A solder based self assembly process to form metal embossing on three dimensional structures

Conventional lithography techniques are limited to two-dimensional (2D) patterning and virtually rules out writing on the sides of the three dimensional (3D) structures. The advantages of patterned 3D micro-scale structures, in general are many, however existing technology does not provide cost-effective and robust schemes. Self assembly is seen as an attractive strategy for constructing highly parallel 3D micron scale structures that is described previously in our laboratory. Self assembly, in general is driven by spontaneous formation of structures to reach a minimum possible energy state. In this paper, we focus on forming different micro scale patterns on the sides of the 3D structures using a novel and robust solder based self assembly (SBSA) process. The SBSA method is demonstrated to obtain oblique patterns on the 3D side faces. SBSA includes planar techniques such as metal deposition, electroplating, wet-etching, optical-lithography, and a robust dip soldering method. Three different designs are patterned on the sides of the 3D structures in the form of inlaid designs, also called as damascene structures. Slight modification to the above process provides same designs in the form of emboss structures on 3D sides. The lowest feature size of these emboss and damascene structures are in the lower micron range. The desired designs are metal patterned on the 2D sacrificial layer. The 2D patterns are then dip soldered using a low melting point solder alloy. The etching of sacrificial layer frees the metal faces from the substrate except the central metal face that remains attached to the substrate. Finally, the patterned metal faces are self folded via solder reflow method to form patterned 3D faces which are filled with void-free solder. The folding yield of emboss and damascene patterns are compared and high resolution SEM images are analyzed.