VOLTAGE-TUNABLE PIEZOELECTRICALLY-TRANSDUCED SINGLE-CRYSTAL SILICON RESONATORS ON SOI SUBSTRATE Gianluca Piazza, Reza Abdolvand, and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, GA 30332-0250, USA Email:
[email protected]; Tel: 1-404-894-9496; Fax: 1-404-894-4700 ABSTRACT This paper reports on a new class of high-Q single crystal silicon (SCS) resonators that are piezoelectrically actuated and sensed, and have voltage-tunable center frequencies. The resonating element is made out of the SCS device layer of a SOI wafer. In a unique manner, piezoelectric transduction was integrated with capacitive fine-tuning of the resonator center frequencies to compensate for any process variations. A quality factor of 6,200 was measured for the 1.7MHz 1st resonance mode of a clamped-clamped beam resonator in 50mTorr vacuum. A 200µm long, 4.2µm thick beam was operated in its higher order modes and demonstrated a Q of 5,300, 3,000, and 2,400 in its 3rd (3.3MHz), 4th (4.9MHz), and 5th (6.7MHz) flexural modes, respectively. A 6kHz tuning range was measured for a 719kHz resonator by applying a DC voltage in the range of 0-20V. 1. INTRODUCTION Advanced consumer electronics such as miniature radios and wristwatch cellular phones pose severe limitations on the size and cost of the frequency selective units contained therein. MEMS resonators are receiving increased attention as building blocks for on-chip integrated filter and frequency references to replace bulky, off-chip ceramic and SAW devices. Several MEMS resonators with capacitive transduction mechanisms have been reported in literature [1-4], showing high mechanical quality factors and optimal performances in the IF and VHF range. However, in order to reduce the motional resistance of the capacitive resonators for higher frequency applications, sub-100nm capacitive gap spacing is needed, which can complicate the fabrication process. The piezoelectric Film Bulk Acoustic Resonators (FBAR) [5,6] have smaller motional resistance compared to their capacitive counterparts, and hence are suitable for UHF applications. However, they are larger in size, have lower quality factors, and do not have voltage tunable center frequencies. This paper reports on a new and simple fabrication technique utilizing SOI wafers to implement single crystal silicon (SCS) resonators that combine the advantages of piezoelectric and capacitive resonators. The resonating element is substantially made out of SCS, which has a higher inherent mechanical quality factor than bulk piezoelectrics and deposited thin films, whereas actuation and sensing is achieved by piezoelectric means. The high electromechanical coupling offered by the thin zinc oxide film provides for small equivalent motional resistance, hence improving signal to noise ratio. Similar clampedclamped resonant beams were previously demonstrated in [7] using SiO2 as the resonating element, but low quality
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factors were reported. The technology described in this paper enables to reach quality factors as high as 6,200 at a resonance frequency of 1.7 MHz and is capable of actuating higher order modes of the same beam resonators. A unique feature of this fabrication process, intrinsically related to the choice of the SOI wafer, is the ability to combine piezoelectric actuation mechanisms with electrostatic finetuning of the center frequency of the resonator. By applying a DC voltage between the handle layer of the SOI wafer and the resonator body, it is possible to introduce electrical stiffness through the action of the capacitance and reduce the effective stiffness of the beam. 2. RESONATOR DESIGN Figure 1 shows the schematic diagrams of the two-port clamped-clamped beam resonators described in this paper. Sense Electrode ZnO Film
(a)
Drive Electrode
L Ts
W
Oxide Handle Layer Device Layer ZnO is etched away (b)
Tuning Capacitor Fig. 1: Voltage-tunable, piezoelectrically-transduced SCS resonators: (a) basic configuration, (b) Q-enhanced configuration.
In the basic resonator design (Fig. 1a), the piezoelectric layer is deposited along the whole length of the beam and is strategically sandwiched between the top aluminum electrodes and the bottom low resistivity SCS substrate. The elimination of the bottom metal electrode conventionally used for piezoelectric devices was introduced in order to reduce the number of stacked layers, which could ultimately affect the mechanical quality factor of the resonator. In the Q-enhanced design (Fig. 1b), the ZnO layer is etched in the middle span of the beam, therefore reducing the effective covering of the resonator body by the piezoelectric and enhancing the mechanical Q of the resonator. Such a solution has resulted in ~100% increase in the Q of the resonators as confirmed by the experimental results.
