Optimization Design And Fabriation Of Unidirectional MEMS Inertial Switches

Inertial switches based on the Micro-Electro-Mechanical System (MEMS), also called shock sensors or threshold accelerometers, have been a major focus for research in recent years, because they have great potential to be widely used in toys, accessories, automotive and health monitoring of products and storage due to their smaller size, lower cost, better functionality and reliability than conventional mechanical ones. Moreover, they can replace the complicated accelerometer systems for sensing and actuation with much less cost, since they have simpler structure and interface circuit, lower cost and less power consumption. What’s better, the use of the inertial switches can eliminate the risk of mis-operation by an electromagnetic noise.Based on surface micromachining technology, unidirectional MEMS inertial switches were designed and fabricated, which consists of two main parts: the proof mass suspended by the conjoined serpentine springs as the movable electrode, and the upper cross beam as the stationary electrode. Besides, Design I uses a compliant stationary electrode and Design II uses a movable contact point as the contact-enhancing mechanism, respectively.In Design I, there is a contact point applying concentrated load on the compliant stationary electrode when shock occurs, enhancing its deformation. As a result, the switch-on time is prolonged.In Design II, there is a movable contact point suspended by spiral springs in the center of the proof mass, whose deformation makes the switch-on time even longer. Besides, it also reduces the bouncing effect and makes the switch-on signals more continuous under quasi-static accelerations.The mass-spring-damping system was modeled, and the spring constant and damping coefficients are calculated theoretically, based on which the Simulink model was built. Combined with ANSYS finite element analysis, it was used to simulate the responses of the MEMS inertial switches under dynamic accelerations and quasi-static accelerations. ANSYS was also used for modal analysis to compare the vertical sensitivity with lateral sensitivity. When the gap between electrodes is given, the threshold acceleration is determined by the natural frequency. The dependences of the threshold acceleration on the natural frequency and the pulse width of the half-sine acceleration were investigated by simulation, which were in agreement with the analysis of the half-sine shock response spectrum. By multi-layer electroplating technology, the thickness of the proof mass can be easily adjusted while the plane geometry remains the same, in order to obtain switches with different thresholds. The threshold accelerations of three different switches (DesignI100μm, DesignII100μm, 50μm) were simulated as 64g, 56g and 133g. Most importantly, the dynamic simulation confirmed the contact-enhancing mechanism. The switch-on times of Design I and Design II can reach 39μs and 100μs, respectively.The low-cost and convenient multi-layer electroplating technology enables high aspect ratio patterning in thick photoresists and quasi-3D structure. Electroplated metal Ni was used as the structure material, which has better mechanical properties than silicon which suffers from brittle fracture under large shock. The fabricated and packaged devices were 3.0×3.0×1.0mm3, smaller than the reported.The inertial switches were tested by drop hammer experiments. Test results indicate that the threshold accelerations of the three switches were 70g, 63g, and 145g. The contact effect was improved significantly and stable switch-on times of 30μs (Design I) and over 50μs (Design II) were observed under half-sine wave accelerations with 1ms duration, in agreement with the dynamic simulation.