An optimized MEMS-based electrolytic tilt sensor

A tilt sensor has been realized based on electrolytic solution, which is filled in a micromachined 400 μm-deep cavity and responded relative to tilt angle due to the gravity. The device consists of anisotropically etched deep cavity to contain electrolytic solution and Au/NiCr metal electrodes to detect the electrical signal formed on non-planar surfaces from the substrate surface to the bottom of the cavity. The tilting motion of the device changes the immersed area of the electrodes in the electrolytic solution and the unbalanced ratio of the elecrical siganal is monitored by the change in its resistance. Injected electrolyte solution should be carefully determined because it must have not only small resistance but also small viscosity. Potassium hydroxide (KOH) solution is chosen as the electrolyte due to the relatively high ionic conductance and it is diluted with methanol for an optimized concentration considering the viscosity. The fabricated electrolytic tilt sensor is excited by an alternating current to prevent electrolysis in the electrolyte-filled cavity and measured by Wheatstone bridge circuit to read the variation of resistance with respect to an incoming inclination. Experiments reveal that the resistance of KOH electrolyte is smallest at 5 wt% in its concentration and 800 Hz in the driving frequency. The tested Micro Electro Mechanical Systems (MEMS)-based electrolytic tilt sensor with KOH solution shows a resolution of approximately 50 mV per 1° of inclination angle change and operating angle range of ±60°. The measured output characteristic of the fabricated MEMS-based electrolytic tilt sensor sustains comparison with other conventional electrolytic tilt sensors. Moreover, MEMS-based sensor prevails in its small size, low cost and possible mass production.