Lowest Noise Low CSWAP 6 DoF MEMS IMU
R. Mark Boysel and Louis J. Ross, Motion Engine, Inc., Canada
Location: Big Sur
MEMS inertial sensors are being used for increasingly advanced applications with the aid of complex sensor fusion solutions. These solutions are necessary due to the inherent performance limitations found in inexpensive, consumer and even many industrial grade MEMS sensor system platforms. Low cost MEMS sensors, particularly gyroscopes, used in indoor positioning and navigation applications have limited accuracy due to drift caused by thermal mechanical noise, measured as angular random walk (ARW) for gyroscopes and velocity random walk (VRW) for accelerometers. The ARWs of these multi-axis consumer devices are on the order of 0.5 deg/rt-hr or greater, and the VRWs are on the order of 100 ?g/rt-Hz or more. Some low noise single-axis gyroscopes used in tactical and navigation applications reach ARWs around 0.02 deg/rt-hr and some single axis accelerometers reach VRWs around 3-10 ?g/rt-Hz, but these cost hundreds or thousands of dollars and multiple sensors must be integrated and aligned in packages to produce a full 6 Degrees of Freedom (6 DoF) IMU.
In this paper we report on the commercial production of a low CSWAP (cst, size, weight, and power) single die MEMS 6 DoF IMU with the lowest thermomechanical noise (ARW as low as 0.005 deg/rt-hr, VRW as low as 0.1 ?g/rt-Hz) of any 6 DoF MEMS IMU that can be fabricated in high volume using a standard semiconductor production process. The low ARW will provide 100 times less drift than current consumer IMUs and will improve the accuracy of current indoor navigation and other novel motion sensing applications while opening up new applications for which no cost-effective motion sensing solution has been available. An example of an application needing such accuracy would include head tracking for Virtual Reality/Augmented Reality (VR/AR) headsets. Furthermore, the low noise IMU introduces a cost-effective MEMS IMU option for tactical and navigation applications currently dominated by more expensive Fiber Optic Gyros (FOGs) and single axis MEMS gyros and single or multi-axis accelerometers integrated into multi-axis IMUs.
The mechanical ARW of a vibratory gyroscope can be expressed as ARW=(1/A)*(kT/MQ?)^0.5, where A is the drive amplitude, M is the mass, ? is the resonant frequency, and Q is the quality factor. Similarly, the VRW of an accelerometer can be expressed as VRW=(4kT?/MQ)^0.5. Historically the proof masses of MEMS gyroscopes and accelerometers have been small and thin because they are typically fabricated in the same thin film layer as the comb capacitors used for driving and sensing. These thin (on the order of 1-40 ?m) masses are on the order of a few micro-grams which consequently lead to the high ARW and VRW of consumer devices. To achieve lower ARW/VRW researchers have pursued ever higher Qs (on the order of hundreds of thousands or millions), and in some cases higher frequencies. However, these approaches inevitably lead to high vacuum packaging costs and complex high frequency circuitry.
Motion Engine has taken a novel alternative approach by developing an all-silicon bulk micromachined IMU with thick, heavy proof masses that is fabricated in a commercial MEMS foundry using a three wafer fusion bonding process. Planar drive and sense electrodes are fabricated in the top and bottom wafers, and the proof masses are fabricated in a central Silicon-On-Insulator MEMS wafer. The high temperature fusion bonding produces a silicon-silicon bond and getters residual oxygen and water vapor, providing a low pressure environment for the sensor without the need for getters and eliminating the need for additional expensive vacuum packaging. Electrical connections are made to the bottom electrodes, MEMS transducer, and top electrodes and brought up through the three-wafer stack to top bond-pad connections using MEI’s proprietary “3DS” (3D System) architecture. The 3DS architecture and process flow allow on-chip integration of other sensors like pressure and magnetometer sensors which will enable the development of IMUs with greater than six degrees of freedom. Furthermore, the 3DS architecture enables scaling of the IMU to navigation grade through the ability to integrate multiple proof masses on the same piece of silicon.
Motion Engine has fabricated 6 DoF inertial sensors with gyroscope Q-factors in the 4000 – 5000 range. Because the mass is large, >1 milligram, the Q-factor is adequate to exceed the ARW of higher Q, but lower mass devices. The resonant frequencies of the proof masses are around 10 kHz. This combination of mass, Q, and frequency translates into a mechanical ARW as low as 0.005 deg/rt-hr and VRW~0.1 ?g/rt-Hz. Based upon surveys of other IMUs we find a relationship between ARW and Bias Stability that indicates that this ARW should correspond to a Bias Stability of approximately 0.1-0.2 deg/hr, approaching navigation grade performance. The MEMS sensor is designed to operate in matched, or near-matched mode to take advantage of both the high sensitivity and low noise. A separate higher-compliance (lower resonant frequency) damped proof mass is used for 3-axis acceleration measurements.
Motion Engine is developing complementary low noise drive/sense electronics to take advantage of the low mechanical noise and high sensitivity of the MEMS sensor and maintain overall low ARW and VRW. In this talk we will describe the sensor architecture and operation, and report on the testing and test results and status of the custom IC development.