rmenian Journal of Physics, 2009, vol. 2, issue 1, p. 64-70 Ferroelectrics in agile microwave components Spartak Gevorgian Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden and Microwave and High Speed Electronics Research Center, Ericsson B, 431 84 Moelndal, Sweden Received February 6, 2009 bstract brief review of microwave applications of ferroelectrics is given. The research results of the last year in this field at Chalmers University are summarized. Keywords: epitaxial thin film ferroelectrics, microwave application, thin film varactor, bulk acoustic resonator PCS number: 77.84.-s, 84.40.Dc 1. Introduction Ferroelectrics, especially perovskites, are truly multifunctional materials widely used in electronic, optical, acoustic and magnetic components. The first attempts of microwave applications date back to early 1950s. In the past most of the developed microwave devices have been based on bulk ceramics such as BaTi 3, SrTi 3 and their solid solutions - Ba x Sr 1-x Ti 3. However, the high microwave losses associated with the poor quality of the used ceramics limited their applications in practical microwave devices. With the advances in modern technology the quality of the epitaxial thin film ferroelectrics has been substantially improved making them qualified for industrial applications. lthough there are many other perovskite systems with promising microwave performances Ba x Sr 1-x Ti 3 still remains the workhorse for the agile microwave applications since it is more studied both in terms of electrical performance, manufacturability and reliability. Thin film varactors and bulk acoustic resonators are the basic components that are used in modern microwave circuits and systems. Ferroelectric varactors have superior performances, in comparison with the competing semiconductor analogues, both in terms of the Q-factor and tunability; especially at frequencies above 10-20 GHz. part from the high tuning speed they have extremely low leakage currents offering design flexibilities and low control power consumption. Today there are several small, and well established companies marketing microwave components based on ferroelectrics. Nevertheless the full potential, especially their multifunctionality, of these materials is not yet employed. The next sections give a brief summary of some of the recent results obtained at Chalmers University. 2. Ferroelectrics: features attractive for microwave applications Ferroelectrics, as high-permittivity materials, are widely used in high-density commercial decoupling capacitors. coustoelectronic transducers are the other
rmenian Journal of Physics, 2009, vol. 2, issue 1 traditional components where the ferroelectric piezoelectrics are used. n the other hand the DC electric-field-dependent dielectric permittivity is the most important feature of some perovskites that makes them attractive for agile microwave components, such as varactors, tunable delay lines, phase shifters, tunable filters, etc. For these applications the complex metal oxides, known as perovskites, such as Titanates (CaTi 3, BaTi 3, etc.), Tantalates (KTa 3 etc.), Niobates (KNb 3 etc.) etc. are the most considered ferroelectrics. They are characterized by a common chemical formula, B 3, and have the same crystal structure, Fig.1a. bove the polar-to-non-polar phase transition (Curie) temperature, T c, their crystal lattice has a cubic structure. In this phase the crystal has no spontaneous polarization. Its permittivity is rather high, DC field, temperature and strain dependent. Below Curie temperature the centers of the positive and negative charges shift, Fig.1a, and the crystal is characterized by spontaneous polarization. ne for the surfaces of a macroscopic crystal is charged positively, while the opposite surface is charged negatively. Below Curie temperature the crystal is in a polar phase characterized by hysteresis loop, Fig.1b, and piezoelectric effect. In this phase the DC-dependent permittivity has butterfly shape, Fig.1d, and the ferroelectrics are used in nonvolatile memory cells and piezoelectric transducers. bove Curie temperature the crystal is in a paraelectric phase with a bell-shaped dependence of the permittivity on the applied DC field, Fig.1e. This phase is preferable for most of the agile microwave applications since it has no hysteresis, Fig.1c, and the permittivity of the crystal may be given unambiguously via the applied DC field. ε(t) B B Nonvolotile mempry T C Tunable microwave devices T P +P s P +P r -E c +E c E -P s -P r ε(e) ε(e) -E c +E c E E Fig.1 Temperature dependences of the permittivity (a), DC field-dependent polarization (b) and permittivity (c) of a typical ferroelectric below and above the Curie temperature. 65
