Towards High-Temperature MEMS: Two-Step Annealing Suppressed Recrystallization in Thin Multilayer Pt-Rh/Zr Films

Platinum-based thin films are widely used to create microelectronic devices operating at temperatures above 500 degrees C. One of the most effective ways to increase the high-temperature stability of platinum-based films involves incorporating refractory metal oxides (e.g., ZrO2, HfO2). In such structures, refractory oxide is located along the metal grain boundaries and hinders the mobility of Pt atoms. However, the effect of annealing conditions on the morphology and functional properties of such multiphase systems is rarely studied. Here, we show that the two-step annealing of 250-nm-thick Pt-Rh/Zr multilayer films instead of the widely used isothermal annealing leads to a more uniform film morphology without voids and hillocks. The composition and morphology of as-deposited and annealed films were investigated using X-ray diffraction and scanning electron microscopy, combined with energy-dispersive X-ray spectroscopy. At the first annealing step at 450 degrees C, zirconium oxidation was observed. The second high-temperature annealing at 800-1000 degrees C resulted in the recrystallization of the Pt-Rh alloy. In comparison to the one-step annealing of Pt-Rh and Pt-Rh/Zr films, after two-step annealing, the metal phase in the Pt-Rh/Zr films has a smaller grain size and a less pronounced texture in the <111> direction, manifesting enhanced high-temperature stability. After two-step annealing at 450/900 degrees C, the Pt-Rh/Zr thin film possessed a grain size of 60 +/- 27 nm and a resistivity of 17 x 10(-6) Omegam. The proposed annealing protocol can be used to create thin-film MEMS devices for operation at elevated temperatures, e.g., microheater-based gas sensors.

Automated summary

Platinum-based thin films are commonly used in microelectronic devices operating at temperatures higher than 500 degrees C. Adding refractory metal oxides, such as ZrO2 and HfO2, can improve the high-temperature stability of these films. This study demonstrates that a two-step annealing process for Pt-Rh/Zr multilayer films results in a more uniform film morphology and enhanced high-temperature stability compared to the commonly used one-step annealing.