KR20040058708A - Method for fabricating CMOS image sensor with spacer etching buffer nitride layer - Google Patents

Abstract

PURPOSE: A method for manufacturing a CMOS image sensor is provided to reduce dark current by using a spacer etch buffer nitride layer for protecting the surface of a photodiode. CONSTITUTION: A field oxide layer(32) is formed on an epitaxial layer(31) to define an active region and a field region. Gates(33a,33b) are formed on the active region. An oxide layer(35) is formed on the epitaxial layer between the field oxide layer and the gates. A buried doping region(37) for a photodiode is formed in the epitaxial layer. A spacer etch buffer nitride layer(39) is formed on the resultant structure. Then, a spacer(41) is formed at both sidewalls of the gates. The residual buffer nitride layer on the photodiode is removed.

Description

Method for fabricating CMOS image sensor with spacer etching buffer nitride layer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a CMOS image sensor. In particular, in an etching process for forming a spacer using a spacer etching buffer nitride film, a CMOS image having a dark current reduced by protecting a surface of a photodiode It relates to a method for manufacturing a sensor.

In general, an image sensor is a semiconductor device that converts an optical image into an electrical signal. A charge coupled device (CCD) is a device in which individual metal-oxide-silicon (MOS) capacitors are very close to each other. It is a device in which charge carriers are stored and transported in a capacitor while being positioned, and the CMOS image sensor uses a CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. It is a device that adopts a switching method that makes a transistor and sequentially detects output using the transistor.

As is well known, an image sensor for implementing a color image has an array of color filters on the light sensing portion that receives and receives light from the outside to generate and accumulate photocharges. The color filter array (CFA) consists of three colors: red, green, and blue, or three colors: yellow, magenta, and cyan. It is made of collar.

In addition, the image sensor is composed of a light sensing part for detecting light and a logic circuit part for processing the detected light as an electrical signal to make data. The ratio of the area of the light sensing part to the overall image sensor element is increased to increase the light sensitivity. Efforts have been made to increase the fill factor, but these efforts are limited in a limited area because the logic circuit part cannot be removed. Therefore, a condensing technology has emerged to change the path of light incident to a region other than the light sensing portion to raise the light sensitivity, and to collect the light sensing portion. For this purpose, the image sensor forms microlens on the kali filter. I'm using the method.

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. To the reset transistor 103, the drive transistor 104 to act as a source follower buffer amplifier, and the select transistor 105 to address the switching role. It is composed. Outside the unit pixel, a load transistor 106 is formed to read an output signal.

1B to 1G are cross-sectional views illustrating a manufacturing process for forming such unit pixels with a transfer transistor and a reset transistor as the center. Referring to FIG. 1B, first, as shown in FIG. 1B, a high concentration of p-type substrate 10 is shown. A low concentration p-type epi layer 11 is formed. The reason for using the low concentration epi layer 11 is to increase the depth of the depletion layer of the photodiode to improve the characteristics, and also because the high concentration substrate can prevent cross talk between unit pixels to be.

Next, a field insulating film 12 defining an active region and a field region is formed in a predetermined region of the epi layer using a thermal oxide film. Next, a gate insulating film (not shown), a gate polysilicon 13, and a tungsten silicide 14 are deposited on the active region and patterned to form gates of the transfer transistor 13a and the reset transistor 13b. Although not shown in FIG. 1B, the gate electrodes of the drive transistor Dx and the select transistor Sx are also patterned.

Next, as shown in FIGS. 1C to 1D, after forming the first mask 15 to open the region where the photodiode is to be formed, an n-type ion implantation region 16 for photodiode is performed by performing a high energy ion implantation process. ) Is formed in the epi layer 11 between the transfer transistor 13a and the field insulating film 12. A low energy ion implantation process is continuously performed to form a p-type ion implantation region 17 for photodiode between the n-type ion implantation region 16 and the surface of the epi layer. Through this process, a low voltage buried photo diode (LVBPD) is completed. Typically, an oxide film or the like is used to protect the surface of the semiconductor substrate from the ion implantation process using high energy when the ion implantation process is performed, but not illustrated in FIGS. 1C to 1D.

