Thickness Mode Resonator with Silicon Stack for Frequency Stability
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Solution Overview
Problem
Current high-frequency resonators, such as FBARs, are large and expensive, and while they have good characteristics for filter applications, they are less suited for oscillators due to low quality factors and significant temperature coefficient of frequency, necessitating a small, high-performance resonator with improved temperature characteristics for future communication systems.
Innovation Solution
A thickness mode overtone resonator is designed with a substrate cavity, a piezoelectric film stack, and a positive temperature coefficient stack comprising a doped silicon layer, which balances the negative temperature coefficient of the piezoelectric film stack to achieve a zero temperature coefficient resonator, optimized for high-frequency oscillator applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If FBAR resonators are used for high-frequency applications, then the operating frequency can be increased above 1 GHz, but the quality factor decreases and temperature coefficient of frequency increases
Solution Approach 1:
The patent uses a composite structure combining piezoelectric material (AlN) with a silicon-based positive temperature coefficient stack. This composite design allows the resonator to achieve both high operating frequency and improved quality factor by leveraging the complementary properties of different materials to compensate for each other's deficiencies.
Solution Approach 2:
The patent changes the physical parameters of the resonator by introducing a positive temperature coefficient stack with specific thickness (multiple of half wavelength) and doping configuration. This parameter modification transforms the temperature coefficient characteristic from negative to positive, enabling compensation and improvement of overall frequency stability and quality factor.
2Speed
If FBAR resonators are used for high-frequency applications, then the operating frequency can be increased above 1 GHz, but the temperature coefficient of frequency becomes significantly larger
Solution Approach 1:
The patent exploits thermal expansion principles by introducing a positive temperature coefficient stack that expands or contracts in response to temperature changes. This thermal response is designed to compensate for the negative temperature coefficient of the piezoelectric layer, thereby stabilizing the overall frequency against temperature variations.
Solution Approach 2:
The patent modifies the temperature coefficient parameter by doping the silicon layer to create a positive temperature coefficient effect. This parameter change transforms the temperature dependency from harmful to beneficial, allowing the resonator to maintain stable frequency operation across temperature ranges.
3Ease of manufacture
If conventional resonator designs are used, then manufacturing simplicity is maintained, but the resonator size increases and performance deteriorates
Solution Approach 1:
The patent transitions from two-dimensional surface acoustic wave resonators to three-dimensional thickness mode resonators. By utilizing the thickness dimension of the piezoelectric film and the silicon stack, the design achieves higher frequency operation in a smaller footprint while maintaining compatibility with standard semiconductor manufacturing processes.
Solution Approach 2:
The patent replaces the mechanical acoustic wave propagation mechanism of conventional resonators with a thickness mode vibration mechanism in a vertically stacked structure. This substitution enables miniaturization while preserving ease of manufacture through standard thin-film deposition and doping 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
The resonator achieves high performance, stability, and reduced temperature dependency, enabling efficient operation in high-speed communication systems with improved quality factors and frequency stability.
Implementation Method 1
a piezoelectric layer 18... the vibration of the air gap type FBAR 10 is mainly in the thickness mode
Implementation Method 2
a positive temperature coefficient stack that includes at least a silicon layer... the silicon layer is doped to configure the positive temperature coefficient stack to have a positive temperature coefficient
Implementation Method 3
a thickness mode overtone resonator... optimized for high-frequency oscillator applications... vibrating in a thickness direction
Data Source
AI summary
A resonator that includes a substrate with a cavity that extends in a principal surface thereof and a vibrating resonator above the principal surface of the substrate and including bottom and top electrodes with a piezoelectric layer disposed therebetween. Moreover, a silicon dioxide layer is provided above the substrate and below the vibrating resonator to cover the cavity of the substrate, and a silicon layer is provided between the silicon dioxide layer and the vibrating resonator. The bottom electrode, the top electrode and the piezoelectric layer of the vibrating resonator each have a thickness configured to accommodate substantially a half wavelength λ/2 of the resonator, and the silicon layer has a thickness that accommodates substantially multiple of the half wavelength λ/2 of the resonator.