In a development that offers potential to the semiconductor industry, American researchers have succeeded in measuring computer chip features 30 times smaller than the wavelength of light.
The National Institute of Standards and Technology’s Richard Silver said, as a rule, optical microscopes can't see features smaller than the wavelength of light in enough detail to make accurate measurements.
“However, light does scatter when it strikes so-called subwavelength features and patterned arrangements of such features,” said Silver.
"Historically, we would ignore this scattered light because it did not yield sufficient resolution. Now we know it contains helpful information that provides signatures telling us something about where the light came from."
He said they were able to measure etched lines on a wafer that were as thin as 16 nm with an accuracy of 1 nm. They confirmed their measurements with an atomic force microscope, which achieves sub-nanometer resolution, but is considered too slow for online quality-control measurements.
The technique, called scatterfield imaging, involves the methodical illumination of a sample with polarised light from different angles. The data is gathered in slices, which together image the volume of scattered light above and into the sample. These slices are analysed and reconstructed to create a three-dimensional representation, similar to a CT scan, except that the slices are collections of interfering waves, not cross-sectional pictures.
"It's the ensemble of data that tells us what we're after," said project leader Bryan Barnes." We may not be able see the lines on the wafer, but we can tell you what you need to know about them—their size, their shape, their spacing."
Scatterfield imaging has critical prerequisites that must be met before it can yield useful data for high-accuracy measurements of exceedingly small features. Key steps entail detailed evaluation of the path light takes as it beams through lenses, apertures and other system elements before reaching the sample. The path traversed by light scattering from the specimen undergoes the same level of scrutiny. These steps are akin to error mapping so that recognised sources of inaccuracy are factored out of the data.
Using their own wave analysis software, the NIST team have assembled an indexed library of light-scattering reference models. So once a specimen is scanned, the team relies on computers to compare their real-world data to models and to find close matches. From there, succeeding rounds of analysis homes in on the remaining differences, reducing them until the only ones that remain are due to variations in geometry such as irregularities in the height, width, or shape of a line.
Image courtesy: NIST.