Direct Nanopatterning With Solid Ionic Stamping
Authors: Hsu Keng H. , Ferreira Placid M. , Fang Nicholas 
Published: April 2008

Among the subtractive fabrication techniques nanoimprint lithography followed by metal etching processes3,4,5 provides features with size down to tens of nanometers6,7. With the aid of high strength tool, the resolution of electrochemical machining (ECM) has been pushed to sub-hundred nanometer regime4. As much as the high resolution it is capable of, nanoimprint lithography followed by metal etching processes bears the multi-step, complex lithography processes that require stringent process environment control and high-cost equipments. Similarly, the pattern dimension fidelity and pattern geometry of the transferred feature is limited by the current density distribution in the liquid-state electrolyte and its physical properties. Effort has been on developing patterning techniques and logic devices that are based on the ionic mass transport property in solid electrolytes. A quantized conductance atomic switch that operates at 1MHz with 0.6V of driving potential has been developed wherein silver mobile atoms bridges and opens the tunneling gap between Pt and silver sulfide wires when driven by a gate potential8. Nanopatterning techniques utilizing local metal cluster deposition and dissolution have also been developed to achieve sub-hundred nanometer line writing and dot deposition with scanning probe microscopy9,10,11. Here we present a novel solid state ionic subtractive stamping technique which provides nanoscale patterning of metallic features with high resolution. Developed based upon a single-step electrochemical material dissolution process in ambient conditions, this technique offers high throughput and high fidelity in metal pattern transfer at nanoscale, as well as the flexibility to be used for various kinds of metals and to be integrated with other nano-fabrication techniques for applications such as chemical sensors and photonic structures. Shown in Figure 1 is a model of ionic migration of silver species in a solid-state ionic conductor, silver sulfide. When subjected to an electric field applied across a silver-silver sulfide interface through anode and cathode attached to them respectively, in achieving the equilibrium of the electrochemical potential of silver atoms in the silver substrate and that in silver sulfide, silver atoms in the substrate oxidize into mobile ions and electrons. These mobilized silver ions then move freely from the interface through the conduction channels in the silver sulfide bulk towards the cathode. Upon receiving electrons when reaching cathode, silver ions reduce back to atoms and deposit on the interface between the cathode and Ag2S. The oxidation at the interface between anode and Ag2S is an ideal tool for surface micromachining in that mass transport only occurs at the portion of the surfaces of anode where actual physical contact exists, making it an ideal tool for pattern transfer. In our preliminary experiments, silver sulfide and silver substrate were chosen and stamping apparatus was built to perform solid state ionic subtractive stamping. Stamping was performed with the chronoamperometry operation mode of the Potentiostat for chosen potentials. Stampings were also run with a fixed potential of 0.3 V but different lengths of time for stamping rate analysis. Shown in figure 2 are the SEM images of the silver sulfide stamp and the produced silver feature. The lateral resolution achieved is 120nm for line width. Shown in figure 3 are the stamping depths measured at different time steps of a stamping process and the calculated stamping rates at different time steps. The silver removal rate throughout the stamping process is found to remain the same. The constant stamping rate suggests constant ionic conduction which means constant ionic conductivity-the ionic conductivity of silver sulfide remains constant irrespective of silver concentration change, or the composition of the silver sulfide stamp. This is in good agreement with Hebb and Wagner12,13 in their electrochemical measurements of silver sulfide which states that ionic conductivity of silver sulfide is almost independent of composition, given the structure of β-form silver sulfide is quite open and the considerable freedom in the disposition of silver ions. The rough surface of the generated features seen in figure 2 is thought to be due to the small depth of the pattern on the silver sulfide stamp which causes undesirable etch of silver and pulling of silver grains; the characterization and optimization of it is currently being investigated. To conclude, we have demonstrated a unique technique to pattern metal with sub-micron resolution in a high-throughput stamping process. The process is a solid-state, room temperature process that is highly compatible with a large variety of process technologies. In our initial attempt, a lateral resolution of 120nm is achieved.