Tunable Piezoelectric MEMS Resonators for Real ... - Semantic Scholar
Tunable Piezoelectric MEMS Resonators for Real-Time Clock Diego Emilio Serrano, Roozbeh Tabrizian and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, GA USA [email protected]; [email protected]; [email protected] Abstract—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 ºC. The devices exhibit low motional impedance that is completely independent of the tuning potential.
I.
INTRODUCTION
Advances in wireless communications and computer processing applications have led to the steady growth of the electronics timing market, which is currently at the $5 billion mark. This niche is mainly composed of resonators, oscillators and clock generators, whose implementation is dominated by quartz crystal technology. This has been partly due to historical reasons [1], but mainly because quartz exhibits excellent temperature-stable characteristics and reasonable quality factor values, which are desired for high-accuracy applications [2]. In recent years, silicon-based MEMS resonators are becoming strong contenders in the timing applications market as a result of their small form factors, high fQ products and the possibility of direct integration with interface circuits [3]. However, most research efforts in this field are presently being invested in high-frequency devices in the MHz to GHz range, leaving conventional tuning fork quartz resonators as the prevailing devices for real-time clock (RTC) applications. The low-frequency operation (32.768 kHz) and precise fabrication steps required for the implementation of quartz tuning forks make these structures large in size with surface areas in the millimeter range [4]. Additionally, to achieve vacuum levels necessary for high-Q performance, quartz resonators require metallic encapsulation, which further complicates their manufacturing and increases cost. At such low frequencies, silicon microresonators operating in flexural modes can be utilized to overcome these complications; however, these devices exhibit large temperature coefficients of frequency (TCF) of about -30
ppm/ºC, so passive compensation techniques [5, 6] and frequency-tuning methods are required to counteract this effect [7]. Several techniques for passive temperature compensation of frequency in flexural silicon MEMS resonators have been reported. For example, thermally-dependent axial loads can be applied to a device to counteract the effects of the temperature coefficient of Young’s modulus (TCE) [8]. However, this type of approach requires a significant amount of additional area to accommodate for the compensation structure. Another common technique is the use of composite resonators, where different materials with opposite values of TCE are stacked to cancel out or minimize the overall TCF [9]. Nevertheless, interfacial material losses tend to degrade the quality factor which compromises the device performance. Hence, in applications where minimal size and maximum performance are a necessity, electrical frequency tuning tends be a better option, as long as the device offers enough tuning range to enable compensation for both temperature and process variations. As of today, very few 32 kHz MEMS resonators have been reported [10, 11]. These devices rely on electrostatic actuation and sensing; thus, they require a polarization voltage VP to operate. To achieve low motional impedances, high aspect ratio capacitive gaps and/or large VP values are necessary, both of which severely compromise power handling. Furthermore, electrostatic tuning is needed to compensate for TCF; hence, the polarization voltage of the device must be used to adjust the frequency utilizing a temperature compensation scheme. This in turn changes the motional impedance of the device, complicating the implementation of frequency-stable oscillators with a wide operating temperature range. In order to overcome these limitations, thin-film piezoelectric-on-SOI flexural resonators can be utilized to enable piezoelectric actuation as well as electrostatic frequency tuning [12]. Small form factor, highly-tunable lowfrequency devices can be realized by carefully designing their geometry as well as the size and shape of their drive and sense electrodes. This paper reports on the optimization, implementation, and characterization of such structures utilizing a CMOS-compatible process flow that is also compatible with high frequency piezoelectric bulk acoustic resonators.
II.
TUNABLE PIEZOELECTRIC MEMS RESONATOR
A. Design and Operating Principle Figure 1 shows a schematic diagram of the proposed 32 kHz piezoelectrically-actuated, capacitively-tunable resonator. This novel structure is composed of an external frame of four clamped-clamped beams anchored at the corners to the surrounding substrate. The beams are mechanically coupled to each other through a suspended rigid plate that provides sufficient structural support without compromising device compliance. More importantly, the plate contributes additional mass to achieve low-frequency operation and a large capacitive electrode area for increased electrostatic tuning.
