Recessed-Finger Lithium Niobate XBAR for Wideband RF Filtering

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Solution Overview

Problem

Current RF filters using acoustic wave resonators, such as SAW and BAW resonators, are not well-suited for higher frequency communications bands proposed for future wireless networks, particularly those above 3 GHz, due to limitations in performance parameters like insertion loss, rejection, and bandwidth.

Innovation Solution

The development of transversely excited film bulk acoustic resonators (XBARs) using rotated Y-X cut lithium niobate, which incorporates a piezoelectric plate with specific Euler angles and a thin film conductor pattern, allowing for improved piezoelectric coupling and frequency performance, enabling the design of high-frequency filters with wider bandwidths.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional SAW and BAW resonators are used, then device structure is simple and manufacturing is easier, but frequency performance and bandwidth are limited for applications above 3 GHz

Engineering Contradiction:

Improvefrequency performanceVSAvoidresonator structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent employs rotated Y-X cut lithium niobate substrate combined with acoustic Bragg reflector layers to create a composite resonator structure. This composite approach enables enhanced piezoelectric coupling and achieves frequency performance suitable for millimeter-wave applications while maintaining structural integrity

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent utilizes specific Euler angles (α=0°, β=30°, γ=0°) for the lithium niobate crystal orientation to optimize piezoelectric coupling coefficients. By changing the crystal cut parameters and orientation angles, the resonator achieves improved frequency performance and bandwidth for high-frequency communications

Inventive Principle:

Principle #35Parameter changes

2Speed

If higher frequency bands are targeted for future wireless networks, then communication bandwidth and data rates improve, but conventional resonators exhibit increased insertion loss and reduced rejection

Engineering Contradiction:

Improvefrequency bandVSAvoidinsertion loss

Core Design Contradiction:

SpeedVSLoss of energy

Solution Approach 1:

The patent optimizes the lithium niobate crystal orientation with Euler angles of α=0°, β=30°, γ=0° to maximize piezoelectric coupling at high frequencies. This parameter optimization reduces energy loss and maintains low insertion loss even in millimeter-wave frequency bands

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The acoustic Bragg reflector serves as an intermediary structure that confines acoustic energy within the resonator cavity, reducing energy leakage and minimizing insertion loss at high frequencies where conventional resonators suffer from increased losses

Inventive Principle:

Principle #24Intermediary (Mediator)

3Speed

If higher frequency bands are used, then communication capacity increases, but resonator rejection and selectivity deteriorate

Engineering Contradiction:

Improvefrequency bandVSAvoidfrequency selectivity

Core Design Contradiction:

SpeedVSMeasurement precision

Solution Approach 1:

The combination of rotated Y-X cut lithium niobate with acoustic Bragg reflector creates a composite resonator that maintains sharp frequency selectivity and high rejection ratios even in millimeter-wave bands, overcoming the deterioration seen in conventional resonators

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The acoustic Bragg reflector acts as a mediator that enhances frequency selectivity by creating acoustic standing waves with well-defined resonances, enabling precise frequency discrimination and high rejection of out-of-band signals at higher frequency bands

Inventive Principle:

Principle #24Intermediary (Mediator)

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

XBARs provide enhanced performance by achieving higher electromechanical coupling coefficients and wider bandwidths, making them suitable for millimeter-wave frequencies, thereby addressing the limitations of existing filters in higher frequency communications.

Implementation Method 1

improved piezoelectric coupling

Methodology Applied

Scientific EffectPiezoelectric coupling: Piezoelectric Effect

Implementation Method 2

transversely excited film bulk acoustic resonators

Methodology Applied

Scientific EffectMechanical vibration: Vibration

Implementation Method 3

film bulk acoustic resonators

Methodology Applied

Scientific EffectBulk acoustic wave propagation: Sound

Data Source

PatentUS11689185B2Solidly-mounted transversely-excited film bulk acoustic resonator with recessed interdigital transducer fingers using rotated y-x cut lithium niobate

Publication Date: 2023.06.27 MURATA MFG CO LTD

AI summary

Acoustic resonator devices, filters, and methods are disclosed. An acoustic resonator includes a substrate, a lithium niobate plate having front and back surfaces, wherein Euler angles of the lithium niobate plate are [0°, β, 0°], where β is greater than or equal to 0° and less than or equal to 60°, and an acoustic Bragg reflector between the surface of the substrate and the back surface of the lithium niobate plate. An interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate. At least one finger of the IDT is disposed in a groove in the lithium niobate plate.