LN/LT FBAR Cavity Structure for Mass-Production Bandwidth Gains

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Solution Overview

Problem

The challenge lies in the lack of a suitable manufacturing method for lithium niobate (LN) or lithium tantalate (LT) film bulk acoustic resonator (FBAR) structures suitable for mass production, as conventional deposition processes fail to produce high-quality piezoelectric material layers, and forming a bottom electrode is difficult in existing FBAR structures.

Innovation Solution

A film bulk acoustic resonator structure is developed, comprising a bottom cap wafer with a lithium niobate or lithium tantalate piezoelectric layer, a bottom electrode, and a top electrode, where the method involves forming a sacrificial island, bonding a bottom cap wafer, and removing the island to create a cavity, enabling precise patterning and minimizing parasitic capacitance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional PVD or CVD processes are used to form piezoelectric material layers, then the manufacturing process is simple and compatible with existing FBAR/BAW filter manufacturing, but high-quality LT and LN piezoelectric material layers cannot be obtained

Engineering Contradiction:

Improvequality of piezoelectric material layerVSAvoidmanufacturing process complexity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The manufacturing process is divided into two independent stages: (1) forming the piezoelectric material layer using molten crystal pulling method to ensure high quality, and (2) forming the bottom electrode and FBAR structure using conventional PVD/CVD processes. This segmentation allows each stage to use the most appropriate method for its specific requirements.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The piezoelectric material layer is pre-formed using molten crystal pulling method before the main FBAR structure fabrication. This preliminary action ensures high crystal quality is achieved before subsequent processing steps, avoiding the need to form the piezoelectric layer during the conventional PVD/CVD process.

Inventive Principle:

Principle #10Preliminary action

2Ease of manufacture

If the LN layer is directly formed on the SiO2 layer without a metal layer, then the structure is simple, but it is difficult to form a bottom electrode of the LN or LT FBAR structure

Engineering Contradiction:

Improvestructure simplicityVSAvoidbottom electrode formation difficulty

Core Design Contradiction:

Ease of manufactureVSDevice complexity

Solution Approach 1:

A metal layer is pre-formed on the SiO2 layer before the LN piezoelectric layer is deposited. This preliminary metal layer serves as the bottom electrode, eliminating the complexity of forming the bottom electrode after LN layer deposition and providing a straightforward fabrication sequence.

Inventive Principle:

Principle #10Preliminary action

3Reliability

If LN or LT materials are used in FBAR structures, then bandwidth and electromechanical coupling are improved, but mass production manufacturing methods are not yet available

Engineering Contradiction:

Improvebandwidth and electromechanical couplingVSAvoidmass production capability

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The manufacturing process is segmented into: (1) forming piezoelectric material layer via molten crystal pulling, (2) forming bottom electrode and patterned structures via PVD/CVD, (3) bonding cap wafer, and (4) releasing cavity. This segmentation enables each step to be optimized independently for both quality and scalability to mass production.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention uses conventional PVD and CVD equipment and processes, which are already widely available in existing FBAR/BAW filter manufacturing lines. This universality allows LN/LT FBAR structures to be manufactured using existing industrial infrastructure, enabling mass production without requiring specialized new equipment.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 allows for the production of FBARs with higher bandwidth and reduced area, leveraging the superior electromechanical coupling and dielectric constants of LN and LT materials, resulting in more efficient and cost-effective resonators compared to those using aluminum nitride.

Implementation Method 1

lithium niobate (LN) or lithium tantalate (LT) piezoelectric crystal materials have been used in the large-scale industrial manufacturing of surface acoustic wave (SAW) filters

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

film bulk acoustic resonator (FBAR) structure

Methodology Applied

Scientific EffectAcoustic resonance: Resonance

Data Source

PatentUS11533039B2Lithium niobate or lithium tantalate FBAR structure and fabricating method thereof

Publication Date: 2022.12.20 SHENZHEN NEWSONIC TECH CO LTD

AI summary

A film bulk acoustic resonator (FBAR) structure includes, a bottom cap wafer, a piezoelectric layer disposed on the bottom cap wafer, the piezoelectric layer including lithium niobate or lithium tantalate, a bottom electrode disposed below the piezoelectric layer, and a top electrode disposed above the piezoelectric layer. Portions of the bottom electrode, the piezoelectric layer, and the top electrode that overlap with each other constitute a piezoelectric stack. The FBAR structure also includes a cavity disposed below the piezoelectric stack. A projection of the piezoelectric stack is located within the cavity.