- Email: [email protected]
Micromachined Piezoelectric-on-Silicon Thickness Extensional Mode Resonators
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 120 (2015) 1007 – 1010
EUROSENSORS 2015
Micromachined Piezoelectric-on-Silicon Thickness Extensional Mode Resonators Nicole Weckmana*, Ashwin Seshiaa a
Nanoscience Centre, Department of Engineering, University of Cambridge, 11 JJ Thomson Avenue, Cambridge CB3 0FF, UK
Abstract This paper describes characterization of a thickness extensional mode in a piezoelectrically driven MEMS composite AlN-on-Si membrane resonator. The measured resonant frequency for the mode closely matches the analytical prediction for the fundamental half-wavelength thickness extensional mode through the entire thickness of the composite membrane. The mode has been measured across a variety of devices corresponding to differing membrane geometries and boundary conditions. For colocated devices the measured frequency of the composite membrane thickness extensional mode is found to be highly reproducible, with a standard deviation of 0.5% or less in the measured frequency device to device. Finally, the mode is compared in vacuum, air, and water for one device. Only very slight damping occurs transitioning from vacuum to air, however operation in water results in the response being significantly damped. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: bulk acoustic resonator; piezoelectric; thickness extensional mode; damping
1. Introduction Due to their common use as RF-front-end filters in mobile handsets, the production of film bulk acoustic resonators (FBARs) using thin piezoelectric films has reached maturity [1] and enabled the fabrication of piezoelectrically transduced MEMS resonators for a broader range of sensing applications [2]. However their main application remains as part of the multi-billion dollar reference oscillator industry, where their quality factor, small size, and high volume manufacturing capability provide performance and cost advantages over alternatives. While
* Corresponding author. Tel.: +44 1223 7603040 E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.675
1008
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
often the thickness extensional mode of the thin piezoelectric film itself is monitored, presented here is the measurement of the thickness extensional mode through the entire membrane stack of a variety of devices that encompass different geometries and electrode configurations but have the same membrane thickness building upon previous work in this area [1, 3-6]. An example of the membrane stack and anticipated thickness extensional mode as modeled in COMSOL are shown in Figure 1.
Fig. 1. A diagram depicting the cross section of the devices (left) and a thickness extensional mode predicted by COMSOL at 360 MHz (right).
The membrane of the devices consists of a 10μm thick silicon membrane with a 0.5μm thin film of c-axis oriented aluminum nitride on the top side of the device, and a top electrode of 1μm thick aluminum as shown in Figure 1. The lateral membrane dimensions and the geometry of the devices are defined by a trench etched on the backside of the device. The device geometry is either circular or rectangular with the rectangular membranes having either a single electrode or a double electrode. Examples of the variety of device designs exhibiting this thickness extensional mode can be seen in the optical images in Figure 2. 1.1. Predicted frequency of thickness extensional mode The resonance frequency of the thickness extensional mode presented here is defined by the total thickness of the membrane including the silicon, the piezoelectric layer, and the electrodes. The resonance frequency of the mode can be estimated using the following equation [7]:
f0
1 2t
c 33
U
(1)
Using a membrane thickness of 11.5μm and approximating the material properties of the stack with those of silicon, the predicted frequency for the thickness extensional mode of the devices described herein is 325 MHz. However, taking into account the +/- 1 μm variability in the thickness of the silicon layer of the membrane due to tolerances in the fabrication process, the predicted frequency could range from 299-357 MHz depending on the device fabrication. Another important metric for evaluating the performance of the resonator is the quality factor, Q, which can be calculated based on the measured resonance frequency and the phase change at the resonance frequency using the following [3]:
Q
fr 2
§ wM · ¨¨ ¸¸ © wf ¹
(2)
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
Fig. 2. Optical microscope image (5x magnification) of various devices that have exhibited the thickness extensional mode. The mode has been measured in circular, square, and rectangular devices. All devices are membrane based with devices having either one or two top electrodes.
