High temperature strain gages based on reactively sputtered AlN x thin films

Abstract

Thin film strain sensors based on reactively sputtered aluminum nitride are being developed for a variety of advanced aerospace applications, where the measurement of both static and dynamic strain is required at elevated temperatures. The non-stoichiometric AlN x thin films are particularly attractive for strain sensor applications at elevated temperatures since they exhibit a relatively large gage factor G, and a relatively low temperature coefficient of resistance (TCR), and they are electrically stable at high temperature, “c” axis oriented (0002) AlN x thin films were prepared by reactive r.f. sputtering from high purity aluminum targets. By varying the nitrogen content in the plasma, AIN x films useful for strain gage applications were produced. The resulting films exhibited room temperature resistivities in the range 1 × 10 −3 Ω cm to 5 × 10 2 Ω cm, were semi-transparent in the visible spectrum (optical band gaps in the range 4.6–5.8 eV) and tested “p” type by hot probe. TCRs ranged from +825 to −1200 ppn °C −1 after repeated thermal cycling to 1100 °C, depending on the nitrogen content in the film and the room temperature resistivity. Sputtering parameters were adjusted to yield a minimum value of +109 ppm°C −1. Large positive gage factors were measured at room temperature for all semiconducting AlN x films and the films exhibited a nearly linear piezoresistive response with little or no hysteresis when cycled from tension to compression. Gage factors on the order of 15 were realized for the AlN x films (gage factors of 2 are normally observed for refractory metal alloys). Annealing the AlN x films in argon at temperatures up to 1100 °C had minimal effect on the gage factor. This suggests that the piezoresistive response was more dependent on the defect structure of the as-deposited films than on the resistivity or subsequent thermal processing. Both the TCR and gage factor were correlated with the as-deposited resistivity and the nitrogen content in the films. The relationship between processing parameters and properties of these A1N x films is reviewed here, and prospects of using such films as high temperature strain sensors are discussed.