Fabrication and modal characterisation of large‐area polymer membranes for acoustic micro‐electromechanical systems devices

Abstract

Bilayer polymer/metal suspended membranes made of poly(methyl methacrylate) (PMMA) and aluminium (Al) have been fabricated using a wet transfer technique where a polyelectrolyte (polydiallyldimethylammonium chloride - PDAC) has been employed as sacrificial layer to facilitate the detachment of the thin PMMA/Al layers from the substrate holders. In this way, free-standing PMMA/Al membranes with outstanding diameter (3.5 mm) to thickness (400 nm) ratios of ~10 4 have been obtained. The membranes have been actuated mechanically and electrothermally and their modal behaviour characterised using laser Doppler vibrometry. The first five modes of vibrations have been detected in the range of 2-25 kHz. The second and fifth degenerate modes have been seen to split, probably due to non-uniform tension or mass density. The membranes can achieve vibration amplitudes in the order of few tens of microns. When performing electro-thermal actuation, it has been observed that the amplitude of a single mode can be tuned by controlling the path of the electric current across the membrane.