Search

Abstract: This paper demonstrates spherical vapor cells intended to be used in chip-scale atomic devices. A micro glass blowing process is introduced, in which multiple glass spheres are simultaneously shaped on the top of a silicon wafer and subsequently filled with rubidium. In the presented fabrication process, an array of cylindrical cavities is first etched in silicon. Next, a thin glass wafer is anodically bonded to the silicon wafer. The bonded wafers are then placed inside a furnace set to 850 C. At this elevated temperature, the viscosity of the glass is decreased and the heated trapped gas in the cavities expands, thus causing the glass to be blown into spherical cells. Microscopic alkali vapor cells are achieved by evaporation of through a small glass nozzle into the cell cavities. The cells are then sealed by anodic bonding. The fabricated cells are characterized and the presence of rubidium vapor inside the cells is verified by observing an absorption spectrum. [Copyright &y& Elsevier]

We present a long-term bias drift compensation algorithm for high quality factor (Q-factor) MEMS rate gyroscopes using real-time temperature self-sensing. This approach takes advantage of linear temperature dependence of the drive-mode resonant frequency for self-compensation of temperature-induced output drifts. The approach was validated using a vacuum packaged silicon Quadruple Mass Gyroscope (QMG), with signal-to-noise ratio (SNR) enhanced by isotopic Q-factors of 1.2 million. Owing to the high Q-factors, measured frequency resolution of 0.01ppm provided a temperature self-sensing precision of 0.0004°C, on par with the state-of-the-art MEMS resonant thermometers. The real-time self-compensation yielded a total bias error of 2°/h and a scale-factor error of 700ppm over temperature range of 25–55°C. The presented approach enabled repeatable long-term rate measurements required for MEMS gyrocompassing applications with a milliradian azimuth precision. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports a new dual mass vibratory MEMS z-axis rate gyroscope architecture that prioritizes the sense-mode quality factor and provides improved ordering of the mechanical vibrational modes. The proposed linearly coupled, dynamically balanced anti-phase sense-mode design minimizes substrate energy dissipation to maximize the quality factor. The levered drive-mode mechanism structurally forces the anti-phase drive-mode motion of the symmetrically decoupled tines eliminating the lower frequency spurious mode and providing true mechanical rejection of external shocks and accelerations. SOI prototypes were characterized in a vacuum chamber demonstrating drive-mode quality factor of 67,000 and ultra-high sense-mode quality factor of 125,000. A vacuum packaging technology was introduced and demonstrated a ceramic package-level sealed gyroscope with a quality factor on the order of 100,000. The high quality factor and mode ordering of the new design provide a path toward ultra-high scale factor without compromising the sensitivity to external accelerations. [Copyright &y& Elsevier]

Abstract: This paper presents a new gyroscope design concept with a multi-degree of freedom (DOF) sense mode enabling interchangeable operation in robust or precision modes. This is accomplished using a complete 2-DOF coupled system which, unlike previous multi-DOF designs based on dynamic vibration absorbers, allows for the specification of the sense mode resonances and coupling between the masses independent of operational frequency. The robust mode corresponds to operation between the 2-DOF sense mode resonant peaks providing a response gain and bandwidth controlled at the design level by frequency spacing; the precision mode, however, relies on mode-matching the drive to one of the sense mode resonant frequencies where larger response gains are achieved through vacuum operation. The complete 2-DOF sense mode provides the gyroscope with distinct advantages over similar devices based on 1-DOF systems: the robust mode introduces a gain advantage versus conventional mismatched operation, while the precision mode gain-bandwidth is larger for the same pressures. Experimental rate characterization of a silicon-on-insulator prototype in air for both robust and precision modes demonstrated scale factors of 0.282 and 0.690mV/°/s, respectively, while the scale factor improvement due to precision mode operation is projected to increase by 40dB for operation in 1mTorr vacuum. [Copyright &y& Elsevier]

Abstract: This paper reports on the design and implementation of high efficiency, nonmetallic reflectors integrated on the sidewalls of micromachined cavities. Due to shadowing from deposition within a cavity, significant variation in the thicknesses of the dielectric thin films composing the reflectors are encountered when the layers are deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). These gradients in thickness limit the performance of the reflector at the intended design wavelength. An optimized design procedure is described to maximize the performance of the reflector at the absorption wavelength of of 795nm for use in micromachined atomic vapor cells. The reflector design is based on multiple shifted quarter wave Bragg reflectors in series, which extends the reflective bandwidth for increased robustness to film thickness gradients. The extended reflectance range maintains high reflection at the design wavelength despite greater than 70% decrease in film thickness across the reflector surface. The reflector technology is ideally suited for use in atomic MEMS vapor cell applications by achieving high reflectance while maintaining light polarization. We demonstrate less than 2dB of return loss with circular polarization ellipticity maintained to . [Copyright &y& Elsevier]

