Close infrared thermography using an intensified CCD camera: application in nondestructive high resolution evaluation of electrothermally actuated MEMS

摘要

This communication proposes the description of an optical method for thermal characterization of MEMS devices. The method is based on the use of an intensified CCD camera to record the thermal radiation emitted by the studied device in the spectral domain from 600 nm to about 850 nm. The camera consists of an intensifier associated to a CCD sensor. The intensification allows for very low signal levels to be amplified and detected. We used a standard optical microscope to image the device with sub-micron resolution. Since, in close infrared, at very small scale and low temperature, typically 250°C for thermal MEMS (Micro-Electro-Mechanical Systems), the thermal radiation is very weak, we used image integration in order to increase the signal to noise ratio. Knowing the imaged materials emissivity, the temperature is given by using Planck's law. In order to evaluate the system performances we have made micro-thermographies of a micro-relay thermal actuator. This device is an "U-shape" Al/SiO2 bimorph cantilever micro-relay with a gold-to-gold electrical contact, designed for secured harsh environment applications. The initial beam curvature resulting from residual stresses ensures a large gap between the contacts of the micro-relay. The current flow through the metallic layer heats the bimorph by Joule effect, and the differential expansion provides the vertical displacement for contact. The experimental results are confronted to FEM and analytical simulations. A good agreement was obtained between experimental results and simulations.