a 3mhz spoke gyroscope with wide bandwidth and ... - Semantic Scholar
A 3MHZ SPOKE GYROSCOPE WITH WIDE BANDWIDTH AND LARGE DYNAMIC RANGE Wang-kyung Sung, Milap Dalal, and Farrokh Ayazi Georgia Institute of Technology, USA ABSTRACT This paper reports on the design and characterization of a high-frequency silicon bulk acoustic wave (BAW) spoke gyroscope operating in air. The gyroscope described here operates at 3.12MHz in a near mode-matched condition (without tuning) and has a –1dB bandwidth of ~1.5kHz. The device has a linear full-scale range in excess of 30,000˚/sec with a sensitivity of 15.0µV/˚/sec using a 10V DC polarization voltage. The wide bandwidth and large dynamic range of this device are beneficial for applications requiring rapid motion sensing.
INTRODUCTION Micromachined vibratory gyroscopes [1] are increasingly used in applications that require large dynamic range and large bandwidth such as gaming controllers and smart user interfaces. Their popularity has grown in large part due to their low cost, small size, robustness and low power consumption, attributes which had been hardly achievable with conventional gyroscopes. Vibratory micro-gyroscopes operating at mode-matched condition [2-4] use two resonance modes of a structure with identical frequencies to amplify the Coriolis force induced vibration by the mechanical quality factor of the sense mode. However, because of imperfections in the fabrication process, the native drive and sense resonance modes of these devices may exhibit a frequency separation. DC potentials are thus applied in a calibration step to adjust the electrical stiffness of the drive and sense resonance modes and make their pass-bands overlap. In this work, a spoke gyroscope design is introduced to increase the resonator bandwidth such that it is larger than the worst-case frequency separation caused by fabrication errors. A wide band-pass filter response is hence created for the gyroscope, as a result of which the bandwidth and dynamic range of the sensor are increased. Similar to the perforated high frequency bulk acoustic wave (BAW) disk gyroscope [5-6], the spoke gyroscope operates in the MHz range.
DESIGN Figure 1 shows a schematic view of the center-supported capacitive silicon BAW spoke gyroscope. Due to the anisotropic nature of (100) single crystal silicon (SCS) substrate, the device is
designed to operate at its degenerate secondary elliptical mode, in which the drive and sense modes are spatially 30º apart [5]. The spoke gyroscope uses a geometry that replaces a concentric ring section of a disk with a network of spokes and beams. At resonance, the outer region that retains the continuous disk-like shape undergoes a bulk acoustic mode of vibration while the region with spokes exhibits a flexural mode (Fig. 2). Energy losses associated with the flexural mode of vibration (e.g. thermoelastic damping) serve to reduce the overall quality factor (Q) of the gyroscope. As a result, the relatively low Q (1,000-2,000) of the spoke gyroscope at high resonance frequency creates a large bandwidth and a fast response time without needing vacuum. Additionally, the –3dB bandwidth of the resonance mode is now larger than the frequency split caused by fabrication errors between the two degenerate modes. Inner and outer spokes are arranged 30˚ and 15˚ apart respectively to minimize excessive frequency separation between the drive and sense modes. In addition, 10µm release holes are included to facilitate the structural release process. The release holes are equally-spaced and oriented in the same way as the spokes. Polarization electrode Sense electrode
Drive electrode
Ωz Figure 1: Schematic diagram of the capacitive silicon BAW spoke gyroscope. An ANSYS electromechanical harmonic simulation was executed to approximate the sensitivity and dynamic range of the gyroscope. First, a worst-case frequency separation of 2kHz was intentionally created between the two secondary elliptical modes by adjusting the electrical stiffness in the simulation environment to separate the drive and sense peaks. Afterward, the drive mode was excited to
a vibration amplitude of 10nm (Fig. 2) while a DC polarization voltage of 10V was applied to the resonating body of the gyroscope. A Q of 1,000 was assumed for the drive and sense modes. Simulated rotation rates were applied, and the Coriolis-induced output current detected at a single sense electrode was plotted in Fig. 3. The device exhibits a very linear dynamic range in excess of 30,000˚/sec with a sensitivity of 2.73pA/˚/sec (per electrode). The overall rate sensitivity of the device can be increased by a factor of 6 through differential sensing and connecting the in-phase sense electrodes.
