I. Introduction
Over the past several decades, microelectromechanical systems (MEMS) technology has developed rapidly. Numerous MEMS applications have been demonstrated in a wide spectrum of business sectors such as manufacturing, electronics, automotive, information technology, and chemical and biomedical industries. Nowadays, most MEMS devices are based on the silicon- and glass-based processes. Although silicon- and glass-based microfabrications are still mainstream MEMS techniques, the construction materials are being extended from silicon and glass to polymers. In comparison to silicon- and glass-based microfabrication, polymer-based microfabrication, e.g., that with polymethylmethacrylate (PMMA), shows advantages in cost (e.g., 0.2–2 for polymer versus 15–20 for silicon and 10–20 for glass), simplicity in microfabrication procedures, and less consumption of harmful chemicals [1]. More importantly, polymer is transparent and flexible, which eliminates the limitations of optical barriers and mechanical stiffness of silicon in many applications. In addition, fabrication of polymer-based devices does not suffer from the geometrical design limitations and does not involve the usage of many harmful chemicals required by the silicon- and glass-based processes. Thus, microfabrication using polymer as construction materials emerges as a very promising technique for making low-cost and flexible microdevices [2]–[4].