CMOS MEMS Microphone Die with Bottom Etchant Access
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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, mass-related resonant frequency issues, and stiction problems, which complicate the integration of CMOS and MEMS structures on a single die.
Innovation Solution
The introduction of an etchant through a hole in the bottom of the wafer, use of multiple alternating layers of metal and dielectric material with metal vias, structural supports to prevent top layer bowing, and offset metallic layers to limit movement and reduce stiction, allowing for quicker etchant penetration and improved structural stiffness and mass reduction.
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 and resonant frequency decreases
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 achieve rigidity through the distributed metallic layers while the dielectric spacers reduce overall mass compared to solid metal construction, thereby maintaining resonant frequency performance.
Solution Approach 2:
The patent employs composite construction by alternating metallic layers with dielectric material layers. This composite approach provides the necessary structural stiffness from the metallic components while the dielectric material reduces density and mass, resolving the contradiction between strength and weight in the MEMS resonator structure.
2Ease of manufacture
If etchant is introduced from the top side through holes in the top layer, then dielectric material can be removed, but sealing the holes is complex and risks introducing sealing material into the chamber
Solution Approach 1:
The patent inverts the conventional etching approach by introducing the etchant from the bottom side of the device rather than from the top. This reversal eliminates the need to create and seal holes in the top structural layer, simplifying the manufacturing process and avoiding the risk of contaminating the sealed chamber with sealing materials.
Solution Approach 2:
The patent extracts the etchant access function from the top surface and relocates it to the bottom surface. By removing the requirement for top-surface holes and their associated sealing complexity, the design achieves dielectric removal capability without the harmful side effects of complex sealing procedures.
3Reliability
If a cap wafer is attached to create an airtight chamber, then sealing is achieved, but access to bonding pads becomes challenging 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 capping from the top. This allows bonding pads on the top surface to remain easily accessible while still achieving reliable sealing through the bottom attachment structure, resolving the contradiction between chamber integrity and electrical access.
4Productivity
If vHF etchant is used to remove dielectric material, then release is achieved, but the etchant may take considerable time to reach the interior of wide plate structures
Solution Approach 1:
The patent changes the dimensional approach to etchant delivery by accessing the dielectric material from the bottom dimension rather than attempting to penetrate through the top surface. This dimensional change allows the etchant to reach interior regions of wide plate structures much faster, dramatically reducing etching time while maintaining complete dielectric removal.
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, minimizes stiction, and increases the effectiveness of MEMS structures by allowing for more precise control over movement and reducing the risk of structural interference.
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
Implementation Method 3
the top layer may bow inward because of the vacuum within the chamber
Implementation Method 4
when the surfaces of mechanical components of a MEMS structure come into contact with one another, adhesive surface forces, commonly known as 'stiction,' can cause the surfaces to become stuck
Data Source
AI summary
The claim invention is directed at a MEMS microphone die fabricated using CMOS-based technologies. In particular, the claims are directed at various aspects of a MEMS microphone die having anisotropic springs, a backplate, a diaphragm, mechanical stops, and a support structure, all of which are fabricated as stacked metallic layers separated by vias using CMOS fabrication technologies.