Global optimization in inverse problem of scatterometry

International audienceIn the current work, we consider the inverse problem in scatterometry which consists in determining the feature shape from an experimental ellipsometric signature. The reformulation of the given nonlinear identification problem was considered as a parametric optimization problem using the Least Square criterion. In this work, a design procedure for global robust optimization is developed using Kriging and global optimization approaches. Robustness is determined by Kriging model to reduce the number of real functional calculations of Least Square criterion. The technical of the global optimization methods is adopted to determine the global robust optimum of a surrogate model

An inverse ellipsometric problem for thin film characterization: comparison of different optimization methods

International audienceIn this paper, an ill-posed inverse ellipsometric problem for thin film characterization is studied. The aim is to determine the thickness, the refractive index and the coefficient of extinction of homogeneous films deposited on a substrate without assuming any a priori knowledge of the dispersion law. Different methods are implemented for the benchmark. The first method considers the spectroscopic ellipsometer as an addition of single wavelength ellipsometers coupled only via the film thickness. The second is an improvement of the first one and uses Tikhonov regularization in order to smooth out the parameter curve. Cross-validation technique is used to determine the best regularization coefficient. The third method consists in a library searching. The aim is to choose the best combination of parameters inside a pre-computed library. In order to be more accurate, we also used multi-angle and multi-thickness measurements combined with the Tikhonov regularization method. This complementary approach is also part of the benchmark. The same polymer resist material is used as the thin film under test, with two different thicknesses and three angles of measurement. The paper discloses the results obtained with these different methods and provides elements for the choice of the most efficient strategy

Dispositif de focalisation de lumière à des dimensions sub-longueur d'onde à fort rendement

The general field of the invention is that of devices for focusing light with sub-wavelength dimensions comprising at least one focusing structure comprising a metal film comprising a first aperture passing through the film and of dimensions of an order of magnitude that are smaller than the wavelength of use of the focusing device. In the devices according to the invention, the focusing structure comprises at least one optical cavity disposed on the aperture so that, when the structure is illuminated by an optical flux at the wavelength of use of the device, a significant part of this flux is concentrated on the aperture by said cavity. Several embodiments are described using various types of cavity that may comprise plasmon reflectors.Le domaine général de l'invention est celui des dispositifs de focalisation de lumière à des dimensions sub-iongueur d'onde comprenant au moins une structure de focalisation comportant un film métallique comportant une première ouverture traversant le film et de dimensions d'un ordre de grandeur inférieures à la longueur d'onde d'utilisation du dispositif de focalisation. Dans les dispositifs selon l'invention, la structure de focalisation comporte au moins une cavité optique disposée sur l'ouverture de façon que, lorsque la structure est éclairée par un flux optique à la longueur d'onde d'utilisation du dispositif, une partie importante de ce flux est concentrée sur l'ouverture par ladite cavité. Plusieurs modes de réalisation sont décrits utilisant différents types de cavité pouvant comporter des réflecteurs à plasmon

Fabry–Pérot-type enhancement in plasmonic visible nanosource

International audienceStarting from thin film planar technology, we designed and fabricated a visible optical source thatproduces a localized bright spot with nanometric dimensions. The structure consists of excitingsurface plasmons through the illumination of a subwavelength hole in a silver film and in confiningthem at the vicinity of the aperture by surrounding the hole of Bragg mirrors resonant with theplasmons generated. Both finite element method computations and experimental results evidence theperformances of this device that could find applications, for example, in nanolithography or opticaldata storage

Application of the Kriging method to the reconstruction of ellipsometric signature

International audienceThe inverse problem in scatterometry which consists in determining the feature shape from an experimental ellipsometric signature is very difficult to solve. On one hand, the problem is ill-posed; on the other hand, due to equipment limitation and the presence of noise in the measurement, the number of experimental signature acquisition is limited to a few measurements. The efficient resolution of the inverse problem requires a more comprehensive signature. To deal with this problem, we use a new approach based on the Kriging interpolation method to enrich the number of usable data. This method is inherently providing the best linear unbiased optimal estimation

Nanometer scale light focusing with high cavity-enhanced output

International audienceEfficient nanoscale optical focusing is demonstrated in double-structured thin metallic films supporting both surface plasmons and cavity eigenmodes. Numerical simulations show that efficient mode coupling can dramatically enhance the transmission of a subwavelength slit. For a spot size of ∼λ/10, the cavity induced enhancement is about an order of magnitude, leading to a final structure output (transmitted power/incident power) of ∼30%. Results are discussed in terms of cavity-slit mode matching and photon trapping within the cavity

Limits of model-based CD-SEM metrology

International audienceAlthough the critical dimension (CD) is getting smaller following the ITRS roadmap, the scanning electron microscope (CD-SEM) is still the most general purpose tool used for non-destructive metrology in the semiconductor industry. However, we are now dealing with patterns whose dimensions are of the same order of magnitude as the electron interaction volume and therefore, the usual edge-based metrology methods fail. Like scatterometry has extended the resolution of optical imaging metrology through complex modeling of light-matter interaction, some electrons-matter simulation models have been proposed. They could be used to improve accuracy and precision of CD-SEM metrology. However, these model-based approaches also face to fundamental limits mainly due to probe size with respect to the considered structure and noise. This paper analyses these limits assuming the model is perfect and the microscope has no systematic defect. In this simulation study, we have used the model proposed by D. Nyyssonen, assuming to perfectly represent the SEM effects in the image. The feature of interest is limited to isolated trapezoidal lines with various CD, sidewall angles (SWA) and heights. We have carried out the study with several beam energies, tilts and probe sizes. Surprisingly enough, sensitivity analysis shows that with typical noise amplitude, sidewall angle can be determined with a reasonable precision using SEM images. Single tilted beam SEM images can also bring advantage to measure patterns height. Since these precision figures depend on the geometries, we provide useful graphs giving the ultimate precision for various dimensions (CD, height, SWA)

Surface plasmon interference printing in a photosensitive azo-dye polymer film

International audienc