# Patent

## Title
Image sensor applied with device isolation technique for reducing dark signals and fabrication method thereof

## Abstract

The present invention relates to an image sensor applied with a device isolation technique for reducing dark signals and a fabrication method thereof. The image sensor includes: a logic unit; and a light collection unit in which a plurality of photodiodes is formed, wherein the photodiodes are isolated from each other by a field ion-implantation region formed under a surface of a substrate and an insulation layer formed on the surface of the substrate.

## Background
The present patent application is a Continuation Application of application Ser. No. 11/890,991, filed Aug. 8, 2007 now U.S. Pat. No. 7,507,614, which is a Divisional of application Ser. No. 10/882,846, filed Jun. 30, 2004 now abandoned.


FIELD OF THE INVENTION
The present invention relates to an image sensor; and, more particularly, to an image sensor applied with a device isolation technique for reducing dark signals and a fabrication method thereof.
DESCRIPTION OF RELATED ARTS
As known, a device isolation technology has been employed to electrically isolate individual devices such as a transistor and a capacitor during fabrication of a semiconductor integration circuit. Among various techniques of the device isolation technology, a local oxidation of silicon (LOCOS) technique and a shallow trench isolation (STI) technique are commonly adopted.
The LOCOS technique is a method of forming a nitride layer-based mask pattern on an active region of a silicon substrate and thermally oxidating the silicon substrate with use of the mask pattern as a mask. However, the LOCOS technique has disadvantages that an oxide layer is formed in a wide area and a bird's beak phenomenon occurs at an interfacial surface between the oxide layer and the silicon substrate. Thus, it is limited to apply the LOCOS technique to highly integrated devices. As a result of this limitation, the STI technique is more widely employed in highly integrated devices since the STI technique forms a device isolation region by forming a shallow trench in a substrate and then burying an oxide layer into the trench.
Meanwhile, an image sensor is a semiconductor device that converts an optical image into an electric signal. Among various types of the image sensor, a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor are widely distributed. Such an image sensor has photodiodes within a unit pixel, and the photodiodes formed in neighboring unit pixels need to be individually isolated through the use of a device isolation layer.
The device isolation layer formed by a LOCOS technique or a STI technique for isolating each photodiode is impaired by numerous defects created by stress generated at a boundary region between a field region and an active region. These defects severely affect a dark characteristic, which is one of the most important characteristics of an image sensor. Therefore, it is strongly necessary to develop an effective device isolation technology applicable to a specific characteristic of an image sensor.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an image sensor applied with a device isolation technique for reducing dark signals and a fabrication method thereof.
In accordance with an aspect of the present invention, there is provided an image sensor, including: a logic unit; and a light collection unit in which a plurality of photodiodes is formed, wherein the photodiodes are isolated from each other by a field ion-implantation region formed under a surface of a substrate and an insulation layer formed on the surface of the substrate.
In accordance with another aspect of the present invention, there is also provided a complementary metal oxide semiconductor (CMOS) image sensor, including: a logic unit; and a pixel array unit in which a plurality of photodiodes is formed, wherein the photodiodes are isolated from each other by a field ion-implantation region formed under a surface of a substrate and an insulation layer formed on the surface of the substrate.
In accordance with still another aspect of the present invention, there is also provided a method for forming a device isolation structure of an image sensor including a light collection unit and a logic unit, including the steps of: forming an insulation layer pattern on a substrate in a field region of the light collection unit; forming an insulation layer for device isolation in a field region of the logic unit by performing one of a LOCOS technique and a STI technique; and forming a field ion-implantation region under a surface of the substrate in the field region of the light collection unit.



