A semiconductor device using an SOI (Silicon On Insulator) substrate is advantageous over a semiconductor device using a Si substrate in an operation speed and energy saving, and, even in the field of a photoelectric conversion device such as an image sensor, it is proposed to utilize the SOI substrate. Here, the “SOI substrate” refers to a substrate in which an SiO2 layer is inserted between a Si substrate and a surface Si layer. The “surface Si layer” may contain an impurity.
An image sensor such as a CCD image sensor or a CMOS image sensor is a sensor in which photoelectric conversion elements such as photodiodes are arranged in a matrix, and each photoelectric conversion element functions as a pixel. In recent years, the opportunity of performing shooting and observing an object with high definition and high resolution has been increasing, and a high-density image sensor in which photoelectric conversion elements are arranged in high density has been increasingly proposed and developed.
As the photoelectric conversion elements are more densely arranged, the area of a light receiving surface of each photoelectric conversion element is inevitably reduced. Since as the area of the light receiving surface is reduced, the amount of light per unit time is reduced, it is necessary to increase the light sensitivity of each photoelectric conversion element. However, there is a limitation in it.
Furthermore, a major cause of the area of the light receiving surface becoming smaller beyond necessity as density increases is the area occupied by wiring for feeding signals to the individual photoelectric conversion elements and the drive elements thereof and for applying a predetermined voltage to a predetermined portion of the image sensor. In general, for ease of manufacturing, in order for the resistance of the wiring to be kept low, the width of the wiring is designed to be maximized. Hence, the ratio of the area occupied by the wiring on the surface of the image sensor is increased as the photoelectric conversion elements are more densely arranged. In order to avoid it, it is proposed not to reduce the resistance by increasing the width of the wiring but to reduce the resistance by increasing the thickness of the wiring, and it becomes commercially practical; however, this increases the number of manufacturing steps to cause cost increase.
As an example of a photoelectric conversion module on an SOI substrate, there is a photoelectric conversion module in which a second substrate where a photoelectric conversion portion having a plurality of photoelectric conversion elements on the surface Si layer of the SOI substrate is provided and a drive circuit is provided is adhered to the SOI substrate such that the drive circuit and the photoelectric conversion portion face each other. In recent years, as an idea of achieving both high light sensitivity and high density on the photoelectric conversion module described above, a large number of so-called backside illumination type image sensors in which, from the opposite side of an entrance direction to the photoelectric conversion element of a general image sensor, that is, from the backside of the Si substrate, light is transmitted through an SiO2 layer to enter the photoelectric conversion element are proposed, and part of them become commercially practical.
Although backside illumination is adopted because it is possible to reduce the influence of wiring, since light is made to enter the photoelectric conversion element through the Si substrate, it is necessary to provide an idea that light of any color (wavelength) is made to efficiently enter the light receiving surface of the corresponding photoelectric conversion element. One proposal is to remove the Si substrate from the backside by CMP (Chemical Mechanical Polishing) or wet etching, to reduce the thickness of the Si substrate as much as possible and to suppress the absorption of light by the Si substrate.
Since the Si substrate is relatively thick, it is ground to a predetermined thickness by CMP, and thereafter wet etching is performed to remove a so-called damage layer caused by CMP. Hence, a large amount of time is taken to limit the efficiency of production, and thus it becomes a cause of cost increase.
Non Patent Literature 1 discloses a Si wafer wet etching technology using highly concentrated fluonitric acid. Although fluonitric acid is strongly acid to limit the materials of a container for transportation or storage, a container at the time of use, a pipe and the like, and is currently used with its acid concentration reduced to some degree, among fluonitric acids disclosed in Non Patent Literature 1, there is a fluonitric acid that has a high etching rate of 800 μm/min for a Si wafer. When such a fluonitric acid is used, there is a possibility that the thickness of the backside can be rapidly reduced to a predetermined thickness without mechanical polishing. Then, a backside illumination type image sensor is manufactured by the wet etching technology described above, and thus it is possible to expect significant increase in production efficiency.