BAW Resonator Transfer Structure With Dielectric Protection Layer
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Solution Overview
Problem
Conventional bulk acoustic wave resonators using polycrystalline piezoelectric thin films face degradation issues at frequencies above 3 GHz, as the quality of these films degrades quickly with decreasing thickness, limiting their performance in high-frequency applications.
Innovation Solution
The use of single crystalline or epitaxial piezoelectric thin films grown on compatible substrates, combined with advanced transfer processes and energy confinement structures, to create high-quality bulk acoustic wave resonators capable of operating at frequencies up to 5 GHz and beyond, utilizing techniques such as sacrificial layer transfer, cavity bond transfer, and solidly mounted transfer processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If polycrystalline piezoelectric thin films are used in bulk acoustic wave resonators, then manufacturing is simpler and cost is lower, but film quality degrades quickly as thickness decreases below 0.5 um, limiting operation to frequencies up to 3 GHz
Solution Approach 1:
The patent changes the fundamental parameter of crystalline structure from polycrystalline to single crystal, enabling the piezoelectric film to maintain high quality at thicknesses below 0.5 um. This parameter change allows the film to support higher frequencies (5 GHz and above) while preserving manufacturing feasibility through established single crystal growth techniques on compatible substrates.
Solution Approach 2:
The patent employs a composite structure combining single crystal piezoelectric thin films with compatible crystalline substrates. This composite approach leverages the superior piezoelectric properties of single crystals while using the substrate to provide mechanical support and enable controlled thinning to achieve the required thickness for high-frequency operation.
2Reliability
If single crystal piezoelectric thin films are used to achieve high-frequency operation above 5 GHz, then film quality and piezoelectric performance are maintained, but manufacturing complexity increases due to transfer process challenges
Solution Approach 1:
The patent applies preliminary action by growing the single crystal piezoelectric thin film on a compatible crystalline substrate before the resonator fabrication process. This preliminary growth step ensures high crystal quality and piezoelectric performance are established early, and the subsequent transfer process merely relocates the pre-formed high-quality film rather than attempting to grow it in the final configuration.
Solution Approach 2:
The patent uses a sacrificial layer as an intermediary element in the transfer process. The sacrificial layer is deposited over the single crystal piezoelectric film, bonded to a support wafer, and then removed to release and transfer the piezoelectric film to the final resonator structure. This intermediary approach simplifies the complex transfer operation by providing a temporary handling medium.
3Speed
If piezoelectric thin film thickness is reduced to achieve higher resonant frequencies, then operating frequency increases, but film quality degradation occurs with polycrystalline materials
Solution Approach 1:
The patent changes the crystalline structure parameter from polycrystalline to single crystal, which fundamentally alters how the material responds to thickness reduction. Single crystal structure maintains atomic order and piezoelectric properties even at very thin dimensions, enabling the film to achieve the reduced thickness required for 5 GHz and above operation without quality degradation.
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
These methods enhance the quality factor and electromechanical coupling of RF filters, enabling reliable operation at high frequencies while maintaining cost-effectiveness and simplicity in manufacturing.
Implementation Method 1
Bulk acoustic wave resonators (BAWR) using crystalline piezoelectric thin films
Implementation Method 2
The sacrificial layer can then be etched to make the air reflection cavity at one side of the BAW resonator
Data Source
AI summary
A method and structure for a transfer process for an acoustic resonator device. In an example, a bulk acoustic wave resonator (BAWR) with an air reflection cavity is formed. A piezoelectric thin film is grown on a crystalline substrate. Patterned electrodes are deposited on the surface of the piezoelectric film. An etched sacrificial layer is deposited over the electrodes and a planarized support layer is deposited over the sacrificial layer. The device can include a dielectric protection layer (DPL) that protects the piezoelectric layer from etching processes that can produce rough surfaces and reduces parasitic capacitance around the perimeter of the resonator when the DPL’s dielectric constant is lower than that of the piezoelectric layer. The DPL can be configured between the top electrode and the piezoelectric layer, between the bottom electrode and the piezoelectric layer, or both.