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Microsyst TechnolDOI 10.1007/s00542-014-2319-0

TECHNICAL PAPER

Piezoelectric thin films for double electrode CMOS MEMS surface acoustic wave (SAW) resonator

Aliza Aini Md Ralib · Anis Nurashikin Nordin · AHM Zahirul Alam · Uda Hashim · Raihan Othman

Received: 12 July 2013 / Accepted: 2 September 2014 © Springer-Verlag Berlin Heidelberg 2014

electromechanical coupling coefficients and are more effi-cient than AlN thin films.

1 Introduction

The future paradigm of Internet of Things (IoT) predicts connectivity between a plethora of devices be it comput-ers, personal communication devices, cars, machine tools, living beings and seemingly passive things such as gar-ments, food and drugs (De Guglielmo et al. 2014). To achieve such ubiquitous connectivity, a sort of ‘digital skin’ formed using wireless sensor/actuator network, is required to translate physical feedback into digital data. At the heart of these wireless sensor/actuator networks are mobile radio frequency (RF) modules consisting of RF integrated circuits (RFICs) and a multitude of passive components, amongst them surface acoustic wave (SAW) devices (Rup-pel et al. 2002a). Driven by the success of the wireless technology business and marked progress in the submicron semiconductor fabrication techniques, SAW technology has progressed to GHz range in recent years (Ruppel et al. 1993). SAW devices have emerged as the most unique of RF passive components, having advantages of being small, rugged, lightweight and easily reproducible (Hunter et al. 2002; Ruppel et al. 1993, 2002a; Weigel et al. 2002). These advantages have made SAW devices a key component in RF communication systems with innovative applications ranging from correlative SAW signal processing systems, satellite receivers, remote control units, keyless entry sys-tems, television sets to identification tags (Campbell 1998; Hikita et al. 2000; Springer et al. 1998). Other emerging applications of SAW resonators include gas sensors (Sadek et al. 2006), biosensors (Xu 2006), chemical (Nomura and Takebayashi 1998), temperature and pressure sensors (Buff

Abstract CMOS integration for RF-MEMS is desired to yield compact, low-power and portable devices. In this work, we illustrate the usage of double electrode CMOS SAW resonators using both ZnO and AlN as its piezoelec-tric material. Double electrode transducers were chosen, as they are better at suppressing undesired acoustic reflec-tions compared to single electrodes. The structure and dimension of the device is based on 0.35 μm CMOS pro-cess where the IDTs are fabricated using standard CMOS fabrication process. 2D Finite element modeling of the CMOS SAW resonator using COMSOL Multiphysics® is presented. Two-step eigenfrequency and frequency domain analyses were performed. The acoustic velocities gen-erated are 3,925 and 5,953 m/s for ZnO and AlN CMOS SAW resonator respectively. Higher acoustic displacement and surface potential were observed in ZnO compared to AlN. It can be concluded that ZnO thin films have higher

A. A. Md. Ralib · A. N. Nordin (*) · A. Z. Alam Department of Electrical and Computer Engineering, International Islamic University Malaysia, Kuala Lumpur, Malaysiae-mail: anisnn@iium.edu.my

A. A. Md. Ralib e-mail: aliza.aini@gmail.com

A. Z. Alam e-mail: zahirulalam@iium.edu.my

U. Hashim Nano fabrication Laboratory, Institute of Nanoelectronic Engineering, Universiti Malaysia Perlis, Perlis, Malaysiae-mail: uda@unimap.edu.my

R. Othman Science Engineering Department, International Islamic University Malaysia, Kuala Lumpur, Malaysiae-mail: raihan@iium.edu.my

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et al. 1997). The significance of these devices in this indus-try can be observed by the large numbers of the worldwide production of these devices, where approximately 3 billion acoustic wave filters are used annually, primarily in mobile cell phones and stations (Ruppel et al. 2002a). A major issue of SAW resonator is that they are non-compatible with standard integrated circuit (IC) processing techniques, making them bulky, off-chip components which are exter-nally connected to RF circuits via bond wires. Recently, we successfully fabricated on-chip surface acoustic wave (SAW) resonators using standard CMOS processing cou-pled with MEMS fabrication techniques (Nordin and Zaghloul 2007). A major fabrication challenge of manu-facturing submicron electrodes to produce RF resonators was overcome by utilizing the high-tech capabilities of IC foundries to fabricate these thin high aspect ratio metal lines. While functioning, these resonators are plagued by low quality factors (Qs) and high losses.

