Hinged MEMS Diaphragm Stress Management
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Solution Overview
Problem
Fabrication-induced stresses and fractures in micro-electro-mechanical system (MEMS) devices, particularly in directional microphones and micromirrors, lead to poor yield and performance due to stress concentration at rotational axes and challenges in achieving high sensitivity and low damping.
Innovation Solution
The development of interdigitated comb-fin structures that protect the rotational axis from fabrication-induced stresses while providing effective linear capacitive actuation and sensing, utilizing a substrate with a sacrificial oxide layer and polysilicon layers patterned with stiffeners and ribs to enhance capacitance and reduce stress-related failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If parallel plate electrodes are used for capacitive sensing, then capacitance is increased, but viscous damping increases due to air flow between diaphragm and electrode
Solution Approach 1:
The patent segments the continuous air gap between the diaphragm and backplate by introducing holes through the backplate. This divides the air space into discrete regions, allowing capacitance to be maintained through the electrode structure while reducing viscous damping by enabling air to flow through the holes rather than being squeezed across the entire gap area.
Solution Approach 2:
The backplate is designed with a porous structure containing multiple holes. This porous configuration allows air to pass through during diaphragm motion, significantly reducing viscous damping forces while maintaining the capacitive sensing function through the interdigitated electrodes positioned around the holes.
2Measurement precision
If bias voltage is increased to improve sensitivity, then capacitive sensitivity increases, but gap collapse occurs due to attractive electrostatic force
Solution Approach 1:
The continuous gap is segmented into multiple smaller gaps by the holes in the backplate. This segmentation distributes the electrostatic attractive force across multiple smaller regions, preventing the catastrophic gap collapse that would occur in a single large gap under high bias voltage, while still maintaining high capacitive sensitivity.
Solution Approach 2:
The patent changes the geometric parameters of the system by introducing holes with specific dimensions and patterns. This modifies the electric field distribution and mechanical stress distribution, allowing the system to operate at higher bias voltages for improved sensitivity without experiencing gap collapse.
3Adaptability or versatility
If rotational support is added to enable pivoting diaphragm, then directional sensitivity is improved, but stress concentration at pivot axis increases leading to fractures
Solution Approach 1:
The patent uses thin film structures for the diaphragm and supporting elements. These flexible thin films can accommodate the rotational motion required for directional sensitivity while distributing mechanical stresses more evenly, reducing stress concentration at the pivot axis and preventing fabrication-induced fractures.
Solution Approach 2:
The patent employs composite material structures combining different layers and materials with complementary properties. The composite construction provides both the mechanical strength needed to prevent fracture at the pivot axis and the flexibility required for rotational motion, thereby maintaining directional sensitivity while improving reliability.
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 approach enables the creation of lighter-weight, rapidly movable rotational structures with improved fabrication yield, reliability, and increased capacitance, addressing the limitations of existing MEMS technologies by reducing stress-induced cracks and enhancing signal sensitivity.
Implementation Method 1
The space between the conducting electrode and the moving diaphragm may then be opened through a release etch
Implementation Method 2
The fabrication of parallel plate electrodes is a common way to achieve capacitive sensing for microphones and other planar displacement sensors
Implementation Method 3
depositing a conductive material, such as silicon or metal, on top of the oxide
Data Source
AI summary
A method of forming a micromechanical structure comprising, forming a sacrificial layer on a surface and walls of a trench in a substrate; depositing a structural layer over the sacrificial layer, extending into the trench, selectively etching the structural layer to define a pattern having a boundary, at least a portion of the structural layer overlying a respective portion of the trench being removed and at least a portion of the structural layer extending into the trench being preserved at the boundary; and removing at least a portion of the sacrificial layer from underneath the structural layer, prior to removal of at least a portion of the sacrificial layer extending into the trench at the structural boundary. A micromechanical structure formed by the method is also provided.