KR100748345B1 - Image sensor with improved light sensitivityy and fabricating method of the same - Google Patents

Abstract

The present invention relates to an image sensor, and in particular, to provide an image sensor suitable for suppressing the generation of residual electrons in the lower portion of the photodiode and a manufacturing method thereof, the present invention comprises a first conductive semiconductor layer; A field insulating film disposed locally on the semiconductor layer; A gate electrode disposed on the semiconductor layer away from the field insulating film; A first impurity region for a photoconductor of a second conductivity type disposed under the semiconductor layer between the field insulating layer and the gate electrode, the central portion having a lower profile relatively deeper than a peripheral portion; And a second impurity region for a photodiode of a first conductivity type disposed above the first impurity region.

In addition, the present invention provides a method of manufacturing the above-described image sensor.

Isotropic profile, Rp, charge transport efficiency, charge capacity, dynamic range, image sensor, photodiode.

Description

Image sensor with improved light sensitivityy and fabricating method of the same}             

1 is a unit pixel circuit diagram of a conventional CMOS image sensor;

2 is a cross-sectional view showing an image sensor according to the prior art,

3A to 3C are cross-sectional views illustrating an image sensor manufacturing process according to an embodiment of the present invention;

4 is a cross-sectional view showing an image sensor of the present invention finally completed;

5 is a plan view of FIG. 4;

6 is a plan view of FIG. 3B;

Figure 7 is a schematic cross-sectional view comparing the charge transport efficiency according to the present invention and the prior art.

Explanation of symbols on the main parts of the drawings

40: semiconductor layer 41: field insulating film

42: gate insulating film 43: gate electrode

44: N-type impurity region for photodiode                 

45: spacer

46 P-type impurity region for photodiode

47: sensing diffusion area 100: first area

200: third region 300: second region

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor, and more particularly, to an image sensor capable of improving light sensitivity and a manufacturing method thereof.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. In a double charge coupled device (CCD), individual metal-oxide-silicon (MOS) capacitors are very different from each other. A device in which charge carriers are stored and transported in a capacitor while being located in close proximity, and CMOS (Complementary MOS) image sensor is a CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. Is a device that employs a switching method that creates MOS transistors by the number of pixels and sequentially detects the output using them.

In manufacturing such various image sensors, efforts are being made to increase the photo sensitivity of the image sensor, one of which is condensing technology. For example, a CMOS image sensor is composed of a photodiode for detecting light and a portion of a CMOS logic circuit for processing the detected light into an electrical signal to make data. To increase light sensitivity, the ratio of the photodiode to the total image sensor area is increased. Efforts have been made to increase (usually referred to as Fill Factor).

FIG. 1 is a unit pixel circuit diagram of a conventional CMOS image sensor, and a submicron CMOS Epi process is applied to increase sensitivity and reduce cross talk between unit pixels.

The unit pixel is composed of one low voltage buried photodiode and four NMOS transistors. Unlike the conventional photo gate structure, the low voltage buried photodiode has a polysilicon with a light sensing region. Not only is it covered, it has excellent light sensitivity for short wavelength blue light as well as increase the depth of depletion in the light sensing area, so the light sensitivity for long wavelength red or infrared light is also excellent. On the other hand, the low-voltage buried photodiode structure allows photogenerated charges in the photosensitive area to be completely transported to the Floating Sensing Node, which significantly increases the charge transfer efficiency. There are advantages to it.

In addition, the transfer gate (Tx), that is, the gate electrode and the reset gate (Rx), which transfer photocharges among the four transistors, is caused by a voltage drop due to a positive threshold voltage. In order to prevent the loss of charge transport efficiency, the NMOS transistor has a negative threshold voltage. In this case, the gate electrode, the reset gate, and the floating sensing node, that is, the sensing diffusion, are omitted by omitting N-LDD ion implantation. The overlap capacitance with the area FD can be reduced, so that the potential change of the floating sensing node can be amplified according to the amount of charge carried. (Δ V-ΔQ / C)

Meanwhile, the drive gate (Sx) serving as a source follower is composed of a general submicron NMOS transistor. This structure was constructed with minimal changes to the submicron CMOS Epi process, and the thermal cycle was designed to be completely unchanged. On the other hand, a color filter array composed of red, green, blue, or yellow, magenta, cyan, and the like on a unit pixel array for implementing a color image. (Color Filter Array) The process of forming.

