Using Thermochemical Nanolithography (TCNL), researchers have developed a new way to fabricate nanometer-scale ferroelectric structures directly on flexible plastic substrates that would be unable to withstand the processing temperatures normally required to create such nanostructures. The technique, which uses a heated Atomic Force Microscope (AFM) tip to produce patterns, could facilitate high-density, low-cost production of complex ferroelectric structures for energy harvesting arrays, sensors and actuators in Nano-Electromechanical Systems (NEMS) and Micro-Electromechanical Systems (MEMS).
“We can directly create piezoelectric materials of the shape we want, where we want them, on flexible substrates for use in energy harvesting and other applications,” said Nazanin Bassiri-Gharb, co-author of the paper and an assistant professor in the School of Mechanical Engineering at the Georgia Institute of Technology. “This is the first time that structures like these have been directly grown with a CMOS-compatible process at such a small resolution. Not only have we been able to grow these ferroelectric structures at low substrate temperatures, but we have also been able to pattern them at very small scales.”
The researchers have produced wires approximately 30 nanometers wide and spheres with diameters of approximately 10 nanometers using the patterning technique. Spheres with potential application as ferroelectric memory were fabricated at densities exceeding 200 gigabytes per square inch — currently the record for this perovskite-type ferroelectric material.
Ferroelectric materials are attractive because they exhibit charge-generating piezoelectric responses an order of magnitude larger than those of materials such as aluminum nitride or zinc oxide. The polarization of the materials can be easily and rapidly changed, giving them potential application as random access memory elements. But the materials can be difficult to fabricate, requiring temperatures greater than 600 degrees Celsius for crystallization.
The thermochemical nanolithography process, which was developed at Georgia Tech in 2007, addresses those challenges by using extremely localized heating to form structures only where the resistively-heated AFM tip contacts a precursor material. A computer controls the AFM writing, allowing the researchers to create patterns of crystallized material where desired. To create energy-harvesting structures, for example, lines corresponding to ferroelectric nanowires can be drawn along the direction in which strain would be applied.
The heat from the AFM tip crystallizes the amorphous precursor to make the structure. The patterns are formed only where the crystallization occurs. Ultimately, arrays of AFM tips under computer control could produce complete devices, providing an alternative to current fabrication techniques.