Image sensor with enlarged outward appearance of microlens and method for fabricating the same

An image sensor with an enlarged outward appearance of a microlens and a method for fabricating the same are provided. The image sensor includes: a plurality of microlenses formed on a semiconductor substrate with a certain spacing distance; and a protection layer formed over the microlenses, wherein the protection layer includes a first oxide layer which is formed by a plasma enhanced chemical vapor deposition (PECVD) method and a second oxide layer which is formed by a spin on glass (SOG) method over the first oxide layer to maintain sufficient step coverage over chasms between the microlenses.

FIELD OF THE INVENTION

The present invention relates to an image sensor and a method for fabricating the same; and, more particularly, to an image sensor including a protection layer for a microlens and a method for fabricating the same.

DESCRIPTION OF RELATED ARTS

An image sensor is a semiconductor device which converts an optical image to an electric-signal, and a charge coupled device (CCD) and a complementary metal-oxide-silicon (CMOS) are examples of the image sensor.

The CCD is a semiconductor device, which transfers and stores charge carriers into metal-oxide-silicon (MOS) capacitors located very closely to each other.

On the other side, the CMOS image sensor is a semiconductor device which employs a switching method. The switching method forms MOS transistors in the same number as pixels by utilizing CMOS technology which uses a control circuit and a signal processing circuit as a peripheral circuit, and employs the MOS transistors to detect outputs one by one.

Currently, the CMOS image sensor is widely used in portable image sensing devices due to its advantages such as high degree of integration and low operation voltage.

FIG. 1is a simplified layout of a conventional image sensor pixel that has a common active region with a pinned photodiode and four transistors.

InFIG. 1, a reference numeral100represents the simplified layout diagram of the conventional 4T pixel used in typical CMOS image sensors. Many such pixels are arranged in an array forming rows and columns in the actual image sensor, but for the simplicity of the drawing only one is shown in a greater detail. An active region101contains active elements of the pixel; those are, a pinned photodiode102, a transfer gate103, a floating diffusion (FD) node104, a reset gate106, a drain bias node118, a drive transistor gate113serving as a source follower (SF), a source-drain region119, an address select transistor gate114, and an address select transistor120.

The area outside of the enclosed active region101is a shallow trench isolation (STI) region that is filled with thick isolation silicon dioxide. Also, inFIG. 1, the multilevel metal interconnects present in the pixel have been for clarity of the drawing omitted and replaced schematically by lines. A first horizontal line117is a row address line connected to a first contact116of the address select transistor gate114, a second horizontal line111is a row transfer line connected to a second contact112of the transfer gate103, and a poly-silicon bus105supplies a row reset signal to the reset gate106of the pixel. A first column line108provides the Vdd bias via third contact109to the drain bias node118, while a second column line107carries the output signal from a source of the address select transistor120via fourth contact115to the column signal processing circuits located at the periphery of the array. The FD node104is connected to the drive transistor gate113via interconnect110. The addressing signals are supplied to the pixels also from the periphery of the array through the first horizontal line117, the fourth contact115and the poly-silicon bus105.

On the other hand, a color filter and a microlens are sequentially formed in layers over the top surface of the pixel with the above layout.

Due to the decreasing design rule, areas of this type of unit pixels are being scaled down, causing the unit elements to be disposed in a layered structure.

To prevent a decrease of light sensitivity caused by a reduced photodiode region which is a light receiving region, the microlens is formed in a convex or a concave form to maximally focus the light upon the photodiode layer.

FIG. 2is a top view showing a conventional layout of unit pixels for three colors.

Referring toFIG. 2, two green color unit pixels ‘G’s, a red color unit pixel ‘R’ and a blue color unit pixel ‘B’ are laid out.

Typically, a CMOS image sensor embodies only one color per one pixel, and expresses each of the R, G and B colors assigned to each pixel through an interpolation method.

As mentioned above, the CMOS image sensor contains one microlens on each of the unit pixels to focus the incident light upon the photodiode formed inside a silicon substrate.

FIG. 3is a simplified view illustrating a formation process of a conventional microlens.

As shown, a microlens ML is made of a polymer such as photoresist PR. The microlens ML is formed through patterning by a photo lithography process and flowing by a baking process at a high temperature.

There should be a certain spacing distance between a microlens and an adjacent microlens because of the above mentioned flow process characteristic. If a spacing distance is less than the certain spacing distance, the adjacent microlenses stick together during the flow process, resulting in deformed microlenses.

