Micromachined tunable devices based on silicon integrated BaxSr1-xTiO3 thin films

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.

Piezoelectric thin films for bulk acoustic wave resonator applications

Roman Lanz

Bandpass filters for microwave frequencies realized with thin film bulk acoustic wave resonators (FBAR) are a promising alternative to current dielectric or surface acoustic wave filters for use in mobile telecommunication applications. With equivalent performance, FBAR filters are significantly smaller than dielectric filters and allow for a larger power operation than SAW filters. In addition, FBARs offer the possibility of on-chip integration, which will result in substantial volume and cost reduction. The first passive FBAR devices are now appearing on the market. They mainly cover needs in miniaturized RF-filters for the new bands around 2 GHz. A FBAR is essentially a thin piezoelectric plate sandwiched between two electrodes and acoustically isolated from the environment for energy trapping purposes. If the isolation is effectuated by an acoustic Bragg reflector, one speaks of solidly mounted resonators (SMR). Piezoelectric aluminum nitride (AlN) thin films are predominantly used in the emerging FBAR technology because AlN exhibits a sufficient electromechanical coupling coefficient kt2 , low acoustic losses at microwave frequencies, a low temperature coefficient of frequency, and its chemical composition is compatible with CMOS requirements. This thesis has two research directions. In the first part, FBAR structures based on AlN thin films were investigated for applications at X-band frequencies (7.2-8.5 GHz), i.e. operating at much higher frequencies than the ones used for present products. The goal was to identify property limitations related to such high frequencies, and to demonstrate to industry high performing SMR filters at 8 GHz. In the second part, a new material for FBAR devices was studied. The motivation is that AlN allows for a restricted filter bandwidth only, limited by its coupling factor of maximal 7%. Monocrystalline KNbO3 appears as an ideal alternative with its high coupling factor kt2 of 47%, and relatively large sound velocity of 8125m/s for longitudinal waves along the [101] direction. Piezoelectricity of KNbO3 films grown on electrodes has never been characterized. Single crystal results indicate that the optimal film texture would be (101). In this thesis, the growth of KNbO3 films on Pt electrodes was studied with the goal to achieve this texture uniformly, and to characterize piezoelectric properties. X-band FBAR's were first studied with numerical simulations based on a one-dimensional theory of the thickness-extensional bulk acoustic wave (BAW). Thickness, acoustic properties and electrical conductivity of the electrodes have a large impact on the resonator characteristics. There are conflicting requirements with respect to optimum acoustic and electrical properties of the electrode materials. An optimum thickness was calculated for 8GHz FBARs that use Pt bottom and Al top electrodes. The characteristics of ladder filters have been calculated based on the impedances of single resonators. The adjustable filter parameters, i.e. the areas of series and shunt resonators, frequency de-tuning between series and parallel resonators, and number of π-sections were screened for a process window offering maximum filter bandwidth with lowest ripple and low insertion loss for a given out-of-band rejection. An important result of the numerical simulations was that the bandwidth of ladder filters can be doubled by de-tuning the series and parallel resonators by more (1.3 times) than the difference of resonance and anti-resonance frequency. This also leads to a flatter passband while keeping the ripples below ±0.2dB. Solidly mounted resonators and filters were fabricated using an acoustic multilayer reflector consisting of AlN and SiO2 λ/4 layers. All films were sputter deposited in a high vacuum sputter cluster system with 4 process chambers. The films were patterned using standard photolithography and dry etching processes. The SMR exhibited a strong and spurious-free resonance at 8GHz with a high quality factor of 360 and electromechanical coupling coefficient of 6.0%. The temperature coefficient of frequency was -18ppm/K, and the voltage coefficient of frequency was -72ppm/V. Passband ladder filters with T- and π-topology consisting of 3 to 14 SMR were successfully demonstrated with a center frequency of 8GHz. These filters were optimized for maximum bandwidth and exhibited an insertion loss of 5.5dB, a rejection of 32dB, a 0.2dB bandwidth of 99MHz (1.3%), and a 3dB bandwidth of 224MHz (2.9%). There was good correspondence between measured and simulated filter and resonator characteristics. For perfect agreement, parasitic elements needed to be taken into account. These were a series resistance of 5Ω and a parallel conductance of 2mS in case of single resonators. The series resistance can be explained with resistive losses in the electrodes, whereas the parallel conduction was due to conduction along the surface. For π-filters, an additional series inductance of 100pH was needed to obtain a satisfactory fit. This inductance increased the out-of band rejection and insertion loss. Besides the group delay variation, all industrial specifications were met. KNbO3 was in-situ sputter deposited at 500 to 600°C using a rf magnetron source. A dedicated sputter chamber with load-lock and oxygen resistant substrate heater was built for this purpose. The high volatility of potassium oxide requires a potassium enrichment of the target. Targets with several excess concentrations (in the form of K2CO3) were studied. Stoichiometric KNbO3 films were obtained with targets containing 25 and 40% excess K. Zero and 10% excess yielded K deficient films, whereas 100% and 200% excess K led to highly unstable targets with K accumulation on the target surface, resulting in K rich second phases. The potassium-to-niobium ratio in the films depends strongly on sputter pressure and substrate temperature. Dense films, nucleated with cubic {100} texture, were obtained on platinized silicon substrates with a 10nm thick IrO2 seed layer at substrate temperatures of 520°C. At lower temperatures the films were amorphous, and at higher temperatures the films were composed of individual and facetted KNbO3 grains. The cubic high-temperature {100} texture results in a mixed (101)/(010) texture in the orthorhombic room temperature phase. The measured relative permitivity of 420 indicates that both orientations are equally present. Micro-Raman confirms the orthorhombic line splitting. Piezoelectrical and ferroelectrical activity were verified by means of a piezoelectric sensitive atomic force microscope. A very large piezoelectric activity was observed on some of the grains, and the polarization could be switched on most of the grains. However, the average d33,f = e33/c33, as measured by means of laser interferometry, showed a modest value of 24pm/V. The effective coupling factor is derived as kt2=2.8%, which is small relative to the theoretical value of 47%. The high dielectric constant and the absence of piezoelectric activity along the [010] direction are responsible for the reduction of the kt2 factor. Film roughness, complexity of deposition process and open poling issue make KNbO3 integration into BAW devices a difficult task.

