# Patent

## Title
Backside illuminated image sensor

## Abstract

A backside illuminated image sensor includes a photodiode, formed below the top surface of a semiconductor substrate, for receiving light illuminated from the backside of the semiconductor substrate to generate photoelectric charges, a reflecting gate, formed on the photodiode over the front upper surface of the semiconductor substrate, for reflecting light illuminated from the backside of the substrate and receiving a bias to control a depletion region of the photodiode, and a transfer gate for transferring photoelectric charges from the photodiode to a sensing node of a pixel.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention claims priority of Korean patent application number 2007-0065368, filed on Jun. 29, 2007, which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
The present invention relates to an image sensor, and more particularly, to a backside illuminated image sensor, which can be applied to an active pixel sensor (APS) such as a complementary metal oxide semiconductor (CMOS) image sensor or a charge coupled device (CCD) image sensor.
An image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor includes a light receiving part (generally referred to as a photodiode) for sensing light, and a logic circuit part for processing the sensed light into electrical signals.
A pixel of an image sensor includes a photodiode for receiving light to generate photoelectric charges, and a charge transfer gate for transferring the photoelectric charges to a sensing node of the pixel.
A conventional image sensor has a front illumination configuration, in which a photodiode is formed below a substrate surface and logic circuits are formed on the substrate, so that light is illuminated on the top surface of the substrate. However, the photo response characteristic of the photodiode is poor because multiple upper layers formed on the photodiode cause light loss. Also, it is difficult to covert incident light flux into photoelectric charges because the penetration depth of photons is large.
To overcome these limitations, a backside illuminated image sensor has been proposed which illuminates a substrate from the rear surface thereof.
FIG. 1 is a sectional view of a conventional backside illuminated image sensor disclosed in US Patent Publication No. 2006-0068586A1.
Referring to FIG. 1, an n-well 120 serving as a junction cathode is formed on a p-type silicon 130 by performing a predetermined process on a silicon-on-insulator (SOI) wafer having a silicon/buried oxide/silicon structure, and a logic circuit (not shown) and metal lines 150 are formed thereon. A supporting substrate 140 is attached, and the silicon on the rear surface of the SOI wafer is polished until the buried oxide layer. An anti-reflecting layer 220 and a microlens 230 are formed on the resulting structure. Therefore, photons are incident from the rear surface of the substrate. The conventional backside illuminated image sensor further includes a p-type ion injection region 125 for preventing cross-talk, and first and second insulation layers 160A and 160B.
However, a metal reflector 240 must be separately provided at a position corresponding to the photodiode, in order to reduce signal loss caused when long-wavelength light passes through the silicon (substrate) whose thickness is reduced. Therefore, a metal process must be added or limitations must be imposed on a metal layout.
Additionally, in the convention backside illuminated image sensor, the internal potential of the photodiode is determined according to processing conditions (doping concentrations, depths, etc.) and the width of the depletion region is determined. Accordingly, when a depletion region is not formed near the rear surface of the substrate, the cross-talk is generated. That is, photoelectric charges generated around the rear surface of the substrate do not reach the depletion region of the photodiode and move to a neighboring pixel. Also, sensitivity to short wavelength is poor.
SUMMARY OF THE INVENTION
Embodiments of the present invention are directed to providing a backside illuminated image sensor that does not require a separate metal reflective layer.
Embodiments of the present invention are also directed to providing a backside illuminated image sensor, which can control the width of a depletion region of a photodiode, thereby improving cross-talk characteristics.
Embodiments of the present invention are further directed to providing a backside illuminated image sensor with improved efficiency and timing margin in transferring photoelectric charges generated by a photodiode to a sensing node of a pixel.
In accordance with an aspect of the present invention, there is provided a backside illuminated image sensor including a photodiode, formed below the top surface of a semiconductor substrate, for receiving light illuminated from the backside of the semiconductor substrate to generate photoelectric charges, a reflecting gate, formed on the photodiode over the front upper surface of the semiconductor substrate, for reflecting light illuminated from the backside of the substrate and receiving a bias to control a depletion region of the photodiode, and a transfer gate for transferring photoelectric charges from the photodiode to a sensing node of a pixel.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a conventional backside illuminated image sensor disclosed in US Patent Publication No. 2006-0068586.
FIG. 2 is a sectional view of a backside illuminated image sensor in accordance with an embodiment of the present invention.
FIG. 3 is a sectional view of a backside illuminated image sensor in accordance with another embodiment of the present invention.



