Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-05-03
2003-04-29
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R
Reexamination Certificate
active
06556104
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to surface acoustic wave (SAW) devices and more particularly to a SAW device having improved performance characteristics for application to RF filtering for wireless communications.
BACKGROUND OF THE INVENTION
High frequency surface acoustic wave (SAW) devices are widely used in wireless communications products, particularly as radio frequency (RF) filters for transmit and receive operations. Such filters often utilize resonant SAW devices formed on single crystal piezoelectric substrates as components to generate the desired filtering function. One single crystal piezoelectric substrate, which is commonly used for RF filters, and which has some desirable characteristics for such filters, is lithium tantalate (LiTaO
3
). The performance characteristics of any crystal substrate vary with the selected wave propagation direction, which can be defined in terms of Euler angles. A particularly desirable cut for certain applications is described by Ueda et. al. in U.S. Pat. No. 6,037,847 and U.S. Pat. No. 5,874,869. U.S. Pat. No. 6,037,847 teaches the use of LiTaO
3
with Euler angles (&lgr;,&mgr;,&thgr;) such that &lgr; and &thgr; fixed (set at zero), and &mgr; varied depending on the metalization type and thickness used. For an electrode pattern containing Al as a primary component and forming a resonator with thickness in the range of 0.03-0.15 times a wavelength &Lgr; (i.e. 3% &Lgr; to 15% &Lgr;), the preferred rotation angle &mgr; is greater than −51°, which corresponds to 39°-rotated YX-cut, and less than −44°, which corresponds to 46°-rotated YX-cut (the angle of rotation of Y-cut is determined as &mgr;′=&mgr;+90°). Additional restrictions are presented indicating that the range of Euler angles with rotational angle &mgr; centered on −48° (42°-rotated YX-cut) is preferred. For electrode patterns having Cu as a primary component, with electrode thickness of 0.9% &Lgr; to 4.5% &Lgr;, a rotational angle &mgr; greater than −51° but less than −44° is selected. For electrode patterns containing Au as a primary component and having thickness in the range of 0.4% &Lgr; to 2.1% &Lgr;, a rotational angle &mgr; greater than −51° but less than −44° is selected. As a result, Ueda '847 uses a rotational angle &mgr; in ranges greater than −51° but less than −44°. U.S. Pat. No. 5,874,869 teaches the use of LiTaO
3
with Euler Angles &lgr; and &thgr; fixed (nominally zero), and &mgr; in a range between −50° and −48° for multi-mode SAW devices with a range of specific device design characteristics.
While the Ueda '847 and '869 patents do not specifically state values for Euler angles &lgr; and &thgr;, the description of piezoelectric substrate having an orientation rotated about an X-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with a rotational angle in a specified range, and the direction of propagation of the surface acoustic wave set in the X-direction would lead one skilled in the art to appreciate that the first Euler angle &lgr; and the third Euler angle &thgr; are equal to zero.
SAW devices built on the aforementioned orientations of LiTaO
3
utilize leaky surface acoustic waves (LSAW). A leaky wave has higher propagation velocity, as compared to SAW, which is an advantageous feature for high-frequency SAW devices. Though normally a leaky wave propagates along crystal surface with non-zero attenuation, caused by radiation of bulk acoustic waves into the bulk of crystal, under certain conditions this attenuation tends to zero. One class of leaky waves having negligible attenuation is quasi-bulk waves. With a free surface of a crystal, the mechanical boundary condition can be satisfied for a bulk wave propagating along the boundary plane and polarized in this plane, thus called horizontally polarized wave. In any crystal, orientations in which one of bulk waves satisfies mechanical boundary conditions, form lines in crystallographic space defined by three Euler angles. For LiTaO
3
, such orientations were previously discussed in a publication by N. F. Naumenko, Sov. Phys.-Crystallography 37, pp. 220-223, 1992. In particular, it was found that one of these orientations is known as 36°-rotated YX-cut, Euler angles (0°, −54°, 0°). This is a symmetric orientation characterized by the propagation direction parallel to X axis and a normal to the boundary plane lying in the plane of reflection symmetry YZ of LiTaO
3
. The fast shear bulk wave propagating along X-axis and polarized in the plane of 36°-rotated YX-cut is strongly piezoelectrically coupled with the electric field component along X-axis, due to proximity of the corresponding effective piezoelectric module to its absolute maximum for LiTaO
3
. As to the promising characteristics of 36°-rotated YX-cut for application in SAW devices, reference should be made to K. Nakamura et al., Proc. 1977 IEEE Ultrasonics Symposium, pp. 819-822.
Electrical boundary conditions change the nature of the bulk wave and make it quasi-bulk with propagation velocity slightly lower than that of the bulk wave. The effect of mass loading and electric boundary conditions in an electrode pattern disposed on the surface of 36°-rotated YX cut results in increasing attenuation or propagation loss. However, as described in U.S. Pat. No. 6,037,847 to Ueda et al., orientation with nearly zero propagation loss does not disappear but continuously moves from 36°YX to 42°YX cut while Al electrode thickness increases from zero to 0.08&Lgr;. Similarly, orientations with zero LSAW attenuation were found for electrode patterns containing Cu or Au as a primary component, as functions of metal thickness. According to the detailed description of a method used for evaluation of propagation loss due to scattering of LSAW into slow shear bulk waves, reported by Hashimoto (K. Hashimoto et al., Proc. 1997 IEEE Ultrasonics Symposium, pp. 245-254), minimum propagation loss at the lower edge of a stopband of Bragg's reflection, which corresponds to the resonant frequency of LSAW resonator, was chosen as a criterion of optimizing cut angle. However, propagation loss is a function of frequency. Thus, it is desirable to minimize its average value in a bandwidth. As will be seen, the present invention minimizes propagation loss simultaneously at resonant (fr) and anti-resonant (fa) frequencies.
To explain the effect of propagation loss on a filter performance, reference is now made to
FIG. 1
, which is an example of a ladder filter, comprising three shunt (R
4
,R
5
, R
6
) and three series (R
1
, R
2
, R
3
) resonant SAW structures and utilizing 42°-rotated YX-Cut LiTaO
3
substrate. For the devices under consideration, resonant SAW structures are used as both series and as parallel (shunt) components within a composite device structure, which may include lattice-like regions. In ladder filters it is common to have the anti-resonant frequency of the shunt elements approximately equal to the resonant frequency of the series elements. The lower passband edge of a filter is then determined by propagation loss at the resonant frequency of the shunt elements and the upper passband edge is determined by the propagation loss at the anti-resonance of the series elements. Thus, the propagation loss at both frequencies, resonant and anti-resonant one, are significant and it is desirable that they be simultaneously minimized.
FIG. 2
shows propagation loss at resonant and anti-resonant frequencies calculated for 42°-rotated YX cut LiTaO
3
with Al as electrode material, as functions of electrode thickness normalized to LSAW wavelength, h/&Lgr;. These and other calculations were made with material constants of LiTaO
3
reported by Taziev (R. M. Taziev et al., Proc. 1994 IEEE Ultrasonics Symposium, pp.415-419), though it was found that the results do not change significantly if another set of material constants is used, for example, the constants reported by Kovacs (G. Kovacs et al. Proc. 1990 IEEE Ultrasonics Sympo
Abbott Benjamin P.
Naumenko Natalya F.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Sawtek Inc.
Summons Barbara
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