MEMS Pressure Sensor Thermal Stress Reduction via CTE Matching
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Solution Overview
Problem
MEMS pressure sensors face thermal-induced stress due to mismatched thermal expansion coefficients between silicon devices and housing materials, which affects bonding strength and stability, especially in medium-pressure applications where low modulus bonding materials are inadequate.
Innovation Solution
A plastic housing with low CTE fillers and a polymer-based adhesive with reduced CTE, mixed to match the silicon MEMS sensor's expansion coefficient, along with a glass transition temperature above the operating range, are used to minimize thermal stress and ensure strong bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a stronger bonding material with higher modulus is used for medium pressure sensors, then bonding strength is improved, but bonding stress increases due to CTE mismatch
Solution Approach 1:
The patent changes the physical-chemical parameters of the bonding material by incorporating silica fillers into the epoxy adhesive. This modification adjusts the adhesive's CTE to better match the silicon MEMS device, reducing thermal expansion mismatch and consequently lowering bonding stress while maintaining adequate bonding strength for medium pressure applications.
Solution Approach 2:
The patent uses a composite bonding material consisting of epoxy adhesive mixed with silica fillers. This composite structure combines the adhesive properties of epoxy with the low CTE characteristics of silica, creating a bonding material that simultaneously provides strong adhesion and reduced thermal stress through CTE matching.
2Stress or pressure
If low modulus silicone bonding material is used, then bonding stress is reduced, but bonding strength becomes insufficient for medium pressure sensors
Solution Approach 1:
The patent employs a composite epoxy-silica bonding material that overcomes the limitations of pure low modulus silicone. The epoxy matrix provides strong bonding capability while silica fillers contribute to CTE matching, achieving both adequate bonding strength and reduced thermal stress for medium pressure sensor applications.
Solution Approach 2:
The patent modifies the bonding material parameters by adjusting the epoxy adhesive formulation with silica fillers to achieve optimal CTE matching. This parameter adjustment allows the bonding material to maintain lower bonding stress while simultaneously providing sufficient bonding strength, unlike low modulus silicone which lacks adequate strength.
3Strength
If bonding area is increased to improve bonding strength, then bonding strength is improved, but device size increases which is not practical for shrinking MEMS sensors
Solution Approach 1:
The patent changes the material properties of the adhesive by incorporating silica fillers to optimize CTE matching. This material parameter change allows for reduced bonding area while maintaining bonding strength, as the improved CTE compatibility reduces stress concentration and enhances bond reliability per unit area.
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 reduces thermally-induced stress and provides stable, reliable sensor outputs with improved sealing and bonding strength, suitable for medium-pressure sensors.
Implementation Method 1
When the MEMS device and housing are bonded together with an adhesive, the bonding stress induced by the CTE differences can affect the MEMS output
Implementation Method 2
A piezo-resistive Wheatstone bridge circuit formed into the diaphragm, changes its resistance responsive to diaphragm deflection
Data Source
AI summary
Thermally-induced stress on a silicon micro-electromechanical pressure transducer (MEMS sensor) is reduced by attaching the MEMS sensor to a plastic filled with low CTE fillers that lowers the plastic's coefficient of thermal expansion (CTE) to be closer to that of silicon. The MEMS sensor is attached to the housing using an epoxy adhesive/silica filler mixture, which when cured has a CTE between about ten PPM/° C. and about thirty PPM/° C. in order to match the housing CTE. The adhesive also has a glass transition temperature (Tg) above the operating temperature range. This design provides good sealing of the sensor and stable sensor outputs.