الفهرس | Only 14 pages are availabe for public view |
Abstract Micro-beams and micro-plates fabricated by deposition suffer from stiffening and curling, resulting from residual stresses initiated during fabrication. Stiffening and curling affect the performance of these microstructures in MEMS applications. Reducing response time is also favourable in MEMS applications such as RF MEMS switches. Previous literature shows that it is challenging to reduce both phenomena simultaneously. This thesis proposes a new design concept aiming at reducing both stiffening and curling. In addition, reducing switching time is also taken into consideration. First, simplified model for a fixed-fixed beam is studied to examine the factors affecting stiffening, and curling. The analytical solution of the beam model depicts that stiffening can be decreased by increasing the ratio between the second moment of area of the beam cross-section and its crosssectional area. In addition, increasing this ratio increases the critical buckling temperature. Thus, extending the operational temperature range of the MEMS device. This simple beam model cannot capture the plate effects. Thus, a Finite Element model is developed. The Finite Element model is verified with previous results in the literature. A preliminary parametric size optimization is done on three well known configurations of flat micro-plates with fixed-fixed supports. The objective is to study the effect of in-plane (2D) structure enhancement in reducing stiffening, curling, and increasing the natural frequency. The higher is the natural frequency, the lower the switching time. For comparison purposes, same volume is set as a constraint for all three designs. Compared to conventional rectangular micro-plate, a reduction of 34% in stiffening for configuration 2, and 44% in curling for configuration 3 are achieved. configuration 1 showed the maximum fundamental natural frequency. Thus, it is predicted to have the lowest switching time. Moreover, configuration 2 ix showed the maximum critical buckling temperature. The effect of changing micro-plate material is also studied. The next step after these preliminary simulations is to test the outputs of the analytical model i.e. using designs of micro-plates with high ratio of second moment of area to the cross-sectional area. In structural design, many cross-sections are known to have a high previously mentioned ratio. This thesis focuses on ribbed and corrugated sections, as they can be easily manufactured in MEMS fabrication technology. Results of the Finite Element simulations for ribbed switch show that specific designs of ribbed micro-plates are superior to conventional flat ones in many aspects. Stiffening and curling are both reduced by about 50% and 64%, respectively. Critical buckling temperature is increased. The sensitivity of stiffness to temperature variations is reduced by 50%. The fundamental natural frequency is increased by 34%. For the same bending stiffness, the mass of the ribbed micro-plate is reduced and hence a faster response can be achieved. The second design proposal is to use corrugated micro-plate as a remedy to residual stress problems. The micro-plate is double corrugated in both longitudinal and transverse directions. A parametric study is implemented to study the effect of number of corrugations in both directions and corrugation height on the micro-plate performance as compared to a flat one with the same stiffness. Proposed corrugations result in reducing stiffening, and curling, by 85%, and 73%, respectively. A selected corrugated micro-plate achieved a reduction of 39% in switching time. Three dimensional designs used in this thesis (ribbed and corrugated micro-plates) show better results than two dimensional designs in reduci ng the problems arising from residual stresses. The three-dimensional design concept can be applied on any micro- x plate device such as resonators, pressure sensors, and micromirrors in order to improve the reliability of these devices. Thus, corrugation concept is applied as a case study on a previously designed radiofrequency switch in literature. Significant reduction in stiffening and curling is achieved that can reach 90% and 95% in stiffening and curling, respectively. |