Silicon mass-flow sensors

R 18] [A 1] Each module is equipped with a heater (H3-H8) and a fluidic cooling (C03-C06). Temperature sensors integrated in the modules deliver the sensor signals for the heater control. Fluidic data such as flow and pressure are measured integrally outside the micro structured devices by laboratory-made flow sensors manufactured by silicon machining. The micro structured pressure sensor can tolerate up to 10 bar at 200 °C with a small dead volume of only 0.5 pi. The micro structured mass flow sensor relies on the Coriolis principle and is positioned behind the pumps in Figure 4.59 (FIC). For more detailed information about the product quality it was recommended to use optical flow cells inline with the chemical process combined with an NIR analytic or a Raman spectrometer. 

A wide range of manufacturers use various technologies (heated wire, ceramics, silicon). The market for mass-flow sensors was estimated as about 800 million in 2000. 

For discrete micromechanical devices that do not include integrated electronics, the situation is different Platinum is an excellent choice for sensors based on a thermal measurement principle and is used in very high volumes, for example, in the air mass flow sensor chip produced at Bosch. Here, a platinum thin film serves as both a heater and a temperature sensor on a thin dielectric membrane consisting of silicon oxide and silicon nitride. The advantages of platinum as a thin film for thermal sensors compared to, for example, polysilicon are as follows ... 

From the vantage point of microfluidics, the structures developed by Petersen et al [33] are the most appropriate. More recently, Baltes and coworkers combined CMOS circuitry with the microfabrication of sensors to construct a thermal mass flow system based on thin-film pyrometers [66]. As free standing mass flow sensors, they have attractive features. However, all of these silicon-based devices operate at relatively high temperatures in the 100-200 °C range. This elevated temperature limits their potential application in more complex microfluidic systems. The ideal flow sensor would be a very-low-temperature element that could be used on the walls of the microchannel. 

It provides a more direct approach for temperature and pressnre compensation than other presently-available mass flow sensors requiring measurement of temperature and pressure. For some gas mixtures of varying composition, mass flow is indicated accurately (e.g. CO2 and He) without calibration corrections. Because it can be fabricated by conventional thin film deposition and silicon processing techniques. It offers the possibility of lower cost and broader applications than present conunercially available gas flow sensors. 

Enoksson, P. Stemme, G. Stemme, E. A silicon resonant sensor structure for Coriolis mass flow measurement. J. MEMS 1997, 6 (2), 119-125. 

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