Capacitive MEMS Devices Research Articles

A Novel Design for Enhancing the Sensitivity of a Capacitive MEMS Device

Sensitivity of a capacitive MEMS device mainly depends on gap variation across the inter-digitated electrodes for an applied acceleration. In conventional design, gap variations across the inter-digitated electrodes are typically due to the displacement of the movable electrodes attached to a movable mass, whereas the fixed electrodes remain static. In this paper, a novel design of secondary-mass-spring assembly is proposed to tilt the fixed electrodes along with the displacement of the moving electrode for applied acceleration such that the capacitive sensitivity is enhanced. The proposed structure is fabricated by a SOIMUMP’s process. An experimentally measured values of the resonant frequency of the secondary mass and the proof mass are 1.06 and 2.68 kHz, respectively. The static response obtained using COMSOL Multiphysics simulation for applied body load of 1 g along Y-axis gives proof mass displacement sensitivity of 41.8 nm/g, secondary mass tilting of $0.000147^{\circ }/g$ , and capacitive sensitivity improvement of 17.6%. Also, non-linearity of the output response is within ±2.5% for applied acceleration of $\pm 15~g$ along the Y-axis. The differential arrangement of inter-digitated electrodes nullifies the cross-axis sensitivity. Differential capacitance increment of more than 50% is reported in the frequency range of 800 to 1200 Hz making it suitable for wideband energy harvesting applications. [2018-0010]

Characterization of Sandia Capacitive Comb-finger MEMS Accelerometers

This paper discusses various methods for testing the performance of MEMS capacitive comb-finger accelerometers manufactured by Sandia National Laboratories. The use of Capacitive MEMS devices requires complex circuits for measurement of capacitance and the Sandia MEMS accelerometer’s capacitance changes in a very small femto-farad (fF) range. The performance of accelerometer is tested using Analog Devices AD7747 sigma-delta capacitance to digital converter. The response of a MEMS capacitive accelerometer to various tests is useful for testing and characterization and investigate it’s suitability for various applications.

Characterization of Capacitive Comb-finger MEMS Accelerometers

This paper discusses various methods for testing the performance of MEMS capacitive comb-finger accelerometers manufactured by Sandia National Laboratories. The use of Capacitive MEMS devices requires complex circuits for measurement of capacitance. Sandia MEMS accelerometer’s capacitance changes in a very small femto-farad (fF) range. The performance of accelerometer is tested using Analog Devices AD7747 sigma-delta capacitance to digital converter. The response of a MEMS capacitive accelerometer to various tests is useful for testing and characterization and investigate it’s suitability for various applications

Investigation of Charge Relaxation in Silicon Nitride for the Reliability of Electrostatically Driven Capacitive MEMS Devices

Dielectric charging/discharging in the real-life electrostatic microelectromechanical system device is proposed to be evaluated by the capacitance-voltage response for an analogous metal-insulator-semiconductor (MIS) structure. An analytical model based on this approach has been established. In the experiment, the relaxation behaviors of trapped charges in silicon nitride of MIS structure have been systematically investigated. For both positive and negative dc bias polarities, fast and slow discharge stages were clearly observed in the early and late charge relaxation processes, respectively. The discharge ratio (DR) was found to depend on both the bias polarity and magnitude. It was shown that negative dc bias would cause a high hole injection level but low DR, whereas positive dc bias would lead to a low electron injection level but high DR. Moreover, the DR will increase with the dc bias voltage. It was further found that the hole relaxation reaches the steady state faster than the electron relaxation. To explain the experimental results, we pointed out that the traps close to the interface play an important role in dielectric charging and discharging process.

A capacitive CMOS–MEMS sensor designed by multi-physics simulation for integrated CMOS–MEMS technology

This paper reports the design and evaluation results of a capacitive CMOS–MEMS sensor that consists of the proposed sensor circuit and a capacitive MEMS device implemented on the circuit. To design a capacitive CMOS–MEMS sensor, a multi-physics simulation of the electromechanical behavior of both the MEMS structure and the sensing LSI was carried out simultaneously. In order to verify the validity of the design, we applied the capacitive CMOS–MEMS sensor to a MEMS accelerometer implemented by the post-CMOS process onto a 0.35-µm CMOS circuit. The experimental results of the CMOS–MEMS accelerometer exhibited good agreement with the simulation results within the input acceleration range between 0.5 and 6 G (1 G = 9.8 m/s2), corresponding to the output voltages between 908.6 and 915.4 mV, respectively. Therefore, we have confirmed that our capacitive CMOS–MEMS sensor and the multi-physics simulation will be beneficial method to realize integrated CMOS–MEMS technology.

Effect of Length-scale Parameter on Pull-in Voltage and Natural Frequency of a Micro-plate

This paper deals with the effect of the intrinsic material length-scale parameter on the stability and natural frequency of a rectangular micro-plate for two different cases; fully clamped and fully simply supported. A variational formulation based on Hamilton ’s principle and the modified couple stress theory is used to obtain the nonlinear governing equation of a micro-plate incorporating the stretching effect. In the static case, the nonlinear governing equation is solved using the step-by-step linearization method (SSLM) and in the dynamic case, is integrated using fourth-ordered Runge-Kutta method. The static and dynamic pull-in parameters, limiting the stability regions of capacitive MEMS devices, are calculated and compared to those obtained by the classical theory. The numerical results reveal that the intrinsic size dependence of materials is more significant for smaller thicknesses and in this case, the stretching effect can be neglected.

Reliability Aspects of Capacitive MEMS Devices

Capacitive MEMS device forms a key building block for many applications, including sensors, RF applications, user interface technologies, transducers, MEMS displays. In this paper, some important concepts of capacitive MEMS reliability physics will be discussed along with the methodology to evaluate it. Packaging effects will also be covered, as investigated under controlled environment in a dedicated vacuum chamber.

Transient dynamics of a MEMS variable capacitor driven with a Dickson charge pump

In the actuation of electrostatically controlled capacitive MEMS devices, drive voltages over the supply voltage are commonly required. In order to obtain such voltages, charge pumps are often used. This paper presents and discusses two methods to drive capacitive MEMS devices using IC-compatible Dickson charge pumps designed to minimize power consumption. Drive circuits include voltage shaping to minimize charge injection in order to provide better reliability. The main design constraints to integrate them in state of the art IC technologies are identified and quantified. The required charge pumps are also optimized to provide an efficient use of its total capacitance. The transient dynamics of charge pumps connected to a RF MEMS variable capacitor are also analysed and discussed. Finally, the proposed methods are tested and validated through experimental results.