The development of compact radiation sources for extreme ultraviolet (EUV) radiation has enabled a multitude of lab-size applications, especially in the field of metrology, that were previously only possible at synchrotron facilities. Plasma-based and high-harmonic generation (HHG) sources are widely used and well characterized at their application specific wavelength ranges in the EUV regime. The spectrum of the emitted radiation from these sources extends to a broader wavelength range, even into the visible and infrared bands and their full spectral composition is not sufficiently characterized. Therefore, a broadband spectral characterization of the source emission is of utmost importance for the investigation of photon-induced processes and metrology. In this study, the authors present a unique setup and corresponding measurement results for the high-resolution broadband spectral characterization of radiation sources covering the extensive wavelength range from 5nm to 1μm. For the vacuum wavelength range (5nm to 200nm) the setup employs a combination of three flat-field diffraction gratings with varying line density as dispersive elements. Higher diffraction orders are filtered out by a selection of thin film filters. The wavelength range above 200nm is measured with two Czerny-Turner spectrometer modules. The resulting spectra are combined to obtain the full spectrum without any contributions from higher diffraction orders. Here, the overall design, wavelength calibration, and the relative and absolute intensity calibration of the measured spectra are presented.
Fast and non-destructive non-imaging metrology of nanostructures is crucial for the development of integrated circuits and for the corresponding in-situ metrology within fabrication processes. Stochastic variations related to the gratings local period (line edge roughness, LER) and line width (line width roughness, LWR) are of special interest due to their key role for the minimal achievable structure size. Non-imaging metrology approaches taking these statistic variations into account are quite limited. For scatterometry, models predict a change of the grating’s diffraction efficiency according to a DebyeWaller factor but only in the non-zeroth diffraction orders. The authors perform simulations of nanoscale gratings that suggest an influence of LER and LWR on the reflectance (zeroth diffraction order efficiency) which motivate an extended study on LER and LWR measured by spectrally resolved EUV reflectometry here described as EUV spectrometry. The authors present reconstruction results of nanoscale gratings measured with a compact spectrometer utilizing extreme ultraviolet (EUV) radiation emitted by a discharged-produced plasma (DPP) EUV source. The use of two sequential spectrographs, one for the reference measurement of the source spectrum, the other one for the measurement of the spectrum after sample interaction, combined within the experimental setup allows to measure the broadband reflectance with 2% relative uncertainty of samples under various grazing incidence angles. The method offers a proven sub-nm reconstruction accuracy for critical grating parameters. Within the presented study, the measured samples are dedicated test samples, fabricated to exhibit well-defined LER and LWR at different grating periods and linewidths. In addition, the samples are also cross-characterized by the Physikalisch-Technische Bundesanstalt (PTB, Berlin). Experimental and simulative results are discussed to derive approaches to include LER and LWR as parameters in the physical model for reconstruction.
Any modeling of an interaction between photons and matter is based on the optical parameters. The determination of these parameters, also called optical constants or refractive indices, is an indispensable component for the development of new optical elements such as mirrors, gratings, or lithography photomasks. Especially in the extreme ultraviolet (EUV) spectral region, existing databases for the refractive indices of many materials and compositions are inadequate or are a mixture of experimentally measured and calculated values from atomic scattering factors. Synchrotron radiation is of course ideally suited to verify such material parameters due to the tuneability of photon energy. However, due to the large number of possible compounds and alloys, the development of EUV laboratory reflectometers is essential to keep pace with the development of materials science and allow for inline or on-site quality control. Additionally, optical constants are also essential for EUV metrology techniques that aim to achieve dimensional reconstruction of nanopatterned structures with sub-nm resolution. For this purpose, we studied a TaTeN grating created on an EUV Mo/Si multilayer mirror, to mimic a novel absorber EUV photomask. We present here a first reconstruction comparison of these structures, measured by EUV scatterometry at the electron storage ring BESSYII and with a laboratory setup of a spectrally-resolved EUV reflectometer developed at RWTH Aachen University. Both approaches differ in several aspects reaching from setup size to spectral quality (brilliance, bandwidth and coherence) as well as the measured and simulated data.
Background: In the extreme ultaviolet (EUV) lithography process the performance of the photoresist is a crucial factor regarding the quality and critical dimensions of the fabricated structures.
Aim: The characterization of the latent image structures in photoresists during the process steps before the development of the resist is key to understand the relation between the material of the resists, the selection of process parameters, and the resulting quality of fabricated structures.