Next, as shown in FIGS. 1E to 1F, a spacer forming insulating film 18 is deposited on the entire structure to form a spacer 18 composed of a nitride film or an oxide film on both sidewalls of the gate electrode of the transistor. Subsequently, as shown in FIG. 1F, a front dry etching process is performed to form spacers 18 on both sidewalls of the gate electrode.

In this case, the surface of the photodiode is damaged in the front dry etching process, and defects occur in the crystal lattice structure. This defect acts as a source of dark current. The dark current is caused by electrons moving from the photodiode to the floating diffusion region even in the absence of light. The dark current is mainly caused by various defects (line defects, point defects, etc.) existing at the edge of the active region. It is reported that it originates from a bonding bond, and dark currents can cause serious problems in low illumunation environments.

Next, as shown in FIG. 1G, a second mask 19 for forming the floating diffusion region 20 and the source / drain region 21 is formed and an n-type ion implantation process is performed. Next, the normal subsequent process is performed to finish the unit pixel manufacturing process.

In the prior art, the surface of the photodiode is damaged when the entire surface is etched to form the spacer, and the dangling bonds, etc., present on the damaged silicon surface act as a source that can cause dark current. There was this.

In addition, the light incident on the photodiode passes through the insulating film (mainly an oxide film) and enters the epi layer. When light enters a material having a small reflection coefficient, such as an oxide film, into a material having a large reflection coefficient, such as an epi layer, a short wavelength such as blue light is used. There was a problem that the light has a low light sensitivity due to reflection.

The present invention is to solve the above-mentioned conventional problems, by using a spacer etching buffer nitride film to protect the surface of the photodiode in the front etching process for forming the spacer to reduce the dark current, and also to the spacer etching remaining on the photodiode surface It is an object of the present invention to provide a method for manufacturing a CMOS image sensor using a buffer nitride film to improve characteristics of short wavelength light.

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

1B to 1G are cross-sectional views illustrating a method of manufacturing a CMOS image sensor according to the prior art;

2A through 2F are cross-sectional views illustrating a method of manufacturing a CMOS image sensor according to an embodiment of the present invention;

3A to 3C are graphs showing optical characteristics when light having a short wavelength is incident on a photodiode in the CMOS image sensor according to an embodiment of the present invention.

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

30 substrate 31 epi layer

32: field smoke screen

33a: transfer transistor gate polysilicon

33b: Reset Transistor Gate Polysilicon

34: tungsten silicide 35: oxide film for ion implantation

36: first mask 37: deep n-type ion implantation region

38: p-type ion implantation region 39: spacer etching buffer nitride film

40 oxide film for spacer formation 41 spacer

42: second mask 43: floating diffusion region

44: source / drain area

The present invention for achieving the above object, in the method for manufacturing a CMOS image sensor comprising a low voltage buried photodiode and a transistor, a field insulating film defining an active region and a field region in a predetermined region of the epi layer formed on the substrate Forming a gate of a transistor on the epitaxial layer of the active region; Forming an ion implantation oxide film on an epitaxial layer between the field insulating film and the gate of the transistor and forming a doped region for the low voltage buried photodiode in an epitaxial layer underneath; Forming a spacer etch buffer nitride film on the entire structure, and forming an oxide film for forming a spacer on the spacer etch buffer nitride film; Performing surface etching to form spacers on both sidewalls of the transistor; And removing the spacer etch buffer nitride film remaining on the surface of the photodiode by wet etching, and forming a floating diffusion region on the other side of the transfer transistor.

The present invention relates to a method of manufacturing a CMOS image sensor, and more particularly, to a method of manufacturing a CMOS image sensor in which a dark current is reduced by protecting a surface of a photodiode using a spacer etching buffer nitride film during an etching process for forming a spacer. .

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 2F are views illustrating a manufacturing process of a CMOS image sensor according to an embodiment of the present invention with reference to a transfer transistor and a reset transistor, and a CMOS image sensor according to an embodiment of the present invention will be described with reference to the figure. do.

First, as shown in FIG. 2A, a low concentration p-type epitaxial layer 31 is formed on the high concentration p-type substrate 30. The reason why the low concentration epitaxial layer 31 is used is to increase the depth of the depletion layer of the photodiode to improve the characteristics, and the high concentration substrate can prevent cross talk between unit pixels. to be.