The operation principle of obtaining an output from such a unit pixel is as follows.

end. Turn off Tx, Rx, Sx. The low voltage buried photodiode is then fully depletion.

I. Photogenerated charge is collected in a low voltage buried photo diode.

All. After a proper integration time, the Rx is turned on to reset the floating sensing node first.

la. The unit pixel is turned on by turning on Sx.

hemp. Measure the output voltage (V1) of the source follower buffer. This value simply means the CD level shift of the Floating Sensing Node (FD).

bar. Turn on Tx.

four. All photogenerated charges are shipped in FD.

Ah. Turn off Tx.

character. Measure the output voltage (V2) of the source follower buffer.

car. The output signals V1-V2 are the result of the photocharge transport resulting from the difference between V1 and V2 and are pure signal values without noise. This method is called CDS (Corelated Double Sampling).

Ka. Repeat the process of 'a' to 'tea'. However, the low voltage buried photodiode is fully depleted during the 'dead' process.

2 is a cross-sectional view showing an image sensor according to the prior art.

Referring to FIG. 2, the conventional image sensor is separated from the field insulating layer 11 and the field insulating layer 11 formed locally in the semiconductor layer 10 in a shallow trench isolation (STI) or LOCOS (LOCal Oxidation of Silicon) structure. The gate electrode 13 formed on the semiconductor layer 10, for example, the transfer gate Tx and the gate insulating layer 12 and the gate electrode 13 sidewalls formed at the contact interface between the gate electrode 13 and the semiconductor layer 10. A high concentration N-type contacting the spacer 15 formed at the lower side, the grape diode PD formed under the semiconductor layer 10 in contact with one side of the gate electrode 13 and the field insulating film 11, and the other side of the gate electrode 13. and the (n +) sensing diffusion region 17 (FD). The photodiode PD is an N-type impurity for a photodiode doped with a low concentration of N-type impurity through deep ion implantation under the semiconductor layer 10. Formed on the surface of the semiconductor layer 10 above the region 14 (hereinafter referred to as n-region) and the n-region 14. A buried photodiode including a high concentration of a photodiode (referred to as 16, or less area P0) for the P-type impurity regions.

Here, the semiconductor layer 10 includes a structure in which a high concentration P ++ substrate and a P-Epi layer are stacked, and the P-Epi layer usually has a depth of 7 μm.

On the other hand, the conventional image sensor described above has a lower profile of a rectangular structure, the photodiode during the electron transport from the photodiode PD such as 'D' to the sensing diffusion region FD during operation of the image sensor ( Electrons located outside the concave boundary such as 'X' at the bottom of the photodiode (PD) such as 'A', 'B' and 'C', which are not fully depletion in PD) The phenomenon of not being transported was confirmed by the light simulation results.

Therefore, there is a fear of the dark signal generated by these residual electrons, for example, when the image data of the current frame is processed, the residual electrons in the previous frame contribute to the image signal, thereby distorting the actual image and There is a possibility of deformation.

In addition, due to the non-depletion effect (Non fully depletion effect), the charge capacity of the image sensor is lowered, so that a larger fill factor may be obtained when the dynamic range of the same image sensor is realized. As the fill factor is required, it acts as a limiting factor in the product design, so that the increase of the area of the photodiode is inevitable.

The present invention proposed to solve the above problems of the prior art, an object thereof is to provide an image sensor and a method of manufacturing the same suitable for suppressing the generation of residual electrons in the lower portion of the photodiode.

In order to achieve the above object, the present invention, the first conductive semiconductor layer; A field insulating film disposed locally on the semiconductor layer; A gate electrode disposed on the semiconductor layer away from the field insulating film; A first impurity region for a photoconductor of a second conductivity type disposed under the semiconductor layer between the field insulating layer and the gate electrode, the central portion having a lower profile relatively deeper than a peripheral portion; And a second impurity region for a photodiode of a first conductivity type disposed above the first impurity region.

In addition, to achieve the above object, the present invention comprises the steps of: forming a gate electrode in a region away from the field insulating film on the semiconductor layer where the field insulating film is locally formed; Forming an insulating film for forming a photodiode along the profile in which the gate electrode is formed; Selectively etching the insulating layer to open a photodiode predetermined region between the field insulating layer and the gate electrode, the center portion of which exposes the surface of the semiconductor layer, and the peripheral portion of the insulating layer having an isotropic profile step; An ion implantation is performed to form a first impurity region for a photodiode of a second conductive type under the photodiode predetermined region, and the remaining profile of the insulating layer is transferred so that its center portion is relatively relative to the periphery of the lower portion of the photodiode. Deepening into; And performing ion implantation to form a second impurity region for a photodiode of a first conductivity type on a surface of the semiconductor layer above the first impurity region.