DESCRIPTION OF SPECIFIC EMBODIMENTS
Hereinafter, backside illuminated image sensors in accordance with embodiments of the present invention will be described in detail with reference to the accompanying drawings, so that those having ordinary skill in the art can easily implement the present invention.
FIG. 2 is a sectional view of a backside illuminated image sensor in accordance with an embodiment of the present invention.
Referring to FIG. 2, a photodiode having an n-type doped region 204 and a p-type doped region 206 is formed below the top surface of a p-type silicon substrate 202. The p-type doped region 206 improves a dark current on the surface of silicon and is commonly referred to as a pinning layer. The photodiode may be configured only the n-type doped region 204, with the p-type doped region 206 being omitted.
The substrate 202 may employ another semiconductor material instead of silicon.
A microlens 244 is formed on the backside of the substrate 202. The microlens 244 converges light illuminated from the backside of the substrate 202 to the photodiode. An insulation layer 242 is formed between the microlens 244 and the substrate 202. The insulation layer 242 includes one of an oxide layer, a nitride layer, and a stack structure thereof. The oxide layer may include one layer selected from the group consisting of a borophosphosilicate Glass (BPSG) layer, a phosphosilicate glass (PSG) layer, a borosilicate glass (BSG), an un-doped silicate glass (USG) layer, a tetra ethyl ortho silicate (TEOS) layer, a high density plasma (HDP) layer, and a silicon oxide (SiO2) layer. The nitride layer may include a silicon nitride (SixNy) layer, where x and y are natural numbers, or a silicon oxynitride (SixOyNz) layer, where x, y and z are natural numbers. The insulation layer 242 can function as an anti-reflecting layer. A color filter for realizing a color image can be formed between the microlens 244 and the insulation layer 242.
While logic circuits including a transfer gate 210 are formed on the substrate using a conventional method. Also, first and second reflecting gates 212 and 214 are further formed on the photodiode. The first and the second reflecting gates 212 and 214 have a stacked structure of a polysilicon layer and a metal silicide layer. The metal silicide layer may include a tungsten silicide layer. A metal layer such as a tungsten layer may be used instead of the metal silicide layer. Also, the reflecting gates may include only a metal silicide layer or a metal layer, without the polysilicon layer. Thus, the reflecting gates may be formed of a conductive material having high reflectivity.
The reflecting gates control the size of the photodiode depletion region by receiving a bias immediately before an integration time. That is, while the conventional image sensor determines a depletion region based on the processing conditions of the photodiode, the image sensor in accordance with the embodiment of the present invention can improve the cross-talk characteristic because the depletion region can be expanded up to near the backside of the substrate through the reflecting gates.
Due to the reflecting gates, the depletion region may be formed up to near the backside of the substrate. Because it is possible to accumulate photoelectric charges generated around the backside of the substrate when short-wavelength (blue wavelength) light is illuminated on the backside of the substrate, light sensitivity to short wavelengths is improved.
The bias applied to the reflecting gates may employ a negative bias generated using a circuit such as a negative charge pump. The negative charge pump may be integrated within the image sensor chip.
Since the reflecting gates include the metal silicide layer which can prevent light transmission and reflect light, signal loss of long-wavelength light illuminated from the backside of the substrate can be reduced. Accordingly, there is no need to provide a separate metal reflective layer at the top surface of the substrate, and there are no restrictions imposed on a metal layout.
Since the reflecting gates cover the photodiode, it protects the photodiode from plasma damage during etching of the gate and damage inflicted during other processes.
When the transfer gate 210 is turned on, the photoelectric charges accumulated in the photodiode are transferred to the sensing node of the pixel, that is, an N+ floating diffusion region 224. An N+ buffer region 222 is formed below the top surface of the substrate between the photodiode and an edge of the transfer gate in order to accumulate the photoelectric charges. The N+ buffer region 222 functions to improve photoelectric charge transfer efficiency and timing margin. The photodiode, with the N+ buffer region 222 being omitted, may be formed along the edge of the transfer gate. In this case, the photodiode region can be expanded by the amount that the N+ buffer region 222 would take up.
FIG. 3 is a sectional view of a backside illuminated image sensor in accordance with another embodiment of the present invention.
Like reference numerals refer to like elements in FIGS. 2 and 3, and description thereof will be omitted.
Referring to FIG. 3, first and second reflecting gates 212 and 214 and a transfer gate (Tx) partially overlapped, with an insulation layer 302 being interposed therebetween and the buffer region 222 of FIG. 2 being omitted.
As illustrated in FIG. 3, the buffering doped region is omitted, and the first and the second reflecting gate 212 and 214 and the transfer gate (Tx) are partially overlapped, whereupon space for the photodiode can be secured and the degradation of the photoelectric charge transfer efficiency and timing margin can be prevented.
As described above, the improved backside illuminated image sensor having the reflecting gates may be applied to both 4-transistor (4T) pixel and 3T pixel, which are well known to those skilled in the art. That is, the reflecting gates are added to a 4T pixel or a 3T pixel. Also, the image sensor may be applied to a scheme in which two or more photodiodes share one floating diffusion circuit and pixel circuit.
Also, a typical correlated double sampling (CDS) process for removing thermal noise in capacitor (kTC) noise may be applied to the above-described improved backside illuminated image sensor having the reflecting gates.
The present invention may be applied to a CCD image sensor as well as a CMOS image sensor manufactured using a CMOS fabrication technology.
By employing a backside illuminated structure, the present invention described above does not require consideration of a path for incident light when drawing up a layout for a logic circuit and metal lines at the front surface of the substrate.
Also, by controlling the width of a depletion region using a control bias in addition to an intrinsic potential of a photodiode, the present invention can improve photoelectric charge generation efficiency, light sensitivity to short wavelength, and cross-talk characteristic.
Furthermore, since the reflecting gates include a metal silicide layer or a metal layer as a gate material, signal loss of long-wavelength light illuminated from the backside of the substrate can be reduced. Thus, there is no need to provide a separate metal reflective layer on the front surface of the substrate.
Moreover, since the reflecting gates cover the top of the photodiode, the photodiode can be protected from damage during processing.
While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

