A flexible or wearable device that can be attached to a human body and carried easily, such as a foldable smartphone, with a flexible display and battery has been under development. In this study, the selective transfer of liquid metal on flexible substrate using a pulsed laser to pattern electrodes and repair damaged electrodes have been studied. Liquid metal is more suitable to apply as electrodes for flexible device than solid-state metal, which is easily broken when the substrate is stretched or bent.
The manufacturing method has four merits. First, this process is simple because no additional instrument, such as a nozzle, pen, mask, or mold, is used. Second, the process is efficient because the laser has a lower energy density; they also make large-area transfer and patterning possible. Third, because the process is precise, the method fabricates a finer line width pattern. Fourth, the process can be used to repair fabrication in that the process makes selective transfer possible. Liquid metal in this study are required to be nontoxic and in a liquid state at room temperature because flexible and wearable devices need to be worn or attached to a human body: gallium-indium alloy is suitable as it meets the above two conditions. Among gallium-indium alloys, eutectic gallium indium (EGaIn), which is composed of 75% gallium and 25% indium, has been chosen because of the higher boiling temperature. In a laser process, a higher boiling point is more appropriate because laser irradiation locally and sharply heats the materials.
To pattern and repair electrodes, while not damaging heat-sensitive receiver substrates, laser-induced forward transfer (LIFT) was performed. In the LIFT process, when only liquid metal is coated on donor substrate, it is ablated by direct laser irradiation easily. To avoid direct laser irradiation and protect liquid metal, a dynamic release layer (DRL) or sacrificial layer is deposited between donor substrate and donor layer. Intact liquid metal can be transferred because the DRL rather than liquid metal is liquefied and vaporized. Liquid metal cannot be coated well on gold film because of poor adhesion between the liquid metal and gold. To improve the adhesion between the liquid metal and DRL, mr-APS1, which is adhesion layer, is added between them.
In first experiment, nanosecond laser (λ=266 nm, τ=20 ns, repetition rate = 30 kHz, full width at half maximum (FWHM) = 10 μm and scanning speed = 300 mm/s) is used. For fast fabrication and finer liquid-metal transfer, thickness of the DRL was selected to be 15 nm considering penetration depth. As a result, liquid metal that had minimum diameter similar to that of the laser beam was transferred. In line patterning, a 40 um line width and 80 Ω resistivity is measured as a result of the laser irradiation considering heat diffusion region. In addition, liquid metal was transferred between two gold electrodes for application to repair fabrication. A resistivity of 50 Ω was measured. Because of the oxide film surrounding the liquid metal, the resistivity of the liquid-metal pattern was higher than the solid metal pattern. The oxide film blocked current flow. To remove oxide film, HCl should be added in the middle of the LIFT.
The purpose of the second experiment was to resolve a problem of debris surrounding the liquid-metal transfer because of the nanosecond laser heat diffusion effect. A femtosecond laser (λ=1030 nm, τ=350 ns, repetition rate = 500 Hz, FWHM = 15 μm, scanning speed = 10 mm/s and objective lens = NA 0.14, 5x), which has a very short laser irradiation time compared with a few nanoseconds heat relaxation time, was used. In the femtosecond LIFT process, there was no donut-shaped liquid metal, unlike the nanosecond laser process, despite the high energy density. In addition, the liquid-metal line was rarely damaged with a high energy density. When the energy density was controlled, the size of the liquid metal could be controlled in the femtosecond LIFT process. Furthermore, in this femtosecond laser instrument, repetition rate is not fixed; therefore a more precise or faster LIFT process is possible according to the control scanning speed. Furthermore, the objective lens, which has higher NA and magnification, made a finer line width and smaller diameter liquid metal possible.
With no heat damage to the liquid metal and receiver substrate, this LIFT fabrication can be possible to a heat-sensitive receiver substrate. In the future, with liquid-metal patterning and transfer using processes such as LIFT, flexible and wearable device will be developed.