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EP-3796555-B1 - TRANSDUCER STRUCTURE FOR AN ACOUSTIC WAVE DEVICE

EP3796555B1EP 3796555 B1EP3796555 B1EP 3796555B1EP-3796555-B1

Inventors

  • BALLANDRAS, SYLVAIN
  • COURJON, EMILIE
  • BERNARD, FLORENT

Dates

Publication Date
20260506
Application Date
20190918

Claims (15)

  1. A transducer structure (100, 200, 300, 408, 410) for an acoustic device comprising: a piezoelectric layer (104), a pair of inter-digitated comb electrodes (108, 110, 412, 414), comprising a plurality of electrode means (112_i, 114_j, 418, 420) with a pitch p satisfying the Bragg condition given by p= λ / 2, λ being the operating acoustic wavelength of said transducer, wherein the inter-digitated comb electrodes (108, 110, 412, 414) are embedded in the piezoelectric layer (104) and the acoustic impedance of the electrode means (112_i, 114_j, 418, 420) is less than the acoustic impedance of the piezoelectric layer (104), characterized in that the inter-digitated comb electrodes (108, 110, 412, 414) are configured to excite a shear wave mode predominantly occurring within the electrode means (112_i, 114_j, 418, 420) compared to the piezoelectric layer (104), said shear wave mode having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer (104).
  2. Transducer structure according to claim 1, wherein the aspect ratio a/p, with "a" the width and "p" the pitch of the electrode means (112_i, 114_j, 418, 420), is comprised between 0,3 and 0,75, in particular between 0,4 and 0,65.
  3. Transducer structure according to claim 1 or 2, wherein the piezoelectric layer is provided over a base substrate (106)).
  4. Transducer structure according to claim 3, wherein the thickness of the embedded electrode means (112_i, 114_j, 418, 420) is less or equal to the thickness of the piezoelectric layer (104).
  5. Transducer structure according to claim 3 or 4, wherein the thickness t e of the electrode means satisfies λ > t e > 0,1 * λ .
  6. Transducer structure according to one of claims 3 to 5, wherein the acoustic impedance of the base substrate (106) is within plus or minus 25% of the acoustic impedance of the piezoelectric layer (104).
  7. Transducer structure according to any one of the previous claims 1 to 6, further comprising a Bragg mirror (204) underneath the piezoelectric layer (104) and/or the electrode means.
  8. Transducer structure according to any one of the previous claims 1 to 7, further comprising a covering layer (302) on top of the embedded electrode means (112_i, 114_j, 418, 420) and the piezoelectric layer (104).
  9. Transducer structure according to one of claims 1 to 8, wherein the embedded electrodes are filled into pyramidal or V- or U-shaped grooves in the piezoelectric layer and/or wherein the side walls of the grooves have a convex or concave shape.
  10. Transducer structure according to one of claims 1 to 9, wherein the electrode means is made of a material that is lighter than Manganese, in particular Aluminum or an aluminum alloy comprising Cu, Si or Ti.
  11. Transducer structure according to one of claims 1 to 10, wherein the piezoelectric layer is Lithium Tantalate or Lithium Niobate.
  12. Transducer structure according to claim 11, wherein the base substrate and/or the covering layer (302) is Silica, quartz, fused quartz or glass or LiTaO 3 or LiNbO 3 .
  13. Acoustic wave device (400) comprising at least one transducer structure (100, 200, 300, 408, 410) according to anyone of the claims 1- 12, wherein the device (400) is an acoustic wave resonator, and/or an acoustic wave filter and/or an acoustic wave sensor and/or a frequency source.
  14. Method of using a transducer structure according to one of claims 1 to 13, comprising a step of applying an alternating potential to the two interdigitated electrodes configured to excite a shear wave mode predominantly occurring within the electrode means compared to the piezoelectric layer, said shear wave mode having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer.
  15. Method according to claim 14, wherein the transducer structure is part of a filter, in particular a ladder filter and/or impedance filter and/or coupling filter, or a resonator or a delay line or a sensor.

