"Hybrid imaging technology" (HIT) is the name of a discipline in which the advancement of electronic image sensors is pursued via hybridization of charge-coupled-device (CCD) and complementary metal oxide/semiconductor (CMOS) circuitry. The guiding principle of HIT is to combine CCD and CMOS components into units that afford capabilities that neither CCD nor CMOS circuitry can provide by itself. HIT can be applied to advantage in almost any situation in which there are requirements for very high imaging quality and low power dissipation. Applications can include portable video and portable digital still cameras, remote surveillance cameras, and low-power cameras in spaceborne and terrestrial scientific instrumentation.

HIT merges the exceptional quantum efficiencies, fill factors, broad spectral responses, and very low noise levels of CCDs with the low power levels, system-integration capabilities, and low costs of CMOS-based active-pixel sensor (APS) circuits to create the next generation of high-performance image sensors. Some of the related research outside HIT has included attempts to merge CCD and CMOS components at the device-fabrication-process level. Although these attempts have yielded working image sensors, the devices have exhibited poor image quality and high noise because of lack of optimization of CMOS processes. In turn, the lack of optimization has been due to a basic incompatibility between CCD and CMOS processes as they relate to processing temperatures and to required oxide thicknesses for CMOS transistors.

An essential element of the HIT approach is that no attempt is made to unite CCD and CMOS devices at the device-fabrication-process level; instead, the CCD and CMOS components of a given device are fabricated in separate CCD and CMOS processes and then joined mechanically and electrically (hybridized) by bump bonding. This element of HIT makes it possible to avoid costly process development. This approach also makes it possible to optimize the CCD and the CMOS parts independently, in such a way as to maximize the overall performance of the resulting image sensor in a highly miniaturized format.

Another advantage of HIT is that it enables the reuse of CCD imaging devices and CMOS readout circuitry without need for costly refabrication. A supply of unhybridized components can be maintained so that combinations of components can be selected to satisfy requirements in specific applications.

The imager integrated-circuit chip of an HIT image sensor is essentially a CCD chip, except that the on-chip amplifier usually found in such a device has been replaced by either a floating diffusion or a floating gate output node. The companion CMOS chip must contain a charge-to-voltage conversion amplifier similar to an operational amplifier configured as a charge integrator. Depending on the application, the CMOS chip could also contain additional circuitry to perform such functions as correlated double sampling and analog-to-digital conversion. Matching bump-bond pads are formed on the CCD imager and CMOS chips during their respective fabrication processes. Indium bumps are deposited on the pads, and the chips are joined by standard bump-bonding techniques.

This work was done by Mark V. Wadsworth of Caltech for NASA's Jet Propulsion Laboratory. NPO-20542

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Hybrid imaging technology

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