BRIEF DESCRIPTION OF THE DRAWING(S)
The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross-sectional view showing a device isolation structure in an image sensor for reducing dark signals in accordance with a first preferred embodiment of the present invention;
FIG. 2 is a cross-sectional view showing a complementary semiconductor metal oxide semiconductor (CMOS) image sensor having a device isolation structure in accordance with a preferred embodiment of the present invention; and
FIGS. 3A to 3F are cross-sectional views illustrating a method for forming a device isolation structure in an image sensor in accordance with the preferred embodiment of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view showing a device isolation structure in an image sensor in accordance with a preferred embodiment of the present invention.
As shown, the image sensor is divided into a light collection unit A in which photodiodes are formed and a logic unit B in which typical complementary metal oxide semiconductor (CMOS) devices are formed. In the light collection unit A, there is a plurality of photodiodes 10A and 10B which generate photo-charges after collecting lights. Herein, the reference numbers 10A and 10B will be referred to as a first and a second photodiodes, respectively. A field ion-implantation region 20 is formed beneath a predetermined surface of a silicon substrate 100 disposed between the first photodiode 10A and the second photodiode 10B. An insulation layer 30 is formed on the predetermined surface of the silicon substrate 100. The field ion-implantation region 20 and the insulation layer 30 isolate the adjacently allocated first and second photodiodes 10A and 10B. If the silicon substrate 100 is a first conductive type, the first and the second photodiodes 10A and 10B are a second conductive type and the field ion-implantation region 20 is the first conductive type. Also, in the logic unit B, a conventional local oxidation of silicon (LOCOS) technique and/or a shallow trench isolation (STI) technique is performed to form an insulation layer 60, which is a device isolation layer.
As described above, in order to reduce dark signals of an image sensor, the light collection unit A is applied with a specialized device isolation technique, while the logic unit B is applied with the conventional LOCOS and/or STI technique. More specifically, the LOCOS technique and the STI technique are compulsorily accompanied with a thermal oxidation process. Thus, defects are created by stress generated at an interfacial surface between a field region and a device isolation region, thereby resulting in a problem of dark signals. However, in this preferred embodiment, generation of dark signals is reduced since the device isolation layer is formed by performing the ion-implantation process which does not require the thermal oxidation process and depositing the insulation layer.
FIG. 2 is a cross-sectional view showing a CMOS image sensor with a device isolation structure in accordance with the preferred embodiment of the present invention.
As shown, the CMOS image sensor is divided into a pixel array unit C in which photodiodes are formed and a logic unit D in which typical CMOS devices are formed. In the pixel array unit C, a photodiode 10 for generating photo-charges after collecting lights and a transfer gate Tx for transferring the photo-charges generated from the photodiode 10 to a sensing node 40 are formed. Also, a field ion-implantation region 20 and an insulation layer 30 isolate the devices in the pixel array unit C, while a device isolation layer 60A formed in the logic unit D by employing a STI technique or a LOCOS technique isolates the typical CMOS devices from each other. Hence, it is possible to reduce the number of defects generated at an interface 200 between a field region and an active region and further to reduce the number of dark signals.
Meanwhile, in consideration of process specifications such as ion-implantation energy, the insulation layer 30 for device isolation should have a predetermined thickness. That is, ion-implantation energy specified in a currently applied image sensor process is determined under consideration of a thickness of a device isolation layer formed by a LOCOS technique. Thus, the insulation layer 30 should be formed with a predetermined thickness identical to the thickness of a field oxide layer growing towards an upper surface of a silicon substrate through the use of a LOCOS technique in order not to change subsequent process conditions such as ion-implantation energy. Also, when the above specific device isolation structure is formed on the basis of the preferred embodiment, it is necessary to minimize changes in currently applied process conditions. Detailed descriptions on a method for forming the specific device isolation structure without changing the currently applied process conditions will be provided in the following.
FIGS. 3A to 3F are cross-sectional views illustrating a method for forming the device isolation structure in an image sensor in accordance with the preferred embodiment of the present invention.
Referring to FIG. 3A, an oxide layer 302 and a nitride layer 303 are deposited on a silicon substrate 301. Herein, the oxide layer 302 is deposited to a thickness ranging from about 2000 Å to about 2500 Å, and the nitride layer 303 is deposited to a thickness ranging from about 1000 Å to about 1500 Å. A first device isolating mask pattern 304 for opening an active region of a light collection unit and an entire region of a logic unit is formed on the above resulting structure.
Specifically, the nitride layer 303 is a layer used to make sidewalls of a subsequently formed oxide layer pattern inclined. Also, the oxide layer 302 is grown through a thermal oxidation process, which makes impurities existing within the silicon substrate 301 diffused externally. As a result of this external diffusion, it is possible to obtain a gettering effect. Also, the oxide layer 302, which will be used as the insulation layer 30 for device isolation as depicted in FIG. 1, should be sufficiently thick in consideration of subsequent processes, e.g., an ion-implantation process. For instance, since a typical thickness of a field oxide layer grown towards an upper surface of a silicon substrate by employing a LOCOS technique is about 2200 Å, it is required to form the oxide layer 302 at least with such thickness. The first device isolating mask pattern 304 is a photoresist PR formed by a conventional photolithography process.
FIGS. 3B and 3C are cross-sectional views showing an etched state of the nitride layer 303 and the oxide layer 302 in the opened active region by using the first device isolating mask pattern 304. Particularly, FIG. 3B is a cross-sectional view showing the etched state of the nitride layer 303 and the oxide layer 302 after a dry etching process. On the other hand, FIG. 3C is a cross-sectional view showing the etched state of the nitride layer 303 and the oxide layer 302 after a wet etching process.
Preferably, the dry etching process is performed such that a lateral profile of the oxide layer 302 is patterned to have an inclination in an angle of about 60°, which is denoted as the reference symbol B in FIG. 3B. The reason for this inclination is to prevent generation of remnants in a conductive layer, e.g., a polysilicon layer for use in a gate electrode. Also, it is preferable to form an undercut, denoted as the reference symbol A, at a bottom portion of each sidewall of the nitride layer 303 patterned by the dry etching process. The reason for forming the undercut is to round an upper part of the oxide layer 302 during the subsequent wet etching process. In addition, during the dry etching process, the oxide layer 302 is made to remain in a thickness ranging from about 600 Å to about 700 Å in order to prevent the silicon substrate 301 from being damaged by a plasma used during the dry etching process. This remaining oxide layer 302 is denoted as the reference symbol C in FIG. 3B.
With reference to FIG. 3D, the first device isolating mask pattern 304 and the nitride layer 303 are removed. Then, a pad oxide layer 305 and a pad nitride layer 306 are deposited for device isolation in the logic unit. At this time, the pad oxide layer 305 has a thickness ranging from about 100 Å to about 200 Å, while the pad nitride layer 306 has a thickness ranging from about 1000 Å to about 1500 Å. Thereafter, a second device isolating mask pattern 307 opening a field region in the logic unit is formed on the above resulting structure.
Referring to FIG. 3E, the second device isolating mask pattern 307 is removed, and a field insulation layer 308 is grown in the logic unit by performing a thermal process. The pad nitride layer 306 and the pad oxide layer 305 are removed thereafter.
FIGS. 3D and 3E disclose the steps of forming the device isolation layer in the field region of the logic unit by employing the conventional LOCOS technique. However, in addition to the LOCOS technique, it is still possible to form the device isolation layer in the logic unit by a conventional STI technique.
Referring to FIG. 3F, a selective ion-implantation process is performed only to the field region in which the oxide layer 302 remains in the light collection unit. From the selective ion-implantation process, a field ion-implantation region 309 is formed under a surface of the silicon substrate 301 disposed below the oxide layer 302 in the light collection unit.
On the basis of the preferred embodiment of the present invention, the light collection unit is applied with a specialized device isolation technique for forming the field ion-implantation region and the oxide layer. On the contrary, the logic unit is applied with the conventional LOCOS and/or STI technique. As a result, it is possible to prevent defect generations caused by stress in the light collection unit, thereby reducing dark signals. Therefore, it is further possible to improve a low light level characteristic of an image sensor and increase yields. Moreover, since the device isolation structure can be obtained with minimum changes in the conventionally employed processes, there is an additional effect of shortening periods and reducing costs for development and mass-production of image sensors.
The present application contains subject matter related to the Korean patent application No. KR 2003-0091843, filed in the Korean Patent Office on Dec. 16, 2003, the entire contents of which being incorporated herein by reference.
While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

