Testing time for thin films

The challenge of determining whether thin films – some no thicker than a single molecule – are strong enough for a growing number of important technology jobs just got easier and quicker.

The challenge of determining whether thin films – some no thicker than a single molecule – are strong enough for a growing number of important technology jobs just got easier and quicker thanks to an inexpensive testing method developed by a team at the National Institute of Standards and Technology (NIST) in the US.

Useful for evaluating all types and combinations of materials, the new method measures and analyses the strength and stiffness of a thin-film sample in about 2 seconds, as compared with several minutes for indentation and other conventional approaches.

In addition, the technique accommodates high-throughput testing, so that hundreds or even a few thousand systematically varying samples can be tested in rapid succession.

Called SIEBIMM (for strain-induced elastic buckling instability for mechanical measurements), the new method builds on the science of buckling, which for most of its 400 years has been concerned with crumbling buildings or crumpling of the Earth’s crust.

The method entails mounting a postage-stamp-sized assortment of incrementally varying thin films on a strip of silicone rubber about the size of a Band-Aid. The combination of sample array and soft substrate are placed on a custom-built stage that can be stretched or compressed.

Subjected to a gradually increasing force that stretches or squeezes, a sample becomes unstable and buckles, wrinkling like a piece of corrugated cardboard. Situated beneath the stage, a laser beams through the sample and a camera captures the light scattered at this critical point of instability.

From the resulting diffraction pattern, the buckling wavelength, or distance between the peaks of adjacent wrinkles, is determined. Through a series of mathematical calculations, the buckling wavelength can be related directly to the elastic modulus of the sample, which corresponds to the strength of the material.

Accelerated testing using the new equipment could spur progress in a large variety of existing and emerging technology areas that rely on thin-film advances for improved performance or enhanced protection. Examples include semiconductors, solar cells, fuel cells, coatings, magnetic storage devices and prospective nanotechnology devices.

For films less than 1 micrometer thick, mechanical-property measurements made with existing tools often yield relative values, which can blur predictions of how different films will perform. In contrast, the new method yields quantitative measurement results that permit definitive comparisons between samples.

NIST and IBM collaborators have already used the technique to assess the strength of polymer and ceramic films ranging from a few nanometres to a micrometre in thickness. One pilot-tested film was a ceramic material dotted with nanometre-scale pores. Such nanoporous films are being developed to insulate devices and layers on future-generation integrated circuits.

While the nanopores in the so-called low-dielectric-constant (low-k) films improve their effectiveness as electrical insulators, the tiny holes also can compromise the films’ strength. A major concern is whether the nanoporous films can withstand the rigors of the chemical mechanical polishing process used to smooth each layer in a chip.

Using the desk-top testing platform, the team evaluated a battery of low-k films that varied in porosity, from samples with no pores to samples in which pores made up half the volume. After comparing the results with those obtained with the widely used nanoindentation method, the team concluded that the NIST-developed approach provided an inexpensive, fast, and highly effective technique for evaluating new varieties of low-k materials.

Christopher Stafford, a NIST polymer scientist, suggests other applications for the new equipment include evaluations of new photoresist masks that will be used to print chips with the smaller-wavelength ultraviolet light sources that the semiconductor industry is now implementing. It also should be useful for assessing the mechanical properties of nanotechnology devices made with still-experimental methods, such as nanoimprint lithography in which nanometre-scale features are stamped into a substrate.

“This simple technique can provide invaluable information concerning the mechanics of nanostructured materials and ultrathin polymer films,” said Stafford.