604 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL 21, NO 11, NOVEMBER 2011 Development of High-Quality FBAR Devices for Wireless Applications Employing Two-Step Annealing Treatments Eunju Lee, Linh Mai, and Giwan Yoon, Member, IEEE Abstract—In this letter, a new two-step annealing technique is presented that can more effectively improve the resonance characteristics of the film bulk acoustic wave resonator (FBAR) devices in terms of return loss, -factor, and effective electromechanical coupling coefficient ( ) In the case of the SMR-type FBAR devices, the use of this approach has considerably improved the resonance performance ( 8000 of Q-factor value, 2% of ) at the operating frequency of GHz, as compared to the conventional annealing techniques Index Terms—Film bulk acoustic wave resonator (FBAR) devices, high quality factor, resonance characteristics, two-step annealing, ZnO films I INTRODUCTION ECENTLY, a considerable technology progress in microelectronics has enabled most of the RF components to be integrated into a single-chip or transceiver of the wireless mobile systems Unfortunately, the portion alone of the RF filter has been still used as an off-chip type This is largely because the conventional RF filters can hardly be integrated compatibly with the current Si-based CMOS technologies [1] From this standpoint, the film bulk acoustic wave resonator (FBAR) devices and their process technologies are most likely to be a very promising candidate to resolve the above issue, mainly due to the high compatibility of the materials, device structures, and fabrication processing with the current CMOS technology Thus, the FBAR device technology has a strong potential for more extensive application of the RF/IF filters, duplexers and voltage-controlled oscillators [2]–[4] In this work, a new FBAR device fabrication method has been studied that can more effectively enhance the resonance characteristics The proposed technique appears to be very useful and promising especially for the solidly mounted resonator (SMR)type FBAR devices [5] R Manuscript received February 18, 2011; accepted September 02, 2011 Date of publication October 06, 2011; date of current version November 09, 2011 This work was supported in part by the Basic Science Research Program through National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090063076) E Lee and G Yoon are with the Electrical Engineering Department, the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea (e-mail: celkaist@gmail.com; gwyoon@kaist.ac.kr) L Mai is with the School of Engineering, International University, Vietnam National University-HCMC, Ho Chi Minh City, Vietnam (e-mail: mlinh@hcmiu.edu.vn) Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/LMWC.2011.2168200 Fig (a) 3-D and 2-D schematic diagrams of the FBAR devices, (b) Crosssectional SEM image of the fabricated FBAR device, and (c) Schematic description of the two-step annealing concept proposed in this work II DEVICE DESIGN AND FABRICATION A two-step thermal annealing treatment was performed on the SMR-type FBAR devices particularly with Co or Al electrodes Fig shows the schematic diagrams (a) and the cross-sectional SEM image of the FBAR device (b), accompanied by the concept of the two-step annealing process employed in this work (c) The two-step annealing process includes both the so-called Bragg reflector-annealing step [6] and the post-annealing step [7] The former step is the thermal annealing treatment performed just on the acoustic Bragg reflectors prior to the deposition of the bottom electrode, and the latter step is the annealing treatment performed immediately after the top electrode deposition was completed Overall, the most significant improvement has been obtained by the proposed two-step annealing technique rather than by either Bragg reflector-annealing or the post-annealing alone The FBAR devices were fabricated using a thin-film deposition technique, as shown in Fig The fabrication of the SMRtype FBAR devices is classified largely into four stages The first stage, namely Bragg reflector deposition, was carried out /W multi-layers as follows The acoustic Bragg reflector of was formed by the thin-film deposition on the 4-inch p-type (100) silicon (Si) wafer with the 6000 Å-thick thermally oxlayer Here, four more layers were additionally idized -thick tungsten formed by the alternate-deposition of 0.6 1531-1309/$26.00 © 2011 IEEE LEE et al.: DEVELOPMENT OF HIGH-QUALITY FBAR DEVICES FOR WIRELESS APPLICATIONS (W) and 0.6 -thick silicon dioxide ( ) in a magnetron sputtering system The second stage was named the Bragg reflector annealing process Immediately after the Bragg