Patent Grant
9450563
This application is a National Stage of International patent application PCT/EP2011/067728, filed on Oct. 11, 2011, which claims priority to foreign French patent application No. FR 1058404, filed on Oct. 15, 2010, the disclosures of which are incorporated by reference in their entirety.
The field of the invention is that of electromechanical devices using acoustic wave propagation through piezoelectric or electrostrictive layers to create electrical filter functions and, in addition, ensure electrical isolation between the input and output of the component.
Such devices have operating frequencies of the order of a few hundred MHz to a few GHz, and are used in radiofrequency transmission circuits (mobile phone, radio link, wireless exchange of data, etc.) signal processing or in sensor systems.
More specifically, the field of the present invention relates to electrical isolation, the input and output of a component with a view mainly to achieving an impedance and/or mode conversion in acoustic filters, therefore filters using acoustic resonators, mainly bulk wave (Bulk Acoustic Wave or BAW) resonators. These filters are located in the radiofrequency stage of audio transmission systems, notably in mobile phone systems.
To ensure the connection between the part of the chain referenced to ground and the differential part, it is possible to use an element external to the filter, inserted between the filter and amplifier or between the antenna and filter, called a balun (for Balanced/Unbalanced). Baluns are systems which take up a lot of space and introduce losses. Baluns can indeed be built using transmission line sections which should have centimeter dimensions at the currently used frequencies for mobile telephony (on the order of a few GHz), or more generally using magnetic windings similar to transformers, therefore requiring large areas (a few mm2), and having non-negligible resistive losses.
CRF (Coupled Resonator Filter) filters, acoustically coupled filters, have already been proposed, enabling the mode conversion to be made G. G. Fattinger, J. Kaitila, R. Aigner and W. Nessler, Single-to-balanced Filters for Mobile Phones using Coupled Resonator BAW Technology, 2004 IEEE Ultrasonics Symposium, pp. 416-419, because by construction the input and output of the filter are electrically insulated: R. Thalhammer, M. M. Handtmann, J. Kaitila, W. Nessler, L. Elbrecht, Apparatus with acoustically coupled BAW resonators and a method for matching impedances, US patent 2009/0096549 A1, April 2009
Likewise, it is possible to stack resonators R1 and R2 of different thicknesses M. L. Franck, R. C. Ruby, T. Jamneala, Bulk Acoustic Resonator Electrical Impedance Transformers, US patent 2009/0273415 A1, November 2009, as indicated in
Moreover, in order to provide a filter with sufficient selectivity, it is often necessary to use two filter sections that, for reasons of technological complexity, are often connected at the level of the lower resonators as shown in
where epiezo is the thickness of the piezoelectric layer, ∈piezo the dielectric constant of the piezoelectric material used, f0 the central frequency of the filter, all these quantities being common to the lower resonators, and Sres the corresponding area of each resonator. For this reason, electrical reflections are created inside the filter, which reflections degrade the transmission of the filter, and thus its performance.
In this context, the subject of the present invention is a solution allowing impedance conversion and/or mode conversion to be achieved using acoustically coupled resonators. This conversion is obtained while electrical isolation between the input and the output of the filter is ensured, the isolation itself being obtained via the propagation of acoustic waves between the resonators of the filter. According to the invention, the guiding of said waves allows better channeling of the waves to be achieved, and therefore resonators with different geometries to be used. The conversion according to the invention also allows the technological complexity of a CRF filter to be avoided, while making it possible to achieve a wider range of impedance ratios and limiting the risk of internal electrical reflections in the case of high order filters.
More specifically, the subject of the present invention is an acoustic wave bandpass filter comprising at least an input first acoustic wave resonator with an output surface, and an output second acoustic wave resonator with an input surface, said resonators being coupled to each other along a set direction, the input and output surfaces being substantially opposite, characterized in that it furthermore comprises at least one first phononic crystal structure between said input and output resonators and/or a second phonic crystal structure at the periphery of said resonators so as to guide the acoustic waves, generated by said input resonator, toward said output resonator along said set direction, the resonators ensuring an impedance conversions and/or a mode conversion.
