Water-Soluble Sacrificial Layers for Surface Micromachining

This manuscript describes the use of water‐soluble polymers for use as sacrificial layers in surface micromachining. Water‐soluble polymers have two attractive characteristics for this application: 1) They can be deposited conveniently by spin‐coating, and the solvent removed at a low temperature (95–150 °C), and 2) the resulting layer can be dissolved in water; no corrosive reagents or organic solvents are required. This technique is therefore compatible with a number of fragile materials, such as organic polymers, metal oxides and metals—materials that might be damaged during typical surface micromachining processes. The carboxylic acid groups of one polymer—poly(acrylic acid) (PAA)—can be transformed by reversible ion‐exchange from water‐soluble (Na+ counterion) to water‐insoluble (Ca2+ counterion) forms. The use of PAA and dextran polymers as sacrificial materials is a useful technique for the fabrication of microstructures: Examples include metallic structures formed by the electrodeposition of nickel, and freestanding, polymeric structures formed by photolithography. Wetter is better! Water‐soluble sacrificial layers are useful for the rapid fabrication of all‐polymer microsystems, and extend the capabilities of conventional micromachining. A reversible ion‐exchange reaction with calcium ions renders the initially water‐soluble polymer insoluble in water. To illustrate the capabilities of this method, the SEM image shows a microfabricated bridge made of epoxy, stretching over a sheet of plastic.