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This paper describes a new pillar-shaped via structure in a Cu-polyimide multilayer substrate and its novel fabrication process. forms a fine rectangular via conductor by pattern electroplating using a thick positive photoresist. Furthermore, a flat polyimide dielectric layer is formed only by the photolithographic process using a photosensitive polyimide precursor. The resulting pillar-shaped via conductor is 30 pm square and 25 thick. The area occupied by this via conductor is 25 % smaller than a conventional via not filled with copper conductor, the interconnection density is twice as high, and the thermal resistance is 50 % less. This new via structure is suitable for high-speed signal transmission in a Cu-polyimide multilayer substrate.

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A compact 40-Gb/s optical receiver module with an MUconnector interface has been developed. Its packaging has three main technical features. (I) Coplanar waveguide (CPW) patterns of the waveguide photodiode (WG-PD) and of the preamplifier IC in the facing a m of the flip-chip structure are optimized for impedance matching. (2) A film carrier is used to connect the preamplifier IC to an electrical coaxial connector for electrical signal output. (3) An MU-connector is used as the optical interface to reduce the module size. Optimum design enabled a module size of 14.0 mm wide, 40.4 mm long, and 9.65 mm high. Measurements showed a 3-dB down bandwidth of the optical/electrical response of at least 50 GHz and a clear open eye pattern for a 40-Gbls non-returnto-zero (NU) signal input. This optical receiver module is suitable for large-capacity communication network systems.

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