According to one embodiment of the invention, each resonator comprises at least one layer of piezoelectric material or of electrostrictive material, and at least one electrode.
According to one embodiment of the invention, the respective input and output surfaces of said second and first resonators are perpendicular to the set direction, this embodiment being particularly advantageous for optimizing the coupling between the resonators.
According one embodiment of the invention, said output surface of said input resonator has a different dimension from that of said input surface of said output resonator.
The first and/or second phononic crystal structures make it possible to converge or diverge (and therefore guide) the acoustic waves from one surface toward the other, depending on their respective dimensions.
According to one embodiment of the invention, the filter comprises a first set of input resonators and a second set of output resonators, a first phononic crystal structure being placed between each set.
According to one embodiment of the invention, the filter comprises more than two coupled resonators, a first phononic crystal structure being placed between each pair of resonators.
According to one embodiment of the invention, the first phononic crystal structure is an acoustic lens structure, said first structure thus ensuring a guiding function and a coupling function.
According to one embodiment of the invention, the second phononic crystal structure ensures a mirror function for said acoustic waves.
According to one embodiment of the invention, the first phononic crystal structure is an acoustic wave coupling structure, the acoustic wave transmission coefficient of the second phononic crystal structure being much smaller than that of the first phononic crystal structure.
According to one embodiment of the invention, the filter comprises at least one input resonator connected to a first potential and to ground, a first output resonator connected to a second potential and to ground, and a second output resonator connected to the inverse of the second potential and to ground, so as to ensure an impedance conversion and an acoustic mode conversion between the input resonator and the output resonators.
According to one embodiment of the invention, the phononic crystal structure is located inside the layer of piezoelectric or electrostrictive material.
According to one embodiment of the invention, the phononic crystal structure comprises features on the surface of the layer of piezoelectric or electrostrictive material.
According to one embodiment of the invention, the features of the phononic structure are produced with at least one of the following materials: SiO2, SiN, Mo, W, AlN.
According to one embodiment of the invention, the phononic crystal structure is one-dimensional or two-dimensional or even three-dimensional.
According to one embodiment of the invention, the piezoelectric material is a material chosen from AlN, LiNbO3, ZnO, PZT and quartz, etc.
According to one embodiment of the invention, the electrostrictive material is a material chosen from BaSrTiO3, SrTiO3 and BaTiO3.
According to one embodiment of the invention, the phononic crystal structure comprises atomic inclusions and/or diffused species.
According to one embodiment of the invention, the inclusions are obtained by implanting hydrogen atoms.
According to one embodiment of the invention, the phononic crystal structure comprises holes.
According to one embodiment of the invention, the filter comprising a plurality of first phononic crystal structures, certain of the first phononic crystal structures of the filter have acoustic wave attenuation coefficients different from the other first structures.
According to one embodiment of the invention, the resonators are bulk wave resonators.
According to one embodiment of the invention, the resonators are Lamb wave resonators and comprise an upper electrode and a lower electrode.
According to one embodiment of the invention, the resonators are surface wave resonators and comprise electrodes positioned on the surface of the piezoelectric material.
The invention will be better understood and other advantages will appear on reading the following non-limiting description, and by virtue of the appended figures, in which:
To make the filter of the present invention, it is proposed to place input and output acoustic resonators such as: Lamb wave or SAW (Surface Acoustic Wave), or BAW (Bulk Acoustic Wave)—such as SMR (Solidly Mounted Resonator) or FBAR (Film Bulk Acoustic Resonator) or HBAR (High-overtone Bulk Acoustic Resonator)—resonators side by side on the same substrate.
Generally, the term “acoustic resonator” is understood to mean a resonant acoustic cavity that optionally comprises one or more electrodes. When a resonant cavity is not associated with an electrode, one or more inclusions are removed from the phononic crystal in set places in order to generate the desired acoustic waves.
