Speakers
Description
Laser sintering of metal micro- and nanoparticles commercial pastes is a promising route for fabrication of high-resolution conductive patterns on temperature-sensitive substrates. However, the narrow processing window between insufficient sintering and substrate damage remains a major challenge, particularly for copper-based materials prone to oxidation and pore formation.
In this work, high speed infrared (IR) thermography is employed as a non-contact diagnostic and control tool to monitor and optimize the laser sintering of approximately 100 µm-wide copper-based conductive lines deposited on an indium tin oxide (ITO)-coated silicon substrate. Sintering is performed using a continuously moving CO2 laser, while surface temperature fields are captured in real time using a FLIR X6981 infrared camera. The high spatial and temporal resolution of the IR measurements enables direct observation of transient thermal profiles generated by the relative motion between the laser spot and the printed line. To account for transient changes in surface emissivity associated with solvent evaporation and progressive oxidation during sintering, the raw FLIR camera data were post-processed using an in-house scripting routine. This correction enabled dynamic adjustment of the emissivity parameter during heating, improving the accuracy and consistency of the extracted temperature fields across the different sintering regimes. Elliptical temperature distributions are consistently observed, reflecting the combined effects of laser scanning speed, spot size, and thermal diffusion within the copper paste and substrate.
Surface temperature mapping is used to delineate three distinct processing regimes: unsintered regions, well-sintered conductive tracks, and substate-damage regions. A critical upper temperature threshold of approximately 230°C is identified, beyond which cracking for the ITO-coated silicon substrate occurs. By avoiding local overheating and inhomogeneous sintering, the thermograph-guided approach enables systematic exploration and optimization of the laser processing window.
Electrical conductivity, porosity, and defect formation are correlated with the measured thermal histories. For the printed lines, conductivity initially improves with increasing laser power due to enhanced particle necking and densification. However, further increases in energy input led to rising resistivity because of excessive pore formation, oxidation, and localized overheating. Minimum contact resistivity values as low as 7mΩcm2 are achieved under optimized conditions that promote dense neck formation without inducing thermal damage.
Quantitative thermal metrics, including peak intensity, heating and cooling rates, effective sintering temperature, and dwell time, are extracted from the IR data using FLIR Research Studio software. These metrics are indirectly correlated with cross sectional scanning electron microscopy SEM and an analytical thermal model to assess in depth-wise sintering and thermal diffusion, as well as with porosity measurements and electrical characterization. The analysis highlights the dominant roles of laser energy density and the ratio of laser spot size to line width line width in determining sintering uniformity and electrical performance.
Finally, a set of predictive metrics based on effective energy density, total energy input, and energy transfer rate is proposed to guide process optimization. Overall, this study demonstrates that high-speed thermography provides an effective and scalable methodology for mapping, controlling, and optimizing temperature fields during laser sintering of cooper-based conductive lines.
