Patterned Cavity Walls for Precise XBAR Frontside Membrane Release

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Solution Overview

Problem

Existing RF filters, particularly those using Transversely-Excited Film Bulk Acoustic Resonators (XBARs), face challenges in efficiently fabricating flexible membrane configurations due to the limitations of the backside etch option (BSMR) and the inefficiencies of the front-side etch option (FSMR) in controlling membrane release and design flexibility.

Innovation Solution

The implementation of a front-side etch option (FSMR) using ion-milling to create elongated holes and metal fences as etch stops, allowing for patterned cavity walls that define the membrane area, while using a substrate as the vertical etch stop, enabling efficient and flexible membrane configuration fabrication.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional acoustic wave resonators are used, then existing manufacturing processes can be maintained, but frequency capability and bandwidth are insufficient for future communication networks

Engineering Contradiction:

Improvefrequency capabilityVSAvoidbandwidth

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent changes the fundamental operating parameters of the resonator by transitioning from surface acoustic wave (SAW) or bulk acoustic wave (BAW) modes to transversely-excited film bulk acoustic wave (XBAR) mode. This parameter change enables operation at higher frequencies (above 3 GHz) and wider bandwidths, directly addressing the limitation of conventional resonators for 5G NR applications.

Inventive Principle:

Principle #35Parameter changes

2Productivity

If front-side membrane release process is used, then manufacturing efficiency and flexibility are improved, but membrane configuration control is challenging

Engineering Contradiction:

Improvemanufacturing efficiencyVSAvoidmembrane configuration

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent applies preliminary action by forming the cavity structure and releasing the membrane from the front side before final device assembly. This allows the membrane to be released and configured in its final position with precise control over its geometry and orientation, while maintaining manufacturing efficiency through a streamlined process.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent introduces a new dimensional approach by accessing the membrane from the front side (top surface) rather than the back side. This dimensional change enables precise control over membrane configuration during the release process, allowing for complex membrane geometries to be formed with high precision while maintaining manufacturing efficiency.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Speed

If higher frequency operation is implemented, then communication bandwidth is improved, but insertion loss and power handling become more difficult to optimize

Engineering Contradiction:

ImprovefrequencyVSAvoidinsertion loss

Core Design Contradiction:

SpeedVSLoss of energy

Solution Approach 1:

The patent employs composite material structures including piezoelectric films deposited on sacrificial substrates, with multiple layers having different acoustic and mechanical properties. This composite approach enables optimization of electromechanical coupling and reduction of energy loss at high frequencies, simultaneously improving frequency capability and minimizing insertion loss.

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

This approach enhances the efficiency and control of membrane release, maintaining the advantages of FSMR while allowing for the flexibility of BSMR patterning on blank substrates, improving the reliability and consistency of XBAR fabrication.

Implementation Method 1

the etch-stop material layer defining a depth of the cavity when the cavity is etched through the piezoelectric plate

Methodology Applied

Scientific EffectEtch-stop:

Implementation Method 2

A piezoelectric plate is attached to a front surface of the substrate with an intervening oxide layer

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS12620961B2Patterned cavity walls for transversely-excited film bulk acoustic resonator frontside membrane release

Publication Date: 2026.05.05 MURATA MFG CO LTD

AI summary

Acoustic resonator devices and methods are disclosed. An acoustic resonator device includes a substrate having a front surface and an intervening oxide layer on the front surface and having a cavity. A thickness of the intervening oxide layer defines a depth of the cavity, and the substrate has vertical etch-stop material for etching the intervening oxide layer. Lateral fences formed in the intervening oxide layer define a perimeter of the cavity. The lateral fences has a lateral etch-stop material for etching the intervening oxide layer. A single-crystal piezoelectric plate has a back surface attached to the front surface of the intervening oxide layer except for a portion of the piezoelectric plate forming a diaphragm that spans the cavity. An interdigital transducer is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm.