MEMS Sonic Sensor Through Hole Vertical Wall Etching
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Solution Overview
Problem
Conventional MEMS sonic sensors face challenges in downsizing due to tapered through holes in silicon substrates, leading to increased reflection and resistance of sound waves, which results in lowered sensitivity and acoustic performance.
Innovation Solution
A method is developed to form through holes in silicon substrates with vertical inner walls using a rhombic opening pattern on the reverse surface, allowing for the use of a (110) crystal face and an alkaline etchant, which reduces the reflection of sound waves and enhances vibration transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a through hole with inclined inner wall (Si 111 crystal face) is formed in silicon substrate, then the diaphragm structure can be realized, but the MEMS sensor chip size increases
Solution Approach 1:
The patent changes the crystal orientation parameter of the silicon substrate from conventional (100) to (110), which fundamentally alters the etching behavior and inner wall geometry of the through hole. This parameter change enables vertical inner walls while maintaining the required diaphragm structure
Solution Approach 2:
Instead of accepting the conventional tapered hole geometry and working around it, the patent inverts the approach by selecting a different crystal orientation that naturally produces the desired vertical wall geometry, turning the problem inside out
2Area of stationary object
If the MEMS sensor chip is downsized, then cost is reduced, but sensitivity is lowered due to sound wave reflection at inclined planes
Solution Approach 1:
By changing the substrate crystal orientation to (110), the patent eliminates the inclined planes that cause sound wave reflection, thereby maintaining sensitivity even in downsized chips
Solution Approach 2:
The patent converts the potential harm of downsizing (which would normally reduce sensitivity) into a benefit by using the (110) crystal orientation to eliminate reflection issues, allowing small chips to maintain high sensitivity
3Area of stationary object
If the MEMS sensor chip is downsized, then cost is reduced, but acoustic performance deteriorates due to narrowed sound wave path
Solution Approach 1:
The patent changes the through hole geometry parameter from tapered to vertical by using (110) crystal orientation, which maintains the sound wave path openness even in downsized chips, preserving acoustic performance
4Ease of manufacture
If anisotropic etching with alkaline etchant is used on (100) silicon substrate, then through hole can be formed, but tapered cross section is produced
Solution Approach 1:
The patent changes the substrate crystal orientation parameter from (100) to (110), which fundamentally changes the etching geometry produced by alkaline etchants, resulting in vertical walls instead of tapered cross sections
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 downsizing of MEMS sensors while maintaining or improving sensitivity and acoustic performance by minimizing the tapered sections within the through holes, allowing for better sound wave transmission and reducing stress localization.
Implementation Method 1
a silicon substrate having a (110) crystal face as a principal plane is subjected to anisotropic etching with an alkaline etchant
Data Source
AI summary
A MEMS device, including: a substrate having a first principal plane and a second principal plane opposite to the first principal plane; a through hole formed in the substrate; and a vibrating film formed over the first principal plane so as to cover the through hole. The first principal plane and the second principal plane are both a (110) crystal face; and the through hole has a substantially rhombic shape on the second principal plane.