Electronic device including langasite structure compounds...

Details Electronic device including langasite structure compounds... Electronic device including langasite structure compounds...

2001-05-03

2002-07-23

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S31300R, C310S330000

Reexamination Certificate

active

06424081

ABSTRACT:

FIELD OF THE INVENTION
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.
BACKGROUND OF THE INVENTION
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 “material Q factor” 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 Q
max
·f factor. For example, for the commonly used BAW and SAW crystal cuts:
(
Q
max
·f
)
BAW
=1.6×10
13
Hz for AT and SC cuts
(
Q
max
·f
)
SAW
=1.1×10
13
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 (La
3
Ga
5
SiO
14
, LGS) was first reported in Russia back in 1980 with a Ca
3
Ga
2
Ge
4
O
14
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 “disordered”) 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 (La
3
Nb
0.5
Ga
5.5
O
14
, 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 (Ca
3
Ga
2
Ge
4
O
14
). 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 (NaSr
2
GaGe
5
O
14
). 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 (LaSr
2
Fe
3
Ge
3
O
14
). 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

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