INVESTIGATION OF THIN ALN FILMS FOR PIEZOLAYER-FIELD EFFECT TRANSISTOR APPLICATIONS

In our work we have developed pressure-sensitive field-effect transistors (FFTs) by use of r.f.-sputtered AlN thin films. The AlN coatings were optimized with regard to a dense microcrystalline structure and a preferred (002) orientation in order to obtain excellent piezoelectric properties. Therefore the substrate etching conditions before and the reactive gas flow during the deposition were systematically varied as was the coating thickness. Vickers microhardness testing and scratch adhesion investigations were applied in view of the mechanical coating properties. The film structure and the coating thickness were analysed by means of scanning electron microscopy. X-ray diffraction techniques were applied for texture evaluation. For realization of the pressure-sensitive FETs the deposition of optimized AlN piezoelectric films was included in a standard n-channel metal-oxide-semiconductor process. To pattern the AlN films. two different methods have been investigated: wet etching in phosphorous acid and the lift-off technique. Finally the sensors produced were tested under various conditions. The sensor characteristics show good linearity of the pressure response. The sensitivity can be biased by variation in the electrical parameters such as gate voltage or drain-source current. Additionally the geometry of the AlN films affects the sensitivity as does the coating thickness. However, owing to the long-time drift of the output signal under a constant weight load the sensor is eligible in particular for dynamic applications such as accelerometers, whereas for a long-time static pressure measurement a different design should be developed.