Design and fabrication of a silicon-based MEMS acceleration switch working lower than 10 g

Abstract

This paper reports a low-g MEMS acceleration switch with threshold acceleration below 10 g. The proposed switch is made of single-crystalline silicon for high thermal stability and stressfree structure. A vertical operation type is adopted to enable fine control of the contact surface during the fabrication process. The switch contains displacement-restricting structures in all directions for impact resistance and is packaged with anodic bonding process. The fabricated switches had an average proof mass, initial gap, and spring constant of 307.38 µg, 6.39 µm, and 3.29 N m −1 , respectively. Height profile of the free-hanging proof mass was measured to show that the switch does not suffer from stress problems. In the electrostatic operation test, the contact resistance of the switch was varied with contact force and the minimum value was estimated to be 8.5 Ω. The response time of the switch was measured to be shorter than 1.2 ms. The fabricated switch operated more than 10 000 cycles without failure. For the thermal stability test, the switch was heated at 80 °C for 6 h and the switch operated successfully over 200 times. In the rotation-table experiment, the switch operated at 6.61 g and error analysis was carried out in the consideration of tangential force generated during the rotation-table experiment. From the experimental values, the tangential force was calculated as 2.375 µN and the resulting reduction in the initial switching gap was simulated as 0.32 µm. The reduced threshold acceleration thus was estimated to be 6.62 g, which agrees very well with the measured threshold acceleration value of 6.61 g.

Recent Advancements in Inertial Micro-Switches

“…On the other hand, SOI technology can accurately control the structural thickness, thus reducing the threshold deviation caused by fabrication errors [19,70,71]. Hwang et al [70] reported a low-g switch based on SOI wafers. The designed and measured threshold values were 6.62 and 6.61 g, respectively.…”

Analysis on Chattering Phenomena by the Tilt of the Proof Mass in MEMS Switch

“…(1) Table 1 shows the values used in the numerical analysis. Those values are from the original design of the low-g MEMS acceleration switch [5] and the input force was set to 12 g (g is the gravitational acceleration). …”

“…The cold launch missiles which the main motor ignites after ejection, and the multi-stage rocket motors can prevent unexpected ignition if they use the acceleration signal as a safety condition. The proof mass of low-g MEMS acceleration switch developed by Kim's group (Figure 1) [5] moves in the z-direction to minimize the installation error. However, when using a MEMS acceleration switch in missiles, noise forces would act in the offaxis directions.…”

Research Progress of MEMS Inertial Switches

“…Field et al [ 61 ] studied a low-g switch with a threshold value of 25 g. The fabrication was based on a silicon-on-insulator (SOI) wafer, and the sensing direction was along the thickness of the wafer. Hwang et al [ 62 ] designed a low-g MEMS acceleration switch with a threshold acceleration of 6.61 g. The structural material was single-crystal silicon, the structure was stress-free, and the stability was good at high temperature. However, common SOI wafers have an unexpected disadvantage in that the balance of structure is hard to maintain, mainly due to over-etching by poor etching verticality, which causes quality deviation, shifts in the center of mass and the contact area becoming smaller, and the mass tilting or rotating when it moves along the shock direction, potentially leading to contact uncertainties.…”