Abstract
The dynamic characterization of a set of gold micro beams by electrostatic excitation in presence of residual stress gradient has been studied experimentally. A method to determine the micro-cantilever residual stress gradient by measuring the deflection and curvature and then identifying the residual stress model by means of frequency shift behaviour is presented. A comparison with different numerical FEM models and experimental results has been carried out, introducing in the model the residual stress of the structures, responsible for an initial upward curvature. Dynamic spectrum data are measured via optical interferometry and experimental frequency shift curves are obtained by increasing the dc voltage applied to the specimens. A good correspondence is pointed out between measures and numerical models so that the residual stress effect can be evaluated for different configurations.
Highlights
Micro-electromechanical systems (RF-MEMS) have recently demonstrated advances in the field of radio-frequency in the realization of tuneable circuits including phase shifters, filters and matching networks
Residual stress/strain becomes a parameter of fundamental importance in micro-structure, surface micromachining and improving reliability of micro-devices [3,4,5]
Residual stresses vary along the beam thickness because of the difference on percentage of diffused chromium. Another feature considered as an origin of internal stresses within the gold beam is the difference in the coefficient of thermal expansion (CTE) between the gold beam and the photoresist sacrificial layer [2]
Summary
Micro-electromechanical systems (RF-MEMS) have recently demonstrated advances in the field of radio-frequency in the realization of tuneable circuits including phase shifters, filters and matching networks. Due to its low electrical loss and chemical inertness, gold is the most common material for fabricating these structures Despite these benefits, the high susceptibility to relaxation effects of gold often introduce much residual stress into these RF-MEMS structures [16]. Residual stresses vary along the beam thickness because of the difference on percentage of diffused chromium Another feature considered as an origin of internal stresses within the gold beam is the difference in the coefficient of thermal expansion (CTE) between the gold beam and the photoresist sacrificial layer [2]. This introduces a stress gradient for any temperature variation during the process. Experimental images of the specimen were analyzed on the basis of the IRST fabrication process technical memo in order to extrapolate a simplified profile, being the real profile quite irregular (Figure 2)
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