EUV Reflectivity Research Articles

Beryllium-based multilayer X-ray optics

The article provides a review of the current state of affairs in the field of physics and technology of multilayer beryllium-containing mirrors intended for projection lithography and solar corona studies in the extreme ultraviolet (EUV) region. The methods of synthesizing and studying beryllium-containing multilayer mirrors are described. The results of recent studies on the internal structure and EUV reflection coefficients are given for Mo/Be, Mo/Si, Be/Al, and Be/Mg multilayer mirrors. The effect of Si and Be interlayers on the reflectivity is explained. Avenues for further research on beryllium-containing mirrors are discussed.

Modeling multilayer coating profiles with defects on EUV collector with grating

Extreme ultraviolet lithography (EUVL) is recognized as a leading technology in next-generation lithography. Achieving spectral purification while ensuring extreme ultraviolet (EUV) reflectance is one of the key technologies for industrializing EUVL. An EUV collector mirror with phase grating can be used in the spectral purification of an EUVL source. However, it also induces a considerable loss of EUV. We propose a deposition model for calculating the multilayer coating profile on the surface of an EUV collector with grating based on a geometric line tracing method. In addition, it also analyzes the coating defects that influence the EUV reflectance, and the evolution of the coating defects with the grating positions. Experimental results reveal that the model accurately predicts the multilayer coating profile deposited on the surface of the collector, which helps improve how EUV collector is deposited.

Free-standing carbon nanotube films for extreme ultraviolet pellicle application

To enable high volume manufacturing with extreme ultraviolet (EUV) lithography, a pellicle membrane is needed to protect the reticle from particles at EUV source powers beyond 250 W. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability, and chemical resistance to the scanner environment is extremely challenging; yet, these properties are required to realize next-generation EUV pellicle solutions. Free-standing carbon nanotube (CNT) film as an alternative next-generation core pellicle material is proposed. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (∼0.4 % half range), mechanical stability (maximum deflection ∼0.08 mm at 2 Pa), thermal stability (no transmission change under greater than 250 W equivalent EUV power in vacuum), and scalability to a full pellicle size (∼15 × 12 cm2). The capability of the CNT membrane to withstand high EUV power in the presence of H2 for a limited time is demonstrated. Other CNT membrane properties are presented that are important for the pellicle application: low EUV scattering, low EUV reflectivity, and sufficient transmission to enable through-pellicle inspection with DUV light or electrons. The ability of the CNT film to stop particles is tested. The influence of hydrogen at higher EUV powers and prolonged exposures on the lifetime of the CNT pellicle remains the current research focus. Approaches for coating the free-standing CNT films for protection are discussed.

Study of oxidation processes in Mo/Be multilayers

The results of an investigation on oxidation processes in Mo/Be multilayer nanofilms are presented. The films annealed both in ambient atmosphere and in vacuum. The extreme ultraviolet (EUV) and X-ray reflectivity of the samples at 11.34 and at 0.154 nm respectively were measured before and after the treatment. No noticeable changes in film thicknesses and boundaries were observed during the annealing at temperatures up to 300°C. An oxidation mechanism of the nanofilms Mo/Be is established and the activation energy of the oxidation process is estimated to be 38 kJ/mol. To determine an absolute quantity of oxygen in the oxidized layers, a simple technique based on the EUV reflectivity data is proposed, and the range of its applicability is subsequently analysed.

Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity

Extreme ultraviolet (XUV) transient reflectivity around the germanium M_(4,5) edge (3d core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of 3.2 ± 0.2 ps attributed to the X−L intervalley scattering and a hot hole relaxation rate of 600 ± 300 fs ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4–5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of 66° with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment (66°–70°) for the range of XUV energies used. This work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.

Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors.

