Optical ellipsometric scatterometry techniques, such as scatterometry based on conventional spectroscopic ellipsometry (SE), have currently become one of the most important approaches for in-line metrology of geometric parameters and nanostructures in high-volume-production nanomanufacturing, due to their attractive advantages of high throughput, low cost, non-contact, and non-destruction. Optical ellipsometric scatterometry is essentially model-based metrology, which involves two key issues - how to collect highly precise ellipsometric scattering data, and how to rapidly and accurately reconstruct the profiles of nanostructures from the measured data. The former issue involves the development of specific ellipsometric scatterometry tools, while the latter involves the theory and method of computational metrology for nanostructures. In order to collect highly precise ellipsometric scattering data, we have developed three kinds of ellipsometric scatterometry instruments for different metrology requirements of practical nanostructures.
(1) Development of a broadband Mueller matrix ellipsometer
Mueller matrix ellipsometry (MME), sometimes also referred to as Mueller matrix polarimetry (MMP) or generalized ellipsometry, can change three measurement conditions, i.e., the wavelength, the incidence, and azimuthal angles. It can also provide up to 16 quantities of a 4 by 4 Mueller matrix in each measurement. Compared with conventional SE-based scatterometry, which only obtains two ellipsometric parameters, MME-based scatterometry can thereby achieve much more useful information about the sample. By choosing an appropriate configuration of the three measurement conditions and by fully exploring the rich information contained in the collected Mueller matrices, MME-based scatterometry is expected to achieve a higher measurement accuracy. We have developed a broadband Mueller matrix ellipsometer under the support of the National Natural Science Foundation and the National Instrument Development Specific Project of China.
(2) Development of a Mueller matrix imaging ellipsometer
Although optical ellipsometric scatterometry techniques have achieved wide applications in high-volume-production nanomanufacturing for nanostructure metrology, they are inherently limited by the size of the illumination spot, which limits the lateral resolution of the instrument as well as the efficiency in constructing a map of the sample over a large area. Aiming at these issues, we have developed a prototype of the Mueller matrix imaging ellipsometer (MMIE) by introducing conventional imaging techniques to optical ellipsometric scatterometry. The experimental results have demonstrated that we can realize Mueller matrix measurement and analysis for nanostructures with a pixel-sized illumination spot by using MMIE. We can also directly construct parameter maps of nanostructures over a large area with pixel-sized lateral resolution by performing parallel ellipsometric analyses for all the pixels of interest.
(3) Development of a Mueller matrix ellipsometer with scatterfield tomography
Both the MME and MMIE are used only for the metrology of periodic nanostructures. They are not suitable for the metrology of much more complex nanostructures, especially aperiodic nanostructures or nanostructures with limited periods corresponding to the illumination spot size. In order to deal with this issue, we are developing a novel instrument based on our previously developed ellispometric scatterometry techniques. We call this novel instrument as Mueller matrix ellipsometer with scatterfield tomography (MMEST). By combining the diffractive tomography techniques, MMEST can collect the scatterfield information about the sample at different incidence angles over a large range at a high speed. Therefore, MMEST can acquire much more information in comparison with the MME and MMIE techniques, which can only obtain the scatterfield information in a specific direction, namely the direction of the zeroth-order diffraction. By fully exploring the collected scatterfield information, it is expected that MMEST can realize the measurement of aperiodic nanostructures.
(4) Applications to nanostructure metrology
Based on the above in-house developed ellipsometric scatterometry instruments, we have conducted a lot of experiments involving some typical nanostructures in cooperation with several research institutions and corporations at home and abroad. The measured nanostructures include e-beam patterned structures with small critical dimensions, photoresist structures with natural line edge roughness, nanopillar arrays with a high-aspect ratio, and nanoimprinted structures with nonuniform residual resist layers. As an example, for the nanoimprinted structures, we first found the depolarization effect induced by the residual layer thickness variation. After incorporating depolarization effects, not only could improved measurement accuracy be achieved for the line width, line height, and residual layer thickness measurement, but the residual layer thickness variation could also be directly determined over the illumination spot.