Figure 1. Small form factor piezoelectrically-transduced electrostatically tunable 32 kHz resonator.
and
The resonator was implemented on an AlN-on-SOI substrate to allow piezoelectric transduction. A pair of drive and sense signal electrodes were carefully defined on top of each tether by etching the AlN and top molybdenum layers of the piezoelectric stack. The bottom molybdenum, which is in electrical contact with the single crystal silicon (SCS) device layer, serves as the ground electrode. By applying an AC voltage between each pair of drive and ground electrodes, shear stress is generated over the beams by transverse piezoelectric coupling, which is determined by the d31 coefficient of AlN. This stress produces a bending moment that excites the structure into its first out-of-plane flexural mode (Fig. 2). The displacement, amplified by the quality factor of the structure, can be sensed by collecting the charge generated at the opposite pair of signal electrodes. Exciting a vertical mode rather than a lateral vibration is advantageous because the frequency value of a flexural structure is highly dependent on the dimensions along the displacement axis. The displacement magnitude of in-plane modes is determined by
Figure 2. ANSYS® modal simulation results of fundamental out-of-plane flexural mode of piezoelectric 32 kHz resonator.
lithography, making the resonance frequency highly prone to process variations. In the case of out-of-plane modes, displacement occurs along the device thickness, which can be more precisely controlled through deposition steps. Although the structure does not require a polarization voltage to resonate, electrical access to the handle layer enables it to be used as a DC tuning electrode. Because the device layer is directly connected to ground, any potential applied to the handle layer will cause an electrostatic spring softening effect that can be utilized to tune the central operating frequency to compensate for temperature and process variations. B. Device Optimization Different design techniques were utilized to optimize the device motional impedance, frequency-tuning range, size and manufacturability. Single clamped-clamped beams can be used to implement a simpler version of the resonators proposed in this work; however, their length has to be greatly increased in order to achieve low-frequency operation, which significantly reduces the structural stiffness. This results in devices with large surface-area-to-volume ratios that are highly prone to stiction. To overcome this issue, the presented design takes advantage of mass loading, provided by a rigid plate rather than a decrease in stiffness, to achieve the target resonance frequency. This makes the structure more robust without compromising the overall area. In piezoelectrically-transduced resonators, the motional impedance is inversely proportional to the square of the electromechanical coupling. To maximize this value and achieve low impedance, the signal electrodes should be carefully sized and shaped to guarantee that, at no point of the covered area, the induced stress experiences a change in sign; this avoids charge cancellation and maximizes coupling. It can be shown that for clamped-clamped beams operating in their fundamental mode, the electrodes should cover up to 1/4th of the beam length in order to achieve maximum electromechanical transduction [13]. For the presented design, FEM simulations were used to sweep the electrode length and verify these results. As shown in Figure 3, the optimal point occurs when the electrode is 22.5% of the beam length rather than 25%. This is due to the undercut of the device layer, which changes the effective beam anchor location and hence its maximum and minimum stress points.
Figure 3. Simulation results for electrode size optimization. Ideal size is slightly smaller than theoretical value due to undercut of device layer.
III.
FABRICATION PROCESSS
For compatibility purposes, these low-freequency structures were implemented with a similar process flow utilized for high-frequency thin-film piezo-on-substratte (TPoS) bulkacoustic wave resonators [14]. SIMOX S SOI wafers with device layer of 1.5 µm and buried oxide (BO OX) of 1 µm, were used as the starting substrate. A stack of Mo/AlN/Mo was deposited on the silicon substrate withh thicknesses of 0.1/0.5/0.1 µm, respectively. The top Mo layyer was patterned to define the top signal electrodes (Fig. 4a)), followed by the etching of the AlN layer to provide access to the bottom ground electrode. AlN is also removed from the center portion of the beams and the rigid plate to reduce Q damping (Fig. 4b). Lateral trenches and release holes were then patterned on the Si device layer to delimit the structure geeometry (Fig. 4c). Access to the handle layer to define tuning ellectrodes was also attained in this step. Finally, the device was released in hydrofluoric acid (HF), leaving a capacitivee gap between the structure and the handle layer (Fig. 4d).
