This invention relates to electronic devices, such as resonators and filters, and more particularly to such devices including a Langasite structure compound and associated methods.
Bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices are two key components in today""s wireless electronic systems. These devices serve the two major functions of signal processing and frequency control. The signal processing function involves filtering of electrical signals which typically have a frequency ranging from several MHZ up to several GHz and a fractional passband from as low as less than a few hundredths of a per-cent up to tens of a per-cent.
The frequency control function involves generating a precise clock signal or a frequency source whose frequency ranges between several MHZ up to several hundred MHZ. Passive BAW and SAW filters as well as BAW and SAW resonator based clocks and oscillators have been, and will continue to be, the mainstay for these signal processing and frequency control applications.
BAW and SAW filters and resonators are electromechanical devices operated based upon the piezoelectric effect. The piezoelectric materials used for BAW and SAW devices are predominantly of single crystal form. Fundamentally the performance of acoustic wave devices depends on the piezoelectric crystal""s electromechanical coupling strength, its inherent acoustic loss, and its temperature stability.
Another material property of interest for BAW and SAW device construction is the acoustic velocity. The merit of acoustic velocity depends on desired application. For example, higher velocity crystals allow fabrication of devices with higher operating frequencies. On the other hand, for certain SAW filter constructions, namely the ones involving classical transversal filters, a higher velocity crystal substrate may suffer from a larger required device size.
The electromechanical coupling strength dictates the efficiency of energy conversion from electrical to acoustic energy and vice versa, and is thus important to the device insertion loss. The inherent acoustic loss also affects the device insertion loss. Perhaps more importantly the inherent acoustic loss manifests itself into affecting the fidelity of the BAW and SAW resonators in the form of the resonance quality factor Q. This has a direct bearing on the frequency stability of the oscillator constructed using the resonator. A xe2x80x9cmaterial Q factorxe2x80x9d has long been recognized in the field of crystal (BAW) resonators and oscillators, and later adapted by workers in the SAW resonator field.
The maximum material Q, established empirically, is inversely proportional to the device frequency. For a given piezoelectric material, this corresponds to a constant Qmaxxc2x7f factor. For example, for the commonly used BAW and SAW crystal cuts:
(Qmaxxc2x7f)BAW=1.6xc3x971013 Hz for AT and SC cuts
(Qmaxxc2x7f)SAW=1.1xc3x971013 Hz for ST cut
The temperature stability of the piezoelectric crystal dictates how stable, typically in terms of device frequency in parts per million, an acoustic device performs with changing ambient temperature.
The compound Langasite (La3Ga5SiO14, LGS) was first reported in Russia back in 1980 with a Ca3Ga2Ge4O14 type structure. It was found then to have attractive laser, electromechanical and acoustic properties. Interest in LGS has grown in recent years for acoustic device applications. LGS has the same point group (32) symmetry as quartz. Similar to quartz, it has temperature compensated crystal orientations suitable for building temperature-stable BAW and SAW devices.
In comparison with quartz it has the advantage of higher electromechanical coupling strength. With a slower acoustic velocity, it has the potential for miniaturized wideband SAW filters suitable for hand-held mobile wireless devices, for example. LGS was also cited for its potential of lower acoustic loss due to the heavier atomic species of La and Ga, although LGS actually has higher acoustic loss than quartz due to its disordered structure.
Langasite is not unique with these attractive properties. It is just one crystal belonging to a very large family of crystals which have the same structure, and which are called the Langasite family compounds. In fact, compounds within this family typically have quite similar properties. In other words, they are non-centro-symmetric and thus piezoelectric. But they do have some variation due to the difference in composition of each compound. The constants that can be affected by composition include the lattice constant, thermal expansion coefficient, acoustic velocity, dielectric constant, and electromechanical coupling constant, as well as the temperature coefficients of all these constants. These variations, in general, are small (within a factor of 2 or less) but still can have a very significant effect on the device performance.
The Langasite structure is very complex for anhydrous compounds. It has four distinct cation sites. They include three dodecahedral (Site A), one octahedral (Site B), three large tetrahedral (Site C) and two small tetrahedral (Site D) sites. Each site can only accommodate a certain size and charge of the cations. Even with this constraint, nearly one hundred combinations of the cation composition are possible within the structure. Each combination must satisfy the charge neutrality requirement. In almost all the cases, it is necessary to fit a specific site with more than one type of element with different ionic charges in order to satisfy the charge neutrality. This kind of charge balance process creates disorder for the particular site and thus the whole crystal.
