1. Field of the Invention
The present invention relates to a solid-state image sensor which includes a plurality of pixel cells that generate signal charges in accordance with incident light.
2. Description of the Related Art
A gettering technique has been a known technique to form a gettering layer on the reverse side of a semiconductor wafer so as to collect and capture metallic contaminants present thereon. The background of such a gettering technique is described in LSI Handbook, Japanese Institute of Electronics, Information and Communication Engineers, first printing of first edition, pp 358-364, published by Ohm-sha.
Also, Japanese Unexamined Patent Application Publication No.2002-43557 describes an example in which the gettering technique is applied to a solid-state image sensor. Herein, gettering layers are stacked outside a well (mainly under the well) that surrounds a pixel region.
If a solid-state image sensor is contaminated with metal or the like, in general, a dark output occurs from the contaminated area, thereby reducing the S/N of an image signal.
In particular, the metal contamination is conspicuous in a pixel region formed in an epitaxial layer. Contaminants of such metallic impurities include metals in a material gas for epitaxial growth or those used for processing equipment (such as gas conduits). The dark output derived from the metallic impurities directly acts upon the pixel region in the epitaxial layer, whereby the S/N of an image signal is significantly reduced.
In this context, the inventors of the present application made experiments on contamination with iron in the processing step, which is a main factor of the dark output and often used in processing equipment. In the experiments, solid iron was dissolved on a silicon substrate which had been heated to 900° C. The result was that the substrate was contaminated with the iron from its surface up to a depth of 5 μm, which may cause degradation in device characteristics such as the dark output.
According to the experiment results, it can be assumed that the metal contamination occurring in the processing steps of the solid-state image sensor is to have direct effects on the pixel region in the vicinity of the surface.
However, the gettering layer is formed on the reverse side of the substrate or under the well by the aforementioned conventional gettering technique. This causes a problem that gettering capability is not sufficient in the pixel region which is intensively contaminated with metal because the gettering layer is substantially spaced away from the pixel region (the substrate surface).
With the progress of device miniaturization of the solid-state image sensor, in particular, the overall gettering capability reduces as temperature in the processes lowers, so that the contamination cannot be sufficiently eliminated from the pixel region.