KR20040058692A - CMOS image sensor with shield layer protecting surface of photo diode and method for fabricating thereof - Google Patents

Abstract

PURPOSE: A CMOS(Complementary Metal Oxide Semiconductor) image sensor and a manufacturing method thereof are provided to protect a surface of a photodiode by forming sequentially an oxide layer and a nitride layer on the photodiode. CONSTITUTION: A CMOS image sensor includes a heavily doped p-type substrate, a lightly doped p-type epitaxial layer, a doping region for a photodiode, an oxide layer and a nitride layer. The epitaxial layer(22) is formed on the substrate(21). The doping region for a photodiode is formed in the epitaxial layer. The doping region includes an n-type ion-implanted region(27) and a p-type ion-implanted region(29). The oxide layer(24) and the nitride layer(25) are sequentially formed on the epitaxial layer corresponding to the doping region to prevent surface damage of the doping region.

Description

CMOS image sensor with shield layer protecting surface of photo diode and method for fabricating

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CMOS image sensor and a method of manufacturing the same. In particular, an oxide film and a nitride film are laminated and formed on an epitaxial layer including a photodiode doped region, thereby preventing the surface of the photodiode from damage generated in various subsequent steps. The present invention relates to a CMOS image sensor and a method for manufacturing the same, wherein the protection prevents the occurrence of an electronic trap and improves the characteristics in low light.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. Among them, a charge coupled device (CCD) includes individual metal-oxide-silicon (MOS) capacitors. A device in which charge carriers are stored and transported in a capacitor while being in close proximity to each other. Complementary MOS image sensors use CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. A device employing a switching scheme that creates MOS transistors as many as pixels and sequentially detects outputs using the MOS transistors.

CCD (charge coupled device) has many disadvantages such as complicated driving method, high power consumption, high number of mask process steps, complicated process, and difficult to implement signal processing circuit in CCD chip. In order to overcome such drawbacks, the development of a CMOS image sensor using a sub-micron CMOS manufacturing technology has been studied in recent years. The CMOS image sensor forms an image by forming a photodiode and a MOS transistor in a unit pixel and sequentially detects signals in a switching method, and implements an image by using a CMOS manufacturing technology, which consumes less power and uses 30 to 40 masks as many as 20 masks. Compared to CCD process that requires two masks, the process is very simple, and it is possible to make various signal processing circuits and one chip, which is attracting attention as the next generation image sensor.

FIG. 1A is a circuit diagram showing a unit pixel composed of one photodiode PD and four MOS transistors in a conventional CMOS image sensor, and includes a photodiode 100 for generating photocharges by receiving light. The transfer transistor 101 for transporting the photocharges collected from the photodiode 100 to the floating diffusion region 102 and resets the floating diffusion region 102 by setting the potential of the floating diffusion region to a desired value and discharging electric charges. A reset transistor (103), a drive transistor (104) serving as a source follower buffer amplifier, and a select transistor (105) for addressing (switching). It is composed. Outside the unit pixel, a load transistor 106 is formed to read an output signal.

FIG. 1B illustrates a cross-sectional structure of a photodiode and a transfer transistor 101 in the unit pixel of the image sensor illustrated in FIG. 1A. The gate electrode 16 of the transfer transistor among the four transistors constituting the unit pixel is shown in FIG. ) And the remaining transistors are not shown.

Referring to this point, first, a low concentration p-type epitaxial layer 12 epitaxially grown on a relatively high concentration p-type semiconductor substrate 11 is shown, and inside the p-type epitaxial layer 12 A field oxide film 13 defining an active region and a field region is formed using a trench structure. Spacers 17 are formed on both sidewalls of the gate electrode 16 of the illustrated transfer transistor.

The p-type ion implantation region 14 constituting the p / n / p-type photodiode has one side aligned with the spacer 17 and the other side aligned with the device isolation layer 13 so as to be constant from the surface of the p-type epilayer 12. The n-type ion implantation region 15 is formed deeply in the epi layer below the p-type ion implantation region 14, and one side of the n-type ion implantation region 15 is the gate electrode 16. The other side is aligned with the device isolation film 13. As described above, the p-type ion implantation region 14 formed near the surface of the semiconductor substrate, the n-type ion implantation region 15 and the p-type epitaxial layer 12 located below the p / n / p photodiode are formed. It will play a role.

