July 18, 2011 — Georgia Institute of Technology (Georgia Tech)’s thermochemical nanolithography (TCNL) is used to fabricate nanometer-scale ferroelectric structures directly on flexible plastic substrates without the conventional aggressive processing temperatures that would destroy these substrate materials.
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| Photo 1. Suenne Kim, Georgia Tech postdoctoral fellow, holds a sample of flexible polyimide substrate used in the heated AFM TCNL research. Assistant professor Nazanin Bassiri-Gharb and graduate research assistant Yaser Bastani are also featured. Credit: Gary Meek. |
TCNL uses a heated atomic force microscope (AFM) tip to produce patterns. The AFM-based litho process could build high-density, low-cost, complex ferroelectric structures, enabling energy harvesting arrays, sensors, and nano-electromechanical systems (NEMS) and micro-electromechanical systems (MEMS).
The piezoelectric materials can be made into precise shapes and deposited accurately on a flexible substrate, said Nazanin Bassiri-Gharb, assistant professor, Georgia Tech School of Mechanical Engineering. The structures were "directly grown with a CMOS-compatible process" at the smallest resolution acheived to-date, Bassiri-Gharb adds, pointing out that lower-temperature processing isn’t the only benefit to the process. Wires were built to 30nm wide; spheres were made with 10nm diameters.
The researchers envision ferroelectric memory applications, depositing spheres at densities exceeding 200 gigabytes per square inch, said Suenne Kim, a postdoctoral fellow in laboratory of Professor Elisa Riedo in Georgia Tech’s School of Physics.
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| Image 1. The topography (by AFM) of a ferroelectric PbTiO3 (PTO) line array crystallized on a 360nm-thick precursor film on polyimide. Scale bar = 1um. Credit: Suenne Kim. |
Piezoelectric materials often require 600°C+ crystallization temperatures, making flexible substrates (i.e., for energy harvesting) incompatible with processing. Chemical etching produces grain sizes as large as the nanoscale features the researchers would like to produce; physical etch damages structures, reducing the piezoelectric materials’ special properties.
The thermochemical nanolithography process, under development at Georgia Tech starting in 2007, uses very localized heating to form structures where the resistively heated AFM tip contacts a precursor material. The sol-gel precursor is applied in standard spin coating, then heated to 250°C to remove organic solvents. The AFM "writes" crystallized material in a computer-controlled pattern.
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| Image 2. Scanning electron microscope (SEM) image of a large Pb(ZrTi)O3 (PZT) line array crystallized on a 240nm-thick precursor film on a platinized silicon wafer. Credit: Yaser Bastani. |
The researchers have used polyimide, glass and silicon substrates, but in principle, any material able to withstand the 250°C precursor heating would be a viable substrate. Structures have been made from PZT and PTO.
The heated AFM tips were provided by William King, a professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign.
Researchers will now combine AFM tips in to arrays to pattern large areas. They also plan to improve upon the heated AFM tips for longer operation. Both development areas hint at commercialization, as an alternative to photolithography at the nano scale, avoiding cleanroom and vacuum environments.
Researchers also must study "the growth thermodynamics of these ferroelectric materials," adds Bassiri-Gharb. They will examine materials properties at bulk, micron, and nano scale. "We need to understand what really happens to the extrinsic and intrinsic responses of the materials at these small scales."
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| Photo 2. Georgia Tech researchers display samples of materials on which ferroelectric nanostructures have been fabricated by thermochemical nanolithography. They are (l-r) graduate research assistant Yaser Bastani with silicon, assistant professor Nazanin Bassiri-Gharb with polyimide and postdoctoral fellow Suenne Kim with glass. Credit: Gary Meek. |
The research was sponsored by the National Science Foundation and the U.S. Department of Energy. In addition to the Georgia Tech researchers, the work also involved scientists from the University of Illinois Urbana-Champaign and the University of Nebraska Lincoln.
Results were reported July 15 in the journal Advanced Materials. Access it here: http://onlinelibrary.wiley.com/doi/10.1002/adma.201101991/abstract
In addition to those already mentioned, the research team included Yaser Bastani from the G.W. Woodruff School of Mechanical Engineering at Georgia Tech, Seth Marder and Kenneth Sandhage, both from Georgia Tech’s School of Chemistry and Biochemistry and School of Materials Science and Engineering, and Alexei Gruverman and Haidong Lu from the Department of Physics and Astronomy at the University of Nebraska-Lincoln.
Courtesy of John Toon, [email protected].