Said Emre Alper

A single-crystal silicon symmetrical and decoupled MEMS gyroscope on an insulating substrate

This paper presents a single-crystal silicon symmetrical and decoupled (SYMDEC) gyroscope implemented using the dissolved wafer microelectromechanical systems (MEMS) process on an insulating substrate. The symmetric structure allows matched resonant frequencies for the drive and sense vibration modes for high-rate sensitivity and low temperature-dependent drift, while the decoupled drive and sense modes prevents unstable operation due to mechanical coupling, achieving low bias-drift. The 12-15-/spl mu/m-thick single-crystal silicon structural layer with an aspect ratio of about 10 using DRIE patterning provides a high sense capacitance of 130 fF, while the insulating substrate provides a low parasitic capacitance of only 20 fF. A capacitive interface circuit fabricated in a 0.8-/spl mu/m CMOS process and having a sensitivity of 33 mV/fF is hybrid connected to the gyroscope. Drive and sense mode resonance frequencies of the gyroscope are measured to be 40.65 and 41.25 kHz, respectively, and their measured variations with temperature are +18.28 Hz//spl deg/C and +18.32 Hz//spl deg/C, respectively, in -40/spl deg/C to +85/spl deg/C temperature range. Initial tests show a rate resolution around 0.56 deg/s with slightly mismatched modes, which reveal that the gyroscope can provide a rate resolution of 0.030 deg/s in 50-Hz bandwidth at atmospheric pressure and 0.017 deg/s in 50-Hz bandwidth at vacuum operation with matched modes.

A SYMMETRIC SURFACE MICROMACHINED GYROSCOPE WITH DECOUPLED OSCILLATION MODES

This paper reports a new symmetric gyroscope structure that allows both matched resonant frequencies for the drive and sense vibration modes for better sensitivity, and also decoupled drive and sense oscillation modes for preventing unstable operation due to mechanical coupling and achieving a low zero-rate output drift. The symmetry and decoupling features are achieved at the same time with a new suspension beam design. The gyroscope structure is designed using a standard three-layer polysilicon surface micromachining process (MUMPS) and simulated using the MEMCAD software. The drive and sense mode resonant frequencies of the fabricated device are measured as 28,535 and 30,306 Hz, respectively, which are in agreement with the finite element simulations. The small mismatch is due to the unsymmetric distribution of the etch holes, which can be eliminated with a proper design. When the resonant frequencies are closely matched, the rate sensitivity of the gyroscope is amplified by the mechanical quality factor of the sense resonant mode. The new suspension beam structure also provides very high quality factors for the drive and sense modes, such as 10,400, when operated under 10 mTorr vacuum level. Even though the performance of the fabricated sensor is limited due to large parasitic capacitances between the mechanical structure and the substrate, measurements, and calculations show that the sensor can still sense angular rates as small as 1.6°/s under vacuum. This sensitivity can be enhanced by at least an order of magnitude if the parasitic capacitances could have effectively been eliminated. The advantage of the new structure can be combined with advanced, high-aspect ratio fabrication processes to obtain very sensitive micromachined gyroscopes.

Quadrature-Error Compensation and Corresponding Effects on the Performance of Fully Decoupled MEMS Gyroscopes

This paper presents experimental data about the sources of the quadrature error in a fully decoupled microelectromechanical systems gyroscope and demonstrates the extent of performance improvement by the cancellation of this error. Quadrature sources including mass, electrostatic-force, and mechanical-spring imbalances have been compared by FEM simulations, and spring imbalance has been found as the dominant source of the quadrature error. Gyroscopes have been designed with intentional spring imbalances and fabricated with a SOI-based silicon-on-glass fabrication process, the resulting quadrature outputs of the fabricated gyroscopes have been measured, and their agreement with FEM simulations has been verified. Next, it has been experimentally shown that the electrostatic nulling of the quadrature error with closed-loop control electronics improves the bias instability and angle random walk (ARW) of a fully decoupled gyroscope up to ten times. Moreover, the quadrature cancellation improves the scale-factor turn-on repeatability about four times and linearity about 20 times, reaching down to 119 and 86 ppm, respectively. Finally, the quadrature cancellation allows operating the gyroscope with higher drive-mode displacement amplitudes for an increased rate sensitivity. With this technique, outstanding bias instability and ARW performances of 0.39°/h and 0.014 °/√h, respectively, have been achieved.

