Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-09-25
2004-01-13
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C333S193000
Reexamination Certificate
active
06677696
ABSTRACT:
The present invention is directed to an electronic surface acoustic wave (SAW) component on, in particular, a lithium tantalate
iobate substrate that works with acoustic waves. Without this being respectively repeated, the invention can also be applied for potassium niobate and similar materials employed a substrate crystal for such components.
It is important for the employment and the operation of such a component, particularly for handsets of the mobile radiotelephone field, to preferably design the input/output filter (directly connected to the antenna) such that, on the one hand, it exhibits high power compatibility (for example, 2 Watts offered power) and, on the other hand, particularly has an extremely low insertion attenuation.
An applicable surface acoustic wave filter is disclosed, for example, by DE-A-2802946. It is essentially composed (see
FIG. 7
) of a piezoelectric substrate lamina
1
such as, for example, lithium tantalate on the selectively oriented substrate surface
2
of which electrode structures
12
such as inter-digital structures and the like are applied. LSAW waves (leaky surface acoustic waves) or HVPSAW(high velocity pseudo) waves proceed between these structures during operation (also see Ultrasonic Symp. Proc. 1994, pp. 281-286). These electrode structures and propagating waves
13
serve for the selective/filtering electrical signal transmission.
It is known for such a reflector/electrode structure (merely referred to overall below as electrode structures) having, for example, interdigital electrode fingers, reflector fingers and the like (also merely referred to overall as fingers below), that it is not only the sequence of the arrangement of the fingers
112
and their spacings that must be adhered to but that a predetermined dimension must also be adhered to (preferably within a range of tolerance) for the thickness (height) of the fingers, namely for adequately high reflectivity of these fingers.
For a high power compatibility, it is known from the Prior Art to provide special techniques for the electrode structures applied on the substrate surface. In general, these electrode structures are composed of, for example, photolithographically structured aluminum. Such structures of pure aluminum are relatively unstable in a number of ways. For enhancing the power compatibility, the aluminum has been alloyed with copper or a sandwich structure of aluminum and copper has been provided. What is thereby disadvantageous is that corrosion occurs given such a combination of materials. Adding titanium to the aluminum leads to higher electrical resistance of an electrode structure composed of this combination. Another approach that has been pursued is to apply the aluminum on the substrate with [
11
]-texture, a prior nucleation of the deposition surface thereof being required for this purpose. This not only causes higher technological expense, the reproducibility of such a textured aluminum layer leaves much to be desired. Epitaxial growth of the aluminum of an electrode structure is a distinctly expensive manufacturing method. A small grain structure of the aluminum for enhanced power compatibility can also be produced by sputtering the aluminum under correspondingly selected conditions. Disadvantageously, however, the photolithographic lift-off technique that is otherwise advantageous (and is preferably employed for the invention) can thereby not be applied for forming the electrode structure.
The desirable, low insertion attenuation for the component has also been mentioned above. It is known from the Prior Art (DE-A-19641662 and DE-C-2802946) to achieve low insertion attenuation of the surface acoustic wave filter in that a crystal of a section (red y) rotated by an angle &thgr; is employed as substrate lamina. The substrate lamina
1
has a surface
2
to which an axis system x
1
, x
2
, x
3
, with x
2
as the normal N of this surface and with axes x
1
and x
3
lying in this surface are allocated. The known orientation of the crystallographic axis system x, y, z is such that the x-axis, coinciding with the axis x
1
, lies in the plane of the crystal section, i.e. in the substrate surface, and this crystallographic x-axis and the direction of the wave propagation
13
in the component are aligned parallel to one another. The y-axis of the crystal in the Prior Art resides on the substrate surface obliquely relative to the normal N thereof in the dimension of the rotational angle &thgr; corresponding to the rotation (red y). The z-axis therefore assumes the same angle &thgr; relative to the x
1
-x
3
plane, i.e. to the substrate surface. The aforementioned publications specify an angular range between 38° and 46°, on the one hand, and between 33° through 39°, on the other hand for an angle &thgr; for the lithium tantalate. Angles with 66° through 74° and 41 ° are known for the lithium niobate.
SUMMARY OF THE INVENTION
An object of the present invention is to specify a concept of an applicable surface acoustic wave component (filter) that comprises the required power compatibility as presented above and that also preferably has minimized insertion attenuation.
According to the present invention, a surface acoustic wave component is provided comprising an electrode structure formed of fingers having aluminum as at least a principal material constituent on a surface with a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of lithium tantalate. The surface is a crystal section such that the surface charges electrically positive given heating of the substrate lamina as a result of a pyroelectric effect. A crystal section is rotated around an x
1
=x-axis as a direction of wave propagation with an angle &Dgr;=(180°+&thgr;) between a positive y-axis and the surface normal, wherein &thgr; is a known angle for low leakage wave losses for lithium tantalate crystal sections.
Also according to the invention, a surface acoustic wave component is provided having electrode structures formed of fingers of aluminum as at least a principal material constituent on a surface having a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of lithium niobate. The surface is a crystal section such that said surface charges electrically positive given heating of the substrate lamina as a result of a pyroelectric effect. A crystal section is rotated around an x
1
=x-axis as a direction of the wave propagation with an angle &Dgr;=180°+&thgr; between a positive y-axis and the surface normal, wherein &thgr; is a known angle for low leakage wave losses for lithium niobate crystal sections.
Also according to the invention, a surface acoustic wave component is provided comprising electrode structures having fingers formed of aluminum as at least a principal material constituent on a surface having a surface normal of a pyroelectric and piezoelectric crystal substrate lamina of potassium niobate. The surface is a crystal section such that the surface charges electrically positive given heating of the substrate lamina as a result of the pyroelectric effect. A crystal section :is rotated around the x
1
=x-axis as a direction of wave propagation with an angle &Dgr;=180°+&thgr; between a positive.y-axis and the surface normal, wherein &thgr; is a known angle for low leakage wave losses for potassium niobate crystal sections.
The invention is described below on the basis of an example with lithium tantalate monocrystal substrate lamina (without being thereby considered limited thereto).
For achieving this object, a crystallographic orientation of the surface of the employed substrate lamina provided for the electrode structures 12 is selected that deviates significantly from the Prior Art. The +z-axis, which is also the pyroelectrical axis, resides at an angle &Dgr; relative to the x
1
-x
2
plane, i.e. it is directed into the substrate lamina relative to the substrate surface
11
(so that the negative z-axis points up away from the substrate surface in FIG.
1
). T
Budd Mark O.
EPCOS AG
Schiff & Hardin & Waite
LandOfFree
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