The spacing distance between the microlenses implies that the incident light is not utilized 100%. For example, if the size of a unit pixel is 3 μm×3 μm and a microlens is formed 0.2 μm smaller than the unit pixel, an area coverage percentage becomes approximately 87% (i.e., 2.8×2.8/3.0×3.0). That is, 13% of the incident light is wasted in this case.

The above kind of limitations induce even more serious limitations as the sizes of unit pixels are being scaled down due to the large integration scale of CMOS image sensors.

Meantime, a protection layer is used to resolve disadvantages such as scratches caused by external impacts. The protection layer is formed by employing low temperature oxide (LTO) to prevent deformation of the microlens.

FIG. 4is a cross-sectional view illustrating a conventional microlens with a protection layer.

As shown, a layer of LTO is formed on top of a microlens ML as a protection layer. The LTO layer is formed by a plasma enhanced chemical vapor deposition (PECVD) method at a temperature of 150° C. Herein, a reference denotation ‘W’ expresses the size of the outward appearance of the microlens ML.

Typically, an LTO layer not only functions as a protection layer, but the LTO layer functions to enlarge an outward appearance of a microlens.

On the other hand, although the LTO layer formed at a low temperature of 150° C. by the PECVD method prevents deformation of the microlens ML, the LTO layer is not so superior in step coverage.

Due to the above characteristic, the LTO layer is formed in a uniform thickness on top of the microlens ML, but a desired thickness of the LTO layer cannot be maintained on the lateral sides of the microlens ML.

Because of the unsatisfactory step coverage, the original size of the microlens ML and the size of the outward appearance of the microlens ML with the LTO layer are practically identical.

Therefore, a process technology development is needed to prevent microlens deformation with a low temperature formation process and maintain a superior step coverage characteristic.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an image sensor capable of preventing microlens deformation during a low temperature process and maintaining a superior step coverage characteristic and a method for fabricating the same.

In accordance with an aspect of the present invention, there is provided an image sensor, including: a plurality of microlenses formed on a semiconductor substrate with a certain spacing distance; and a protection layer formed over the microlenses, wherein the protection layer includes a first oxide layer which is formed by a plasma enhanced chemical vapor deposition (PECVD) method and a second oxide layer which is formed by a spin on glass (SOG) method over the first oxide layer to maintain sufficient step coverage over chasms between the microlenses.

In accordance with another aspect of the present invention, there is provided an image sensor, including: a plurality of microlenses formed on a semiconductor substrate with a certain spacing distance; and a protection layer formed over the microlenses, wherein the protection layer includes a first oxide layer which is formed by a plasma enhanced chemical vapor deposition (PECVD) method and a second oxide layer which is formed by a chemical vapor deposition (CVD) method over the first oxide layer to maintain sufficient step coverage over chasms between the microlenses.

In accordance with still another aspect of the present invention, there is provided a method for forming an image sensor, including: forming a plurality of microlenses on a substrate; forming a first oxide layer on the microlenses by employing a plasma enhanced chemical vapor deposition (PECVD) method; and forming a second oxide layer over the first oxide layer by employing a spin on glass (SOG) method to maintain sufficient step coverage over chasms between the microlenses.

In accordance with further aspect of the present invention, there is provided a method for forming an image sensor, including: forming a plurality of microlenses on a substrate; forming a first oxide layer on the microlenses by employing a plasma enhanced chemical vapor deposition (PECVD) method; and forming a second oxide layer over the first oxide layer by employing a chemical vapor deposition (CVD) method to maintain sufficient step coverage over chasms between the microlenses.

DETAILED DESCRIPTION OF THE INVENTION

An image sensor with an enlarged outward appearance of a microlens and a method for fabricating the same in accordance with certain embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 5is a cross-sectional view illustrating enlarged outward appearances of microlenses caused by a flowable protection layer in accordance with a specific embodiment of the present invention.

Referring toFIG. 5, although not illustrated, light receiving elements such as photodiodes, various transistors, metal wirings and color filters are formed on a substrate SUB. Also, microlenses ML are formed on the above resulting substrate structure, and a protecton layer of low temperature oxide (LTO) is formed over the microlenses ML.

According to the specific embodiment of the present invention, it is targeted to provide the LTO layer that can be formed by a low temperature process with superior step coverage. Thus, superior step coverage over a chasm between the microlenses ML is maintained and an outward appearance can be enlarged up to a length of ‘W′’.

Therefore, the protection layer in this specific embodiment of the present invention is structured in layers of either: an LTO layer by a plasma enhanced chemical vapor deposition (PECVD) method and an LTO layer by a spin on glass (SOG) method with a superior step coverage characteristic; or an LTO layer by a PECVD method and an LTO layer by a chemical vapor deposition (CVD) method which utilizes silicon hydride (SiH4) and hydrogen peroxide (H2O2) as sources with a superior step coverage characteristic.

