Failsafe wafer-level packaging of a piezoelectric MEMS actuator

Micro-electro-mechanical systems (MEMS) technology can offer a viable alternative to realize miniaturized and less expensive actuators for deformable mirror in adaptive optics for high resolution retinal imaging. However, during fabrication of such devices, functional multilayered thin films are generally deposited at elevated temperatures. These films are therefore subjected to residual stresses which may result in bending of the structure. The bending thus occurred may lead to failure at interfaces between films. A successful fabrication of device therefore relies on the engineering justification of multi-structured device design and growth parameters used in fabrication. In this paper, we present the design of a piezoelectric (ceramic) thin film based MEMS actuator for deformable mirror used in retinal imaging. A proto-type piezoelectric thin film actuator has been fabricated epitaxially using Pt/PZT/SRO/Pt/γ-Al₂O₃/Si structure. Advanced 3D finite element simulations were conducted to correlate the bending of fabricated structure with residual stresses. A smart alternative design was also proposed employing an extra layer of aluminium in the diaphragm region. Simulation results predict a failsafe structure when the thickness of extra Al-layer is tailored to an optimal thickness. The outcome of this research can be used to overcome the challenge encountered (bending due to residual stresses) to obtain a failsafe wafer-level packaged MEMS actuator for deformable mirror.