An Electro Thero-Actuated Bidirectional Bistable Mems Relay And Its Integrated Micro Fabrication

With the rapid development of microelectromechanical systems (MEMS), microrelays with smaller dimensions have great potentials to the application to the industrial fields such as electric toys, communication systems, consuming electrons. Recently, MEMS-based relays are investigated by more and more researchers all over the world. Versatile MEMS relays have a series of advantages which include not only microscale dimesions, low power consumptions, but also fast response speeds, good dependabilities. What's more, microrelays have a good compatibility with monolithic integration with IC by post-processing and can subsequently enlarge the possibilities of industrial applications in a low-cost manner. Therefore, it is a necessary current to develop a type of smaller, faster, lower power consumption and higher efficient MEMS relay.At present, prepared were a variety of MEMS relays in the labs based on electrostatic, electromagnetic and electrothermal actuations. Silicon-based electrostatic MEMS relay is a pioneer of miniaturized relay depending on the integrated circuit process. Rapid response is the outstanding property of electrostatic MEMS relay. But it is only applied in high voltage devices (e.g. 20-150V) which are extremely restricted because of the low drive capability. While magnetic MEMS relay can generate larger forces with high nonlinearity at lower power consumption, the MEMS community generally has not taken to its use because of the complications of electromagnetic-induced coils and the near impossibility of shielding neighboring devices from actuator crosstalk. So, the major disadvantage of magnetic actuation is the lack of a mature technology. As for electrothermal MEMS relay, they are very attractive not only because of large deflection and force but also because of low power consumption. Similarly, magnetic actuation is usually long range and capable of producing large displacement or deflections. Theoretically, these two actuating methods become compatible with each others due to the similar physical performance.In addition, a bistable mechanism (BM) is a favorable feature of MEMS actuators, in particular for application in RF switches and microrelays. A bistable micromechanism is defined as a mechanism that has two stable positions within its range of motion. A primary advantage of BM is that it only requires energy input to switch the mechanism from one state to the other, but input energy is not required to maintain the mechanism in either of the stable equilibrium positions. This behavior of BM is of particular importance at the microlevel when power consumption is often a major concern.So as to make use of the common advantages of both electrothermal actuation and magnetostatic latching, a novel torsion/cantilever-based bidirectional bistable MEMS relay for low power consumption is presented in this dissertation. The proposed microrelay consists of a pair of electrothermal actuator, a torsion/cantilever bistable mechanism, and an external permanent magnet. The electrothermal actuator, which is comprised in a nickel heater as a passive layer, and a polymeric active layer, is employed to provide a driving force at the free end of the cantilever beam. Furthermore, the magnet bistable mechanism includes a magnetic circuit, a clamped-clamped torsional flexure, and a cantilever beam with two free ends. The micromechanism features a low-stress low-stiffness permalloy cantilever beam, suspended in air by two supporting torsional flexures at its two sides. A uniform static magnetic field was provided by a permanent magnet beside the supporting anchors for latching purposes. The free-free cantilever forms a symmetrical rocker lever swinging along the torsional flexures as a combined result of the electrothermal torque and the magnetostatic torque. The two supporting flexures act as the rotation axis. Once switched, it will be latched by the external magnetic field and remains in the current state until the counterpart trigger is reenergized in an opposite direction. Consequently, the bistabilities of the MEMS relay are implemented the combined influence of the actuating force and the magnetostatic latching.The main contributions of the paper consist of four types of endeavors to achieve the integrated MEMS relay with bidirectional motions.Firstly, a novel design for out-of-plane polymeric multimorph electrothermal actuators with sandwiched heaters is proposed. In order to deduce the actuating displacement and temperature distributions along the bimorph cantilever, an electro-thermo-mechanical analytical model is presented. Moreover, numerical simulation and structure optimization of the designed thermal actuators have been completed by the combination with ANSYS and MATLAB software.Secondly, a novel bidirectional bistable mechanism based on a leveraged torsion/cantilever system is designed, simulated and optimized by theoretical and numerical analysis, respectively. In order to decrease the stiffness coefficient of the bistable mechanism, three types of support configurations of the micromechanism are presented, e.g. a crisscross type, a ring type and a diamond type. Meanwhile, the geometric dimensions have been optimized by the simulation analysis with ANSYS multiphysical fields. Thirdly, since sacrificial-layer (SL) technique in S-LIGA process is one of the primary techniques to obtain movable suspension parts of MEMS devices, we have thoroughly investigated a multimaterial-compatible non-silicon surface micromachining technology. As for one of the stratagems for sacrificial-layer technique, we developed an effective solution to selective etching of ultra-thick copper sacrificial layer for realization of 3-D suspended metal microstructure. The total height of sacrificial Cu laminations can reach more than 100 microns. The complexion-based etching solution to ultra-thick Cu SL has some unique features in high selective etching ratio, low-cost, high efficiency, time-saving, and compatible with MEMS process. What'more, a serials of intractable process problems has been improved, such as etching of seeder layers, strategy for palanrization of PR, efficient wet etching of Cu layer, and low-temperature release of low-stiffness microstructures. Based on the above-mentioned endeavors, the bistable microrelay has been fabricated successfully by the laminated thick Cu sacrificial layer process on glass substrate.Finally, static and dynamical performances of the fabricated three classes of MEMS devices, e.g. thermal actuators, bistable mechanisms and microrelays, have been characterized. The 3-D height contours of these devices were illuminated with a WYKO optical profiling system. As for BMs, test results show that the stiffness coefficients of three support structures have a good consistence in theoretic calculation, simulation and experiments, respectively. Among these different support types, the stiffness coefficient of ring-shaped BM is minimum with a value of 1.587μN/μm, which has more compliant than others. Meanwhile, bistabilities of the microrelay were implemented successfully with no power consumption when keeping the Close/On states. The dimensions of the fabricated prototype are 5.2mm×2.2mm×120μm. when applied to an 80mA current pulse with a 10% duty cycle, the response time of the microrelay is proximately 22.5ms, and the exciting power is consumed to be 60mW at one DC pulse.In a word, a bidirectional bistable MEMS actuator with out-of-plane motions has been developed in this paper. The bistabilities of the prepared compliant microrelay were implemented successfully. The proposed MEMS device has low power consumptions and fast response time, which has a potential to the application in the flied of lower power consumption devices.