Metal Deposition Deep into Microstructure by Electroless Plating

Metal microfabrication has been actively studied and used in various fields because of the unique optical, electrical, and catalytic properties of the fine structures that it can produce. 1–6) The ability to simultaneously and precisely fabricate a number of micro-objects over a wide region is essential to the practical application of nanotechnology. In these processes, plating methods are occasionally used for metal deposition. Plating is a conventional method of depositing metal onto substrates, 7) and can be used not only with conductors, but also with insulators such as plastics and ceramics. Among known plating techniques, electroless plating is commonly used for metal deposition on insulators because it does not require any external electric field. Electroless plating is chemically performed with a solution, and it results in a uniform metal deposition over the entire surface area of micro-objects. 8–10) With this method, it is possible to deposit metal effectively even though the area is limited to less than a micrometer in width. 11,12) Moreover, a number of micrometer-scale metal patterns can be fabricated simultaneously over a very wide area much more easily than with sputtering or vacuum evaporation, which require large, complex equipment. These techniques have been long studied, and are used widely for metal fabrication processes. However, it is generally very difficult, using these methods, to deposit metal on a deep or normally occluded part of a structure, such as the inner wall of a long tubular structure. In this letter, we demonstrate electroless plating on three types of structure. First, gold was deposited on the inner wall of a fused-silica capillary, which is difficult to achieve by other methods. Second, silver was deposited onto glass beads. Third, gold was deposited inside a complex concave structure formed by polystyrene microbeads sandwiched between glass plates. As mentioned above, although it is generally difficult to coat metals onto the inside of a concave structure using other methods (such as vacuum evaporation and sputtering), with electroless plating the plating solution containing metal ions naturally goes into the inside of even a fine, complex structure, and therefore all surfaces contacting the plating solution are uniformly coated with metal. To determine the main features of our process and to verify its effectiveness, we first investigated the relationship between the amount of gold deposited onto the surface of a polystyrene substrate at room temperature (295 K) by measuring the transmission spectrum. We prepared 0.024 M tetra-chloro auric acid (HAuCl4), 0.75 M sodium hydroxide (NaOH), and 0.086 M sodium chloride (NaCl) in water as a gold ion solution, and 0.5 vol % glycerol (C3H8O3) in water as a reduction agent. Samples were obtained by terminating the reaction at 9, 12, 15, and 18 min from mixing the plating solution and the reduction agent. The transmission spectra were measured from 300 to 800 nm for these samples by an absorptiometer (Shimadzu, UV2500PC), and are shown in Fig. 1. For reaction times of less than 9 min, no noticeable deposition of gold was observed. A region with a lower transmission compared to the others was found from 500 to 600 nm; this was due to the local plasmon-mode resonance absorption of gold particles. 13) The