## Claims

1. A method for fabricating an image sensor, the method comprising:
forming an insulation layer pattern on a substrate in a light collection unit region of the image sensor;
forming an insulation layer configured to provide device isolation in a logic unit region of the image sensor; and
forming a field ion-implantation region under a surface of the substrate in the light collection unit region.

2. A method for fabricating an image sensor, the method comprising:
forming an oxide layer on both a logic unit portion and a pixel array unit portion of the image sensor such that the oxide layer formed on the logic unit portion has a first thickness and the oxide layer formed on the pixel array unit portion has a different second thickness;
forming an ion-implantation region under the oxide layer in the pixel array unit portion; and
forming an insulation layer in the logic unit portion.

3. The method of claim 1, wherein said forming an insulation layer pattern comprises forming the insulation layer pattern with inclined sidewalls.

4. The method of claim 1, wherein said forming an insulation layer pattern comprises:
stacking an oxide layer and a nitride layer on the substrate;
forming a mask pattern on the nitride layer, wherein the mask pattern opens an active region of the light collection unit region;
applying a dry etching process to the nitride layer and the oxide layer while using the mask pattern as an etch mask;
after said applying a dry etching process to the nitride layer and the oxide layer, applying a wet etching process to the nitride layer and the oxide layer while using the mask pattern as an etch mask; and
removing the mask pattern and the nitride layer.

