Mechanical modeling and characterization of suspended cooled silicon bolometers for sub-millimeter and millimeter waves polarization detection

Silicon bolometers for space and astronomy applications, fabricated in standard CMOS-SOI technology are now successfully used as cryogenic detectors working at very low temperature, typically in the range of 0.05–0.1 K. They feature a remarkably high electromagnetic absorption, high temperature sensitivity and low noise. However, the mechanical behavior of suspended silicon bolometers results from the fabrication process parameters and a good understanding of these mechanisms is necessary to better control their deformation. In this work, silicon bolometer pixels with a pitch of 1200 μm and 500 μm for millimeter-wave (mm-wave) polarization detection have been fabricated and their mechanical behavior is investigated at room temperature and cryogenic temperature. First, a mechanical model was developed based on simulated and experimental deformations at room temperature of multi-layer cantilever test structures with different Young’s modulus and thickness (Ei, hi). The actual multi-layer suspended structures are modeled as an equivalent composite layer with an effective Young’s modulus (Eeff), an effective thickness (heff) and residual stresses (σ0, σ1). The residual stress values are positive, corresponding to a tensile stress in the fabricated multilayer stack. The impact of the a-Si passivation thickness on the total stress is discussed. The equivalent model is used in the simulation of the full pixel structure and results in excellent agreement with optical measurements of the deflection at room temperature. At cryogenic temperatures, mechanical deformations can hardly be measured, so the mechanical behavior of a 500-μm pixel was simulated at 0.1 K assuming that tensile residual stresses coming from defects are independent on temperature, and a good mechanical stability of the pixel was obtained. The optical performance simulation of this 500-μm pixel is discussed and showed that the mechanical deformations result in a degradation of the Noise Equivalent Power (NEP) from 1.59 × 10−18 to 1.05 × 10-17 W/Hz1/2 for an optical load of 6 × 10−15 W at 0.1 K.