MEMS cantilevers for dynamic strain studies of 2D materials

Abstract

Photoluminescence properties of transition metal dichalcogenide (TMDC) 2D materials are known to vary sensitively with strain. There have been a few attempts to create tunable mechanical strain using silicon MEMS techniques, but these initial devices were limited in the maximum strain achievable, ability to operate in cryogenic environments, or ability to generate strain dynamically. This project is developing microelectromechanical systems (MEMS) for dynamic electronic control of external strain in 2D materials. These electrostatically actuated cantilevers are intended for either vertical or lateral continuous deflection or full snap-down (or to the side for lateral case), with up to 2 micron of vertical travel and a range of 2-10 micron for the lateral cantilever. According to geometrical calculations, these cantilevers could deliver up to 5.9% strain with vertical motion, and up to 33.3% strain with lateral motion. Such strain is more than sufficient to transition some 2D semiconductors from a direct to an indirect bandgap material and fully modulate light emission processes. In this project test coupons with vertically and laterally actuated cantilevers were created using silicon-on-insulator wafers. The benefit of making use of SOI wafers is a thick device layer leading to robust cantilevers that will survive the flake transfer, and an easy fabrication process compared with previous technologies of MEMS fabrication. It has been shown that thicker device layer SOI wafers could reduce the stress-induced elevation of the cantilevers above the substrate plane after release etch. BOX layer thickness of 2 micron allows for reasonably low actuation voltage while keeping the vertical travel long enough for inducing considerable strain. In order to eventually operate the chip in the cryostat for cryogenic characterization of the strained material, highly Boron-doped SOI wafers have been purchased. Gold assisted exfoliation and dry transfer techniques have been successfully used for transferring 2D TMDC over the MEMS platform for PL measurements. Based on PL measurements and comparing with literature data reported about WSe2, we could confirm about 0.031% of strain that could be induced at room temperature with MEMS actuation. This is lower than the geometric lengthening we predict, which we attribute to poor adhesion and sliding of the flake over the cantilever. This suggests that future work may profit from a mechanism to clamp the flake on the cantilever as well as the anchored surrounding substrate, in order to increase the amount of strain the MEMS can deliver to the flake.