Highly Flexible Piezoelectric Thin Film MEMS Resonators For Wearable Electronic Devices

With the rise of wearable electronics and advanced biosensors,flexible electronic devices and systems with critical features,including small size,light weight,flexibility,and even stretchability,have been investigated intensively in recent years.Presently,flexible electronics have been found in a variety of applications,e.g.,flexible wearable devices,electronic skin,flexible displays,etc.Most of the researches on flexible electronic devices are mainly about the flexible organic or inorganic transistors,but little research has been reported on flexible film bulk acoustic resonators(FBARs)that can operate at higher frequencies(beyond 1 GHz operation frequency).The FBARs,as typical RF MEMS devices,are traditionally used as the basic building blocks for the modern RF filters and oscillators.FBAR-based electronic systems have also been widely used in the biochemical sensing and actuating domains.Therefore,with the flexible FBARs,flexible electronics would found a much wider range of applications.In this thesis,the Flex MEMS technology was utilized to fabricate the flexible FBARs with prominent electrical performance and mechanical flexibility.Our major research achievements and contents are below:1.The equivalent circuit model and finite element model(FEM)are constructed to determine structure of the FBAR.The neutral mechanical plane method is introduced to place the FBAR at the mechanical neutral plane using two polyimide thin films.2.The methods of transfer-printing device onto the non-adhesive substrate and fixing the device to silicon wafer with anchors are proposed.A typical type of elastomer stamp with hundreds of micro-pyramid structures on its surface is developed to retract the FBAR from silicon wafer and then print it onto the fully cured polyimide substrate.A type of special anchor structure is proposed for the purpose of tethering the FBAR to the silicon wafer and facilitating the FBAR's transfer printing.3.To evaluate the mechanical flexibility and durability of the flexible FBAR,its electrical properties are characterized under various bending radii states.The flexible device remains fully functional even when bent into a radius of 0.5 mm and after 2000 bending cycles.The introduction of our flexible device provides innovative opportunities for designing flexible wireless communication systems based on gigahertz resonators.Besides,this method also offers a viable approach for the fabrication of many other flexible MEMS devices.