Laser fabrication and performance of NiCr strain gauges

Abstract

Increasing demand for smart devices has spurred the development of advanced sensors with smaller and more adaptable form factors. The integration of thin film technology into sensors such as strain gauges has the potential to reduce their size and allow their use in flexible applications. Laser patterning is a promising choice to replace the conventional lithography-based method for flexible microelectronic fabrications, due to its low process complexity, short development cycle for custom sensor designs and the ability to pattern on three-dimensional (3D) surfaces with high structure resolution. The laser ablation process of NiCr film was investigated in detail by the study of the influence of laser parameters and theoretical analysis. The average power, repetition frequency, scanning speed, and pulse overlap were found to be significant parameters in controlling the quality and dimensional accuracy of ablated microchannels. An empirical threshold fluence was derived from an analytical analysis of the ablation process. This threshold value allows for predicting the geometry of the ablated structure based on the selected processing parameters. In addition, a numerical model was created in COMSOL Multiphysics to analyze the material removal process and the final geometry of the ablated structure with different laser parameter sets. Generally, ablation with a higher frequency and slower speed results in a smoother bottom surface of the ablated microchannel. The findings can be utilized to establish a process map that allows for the selection of parameters according to dimensional and structural requirements. The laser patterning process was used to fabricate a 1025 Ω flexible thin film strain gauge using NiCr film. The characteristics and dynamic tensile response of the laser fabricated strain gauge were evaluated. The strain gauges exhibit sensitivity comparable to commercially available NiCr-based strain gauges and can reliably survive 106 cycles up to 1750 με. Several failure mechanisms were identified, and these findings provide a guide to diagnose thin film strain gauge failures. Strain gauges fabricated by this unconventional technique have shown their potential for use in long-term dynamic load sensing applications.