CMOS image sensor and method for fabricating the same

A CMOS image sensor and a method for fabricating the same are disclosed, in which light that transmits through a microlens is prevented from being beyond a photodiode region to minimize loss of incident light and to improve low illumination characteristics of the CMOS image sensor. The CMOS image sensor includes a semiconductor substrate including a transistor region and a photodiode region, a gate electrode formed on the semiconductor substrate corresponding to the transistor region, an interlayer dielectric layer formed on an entire surface of the semiconductor substrate including the gate electrode, a microlens formed over the interlayer dielectric layer to condense light, and a metal barrier formed in the interlayer dielectric layer to surround a portion of the interlayer dielectric layer corresponding to the photodiode region and to reflect light in the photodiode region that transmits through the microlens but goes beyond the photodiode region.

This application claims the benefit of the Korean Patent Application No. 10-2004-0116522, filed on Dec. 30, 2004, which is hereby incorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same, and more particularly, to a CMOS image sensor and a method for fabricating the same in which a metal barrier is formed to surround a periphery of a photodiode region so as to reduce loss of light that transmits through a microlens, thereby improving light-receiving efficiency of the photodiode region.

2. Discussion of the Related Art

Generally, a CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors. The MOS transistors are formed in a semiconductor transistor to correspond to the number of unit pixels using CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits. That is, to display images, the CMOS image sensor sequentially detects the electrical signals of the respective unit pixels in the switching mode by forming a photodiode and MOS transistors in each unit pixel.

Since the CMOS image sensor uses CMOS technology, it has advantages in that low power consumption is required and process steps are simplified due to a relatively small number of photo processes. Also, the CMOS image sensor has an advantage in that a control circuit, a signal processing circuit, and an analog-to-digital converter can be integrated in one chip to easily obtain a slim sized device.

FIG. 1is a structural sectional view illustrating incident light to a photodiode region in a related art CMOS image sensor.

As shown inFIG. 1, the related art CMOS image sensor includes a semiconductor substrate10including a photodiode region10a, a device isolation film11formed in the semiconductor substrate10to isolate devices from each other, a gate electrode21formed on a transistor region of the semiconductor substrate10, source and drain regions12formed in the semiconductor substrate10at both sides of the gate electrode21, an interlayer dielectric layer20formed on an entire surface of the semiconductor substrate10including the gate electrode21, a first metal line40formed on the interlayer dielectric layer20, a first contact30formed in the interlayer dielectric layer20to electrically connect the gate electrode21with the first metal line40, an inter-metal dielectric layer50formed on an entire surface of the interlayer dielectric layer20, a second metal line70formed on the inter-metal dielectric layer50, a second contact60formed in the inter-metal dielectric layer50to electrically connect the first metal line40with the second metal line70, an oxide layer80formed on the inter-metal dielectric layer50including the second metal line70, a nitride layer90formed on the oxide layer80, and a microlens100formed on the nitride layer90to condense light.

The aforementioned related art CMOS image sensor condenses light through the microlens100and transfers the condensed light to the photodiode region10aof the semiconductor substrate10such that the condensed light is stored as an electrical signal in the photodiode region10a. Therefore, a focal plane of the microlens100serves as a main factor that determines low illumination characteristics of the CMOS image sensor.

However, the related art CMOS image sensor has a drawback. If the focus of the microlens100is not exact, loss of the light condensed by the microlens100may occur. That is, as shown inFIG. 1, if the distance between the microlens100and the photodiode region10ais longer than the originally designed distance, or if the focal distance of the microlens100becomes shorter than intended without being uniformly maintained, the light condensed by the microlens100may partially be beyond the photodiode region10a, thereby causing loss of the light.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a CMOS image sensor and a method for fabricating the same that substantially obviates one or more problems due to limitations and disadvantages of the related art.

An advantage of the present invention is to provide a CMOS image sensor and a method for fabricating the same in which a metal barrier is formed to surround a periphery of a photodiode region to reflect light in the photodiode region that may be lost from the photodiode region, thereby improving a light-receiving efficiency of the photodiode region and low illumination characteristics.

