MEMS Thermal Mass Flow Sensor with Porous Silicon Self-Cleaning
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Solution Overview
Problem
Conventional thermal mass flow sensors face issues with low flow measurement accuracy, high power consumption, and malfunction in dirty fluid environments due to fragility, heat conduction limitations, and particle trapping, which affects their reliability and productivity.
Innovation Solution
A MEMS thermal mass flow sensor with a porous silicon layer that emits acoustic waves for self-cleaning and provides thermal isolation, combined with a capacitive force to remove foreign materials, and a Wheatstone bridge circuit for constant temperature control, enhancing measurement accuracy and robustness in dirty fluid conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a thin film membrane is used for thermal isolation in thermal mass flow sensors, then measurement accuracy is improved, but the device becomes fragile and malfunctions frequently in dusty or smoked fluids
Solution Approach 1:
The patent applies porous silicon material to create a porous membrane structure that provides thermal isolation for accurate flow measurement while being more robust than thin film membranes. The porous structure allows the membrane to withstand dusty and smoked fluid environments without frequent malfunction, resolving the contradiction between measurement precision and reliability.
2Measurement precision
If opening slots are constructed on the membrane surface to block horizontal heat conduction, then thermal isolation is improved, but particles are trapped leading to sensor malfunction
Solution Approach 1:
The patent replaces opening slots with a porous membrane structure that achieves thermal isolation through the porous material's inherent low thermal conductivity. This eliminates the particle trapping problem associated with slots while maintaining the thermal isolation function, thus improving reliability without sacrificing measurement precision.
3Measurement precision
If opening slots are placed on the membrane, then thermal isolation is enhanced, but the sensor cannot be applied to liquid measurement due to reduced thermal resistance
Solution Approach 1:
The porous membrane structure provides effective thermal isolation that prevents heat conduction to the substrate even when used with liquid measurements. The porous structure's low thermal conductivity compensates for the higher thermal conductivity of liquids compared to gases, enabling the sensor to be applied to both gas and liquid flow measurements, thus enhancing adaptability.
4Productivity
If conventional thermal mass flow sensors are used in dirty fluid environments, then flow measurement is performed, but the sensors malfunction due to particle accumulation on the surface
Solution Approach 1:
The porous membrane structure prevents particle accumulation on the sensor surface by allowing particles to pass through the porous structure rather than accumulating on a solid surface. This maintains sensor performance and enables continuous operation in dirty fluid environments, resolving the contradiction between productivity and reliability.
Solution Approach 2:
The patent incorporates ultrasonic vibration to mechanically remove accumulated particles from the sensor surface. This active cleaning mechanism ensures continuous operation in dirty environments by preventing particle buildup that would otherwise cause malfunction, maintaining both productivity and reliability.
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 sensor achieves superior flow measurement performance with self-cleaning capabilities, improved reliability, and reduced particle sticking, enabling operation in dirty environments with increased manufacturing simplicity and reduced failure rates.
Implementation Method 1
The active membrane or the porous silicon device is capable of generating a self-cleaning surface wave, such as ultrasonic acoustic wave emitted from porous silicon
Implementation Method 2
generating a self-cleaning surface wave, such as ultrasonic acoustic wave emitted from porous silicon
Implementation Method 3
the region having poor heat conductivity is made of porous silicon or porous silicon dioxide
Implementation Method 4
The active region of membrane usually consists of heating elements and sensing elements such as resistance temperature detector (RTD) or thermopiles
Data Source
AI summary
The current invention generally relates to Micro Electro Mechanical Systems (MEMS) thermal mass flow sensors for measuring the flow rate of a flowing fluid (gas/liquid) and the methods of manufacturing on single crystal silicon wafers. The said mass flow sensors have self-cleaning capability that is achieved via the modulation of the cavity of which the sensing elements locate on the top of the cavity that is made of a silicon nitride film; alternatively the sensing elements are fabricated on top of a binary silicon nitride/conductive polycrystalline silicon film under which is a porous silicon layer selective formed in a silicon substrate. Using polycrystalline silicon or the sensing elements as electrodes, an acoustic wave can be generated across the porous silicon layer which is also used for the thermal isolation of the sensing elements. The vibration or acoustic energy is effective to remove foreign materials deposited on top surface of the sensing elements that ensure the accuracy and enhance repeatability of the thermal mass flow sensing.