trenches are etched through the device layer of SOI wafer to define the spoke structure and release holes, and a 200nm oxidation is done to create a capacitive gap between the vibrating mass and electrodes. The trenches are refilled with LPCVD polysilicon after doping and 200nm sacrificial oxide (SACOX) is patterned from the top surface (Fig. 4(a)). The second LPCVD polysilicon is deposited, doped, and patterned to define the electrode pads. The final step of the fabrication is a timed release in hydrofluoric acid (HF), leaving a central buried oxide support layer underneath the spoke structure (Fig. 4(b)).
(a)
(b)
Figure 2: ANSYS harmonic simulation of 1.12mm diameter (100) SCS BAW spoke gyroscope showing secondary elliptical drive mode (left) at 3.1815MHz and sense mode (right) at 3.1795MHz with ±10nm deformation.
Figure 4: Fabrication process flow of SCS BAW spoke gyroscope.
Figure 5: SEM view of 60µm thick silicon BAW spoke gyroscope. Figure 3: ANSYS simulation showing sensitivity and dynamic range of 1.12mm diameter (100) SCS spoke gyroscope. The device has a rate sensitivity of 2.73pA/˚/sec per electrode and remains linear with applied rotation rate as high as ±30,000˚/sec.
(a)
(b)
FABRICATION The capacitive BAW spoke gyroscopes were fabricated on 60µm thick silicon-on-insulator (SOI) wafers using the HARPSS™ process [2]. The basic outline of the process is shown in Fig. 4. The fabrication starts from patterning the oxide mask created by thermal oxidation and PECVD. Deep
Figure 6: SEM view of (a) trench-refilled poly-electrode and 200nm air gap, (b) poly-trace. SEM images of the fabricated device are shown in Figs. 5 and 6. A suspended polysilicon trace is connected to the center of the mass to provide a DC
polarization voltage to the vibrating structure (Fig. 6(b)).
MEASUREMENT RESULTS The spoke gyroscope was affixed to a printed circuit board (PCB) and driven open-loop using an Agilent N4395A network analyzer at a constant vibration amplitude of 0.6nm. The output sense electrode was connected to a TI OPA657 discrete trans-impedance amplifier front-end with a feedback resistance of 33k. Additional voltage amplifiers were added after the trans-impedance stage to provide supplementary gain to compensate for the insertion loss of the device as well as prevent any loading from the measurement equipment that would affect the output sense signal. The frequency response of several devices were tested in air, each showing the expected wide bandpass response of the gyroscope. Figure 7 shows the measured frequency response of one prototype, measured in air, exhibiting a large –3dB bandwidth of 2.87kHz at a frequency of ~3.12MHz. No electronic tuning was performed on this device (all the electrodes around the disk were tied to VP with the exception of drive and sense electrodes). The –1dB bandwidth of the gyroscope was measured on the network analyzer to be over 1.5kHz, suggesting that the rate sensitivity of the device will remain constant across a large operational bandwidth of at least 1 kHz. Figure 8 shows the zero rate output (ZRO) or the quadrature signal of the measured device, along with the drive signal applied to the gyro. Following the ZRO measurement, different rotation signals were applied to the spoke gyroscope using an Ideal Aerosmith rotation table, and the amplitude modulated sense current was amplified and demodulated using the input drive signal and an Analog Devices AD835 four-quadrant mixer to extract the Coriolis signal. Figure 9 shows the measured rate sensitivity of a spoke gyroscope. The linear scale factor of this sensor was measured to be ~15.0µV/˚/sec. Although a large dynamic range of ~30,000˚/sec was simulated, the measurement was capped at 500˚/sec because the rate table could not support rotation rates in excess of this value. Preliminary temperature sensitivity measurements were performed on an unpackaged device over a range of -20˚C to 70˚C (Fig. 10). The bandwidth and bandpass response of the gyroscope remained relatively constant over this temperature range, although some distortion can be seen in the pass band at lower temperatures. The gyroscope showed a frequency response dependency of ~ -27ppm/˚C.