2. Characterization of Devices The devices are characterized using an Agilent Network Analyzer in a standard 1-port configuration. A response peak can be identified within the range of predicted frequencies as calculated above, and is measured on all devices in air using the network analyzer. The response peak occurs at an average frequency of 346.19 MHz across all the devices on 6 chips and the typical device has a Q of about 250. The geometry, lateral dimensions, and electrode configurations have little effect on the frequency of the measured mode as can be seen by the frequencies listed in Table 1 for all devices tested. Table 1. Frequency (MHz) of thickness extensional mode in air Lateral dimensions (μm)
Chip 1
Chip 2
Chip 3
Circle
900
367.53
341.05
345.18
Square
900
367.61
340.77
344.47
Square
900
367.38
340.34
344.61
Chip 4
Chip 5
Chip 6
Rectangle
860x880
345.71
329.49
349.09
Rectangle
860x880
345.65
330.53
348.84
Rectangle
860x880
345.71
347.51
As can be seen in the table, the resonant frequency of the thickness extensional mode is highly reproducible across all co-located devices, with a standard deviation of less than 0.3% of the frequency. There is slightly more variation in the frequency when measured on independent chips, up to a standard deviation of 3.6% of the average frequency. This would be indicative of less than a 0.5μm change in the thickness of the membrane between different chips which is well within the manufacturing tolerances of the fabrication process used. In order to further evaluate the performance of the device, and the range of potential applications, the response for one of the square membrane devices was compared in vacuum, air, and water. While the frequency response of the resonator does not change significantly transitioning from vacuum to air, it disappears almost entirely in water as can be seen in Figure 3.
1009
1010
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
Fig. 3. The thickness extensional mode in air (left) and water (right) measured using a network analyzer.
The severe damping in liquid is characteristic of a mode such as a thickness extensional mode that dissipates energy by radiating longitudinal waves into the medium. This behaviour makes the application of this device for liquid sensing unfeasible. However due to the retention of a quality factor of around 250 in air, sensing in gas environments or applications as filters will be explored. 3. Conclusion A thickness extensional mode of a piezoelectrically transduced MEMS resonator has been predicted at a frequency of 325 MHz and measured at a frequency of 346 MHz. The mode is consistently reproduced on multiple devices and multiple chips with minimal variation in the resonance frequency. While high damping in liquid prevents applications in liquid sensing, a typical Q of 250 in air shows promise for gas sensing or for filter applications. Acknowledgements This project is funded by BBSRC. References [1] R. Ruby, M. Small, F. Bi, D. Lee, L. Callaghan, R. Parker, S. Ortiz, Positioning FBAR Technology in the Frequency and Timing Domain, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 59 (2012) 334-345. [2] H. Zhang, W. Pang, M.S. Marma, C.Y. Lee, S. Kamal-Bahl, E.S. Kim, C.E. McKenna, Label-free detection of protein-ligand interactions in real time using micromachined bulk acoustic resonators, Applied Physics Letters, 96 (2010). [3] K.M. Larkin, J.S. Wang, Acoustic bulk wave composite resonators, Appl. Phys. Lett., 38 (1981) 125-127. [4] T.W. Grudkowski, J.F. Black, T.M. Reeder, D.E. Cullen, R.A. Wagner, Fundamental-mode VHF/UHF miniature acoustic resonators and filters on silicon, Appl. Phys. Lett., 37 (1980) 993-995. [5] K. Nakamura, H. Sasaki, H. Shimizu, ZnO/SiO2-diaphragm composite resonator on a silicon wafer, Electronic Letters, 17 (1981) 507-509. [6]T. Fujikura, O. Matsuda, D.M. Profunser, O.B. Wright, J. Masson, S. Ballandras, Real-time imaging of acoustic waves on a bulk acoustic resonator, Appl. Phys. Lett, 93 (2008). [7] H.F. Tiersten, D.S. Stevens, An analysis of thickness-extensional trapped energy resonant device structures with rectangular electrodes in the piezoelectric thin film on silicon configuration, J. Appl. Phys., 54 (1983) 5893-5910.
