IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007
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Design and Fabrication of ZnO-Based FBAR Microwave Devices for Mobile WiMAX Applications Linh Mai, Jae-Young Lee, Van-Su Pham, and Giwan Yoon
Abstract—In this letter, we present the design and fabrication of a novel ZnO-based film bulk acoustic wave resonator (FBAR) microwave devices. The novel FBAR devices employ a new-type of Bragg reflector with very thin chromium (Cr) layer formed films. The Cr layer seems to enhance the between SiO2 and layers. The novel FBAR devices adhesion between SiO2 and show good return losses ( 11 ) and high -factors at the frequency range of 2.7–3.0 GHz. This approach will be very helpful for mobile worldwide interoperability for microwave access applications. Index Terms—Bragg reflector, film bulk acoustic wave resonator (FBAR), -factor, resonator, return loss, worldwide interoperability for microwave access (WiMAX).
I. INTRODUCTION
W
ORLDWIDE interoperability for microwave access (WiMAX) technology has recently attracted great attention mainly because it can bring the wireless and internet revolutions to portable devices worldwide. In particular, the 2.3–3.6 GHz frequency band can be assigned for the mobile broadband WiMAX applications [1]. From the microwave component/device point of view, the film bulk acoustic wave resonator (FBAR) devices have also attracted much attention as a promising novel filter technology mainly because it may be fully integrated with the conventional complementary metal oxide semiconductor/radio frequency integrated circuit (CMOS/RFIC) fabrication technologies, eventually allowing for the realization of a single-chip radio or a transceiver in the future. Typically, the FBAR devices consist of a piezoelectric film sandwiched between top and bottom metal electrodes formed on top of a Bragg reflector [2]. When a radio frequency (RF) signal is applied across the FBAR device, it produces a resonance [3]. Thus, the FBAR devices are expected to be very useful to fabricate the RF filters for WiMAX applications. The Bragg reflector (BR) sandwiched between the resonating part and substrate plays an important role in suppressing any
Manuscript received June 16, 2007; revised August 3, 2007. This work was supported by the Korea Science and Engineering Foundation (KOSEF) under ERC Program through the Intelligent Radio Engineering Center (IREC) at ICU, Korea. The authors are with the Communication Electronics Laboratory, School of Engineering, Information and Communications University (ICU). 119, Munjiro, Yuseong-gu, Daejeon 305-732, Korea (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2007.910495
possible transfer of the acoustic energy generated from the resonating part into the substrate. This allows the FBAR devices to have even higher quality factor ( ). From this standpoint, the fabrication of the high quality Bragg reflector is considered very important to yield superior device performances. Although some efforts [4]–[7] have been made to improve the FBAR characteristics, few studies have been focused to enhance the quality of the Bragg reflectors, let alone the new-type Bragg reflectors proposed in this work. In this work, a feasibility study of the FBAR devices for the mobile WiMAX applications has been presented. For this study, a novel device fabrication technique has been employed that could improve the resonance characteristics of the FBAR devices. Particularly, the design and fabrication of the novel FBAR devices have been focused on the formation of the very thin Cr adhesion layer between SiO and W films in the Bragg reflector as well as the thickness control of the bottom electrode placed on top of the Bragg reflector. As a result, the resonance frequency peaks appear at 2.7–3.0 GHz and excellent resonance characteristics are observed in terms of return loss and -factor. II. DESIGN AND EXPERIMENT SEM cross-sectional images of the Bragg reflectors used for the FBAR device are shown in Fig. 1. The FBAR device consists of a multilayered Bragg reflector on Si substrate and a piezoelectric (ZnO) film sandwiched between Al top electrode and Al bottom electrode where the bottom electrode was designed to act as a floating ground plane with various thicknesses. The FBAR devices were fabricated on three 4-inch, p-type Si wafers (named N1, N2, and N3, respectively) as follows. First, a multilayered Bragg reflector was prepared by depositing thin film layers of SiO , Cr, W, SiO , Cr, W, and SiO in sequence. The SiO layers (0.6 m-thick) were deposited by a chemical vapor deposition (CVD) technique. The Cr (0.03 m-thick) and W (0.6 m-thick) layers were deposited using a sputtering technique. Then, 0.3, 0.8, and 1.2 m-thick Al bottom electrodes were formed by deposition on the three wafers (N1, N2, and N3, respectively), followed by 1.2 m-thick ZnO film deposition. Finally, the deposition and patterning of the top Al electrodes (0.2 m-thick) on the ZnO film completed the FBAR device fabrication. The geometry of the FBAR devices has been studied by performing electromagnetic simulations with advanced design system (ADS) software. The relationship between parameters such as effective dielectric constant and thickness of piezoelectric film, dimension of top electrodes, and line width were optimized as a function of frequency. As a result, the resonators with
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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 17, NO. 12, DECEMBER 2007
Fig. 1. (a) Cross-sectional view of ZnO-based FBAR device, and their crosssectional SEM images with Al bottom thickness of (b) 0.3 m, (c) 0.8 m, and (d) 1.2 m.
