Direct structural characterisation of line gratings with grazing incidence small-angle x-ray scattering

Virtual X-ray critical dimension metrology via Monte Carlo simulation

X-ray critical dimension (XCD) metrology is a highly promising technique for achieving sub-nanometer precision in critical dimension measurements at advanced nodes of integrated circuit manufacturing. Compared to XCD experiments utilizing synchrotron radiation sources, those employing compact X-ray sources encounter challenges like extended testing time and increased uncertainty. To evaluate the influence of experimental conditions on measurement results, we developed an ab initio virtual X-ray critical dimension metrology via a Monte Carlo simulation (MC-VXCD). Through calibrating the system parameters of the MC-VXCD to a home-built compact XCD instrument, we achieved excellent consistency between virtual and actual measurement results. The virtual instrument effectively estimated measurement errors stemming from the reduced exposure time, which significantly influences the measurement accuracy and throughput. Furthermore, through the MC-VXCD, we establish the connection between the application scenarios of the XCD metrology and the geometry of XCD instruments, offering a versatile platform for the system design, experimental configuration optimization, data analysis, etc., in XCD metrology.

A new sample chamber for hybrid detection of scattering and fluorescence, using synchrotron radiation in the soft x-ray and extreme ultraviolet (EUV) spectral range

Smaller and more complex nanostructures in the semiconductor industry require a constant upgrade of accompanying metrological methods and equipment. A central task for nanometrology is the precise determination of structural features of gratings in the nanometer range as well as their elemental composition. Scatterometry and x-ray fluorescence in the soft x-ray and extreme ultraviolet spectral ranges are ideally suited to this task. We here present a new, compact measurement chamber that can simultaneously detect the elastically scattered signal and the fluorescence, originating from nanoscale grating samples. Its geometry enables detecting scattered intensity over a wide angular range with a variable angle of incidence. We show first experiments on industry-relevant test structures from the commissioning process alongside the specifications of the setup, located at PTB's soft x-ray radiometry beamline at the synchrotron radiation facility BESSY II in Berlin.

Using small-angle scattering to guide functional magnetic nanoparticle design

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

Controllable CaF2 Nanosized Stripe Arrays on Si(001) Studied by X-ray and Electron Diffraction

Adding uniaxial in-plane anisotropy to the otherwise four-fold Si(001) surface has for a long time been known to be possible via epitaxial deposition of a single atomic layer of calcium fluoride (CaF2), which forms an array of micron-long (110) oriented parallel stripes when the substrate temperature during the growth is kept in the range of 700–800 °C. As shown in the present paper, a fine control over dimensions and periodicity of the stripe array is possible through the introduction of a two-stage growth process at which the (110) orientation of the fluorite layer is settled at the high-temperature nucleation stage, while the stripes of controllable dimensions are formed at the second stage. By varying the substrate temperature at the second growth stage in the range of 800–400 °C, the stripe arrays with a periodicity from above 30 nm to below 10 nm can be fabricated with the height variation changing accordingly. Such variability can be of use in the applications in which the striped fluorite surface is used to influence the anisotropy of other functional (e.g., magnetically ordered or organic) materials grown on top. While large CaF2 stripes can be easily characterized by direct space techniques such as atomic force microscopy, the study of the shape and in-plane correlation between the stripes of a much smaller size is most effectively achieved through the use of grazing incidence reciprocal space techniques applied in the present paper. The discussed universal approach to 3D reciprocal space mapping utilizing scattering of X-rays and high-energy electrons offers a complementary way to study samples with arrays of long and narrow one-dimensional stripes at their surface.