rmenian Journal of Physics, 2009, vol. 2, issue 1 Varactors are the basic microwave components where ferroelectrics in paraelectric phase are used. They may be based on ceramic (bulk, film) and single crystal (bulk, epitaxial film) films and may have parallel-plate and coplanar-plate designs, Fig.2. Typically the thin (epitaxial and ceramic) films have better performances in comparison with the bulk ceramics and thick films and they offer integration flexibility. Table 1. Technology comparisons Technology Power consumption Bias Speed Q-factor at 10 GHz Semiconductor Magnetic Schottky (Gas) <1 mw <5 V < 1 ns 200 HBV (Gas) < 1mW <20 V < 5 ns 40 brupt p-n junction (Si) <5 mw <30 V < 10 ns 30 P-I-N diode <0.1mW <10 V <1 μs FET 1 ns YIG (variable permeability, ferromagnetic resonance) Remnant magnetization Magneto-static (spin) wave High Low (coil) (coil) <5 ms >3000 <5 ms - Low - <5 ms Low Thin film Negligible <30 V < 1 ns >100 Ferroelectric Thick film Negligible <1000 V < 10 ns <100 Bulk Negligible <15 kv < 1μs >500 Liquid crystal Negligible < 40 V < 10 ms <20 ptical Mechanical Photoconductivity Fiber-optical Bulk MEM varactor Piezotransducer <10 mw <10 mw High (LD, LED) (LD, LED) (motor/c oil) 10 fs -10 ms 10 fs -10 ms < 10 > 1ms >1000 Negligible <50 V > 10 μs >200 Negligible >100 V >100 μs >500-66
rmenian Journal of Physics, 2009, vol. 2, issue 1 The field, E, dependent tenability, T(E), of a varactor is defined as T(E)=[C(0)- C(E)]/ C(0)] where C(0) and C(E) are correspondingly varactor capacitances at zero and given (E) DC bias fields. The parallel-plate varactor offers higher tenability at low applied DC voltages but has more complex fabrication process as compared with the coplanar-plate design. brief comparison of ferroelectric varactors with the competing technologies is given in Table 1. The main distinguishing features of the ferroelectric varactors are high Q-factor, extremely low power (leakage current) required for tuning, ultimate tuning speed and simple fabrication processes. The fabrication of the ferroelectric films requires relatively high temperatures (<700C) which imposes limitations in the selection of the substrates. The industry-relevant large-area wafers (>4 ) that meet this requirement include silicon, fused silica, sapphire, etc. Ferroelectric Substrate Ferroelectric Substrate a) b) Fig.2. Parallel-plate (a) and coplanar-plate (b) ferroelectric varactors. 3. Recent results The recent research activities have been focused on the development of microwave devices and circuits based on new materials/physical phenomena. It includes synthesis of the thin films, modeling, fabrication and experimental characterization of components and circuits based on ferroelectrics. Given below are some of the last year results obtained at Chalmers University. 3.1. Varactors Ferroelectric varactors are optimized for microwave circuit applications. The Q f(ghz) product of the developed varactors is more than 1000 at frequencies above 20 GHz. The tunability is more than 40%. These state-of-the-art performances are much better than that of the semiconductor analogues. The experimentally estimated time to failure is more than 10 years. Fig.3a shows a circuit application example of a parallel-plate varactors, while Fig.3b shows the dependence of the varactor Q-factor on the composition (x) of the used ferroelectric Ba x Sr 1-x Ti 3 films [1]. The varactors of this type are used in different microwave devices. n example of Voltage-Controlled scillator (VC) based on these varactors is considered below. 67
rmenian Journal of Physics, 2009, vol. 2, issue 1 Top plates of varactors Q-factor 120 100 80 60 20 GHz comp-0v 14:06:18 11/09/2008 15 GHz Bottom plates a) 40 25 GHz 10 GHz 20 0 0,2 0,3 0,4 0,5 0,6 0,7 0,8 Ba content (x) b) Fig.3. Micro-photo of a three terminal ferroelectric varactor (a) and composition (x) dependence of the Ba x Sr 1-x Ti 3 -based varactors at zero DC bias and for different frequencies. 