Figure 5 shows an SEM view of a fabricated 32 kHz resonators with a center plate areaa of only 250x250 µm2. Structures with different plate areaas where implemented to analyze the trade-offs between performance and area; however, the results presented in this work are only related to the aforementioned structure.
Figure 5. SEM view of 32 kHz tunable piezzoelectric MEMS resonator.
IV. (a) Deposit Mo/AlN/Mo stack and pattern toop electrode
NT RESULTS MEASUREMEN
The devices were characterrized in an open-loop configuration. Figure 6 shows the frequency response of a resonator after three different tuning potentials were applied. It can be seen that both the motionaal impedance and the Q remain constant even when the DC voltage v changes. This is a clear advantage over most capacitiv vely-transduced resonators since it simplifies the design of the in nterface circuit required to build up a temperature stable oscillattor.
(b) Etch AlN to grant access to ground ellectrode
Frequency Response – 32 3 kHz Resonator
fine geometry (c) Pattern Si trenches and release holes to defi
Figure 6. Resonator frequency response. Insertion loss and quality factor ge. are independent of the applied tuning voltag (d) Device release by etching BOX layerr in HF Single Crystal Silicon
Silicon n Dioxide
Aluminum Nitride
Molybd denum
Figure 4. Fabrication process flow diagram of AlN-onn-SOI resonator.
To compensate for frequency changes with temperature, the device requires a wide frequenccy tuning range. Figure 7 shows that by applying voltages no n larger than 4 V, this particular design can achieve more than t 3100 ppm of tuning. The pull-in voltage of this structure is close to 6 V, meaning a if needed. This that up to 6500 ppm is theoretically achievable extra margin can be use to counterract frequency deviations due to process variations.
A linear TCF of -27.8 ppm/ºC was measuured for the same device at a constant DC potential. The tuningg voltage was then adjusted at each measured temperature pooint to bring the frequency back to its original value. Full T TCF compensation m -25 to 100 ºC. was achieved for temperatures ranging from Figure 8 shows the temperature characterisstic of the device before and after calibration.
ACKNOWLEDGM MENT This work was supported by Integrated Device Technology (IDT), Inc. The authorss would like to thank the staff at the Nanotechnology Researrch Center (NRC) at the Georgia Tech for fabrication assistan nce. REFERENCE ES [1]
Frequency Tuning Characteristic – 32 kHz k Resonator
[2]
[3] [4] [5]
[6]
Figure 7. Resonator frequency tuning characteristic. T The device achieves 3100 ppm of frequency tuning for voltages no larger tthan 4 V.
[7]
[8]
CONCLUSION This work demonstrates the implemenntation of tunable piezoelectric MEMS resonators with form ffactors of at least one order of magnitude smaller than currennt quartz crystals. The devices operate in an out-of-planee flexural mode, allowing for accurate frequency control duuring fabrication. 3100 ppm of frequency tuning was achievved for full TCF compensation from -25 to 100 ºC utilizing nno more than 4 V. The quality factor and motional impedancce of the devices remain constant regardless of the tuning pottential, facilitating the implementation of temperature stable osscillators for realtime clock applications.
[9]
[10] [11] [12]
[13] [14]
Figure 8. Resonator temperature response before and after calibration. Tuning range is sufficient for full TCF compensation from -25 to 100 ºC.
