XBAR IDT Busbar Layout for Lower Diaphragm Stress
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Solution Overview
Problem
Current RF filters using acoustic wave resonators are not well-suited for higher frequencies and wider bandwidths required in future communications networks, particularly for 5G NR standards and millimeter wave communication bands, due to limitations in handling transmit power and achieving optimal performance parameters like insertion loss, rejection, and bandwidth.
Innovation Solution
The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) design, which includes an interdigital transducer on a thin floating layer of single-crystal piezoelectric material with busbars positioned off the diaphragm to reduce stress, enabling high electromechanical coupling and frequency capability, is used to create RF filters suitable for frequencies above 3 GHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If busbars are positioned on the diaphragm, then electrical connection is simplified, but diaphragm stress increases reducing reliability
Solution Approach 1:
The busbars are extracted from the diaphragm structure and positioned on the substrate beneath the resonator. This separation removes the stress-inducing electrical connections from the diaphragm while maintaining electrical connectivity through the substrate, thus resolving the contradiction between manufacturing simplicity and diaphragm reliability.
2Ease of manufacture
If existing acoustic wave resonator designs are used, then manufacturing is straightforward, but performance at higher frequencies and wider bandwidths is insufficient
Solution Approach 1:
The invention changes the fundamental operating parameters by using transversely-propagating acoustic waves instead of vertically-propagating waves, and by employing a laterally-polished substrate to enable millimeter-wave frequencies. These parameter changes allow the resonator to achieve higher frequencies and wider bandwidths while maintaining a manufacturable structure.
3Loss of energy
If conventional RF filter designs are used, then insertion loss is acceptable, but rejection and isolation performance deteriorate at higher frequencies
Solution Approach 1:
The invention uses a composite structure combining a laterally-polished semiconductor substrate with a piezoelectric resonator layer. This composite approach enables superior acoustic wave confinement and electromechanical coupling, achieving both low insertion loss and high rejection/isolation performance at millimeter-wave frequencies simultaneously.
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 enhances RF filter performance by achieving high piezoelectric coupling (>20%) and enabling the design of microwave and millimeter-wave filters with appreciable bandwidth, addressing the limitations of existing technologies in handling higher frequencies and wider bandwidths.
Implementation Method 1
An interdigital transducer (IDT) is formed on the piezoelectric plate and is configured to convert an electrical signal to a shear acoustic mode
Implementation Method 2
A cavity is formed in the substrate and the IDT is disposed over the cavity such that the IDT excites a shear acoustic mode in the piezoelectric plate
Data Source
AI summary
Acoustic resonators are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces. The back surface is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the piezoelectric plate. The IDT includes: a first busbar and a second busbar disposed on respective portions of the piezoelectric plate other than the diaphragm; a first set of elongate fingers extending from the first bus bar onto the diaphragm; and a second set of elongate fingers extending from the second bus bar onto the diaphragm, the second set of elongate fingers interleaved with the first set of elongate fingers.