Colour sensors based on active interference filters using silicon-compatible materials

Multilayer stacks of dielectric materials and polycrystalline silicon (with photodiodes therein) deposited on a silicon substrate can be used to result in an 'active' structure that filters and detects light in its 'active' regions at the same time. Such a structure can be employed to design optical detectors with a programmable spectral response by weighted summing of the photocurrents supplied by polysilicon and monocrystalline silicon photodiodes. Such thin-film colour sensors provide improved response flexibility and fabrication compatibility with standard microelectronic processing. Furthermore, such a realization facilitates the implementation of on-chip smart colour-imaging sensors and features efficient area occupation by vertical stacking of the colour detectors. This paper presents the design methods, the fabrication and the characterization of such a device. Results of the measured optical properties of the available silicon-compatible materials are also presented, together with the practical considerations required to implement photodetectors in polysilicon. The measurement results of a fabricated colour sensor indicate that sufficient spectral selectivity can be achieved. (C) 1997 Elsevier Science S.A.