P. Rako, "Making Oscillator Selection Crystal Clear," EDN, pp. 28-37, Feb. 19, 2009. velopment of MEMS and crystal C. S. Lam, "A review of the recent dev oscillators and their impacts on th he frequency control products industry," in IEEE International Ultrrasonics Symposium, 2008, pp. 694-704. ng and spectral processing," in F. Ayazi, "MEMS for integrated timin Proc. IEEE Custom Integrated Circuits Conference, 2009, pp. 65-72. FC-125 - Low Profile SMD kHz Range Crystal Unit. EPSON Toyocom oyocom.co.jp/ [Online]. Available: http://www.epsonto A. K. Samarao, G. Casinovi, an nd F. Ayazi, "Passive TCF Compensation in High Q Silicon Miicromechanical Resonators," in IEEE International Conference on Micro Electro Mechanical Systems, Hong Kong, 2010, pp. 116-119. R. Tabrizian, G. Casinovi, and F. Ayazzi, "Temperature-Stable High-Q AlN-on-Silicon Resonators with Embed dded Array of Oxide Pillars," in Solid-State Sensors, Actuators, and Microsystems M Workshop, Hilton Head, 2010, pp. 100-101. K. Sundaresan, G. Ho, S. Pourkamali, and F. Ayazi, "Electronically k Acoustic Resonator Reference Temperature Compensated Silicon Bulk Oscillators," IEEE Journal of Solid-Sta ate Circuits, vol. 42, pp. 14251434, 2007. H. Wan-Thai, J. R. Clark, and C. T. C. Nguyen, "Mechanically de micromechanical resonators," temperature-compensated flexural-mod in Tech. Dig. IEEE International Electtron Devices Meeting, 2000, pp. 399-402. B L. Hyung Kyu, J. C. R. Melamud, S. A. Chandorkar, K. Bongsang, Salvia, G. Bahl, M. A. Hopcroft, and d T. W. Kenny, "TemperatureInsensitive Composite Micromechanicaal Resonators," IEEE Journal of Microelectromechanical Systems, vol. 18, 1 pp. 1409-1419, 2009. K. R. Cioffi and H. Wan-Thai, "32KH Hz MEMS-based oscillator for low-power applications," in Proc. IEEE I International Frequency Control Symposium and Exposition, 200 05, pp. 551-558. G. Ho and F. Ayazi, "Low Frequenccy Process-Variation-Insensitive Temperature-Stable Micromechanicaal Resonators," US Patent 7,859,365, Dec. 28, 2010. G. Piazza, R. Abdolvand, G. Ho, an nd F. Ayazi, "PiezoelectricallyTransduced, Capacitively-Tuned, Hiigh-Q Single-Crystal Silicon Micromechanical Resonators on SOI Wafers," W Sensors and Actuators A: Physical, vol. 111, pp. 71-78, 2004. D. L. DeVoe, "Piezoelectric thin film micromechanical beam resonators," Sensors and Actuators A: Physical, vol. 88, pp. 263-272, 2001. W. Pan and F. Ayazi, "Thin-Film Piezoelectric-on-Substrate nd TCF Reduction," in IEEE Resonators with Q Enhancement an International Conference on Micro Electro Mechanical Systems, 2010, pp. 104-107.
I.
INTRODUCTION
Advances in wireless communications and computer processing applications have led to the steady growth of the electronics timing market, which is currently at the $5 billion mark. This niche is mainly composed of resonators, oscillators and clock generators, whose implementation is dominated by quartz crystal technology. This has been partly due to historical reasons [1], but mainly because quartz exhibits excellent temperature-stable characteristics and reasonable quality factor values, which are desired for high-accuracy applications [2]. In recent years, silicon-based MEMS resonators are becoming strong contenders in the timing applications market as a result of their small form factors, high fQ products and the possibility of direct integration with interface circuits [3]. However, most research efforts in this field are presently being invested in high-frequency devices in the MHz to GHz range, leaving conventional tuning fork quartz resonators as the prevailing devices for real-time clock (RTC) applications. The low-frequency operation (32.768 kHz) and precise fabrication steps required for the implementation of quartz tuning forks make these structures large in size with surface areas in the millimeter range [4]. Additionally, to achieve vacuum levels necessary for high-Q performance, quartz resonators require metallic encapsulation, which further complicates their manufacturing and increases cost. At such low frequencies, silicon microresonators operating in flexural modes can be utilized to overcome these complications; however, these devices exhibit large temperature coefficients of frequency (TCF) of about -30
ppm/ºC, so passive compensation techniques [5, 6] and frequency-tuning methods are required to counteract this effect [7]. Several techniques for passive temperature compensation of frequency in flexural silicon MEMS resonators have been reported. For example, thermally-dependent axial loads can be applied to a device to counteract the effects of the temperature coefficient of Young’s modulus (TCE) [8]. However, this type of approach requires a significant amount of additional area to accommodate for the compensation structure. Another common technique is the use of composite resonators, where different materials with opposite values of TCE are stacked to cancel out or minimize the overall TCF [9]. Nevertheless, interfacial material losses tend to degrade the quality factor which compromises the device performance. Hence, in applications where minimal size and maximum performance are a necessity, electrical frequency tuning tends be a better option, as long as the device offers enough tuning range to enable compensation for both temperature and process variations. As of today, very few 32 kHz MEMS resonators have been reported [10, 11]. These devices rely on electrostatic actuation and sensing; thus, they require a polarization voltage VP to operate. To achieve low motional impedances, high aspect ratio capacitive gaps and/or large VP values are necessary, both of which severely compromise power handling. Furthermore, electrostatic tuning is needed to compensate for TCF; hence, the polarization voltage of the device must be used to adjust the frequency utilizing a temperature compensation scheme. This in turn changes the motional impedance of the device, complicating the implementation of frequency-stable oscillators with a wide operating temperature range. In order to overcome these limitations, thin-film piezoelectric-on-SOI flexural resonators can be utilized to enable piezoelectric actuation as well as electrostatic frequency tuning [12]. Small form factor, highly-tunable lowfrequency devices can be realized by carefully designing their geometry as well as the size and shape of their drive and sense electrodes. This paper reports on the optimization, implementation, and characterization of such structures utilizing a CMOS-compatible process flow that is also compatible with high frequency piezoelectric bulk acoustic resonators.
