BAW Resonator Stack Formation for Crystal Orientation Stability
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Solution Overview
Problem
The manufacturing of Bulk Acoustic Wave (BAW) resonators faces challenges due to crystal grain orientation changes in the piezoelectric layer, leading to decreased electromechanical coupling factor and Q value, and the process is inflexible, making it difficult to address mutual interference between wireless frequency bands, especially in 5G applications.
Innovation Solution
A method is developed to form a bulk acoustic wave resonance device by forming a piezoelectric layer on a flat substrate, allowing separate processing of the second substrate and active layers, including a cavity pretreatment layer to reduce leaky waves and improve acoustic reflection, and using sacrificial and intermediate layers to enhance crystal quality and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the piezoelectric layer is formed directly on the electrode layer and substrate in a stacked structure, then the manufacturing process is simplified, but the crystal grains in the piezoelectric layer suffer from significant orientation changes, decreasing the electromechanical coupling factor and Q value
Solution Approach 1:
The piezoelectric layer is divided into multiple sub-layers (first piezoelectric sub-layer and second piezoelectric sub-layer) with different crystal orientations. This segmentation allows each sub-layer to maintain its own optimal crystal grain orientation, preventing the significant orientation changes that would occur in a single uniform layer, thereby maintaining high electromechanical coupling factor and Q value while enabling flexible manufacturing.
Solution Approach 2:
Different regions of the piezoelectric layer are assigned different crystal orientations tailored to their specific functional requirements. The first piezoelectric sub-layer has crystal grains oriented in a first direction optimized for its position, while the second piezoelectric sub-layer has crystal grains oriented in a second direction optimized for its position. This local optimization maintains high performance throughout the device while enabling the stacked manufacturing approach.
2Reliability
If a conventional BAW resonator structure is used, then high Q value and electromechanical coupling factor can be achieved, but the process is inflexible and cannot effectively address mutual interference between wireless frequency bands
Solution Approach 1:
The piezoelectric layer structure is made dynamically adaptable through the use of multiple sub-layers with different crystal orientations. This dynamic structure can be configured to optimize performance for different frequency bands and operational requirements, allowing the same basic device architecture to adapt to various wireless communication standards and frequency allocations, thereby achieving both high reliability and frequency band versatility.
Solution Approach 2:
The crystal orientation parameters of the piezoelectric layer are changed across different sub-layers to optimize performance for different operational conditions. By varying the crystal grain orientation directions between the first and second piezoelectric sub-layers, the device can be tuned for different frequency bands and operational modes, achieving high Q value and electromechanical coupling factor across multiple wireless frequency bands without requiring completely different device architectures.
3Ease of manufacture
If the piezoelectric layer is formed on a flat substrate without intermediate structures, then the manufacturing process is simpler, but leaky waves are not effectively reduced and acoustic reflection is insufficient
Solution Approach 1:
An intermediate layer is introduced between the piezoelectric layer and the substrate to serve as an acoustic reflection interface. This intermediary layer has acoustic impedance characteristics that are optimized for reflecting acoustic waves back into the resonator cavity, thereby reducing leaky wave losses. The intermediate layer acts as a mediator that improves acoustic confinement without significantly complicating the manufacturing process, as it can be formed using standard thin-film deposition techniques.
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 method improves the electromechanical coupling factor and Q value of the resonator device by maintaining consistent crystal grain orientation, reducing leaky waves, and allowing for flexible manufacturing processes, effectively addressing frequency interference issues.
Implementation Method 1
an acoustic wave is generated by the piezoelectric layer under the alternating voltages with different polarities
Implementation Method 2
the acoustic wave within the resonator propagates along a direction perpendicular to the piezoelectric layer. To form resonance, the acoustic wave requires to be totally reflected on an upper surface of an upper metal electrode and on a lower surface of a lower metal electrode to form a standing acoustic wave
Data Source
AI summary
A method for forming a bulk acoustic wave resonance device is provided, including forming a first stack, wherein forming the first stack includes: providing a first substrate; forming a piezoelectric layer on the first substrate; forming a first electrode layer on the piezoelectric layer; and forming a cavity pretreatment layer on the piezoelectric layer, wherein a first side of the first stack corresponds to a side of the first substrate, and a second side of the first stack corresponds to a side of the cavity pretreatment layer; forming a second stack, wherein forming the second stack includes providing a second substrate; joining the first stack and the second stack, wherein the second stack is disposed at the second side of the first stack; removing the first substrate; and forming a second electrode layer at the first side of the first stack and in contact with the piezoelectric layer.