LiTaO3/Silicon Composite Wafers for the Fabrication of Low Loss Low TCF High Coupling Resonators for Filter Applications

International audienceSAW devices are widely used for radio-frequency (RF) telecommunication filtering and the number of SAW filters, resonators or duplexers is still increasing in RF stage of cellular phones. Therefore, a strong effort is still dedicated to reduce as much as possible their sensitivity to environmental parameter and more specifically to temperature. Bounding processes have been developed at FEMTO-ST and CEA-LETI using either Au/Au or direct bonding techniques for the fabrication of composite wafers combining materials with very different thermoelastic properties, yielding innovative solutions for about-zero temperature coefficient of frequency (TCF) bulk acoustic wave devices. In the present work, this approach has been applied to (YXl)/42∘ lithium tantalate plates, bounded onto (100) silicon wafers and thinned down to 25μm. The leading idea already explored by other groups as mentioned in introduction consists in impeding the thermal expansion of the piezoelectric material using silicon limited expansion. 2GHz resonators have been built on such plates and tested electrically and thermally, first by tip probing. A dramatic reduction of the TCF is observed for all the tested devices, allowing to reduce the thermal drift of the resonators down to a few ppm.K-1 within the standard temperature range. We then propose an analysis of the frequency-temperature behavior of the device to improve the resonator design to use these wafers for industrial applications

Fabrication of a 50 MHz annular array transducer for opthtalmology and development of its driving electronics

National audienceModern ophthalmologic transducers consist in one channel pre-focused transducers operating at frequencies ranging from 25 to 50 MHz. This notably reduces the degree of freedom of physicians to accurately observed various regions of the ocular cavity. We develop then an annular array operating at 50 MHz for high resolution dynamic and its driving electronics. At first, we present the technological fabrication of a 6 channel annular array built on a piezopelectric ceramic to address that demand. The devices are batch-processed on a 2 wafer in order to obtain several probes on a single substrate. The rings are connected by means of electroformed pads and each channel is separated using a femto-laser ablation process. This innovative approach allows for an efficient addressing of each channel considering requirements concerning the final packaging of the probe. Several probes have been fabricated and tested. Although this approach is not completely new, we show that very thin kerfs can be achieved, compatible with the fabrication of the whole probe. A dedicated electronics has been developed to control the probes. The basic principle of the electronics consists in using a micro-controler ADuC7026 which mainly sequences the whole system operation. It drives first a Direct Digital Synthesis (DDS AD9954) circuit to create a high frequency clock. An Erasable Programmable Logic Device (EPLD) is then cadenced using this clock to build the transducer driving signal, consisting in a simple period voltage alternation. Once this signal generated, it enters the amplifier stage consisting in generating high voltage signal (80 Vp-p) in order to control each annular ring (6 channels). A switch is used to isolate the emitting and the receiving parts of the system. The devices and the driving electronics have been successfully fabricated and tested. As expected, their operating frequency of the probe is 50 MHz. We have also used a axi-symmetric finite element analysis to simulate the implemented devices and a comparison between the theoretical and experimental admittance measurements are used to optimize the probe shape

Improvement of textured AlCu with Ta underlayer on LiNbO3 substrate

International audienceImproving the lifetime of Surface Acoustic Wave filters requiresreduction of electromigration effects. We report on the growthmechanism of AlCu2% film deposited by E-Beam Physical VaporDeposition technique. State of the art shows that the introductionof an underlayer that alters the surface free energy can change afilm's growth mode or improve the earliest stages of filmformation. In this work, this will be achieved by using anultra-thin (&lt;5nm) Ta underlayer between the LiNbO3 substrate andthe AlCu2% film. The effects of Ta underlayer on the microstructureof AlCu2%/LiNbO3 films is investigated. An increase in the surfaceenergy of the LiNbO3 substrate is observed with the addition of1.3nm thick Ta underlayer. Crystalline quality of AlCu2% thin filmwith a Ta underlayer is measured with XRD techniques. Apost-deposition heat processing is also used to recrystallize thefilms. The AlCu2%/1.3Ta/LiNbO3 film annealed at 250°C improvedthe<br /&gtelectrode quality with the increasing of 16% of grain size

Prediction and Measurement of Boundary Waves at the Interface Between LiNbO3 and Silicon