The ability to observe charge localization in photocatalytic materials on the ultrafast time scale promises to reveal important correlations between excited state electronic structure and photochemical energy conversion. Of particular interest is the ability to determine hole localization in the hybridized valence band of transition metal oxide semiconductors. Using femtosecond extreme ultraviolet reflection absorption (XUV-RA) spectroscopy we directly observe the formation of photoexcited electrons and holes in Fe2O3, Co3O4, and NiO occurring within the 100 fs instrument response. In each material, holes localize to the O 2p valence band states as probed at the O L1-edge, while electrons localize to metal 3d conduction band states on this same time scale as probed at the metal M2,3-edge. Chemical shifts at the O L1-edge enable unambiguous comparison of metal-oxygen (M-O) bond covalency. Pump flux dependent measurements show that the exciton radius is on the order of a single M-O bond length, revealing a highly localized nature of exciton in each metal oxide studied.

Characterization of periodic extreme ultraviolet multilayer based on multi-objective evolutionary algorithm

In order to refine the layered structure of extreme ultraviolet multilayers, a multi-objective evolutionary algorithm which is post-hybridized with the standard Levenberg-Marquardt algorithm is applied to analyze the grazing incidence X-ray reflectivity (GIXR) and the normal incidence extreme ultraviolet reflectance (EUVR). In this procedure, the GIXR data and EUVR data are simultaneously fitted as two objectives, and the high sensitivities of these two sets of data to layer thicknesses and densities are combined. This set of mathematical procedures is conducive to obtain a more correct model of periodic multilayers which can simultaneously describe both GIXR and EUVR measurements. As a result, the layered structure of Mo/Si multilayers with a period of about 7.0 nm is obtained.

Removal of Tin from Extreme Ultraviolet Collector Optics by In-Situ Hydrogen Plasma Etching

Extreme ultraviolet (EUV) lithography produces 13.5 nm light by irradiating a droplet of molten Sn with a laser, creating a dense, hot laser-produced plasma and ionizing the Sn to the + 8 through + 12 states. An unwanted by-product is deposition of Sn debris on the collector optic, which focuses the EUV light emitting from the plasma. Consequently, collector reflectivity is degraded. Reflectivity restoration can be accomplished by means of Sn etching by hydrogen radicals, which can be produced by an H2 plasma and etch the Sn as SnH4. It has previously been shown that plasma cleaning can successfully create radicals and restore EUV reflectivity but that the Sn removal rate is not necessarily limited by the radical density. Additionally, while Sn etching by hydrogen radicals has been shown by multiple investigators, quantification of the mechanisms behind Sn removal has never been undertaken. This paper explores the processes behind Sn removal. Experiments and modeling show that, within the parameter space explored, the limiting factor in Sn etching is not radical flux or SnH4 decomposition, but ion energy flux. Thus the removal is akin to reactive ion etching.

Extended model for the reconstruction of periodic multilayers from extreme ultraviolet and X-ray reflectivity data

An extended model for the reconstruction of multilayer nanostructures from reflectometry data in the X-ray and extreme ultraviolet ranges is proposed. In contrast to the standard model approach, where the transitional region is defined in advance as a specific function, the transition layer is sought as a linear combination of several functions at once in the extended model. This allows one to describe a much wider class of multilayer structures with different dominant physical mechanisms for the formation of transition regions. The extended model occupies an intermediate position between the classical model approach and the so-called model-free methods. The efficiency of the described method is illustrated in detail in numerical simulations and in a real experiment on the annealing of a multilayer Mo/Be mirror.

Low-stress and high-reflectance Mo/Si multilayers for extreme ultraviolet lithography by magnetron sputtering deposition with bias assistance.

To explore the potential of achieving low-stress and high-reflectance Mo/Si multilayers deposited by conventional magnetron sputtering with bias assistance, we investigated the effects of varying Ar gas pressure, substrate bias voltage, and a bias-assisted Si ratio on the stress and extreme ultraviolet (EUV) reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on an Si-on-Mo interface, only the final part of the Si layer was deposited with bias assistance. Bias voltage has strong influence on the stress. The compressive stress of Mo/Si multilayers can be reduced remarkably by increasing bias voltage due to the increase of Mo-on-Si interdiffusion and postponement of Mo crystallization transition. Properly choosing gas pressure and a bias-assisted Si ratio is critical to obtain high EUV reflectance. Appropriately decreasing gas pressure can reduce the interface roughness without increasing interdiffusion. Too much bias assistance can seriously reduce the optical contrast between Mo and Si layers and lead to a remarkable decrease of EUV reflectance. Thus, by appropriately choosing gas pressure, bias voltage, and a bias-assisted Si ratio, the stress values of Mo/Si multilayers can be reduced to the order of -100 MPa with an EUV reflectance loss of about 1%.

