Announcing a new publication from Opto-Electronic Manufacturing; DOI 10.29026/oem.2026.260002 .
Copper interconnects carry electrical power and signals in modern electronic devices, but fabricating them with both fine feature sizes and high electrical conductivity often requires complex manufacturing processes. Researchers at Huazhong University of Science and Technology have developed a mask-free femtosecond laser direct writing (FsLDW) method that combines a tailored particle-free copper precursor ink with a pre-drying step. Under ambient conditions, the method produces dense copper micropatterns with line widths below 10 μm and a resistivity as low as 9.34 μΩ·cm. It is also compatible with flexible substrates, wafer-scale processing, and curved surfaces.
Copper wiring is essential to modern electronics. These microscopic conductive paths connect different components and carry electrical power and signals in devices such as sensors, flexible circuits, and integrated electronic systems. Copper is widely used because it combines high electrical conductivity with relatively low material cost. However, producing very small copper patterns often requires photolithography, metal deposition, and etching. Although these processes can provide high precision, they usually require multiple fabrication steps and specialized equipment, while their applicability to nonplanar surfaces remains limited.
Laser direct writing offers a simpler, mask-free alternative. In this approach, a copper-containing precursor ink is first coated onto a surface. A computer-controlled laser beam then scans along a programmed path and converts only the illuminated regions into metallic copper. The unexposed material is removed by rinsing, leaving the desired copper pattern behind. Because the pattern is defined digitally, a new design can be produced by changing the scanning program rather than preparing a new physical mask. This makes the process attractive for customized manufacturing and for substrates that are difficult to handle using conventional planar techniques. Femtosecond lasers are especially suitable for this task because their extremely short pulses concentrate energy within a small focal region and limit heat spreading into the surrounding material. The resulting rapid heating and cooling can promote copper formation, particle coalescence, and solidification. Even so, previously reported methods often produced porous copper structures with relatively high electrical resistance. Residual solvent could vaporize suddenly during laser exposure and create micro-explosions, while uncontrolled copper grain growth could prevent the formation of a dense and continuous conductor.
To address these problems, the researchers combined a specially formulated copper precursor ink with a pre-baking step and optimized laser parameters. The ink formulation was tailored to lower the activation barrier for precursor decomposition and control copper grain growth, while a pre-drying step removed the volatile solvent before femtosecond laser irradiation. By systematically tuning the laser power and scanning speed, the researchers produced dense, low-porosity copper microstructures with sub-10-μm linewidths and a minimum resistivity of 9.34 μΩ·cm under ambient conditions. The researchers also demonstrated the method's broad substrate compatibility, enabling copper patterning on glass, flexible polyimide films, a 2-inch silicon wafer, and a stainless-steel hemispherical surface. To further validate its practical utility, the researchers fabricated a SnO2 ultraviolet photodetector using sequential FsLDW steps, with the copper patterns serving as both electrodes and conductive pathways for electrical biasing and signal readout. This device-level demonstration shows that the approach can extend beyond material patterning to the fabrication of functional microsystems.
The research group of Prof. Wei Xiong and Associate Prof. Hui Gao from Huazhong University of Science and Technology introduces an FsLDW method for producing dense, low-porosity, and highly conductive copper micropatterns without conventional masks. The method is particularly well suited for applications requiring fine conductive features, rapid design iteration, and direct patterning on flexible, large-area, or curved substrates.
Its most immediate value lies in simplifying the fabrication of customized electronic devices. Because copper patterns are defined by digitally programmed laser paths, circuit layouts can be changed by updating the design file rather than fabricating a new physical mask. This reduces the time and effort required to move from design to prototype. This process can also produce fine copper interconnects directly on substrates made of different materials and with different geometries, including flexible and curved substrates, where conventional lithography may be difficult or inefficient. These capabilities could expand the use of copper interconnects in devices built on flexible, curved, or otherwise unconventional surfaces. Overall, mask-free processing, digital programmability, and broad substrate compatibility provide a practical route for directly fabricating copper interconnects across a wide range of circuit layouts, substrate materials, and surface geometries.
Further research will focus on improving manufacturing throughput and device reliability. Key priorities include increasing processing speed, further reducing the resistivity of the as-fabricated copper without relying on high-temperature post-treatment, and evaluating long-term performance under repeated bending, humid conditions, thermal cycling, and prolonged electrical operation. The integration of multifocal parallel processing and high-speed scanning could improve throughput, while sequential femtosecond laser writing of conductive and functional materials may enable electrodes, interconnects, and active components to be fabricated on the same laser-processing platform. These developments could help transition the method from laboratory-scale copper patterning to the direct fabrication of customized functional microsystems on planar, flexible, and curved surfaces.
Keywords: femtosecond laser direct writing, copper micropatterns, metallization, conductive interconnects
