polyimide

H. Yousef, M. Lindeberg, K. Hjort
Department of Engineering Sciences, Uppsala University, Box 534, 75121 Uppsala, Sweden

Abstract

As the call for higher wiring density in packaging and interconnection technologies rapidly evolves, the need for smaller dimensions in vias and interconnects must be met. The frontier of advanced high aspect ratio technologies is today often found within microelectronics and MEMS. The process described in this paper stems from advanced MEMS and allows micromachining of deep, vertical vias in polyimide based foils and flexible-PCBs. The process is superior with respect via throughput and size compared with traditional via manufacturing techniques such as chemical etching, drilling, dry etching and laser ablation.

The technique makes use of ion irradiation to enhance the selectivity and directionality of the chemical etching technique. Within the areas exposed to the ion irradiation, small sub-micron pores (capillaries) are created, one for every ion. If etching is prolonged, the pores become merged. Conventional electrodeposition from a metallic seed layer is used to fill these structures with metal. The smallest achievable size of the vias is only limited by the resolution of the mask; vias of below 10 μm in diameter can readily be achieved in a 75 μm thick polyimide foil. It is possible to obtain two inherently different types of via structures using this process: (1) conventional solid vias, and (2) vias consisting of bundles of sub-micron wires (having a specified metal density between 0.2 and 20%). As the individual sub-micron wires have aspect ratios of several hundreds, this allows fabrication of truly vertical via structures, allowing ultra high-density wiring.