Spontaneous Registration of Sub-10 nm Features Based on Subzero Celsius Spin-Casting of Self-Assembling Building Blocks Directed by Chemically Encoded Surfaces

For low-cost and facile fabrication of innovative nanoscale devices with outstanding functionality and performance, it is critical to develop more practical patterning solutions that are applicable to a wide range of materials and feature sizes while minimizing detrimental effects by processing conditions. In this study, we report that area-selective sub-10 nm pattern formation can be realized by temperature-controlled spin-casting of block copolymers (BCPs) combined with submicron-scale-patterned chemical surfaces. Compared to conventional room-temperature spin-casting, the low temperature ( e.g., -5 °C) casting of the BCP solution on the patterned self-assembled monolayer achieved substantially improved area selectivity and uniformity, which can be explained by optimized solvent evaporation kinetics during the last stage of film formation. Moreover, the application of cold spin-casting can also provide high-yield in situ patterning of light-emitting CdSe/ZnS quantum dot thin films, indicating that this temperature-optimized spin-casting strategy would be highly effective for tailored patterning of diverse organic and hybrid materials in solution phase.

Element sensitive reconstruction of nanostructured surfaces with finite elements and grazing incidence soft X-ray fluorescence

The geometry of a Si3N4 lamellar grating was investigated experimentally with reference-free grazing-incidence X-ray fluorescence analysis. While simple layered systems are usually treated with the matrix formalism to determine the X-ray standing-wave field, this approach fails for laterally structured surfaces. Maxwell solvers based on finite elements are often used to model electrical field strengths for any 2D or 3D structures in the optical spectral range. We show that this approach can also be applied in the field of X-rays. The electrical field distribution obtained with the Maxwell solver can subsequently be used to calculate the fluorescence intensities in full analogy to the X-ray standing-wave field obtained by the matrix formalism. Only the effective 1D integration for the layer system has to be replaced by a 2D integration of the finite elements, taking into account the local excitation conditions. We will show that this approach is capable of reconstructing the geometric line shape of a structured surface with high elemental sensitivity. This combination of GIXRF and finite-element simulations paves the way for a versatile characterization of nanoscale-structured surfaces.

Characteristic diffuse scattering from distinct line roughnesses

Lamellar gratings are widely used diffractive optical elements; gratings etched into Si can be used as structural elements or prototypes of structural elements in integrated electronic circuits. For the control of the lithographic manufacturing process, a rapid in-line characterization of nanostructures is indispensable. Numerous studies on the determination of regular geometry parameters of lamellar gratings from optical and extreme ultraviolet (EUV) scattering highlight the impact of roughness on the optical performance as well as on the reconstruction of these structures. Thus, a set of nine lamellar Si gratings with a well defined line edge roughness or line width roughness were designed. The investigation of these structures using EUV small-angle scattering reveals a strong correlation between the type of line roughness and the angular scattering distribution. These distinct scattering patterns open new paths for the unequivocal characterization of such structures by EUV scatterometry.

Reconstructing detailed line profiles of lamellar gratings from GISAXS patterns with a Maxwell solver

Laterally periodic nanostructures have been investigated with grazing-incidence small-angle X-ray scattering (GISAXS) by using the diffraction patterns to reconstruct the surface shape. To model visible light scattering, rigorous calculations of the near and far field by numerical solution of Maxwell's equations with a finite-element method are well established. The application of this technique to X-rays is still challenging, owing to the discrepancy between the incident wavelength and the finite-element size. This drawback vanishes for GISAXS because of the small angles of incidence, the conical scattering geometry and the periodicity of the surface structures, which allows a rigorous computation of the diffraction efficiencies with sufficient numerical precision. To develop metrology tools based on GISAXS, lamellar gratings with line widths down to 55 nm were produced by state-of-the-art electron-beam lithography and then etched into silicon. The high surface sensitivity of GISAXS in conjunction with a Maxwell solver allows the detailed reconstruction of the grating line shape for thick non-homogeneous substrates as well. The reconstructed geometric line-shape models are statistically validated by applying a Markov chain Monte Carlo sampling technique which reveals that GISAXS is able to reconstruct critical parameters like the widths of the lines with sub-nanometre uncertainty.

