Monolithic silicon micromechanical metastructures for compliant MEMS devices.

Mechanical metastructures (MMs) have enabled exotic and programmable mechanical properties in artificial materials, which have attracted considerable interest for applications like energy harvesting, energy absorption, sensing, and biomedical devices. Although MMs have already been realized based on materials including metals, polymers, and ceramics, there has been no demonstration of MMs with single crystalline silicon (Si) in microscale, which is limited by precise 3D manufacturing technologies on monolithic Si substrates. By actively engineering the mechanical properties of Si microstructures with an MM design, we have created the possibility for a broader range of functionalities in Si-based microelectromechanical systems (MEMS) and microchips. In this study, we report micromechanical metastructures (µMMs) fabricated with monolithic silicon materials on a wafer-scale. Freestanding µMMs were fabricated and their mechanical properties were characterized using a microprobe station. With a rational design of unit cell geometries, we achieve negative Poisson's ratio (-0.25 to -1.0), maximum strain up to 20% both tensile and compressive, and an effective Young's modulus of 5 MPa to 13 MPa. Moreover, we have demonstrated transfer of large areas of Si µMMs (5 mm by 2.5 mm) into flexible polymer substrates to achieve hybrid structures, which exhibit compliant mechanical properties with an effective Young's modulus down to 0.42 MPa. The embedded µMMs remain electrically conductive with applied tensile strain up to 20%, implying potential for flexible and wearable MEMS devices.