BackgroundA key element of semiconductor fabrication is the precise deposition of thin films. Among other aspects, the quality of interfaces between different materials plays a crucial role for the success of further processing steps.AimWe here present a combined quantitative study of soft X-ray reflectometry measurements compared to scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (STEM-EDX) on stacked thin film samples of silicon and silicon-germanium (SiGe).ApproachThe thin film structures feature two distinct germanium concentrations in the SiGe layers and are produced for complementary field-effect transistor applications. We use synchrotron-based, angle-, and energy-resolved broadband reflectance to investigate the sharpness of the layer interfaces, which is accessible through rigorous modeling of the acquired data. Complementary, the samples are investigated using STEM-EDX on thin lamellas across the interfaces, which give a direct representation of the interface sharpness through the varying germanium content.ResultsLayer thicknesses and interface properties are studied with the two methods. As a side-product of the measurement, the optical constants of the different SiGe compounds are determined and reported.ConclusionsWe find a very high correlation of the retrieved values between both methods and discuss their comparability and limits.
Richard Ciesielski, Roger Loo, Yosuke Shimura, Janusz Bogdanowicz, Antonio Mani, Christoph Mitterbauer, Vinh-Binh Truong, Michael Kolbe, Victor Soltwisch
A key element of semiconductor fabrication is the precise deposition of thin films. Amongst other aspects, the quality of interfaces between different materials plays a crucial role for the success of further process steps. We here present soft x-ray reflectometry measurements on stacked thin film samples of silicon and silicon-germanium in various concentrations as they are produced for complementary field-effect transistor (CFET) applications. Synchrotron-based, angle- and energy-resolved broadband reflectance data sets can be modeled using a matrix-method approach that describes reflection, absorption, and diffuse scattering off the interfaces. This method is often used to determine the optical constants of materials in the EUV spectral region as parameters of the fitting procedure. We here show that the method is equally well suited to investigate roughness and layer intermixing between different deposited materials. These roughness parameters alongside the actual thicknesses of the individual layers also result from the physical modelling of the measured data. The method is inherently non-destructive and very sensitive, down to approximately 50nm depth and as such gives valuable information. To further qualify our findings, we compare the data to scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (STEM-EDX) to give insight into the atomic structure at the interfaces.
Knowledge of optical constants in the vacuum ultraviolet (VUV) range is vital for the semiconductor industries to develop new materials with the required optical properties for lithography. However, the optical constants for most materials in this range are not precisely known because corresponding measurements are demanding in many regards. Measuring the s- and p-polarized reflectance for different materials from 36 nm- 220 nm, we have calculated the optical constants and studied the effect of polarization in the sensitivity of the optical constant determination in the VUV spectral range
Optical constants of materials are essential for predicting and interpreting optical responses, which is crucial when designing new optical components. Although accurate databases of optical constants are available for some regions of the electromagnetic spectrum, for the vacuum ultraviolet (VUV), the extreme ultraviolet (EUV), and soft x-ray spectral ranges, the available optical data suffer inconsistencies, and their determination is particularly challenging. Here, we present a selected example of ruthenium (Ru) for the determination of optical constants from the VUV to the soft x-ray spectral range using reflectivity measurements performed with synchrotron radiation. The subtleties of reflectivity measurements are discussed for a large wavelength range, from 0.7 to 200 nanometers.
X-ray fluorescence techniques in special operation modes can provide valuable quantitative insights for semiconductor related applications and can be made compatible to typical sizes of homogeneously structured metrology pads. As their dimensions are usually in the order of several 10 μm per direction, it must be ensured that no adjacent regions are irradiated or that no x-ray fluorescence radiation from adjacent areas reaches the detector. As this can be realized by using small excitation beams, a multitude of information can be retrieved from such XRF data. In addition to elemental composition, including sensitivity to sub-surface features, one can derive quantitative amounts of material and even dimensional properties of the nanostructures under study. Here, we show three different approaches for studies related to semiconductor applications that are capable to be performed on real world dies with commonly sized metrology pads.
EUV scatterometry can retrieve geometrical information from nanoscale grating structures through elastic scattering of EUV radiation and the evaluation of the diffraction intensities. Its geometry and energy range place it in between grazing incidence x-ray scattering (GISAXS) and optical critical dimension (OCD). PTB recently commissioned a new scatterometry setup for the EUV and soft x-ray region that can address sample areas below 100 × 100 μm size by using a comparably steep, grazing angle of incidence of up to 30°. At the same time, the full cone of exit angles of 30° can be detected such that also the higher orders can be recorded in scatterometry measurements. It has been commissioned at PTB’s monochromatic soft x-ray beamline at the synchrotron radiation facility BESSY II and can also be used for simultaneous x-ray fluorescence detection. Its great tunability and energy resolution allows to scan across absorption edges of the relevant semiconductor materials to increase the contrast between different materials. The nanoscale geometry of modern transistor designs features different materials and structure sizes in the single digit nanometer range. Using the information wealth of spectrally resolved scatterometry measurements from the new setup, we present data and first geometrical reconstructions of selected, complex, industry-relevant design studies. The geometrical reconstruction of these structures relies on precise measurements, modelling of the scattering process, and statistical data evaluation methods.
The interaction of matter and light can be described based on optical constants, shortly called δ&β. These constants provide the fundamental basis for the design of any optical system. In the Extreme Ultraviolet (EUV) spectral range, however, the existing data for many materials or compounds is very sparse, non-existent or exhibit considerable discrepancies between different sources. This is further complicated since the scaling effects stipulate the optical response of a thin film to differ from bulk. Oxidation, impurities or interdiffusion significantly affect the optical response of a system to EUV radiation. For this reason, the Physikalisch-Technische Bundesanstalt (PTB) is establishing a new database in cooperation with other European partners. This database, designated as the Optical Constants Database (OCDB) can be accessed online freely (OCDB.ptb.de). This data collection shall support further development of various fields from new metrological techniques, like EUV scatterometry to computational lithography in the EUV. This is demonstrated exemplarily here by the interplay between δ&β and the dimensional parameters with respect to a structured TaTeN EUV photomask. It is equally important either direction, to derive structure parameters from the measured EUV scattering as vice versa to predict the EUV response from the geometrical structure. In addition, the impact of varying δ and β on the expected imaging performance will be investigated by simulating typical lithographic image metrics like Critical Dimension (CD), best focus position, image contrast (NILS) and non-telecentricity for the imaging of through pitch L/S and 16 nm vertical Lines with 32 nm pitch in a NA=0.55 scanner for TaTeN mask absorber as typical representatives of high-k absorber materialsand as an example of the effect on imaging simulation.
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.
KEYWORDS: Perovskite, Modulation, Luminescence, Optoelectronics, Process control, Tandem solar cells, Solar energy, Solar cells, Semiconductors, Quantum efficiency
Development of efficient absorber masks and highly reflective mirrors in the EUV spectral range is a key challenge for upcoming lithography techniques in semiconductor technology. There is an improved need to precisely know the optical constants of the materials at hand for specialized applications such as phase shifting absorber masks. A further field of application is the interpretation and accompanied modeling of scattering data of complex nanostructures. At PTB, we measured the spectral reflectance of thin film samples in the angular range from normal incidence to grazing incidence in the range of 10nm to 20nm using PTB’s lubrication-free Ellipso-Scatterometer at the soft x-ray beamline at the electron storage ring BESSY II. This allowed us to determine the optical constants of a variety of metals, semimetals and their alloys from model fits based on Fresnel’s equations for layered material stacks.
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