Microfabrication has greatly matured and proliferated in use amongst many disciplines. There has been great interest in micromachined flow sensors due to the benefits of miniaturization: low cost, small device footprint, low power consumption, greater sensitivity, integration with on-chip circuitry, etc. This paper reviews the theory of thermal flow sensing and the different configurations and operation modes available. Material properties relevant to micromachined thermal flow sensing and selection criteria are also presented. Finally, recent applications of micromachined thermal flow sensors are presented. Detailed tables of the reviewed devices are included.

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Micromachined Thermal Flow Sensors—A Review

A module may be provided with at least one opening, the opening being an endpoint of a microfluidic channel that passes through at least part of the module. A set of multiple such modules may be arranged into an arrangement of modules, which may be coupled together using one or more coupling mechanisms included on each module. The arrangement of modules may fit within a regular polyhedral grid, and each module within the arrangement of modules may have a form suitable for arrangement of the modules within the regular polyhedral grid. Fluid may then flow through at least a subset of the arrangement of modules via the microfluidic channel of each module of the subset of the arrangement of modules. Some modules may include sensors, actuators, or inner microfluidic channel surface coatings. The arrangement of modules may form a microfluidic circuit that can perform a microfluidic circuit function.

https://www.pnas.org/content/pnas/111/42/15013.full.pdf

Discrete elements for 3d microfluidics

This paper presents an overview and development of 2-D micromachined thermal wind sensors. It first compares different types of wind sensors, then gives a brief introduction to three basic measurement principles of the thermal wind sensors: hot-wire or hot-film, calorimetric, and time of flight. In order to have a good understanding of development of the 2-D micromachined thermal wind sensors, they are first categorized into thermoresistive, thermoelectric, and thermoelectronic wind sensors based on their operations. Then, each category is further divided into different subcategories, according to their measuring principles and sensor configurations. In Section IV , different materials, fabrication processes, packaging techniques, and control modes of the 2-D micromachined thermal wind sensors are analyzed. This paper also evaluates the core technologies on how to improve the sensitivity, measurement range, and power consumption of the thermal wind sensors.

2-D Micromachined Thermal Wind Sensors—A Review

Flow sensing in hostile environments is of increasing interest for applications in the automotive, aerospace, and chemical and resource industries. There are thermal and non-thermal approaches for high-temperature flow measurement. Compared to their non-thermal counterparts, thermal flow sensors have recently attracted a great deal of interest due to the ease of fabrication, lack of moving parts and higher sensitivity. In recent years, various thermal flow sensors have been developed to operate at temperatures above 500 °C. Microelectronic technologies such as silicon-on-insulator (SOI), and complementary metal-oxide semiconductor (CMOS) have been used to make thermal flow sensors. Thermal sensors with various heating and sensing materials such as metals, semiconductors, polymers and ceramics can be selected according to the targeted working temperature. The performance of these thermal flow sensors is evaluated based on parameters such as thermal response time, flow sensitivity. The data from thermal flow sensors reviewed in this paper indicate that the sensing principle is suitable for the operation under harsh environments. Finally, the paper discusses the packaging of the sensor, which is the most important aspect of any high-temperature sensing application. Other than the conventional wire-bonding, various novel packaging techniques have been developed for high-temperature application.

Thermal Flow Sensors for Harsh Environments.

This study develops a MEMS-based low-cost sensing platform for sensing gas flow rate and flow direction comprising four silicon nitride cantilever beams arranged in a cross-form configuration, a circular hot-wire flow meter suspended on a silicon nitride membrane, and an integrated resistive temperature detector (RTD). In the proposed device, the flow rate is inversely derived from the change in the resistance signal of the flow meter when exposed to the sensed air stream. To compensate for the effects of the ambient temperature on the accuracy of the flow rate measurements, the output signal from the flow meter is compensated using the resistance signal generated by the RTD. As air travels over the surface of the cross-form cantilever structure, the upstream cantilevers are deflected in the downward direction, while the downstream cantilevers are deflected in the upward direction. The deflection of the cantilever beams causes a corresponding change in the resistive signals of the piezoresistors patterned on their upper surfaces. The amount by which each beam deflects depends on both the flow rate and the orientation of the beam relative to the direction of the gas flow. Thus, following an appropriate compensation by the temperature-corrected flow rate, the gas flow direction can be determined through a suitable manipulation of the output signals of the four piezoresistors. The experimental results have confirmed that the resulting variation in the output signals of the integrated sensors can be used to determine not only the ambient temperature and the velocity of the air flow, but also its direction relative to the sensor with an accuracy of ± 7.5° error.

A MEMS-Based Flow Rate and Flow Direction Sensing Platform with Integrated Temperature Compensation Scheme

A calorimetric sensor for detection of gas velocity and flow direction was constructed by sacrificial porous silicon micromachining technique. The sensing element contains four temperature sensing resistors arranged symmetrically around a central filament heater. Finite volume flow simulations (FVM) for velocity vector and temperature distribution around the system elements led to the conclusion that the laminar flow conditions, essential for accurate calorimetric sensing, can be maintained by adequate selection of device dimensions and channel depth. Predictions by the flow and thermal simulations were aimed at defining the optimum geometric design. The results of functional tests performed on the fabricated structure were interpreted in terms of changes of parameters in the thermal equivalent circuit of the device.

G. Légrádi

Papers

Thermal characterisation of a direction dependent flow sensor