KR20020048704A - Image sensor formation method capable of reducing node capacitance of floating diffusion area - Google Patents

Abstract

PURPOSE: A fabrication method of an image sensor is provided to comparably improve a sensitivity by a light energy by reducing node capacitance of a floating diffusion region. CONSTITUTION: A gate insulating layer(41) and a gate electrode(42) are firstly formed on a first conductive-type semiconductor substrate(40). A second conductive-type first impurity region(43) is secondly formed on one side of the gate electrode(42) in the substrate(40) so as to form a photodiode, then an insulating layer is formed on the entire surface of the resultant structure. Insulating spacers(44A,44B) are thirdly formed on both sidewalls of the gate electrode(42) by a tilt etching. At this time, the first insulating spacer(44A) relatively having a wide width is formed on one sidewall near to a defined floating diffusion region and the second insulating spacer(44B) relatively having a narrow width is formed on the other sidewall, thereby increasing an off-set distance between the gate electrode(42) and the floating diffusion region so that gate overlapping capacitance is reduced.

Description

Image sensor formation method capable of reducing node capacitance of floating diffusion area}

TECHNICAL FIELD The present invention relates to the field of image sensor manufacturing, and more particularly, to an image sensor manufacturing method capable of reducing the node capacitance of a floating diffusion region.

An image sensor is an apparatus that converts optical information of one or two dimensions or more into an electrical signal. The types of image sensors are broadly classified into imaging tubes and solid-state imaging devices. Imaging tubes are widely used in measurement, control, and recognition using image processing technology centered on televisions, and applied technologies have been developed. There are two types of commercially available solid-state image sensors, a metal-oxide-semiconductor (MOS) type and a charge coupled device (CCD) type.

CMOS image sensor is a device that converts an optical image into an electrical signal by using CMOS fabrication technology, and adopts a switching method in which MOS transistors are made by the number of pixels and the outputs are sequentially detected using the same. The CMOS image sensor is simpler to drive than the CCD image sensor, which is widely used as a conventional image sensor, and can realize various scanning methods, and can integrate a signal processing circuit into a single chip, thereby miniaturizing the product. The use of compatible CMOS technology reduces manufacturing costs and significantly lowers power consumption.

1 is a circuit diagram showing a unit pixel of a CMOS image sensor composed of four transistors and two capacitance structures, and a unit pixel of a CMOS image sensor composed of a photodiode (PD) as an optical sensing means and four NMOS transistors. . Of the four NMOS transistors, the transfer transistor Tx transmits a signal for transferring the photocharge generated in the photodiode PD to the floating diffusion region FD, and the reset transistor Rx supplies the floating diffusion region FD. The drive transistor Dx serves as a source follower, and the select transistor Sx receives a pixel data enable signal and receives a pixel to reset the voltage to the voltage V _DD level. It is responsible for transmitting the data signal to the output.

FIG. 2 is a cross-sectional view illustrating a structure of a photodiode, a gate electrode, and a floating diffusion region of an image sensor unit pixel as shown in FIG. 1, and a p-type epitaxial layer 21 and device isolation on a p-type semiconductor substrate 20. Forming a field oxide film (FOX) for the formation, forming a gate insulating film 22 and a gate electrode 23 on the epitaxial layer 21, and forming a photo in the epitaxial layer 21 at one end of the gate electrode 23. The n-type impurity region 24 of the diode is formed, the insulating film spacer 25 is formed on the sidewall of the gate electrode 24, and then the p of the photodiode in the epitaxial layer 21 on the n-type impurity region 22 is formed. The type impurity region 26 is formed, and the floating diffusion region 27 spaced apart from the photodiode with the gate electrode 24 interposed therebetween is shown.

Operation of the image sensor unit pixel configured as described above is performed as follows. Initially, the unit pixel is reset by turning on the reset transistor Rx, the transfer transistor Tx, and the select transistor Sx. At this time, the photodiode PD starts to deplete, and carrier accumulation occurs, and the floating diffusion region is charged and stored up to the supply voltage VDD. The transfer transistor Tx is turned off, the select transistor Sx is turned on, and the reset transistor Rx is turned off. In this operation state, after reading the output voltage V1 from the unit pixel output terminal SO and storing it in the buffer, the transfer transistor Tx is turned on to move the carriers of the capacitance Cp changed according to the light intensity to the capacitance Cf. The output voltage (V2) is read from the output terminal (Out) again and the analog data for V1-V2 is converted into digital data, so one operation cycle for the unit pixel is completed.

As the integration degree of CMOS image sensor increases, the photodiode region and floating diffusion region become smaller at the same time, and thus the sensitivity of the image sensor is lowered by decreasing the amount of electrons moving through the transfer gate to the floating diffusion region. There is this.

The present invention for solving the above problems is to provide an image sensor manufacturing method that can improve the sensitivity of the highly integrated image sensor .

