Study Of Micromachined Inertial Sensors Based On A Novel Comb-bar Capacitor Scheme

A novel comb-bar capacitor is proposed on the basis of the study of traditional bar capacitor based inertial sensors, and the study of comb-bar capacitor based MEMS accelerometers and gyroscopes is presented in this paper, specific work including: the structural design for an accelerometer and gyroscope, the process flow design for the fabrication, the design of the interface circuits, performance tests of the prototypes, and the analysis and optimization of the accelerometer and gyroscope.(1) Compared to the traditional bar capacitor structure, the bar electrodes of the comb-bar structure is only etched for a few tens of micrometers instead of being cut though, which increases the proof masses of the accelerometer and gyroscope efficiently, resulting in higher mechanical sensitivities and lower mechanical-thermal noises. For this typical design, the proof masses are increased by 54% both for the accelerometer and gyroscope. As a result, the mechanical sensitivities have been increased by 54% and 92%, and the mechanical-thermal noises have been reduced by 35% and 48% respectively within the same geometry. Furthermore, the bar electrodes and the structure are released separately, which lowers the damages caused by the lag effect and notching effect during DRIE greatly, resulting in higher fabrication quality and yield rates.(2) An analysis model for the mechanical sensitivity of the electrostatically driven bar capacitor based frame gyroscope is built, which reveals that, given fixed chip area and driving voltages, the mechanical sensitivity is maxed when the areas of the outer frame and the inner proof mass are equal, thus the performance of the gyroscope is optimized. An equal width, low aspect-ratio design principle for both spring beams and bar electrodes of the driving motion and sensing motion is proposed to deal with the large dimension errors caused by the lag effect, notching effect and edge effect of the thick photoresist during DRIE, thus the deviation of the difference between the driving and sensing resonant frequencies, which is caused by the the relatively large dimension error and fabrication ununiformity, is minimized. Test results show that: the standard deviation for resonant frequencies of gyroscopes from the same batch has been reduced by more than 79%; With the maximal frequency difference 132Hz of the tested gyroscope resonant frequencies from the same batch caused by process tolerances, the average value for the difference between the driving and sensing resonant frequencies is 7.5Hz, which is close to the designed value 6Hz, and the standart deviation for the difference is 16.5Hz, which implies that the driving and sensing resonant frequencies keep matched after fabrication.(3) Test results of the accelerometer and gyroscope show that the SNR of the capacitor sensing carrier has been increased by around 13dB after passing by a single input—double differential output CV circuit, which is because the correlated common noises are canceled after the differtial amplifier. Noise analysises for the accelerometer and gyroscope are presented and analysis results are of the same order of magnitude as the tested values. An optimization for the gyroscope is carried out according to the noise analysis results, and the noise floor of the gyroscope at 1Hz hasbeen decreased from 0.023 o/s/Hz^1/2 to 0.0079 o/s/Hz^1/2 .The accelerometer and gyroscope both achieve high linearity as a result of sensing displacement by the varying overlapped area: the linearity for the accelerometer is 99.997% within±lg input acceleration, and the ratio of the quadratic term coefficient and the linear term coefficient of the quadratic fit line is 159ppm for±30g input accelerotion; the linearity for the gyroscope is 99.995% within±43°/s input angular rate, and the ratio of the quadratic term coefficient and the linearterm coefficient of the quadratic fit line is 203ppm for±200°/s input angular rate. Themain system damping of the accelerometer and gyroscope is slide-film damping, thus low damping coefficients and high quality factors are achieved at atmospheric pressure, and the novel comb-bar capacitor have ensured larger proof masses for the inertial sensors, as a result, the accelerometer and gyroscope have achieved high performances at atmospheric pressure: The bias instability is 0.3mg, the white noise floor is 0.348 mg/Hz^1/2 and the dynamic range is±30g for the accelerometer; Thebias instability is 21.6°/h, and the noise floor at 1Hz is better than 0.01°/s/Hz^1/2 forthe gyroscope. The linearity, bias instability and noise floor of the comb-bar structure gyroscope are better than the MEMS gyroscopes working at atmospheric pressure reported in recent years.