Interferometric characterization of MOEMS devices in cryogenic environment for astronomical instrumentation

摘要

Next generation of infra-red astronomical instrumentation for space telescopes as well as ground-based extremely large telescopes requires MOEMS devices with remote control capability and cryogenic operation, including programmable multi-slit masks for multi-object spectroscopy (MOS). For the complete testing of these devices, we have developed in parallel and coupled a high-resolution Twyman-Green interferometer and a cryogenic-chamber for full surface and operation characterization. The interferometer exhibits a nanometer accuracy by using phase-shifting technique and low-coherence source. The cryogenic-chamber has a pressure as low as 10e-6 mbar and is able to cool down to 60K. Specific interfaces minimizing stresses for vacuum and cryo have been set. Within the framework of the European program on Smart Focal Planes, micro-mirrors have been selected for generating MOEMS-based slit masks. A first 5×5 micro-mirror array (MMA) with 100×200μm~2 mirrors was successfully fabricated using a combination of bulk and surface silicon micromachining. They show a mechanical tilting angle of 20° at a driving voltage below 100V, with excellent surface quality and uniform tilt-angle. The mirrors could be successfully actuated before, during and after cryogenic cooling. The surface quality of the gold coated micro-mirrors at room temperature and below 100K, when they are actuated, shows a slight increase of the deformation from 35nm peak-to-valley to 50nm peak-to-valley, due to CTE mismatch between silicon and gold layer. This small deformation is still well within the requirement for MOS application.