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When an AC voltage is applied to the drive electrode, the active piezoelectric film is producing a distributed moment, which causes the beam to deflect. Such a deformation is sensed by the piezoelectric material on the opposite side of the beam. The admittance model [8] of a clamped-clamped piezoelectric beam resonator was taken into account during the design phase. The metal electrodes were strategically terminated at the inflection point of the beam mode shape to maximize the electromechanical coupling factor of the resonator (the electrodes are laid over the area under the same type of stress).
and a power of 300W were used as deposition parameters. The piezoelectric film has a thickness of 0.3µm and shows strong c-axis orientation, as confirmed by the XRD data shown in Fig. 3. Zinc oxide was patterned by wet etching using ammonium chloride (NH4Cl). NH4Cl was selected because it has a very slow etch rate (50Å/s) and enables the definition of small features without severe lateral undercut. Sputter Parameters RF Power Temperature Ar/(Ar+O2) flow Pressure ZnO thickness
(002) plane
3. RESONATOR FABRICATION
Values 300 W 250 °C 50% 6 mTorr 0.3µm
Figure 2 shows a brief process flow for the voltagetunable piezoelectrically-transduced SCS resonators. The choice of the SOI substrate was made to combine the advantages of SCS (i.e., stress free and high mechanical quality factor) with the ability of electrostatic fine-tuning of the resonator center frequency. The buffer oxide layer of the SOI wafer creates the capacitive gap used for electrostatic tuning of the resonator stiffness. 1. 4µm wide trenches are etched in the device layer. 2. Buffer oxide is etched in HF to create a cavity underneath the beam. 3. ZnO is deposited from a Zinc Oxide target using RF sputtering. 4. ZnO is patterned by wet etching in NH4Cl @ 55°C. 5. Aluminum top electrodes are patterned by lift-off.
Fig. 2: Fabrication process flow for the voltage-tunable, piezoelectrically-transduced SCS resonators.
The fabrication process has three masks. The resonator body is defined by etching shallow trenches (4µm wide) into the device layer and landing on the buffer oxide layer of the SOI substrate. The device layer of the selected SOI is ptype, low resistivity, orientation with a nominal thickness of 4µm. The beam thickness is defined by the thickness of the device layer. A cavity is opened underneath the beam by isotropic etching of the buffer oxide layer in HF/H2O. This unconventional step, aimed at the release of the structures in an intermediate step of the process, was made necessary by the presence of ZnO, which is easily attacked by any type of acid [9]. The cavity underneath the beam provides for the 1µm gap that is used for capacitive fine-tuning of the micro-beam center frequency. The active piezoelectric film is sputter-deposited on the silicon substrate. Zinc oxide was selected because of its well-established process recipe [10] and ease of integration with current microelectronics. The low-temperature fabrication process makes these devices post-CMOS compatible. A temperature of 250ºC for the silicon substrate, a pressure of 6mTorr, an Ar to O2 mix ratio of 0.5,
Fig. 3: XRD pattern for ZnO deposited on SCS substrate by RF sputtering.
The aluminum top electrode (1000 Å) is defined by the third mask using lift-off. The thickness of the ZnO and Al layers have been kept small to avoid any detrimental effects on the quality factor and resonance frequency of the resonators due to stacked layers of different materials. Figure 4 shows top-view of a SCS piezoelectricallytransduced clamped-clamped beam resonator. The input and output signal pads are electrically isolated from the silicon substrate by the piezoelectric ZnO film, which exhibits a resistivity value higher than 108 Ω·cm if an Ar to O2 mix ratio of 0.5 is used during the deposition phase. Figure 5 is a close-up view of a SCS piezoresonator, showing the zinc oxide film sandwiched between the low resistivity silicon and the top aluminum electrodes. ZnO was etched away in the middle of the beam to increase the quality factor of the resonator. Ground Electrode Resonator Output Signal Resonator Input Signal
Clamped-Clamped SCS Piezoelectric Resonator
Fig. 4. SEM view of a SCS resonator piezoelectric resonator.
