A Piezoresistive MEMS Memory Device Using a Buckled Beam Jerry A. Yang*, Pranoy Deb Shuvra**, Ji-Tzuoh Lin**, Kevin Walsh**, Shamus McNamara** and Bruce Alphenaar** * University of Texas at Austin, Austin, TX, USA, jerryyang747@utexas.edu ** University of Louisville, Louisville, KY, USA, kevin.walsh@louisville.edu ABSTRACT Microelectronics and microelectromechanical systems (MEMS) have given rise to a large class of new de- vices, from ultra-miniature sensors for cell phones and automobiles to complex memory devices for computers. Recent research in memory devices has largely focused on designing new non-volatile forms of electronic mem- ory, yet a MEMS-based memory device has not been proposed in the literature. This work presents a novel MEMS memory device using an asymmetrical, bistable buckled beam. Relying on well-known MEMS structures and the piezoresistive effect, the MEMS memory model was first conceptually designed, then optimized for sig- nal strength using CoventorWare finite-element model- ing. In the optimization process, various aspects of the devices geometry were ranked in order of magnitude of influence on the devices output signal strength, then op- timized based on their importance. Simulation results indicate that the optimized device can generate signals up to 5.5uV with a supply voltage of 2.5V, a 27.5x im- provement over the initial design. The length and width of the beam were found to be the most influential fac- tors in controlling the signal output: increases in beam width lead to significant increases in signal when paired with the corresponding beam lengths. However, large beam widths caused the beam to buckle into higher- order modes when the beam was short, leading to sharp decreases in signal. Other geometric factors had only minor impacts on signal strength. The MEMS memory device paves the way for future low-power, radiation- hard MEMS memory models. Keywords: MEMS, memory, optimization, modeling 1 INTRODUCTION As computers become increasingly powerful, inno- vative memory types and architectures are required to store and process the increasingly complex tasks com- puters complete. Memory devices must be small and use little power, yet be insensitive to radiation and scalable to industry standards. Recent research in nonvolatile memory has generated numerous unique approaches and designs, from field-effect transistor (FET) devices [1] made from organic materials to flexible all-carbon de- vices [2]. Other non-volatile memory research has also focused on electrochemical and resistive switches [3]. One field that has not contributed to the plethora of current non-volatile memory devices is microelectrome- chanical systems (MEMS). The rapid growth of MEMS applications has caused a proliferation of MEMS devices such as pressure sensors, gyroscopes, accelerometers, mi- crofluidic devices, RF devices and more [4] [5]. Yet there have only been a few MEMS memory devices reported in the literature. Reference [6] showcases a MEMS mem- ory design using a suspended gate MOSFET, and [7] describes a cantilever device that uses vibrational de- actuation for high-temperature operation. While the MOSFET device in [6] is easily scalable and the can- tilever in [7] is robust under extreme temperature con- ditions, neither device has been shown to be low-power or radiation-hard. This paper proposes a novel MEMS memory device utilizing a bistable buckled beam. The paper first de- scribes the theoretical development and operation of the model, then proceeds with the models geometric opti- mization. Several trends discovered in the optimization process are discussed, and limitations of the optimized device model and future directions are considered. 2 DEVICE MODEL 2.1 Development The first MEMS memory model proposed was a sili- con cantilever beam that had a symmetric base relative to the beam axis, first reported in [8]. The state of the memory device would be determined by the piezoresis- tance of the base when the beam was bent one way or the other. Due to the symmetry of the device, there was a negligible piezoresistance between the two states. To increase the difference in piezoresistance of the de- vice, the base was widened on one side of the beam, cre- ating an asymmetrical base. However, the device was not bistable: without constant actuation, the cantilever would return to its upright (non-bent) position. The current model uses a lateral bistable buckled beam de- rived from [9]. To ensure that the piezoresistance of the device would be different in both buckled states of the beam, one side of each of the bases was thickened as shown in Fig. 1. Since silicon has a high piezoresistivity TechConnect Briefs 2019, TechConnect.org, ISBN 978-0-9988782-8-7 330