* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download „Thin Film Electroacoustic Devices“
Survey
Document related concepts
Terahertz metamaterial wikipedia , lookup
Negative-index metamaterial wikipedia , lookup
Metamaterial wikipedia , lookup
Metamaterial cloaking wikipedia , lookup
Semiconductor device wikipedia , lookup
Metamaterial antenna wikipedia , lookup
Multiferroics wikipedia , lookup
Nanogenerator wikipedia , lookup
History of metamaterials wikipedia , lookup
Piezoelectricity wikipedia , lookup
Tunable metamaterial wikipedia , lookup
Acoustic tweezers wikipedia , lookup
Transcript
„Thin Film Electroacoustic Devices“ Klausurtagung in Pappenheim, 16. – 19. Feb. 2004, Vortrag von Matthias Bickermann the only means to extract energy out of the medium is dielectric resonance. 1. Introduction Mobile communication applications have boosted the development of frequency control devices in the GHz range. Basic passive devices in this area are SAW and BAW devices used as narrow band pass filters for frequency selection. In those devices, acoustic waves are excited by an electrical field, and only waves resonant to the device are sensed. There are two major concepts to increase the filter frequency: To further reduce device structure or to employ novel thin-film materials and acoustic wave modes for higher wave velocities. Both concepts rely on advances in nanotechnology. 2. Basic Principles In a piezoelectric medium, the stress T and the electric displacement D depend linearly on both the strain S and the electric field E as: (1) Tij = cijkl Skl + ekij Ek (2) Dj = eijk Sjk + εij Ej Devices as shown in Fig. 1a are used as narrow band pass filters. If supplied with a frequency mix, only the resonant frequency will pass. The resonance is given by the acoustic wave velocity v and the thickness of the piezoelectric material h. Very homogeneous thickness and high material quality (single-crystalline material or textured needles) are required to obtain good electromechanical coupling kh defined as (4) kh = K2 / (1+K2) The acoustic waves traveling through the device are referred to as Bulk Acoustic Waves (BAW). Novel Thin Film Bulk Acoustic Resonators (FBAR) can also be integrated on standard IC technologies. A pioneer of discrete FBAR devices is Agilent Technologies, but also Epcos in Germany has started mass production of those devices recently. h where εij, ekij and cijkl are the permittivity, piezoelectric, and stiffness tensors, respectively. In the static case these equations describe mechanical displacement in the material as a result of application of a static electrical field. Applying an electromagnetic wave to the material, however, results in the formation of an acoustic wave having a phase velocity of V in z a) electrodes, V y piezoelectric film in h (3) v= ( c 1+ K2 ρ ) with K = e2 ε cE where K2 is called piezoelectric coupling constant (ρ is the material density). Eq. 3 holds also for any non-piezoelectric material, but K = 0 in this case. Note that in piezoelectric materials, v depends on both material tensors and the direction of propagation of the acoustic wave. In the following we consider two types of excitations by which acoustic waves can be generated. 3. Piezoacoustic excitation a) Thickness excitation If an externally generated electrical field is applied to a device shown in Fig. 1a, the acoustic wave and the electric field both will propagate in zdirection. As the electromagnetic wave travels much faster (about 108 m/s) than the acoustic wave (103...104 m/s), the piezoelectrically coupled electric field is assumed to be quasistatic. Hence the electric displacement vector Dz vanishes, and b) d Fig. 1. a) device operating in thickness excitation mode. b) device operating in lateral field excitation mode. b) Lateral field excitation In this excitation mode, the electric field is in the plane perpendicular to acoustic propagation as shown in Fig. 1b. An electric field can excite an acoustic wave normal to its direction because the piezoelectric matrix e couples electrical and mechanical fields of different orientations. The internally generated electric displacement vector D is parallel to the externally generated