Analysis of fringing effects in MEMS capacitive pressure sensors with vertically offset guard rings

Abstract In MEMS capacitive pressure sensors (CPSs), fringing effects have a pronounced influence on the capacitance output characteristics. Conventional fringing-effect suppression relies on coplanar guard rings, where the inherent in-plane gap restricts the effective suppression of fringing fields. To overcome this limitation, a vertically offset guard ring configuration is proposed. By eliminating the in-plane gap and retaining only a small vertical gap, the proposed design further weakens fringing effects and reduces capacitance error. A mathematical model and a finite element model are established to systematically investigate the fringing effects in CPSs, with a comparative analysis of conventional coplanar and vertically offset guard ring configurations. The overall performance of the sensor incorporating the vertically offset guard ring is evaluated. The results indicate that the downward-offset guard ring provides significantly superior suppression of fringing effects compared with the conventional coplanar guard ring. Under the investigated sensor configuration, the fringing capacitance reaches 76.8 fF in the absence of a guard ring, corresponding to a fringing capacitance error ratio of 44.2%, which further increases when the silicon-substrate electrode is taken into account. In contrast, with the downward-offset guard ring employed, the fringing capacitance is reduced to 0.39 fF, and the corresponding error ratio decreases to only 0.22%, exhibiting close agreement with the ideal capacitance model.