In the conventional image sensor configured as described above, there is a problem that the surface A of the epi layer corresponding to the photodiode doping region is damaged in various subsequent steps. That is, an etching process for patterning the gate electrode 16 of the transfer transistor, a front etching process for forming the spacers 17, an ion implantation process for forming the doped regions 14 and 15 for photodiodes, In the chemical cleaning process performed after various etching processes, the surface A of the epi layer corresponding to the photodiode doping region is damaged, causing a dislocation in the silicon lattice structure existing on the surface A of the epi layer. It was.

Since the misalignment part of the silicon lattice structure serves as an electron trap for trapping electrons, it serves as a main source of deterioration of low light characteristics of the CMOS image sensor.

That is, in the low light environment, since the amount of light incident on the photodiode is small, the amount of charge photoelectrically converted in the photodiode must be small, but the electrons trapped in the electron trap as described above are transferred to the image via the transfer transistor 16. By being used for reproduction, the characteristics of the image sensor are degraded in a low light environment.

The present invention is to solve the above-described problems, the CMOS image sensor to improve the low light characteristics by reducing the damage of the surface of the photodiode by performing a subsequent process after the oxide film and the nitride film is laminated and formed on the epi layer surface And it aims to provide a manufacturing method thereof.

1A is a circuit diagram showing a unit pixel of a CMOS image sensor composed of four transistors and a photodiode;

Figure 1b is a cross-sectional view showing the configuration of a unit pixel around the photodiode and the transfer transistor in the CMOS image sensor according to the prior art,

Figures 2a to 2e is a cross-sectional view showing a manufacturing process of the CMOS image sensor according to an embodiment of the present invention.

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

21: p-type substrate

22: p-type epi layer

23 trench isolation film

24: oxide film

25: nitride film

26: gate of the transfer transistor

27: n-type ion implantation region

28: spacer

29: p-type ion implantation region

30: floating diffusion area

The present invention for achieving the above object, a relatively high concentration of the first conductivity type substrate; A low concentration first conductive epitaxial layer formed on the substrate; A doped region for photodiode formed in the epi layer; An oxide film formed on the epi layer corresponding to the doped region for the photodiode; And a nitride film formed on the oxide film.

The present invention also provides a method for manufacturing a CMOS image sensor comprising a photodiode and a transfer transistor, comprising: forming an epitaxial layer of a first conductivity type having a low concentration on a relatively high concentration of a first conductivity type substrate; Forming a trench isolation layer in the epi layer; Stacking an oxide film and a nitride film on the epi layer; Removing the oxide film and the nitride film of a predetermined region and forming a transfer transistor on the epi layer; And forming a doped region for photodiode between the device isolation layer and the transfer transistor.

According to the present invention, in a CMOS image sensor using a relatively high concentration of substrate and a low concentration of epi layer, an oxide film and a nitride film are laminated and formed on the epi layer, and the surface of the photodiode is protected from damage to protect the surface of the photodiode from damage. It is an improved invention. That is, the surface of the photodiode is protected from damage by performing a gate polysilicon etching process, a spacer forming process, an ion implantation process, and a cleaning process in a state where an oxide film and a nitride film are stacked on the epitaxial layer.

Hereinafter, the most preferred embodiments of the present invention will be described with reference to the accompanying drawings so that those skilled in the art can easily implement the technical idea of the present invention.

2A to 2E illustrate a structure of a transfer pixel and a photodiode among unit pixels including four transistors and one photodiode. In the CMOS image sensor, four transistors and A unit pixel array is formed by providing a plurality of unit pixels composed of one photodiode.

