Durchflusssensoren aus Kunststoff für sehr kleine Volumenströme auf der Basis des AMANDA-Verfahrens

Abstract

A micromechanical flow sensor for liquids and gases from polymer has been developed, manufactured, and characterized. For this purpose, various methods of flow measurement were examined with a view to both the possibilityof micro-manufacture by the AMANDA process and to the specifications required by industry. A few interesting implementation possibilities were elaborated in the conceptual design phase which are described in detail in this paper. The principle of thermoanemometry, which uses as a measurement signal the power output as a function of flow, of a body electrically heated to the surrounding fluid, was used for the practical design of the sensor. Due to the use of different polymers in manufacturing the sensor, a temperature-independent design of the functional element had to be developed. For this reason, a support structure shaped like a reed was developed, which is attached to one side of the housing above the fluid channel. A meandering conductor strip was applied to this reed above the freely movable section. To protect the conductor strip, a layer identical to the geometry of the support structure and its thickness was used. As a result of these design measures, a temperature-compensated functional element was constructed which prevents unwanted additional thermal expansion of the conductor strip, which would falsify the measured signal. Fifty flow sensors were produced in parallel. The housings were made by hot embossing of polysulfone with integrated fluid channels. The functional elements were produced by photolithographic patterning of membranes and by a vapor coating process. The support structure and the protective coating both consist of 1.2 mm thick polyimide. The width of the support structure was adapted to the respective width of the fluid channel. The conductor strip located between the two membranes is made of 2 μm wide and 100 nm high platinum. After connecting the sensor components, which are produced in batches, and subsequent singulization of the sensors, the flow sensor outer dimensions are 5.5 X 4.5 x 1.2 mm 3 . Fluidic tests were carried out with water and nitrogen to characterize the sensor. For water, a minimum volumetric flow rate of 0.1 μl/min was detected for a fluid temperature increase of 6°C. The response time of the sensor was measured to be 2.5 ms. Due to the short response times, a sensor electronic system operating in pulse mode was used, which measures the flow velocity and the fluid temperature influencing the measured signal on the same conductor strip. The independence of temperature of the measured signal achieved by the measurement of the fluid temperature and by the newly developed sensor design was confirmed in measurements. To determine the temperature distribution in the sensor element, FEM calculations were carried out, whose results were confirmed by means of infrared thermal imaging. In this way, a computer-supported method of sensor optimization has been developed. After the successful demonstration of the feasibility of the concept, a comparison was made with other flow sensors produced by micromechanical techniques. Because of the low measurable volumetric flow rates accompanied by minor temperature increases in the fluid to be measured, the flow sensor developed in this activity can be used, e.g., in drug dosage or in bioanalytics.