Towards volume medium-accuracy MEMS gyro manufacturing: Challenges and solutions

Abstract

Stepping up from a working prototype to a volume production with adequate yield is a challenge for every type of MEMS hardware. That especially applies to precise inertial sensors, among which gyros have always been of special attention. This paper provides an overview of development of medium accuracy MEMS gyro and associated steps that led to a running manufacturing facility with current production capacity of thousands of MEMS tri-axial systems a year. The core technology behind all of our IMUs and gyrometers is a multi-layer silicon gyrochip - a 3D structure assembled using WLP (wafer level packaging) technology with a deeply vacuumed cavity inside that contains a vibrating structure. Reliable and proven silicon wafer anodic bonding process guarantees long lasting vacuum without use of inner getters - numbers for vacuum reliability inside the gyrochip through indirect Q-factor measurements over time are given. An all silicon structure also provides significantly better temperature linearity compared to common SOI (silicon on insulator) gyro assemblies. The top silicon layer contains feed throughs for gyro excitation and pick-off that are manufactured using novel TSI <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">™</sup> (through silicon insulator) technology. The gyrochip dimensions are 12.6×4.6×1 mm and 3 gyrochips are packaged into a 3-axis gyrometer via glass substrate with overall volume of the sensor module just below 4 in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Statistics for angular rate performance show in-run bias stability better than 10 deg/hour, angular random walk below 0.1 deg/sqrt(h) and quite good bias repeatability. The paper talks about problems that were successfully solved in short-loop and pilot gyrochip manufacturing, such as selection of proper type of VIA <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">™</sup> trenches and each wafer thickness to minimize parasitic capacitances, reliable bonding of thin silicon wafers, testing of dies (for single sliced dies and automated wafer level testing equipment). For the 3-axis system manufacturing, issues of silicon gyrochips mounting through glass substrates, overall vibration and thermal analysis and shock reliable design are discussed. Examples of performance are also given. Currently, the overall production yield of the gyrochips at the MEMS foundry is close to 50%, and the yield from a successful batch is close to 70%. The yield from the 3-axis system (IMU or gyrometer) manufacturing line is close to 80%. Those numbers are still increasing and have potential to get to 90's digits in one year.