For example, LGS has three La ions in the dodecahedral site, one Ga ion in the octahedral site, three Ga ions in the large tetrahedral site and finally one Ga ion and one Si ion in the small tetrahedral sites. The locations of both Ga and Si ions are totally random (or xe2x80x9cdisorderedxe2x80x9d) within the smaller tetrahedral site. Since Ga is 3+ charged and Si is 4+ charged, there is a disorder of ionic charge. In addition, since Ga and Si have a difference in ionic size, mass and density, this creates additional disorder in the lattice of the crystal. Another example is Langanite (La3Nb0.5Ga5.5O14, LGN) where the disorder is located at the single octahedral sites. In this case, half of the octahedral sites are occupied by Nb ions, and the other half occupied by Ga ions. Thus the charge difference is even higher than LGS with Nb 5+ charged and Ga 3+ charged. A third example is CGG (Ca3Ga2Ge4O14). Here the disorder is located at the large tetrahedral site where ⅔ of the sites are occupied by Ge with a 4+ charge and ⅓ of the sites are occupied by Ga with a 3+ charge.
A fourth example is NSGG (NaSr2GaGe5O14). Here the disorder is located at the dodecahedral site where ⅔ of the sites are occupied by Sr with a 2+ charge and ⅓ of the sites are occupied by Na with a 1+ charge.
A fifth example is LSFG (LaSr2Fe3Ge3O14). Here the disorder occurs in two different sites. The first one is the dodecahedral site where ⅓ of the sites are occupied by La with a 3+ charge and ⅔ of the sites are occupied by Sr with a 2+ charge. The second one is the large tetrahedral site where ⅔ of the sites are occupied by Fe with a 3+ charge and ⅓ of the sites are occupied by Ge with a 4+charge.
Structure disorder may not be a desirable feature for crystals to be used in certain acoustic and optical applications. The classic example is glass. Glass is totally disordered from a structural point of view. Even though it has good optical transmission, it is not a good laser host because the local disorder of the lazing element causes non-homogeneous broadening of the emission and a lower gain cross-section.
The problem of disorder for acoustic applications is the typically high acoustic loss. Disorder induces high acoustic friction due to incoherent phonon scattering. Low acoustic loss may, however, be a highly desirable property for both resonator and filter applications. To enhance the crystal performance, it may be desirable to have a perfectly ordered structure. In other words, each site in the lattice structure will have only one specific ion located in it and not a mixture of multiple ions.
It should be noted that, despite the disordered structure, high quality single crystal Y-cut Langasite isomorphs LGN and LGT (La3Ta0.5Ga5.5O14) have already been demonstrated to show higher material Q than quartz, with Qmaxxc2x7f product reaching as high as (Qmaxxc2x7f)LGN PAW=2.2xc3x971013 Hz and (Qmaxxc2x7f)LGT BAW=2.9xc3x971013 Hz.
In the case of the Langasite structure compounds, essentially all the known La containing compositions have disorder structures in at least one cation site. Some of the examples include LGS, LGN and LGT. However, there is one exception, LTG (La3TiGa5O14), which has a totally ordered structure. This, in fact, may be the most ideal composition for the La containing Langasite compound from both a structure and composition point of view. This compound can be synthesized by solid state sintering reaction and is thermodynamically stable.
Applicants have tried to grow a single crystal of LTG, but found that it is not possible to grow it directly from the melt, because of the reduction of Ti4+ to Ti3+ under the growth conditions where the iridium crucible is stable. As a consequence, there were not sufficient 4+ charge ions in the melt to produce LTG.
Even though charge neutrality may be the most important factor controlling the composition of Langasite structure compounds, it is not the only factor. The ionic size and also the thermal stability should also be considered to make the composition compatible. The choice of cations to fit into any specific site is a very difficult task with no guarantee that the selected combination will work. The reason is that there is not sufficient data to predict its thermodynamic properties. Unless the selected composition has the lowest free energy, the compound will not exist. The only way to prove its existence is to actually synthesize the compound according to the proposed composition. When the composition is properly selected, it is possible to fit each cation into a specific site with a total balance of electric charge.
