Facile fabrication of low-cost passive wireless humidity sensor for smart packaging via all-laser processing of metalized paper

Abstract

We have developed a scalable and rapid process for creating a low-cost humidity sensing transponder for wireless monitoring of moisture level within packaged goods. The sensor is composed of a moisture sensitive interdigitated capacitor connected to a planar spiral coil, forming a parallel LC circuit whose resonant frequency is a function of environment humidity. The fabrication process is composed of a series of laser processing steps performed on a commercially available metallized paper which included selective metal etching and welding using a Nd-YAG (1.06μm) laser and cutting the paper substrate by a CO2 (10.6μm) laser. Selective laser ablation of the 10μm laminated Al film on the paper substrate was found to provide a unique one-step patterning of the conductive film on the paper substrate into the desired electrical patterns while simultaneously creating high surface area Al2O3 nanoparticles within the ablated regions. The intrinsic humidity-responsive characteristics of the laser induced metal oxide nanostructures provided a unique humidity sensitivity to the wireless sensor with a 10 folds higher performance than a similar LC resonant sensor that was prepared by conventional photolithography-based processes on a PCB substrate. The frequency change of the sensor was observed to be a linear function of RH (within the range of 0−85%RH), with an average sensitivity of -87 kH/RH and good reversibility and repeatability.