Figure 7: Frequency response of a 1.12mm diameter spoke gyroscope with uniform 10µm diameter release holes in air.
Figure 8: Measured ZRO of the spoke gyroscope.
15.0µV/˚/sec
3.56mVpeak-to-peak
Figure 9: Measured rate response of 1.12mm diameter (100) SCS spoke gyroscope in air, tested to 500˚/sec. The sensitivity of the device was measured to be 15.0µV/˚/sec. The inset is the demodulated output signal from the device with the rotation rate of 250˚/sec.
CONCLUSION TCF= -27 ppm/˚C
A 3.12MHz BAW spoke gyroscope was designed and fabricated on a 60µm thick substrate. The device provided a wide –1dB bandwidth of 1.5kHz and a linear dynamic range simulated to go as large as ~30,000˚/sec. The device operates in air with a low DC polarization voltage of 10V, eliminating the need for vacuum packaging and post-fabrication tuning. Table 1. Performance summary for the 1.12mm diameter BAW spoke gyroscope. Device Parameter Value 3.12MHz (Measured) Operation frequency 3.18MHz (ANSYS) Device thickness 60µm Capacitive gap 200nm Polarization voltage 10V –3dB bandwidth 2.867kHz –1dB bandwidth 1.5kHz Rate sensitivity 15.0µV/˚/sec (Measured) Dynamic range 30,000˚/sec (ANSYS)
ACKNOWLEDGEMENTS
Figure 10: (top) Temperature coefficient of frequency in air for a 1.12mm diameter (100) SCS spoke gyroscope with uniform 10µm diameter release holes. (bottom) Measured frequency response of device at each marker from -20˚C-70˚C.
DISCUSSION
The authors wish to thank Qualtré Inc. for supporting this project. In addition, we would like to acknowledge Microelectronics Research Center (MiRC) at Georgia Institute of Technology for fabrication assistance, Ashwin Samarao and Xin Gao for assistance with device processing, and Dr. M. Faisal Zaman for simulation support and discussion.
REFERENCES [1]
The design specifications and measurement results of the BAW spoke gyroscope are summarized in Table 1. Although the lower Q of the resonant modes results in a lower rate sensitivity than what was reported in [5-6], the spoke gyroscope offers a higher dynamic range and large bandwidth while operating in air at a low DC polarization voltage of 10V. Furthermore, electronic tuning of the gyroscope was completely eliminated. Although the measured device operated under mode-coupled condition, simulations show that the sensitivity of the spoke gyroscope will remain relatively constant for a peak separation of up to 2kHz. The sensitivity can be improved by reducing the capacitive gap size and increasing the device thickness to augment the capacitive area, as well as increasing the drive amplitude. In addition to these changes, the input referred noise of the interface electronics can be reduced by interfacing the device with an application specific integrated circuit (ASIC).
[2] [3]
[4]
[5]
[6]
N. Yazdi, F. Ayazi, and K. Najafi, "Micromachined inertial sensors," Proc. IEEE, vol. 86, no. 8, pp. 1640-1659, 1998. F. Ayazi and K. Najafi, "A HARPSS polysilicon vibrating ring gyroscope," J. Microelectromech. Syst., vol. 10, no. 2, pp. 169-179, 2001. M. F. Zaman, A. Sharma, and F. Ayazi, "High Performance Matched-Mode Tuning Fork Gyroscope," IEEE Intl. Conf. on Microelectromech. Syst. 2006, pp. 66-69. M. F. Zaman, A. Sharma, and F. Ayazi, "The Resonating Star Gyroscope: A Novel Multiple-Shell Silicon Gyroscope With Sub-5 deg/hr Allan Deviation Bias Instability," J. Sensors, vol. 9, no. 6, pp. 616-624, 2009. H. Johari and F. Ayazi, "Capacitive Bulk Acoustic Wave Silicon Disk Gyroscopes," Intl. Electron Devices Meeting 2006, pp. 513-516. H. Johari and F. Ayazi, "High-frequency capacitive disk gyroscopes in (100) and (111) silicon," IEEE Intl. Conf. on Microelectromech. Syst. 2007, pp. 47-50.
