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True 3D Through Hole Interconnection Formed by Femtosecond Laser Irradiation/Wet Etching and Molten Metal Suction Method
Keywords: true 3D through hole interconnection, curved-holes and branched-holes, motlten metal suction method
Recently there have been outstanding technological advances in making through hole interconnections (THIs), which can be applied to Micro Electro Mechanical Systems (MEMS) or high-density packaging. For this purpose, dry etching technique i.e. Deep Reactive Ion Etching (DRIE) is often used, because this technique makes it possible to form high aspect ratio through holes connecting from one side of a wafer to the other vertically. Due to the demand of miniaturization of devices, higher-density and smaller packaging, THIs are believed to be one of the most promising candidates for these. We would like to propose true 3-dimensional thorough holes forming process using femtosecond laser irradiation/Wet etching and Molten Metal Suction Method (MMSM). It enables to form complicated holes such as curved-holes or branched-holes which can never be done by DRIE and gives us much more freedom in the design of THIs. Our process is outlined below. In this study, Ti Sapphire laser was used as a light source, and quarts, sapphire and borosilicate glasses were used as a substrate. First, femtosecond laser beam is focused into a substrate according to 3-dimnesional micro patterns and laser focused areas change its material structures by multi photon absorption. Second, the laser-irradiated areas are removed by wet etching. Since the etching rate of the areas is much faster than non-irradiated area, 3-dimensional through holes are obtained successfully. Finally a conductive material is filled into the through holes by MMSM, which can fill a metal into even high aspect ratio holes applying differential pressure between through holes and MMSM chamber. In this experiment, Gold-Tin (Au:80wt%-Sn:20wt%) solder was used. Some characteristics such as air-tightness and conductivity of them are examined. It was confirmed that this process can be applicable to MEMS and high density packaging which needs not only electrical connections but also air tightness.
Osamu Nukaga,
Fujikura Ltd.
Sakura, Chiba 285-8550,

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