Interface morphology of Mo/Si multilayer systems with varying Mo layer thickness studied by EUV diffuse scattering.

We investigate the influence of the Mo-layer thickness on the EUV reflectance of Mo/Si mirrors with a set of unpolished and interface-polished Mo/Si/C multilayer mirrors. The Mo-layer thickness is varied in the range from 1.7 nm to 3.05 nm. We use a novel combination of specular and diffuse intensity measurements to determine the interface roughness throughout the multilayer stack and do not rely on scanning probe measurements at the surface only. The combination of EUV and X-ray reflectivity measurements and near-normal incidence EUV diffuse scattering allows to reconstruct the Mo layer thicknesses and to determine the interface roughness power spectral density. The data analysis is conducted by applying a matrix method for the specular reflection and the distorted-wave Born approximation for diffuse scattering. We introduce the Markov-chain Monte Carlo method into the field in order to determine the respective confidence intervals for all reconstructed parameters. We unambiguously detect a threshold thickness for Mo in both sample sets where the specular reflectance goes through a local minimum correlated with a distinct increase in diffuse scatter. We attribute that to the known appearance of an amorphous-to-crystallization transition at a certain thickness threshold which is altered in our sample system by the polishing.

Imaging performance improvement of coherent extreme-ultraviolet scatterometry microscope with high-harmonic-generation extreme-ultraviolet source

In extreme-ultraviolet (EUV) lithography, the development of a review apparatus for the EUV mask pattern at an exposure wavelength of 13.5 nm is required. The EUV mask is composed of an absorber pattern and a Mo/Si multilayer on a glass substrate. This mask pattern has a three-dimensional (3D) structure. The 3D structure would modulate the EUV reflection phase, which would cause focus and pattern shifts. Thus, the review of the EUV phase image is also important. We have developed a coherent EUV scatterometry microscope (CSM), which is a simple microscope without objective optics. The EUV phase and intensity images were reconstructed with diffraction images by ptychography. For a standalone mask review, the high-harmonic-generation (HHG) EUV source was employed. In this study, we updated the sample stage, pump-laser reduction system, and gas-pressure control system to reconstruct the image. As a result, an 88 nm line-and-space pattern and a cross-line pattern were reconstructed. In addition, a particle defect of 2 µm diameter was well reconstructed. This demonstrated the high capability of the standalone CSM, which can hence be used in factories, such as mask shops and semiconductor fabrication plants.

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time ( τ ≲ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature ( T e ≈ 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing.

Structure and extreme ultraviolet performance of Si/C multilayers deposited under different working pressures.

Narrow bandwidth Si/C multilayer mirrors are fabricated and characterized for the Z-pinch plasma diagnostic at a wavelength of 16.5nm. To reduce the large stress of the multilayer and maintain a practical reflectivity, different working pressures, from 0.13Pa to 0.52Pa, are optimized during the deposition. The grazing incidence x-ray reflectometry (GIXR) measurement and the fitting results indicate that an interlayer was formed at the interfaces, while both the interlayer thickness and interface widths increase with larger working pressure. The surface roughness of the multilayers also increases from 0.13nm at 0.13Pa to 0.29nm at 0.52Pa, as revealed by the atomic force microscope (AFM) measurements. The multilayer stress decreases from -682MPa to -384 MPa as the working pressure increases from 0.13Pa to 0.52Pa, respectively. The experimental extreme ultraviolet (EUV) reflectivity of the samples with 20 bilayers gradually decreased from 26.3% to 18.9% with increased working pressure. The bandwidth of the reflection peak remains similar for the different samples with a full width half-maximum (FWHM) value of around 0.87nm. A maximum EUV reflectivity of 33.2% and a bandwidth of 0.64nm were achieved by the sample with 50 bilayers fabricated under a working pressure of 0.13Pa.