II.
TUNABLE PIEZOELECTRIC MEMS RESONATOR
A. Design and Operating Principle Figure 1 shows a schematic diagram of the proposed 32 kHz piezoelectrically-actuated, capacitively-tunable resonator. This novel structure is composed of an external frame of four clamped-clamped beams anchored at the corners to the surrounding substrate. The beams are mechanically coupled to each other through a suspended rigid plate that provides sufficient structural support without compromising device compliance. More importantly, the plate contributes additional mass to achieve low-frequency operation and a large capacitive electrode area for increased electrostatic tuning.
Figure 1. Small form factor piezoelectrically-transduced electrostatically tunable 32 kHz resonator.
and
The resonator was implemented on an AlN-on-SOI substrate to allow piezoelectric transduction. A pair of drive and sense signal electrodes were carefully defined on top of each tether by etching the AlN and top molybdenum layers of the piezoelectric stack. The bottom molybdenum, which is in electrical contact with the single crystal silicon (SCS) device layer, serves as the ground electrode. By applying an AC voltage between each pair of drive and ground electrodes, shear stress is generated over the beams by transverse piezoelectric coupling, which is determined by the d31 coefficient of AlN. This stress produces a bending moment that excites the structure into its first out-of-plane flexural mode (Fig. 2). The displacement, amplified by the quality factor of the structure, can be sensed by collecting the charge generated at the opposite pair of signal electrodes. Exciting a vertical mode rather than a lateral vibration is advantageous because the frequency value of a flexural structure is highly dependent on the dimensions along the displacement axis. The displacement magnitude of in-plane modes is determined by
Figure 2. ANSYS® modal simulation results of fundamental out-of-plane flexural mode of piezoelectric 32 kHz resonator.
lithography, making the resonance frequency highly prone to process variations. In the case of out-of-plane modes, displacement occurs along the device thickness, which can be more precisely controlled through deposition steps. Although the structure does not require a polarization voltage to resonate, electrical access to the handle layer enables it to be used as a DC tuning electrode. Because the device layer is directly connected to ground, any potential applied to the handle layer will cause an electrostatic spring softening effect that can be utilized to tune the central operating frequency to compensate for temperature and process variations. B. Device Optimization Different design techniques were utilized to optimize the device motional impedance, frequency-tuning range, size and manufacturability. Single clamped-clamped beams can be used to implement a simpler version of the resonators proposed in this work; however, their length has to be greatly increased in order to achieve low-frequency operation, which significantly reduces the structural stiffness. This results in devices with large surface-area-to-volume ratios that are highly prone to stiction. To overcome this issue, the presented design takes advantage of mass loading, provided by a rigid plate rather than a decrease in stiffness, to achieve the target resonance frequency. This makes the structure more robust without compromising the overall area. In piezoelectrically-transduced resonators, the motional impedance is inversely proportional to the square of the electromechanical coupling. To maximize this value and achieve low impedance, the signal electrodes should be carefully sized and shaped to guarantee that, at no point of the covered area, the induced stress experiences a change in sign; this avoids charge cancellation and maximizes coupling. It can be shown that for clamped-clamped beams operating in their fundamental mode, the electrodes should cover up to 1/4th of the beam length in order to achieve maximum electromechanical transduction [13]. For the presented design, FEM simulations were used to sweep the electrode length and verify these results. As shown in Figure 3, the optimal point occurs when the electrode is 22.5% of the beam length rather than 25%. This is due to the undercut of the device layer, which changes the effective beam anchor location and hence its maximum and minimum stress points.
