Author:  Xiuguo Chen     2014-09-04

Nanoimprint lithography (NIL), in which features on a prepatterned mold are transferred directly into a polymer material, represents a promising technique with the potential for high resolution and throughput as well as low cost. In order to control NIL processes to achieve good fidelity, accurate characterization of structural parameters of nanoimprinted resist patterns is highly desirable. These parameters usually include not only the critical dimension, sidewall angle and feature height, but also the residual layer thickness. Although both scanning electron microscopy (SEM) and atomic force microscopy (AFM) can provide high precision data, they are in general time-consuming, expensive, complex to operate, and problematic in realizing in-line integrated measurement.

Recently, Prof. Shiyuan Liu’s research group in WLNO proposed to apply Mueller matrix ellipsometry (MME) to realize accurate characterization of nanoimprinted resist patterns. Compared with conventional spectroscopic ellipsometry, which only has two changeable measurement conditions, i.e., the wavelength and the incidence angle, and can only obtain two ellipsometric angles in each measurement, MME has more than one measurement condition, i.e., the azimuthal angle, and can provide up to 16 quantities of a 4 ´4 Mueller matrix in each measurement. Consequently, MME can acquire much more useful information about the sample. Supported by the National Instrument Development Specific Project of China, Prof. Liu’s group has developed an in-house broad-band Mueller matrix ellipsometer with theory and methodology of computational metrology for nanostructure measurement. Together with Prof. Zhimou Xu’s research group in WLNO, they have performed experiments to demonstrate that not only more accurate quantification of the line width, line height, sidewall angle, and residual layer thickness of nanoimprinted resist patterns can be achieved, but also the residual layer thickness variation over the illumination spot can be directly determined, when performing MME measurements in the optimal configuration and meanwhile incorporating depolarization effects into the optical model.

The work was funded by the National Natural Science Foundation of China (Grant Nos. 91023032 and 51005091), the National Instrument Development Specific Project of China Grant No. 2011YQ160002), and the Program for Changjiang Scholars and Innovative Research Team in University of China. The work was published in Applied Physics Letters [103(15), 151605 (2013)] and Optics Express [22(12), 15165-15177 (2014)].