The use of sacrificial layers is a integral part of surface micromachining technology. Currently, silicon dioxide is the most widely used sacrificial layer material. However, since hydrofluoric acid is needed to remove oxide sacrificial layers, many acid-sensitive metals such as aluminum and chromium are not compatible with surface micromachining schemes. Phase I studies demonstrated that a new family of sacrificial layers materials termed alkali-soluble polyimides, or ASI's can be used to fabricate metal microdevices without the usual problems of metal attack. ASI's were originally developed as improved lift-off coatings for integrated circuit manufacturing processes. They exhibit excellent high temperature stability an chemical resistance and, most importantly, they can be removed by mild, noncorrosive enchants. Removal occurs rapidly without attendant swelling that can jeopardize the integrity of overlying structures. Phase II will investigate the basic relationships between polymer structure, etch bath chemistry, and a host of processing variables to understand how they control the behavior of ASI materials. The knowledge base will be invaluable for developing successful ASI-based sacrificial layer products during Phase III. These new products are expected to become an important part of the polyimides business which is currently serving the microelectronics and optoelectronics industries.