To counteract this, we propose the design of an inte-grated SAW resonator formed using double electrode trans-ducers and reflectors to improve the quality factors. Com-parative analysis of the SAW resonator using single and double electrode transducers had been reported earlier by (Stefanescu et al. 2011) for GHz application. Single IDT electrodes are reflective in nature, generating unwanted spurious signals. Double electrode transducers on the other hand, are less reflective than single electrode transducers, exhibiting lower insertion losses (Stefanescu et al. 2011). Finite element modeling of single IDTs versus double elec-trode IDTs has also been reported by (Ralib et al. 2013). The simulated results shows higher quality factor in the order of thousands (Q = 26,845.3) for double electrode CMOS SAW resonator at resonance of 0.806 GHz com-pared to single electrode CMOS SAW resonator (Ralib et al. 2013).

Other than the design, the quality of the piezoelectric material can be crucial to enhance the performance of the SAW resonator. The efficiency of the conversion from elec-trical to mechanical waves is evaluated by electromechanical coupling coefficient, k2. Typically SAW devices use quartz (Tanski 1979; Baron et al. 2010), LiNbO3 (Nakanishi et al. 2010; Ramli and Nordin 2011) (Bu et al. 2004) and LiTaO3 (Kushibiki et al. 2000) piezoelectric wafers as their substrate which have high k2 but have limitations for integration with circuits (Ruppel et al. 2002b). Thin film piezoelectric mate-rials play an essential role in micro-electro-mechanical sys-tem (MEMS) SAW resonators to achieve single chip trans-ceiver implementations. Efforts have been done to realize silicon based SAW resonators that have less lossy interfac-ing (Kaletta et al. 2013; Vellekoop et al. 1994; Baca et al. 1999; Neculoiu et al. 2009). Non-ferroelectric piezoelectric materials such as ZnO and AlN are highly silicon compat-ible making them suitable for RF-MEMS applications. Low

deposition temperature of piezoelectric thin films (<400 °C) is necessary for compatibility with electronic circuits. Exam-ples of devices using piezoelectric thin films include a 5 GHz, two-port SAW resonator using AlN (Neculoiu et al. 2009). High quality ZnO thin films deposited at low temper-atures have also been reported to produce acoustic wave res-onators with low insertion losses (Ralib and Nordin 2013).

This paper reports the comparative analysis of two dif-ferent piezoelectric materials: ZnO and AlN for double electrode CMOS MEMS SAW resonators. This paper is organized as follows. Section 2 explains the design concept of CMOS MEMS SAW Resonator and double electrodes. Section 3 presents the simulation analysis, results and dis-cussion. A finite element simulation of the CMOS SAW resonator is conducted using COMSOL Multiphysics®. The performance of the CMOS SAW resonator using both ZnO

Fig. 1 a Cross section b 3D model of two-port double electrode CMOS SAW resonator

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and AlN were compared via the simulation results. Finally, the conclusion is given in Sect. 4.

2 Design concept

In our work, the CMOS SAW resonator consists of piezo-electric thin film, interdigital transducers (IDT) and reflec-tors as shown in Fig. 1 (Nordin and Zaghloul 2007). The design uses two CMOS metal layers: Metal 1 for the ground shield and Metal 2 for the IDTs and reflectors. The presence of reflectors create standing waves within the cav-ity to reduce the loss of the acoustic waves propagating outwards (Nordin and Zaghloul 2007).

The velocities of acoustic waves are 105 times slower than electromagnetic waves. This means that at a specific fre-quency, acoustic waves have smaller wavelengths compared to electromagnetic waves; allowing small-sized acoustic reso-nators to be formed (Rajesh 2013). Acoustic energy in SAW devices is maintained close to the surface in the range of the acoustic wavelength (Länge et al. 2008). When a voltage is applied between alternately connected electrodes, it launches acoustic waves that propagate through piezoelectric material. The arrival of the acoustic wave at the output transducer is delayed a few microseconds later. The wave vibrates to the output IDT and generates corresponding output voltage which is a delayed and attenuated version of input voltage (Weigel et al. 2002). Efficient transducer operation is achieved when all vibrations interfere constructively when excited at the resonance frequency, f (Rocha-Gaso and March-Iborra 2009). Based on Eq. 1, the periodic distance (λ) of the IDT deter-mines the resonant frequency (fr) of the CMOS SAW resona-tor; where v is the assumed acoustic velocity.

Figure 2, illustrates the schematic of a 2-port double electrode SAW resonator. The IDT topology is important

(1)v = fr�

to ensure that the surface acoustic wave is properly excited and unwanted reflections are minimized. To study the effect of unwanted reflections in IDTs, the reflection of a single strip is first studied.