In order to achieve a filter function and ensure an impedance and/or acoustic mode conversion, it is advantageously possible to couple two resonators of different sizes. The acoustic coupling thus produced does not then lead to internal electrical reflections, and, as a result, filter performance is improved. It is moreover proposed to insert a phononic crystal structure between at least one input resonator and at least one output resonator, in order to control the amount of energy exchanged between the two resonators, and therefore the bandwidth of the final component.
The impedance conversion makes it possible to provide, for example, a component having a characteristic input impedance different from the output impedance. A conventional example is a filter with an input matched to 50 ohms and an output matched to 200 ohms (these values being set by the dimensions of the input and output resonators: the area for BAW resonators or the length and the number of interdigitated combs for Lamb wave or SAW resonators).
Moreover, the mode conversion may consist in providing an input resonator, one electrode of which is referenced to ground, and the other of which is referenced to the input signal. At the output, there are thus two resonators, either acoustically out-of-phase by 180° or connected back to back, so as to generate two signals of opposite signs in the downstream circuit.
According to a first embodiment of the invention, a phononic crystal is placed coupling the resonators together, and waves are guided using another phononic crystal designed to reflect almost all of the waves and thus confine the latter in or between the resonators, as illustrated in
According to a second embodiment of the invention, it is possible to configure the first phononic crystal structure so as to provide it with a lens function with respect to the acoustic waves, such a lens structure is shown in
According to a third embodiment of the invention, the filter may advantageously comprise coupled resonators. The addition of supplementary resonators improves the selectivity of the filter.
The present invention makes it possible to envision simultaneous impedance and mode conversion since it is possible to guide the waves toward two resonators rather that to only one. Depending upon the technology used, it is also possible to electrically connect the two output resonators, either directly or via an inverter, as shown more specifically in
It will be recalled that the present invention can be applied to both bulk wave resonators and Lamb wave resonators, and to surface wave resonators, as well. For this reason examples of embodiments are given hereafter.
In the context of surface wave resonators and in the context of the previously described second embodiment of the invention,
It will be recalled that whatever the embodiment of the invention, it is possible to use, indifferently, one-, two-, or three-dimensional phononic crystals provided the first phononic crystal structure used fulfills its mirror or acoustic lens function.
The present invention thus makes it possible to achieve the impedance conversion without the mode conversion. It also takes up less space (and probably introduces fewer losses) than the balun technique (electrical coupling). Compared with CRFs, the manufacturing technology is much simpler. Indeed, such devices can be manufactured with just four mask levels.
It should be noted that in the context of the present invention, and in contrast to the prior art, it is notably proposed to use variations in resonator areas rather than variations in thicknesses, areas being, as a parameter, easier to control.
Examples of embodiments of filters according to the present invention are described below.
According to this example, the filter comprises BAW resonators formed from a 2 μm layer of aluminum nitride (AlN) between two molybdenum (Mo) electrodes each having a thickness of 200 nm. This AlN layer is suspended on top of a conventional silicon substrate making it possible, more precisely, to produce FBAR (Film Bulk Acoustic Resonator) resonators.
To obtain an impedance of 50Ω for the input resonator, and of 100Ω for the output resonators, the resonator areas can be calculated using the following equation:
With:
In addition, it is known that:
With:
To obtain a 50Ω impedance, a capacitance of C=2 pF is recommended and therefore an area of S=48,600 μm2. Likewise, to obtain a 100Ω impedance, it is sought to obtain the same characteristics divided by 2, meaning a capacity of C=1 pF and an area of S=24,300 μm2.
To produce the phononic crystal structure CP1 as illustrated in
In this case, the phononic crystal has a stop-band between 1.57 and 1.64 GHz.
Advantageously, this filter may comprise a second phononic crystal structure CP2, as illustrated in
It is equally possible, with this exemplary filter, to achieve mode conversion by making electrical connections between the lower and upper electrodes of the two right-hand side resonators.
This first example is a filter formed from SMR (Solidly Mounted Resonators) resonators on a Bragg mirror, these resonators being coupled by an acoustic lens consisting of a two-dimensional phononic crystal formed by silica pads placed in the space between the resonators on the piezoelectric layer. The silica pads could also be placed under and, advantageously, on and under the piezoelectric layer.