Figure 3. Simulation results for electrode size optimization. Ideal size is slightly smaller than theoretical value due to undercut of device layer.
III.
FABRICATION PROCESSS
For compatibility purposes, these low-freequency structures were implemented with a similar process flow utilized for high-frequency thin-film piezo-on-substratte (TPoS) bulkacoustic wave resonators [14]. SIMOX S SOI wafers with device layer of 1.5 µm and buried oxide (BO OX) of 1 µm, were used as the starting substrate. A stack of Mo/AlN/Mo was deposited on the silicon substrate withh thicknesses of 0.1/0.5/0.1 µm, respectively. The top Mo layyer was patterned to define the top signal electrodes (Fig. 4a)), followed by the etching of the AlN layer to provide access to the bottom ground electrode. AlN is also removed from the center portion of the beams and the rigid plate to reduce Q damping (Fig. 4b). Lateral trenches and release holes were then patterned on the Si device layer to delimit the structure geeometry (Fig. 4c). Access to the handle layer to define tuning ellectrodes was also attained in this step. Finally, the device was released in hydrofluoric acid (HF), leaving a capacitivee gap between the structure and the handle layer (Fig. 4d).
Figure 5 shows an SEM view of a fabricated 32 kHz resonators with a center plate areaa of only 250x250 µm2. Structures with different plate areaas where implemented to analyze the trade-offs between performance and area; however, the results presented in this work are only related to the aforementioned structure.
Figure 5. SEM view of 32 kHz tunable piezzoelectric MEMS resonator.
IV. (a) Deposit Mo/AlN/Mo stack and pattern toop electrode
NT RESULTS MEASUREMEN
The devices were characterrized in an open-loop configuration. Figure 6 shows the frequency response of a resonator after three different tuning potentials were applied. It can be seen that both the motionaal impedance and the Q remain constant even when the DC voltage v changes. This is a clear advantage over most capacitiv vely-transduced resonators since it simplifies the design of the in nterface circuit required to build up a temperature stable oscillattor.
(b) Etch AlN to grant access to ground ellectrode
Frequency Response – 32 3 kHz Resonator
fine geometry (c) Pattern Si trenches and release holes to defi
Figure 6. Resonator frequency response. Insertion loss and quality factor ge. are independent of the applied tuning voltag (d) Device release by etching BOX layerr in HF Single Crystal Silicon
Silicon n Dioxide
Aluminum Nitride
Molybd denum
Figure 4. Fabrication process flow diagram of AlN-onn-SOI resonator.
To compensate for frequency changes with temperature, the device requires a wide frequenccy tuning range. Figure 7 shows that by applying voltages no n larger than 4 V, this particular design can achieve more than t 3100 ppm of tuning. The pull-in voltage of this structure is close to 6 V, meaning a if needed. This that up to 6500 ppm is theoretically achievable extra margin can be use to counterract frequency deviations due to process variations.
A linear TCF of -27.8 ppm/ºC was measuured for the same device at a constant DC potential. The tuningg voltage was then adjusted at each measured temperature pooint to bring the frequency back to its original value. Full T TCF compensation m -25 to 100 ºC. was achieved for temperatures ranging from Figure 8 shows the temperature characterisstic of the device before and after calibration.
ACKNOWLEDGM MENT This work was supported by Integrated Device Technology (IDT), Inc. The authorss would like to thank the staff at the Nanotechnology Researrch Center (NRC) at the Georgia Tech for fabrication assistan nce. REFERENCE ES [1]
Frequency Tuning Characteristic – 32 kHz k Resonator
[2]
[3] [4] [5]
[6]
Figure 7. Resonator frequency tuning characteristic. T The device achieves 3100 ppm of frequency tuning for voltages no larger tthan 4 V.