Grazing-incidence small-angle X-ray scattering (GISAXS) on small periodic targets using large beams

Grazing-incidence small-angle X-ray scattering (GISAXS) is often used as a versatile tool for the contactless and destruction-free investigation of nano-structured surfaces. However, due to the shallow incidence angles, the footprint of the X-ray beam is significantly elongated, limiting GISAXS to samples with typical target lengths of several millimetres. For many potential applications, the production of large target areas is impractical, and the targets are surrounded by structured areas. Because the beam footprint is larger than the targets, the surrounding structures contribute parasitic scattering, burying the target signal. In this paper, GISAXS measurements of isolated as well as surrounded grating targets in Si substrates with line lengths from 50 µm down to 4 µm are presented. For the isolated grating targets, the changes in the scattering patterns due to the reduced target length are explained. For the surrounded grating targets, the scattering signal of a 15 µm × 15 µm target grating structure is separated from the scattering signal of 100 µm × 100 µm nanostructured surroundings by producing the target with a different orientation with respect to the predominant direction of the surrounding structures. As virtually all litho-graphically produced nanostructures have a predominant direction, the described technique allows GISAXS to be applied in a range of applications, e.g. for characterization of metrology fields in the semiconductor industry, where up to now it has been considered impossible to use this method due to the large beam footprint.

Characterization of the shape and line-edge roughness of polymer gratings with grazing incidence small-angle X-ray scattering and atomic force microscopy

Grazing-incidence small-angle X-ray scattering (GISAXS) is increasingly used for the metrology of substrate-supported nanoscale features and nanostructured films. In the case of line gratings, where long objects are arranged with a nanoscale periodicity perpendicular to the beam, a series of characteristic spots of high-intensity (grating truncation rods, GTRs) are recorded on a two-dimensional detector. The intensity of the GTRs is modulated by the three-dimensional shape and arrangement of the lines. Previous studies aimed to extract an average cross-sectional profile of the gratings, attributing intensity loss at GTRs to sample imperfections. Such imperfections are just as important as the average shape when employing soft polymer gratings which display significant line-edge roughness. Herein are reported a series of GISAXS measurements of polymer line gratings over a range of incident angles. Both an average shape and fluctuations contributing to the intensity in between the GTRs are extracted. The results are critically compared with atomic force microscopy (AFM) measurements, and it is found that the two methods are in good agreement if appropriate corrections for scattering from the substrate (GISAXS) and contributions from the probe shape (AFM) are accounted for.

Determining the shape and periodicity of nanostructures using small-angle X-ray scattering

The semiconductor industry is exploring new metrology techniques capable of meeting the future requirement to characterize three-dimensional structure where the critical dimensions are less than 10 nm. X-ray scattering techniques are one candidate owing to the sub-Å wavelengths which are sensitive to internal changes in electron density. Critical-dimension small-angle X-ray scattering (CDSAXS) has been shown to be capable of determining the average shape of a line grating. Here it is used to study a set of line gratings patternedviaa self-aligned multiple patterning process, which resulted in a set of mirrored lines, where the individual line shapes were asymmetric. The spacing between lines was systematically varied by sub-nm shifts. The model used to simulate the scattering was developed in stages of increasing complexity in order to justify the large number of parameters included. Comparisons between the models at different stages of development demonstrate that the measurement can determine differences in line shapes within the superlattice. The shape and spacing between lines within a given set were determined to sub-nm accuracy. This demonstrates the potential for CDSAXS as a high-resolution nanostructure metrology tool.

Traceable GISAXS measurements for pitch determination of a 25 nm self-assembled polymer grating

The feature sizes of only a few nanometres in modern nanotechnology and next-generation microelectronics continually increase the demand for suitable nanometrology tools. Grazing-incidence small-angle X-ray scattering (GISAXS) is a versatile technique to measure lateral and vertical sizes in the nanometre range, but the traceability of the obtained parameters, which is a prerequisite for any metrological measurement, has not been demonstrated so far. In this work, the first traceable GISAXS measurements, demonstrated with a self-assembled block copolymer grating structure with a nominal pitch of 25 nm, are reported. The different uncertainty contributions to the obtained pitch value of 24.83 (9) nm are discussed individually. The main uncertainty contribution results from the sample–detector distance and the pixel size measurement, whereas the intrinsic asymmetry of the scattering features is of minor relevance for the investigated grating structure. The uncertainty analysis provides a basis for the evaluation of the uncertainty of GISAXS data in a more general context, for example in numerical data modeling.

