In-Plane High-Sensitivity Capacitive Accelerometer in a 3-D CMOS-Compatible Surface Micromachining Process

This paper proposes a novel design for a 3-D, high-sensitivity, single-axis lateral capacitive accelerometer. The accelerometer utilizes the entire area of the sensor for both sensing and the proof mass, which mitigates the tradeoffs often needed in conventional 2-D designs. The accelerometer structure is optimized to target the highest possible performance. Innovative Z-shaped supporting beams are introduced to limit the vertical displacement within the transducer’s submicron gap. The design was fabricated in a novel 3-D complementary metal–oxide–semiconductor-compatible surface micromachining process. This process allows the use of non-conductive materials with attractive mechanical properties to build capacitive microelectromechanical system (MEMS) devices. Polyimide is used as a core structural layer which is combined with other materials such as silicon nitride or silicon carbide to build the final sensors. The photolithography steps are limited to four, and the number of materials used is also limited to four, in order to keep the process feasible. The overall thermal budget is 300 °C, which enables above-IC MEMS integration. While the used materials provide good results, this process is not limited to them, and other materials can be used, if needed. The fabricated accelerometer measures
$500 \times 500 \,\,\mu \text{m}^{2}$
and achieves 58 fF/g sensitivity in a ±4 g range in an open-loop system, yielding a competitive sensitivity per unit area. [2018-0125]