International audienceInterface acoustic waves (IAWs) propagate along the boundary between two perfectly bonded solids. For a leakage-free IAW, all displacement fields must be evanescent along the normal to the boundary inside both solids, but leaky IAWs may also exist depending on the selected combination of materials. When at least one of the bonded solids is a piezoelectric material, the IAW can be excited by an interdigital transducer (IDT) located at the interface, provided one can fabricate the transducer and access the electrical contacts. We discuss here the fabrication and characterization of IAW resonators made by indirect bonding of lithium niobate onto silicon via an organic layer. In our fabrication process, IDTs are first patterned over the surface of a Y-cut lithium niobate wafer. A thin layer of SU-8 photo-resist is then spun over the IDTs and lithium ni-obate to a thickness below one micrometer. The SU-8-covered lithium niobate wafer then is bonded to a silicon wafer. The stack is subsequently cured and baked to enhance the acoustic properties of the interfacial resist. Measurements of resonators are presented, emphasizing the dependence of propagation losses on the resist properties. Comparison with theoretical computations based on periodic finite element/boundary element analysis allows for explanation of the actual operation of the device

Prediction and measurement of boundary waves at the interface between LiNbO3 and Silicon

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On the Modeling of Electrical Response of SAW Resonator-based Sensors Versus Temperature

International audienceSurface acoustic wave (SAW) resonators built on Langasite (LGS) are capable to withstand temperature in excess of 900∘ C and demonstration of wireless interrogation of packaged sensors up to 700∘ C has been achieved for several tens of hours. These promising results emphasize the need for an accurate characterization of the raw material in order to design SAW resonators with a high level of confidence in the prediction, particularly concerning the temperature coefficient of frequency (TCF). Several data set have been published for LGS, offering prediction capabilities but also a significant level of data dispersion. Therefore, the evaluation of the effective thermal properties of SAW under periodic gratings turns out less robust than expected. Based also on published data and on measurements achieved within the SAWHOT project, harmonic admittance calculations have been achieved for deriving the evolution of mixed matrix parameters allowing for accurate SAW device simulation at any temperature. Adjusting the temperature coefficients then yield improved sets of material coefficients for design purpose. Using these data, we have demonstrated the possibility to develop a differential temperature sensor operating at temperature up to 600°C

A LN/Si-Based SAW Pressure Sensor

Surface Acoustic Wave (SAW) sensors are small, passive and wireless devices. We present here the latest results obtained in a project aimed at developing a SAW-based implantable pressure sensor, equipped with a well-defined, 30 &mu;m-thick, 4.7 mm-in-diameter, Lithium Niobate (LN) membrane. A novel fabrication process was used to solve the issue of accurate membrane etching in LN. LN/Si wafers were fabricated first, using wafer-bonding techniques. Grinding/polishing operations followed, to reduce the LN thickness to 30 &mu;m. 2.45 GHz SAW Reflective Delay-Lines (R-DL) were then deposited on LN, using a combination of e-beam and optical lithography. The R-DL was designed in such a way as to allow for easy temperature compensation. Eventually, the membranes were etched in Si. A dedicated set-up was implemented, to characterize the sensors versus pressure and temperature. The achieved pressure accuracy is satisfactory (&plusmn;0.56 mbar). However, discontinuities in the response curve and residual temperature sensitivity were observed. Further experiments, modeling and simulations were used to analyze the observed phenomena. They were shown to arise essentially from the presence of growing thermo-mechanical strain and stress fields, generated in the bimorph-like LN/Si structure, when the temperature changes. In particular, buckling effects explain the discontinuities, observed around 43 &deg;C, in the response curves. Possible solutions are suggested and discussed

Acoustic resonator based on periodically poled lithium niobate ridge

International audienceThe constant improvement of industrial needs to face modern telecommunication challenges leads to the development of novel transducer principles as alternatives to SAW and BAW solutions. The main technological limits of SAW (short-circuit between electrodes) and BAW (precise thickness control) solutions can be overcome by a new kind of transducer based on periodically poled ferroelectric substrate. The approach proposed in this paper exploits a ridge structure combined with a periodically poled transducer (PPT), allowing for the excitation of highly coupled modes unlike previously published results on planar PPTs. High-aspect-ratio ridges showing micrometer dimensions are achieved by dicing PPT plates with a diamond-tipped saw. An adapted metallization is achieved to excite acoustic modes exhibiting electromechanical coupling in excess of 15% with phase velocities up to 10 000 m*s-1. Theoretical predictions show that these figures may reach values up to 20% and 18 000 m*s-1, respectively, using an appropriate design