Abstract: This paper reports a new MEMS vibratory rate gyroscope designed with increased robustness to fabrication imperfections and variations in environmental conditions. The proposed architecture utilizes a single degree-of-freedom (DOF) drive-mode and a fully coupled 2-DOF sense-mode. The drive-mode operational frequency and the sense-mode bandwidth can be set independently, relaxing the tradeoff between the gain, die size, and detection capacitance inherent to the previously reported robust gyroscopes with dynamic vibration absorber (DVA) architecture of the 2-DOF sense-mode. Prototypes with 2.5kHz operational frequency were extensively characterized in air and demonstrated sense-mode 3dB bandwidth of 250Hz. The uncompensated temperature coefficients of bias and scale factor were 313(/h)/C and 351ppm C, respectively. Using off-chip detection electronics, the rate sensitivity was 28 V/(/s), ARW – 0.09(/s)/, bias instability – 0.08(/s), and ARRW – 0.03(/s) . Detailed time domain, Allan variance, and probability density function analysis revealed that both ARW and ARRW are of Gaussian type and Coriolis-uncorrelated, showing stable in-operational noise performance of the reported gyroscope. [Copyright &y& Elsevier]

• Structural asymmetries of the vibratory gyroscope are induced mainly by fabrication errors. • Structural imperfections limit the sensor performance by affecting the signal-to-noise ratio, resolution, zero rate output and bias stability. • Mechanical ablation of interconnecting spokes in concentric ring suspension of a gyroscope enabled coarse (>50 Hz) tuning and fine-tuning (<1 Hz) of resonant frequencies. • Mechanical trimming allows for permanent tuning of the gyroscope's vibratory modes without introducing additional noise. • Electrostatic tuning allows for gyroscope frequency split reduction to the level of mHz after packaging of the device. This paper reports mechanical ablation techniques for permanent frequency tuning of the primary wineglass modes in planar gyroscopes with concentric ring suspension. Three different approaches for frequency tuning are discussed: (1) selective mass removal, (2) stiffness reduction via thinning of suspension elements, and (3) stiffness reduction by removal of interconnecting spoke elements between concentric rings. A predictive tuning algorithm is developed for identification of ablation locations to guide the trimming process. A dual-mass Dynamically Amplified Gyroscope (DAG) design is used to demonstrate the tuning approach. It is shown that introduction of "side-spokes" to the concentric ring system of DAG enables coarse (> 50 Hz) tuning, as well as fine-tuning (< 1 Hz) of resonant frequencies, which is performed by means of mechanical ablation with focused ion beam (FIB). Finally, a comparison of the sensor noise characteristics using active electrostatic tuning and mechanical ablation is presented. [ABSTRACT FROM AUTHOR]

This paper presents a study on dynamics of a dual-mass MEMS vibratory gyroscope in presence of fabrication imperfections and reports a method for precision electrostatic frequency tuning of the operational modes. A number of multi-mass MEMS gyroscopes have emerged in recent years pursuing different goals, such as dynamically balanced structure, increased bandwidth, and dynamic amplification. Along with many perceived advantages of multi-mass devices, several challenges associated with mode-matching in a system with increased number of degrees-of-freedom (DOF) have to be considered. This work shows that it is possible to apply the DC tuning techniques, similar to tuning a conventional single-mass gyroscope, to achieve the precision tuning in a dual-mass sensor, without losing advantages of increased DOF of the system. The presented frequency trimming technique is based on assessing the modes mismatch and cross-coupling between modes by means of fitting the experimental frequency response curves to the analytical solutions of the dual-mass system in presence of imperfections. The tuning algorithm involves two steps. First, the stiffness mismatch along the two axes and the anisoelasticity angles α and β are identified, then the tuning DC voltages for modification of diagonal, off-diagonal, and coupling terms in the stiffness matrix are chosen. The method of electrostatic tuning was validated through the experimental characterization of a dual-mass dynamically amplified gyroscope, where the coupling between the two operational modes was minimized and frequency split was reduced from 26 Hz down to 50 mHz, resulting in 17.5× increase in the gyroscope scale factor and significantly improved noise characteristics. The presented electrostatic compensation method is suitable for both off-line and on-line calibration. [ABSTRACT FROM AUTHOR]

Abstract: We report detailed characterization of a vacuum sealed rate integrating silicon MEMS gyroscope. The new gyroscope utilizes geometrically symmetric, dynamically balanced quadruple mass architecture, which provides a combination of maximized quality (Q) factors and isotropy of both the resonant frequency and damping. The vacuum sealed SOI prototype with a 2kHz operational frequency demonstrated virtually identical X- and Y-mode Q-factors of 1.2 million. Due to the stiffness and damping symmetry, and low energy dissipation, the gyroscope can be instrumented for direct angle measurements with fundamentally unlimited rotation range and bandwidth. Experimental characterization of the mode-matched gyroscope operated in whole-angle mode confirmed linear response in a ±450°/s range and 100Hz bandwidth (limited by the experimental setup), eliminating both bandwidth and range constraints of conventional open-loop Coriolis vibratory rate gyroscopes. [Copyright &y& Elsevier]

Abstract: This paper reports on a method for improving the optical efficiency of micromachined reflectors integrated in rubidium vapor cells for application in atomic MEMS sensors. A hybrid bulk micromachining and multilayer PECVD thin film process is used to form the Bragg reflectors on angled sidewalls, which redirect laser light through the vapor cell and back toward the plane of the source with reduced optical power loss. Integrated thin film Bragg reflectors are shown to improve the reflectance of the micromachined surface by almost three times, resulting in an optical return efficiency of two paired dielectric reflectors that is as much as eight times higher then uncoated silicon reflectors. Absorption of the absorption line at a wavelength of 795nm in a 1 vapor cell is demonstrated experimentally by use of two integrated thin film reflectors with a total optical return efficiency approaching 40%. [Copyright &y& Elsevier]