Nondestructive evaluation of MEMS devices by laser confocal measurements

This paper proposes an innovative nondestructive evaluation (NDE) setup for MEMS mechanical deformation measurements. The setup is based on an active laser confocal displacement meter, and a computer controlled motor-driven 3-dimension (3D) stage. The out-of-plane measurement resolution of the scanning confocal setup is 0.1 μm. The maximum measurement range is 0.3 mm out-of-plane and 60 mm×60 mm in plane, respectively. The principle of the measurement method is introduced briefly. A multilayer piezoelectric acoustic MEMS membrane was fabricated and measured by using this setup. The profile of membrane was estimated. Nonlinear response feature of the membrane to direct current bias voltage was observed. The residual stresses of individual layer of the membrane were estimated according to Stoney´s theory. The test results gave quantificational description of MEMS membrane deformation, which can guide the optimal design of the acoustic devices. The laser confocal technique proved to be a valuable micro-NDE characterization tool.