Piezoelectric MEMS Resonator With Wafer-Level Hermetic Packaging
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Solution Overview
Problem
Existing MEMS resonator devices face challenges such as costly and complex manufacturing processes, large dimensions, and suboptimal performance due to the lack of integration of a protective package, which affects their electrical properties and parasitic capacitance.
Innovation Solution
A piezoelectric MEMS resonator device is manufactured using a SOI wafer with a monocrystalline silicon structural substrate, epitaxial silicon layer, and piezoelectric material, integrated with a wafer-level package and cap to form a hermetic structure, enabling precise control of gap dimensions and electrical performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a SOI substrate with pre-formed buried cavity is used, then the resonator structure can be formed, but the manufacturing process becomes costly and complex
Solution Approach 1:
The patent applies preliminary action by forming the cavity structure during the wafer fabrication process itself, before final assembly. The cavity is created through epitaxial growth with sacrificial layers that are removed to form the resonator cavity, eliminating the need for post-fabrication cavity formation and reducing manufacturing complexity.
Solution Approach 2:
The SOI wafer structure serves multiple functions: it provides the mechanical support substrate, defines the resonator cavity through epitaxial layer removal, and integrates the piezoelectric film deposition surface. This multi-functionality reduces the need for separate components and simplifies the overall manufacturing process.
2Ease of manufacture
If no package is integrated at the microelectromechanical structure level, then manufacturing is simpler, but external metal or ceramic caps are required increasing overall dimensions
Solution Approach 1:
The patent merges the protective package function with the microelectromechanical structure by integrating the cavity formation and packaging into a single wafer-level structure. The epitaxial silicon layer and sacrificial oxide combination creates both the protective enclosure and the resonator cavity simultaneously, eliminating the need for separate external caps.
Solution Approach 2:
The resonator structure is nested within the epitaxial silicon layer, which itself is contained within the SOI wafer substrate. The sacrificial oxide layer is nested within the epitaxial structure, creating a hierarchical nested arrangement that achieves compact packaging while maintaining structural integrity.
3Reliability
If external metal or ceramic caps are used, then the resonator is protected, but parasitic capacitance increases and performance deteriorates
Solution Approach 1:
The patent applies local quality by using different materials in different regions: the epitaxial silicon layer provides mechanical protection and structural support, while the sacrificial oxide removal creates a vacuum or inert-filled cavity that minimizes parasitic capacitance. This localized material selection optimizes both protection and electrical performance.
Solution Approach 2:
The patent creates an inert environment by removing the sacrificial oxide to form a vacuum or inert gas-filled cavity within the resonator structure. This inert atmosphere minimizes parasitic capacitance and protects the resonator elements from environmental degradation, achieving both protection and low parasitic capacitance.
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 provides a compact, low-power, and temperature-stable resonator device that can replace traditional quartz resonators, with improved electrical performance and reduced manufacturing complexity, suitable for RTC applications and other frequency reference uses.
Implementation Method 1
MEMS resonator devices include structures obtained using micromachining techniques, which, as a result of appropriate stimuli (via electrical biasing signals), are induced to vibrate at their natural resonance frequency
Implementation Method 2
MEMS resonator devices of a capacitive type are known, in which the mobile element forms, with a fixed electrode structure coupled thereto, a capacitor, and in which resonance vibration of the mobile element causes a variation of capacitance of this capacitor, which is converted into an output signal at the desired operating frequency
Data Source
AI summary
A microelectromechanical resonator device has: a main body, with a first surface and a second surface, opposite to one another along a vertical axis, and made of a first layer and a second layer, arranged on the first layer; a cap, having a respective first surface and a respective second surface, opposite to one another along the vertical axis, and coupled to the main body by bonding elements; and a piezoelectric resonator structure formed by: a mobile element, constituted by a resonator portion of the first layer, suspended in cantilever fashion with respect to an internal cavity provided in the second layer and moreover, on the opposite side, with respect to a housing cavity provided in the cap; a region of piezoelectric material, arranged on the mobile element on the first surface of the main body; and a top electrode, arranged on the region of piezoelectric material, the mobile element constituting a bottom electrode of the piezoelectric resonator structure.