As illustrated in
It is thus necessary to have four mask levels. The phononic crystal could thus consist of molybdenum contacts to avoid a few technological steps, but it is then necessary to make sure that there is no electrical coupling between the metal pads and the resonators.
One variant of the first example consists in achieving the impedance conversion by acting at the same time on the area and the thickness of the resonators so as to limit their footprint. For example, by reducing the thickness of the piezoelectric layer, the area required to achieve a given impedance would be smaller, which would lead to the minimization of the area occupied by the component. This reduction can be achieved by inserting steps of lithography, etching of the AlN, and “stripping” between the deposition of the AlN and the etching of the accesses to the lower electrode.
In this example, the filter allows the impedance and mode conversion to be achieved, with a cross-connected output. The resonators are coupled via a two-dimensional phononic crystal consisting of cylindrical air inclusions (holes) in an aluminum nitride (AlN) membrane, and produced according to the method developed by G. Piazza and his team: N. K. Kuo, C. J. Zuo, G. Piazza, Demonstration of Inverse Acoustic Band Gap Structures in AlN and Integration with Piezoelectric Contour Mode Wideband Transducers, 2009 Solid-State Sensors, Actuators and Microsystems symposium, pp. 10-13, 2009, N. Sinha, R. Mahameed, C. Zuo, M. B. Pisani, C. R. Perez and G. Piazza, Dual-beam actuation of piezoelectric AlN RF MEMS switches monolithically integrated with AlN contour-mode resonators, Solid-State Sensors, Actuators and Microsystems Workshop, Hilton Head Island, pp. 22-25, Jun. 1-5, 2008.
As illustrated in
A wet etching operation of the layer of piezoelectric material AlN is then carried out, using KOH in order to define the accesses to the lower electrodes, as illustrated in
The phononic crystal structures are then produced by dry etching of the AlN as shown in
The upper electrodes ES1 and ES2 are then produced as shown in
Finally, the freeing of the membrane is carried out in order to define the FBAR resonators, using a xenon difluoride (XeF2) etch, as shown in
The third example is an impedance conversion filter consisting of FBAR resonators produced on a thin zinc oxide (ZnO) film. Wave propagation is achieved through a silicon membrane. The phononic crystals consist of air holes in the silicon membrane. The method is based of the method for producing phononic crystals, developed by Mohammadi: S. Mohammadi, A. A. Eftekhar, A. Adibi, US patent 2009/0295505 A1, December 2009.
In a first step, starting with an SOI (Silicon-On-Insulator) substrate consisting of a silicon substrate S comprising a SiO2 layer S0; buried and therefore covered by a silicon layer S′, with a Si thickness of 15 μm, and a gold layer Mi with a thickness of 100 nm is deposited and the lower electrodes of the resonators are defined in this layer via lithography, as shown in
During a second step, the deposition of a layer of piezoelectric material Piézo of ZnO with one micron thickness, is carried out, in which features are defined, as illustrated in
During a third step, a second metalization layer, MS, of aluminum with a 200 nm thickness, is deposited, and the features of the upper electrodes are defined via lithography, as illustrated in
The phononic crystal structures CP1 and CP2 are produced within the silicon via lithography, etching and stripping, as illustrated in
Finally, the back side of the membrane is freed in order to produce an FBAR filter, via an etching method which could advantageously be DRIE etching “Deep Reactive Ionic Etching”, as illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 58404 | Oct 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/067728 | 10/11/2011 | WO | 00 | 5/6/2013 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2012/049174 | 4/19/2012 | WO | A |
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| 20090273415 | Frank et al. | Nov 2009 | A1 |
| 20090295505 | Mohammadi et al. | Dec 2009 | A1 |
| Number | Date | Country |
|---|---|---|
| 2352101 | Jan 2001 | GB |
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| Number | Date | Country | |
|---|---|---|---|
| 20130214878 A1 | Aug 2013 | US |
