XBAR Frontside Etch Using Polysilicon Sacrificial Cavity
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Solution Overview
Problem
Current RF filters, particularly those using acoustic wave resonators, are not well-suited for higher frequencies and wider bandwidths required in future communication networks, such as the 5G NR standard, which necessitates the development of more effective filtering solutions for frequencies above 3 GHz.
Innovation Solution
The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) with a front-side etch process using a sacrificial polycrystalline silicon layer to form a predefined diaphragm area, allowing for the creation of a controlled cavity structure that enhances the resonator's performance and stability, particularly in high-frequency applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional acoustic wave resonators are used, then the filter can operate at lower frequencies, but the filter cannot achieve the higher frequencies and wider bandwidths required for future communication networks
Solution Approach 1:
The patent changes the physical parameters of the resonator by transitioning from surface acoustic wave (SAW) to film bulk acoustic wave (FBAR) technology, and further to XBAR configuration. This involves changing the wave propagation mode from surface to bulk, modifying the resonator geometry to achieve higher frequency operation while maintaining performance stability through controlled parameter optimization
Solution Approach 2:
The patent employs composite material structures including piezoelectric films bonded to substrate, with carefully selected material combinations to achieve both high frequency capability and stable performance. The use of multiple functional layers with different acoustic and mechanical properties enables simultaneous optimization of frequency response and reliability
2Manufacturing precision
If the diaphragm area is not predefined, then the manufacturing process is simpler, but the resonator performance and stability deteriorate
Solution Approach 1:
The patent applies preliminary action by forming a sacrificial polysilicon layer before the main resonator fabrication. This sacrificial layer defines the future diaphragm area and is removed later to create the acoustic cavity. The preliminary structuring enables precise diaphragm area control while managing fabrication complexity through staged processing
Solution Approach 2:
The sacrificial polysilicon layer acts as an intermediary element that temporarily occupies the cavity space during fabrication. It mediates between the manufacturing process requirements and the final device structure, enabling precise cavity formation without direct complex patterning of the final resonator structure
3Manufacturing precision
If no sacrificial layer is used, then the fabrication process is shorter, but the cavity formation precision and resonator performance are compromised
Solution Approach 1:
The sacrificial polysilicon layer is deposited and patterned in advance to define the precise cavity geometry before the resonator structure is built. This preliminary action ensures high cavity formation precision while allowing parallel processing of other resonator components, thereby managing the impact on fabrication throughput
Solution Approach 2:
The sacrificial layer is selectively removed (taken out) after serving its defining purpose. This extraction creates the acoustic cavity with high precision without requiring direct patterning of the cavity boundaries in the final structure, thus improving cavity precision while limiting the increase in overall process steps
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 resonator provides high electromechanical coupling and frequency capability, enabling improved performance in RF filters for higher frequency bands, including those above 3 GHz, by maintaining a stable diaphragm area and reducing resistive and acoustic losses.
Implementation Method 1
a piezoelectric material 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... excites a shear primary acoustic wave in the piezoelectric diaphragm
Data Source
AI summary
An acoustic resonator is fabricated by etching a recess in a silicon thermal oxide (TOX) upper layer of a silicon substrate and filling the recess with sacrificial polysilicon. A surface of the silicon TOX layer and the sacrificial polysilicon-filled recess are planarized. A back surface of a single-crystal piezoelectric plate is bonded to the planarized surface of the silicon TOX layer. Openings are formed through the piezoelectric plate and an interdigital transducer (IDT) is formed on a front surface of the piezoelectric plate such that interleaved fingers of the IDT are disposed over the sacrificial polysilicon-filled recess. The sacrificial polysilicon is removed from the recess to form a cavity such that a portion of the piezoelectric plate forms a diaphragm spanning the cavity and the interleaved fingers of the IDT are disposed on the diaphragm.