(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to fabricate a stripe ph todiode element for an image sensor cell.
(2) Description of Prior Art
Active pixel, or image sensor cells, are usually comprised with a photodiode element, used to collect photon energy for the cell. The efficiency of the photon collector, or the photodiode element, usually comprised of a N/P diode, is directly related to the depletion region of this diode. The depletion region in turn is a function of the doping level of the components of the N/P photodiode element, as well as the area of the photodiode. However trends to micro-miniaturization have resulted in decreasing space allotted for the image censor cell, and thus smaller, less efficient photodiode elements are used.
Prior art, such as Merrill, in U.S. Pat No. 5,789,774, as well as Lee et al, in U.S. Pat. No. 5,904,493, describe methods of fabricating active pixel cells, however these prior arts do not describe active pixel cells featuring a stripe photodiode design, and the method used to fabricate this type of photodiode element, now described in the present invention.
It is an object of this invention to fabricate an image sensor cell featuring a photodiode element configured with a stripe, or serpentine type, design.
It is another object of this invention to fabricate the stripe photodiode element by forming a serpentine type, N type region, in a P well region.
It is still another object of this invention to increase the depletion region of the stripe photodiode element via reducing the doping concentration of the N type region of the stripe photodiode element.
It is still yet another object of this invention to design the width of the N type serpentine like region nearly equal to the depletion width of the junction, so that vertical feature of the photodiode structure is fully depleted.
In accordance with the present invention a method of fabricating a stripe photodiode element, for an image sensor cell, featuring a low dopant concentration, N type serpentine type region, in a P well region, is described. After forming a P well region, in a top portion of a P type semiconductor substrate, a serpentine like, N type region is formed in a top portion of the P well region, via conventional photolithographic masking, and ion implantation procedures, creating the stripe photodiode element. A low doping concentration is used for the N type serpentine type region, allowing an increased depletion region to be achieved. In addition the width of the N type serpentine type region is designed with a width equal to the Width of the photodiode depletion region. A heavily doped P type region, used for a low resistance contact region, is formed in the P well region of the photodiode element, while a heavily doped N type region is formed in the N type serpentine like region, again for use as a low resistance contact region.
This invention will describe a stripe design, as well as a process for fabricating, a stripe photodiode element, featuring a serpentine like, lightly doped N type region, located in a P well region. The use of this type of photodiode element, comprised with serpentine like N type region, in a P well region, increases the area of the photon collector, via use of the three dimensional interface between the elements of the diode. In addition to the increased area of the photodiode element, the concentration of the N type serpentine pattern is reduced, also allowing an increased depletion region to result. The result of the additional collector, or photon collector area, as well as the increase in depletion region as a result of a reduction in doping levels, translates to increased quantum efficiency, for the photodiode element, when compared to counterparts fabricated with two dimensional photodiode structures, and fabricated with higher doping levels for the photodiode regions.
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