Patterning Metallic Nanostructures Using Solid Electrolyte Stamps
7 Mar, 2007 11:11 am
Metallic structures are ubiquitous in micro and nanotechnology. From the interconnects that transmit signals in your i-Pod, cell phone and laptop, to the active elements or electrodes sensors in your home, workplace and clinics, they help keep you entertained, safe and healthy.
Such metallic structures are also becoming increasingly important in emerging fields related to sub-wavelength optics such as plasmonics (see http://en.wikipedia.org/wiki/Plasmonics). Because metallic structures are such an integral part of ever-shrinking micro and nano devices and systems, it is of critical importance to be able to economically manufacture them at these length scales. Unfortunately, current approaches to making such sub-micron sized metallic structures are expensive because they require multiple steps, in highly controlled and clean environments, on very expensive equipment.
The solid-state superionic stamping (S4) process is a very simple and robust approach to making these extremely small patterns. The basic principle of this process resembles the chemistry inside a battery cell. Many battery and fuel cells use solid electrolyte – ionic conductors in which charge is transported by mobile ions (atoms having a deficit or excess of electrons) instead of electrons. In a battery cell, one uses this ionic mobility to transfer charges. The mass transferred is a by-product. It is quite the reverse in our manufacturing process. We exploit ionic mobility to transport mass; the resulting current flow is a side effect that we use to monitor the mass transport process.
For our process, we create a relief pattern in a very smooth, plane surface of an ionic conductor (to be accurate, we use silver sulfide, a mixed conductor, that conducts charge by both ions and electrons) as a stamp. We bring this stamp into contact with a metallic film (we used a silver film deposited on glass) and apply a voltage across the contact interface, with the stamp being connected to the cathode and the metal film as the anode. Thanks to the high mobility of silver ions in silver sulfide, the stamp behaves like a silver sponge. A small electrical potential causes it to “soak-up” or dissolve silver at all contact points on the interface, drawing it into the stamp in the form of ions that drift towards the cathode. Eventually only the silver in the recessed or relieved portion of the stamp (i.e., silver that does not come in contact with the electrolyte), is left behind on the glass.
With this simple process, we were able to pattern tiny metal structures with features more than a 1000 times smaller than your hair! Some of the patterns we “stamp out” have features smaller than 50 nanometers (1 nanometer is one billionth of a meter). This process has the potential to be fast (we observed dissolution rates of 4 nanometer per sec at the interface). It can also be economical because a stamping process can have a large stamping area and simultaneously produce a large number of structures in one stamping operation. Further the stamp, which the most expensive part of the system, can be used for many such stamping operations. Finally, since this process uses only solids and no liquids, it replicates with high dimensional fidelity and does not contaminate the substrate it patterns. While this process has many advantages, it also presents challenges. For example, we don’t fully understand the transport phenomena at the aforementioned interface. We also expect challenges in making large-area stamps (for high production rates – something that industrial applications need) and making even contact between the stamp and the substrate over these large areas. However, we are very optimistic these are solvable and that S4 will contribute to an environmentally-benign approach to affordable and accessible nanofabrication.
Keng H. et. al, Nanoletters, Feb 14, 2007