MEMS Microphone Fabrication Using Horizontal Sacrificial Layer Cavities
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Solution Overview
Problem
Conventional MEMS microphone fabrication methods face challenges due to large step thicknesses resulting from thick sacrificial and polysilicon layers, which complicate photolithography and etching processes.
Innovation Solution
A method involving a silicon substrate with formed cavities and acoustic holes, a sacrificial layer, and a polysilicon layer, where the sacrificial layer is partially removed to create a back chamber and hollow chamber, reducing the overall stack thickness and simplifying the fabrication process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the sacrificial layer thickness is increased to 3-4 μm and baseplate thickness to 2-3 μm to meet higher performance demands, then the microphone performance is improved, but the step thickness becomes large which imposes very stringent requirements on photolithography and etching
Solution Approach 1:
The patent introduces a horizontal cavity structure within the sacrificial layer, transforming the traditional vertical stacking approach into a three-dimensional configuration. This allows the sacrificial layer to serve dual purposes: maintaining the required thickness for performance while creating internal cavities that reduce the effective step height, thereby relaxing photolithography and etching precision requirements.
Solution Approach 2:
The patent embeds cavities within the sacrificial layer structure, creating a nested configuration where hollow spaces are contained within the solid sacrificial material. This nesting approach allows the sacrificial layer to maintain its overall thickness for performance requirements while the internal cavities reduce the net step thickness that challenges fabrication precision.
2Reliability
If the sacrificial layer and polysilicon layer thicknesses are increased to meet performance demands, then the microphone performance is improved, but the overall stack thickness becomes large which complicates the fabrication process
Solution Approach 1:
The patent transitions from a purely vertical layer stack to a three-dimensional structure with horizontal cavities embedded within the sacrificial layer. This dimensional change allows the structure to achieve the required acoustic performance with thicker layers while the cavities create void spaces that reduce the effective vertical footprint, simplifying the overall fabrication process.
Solution Approach 2:
The patent divides the sacrificial layer into multiple functional regions by introducing cavities, separating the structure into solid portions (for structural integrity and acoustic function) and hollow portions (for reducing step thickness). This segmentation allows each region to be optimized independently, reducing overall fabrication complexity while maintaining performance.
Data Source
AI summary
A preparation method for a micro-electromechanical systems (MEMS) microphone includes the steps of: providing a silicon substrate having a silicon surface; forming an enclosed cavity in the silicon substrate; forming a plurality of spaced apart acoustic holes in the silicon substrate, each acoustic hole having two openings, one of which communicating with the cavity and the other one located on the silicon surface; forming a sacrificial layer on the silicon substrate, which includes a first filling portion, a second filling portion and a shielding portion; forming a polysilicon layer on the shielding portion; forming a recess in the silicon substrate on the side away from the silicon surface; and removing the first filling portion, the second filling portion and part of the shielding portion so that the recess is brought into communication with the cavity to form a back chamber, and that the polysilicon layer, the remainder of the shielding portion and the silicon substrate together delimit a hollow chamber, the hollow chamber communicating with the opening of the plurality of acoustic holes away from the cavity, completing the MEMS microphone.