[7]
[8]
CONCLUSION This work demonstrates the implemenntation of tunable piezoelectric MEMS resonators with form ffactors of at least one order of magnitude smaller than currennt quartz crystals. The devices operate in an out-of-planee flexural mode, allowing for accurate frequency control duuring fabrication. 3100 ppm of frequency tuning was achievved for full TCF compensation from -25 to 100 ºC utilizing nno more than 4 V. The quality factor and motional impedancce of the devices remain constant regardless of the tuning pottential, facilitating the implementation of temperature stable osscillators for realtime clock applications.
[9]
[10] [11] [12]
[13] [14]
Figure 8. Resonator temperature response before and after calibration. Tuning range is sufficient for full TCF compensation from -25 to 100 ºC.
P. Rako, "Making Oscillator Selection Crystal Clear," EDN, pp. 28-37, Feb. 19, 2009. velopment of MEMS and crystal C. S. Lam, "A review of the recent dev oscillators and their impacts on th he frequency control products industry," in IEEE International Ultrrasonics Symposium, 2008, pp. 694-704. ng and spectral processing," in F. Ayazi, "MEMS for integrated timin Proc. IEEE Custom Integrated Circuits Conference, 2009, pp. 65-72. FC-125 - Low Profile SMD kHz Range Crystal Unit. EPSON Toyocom oyocom.co.jp/ [Online]. Available: http://www.epsonto A. K. Samarao, G. Casinovi, an nd F. Ayazi, "Passive TCF Compensation in High Q Silicon Miicromechanical Resonators," in IEEE International Conference on Micro Electro Mechanical Systems, Hong Kong, 2010, pp. 116-119. R. Tabrizian, G. Casinovi, and F. Ayazzi, "Temperature-Stable High-Q AlN-on-Silicon Resonators with Embed dded Array of Oxide Pillars," in Solid-State Sensors, Actuators, and Microsystems M Workshop, Hilton Head, 2010, pp. 100-101. K. Sundaresan, G. Ho, S. Pourkamali, and F. Ayazi, "Electronically k Acoustic Resonator Reference Temperature Compensated Silicon Bulk Oscillators," IEEE Journal of Solid-Sta ate Circuits, vol. 42, pp. 14251434, 2007. H. Wan-Thai, J. R. Clark, and C. T. C. Nguyen, "Mechanically de micromechanical resonators," temperature-compensated flexural-mod in Tech. Dig. IEEE International Electtron Devices Meeting, 2000, pp. 399-402. B L. Hyung Kyu, J. C. R. Melamud, S. A. Chandorkar, K. Bongsang, Salvia, G. Bahl, M. A. Hopcroft, and d T. W. Kenny, "TemperatureInsensitive Composite Micromechanicaal Resonators," IEEE Journal of Microelectromechanical Systems, vol. 18, 1 pp. 1409-1419, 2009. K. R. Cioffi and H. Wan-Thai, "32KH Hz MEMS-based oscillator for low-power applications," in Proc. IEEE I International Frequency Control Symposium and Exposition, 200 05, pp. 551-558. G. Ho and F. Ayazi, "Low Frequenccy Process-Variation-Insensitive Temperature-Stable Micromechanicaal Resonators," US Patent 7,859,365, Dec. 28, 2010. G. Piazza, R. Abdolvand, G. Ho, an nd F. Ayazi, "PiezoelectricallyTransduced, Capacitively-Tuned, Hiigh-Q Single-Crystal Silicon Micromechanical Resonators on SOI Wafers," W Sensors and Actuators A: Physical, vol. 111, pp. 71-78, 2004. D. L. DeVoe, "Piezoelectric thin film micromechanical beam resonators," Sensors and Actuators A: Physical, vol. 88, pp. 263-272, 2001. W. Pan and F. Ayazi, "Thin-Film Piezoelectric-on-Substrate nd TCF Reduction," in IEEE Resonators with Q Enhancement an International Conference on Micro Electro Mechanical Systems, 2010, pp. 104-107.
