Multi-Pitch XBAR IDT Layout for Spurious Mode Suppression
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Solution Overview
Problem
Existing RF filters using acoustic wave resonators are not well-suited for higher frequencies and bandwidths required by future communications networks, particularly in 5G NR and WiFi bands, due to spurious modes that affect performance.
Innovation Solution
Employing a transversely-excited film bulk acoustic resonator (XBAR) with a multi-pitch interdigital transducer (IDT) to reduce spurious modes by varying the pitch along the length of the IDT, thereby enhancing the filter's performance at higher frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional uniform-pitch IDT is used in an XBAR resonator, then the device structure is simple and easy to manufacture, but spurious modes appear that degrade filter performance at higher frequencies
Solution Approach 1:
The IDT is designed with different pitch values in different sections along its length. The first section has a first pitch and the second section has a second pitch that differs from the first. This local variation in pitch suppresses spurious modes by disrupting the uniformity that allows them to form, while maintaining overall device functionality. Each section's pitch is optimized for specific frequency ranges, improving overall filter performance.
Solution Approach 2:
The IDT is divided into multiple sections, each with its own pitch value. This segmentation allows different portions of the IDT to operate at different frequency ranges, with the first section handling lower frequencies and the second section handling higher frequencies. This segmentation effectively suppresses spurious modes that would otherwise appear in a uniform-pitch design.
2Reliability
If the IDT pitch is varied to suppress spurious modes, then filter performance improves, but the manufacturing complexity and design difficulty increase
Solution Approach 1:
The pitch parameter of the IDT is changed across different sections to suppress spurious modes. The first section uses a first pitch value and the second section uses a second pitch value, creating a controlled parameter variation that disrupts spurious mode formation. This parameter change is implemented in a systematic way that balances performance improvement with manufacturing feasibility.
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 multi-pitch IDT effectively suppresses spurious modes, improving the filter's performance by up to 8 dB reduction in peak admittance of spurious modes, making it suitable for high-frequency applications.
Implementation Method 1
An interdigital transducer (IDT) is formed on the piezoelectric layer... A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm
Implementation Method 2
XBAR resonators provide very high electromechanical coupling and high frequency capability... The multi-pitch IDT effectively suppresses spurious modes, improving the filter's performance
Data Source
AI summary
There are disclosed acoustic resonators. An acoustic resonator includes a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to a surface of a substrate except for a portion of the piezoelectric plate forming a diaphragm spanning a cavity in the substrate. A conductor pattern on the front surface includes a multi-pitch interdigital transducer (IDT) with interleaved fingers of the IDT disposed on the diaphragm.