first P well 2 deep in the epitaxy wafer; Forming a photodiode by doping an N-type impurity (7) on the surface of the epitaxy wafer, and doping a P-type impurity at a high concentration to form a high surface P + layer (8); And forming a channel stop layer 5 through double ion implantation from the surface to the N-type substrate.

Looking at the manufacturing method of a CCD image sensor according to an embodiment of the present invention. 4A to 4I illustrate a method of manufacturing a CCD image sensor according to the exemplary embodiment of FIG. 3.

First, P-type impurities are implanted into the N-type epitaxy wafer as shown in FIG. 4A with energy of several mega electron volts (MeV), so that the first P well 2 is formed on the lower end surface of the N-type wafer as shown in FIG. 4B. ).

Subsequently, as shown in FIG. 4C, a P-type impurity is implanted into one surface of the N-type wafer to form a second P well 3.

Subsequently, as shown in FIG. 4D, the vertical transfer channel 4 is formed by doping N-type impurities on the second P well 3.

Subsequently, as shown in FIG. 4E, the P-type impurity is doped on the other surface of the N-type wafer to form the channel stop layer 5 to a predetermined depth.

Subsequently, as shown in FIG. 4F, the second channel stop layer 5-a is deeply and vertically deep down to the N type substrate 1 by implanting P-type impurities into the channel stop layer 5. .

Subsequently, as shown in FIG. 4G, a P-type impurity is doped on one surface of the vertical transfer channel 4 to form a transfer gate 6.

Next, as shown in FIG. 4H, an N-type impurity is doped on the surface of the wafer between the transfer gate 6 and the channel stop layer 5 to form an N-type photodiode 7 region.

Here, by doping the N-type photodiode 7 from the first P well 2 to a predetermined depth, an epitaxial layer (1-a) between the N-type photodiode 7 and the first P well 2. ) Is formed.

Finally, as shown in FIG. 4I, a high concentration of P-type impurities is doped on the surface of the N-type photodiode 7 to form a region of high concentration surface P + layer 8.

In the CCD image sensor formed by the above manufacturing method, the N-type wafer is formed by using an N-type epitaxy wafer (Epi-Wafer) in which a low concentration epitaxy layer is grown. Both the epitaxy layer (1-a) and the first P well (2) can be formed at low concentrations, making it possible to create a CCD image sensor that can be fully depleted even at low voltages during OFD operation, thereby solving the existing problems. have.

In addition, in the process of forming the channel stop layer 5 of the present invention, the pixel structure of the CCD image sensor which is not subjected to the process of forming the second channel stop layer 5-a of FIG. 4F is shown in FIG. 5. The channel stop layer 5 is not deeply formed to the N-type substrate 1 but is formed to the same depth as before. However, in such a case, it is impossible to obtain a perfect high quality image.

Hereinafter, the problem according to the degree of isolation of the channel stop layer 5 will be described with reference to FIG. 6, which shows a doping profile in which the channel stop layer 5 is identified by the A-A 'cross-section.

First, the curve shown by the dotted line shows the doping profile when the channel stop layer 5 is formed in the same structure as the embodiment according to the present invention with a low depth as in the prior art, and the curve indicated by the solid line is shown in the present invention. Figure 1 illustrates a doping profile according to one embodiment. As shown in the figure, the solid line is made of P-type impurities, while the dotted line is lowered down a lot of horizontal lines, and it can be seen that the area indicated by the circle is an epitaxy layer (1-a) formed of N-type impurities.

In the case of the dotted line, a signal charge photoelectrically converted through the epitaxy layer 1-a leaks to an adjacent cell, and the leakage current appears as noise in the captured image, thereby reducing the image quality. There is a problem that is reduced.

Therefore, the present invention completely isolates adjacent cells by the channel stop layer 5 in order to realize a higher quality CCD image sensor, and simultaneously protects the signal charge of the vertical transmission channel 4 during OFD operation. By forming the second P well 3 in the same manner as the purpose, it is possible to form a floating epitaxial layer (1-a) to increase the area of the photodiode and reduce the loss due to leakage current This is a more preferred example of the present invention.

In the process of forming the first P well 2, a P-type impurity penetrates into the epitaxial layer 1-a and enters the epitaxial layer 1-a by applying a high voltage of several mega volts. It may be formed between the N-type substrate (1). At this time, the thickness of the first P well 2 is reduced to reduce the area of the first P well 2 region, and the photodiode is formed at the same depth as the conventional P well 2, thereby forming the first P well 2 and the photodiode. An epitaxy layer 1-a is formed therebetween.

Referring to the net doping profile (Net Doping Profile) of Figure 7 and the vertical potential profile of Figure 8 identified by the B-B 'cross-section of the epitaxy layer (1-a) formed as follows.

As shown in FIG. 7, the photodiode region is formed by adding a photodiode region composed of an N-type photodiode 7 doped with N-type impurities and an epitaxy layer 1-a formed up to a P well. It can be seen that a positive signal can be stored.

In addition, the signal charge is stored in the region where the signal charge is stored in the conventional CCD image sensor structure indicated by the dotted line as shown in FIG. 8 and in the CCD image sensor structure in which the epitaxy layer (1-a) is formed by the present invention indicated by the solid line. When comparing the stored areas, it can be seen that the CCD image sensor according to the present invention can store more signal charges.