INTRODUCTION Micromachined vibratory gyroscopes [1] are increasingly used in applications that require large dynamic range and large bandwidth such as gaming controllers and smart user interfaces. Their popularity has grown in large part due to their low cost, small size, robustness and low power consumption, attributes which had been hardly achievable with conventional gyroscopes. Vibratory micro-gyroscopes operating at mode-matched condition [2-4] use two resonance modes of a structure with identical frequencies to amplify the Coriolis force induced vibration by the mechanical quality factor of the sense mode. However, because of imperfections in the fabrication process, the native drive and sense resonance modes of these devices may exhibit a frequency separation. DC potentials are thus applied in a calibration step to adjust the electrical stiffness of the drive and sense resonance modes and make their pass-bands overlap. In this work, a spoke gyroscope design is introduced to increase the resonator bandwidth such that it is larger than the worst-case frequency separation caused by fabrication errors. A wide band-pass filter response is hence created for the gyroscope, as a result of which the bandwidth and dynamic range of the sensor are increased. Similar to the perforated high frequency bulk acoustic wave (BAW) disk gyroscope [5-6], the spoke gyroscope operates in the MHz range.
DESIGN Figure 1 shows a schematic view of the center-supported capacitive silicon BAW spoke gyroscope. Due to the anisotropic nature of (100) single crystal silicon (SCS) substrate, the device is
designed to operate at its degenerate secondary elliptical mode, in which the drive and sense modes are spatially 30º apart [5]. The spoke gyroscope uses a geometry that replaces a concentric ring section of a disk with a network of spokes and beams. At resonance, the outer region that retains the continuous disk-like shape undergoes a bulk acoustic mode of vibration while the region with spokes exhibits a flexural mode (Fig. 2). Energy losses associated with the flexural mode of vibration (e.g. thermoelastic damping) serve to reduce the overall quality factor (Q) of the gyroscope. As a result, the relatively low Q (1,000-2,000) of the spoke gyroscope at high resonance frequency creates a large bandwidth and a fast response time without needing vacuum. Additionally, the –3dB bandwidth of the resonance mode is now larger than the frequency split caused by fabrication errors between the two degenerate modes. Inner and outer spokes are arranged 30˚ and 15˚ apart respectively to minimize excessive frequency separation between the drive and sense modes. In addition, 10µm release holes are included to facilitate the structural release process. The release holes are equally-spaced and oriented in the same way as the spokes. Polarization electrode Sense electrode
Drive electrode
Ωz Figure 1: Schematic diagram of the capacitive silicon BAW spoke gyroscope. An ANSYS electromechanical harmonic simulation was executed to approximate the sensitivity and dynamic range of the gyroscope. First, a worst-case frequency separation of 2kHz was intentionally created between the two secondary elliptical modes by adjusting the electrical stiffness in the simulation environment to separate the drive and sense peaks. Afterward, the drive mode was excited to
a vibration amplitude of 10nm (Fig. 2) while a DC polarization voltage of 10V was applied to the resonating body of the gyroscope. A Q of 1,000 was assumed for the drive and sense modes. Simulated rotation rates were applied, and the Coriolis-induced output current detected at a single sense electrode was plotted in Fig. 3. The device exhibits a very linear dynamic range in excess of 30,000˚/sec with a sensitivity of 2.73pA/˚/sec (per electrode). The overall rate sensitivity of the device can be increased by a factor of 6 through differential sensing and connecting the in-phase sense electrodes.
trenches are etched through the device layer of SOI wafer to define the spoke structure and release holes, and a 200nm oxidation is done to create a capacitive gap between the vibrating mass and electrodes. The trenches are refilled with LPCVD polysilicon after doping and 200nm sacrificial oxide (SACOX) is patterned from the top surface (Fig. 4(a)). The second LPCVD polysilicon is deposited, doped, and patterned to define the electrode pads. The final step of the fabrication is a timed release in hydrofluoric acid (HF), leaving a central buried oxide support layer underneath the spoke structure (Fig. 4(b)).
