A comprehensive study of ion track enabled high aspect ratio microstructures in flexible circuit boards

Abstract

A process to form deep, vertical and high aspect ratio microstructures of solid as well as porous nature is presented. The process is capable of producing regions with perpendicular sub-micron metal wire connections, with a regulated effective metal density at numerous, arbitrarily specified locations. The structures are created in a two-metallic-layer polyimide laminate, i.e. a flexible printed circuit board. The high aspect ratio of the process is indebted to ion track technology. The laminate is irradiated with heavy ions creating a vertical damage anisotropy (individual ion tracks) in the polymer layer. Apertures in the front metallic layer define the geometry and the positions of the vertically projected structures. The tracks are selectively developed forming nanometer wide pores, which after prolonged etching grow in diameter and eventually merge creating fully opened cavities. Metallic structures have been replicated in these pores/cavities by electrodeposition of nickel and copper. We have fabricated open and dense clusters of separated micron or sub-micron sized wires as well as solid structures. Highly vertical, through hole microvias in average 39 μm wide, with a pitch of 100 μm have been fabricated. The smallest structures obtained were 25 × 25 μm square columns. The process appears promising for ultra-high density via batch production and has a strong potential of further miniaturising via dimensions.