The protection layer formation process mentioned above is described in detail through an embodiment below.

FIG. 6is a cross-sectional view illustrating microlenses on which a protection layer is formed by an SOG method in accordance with a specific embodiment of the present invention.

Referring toFIG. 6, microlenses ML are formed on a substrate SUB, and then a protection layer LTO is formed by an SOG method over the microlenses ML to maintain sufficient step coverage over chasms among the microlenses ML. The protection layer LTO includes a material such as LTO.

When forming the protection layer LTO, a first protection layer LTO1is first formed by a PECVD method on the upper portion of the individual microlens ML, and then a second protection layer LTO2is formed by the SOG method to sufficiently cover the chasms. Due to an application characteristic of fluidity of the SOG method, the second protection layer LTO2is concentrated at the topologically lower regions (i.e., chasms) as indicated by a reference denotation ‘B’, and thinly formed on the upper portion of the individual microlens ML as indicated by a reference denotation ‘A’.

The protection layer formation process inFIG. 6is described in further detail as follows. Herein, only one microlens ML is described below for the convenience of description.

First, the first protection layer LTO1is formed by the PECVD method and then the second protection layer LTO2is formed by the SOG method on the microlens ML. Herein, a chemical used in the SOG method should have extremely low viscosity. That is, the second protection layer LTO2with a viscosity value ranging from approximately 1.0 to approximately 2.5 is used. The second protection layer LTO2includes a silicate SOG layer that is formed by employing pure silicon dioxide (SiO2). A typical SOG layer is formed by employing a material including carbon or hydrogen to maintain stability and inner uniformity. In this embodiment, it is preferable to use the silicate SOG layer formed by employing pure SiO2in consideration of a light characteristic of the SOG layer. However, considering light transmittance, an SOG layer formed by employing siloxane or hydrogen silsesquioxane can be used as well.

Preferably, an SOG layer thickness on a flat wafer is set in a range of approximately 100 Å to approximately 2000 Å in accordance with a typical microlens form standard. After the formation of the SOG layer (i.e., the second protection layer LTO2), a baking process is performed at a temperature in a range of approximately 100° C. to approximately 250° C. to remove moisture within the SOG layer.

FIG. 7is a cross-sectional view illustrating microlenses with a protection layer formed by a CVD method which utilizes SiH4and H2O2as sources.

Referring toFIG. 7, microlenses ML are formed on a substrate SUB, and then a protection layer LTO is formed over the microlenses ML by a CVD method which utilizes SiH4and H2O2as sources to maintain sufficient step coverage over a chasm between the microlenses ML. The protection layer LTO includes a material such as LTO.

Although not illustrated, when forming the protection layer LTO, a first protection layer is thinly formed by a PECVD method on the upper portion of the individual microlens ML, and then a second protection layer is formed by the CVD method which utilizes SiH4and H2O2as sources to sufficiently cover the chasm. Due to an application characteristic of fluidity of the CVD method which utilizes SiH4and H2O2as sources, the second protection layer is concentrated at the topologically lower regions (i.e., chasms).

In more detail of the protection layer formation process inFIG. 7, the first protection layer which includes LTO is formed on the microlenses ML by the PECVD method and then the second protection layer which also includes LTO is formed by the CVD method which utilizes SiH4and H2O2as sources. Herein, during performing the CVD method, SiH4reacts with H2O2at a temperature in a range of approximately 0° C. to approximately 100° C. to form a Si(OH)4layer on the microlenses ML.

However, since the Si(OH)4layer exhibits a fluidity characteristic in a quasi-liquid state, the Si(OH)4layer is formed thick over the chasm and formed thin on the upper portion of the individual microlens ML. A baking process is then performed at a temperature in a range of approximately 50° C. to approximately 250° C. to remove a —OH functional group and as a result, a SiO2layer is formed.

The above specific embodiments of the present invention showed that prevention of microlens deformation with a low temperature process and an enlarged outward appearance of a microlens can be achieved through: forming a first protection layer by a typical method, e.g., a PECVD method; and then forming a second protection layer by an SOG method or a CVD method which utilizes SiH4and H2O2as sources, maintaining sufficient step coverage over chasms among a plurality of microlenses. The first protection layer can include a material such as LTO, and the second protection layer can include a material such as silicate SOG layer or LTO.

The present application contains subject matter related to the Korean patent application No. KR 2004-59459, filed in the Korean Patent Office on Jul. 29, 2004, the entire contents of which being incorporated herein by reference.

While the present invention has been described with respect to certain 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.