Wafer-Level-Packaged HARPSS+ MEMS Platform: Integration of Robust Timing and Inertial Measurement Units (TIMU) on a Single Chip
Haoran Wen, Anosh Daruwalla, Yaesuk Jeong, Pranav Gupta, Jaehoo Choi, Chang-Shun Liu, Farrokh Ayazi, Georgia Institute of Technology
Location: Big Sur
Novelty / progress claims:
This paper presents a high aspect-ratio poly- and single-crystal silicon RIE plus wet-etching (HARPSS+) fabrication platform that enables single-chip integration and wafer-level-packaging of robust timing and inertial sensors. The HARPSS+ platform is the first highly developed process providing high-efficiency nano-gap capacitive transduction in vertical, horizontal, and 54°-slanted orientations. The platform also provides wafer-level-packaging of MEMS devices with through-silicon via and built-in cavities with the capability of achieving different cavity pressures in a single bonding step. Single-chip timing and inertial measurement units (TIMU) are fabricated with high yield through the HARPSS+ platform, which for the first time, features 3-axis high-frequency mode-matched gyroscopes with complete quadrature cancellation capabilities enabled by the slanted electrodes. High-bandwidth 3-axis accelerometers are used on the TIMU with high sensitivity provided by the nano-gaps. A high-Q BAW resonator is included as internal frequency reference for self-contained timing and inertial measurement, along with on-chip heaters for ovenization capability. Each device is packaged in cavities with different pressure levels ranging from atmosphere to sub-10 Torr for optimized operation through one single bonding step. The nano-gap-enabled high-frequency devices have high environmental robustness and small form-factor with the overall TIMU being 4.5mm?5.5mm?1mm. Fabricated TIMU demonstrates high-performance with sub-10°/h gyroscopes and sub-100?g accelerometers. Low power chip-level ovenization is achievable with the on-chip heaters owing to the small chip size and vacuum packaging.
Background / significance of work:
MEMS IMUs have been widely used in personal electronics due to their small size and low cost, and they have enabled a large variety of applications such as gaming and automotive control. However, the growing interests in miniature inertial navigation systems ask for robust high-performance IMUs that are still not commercially available. Most commercial MEMS IMUs use low-frequency flexural devices that are vulnerable to random vibration and shock. This causes their performance to degrade in the presence of environmental nonidealities and makes them unsuitable for high-end navigation applications. High-frequency gyroscopes operating at megahertz range provide a good solution to environmental robustness with their frequencies outside of the environmental vibration spectrum. However, they have large stiffness and small displacements, and building highly efficient transducers for actuation, readout, and tuning of the high-frequency devices are challenging. In additional, for single-chip IMU with planar pitch/roll gyros, quadrature tuning is crucial, which can be most effectively addressed using slanted electrode. For single-chip IMUs, packaging pressure is also critical with the resonant devices (gyros, resonators) favoring low pressure to minimize air damping and stationary devices (accels) requiring higher pressure for stable operation. HARPSS+ process presented in this work offers a universal solution to the challenges mentioned above, making it a powerful platform for high-performance high-robustness sensor fusion.
The HARPSS+ process combines CVD, RIE, and silicon anisotropic-wet-etching procedures, allowing planar as well as transverse structural engineering, with nano-scale transduction gaps defined by thermal oxidation of silicon. Stacking and eutectic bonding a capping wafer with built-in through-silicon vias and deep cavities to the device base wafer realizes 3D integration for packaged MEMS sensor units. Different package pressures can be obtained simultaneously on a single chip with HARPSS+. Better vacuum is achieved in cavities with larger volume-to-boundary ratio for pressure caused by post-bonding outgassing, and atmosphere cavity is achieved by introducing vent holes. The advanced platform enables advanced sensor designs. High-frequency 3-axis planar gyroscopes with full quadrature tuning capabilities and all resonant modes above 200kHz are achieved for the first time with nano-scale HARPSS+ gaps for tuning, actuation, and signal readout. The gyros are in a vacuum cavity along with a cross-sectional Lamé mode timing resonator and integrated heaters. High-bandwidth 3-axis accelerometers are fabricated on the same die in an atmosphere cavity. The atmosphere operation eliminates the need for additional dampers, significantly reducing the accelerometer size and area, which allows center-supported designs for stress relief without increasing stiction risks.
The versatility and yield of the HARPSS+ platform is verified through fabrication and characterization of TIMUs with the advanced designs. The process is performed on a 40?m (100) SOI wafer with capacitive gaps of ~270nm. All devices on the TIMUs are found functional with high yield and no stiction. Sufficient tuning is achieved for quadrature cancellation and mode-matching over process variations for all 3-axis gyros with the nano-gaps. The gyros show mode-matched sense mode Qs of 180k at 5.4MHz for yaw gyro and 4.5k at 0.7MHz for pitch/roll gyro, corresponding to a cavity pressure of ~5 Torr. The gyroscopes are also fabricated in the same batch on separated dies with smaller cavity designs, showing Qs of 1.5k for pitch/roll gyros and 130k for yaw gyros, which corresponds to a cavity pressure of ~16 Torr.
Interfacing the gyros with discrete electronics shows scale-factors of 2.7nA/°/s and 150pA/°/s, ARW of 0.23°/?h and 0.6°/?h, and bias instability of 8.7°/h and 17.9°/h for yaw and pitch/roll gyros, respectively. The lowest resonant frequencies of the accelerometers are measured to be above 10kHz, indicating a large bandwidth. Interfacing accelerometers with off-the-shelf ICs gives scale-factors of 28.19mV/g and 70.29mV/g, VRW of 218?g/?h and 85?g/?h, and bias instability of 60?g and 82?g for X/Y and Z accels, respectively. Cross-axis sensitivity of the 3-axis accelerometers are measured to be less than 4%, which is limited by manual alignment errors in the testing setup. The timing resonator shows a high Q of 47k at a resonant frequency of 85MHz.
Efficiency of the on-chip heater is verified by monitoring the frequency of nearby gyroscope whiling changing the voltage applied across the heater. Results show that ovenization temperature of 85°C can be achieved for the TIMU die with a power consumption less than 500mW when the chip is directly attached to the testing PCB board without any heat isolations. Adding a heater control loop will allow chip-level ovenization, which will further improve the overall stability of the TIMU.