XBAR Interdigital Transducer Layout for High-Frequency Spurious Mode Control

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Solution Overview

Problem

Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications bands, such as those proposed for future wireless networks, due to issues with spurious modes and performance limitations at frequencies above 3 GHz.

Innovation Solution

The development of transversely-excited film bulk acoustic resonators (XBARs) and solidly-mounted transversely-excited film bulk acoustic resonators (SM XBARs) with varying pitch and mark configurations in interdigital transducers (IDTs) to excite primary acoustic modes while suppressing spurious modes, using piezoelectric materials like lithium niobate and aluminum nitride, and incorporating acoustic Bragg reflectors for improved frequency selectivity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional acoustic wave resonators are used, then the filter can operate at lower frequencies, but spurious modes and performance limitations occur at frequencies above 3 GHz

Engineering Contradiction:

Improvefilter performance at high frequenciesVSAvoidspurious modes

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent changes the geometric parameters of the IDT fingers (width, spacing, length) to optimize the excitation of acoustic modes. By adjusting these parameters, the resonator is tuned to excite only the desired fundamental mode while suppressing spurious modes, enabling reliable operation at frequencies above 3 GHz

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs composite material structures including piezoelectric films deposited on substrate layers with specific acoustic properties. This composite construction allows control over acoustic wave propagation characteristics, enhancing fundamental mode excitation while attenuating spurious modes through material property optimization

Inventive Principle:

Principle #40Composite materials

2Measurement precision

If the IDT finger configuration is optimized for fundamental mode excitation, then frequency selectivity is improved, but the design complexity increases

Engineering Contradiction:

Improvefrequency selectivityVSAvoidIDT design complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent applies local quality by creating regions within the IDT with different finger configurations. Specific zones have optimized finger widths and spacings to excite the fundamental mode, while other regions are designed to suppress spurious modes. This localized optimization achieves high frequency selectivity without requiring complete redesign of the entire IDT structure

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The IDT is segmented into multiple sections with progressively varying finger parameters. This segmentation allows each section to contribute differently to the overall acoustic excitation, with earlier sections initiating the fundamental mode and later sections refining the frequency selectivity while suppressing harmonics

Inventive Principle:

Principle #1Segmentation

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

These resonators effectively reduce spurious modes and enhance frequency selectivity, enabling the use of RF filters in higher frequency bands up to 28 GHz, improving communication system performance by providing better rejection and insertion loss characteristics.

Implementation Method 1

using piezoelectric materials like lithium niobate and aluminum nitride

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

incorporating acoustic Bragg reflectors for improved frequency selectivity

Methodology Applied

Scientific EffectBragg reflection: Bragg Diffraction

Implementation Method 3

transversely-excited film bulk acoustic resonators (XBARs)

Methodology Applied

Scientific EffectAcoustic wave propagation: Sound

Data Source

PatentUS11206009B2Transversely-excited film bulk acoustic resonator with interdigital transducer with varied mark and pitch

Publication Date: 2021.12.21 MURATA MFG CO LTD

AI summary

Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate. The IDT is configured to excite a primary acoustic mode in the diaphragm in response to a radio frequency signal applied to the IDT. A pitch between adjacent fingers of the IDT is varied along one or both of an aperture of the IDT and a length of the IDT.