Composite MEMS Flow Sensor on Silicon-on-Insulator
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Solution Overview
Problem
Current micromachined silicon flow sensors face challenges in accurately measuring liquid flow rates due to instability and limited dynamic range, especially in high-pressure applications, and existing solutions are either costly or lack the necessary precision for medical applications like precise drug delivery.
Innovation Solution
A composite flow sensor integrating calorimetric, anemometric, and time-of-flight measurement principles on a silicon-on-insulator device with platinum and doped polysilicon sensing elements, capable of measuring both volumetric and mass flow rates with fast response times and low power consumption, while being robust against pressurized flows and easily manufacturable.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a syringe pump with delivery scale and timer is used for metering, then measurement precision is improved, but device complexity and size increase making it impractical for portable applications
Solution Approach 1:
The patent replaces the mechanical syringe pump and scale system with a microelectromechanical thermal flow sensor that uses thermal conduction measurements to determine flow rate, eliminating the need for bulky mechanical components while maintaining measurement capability
Solution Approach 2:
The patent introduces a thermal mediator (the fluid itself) to transfer heat from the heated element to the sensing elements, using the fluid's thermal properties as an intermediary to measure flow rate without direct mechanical interaction
2Measurement precision
If a silicon nitride membrane with through holes is used for gas flow sensing, then measurement precision is improved, but reliability deteriorates when applied to liquid flow due to capillary effects and trapped liquid instability
Solution Approach 1:
The patent changes the membrane material from silicon nitride to silicon oxide, altering the material's wetting properties and thermal conductivity to prevent capillary trapping of liquid and improve reliability in liquid flow applications
Solution Approach 2:
The patent uses a thin silicon oxide membrane that is selectively removed at through-holes to create a flexible, liquid-wettable surface that allows liquid to pass through without being trapped, maintaining sensor functionality in liquid environments
3Reliability
If the sensor is placed outside the flow channel separated by tube wall, then reliability is improved by avoiding membrane deformation, but device complexity and cost increase
Solution Approach 1:
The patent segments the sensor into two functional parts: a robust pressure-sensing membrane that withstands liquid pressure, and a separate thermal sensing elements that measure flow rate, allowing each component to be optimized for its specific function without compromising the other
4Strength
If a thick membrane is used to withstand high pressure, then strength is improved, but measurement precision deteriorates due to reduced thermal sensitivity
Solution Approach 1:
The patent applies different material properties to different regions: the membrane is made thick and strong for pressure withstanding, while the thermal sensing elements are positioned locally where thermal sensitivity is maximized, and the membrane material is changed to silicon oxide which has better thermal properties for sensing
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 provides precise, reliable, and cost-effective measurement of liquid flow rates with enhanced robustness and sensitivity, suitable for microfluidic applications, including medical drug delivery, with reduced uncertainties and improved accuracy across varying fluid properties.
Implementation Method 1
The sensor senses the medium flow rate mostly using the thermal calorimetric principle where a micro heater provides a constant temperature or constant power
Implementation Method 2
the sensing elements located at up and down stream of the micro heater shall measure the temperature changes associated with mass flow rate of a flow medium
Implementation Method 3
the cavity underneath the membrane serves as the thermal isolation for the better signal to noise ratio
Data Source
AI summary
The present invention disclosed a micromachined composite silicon flow sensor that is comprised of calorimetric flow sensing elements, time-of-flight sensing elements as well as independent temperature sensing element on a silicon-on-insulator device where the device layer is used for the thermal isolation membrane. The disclosed composite silicon flow sensor can measure mass flowrate, volumetric flowrate and flow medium temperature simultaneously, from which a full spectrum of flow parameters including flow pressure can be obtained. The sensor can be further used to alert any changes in physical properties of flow medium during operation. The disclosed manufacture process details the micromachining process of making such a sensor.