XBAR Membrane Release Using a Sacrificial Tub to Reduce Spurious Modes
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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 by future communication networks, particularly for 5G NR standards which include bands above 3 GHz and millimeter wave frequencies, due to limitations in existing technologies such as SAW and BAW resonators.
Innovation Solution
The development of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with improved etch fabrication techniques using a silicon nitride sacrificial layer or tub, allowing for front-side or back-side etch membrane release, which reduces spurious modes and enables high electromechanical coupling and frequency capability suitable for frequencies above 3 GHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional SAW and BAW resonators are used, then existing RF filter technologies are available, but they are not well-suited for higher frequencies and wider bandwidths required by future communication networks
Solution Approach 1:
The patent employs parameter changes by modifying the resonator structure from conventional SAW/BAW to XBAR configuration, changing the acoustic wave propagation mode from surface or bulk waves to transverse excitation mode. This structural parameter change enables the resonator to operate effectively at higher frequencies (above 3 GHz) and wider bandwidths, making it suitable for 5G NR standards while maintaining reliable performance
2Ease of manufacture
If traditional etch fabrication techniques are used, then existing manufacturing processes are available, but spurious modes are not effectively reduced
Solution Approach 1:
The patent applies the extraction principle by removing the sacrificial tub material through selective etching processes. This extraction of the sacrificial structure creates the necessary cavity geometry that suppresses spurious acoustic modes while maintaining the integrity of the piezoelectric plate and underlying substrate, thereby reducing harmful spurious modes without complicating the fabrication process
Solution Approach 2:
The sacrificial tub serves as an intermediary element during fabrication. It is temporarily introduced to define the cavity geometry, then selectively removed through etching. This intermediary approach allows precise control over the cavity shape and size, which is critical for suppressing spurious modes, while keeping the fabrication process manageable through standard semiconductor manufacturing techniques
3Strength
If complex substrate structures are used, then structural support is provided, but substrate complexity increases
Solution Approach 1:
The patent simplifies the substrate structure by extracting or removing the sacrificial tub material through selective etching. This leaves a clean cavity geometry with minimal additional structural elements, providing necessary structural support through the piezoelectric plate and substrate combination while reducing overall device complexity compared to approaches requiring complex multi-layer substrate structures
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
XBARs provide enhanced performance by reducing spurious modes and achieving high frequency capability, making them suitable for advanced communication bands, including 5G NR frequencies, with improved fabrication techniques that simplify the process and reduce substrate complexity.
Implementation Method 1
a piezoelectric plate bonded to a substrate
Implementation Method 2
Current RF filters using acoustic wave resonators are not well-suited for higher frequencies and wider bandwidths required by future communication networks, particularly for 5G NR standards which include bands above 3 GHz and millimeter wave frequencies
Implementation Method 3
Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with improved etch fabrication techniques
Data Source
AI summary
An acoustic resonator device is formed using a sacrificial layer and a front side etched cavity by forming a recess in a silicon substrate with a trap-rich top layer and filling the recess with sacrificial silicon nitride. A bonding oxide (BOX) layer is formed over the trap-rich layer and the sacrificial silicon nitride filled recess and a piezoelectric plate is bonded to the BOX layer. The sacrificial silicon nitride is then removed to form a cavity by using an etchant introduced through holes in the piezoelectric plate and BOX layer without removing the BOX layer from over the cavity.