XBAR Filter Structure With Trap-Rich Silicon for High-Frequency RF

Solution Overview

Problem

Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications networks, particularly those above 3 GHz, due to limitations in performance parameters such as insertion loss, rejection, and bandwidth.

Innovation Solution

A transversely-excited film bulk acoustic resonator (XBAR) is developed, utilizing a piezoelectric film bonded to a high resistivity silicon substrate with a trap-rich layer, which enhances the acoustic wave resonator's performance by reducing substrate conductivity and increasing piezoelectric coupling, enabling effective operation at higher frequencies.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional acoustic wave resonators are used for RF filters, then the filters can operate at lower frequencies, but they cannot achieve satisfactory performance at higher frequencies above 3 GHz due to limitations in insertion loss, rejection, and bandwidth

Engineering Contradiction:

Improvefilter performanceVSAvoidoperating frequency

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The patent changes the substrate resistivity parameter from conventional low resistivity silicon to high resistivity silicon (greater than 1000 ohm-cm), which fundamentally alters the electrical characteristics and enables high-frequency operation. This parameter change reduces substrate losses and improves Q-factor, allowing the resonator to operate effectively above 3 GHz while maintaining good insertion loss and rejection characteristics

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs a composite structure combining piezoelectric film (such as aluminum nitride or zinc oxide) with high resistivity silicon substrate. This composite material system leverages the high piezoelectric coupling of the film material and the low loss properties of the high resistivity substrate, achieving both high-frequency operation and satisfactory filter performance with improved insertion loss and bandwidth

Inventive Principle:

Principle #40Composite materials

2Device complexity

If the piezoelectric film is bonded directly to the silicon substrate, then the structure is simple, but the substrate conductivity causes increased insertion loss and reduced Q-factor at higher frequencies

Engineering Contradiction:

ImprovestructureVSAvoidinsertion loss

Core Design Contradiction:

Device complexityVSLoss of energy

Solution Approach 1:

The patent changes the substrate resistivity parameter from conventional low resistivity silicon to high resistivity silicon (greater than 1000 ohm-cm), which fundamentally alters the electrical characteristics and enables high-frequency operation. This parameter change reduces substrate losses and improves Q-factor, allowing the resonator to operate effectively above 3 GHz while maintaining good insertion loss and rejection characteristics

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent introduces an intermediary layer (such as a dielectric layer or trap-rich layer) between the piezoelectric film and the silicon substrate. This intermediary layer acts as a barrier that reduces the interaction between the piezoelectric field and the conductive substrate, thereby reducing parasitic losses and improving Q-factor at high frequencies while maintaining structural integrity

Inventive Principle:

Principle #24Intermediary (Mediator)

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 XBAR design improves filter performance by reducing insertion loss and increasing bandwidth, making it suitable for frequencies up to 28 GHz, thereby addressing the limitations of existing RF filters in future communication systems.

Implementation Method 1

A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

utilizing a piezoelectric film bonded to a high resistivity silicon substrate with a trap-rich layer, which enhances the acoustic wave resonator's performance by reducing substrate conductivity

Methodology Applied

Scientific EffectElectrical resistivity: Electrical Resistance

Implementation Method 3

A transversely-excited film bulk acoustic resonator (XBAR) is developed, utilizing a piezoelectric film bonded to a high resistivity silicon substrate with a trap-rich layer, which enhances the acoustic wave resonator's performance

Methodology Applied

Scientific EffectAcoustic wave resonance: Resonance

Data Source

PatentUS11984868B2Filter using piezoelectric film bonded to high resistivity silicon substrate with trap-rich layer

Publication Date: 2024.05.14 MURATA MFG CO LTD

AI summary

Acoustic resonator devices and filters are disclosed. An acoustic resonator includes a substrate having a trap-rich region adjacent to a surface and a single-crystal piezoelectric plate having parallel front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The single-crystal piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode within the diaphragm.