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

2 is a cross-sectional view showing a photodiode region and its periphery of a conventional image sensor;

3 is an explanatory diagram showing a node capacitance component of a floating diffusion region;

4A to 4D are cross-sectional views of an image sensor manufacturing process according to an embodiment of the present invention;

5A to 5C are cross-sectional views of an image sensor manufacturing process according to another embodiment of the present invention.

* Description of reference numerals for the main parts of the drawings *

40, 50: semiconductor substrate 41, 51: gate insulating film

42, 52: gate electrode 43, 53: n-type impurity region

44: oxide film 44A, 44B, 54, 56: oxide film spacer

45, 57: n-type floating diffusion region 46, 55: p-type impurity region

The present invention for achieving the above object, the first step of forming a gate insulating film and a gate electrode on a semiconductor substrate of the first conductivity type; A second step of forming a first impurity region of a second conductivity type to form a photodiode in the semiconductor substrate at one end of the gate electrode; A third step of forming an insulating film on the entire structure; While forming the insulating film spacers on both side walls of the gate electrode by inclining the entire surface of the insulating film, a wide first insulating film spacer is formed on one side wall close to the floating diffusion region, and the other side wall is close to the photodiode region. A fourth step of forming a narrow second insulating film spacer; Forming a floating diffusion region of a second conductivity type in the semiconductor substrate at the other end of the gate electrode with the first insulating layer spacer interposed therebetween; And forming a second impurity region of a first conductivity type to form a photodiode in the semiconductor substrate above the first impurity region.

In addition, the present invention for achieving the above object, a first step of forming a gate insulating film and a gate electrode on a first conductive semiconductor substrate; A second step of forming a first impurity region of a second conductivity type to form a photodiode in the semiconductor substrate at one end of the gate electrode; A third step of forming a first impurity region of a second conductivity type forming a photodiode in the semiconductor substrate at one end of the gate electrode; Forming a first insulating film spacer on both sidewalls of the gate electrode; A fifth step of forming a second impurity region of a first conductivity type forming a photodiode in the semiconductor substrate on the first impurity region; A sixth step of forming a second insulating film spacer on the first insulating film spacer; And a seventh step of forming a floating diffusion region by implanting impurities of a second conductivity type into the semiconductor substrate at the other end of the gate electrode.

The present invention is characterized by improving the sensitivity of the image sensor by increasing the voltage change by reducing the capacitance of the n-type floating diffusion region node, which is used as a charge sensing node structure in an image sensor using CMOS. . That is, in order to move the electrons generated by the light energy in the photodiode to the floating diffusion region very effectively, as shown in Equation 1, the capacitance C _f of the floating node is reduced to make the voltage change ΔV very large. An image sensor manufacturing method can be provided.

In Equation 1, q represents the amount of charge and N represents the number of electrons to move.

3 is a cross-sectional view illustrating a structure of a photodiode of a pixel of an image sensor unit, a gate electrode of a transfer transistor, and a floating diffusion region, and a p-type epitaxial layer 31 on a p-type semiconductor substrate 30 and a field oxide film for device isolation. (FOX) is formed, the gate insulating film 32 and the gate electrode 33 are formed on the epitaxial layer 31, and the n-type photodiode in the epitaxial layer 31 at one end of the gate electrode 33 is formed. The impurity region 34 is formed, and the insulating film spacer 35 is formed on the sidewall of the gate electrode 34, and then the p-type impurity region of the photodiode is formed in the epitaxial layer 31 on the n-type impurity region 32. 36, the floating diffusion region 37 spaced apart from the photodiode with the gate electrode 34 interposed therebetween, and the various capacitance components A, B, C, which contribute to the capacitance of the floating diffusion region. D) is showing.

Among the various capacitance components A, B, C, and D contributing to the capacitance of the floating diffusion region, only the gate overlap capacitance A can be reduced in size without changing the structure of the conventional image sensor. Accordingly, the present invention reduces the capacitance of the floating diffusion region by reducing the overlap area between the gate electrode and the floating diffusion region of the transfer transistor.

Hereinafter, a method of manufacturing an image sensor according to an embodiment of the present invention will be described with reference to FIGS. 4A to 4D.

First, as shown in FIG. 4A, a field oxide film FOX is formed on a p-type semiconductor substrate 40, and a gate insulating layer 41 and a gate electrode 42 are formed on the semiconductor substrate 40. The n-type impurity region 43 of the photodiode is formed in the semiconductor substrate 40 at one end of the gate electrode 42, and the oxide film 44 is formed as an example of the insulating film on the entire structure.