ScienceDirect Procedia Engineering 120 (2015) 1007 – 1010
EUROSENSORS 2015
Micromachined Piezoelectric-on-Silicon Thickness Extensional Mode Resonators Nicole Weckmana*, Ashwin Seshiaa a
Nanoscience Centre, Department of Engineering, University of Cambridge, 11 JJ Thomson Avenue, Cambridge CB3 0FF, UK
Abstract This paper describes characterization of a thickness extensional mode in a piezoelectrically driven MEMS composite AlN-on-Si membrane resonator. The measured resonant frequency for the mode closely matches the analytical prediction for the fundamental half-wavelength thickness extensional mode through the entire thickness of the composite membrane. The mode has been measured across a variety of devices corresponding to differing membrane geometries and boundary conditions. For colocated devices the measured frequency of the composite membrane thickness extensional mode is found to be highly reproducible, with a standard deviation of 0.5% or less in the measured frequency device to device. Finally, the mode is compared in vacuum, air, and water for one device. Only very slight damping occurs transitioning from vacuum to air, however operation in water results in the response being significantly damped. © Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ©2015 2015The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
Keywords: bulk acoustic resonator; piezoelectric; thickness extensional mode; damping
1. Introduction Due to their common use as RF-front-end filters in mobile handsets, the production of film bulk acoustic resonators (FBARs) using thin piezoelectric films has reached maturity [1] and enabled the fabrication of piezoelectrically transduced MEMS resonators for a broader range of sensing applications [2]. However their main application remains as part of the multi-billion dollar reference oscillator industry, where their quality factor, small size, and high volume manufacturing capability provide performance and cost advantages over alternatives. While
* Corresponding author. Tel.: +44 1223 7603040 E-mail address: [email protected]
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi:10.1016/j.proeng.2015.08.675
1008
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
often the thickness extensional mode of the thin piezoelectric film itself is monitored, presented here is the measurement of the thickness extensional mode through the entire membrane stack of a variety of devices that encompass different geometries and electrode configurations but have the same membrane thickness building upon previous work in this area [1, 3-6]. An example of the membrane stack and anticipated thickness extensional mode as modeled in COMSOL are shown in Figure 1.
Fig. 1. A diagram depicting the cross section of the devices (left) and a thickness extensional mode predicted by COMSOL at 360 MHz (right).
The membrane of the devices consists of a 10μm thick silicon membrane with a 0.5μm thin film of c-axis oriented aluminum nitride on the top side of the device, and a top electrode of 1μm thick aluminum as shown in Figure 1. The lateral membrane dimensions and the geometry of the devices are defined by a trench etched on the backside of the device. The device geometry is either circular or rectangular with the rectangular membranes having either a single electrode or a double electrode. Examples of the variety of device designs exhibiting this thickness extensional mode can be seen in the optical images in Figure 2. 1.1. Predicted frequency of thickness extensional mode The resonance frequency of the thickness extensional mode presented here is defined by the total thickness of the membrane including the silicon, the piezoelectric layer, and the electrodes. The resonance frequency of the mode can be estimated using the following equation [7]:
f0
1 2t
c 33
U
(1)
Using a membrane thickness of 11.5μm and approximating the material properties of the stack with those of silicon, the predicted frequency for the thickness extensional mode of the devices described herein is 325 MHz. However, taking into account the +/- 1 μm variability in the thickness of the silicon layer of the membrane due to tolerances in the fabrication process, the predicted frequency could range from 299-357 MHz depending on the device fabrication. Another important metric for evaluating the performance of the resonator is the quality factor, Q, which can be calculated based on the measured resonance frequency and the phase change at the resonance frequency using the following [3]:
Q
fr 2
§ wM · ¨¨ ¸¸ © wf ¹
(2)
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
Fig. 2. Optical microscope image (5x magnification) of various devices that have exhibited the thickness extensional mode. The mode has been measured in circular, square, and rectangular devices. All devices are membrane based with devices having either one or two top electrodes.