The present invention implements a photodiode having a concave profile at the bottom instead of a conventional image sensor having a rectangular profile, thereby suppressing the generation of residual electrons during image signal processing to implement a more accurate image and at the same time complete the photodiode. It is a technical feature to facilitate the depletion to have a high light sensitivity.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention. 3A to 3C are cross-sectional views illustrating an image sensor manufacturing process according to an embodiment of the present invention, FIG. 4 is a cross-sectional view illustrating an image sensor of the present invention, and FIG. 5 is a view showing FIG. 4. 6 is a plan view of FIG. 3B, and FIG. 7 is a schematic cross-sectional view comparing charge transport efficiency according to the present invention and the prior art.

Referring to FIG. 4, the image sensor of the present invention includes a P-type semiconductor layer 40 in which a high concentration P ++ substrate and a P epi layer are stacked, a field insulating film 41 disposed locally on the semiconductor layer 40, A gate electrode, for example, a transfer gate Tx and a field insulating film disposed on the semiconductor layer 40 separated from the field insulating film 41 and having a gate insulating film 42 below and a spacer 45 on the sidewall thereof. An impurity region for an N-type photodiode disposed under the semiconductor layer 40 between the gate electrode 43 and the central portion 100 having a lower profile lower than that of the peripheral portions 200 and 300. 44, hereinafter referred to as n-region, and a P-type photodiode impurity region 46 (hereinafter referred to as P0 region) disposed above the n-region 44, High concentration N-type (n +) sensing diffusion region FD disposed to be in contact with gate electrode 43 on the other side in contact with photodiode PD It includes.

In this case, the n-region 44 is formed between the first region 100 including the above-described central portion and the second region 300 including the above-described peripheral portion, and the first region 100 and the second region 300. A third region 200 having a predetermined inclination, i.e., an isotropic profile, wherein the lower profile of the n-region 44 comprising them is concave.

Therefore, it is possible to solve the problem that the electrons in the region corresponding to the lower portion of the second region 300 of the n-region 44 of the present invention mentioned in the prior art remain without being completely depleted. ) And other areas, that is, the ratio of the area between the second and third areas 300 and 200 is about 3: 2. As described above, the epi layer has a depth of about 7 μm, so The region 100 is preferably disposed at a distance of 1 μm from the epi layer in consideration of diffusion in subsequent processes.

The plan view of FIG. 5 shows the doping profile of impurities for the first to third regions 100, 200 and 300 described above, where 'Y' corresponds to an edge in a conventional rectangular structure and is not transported electrons. In the present invention, since this portion is excluded from the photodiode PD, the above-mentioned conventional problem can be overcome, and Rp (Projected range) in the first region 100 including the central portion is the largest. Able to know.

The manufacturing process of the above-described image sensor will be described later.

First, as shown in FIG. 3A, a field insulating film 31 having a shallow trench isolation (STI) or a LOCal oxide of silicon (LOCOS) structure is formed in a P-type semiconductor layer 30, where the semiconductor layer 30 is formed. Since the high concentration of the P + + layer and the P- epi layer is used as a stack, it is omitted for the sake of simplicity of the drawings.

Subsequently, the gate insulating film 32 and the gate electrode 33 are formed on the semiconductor layer 30 separated from the field insulating film 41, and then the insulating film 47 for forming the photodiode along the profile in which the gate electrode 43 is formed. To form.

Here, the lower profile of the n-region formed under the semiconductor layer 40 is determined according to the thickness of the insulating layer 47 remaining during ion implantation for subsequent n-region formation. Therefore, it is preferable to optimize the insulating film 47 in the thickness range of 1000 micrometers-1500 micrometers.

Next, as shown in FIG. 3B, when the photoresist pattern 48 is formed on the insulating film 47, the insulating film 47 corresponding to the central portion of the predetermined region where the photodiode is to be formed is opened.

FIG. 6 is a plan view of forming the photoresist pattern 48. As shown in FIG. 6, the area of the predetermined area PD-O opened in the region where the photodiode predetermined region overlaps with the insulating layer, that is, the width and the opening thereof. Instead, the area of the planned region PD-NO, ie, the width ratio, in which the insulating film 47 will remain is 3: 2.