3 layout patterns (pattern 1, 2, and 3) were designed for testing the resonance characteristics. The microwave characterization was performed using a probe station and HP 8722D network analyzer to measure the return loss parameters.
III. RESULTS AND DISCUSSION The schematic structure of the FBAR devices are shown in Fig. 1 and three different top electrode patterns of the FBAR devices are shown in Fig. 2. In this work, the FBAR devices have three parts: resonator part (Al/ZnO/Al), Bragg reflector part (SiO /W/Cr/SiO /W/Cr/SiO /Si), and Si substrate part. The fabricated FBAR devices were measured at the WiMAX band frequency range. The three FBAR patterns 1, 2, and 3 (with areas of 181600, 191600, and 102100 m , respectively) were designed for 2nd order resonance at about 3.0 GHz. Fig. 2 compares return loss characteristics of the FBAR devices with different patterns 1, 2, and 3 (Fig. 2(a), (b), and (c), respectively) with different thickness of Al bottom electrode (0.3, 0.8, values of the two and 1.2 m-thick) for each pattern. The FBAR devices fabricated on N2 and N3 samples show almost the same increasing trend in comparison with that of resonators on N1. From the measurement results, the FBAR devices with values. the thicker bottom Al electrodes show the larger In Fig. 2(a), at the resonance points, the FBAR devices on N1 sample have the smallest ( 19.03 dB). Meanwhile, the values of N2 and N3 samples are 23.61 dB and 30.37 dB,
Fig. 2. Return loss characteristics versus operating frequency for various bottom electrode thicknesses: (a) Pattern 1, (b) Pattern 2, and (c) Pattern 3.
respectively. All the extracted values of patterns are summarized in Table I.
of the three FBAR
MAI et al.: DESIGN AND FABRICATION OF ZNO-BASED FBAR MICROWAVE DEVICES
TABLE I RETURN LOSS VALUES OF ELECTRODE PATTERNS
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Q
Also from Fig. 2, the FBAR devices fabricated on samples values at resonance frequencies N1, N2, and N3 show high of 2.9, 3.0, and 2.7 GHz, respectively. The variation seems to be attributed largely to the deposition process of the multiple-layered thin films, potentially affecting the device performances. Therefore, the interface bonding/stability and sharpness, grain size, contamination and other film properties must be taken into account in the fabrication process. Reported elsewhere [4] and [7], the quality of the multilayered Bragg reflector may have an impact on the FBAR characteristics. In the as-deposited SiO /W multilayer, some physical defects-like imperfections may exist and/or some poor adhesions at interfaces may occur between the physically deposited films, hence degrading the device performances. Chromium (Cr) is one of the most widely used materials for the adhesion enhancement in thin-film technology [8]. In this work, Cr was selected based not only on its good bond-forming abilities, but also because it has the same crystal structure as W. The Cr adhesion layers are additionally added thin films (0.03 m-thick) with an expectation of a possible adhesion improvement between SiO and W layers. The fabricated FBAR devices were observed to resonate at 2.7–3.0 GHz with values. Based on our findings, the FBAR reasonably good devices appear to be very useful for the 2.7–3.0 GHz mobile WiMAX applications. The performance of the FBAR devices can be determined by the figure of merit (FOM) in terms of -factor [9]. Based on the definition reported elsewhere [10], the series/parallel resonance -factors ( ) were calculated as follows:
(1)
According to (1) that uses the local extrema in the slope of the input impedance phase as a function of the frequency for the resonator pattern 1, 2, and 3, the series and parallel freas a function of the quencies ( and ) and the slope of frequency are obtained. As a result, the values of FOM of the FBAR components were achieved and shown in Table II.
