MEMS Pressure Sensor Resonant Frequency Shift
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Solution Overview
Problem
Existing MEMS pressure sensors are unreliable in measuring absolute pressure, particularly at low pressures, due to unstable motional resistance and limited sensitivity, which is critical for maintaining vacuum conditions in micro cavities of resonant MEMS devices.
Innovation Solution
A MEMS pressure sensor that utilizes a resonant frequency shift caused by gas pressure as an additional spring, with narrow etch slits and plate perforations to prevent gas escape, allowing for sensitive pressure measurement by monitoring the resonant frequency, which is more stable and sensitive to low pressures than existing solutions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing MEMS pressure sensors are used to measure absolute pressure, then pressure measurement is possible, but the measurement is unreliable particularly at low pressures due to unstable motional resistance and limited sensitivity
Solution Approach 1:
The patent replaces the mechanical resonance-based pressure sensing mechanism with a capacitive sensing mechanism. Instead of measuring pressure through resonant frequency shifts affected by gas damping, the invention uses a capacitive sensor that directly measures pressure-induced changes in capacitance, eliminating the instability of motional resistance and providing reliable measurements at low pressures.
Solution Approach 2:
The patent changes the measurement parameter from motional resistance (which is unstable) to capacitance (which provides stable and sensitive measurements). By measuring capacitance changes rather than resistance changes, the system achieves improved reliability and precision, particularly in the low pressure range where previous sensors failed.
2Reliability
If a resonant MEMS device is operated in vacuum, then the Q factor and performance are maintained, but integration of a pressure sensor within the same micro cavity is challenging
Solution Approach 1:
The patent implements a multi-functional MEMS device where the same micro cavity structure serves both as the resonator housing and as the pressure sensor housing. The capacitive pressure sensor is integrated within the same micro cavity, allowing the device to simultaneously maintain vacuum conditions for high Q-factor operation and provide accurate pressure sensing without requiring separate sensing chambers or additional complex structures.
3Measurement precision
If pressure sensors are made more sensitive to low pressures, then measurement precision improves, but the device complexity increases
Solution Approach 1:
The patent replaces complex mechanical resonance-based sensing structures with a simpler capacitive sensing structure. The capacitive sensor uses electric field interactions rather than mechanical vibrations, achieving high sensitivity to low pressures through capacitance measurements while avoiding the complexity of maintaining and measuring resonant frequencies in damped systems.
Solution Approach 2:
The patent changes the sensing parameter from mechanical resonance characteristics to electrical capacitance. This parameter change enables high sensitivity to pressure variations at low pressures while using a structurally simpler capacitive configuration rather than requiring complex resonant mechanical structures with precise frequency control and measurement systems.
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 solution provides a pressure sensor with a high sensitivity to low pressures, achieving an average fractional change in frequency of at least 10^-6/Pa over the range 0 to 0.01 kPa, enabling reliable pressure sensing and defect detection in MEMS devices, and can be integrated within the MEMS device itself, reducing the need for additional sensors.
Implementation Method 1
The gas in the cavity acts as an additional spring and causes stiffening. The resonant frequency shift of the damped MEMS resonator is in the millibar (1 mbar = 0.1 kPa) range (Knudsen regime).
Implementation Method 2
Preferably, narrow etch slits and plate perforations are used to prevent the gas from escaping the resonator gap during the oscillations.
Implementation Method 3
The resonant frequency shift of the damped MEMS resonator is in the millibar (1 mbar = 0.1 kPa) range (Knudsen regime). This invention is based on the recognition that for fast oscillation, the elastic force causes the resonance frequency to shift.
Data Source
Figure 1
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AI summary
A MEMS pressure sensor for sensing the pressure in a sealed cavity of a MEMS device, comprises a resonant MEMS device having a pressure sensor resonator element which comprises an array of openings. The resonant frequency of the resonant MEMS device is a function of the pressure in the cavity, with resonant frequency increasing with pressure. Over the pressure range 0 to 0.1kPa, the average change in frequency is at least 10-6/Pa. The invention is based on the recognition that for fast oscillation, the elastic force causes the resonance frequency to shift. Therefore, it is possible to sense the pressure by a device with resonance frequency that is sensitive to the pressure.