Figure 3 shows the SAW reflection at a strip based on the unit cell of an IDT. Some portions of incident SAW is reflected because of impedance mismatch between the IDT and the piezoelectric material (Hashimoto 2000). A+ and A− denote reflected SAW at the left and the right of each IDT and can be described as shown in (2) where r− and r+ are the reflection coefficients and β is the wavenumber. The reflection coefficient per strip, Γs is exhibited in (3) (Hashi-moto 2000).

(2)A− = r−Aine−2jβ2(L−

ω/2)

A+ = r+Aine−j(β2L+β1ω)

Fig. 2 Top view of 2-two SAW resonator and the design parameter

Fig. 3 Surface acoustic wave incidence and reflection at a strip (Hashimoto 2000)

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In single electrode configurations, the IDT has undesir-able internal reflections that degrade the performance of the SAW resonator. Each IDT reflects weakly, but the effect of unwanted reflections become crucial when they are added in phase as shown in Fig. 4a (Campbell 1998). Double electrodes however can suppress reflections at resonance frequency as shown in Fig. 4b (Hashimoto 2000). Reflec-tions due to electrodes 1 and 2 are cancelled by reflections from electrode 3 and 4 that are 180o out of phase from the first reflections (De Vries et al. 1972). Thus, double elec-trodes are almost nonreflective and have very small reflec-tions (Morgan 2007). Double electrode configurations are widely used when the resonance frequency need to be con-trolled accurately (Hashimoto 2000). However the double electrode limits the maximum resonance frequency because of the reduction of electrode pitch from (λ/4) to (λ/8) (Mor-gan 2007).

While reflections are undesired in IDTs, total reflection is needed for the reflectors to contain the acoustic wave within a cavity and to create standing waves. Like the IDTs, each reflector strip creates reflection and as shown in Fig. 2, total reflection is created at point Lp, also known as the effective penetration length. Maximum reflection of the acoustic waves by the reflectors is desired to improve the quality factor (Q) of the device and to minimize losses due to escaping acoustic waves to the left and to the right of the IDTs. To achieve this, both the distance between the reflectors, L and Lc need to be located an integer num-ber half wavelength apart as shown in Eq. 4 to maximize reflection (Nordin 2008). A large aperture (W) also reduces diffraction and enhances the performance of the SAW resonator.

(3)Γs =A+ + A−

AIN

∼= 2jr − sin(β1ω)e(−2jβ2L)

(4)Lc = 0.5(n − 1)�

3 ZnO versus AlN as piezoelectric thin films

Proper choice of piezoelectric material is crucial to ensure satisfactory performance of the CMOS SAW resonator. The piezoelectric material is chosen based on its CMOS compatibility, ease of deposition, electromechanical and piezoelectric properties (Md-Ralib et al. 2012). Non-fer-roelectric piezoelectric materials such as ZnO and AlN have high k2, and are highly compatible with CMOS pro-cessing making them attractive for SAW applications. ZnO and AlN are wurtzite-structured thin film piezoelec-tric materials with the polar direction giving piezoelectric response along crystallographic direction (Bassiri-Gharb 2008). Both ZnO and AlN are non-ferroelectric materials do not require poling or post-deposition annealing (Md-Ralib et al. 2012). To achieve high electromechanical cou-pling coefficient, we explore the usage of AlN and ZnO thin films as the piezoelectric layer. Table 1 summarizes the properties of ZnO and AlN piezoelectric material

Fig. 4 Internal reflection of surface acoustic wave a single electrode b double electrode

Table 1 ZnO and AIN properties

Unit Symbol AIN ZnO

Density kg/m3 ρ 3,260 5,675

Piezoelectric constants C/m2 e15 −0.48 −0.8

e31 −0.45 −0.57

e33 1.55 1.32

Elastic constants GPa C11 345 210

C12 125 121

C13 120 105

C33 395 211

C44 118 43

C66 110 45

Relative permittivity ε 11 9 8.6

ε 33 11 10

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Table 2 Design parameters of simulated CMOS SAW resonator

Parameters ZnO SAW resonator AlN SAW resonator

Wavelength, λ (μm) 3.48 3.48

Width of aperture, W (μm) 150 150

Metal 2 width (λ/8) (μm) 0.435 0.435

Distance between input and output IDT, Lc (μm) 1.74 1.74

Distance between reflector and transducer, Lg (μm) 3.48 3.48

Piezoelectric thickness (μm) 2.0 2.0

Fig. 5 Deformed shape plot of the resonance surface acoustic wave mode for a AIN CMOS SAW resonator b ZnO CMOS SAW resonator

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that are used in FEM simulation. Each of the stress ten-sor components (σ) is a linear function of the strain tensor components (ε) as shown in (5) where Cijkl is the elastic stiffness coefficient (Morkoç and Özgür 2009). Only five

stiffness constants exist due to additional symmetry as shown in Table 1. Piezoelectric effect occurs when there is a presence of stress and consequently electric polariza-tion is produced. The relationship can be expressed in (6)

Fig. 6 Electric potential distri-bution for a AIN CMOS SAW resonator b ZnO CMOS SAW resonator

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using piezoelectric strain coefficient, eijk and piezoelectric stress coefficient dijk. Only three independent components (e31, e33 and e15) exist in hexagonal wurtzite phase which characterize the piezoelectric crystals (Morkoç and Özgür 2009). The piezoelectric stress coefficient dijk directly affects k2.