Another advantage of the present invention is to provide a CMOS image sensor and a method for fabricating the same in which a metal contact electrically connecting a gate electrode with a metal line is formed of the same material as that of a metal barrier so that the metal contact and the metal barrier can simultaneously be formed by one process without a separate additional process.

To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a CMOS image sensor according to the present invention includes a semiconductor substrate including a transistor region and a photodiode region, a gate electrode formed on the semiconductor substrate corresponding to the transistor region, an interlayer dielectric layer formed on an entire surface of the semiconductor substrate including the gate electrode, a microlens formed over the interlayer dielectric layer to condense light, and a metal barrier formed in the interlayer dielectric layer to surround a portion of the interlayer dielectric layer corresponding to the photodiode region and to reflect light in the photodiode region that transmits through the microlens but goes beyond the photodiode region.

In another aspect of the present invention, the CMOS image sensor further includes a metal line formed between the interlayer dielectric layer and the microlens, and a contact formed in the interlayer dielectric layer to electrically connect the gate electrode with the metal line.

In another aspect of the present invention, the metal barrier and the contact are formed of the same metal.

In another aspect of the present invention, the metal barrier and the contact are formed of W or Al.

In another aspect of the present invention, a method for fabricating a CMOS image sensor includes forming a gate electrode on a transistor region of a semiconductor substrate including the transistor region and a photodiode region, forming an interlayer dielectric layer on an entire surface of the semiconductor substrate including the gate electrode, forming a microlens over the interlayer dielectric layer to condense light, and forming a metal barrier in the interlayer dielectric layer to surround a portion of the interlayer dielectric layer corresponding to the photodiode region and to reflect light to the photodiode region that transmits through the microlens but goes beyond the photodiode region.

In another aspect of the present invention, the method further includes forming a metal line between the interlayer dielectric layer and the microlens, and forming a contact in the interlayer dielectric layer to electrically connect the gate electrode with the metal line.

In another aspect of the present invention, the metal barrier and the contact are formed of the same metal.

In another aspect of the present invention, the metal barrier and the contact are formed of W or Al.

In another aspect of the present invention, the metal barrier and the contact are simultaneously formed.

In another aspect of the present invention, the metal barrier and the contact are simultaneously formed by forming a photoresist pattern on the interlayer dielectric layer, selectively etching the interlayer dielectric layer using the photoresist pattern as a mask to respectively form a metal barrier hole and a contact hole, and respectively forming the metal barrier and the contact in the metal barrier hole and the contact hole.

In another aspect of the present invention, the interlayer dielectric layer is etched by an anisotropic etching process.

DETAILED DESCRIPTION OF THE INVENTION

As shown inFIG. 2, the CMOS image sensor according to the present invention includes a semiconductor substrate10. The semiconductor substrate10is defined by a transistor region, an active region including a photodiode region10a, and a device isolation region that isolates devices from each other. The device isolation region is provided with a device isolation film11formed by a shallow trench isolation (STI) to isolate devices from each other. The transistor region is provided with a transistor that includes a gate electrode21and source and drain regions12. The photodiode region10ais provided with a photodiode (not shown) that generates charges depending on incident light. The transistor transmits and receives an electrical signal to and from a first metal line40. An interlayer dielectric layer20is formed between the first metal line40and the transistor. The first metal line40is electrically connected with the gate electrode21and the source and drain regions12through a first contact30formed in the interlayer dielectric layer20.

Also, a metal barrier110ais formed in the interlayer dielectric layer20to surround a portion of the interlayer dielectric layer20corresponding to the photodiode region10a. As shown inFIG. 2, light that transmits through a microlens100but is lost from the photodiode region10adue to a focal error of the microlens100is reflected in the photodiode region10aby the metal barrier110a. Therefore, the lost light enters the photodiode region through the metal barrier110a. As a result, the light that transmits through the microlens is efficiently used and low illumination characteristics of the CMOS image sensor are improved.

