BAW Resonator Backside Coating for Passband Ripple Suppression
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Solution Overview
Problem
Bulk acoustic wave (BAW) resonators experience passband ripple due to acoustic energy leakage into the substrate, causing increased insertion loss and reflections, which are exacerbated when the substrate thickness is an odd multiple of quarter wavelengths, leading to suboptimal performance.
Innovation Solution
Applying a lossy material with an optimal acoustic impedance, such as epoxy, to the polished substrate backside to absorb and scatter leaked acoustic energy, thereby reducing passband ripple by creating a well-defined boundary condition and improving isolation from the substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a Bragg mirror with multiple high and low impedance layers is used to achieve high reflection coefficient and isolate the BAW resonator from the substrate, then the acoustic isolation is improved, but the device complexity and manufacturing cost increase significantly
Solution Approach 1:
The invention extracts the essential function of acoustic isolation from the complex multi-layer Bragg mirror structure and implements it through a simplified single-layer coating on the substrate backside. This coating has acoustic impedance between 1-100×10^6 kg/m²s, which is sufficient to prevent substrate thickness effects without requiring multiple alternating high and low impedance layers.
Solution Approach 2:
The invention changes the acoustic impedance parameter of the substrate backside coating to a specific range (1-100×10^6 kg/m²s) that effectively prevents the substrate thickness effect. This parameter optimization allows a single layer to replace multiple layers while achieving the same isolation performance.
2Manufacturing precision
If the substrate thickness is reduced to ≤400 μm to minimize passband ripple, then the passband linearity is improved, but the mechanical strength and handling robustness of the substrate decrease
Solution Approach 1:
The invention introduces an intermediary acoustic impedance coating layer on the substrate backside that mediates between the substrate and the acoustic waves. This coating absorbs and scatters leaked acoustic energy, preventing reflections that cause passband ripple, thereby allowing thinner substrates to be used without compromising mechanical integrity.
3Manufacturing precision
If a lossy material coating is applied to the substrate backside to absorb scattered acoustic energy, then the passband ripple is reduced, but the insertion loss increases due to energy absorption
Solution Approach 1:
The invention optimizes the acoustic impedance parameter of the coating material to fall within the range of 1-100×10^6 kg/m²s. This parameter optimization ensures that the coating provides sufficient acoustic isolation to prevent substrate thickness effects while minimizing energy absorption that would increase insertion loss.
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
The solution effectively reduces passband ripple, enhances passband linearity, and adheres to insertion loss specifications by attenuating reflected acoustic energy, with the optimal substrate thickness being ≤400 μm for maximum effectiveness.
Implementation Method 1
Applying a lossy material with an optimal acoustic impedance, such as epoxy, to the polished substrate backside to absorb and scatter leaked acoustic energy
Implementation Method 2
Applying a lossy material with an optimal acoustic impedance, such as epoxy, to the polished substrate backside to absorb and scatter leaked acoustic energy
Implementation Method 3
a Bragg mirror for solidly mounted resonators (SMR) of alternating high and low acoustic impedance materials designed to be one-quarter wavelength thick (λL/4) at the operating frequency
Data Source
AI summary
A bulk acoustic wave (BAW) resonator is constructed to reduce phase and amplitude ripples in a frequency response. The BAW resonator is fabricated on a substrate 400 μm thick or less, preferably approximately 325 μm, having a first side and a polished second side with a peak-to-peak roughness of approximately 1000 A. A Bragg mirror having alternate layers of a high acoustic impedance material, such as tungsten, and a low acoustic impedance material is fabricated on the first side of the substrate. A BAW resonator is fabricated on the Bragg mirror. A lossy material, such as epoxy, coats the second side of the substrate opposite the first side. The lossy material has an acoustic impedance in the range of 0.01× to 1.0× the acoustic impedance of the layers of high acoustic impedance material.