FBAR Electrode Perforations for Lateral Wave Suppression
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Solution Overview
Problem
Film bulk acoustic resonators (FBARs) face performance issues due to spurious resonances caused by unwanted minor resonances near the fundamental frequency, leading to impedance ripples that affect filter performance.
Innovation Solution
The introduction of perforations and posts in the electrode and piezoelectric layers creates irregular acoustic boundary conditions, effectively suppressing spurious waves by reflecting lateral waves and improving the structural robustness of the resonator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If FBAR stack structure is used with free surfaces, then fundamental resonant frequencies are sharply defined, but spurious resonances occur due to lateral wave reflections from edges
Solution Approach 1:
The electrode layers are segmented by introducing periodic perforations (holes) through them, dividing the continuous conductive layers into periodic structures. This segmentation disrupts the propagation path of lateral acoustic waves while maintaining the electrical functionality of the electrodes, thereby suppressing spurious resonances without compromising fundamental resonance characteristics
Solution Approach 2:
Acoustic matching layers are introduced as intermediary structures between the FBAR stack and the surrounding environment. These layers have acoustic impedance values that gradually transition between the piezoelectric film and the external medium, reducing acoustic reflections at interfaces that would otherwise generate spurious lateral waves
2Ease of manufacture
If FBAR structure with cavity is used, then device integration is improved, but spurious resonances are caused by reflections from cavity edges
Solution Approach 1:
The cavity structure is modified by introducing periodic perforations through the electrode layers above the cavity. This creates a segmented structure that disrupts lateral wave reflections from the cavity edges while maintaining the cavity's beneficial integration properties
Solution Approach 2:
The acoustic impedance parameters are modified by introducing periodic variations through the perforated electrode structure. This creates impedance variations that scatter and suppress lateral acoustic waves reflecting from the cavity edges, reducing spurious resonances
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 enhances the filter performance of FBARs by reducing spurious resonances, leading to sharper defined fundamental resonant frequencies and improved impedance characteristics.
Implementation Method 1
the piezoelectric thin film layer converts the electrical energy of the signal into mechanical energy
Implementation Method 2
The bulk waves reflect from the free surfaces at the top and the bottom of the stack, yielding sharply defined fundamental resonant frequencies
Implementation Method 3
The arrangement of perforations, posts, and/or beams suppress resonance of spurious waves, e.g., by reflecting lateral waves in the FBAR
Data Source
AI summary
Devices and processes for preparing devices are described for reducing resonance of spurious waves in a bulk acoustic resonator. A first electrode is coupled to a first side of a piezoelectric layer and a second electrode is coupled to a second side of the piezoelectric layer. The piezoelectric layer is configured to resonate in response to an electrical signal applied between the first electrode and the second electrode. Perforations in the first electrode, the piezoelectric layer and/or the second electrode, and/or posts or beams supporting the second electrode, reduce resonance of spurious waves.