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NIST researchers validate new method for investigating nanoscale pattern shape evolution

NIST researchers successfully demonstrated the application of scatterometry to monitor, in situ, pattern profiles made by thermal embossing nanoimprint lithography. In addition to obtaining real-time measurement of pattern profiles, researchers also gained new insight into the evolution of patterns imprinted in polymers of different molecular weight.

Thermal embossing nanoimprint lithography uses a mold to directly stamp a nanoscale pattern into a polymer under heat and pressure. This simple process is a low-cost alternative to photolithography, with high throughput and high resolution. Consequently, the researchers are studying the process to better understand how the shape of the imprinted pattern evolves during thermal annealing. This provides important information about structural stability and decay mechanisms that impact the performance of imprinted structures, enabling optimization of the imprint process. 

To probe this process, the researchers in the Optical Technology, Surface and Microanalysis Science, and Polymer Divisions collaborated to use scatterometry, which combines spectroscopic ellipsometry with rigorous coupled wave analysis to extract topographical information from the optical measurement. Thus, the researchers could use scatterometry to obtain, in situ, the same information about pattern height and shape as the traditional ex situ methods, such as AFM, SEM, and spectral x-ray reflectivity. This technique also provides a complete record of pattern evolution during annealing of a single sample; whereas, ex situ methods require time-consuming preparation of multiple samples, annealed for various times, to build up a picture of the pattern decay. Additionally, scatterometry may produce more sensitive measurements of nanoscale features in the pattern cross-section.

The researchers had expected that patterns imprinted in polymers of high molecular weight to change relatively slowly due entanglement of longer polymer chains. However, they observed that the patterns in low molecular weight polymer initially appeared more resistant to change, while the patterns in high molecular weight polymer showed a much faster initial relaxation, consistent with measurements made using ex situ techniques. Furthermore, the scatterometry measurements indicated subtle differences in shape between the annealed high and low molecular weight polymers that are difficult to discern using other methods.

For further information, see the article, Scatterometry for in situ measurement of pattern reflow in nanoimprinted polymers.

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 Line profiles as a function of time
Line profiles of a pattern made by nanoimprint lithography as measured by scatterometry during the annealing process.


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