(a)
(b)
Figure 2: ANSYS harmonic simulation of 1.12mm diameter (100) SCS BAW spoke gyroscope showing secondary elliptical drive mode (left) at 3.1815MHz and sense mode (right) at 3.1795MHz with ±10nm deformation.
Figure 4: Fabrication process flow of SCS BAW spoke gyroscope.
Figure 5: SEM view of 60µm thick silicon BAW spoke gyroscope. Figure 3: ANSYS simulation showing sensitivity and dynamic range of 1.12mm diameter (100) SCS spoke gyroscope. The device has a rate sensitivity of 2.73pA/˚/sec per electrode and remains linear with applied rotation rate as high as ±30,000˚/sec.
(a)
(b)
FABRICATION The capacitive BAW spoke gyroscopes were fabricated on 60µm thick silicon-on-insulator (SOI) wafers using the HARPSS™ process [2]. The basic outline of the process is shown in Fig. 4. The fabrication starts from patterning the oxide mask created by thermal oxidation and PECVD. Deep
Figure 6: SEM view of (a) trench-refilled poly-electrode and 200nm air gap, (b) poly-trace. SEM images of the fabricated device are shown in Figs. 5 and 6. A suspended polysilicon trace is connected to the center of the mass to provide a DC
polarization voltage to the vibrating structure (Fig. 6(b)).
MEASUREMENT RESULTS The spoke gyroscope was affixed to a printed circuit board (PCB) and driven open-loop using an Agilent N4395A network analyzer at a constant vibration amplitude of 0.6nm. The output sense electrode was connected to a TI OPA657 discrete trans-impedance amplifier front-end with a feedback resistance of 33k. Additional voltage amplifiers were added after the trans-impedance stage to provide supplementary gain to compensate for the insertion loss of the device as well as prevent any loading from the measurement equipment that would affect the output sense signal. The frequency response of several devices were tested in air, each showing the expected wide bandpass response of the gyroscope. Figure 7 shows the measured frequency response of one prototype, measured in air, exhibiting a large –3dB bandwidth of 2.87kHz at a frequency of ~3.12MHz. No electronic tuning was performed on this device (all the electrodes around the disk were tied to VP with the exception of drive and sense electrodes). The –1dB bandwidth of the gyroscope was measured on the network analyzer to be over 1.5kHz, suggesting that the rate sensitivity of the device will remain constant across a large operational bandwidth of at least 1 kHz. Figure 8 shows the zero rate output (ZRO) or the quadrature signal of the measured device, along with the drive signal applied to the gyro. Following the ZRO measurement, different rotation signals were applied to the spoke gyroscope using an Ideal Aerosmith rotation table, and the amplitude modulated sense current was amplified and demodulated using the input drive signal and an Analog Devices AD835 four-quadrant mixer to extract the Coriolis signal. Figure 9 shows the measured rate sensitivity of a spoke gyroscope. The linear scale factor of this sensor was measured to be ~15.0µV/˚/sec. Although a large dynamic range of ~30,000˚/sec was simulated, the measurement was capped at 500˚/sec because the rate table could not support rotation rates in excess of this value. Preliminary temperature sensitivity measurements were performed on an unpackaged device over a range of -20˚C to 70˚C (Fig. 10). The bandwidth and bandpass response of the gyroscope remained relatively constant over this temperature range, although some distortion can be seen in the pass band at lower temperatures. The gyroscope showed a frequency response dependency of ~ -27ppm/˚C.
Figure 7: Frequency response of a 1.12mm diameter spoke gyroscope with uniform 10µm diameter release holes in air.
