Membrane-less in-plane bulk-micromachined thermopiles for energy harvesting

This paper describes the process development for film-based thermopiles suited for energy harvesting on low-temperature heat sources. The patented design [1], which consists of a silicon frame with alternately doped poly-Si (or poly-SiGe) legs bridging the gap between the hot and cold sides of the frame, can be fabricated at low cost using contact lithography on standard Si wafers. A generic process flow was developed to fabricate thermopile structures on membranes of SiO"2/Si"3N"4, Si"3N"4 and low-stress Si"xN"y as well as self-supported thermopiles. The mechanical properties of poly-Si based structures with Al interconnects are compared for a variety of membrane supports. Fabricating the thermopile structure without silicon nitride membrane to eliminate parasitic heat flow through the membrane is also found to improve the mechanical stability considerably as it improves yield and shows minimum deformation.