Georgia Tech post-doctoral fellow Suenne Kim holds a sample of flexible polyimide substrate used in research on a new technique for producing ferroelectric nanostructures that normally couldn’t tolerate the high temperatures required for the process. Assistant professor Nazanin Bassiri-Gharb points to a feature on the material, while graduate research assistant Yaser Bastani observes. Photo Credit: Gary Meek

Using a technique known as thermochemical nanolithography, researchers have developed a way to fabricate nanometer-scale ferroelectric structures directly on flexible plastic substrates that normally would be unable to withstand the processing temperatures required in such a process.

The technique, which uses a heated atomic force microscope 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 and micro-electromechanical systems.

“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,” says Nazanin Bassiri-Gharb, co-author of the paper published in the journalAdvanced Materials, and an assistant professor in the School of Mechanical Engineering at the Georgia Institute of Technology, Atlanta.

“This is the first time that structures like these have been directly grown with a CMOS-compatible process at such a small resolution,” Bassiri-Gharb says. “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 GB/in2- currently the record for this perovskite-type ferroelectric material.

Ferroelectric materials are attractive because they exhibit charge-generating piezoelectric responses on 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.

However, the materials can be difficult to fabricate, requiring temperatures greater than 1,112°F (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 developed at Georgia Tech in 2007 addresses those challenges by using extremely localized heating to form structures only where the resistively heated atomic force microscope (AFM) tip contacts a precursor material. A computer controls the AFM writing, allowing the researchers to create patterns of crystallized material where desired.

“The heat from the AFM tip crystallizes the amorphous precursor to make the structure,” Bassiri-Gharb says. “The patterns are formed only where the crystallization occurs.”