Ion track enabled multiple wire microvia interconnects in printed circuit boards

As the call for higher wiring density in packaging and vertical microvia interconnections (microvias) rapidly evolves, the need for smaller lateral dimensions in printed circuit boards (PCB) microvias must be met. The ion track lithography described in this paper allows for high throughput micromachining of small, deep, vertical microvias in flexible PCB and all-polymer laminates. Ion track lithography makes use of swift heavy ion irradiation to enhance the selectivity and directionality of chemical etching. 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. Electrodeposition from a metallic seed layer is used to fill these structures with metal. The lithography masks define either the areas where the ion tracks are developed or where the tracks are metallized. The smallest achievable size of the microvias is only limited by the resolution of the mask; microvias below 10 mu m in diameter can also be achieved also in thick polyimide foils. Since each impinging ion forms one track, the foil's porosity can be controlled by adjusting the irradiation dose, as well as by etching the pores to a suitable size. Depending on the porosity and material, the resultant metallized microvia consists of either individual or interlaced wires (like strands in a bundle wire), or is a solid. As an individual sub-micron wire may have an aspect ratio of several hundreds, this allows for the fabrication of truly vertical microvia structures, allowing ultra-high density microvia batch production. Demonstrator microstructures with highly vertical microvias have been fabricated in foils up to 125 mu m thickness. Several components integrated in flexible PCB have been presented by us, e.g. magnetoresistive sensors, thermopile IR-sensors and microwave components like inductor elements. (C) 2007 Elsevier B.V. All rights reserved.