It should be mentioned that the oxide undercut extending into the clamping region of the beams causes a downshift in the resonance frequency of the smaller beams.
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A thorough FEM analysis was conducted using ANSYS to predict the effect of the oxide undercut as well as the deposited thin films on the resonance frequency of the fabricated resonators. The following section will present the measurement results and their comparison with ANSYS simulation results.
middle, as shown in Fig. 1a) showed Q’s of about two times smaller than the ones without ZnO (in the order of 3,000).
ZnO is etched in the middle of the beam
Q ~ 6,200 at 1.72 MHz
Fi t M d
Sense Electrode ZnO
Fig. 6: Response of a 100µm long, 20µm wide SCS piezoresonator showing a Q of ~6,200 at a resonance frequency of 1.72 MHz.
Drive Electrode Fig. 5: Close-up view of a 100µm long piezoresonator with ZnO etched in the middle area of the beam.
4. MEASUREMENT RESULTS The fabricated microresonators were tested in a custom-built vacuum chamber capable of pressures as low as 10µTorr. A low noise JFET source-follower with a gain stage was used to interface with the resonators. The frequency spectra of the resonators were captured using Agilent 4395A network analyzer. As expected, higher quality factors were obtained from the SCS piezoresonators in which the zinc oxide is etched away from the middle of the beam. Figure 6 shows a typical frequency response taken from the network analyzer for a 100µm long, 20µm wide clamped-clamped beam resonator. This MEMS resonator has a center frequency of 1.72 MHz and shows a quality factor of 6,200 at a pressure of 50mTorr. Clamped-clamped beams of identical dimensions in the basic configuration (with ZnO in the
4.1 Higher Order Modes The fabricated SCS piezoresonators were operated in their higher order flexural modes to achieve higher frequencies. The responses of a 200µm long beam in its 3rd and 4th modes are shown in Fig. 7. A Q of 5,300 at 3.29MHz was measured for the third resonance mode, with no substantial decrease from the first mode quality factor. The quality factors for the 4th (4.87MHz) and the 5th (6.7MHz) modes are approximately halved; a Q of ~3,000 was recorded for the fourth mode and a Q of 2,400 for the fifth mode. The relatively high values of quality factor measured for these devices in the 1-10 MHz range, when compared to the values reported in [7], confirm the optimal choice of SCS as the resonator structural element. Actuation voltages as low as 0.7mV (minimum value enabled by the network analyzer) can excite the microresonators, showing a dynamic range of at least 45dB.
Q~3,000 @ 4.87 MHz Fourth Mode
Q~5,300 @ 3.29 MHz Third Mode
Beam Size
Mode
fo (MHz)
ANSYS fo (MHz)
Q
L (µm)
Ts (µm)
1
0.721
0.730
5,400
3
3.29
3.31
5,300
200
4.2
4
4.87
4.98
3,000
5
6.70
6.91
2,400
Fig. 7: Response of 200 µm long resonator actuated in its third and fourth resonance modes. The table summarizes the measurement results and compare with FEM analysis results in which the effect of buffer oxide undercut on the resonant frequency was taken into account.
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It is worth noting that the relative position of the resonance and anti-resonance peaks varies with the excited mode shapes. Depending on the mode shape of the resonator, the direction of the electrical current iout (shown in Fig. 8) generated on the sense terminal of the beam could be either in phase or out of phase with the current flowing in the input terminal (iin). This makes the feedthrough capacitance, CFT, to act like a positive or a negative capacitor, respectively, causing a change in the position of the antiresonance peak with respect to the resonance peak (can appear either before or after the main resonance peak).
