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 (~1.3 mum) have been successfully transferred from one wafer to another to form a freestanding membrane encapsulating over wide and shallow cavities (< 5 mum deep and 2000 times 2000 mum² square) with fine alignment (< 3.0 mum) and 87% yield. Transferred membranes may be a composite of parylene/metal/parylene, contain through-hole patterns of diverse size (5 times 5 ~ 2000 times 2000 mum²), 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-layer 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.