Optimized Cut of LiTaO 3 for Resonator Filters with Improved Performance Natalya Naumenko 1 , Benjamin Abbott 2 1 Moscow Steel and Alloys Institute, 117936, Leninski prosp.,4. Moscow, Russia 2 SAWTEK Inc., 1818 South Hwy 441, Apopka, FL 32703 Abstract – In resonator filters, it is often desirable to minimize propagation loss simultaneously at resonant and anti-resonant frequencies. Using this criterion, we found an optimal dependence of rotation angle on electrode thickness in wavelengths, in rotated YX cuts of LiTaO 3 with Al grating. In particular, 48°YX cut was found to be optimal for resonator filters with thick Al electrodes, about 10% wavelength. With rotation angle set to be optimal for the specified electrode thickness, the lines of non- symmetric orientations were located, along which propagation losses at resonant and anti-resonant frequencies do not exceed 0.003 dB/Λ, electromechanical coupling coefficient is higher than 8% and power flow angle does not exceed 5 o . I. INTRODUCTION A performance of a resonator-type SAW filter comprising piezoelectric substrate with periodic electrode gratings, forming resonators, is effected by propagation loss in a substrate. In rotated Y-cuts of LiTaO 3 , X-propagation, widely used as a substrate in RF SAW filters, leaky surface acoustic wave (LSAW) is utilized. The propagation loss resulting from radiation of bulk waves into the bulk of the substrate is a dominant loss mechanism in RF SAW filters. This loss depends on substrate orientation and details of filter structure, such as metalization ratio and electrode thickness normalized to wavelength. The 42 o -rotated YX-cut of LiTaO 3 was found to be a particularly desirable orientation if Al is used as an electrode material, and electrode thickness varies between 8% and 10% [1]. According to the description of a method used for evaluation of propagation loss due to scattering of LSAW into slow shear bulk waves, 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, and in many applications it is desirable to minimize its average value in a stopband. Resonant SAW structures are often used as both series and as parallel (shunt) components within a composite device structure, which may include lattice-like regions. 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 one, are significant and desirable to be minimized. Using the criterion of simultaneous minimization of propagation loss at resonant and anti-resonant frequencies, we found an optimal dependence of rotation angle μ’ upon the normalized electrode thickness h/Λ, in μ’-rotated YX cuts of LiTaO 3 . Utilizing of these orientations in ladder filters provides better performance, compared to orientations optimized for minimum propagation loss at resonant frequency. In particular, more symmetric frequency response and better shape factor can be obtained. II. OPTIMIZED CUT ANGLE AS FUNCTION OF ELECTRODE THICKNESS A leaky wave propagates along crystal surface with non-zero attenuation, caused by radiation of bulk acoustic waves into the bulk of crystal. However, under certain conditions this attenuation tends to zero. One class of leaky waves having negligible 0-7803-7582-3/02/$17.00 (c) 2002 IEEE 2002 IEEE ULTRASONICS SYMPOSIUM-385