2D Resonant Rod RF Filter Structure for High kt2 Tuning

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Solution Overview

Problem

Current RF filters based on aluminum nitride film-bulk-acoustic-resonators (FBARs) face challenges in achieving high electromechanical coupling coefficients and lithographic frequency tunability, especially for emerging IoT and 5G applications, due to the complexity of controlling resonance frequency and the limitations of two-dimensional-mode-resonators (2DMRs) in attaining suitable kt2 values for wideband applications.

Innovation Solution

The development of two-dimensional resonant rods (2DRRs) that utilize a forest of locally resonant rods in a profiled AlN layer, sandwiched between a bottom un-patterned metal plate and a top metallic grating, allowing for high kt2 values and lithographic frequency tunability, enabling the excitation of dilatational modes and acoustic energy confinement within the rods.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If two-dimensional-mode-resonators (2DMRs) with metallic gratings are used, then lithographic frequency tunability is achieved, but the electromechanical coupling coefficient (kt2) is limited to approximately 5%

Engineering Contradiction:

Improvelithographic frequency tunabilityVSAvoidelectromechanical coupling coefficient

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The resonator structure is segmented into vertically stacked layers with distinct functions: a bottom un-patterned metal plate for electrical grounding and capacitance, a piezoelectric layer for active resonance, and a top metallic grating for mode confinement. This segmentation allows each layer to be optimized independently, achieving both high kt2 and frequency tunability

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention transitions from two-dimensional planar resonators to three-dimensional vertically-stacked structures. The bottom un-patterned metal plate extends across the entire device area, providing enhanced capacitance and electrical grounding in the vertical dimension, while the top grating confines modes laterally, achieving both high coupling and frequency control

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Reliability

If the resonance frequency of FBARs is set by AlN plate thickness, then high kt2 (approximately 7%) is achieved, but lithographic frequency tunability requires large fabrication complexity

Engineering Contradiction:

Improveelectromechanical coupling coefficientVSAvoidfabrication complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The resonance frequency is controlled by changing the lateral dimensions (width and length) of the piezoelectric layer through standard lithographic patterning, rather than changing the thickness. This parameter change approach enables frequency tuning using conventional fabrication processes with typical lithographic resolutions, avoiding the need for complex post-fabrication thickness modification steps

Inventive Principle:

Principle #35Parameter changes

3Quantity of substance

If an un-patterned bottom metal plate is used, then large capacitance values and optimal AlN crystalline orientation are achieved, but mode confinement becomes difficult

Engineering Contradiction:

Improvecapacitance valueVSAvoidmode confinement

Core Design Contradiction:

Quantity of substanceVSEase of operation

Solution Approach 1:

The metallic top layer is segmented into a grating structure with periodic conductive strips separated by gaps. This segmentation creates lateral confinement through the grating effect while maintaining the un-patterned bottom plate for optimal capacitance and crystalline orientation. The grating periodicity is designed to match the acoustic wavelength for effective mode confinement

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The device uses a composite structure combining piezoelectric material (AlN) with metallic layers (un-patterned bottom plate and patterned top grating). The piezoelectric layer provides both the resonant active region and the medium for acoustic wave propagation, while the metallic layers provide electrical grounding, capacitance, and mode confinement through their geometric configuration

Inventive Principle:

Principle #40Composite materials

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

2DRRs achieve an electromechanical coupling coefficient exceeding 7.4%, enabling efficient lithographic frequency tunability and relaxed lithographic resolution, making them suitable for monolithic integrated wideband filters in next-generation RF front-ends.

Implementation Method 1

A resonator device comprises a piezoelectric layer suspended from a substrate... a top electrode comprising a metal grating... to excite one or more dilatational modes of mechanical vibration in the rods

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

two-dimensional resonant rods (2DRRs)... excitation of an array of locally resonant rods... showing an electromechanical coupling coefficient exceeding 7.4%

Methodology Applied

Scientific EffectResonance: Resonance

Data Source

PatentUS12255603B2Two dimensional rod resonator for RF filtering

Publication Date: 2025.03.18 NORTHEASTERN UNIV (US)

AI summary

A microelectromechanical resonator device is provided having two-dimensional resonant rods. The resonator device has a piezoelectric layer formed with a plurality of alternating rods and trenches. A bottom electrode is in contact with a bottom surface of the piezoelectric layer. A top electrode metal grating of conductive strips is aligned in contact with corresponding rods of the piezoelectric layer.