Jul 15, 2004
Thin-film polymers buckle up for measurement
Researchers at the National Institute of Standards and Technology (NIST) and IBM Research, both in the US, have measured the stiffness of extremely thin polymer films by causing the films to buckle. Their new SIEBIMM (strain-induced elastic buckling instability for mechanical measurements) technique takes just a couple of seconds, is relatively cheap and doesn't require material-specific modelling.
"We have developed a straightforward combinatorial and high-throughput (C&HT) tool that practically any laboratory - academic or industrial - can use with only modest investment in equipment," Chris Stafford of NIST told nanotechweb.org. "This technique promises to be an important nanometrology tool, since it can measure the modulus of ultra-thin films of polymers - and other materials - with thickness down to 5 nm."
To carry out the technique Stafford and colleagues transferred a thin film of the test material onto a piece of polydimethylsiloxane (PDMS). They mounted the resulting sample on a strain stage and applied compression. The stiff upper film and soft underlying PDMS substrate required different amounts of energy to deform them, and this resulted in the sample buckling in order to minimize the total strain energy.
By measuring the wavelength of this buckling and using the known properties of the substrate, the scientists were able to calculate the elastic modulus of the film. They tested polystyrene films from 30 to 240 nm thick. For films less than 30 nm thick the researchers say the limitation is measuring the film thickness accurately, rather than the technique itself.
"Our current work aims to measure the nanomechanics of polymer film systems where the thickness compares to the dimensions of individual polymer chains," said Stafford. "This knowledge is critical for the advancement of emerging technologies areas such as nanoimprint lithography and MEMS/NEMS, where polymer materials will be used in geometries commensurate with these dimensions."
The researchers have also used the technique to test the nanoporous films that are under development for next-generation low-k dielectric materials for semiconductor applications. The modulus of these materials is strongly linked to their resilience to the chemical-mechanical polishing technique used during semiconductor processing. "Mechanical analysis of such films is problematic for other techniques like nanoindentation," said Stafford.
The researchers reported their work in Nature Materials.
About the author
Liz Kalaugher is editor of nanotechweb.org.