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 µm-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 µm. 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 (±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 °C, in the response curves. Possible solutions are suggested and discussed.

Acoustic resonator based on periodically poled lithium niobate ridge.

The 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.

Acoustic wave filter based on periodically poled lithium niobate.

Solutions for the development of compact RF passive transducers as an alternative to standard surface or bulk acoustic wave devices are receiving increasing interest. This article presents results on the development of an acoustic band-pass filter based on periodically poled ferroelectric domains in lithium niobate. The fabrication of periodically poled transducers (PPTs) operating in the range of 20 to 650 MHz has been achieved on 3-in (76.2-mm) 500-µm-thick wafers. This kind of transducer is able to excite elliptical as well as longitudinal modes, yielding phase velocities of about 3800 and 6500 ms(-1), respectively. A new type of acoustic band-pass filter is proposed, based on the use of PPTs instead of the SAWs excited by classical interdigital transducers. The design and the fabrication of such a filter are presented, as well as experimental measurements of its electrical response and transfer function. The feasibility of such a PPT-based filter is thereby demonstrated and the limitations of this method are discussed.

Prediction and measurement of boundary waves at the interface between LiNbO3 and silicon.

Interface 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 niobate 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.