Laser micro-machining of hydrophobic-hydrophilic patterns for fluid driven self-alignment in micro-assembly

Fluid driven self-alignment is a low cost alternative to fast but relatively inaccurate robotic pickand-place assembly of micro-fabricated components. This fluidic self-alignment technique relies on a hydrophobic-hydrophilic pattern on the surface of the receiving substrate, which confines a fluid to a receptor site. When a micro-component is dropped on the fluid capillary forces drive the assembly process, resulting in accurate positioning of the part relative to the site. This paper demonstrates the advantages of the use of an ultra short pulse laser, with pulse durations in ps regime, to create receptor sites (ranging from $110x110 \mu m^2$ up to $5x5mm^2$) from which liquid spreading is stopped by a sharp geometrical modification around the site. It was found, by video based optical contact angle measurement, that the volume of water that is pinned on the receptor site increases with increasing angle of the edges of the receptor site. In addition, it was found, by using a robotic microassembly system, that the success rates of self-alignment of $110x110 \um m^2$ parts, as well as $200x200 \um m^2$ parts, on the receptor sites is 100% if angle of the edges of the receptor site are sharp, and the height of the receptor site is well over the initial surface roughness of the substrate.