Comparative Study of Line- and Dot-Mode Continuous-Wave Green Laser Sintering of CuO Ink on Substrates with Different Thermal Conductivity.

Ink-based fabrication of copper (Cu) interconnects and electrical pads has attracted increasing attention for a wide range of applications. In this study, we investigate the use of copper oxide (CuO) nanoparticle ink combined with laser sintering to produce conductive Cu pads on various substrates with different thermal conductivities, including glass, polyimide (PI), and poly(ethylene terephthalate) (PET) films. A 532 nm continuous-wave (CW) laser was employed in both CW and modulated-power (dot-mode) configurations to reduce CuO to metallic Cu. Our findings reveal that the degree of laser irradiation overlap plays a critical role in selective-area sintering, affecting both line- and dot-based processing. Optimal overlap conditions depend on several factors, such as ink layer thickness, laser scanning speed, substrate thermal conductivity, and glass transition temperature. For CuO layers with a thickness of more than 5.02 μm, slight overlap of laser irradiation is necessary for effective pad formation. In contrast, thinner layers (≤3.24 μm) typically do not require overlap due to the enhanced heat transfer efficiency. The resulting Cu patterns exhibited a resistivity of approximately 3.50 μΩ·cm, which is about twice that of bulk Cu on both glass and PI substrates. However, achieving conductivity on PET proved difficult due to substrate damage caused by its low thermal conductivity and glass transition temperature. This challenge was addressed by predepositing a thin silver (Ag) interlayer prior to CuO ink deposition. While both line- and dot-based sintering methods showed comparable electrical performance, dot-based irradiation offered advantages in reducing surface porosity and preserving edge integrity during sintering.