Design and fabrication of a MEMS-based electrolytic tilt sensor

Tilt sensors are applied in a wide range of applications: antenna positioning, avionics, slope or tilt monitoring, vehicle attitude monitoring and so on. It is known the electrolytic type is one of the most enduring tilt sensors to date. One can find the reason with the fact that it is unique in its structure and applicable to both narrow and wide angular range measurements, while keeping compact size and high accuracy. A MEMS-based electrolytic tilt sensor could have many advantages, such as low cost, possible mass production by using batch process, low power consumption, good repeatability and reliability. This paper reports designing and fabrication of a MEMS-based electrolytic tilt sensor. Fig. 1 (a) and (b) show the schematic view and operation principle of the MEMS-based double-axis electrolytic tilt sensor, respectively. An electrically conductive electrolyte is filled in bulk-Si cavity prepared by anisotropic KOH etching and sealed by top glass wafer. When the tilt sensor is leveled, both the positive and negative electrodes are equally immersed in the electrolyte, giving balanced signal outputs between the positive and negative electrodes with respect to the common electrode. As the sensor tilts the immersed surface area of the electrodes will change, increase for one electrode and decrease for the other. The unbalanced ratio in the resistance is directly proportional to the angle of tilt. Among technologies to form metal patterns along the nonplanar surface, shadow mask technique has been applied to prepare metal electrodes into the deeply etched silicon cavity^[1][2]. Fig. 2 shows the fabrication sequence. A 300 μm-deep cavity was formed by KOH etching (40wt% at 85°C) using 300 nm-thick LPCVD Si₃N₄ layer as the mask. After further addition of 500nm-thick SiO₂ by PECVD, metal electrodes (Au/NiCr = 200 nm/50 nm, 100 μm width) are deposited by thermal evaporation with the fabricated shadow mask on top of the process wafer. KCl solution (electrolyte) was injected using Ovation® Bionatural Pipette (no. 1057-0002) from VistaLab Technologies, Inc. that has accuracy of ± 9% at 0.2 μl. The amount of KCl solution was precisely controlled by a digitally-controlled plunger which has the dispensing increments of 0.002 μl, the volume range of 0.2-2 μl. Finally, a cover glass was bonded to seal the electrolyte in the cavity. Fig. 3 shows the fabricated device, (a) and (b) SEM images of Au electrodes formed on nonplanar surfaces using the shadow mask and (c) completed device. The electrodes formed in 300 μm-deep cavity are wider than those in 100 μm-deep cavity due to the diffusion of metal molecules during the evaporation.