1. Technical Field
The invention relates to a back-illuminated type imaging device in which light is illuminated from a back-surface side of a semiconductor substrate and charges that are generated in the semiconductor substrate in response to the light are read from a front-surface side of the semiconductor substrate to perform imaging.
2. Description of the Related Art
The following back-illuminated type imaging device has been proposed. That is, light is illuminated from a back-surface side of a semiconductor substrate. The back-illuminated type imaging device stores charges that are generated in the semiconductor substrate in response to the light, into charge storage areas formed on a front-surface side of the semiconductor substrate. The back-illuminated type imaging device outputs signals corresponding to the charges stored in the charge storage areas to the outside by a CCD or CMOS circuit formed on the front-surface side of the semiconductor substrate so as to perform imaging.
In order to absorb almost all of visible light, the semiconductor substrate (a photoelectric conversion area) the back-illuminated type imaging device, the semiconductor substrate (a photoelectric conversion area) is required to have a thickness of about 10 μm. For this reason, when the back-illuminated type imaging device is fabricated, at first structures such as the charge storage areas and the CCD are first formed on the front surface of the thick semiconductor substrate. Thereafter, a wiring layer is thereafter formed on the structures, and a supporting substrate is bonded thereto via an adhesive layer. Then, the semiconductor substrate is etched from the back-surface side until the thickness becomes 10 μm, for example. After the etching, structures such as color filters and microlenses are formed on the back surface of the semiconductor substrate so as to be aligned with the structures formed on the front-surface side of the semiconductor substrate.
In the back-illuminated type imaging device formed in such a manner, it is necessary to form a pad on the back-surface side or front-surface side of the semiconductor substrate so as to be connected to the wiring layer formed on the front-surface side of the semiconductor substrate. A general image sensor that is presently in wide use has such a structure that a pad opening is provided on a light incidence side. In accordance with this structure, test equipments for testing the functions of chips in the wafer are installed in a fabrication line. For this reason, if the pad opening is formed on the front-surface side of the semiconductor substrate in the back-illuminated type imaging device, it becomes necessary to drastically change or modify the test equipments, which may increase the fabrication cost.
Therefore, similarly to the general image sensor, various methods for providing the pad opening on the light incidence side (the back-surface side) have been proposed for the back-illuminated type imaging device. One method of them is to form a through hole from the back-surface side of the semiconductor substrate, thereby exposing from the back surface the pad connected to the wiring layer formed on the front-surface side (for example, see JP 2005-285814 A and JP 2006-19653 A). However, such a method has the following problems. That is, if the through hole is formed before the color filter or the microlens are formed on the back-surface side, a material for the color filter or the microlens may be left in the through hole and cannot be removed in subsequent steps. Or, since the substrate on which the material is to be deposited has a large step formed thereon, the thickness of the deposited material is not even, which may cause a fixed pattern noise that appears as oblique lines in an image. Moreover, even if the through hole is formed after the color filter or the microlens is formed, it is necessary to remove a photoresist that defines the through hole without causing any harm to the color filter or the microlens, which is very troublesome. In particular, as described above, in the semiconductor substrate having a thickness of 10 μm, the above problems may become conspicuous, and it becomes more difficult to perform such a removal work.
In light of the above, as disclosed in JP 2006-339566 A, the following method may be conceived. That is, a trench is formed in the semiconductor substrate so as to extend from the front surface of the semiconductor substrate to reach the back surface thereof; the trench is filled with a conductive material, and a pad is formed on the conductive material. It can be said that in principle, it is possible to fill the trench corresponding to the pad with the conductive material. However, usually, the pad has a size of 100 μm×100 μm; therefore, in order to form a trench having substantially the same size and fill the trench with conductive material, if a CVD process is used, it is necessary to form a layer of the conductive material to a thickness equal to or larger than 50 μm and then to remove the same thickness by an anisotropic etching process. Such a process step cannot be said to be practical as a semiconductor fabrication process.
If the size of the trench is decreased, it may not cause any problem in view of a fabrication process. However, in such a case, there arises a problem that the resistance of the conductive material filled in the trench may increase, which may affect the device characteristics.