A complementary metal oxide/semiconductor (CMOS) image detector now undergoing development is designed to exhibit less cross-talk and greater full-well capacity than do prior CMOS image detectors of the same type. Imagers of the type in question are designed to operate from low-voltage power supplies and are fabricated by processes that yield device features having dimensions in the deep submicron range.
Because of the use of low supply potentials, maximum internal electric fields and depletion widths are correspondingly limited. In turn, these limitations are responsible for increases in cross-talk and decreases in charge-handling capacities. Moreover, for small pixels, lateral depletion cannot be extended. These adverse effects are even more accentuated in a back-illuminated CMOS imager, in which photogenerated charge carriers must travel across the entire thickness of the device.
The figure shows a partial cross section of the structure in the device layer of the present developmental CMOS imager. (In a practical imager, the device layer would sit atop either a heavily doped silicon substrate or a thin silicon oxide layer on a silicon substrate, not shown here.) The imager chip is divided into two areas: area C, which contains readout circuits and other electronic circuits; and area I, which contains the imaging (photodetector and photogenerated-charge-collecting) pixel structures. Areas C and I are electrically isolated from each other by means of a trench filled with silicon oxide.
The electrical isolation between areas C and I makes it possible to apply different supply potentials to these areas, thereby enabling optimization of the supply potential and associated design features for each area. More specifically, metal oxide semiconductor field-effect transistors (MOSFETs) that are typically included in CMOS imagers now reside in area C and can remain unchanged from established designs and operated at supply potentials prescribed for those designs, while the dopings and the lower supply potentials in area I can be tailored to optimize imager performance.
In area I, the device layer includes an n+-doped silicon layer on which is grown an n–-doped silicon layer. A p–-doped silicon layer is grown on top of the n–-doped layer. The total imaging device thickness is the sum of the thickness of the n+, n, and p– layers. A pixel photodiode is formed between a surface n+ implant, a p implant underneath it, the aforementioned p– layer, and the n and n+ layers. Adjacent to the diode is a gate for transferring photogenerated charges out of the photodiode and into a floating diffusion formed by an implanted p+ layer on an implanted n-doped region. Metal contact pads are added to the back-side for providing back-side bias.
The n– and p– doping concentrations are chosen such that everywhere in area I, a depletion region exists between the n– and p– layers. This depletion region enables electrical isolation between the several front (top) doped regions and the back (bottom) n and n+ layers. Consequently, the bias potentials applied to the top of the diode and the adjacent transfer gate can be different from the bias applied to the bottom. Thus, while CMOS-compatible potentials (e.g., 3 V) are applied at the top, the bottom of the structure can be biased to greater potential (e.g., 5 V) via the back-side metal contact pads to completely deplete the photodiode. The resulting depletion region is indicated in the figure as the area enclosed by the dashed outline. Complete depletion of the photodiode results in collection of charge carriers (holes in this case) under the influence of an electric field, and hence, a significant reduction of cross-talk. Complete depletion also increases the charge-storage volume, and, hence, the charge-handling capacity. Thus, the structure described here provides for large depletion width around each photodiode, independent of the CMOS power-supply voltage and pixel size.
This work was done by Bedabrata Pain and Thomas J. Cunningham of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-45964, volume and number of this NASA Tech Briefs issue, and the page number.