Patent Application
20140048898
This application claims the priority of Chinese patent application number 201210290693.3, filed on Aug. 15, 2012, the entire contents of which are incorporated herein by reference.
The present invention relates in general to image sensors, and more particularly, to a backside illuminated (BSI) CMOS image sensor.
Digital cameras are a kind of modern electronic products and are widely used. A digital camera typically incorporates an image sensor for converting light to electric charges. According to their different operating principles, image sensors are classified into charge-coupled device (CCD) sensors and complementary metal oxide semiconductor (CMOS) sensors. CMOS image sensors are fabricated using conventional CMOS technologies and therefore can be integrated with peripheral circuitries during the fabrication.
Conventional CMOS image sensors typically rely on a front-side illumination (FSI) technology to provide pixels for a pixel array. In the FSI technology, incident light enters a pixel from the front side and then reaches a photo-sensing area. In other words, before reaching the photo-sensing area, the incident light must pass through several dielectric layers and metal layers. Such design leads to many problems of the conventional CMOS image sensors, such as low quantum efficiency, severe cross talk and dark current.
In order to solve the above-mentioned problems, a CMOS image sensor adopting a backside illumination design, also referred to as a backside illuminated (BSI) CMOS image sensor, has been proposed in the prior art. Different from the FSI sensors discussed above, the BSI CMOS image sensor is formed on a front side of a silicon substrate, while color filters and microlenses are arranged on a back side of the substrate, thereby allowing incident light to enter the sensor from its back side. This BSI CMOS image sensor has a number of advantages compared with conventional FSI ones, such as less light loss and better quantum efficiency.
Nevertheless, the BSI CMOS image sensor still has a serious problem of color cross-talk, and therefore an existing BSI CMOS image sensor typically includes an additional metal shielding layer for solving this problem of color cross-talk. However, adding the metal shielding layer further causes a problem of photon cross-talk.
The present invention is directed to the provision of a BSI CMOS image sensor which can solve the problem of the photon cross-talk of the existing BSI CMOS image sensor.
To achieve the above objective, there is provided a BSI CMOS image sensor including: a substrate having a front side and a back side, the substrate including a photodiode formed therein, the photodiode being proximate the back side of the substrate; a metal shielding layer covering the back side of the substrate, the metal shielding layer including an opening formed therein, the opening being arranged in correspondence with the photodiode; and a light-absorbing layer formed on each side face of the opening.
Optionally, the opening is vertically aligned with the photodiode.
Optionally, the BSI CMOS image sensor further includes a high-k dielectric layer between the device substrate and the metal shielding layer.
Optionally, a portion of the high-k dielectric layer is exposed in the opening.
Optionally, the light-absorbing layer is a nitride layer.
Optionally, the light-absorbing layer is a silicon oxynitride layer, a silicon nitride layer, a titanium nitride layer, or a tantalum nitride layer.
Optionally, the light-absorbing layer has a thickness of 200 Å to 700 Å.
Optionally, the light-absorbing layer is formed by a PECVD process or a furnace process.
Optionally, an angle between the side face and a bottom face of the opening is greater than 90 degrees and smaller than 180 degrees.
Optionally, a top face of the metal shielding layer is also covered by the light-absorbing layer.
Optionally, the metal shielding layer is made of aluminum or tungsten.
By forming a light-absorbing layer on both side faces of each opening, the present invention is capable of preventing the occurrence of photon cross-talk and hence improving imaging quality of the BSI CMOS image sensor.
a to 4c are cross-sectional views schematically illustrating structures after steps of the method of
The present invention will be described in detail below by way of specific embodiments with reference to the accompanying drawings. Advantages and features of the invention will be readily apparent upon reading of following description and appended claims. It is noted that drawings are provided in a very simplified form and are solely for enhancing convenience and clarity of the description of the specific embodiments.
As indicated above in the Background, although the existing BSI CMOS image sensor has addressed the issue of color cross-talk, its inclusion of the metal shielding layer further leads to the problem of photon cross-talk. In order to solve this problem, the inventors of this invention had an in-depth study on this subject and identified the cause of the occurrence of photon cross-talk in the existing BSI CMOS image sensor, which is described in detail below.
The openings 14 in the metal shielding layer 13 may be formed by the following steps: forming the metal shielding layer 13; and forming the openings 14 in the metal shielding layer 13 by an etching process.
Due to the nature (or inherent feature) of the etching process, each of the formed openings 14 has an inverted-trapezoid shape (as described per the profile and orientation of the opening shown in
Based on this finding of the inventors, there is provided a BSI CMOS image sensor which can address the issue discussed above and is described below by way of non-limitative exemplary embodiments with reference to the accompanying drawings.
In this embodiment, the metal shielding layer 23 is comprised of aluminum, while in other embodiments the metal shielding layer 23 may also be made of copper, titanium, or the like. An angle between the side face 241 and the bottom face 242 of the opening 24 is greater than 90 degrees and smaller than 180 degrees.
The light-absorbing layer 25 is able to prevent light from being reflected by the side face 241 of the opening 24 into another photodiode 22 other than the photodiode 22 corresponding to the opening 24, thereby preventing the occurrence of photon cross-talk and improving imaging quality of the BSI CMOS image sensor.
