Photosensitive Material Enabling Direct Fabrication of Filigree 3D Silver Microstructures via Laser-Induced Photoreduction

Dear Editor Laser-induced photoreduction (LPR) as a direct fabrication technique that promises to be one of the most versatile routes for fabricating highly conductive 3D metallic microstructures on-chip (e.g., metamaterials, electro-mechanical systems, and high-frequency components like antennas). This technology has the potential to directly fabricate circuits on elastic and bendable substrates as well as antennas on complementary metal-oxide-semiconductor (CMOS) chips or on substrates with considerable topography. However, the fabrication of three-dimensional (3D) structures of high quality remains challenging. Here, a novel photosensitive material is used for the additive fabrication of filigree 3D conductive silver microstructures of almost arbitrary geometry via LPR. The material is based on silver perchlorate and gelatine solution. Structures fabricated with this material have a resistivity on the order of 10 Ωm, a material density of approximately 95%, and consist of almost 100 wt% silver. As a first functional component, a chiral metamaterial is presented. Owing to the high demand for metallic microstructures, several techniques have been developed for fabricatingthree-dimensional (3D) metallic microstructures. Usually, these structures are fabricated via indirect methods. First, a template is manufactured using subtractive micromachining or additive microfabrication. Thereafter, the metallic structure is electrochemically grown inside the template, and the template is removed subsequently. Although this method produces structures of outstanding quality, it has two crucial disadvantages: limited freedom of design and difficulty with on-chip fabrication. Considering this, it would be useful to have a direct method for fabricating arbitrary microstructures on arbitrary substrates, such as circuits on elastic and bendable substrates and antennas on CMOS chips or substrates. Further, this technique would be able to produce microstructures with substantial topography. Most existing direct methods that enable the fabrication of 3D metallic microstructures either require a conductive substrate (e.g., electro-hydrodynamic printing) or have been slow thus far (e.g., electron beam-induced deposition). Direct laser writing (DLW) via multiphoton absorption can quickly fabricate almost arbitrary and highly accurate microstructures without the aforementioned drawbacks. DLW uses a laser beam to selectively harden a photoresist via polymerisation. LPR, similar to DLW, exploits multiphoton absorption but employs photo-reducing agents that reduce the metal precursors. The fundamental building block of the microstructure is formed via consequent nucleation, growth, and agglomeration (Fig. 1). Some research groups have exploited this mechanism to fabricate