LiNbO3 Rayleigh Wave Filter Structure for Sezawa Mode Suppression
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Solution Overview
Problem
In communication devices using carrier aggregation, band-pass filters with Rayleigh waves on LiNbO3 substrates face degradation due to the excitation of higher-order modes like Sezawa waves, which overlap with the pass band of higher-frequency filters, making it difficult to achieve good filtering characteristics for both low-frequency and high-frequency bands.
Innovation Solution
The elastic wave device incorporates a silicon oxide film with a thickness of about 33% or less of the wavelength, IDT electrodes with a Pt film, and additional layers such as a low-electrical-resistance layer and diffusion barrier to reduce the fractional bandwidth of Sezawa waves and improve filter characteristics, while maintaining good Rayleigh wave filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a band-pass filter using Rayleigh waves on a LiNbO3 substrate is used for low-frequency filtering, then good filtering characteristics are achieved, but higher-order modes like Sezawa waves are excited which degrade the filter characteristics of high-frequency filters
Solution Approach 1:
The patent changes the physical parameters of the IDT electrodes by optimizing the Pt film thickness to a specific range (50-150 nm) and adjusting the silicon oxide film thickness to 33% or less of the wavelength. These parameter changes suppress the excitation of Sezawa waves while maintaining Rayleigh wave filtering performance, thus resolving the contradiction between low-frequency filtering effectiveness and higher-order mode suppression.
Solution Approach 2:
The patent employs a composite structure consisting of multiple layers including Pt film, silicon oxide film, and LiNbO3 substrate. This composite material structure enables simultaneous optimization of Rayleigh wave propagation and Sezawa wave suppression, achieving both good low-frequency filtering characteristics and reduced higher-order mode excitation.
2Reliability
If the thickness of Pt film is increased to improve Rayleigh wave filtering, then filtering characteristics improve, but the fractional bandwidth of Sezawa waves increases
Solution Approach 1:
The patent identifies and optimizes the Pt film thickness parameter to a specific range (50-150 nm) that simultaneously achieves good Rayleigh wave filtering characteristics and suppresses Sezawa wave fractional bandwidth. This precise parameter control resolves the contradiction between improving filtering performance and preventing higher-order mode excitation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration effectively reduces the fractional bandwidth of Sezawa waves to 0.05% or less, minimizing the degradation of filter characteristics in higher-frequency filters and enhancing the performance of both low-frequency and high-frequency band-pass filters.
Implementation Method 1
an elastic wave resonator that uses Rayleigh waves propagating on a piezoelectric substrate made of LiNbO3
Implementation Method 2
uses Rayleigh waves propagating on a piezoelectric substrate made of LiNbO3
Implementation Method 3
the excitation of not only Rayleigh waves but also higher-order modes, such as Sezawa waves
Data Source
AI summary
An elastic wave device includes a first filter including an elastic wave resonator and a second filter connected to an antenna common terminal via a common node. When a first pass band of the first filter and second pass bands of the second filters are F1 and F2, respectively, F1<F2. The first filter includes at least one elastic wave resonator, and the elastic wave resonator uses Rayleigh waves propagating on a piezoelectric substrate made of LiNbO3, IDT electrodes including a Pt film on a piezoelectric layer made of LiNbO3, and a silicon oxide film covering the IDT electrodes. The thickness of the silicon oxide layer is about 33% or less of the wavelength of the IDT electrodes.