EPFL2004

Polarization response of Perovskite films for microwave tunable applications

Konstantin Astafiev

Ferroelectric thin films are widely studied nowadays as potential replacements for semiconductors in modern tunable microwave devices such as tunable filters, phase-shifters, frequency mixers, power dividers, etc. Recent progress in the deposition of complex oxide films shows promise of significant improvements in the quality of ferroelectric thin films from the point of view of dielectric loss and electrical tunability. However, there still exist certain problems which limit a wide penetration of ferroelectric thin films into the market. The most crucial issue which needs further investigation is the difference in polarization response between ferroelectric thin films and bulk materials. In comparison to bulk materials, thin films show lower dielectric constants, and consequently a smaller electrical tunability, and much higher dielectric losses at microwave frequencies. Another important question which has rarely been investigated is the problem of the reliability of tunable devices based on ferroelectric films. Therefore, a systematic, experimental investigation to understand the physics behind these problems is a task of primary importance. The present work is devoted to the detailed theoretical and experimental investigation of the polarization response in SrTiO3 and KTaO3 thin films at radio and microwave frequencies over a wide range of temperatures and applied bias fields. Specifically, the following has been performed: The polarization response of SrTiO3 and KTaO3 ferroelectric thin films has been investigated at radio frequencies over a wide temperature range using a planar capacitor structure. The sensitivity of the dielectric properties to the deposition technique, conditions of films growth and type of the substrate used, and hence to the microstructure of the films, is clearly demonstrated. While SrTiO3 is paraelectric in bulk form, the appearance of a ferroelectric phase in SrTiO3 thin films on LaAlO3 substrate has been experimentally documented, with a transition temperature lying in the range of 90-120K. It is theoretically shown in terms of Landau theory, that the behavior of the SrTiO3 thin films can not be attributed to the effect of the film/substrate mechanical coupling. The ferroelectricity of SrTiO3 thin films on LaAlO3 substrates has been investigated in detail. The hysteresis loops have been monitored in a wide temperature range using thin film planar capacitors, the driving field being predominantly in the plane of the film. The microwave dielectric loss in perovskite ferroelectric materials has been theoretically investigated. A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 materials at microwave frequencies has been developed. Experimental investigations of the dielectric loss at microwave frequencies in SrTiO3 and KTaO3 thin films have been performed using a half-wave length microstrip resonator with a resonant frequency of around 8GHz. The field and temperature dependences of the loss tangent for the SrTiO3 and KTaO3 thin-film based planar capacitors have been measured. The impact of electrical stressing on the microwave performance of SrTiO3 films deposited onto different substrates (MgO and LaAlO3) has been investigated. The most important characteristics of tunable microwave components, including tunability and loss tangent, have been monitored. The influence of the addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of electrostatic considerations. Calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed using three different models. On the basis of the results obtained, a figure of merit for the composite material has been evaluated.

