BAW Resonator Electrode Edge Isolation for Higher Q and Kt2
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Solution Overview
Problem
Conventional Bulk Acoustic Wave (BAW) resonators face issues with reduced effective electromechanical coupling coefficient (Kt2) and quality factor (Q) due to cracks in the piezoelectric layer, which decrease electrostatic discharge robustness and increase lateral acoustic energy loss, especially at the connection edges where the electrodes intersect.
Innovation Solution
A gap or dielectric layer is introduced between one of the electrodes and the piezoelectric film in the tapered area, reducing electrical field strength in poorly formed regions and minimizing lateral acoustic energy loss by acting as a low capacitance capacitor, thereby enhancing the Kt2 and Q factor of the resonator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the top electrode extends beyond the acoustic mirror contour to connect with pads, then electrical connection is achieved, but cracks form in the piezoelectric layer at the connection edge
Solution Approach 1:
A dielectric layer is introduced as an intermediary between the top electrode and the piezoelectric layer at the connection edge region. This dielectric layer acts as a mediator that prevents direct contact, thereby eliminating the formation of cracks in the piezoelectric layer while still allowing electrical connection to be established through alternative paths.
2Strength
If the bottom electrode spans the entire cavity, then mechanical support is provided, but effective electromechanical coupling coefficient is reduced due to substrate contact
Solution Approach 1:
The bottom electrode is segmented into two distinct regions: a first portion that contacts the substrate and provides mechanical support, and a second portion that is suspended over the cavity and provides electromechanical coupling. This segmentation allows each region to perform its specific function optimally without compromising the other.
3Area of stationary object
If the piezoelectric layer is deposited over sharp electrode edges, then complete coverage is achieved, but the layer cracks due to following the terrain
Solution Approach 1:
A dielectric layer is deposited beforehand in the connection edge region where the top electrode extends beyond the acoustic mirror. This pre-deposited dielectric layer cushions the piezoelectric layer from the sharp edges of the top electrode, preventing cracks while still allowing the piezoelectric layer to achieve complete coverage over the electrode structure.
4Device complexity
If lateral acoustic energy escapes into the substrate, then device simplicity is maintained, but quality factor decreases due to energy loss
Solution Approach 1:
The acoustic mirror structure is extracted and extended into the substrate to create a deeper acoustic isolation barrier. By taking out the acoustic mirror function and integrating it into the substrate structure, lateral acoustic energy escape is prevented while maintaining relative device simplicity.
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 minimizes the negative impact on the resonator's performance by reducing voltage drop in cracked regions and improving electrostatic discharge robustness, leading to improved Kt2, Q factor, and reduced lateral energy loss.
Implementation Method 1
thin film Bulk Acoustic Wave (BAW) resonators utilizing the thickness longitudinal resonance of a piezoelectric (PZ) film
Implementation Method 2
The acoustic mirror provides acoustical isolation between the BAW structure and a substrate
Data Source
AI summary
An acoustic resonator with improved quality factor and electro-mechanical coupling is disclosed. In one embodiment, the acoustic resonator includes an acoustic mirror formed on the top surface of a substrate or in the substrate, a first electrode having a end portion, formed on the acoustic mirror, a piezoelectric layer formed on the first electrode; and a second electrode formed on the piezoelectric layer, where at least one of the first electrode and the second electrode and the piezoelectric layer define a gap in a region that overlaps the end portion of the first electrode. In one embodiment, a dielectric film is deposited on the surface of the end portion of the first electrode to form completely planarized surface before the piezoelectric layer deposition. In another embodiment, a gap between the second electrode and the piezoelectric layer, so that the piezoelectric coupling in the end portion area of the first electrode is minimally contributed into the whole resonator.