Single Crystal Silicon Electrodes for Hermetic MEMS Sealing
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Solution Overview
Problem
Current capacitive sensors face challenges such as thermal mismatch due to different materials on top and bottom surfaces, difficulty in hermetic sealing, and exposure of wirebond pads to harsh media, which existing fabrication methods do not adequately address.
Innovation Solution
The use of single crystal silicon electrodes with insulating trenches and a dielectric material to form capacitors, where the pressure port is located on the opposite side of the device from electrical contacts, eliminating the need for metal and reducing thermal mismatch and exposure issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal electrodes are used, then electrical interconnects can be formed, but hermetic sealing becomes difficult and thermal mismatch increases
Solution Approach 1:
The patent changes the material parameter from metal to single crystal silicon for electrodes. This material substitution enables hermetic sealing by allowing high temperature fusion bonding between silicon layers while eliminating the thermal mismatch problems associated with metal electrodes. The single crystal silicon material maintains electrical conductivity while providing compatibility with silicon dioxide and silicon nitride layers for hermetic sealing.
Solution Approach 2:
The patent employs a composite structure combining single crystal silicon electrodes with silicon dioxide and silicon nitride layers. This composite approach allows the silicon electrodes to provide electrical interconnects while the silicon dioxide and nitride layers provide hermetic sealing, creating a multi-material system that achieves both electrical functionality and hermetic integrity.
2Ease of manufacture
If different materials are used on top and bottom surfaces, then fabrication is easier, but thermal mismatch occurs
Solution Approach 1:
The patent applies homogeneity by using single crystal silicon material for both top and bottom electrode surfaces. This uniform material composition eliminates thermal mismatch between different materials while maintaining fabrication feasibility through standard silicon processing techniques. The consistent material properties throughout the device structure prevent differential thermal expansion and stress issues.
3Device complexity
If pressure port and wirebond pads are on the same surface, then device structure is simplified, but wirebond pads are exposed to harsh media
Solution Approach 1:
The patent resolves this contradiction by moving the wirebond pads to a different spatial dimension (the bottom surface) separate from the pressure port opening on the top surface. This dimensional separation allows the wirebond pads to be protected from harsh media while maintaining structural simplicity through the use of single crystal silicon layers and standard interconnect 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 design enables hermetic sealing, reduces thermal mismatch, and protects wirebond pads, enhancing the accuracy and durability of capacitive micro-electro-mechanical sensors while allowing for precise sensing of pressure, acceleration, and resonance.
Implementation Method 1
at least one electrode is defined in the second layer by an insulating trench of dielectrical material that extends from the top surface to the bottom surface of the layer
Implementation Method 2
The at least one electrode in the first layer and the at least one electrode in the second layer together define a capacitor. The sensors preferably sense at least; one of pressure, acceleration, angular rate or resonance.
Data Source
Figure 1A~1B
Figure 2A~2C
Figure 3A~3B
AI summary
The devices presented herein are capacitive sensors with single crysta silicon on all key stress points. Isolating trenches are formed by trench and refill forning dielectrically isolated conductive silicon electrodes for drive, sense and guards, For pressure sensing devices according to the invention, the pressure port is opposed to the e ectrical wire bond pads for ease of packaging. Dual-axis accelerometers measuring in plane acceleration and out of plane acceleration are also described. A third axis in plane is i easy to achieve by duplicating and rotating the accelerometer 90 degrees about its out of plane axis Creating resonant structures, angular rate sensors, bolometers, and many other structures are possible with this process technology. Key advantages are hermeti ity, vertical vias, vertical and horizontal gap capability, single crystal materials, wafejr level packaging, small size, high performance and low cost.