Static and dynamic micro deformable mirror characterization by phase-shifting and time-averaged interferometry

Since micro deformable mirrors based on Micro-Opto-Electronico-Mechanical Systems (MOEMS) technology would be essential in next generation adaptive optics system, we are designing, realizing and characterizing blocks of this key-component. An in-house designed tiltable mirror (170*100 μm2) has been processed by surface micromachining in the Cronos foundry, and a dedicated characterization bench has been developed for the complete analysis of building blocks as well as operational deformable mirrors. This modular Twyman-Green interferometer allows high in-plane resolution (4μm) or large field of view (40mm). Out-of-plane measurements are performed with phase-shifting interferometry showing highly repeatable results (standard deviation<5nm). Features such as optical quality or electro-mechanical behavior are extracted from these high precision three-dimensional component maps. Range is increased without loosing accuracy by using two-wavelength phase-shifting interferometry authorizing large steps measurements such as 590 nm print-through steps caused by the Cronos process. Dynamic analysis like vibration mode and cut-off frequency is realized with time-averaged interferometry. Rotation mode frequency of 31±3kHz of the micro tiltable mirror, and a resonance with a tuned damping at 1.1kHz of the commercial OKO deformable mirror are revealed.