5. The method of claim 4, wherein said applying a dry etching process to the nitride layer and the oxide layer forms an undercut at a bottom portion of each sidewall of the nitride layer.

6. The method of claim 4, wherein said applying a dry etching process to the nitride layer and the oxide layer results in the oxide layer having a thickness ranging from about 600 Å to about 700 Å.

7. The method of claim 4, further comprising using a thermal process to grow the oxide layer.

8. The method of claim 2, wherein said forming an oxide layer comprises:
forming a nitride layer on the oxide layer;
forming an isolation mask pattern only on the nitride layer formed over the pixel array unit portion;
etching the oxide layer and the nitride layer using the isolation mask pattern; and
removing the isolation mask pattern and the nitride layer.

9. The method of claim 2, wherein said forming an ion-implantation region comprises using a selective ion-implantation process to form the ion-implantation region.

10. The method of claim 2, further comprising:
forming a plurality of photodiodes in the pixel array unit portion, wherein said forming an ion-implantation region comprises using a selective ion-implantation process to isolate the plurality of photodiodes from each other.

11. The method of claim 2, further comprising:
forming the logic unit portion and the pixel array unit portion in a substrate having a first conductive type; and
forming a plurality of photodiodes having a second conductive type in the pixel array unit portion, wherein said forming an ion-implantation region comprises using a selective ion-implantation process to form the ion-implantation region having the first conductive type to isolate the plurality of photodiodes from each other.

12. The method of claim 2, wherein said forming an insulation layer in the logic unit portion comprises:
forming a pad oxide layer in the logic unit portion;
forming a pad nitride layer on the pad oxide layer;
forming an isolation mask pattern on the pad nitride layer;
using the isolation mask pattern to form the insulation layer via a thermal process; and
removing the pad oxide layer and the pad nitride layer.

13. The method of claim 2, wherein said forming an insulation layer in the logic unit portion comprises:
forming a pad oxide layer in the logic unit portion;
forming a pad nitride layer on the pad oxide layer;
forming an isolation mask pattern on the pad nitride layer;
using the isolation mask pattern to form the insulation layer via a local oxidation of silicon (LOCOS) technique; and
removing the pad oxide layer and the pad nitride layer.

14. The method of claim 2, wherein said forming an insulation layer in the logic unit portion comprises:
forming a pad oxide layer in the logic unit portion;
forming a pad nitride layer on the pad oxide layer;
forming an isolation mask pattern on the pad nitride layer;
using the isolation mask pattern to form the insulation layer via a shallow trench isolation (STI) technique; and
removing the pad oxide layer and the pad nitride layer.

15. The method of claim 2, wherein said forming an insulation layer in the logic unit portion comprises using a thermal process to grow an oxide layer of the insulation layer.

16. The method of claim 15, wherein the oxide layer of the insulation layer is grown to a thickness that accounts for ion-implantation energy to be used by a subsequent ion-implantation process.

17. A method for fabricating an image sensor, the method comprising:
forming a plurality of devices in a logic unit portion of a substrate for the image sensor;
forming a plurality of photodiodes in a light collection portion of the substrate;
using a thermal oxidation process to isolate devices from the plurality of devices;
after said using a thermal oxidation process, using an ion-implantation process to isolate photodiodes from the plurality of photodiodes.

18. The method of claim 17, wherein said using a thermal oxidation process comprises forming an insulation layer pattern having inclined sidewalls.

19. The method of claim 18, wherein said forming an insulation layer pattern comprises:
stacking an oxide layer and a nitride layer on the substrate;
forming a mask pattern on the nitride layer, wherein the mask pattern opens an active region of the light collection portion;
applying a dry etching process to the nitride layer and the oxide layer while using the mask pattern as an etch mask;
after said applying a dry etching process to the nitride layer and the oxide layer, applying a wet etching process to the nitride layer and the oxide layer while using the mask pattern as an etch mask; and
removing the mask pattern and the nitride layer.

20. The method of claim 19, wherein said applying a dry etching process to the nitride layer and the oxide layer forms an undercut at a bottom portion of each sidewall of the nitride layer.

21. The method of claim 19, wherein said applying a dry etching process to the nitride layer and the oxide layer results in the oxide layer having a thickness ranging from about 600 Å to about 700 Å.

22. The method of claim 17, wherein:
the substrate has a first conductive type;
the plurality of photodiodes have a second conductive type; and
an ion-implantation region formed by the ion-implantation process has the first conductive type and isolates the plurality of photodiodes from each other.