XBAR Resonator Reflector Layout for Acoustic Loss Reduction

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Solution Overview

Problem

Existing RF filters using acoustic wave resonators, such as XBARs, face challenges in achieving high performance at higher frequencies and wider bandwidths required for future communication systems, particularly in bands like n77 and n79.

Innovation Solution

The integration of reflector elements with optimized pitch and mark configurations in the interdigital transducer (IDT) of XBARs helps to confine acoustic energy and reduce losses, thereby enhancing the Q-factor and overall performance of the filters.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If reflector elements are added to the IDT structure, then the Q-factor is improved, but the device complexity increases

Engineering Contradiction:

ImproveQ-factorVSAvoidIDT structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The IDT structure is segmented into functional zones: active IDT fingers for signal generation and separate reflector elements for acoustic confinement. This segmentation allows the reflector elements to be independently optimized for reducing acoustic energy loss without affecting the core IDT operation, thereby improving Q-factor while managing complexity through modular design

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Reflector elements are positioned upstream of the active IDT region to pre-confine acoustic energy before it propagates through the piezoelectric film. This preliminary action prevents acoustic energy from escaping at the edges, thereby improving Q-factor by reducing energy loss before the main signal processing occurs

Inventive Principle:

Principle #10Preliminary action

2Reliability

If the pitch of reflector elements is optimized, then acoustic energy confinement is improved, but the manufacturing precision requirements increase

Engineering Contradiction:

Improveacoustic energy confinementVSAvoidreflector element pitch precision

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The pitch of reflector elements is optimized to specific values (e.g., 1.5p where p is the IDT finger pitch) to create constructive interference patterns that confine acoustic energy. By establishing standardized pitch ratios, the design transforms a potentially complex precision requirement into a scalable parameter relationship that can be manufactured with conventional precision while achieving effective acoustic confinement

Inventive Principle:

Principle #35Parameter changes

3Reliability

If the mark of reflector elements is optimized, then the normalized Q-factor is improved, but the device complexity increases

Engineering Contradiction:

Improvenormalized Q-factorVSAvoidreflector element configuration

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

Reflector elements are designed with specific mark (width) dimensions optimized for their local function of acoustic confinement. The mark is typically set to a fraction of the acoustic wavelength to create appropriate impedance mismatch for reflection. This local optimization of the mark parameter improves Q-factor by enhancing acoustic energy reflection at the reflector locations without requiring complex variations throughout the structure

Inventive Principle:

Principle #3Local quality

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

The use of reflector elements in XBARs significantly improves the normalized Q-factor, leading to better insertion loss and rejection characteristics, which are critical for maintaining filter performance across the desired frequency ranges.

Implementation Method 1

a thin plate of piezoelectric material bonded to a substrate

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

an interdigital transducer (IDT) including a first set of parallel fingers extending from a first busbar and a second set of parallel fingers extending from a second busbar, wherein the first and second sets of parallel fingers are interleaved

Methodology Applied

Scientific EffectSurface acoustic wave generation: Surface Acoustic Wave

Implementation Method 3

a first reflector element and a second reflector element disposed on the piezoelectric plate outside of a periphery of the IDT

Methodology Applied

Scientific EffectAcoustic reflection: Reflection

Data Source

PatentUS12341490B2Low loss transversely-excited film bulk acoustic resonators and filters

Publication Date: 2025.06.24 MURATA MFG CO LTD

AI summary

An acoustic resonator device includes a portion of a piezoelectric plate is a diaphragm spanning a cavity in a substrate. A conductor pattern on a surface of the piezoelectric plate includes an interdigital transducer (IDT) with a first busbar, a second busbar, and a plurality of interleaved fingers extending alternately from the first and second busbars, first and second reflector elements proximate and parallel to a first finger of the interleaved fingers, and third and fourth reflector element proximate and parallel to a last finger of the interleaved fingers. Overlapping portions of the interleaved fingers and the first to fourth reflector elements are on the diaphragm. pr1 is a center-to-center distance of the first and second reflector elements and a center-to-center distance of the third and fourth reflector elements, p is a pitch of the interleaved fingers, and 1.1p≤pr1≤1.5p.