Emilie Courjon

Fabrication of High Frequency Bulk Acoustic Wave Resonator Using Thinned Single-Crystal Lithium Niobate Layers

Bulk acoustic waves excited in thin piezoelectric films have revealed their capabilities for addressing the problem of high frequency RF filters (above 1 GHz). In this paper, we propose an alternative to thin film deposition consisting in single crystal wafers bonded on a substrate (for instance silicon or glass) and thinned, allowing for plate thickness close to 10 μ m. This has been achieved on 3 inches wafers and allows for an accurate selection of the wave characteristics. More, the properties of the piezoelectric material are found conform with tabulated values, enabling one to reliably design any passive signal processing device.

P1H-1 LiNbO3-LiNbO3 High Overtone Bulk Acoustic Resonator Exhibiting High Q.f Product

Bulk acoustic waves excited in thin piezoelectric films have revealed their capabilities for addressing the problem of high frequency RF filters and frequency sources (above 1 GHz). In this paper, we propose an alternative to thin film deposition consisting in single crystal wafers bonded on substrate (high quality) and thinned down, allowing for plate thickness close to 30mum. This has been achieved on 3 inches wafers and allows for an accurate selection of the wave characteristics. Harmonic bulk acoustic resonators have been built using this techniques, combining a thinned Y+36 LiNbO3 plate and a Y+36 LiNbO3 wafer. High quality resonances have been measured on an extended frequency range, allowing for QF products in excess of 8times1013 without any ambiguity.

Fabrication of periodically poled domains transducers on LiNbO3

The development of piezoelectric transducers based on periodically poled ferroelectrics domains is investigated. Optical quality Z-cut LiNbO3 wafers have been used for the fabrication of test devices operating in the range 7-70 MHz. The fabrication process is detailed and characterization results are reported. A very good agreement between theoretical predictions achieved by periodic finite element analysis and experiments is observed, allowing for a precise identification of the excited modes

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.

Laterally coupled narrow-band high overtone bulk wave filters using thinned single crystal lithium niobate layers

This work is devoted to the development of laterally coupled filters built on compound single crystal substrates. High overtone bulk acoustic resonators are built on LiNbO3 thinned films bonded on thick LiNbO3 or Quartz substrates. Two resonators are coupled via a narrow gap between their electrodes, yielding the possibility for their modes to interact and to produce coupled mode resonance conditions. The coupling efficiency is shown to be dependent on the frequency (wavelength), yielding well defined coupled-mode filters at intermediate frequencies (300–800 MHz) and single mode transfer functions above 1 GHz. The implementation of an oscillator stabilized by such a filter at 1.7 GHz is demonstrated.

Prediction and measurement of boundary waves at the interface between LiNbO 3 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.

Frequency sources and filters applications using high overtone bulk acoustic resonators exhibiting high Q.f product

Bulk acoustic waves excited in thin piezoelectric films have revealed their capabilities for addressing the problem of high frequency RF filters and frequency sources (above 1 GHz). In this paper, we propose an alternative to thin film deposition consisting in single crystal wafers bonded on substrate (high quality) and thinned down, allowing for plate thickness close to 30 mum. This has been achieved on 3 inches wafers and allows for an accurate selection of the wave characteristics.

A comprehensive model of the electrical response of SAW devices submitted to thermal perturbation

A comprehensive model for computing the electrical response of surface acoustic wave (SAW) devices is proposed in this paper. Based on a mixed-matrix analysis, it allows for the prediction of any SAW device admittance/impedance at any temperature. In that purpose, the temperature dependences of the wave velocity of course but also of the reflexion coefficient, directivity, conductance and capacitance are established using polynomial developments. The interest of this approach is illustrated for SAW devices on directive crystal cuts for which dramatic changes of the SAW parameters versus temperature may occur, such as the (YXlt)/48.5°/26.7° Langasite (LGS) cut. First assessment elements are shown to illustrate the importance of accurately designing SAW devices operating on extended temperature range.

A new acoustic resonator concept based on acoustic waveguides using silicon/periodically poled transducer/silicon structures for RF applications

We propose a new acoustic resonator concept based on a periodically poled transducer (PPT) in a piezoelectric substrate (LiNbO3 or LiTaO3), embedded between two guiding substrates in order to create an acoustic waveguide. A resonator operating at 131MHz have been successfully fabricated and used in order to stabilize an oscillator. However the fabricated resonator presents a significant thermal sensitivity. The following experiments have consisted in studying a Si/thinned PPT layer/Si in order to reduce the thermal sensitivity.

An acoustic waveguide using doubly-bonded silicon/thinned PPT/silicon structures for RF applications

In this paper, we present new results on the development of piezoelectric transducers based on periodically poled ferroelectric domains in a lithium niobate plate bonded between two silicon wafers. The fabrication of the periodically poled transducers operating in the range 50 – 500 MHz has been achieved on a 3 inches 500 µm thick wafer. These devices then have been bonded on silicon wafers to fabricate a waveguide. Guided elliptic as well as partially guided longitudinal modes are excited. The experimental responses of the tested devices are compared to predicted harmonic admittances, showing a good agreement between both results and allowing for a reliable analysis of the nature of the excited modes. We also show interesting studies of material combinations used to guide ultrasonic waves. Dispersion properties have also been studied for a structure Si/PPT/Si. Operating points corresponding to a specific thickness/period ratio are found. Therefore a new conception with a Si/thinned PPT/Si structure is fabricated.