Subsequently, the insulating film 47 is etched using the photoresist pattern 48 as an etch mask, but a wet etching process is used such that the etching profile of the insulating film 47 is isotropic, such as '49'.

Subsequently, as the photoresist pattern 38 is removed, the insulating layer is removed from the first region 100, which is the center portion of the predetermined area of the photodiode, and the surface of the semiconductor layer 40 is exposed. The insulating film 47 remains almost at the thickness at the time of formation, and the third region 200 between the first and second regions has a predetermined inclination, that is, an isotropic profile, by the above-described etching process.

Next, as illustrated in FIG. 3C, the ion implantation is performed using the ion implantation mask 50 so that the n-region 44 deep below the semiconductor layer 40 between the field insulating layer 41 and the gate electrode 43. ), The remaining profile of the insulating layer 470 is transferred so that the central portion 100 has a state deeper profile than the peripheral portions 200 and 300 in the lower profile.

Therefore, the first region 100 including the center portion is the highest in the photodiode doping profile, and the second region 300 including the peripheral portion corresponding to the rectangular edge has the lowest Rp.

Subsequently, after the ion implantation mask 50 is removed, the insulating film 47 is removed, a spacer 45 is formed on the sidewall of the gate electrode, and then a P0 region is formed on the surface of the semiconductor layer 40 on the n-region 44. To form 46.

Subsequently, by forming a high concentration N-type (n +) sensing diffusion region 47 (FD), the intermediate forming step of the image sensor as shown in FIG. 4 is completed.

The present invention described above has the following advantages by having the lower portion of the n-region have a concave shape, that is, an isotropic profile.

end. Improved charge transport efficiency and light sensitivity

That is, when the doping concentration of the n-region implanted by the ion implantation process is Qtot, as shown in FIG. 7, the amount of charge actually transported is distributed as Q1 and Q2 in the lower edge of the n-region of the photodiode in the conventional case. 'QA' becomes Qtot-Q1-Q2 excluding Q1 and Q2, and in the present invention, since QA and Q2 do not distribute charge amount and most of them are distributed in an area where charge transport is easy, QB ≒ Qtot, QB> It can be seen that the QA.

Therefore, the light sensitivity and the characteristics of the dynamic region can be improved.

I. The complete depletion of the photodiode can be made easier, thereby minimizing problems such as deformation and distortion of the image data due to residual electrons during image processing.

All. Due to the reduction of the deep n-region (near the bottom edge), leakage current components between adjacent pixels can be suppressed to improve blooming or crosstalk characteristics.

Photodiodes are the key element of an image sensor. Accordingly, the present invention, which changes the doping profile of the photodiode to facilitate complete depletion during the transfer gate operation, can be applied regardless of 0.5 μm, 0.35 μm, 0.25 μm, etc. of the image sensor to which the photodiode is applied. If the above doping profile is required even in the peripheral circuit area can be applied without any difficulty.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

The present invention described above can improve the saturation signal characteristics and dynamic range of the image sensor by widening the surface area of the photodiode occupied in the unit pixel, and ultimately, an excellent effect of greatly improving the performance of the image sensor can be expected. have.

Claims (11)

delete delete delete delete delete delete Forming a gate electrode in a region away from the field insulating film on the semiconductor layer on which the field insulating film is locally formed; Forming an insulating film for forming a photodiode along the profile in which the gate electrode is formed; Selectively etching the insulating film to open a photodiode predetermined region between the field insulating film and the gate electrode, the center portion of which exposes the surface of the semiconductor layer and the peripheral portion of the insulating layer having an isotropic profile ; An ion implantation is performed to form a first impurity region for a photodiode of a second conductive type under the photodiode predetermined region, and the remaining profile of the insulating layer is transferred so that its center portion is relatively relative to the periphery of the lower portion of the photodiode. Deepening into; And Performing ion implantation to form a second impurity region for a photodiode of a first conductivity type on a surface of the semiconductor layer above the first impurity region Image sensor manufacturing method comprising a. The method of claim 7, wherein And the insulating film comprises an oxide film. The method of claim 7, wherein The insulating film is manufactured to an image sensor of claim 1000 to 1500 두께 thickness. The method of claim 7, wherein Method of manufacturing an image sensor, characterized in that using the wet etching in the step of etching the insulating film. The method of claim 7, wherein And etching the insulating film so that an area ratio of the exposed photodiode predetermined region to the remaining insulating film is 3: 2.

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