8 GHz Harmonic Surface Acoustic Wave Ladder Filters with Grooved Al Electrode in LiNbO3 Substrate

3GPP has allocated frequency range 3 (FR3) within the 7-24 GHz band, but no suitable acoustic wave filters have been developed for practical use at these frequencies. Surface acoustic wave (SAW) devices require an interdigital transducer (IDT) wavelength (λ) of 1.2 μm or more due to power handling constraints. Consequently, the upper frequency limit when using a standard 42°YX LiTaO3 (LT) substrate is in 3 GHz range. We previously reported that by using aluminum (Al) grooved electrode in LiNbO3 (LN) substrate, 8.1 GHz harmonic SAW, which is 2.3 times higher frequencies than the fundamental SAW, was excited. This time, a harmonic SAW resonator and ladder filters were fabricated by improving the slope of the sidewalls of the groove Al electrode in the 26°YX LN As a result, an 8.44 GHz resonator using an IDT with a λ of 1.3 μm was obtained. For the first time, 8 GHz ladder filters with a minimum insertion loss of 3.4-3.7 dB and a 3 dB bandwidth of 6.8-11.6%, using an IDT with λ greater than 1.2 μm were also achieved.

I. Introduction

Surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters are the primary RF components used in mobile phones [1][2]. As mobile communications continue to expand, higher frequency bands are needed, and 7-24 GHz range has been allocated by 3GPP as Frequency Range 3 (FR3). The frequency of a SAW device is determined by the phase velocity of the substrate and the wavelength (λ) of the interdigital transducer (IDT). However, due to power-handling constraints, the minimum IDT λ for SAW devices is limited to 1.2 μm, and the maximum frequency for SAW devices on standard 42°YX LiTaO₃ (LT) is 3.2 GHz [3]. There have been reports of high-frequency SAW ladder filters using IDTs with λ of 1.2 μm or less, but due to the above-mentioned reasons, these devices are not suitable for practical use in mobile communications [4].