Bending in VO2-coated microcantilevers suitable for thermally activated actuators

Abstract

The curvature of VO 2-coated silicon microcantilevers was measured as the temperature was cycled through the coating's insulator-to-metal transition ͑IMT͒, which drives the curvature change mainly through the strain generated during this reversible structural transformation. The films were grown by pulsed laser deposition ͑PLD͒ on heated substrates. Cantilever tip displacement was measured for a 130 m long cantilever as the temperature was changed by recording the deflection of a laser beam, and the curvature change and estimated film stress were calculated from this data. A change in curvature of over 2000 m −1 was observed through the narrow temperature range of the IMT, with a maximum rate of ϳ485 m −1 per degree. Estimated recoverable stress was ϳ1 GPa through the transition region. These results suggest applications in actuator devices with reduced dimensions, including submicron lengths, multifunctional capabilities, and possibly with higher operational frequencies than other thermally actuated devices.

Performance of Electro-Thermally Driven ${\rm VO}_{2}$-Based MEMS Actuators

“…The contraction of the VO 2 crystal in the c r direction causes overall contraction of the film in the direction parallel to the substrate as it undergoes the SPT. This contraction can be used to create large bending in bimorph structures, hundreds of times larger than the bending achievable by thermal expansion alone [1], [4].…”

“…Studies of the IMT in VO 2 nanobeams were done recently [21], [22], and in 2010 it was observed that a VO 2 -coated 350 μm-long single crystal silicon (SCS) cantilevers experienced curvatures over 2000 m −1 when heated across VO 2 's T Tr [1]. Such phenomenon was not a result of the difference in thermal expansion coefficients ( α) between the SCS and VO 2 , since the observed strain was much larger than what could be achieved by this mechanism.…”

“…The mechanism consisted on the use of the crystallographic changes of vanadium dioxide (VO 2 ) thin films across its structural phase transition (SPT), which occurs at ∼68°C. When highly oriented VO 2 thin film coatings are deposited on suspended micromechanical structures, the SPT of the VO 2 induces strains up to −0.32% in the bimorph microactuator, which have been used to tune the resonant frequency of simple VO 2 -coated SiO 2 micro-bridges up to −23% [2], and to generate curvatures in VO 2 -coated single crystal silicon (SCS) cantilevers over 2,000 m −1 [1]. Larger curvatures changes were obtained on initially curved single crystal VO 2 microcantilevers (20,000 m −1 ) [3] and 50 nm thick Cr cantilevers coated with polycrystalline VO 2 (14,000 m −1 ) [4]; which were straightened during the phase change.…”

“…I N 2010, a new mechanism for micrometer-sized actuation was introduced [1]. The mechanism consisted on the use of the crystallographic changes of vanadium dioxide (VO 2 ) thin films across its structural phase transition (SPT), which occurs at ∼68°C.…”

Molecular actuators driven by cooperative spin-state switching

“…The differential thermal expansion of the two layers causes the strip to bend when heated or cooled. The deliberate construction of a bimetallic strip analogue (BSA) designed to amplify motion produced by a phase transition has recently been demonstrated for VO 2 , which undergoes a thermally induced metal-insulator transition above room temperature, concomitant with a 0.3% volume change in the material 9,10 . Very recently, micro-actuators were also created from this same material, and actuation was observed as a direct result of the phase transition 11 .…”