Towards Design, Fabrication, Packaging, Integration, and Characterization of High-Performance MEMS Gyroscopes

Abstract

MEMS vibratory gyroscopes are used to measure the angular rate of a body by sensing the Coriolis force induced motion of the sensing element vibrating in a rotating frame of reference. MEMS gyroscopes find use in a wide variety of applications such as smartphones, automobiles, navigation, bio medical instruments, industrial systems, etc. A considerable amount of research has been done over the past two decades towards the improvement of MEMS gyroscope performance in terms of its scale factor, resolution, dynamic range, temperature sensitivity and bandwidth, with a goal of using them in high performance applications. Industrial report published about a year ago predicted the advent of inertial grade MEMS gyroscopes into the market by 2030. Towards this goal, active research is being pursued across the world by solving various issues towards the improvement of signal to noise ratio in MEMS gyroscopes stretching them into the ultimate performance regime at par with their optical counterparts. This work focuses on the complete development of a MEMS gyroscope suitable for high-performance applications. This involves the thorough study of specifications, meticulous design of the sensing structure to extract the required performance parameters, incorporation of fabrication related non-idealities into the design, precise fabrication of the sensor structure with tight tolerance, complete die-level characterization of the sensor, vacuum sealed packaging and integration with electronics to extract the sensor response to applied angular rates. The technique of sensitivity analysis is used to optimize the design and the same is demonstrated on three different designs of MEMS gyroscopes. DRIE process related non-idealities like the slanting and scalloping profiles are incorporated into the modelling of gyroscope structures. Extensive simulations are carried out to study the dependence of the etch profiles on different performance criteria of the MEMS gyroscope. Two designs are finalized and carried forward for fabrication. Combined wet and dry bulk micromachining technique is used to fabricate the sensor chips. Two different fabrication process flows are optimized to realize the two different designs. Novel findings in the area of TMAH based wet bulk micromachining for long duration etching and NH2OH added KOH based wet bulk micromachining at low temperatures for faster etch rates are reported. DRIE is used for precise controlled etching of the 100 μm device layer to realize the gyroscope structure. The fabricated sensor chips are subsequently characterized mechanically to extract the drive and sense resonant frequencies. A novel mechanical characterization set up for multiple sensor chips is discussed. The experimental observations are compared with FEM simulation values. Electrical characterization of the sensor chip involving C-V measurements and drive voltage optimization is also reported. The packaging of the sensor chip is carried out using an 84 pin quad flat leadless ceramic package. This package is sealed in vacuum of a few mTorr. The sealed sensor package is then used for further electronic integration. The electronic integration is carried out in two different ways. The sensor is directly integrated with AD7746 24-bit capacitance-to-digital converter for extracting the sense capacitance variation with angular rate. In the second method, a Trans-Impedance Amplifier circuit and a Lock-in Amplifier are used for real time measurements of sensor response to angular rate variation. A sensitivity of about 9.27 mV/(deg/s) is obtained with a non-linearity of about 0.5% over the entire range of ±440 deg/s. Further, raw data is continuously recorded for about 7 hrs with gyroscope in drive excitation mode and without application of angular rate. This data is used to plot Allan deviation curve which shows a bias instability of <5 deg/hr. The values of the specifications achieved, point towards a high- performance gyroscope even with the sensor packaged separately. The integration of the sensor chip with a C-V conversion ASIC is also briefly discussed for hybrid packaging to achieve even better performance. This thesis therefore presents some novel findings in the area of design, fabrication and characterization of high-performance MEMS gyroscopes