Stress compensation for arbitrary curvature control in vanadium dioxide phase transition actuators

Abstract

Due to its thermally driven structural phase transition, vanadium dioxide (VO2) has emerged as a promising material for micro/nano-actuators with superior volumetric work density, actuation amplitude, and repetition frequency. However, the high initial curvature of VO2 actuators severely obstructs the actuation performance and application. Here, we introduce a “seesaw” method of fabricating tri-layer cantilevers to compensate for the residual stress and realize nearly arbitrary curvature control of VO2 actuators. By simply adjusting the thicknesses of the individual layers, cantilevers with positive, zero, or negative curvatures can be engineered. The actuation amplitude can be decoupled from the curvature and controlled independently as well. Based on the experimentally measured residual stresses, we demonstrate sub-micron thick VO2 actuators with nearly zero final curvature and a high actuation amplitude simultaneously. This “seesaw” method can be further extended to the curvature engineering of other microelectromechanical system multi-layer structures where large stress-mismatch between layers are inevitable.

Single‐Crystalline Vanadium Dioxide Actuators

“…The presence of metastable phases in VO 2 of bimorph actuators leads to complex actuation behaviors and remarkable hysteresis . Third, during the device fabrication, the built‐in residual stress in VO 2 or SL usually generates an evident initial curvature . Finally, the device performances are very sensitive to the quality of interface between the two layers.…”

“…Despite the well‐developed microfabrication technologies, the bimorph actuators suffer from the risk of the fall‐off of SL due to the high humidity or oxidation. Lots of efforts have been made to optimize the quality of VO 2 materials (films and single crystalline VO 2 nanobeams) and SL materials, as well as device fabrication process, which are quite helpful to promote the performance of VO 2 ‐based bimorph actuators . To overcome the intrinsic limits above, a new strategy for the mechanical device basing on a monomorphic VO 2 structure is still demanding.…”

A 0.2 V Micro‐Electromechanical Switch Enabled by a Phase Transition

“…The mechanical properties (e.g., residual stress, density, Young's modulus, and Poisson's ratio) were adopted from previous reports as well as our measured results . In order to simulate the phase transition of the polycrystalline VO 2 film, temperature‐dependent electrical conductivity, thermal conductivity, and effective internal stress of the VO 2 film were obtained by combining the above parameters in the I phase and the M phase with the measured VO 2 resistance–temperature curve . The latent heat of the VO 2 phase transition was integrated into the temperature‐dependent heat capacity of VO 2 …”

“…The main barriers lie in the uncontrolled and nonflat initial curvatures of the VO 2 bimorphs, as well as the chemical vulnerability of VO 2 in the fabrication process. The former obstacle has recently been tackled with a “seesaw” method to balance the initial strains . Based on this method, we present the first VO 2 phase‐transition MEM switch fabricated on a single chip, with ≈0.2 V operating voltages, ≳10 6 operating cycles, and potential applications such as an ultralow power consumption temperature alarm.…”

“…To perform proper switching, the initial (before the VO 2 phase transition) and final (after the transition) cantilever curvatures must be carefully controlled. The “seesaw” method, which utilizes the residual stresses in all involved films for delicate stress compensation and curvature control, was used in the design, resulting in the required thicknesses of Au, Cr, and VO 2 layers to be 95, 36, and 160 nm, respectively. With such a design, at room temperature (when the VO 2 is in the I phase), the cantilever slightly bends upward, so the switch is at the OFF state with an air gap between the moving and the bottom electrodes.…”