reflector formation, the wafer with the five-layer Bragg reflector was divided into five samples (A~E) Then, only the three samples (B, D, and E) were thermally annealed in a dehydrate furnace at 400 /30 min, while keeping the other two samples (A, C) non-annealed In the third stage of the so-called resonator part -thick Co bottom electrodes were fabrication process, the 0.2 -thick Al formed on the samples (A~D) while the 0.2 bottom electrode was formed on the sample E These bottom electrodes can act as a floating ground, which has the semi-in-thick zinc oxide (ZnO) finite ground effect Then, the 1.2 was deposited on all five samples at the same time Three different top electrode patterns (patterns 1, 2, and in the insets of Fig 2) were employed in this work in order to confirm whether the proposed process could work well, regardless of the dimensions, configurations and shapes of the top electrodes The top -thick) was formed on the four samples Co electrode (0.2 (A~D) while the top Al electrode on the sample E As a result, the five FBAR device samples (A~E) were divided into two groups where the first group is the Co-FBAR devices (samples A~D) and the second is the Al-FBAR devices (sample E) The last stage, called the post annealing process, was carried out only on the three resonator samples (C~E) in argon ambient at 200 /120 The thermal treatment conditions of the five samples are summarized in Table I Finally, the parameters for return loss ( ) were extracted from the FBAR devices using a measurement system along with a probe station and HP 8722D network analyzer 605 Fig S measurement results for the three different top electrode patterns of the FBAR device TABLE I THERMAL TREATMENT CONDITIONS FOR FBAR DEVICE SAMPLES III RESULTS AND DISCUSSION The values for return loss ( ) of the Co-FBAR devices for the three patterns were plotted and summarized for the comparison of the annealing effects, as shown in Table II and Fig The resonance characteristics of the three annealed samples, i.e., Bragg reflector annealing only (sample B), post-annealing only (sample C) and two-step annealing (sample D) were compared with the non-annealed sample A On the other hand, the effective electromechanical coupling coefficient (1) and series/parallel -factors (2) are generally used as the figure of merits (FOMs) to estimate the device performance [8] TABLE II RETURN LOSS MEASUREMENT RESULTS FOR THREE DIFFERENT PATTERNS (1) (2) where the and are the series and parallel resonance freis the input impedance quencies, respectively, and also the are plotted as a function of the phase The slopes of the frequency for the different annealing conditions on the FBAR devices with the pattern 1, as shown in Fig The calculated and the effective electromechanical coupling coefficient values for the FBAR devices series/parallel quality factor with pattern are tabulated in Table III First, as shown in Table II, the return losses of the sample B were around 3.18, 1.38, and 0.96 dB better than those of the non-annealed sample A for the three different patterns, respectively This indicates that the resonance characteristics of the FBAR devices (sample B) depend surely on the annealing conditions applied to the Bragg reflectors As a result, it was confirmed that the devices with the Bragg reflectors annealed at 400 /30 could show a good resonator performance [6], [9] Second, the return losses of the sample C were around 4.87, 4.24 and 8.99 dB better than those of the sample A Clearly, 606 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL 21, NO 11, NOVEMBER 2011 Fig Slopes of Z plotted as a function of the frequency for the different annealing conditions on the FBAR devices with pattern TABLE III CALCULATED Q-FACTOR AND ELECTROMECHANICAL COUPLING COEFFICIENTS FOR FBAR DEVICES WITH PATTERN and the Al-FBAR (sample E) The two critical factors that determine the ZnO-based FBAR characteristics are believed to be the quality of both Bragg reflector and the piezoelectric property of the ZnO film as well Comparing the Al-FBAR and Co-FBAR devices with the Bragg reflectors annealed under the same condition, the difference in the device performance appears to be related to the degree of the preferred orientation of the crystal grains of the ZnO film The ZnO/Co structured film was reported to be more strongly oriented toward the -axis perpendicular to the surface of the substrate compared with the ZnO/Al film Thus, the resonance characteristics could be further improved by using the Co electrodes, instead of the Al electrodes [6], [9] As shown in Table II, the return losses of the sample D were around 1.61, 5.13 and 3.51 dB better than those of