Specifically, for example, if neither of two of the openings 24 shown in
Preferably, the first and second openings 24A, 24B are vertically aligned with the first and second photodiodes 22A, 22B, respectively, so as to improve light reception and photoelectric conversion of the photodiodes and hence improve imaging quality of the BSI CMOS image sensor.
In this embodiment, a high-k dielectric layer 26 may be provided between the device substrate 21 and the metal shielding layer 23. The high-k dielectric layer 26 can further optimize the photoelectric conversion of the photodiodes and hence further improve imaging quality of the BSI CMOS image sensor. In addition, the opening 24 may expose a portion of the high-k dielectric layer 26. Specifically, the first opening 24A exposes a portion of the high-k dielectric layer 26 that is right above the first photodiode 22A such that the first photodiode 22A can be further exposed after the portion of the high-k dielectric layer 26 is removed in a subsequent process. Similarly, the second opening 24B exposes a portion of the high-k dielectric layer 26 that is right above the second photodiode 22B such that the second photodiode 22B can be further exposed after the portion of the high-k dielectric layer 26 is removed in a subsequent process.
In this embodiment, the light-absorbing layer 25 is a silicon oxynitride (SiON) layer which preferably has a thickness of 200 Å to 700 Å. For example, the SiON layer may have a thickness of 200 Å, 250 Å, 300 Å, 350 Å, 400 Å, 450 Å, 500 Å, 550 Å, 600 Å, 650 Å, or 700 Å. The SiON layer with such a thickness not only ensures a relatively thin light-absorbing layer 25, thus contributing to the thinness and miniaturization of the BSI CMOS image sensor, but also prevents light from being reflected by the side face 241 of the opening 24 into another photodiode 22 other than the photodiode 22 corresponding to the opening 24, thereby preventing the occurrence of photon cross-talk and improving imaging quality of the BSI CMOS image sensor. Moreover, the light-absorbing layer 25 may be formed by a plasma enhanced chemical vapor deposition (PECVD) process, or alternatively by other semiconductor processes, such as a furnace process.
Furthermore, in other embodiments of the present invention, the light-absorbing layer 25 may be formed of other nitride layers, such as a silicon nitride (SiN) layer, a titanium nitride (TiN) layer or a tantalum nitride (TaN) layer. Preferably, the nitride layer has a thickness of 200 Å to 700 Å and is formed by a PECVD process or a furnace process.
While in the illustrated embodiment, the light-absorbing layer 25 is formed only on the side faces of the opening 24, in other embodiments of the present invention, the light-absorbing layer 25 may also be formed on a top face of the metal shielding layer 23 (as described per the profile and orientation of the opening shown in
In this disclosure there is also provided a method of forming a BSI CMOS image sensor, a flow chart of which is illustrated in
a to 4c are cross-sectional views schematically illustrating structures after steps of the method in this embodiment.
a shows the device substrate 41 which has a front side 411 and a back side 412. Two of the photodiode 42, a first photodiode 42A and a second photodiode 42B, are formed in the device substrate 41 and both proximate the back side 412 of the device substrate 41.
In this embodiment, the device substrate 41 is bonded to a carrier substrate 40, specifically in a manner that the front side 411 of the device substrate 41 is bonded to the carrier substrate 40. Moreover, the backside 412 of the device substrate 41 that is bonded to the carrier substrate 40 is thinned by grinding and wet etching.
b shows a resulting structure after a high-k dielectric layer 46 is formed on the thinned backside 412 of the device substrate 41, a metal shielding layer 43 is formed on the high-k dielectric layer 46, one or more openings 44 (two as shown in the figure) are formed in the metal shielding layer 43 using a dry etching process. The metal shielding layer 43 may be comprised of aluminum or tungsten. An angle between the side face 441 and the bottom face 442 of the opening 44 is greater than 90 degrees and smaller than 180 degrees. The opening 44 exposes a portion of the underlying high-k dielectric layer 46.
b shows two of the openings 44, namely a first opening 44A that corresponds to the first photodiode 42A and a second opening 44B that corresponds to a second photodiode 22B. Preferably, the first and second openings 44A, 44B are vertically aligned with the first and second photodiodes 42A, 42B respectively.
Forming the light-absorbing layers 45 may include: forming a layer of a light-absorbing material, covering both the metal shielding layer 43 (i.e., covering the side faces 441 of the first and second openings 44A, 44B and a top surface of the metal shielding layer 43) and the exposed portions of the high-k dielectric layer 46; and removing the light-absorbing material over the portions of the high-k dielectric layer 46 using a photolithography-and-etching process to form a light-absorbing layer 45 over each of the side faces 441 of the first and second openings 44A, 44B and the top surface of the metal shielding layer 43, or removing the light-absorbing material over the portions of the high-k dielectric layer 46 and the top surface of the metal shielding layer 43 using a photolithography-and-etching process to form a light-absorbing layer 45 over each of the side faces 441 of the first and second openings 44A, 44B.
Forming the light-absorbing layer 45 can prevent light from being reflected by the side face 441 of the opening 24 into another photodiode 22 other than the photodiode 22 corresponding to the opening 24, thereby preventing the occurrence of photon cross-talk and improving imaging quality of the BSI CMOS image sensor.
Whilst there has been described in the foregoing description specific embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications may be made according to the present teachings. Accordingly, it is intended that the appended claims embrace all such modifications and variations as falling within the true scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201210290693.3 | Aug 2012 | CN | national |