Referring to FIG. 2E, the CMOS image sensor according to the exemplary embodiment of the present invention has a relatively high concentration of the p-type substrate 21 and a low concentration of the p-type epi layer 22 formed on the p-type substrate 21. And a trench isolation layer 23 formed in a predetermined region of the p-type epitaxial layer 22 to define an active region and a field region, and formed on the p-type epitaxial layer 22, and the p-type epitaxial layer 22. An oxide film 24 exposing a portion of the oxide film, a nitride film 25 formed on the oxide film 24, a gate electrode 26 of a transfer transistor formed on the exposed p-type epi layer 22, and a transfer transistor gate. Spacers 28 disposed on both sidewalls of the electrode 26, one side of which is aligned with the spacer 28, and the other side of which is aligned with the device isolation layer 23, from the surface of the p-type epi layer 22 toward the semiconductor substrate 21. P0 ion implantation region 29 formed to be expanded and one side thereof are aligned with transfer transistor gate electrode 26 Side, is arranged on the isolation film (23) comprises a floating diffusion region 30 formed at the other side of the n-type ion-implanted region 27 and a transfer transistor 26 formed on a lower p0 ion implantation area (29).

Referring to FIG. 2E, in the structure of the unit pixel of the conventional CMOS image sensor, it can be seen that an oxide film 24 and a nitride film 25 are further stacked and formed on the epi layer 22. ) And the nitride film 25 are formed on the epi layer 22 immediately after the trench device isolation film 23 is formed, and remain even during the ion implantation process to form source / drain regions of each transistor constituting the unit pixel. To prevent damage to the surface of the photodiode. In one embodiment of the present invention, the surface of the photodiode refers to the surface of the epi layer 22 in which the p-type ion implantation region 29 is formed.

However, since the plurality of transistors including the transfer transistor in the unit pixel should have a gate oxide film (not shown) having a predetermined characteristic, the surface of the epi layer 22 corresponding to the region where the gate polysilicon of each transistor is to be formed is formed. The silver oxide film 24 and the nitride film 25 were removed and exposed.

The oxide layer 24 and the nitride layer 25 formed on the epitaxial layer 22 surface in this manner may be etched with the gate polysilicon 26, the front side etching process for forming the spacer 28, and the doped region 27 for photodiode. In the ion implantation step for forming 29) and the cleaning step performed after various etching processes, etc., the photodiode remains on the surface of the photodiode, thereby protecting the surface of the photodiode from damage.

In one embodiment of the present invention, the oxide film 24 was formed to have a thickness of 100 to 300 kPa, and the nitride film 25 was formed to have a thickness of 500 to 1000 kPa.

Next, a method of manufacturing a CMOS image sensor according to an embodiment of the present invention will be described with reference to FIGS. 2A to 2E. 2A through 2E are cross-sectional views illustrating a method of manufacturing a CMOS image sensor in accordance with an embodiment of the present invention, focusing on a photodiode and a transfer transistor. Referring to this, first, as shown in FIG. The low concentration p-type epitaxial layer 22 is grown on the type semiconductor substrate 21.

As shown in FIG. 2A, the reason for using the low concentration p-type epitaxial layer 22 on the high concentration p-type substrate 21 is as follows: First, since there is a low concentration p epi layer, a depletion region of the photodiode is used. Can be increased greatly and deeply to increase the photodiode's ability to collect photocharges. Second, when the p-type epitaxial layer 22 has a high concentration of p ⁺ substrate 21, photocharge irregularities because the charge is quickly recombined before the charge is diffused to neighboring pixel units. It is possible to reduce the change in the transfer function of the photocharge by reducing the diffusion (Random Diffusion).

Next, as shown in FIG. 2A, a trench isolation layer 23 defining an active region and a field region is formed in a predetermined region of the p-type epitaxial layer 22.

Subsequently, as shown in FIG. 2B, an oxide film 24 and a nitride film 25 are laminated on the entire structure including the trench isolation film 23. The oxide film 24 is formed to have a thickness of 100 to 300 GPa, and the nitride film is formed to have a thickness of 500 to 1000 GPa.

Next, as shown in FIG. 2C, a portion of the stacked oxide film 24 and nitride film 25 are removed to expose the surface of the p-type epi layer 22. This is for forming the gate oxide film and the gate polysilicon of the transfer transistor, and since a gate insulating film having a predetermined specificity is required for proper operation of the transistor, a gate insulating film (not shown) and a gate are formed on the exposed epitaxial layer 22. Polysilicon 26 is formed in turn. Conventionally, in the etching process of patterning the formed gate polysilicon into a desired shape, there is a problem in that the surface of the photodiode is damaged, but in one embodiment of the present invention, the oxide film 24 and the nitride film 25 cover the surface of the photodiode. Therefore, damage can be prevented.

