Curved Silicon Electronics
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Curved Silicon Electronics Linlin Wang, Dieter G. Ast, Material Science & Engineering Peeyush Bhargava, Alan T. Zehnder, Theoretical and Applied Mechanics Cornell University Ithaca, NY, 14853-1501 Abstract An electronic imaging system using a curved image sensor can use a faster lens, and cover a greater field of view, than an imaging system using a planar sensor. The simpler lens systems also weight less, a decisive advantage in portable applications. This paper describes a method to fabricate a curved silicon substrate from a flat wafer containing appropriate circuits. To curve the substrate, the processed wafer is diced, by dry-etching from the backside, into 1x1cm tiles. The tiles are separated by 0.5mm gaps, which are bridged, in turn, by a dense array of 45x100µm gold leads formed by electroplating using lithographically defined leads as seeds. Two methods were used to curve the wafer. In the first one, the wafer was bonded with epoxy to a PMMA disk, and then curved by heating the sandwich, under a load of ~ 230gr, for 1.5 hrs at 1300C in a concave metal mold with a radius of curvature of 7.8cm. In the second method, the wafer was put into a curved metal mold, radius 14cm, loaded with 230gr, and heated to 2900C for 2 hrs. The normal and shear strains accommodated by the flexible interconnects were measured by analyzing their deformation. The experimentally measured strains are compared with a model that calculates the deformation required to deform a flat sheet into a spherical surface.
Introduction As illustrated by the human eye, the image of a convex lens converges on a curved surface. Recording images on a planar surface has historical (glass plates) and technical (electronic sensors are made from flat wafers) reasons and requires formation of a planar optical image. A planar image can be generated using a multi-element lens, but the result is a complex and heavy optical system, which still covers only a comparatively limited field of view. Lighter electronic image converters (such as night-vision goggles) with a wider field of view and using faster lenses could be constructed if curved electronic sensors would be available. Ideally, such curved sensor would be built on a curved silicon substrate. But as neither curved substrates, nor processing equipment capable of handling curved
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substrates are available, a more practical method is to curve a finished flat wafer into a hemispherical shape. This method is compatible with modern imaging technology, which generally using a technology in which the sensing element (e.g. HgCdTe) is flipchip mounted to a CMOS silicon chip. Fabrication The silicon wafer is processed as usually to form the desired circuits. The only limitation is that the size of the tiles in which the wafer is eventually separated is a multiple of the die s
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