Polysilicon Pressure Sensor Stability via Material and Structural Changes
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Solution Overview
Problem
High-performance pressure sensors face challenges in stability, particularly in extreme environments, due to large die size, high temperature coefficient of metal layers, and sensitivity to gravity, which affects signal output and mechanical stability.
Innovation Solution
A pressure sensor design utilizing a polysilicon sensing membrane and electrodes with reduced metal layers, polysilicon routing, and a membrane array structure to improve temperature coefficient matching, thermal budget, and mechanical stability, while minimizing hillock effects and gravity sensitivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal layers are used for electrodes and routing, then electrical conductivity is improved, but temperature coefficient matching deteriorates and thermal budget is limited
Solution Approach 1:
The patent changes the material parameter from metal to polysilicon for electrodes and routing layers. This material substitution fundamentally alters the thermal and electrical characteristics, enabling better temperature coefficient matching with the sensing membrane while allowing higher processing temperatures through increased thermal budget.
Solution Approach 2:
The patent employs polysilicon as a composite material solution that combines the desirable properties of both metals (electrical conductivity) and ceramics (thermal stability). The polysilicon layers are deposited using PECVD processes and doped to achieve the required electrical characteristics while maintaining thermal compatibility with the silicon-based sensing membrane.
2Ease of manufacture
If multiple metal layers are used, then pad connection is achieved, but hillock effect occurs due to melting
Solution Approach 1:
The patent changes the material parameter from metal to polysilicon, which has a significantly higher melting point. This eliminates the hillock effect that occurs when metal layers melt during high-temperature processing. The polysilicon can withstand annealing temperatures up to 1100°C without melting or deforming, ensuring electrode stability.
Solution Approach 2:
The patent uses a simplified interconnect structure with fewer layers (one polysilicon routing layer and one metal layer for pads) compared to traditional multi-metal-layer designs. This reduced structure is sufficient for pad connection while avoiding the reliability issues of complex metal stacks during high-temperature processing.
3Strength
If thick membrane is used, then mechanical strength is improved, but gravity sensitivity increases
Solution Approach 1:
The patent changes the membrane thickness parameter to a thin configuration (less than 1 micrometer). This thin membrane design reduces the mass and moment of inertia, thereby minimizing gravity sensitivity and G-sensitive errors. The membrane maintains sufficient mechanical strength through optimized material properties and structural design rather than increased thickness.
Solution Approach 2:
The patent employs a thin polysilicon membrane that functions as a flexible sensing element. The thin film design allows the membrane to respond sensitively to pressure changes while its low mass reduces sensitivity to gravitational and acceleration forces. The membrane is supported by a frame structure that provides mechanical strength without increasing the sensing mass.
4Measurement precision
If large area membrane is used, then pressure sensing capability is improved, but temperature performance deteriorates
Solution Approach 1:
The patent divides the large-area membrane into an array of smaller sensing elements or membrane cells. Each small membrane maintains excellent temperature performance due to its small area, while the array configuration collectively provides sufficient pressure sensing capability. The segmented structure also reduces parasitic effects and improves signal-to-noise ratio.
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
The design enhances sensor stability and performance by reducing thermal expansion mismatch, gravity sensitivity, and chip size, improving signal-to-noise ratio and sensitivity, and preventing membrane stiction.
Implementation Method 1
changes a capacitance between the polysilicon sensing membrane and the one or more polysilicon electrodes
Implementation Method 2
the polysilicon sensing membrane deforms responsive to a stimuli
Data Source
AI summary
A pressure sensor comprises a polysilicon sensing membrane. The pressure sensor further includes one or more polysilicon electrodes disposed over a silicon substrate. The sensor also includes one or more polysilicon routing layers that electrically connects electrodes of the one or more polysilicon electrodes to one another, wherein the polysilicon sensing membrane deforms responsive to a stimuli and changes a capacitance between the polysilicon sensing membrane and the one or more polysilicon electrodes. The sensor also includes one or more vacuum cavities positioned between the polysilicon sensing membrane and the one or more polysilicon electrodes.