A Compact Angular Rate Sensor System Using a Fully Decoupled Silicon-on-Glass MEMS Gyroscope

This paper presents the development of a compact single-axis angular rate sensor system employing a 100- mum-thick single-crystal silicon microelectromechanical systems gyroscope with an improved decoupling arrangement between the drive and sense modes. The improved decoupling arrangement of the gyroscope enhances the robustness of sensing frame against drive-mode oscillations and therefore minimizes mechanical crosstalk between the drive and sense modes, yielding a small bias instability. The gyroscope core element is fabricated by through-etching a 100-mum -thick silicon substrate which is anodically bonded to a recessed glass handling substrate. A patterned metal layer is included at the bottom of the silicon substrate, both as an etch-stop layer and a heat sink to prevent heating- and notching-based structural deformations encountered in deep dry etching in the silicon-on-glass process. The fabricated-gyroscope core element has capacitive actuation/sensing gaps of about 5 mum yielding an aspect ratio close to 20, providing a large differential sense capacitance of 18.2 pF in a relatively small footprint of 4.6 mm times 4.2 mm. Excitation and sensing electronics of the gyroscope are constructed using off-the-shelf integrated circuits and fit in a compact printed circuit board of size 54 mm times 24 mm. The complete angular rate sensor system is characterized in a vacuum ambient at a pressure of 5 mtorr and demonstrates a turn-on bias of less than 0.1 deg/s, bias instability of 14.3 deg/h, angle random walk better than 0.115 deg/radic(h), and a scale-factor nonlinearity of plusmn0.6% in full-scale range of plusmn50 deg/s. [2007-0158].

An Automatically Mode-Matched MEMS Gyroscope With Wide and Tunable Bandwidth

This paper presents the architecture and experimental verification of the automatic mode-matching system that uses the phase relationship between the residual quadrature and drive signals in a gyroscope to achieve and maintain matched resonance mode frequencies. The system also allows adjusting the system bandwidth with the aid of the proportional-integral controller parameters of the sense-mode force-feedback controller, independently from the mechanical sensor bandwidth. This paper experimentally examines the bias instability and angle random walk (ARW) performances of the fully decoupled MEMS gyroscopes under mismatched (~ 100 Hz) and mode-matched conditions. In matched-mode operation, the system achieves mode matching with an error frequency separation between the drive and sense modes in this paper. In addition, it has been experimentally demonstrated that the bias instability and ARW performances of the studied MEMS gyroscope are improved up to 2.9 and 1.8 times, respectively, with the adjustable and already wide system bandwidth of 50 Hz under the mode-matched condition. Mode matching allows achieving an exceptional bias instability and ARW performances of 0.54 °/hr and 0.025 °/√hr, respectively. Furthermore, the drive and sense modes of the gyroscope show a different temperature coefficient of frequency (TCF) measured to be -14.1 ppm/°C and -23.2 ppm/°C, respectively, in a temperature range from 0 °C to 100 °C. Finally, the experimental data indicate and verify that the proposed system automatically maintains the frequency matching condition over a wide temperature range, even if TCF values of the drive and sense modes are quite different.

Pirani Vacuum Gauges Using Silicon-on-Glass and Dissolved-Wafer Processes for the Characterization of MEMS Vacuum Packaging

This paper presents the design and implementation of Pirani vacuum gauges for the characterization of vacuum packaging of microelectromechanical systems (MEMS). Various Pirani vacuum gauges are fabricated with two different standard in-house fabrication processes, namely the silicon-on-glass (SOG) process and dissolved-wafer process (DWP). The Pirani gauges utilize meander-shaped suspended silicon coils as the heaters and two isolated silicon islands in the close proximity of the heater that function as dual-heat sinks to enhance the sensitivity and dynamic range as compared to a microbridge with a single heat sink. The gauges are designed to occupy an area of 4 mm × 1.5 mm. The DWP Pirani gauge fabricated with a structural thickness of 14 mum and a gap of 2 mum shows a measured sensitivity of 4.2×10⁴ (K/W)/Torr in a dynamic range of 10-2000 mTorr. The SOG Pirani gauge fabricated with a structural thickness of 100 mum and a gap of 3 mum shows a lower measured sensitivity of 3.8×10³ (K/W)/Torr in a dynamic range of 50-5000 mTorr; however, the 100 mum-thick structural layer results in a much more robust process against stress-based deformations in suspended silicon compared to the DWP Pirani gauges. Each gauge is used to monitor the pressure of a different packaging approach. The DWP Pirani gauge is used to detect the pressure of a wafer-level vacuum package, where the pressure inside the cavity is measured to be about 2.4 mTorr. The SOG Pirani gauge is used the monitor the pressure inside a hybrid platform package which is vacuum-sealed using a projection welder, where the pressure is measured to be about 1400 mTorr. These measurements verify that the DWP and SOG Pirani gauges can be used for the characterization of wafer-level or hybrid platform vacuum packages for MEMS devices.

