Influence of the AlN/Pt-ratio on the electro-mechanical properties of multilayered AlN/Pt thin film strain gauges at high temperatures

Abstract

In this work, the impact of the AlN/Pt ratio on the strain-sensitive properties of multilayered AlN/Pt thin films is investigated. These thin film systems consisting of 10 bi-layers of AlN and Pt each with a thickness of 3 nm AlN and 7 nm Pt or 5 nm AlN and 5 nm Pt are fabricated, evaluated and compared to those realized with a sequence of 7 nm AlN and 3 nm Pt thin bi-layers. The thin film systems are sputter-deposited on oxidized silicon wafers or sapphire substrates. The influence of different annealing steps at 900 °C up to 24 h in Argon (Ar) atmosphere on the electrical film resistivity and the temperature coefficient of the electrical resistance (TCR) is investigated for these different multilayers using Van-der-Pauw measurements up to 330 °C in air. Furthermore, the impact of the AlN/Pt-ratio on the gauge factor of the thermally stabilized multilayers is determined using a purpose-built measurement setup up to 500 °C in air. Transmission electron microscopy and X-ray diffraction analyses are utilized to examine the microstructure and the crystallographic phase composition of the multilayers before and after thermal loading. Annealing the different multilayers at 900 °C leads to diffusion effects between the AlN and Pt thin films and recrystallizations processes in the Pt sublayers depending on the individual sublayer thickness values. After thermal pre-conditioning at 900 °C for 1 h in Ar the samples were stable even in air up to 500 °C. Finally, it is shown that the TCR as well as the gauge factor of an AlN/Pt multilayer can be tuned through the AlN/Pt-ratio. The highest gauge factor with a value of 4.7 (±0.3) at room temperature is achieved with a multilayer consisting of 5 nm AlN and 5 nm Pt (pure Pt thin film on Cr adhesion promotor: 3.2), while the lowest linear TCR with a value of α = 7.4 ∙ 10−4 K-1 is measured for multilayers with 7 nm AlN and 3 nm Pt sublayers (pure Pt thin film on Cr adhesion promotor: 3.66 ∙ 10-3 K-1).