A Review of the Capacitive MEMS for Seismology

Abstract

MEMS (Micro Electro-Mechanical Systems) sensors enable a vast range of applications: among others, the use of MEMS accelerometers for seismology related applications has been emerging considerably in the last decade. In this paper, we provide a comprehensive review of the capacitive MEMS accelerometers: from the physical functioning principles, to the details of the technical precautions, and to the manufacturing procedures. We introduce the applications within seismology and earth sciences related disciplines, namely: earthquake observation and seismological studies, seismic surveying and imaging, structural health monitoring of buildings. Moreover, we describe how the use of the miniaturized technologies is revolutionizing these fields and we present some cutting edge applications that, in the very last years, are taking advantage from the use of MEMS sensors, such as rotational seismology and gravity measurements. In a ten-year outlook, the capability of MEMS sensors will certainly improve through the optimization of existing technologies, the development of new materials, and the implementation of innovative production processes. In particular, the next generation of MEMS seismometers could be capable of reaching a noise floor under the lower seismic noise (few tenths of ng/ Hz ) and expanding the bandwidth towards lower frequencies (∼0.01 Hz).

Keywords: MEMS; capacitive accelerometer; earthquake; seismology.

Figure 1

Number of scientific papers containing the term “MEMS” in the title from Scopus database (accessed on February 2019).

Figure 2

The accelerometer is a spring-mass system attached to a moving reference frame $xy$; $XY$ is the inertial reference frame. The meaning of m, b, and k is described by Equation (3).

Figure 3

The equivalent circuit of the proof mass accelerometer.

Figure 4

Basic electromechanical transducer with one electrical terminal pair and a single mechanical degree of freedom.

Figure 5

In the plane between electrical power and mechanical power, the capacitive transducers fall in the “sensor” field.

Figure 6

(Left): variable-gap capacitor with parallel electrodes of fixed area; if the gap $x\left(t\right)$ remains small with respect to all areal dimensions, the fringing fields can be neglected. (Right): variable-area capacitor; the air gap is fixed and the area is variable with respect to one degree of freedom. If the gap is small with respect to areal dimensions, fringing can be neglected.

Figure 7

(Left): three-plate variable-gap device; (Right): three-plate variable-area device.

Figure 8

Equivalent circuit of the capacitive MEMS accelerometer.

Figure 9

SEM image showing the details of a capacitive MEMS accelerometer.

Figure 10

Comparison of the Power Spectral Density (PSD) for some MEMS sensors compared with the seismic noise models (red and blue lines from [37]), and with a spectra response of a local earthquake (green line). The red area indicates the target zone desirable for the next generation of MEMS sensors. Figure from [12].

Figure 11

Working scheme of a typical MEMS-based station for seismic monitoring.