Characterization of aligned wafer-level transfer of thin and flexible parylene membranes

This paper reports a wafer-level transfer technique for forming thin, flexible, and freestanding Parylene membranes. Parylene thin films (similar to 1.3 pm) have been successfully transferred from one wafer to another to form a freestanding membrane encapsulating over wide and shallow cavities (< 5 mu m deep and 2000 x 2000 mu m(2) square) with fine alignment (< 3.0 mu m) and 87% yield. Transferred membranes may be a composite of Parylene/metal/Parvlene, contain through-hole patterns of diverse size (5 x 5 similar to 2600 x 2000 mu m(2)), have mild tension (1.14 MPa), and remain freestanding and flat through various standard post-transfer microfabrication processes such as photolithography.. evaporation, and wet etching. They also provide excellent sealing against pressure of up to 20 kPa and long-term stability over repeated deflection. This paper focuses on two areas: 1) the study of issues involving optimum transfer conditions, minimum achievable gap between transferred membranes and device wafers, patterned-film and composite-laver transfer, and aligned transfer, and 2) the characterization of the post-transfer membrane properties, including stress/tension, sealing capability, effects of post-transfer processing, and long-term stability after a repeated deflection.