A single-crystal silicon symmetrical and decoupled gyroscope on insulating substrate

This paper presents a single-crystal silicon symmetrical and decoupled (SYMDEC) gyroscope implemented using dissolved wafer process on an insulating substrate. The symmetric structure allows matched resonant frequencies for the drive and sense vibration modes for high rate sensitivity and low temperature-dependent drift, while the decoupled drive and sense modes prevents unstable operation due to mechanical coupling, achieving a low bias drift. The 12-15 /spl mu/m-thick single-crystal silicon structural layer with an aspect ratio about 10 using DRIE patterning provides a high sense capacitance of 130fF, while the insulating substrate provides a low parasitic capacitance of only 20fF. Drive and sense mode resonance frequencies of the gyroscope are measured to be 39,010 Hz and 38,570 Hz, respectively. Measurement results reveal that the gyroscope provides a rate sensitivity of 0.01 deg/sec in 50 Hz bandwidth at vacuum.

High-Performance SOI-MEMS Gyroscope with Decoupled Oscillation Modes

This paper presents a new, high-performance SOI-MEMS gyroscope with decoupled oscillation modes. The gyroscope structure allows to achieve matched-resonance-frequencies, large drive-mode oscillation amplitude, high sense-mode quality factor, and low mechanical crosstalk, demonstrating a measured noise-equivalent rate of 90 (deg/hr)/Hz1/2even at atmospheric pressure. The angular rate sensitivity of the gyroscope is 100 µ V/(deg/sec) at atmospheric pressure. This value improves to 2.4mV/(deg/sec) at vacuum, for which the measured noise-equivalent rate of the gyroscope reaches to 35 (deg/hr)/Hz1/2. The R2-nonlinearity of the gyroscope is measured to be better than 0.02%. The gyroscope has a low quadrature signal of 70deg/sec and a short-term bias stability of 1.5deg/sec.

An automatically mode-matched MEMS gyroscope with 50 Hz bandwidth

This paper presents the architecture and experimental verification of an automatic mode matching system that uses the phase relationship between the residual quadrature and drive signals in a gyroscope to accomplish and maintain the frequency matching condition. The system also allows controlling the system bandwidth by adjusting the closed loop controller parameters of the sense mode. This study experimentally examines the angle random walk (ARW) and bias instability performances of the fully decoupled MEMS gyroscopes under mismatched (~100Hz) and mode-matched conditions. Moreover, it has been experimentally shown that the performance of the studied MEMS gyroscopes is improved up to 2.4 times in bias instability and 1.7 times in ARW with 50 Hz system bandwidth under the mode-matched condition reaching down to a bias instability of 0.83°/hr and an ARW of 0.026°/√hr.

A wide-bandwidth and high-sensitivity robust microgyroscope

This paper reports a microgyroscope design concept with the help of a 2 degrees of freedom (DoF) sense mode to achieve a wide bandwidth without sacrificing mechanical and electronic sensitivity and to obtain robust operation against variations under ambient conditions. The design concept is demonstrated with a tuning fork microgyroscope fabricated with an in-house silicon-on-glass micromachining process. When the fabricated gyroscope is operated with a relatively wide bandwidth of 1 kHz, measurements show a relatively high raw mechanical sensitivity of 131 μ V( ◦ s −1 ) −1 . The variation in the amplified mechanical sensitivity (scale factor) of the gyroscope is measured to be less than 0.38% for large ambient pressure variations such as from 40 to 500 mTorr. The bias instability and angle random walk of the gyroscope are measured to be 131 ◦ h −1 and 1.15 ◦ h −1/2 , respectively. (Some figures in this article are in colour only in the electronic version)