In more detail, referring to the image taken with the conventional CCD image sensor and the image taken with the CCD image sensor according to the present invention with reference to Figures 9 and 10, conventionally, image loss and epitaxy layer (1) due to lack of signal charge (1) Screen noise due to the leakage current of -a) is seen, but the image captured by the present invention can capture a clear and sensitive image without noise.

As a result, by forming the floating epitaxial layer (1-a) by the above-described process method, it is possible to implement a CCD image sensor having a large photodiode area of low concentration to obtain a high sensitivity image.

In addition, by forming the channel stop layer (5) to the N-type substrate (1), the CCD image can be integrated without the loss and loss of signal charge by blocking the leakage current to the pixels isolated from the adjacent cells and prevent the influx of noise Sensors can be implemented.

In the above description, the CCD image sensor having the channel stop layer 5 formed up to the N-type substrate 1 on one surface of the floating epitaxy layer 1-a has been described as an embodiment. The scope of application of the present invention is not limited thereto.

For example, as mentioned above, the structure of a CCD image sensor having an epitaxy layer 1-a formed between a P-type well and a photodiode, and having a channel stop layer 5 thinly formed on one end surface of the photodiode; You may use a process.

As described above, the present invention forms an epitaxy layer (1-a) between the photodiode and the P well, thereby storing a signal charge in an area where the photodiode and the epitaxy layer (1-a) are added. This makes it possible to capture an image with more signal charge, thereby obtaining a high sensitivity image.

And by using a low concentration of the N-type epitaxy wafer, there is an advantage that can drive the OFD at a low voltage through the epitaxial layer (1-a) of low concentration.

In addition, the channel stop layer 5 completely isolates the adjacent cells through the double ion implantation, thereby blocking the leakage current of the photoelectric conversion signal charge generated through the epitaxy layer 1-a, and introducing noise and There is an advantage to get a clear high-definition image by blocking the leakage.

In addition, P wells are implanted with high energy to form a low concentration epitaxy layer at the bottom of the photodiode, thereby forming a photodiode with a large area of low concentration, thereby providing a high density and high sensitivity CCD. There is an advantage to implement an image sensor.

Claims (8)

In the pixel structure of the CCD image sensor, A first P well (2) which separates the entirety of the N-type epitaxy wafers in which epitaxy of low concentration is grown into an N-type substrate (1) and an N-type epitaxy layer (1-a); A photodiode (7) formed of an N-type impurity layer on a surface of the N-type epitaxy layer (1-a); A high concentration surface P + layer (8) formed above the N-type photodiode region; A second P well (3) formed from an upper portion of the N-type substrate (10) to a predetermined portion of one side of the N-type epitaxy layer (1-a); A vertical transfer channel (4) which is an N-type impurity layer formed from an upper portion of the second P well (3) to an epitaxial wafer surface on the side of the photodiode (7); A transfer gate (6) formed between the photodiode (7) and the vertical transfer channel (4); The high concentration surface P + layer 8, the photodiode 7, the N-type epitaxy layer 1-a, and one side of the first P well 2 are formed on the epitaxy wafer surface such that they are completely isolated from adjacent cells. A pixel structure of a CCD image sensor, characterized in that it comprises a channel stop layer (5) formed deep in the vertical direction to the N-type substrate (1). The method of claim 1, The N-type epitaxy layer (1-a) is surrounded by a channel stop layer (5), a second P well (3), an N-type photodiode (7) and a first P well (2) in all directions, and is floating. Pixel structure of the CCD image sensor, characterized in that consisting of. delete In the process of CCD image sensor, Implanting P-type impurities into a low-doped N-type epitaxy wafer to form a low-concentration N-type epitaxy layer (1-a) and a first P well (2); Implanting P-type impurities on one surface of the N-type epitaxy wafer to form a second P well 3; Forming a vertical transmission channel (4) by doping N-type impurities on the second P well (3); Forming a channel stop layer (5) by implanting P-type impurities on the opposite side surface of the N-type epitaxy layer (1-a) on which the vertical transfer channel (4) is formed; Forming a transfer gate (6) on the contact surface of the N-type epitaxy layer (1-a) and the vertical transfer channel (4); Doping N-type impurities (7) in order on the N-type epitaxy layer (1-a) to form a photodiode; And forming a high concentration surface P + layer (8) formed on top of the N-type photodiode. delete The method of claim 4, wherein The process of forming the first P well 2 is characterized in that the first P well 2 is implanted with energy of several megaelectron bolts (MeV) to ion implant the lower portion of the N-type wafer. Method of manufacturing a CCD image sensor. The method of claim 4, wherein The step of forming the photodiode is formed at a predetermined distance from the position where the first P well 2 is formed, thereby forming an N-type epitaxy layer (1-a) between the first P well 2 and the photodiode. Method for manufacturing a CCD image sensor, characterized in that forming a). The method according to claim 4, The channel stop layer 5 forms a channel stop layer 5 by double implanting the surface of the N-type substrate 1 to block leakage current between adjacent cells. Manufacturing method.

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