Writing circuits and other tiny structures onto flexible plastic substrates could become more practical with a technique developed by researchers at Georgia Institute of Technology. The method could facilitate high-density, low-cost production of complex ferroelectric structures for energy-harvesting arrays, sensors and actuators in nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS). Dubbed thermochemical nanolithography, the technique uses a heated atomic force microscope (AFM) tip to produce patterns, allowing the fabrication of nanometer-scale ferroelectric structures directly on substrates that would otherwise be unable to withstand the temperatures normally required. The topography (via atomic force microscope) of a ferroelectric PTO line array crystallized on a 360-nm-thick precursor film on polyimide. The scale bar corresponds to 1 μm. (Image: Suenne Kim) The Georgia Tech researchers reported their work in the July 15 issue of Advanced Materials. The team also included members from the University of Illinois Urbana-Champaign and the University of Nebraska in Lincoln. “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 assistant professor Nazanin Bassiri-Gharb, co-author of the paper. “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 nm wide and spheres with diameters of ~10 nm using the patterning technique. Spheres with potential application as ferroelectric memory were fabricated at densities exceeding 200 GB/sq in. — currently the record for this perovskite-type ferroelectric material, said Suenne Kim, the paper’s first author and a postdoctoral fellow in the laboratory of Elisa Riedo. Postdoctoral fellow Suenne Kim holds a sample of flexible polyimide substrate used to produce ferroelectric nanostructures. Assistant professor Nazanin Bassiri-Gharb points to a feature on the material, while graduate research assistant Yaser Bastani observes. (Image: Georgia Institute of Technology) 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 changed easily and rapidly, giving them potential application as random access memory elements. But the materials can be difficult to fabricate, requiring temperatures greater than 600 °C for crystallization. Chemical etching techniques produce grain sizes as large as the nanoscale features researchers would like to produce, while physical etching processes damage the structures and reduce their attractive properties. Until now, these challenges required that ferroelectric structures be grown on a single-crystal substrate compatible with high temperatures, then transferred to a flexible substrate for use in energy-harvesting. The thermochemical nanolithography process 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. A scanning electron microscope image shows a large PZT line array crystallized on a 240-nm-thick precursor film on a platinized silicon wafer. (Image: Yaser Bastani) “The heat from the AFM tip crystallizes the amorphous precursor to make the structure,” Bassiri-Gharb explained. “The patterns are formed only where the crystallization occurs.” As a next step, the researchers plan to use arrays of AFM tips to produce larger patterned areas and to improve the heated AFM tips to operate for longer periods. They also hope to understand the basic science behind ferroelectric materials, including properties at the nanoscale. “We need to look at the growth thermodynamics of these ferroelectric materials,” Bassiri-Gharb said. “We also need to see how the properties change when you move from the bulk to the micron scale and then to the nanometer scale. We need to understand what really happens to the extrinsic and intrinsic responses of the materials at these small scales.” Ultimately, arrays of AFM tips under computer control could produce complete devices, providing an alternative to current fabrication techniques. “Thermochemical nanolithography is a very powerful nanofabrication technique that, through heating, is like a nanoscale pen that can create nanostructures useful in a variety of applications, including protein arrays, DNA arrays, and graphenelike nanowires,” Riedo said. “We are really addressing the problem caused by the existing limitations of photolithography at these size scales. We can envision creating a full device based on the same fabrication technique without the requirements of costly cleanrooms and vacuum-based equipment. We are moving toward a process in which multiple steps are done using the same tool to pattern at the small scale.” For more information, visit: www.gatech.edu