An all-polymer airflow sensor using a piezoresistive composite elastomer

This paper presents an all-polymer flexible micromachined flow sensor using a carbon-black based conductive composite elastomer as a piezoresistor. The device is composed of an out-of-plane curved flow sensing element formed using a polyimide film. The conductive composite elastomer combines a low Young's modulus (similar to 1.72 MPa) and a high piezoresistive gage factor (similar to 7.3), making it an ideal material for the sensing application. Moreover, the use of the polyimide film, which can be easily laser micromachined, as the material for device fabrication enables the use of planar micromachining techniques, which minimizes process complexities. The proposed fabrication sequence combines the benefits of the polymeric materials used, while simultaneously enabling a backside interconnect scheme for an array of devices, without additional processing steps. The backside interconnect scheme allows for flow field mapping with minimum interference due to the sensing circuitry. Individual sensors as small as 1.5 mm in length and 0.4 mm in width, with 70 mu m wide and 20-50 mu m thick piezoresistor lines, have been fabricated. Wind tunnel testing demonstrated sensitivities as high as 66 Omega/(m s(-1)). The integration of polyimide films and conductive elastomers into a flow sensing device using the simple planar fabrication technologies discussed is suitable for reduced cost, large area sensor array development, and can also leverage traditional flexible circuit fabrication.