Bulk Acoustic Wave Resonator Mass Adjustment for Q and Flatness

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Solution Overview

Problem

Conventional bulk acoustic wave resonators face issues with mechanical strength and flatness, leading to potential collapse and adverse effects on acoustic wave characteristics, while existing methods for enhancing the Q factor and suppressing spurious modes are inadequate.

Innovation Solution

A method involving the formation of a sacrificial structure mesa on a substrate, followed by the deposition of an insulating layer and a polish layer, with the polish layer serving as both a mass adjustment and frequency tuning structure, to enhance mechanical strength and adjust resonance frequency without compromising flatness.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If the piezoelectric layer material is removed around the periphery of the resonance film to form an annular piezoelectric layer recess, then the Q factor is enhanced, but the mechanical strength of the resonance film deteriorates

Engineering Contradiction:

ImproveQ factorVSAvoidmechanical strength of resonance film

Core Design Contradiction:

Loss of energyVSStrength

Solution Approach 1:

The patent introduces a protruding structure at the periphery of the resonance film that breaks the symmetric annular recess design. This asymmetric protrusion reinforces the mechanical strength at the critical periphery region while preserving the acoustic wave reflection properties that enhance the Q factor.

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The protruding structure is formed during the fabrication process before the resonance film is fully operational. This preliminary structural reinforcement prevents mechanical collapse during subsequent operations and ensures the resonance film maintains its integrity throughout the device lifecycle.

Inventive Principle:

Principle #10Preliminary action

2Loss of energy

If a protruding structure is formed along the edge of the top surface of the bottom electrode, then the Q factor is enhanced, but the flatness of the resonance film deteriorates

Engineering Contradiction:

ImproveQ factorVSAvoidflatness of resonance film

Core Design Contradiction:

Loss of energyVSManufacturing precision

Solution Approach 1:

The protruding structure is localized specifically at the periphery of the resonance film rather than being uniformly distributed. This local modification enhances the Q factor through improved acoustic wave reflection at the boundaries while minimizing the impact on the overall flatness of the resonance film's top surface.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The protruding structure extends in the vertical dimension from the top surface of the bottom electrode, creating a three-dimensional feature that affects acoustic wave propagation without significantly altering the two-dimensional flatness of the resonance film's piezoelectric layer.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

3Loss of energy

If the resonance film is made wider to improve acoustic wave propagation, then the Q factor is enhanced, but the mechanical strength deteriorates due to increased bending

Engineering Contradiction:

ImproveQ factorVSAvoidmechanical strength of resonance film

Core Design Contradiction:

Loss of energyVSStrength

Solution Approach 1:

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The protruding structure introduces a curved or elevated feature at the periphery that modifies the stress distribution across the resonance film. This curvature helps distribute mechanical stresses more evenly, preventing excessive bending in wider resonance films while maintaining acoustic wave propagation characteristics.

Inventive Principle:

Principle #14Spheroidality (Curvature)

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 method significantly enhances the mechanical strength of the resonance film, maintains flatness, and effectively suppresses spurious modes, thereby improving the overall performance of the bulk acoustic wave resonator.

Implementation Method 1

the polish layer serving as both a mass adjustment and frequency tuning structure

Methodology Applied

Scientific EffectMass adjustment:

Implementation Method 2

The piezoelectric layer 92 is formed on the bottom electrode 91. The top electrode 93 is formed on the piezoelectric layer 92

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 3

Since the acoustic wave is propagating and resonating in the resonance film of the bulk acoustic wave resonator

Methodology Applied

Scientific EffectAcoustic wave resonance: Resonance

Data Source

PatentUS11502661B2Method for fabricating bulk acoustic wave resonator with mass adjustment structure

Publication Date: 2022.11.15 WIN SEMICON

AI summary

A method for fabricating bulk acoustic wave resonator with mass adjustment structure, comprising following steps of: forming a sacrificial structure mesa on a substrate; etching the sacrificial structure mesa such that any two adjacent parts have different heights, a top surface of a highest part of the sacrificial structure mesa is coincident with a mesa top extending plane; forming an insulating layer on the sacrificial structure mesa and the substrate; polishing the insulating layer to form a polished surface; forming a bulk acoustic wave resonance structure including a top electrode, a piezoelectric layer and a bottom electrode on the polished surface; etching the sacrificial structure mesa to form a cavity; the insulating layer between the polished surface and the mesa top extending plane forms a frequency tuning structure, the insulating layer between the mesa top extending plane and the cavity forms a mass adjustment structure.