The tolerancing capabilities of the current photolithography processes and microfabrication techniques are inadequate compared to the requirements from production of high-performance inertial sensors. The resulting inherent imperfections in the mechanical structure significantly limits the performance, stability, and robustness of MEMS gyroscopes. Thus, fabrication and commercialization of high-performance and reliable MEMS gyroscopes that require picometer-scale displacement measurements of a vibratory mass have proven to be extremely challenging.
In micromachined vibratory rate gyroscopes, the mode-matching requirement renders the system response very sensitive to variations in systems parameters due to fabrication imperfections and fluctuations in operating conditions. Inevitable fabrication imperfections affect both the geometry and the material properties of MEMS devices, and shift the drive and sense-mode resonant frequencies. The dynamical system characteristics are observed to deviate drastically from the designed values and also from device to device, due to slight variations in photolithography steps, etching processes, deposition conditions or residual stresses. Process control becomes extremely critical to minimize die-to-die, wafer-to-wafer, and lot-to-lot variations.
Fluctuations in the temperature of the structure also perturb the dynamical system parameters due to the temperature dependence of Young's Modulus and thermally induced localized stresses. Temperature also drastically affects the damping and the Q-factor in the drive and sense modes.
Consequently, the mechanical sensing elements of micromachined gyroscopes are required to provide excellent performance, stability, and robustness to meet demanding specifications. Fabrication imperfections and variations, and fluctuations in the ambient temperature or pressure during the operation time of these devices introduce significant errors, which typically require electronic compensation. Closed-loop force-feedback implementations in the sense-mode are known to alleviate the sensitivity to frequency and damping variations, and increase the sensor bandwidth. However, a closed-loop sense-mode required additional feedback electronics, and increases the cost and complexity of both the MEMS device and the electronics. Thus, it is desirable to achieve inherent robustness at the sensing element to minimize compensation requirements.