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High Performance Complex Oxide Thin Film Materials to Enable Switchable Film Bulk Acoustic Resonators (FBAR) for Low-Loss Radio Frequency Devices


OBJECTIVE: To develop molecular beam epitaxy/ chemical vapor deposited (MBE/CVD or MOCVD) low-loss, tunable complex oxide thin film materials to enable compact, switchable FBAR filters operating in the 1-3 GHz frequency range. DESCRIPTION: In modern communication systems, frequency-agile and reconfigurable components are becoming increasingly necessary to cope with a multitude of signal frequencies and modulation formats. Acoustic resonator devices are currently the technology of choice for compact front-end filtering since they are not limited by the inductor resonator quality (Q) factors. Historically, surface acoustic wave (SAW) resonators have been used, which in recent years have given way to Film Bulk Acoustic Resonator (FBAR) technologies. Such acoustic devices exploit piezoelectric materials (such as AlN, ZnO) and when properly designed can have extremely high Q-factors of 600 or more at RF frequencies; hence high-order, low-loss filters can be realized. However, the resonance frequency of an FBAR resonator is determined by the thickness of the piezoelectric material, and is thus fixed. Since an FBAR device has a high Q-factor, it is difficult to pull or de-tune this resonant frequency with external components like varactors unless the devices are heavily loaded, which then degrades the Q. Thus, in order to achieve frequency agility or re-configurability, multiple FBAR filters must be combined with a switching network to route the signal through an appropriate fixed-frequency filter. It has recently been shown that electrostrictive materials as well as materials that show DC-voltage induced piezoelectricity can be utilized as both a switch and a resonator if configured properly [1-5]. This eliminates the need for complex and lossy switching networks in frequency-agile filter networks. This switchable FBAR technology is enabled by perovskite oxide materials, specifically BaSrTiO3 (BST) and SrTiO3, which have been shown to have coupling constants comparable to that of AlN, which is the current material used in FBAR. The quality of FBAR devices is highly sensitive to film quality and crystal orientation. As is well-established in semiconductor technology, molecular beam epitaxy offers the highest quality thin film materials and excellent control over film orientation. It has been recently shown that high-quality perovskite oxide thin films, such as SrTiO3, can be grown by MBE [6]. It is anticipated that by using advanced growth techniques (which are standard to the electronics industry for growing III-V compound semiconductors), such as, Molecular Beam Epitaxy (MBE) and/or Chemical Vapor Deposited (CVD) or Metallo Organic CVD - thin films of BST or SrTiO3, switchable FBAR of unprecedented quality will be obtained. The goal of this STTR is to investigate the feasibility of using of high-quality MBE/CVD electrostrictive oxide films for switchable FBAR devices and filters. PHASE I: The offerer will demonstrate feasibility to deposit high-quality films of BST and SrTiO3 films using MBE/CVD or MOCVD. Thin film quality metrics include the demonstration of high quality stoichiometric films with smooth interfaces and low dielectric losses. The Phase 1 work should also include full characterization of the FBAR process; including (i) identifying the challenges relating to the processing temperatures and (ii) identification of a route to commercial processing. The offerer will design a FBAR device utilizing a switchable perovskite oxide FBAR from 1-3 GHz and predict its performance. PHASE II: The offerer will deposit high quality perovskite oxide based films consistently by MBE/CVD or MOCVD and conducts full characterization of these films. These films will then be optimized and processed into FBAR devices with detailed material analysis and processing characterization performed to understand how material/film orientation affects the FBAR performance. Material deposition parameters will be varied with the goal to evaluate how they impact the FBAR properties. Filters will be fabricated utilizing the FBAR devices and full RF characterization of the filters will be performed including filter shape performance, linearity, harmonics, Q, loss, return loss, group delay, etc. in order to compare with existing technology. The ability to switch on and off will be demonstrated. Initial reliability and lifetime testing will also be performed to baseline the robustness of the process and design. PHASE III DUAL USE APPLICATIONS: Phase III will involve refinements of the filter design and include specifically designed filters and packaging for use in actual systems. Documentation to ISO standards and refinement of the manufacturing processes will be performed and repeated to establish a commercially scalable manufacturing process. Additional reliability testing will be performed to conform to commercial and military standard test practices. This filter technology can be utilized in a number of high frequency communications systems for both the military and commercial use. REFERENCES: [1] K. Morito, Y. Iwazaki, T. Suzuki, and M. Fujimoto,"Electric field induced piezoelectric resonance in the micrometer to millimeter waveband in a thin film SrTiO3 capacitor", J. Appl. Phys. 94, 5199 (2003). [2] S. Tappe, U. Bttger, and R. Waser,"Electrostrictive resonances in Ba0.7Sr0.3TiO3 thin films at microwave frequencies", Appl. Phys. Lett. 85, 624 (2005). [3] A. Noeth, T. Yamada, A. K. Tagantsev, N. Setter, Electrical tuning of dc bias induced acoustic resonances in paraelectric thin films, J. Appl. Phys. 104, 094102 (2008). [4] J. Berge, A. Vorobiev, W. Steichen, S. Gevorgian, Tunable solidly mounted thin film bulk acoustic resonators based on BaxSr1-xTiO3 films, IEEE Microwave Wireless Comp. Lett. 17, 655 (2007). [5] G. N. Saddik, D. S. Boesch, S. Stemmer, R. A. York, DC electric field tunable bulk acoustic wave solidly mounted resonator using SrTiO3, Appl. Phys. Lett. 91, 043501 (2007). [6] J. Son, P. Moetakef, B. Jalan, O. Bierwagen, N. J. Wright, R. Engel-Herbert, S. Stemmer, Epitaxial SrTiO3 films with electron mobilities exceeding 30,000 cm2V-1s-1, Nature Mater. 9, 482 (2010).
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