EPFL2004

High coupling materials for thin film bulk acoustic wave resonators

Janine Conde

Radio frequency (RF) filters based on bulk acoustic wave resonances in piezoelectric thin films have become indispensable components in mobile communications. The currently used material, AlN, exhibits many excellent properties for this purpose. However, its bandwidth is often a limiting factor. In addition, no tuning is possible with AlN. Ferroelectrics would offer both larger coupling to achieve larger bandwidths, and tunability. However, their acoustic properties are not well known, especially in the thin film case. The goal of this thesis is to investigate the potential and identify the limitations of ferroelectric thin films for thickness mode resonators in the 0.5 - 2 GHz range. The Pb(Zrx,Ti1-x)O3 (PZT) solid solution system is the main candidate, since it is known for its large piezoelectric constants and its growth is already well studied. As a main test vehicle, free standing thin film bulk acoustic resonator (TFBAR) structures with Pt/PZT/Pt/SiO2 membranes were successfully fabricated using silicon micro-machining techniques. The main drawback of ferroelectrics is the damping of acoustic waves by domain wall motion both in the RF electric field and in the pressure wave. For this reason films with varying orientations and compositions were investigated. From the device structures the electro-mechanical coupling constants kt2, the quality factors (Q-factors) and several materials parameters have been obtained. High coupling constants have been found for sol-gel Pb(Zr0.53,Ti0.47)O3 films with a {100} texture, kt2 is found to be 0.4 for a 1 µm thick film and 0.8 for a 3.8 µm thick film. However, the Q-factors of these films are low, 18 for the first film and 3 for the second film. The increase of kt2 and the decrease of the Q-factor with frequency indicates that the domains present in these films contribute to these characteristic parameters. It was generally observed that high coupling constant are associated to low Q-factors. This became evident when comparing films with 53/47 composition, where both tetragonal and rhombohedral phases are present, to tetragonal films as well as when comparing {100} textures with (111) textures. Both for the 53/47 composition and for the {100} texture, ferroelastic domain walls are thought to play a bigger role than for tetragonal compositions and (111) textures. The highest figure of merit (FOM) of about 15 was found when combining the composition leading to a high coupling constant (53/47) and the orientation leading to lower losses (111). However the losses even in this film are too high for RF-filter applications. On the other hand, films with low Q-factors but high coupling could prove very useful as transducers for ultrasonic imaging applications, where low Q-factors are desired. The stiffness coefficients of the studied PZT films were shown to be higher than expected from ceramics data. Most likely the stiffness of ceramics always contains domain contributions leading to softening. In contrast, in textured films the variety of domain orientation is very much reduced. In order to reduce losses due the presence of ferroelastic domains three different potential solutions were explored. The first idea was to manipulate the domain populations of the films deposited on silicon by using heat and vacuum treatments. Silicon substrates are known from previous works to be unfavourable for high c-domain fractions. It was discovered that an anneal in vacuum at 550 °C lead to a significant reduction of c-domains in tetragonal 30/70 PZT ({100}). On the contrary, if the sample was subjected to a compressive stress during cooling, the c-domain fraction could be increased. Analysis of the film stress versus temperature curves revealed a trend consistent with theoretical predictions, i.e. a phase boundary between the c/a/c/a and the a1/a2/a1/a2 domain patterns between room temperature and the Curie temperature θC. However, even though this method reveals interesting results, it can not be exploited as a method to achieve a sufficient c-domain population. The second idea explored was the implementation of a high thermal expansion material as a substrate. PZT films deposited on MgO are known to be compressive due to the difference in thermal expansion of the two materials. The compressive stress leads to highly c-axis oriented PZT films. Devices using MgO substrates were fabricated, however difficulties in the micro-machining of the MgO substrate inhibited a complete liberation of the membrane. Nevertheless, preliminary measurements indicate these devices could lead to both high coupling and high Q-factors, suggesting that further detailed study of this method is worthwhile. As a third method for avoiding ferroelastic domains, the uniaxial ferroelectric potassium lithium niobate (KLN) was explored. The unique ferroelectric axis in this material means that only 180° domain walls are present, which can theoretically be removed by poling. This material has been deposited in thin film form using pulsed laser deposition (PLD). KLN thin films with a {001} texture were deposited successfully on Pt/Si substrates. The films were piezoelectric with a d33,f value of around 10 pm/V and a dielectric constant of 250. This is the first time that piezoelectric properties were measured on KLN thin films. A columnar structure has been observed, however the small grain size and the rough surface currently make it difficult to apply this material to TFBAR's.

EPFL2009

High coupling materials for thin film bulk acoustic wave resonators

Janine Conde

EPFL2009

Polarization response of Perovskite films for microwave tunable applications

Konstantin Astafiev

EPFL2004

Piezoelectric thin films for bulk acoustic wave resonator applications

Roman Lanz

EPFL2004