sample E and for the different patterns, respectively Also, the -factor values of the sample D were observed to show better resonance characteristics than those of the sample E, as shown in Table III Therefore, we believe that the combined use of the two-step thermal annealing and the Co electrodes can more effectively improve the resonance characteristics of the FBAR devices as compared to the other cases [6], [9] IV CONCLUSION the post-annealing process alone seems to affect the sandwiched structure and help to further enhance the resonator performance of the FBAR devices Last, the return losses of the sample D were around 10.37, 11.61, and 12.81 dB better than those of the sample A, indicating a significantly more improvement by the proposed two-step annealing method The combined use of both the Bragg reflector-annealing and the post-annealing (namely, two-step annealing) is expected to more effectively improve the resonance characteristics of the FBAR devices These trends of the resonance characteristics could be further supported by the and , as summarized in Table III Parvalues of the , ticularly in the case of the SMR-type FBAR devices employing this two-step annealing approach (sample D), a considerably , as comhigher performance could be achieved at pared to the conventional annealing techniques (samples B, C) Overall, the Bragg reflector-annealing at 400 /30 min, the first step, is considered to eliminate any possibly existing imperfect microstructures and incomplete adhesions in the Bragg reflectors The post-annealing at 200 /120 min, the second step, is likely to get rid of any physical imperfections, such as the incomplete adhesions and micro-defects of the FBAR device itself, further reducing any incomplete microstructures and adhesions in the Bragg reflectors Moreover, the effects of the two-step annealing on the resonance characteristics of the FBAR devices particularly with the Co electrodes were investigated in comparison with those with the Al electrodes, respectively called the Co-FBAR (sample D) We for the first time presented a new two-step annealing method that could more effectively improve the resonance characteristics of the FBAR devices We have also demonstrated that the resonance characteristics of the FBAR devices could be further enhanced in terms of return loss, -factor, and by the optimization of the fabrication processing technique The proposed two-step annealing approach seems very useful and promising especially for the fabrication of the SMR-type multi-layer acoustic Bragg Co-FBAR devices with the W/ reflectors REFERENCES [1] P R Gray and R G Meyer, “Future directions in silicon ICs for RF personal communications,” in Proc IEEE Custom Integr Circuits Conf., 1995, pp 83–90 [2] M Aissi, E Tournier, M A Dubois, G Parat, and R Plana 1, “A 5.4 GHz 0.35 m BiCMOS FBAR resonator oscillator in above-IC technology,” in Proc IEEE ISSCC, 2006, pp 1228–1235 [3] C H Tai, T K Shing, Y D Lee, and C C Tien, “A novel thin film bulk acoustic resonator duplexer for wireless applications,” Tamkang J Sci Eng., vol 7, pp 67–71, 2004 [4] P Bar, A Giry, P Triolet, G Parat, D Pache, P Ancey, and J F Carpentier, “Full-duplex receiver and PA integration with BAW devices,” in Proc SiRF, 2008, pp 9–12 [5] K M Lakin, “Thin film resonator technology,” IEEE Trans Ultrason Ferroelectr Freq Control, vol 52, no 5, pp 707–716, May 2005 [6] M H Yim, D H Kim, D K Chai, and G W 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, pp 465–471, 2004 [7] S Y Chu, W Water, and J T Liaw, “Influence of post deposition annealing on the properties of ZnO films prepared by RF magnetron sputtering,” J Eur Ceram Soc., vol 23, pp 1593–1598, 2003 [8] K M Lakin, G R Kline, and K T McCarron, “High-Q acoustic microwave resonators and filters,” IEEE Trans Microw Theory Tech., vol 41, no 12, pp 2139–2146, Dec 1993 [9] M H Yim, D H Kim, D K Chai, and G W Yoon, “Significant resonance characteristic improvements by combined thermal annealing and Co electrode in ZnO-based FBARs,” Electron Lett., vol 39, pp 1638–1640, Nov 2003 ... electrode patterns of the FBAR device TABLE I THERMAL TREATMENT CONDITIONS FOR FBAR DEVICE SAMPLES III RESULTS AND DISCUSSION The values for return loss ( ) of the Co -FBAR devices for the three patterns... al.: DEVELOPMENT OF HIGH-QUALITY FBAR DEVICES FOR WIRELESS APPLICATIONS (W) and 0.6 -thick silicon dioxide ( ) in a magnetron sputtering system The second stage was named the Bragg reflector annealing. .. the FBAR devices with pattern TABLE III CALCULATED Q-FACTOR AND ELECTROMECHANICAL COUPLING COEFFICIENTS FOR FBAR DEVICES WITH PATTERN and the Al -FBAR (sample E) The two critical factors that determine