Next, as shown in FIG. 2D, an n-type ion implantation region 27 for photodiode is formed. Since one side of the n-type ion implantation region 27 is aligned with the gate 26 of the transfer transistor and the other side is aligned with the device isolation layer 23, an ion implantation mask (not shown) corresponding thereto is manufactured to produce an n-type ion implantation region ( 27). The ion implantation process may also damage the surface of the photodiode, but the damage is prevented due to the presence of the oxide film 24 and the nitride film 25 as described above.

Subsequently, as shown in FIG. 2E, spacers 28 are formed on both sidewalls of the gate electrode 26. A spacer is generally formed by applying an insulating film for spacers and etching the entire surface. The surface of the photodiode may be damaged in such an entire surface etching process, but in one embodiment of the present invention, since the oxide film 24 and the nitride film 25 cover the surface of the photodiode, such damage may be prevented.

Next, the p-type ion implantation region 29 for photodiode is formed, and the p-type ion implantation region 29 is aligned with the spacer 28 and the other side is aligned with the device isolation layer 23. (Not shown) is fabricated to form the p-type ion implantation region 29.

Similarly, since the oxide film 24 and the nitride film 25 cover the surface of the photodiode, the surface of the photodiode can be prevented from being damaged during the ion implantation process for forming the p-type ion implantation region 29. Next, the floating diffusion region 30 is formed on the other side of the transfer transistor.

After the various etching processes described above are performed, the cleaning process may be performed. The cleaning process may also damage the fine crystal lattice on the surface of the photodiode. However, in one embodiment of the present invention, since the oxide film 24 and the nitride film 25 cover the surface of the photodiode, damage can be prevented.

As described above, the present invention is not limited to the above-described embodiments and the accompanying drawings, and the present invention may be variously substituted, modified, and changed without departing from the spirit of the present invention. It will be apparent to those of ordinary skill in Esau.

When the present invention is applied to the CMOS image sensor, it is possible to prevent damage to the surface of the photodiode due to various processes, and to suppress the occurrence of electronic traps, thereby improving the characteristics of the image sensor at low illumination.

Claims (6)

A relatively high concentration of a first conductivity type substrate; A low concentration first conductive epitaxial layer formed on the substrate; A doped region for photodiode formed in the epi layer; An oxide film formed on the epi layer corresponding to the doped region for the photodiode; And Nitride film formed on the oxide film CMOS image sensor comprising a. A relatively high concentration of the first conductivity type substrate and a low concentration of the first conductivity type epi layer formed on the substrate; A trench isolation layer formed in the epi layer; A transfer transistor formed on the epi layer; A doped region for photodiode formed in the epi layer between the device isolation layer and the transfer transistor; An oxide film formed on the epi layer except for a region where the transfer transistor is formed; And Nitride film formed on the oxide film CMOS image sensor comprising a. The method of claim 2, The oxide film has a CMOS image sensor, characterized in that having a thickness of 100 ~ 300Å. The method of claim 2, The nitride film has a thickness of 500 ~ 1000Å CMOS sensor. In the method of manufacturing a CMOS image sensor comprising a photodiode and a transfer transistor, Forming an epitaxial layer of a low concentration of the first conductivity type on a relatively high concentration of the first conductivity type substrate; Forming a trench isolation layer in the epi layer; Stacking an oxide film and a nitride film on the epi layer; Removing the oxide film and the nitride film of a predetermined region and forming a transfer transistor on the epi layer; And Forming a doped region for photodiode between the device isolation layer and the transfer transistor Method of manufacturing a CMOS image sensor comprising a. The method of claim 5, wherein In the step of forming a doped region for the photodiode, A method of manufacturing a CMOS image sensor, characterized in that the ion implantation for forming the doped region for the photodiode is carried out through the oxide film and the nitride film.