On-Chip Tests for the Characterization of the Mechanical Strength of Polysilicon †

Microelectromechanical systems (MEMS) are nowadays widespread in the sensor market, with several different applications. New production techniques and ever smaller device geometries require a continuous investigation of potential failure mechanisms in such devices. This work presents an experimental on-chip setup to assess the geometry- and material-dependent strength of stoppers adopted to limit the deformation of movable parts, using an electrostatically actuated device. A series of comb-finger and parallel plate capacitors are used to provide a rather large stroke to a shuttle, connected to the anchors through flexible springs. Upon application of a varying voltage, failure of stoppers of variable size is observed and confirmed by post-mortem DC–V curves. The results of the experimental campaign are collected to infer the stochastic property of the strength of polycrystalline, columnar silicon films

Nonlinear dynamics of MEMS resonators: numerical modelling and experiments

Numerical modelling of MicroElectroMechanical Systems (MEMS) resonators is attracting increasing interest from the sensors community especially when the nonlinear regime is activated by challenging applications of the device. Here, the dynamic response of a double-ended tuning fork MEMS resonator is studied both in the linear and nonlinear regime. A one Degree Of Freedom (1 dof) model able to predict the frequency response of the device is proposed. Geometric and electrostatic nonlinearities are simulated through a Finite Element Method (FEM) and a Boundary Element Method (BEM) code, respectively. The total damping of the resonator is computed by taking into account both the thermoelastic and the nonlinear fluid contributions. Experimental measurements performed on a resonator fabricated in polysilicon through a commercial surface micromachining process, validate the proposed model showing a very good agreement with theoretical predictions

A MEMS Real-Time Clock with Single-Temperature Calibration and Deterministic Jitter Cancellation

This article presents a real-time clock (RTC) system based on a microelectromechanical system (MEMS) resonator coupled to an integrated circuit (IC) that implements a frequency-compensating machine. The MEMS resonator is built with a standard, industrial-grade polysilicon process characterized by a -30-ppm/K linear temperature coefficient of frequency ( $TCf$ ) and the frequency-drift compensation is entirely carried out within the IC using a fractional frequency division. The large, but deterministic, output jitter (≈1 $mu s_{rms}$ ) is then suppressed down to less than 40 $ns_{rms}$ with a low-power digital-to-time converter (DTC), whose usefulness in this kind of application is then analyzed. With a single-point temperature calibration, a ±8-ppm output frequency stability is demonstrated at ≈800-nA current consumption from a 1.2-V supply

MEMS real-time clocks based on epitaxial polysilicon: System-level requirements and experimental characterization

The purpose of this paper is to assess the feasibility of MEMS-based real-time clocks (RTCs) using conventional polysilicon, without correcting the temperature coefficient of frequency (TCf) through dedicated technological steps. The paper first shows how such a large TCf (-30 ppm/K) is not an issue in terms of maximum frequency correction to achieve with a dedicated electronics: indeed, whatever the TCf, the dominant part of the frequency correction, required to match the 32-kHz RTC target value, is always demanded by the native frequency offset due to etching nonuniformities, and not by temperature changes. This sets the required number of bits of the modulator used to drive a fractional frequency divider that performs the compensation. Instead, requirements in the bit number and refresh rate of the temperature sensor are affected by a large TCf. Nevertheless, the paper shows the possibility to achieve few ppm frequency stability using a 9-bit temperature sensor with a 4-Hz refresh rate. This makes the approach quite competitive against more sophisticated MEMS processes, especially in terms of final cost. Experimental measurements on a MEMS-based resonator coupled to a dedicated integrated circuit are used to support the discussion

Bandwidth vs ZRO Stability Trade-Off in Lissajous Frequency Modulated MEMS Gyroscopes

The research highlights a significant challenge in microelectromechanical system (MEMS) gyroscopes operated under Lissajous frequency modulation (FM). While providing excellent inherent scale-factor stability performance and scale-factor independence of the mode mismatch, this operating principle suffers from a strict requirement in the implementation of the motion amplitude control on the two axes. It is shown how, unless a few-Hz (only) bandwidth is targeted, different amplitude-gain control (AGC) designs are all challenged and give rise to spurious harmonics which, after additional demodulation, may turn into a spurious offset, which undermines the system stability performance