Approach: Spectroscopic EUV reflectometry is a nondestructive metrology technique that measures the broadband reflectance of samples in the EUV spectral range and under grazing incidence angles. The technique offers a combination of high sensitivity to nanoscale structural parameters of periodic structures as well as a high sensitivity to the material composition samples, enabling the characterization of latent images of periodic structures.
Results: Measurements of the reflectance of an EUV-exposed and unexposed photoresist reveal the contrast in optical constants after the resists are treated with a post-exposure bake as well as shrinkage of the resist layer thickness. Based on this data, simulative studies on latent images of periodic grating structures are conducted showing the possibility to extract information on the structure parameters including the latent image profile and surface topography.
Conclusion: Spectroscopic EUV reflectometry shows to be sensitive to the contrast of exposed and unexposed photoresist which commends the technique to be adequate for the characterization of latent images in photoresists.
The authors present a novel approach for the structural characterization of periodic nanostructures using spectrally resolved broadband scatterometry in the extreme ultraviolet (EUV) wavelength range. The implemented metrology method combines 0th and ±1st diffraction order spectrum measurements of a nanograting under broadband illumination from 8 nm to 17 nm for model-based reconstruction of geometrical parameters. For the experimental investigations, a compact stand-alone scatterometer setup is designed and realized. The setup enables measurements of spectrally resolved 0th and ±1st diffraction orders of a grating that is illuminated at various grazing incidence angles. The acquired data serves as a basis for the reconstruction of the grating’s geometry using rigorous optical finite element method (FEM). The method is applied to arrays of lines and spaces with sub-100 nm feature size.
The authors present latent image characterization in photoresists by means of extreme ultraviolet (EUV) spectroscopic reflectometry. The optical constants of photoresists before and after exposure are measured in the EUV spectral range. Latent images are investigated in the form of periodic line gratings. The investigation is performed by the analysis of spectroscopic reflectance curves in the wavelength range from 5 nm to 20 nm at grazing incidence angles. Through an analysis of the reflectance curves based on rigorous electromagnetic modeling, a characterization of parameters of interest of the latent image is evaluated. This includes the latent image profile, surface topography and stochastic-related parameters such as line edge roughness.
In this contribution the accuracy of measurements performed with a stand-alone EUV spectrometer is analyzed. The setup is used to determine optical constants and dimensional characteristics of samples, e.g. ultrathin films or nanoscale gratings. For this purpose, measurements of the broadband EUV reflectance of the samples at variable grazing incidence angles are used to reconstruct sample parameters in a model-based approach. The accuracy of these measurements is a crucial factor for a reliable characterization of samples. We present an overview on the sources of uncertainties in the experimental setup as well as improvements to the setup that improves its accuracy. Additionally, the reconstruction accuracy of the optical constants is analyzed. A focus is put on the influence of the experimental uncertainty and the range of incidence angles used for reflectance measurements.
In this contribution nanoscale gratings are characterized by means of broadband EUV spectroscopy with wavelengths from 10 nm to 15 nm. The study focuses on the specifics of this spectral range that can be beneficial for metrology applications in lithography. Experimental investigations are carried out on fused silica nanoscale line gratings in a stand-alone laboratory-based setup. A corresponding sensitivity study is carried out analyzing the influence of grating parameter variations on EUV reflectance curves. Subsequently, experimental uncertainties are propagated to accuracies of grating parameter extraction. Using rigorous simulations in combination with machine learning, limitations of the technique are discussed regarding industrially relevant gratings. Extending the method through analysis of higher diffraction orders is evaluated.
The authors present studies on laboratory-based spectroscopy in the extreme ultraviolet (EUV) performed on a variety of thin film materials. This work focuses on spectroscopic measurements of EUV reflectivity and transmittance at wavelengths between 10 nm and 15 nm. The presented applications of the technique include the reconstruction of optical constants for novel materials such as EUV photoresists and absorbers, the characterization of EUV pellicles and ultrathin layer systems as well as the characterization of nanostructured surfaces.
The authors report on critical dimension metrology on nanoscale gratings by means of laboratory-based spectroscopic reflectometry in the extreme ultraviolet (EUV). EUV reflectivity spectra of nanoscale gratings under grazing incidence illumination and their dependency on the geometrical grating parameters are discussed. A laboratory-based setup to measure such spectra is introduced and its main features are presented. A nanoscale grating with a grating period below 100 nm, consisting of multiple nanometer-size layers of materials, is experimentally investigated in the setup. The experimental results are consequently compared to a rigorous model fit of the reflectivity and thus the ability to model the grating’s interaction with EUV radiation is shown.
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