Next, an element isolation film is formed in a predetermined region of the epitaxial layer 31, and as the device isolation film, a field insulating film 32 using a thermal oxide film is formed. In an embodiment of the present invention, a field insulating film using a thermal oxide film is used as the device isolation film, but a device isolation film using a trench structure may be used.

Next, a gate insulating film (not shown), a gate polysilicon 33, and a tungsten silicide 34 are sequentially deposited on the active region, and patterned to form a gate of the transfer transistor 33a and the reset transistor 33b. . Although not shown in FIG. 2A, the gates of the drive transistor Dx and the select transistor Sx are also patterned.

Next, as shown in FIGS. 2B to 2C, an ion implantation process for forming a low voltage buried photo diode (LVBPD) is performed, and the surface of the semiconductor substrate is damaged during such an ion implantation process. In order to prevent this, an ion implantation oxide film 35 is formed on the epitaxial layer 31 on which the photodiode doped region is to be formed. The ion implantation oxide film 35 is formed to have a thickness of 100 to 500 kPa.

Next, after forming the first mask 36 to open the region where the photodiode is to be formed, a high energy ion implantation process is performed to transfer the n-type ion implantation region 37 for the photodiode to the transfer transistor 33a and the field. The epitaxial layer 31 is formed inside the epitaxial layer 31 between the insulating films 32. The low energy ion implantation process using the first mask 36 is successively formed to form a p-type ion implantation region 38 for the photodiode between the n-type ion implantation region 37 and the surface of the epi layer 31. . Through this process, a low voltage buried photo diode (LVBPD) is completed.

Next, as shown in FIG. 2D, a spacer etch buffer nitride film 39 is formed on the entire structure including the gate 33a of the transfer transistor, the gate 33b of the reset transistor, and the ion implantation oxide film 35. An oxide film 40 for forming a spacer is formed on the spacer etching buffer nitride layer 39. The spacer etch buffer nitride film 39 is formed to have a thickness of 100 to 500 kPa.

The spacer etch buffer nitride film 39 is to protect the surface of the photodiode in the process of forming the spacer 41 by the front surface of the oxide film 40 for forming a spacer, and a significant over-etching in the front surface etching process. Is performed, the spacer etching buffer nitride film 39 has a considerable etching selectivity compared with the spacer film 40 for forming spacers, so that there is no problem in protecting the surface of the photodiode.

Next, as shown in FIG. 2E, dry etching is performed on the entire surface to form oxide spacers 41 on both sidewalls of the transistor gate electrode. The remaining spacer etch buffer nitride layer 39 is then removed by wet etching. In an embodiment of the present invention, the surface of the photodiode is protected from the entire surface etching process by using the spacer etching buffer nitride film 39. Also, when the spacer etching buffer nitride film 39 is removed, the photo etching is performed by using the wet etching method. Damage to the diode surface is minimized.

Next, as shown in FIG. 2F, after forming the second mask 42 for forming the floating diffusion region and the source / drain region, an n-type ion implantation process is performed to perform the gate transistor 33a and the reset transistor of the transfer transistor. A floating diffusion region 43 is formed between the gate 33b of the transistor, and a source / drain region 44 is formed on the other side of the reset transistor. Next, the CMOS image sensor is completed by performing a normal subsequent process.

In another embodiment of the present invention, the spacer etch buffer nitride layer 39 may be left without being removed. This is to improve optical characteristics of blue light having a short wavelength. In another embodiment of the present invention, the spacer etch buffer nitride film 39 shown in FIG. 2E is left on the surface of the photodiode without being wet-etched to improve optical characteristics. As described above, a method of improving optical characteristics with respect to light having a short wavelength such as blue light by using the spacer etch buffer nitride film 39 remaining on the surface of the photodiode will be described.

3A to 3C show light incident on the photodiode when the spacer etch buffer nitride film 39 remains on the surface of the photodiode and the ion implantation oxide film 35 is formed under the spacer etch buffer nitride film 39. As a graph showing the experimental results for the transmittance of is described with reference to this.

FIG. 3A illustrates a case in which an ion implantation oxide film 35 having a thickness of 200 μs and a spacer etch buffer nitride film 39 having a thickness of 360 to 480 μs are stacked on a surface of a photodiode and in a general case according to the related art (FIG. 3A). is a graph showing the transmittance of light incident on the photodiode, and a case where light having a wavelength of 0.45 mu m or 0.55 mu m is used as the light source.

