Island Edge Coverage By Metal Interconnects for Three Dimensional Circuits
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Island Edge Coverage By Metal Interconnects for Three Dimensional Circuits Rabin Bhattacharya and Sigurd Wagner Department of Electrical Engineering, Princeton University Princeton, NJ 08544, U.S.A. Abstract Three-dimensional integrated circuits is a new and exciting field that has been spurred on by the need for conformal displays and detector arrays. These circuits are fabricated by interconnecting rigid sub-circuit islands made on a flat, deformable substrate. To prepare for interconnects, we apply a patterned deformable sacrificial material over the flat substrate. We then deform the substrate, with the sub-circuit islands and the sacrificial pattern on it, to a spherical surface. Following deformation, we evaporate interconnect metal and then liftoff the sacrificial material, leaving only the metal lines that interconnect the islands. The maximum size for a circuit island to remain intact is proportional to the island’s thickness. The larger the island has to be, the thicker it must be made to prevent it from cracking. For 50 µm thick Kapton E substrates, it was found that 40 µm square SiNx islands must be at least 0.5 µm thick. With such thick islands, sidewall coverage during evaporation of the interconnect metal becomes difficult because of the dome shape of the substrate. Moreover, during substrate deformation, there is delamination that occurs at the island-substrate interface. This delamination forms a gap between the edge of the island, and the substrate. Thick metalization that covers both sidewall and islandsubstrate gap, produces a yield of 97% for 10 micrometer wide aluminum lines. Introduction Flexible electronics is a new and evolving field of both electrical and mechanical engineering. It encompasses many different applications including sensitive skin, foldable displays, and electronic textiles. In particular there has been growing interest in the creation of electronics on conformal surfaces. Such applications have been targeted for photo-sensor arrays for use in small optical imaging devices, and curved displays for use in portable electronics. It has been demonstrated that simple electronic devices and circuits can be integrated onto a spherical shell.1 Such devices can only be a viable technology if they can be interconnected. Presented here is a method for fabricating X-Y matrix metalization on a dome shaped surface. Using similar principles to those used in the fabrication of circuit islands on a spherical surface, a matrix with 58.5 mm long lines and widths down to 4 µm was fabricated on a spherically deformed polymer foil. Experiments The approach taken for fabrication of an X-Y matrix on a spherical dome was to first fabricate all of the electronics on the flat polymer foil, pattern a flexible photoresist interconnect pattern over the electronics, deform the foil together with the electronics and the interconnect pattern, evaporate metal over the pattern, and finally liftoff the pattern. The electronics are built on rigid thin film square islands that allow them to be deformed w
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