A Piezo-Capacitive BAW Accelerometer with Extended Dynamic Range using a Gap-Changing Moving Electrode
Anosh Daruwalla, Haoran Wen, Chang-shun Liu, Hoon Jeong, Farrokh Ayazi, Georgia Institute of Technology
Location: Big Sur
We present for the first time, a frequency-output resonant accelerometer based on the electrostatic softening effect in a bulk acoustic wave (BAW) resonator. The device is fabricated in a combined piezoelectric and capacitive HARPSS platform and uses a moving capacitive electrode with nanoscale gaps (100-200nm) to extend its dynamic range to 120dB. Capacitive IMUs and accelerometers have played an influential role in navigation and high-performance motion sensing since the advent of the inertial MEMS industry. However, most commercial accelerometers have small dynamic range, relatively low resonant frequency and low bandwidth due to their static nature. In our work, a high-frequency accelerometer is fabricated using the piezo-HARPSS process, which relies on the electrostatic frequency tuning of a piezoelectrically-actuated silicon BAW resonator with high stiffness, high resonant frequency, and high Q-factor. The concept of a moving electrode is proposed to enable a self-acting linear range extension for the accelerometer. This novel moving electrode mechanism allows large scale factor for low acceleration range, at the same time reduces nonlinear gap-closing behavior at high acceleration range, thereby improving the dynamic range by a few orders of magnitude.
The BAW accelerometer is comprised of a solid mass resonator operating in its high-frequency bulk mode, on a 60um SOI with a 185nm air-gap tuning electrode. The resonator is actuated to oscillate in a high-frequency BAW mode, the frequency of which will be used as acceleration indicator. Stiff tethers rigidly suspend the resonator at the nodes of the BAW mode. Linear accelerations will cause a small translational displacement of the resonator, resulting in a gap-size change between the resonator and the tuning electrode, as a result, the resonant frequency of the BAW mode will change due to the change in electrical spring softening effect. To decouple actuation and sensing (nano-gap tuning) mechanism and to improve sensor stability, we incorporate thin film piezoelectric transduction for the actuation and readout of the BAW accelerometer.
The BAW resonator used in its 2nd order length extensional mode, serves as a proof mass and forms an electromechanical system together with its anchoring tethers and experiences a uniform deflection when an acceleration is applied. The magnitude of the deflection is determined by the total mass of the BAW resonator and the spring constant of the tethers, and is independent of the BAW resonant mode. The acceleration-induced deflection causes a change in the capacitive gap between the deflected BAW resonator and the tuning electrode, altering the electrostatic spring constant. The accelerometer scale factor is inversely proportional to the 4th order of the gap size (equations not included in this abstract as per instructions), which increases significantly with smaller gap sizes. To achieve very small gap sizes and large sensitivity, HARPSS process is used to fabricate the BAW accelerometer where high aspect-ratio nanoscale capacitive gaps defined by sacrificial thermal oxide are form between the resonator and the tuning electrode.
On the other hand, the sensitivity is inversely proportional only to the first order of the tether stiffness, very strong tethers can be used when using nano-scale tuning gap without compromising the sensitivity, which results in a solid-state, close-to-zero movement sensor with large full-scale range and superior shock survivability.
In order to achieve an extensive linear dynamic range, we introduce a moving tuning electrode mechanism for self-acting scale factor nonlinearity cancellation. The BAW structure is in the form of a fixed electrode-first nano gap-moving electrode-second nano gap-BAW resonator configuration, with the moving electrode being used to expand the linear dynamic range of the device. This moving electrode undergoes a nonlinear displacement under the action of large acceleration and electrostatic force from the fixed electrode, causing the second nano gap to change nonlinearly, which, with proper design, automatically counteracts the nonlinear terms in electrostatic stiffness, resulting in a more linear scale factor. The moving electrode is self-acting and requires no active control electronics or close-loop operation.
Measured and Anticipated Results:
A batch of the prototype accelerometers has been fabricated on the piezo-HARPSS platform with intended dynamic range of 50ug to 50g. Q as high as 6k is measured in vacuum on fabricated devices. Despite the nano-gap, the squeeze film damping is relatively low due to high frequency of operation at 4.4MHz, measuring a Q of 4k in air, also verifying the advantage of using piezoelectric transduction to reduce necessary capacitive transduction areas. The moving electrode functionality has been verified by a frequency tuning of about -4ppm/V of the BAW mode when the resonator and the fixed electrode are kept at 0V. When a second DC voltage is applied to the fixed electrode, the BAW mode frequency further reduces due to the unbalanced electrostatic force on the moving electrode causing it to move closer to the resonator.
The acceleration response of the device was measured with discrete electronics while mounted on a rate table which provides a constant acceleration of up to 10g due to centrifugal force. A scale factor of 0.19Hz/g is measured for an applied tuning voltage of 12V, for up to 10g of acceleration range. The devices were tested in a differential configuration to eliminate 1st order temperature effects and a beat frequency was extracted using a frequency counter. The measured bandwidth for the device was 100kHz, defined by its translational mode frequency, which is significantly larger than conventional static accelerometers with bandwidths in the order of a few hundred Hz.
The basic functionality of moving electrode and linear scale factor have been verified, but smaller than designed scale factor is observed due to the limited tuning voltage being used. The maximum applicable voltage is currently limited by fabrication quality and yield, with problems identified to be the roughness of device sidewall. A higher-temperature process with hydrogen annealing is developed and that can smoothen of sidewalls by removing etching debris as well as reducing scalloping from DRIE processes to enable higher operational voltages and smaller gap sizes of less than 150nm, with which significantly better performance is expected. On-chip ovenization of the device shall also be implemented to further stabilize the temperature of the device, to improve the signal to noise ratio of the output. While the currently measured results prove the claims in this abstract, anticipated results will show improved electromechanical sensitivity and performance of the accelerometer.
Conclusions/Significance of work:
The moving electrode mechanism offers a breakthrough solution to extend the linear dynamic range of MEMS accelerometers. The high-frequency BAW operational mode combined with high stiffness tethers of the device, improves the bandwidth by ~100x as compared to conventional devices. Preliminary characterization has verified the concept and functionality of the design with improvements expected with process optimization. The piezo-HARPSS process developed in this work offers a hybrid MEMS fabrication platform which paves the way for a new branch of piezoelectrically actuated, capacitively tuned MEMS devices, including the presented highly robust BAW accelerometer that is suitable for high-end, harsh-environment inertial measurement applications.