Method for patterning poly(acrylic acid) sacrifical layers for use in solder-based self-assembly
Solder-based self-assembly is a method for micromachining three-dimensional structures on silicon. This process has been used for educational purposes due to the significant experience gained by students interested in semiconductor processing. However, patterning the silicon dioxide sacrificial layer, involves handling hazardous materials such as buffered hydrofluoric acid. To provide a safer alternative to this dangerous etchant, we describe a method for using poly(acrylic acid), a water soluble polymer, as a patternable sacrificial layer in the solder-based self-assembly process. Key to this method, in order of steps; is the use of thermal crosslinking, a partial development technique; oxygen plasma ashing; and a strong alkaline solution to etch the sacrificial layer and reflow the solder. Plasma ashing is important to keep water and alkaline developers from etching the vulnerable poly(acrylic acid) layer too early in the process. This method is able to achieve the goal of promoting the self-assembly of three-dimensional structures on silicon. Beyond its use in our solder-based self-assembly process, this method may also prove useful in any application requiring the use of a patternable sacrificial layer made from poly(acrylic acid), as well as other polymers of a similar nature. One such application is explored, in which the technique for patterning poly(acrylic acid) is used as a potential way to pattern crystal colloidal masks of nanospheres for use in nanosphere lithography. This technique allows for spin-coating, an inexpensive deposition technique, to be used to both form and pattern these nanosphere masks with micro-scale features.