Thin-film Silicon MEMS on Flexible Polyimide Substrate Strip

Hydrogenated amorphous silicon doubly clamped beams and cantilevers have been fabricated on a flexible transparent polyimide (PI) strip with 10µm thickness. A metal counter electrode was patterned on the substrate below the suspended part (which works as top electrode itself). The achieved goal is to demonstrate the fabrication of MEMS devices in large-area flexible substrates for sensor and actuator applications (such as small mass variation measurements based on resonance frequency shift).

The fabrication process comprised

1. a) 500nm SiO₂ layer deposition on a silicon substrate (sacrificial layer later removed for release of the flexible PI from the rigid support;
2. b) coating with a 10µm PI layer;
3. c) patterning a first layer of titanium-tungsten, 150nm, (bottom fixed electrodes, metal lines and contacts);
4. d) patterning a 400nm-thick aluminum layer (sacrificial layer to obtain suspended structures);
5. e) growing a 1µm-thick conformal amorphous silicon layer (suspended beams‘ structural material) and patterning (RIE) the suspended structures and anchors;
6. f) dicing individual devices;
7. f) MEMS release with wet aluminum etchant; and
8. h) flexible device release step by vapor HF etching of SiO₂.

For experimental characterization, a Polytec MSA-500 vibrometer was used for both topography and out-of-plane vibration measurements, including resonance mode shapes analysis. Out-of-plane displacements are measured optically (laser doppler vibrometry on several scanpoints along the structures), while electrostatically actuating the structure (with DC and AC components, and frequency sweeps). Maximum vibration amplitude was observed at the center of the released doubly clamped beams and at the tip of the cantilevers. Both the first and the second resonance mode shapes were found. Some structures were evaluated in vacuum (below 0.1mbar). Electrical resonance peak identification was impaired by noise.

Future work will include verifying the resonance frequency stability, its dependence on the bending of the flexible strip, and redesigning the anchors and metal traces.