Description

The invention relates to acoustic wave devices, in particular to transducer structures for an acoustic wave device. In recent years, surface acoustic wave (SAW) devices have been employed in an increasing number of practical applications, such as filters, sensors and delay lines. In particular, SAW filters are interesting for mobile phone applications due to their ability to form low loss high order bandpass filters without employing complex electrical circuits with unprecedented compactness. Therefore, SAW filters provide significant advantages in performance and size over other filter technologies. In a typical surface acoustic wave device, one or more inter-digitated transducers (IDTs) are formed over a surface propagating substrate and are used to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of the substrate. An inter-digitated transducer (IDT) comprises opposing "electrode combs" with inter-digitated metal fingers disposed on a piezoelectric substrate. A Rayleigh surface acoustic wave develops on the substrate by electrically exciting the fingers. The other wave types, shear and longitudinally polarized wave, travel in the volume and get absorbed, thus requiring optimized metal grating thicknesses to be used for filter applications. Conversely, an electrical signal can be induced across the fingers by a surface acoustic wave propagating in the piezoelectric substrate material beneath the transducer. Examples of SAW devices are shown in US 2011/199168 A1, US 2014/203893 A1, JP H09 167935 A or DE 11 2017 005984 T5. SAW devices commonly use wafers made from a monolithic Quartz, LiNbO3 or LiTaO3 crystals as piezoelectric materials. However, the use of piezoelectric substrates leads to either high sensibility to temperature in the case of LiNbO3 or LiTaO3 or weak electromechanical coupling in the case of Quartz depending on the piezoelectric material used. Furthermore, elastic wave velocities are generally limited by the single crystal material properties, particularly considering phase velocity which remains between 3000 and 4000m/s most of the time. Indeed, in the case of Quartz, Rayleigh surface acoustic waves are the most used modes and their phase velocity ranges from 3000 to 3500 m.s-1. The use of shear waves allows for phase velocity up to 5100 m.s-1. In Quartz, coupling is limited to 0.5%. In the case of Lithium Tantalate, Rayleigh waves exhibit phase velocity in the range 3000-3500 m.s-1 but the mode coupling may reach 2%. Rayleigh waves on Lithium Niobate reach phase velocities up to 3900m.s-1 with a coupling factor of 5.6%, potentially achieving 8% with using a SiO2 passivation layer above the IDTs. Shear waves also called pseudo modes, on LiTaO3 and LiNbO3 are exhibiting radiation leakage, so called leaky modes. In that case, the surface is partially guiding the waves. Therefore, the electrode grating plays a major role in trapping the energy close to the surface. The phase velocity is in the range of 4000-4500 m.s-1 for both materials. Finally, compressional modes can also be excited on LiTaO3 and LiNbO3 substrates along certain crystal cuts but also here, the modes are leaky by nature, thus requiring specific electrode thicknesses vs frequency to minimize leakage effects due to wave radiation into the bulk. One approach to overcome the leakage effects has led to the use of composite substrates. A composite substrate comprises a piezoelectric layer formed over a base substrate. A composite substrate gives a large choice of materials for the base substrate and base substrate materials with a high acoustic wave propagation velocity can be chosen, such as Diamond, Sapphire, Silicon Carbide or Silicon. Similar to optics, the use of such a base substrate leads to the guidance of the mode. Composite substrates can combine strong electro mechanical coupling, i.e. an electromechanical coefficient ks2 larger than 1%, and temperature stability, i.e. a temperature coefficient of frequency (TCF) smaller than 20ppm/K, and can improve the performance of the SAW devices and provide flexibility of design. However, acoustic wave devices are limited to operating frequencies from about 1 to 3 GHz, as for the given phase velocities, the electrode pitch p of the comb electrodes determines the wavelength λ of the acoustic wave given by the relation p=λ/n, with n ≧ 2, generally equal to 2. Operation at frequency above 2 GHz requires metal dimension and thickness of the order or below 100nm, which present stability problems of the structure. Thus, in practice, it is difficult to further miniaturize comb electrodes when higher operating frequencies are required. This is on the one hand due to the need to use higher resolution lithography technology compared to the I-line lithography used today in SAW industry, and on the other hand due to electric losses occurring in the structure. Therefore, to create SAW devices above 3 GHz, a strong technological effort is requir