Effects of Heat Treatment on the Temperature Coefficient of Resistivity of Laser Fabricated NiCr Flexible Sensors

Abstract

Advancements in automation, flexible electronics and embedded sensing systems has increased the demand for specifically designed and customizable sensors. This includes thin film resistive sensors with customizable electrical properties and long-term stability. Nickel chromium (NiCr) alloys are widely used in resistive sensors due to their chemical stability, high resistivity, and low temperature coefficient of resistivity (TCR); however, controlling TCR remains a challenge in novel sensor fabrication methods, such as laser ablation. Laser ablation offers a promising alternative to conventional lithography-based fabrication due to its low process complexity, rapid prototyping capabilities, and compatibility with flexible substrates. In this work, NiCr thin films were heat treated over a range of temperatures and hold times to modify their electrical and microstructural properties. Following heat treatment, the thin film samples were hot pressed between two polyimide sheets and patterned into flexible resistive sensors using UV laser ablation. The laser fabrication process was optimized through investigation of processing parameters, including laser power, number of passes, and environmental factors. The influence of these parameters on feature geometry, resistance control, and fabrication repeatability was evaluated, and laser fabricated sensors were benchmarked against commercially manufactured resistive sensors. Heat treatment was found to significantly reduce the magnitude of TCR, while improving resistance stability. Microstructural and compositional changes were characterized using scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The results indicate that surface compositional changes and oxidation influence electrical properties more than bulk changes in thin films. These findings demonstrate that pre-fabrication heat treatment is an effective strategy for tailoring the TCR of laser fabricating NiCr thin film sensors for flexible sensor applications.