Electroless Deposition and Structuring of Silver Electrodes in Closed Microfluidic Capillaries

The deposition of electrodes as the final step in the microfabrication of a fluidic system avoids incompatibilities with the microfabrication, i.e., high-temperature steps, or the process environment, i.e., CMOS fabrication. The employed strategy to deposit and structure silver (Ag) electrodes in microfluidic capillaries (cross-sectional length less than 10 μm) is presented. First, the adhesion of the Ag layer to the silicon dioxide (SiO₂) surface of the capillary was improved with an intermediate mercapto silane layer. Second, the Ag electrodes were electrolessly deposited with a modified Tollens reagent. The high conductivity of the deposited Ag layer indicated high density and purity. Third, the electrodes were structured by controlling the capillary filling of the electroless solution within the fluidic system using microfluidic stop valves. Experiments in a microfluidic system with a capillary dimension of 3 μm showed successful deposition and microfluidic structuring of the Ag electrode, as well as postdeposition, void-free filling by changing the solution´s surface tension.