XBAR Wafer Bonding for Crack-Resistant Membrane Release
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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 use of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with a novel fabrication process involving wafer-to-wafer bonding, which allows for a frontside membrane release and reduces the risk of cracking during device fabrication, enabling the creation of high-frequency capable RF filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional backside etch methods are used for FBAR fabrication, then the resonators can be formed, but the fabrication process becomes complex and time-consuming with increased risk of cracking
Solution Approach 1:
The patent inverts the traditional fabrication sequence by performing wafer-to-wafer bonding before etching the cavity. In conventional methods, the piezoelectric layer is etched first to form a membrane, then bonded. This patent bonds the piezoelectric layer to the substrate first, then etches through the substrate to release the membrane, eliminating the need for complex release hole formation and reducing cracking risk.
Solution Approach 2:
The bonding operation is performed as a preliminary action before the etching step. By bonding the piezoelectric layer to the substrate prior to cavity formation, the structure is stabilized early in the process, preventing cracking that would occur if bonding were attempted after etching on fragile released membranes.
2Productivity
If higher frequencies are used to increase bandwidth, then communication capacity improves, but existing acoustic wave resonators become unsuitable
Solution Approach 1:
The patent changes the operating frequency parameter to enable XBAR resonators to function at higher frequencies (above 3 GHz) where conventional FBAR and SAW resonators become unsuitable. This frequency parameter change, combined with the novel fabrication method, allows the resonators to maintain reliability while achieving the required bandwidth for future communication networks.
3Productivity
If wafer-to-wafer bonding is performed before etching, then fabrication time is reduced and cracking risk decreases, but the process requires precise alignment
Solution Approach 1:
Alignment marks are formed as a preliminary action on both the piezoelectric layer and substrate before bonding. This ensures that when wafer-to-wafer bonding occurs, precise alignment can be achieved quickly, reducing fabrication time while maintaining the required manufacturing precision.
Solution Approach 2:
Alignment marks serve as intermediary features that facilitate precise bonding between the piezoelectric layer and substrate. These marks provide visual or optical references that enable accurate alignment during the bonding process, reducing both time and precision requirements.
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
This approach results in RF filters with improved performance and reliability, capable of handling higher frequencies and wider bandwidths, while reducing fabrication time and costs associated with traditional backside etch methods.
Implementation Method 1
bonding the first surface of the piezoelectric substrate to a handle wafer
Implementation Method 2
thinning the piezoelectric substrate to the target piezoelectric membrane thickness
Implementation Method 3
An interdigital transducer (IDT) is formed on a thin floating layer or diaphragm of a piezoelectric material
Implementation Method 4
Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters
Data Source
AI summary
An acoustic resonator device is formed using a wafer-to-wafer bonding process by etching recesses into a first surface of a piezoelectric substrate, a depth of the recesses greater than a target piezoelectric membrane thickness; then wafer-to-wafer bonding the first surface of the piezoelectric substrate to a handle wafer using a releasable bonding method. The piezoelectric substrate is then thinned to the target piezoelectric membrane thickness to form a piezoelectric plate and at least one conductor pattern is formed on the thinned piezoelectric plate. The side of the thinned piezoelectric plate having the conductor pattern is bonded to a carrier wafer using a metal-to-metal wafer bonding process and the handle wafer is removed.