In one embodiment of the present invention, the metal barrier110ais formed of the same material as that of the first contact30. This enables the first contact30and the metal barrier110ato be simultaneously formed by one process. The metal barrier110aand the first contact30are preferably formed of W or Al or any metal having excellent electrical conductivity and excellent reflexibility. In another embodiment of the present invention, the metal barrier110amay be formed of a metal different from that of the first contact30. Additionally, the barrier110aand the first contact30may be formed in separate steps.

The CMOS image sensor of the present invention further includes an inter-metal dielectric layer50formed on an entire surface of the interlayer dielectric layer20including the first metal line40, a second metal line70formed on the inter-metal dielectric layer50, a second contact60formed in the inter-metal dielectric layer50to electrically connect the first metal line40with the second metal line70, an oxide layer80formed on the inter-metal dielectric layer50including the second metal line70to protect the device from humidity and scratches, a nitride layer90formed on the oxide layer80, and a microlens100formed on the nitride layer90to condense light.

A method for fabricating the CMOS image sensor according to the present invention will be described with reference toFIG. 3toFIG. 5.

As shown inFIG. 3, the interlayer dielectric layer20is formed on the semiconductor substrate10in which the transistor including the gate electrode21and the source and drain regions12, the photodiode region10aincluding a photodiode that generates charges using incident light, and the device isolation film11are formed. A photoresist is deposited on the interlayer dielectric layer20and then exposed and developed to form a photoresist pattern200.

As shown inFIG. 4, the interlayer dielectric layer20is selectively etched using the photoresist pattern200as a mask to respectively form a metal barrier hole110band a contact hole30a. The metal barrier hole110bis formed to surround a portion of the interlayer dielectric layer20corresponding to the photodiode region10a. Meanwhile, the interlayer dielectric layer20is preferably etched by an anisotropic etching process.

Subsequently, as shown inFIG. 5, a metal layer is deposited on the entire surface of the interlayer dielectric layer20including the metal barrier hole110band the contact hole30a. Then, the metal layer is etched until the interlayer dielectric layer20is exposed, so that the metal barrier110aand the first contact30are respectively formed in the metal barrier hole110band the contact hole30a. The first contact30electrically connects the first metal line40with the transistor. Therefore, the first contact30should have excellent electrical conductivity. The metal barrier110areflects the light that transmits through the microlens100in the photodiode region10aso as not to allow the light to be beyond the photodiode region10a. Therefore, the metal layer is preferably formed of W or Al or any metal having excellent electrical conductivity and excellent reflexibility. The metal layer may be etched by an isotropic etching process.

As shown inFIG. 6, the metal barrier110ais formed around the photodiode region10a. This enables the metal barrier110ato reflect the light that transmits through the microlens100in the photodiode region10aso as not to allow the light to be beyond the photodiode region10a.

Conventional processes may then be performed. Thus, the first metal line40is formed on the interlayer dielectric layer20to correspond to the first contact30. The inter-metal dielectric layer50is formed on the entire surface of the interlayer dielectric layer20including the first metal line40. The second contact60is formed in the inter-metal dielectric layer50. The second metal line70electrically connected with the first metal line40through the second contact60is formed on the inter-metal dielectric layer50to correspond to the second contact60. The oxide layer80and the nitride layer90are sequentially formed on the entire surface of the inter-metal dielectric layer50including the second metal line70to protect the device from humidity and scratches. The microlens100is formed on the nitride layer90to correspond to the photodiode region10a.

As described above, the CMOS image sensor and the method for fabricating the same have the following advantages.

Since the light that transmits through the microlens is prevented from being beyond the photodiode region, it is possible to minimize loss of the incident light and improve low illumination characteristics of the CMOS image sensor. In addition, since the metal barrier that reflects the light in the photodiode region and the contact that electrically connects the metal line with the transistor are formed by one process, it is possible to improve low illumination characteristics without a separate additional process and minimize loss of light, thereby reducing a dark current caused by the loss of light.