## Claims

1. A backside illuminated image sensor, comprising:
a photodiode formed below a front upper surface of a semiconductor substrate, wherein the photodiode is configured to receive light illuminated from a backside of the semiconductor substrate that generates photoelectric charges;
a reflecting gate formed on the photodiode proximate the front upper surface of the semiconductor substrate, wherein the reflecting gate is configured to reflect light illuminated from the backside of the semiconductor substrate, and wherein the reflecting gate is further configured to receive a bias signal to control a depletion region of the photodiode; and
a transfer gate configured to transfer photoelectric charges from the photodiode to a sensing node of a pixel.

2. The backside illuminated image sensor of claim 1, wherein the reflecting gate comprises a metal silicide layer or a metal layer configured to reduce loss of light illuminated from the backside of the semiconductor substrate.

3. The backside illuminated image sensor of claim 1, further comprising a buffer region formed below the front upper surface of the semiconductor substrate between the photodiode and the transfer gate, wherein the buffer region is configured to improve a photoelectric charge transfer efficiency and timing margin in transferring the photoelectric charges from the photodiode to the sensing node.

4. The backside illuminated image sensor of claim 1, wherein the reflecting gate is further configured to receive a negative bias as the bias signal, and wherein the reflecting gate is configured to control a width of the depletion region.

