Optimized Apodization to Suppress Transverse Modes in Guided SAW Resonators
S. Inoue, T. Yokoyama, Hao Dong, Mark Gallagher, M. Solal
Abstract
Quality factor (Q) and electro-mechanical coupling factor (k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of acoustic resonators are keys to achieve low-loss and wideband filters. Transverse modes (TMs) need to be suppressed to avoid spurious responses with minimal degradation of Q and k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Guided SAW (e.g. LT/SiO2/Si) using a shear horizontal (SH) mode makes it difficult to use piston mode configurations to suppress TMs because of a large scattering loss on the edge of electrodes (slow region). Apodization is a well-known technique to reduce TMs. However, without a proper design, it reduces Q and k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> as well as resonator capacitance resulting in a larger resonator size. This paper demonstrates a new apodization scheme able to suppress TMs with minimal reduction in Q, k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and capacitance. First, a simulator combining coupling-of-modes (COM) and scalar potential theory is developed to accurately predict TM responses as well as SAW distributions in resonators. Second, a systematic optimization of an apodization scheme maintaining high Q, k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and large capacitance is conducted using the developed simulator. Finally, guided SAW resonators with the optimized apodization and a piston mode configuration are fabricated on LT/SiO2/Si and the performances are compared. The measurement confirms that the proposed apodization has a similar k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , capacitance density and TM suppression to the piston mode configuration, while its Bode Q is higher. This is probably because the proposed apodization without a slow region has lower scattering losses than the piston mode configuration.