Electret Acoustic Resonator for Precise Frequency Tuning
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Solution Overview
Problem
In the field of mobile telephony, acoustic resonators require precise frequency adjustment, but conventional deposition techniques for piezoelectric materials like AlN achieve only 1% accuracy, necessitating expensive and complex trimming processes, and the application of DC voltage to adjust resonance frequency is not feasible due to battery voltage limitations.
Innovation Solution
Incorporating an electret layer that applies a permanent and constant electric field to the piezoelectric layer, shifting the resonant frequency by adjusting the electric field intensity, allowing for precise frequency adjustment without the need for complex trimming or high voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional deposition techniques (sputtering) are used to form piezoelectric layers and electrodes, then manufacturing process is simple and well-established, but the thickness accuracy achieves at best 1% which is insufficient for 0.1% frequency precision requirement
Solution Approach 1:
The patent applies preliminary action by depositing a sacrificial load layer (SiO2, SiN, or metal) with controlled thickness before the piezoelectric layer, then selectively etching portions of this load layer to adjust the resonant frequency. This preliminary deposition and selective removal approach enables precise frequency control without requiring ultra-precise thickness control during the main piezoelectric layer deposition process.
2Ease of operation
If a DC voltage is applied to the piezoelectric layer to shift resonant frequency, then frequency adjustment is achieved without trimming equipment, but the required voltage (±70V to ±190V) exceeds battery voltage capability in mobile devices
Solution Approach 1:
The patent replaces the electrical field-based frequency adjustment (DC voltage application) with a mechanical/structural approach by selectively removing material from the load layer. This structural modification creates a permanent mechanical change in the resonator's mass distribution and stress state, achieving frequency shift without requiring high voltage electrical fields that would exceed mobile device power constraints.
3Productivity
If localized etching trimming is performed to adjust resonant frequency, then manufacturing yield is considerably increased, but expensive and complex localized etching equipment is required with difficult maintenance
Solution Approach 1:
The patent introduces an intermediary load layer (SiO2, SiN, or metal) that serves as a sacrificial material for frequency adjustment. This intermediary layer mediates between the deposition process and the frequency tuning requirement, allowing standard deposition equipment to create the layer and simpler etching processes to remove portions of it, thereby achieving frequency adjustment without requiring complex localized etching equipment.
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 solution enables precise frequency adjustment of acoustic resonators, improving manufacturing yield and eliminating the need for expensive trimming equipment and high voltage applications, while maintaining material integrity.
Implementation Method 1
acoustic resonators of the SAW type are currently used (for Acoustic Wave surface) or BAW type (for Bulk Acoustic Wave) which use piezoelectric materials such as LiNbO3, LiTaO3 and AlN
Implementation Method 2
an electret is added to the latter. This electret applies a permanent and constant electric field to the layer
Data Source
Figure 1~3
Figure 4A~5
Figure 6A~6C
AI summary
The resonator has electrodes (24, 26) arranged on both sides of a piezoelectric layer (30). An electret layer (32) is provided between the electrodes for applying permanent electric field to the piezoelectric layer, where the piezoelectric layer has electromechanical properties. The electret layer is made of piezoelectric material, crystalline and amorphous, where the electret layer has permanent electric loads (28). A substrate is provided in the cavity. An independent claim is also included for a method for fabricating an acoustic resonator.