5. The backside illuminated image sensor of claim 1, wherein the photodiode comprises a pinning layer configured to reduce a dark current generated proximate the front upper surface of the semiconductor substrate.

6. The backside illuminated image sensor of claim 1, wherein the sensing node comprises a floating diffusion region.

7. The backside illuminated image sensor of claim 1, wherein the sensing node is connected so that it is shared by at least two photodiodes.

8. The backside illuminated image sensor of claim 1, further comprising a negative charge pumping circuit integrated into the backside illuminated image sensor and configured to generate the bias signal applied to the reflecting gate.

9. The backside illuminated image sensor of claim 1, wherein the backside illuminated image sensor comprises a correlated double sampling (CDS) circuit, and wherein the CDS circuit is configured to remove thermal noise in capacitor (kTC) noise of the sensing node.

10. The backside illuminated image sensor of claim 1, wherein the photodiode, the reflecting gate, and the transfer gate comprise CMOS circuit structures.

11. The backside illuminated image sensor of claim 1, wherein the reflecting gate is partially overlapped by the transfer gate, and wherein an insulation layer is interposed between the reflecting gate and the transfer gate.

12. The backside illuminated image sensor of claim 1, further comprising a microlens formed proximate the backside of the semiconductor substrate.

13. The backside illuminated image sensor of claim 1, further comprising an anti-reflecting layer formed proximate the backside of the semiconductor substrate.

14. The backside illuminated image sensor of claim 13, wherein the anti-reflecting layer comprises an insulation layer.

15. The backside illuminated image sensor of claim 13, wherein the anti-reflecting layer comprises at least one of an oxide layer, a nitride layer, or a stacked structure thereof.

16. The backside illuminated image sensor of claim 15, wherein the oxide layer comprises at least one layer selected from the group consisting of: a borophosphosilicate glass (BPSG) layer, a phosphosilicate glass (PSG) layer, a borosilicate glass (BSG), an un-doped silicate glass (USG) layer, a tetraethyl orthosilicate (TEOS) layer, a high density plasma (HDP) layer, and a silicon oxide (SiO2) layer.

17. The backside illuminated image sensor of claim 15, wherein the nitride layer comprises a silicon nitride (SixNy) layer, where x and y are natural numbers, or a silicon oxynitride (SixOyNz) layer, where x, y and z are natural numbers.

18. A pixel of a backside illuminated image sensor, the pixel comprising:
a substrate having a front upper surface and a backside;
a photodiode configured to generate photoelectric charges in response to light received from the backside of the substrate, wherein the photodiode is formed proximate the front upper surface of the substrate; and
a reflecting gate disposed over the photodiode and configured to reflect light received from the backside of the substrate onto a frontside of the photodiode, wherein the reflecting gate is further configured to receive a bias signal to control an extent of a depletion region of the photodiode.

19. The pixel of claim 18, further comprising a transfer transistor configured to transfer the photoelectric charges to a floating diffusion node.

20. The pixel of claim 19, wherein the transfer transistor comprises a transfer gate laterally spaced from the reflecting gate, and wherein the transfer gate and the reflecting gate are disposed at substantially a same distance from the front upper surface of the substrate.

21. The pixel of claim 19, wherein the transfer transistor comprises a buffer region formed in the substrate and vertically aligned with an edge of the reflecting gate.

22. The pixel of claim 20, wherein a portion of the transfer gate overlies the reflecting gate.

23. The pixel of claim 18, wherein the reflecting gate comprises a stacked layer structure.

24. The pixel of claim 23, wherein the stacked layer structure comprises:
a polysilicon layer; and
a metal layer adjacent the polysilicon layer.

25. The pixel of claim 24, wherein the metal layer comprises a tungsten silicide layer.

26. The pixel of claim 24, wherein the metal layer comprises tungsten.

27. The pixel of claim 18, wherein the reflecting gate comprises a single-layer structure.

28. The pixel of claim 27, wherein the single-layer structure comprises a metal silicide layer or metal layer.