Dielectric-Cover XBAR Packaging for Low-Loss RF Filters

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Solution Overview

Problem

Existing RF filters using acoustic wave resonators, such as SAW and BAW resonators, are not well-suited for higher frequencies and bandwidths required by future communications networks, particularly in 5G NR bands n77, n79, and WiFi bands at 5 GHz and 6 GHz, due to high insertion loss and capacitive coupling issues.

Innovation Solution

The use of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with wafer-level packaging (WLP) employing a dielectric cover, such as glass or quartz, to reduce capacitive coupling and insertion loss, thereby improving RF performance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If conventional SAW and BAW resonators are used for RF filters, then manufacturing is simpler, but insertion loss increases and bandwidth is limited at higher frequencies

Engineering Contradiction:

Improveinsertion lossVSAvoiddevice complexity

Core Design Contradiction:

Loss of energyVSDevice complexity

Solution Approach 1:

The patent changes the fundamental operating parameters of the resonator by switching from surface acoustic waves (SAW) or bulk acoustic waves (BAW) to transversely-excited film bulk acoustic resonators (XBAR). This parameter change enables operation at higher frequencies (3 GHz and above) with reduced insertion loss and improved bandwidth, directly resolving the contradiction between energy loss and device complexity.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The XBAR structure employs composite materials including piezoelectric films (such as AlN or GaN) deposited on substrate structures. This composite approach enables the resonator to achieve both low insertion loss and high frequency performance while maintaining manufacturability through established thin-film deposition techniques.

Inventive Principle:

Principle #40Composite materials

2Productivity

If higher frequency communications bands are used to increase bandwidth, then communication capacity improves, but capacitive coupling issues and insertion loss increase

Engineering Contradiction:

Improvecommunication bandwidthVSAvoidinsertion loss

Core Design Contradiction:

ProductivityVSLoss of energy

Solution Approach 1:

The patent addresses the bandwidth-insertion loss contradiction by changing the resonator type to XBAR, which maintains low insertion loss even at higher frequencies (3 GHz and above). The transverse excitation mode and film bulk acoustic wave propagation enable efficient energy transfer at these frequencies, allowing increased communication bandwidth without proportional increases in insertion loss.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If conventional acoustic wave resonators are used, then device structure is simpler, but capacitive coupling across resonators increases at higher frequencies

Engineering Contradiction:

Improvedevice structureVSAvoidcapacitive coupling

Core Design Contradiction:

Device complexityVSObject-generated harmful factors

Solution Approach 1:

The patent resolves the capacitive coupling issue by changing the resonator operating mode to transverse excitation in film bulk acoustic resonators. This parameter change fundamentally alters the electric field distribution and acoustic wave propagation characteristics, reducing parasitic capacitive coupling between adjacent resonators while maintaining a relatively simple device structure suitable for wafer-level packaging.

Inventive Principle:

Principle #35Parameter changes

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 with dielectric covers achieve lower insertion loss and improved bandwidth, making them suitable for high-frequency communications bands, enhancing filter performance and reducing capacitive coupling across resonators.

Implementation Method 1

a piezoelectric material layer... A radio frequency signal applied to the interdigital transducers excites a primary acoustic mode

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

XBAR resonators provide very high electromechanical coupling and high frequency capability... excites a shear primary acoustic wave in the piezoelectric diaphragm

Methodology Applied

Scientific EffectAcoustic resonance: Resonance

Data Source

PatentUS20260031787A1Transversely-excited film bulk acoustic resonators wafer-level packaging using a dielectric cover

Publication Date: 2026.01.29 MURATA MFG CO LTD

AI summary

An acoustic resonator device is provided that includes a substrate having a surface; a piezoelectric layer attached to the surface of the substrate via an intermediate layer, the piezoelectric layer including a portion that is over a cavity in the intermediate layer; a conductor pattern including an interdigital transducer (IDT) on a surface of the piezoelectric layer and having interleaved fingers on the piezoelectric layer; a dielectric layer at least between the interleaved fingers of the IDT; and a dielectric cover over the IDT and the piezoelectric layer, the dielectric cover including a bottom surface, wherein at least a portion of the dielectric cover is attached to a portion of the conductor pattern, wherein the conductor pattern includes at least two metal layers including a first metal layer of the interleaved fingers of the IDT and a second metal layer attached to the dielectric cover.