3.2. Thin-Film Bulk coustic Wave Resonators (TFBR) DC electric field-tuned TFBRs based on paraelectric phase Ba x Sr 1-x Ti 3 are essentially new electronic components [2], [3]. Fig.4a depicts the simplified cross section of a solidly mounted TFBR where two pairs of u/si 2 layers are used as a Bragg reflector deposited on a high resistivity silicon substrate. The paraelectric phase Ba x Sr 1-x Ti 3 is not piezoelectric. However, the applied DC field brakes its centrosymmetricity and for a superimposed microwave b) field it pretends to be as a piezoelectric [4]. The theory of the induced piezoelectric effect, within FP7 EU project NNSTR (www.nanostar-eu.com), was developed recently [5], [6]. Fig.4b shows typical measured DC bias dependences of the resonant frequencies of the series and parallel resonances. s it may be seen the resonant frequencies have the same depedences for increasing and decreasing DC bias, i.e. there is no hysteresis in f(v) dependences. The hysteresisfree performance is associated with the induced nature of the piezoelectricity (in paraelectric phase Ba x Sr 1-x Ti 3 ). s soon as the DC bias is removed the induced pizoelectrisity disappears. The hysteresis-free performance is extremely important for practical applications. The Q-factor of the resonator is about 100-150 and the tunability is about 2%. Further studies are planned to improve the performance. 68
rmenian Journal of Physics, 2009, vol. 2, issue 1 4.2 u l/u Substrate BST Reflector / Bottom el. Resonance frequency (GHz) 4.15 4.1 0 5 10 15 Bias (V) a) b) f s f p Fig.4. Cross section (a) and DC bias-dependent resonant frequencies (b) of the TFBRS. 3.3. Multiproject wafers In an attempt to industrialize the developed ferroelectric technology Chalmers and Ericsson, within a joint project, developed fabrication process of ferroelectrically tunable microwave devices using 4 high-resistivity silicon wafers. The process includes passivation of the HR silicon (to eliminate the losses due to inverted/accumulated surface layers), 0.5 μm thick patterned (custom design) bottom electrodes, deposition of ferroelectric films (by magnetron sputtering) deposition and patterning (custom design) top electrodes. It includes ferroelectric varactors and devices based on them (delay line, phase shifter, VCs, filters, power limiter, pulse shaper, harmonic generator, etc.), distributed (microstrip, coplanar) and passive components (inductor coils, high-density capacitors, resistors, etc.). n example of a wafer with 12 20x20 mm 2 reticles is shown in Fig.5. Each of the reticles may contain different devices designed in different projects. This process was used by Ericsson to develop state-of-the-art millimerewave VC [7]. Fig.6 shows the design of the oscillator where the semiconductor IC chip including the oscillator transistors is flip-chip connected to a silicon carrier with ferroelectric varactors and other passive microwave/dc components. Fig.5. 4 silicon multi-project wafer incorporating tunable and passive microwave components and circuits. 69
rmenian Journal of Physics, 2009, vol. 2, issue 1 Fig.6. VC uses silicon carrier with integrated ferroelectric varactor tunable LC resonators (Project Himission/Ericsson). 4. Conclusions Ferroelectric films for microwave applications are successfully optimized and large number of industry relevant microwave devices is demonstrated. and future activities will include further studies of ferroelectrics for microwave applications, focusing on the optimization and circuit/subsystem applications of the varactors and TFBRs. Multiferroics are the other materials considered for microwave and THz applications. References 1.. Vorobiev and S. Gevorgian, "Composition dependent tunable microwave properties of thin BaxSr1-xTi3 films", Symposium G, European MRS spring meeting, 2009. 2. S. Gevorgian, T. Lewin, H. Jacobsson, and. Vorobiev, Tuneable Bulk coustic Resonator (TFBR), Ericsson B application P19309W1, 2004 PCT/SE2004/001099, July 2004. 3. J. Berge, M. Norling,. Vorobiev, and S. Gevorgian, Field and temperature dependent parameters of the dc field induced resonances in BaxSr1-xTi3-based tunable thin-film bulk acoustic resonators, J. ppl. Phys., vol.103, p. 064508, 2008. 4. S. Gevorgian,. Vorobiev, and T. Lewin, DC field and temperature dependent acoustic resonances in parallel-plate capacitors based on SrTi 3 and Ba 0.25 Sr 0.75 Ti 3 films. Experiment and modeling, J. ppl. Phys., vol.99, pp.124112 (1-11), 2006. 5. I. B. Vendik et al., Modeling tunable bulk acoustic wave resonators based on induced piezoelectric effect in films, J. ppl. Phys., vol.103, p. 014107, 2008. 6.. North et al., Tuning of direct current bias-induced resonances in Ba 0.3 Sr 0.7 Ti 3 micromachined thin-film capacitors, J. ppl. Phys., vol.102, p.114110, 2007. 7. L. spemyr, D. Kuylenstierna, H. Sjöland,. Vorobiev, and S. Gevorgian, 25 GHz and 28 GHz Wide Tuning Range 130 nm CMS VCs with Ferroelectric Varactors, The 2nd IEEE International Workshop on RF Integration Technology, Singapore, Dec. 2007. 70