Cap Electrode MEMS Sensor for Z-Axis Sensing
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Solution Overview
Problem
MEMS devices face challenges in accurately measuring z-axis movement due to difficulties in forming electrodes under the inertial mass, leading to susceptibility to drift mechanisms and inadequate sensitivity for out-of-plane motion detection.
Innovation Solution
A MEMS sensor design featuring a cap electrode formed on a conductive cap that encapsulates the MEMS structure, with a seal ring for attachment to the substrate, allowing for accurate z-axis movement sensing through controlled capacitance changes, using materials like aluminum or its alloys for the electrode and seal ring.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a cap structure is added to protect the MEMS structure from environmental hazards, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent combines the protective cap structure with the sensing electrode function into a single integrated component. The cap electrode serves dual purposes: protecting the MEMS structure from environmental hazards while simultaneously functioning as the sensing electrode for z-axis movement detection, thereby reducing overall device complexity despite adding protection.
Solution Approach 2:
The cap electrode is designed to perform multiple functions: it acts as both a protective enclosure for the MEMS structure and as the active sensing electrode for measuring z-axis displacement. This multi-functional design eliminates the need for separate protective and sensing components.
2Measurement precision
If electrodes are formed under the inertial mass to measure z-axis movement, then measurement precision is improved, but manufacturing precision deteriorates due to difficulty in forming electrodes
Solution Approach 1:
Instead of forming the sensing electrode underneath the inertial mass as in conventional designs, the patent inverts the approach by placing the sensing electrode on the cap structure above the MEMS device. This allows standard planar deposition techniques to be used rather than requiring complex under-cutting or release-hole formation processes.
Solution Approach 2:
The patent transitions the electrode from the traditional planar substrate dimension to the vertical dimension by placing it on the cap structure. This dimensional change allows the electrode to be formed using standard top-down deposition processes rather than requiring complex through-substrate or under-mass formation techniques.
3Device complexity
If conventional z-axis sensing methods are used, then device complexity is reduced, but measurement precision deteriorates due to susceptibility to drift mechanisms
Solution Approach 1:
The cap electrode acts as an intermediary element that provides a stable reference plane for capacitance measurement. By positioning the electrode on the rigid cap structure rather than on the flexible substrate, the system reduces susceptibility to substrate drift and thermal expansion effects while maintaining relatively simple device architecture.
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 cap electrode effectively measures z-axis movement with increased accuracy and reliability, reducing drift mechanisms and enhancing sensitivity for out-of-plane motion detection while protecting the MEMS structure from environmental hazards.
Implementation Method 1
the movable structure and the fixed structures form a capacitor having a capacitance that changes when the movable structure moves relative to the fixed structures
Data Source
AI summary
A MEMS sensor includes a substrate having a MEMS structure movably attached to the substrate, a cap attached to the substrate and encapsulating the MEMS structure, and an electrode formed on the cap that senses movement of the MEMS structure.