Buckling of Nanostructures

AUSTIN, TEXAS—July 24, 2008

Faculty: Paul Ho, Cockrell Family Regents Chair in Engineering, Department of Mechanical Engineering; Co-principal investigator: Rui Huang, assistant professor, Department of Aerospace Engineering

• Agency: National Science Foundation
• Amount: $159,138

This project integrates experiments, theories, and modeling to study buckling mechanical behavior at the nanometer scales. The objective is to correlate experimental observations of buckling nanometer-sized structures to mechanical properties of materials at the nanoscale. As a result, this project will open a new line of methodology for characterization of nanoscale structures and materials.

The buckling phenomenon at the nanoscale is of fundamental interest. As the dimension of material is reduced to the nanoscale of tens of nanometers, about the dimension of 100 atoms or one thousandth the diameter of a human hair, its properties will become stronger, tougher and more difficult to break. This can be observed from the buckling behavior of nanostructures, such as from buckling of vertically aligned carbon nanotubes and polymer nanolines. At UT Austin, a team of two professors, Ho and Huang, with the support from National Science Foundation has carried out a study on buckling of single-crystal silicon nanolines. They developed a novel method by combining electron beam lithography and anisotropic etching to fabricate silicon nanolines with width as small as 20 nanometers.

As illustrated in Fig. 1, silicon nanolines of 200 nm and 40 nm wide have vertical sidewalls almost atomically smooth and a high aspect (height/width) ratio exceeding 15, making them ideally suited for buckling studies. Nanoindentation tests were performed in an atomic force microscope to observe the buckling behavior of the silicon nanolines. Under indentation, the silicon nanolines showed extraordinary toughness as they deformed elastically without plastic deformation to about 8%. This is far beyond the usual range of about 0.5% for bulk materials and nearly approaches the theoretical limit for silicon. Combining with modeling simulation, Professor Ho and his team was able to extend this study beyond buckling to measurements of contact friction at nanoscale. The results clearly demonstrate how the elastic behavior of materials can be substantially extended for nanoscale materials.

Working with Drs. Michael Cresswell and Richard Allen, two research scientists at NIST, the Si nanoline structure was adopted as critical dimension (CD) standard for micro-lithography for microchips. This is based on counting the number of the lattice fringes using a transmission electron microscope. The measurement provides an absolute critical dimension standard for advanced lithography which is important as semiconductor devices are being scaled down into nanometer dimensions. The application for lithography and critical dimensional control was presented in a public conference SEMICON West on July 16, 2007 in San Jose, CA addressing to a large audience from the semiconductor industry.

The research from this project will advance fundamental understanding of mechanical behaviors at the nanoscale, which is critically important in the development of nanostructures for engineering applications, and thus has a broader impact on nanoscience and nanotechnology. Educational activities will be incorporated within the research project to enhance its societal impacts, which include training of graduate and undergraduate students and outreach to underrepresented groups. The research results will be broadly disseminated to the academic community as well as the general public.