Measurement and Isolation of Thermal Stress in Silicon-On-Glass MEMS Structures

摘要

The mechanical stress in silicon-on-glass MEMS structures and a stress isolation scheme were studied by analysis and experimentation. Double-ended tuning forks (DETFs) were used to measure the stress based on the stress-frequency conversion effect. Considering the coefficients of thermal expansion (CTEs) of silicon and glass and the temperature coefficient of the Young's modulus of silicon, the sensitivity of the natural frequency to temperature change was analyzed. A stress isolation mechanism composed of annular isolators and a rigid frame is proposed to prevent the structure inside the frame from being subjected to thermal stresses. DETFs without and with one- or two-stage isolation frames with the orientations 110 and 100 were designed, the stress and natural frequency variations with temperature were simulated and measured. The experimental results show that in the temperature range of -50 degrees C to 85 degrees C, the stress varied from -18 MPa to 10 MPa in the orientation 110 and -11 MPa to 5 MPa in the orientation 100. For the 1-stage isolated DETF of 110 orientation, the measured stress variation was only 0.082 MPa. The thermal stress can be mostly rejected by a stress isolation structure, which is applicable in the design of stress-sensitive MEMS sensors and actuators.