Residual Pressure Measurement in Optical MEMS Fabry-Perot Sensors
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Solution Overview
Problem
Existing optical MEMS pressure sensors face challenges in accurately measuring residual pressure within Fabry-Perot cavities due to gas trapped during silicon wafer bonding, which affects temperature characteristics and stability, and previous methods are either inaccurate or destructive.
Innovation Solution
A residual pressure measurement system using a low-coherence source, 3 dB coupler, MEMS pressure sensor, air pressure chamber, thermostat, pressure control system, cavity length demodulator, and acquisition card, allowing for non-destructive measurement of residual pressure by scanning external pressures at different temperatures and calculating the residual pressure without altering the sensor structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If theoretical calculation methods are used to measure residual pressure, then measurement can be performed, but accuracy is strongly dependent on measurement accuracy of parameters such as cavity radius, diaphragm thickness, diaphragm deflection and Young's modulus
Solution Approach 1:
The patent replaces theoretical mechanical calculation methods with optical measurement methods. By using optical interference principles to directly measure cavity length changes, the system eliminates the need for multiple mechanical parameter measurements (cavity radius, diaphragm thickness, deflection, Young's modulus) and provides more accurate residual pressure measurement through direct optical detection.
2Measurement precision
If focused ion beam drilling is used to measure residual pressure, then pressure dependent resonance frequency can be compared, but the sensor is destroyed and only several sensors can be sampled
Solution Approach 1:
The patent enables the sensor to perform self-diagnosis and self-measurement of residual pressure through optical interference without requiring external destruction or special handling. The sensor structure itself serves as the measurement tool, allowing non-destructive evaluation of multiple sensors from the same batch.
3Measurement precision
If two fibers are placed at different positions of the cavity, then residual pressure can be measured by fitting straight lines, but a special device is required to assemble the fibers with the MEMS chip and it cannot measure packaged sensors
Solution Approach 1:
The patent creates a universal measurement system that can evaluate both bare MEMS chips and packaged optical MEMS pressure sensors using the same optical interference methodology. The system adapts to different sensor configurations without requiring special assembly devices or complex fiber positioning arrangements.
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
Enables accurate, non-destructive measurement of residual pressure in packaged optical MEMS pressure sensors, allowing for long-term monitoring and evaluation of bonding quality and air leakage, with high stability and suitability for various MEMS pressure sensor structures.
Implementation Method 1
a low-coherence source 13, a 3 dB coupler 14, a MEMS pressure sensor 16... light output from the optical fiber 9 is partially reflected for the first time on a reflective coating 6 to form a reflected reference light 10; and the rest light is transmitted to a diaphragm inner surface 7 for a second reflection to form a reflected sensing light 11. The reflected reference light 10 and the reflected sensing light 11 form an interference signal
Implementation Method 2
light output from the optical fiber 9 is partially reflected for the first time on a reflective coating 6 to form a reflected reference light 10; and the rest light is transmitted to a diaphragm inner surface 7 for a second reflection to form a reflected sensing light 11
Data Source
AI summary
The present invention discloses a residual pressure measurement system for a MEMS pressure sensor with an F-P cavity and method thereof, the measurement system includes a low-coherence light source, a 3 dB coupler, a MEMS pressure sensor, an air pressure chamber, a thermostat, a pressure control system, a cavity length demodulator, an acquisition card and a computer. The measurement method comprises: performing cavity length measurement by using the reflecting light by the pressure control system at two temperatures, respectively, so as to calibrate the MEMS pressure sensor and establish a relationship between the absolute phase of a monochromatic frequency and the external pressure; performing linear fitting to the two measurement data to obtain all the external pressure when the cavity length of two measurement data are equal to each other, and substituting the theoretical equation for calculation to obtain the residual pressure under the flat condition of the diaphragm.