NIST, UNC Design New Device for More Accurate Thin Film Measurement
Researchers from NIST (the National Institute of Standards and Technology) and the University of North Carolina have designed a new "instrumented nanoscale indenter" to improve the accuracy of sensitive measurements of thin film surface properties.
NIST, UNC nanoscale indenter flanked by tuning forks
Indenters have a long history in materials research. Johan August Brinell devised one of the first versions in 1900. The concept is to drop or ram something hard onto the test material and gauge the material's hardness by the depth of the dent. But more modern and sensitive devices require nanoscale precision.
The NIST solution is a touchless surface detector that uses a pair of tiny quartz tuning forks – like those used in wrist watches. When the tuning fork gets close to the test surface, the influence of the nearby mass changes the frequency just enough so they can be detected, according to Smith.
The image (at right) shows the tip of the new NIST nanoscale indenter flanked by two tuning forks that provide a stable, noncontact reference relative to the specimen, a piece of single-crystal silicon. Using a pair of tuning forks allows the system to compensate for any tilt.
"We are trying to get the most accurate measurement possible of how far the indenter tip penetrates into the surface of the specimen, and how much force it took to push it in that far. We record this continuously. It's called 'instrumented indentation testing'," Smith said in a statement.
To look at creep in polymers, something the instrument is particularly good at, Smith noted, —that reference point itself is going to be creeping into the polymer just under its own contact force. That's an error you don't know and can't correct for," says Smith.
The nanoindenter uses that frequency shift to "lock" the position of the indenter mechanism at a fixed distance from the test surface, but without exerting any detectable force on the surface itself. "The only significant interaction we want is between the indenter and the specimen -- or at least, to be constant and not deforming the surface. This is a significant improvement over the commercial instruments," he added.
The NIST nanoindenter can apply forces up to 150 millinewtons, taking readings a thousand times a second, with an uncertainty lower than 2 micronewtons, and while measuring tip penetration up to 10 micrometers to within about 0.4 nanometers.
The work appears in Review of Scientific Instruments, titled Development of a precision nanoindentation platform.” Smith’s coauthors include *B.K. Nowakowski, S.T. Smith, L.F. Correa and R F. Cook.