Figure 8: Measured ZRO of the spoke gyroscope.
15.0µV/˚/sec
3.56mVpeak-to-peak
Figure 9: Measured rate response of 1.12mm diameter (100) SCS spoke gyroscope in air, tested to 500˚/sec. The sensitivity of the device was measured to be 15.0µV/˚/sec. The inset is the demodulated output signal from the device with the rotation rate of 250˚/sec.
CONCLUSION TCF= -27 ppm/˚C
A 3.12MHz BAW spoke gyroscope was designed and fabricated on a 60µm thick substrate. The device provided a wide –1dB bandwidth of 1.5kHz and a linear dynamic range simulated to go as large as ~30,000˚/sec. The device operates in air with a low DC polarization voltage of 10V, eliminating the need for vacuum packaging and post-fabrication tuning. Table 1. Performance summary for the 1.12mm diameter BAW spoke gyroscope. Device Parameter Value 3.12MHz (Measured) Operation frequency 3.18MHz (ANSYS) Device thickness 60µm Capacitive gap 200nm Polarization voltage 10V –3dB bandwidth 2.867kHz –1dB bandwidth 1.5kHz Rate sensitivity 15.0µV/˚/sec (Measured) Dynamic range 30,000˚/sec (ANSYS)
ACKNOWLEDGEMENTS
Figure 10: (top) Temperature coefficient of frequency in air for a 1.12mm diameter (100) SCS spoke gyroscope with uniform 10µm diameter release holes. (bottom) Measured frequency response of device at each marker from -20˚C-70˚C.
DISCUSSION
The authors wish to thank Qualtré Inc. for supporting this project. In addition, we would like to acknowledge Microelectronics Research Center (MiRC) at Georgia Institute of Technology for fabrication assistance, Ashwin Samarao and Xin Gao for assistance with device processing, and Dr. M. Faisal Zaman for simulation support and discussion.
REFERENCES [1]
The design specifications and measurement results of the BAW spoke gyroscope are summarized in Table 1. Although the lower Q of the resonant modes results in a lower rate sensitivity than what was reported in [5-6], the spoke gyroscope offers a higher dynamic range and large bandwidth while operating in air at a low DC polarization voltage of 10V. Furthermore, electronic tuning of the gyroscope was completely eliminated. Although the measured device operated under mode-coupled condition, simulations show that the sensitivity of the spoke gyroscope will remain relatively constant for a peak separation of up to 2kHz. The sensitivity can be improved by reducing the capacitive gap size and increasing the device thickness to augment the capacitive area, as well as increasing the drive amplitude. In addition to these changes, the input referred noise of the interface electronics can be reduced by interfacing the device with an application specific integrated circuit (ASIC).
[2] [3]
[4]
[5]
[6]
N. Yazdi, F. Ayazi, and K. Najafi, "Micromachined inertial sensors," Proc. IEEE, vol. 86, no. 8, pp. 1640-1659, 1998. F. Ayazi and K. Najafi, "A HARPSS polysilicon vibrating ring gyroscope," J. Microelectromech. Syst., vol. 10, no. 2, pp. 169-179, 2001. M. F. Zaman, A. Sharma, and F. Ayazi, "High Performance Matched-Mode Tuning Fork Gyroscope," IEEE Intl. Conf. on Microelectromech. Syst. 2006, pp. 66-69. M. F. Zaman, A. Sharma, and F. Ayazi, "The Resonating Star Gyroscope: A Novel Multiple-Shell Silicon Gyroscope With Sub-5 deg/hr Allan Deviation Bias Instability," J. Sensors, vol. 9, no. 6, pp. 616-624, 2009. H. Johari and F. Ayazi, "Capacitive Bulk Acoustic Wave Silicon Disk Gyroscopes," Intl. Electron Devices Meeting 2006, pp. 513-516. H. Johari and F. Ayazi, "High-frequency capacitive disk gyroscopes in (100) and (111) silicon," IEEE Intl. Conf. on Microelectromech. Syst. 2007, pp. 47-50.