2. Characterization of Devices The devices are characterized using an Agilent Network Analyzer in a standard 1-port configuration. A response peak can be identified within the range of predicted frequencies as calculated above, and is measured on all devices in air using the network analyzer. The response peak occurs at an average frequency of 346.19 MHz across all the devices on 6 chips and the typical device has a Q of about 250. The geometry, lateral dimensions, and electrode configurations have little effect on the frequency of the measured mode as can be seen by the frequencies listed in Table 1 for all devices tested. Table 1. Frequency (MHz) of thickness extensional mode in air Lateral dimensions (μm)
Chip 1
Chip 2
Chip 3
Circle
900
367.53
341.05
345.18
Square
900
367.61
340.77
344.47
Square
900
367.38
340.34
344.61
Chip 4
Chip 5
Chip 6
Rectangle
860x880
345.71
329.49
349.09
Rectangle
860x880
345.65
330.53
348.84
Rectangle
860x880
345.71
347.51
As can be seen in the table, the resonant frequency of the thickness extensional mode is highly reproducible across all co-located devices, with a standard deviation of less than 0.3% of the frequency. There is slightly more variation in the frequency when measured on independent chips, up to a standard deviation of 3.6% of the average frequency. This would be indicative of less than a 0.5μm change in the thickness of the membrane between different chips which is well within the manufacturing tolerances of the fabrication process used. In order to further evaluate the performance of the device, and the range of potential applications, the response for one of the square membrane devices was compared in vacuum, air, and water. While the frequency response of the resonator does not change significantly transitioning from vacuum to air, it disappears almost entirely in water as can be seen in Figure 3.
1009
1010
Nicole Weckman and Ashwin Seshia / Procedia Engineering 120 (2015) 1007 – 1010
Fig. 3. The thickness extensional mode in air (left) and water (right) measured using a network analyzer.
The severe damping in liquid is characteristic of a mode such as a thickness extensional mode that dissipates energy by radiating longitudinal waves into the medium. This behaviour makes the application of this device for liquid sensing unfeasible. However due to the retention of a quality factor of around 250 in air, sensing in gas environments or applications as filters will be explored. 3. Conclusion A thickness extensional mode of a piezoelectrically transduced MEMS resonator has been predicted at a frequency of 325 MHz and measured at a frequency of 346 MHz. The mode is consistently reproduced on multiple devices and multiple chips with minimal variation in the resonance frequency. While high damping in liquid prevents applications in liquid sensing, a typical Q of 250 in air shows promise for gas sensing or for filter applications. Acknowledgements This project is funded by BBSRC. References [1] R. Ruby, M. Small, F. Bi, D. Lee, L. Callaghan, R. Parker, S. Ortiz, Positioning FBAR Technology in the Frequency and Timing Domain, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 59 (2012) 334-345. [2] H. Zhang, W. Pang, M.S. Marma, C.Y. Lee, S. Kamal-Bahl, E.S. Kim, C.E. McKenna, Label-free detection of protein-ligand interactions in real time using micromachined bulk acoustic resonators, Applied Physics Letters, 96 (2010). [3] K.M. Larkin, J.S. Wang, Acoustic bulk wave composite resonators, Appl. Phys. Lett., 38 (1981) 125-127. [4] T.W. Grudkowski, J.F. Black, T.M. Reeder, D.E. Cullen, R.A. Wagner, Fundamental-mode VHF/UHF miniature acoustic resonators and filters on silicon, Appl. Phys. Lett., 37 (1980) 993-995. [5] K. Nakamura, H. Sasaki, H. Shimizu, ZnO/SiO2-diaphragm composite resonator on a silicon wafer, Electronic Letters, 17 (1981) 507-509. [6]T. Fujikura, O. Matsuda, D.M. Profunser, O.B. Wright, J. Masson, S. Ballandras, Real-time imaging of acoustic waves on a bulk acoustic resonator, Appl. Phys. Lett, 93 (2008). [7] H.F. Tiersten, D.S. Stevens, An analysis of thickness-extensional trapped energy resonant device structures with rectangular electrodes in the piezoelectric thin film on silicon configuration, J. Appl. Phys., 54 (1983) 5893-5910.