D. E. Serrano

Tunable piezoelectric MEMS resonators for real-time clock

This paper reports on the design, simulation and characterization of small form factor, tunable piezoelectric MEMS resonators for real time clock applications (32.768 kHz). The structures were fabricated on a thin-film AlN-on-SOI substrate to enable piezoelectric actuation of an out-of-plane flexural mode, as well as electrostatic frequency tuning by utilizing the handle layer as a DC voltage electrode. Resonators of only a few hundred of µm in size exhibit greater than 3100 ppm of tuning using voltages no larger than 4 V; this tuning sufficiently compensates for frequency variations across temperature from −25 to 100 °C. The devices exhibit low motional impedance that is completely independent of the tuning potential.

Electrostatically tunable piezoelectric-on- silicon micromechanical resonator for real-time clock

This paper reports on the design, fabrication, and characterization of a small form factor, piezoelectrically transduced, tunable micromechanical resonator for real-time clock (RTC) applications (32.768 kHz). The device was designed to resonate in an out-of-plane flexural mode to simultaneously achieve low-frequency operation and reduced motional resistance in a small die area. Finite element simulations were extensively used to optimize the structure in terms of size, insertion loss, spurious-mode rejection, and frequency tuning. Microresonators with an overall die area of only 350 × 350 μm were implemented on a thin-film AlN on silicon-on-insulator (SOI) substrate with AlN thickness of 0.5 μm, device layer of 1.5 μm, and an electrostatic tuning gap size of 1 μm. A frequency tuning range of 3100 ppm was measured using dc voltages of less than 4 V. This range is sufficient to compensate for frequency variations of the microresonator across temperature from -20°C to 100°C. The device exhibits low motional impedance that is completely independent of the frequency tuning potential. Discrete electronics were used in conjunction with the resonator to implement an oscillator, verifying its functionality as a timing reference.

Hermetic packaging of resonators with vertical feedthroughs using a glass-in-silicon reflow process

A low-cost, hermetic wafer-level packaging solution with negligible parasitics suitable for MEMS resonators is presented. A glass cap with embedded 50µm diameter vertical single-crystal silicon feedthroughs is anodically bonded in vacuum to an SOI wafer prefabricated with mechanical resonators. This hermetic packaging process provides virtually zero-parasitic-capacitance vertical interconnects. A silicon-gold eutectic is used to ensure electrical contacts between the feedthroughs and the device wafer. An embedded wet-etched cavity in the glass cap allows mechanical motion of the resonator as well as the inclusion of a thin-film getter to further enhance and maintain vacuum. Temperature sweeps and quality factor measurements demonstrate the successful packaging of resonators using this technique.

Wafer-level vacuum-packaged triaxial accelerometer with nano airgaps

This paper reports on the design, implementation and characterization of triaxial capacitive accelerometers operating in a low-pressure environment (~1 Torr). Small form-factor devices, with proof-mass area of less than 1mm2, were fabricated on a 40 μm-thick SOI substrate using the HARPSS™ process to attain in-plane and out-of-plane nanoscale capacitive airgaps (~300 nm). Increased sensitivity and stable open-loop operation were simultaneously achieved by using additional damping electrodes. Large electromechanical coupling provided by the deep sub-micron airgaps allow for the design of high-frequency accelerometers (~15 kHz) that yield better shock and vibration immunity. Scale factors of 7.5 mV/g and 8.7 mV/g were measured for the in-plane (X/Y-axis) and out-of-plane (Z-axis) accelerometers, respectively, with measured cross-axis sensitivity of less than 0.5 % for accelerations of up to ±6g.

Substrate-decoupled silicon disk resonators having degenerate gyroscopic modes with Q in excess of 1-million

This paper details a center-supported solid disk resonator in <;100> single-crystalline-silicon (SCS) that uses a novel substrate decoupling feature to achieve ultra-low dissipation gyroscopic modes with small frequency split. The secondary bulk acoustic wave (BAW) elliptic modes (m = 3) of a 2mm diameter substrate-decoupled disk resonator exhibit quality factor (Q) of ~1.3 M with 40 ppm frequency split (as fabricated) at 2.745 MHz. Q-factor remained in excess of 1 million at pressure levels as high as 500 mTorr. The measured temperature behavior of the Q, which is mostly limited by thermoelastic damping (TED), is in very close agreement with FEM predictions.

A dual-mode gyroscope architecture with in-run mode-matching capability and inherent bias cancellation

This paper introduces a novel dual-mode actuation and sensing scheme for readout and calibration of axisymmetric Coriolis resonant gyroscopes. The proposed scheme actuates both gyroscope modes simultaneously with the same in-phase excitation, senses both modes concurrently, and utilizes the sum and difference of the sense signals to demonstrate complete cancellation of the gyroscope bias terms, and provide automatic in-run mode-matching capability. Moreover, the architecture provides twofold enhancement of angular rate sensitivity and signal-to-noise performance, as compared to conventional single-mode excitation of the same gyroscope. We demonstrate, for the first time 45× reduction in temperature drift of bias of a 2.6 MHz 650 μm diameter substrate-decoupled BAW gyroscope and a bias stability of 5.4 °/hr, paving the way towards near-zero-drift gyroscopes.

Single proof-mass tri-axial pendulum accelerometers operating in vacuum

This paper reports on the design, fabrication and characterization of single proof-mass tri-axial capacitive accelerometers coexisting in a low-pressure environment with high-frequency gyroscopes, for the implementation of monolithic 6-degree-of-freedom inertial measurement units. The accelerometers are designed to operate as quasi-static devices (i.e. non-resonant sensors) in mid vacuum levels (1-10 Torr) by increasing squeeze-film air damping through the use of capacitive nano-gaps (<; 300 nm). Reduced die area is achieved utilizing a pendulum-like structure composed of a 450×450×40 μm3 proof-mass anchored to the substrate by a cross-shaped polysilicon spring. The small capacitive gaps, allow for the design of devices with high resonance frequency (~ 15 kHz) that provide large shock and vibration immunity.

Environmentally-robust high-performance tri-axial bulk acoustic wave gyroscopes

This paper reports on the design and characterization of the first commercially-available tri-axial rotation-rate sensor implemented by the use of substrate-decoupled bulk acoustic wave (SD-BAW) gyroscopes. Owed to their high-frequency of operation (4.3 MHz), these devices exhibit significantly reduced dependencies on environmental stimuli such as vibration and shock. The sensor consists of three separate, but identical, wafer-level packaged (WLP) gyros, each of which is oriented along the X, Y and Z axes for 3-axis detection. The three gyro dies are co-integrated in a 7 mm × 7 mm × 2.9 mm cavity package with an application specific integrated circuit (ASIC) responsible for signal actuation, readout, control of the device. With a scale factor of 3 mV/(°/s), the complete system attains a full-scale range (±300 °/s) with non-linearity of 0.05%. Furthermore, integration of all dies in the same package facilitates achieving low cross-axis sensitivity of less than 1%. Values as low as 0.17 °/√h of angle-random walk (ARW) and 10.5 °/h of bias-instability (BI) have been measured; these are dominated by the thermal and 1/f noise of the ASIC, respectively.