electric field vector E, but propagates at acoustic velocity. Lateral field excitation can only exist if the distance between the electrodes d is much larger than the thickness of the piezoelectric material h. The resulting wave is called Surface Acoustic Wave (SAW). The efficiency of such devices is characterized by the piezoelectric coupling constant K2 as defined in Eq. (3). Note that SAW stands for a whole zoo of acoustic wave modes which can be generated by lateral field excitation. Apart from the "true SAW" Rayleigh mode described above, higher-order modes (Sezawa modes) and pseudo-SAWs (bulk modes which are confined near the surface) can occur. Their velocities often exceed the velocity of the Rayleigh mode wave. Excitation of such modes has been extensively investigated in recent years. is lower than 2λ, the wave penetrates into the substrate and dispersion rules apply. 4. Thin-film SAW device designs Fig. 2 shows a sample SAW filter design. The SAW is generated and sensed by interdigital transducers (IDTs) made of a thin structured Al film. The IDTs consist of multiple pairs of fingers. This arrangement favors the propagation of a wave resonant to the structure and orientation of the IDT electrodes. The distance b between two fingers of the same electrode determines the SAW wavelength λ, which in turn defines the center frequency f of the band pass filter as: (5) f=v/λ=v/b Piezoelectric materials and their acoustic wave velocities are summarized in Tab. 1. A device with b = 4 µm (i.e. lithographic structures with 1 µm resolution) made on LiNbO3 has its center frequency at 1 GHz. To go towards higher frequencies, one could either employ a sub-micron lithography or choose a material with higher acoustic wave velocity like e.g. AlN or ZnO. As AlN or ZnO substrates are not available up to now, thin films are widely used. The films are deposited mostly by magnetron sputtering (which is a low-cost technique, but the material quality is poor) or by expensive CVD or MBE techniques. New SAW device designs are proposed and studied extensively in order to further increase the center frequency at a given photolithographic resolution. These designs are based on thin-film layers deposited on substrates with high acoustic velocity. While AlN is the piezoelectric material with highest SAW velocity, diamond and SiC exceed In such devices, the acoustic waves transform and generally become near-surface confined waves with higher acoustic velocity. The "true SAW" wave is confined within a surface layer of about 2λ in thickness; if the thickness h of the film Fig. 2. Schematic diagram of a SAW filter device based on a IDT/AlN/nanocrystalline diamond/Si structure. As also shown in Tab.1, diamond and SiC both exhibit higher acoustic velocities than AlN, which is the piezoelectric material with highest v known by now. These properties are used for novel SAW devices of AlN and ZnO thin films on SIC substrates or diamond films. Fig. 3 shows the basic design of such a device; Fig. 4 shows the dispersion curves of the AlN/diamond structure in regard to phase velocity v and electromechanical coupling coefficient K2. Both values can be increased by over 100% compared to "conventional" AlN devices. Thin-film growth technology and further applications are addressed in the oral talk sheets. Tab. 1: Non-metal materials and cuts with high acoustic velocities. A star (*) denotes non-piezoelectric materials. Material Diamond* SiC* AlN ZnO Sapphire* Al0.53Ga0.47N GaN LiTaO3 LiNbO3 LiTaO3 Quartz Si Orientation (111) (002) (002) (002) (110) (002) (002) 128° YX YZ 77° YZ ST-X (111) Velocity [m/s] 10,200 6,832 5,790 5,700 5,548 5.060 3.693 3.992 3.488 3.254 3.158 2.650 Literature: G.F.Iriarte, AlN Thin Film Electroacoustic Devices. Comprehensive summaries of Uppsala Dissertations from the Faculty of Science and Technology 817 (2003). ISBN 91-5545557-3. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3359. Fig. 3. Dispersion curves of a AlN/diamond thin-film SAW structure. left: Phase velocity v (as calculated and measured), right: coupling coefficient K2 (as calculated). "Mode 0" denotes the Rayleigh-like wave mode, "Mode 1" and "Mode 2" are higher-order Sezawa wave modes.