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 degrees 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.