IMAGE: An image of the designed pseudo-random test sample. (Image credit: aBeam Technologies)
aBeam Technologies (Hayward, CA), Lawrence Berkeley National Laboratory, and Argonne National Laboratory researchers have developed a technology to fabricate test patterns with a minimum linewidth down to 1.5 nm. The fabricated nanostructures are used to test metrological equipment. The designed patterns involve thousands of lines with precisely designed linewidths; these lines are combined in such a way that the distribution of linewidths appears to be random at any location. This pseudo-random test pattern allows nanometrological systems to be characterized over their entire dynamic range.
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The test pattern contains alternating lines of silicon and silicon-tungsten, this results in a pretty good contrast in the metrological systems. The size of the sample is fairly large, approximately 6 x 6 microns, and involves thousands of lines, each according to its designed width. Earlier, aBeam and LBNL reported the capability of fabricating 4 nm lines and spaces using e-beam lithography, atomic layer deposition, and nanoimprint, see http://nanotechweb.org/cws/article/lab/48111.
Sergey Babin, president of aBeam Technologies said, "The semiconductor industry is moving toward a half-pitch of 11 nm and 7 nm. Therefore, metrology equipment should be very accurate, at least one order of magnitude more accurate than that. The characterization of metrology systems requires test patterns at a scale one order smaller than the measured features. The fabrication was a challenge, especially for such a complex pattern as a pseudo-random design, but we succeeded."
Valeriy Yashchuk, a researcher at the Advanced Light Source of LBNL continued, "When you measure anything, you have to be sure that your metrological system produces accurate results, otherwise what kind of results will you get, nobody knows. Qualifying and tuning metrology systems at the nanoscale is not easy. We designed the test pattern that is capable of characterizing nano-metrology systems over their entire dynamic range, resulting in the modulation transfer function, the most comprehensive characteristic of any system."
The test pattern is to be used to characterize almost any nano-metrology system. Experiments were performed using a scanning electron microscope (SEM), atomic force microscope (AFM), and soft x-ray microscopes. For example, comparisons of an ideal test-sample and its SEM microscopy image show imperfections in the microscope that need to be characterized. The power spectral density of the sample is flat; the spectra of the image has a significant cut-off at high frequencies; this is used to characterize the microscope over its dynamic range and show the degradation of the microscope’s sensitivity as soon as the linewidth becomes smaller.
SOURCE: aBeam Technologies; http://www.abeamtech.com/newsletters/press_release_aBeam_020415.pdf