Our activities aim at the development of mechanically active nanostructures showing sensing and actuation functions to pave the way towards a new generation of smart nanosystems. A key issue is the development of reliable processes for the fabrication of nanostructures of smart materials having critical dimensions below 100 nm. Novel in-situ measurement methods are required to characterize their physical properties on the nanometer scale. In these dimensions, scaling as well as coupling effects occur. Their understanding is a prerequisite for the design of nanodevices.
Shape memory films are virtually predestined for applications in the submicrometer regime due to their high work density and favourable scaling properties. Experiments with nanoindenters demonstrate the existence of martensitic phase transformations down to structural dimensions of 10 nm. One possibility for nanomachining of shape memory films is electron beam lithography. The figure shows typical experiments on a double-beam nanoactuator of Ni-Mn-Ga. Measurements of the electrical resistance and deflection demonstrate the shape memory effect by electro-thermal actuation (Joule heating).
|In-situ deflection of the beam tip during an electrical heating cycle and corresponding normalized electrical resistance of a freestanding Ni-Mn-Ga double beam nanoactuator shown in the inset. The electrical resistance signal is interrupted between 1.1 and 1.95mA due to experimental reasons. Inset: Overlay of SEM images showing the nanoactuator after deformation (A) and after subsequent electro-thermal heating (B). The length, width, and thickness of each nanobeam are 8 µm, 200 nm, and 250 nm, respectively .|
For nanoactuation, we also developed SMA/Si bimorph nanostructures as shown below. These actuators may be combined with Si waveguide structures to develop novel active photonic devices such as nanooptical switches or tunable nanooptical devices. For this purpose, a process flow has been developed to simultaneously fabricate double-beam SMA-Si-bimorph cantilevers, Si waveguide structures and grating couplers on a common SOI wafer, with 340 nm device layer thickness and 2 μm SiO2 layer thickness (see Figure below). Fabrication comprises two-step electron beam lithography (EBL) and pattern transfer to Si by reactive ion etching (RIE).
|Top: SMA-Si bimorph nanoactuator with 50 nm Si beam width and 9 μm beam length. The thicknesses of SMA (NiMnGa) and Si layer are 200 and 340 nm, respectively.
Bottom: Schematic of SMA-Si bimorph nanoactuator and tapered Si waveguide. Actuation is in out-of-plane direction due to the combined shape memory and bimorph effect in the evanescent field of the tapered waveguide .
- M. Kohl, M. Schmitt, A. Backen, L. Schultz, B. Krevet, and S. Fähler, Ni-Mn-Ga shape memory nanoactuation, Appl. Phys. Lett. 104, (2014) 043111 (5pp).
- F. Lambrecht, I. Aseguinolaza, V. Chernenko, and M. Kohl, Integrated SMA-Based NEMS Actuator for Optical Switching, Proc. MEMS’16, Shanghai, China (2016). DOI: 10.1109/MEMSYS.2016.7421562