Nanoimprinted comb structures in a low bandgap polymer: thermal processing and their application in hybrid solar cells

In this paper, we investigate conjugated polymer layers structured by nanoimprint lithography toward their suitability for the fabrication of nanostructured polymer/metal sulfide hybrid solar cells. Consequently, we first study the thermal stability of the nanoimprinted conjugated polymer layers by means of scanning electron microscopy and grazing incidence small-angle X-ray scattering, which reveals a reasonable thermal stability up to 145 °C and sufficient robustness against the solvent mixture used in the subsequent fabrication process. In the second part, we demonstrate the preparation of nanostructured polymer/copper indium sulfide hybrid solar cells via the infiltration and thermal decomposition of a mixture of copper and indium xanthates. Although this step needs temperatures of more than 160 °C, the nanostructures are retained in the final polymer/copper indium sulfide layers. The nanostructured solar cells show significantly improved power conversion efficiencies compared to similarly prepared flat bilayer devices, which is based on a distinct improvement of the short circuit current in the nanostructured solar cells.

Order quantification of hexagonal periodic arrays fabricated by in situ solvent-assisted nanoimprint lithography of block copolymers

Directed self-assembly of block copolymer polystyrene-b-polyethylene oxide (PS-b-PEO) thin film was achieved by a one-pot methodology of solvent vapor assisted nanoimprint lithography (SAIL). Simultaneous solvent-anneal and imprinting of a PS-b-PEO thin film on silicon without surface pre-treatments yielded a 250 nm line grating decorated with 20 nm diameter nanodots array over a large surface area of up to 4' wafer scale. The grazing-incidence small-angle x-ray scattering diffraction pattern showed the fidelity of the NIL stamp pattern replication and confirmed the periodicity of the BCP of 40 nm. The order of the hexagonally arranged nanodot lattice was quantified by SEM image analysis using the opposite partner method and compared to conventionally solvent-annealed block copolymer films. The imprint-based SAIL methodology thus demonstrated an improvement in ordering of the nanodot lattice of up to 50%, and allows significant time and cost reduction in the processing of these structures.

Characterization of an in-vacuum PILATUS 1M detector

A dedicated in-vacuum X-ray detector based on the hybrid pixel PILATUS 1M detector has been installed at the four-crystal monochromator beamline of the PTB at the electron storage ring BESSY II in Berlin, Germany. Owing to its windowless operation, the detector can be used in the entire photon energy range of the beamline from 10 keV down to 1.75 keV for small-angle X-ray scattering (SAXS) experiments and anomalous SAXS at absorption edges of light elements. The radiometric and geometric properties of the detector such as quantum efficiency, pixel pitch and module alignment have been determined with low uncertainties. The first grazing-incidence SAXS results demonstrate the superior resolution in momentum transfer achievable at low photon energies.

On the assessment by grazing-incidence small-angle X-ray scattering of replica quality in polymer gratings fabricated by nanoimprint lithography

Grazing-incidence small-angle X-ray scattering (GISAXS) can be used to characterize the replica quality of polymer gratings prepared by thermal nanoimprint lithography (NIL). Here it is shown using GISAXS experiments that a series of NIL polymer gratings with different line quality present characteristic features that can be associated with the level of defects per line. Both stamps and NIL polymer gratings exhibit characteristic semicircle-like GISAXS patterns. However NIL polymer gratings with defective lines exhibit GISAXS patterns with an excess of diffuse scattering as compared to those of the corresponding stamps. In a first approach, this effect is attributed to a reduction of the effective length of the lines diffracting coherently as the number of defects per line increases.