dimensions of the resonator and the amount of the buffer oxide undercut could account for the small mismatches between the theoretical and experimental curves. 5. CONCLUSIONS The design, fabrication and testing of voltage-tunable, piezoelectrically-transduced, high-Q single crystal silicon resonators on SOI substrates were reported in this paper. High mechanical quality factors ranging from 5,400 to 6,000 were demonstrated for the resonators in which the zinc oxide film was etched away in the middle area of the beam. The experimental results confirmed the optimal choice of SCS as the resonating material. Higher order modes were actuated showing the ability of reaching resonance frequencies as high as 6.7 MHz with a quality factor of 2,400. The advantages of piezoelectric transduction mechanisms were combined with electrostatic fine-tuning of the resonance frequency. A tuning range of 6kHz was obtained for a 719kHz resonator by applying a DC voltage in the range of 0-20V. ACKNOWLEDGMENT
RM =
K1M1 2
η Q
LM =
M1
η
2
CM =
η2
η = 2.49 ⋅ d31EPTs
K1
W L
Fig. 8: The simplified equivalent circuit model of the piezoelectrically-transduced micromechanical resonator.
4.2 Electrostatic Fine-Tuning Figure 9 shows the comparison between the measured and the theoretical frequency-tuning characteristic for a 200µm long resonator, obtained by changing the DC voltage applied between the handle layer of the SOI wafer and the body of the resonator from 0 to 20V. This feature, uniquely related to this specific fabrication technology, enables the combination of the piezoelectric transduction mechanism with capacitive fine-tuning of the resonance frequency.
Theoretical Curve Tuning Range of ~ 1% for 719 kHz Resonator
Experimental Curve
This work was supported by DARPA under contract # DAAH01-01-1-R004. Authors would like to thank Gavin K. Ho for the FEM analysis, and the staff at the Georgia Tech Microelectronics Research Center for their assistance. REFERENCES [1] S.Y. No, A. Hashimura, S. Pourkamali, F. Ayazi, “SingleCrystal Silicon HARPSS Capacitive Resonators with Submicron Gap-Spacing”, Solid-State Sensors, Actuator and Microsystems Workshop, June, 2002, pp. 281-284. [2] J.R. Clark, W.-T. Hsu, C.T.-C. Nguyen, “High-Q VHF Micromechanical Countour-Mode Disk Resonators”, IEEE Int. Electron Devices Meeting, Dec., 2000, pp. 493-496. [3] Sunil A. Bhave, et al., “Poly-SiGe: A High-Q Structural Material for Integrated RF MEMS”, Solid-State Sensors, Actuator and Microsystems Workshop, June, 2002, pp. 34-37. [4] R.E. Mihailovic,N.C. MacDonald, “Dissipation measurements of vacuum-operated single-crystal silicon microresonator”, Sensors and Actuators, A 50, September, 1995, pp. 199-207. [5] R. Ruby, et al., “Ultra-miniature High-Q Filters and Duplexers Using FBAR Technology”, IEEE International Solid-State Circuits Conference, 2001, pp. 120-122. [6] K.M. Lakin, “Thin Film Resonators and Filters”, IEEE Ultrasonics Symposium,1999, pp. 895-906. [7] B. Piekarski, D. DeVoe, M. Dubey, R. Kaul, J.Conrad, R. Zeto, “Surface Micromachined Piezoelectric Resonant Beam Filters”, Sensors and Actuators, A 91, 2001, pp.313-320. [8] D.L. DeVoe, “Piezoelectric thin film micromechanical beam resonators’, Sensors and Actutators, A 88, 2001, pp 263-272. [9] M.J. Vellekoop, C.C.G. Visser, P.M. Sarro, A. Venema, “Compatibility of Zinc Oxide with Silicon IC Processing”, Sensors and Actuators, A 23, 1990, pp. 1027-1030. [10] W. Water, S.-Y. Chu, “Physical and structural properties of ZnO sputtered films”, Materials Lettters, 55, 2002, pp. 67-72.
Fig. 9: Electrostatic fine-tuning characteristic for a 719 kHz SCS piezoresonator.
This technique provides an electrostatic tuning range of 6kHz for a 719kHz resonator. The uncertainty in the exact
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