Thick Substrate MEMS Capacitor Reducing Residual Stress
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Solution Overview
Problem
Conventional MEMS devices with thin film structures suffer from residue stress, which degrades their performance due to the inherent limitations in material design and fabrication processes.
Innovation Solution
A MEMS device design utilizing a thick device substrate with a movable element and polysilicon structures, where the distance between the movable element and polysilicon is less than approximately 0.5 μm, forming a capacitor structure with high capacitance and reduced residual stress, and incorporating a robust etch stop layer to prevent undesired etching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a thin film structure is used to form the movable part, then the device can be fabricated using standard semiconductor processes, but residue stress is induced that degrades MEMS device performance
Solution Approach 1:
The patent changes the physical parameter of substrate thickness from thin film to thick substrate (greater than 10 micrometers). This parameter change fundamentally alters the stress distribution characteristics, eliminating residue stress while maintaining compatibility with semiconductor fabrication processes. The thick substrate acts as a stress buffer that prevents the accumulation of residual stresses during fabrication.
Solution Approach 2:
The patent employs a composite structure consisting of a thick device substrate, sacrificial layer, and polysilicon structures. This composite approach allows the thick substrate to provide mechanical stability and stress relief, while the sacrificial layer enables precise formation of the movable element. The combination of these materials and structures resolves the contradiction between ease of manufacture and device performance.
2Reliability
If the distance between the movable element and polysilicon is reduced to less than 0.5 μm, then capacitance is increased, but manufacturing precision requirements are intensified
Solution Approach 1:
The patent applies preliminary action through the sacrificial layer technique. The sacrificial layer is formed first with a precisely controlled thickness (less than 0.5 micrometers), establishing the target gap distance before the movable element is released. This preliminary structure guides the subsequent release process, ensuring that the final gap between the movable element and polysilicon achieves the desired capacitance without requiring ultra-precise control during the release step itself.
Solution Approach 2:
The sacrificial layer serves as an intermediary that temporarily occupies the space where the gap will eventually form. By controlling the thickness of this intermediary layer, the patent indirectly controls the final gap distance with high precision. The sacrificial layer is removed after the structure is formed, leaving behind the precisely controlled gap needed for high capacitance without demanding extreme precision during the critical release process.
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 design enhances the performance of MEMS devices by increasing capacitance and reducing residual stress, resulting in a more robust and efficient mechanical and electrical operation.
Implementation Method 1
forming a capacitor structure with high capacitance
Data Source
AI summary
The present disclosure provides a CMOS MEMS device. The CMOS MEMS device includes a first substrate, a second substrate, a first polysilicon and a second polysilicon. The second substrate includes a movable part and is located over the first substrate. The first polysilicon penetrates the second substrate and is adjacent to a first side of the movable part of the second substrate. The second polysilicon penetrates the second substrate and is adjacent to a second side of the movable part of the second substrate.