Rectangular Membrane MEMS Pressure Sensor Aspect Ratio
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Solution Overview
Problem
MEMS pressure sensors face challenges with insensitivity, inaccuracy, and signal drift due to conflicting requirements of membrane thickness and hermeticity, leading to reduced sensitivity and increased sensor size, as well as limitations in operational pressure range caused by membrane flexibility and touch-point pressure.
Innovation Solution
A surface-micromachined capacitive pressure sensor with a rectangular membrane design having an aspect ratio of 3 or more, allowing for increased signal and operational range while maintaining membrane rigidity, achieved through optimized layer thickness and stress tuning, and integration with CMOS read-out circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the membrane thickness is reduced to increase deflection and sensitivity, then sensitivity is improved, but hermeticity deteriorates
Solution Approach 1:
The patent applies asymmetry by changing the membrane shape from circular or square to rectangular with aspect ratio ≥ 2:1. This asymmetric geometry allows the membrane to achieve greater effective area and deflection for sensitivity while maintaining structural rigidity and hermeticity through the elongated shape that distributes stress differently compared to symmetric designs.
Solution Approach 2:
The patent changes the geometric parameter of the membrane by specifying an aspect ratio of 2:1 or greater. This parameter change enables the membrane to achieve optimal balance between deflection (for sensitivity) and structural integrity (for hermeticity), resolving the contradiction between thin membrane requirements and hermetic sealing requirements.
2Measurement precision
If the sensor area is increased to increase signal, then sensitivity is improved, but touch-point pressure reduces
Solution Approach 1:
The rectangular membrane with aspect ratio ≥ 2:1 creates asymmetric stress distribution under pressure. The elongated shape allows the membrane to sustain higher pressures before touching the substrate because the pressure is distributed over a longer dimension, while still providing increased area for enhanced signal output.
Solution Approach 2:
The patent transitions from considering only the area of the membrane to considering the aspect ratio (length-to-width ratio). By optimizing the dimensional proportions rather than just increasing overall size, the membrane achieves enhanced signal through increased area while maintaining adequate touch-point pressure through favorable length-to-width proportions.
3Measurement precision
If the membrane area is increased to enhance signal, then sensitivity is improved, but sensor size increases
Solution Approach 1:
The rectangular membrane with aspect ratio ≥ 2:1 achieves enhanced signal output through increased effective area while occupying less overall sensor footprint. The asymmetric shape allows the membrane to be elongated in one dimension rather than expanded uniformly, providing more sensing area without proportionally increasing the sensor's lateral dimensions.
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 rectangular membrane design enhances sensitivity and operational range by increasing the sensor area without compromising touch-point pressure, and integration with CMOS read-out circuitry improves signal-to-noise ratio and reduces parasitic capacitances.
Implementation Method 1
MEMS capacitive pressure sensor... deriving an output from a capacitance between the electrode arrangements... the pressure difference between the external pressure and the gauge pressure generates a force on the membrane, which causes the membrane to deflect. This deflection is then measured by piezoresistive, capacitive or optical sensors.
Implementation Method 2
The external pressure is measured because the pressure difference between the external pressure and the gauge pressure generates a force on the membrane, which causes the membrane to deflect.
Data Source
Figure 1~2
Figure 3~4
Figure 5(a)~5(d)
AI summary
A MEMS capacitive pressure sensor has a deformable sensor electrode with a length to width ratio of 3 or more. This enables the flexibility to be kept low, so that the pressure range of operation is increased, while also enabling an increased area to give an increased signal.