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In this blog I would like to share the development of a MEMS-based Coriolis instrument, currently the lowest flow measuring Coriolis mass flow sensor in the world. MEMS is short for Micro Electro Mechanical System.
MEMS technology is similar to semi-conductor technology, but it is applied for sensors and miniature mechanical components instead of electronic chips. Well known applications of MEMS technology are airbag sensors, inkjet heads, pressure sensors, microphones, compasses, accelerometers, gyroscopes and time-base oscillators. For instance, a smartphone contains a lot of MEMS components and thermal flow sensors are widely used in air conditioning systems.
MEMS chips are made from wafers. Wafers are extremely flat circular discs, made from Silicon or glass. A typical wafer has a thickness of 0.5 mm and a diameter of 6 inch. MEMS technology is all about adding layers and removing those layers in certain areas. The layers that are applied can be of very high quality and robust materials. Silicon Nitride is an example of such a material, which is applied by Low Pressure Chemical Vapor Deposition (LPCVD) and it is performed around 800˚C.
Photo-lithography is used to define the areas that need to be removed. In photo-lithography, a layer of photo-resist is deposited on the surface of the wafer. The photo-resist is chemically altered by shining light on its surface and is selectively removed in a development solution.
Most MEMS flow sensors are based on a thermal measurement principle. It has been demonstrated that such sensors are capable of measuring liquid flow down to a few nanoliter per minute. Advantages of these sensors are that they are fast and very stable. A disadvantage is that they need to be calibrated for each specific fluid.
A Coriolis type of flow sensor, i.e. flow sensors containing a vibrating tube in which a mass flow is subjected to Coriolis forces, do not have this problem. The Coriolis forces are directly proportional to the mass flow and independent of temperature, pressure, flow profile and fluid properties since Coriolis flow sensors measure true mass flow.
Coriolis flow meters are mostly used for measuring large flow rates (>1 kilogram per hour), since the relatively weak Coriolis forces are correspondingly harder to detect for small flows. In order to gain enough sensitivity to measure ultra low flows below 2 gram per hour, the sensor size and tube wall thickness needs to be minimized to the extreme, which is not possible by conventional machining of stainless steel.
Here MEMS technology comes into play. A process called “surface channel technology”, which we developed in close collaboration with the University of Twente, allows for the fabrication of tubes with 1 micrometer thin Silicon Nitride walls. The choice of material renders these tubes mechanically stable even at this extremely thin wall thickness.
In the picture below, the principle of operation of the MEMS based Coriolis sensor is explained. The sensor that is built into the demonstration model is based on this technology. The demonstration model can measure and control gas and liquid flowrates from 0.01 up to 2 gram per hour. As an additional advantage of MEMS technology, the Coriolis tube inside the instrument has such small dimensions that the resonance frequency of the tube is in the kHz range. This results in a lower susceptibility to external vibrations than conventional stainless steel Coriolis instruments.
The 'surface channel technology' that is used to create the micro-Coriolis sensor chip allows for other types of sensors as well. Examples are: pressure sensors, density sensors, viscosity sensors and thermal mass flow sensors.