Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-10-25
2003-12-09
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R, C333S195000
Reexamination Certificate
active
06661313
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
In the past decade, surface acoustic wave (SAW) filters became widely used in wireless communication systems operating at radio frequencies (RF). One of the most important requirements of SAW filters for RF applications is low insertion loss, which can be provided by utilizing resonant SAW devices formed on a piezoelectric substrates with high electromechanical coupling coefficient. Single crystal of lithium niobate, LiNbO
3
, is known as a piezoelectric material with the highest electromechanical coupling coefficients, compared to other substrates utilized in SAW devices. High propagation velocity of acoustic waves is also desirable for high frequency devices. Therefore, leaky surface acoustic waves (LSAW), which exist in rotated Y-cuts of lithium niobate (LN), X-propagation, and which combine a large electromechanical coupling coefficient with a high propagation velocity, are very promising for use in low-loss SAW filters.
Though a leaky wave propagates along the surface with a certain attenuation (propagation loss), which is caused by radiation of bulk waves, the propagation loss tends to zero in a 41°-rotated Y-cut substrate, when the surface is free (“open”), and in 64°-rotated Y-cut substrate, when the surface is covered with infinitely thin metal film (“short”). The reference should be made to K. Yamanouchi (K. Yamanouchi et al, J.Appl.Phys., 1972, v.43, pp.856-862), who first reported LSAW characteristics in rotated Y-cuts of LN. He found that a strong effect of changing electrical boundary condition on LSAW velocity results in high piezoelectric coupling of LSAW in the above-mentioned orientations.
In resonant structures with metal electrodes of certain thickness, the optimal rotation angle of a Y-cut, which provides minimum propagation loss, depends on the thickness. Particularly desirable cuts for certain applications are described by Ueda et al. in U.S. Pat. No. 6,037,847. According to FIG. 14 of this '847 patent, an orientation with nearly zero propagation loss continuously moves from 62° YX to 72° YX substrate surface cut while the Al electrode thickness increases from 0.025&Lgr; to 0.085&Lgr;, (i.e. from 2.5% &Lgr; to 8.5% &Lgr;), where &Lgr; is LSAW wavelength. Similarly, orientations with nearly zero propagation loss were found for electrode patterns containing Cu or Au as a primary component, as functions of metal thickness.
A selected cut and propagation direction in any crystal can be defined in terms of Euler angles (&lgr;,&mgr;,&thgr;). The U.S. Pat. No. 6,037,847 teaches the use of LiNbO
3
with Euler angles (&lgr;,&mgr;,&thgr;) such that &lgr; and &thgr; fixed (nominally 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 from 4% &Lgr; to 12% &Lgr;, the preferred rotation angle &mgr; is greater than −24°, which corresponds to a 66°-rotated YX-cut, and less than −16°, which corresponds to a 74°-rotated YX-cut (the angle of rotation of Y-cut is determined as &mgr;′=&mgr;−90°). For electrode patterns having Cu as a primary component, with electrode thickness of 1.2% &Lgr; to 3.6% &Lgr;, a rotational angle &mgr; greater than −24° but less than −16° is selected. For electrode patterns containing Au as a primary component and having thickness in the range of 0.4% &Lgr; to 1.7% &Lgr;, a rotational angle &mgr; greater than −24° but less than −16° is selected. Thus, this patent claims devices using a rotational angle &mgr; in the ranges: greater than −24° but less than −16°.
In this instance, the patent does not specifically state what the Euler angles &lgr; and &thgr; are. However, from 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 [U.S. Pat. No. 6,037,847 Summary of the Invention], it is clear to one skilled in the art that the first Euler angle &thgr; and the third Euler angle &thgr; are equal to zero. 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.
FIG. 1
shows calculated propagation loss at resonant and anti-resonant frequencies for 41°-rotated YX cut and 64°-rotated YX cut of LiNbO
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, with Al as electrode material, as functions of electrode thickness normalized to LSAW wavelength, h/&Lgr;. These and other calculations of the present invention were made with material constants of LiNbO
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reported by Kovacs (G. Kovacs et al. Proc. 1990 IEEE Ultrasonics Symposium, pp.435-438), 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 Warner (A. Warner et al, J.Acoust.Soc.Amer., 1967, v.42, pp.1223-1231). In all calculations, the metalization ratio (electrode width to period of grating structure) is assumed to be 0.5.
In 41° YX of LN, both propagation losses, estimated at resonant frequency (fr) and anti-resonant frequency (fa), increase rapidly with Al electrode thickness. In 64° YX of LN, each propagation loss versus normalized thickness dependence has a minimum with nearly zero loss value. This minimum occurs at about 3% &Lgr; and 1.2% &Lgr; for resonant and anti-resonant propagation loss, respectively. Thus, average propagation loss is minimum at approximately 2.4% &Lgr;.
In many applications it is desirable to minimize propagation loss at the center frequency that does not coincide with the resonant or anti-resonant frequency. For example, 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, is significant and desirable to be simultaneously minimized. This can be achieved if a minimum average propagation loss, (Lr+La)/2, is used as a criterion of optimal cut angle. Here Lr and La are propagation loss values at resonant and anti-resonant frequencies, respectively. Moreover, if propagation loss is minimized at center frequency, in addition to lower insertion loss, the shape factor of the SAW filter can also be improved. Support for such is found in Unites States patent application for “Surface Acoustic Wave Devices Using Optimized Cuts of a Piezoelectric Substrate” having Ser. No. 09/848,714, the disclosure of which is herein incorporated by reference.
FIG. 2
shows average propagation loss versus Al electrode thickness dependencies in three orientations of LiNbO
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, 41°-YX, 64°-X and 74°-YX cuts. It is apparent from this figure that LiNbO
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orientations in the interval from 64°-YX to 74°-YX can provide fairly low propagation loss if the thickness of Al electrode changes between 2.4% and 4.8% &Lgr;. With thicker electrodes, the minimal propagation loss value, which can be obtained in orientations of LiNbO
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from 64°-YX cut to 74°-YX cut, grows with thickness. For example, with increasing Al elect
Abbott Benjamin P.
Naumenko Natalya F.
Sawtek Inc.
Summons Barbara
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