Metal Micro-Nano Structures by Femtosecond Laser Projection Lithography (Invited)

Objective Nanopatterned metal thin films are key functional elements of various nano-enabled devices for a variety of applications. However, the efficient fabrication of large-area micro-nano metal structures remains extremely challenging. In this study, a digital micromirror device is used to modulate the beam of a femtosecond laser into an arbitrary two-dimensional patterned beam, and the ions in the metal solution are reduced to nanoparticles and deposited onto the corresponding metal pattern using the projection of a patterned femtosecond laser. We succeed in the high-speed large-area deposition of gold and silver precious metals, where the processed structures exhibit excellent surface quality and optical properties. This processing method is fast, mild, widely used, and inexpensive, and provides a new means of conducting metal micro-nano patterned manufacturing. Methods A silver ion precursor solution is prepared by adding a suitable amount of aqueous ammonia to an aqueous mixture of silver nitrate (0.1 mol/L) and trisodium citrate (0.05 mol/L) under stirring until a clear solution is formed. The gold-ion precursor solution is prepared by an aqueous mixture of ionic liquid (2.1 mol/L) and tetrachloroauric acid (0.3 mol/L) under stirring until a clear solution is formed. The ionic liquid is prepared slightly differently from that previously reported. Specifically, an excess of glycine (73.2 mmol) is added to an aqueous solution of choline hydroxide (mass fraction of 46%,61 mmol), and the mixture is stirred at room temperature for 24 h. Water is then removed under vacuum at 50 oC. Acetonitrile (60 mL) and methanol (20 mL) are then added to precipitate the unreacted amino acids. The mixture is stirred vigorously overnight and filtered through a Celite filter. The solvents are evaporated under reduced pressure and, if necessary, the residue is redissolved in acetonitrile/methanol. Finally, the purified ionic liquid is dried under vacuum overnight at 60 degrees C and stored under moisture-free conditions until use. Results and Discussions Images are projected onto a commercially available digital micromirror device (DMD), which acts as a digital mask to pattern the femtosecond laser beam into dark and bright regions. Patterning is achieved by switching the individual micromirrors on the DMD to either on or off. Accordingly, the metal structures can be rapidly deposited (Fig.1). The deposition time required for the gold and silver materials and the thickness of the deposition structure are studied, and the maximum thicknesses of the silver and gold materials are 120 nm and 380 nm, respectively. The time required to reach the maximum thickness is different when the power is changed (Figs.3 and 4). It is found that gold and silver reach their shortest deposition time of 170 s and 26 s under 6 mW and 9 mW, respectively. Under a laser power of 3 mW, the pattern exhibits excellent optical properties and its surface is flat (Fig.6). Conclusions We present a projection-based photoreduction technique that can rapidly and photochemically deposit metal structures with smooth planes, solving the difficulty in depositing two-dimensional structures of precious metals with excellent surface quality at high speed. A femtosecond laser is innovatively used as a light source for deposition, making full use of its characteristics, where the power required for light reduction processing is greatly reduced. More specifically, the laser enables reduction of the excessive agglomeration of nanoparticles during deposition and improvement in surface quality. This photoreduction technique is not only simple to operate but also has wide applicability in the fields of microelectromechanical systems, wearable electronics, and bioscience.