An article by B. V. Mill, et al., xe2x80x9cSynthesis, Growth and Some Properties of Single Crystals with the Ca3Ga2Ge4O,14 Structurexe2x80x9d, Proc. 1999 Joint Meeting EFTFxe2x80x94IEEE IFCS, pp.829-834 discloses numerous synthesized Langasite family compositions, among which are the group of A2+3X5+Y3+3Z4+2O14,with A=Ca, Sr, Ba, Pb; X=Sb, Nb, Ta; Y=Ga, Al, Fe, In; Z=Si, Ge. The article identifies nine individual compounds that are grown according to the Czochralski technique, and of these only three were further identified as having a good chance to become piezoelectric materials for digital mobile communications systems and other acoustic applications in the 21st century. These three materials are La3Ga5O14, La3Nb0.5Ga5.5O14 and La3 Ta0.5Ga5.5O14.
An article by H. Takeda, et al., xe2x80x9cSynthesis and Characterization of Sr3TaGa3Si2O14 Single Crystalsxe2x80x9d, Material Research Bulletin, vol. 35 (2000), pp. 245-252, cited previous work of polycrystal STGS by Mill, et al., (Russ. Jour. Inorg. Chem., vol. 43, p.1168 (1998), and disclosed the synthesis and characterization of Sr3TaGa3Si2O14 (STGS). The article further disclosed that STGS resonators were prepared and the piezoelectric properties thereof were determined.
Despite continuing development in the area of Langasite structure compounds for electronic devices, there still exists a need for further development work to identify and produce such compounds with desirable properties and that can be used to produce high frequency electronic devices.
In view of the foregoing background, it is therefore an object of the present invention to provide electronic devices, such as for signal processing or frequency control applications that include a piezoelectric layer based on a Langasite structure that are readily manufacturable and/or which enjoy advantageous operating characteristics.
This and other objects, features and advantages in accordance with the present invention are provided by an electronic device comprising a piezoelectric layer including an ordered Langasite structure compound having the formula A3BC3D2E14, wherein A is calcium, B is selected from the group consisting of niobium and tantalum, C is gallium, D is silicon, and E is oxygen; and at least one electrode connected to the piezoelectric layer. The ordered Langasite structure compound may have a substantially perfectly ordered structure.
In comparison with the established material of choice to-date, quartz, the ordered Langasite structure compound of the present invention enjoys a lower acoustic loss and higher material Q due, possibly due to the perfect ordering and heavy elements. The ordered Langasite structure compound may also enjoy a higher electromechanical coupling factor possibly due to stronger piezoelectric effect resulting from the crystal structure and Nb or Ta in the octahedral sites. These factors may be important for high performance bulk and surface acoustic wave devices, for example. Furthermore, the crystal symmetry of point group 32 may provide temperature compensated orientations with which devices can be manufactured for minimal temperature variation induced frequency and group delay shifts.
The at least one electrode may comprise a plurality of electrodes configured so that the electronic device is a resonator, for example. The plurality of electrodes may be connected to a same face of the piezoelectric layer so that the electronic device is a surface acoustic wave (SAW) resonator. In particular, the plurality of electrodes may comprise first and second interdigitated electrodes in some resonator embodiments.
The plurality of electrodes may also comprise first and second electrodes connected to respective opposing first and second faces of the piezoelectric layer so that the electronic device is a bulk acoustic wave (BAW) resonator.
In other embodiments of the electronic device, the at least one electrode may comprise a plurality of pairs of electrodes configured so that the electronic device is a filter. Each of the plurality of pairs of electrodes may include first and second interdigitated electrodes. Moreover, the plurality of pairs of electrodes may be connected to a same face of the piezoelectric layer so that the electronic device is a surface acoustic wave (SAW) filter. In other embodiments, the plurality of pairs of electrodes may comprise first and second pairs of electrodes connected to respective opposing first and second faces of the piezoelectric layer so that the electronic device is a bulk acoustic wave (BAW) filter.
The ordered Langasite structure compound may be readily producible using a melt pulling crystal growth technique, especially since the components have congruent melting properties. In addition, the ordered Langasite structure compound may have a relatively high thermally stability.
A method aspect of the invention is for making an electronic device. The method may include providing a piezoelectric layer comprising an ordered Langasite structure compound having the formula A3BC3D2E14, wherein A is calcium, B is selected from the group consisting of niobium and tantalum, C is gallium, D is silicon, and E is oxygen; and connecting at least one electrode to the piezoelectric layer. The ordered Langasite structure compound may have a substantially perfectly ordered structure.