(5)σij = Cijklεkl

(6)Pi = eijkεjk = dijkσjk

3.1 Finite element simulation modeling

Table 2 presents the CMOS SAW resonator design param-eters of the resonator implemented in this paper for both ZnO and AlN CMOS MEMS SAW resonator based on 0.35 μm CMOS technology.

Finite element simulations of the CMOS SAW resona-tor were conducted using COMSOL Multiphysics®. The performance of the resonator using both ZnO and AIN were compared via the simulation results. A 2D geometry of

Fig. 7 Frequency response for AlN CMOS SAW resonator where fr = 1.636 GHz

Fig. 8 Frequency response for ZnO CMOS SAW resonator where fr = 1.128 GHz

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CMOS SAW resonator was drawn under the piezoelectric model. Two analyses were applied: eigenfrequency analy-sis and frequency domain. Eigenfrequency analysis was done to estimate the resonance frequency where maximum

displacement occurred. Figure 5 shows the deformations on the surface of the thin film on top the IDTs based on eigen frequency analysis. ZnO and AlN show resonance frequency of 1.128 and 1.636 GHz respectively as shown in Fig. 5.

Fig. 9 Total susceptance vs. simulated frequency in fre-quency domain analysis for AlN CMOS SAW resonator

Fig. 10 Total susceptance vs. simulated frequency in frequency domain analysis for ZnO CMOS SAW resonator

Table 3 Performance evaluation for both CMOS SAW resonator

Parameters ZnO SAW Resonator AlN SAW Resonator

Acoustic velocity, v (m/s) 3,925 5,953

Wavelength, λ (μm) 3.48 3.48

Fundamental Resonant Frequency, fr (GHz) 1.128 1.636

Max displacement (nm) 4.86 2.90

Quality factor, Q 1,030 669

Coupling coefficient, k2 0.43748 0.30164

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Figure 6 exhibits the electric potential distribution and deformation of the SAW resonator using both ZnO and AlN at their respective resonance frequencies. ZnO shows higher electric potential of 7.6548 V compared to 6.6264 V for AlN. Once the mode with highest displacement has been identified, frequency domain analysis was performed to compute the dis-placement based on a user range of input harmonic frequen-cies. Figures 7 and 8 shows the graph for displacement versus frequency applied. Maximum displacement can be observed at 4.86 nm for ZnO that resonates at 1.128 GHz (Fig. 8) com-pared to lower displacement of 2.90 nm for AlN that reso-nates at 1.636 GHz (Fig. 7). Another important parameter to measure the performance of CMOS SAW resonator is qual-ity factor, Q. The quality factor is calculated based on 3 dB bandwidth from the maximum displacement generated. Nar-row bandwidth of ZnO CMOS SAW resonator gives higher quality factor of 1030 compared to AIN CMOS SAW resona-tor; which exhibits quality factor of 669 as shown in Figs. 7 and 8. The electromechanical coupling coefficient, k2 can also be calculated based on FEM simulations as illustrated in (7) where fs and fp are the series and parallel resonance frequency respectively as shown in Figs. 9 and 10. Table 3 highlights the comparative results of ZnO and AlN CMOS SAW resonator. ZnO shows higher k2 compared to AlN. Higher electrome-chanical coupling coefficient is observed when higher acous-tic wave displacement are generated and when there is larger separation between series and parallel resonances.

4 Conclusion

We presented in this paper the mechanical finite element solution of double electrode CMOS MEMS SAW resonator at GHz frequencies. Double electrode designs were chosen due to their superior suppression of undesirable reflections at the IDTs, yielding more efficient devices. Comparative analysis of two different piezoelectric thin films: AlN and ZnO were done using the double electrode design. Devices using ZnO produced higher displacement, k2 and Qs com-pared to AlN. The acoustic velocity generated are 3,925 and 5,953 m/s for ZnO and AlN CMOS SAW resonator respectively, suitable for GHz application devices.

Acknowledgments This research was supported by Exploratory Research Grant Scheme: ERGS 11-009-009 under the Ministry of Higher Education Malaysia.

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