Comparison of Direct and Indirect Laser Ablation of Metallized Paper for Inexpensive Paper-Based Sensors

In this work, we present a systematic study of laser processing of metallized papers (MPs) as a simple and scalable alternative to conventional photolithography-based processes and printing technologies. Two laser-processing methods are examined in terms of selectivity for the removal of the conductive aluminum film (25 nm) of an MP substrate; these processes, namely direct and indirect laser ablation (DLA and ILA), operate at wavelengths of 1.06 mu m (neodymium-doped yttrium aluminum garnet) and 10.6 mu m (CO2), respectively. The required threshold energy for each laser processing method was systematically measured using electrical, optical, and mechanical characterization techniques. The results of these investigations show that the removal of the metal coating using ILA is only achieved through partial etching of the paper substrate. The ILA process shows a narrow effective set of laser settings capable of removing the metal film while not completely burning through the paper substrate. By contrast, DLA shows a more defined and selective removal of the aluminum layer without damaging the mechanical and natural fibular structure of the paper substrate. Finally, as a proof of concept, interdigitated capacitive moisture sensors were fabricated by means of DLA and ILA onto the MP substrate, and their performance was assessed in the humidity range of 2-85%. The humidity sensitivity results show that the DLA sensors have a superior humidity sensing performance compared to the ILA sensors. The observed behavior is attributed to the higher water molecule absorption and induced capillary condensation within the intact cellulose network resulting from the DLA process (compared to the damaged one from the ILA process). The DLA process of MP should enable scalable production of low-cost, paper-based physical and chemical sensing systems for potential use in point-of-care diagnostics and food packaging.