Fabrication and Multiphysics Modeling of MEMS Thermal Flow Sensor

A micromachined thermal flow sensor is presented demonstrating sensing liquid flow rates as low as 2 microliters per minute capable of being used for biomedical applications. These flow sensors rely on the varying electrical resistance of sensors generated by forced convection at different flow rates. The sensor array presented was constructed using microelectromechanical systems (MEMS) techniques including micro-molding, wet etching and dry etching utilizing biocompatible materials. A numerical model was built using COMSOL multi-physics in order to predict and optimize the electrical, thermal and fluid behavior of the sensor, which was verified with experimental data. The construction allowed for multiple thermal flow sensing operational modes. Here, constant current hot film and constant current calorimetric were simulated and tested. A variety of flow sensor geometries were compared to investigate maximum heat transfer to the sensors, thermal insultation, size, sensitivity and range capabilities. The sensor design is such that it is capable of detecting different flow direction and various flow ranges for different fluids. In addition to the performance capabilities outlined, the sensor is relatively inexpensive and should have a long lifetime due to the lack of moving parts.