Andreas Nöth
Thin Film Bulk Acoustic Wave Resonators (TFBARs) had been developed a decade ago and since then were implemented extensively in mobile communications devices. The "heart" of a TFBAR consists of a piezoelectric film that operates as an acousto-electric transducer, stabilizing the transmission at a given predetermined frequency. For reasons such as space economy in hand-held devices, it is of interest to make these TFBARs tunable, so that a single TFBAR is multi-band responsive. This thesis demonstrates for the first time electrically tunable, single-component TFBARs. A theory describing the tuning behavior of dc bias induced acoustic resonances was developed. Then we made the hypothesis that dc bias induced piezoelectric BaxSr1-xTiO3 (BST) thin films – namely, paraelectric, non-piezoelectric films operating under dc bias – can be used to make electrically tunable TFBARs and that the devices can be switched on or off depending on the dc bias state. The devices were then fabricated: We integrated BST lms onto silicon substrates, micromachined the substrate to create the TFBARs and a new type of suspended planar capacitor which were then characterized, analyzed, and modeled, demonstrating successfully the new concept: We developed a theory describing the electrical tuning behavior of the dc bias induced acoustic resonances in paraelectric thin lms in terms of material parameters. The field dependent constitutive piezoelectric equations were derived from the Landau free energy P-expansion by taking the linear and nonlinear electrostrictive terms as well as the background permittivity into account. We considered two modes of excitation for the tuning of the acoustic resonances, namely the thickness excitation (TE) mode and the lateral field excitation (LFE) mode. The tuning behavior of the two types of resonators based on BST thin films was modeled and discussed. For the modeling we calculated the relevant tensor components controlling the tuning of the BST resonators from the available literature data. The fabrication of the membrane-type TFBARs was realized by integrating BST thin films onto silicon substrates and using micromaching technologies. We showed that the developed TFBARs can be switched on or off with a dc bias. At a dc electric field of 615 kV/cm we observed a tuning of -2.4% (-66 MHz) and -0.6% (-16 MHz) for the resonance and antiresonance frequencies of the device, while the resonance frequency at a dc electric field extrapolated to 0 kV/cm was 2.85 GHz. The effective electromechanical coupling factor k2eff of the device increased up to 4.4%. The tuning was non-hysteretic. The Quality-factor (Q-factor) of the device was about 200. The developed micromaching processes for the TFBARs were used to fabricate coplanar BST capacitors on silicon. Micromaching was used to remove the Si substrate under the active area of the device. Comparing this new micromachined coplanar capacitor with conventional non-micromachined capacitors, we demonstrated that the removal of the substrate from the active device area resulted in a reduction of parasitic effects. The micromachined coplanar capacitor showed an increased tunability and a reduced loss tangent in comparison to the non-micromachined capacitor. The micromachined capacitor showed a relative tunability of 37% at a dc electric field of 1100 kV/cm. The loss tangent was 0.08 and 0.06 at zero and maximum dc bias, respectively, at a measurement frequency of 20 GHz. The integration of epitaxial BST thin films on silicon was studied as well because of its potential improvement of the device performance in comparison to devices based on polycrystalline films. Two different electrode/buffer layer systems, YBa2Cu3O7-x (YBCO) / CeO2 / YxZr1-xO2-x/2 (YSZ) and TiN were used for the integration. Epitaxial BST thin films were grown with both structures. For the BST / YBCO / CeO2 / YSZ / Si structure, the BST unit cell was rotated by 45° in the in-plane dimension with respect to the substrate. The BST layer exhibited good structural quality as indicated by a Full Width at Half Maximum (FWHM) of 0.5° of the rocking curve of the BST(002) diffraction peak. For the BST / TiN / Si structure, the BST layer was grown with a cube-on-cube epitaxial relationship on the silicon substrate, thus demonstrating epitaxially grown BST on silicon using conventional bottom electrode that is easily acceptable by the microelectronic industry. However, in this case, the structural quality of the BST layer was reduced in comparison to the BST / YBCO / CeO2 / YSZ / Si structure. The FWHM of the rocking curve of the BST(002) diffraction peak was 2.2°. We established the temperature (T=550 to 600 °C) and pressure (p ≈ 10-7 to 5 × 10-4 Torr) conditions for the growth of epitaxial BST thin films on TiN-buffered Si. At too high temperatures and/or oxygen pressures epitaxial BST thin film growth was impeded due to the oxidation of the TiN layer. By introducing experimental results from the electrical characterization of the micromachined devices with the polycrystalline BST into our developed theory, we could successfully model the tuning behavior of our fabricated TFBARs. The modeling allowed us to de-embed the intrinsic electromechanical properties of a freestanding BST layer. The effect of increasing mechanical load on the tuning performance of the device was modeled and studied experimentally. Under strong mechanical load, the tuning of both resonance and antiresonance frequency was reduced. The effect was attributed to a reduction in the tuning of k2eff of the device and of the sound velocity of the BST layer.
EPFL2009
