Highly Sensitive Capacitive Tilt Sensor Using Liquid Metal Movable Electrode

Capacitive liquid-based tilt sensors offer advantages such as environmental stability, mechanical robustness, and linear output characteristics. However, achieving high sensitivity remains challenging due to the low permittivity of dielectric liquids and the inevitable air gaps between the electrodes. We report on a new design using a liquid metal movable electrode to improve the sensitivity of capacitive tilt sensors. The tilt sensor was fabricated using Galinstan liquid metal as a movable electrode, a Cr/Au (20/200 nm) fixed electrode, and a CF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>/O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma-etched polyimide as fixed dielectric layer. To make the liquid metal electrode moveable, optimization of CF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>/O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma was performed to achieve non-wetting surface against the liquid metal. To assess the durability of the CF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>/O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma-etched polyimide, repeated tilting of the sensor over 100 cycles between -45<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> and 45<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> was performed and there was no sign of the liquid metal wetting on the plasma-etched polyimide. By using liquid metal movable electrode, the proposed tilt sensor showed high sensitivity of 3.67 pF/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula>, several orders of magnitude higher than the previous works.