Next, as shown in FIG. 4B, the oxide film 44 is inclined to be etched in the entire surface to form insulating film spacers 44A and 44B on both side walls of the gate electrode 42, and relatively to one side wall close to the floating diffusion region. A wide first oxide film spacer 44A is formed, and a relatively narrow second oxide film spacer 44B is formed on the other side wall close to the photodiode region. When the oxide film 44 is etched, the inclined etching is performed in the direction of the floating diffusion region in the photodiode region, and the inclination angle is 20 ° to 45 °.

Subsequently, as shown in FIG. 4C, a photoresist pattern PR covering the photodiode region is formed, and a high concentration of impurity ions are implanted into the semiconductor substrate 40 at one end of the relatively wide first oxide spacer 44A. An n-type floating diffusion region 45 is formed.

Next, as shown in FIG. 4D, the photosensitive film pattern PR is removed to form the p-type impurity region 46 of the photodiode in the semiconductor substrate 40 on the n-type impurity region 43.

According to one embodiment of the present invention as described above, the oxide spacer is formed on the sidewall of the transfer gate electrode close to the floating diffusion region to increase the offset distance X between the gate electrode and the floating diffusion region, thereby increasing the gate overlap capacitance. Can be reduced.

Hereinafter, a method of manufacturing an image sensor according to another exemplary embodiment of the present invention will be described with reference to FIGS. 5A to 5C.

First, as shown in FIG. 5A, a field oxide film FOX is formed on a p-type semiconductor substrate 50, and a gate insulating layer 51 and a gate electrode 52 are formed on the semiconductor substrate 50. The first photosensitive film pattern PR1 exposing the photodiode region is formed, and ion implantation is performed to form an n-type impurity region 53 in the semiconductor substrate 50 of the photodiode region.

Next, as shown in FIG. 5B, the first photoresist pattern PR1 is removed, an oxide film is formed as an example of an insulating film on the entire structure, and the surface is etched to form a first oxide film spacer 54 on the sidewall of the gate electrode 52. ), A second photoresist pattern PR2 exposing the photodiode region is formed, and an ion implantation process is performed to form the p-type impurity region 55 in the semiconductor substrate 50 on the n-type impurity region 53. Form.

Subsequently, as shown in FIG. 5C, the second photoresist pattern is removed, an oxide film is formed as an example of an insulating film on the entire structure, and the entire surface is etched to form a second oxide film spacer 56 on the first oxide film spacer 54. After forming the third photoresist pattern PR3 exposing the floating diffusion region, a high concentration impurity ion implantation is performed to form an n-type floating diffusion region 57.

Subsequently, the second photoresist pattern PR3 is removed and a subsequent process is performed.

According to another exemplary embodiment of the present invention as described above, an oxide spacer may be formed on the sidewall of the transfer gate electrode to increase the offset distance Y between the gate electrode and the floating diffusion region, thereby reducing the gate overlap capacitance.

The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible in the art without departing from the technical spirit of the present invention. It will be apparent to those of ordinary knowledge.

According to the present invention as described above, it is possible to greatly reduce the node capacitance of the floating diffusion region, thereby greatly improving the sensitivity of the image sensor due to light energy.

Claims (4)

In the image sensor manufacturing method, Forming a gate insulating film and a gate electrode on the first conductive semiconductor substrate; A second step of forming a first impurity region of a second conductivity type to form a photodiode in the semiconductor substrate at one end of the gate electrode; A third step of forming an insulating film on the entire structure; While forming the insulating film spacers on both side walls of the gate electrode by inclining the entire surface of the insulating film, a wide first insulating film spacer is formed on one side wall close to the floating diffusion region, and the other side wall is close to the photodiode region. A fourth step of forming a narrow second insulating film spacer; Forming a floating diffusion region of a second conductivity type in the semiconductor substrate at the other end of the gate electrode with the first insulating layer spacer interposed therebetween; And A sixth step of forming a second impurity region of a first conductivity type to form a photodiode in the semiconductor substrate above the first impurity region Image sensor manufacturing method comprising a. According to claim 1, The fourth step, And an oblique etching in the photodiode region toward the floating diffusion region. The method according to claim 1 or 2, In the fourth step, An etching method of manufacturing an image sensor characterized by etching at an inclination angle of 20 ° to 45 °. In the image sensor manufacturing method, Forming a gate insulating film and a gate electrode on the first conductive semiconductor substrate; A second step of forming a first impurity region of a second conductivity type to form a photodiode in the semiconductor substrate at one end of the gate electrode; A third step of forming a first impurity region of a second conductivity type forming a photodiode in the semiconductor substrate at one end of the gate electrode; Forming a first insulating film spacer on both sidewalls of the gate electrode; A fifth step of forming a second impurity region of a first conductivity type forming a photodiode in the semiconductor substrate above the first impurity region; A sixth step of forming a second insulating film spacer on the first insulating film spacer; And A seventh step of forming a floating diffusion region by implanting impurities of a second conductivity type into the semiconductor substrate at the other end of the gate electrode Image sensor manufacturing method comprising a.