CMOS MEMS Microphone Die Stiffness and Etching
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Solution Overview
Problem
Current MEMS fabrication techniques using CMOS materials face challenges such as lack of stiffness in metallic layers, curvature issues, difficulty in creating airtight chambers, slow etchant penetration, excessive dielectric removal, high mass leading to lower resonant frequencies, and stiction problems, which complicate the integration of CMOS and MEMS structures on a single die.
Innovation Solution
The introduction of etchant through the bottom of the wafer, use of multiple alternating layers of metal and dielectric with metal vias, structural supports to prevent top layer bowing, and offset vias configurations to manage movement and reduce stiction, allowing for quicker etchant penetration and improved structural stiffness while minimizing mass and height.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If multiple layers of metal are stacked to increase rigidity, then structural stiffness is improved, but mass increases leading to lower resonant frequencies
Solution Approach 1:
The patent divides the continuous metallic structural layer into multiple discrete metallic layers separated by dielectric layers. This segmentation allows the structure to maintain rigidity through multiple connected layers while reducing the total mass compared to a solid thick metal layer, thereby improving resonant frequency performance.
Solution Approach 2:
The patent creates a composite structure by alternating metallic layers with dielectric layers. This composite approach provides the necessary structural stiffness from the metal while the dielectric material reduces overall density and mass, resolving the contradiction between strength and weight.
2Ease of manufacture
If etchant is introduced from the top side to remove dielectric material, then access to dielectric is achieved, but sealing becomes difficult and material may enter chamber
Solution Approach 1:
The patent inverts the conventional approach by introducing the etchant from the bottom side of the substrate rather than from the top. This reversal allows complete removal of sacrificial dielectric material without risking contamination of the MEMS chamber, as the etchant exits through the same bottom path, eliminating the sealing problem.
Solution Approach 2:
The patent extracts the etching process from the top-side approach and relocates it to the bottom side. This extraction separates the etchant introduction path from the MEMS chamber interior, allowing effective dielectric removal while preventing material entry into the chamber.
3Adaptability or versatility
If wide plate structures are used for MEMS, then device functionality is improved, but etchant penetration time increases and excessive dielectric removal occurs
Solution Approach 1:
The patent reverses the etchant introduction direction to access the bottom of wide plate structures. This inversion allows etchant to penetrate upward through the entire thickness of wide structures efficiently, reducing penetration time and enabling complete dielectric removal without excessive lateral etching.
4Reliability
If cap wafer is attached to create airtight chamber, then sealing is achieved, but bonding pads access becomes difficult and die height increases
Solution Approach 1:
The patent inverts the chamber sealing approach by forming the airtight chamber from the bottom side rather than adding a cap from the top. This inversion maintains bonding pad accessibility on the top surface while achieving effective chamber sealing through bottom-side processing and sealing structures.
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 simplifies the fabrication process, enhances structural stiffness, reduces mass, improves etchant penetration, and minimizes stiction, resulting in more effective and efficient MEMS devices with improved resonant frequencies and reduced height.
Implementation Method 1
the sacrificial dielectric material around the MEMS structures is removed using an etchant such as vHF (vapor hydrofluoric acid)
Implementation Method 2
patterned layers of metallic and dielectric materials on another part of the wafer can form complex MEMS structures
Data Source
AI summary
A die is manufactured using complementary metal-oxide semiconductor (CMOS) techniques to create transistors, electrical pathways, and microelectromechanical system (MEMS) structures. The MEMS structures include springs, plates, mechanical stops, and structural supports, which can be combined to form complex MEMS structures including microphones, pressure sensors, accelerometers, resonators, gyroscopes, and the like.