Referring to Fig. 3A, the average value of the light transmittance in the conventional general case and the average value of the light transmittance when light having a wavelength of 0.45 mu m or 0.55 mu m is used as the light source are shown. Show average value.)

Referring to the short wavelength region corresponding to the blue light, when the spacer etch buffer nitride film 39 and the ion implantation oxide film 35 are stacked, it can be seen that the light transmittance is increased compared to the conventional art. In the graph shown in FIG. 3A, the X axis represents the wavelength of light (unit is μm), and the Y axis represents the light transmittance.

FIG. 3B shows a photodiode in which the ion implantation oxide film 35 having a thickness of 300 kPa and the spacer etch buffer nitride film 39 having a thickness of 260-380 kPa are formed on the surface of the photodiode and in a general case according to the prior art. 3C is a graph showing the transmittance of light incident on the photodiode, and FIG. 3C is a case where the ion implantation oxide film 35 having a thickness of 500 mV and the spacer etch buffer nitride film 39 having a thickness of 180 m are formed on the photodiode. In the general case according to the technique, it is a graph showing the transmittance of light incident on the photodiode. 3B to 3C, light having a wavelength of 0.45 μm or 0.55 μm was used as the light source as in FIG. 3A.

3B to 3C also show the average value of the light transmittance in the conventional general case and the average value of the light transmittance when light having a wavelength of 0.45 mu m or 0.55 mu m is used as the light source. 3C, the average value is indicated by a line close to a straight line.)

Referring to FIGS. 3B to 3C, when the spacer etch nitride film 39 and the ion implantation oxide film 35 are stacked, the light transmittance is increased compared to the prior art. .

That is, referring to FIGS. 3A to 3C, when the ion implantation oxide layer 35 and the spacer etch buffer nitride layer 39 are stacked on the surface of the photodiode, light transmittance is improved for light having a short wavelength such as blue light. Can be.

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 the art.

When the present invention is applied to an image sensor, the surface of the photodiode may be damaged to act as a source of dark current, and the optical characteristic may be improved with respect to light having a short wavelength incident on the photodiode.

Claims (4)

In the method of manufacturing a CMOS image sensor comprising a low voltage buried photodiode and a transistor, Forming a field insulating film defining an active region and a field region in a predetermined region of an epi layer formed on the substrate, and forming a gate of a transistor on the epi layer of the active region; Forming an ion implantation oxide film on an epitaxial layer between the field insulating film and the gate of the transistor and forming a doped region for the low voltage buried photodiode in an epitaxial layer underneath; Forming a spacer etch buffer nitride film on the entire structure, and forming an oxide film for forming a spacer on the spacer etch buffer nitride film; Performing surface etching to form spacers on both sidewalls of the transistor; And Removing the spacer etch buffer nitride film remaining on the surface of the photodiode by wet etching, and forming a floating diffusion region on the other side of the transfer transistor Method of manufacturing a CMOS image sensor comprising a. The method of claim 1, The ion implantation oxide film is formed to a thickness of 100 ~ 500Å, the spacer etching buffer nitride film is a method of manufacturing a CMOS image sensor, characterized in that formed in a thickness of 100 ~ 500Å. The method of claim 1, The step of forming the doped region for the low voltage buried photodiode, And n-type implantation and p-type implantation are successively performed using a mask for opening the doped region for the low voltage buried photodiode. In the method of manufacturing a CMOS image sensor comprising a low voltage buried photodiode and a transistor, Forming a field insulating film defining an active region and a field region in a predetermined region of an epi layer formed on the substrate, and forming a gate of a transistor on the epi layer of the active region; Forming an ion implantation oxide film on an epitaxial layer between the field insulating film and the gate of the transistor and forming a doped region for the low voltage buried photodiode in an epitaxial layer underneath; Forming a spacer etch buffer nitride film on the entire structure, and forming an oxide film for forming a spacer on the spacer etch buffer nitride film; Performing surface etching to form spacers on both sidewalls of the transistor; And Forming a floating diffusion region on the other side of the transfer transistor Method of manufacturing a CMOS image sensor comprising a.