TABLE II -FACTORS FOR FBAR SAMPLES
IV. CONCLUSION In this work, a novel type of FBAR device has been proposed and demonstrated to have a high feasibility for mobile broadband WiMAX applications. The proposed FBAR devices use a new-type Bragg reflector consisted of very thin chromium (Cr) layer formed between SiO and W films where the Cr layer enhances the adhesion between SiO and W layers. The FBAR devices were found to resonate at 2.7–3.0 GHz frequency with good return loss values and high -factors. Also, the FBAR devices with the thicker bottom electrodes showed the better performances. Considering that FBAR devices can be fabricated in smaller size, light weight, and high resonance frequency, and excellent resonance characteristics, this new approach appears very helpful for the mobile WiMAX applications. REFERENCES [1] J. G. Andrews, A. Ghosh, and R. Muhamed, Fundamentals of WiMAX Understanding Broadband WiMAX—Understanding Broadband Wireless Networking. Englewood Cliffs, NJ: Prentice-Hall, 2007. [2] S. V. Krishnaswamy, J. F. Rosenbaum, S. S. Horwitz, and R. A. Moore, “Film bulk acoustic wave resonator and filter technology,” in IEEE MTT-S Int. Dig., 1992, pp. 153–155. [3] K. M. Lakin, K. T. McCarron, and R. E. Rose, “Solidly mounted resonators and filter,” in Proc. IEEE Ultrason. Symp., 1995, pp. 905–908. [4] M. Yim, D. H. Kim, D. Chai, and G. Yoon, “Significant resonance characteristic improvements by combined used of thermal annealing and Co electrode in ZnO-based FBARs,” Electron. Lett., vol. 39, pp. 1638–1640, Nov. 2003. [5] M. Yim, D. H. Kim, D. Chai, and G. Yoon, “Effects of thermal annealing of W/SiO multilayer Bragg reflectors on resonance characteristics of film bulk acoustic resonator devices with cobalt electrodes,” J. Vac. Sci. Technol. A, vol. 22, no. 3, pp. 465–471, Jun. 2004. [6] D. H. Kim, M. Yim, D. Chai, and G. Yoon, “Improvements of resonance characteristics due to thermal annealing of Bragg reflectors in ZnObased FBAR devices,” Electron. Lett., vol. 39, no. 13, pp. 962–964, Jun. 2003. [7] L. Mai, H.-I. Song, L. M. Tuan, P. V. Su, and G. Yoon, “A comprehensive investigation of thermal treatment effects on resonance characteristics in FBAR devices,” Microw. Opt. Technol. Lett., vol. 47, no. 5, pp. 459–462, Dec. 2005. [8] S. Franssila, Introduction to Micro Fabrication. New York: Wiley, 2004. [9] K. M. Lakin, G. R. Kline, and K. T. McCarron, “High-Q microwave acoustic resonators and filters,” IEEE Trans. Microw. Theory Tech., vol. 41, no. 12, pp. 2139–2146, Dec. 1993. [10] S. H. Park, B. C. Seo, H. D. Park, and G. Yoon, “Film bulk acoustic resonator fabrication for radio frequency filter applications,” Jpn. J. Appl. Phys., vol. 39, pp. 4115–4119, July 2000.
