An elastic wave device includes a piezoelectric substrate with first and second main surfaces internally facing each other, an elastic-wave element that includes an interdigital transducer electrode provided on or in the first main surface of the piezoelectric substrate, and a first protective film that is provided on the first main surface of the piezoelectric substrate so as to cover the IDT electrode. The IDT electrode includes a main electrode layer made of a metal having a density higher than that of the first protective film. The piezoelectric substrate has a thickness of about 0.35 mm or smaller, and irregularities are located on the second main surface.

The present disclosure relates to acoustic wave devices, and particularly to high quality factor (Q) transducers for surface acoustic wave (SAW) devices. An exemplary SAW device includes an interdigital transducer (IDT) between two reflective gratings to form a resonator. The resonator operates through shear horizontal mode acoustic waves, and therefore suppression of transverse modes (parallel to electrode fingers of the IDT) is desired. A piston mode can be formed in the resonator to suppress transverse modes, which may also increase energy leakage and result in a lower Q. A higher Q is achieved by adding a fast region at an end of one or more of the electrode fingers of the IDT.

A filter (10) includes a series arm circuit (11) and a parallel arm circuit (12). The parallel arm circuit (12) includes a parallel arm resonator (p1), a variable frequency circuit (12T), and a parallel arm resonator (p2) connected in parallel with a circuit in which the parallel arm resonator (p1) and the variable frequency circuit (12T) are connected in series. The variable frequency circuit (12T) includes a parallel arm resonator (p3) and a switch (SW) connected in parallel with the parallel arm resonator (p3), and is configured to change frequencies of a pass band and a frequency of an attenuation pole by switching between an on state and off state of the switch (SW). A resonant frequency and anti-resonant frequency of the parallel arm resonator (p1) are respectively different from a resonant frequency and anti-resonant frequency of the parallel arm resonator (p2).

A filter device includes a substrate having piezoelectricity, a first filter including an IDT electrode disposed on the substrate, a terminal electrode disposed on the substrate, a first wiring electrode disposed on the substrate and connecting the first filter and a terminal electrode, and a dielectric film disposed above the substrate to cover the IDT electrode. At least a portion of the first wiring electrode is not covered with the dielectric film.

A transmission filter in a high frequency module includes serial arm resonators electrically connected in series to a serial arm electrically connecting a shared terminal and a transmission terminal, parallel arm resonators each electrically connected in series to each of parallel arms electrically connecting the serial arm and a ground, a first inductor electrically connected between the ground and a connection end electrically connecting at least the two parallel arm resonators of the parallel arm resonators, and a second inductor electrically connected between the ground and one parallel arm resonator different from the at least two parallel arm resonators of the parallel arm resonators. The second inductor is electromagnetic field coupled to at least one of an antenna side matching element, a transmission side matching element, and a portion of the serial arm in the transmission filter. The first and second inductors obstruct electromagnetic field coupling therebetween.

An acoustic wave device includes a silicon oxide film, a piezoelectric body made of lithium tantalate, and interdigital transducer electrodes stacked on a supporting substrate made of silicon, in which the values of the wave length-normalized film thickness and the Euler angle of the piezoelectric body made of lithium tantalate, the wave length-normalized film thickness of the silicon oxide film, the wave length-normalized film thickness of the interdigital transducer electrodes in terms of aluminum thickness, the propagation direction of the supporting substrate, and the wave length-normalized film thickness of the supporting substrate are set such that represented by Formula (1) for at least one of responses of first, second, and third higher-order modes is more than about 2.4, and T.sub.Si>20.

A filter device having a pass band and a stop band on a lower frequency side than the pass band includes a filter having a pass band including the pass band, a series arm resonator connected in series to the filter, a first inductor directly connected in series to the series arm resonator, and a parallel arm resonator connected between a node on a path connecting the filter and the series arm resonator and the ground. The parallel arm resonator constitutes a resonance circuit having a resonant frequency at which an attenuation pole corresponding to a high frequency end of the first stop band, and the series arm resonator and the inductor constitute a resonance circuit having an anti-resonant frequency on a lower frequency side than the pass band and having a sub-resonant frequency higher than a resonant frequency of the resonance circuit.

An acoustic wave element includes an IDT electrode including a plurality of electrode fingers and exciting a surface acoustic wave, a first substrate including an upper surface on which the IDT electrode is located, the first substrate being configured by a piezoelectric crystal, and a second substrate bonded to a side where a lower surface of the first substrate is located. Either of a first region which continues from the lower surface of the first substrate toward a side where the upper surface is located or a second region which continues from the lower surface of the first substrate toward a side where the second substrate is located is a low resistance region having a resistance value of 510.sup.3 to 510.sup.7.

A sensor system that combines the sensing application of surface acoustic wave (SAW) sensor and sensor signal transfer though the enclosure wall via acoustic means. The sensor system includes SAW sensor placed inside the enclosure and at least one pair of bulk acoustic wave (BAW) transducers, one mounted inside and second outside the enclosure wall, allowing the interrogation of SAW sensor from outside the enclosure. The external BAW transducer converts interrogation electrical pulse into acoustic pulse which travels though the enclosure wall to the internal BAW transducer. The internal BAW transducer converts the interrogation electrical pulse to electrical pulse and transfers it to SAW sensor. The response of the SAW transducer containing series of electric pulses is converted to the series of acoustic pulses by internal BAW transducer which propagates though enclosure wall. The external BAW transducer converts the series of acoustic pulses into series of electrical pulses and is received by the interrogation circuit for processing.

A filter includes a multilayer structure having films configured as bulk acoustic wave resonators; a wiring line connected to the bulk acoustic wave resonators; a cap coupled to the multilayer structure on a bonding line; and the filtering characteristics of the filter being configured through a mutual inductance between the wiring line and the bonding line.