In depth study of molybdenum silicon compound formation at buried interfaces

Angle resolved x-ray photoelectron spectroscopy (ARXPS) has been employed to determine non-destructively the in-depth interface formation during thin film growth. Buried interfaces underneath the nanometer thick layers are probed by identifying the chemical shift of compound materials in photoelectron spectroscopy and using the angular response to quantify the compound amounts from the measured intensities. The thin interfaces in molybdenum-silicon multilayers grown at ambient temperature are investigated. This system is an example of an almost perfect 1D-system, where the interface region is only a small part of the individual layer thicknesses of 3 to 5 nm. Despite the low growth temperature, both the interfaces of this multilayer show layer thickness dependent interface formation. While the silicon-on-molybdenum interface shows a limited interface thickness of 0.4 nm of Mo5Si3, the molybdenum-on-silicon interface shows a more complex evolution. For this interface, the composition of the first 2.0 nm of deposited layer thickness is best described as a molybdenum-silicon compound layer with a molybdenum rich top and a MoSi2 bottom layer. After 2.5 nm of the deposited layer thickness, the molybdenum rich compound at the top has transformed into polycrystalline molybdenum on top of 1.8 nm MoSi2 at the interface. The formation of the 1.8 nm MoSi2 precedes the formation of polycrystalline molybdenum on top. Angle resolved x-ray photoelectron spectroscopy (ARXPS) is shown to be a good tool to study the interface phenomena beneath the nanometer thick top layers. In the case of Mo/Si multilayer mirrors, this ARXPS study shows that the compound formation at the interface accounts for the majority of the extreme ultraviolet reflectance loss.

Improving the EUV reflectivity of Mg/SiC multilayers by inserting Zr barrier layers at the SiC-on-Mg interfaces

In Mg/SiC multilayer deposition, the SiC-on-Mg interfaces were found to be much more diffused than the Mg-on-SiC interfaces. By inserting Zr barrier layers at the SiC-on-Mg interfaces, the diffusion at interface can be suppressed. The Mg/SiC multilayers were deposited by magnetron sputtering method, and were characterized by X-ray reflectometry and reflectometer of National Synchrotron Radiation Laboratory of China, respectively. Results show that 0.5-nm-thick Zr barrier layers can dramatically reduce the interdiffusion at the SiC-on-Mg interfaces.

Deposition and characterization of B4C/CeO2 multilayers at 6.x nm extreme ultraviolet wavelengths

New multilayers of boron carbide/cerium dioxide (B4C/CeO2) combination on silicon (Si) substrate are manufactured to represent reflective-optics candidates for future lithography at 6.x nm wavelength. This is one of only a few attempts to make multilayers of this kind. Combination of several innovative experiments enables detailed study of optical properties, structural properties, and interface profiles of the multilayers in order to open up a room for further optimization of the manufacturing process. The interface profile is visualized by high-angle annular dark-field imaging which provides highly sensitive contrast to atomic number. Synchrotron based at-wavelength extreme ultraviolet (EUV) reflectance measurements near the boron (B) absorption edge allow derivation of optical parameters with high sensitivity to local atom interactions. X-ray reflectivity measurements at Cu-Kalpha (8 keV) determine the period of multilayers with high in-depth resolution. By combining these measurements and choosing robust nonlinear curve fitting algorithms, accuracy of the results has been significantly improved. It also enables a comprehensive characterization of multilayers. Interface diffusion is determined to be a major cause for the low reflectivity performance. Optical constants of B4C and CeO2 layers are derived in EUV wavelengths. Besides, optical properties and asymmetric thicknesses of inter-diffusion layers (interlayers) in EUV wavelengths near the boron edge are determined. Finally, ideal reflectivity of the B4C/CeO2 combination is calculated by using optical constants derived from the proposed measurements in order to evaluate the potentiality of the design.

In situ collector cleaning and extreme ultraviolet reflectivity restoration by hydrogen plasma for extreme ultraviolet sources

Laser-produced Sn plasmas used to generate extreme ultraviolet (EUV) light for lithography cause the release of Sn ions and neutrals in the EUV source chamber. These Sn atoms condense and deposit on the multilayer collector optic, which reduces its ability to reflect EUV light. This lowers the source throughput and eventually necessitates downtime for collector cleaning. In this paper, an in situ plasma-based collector cleaning technique is presented and experimentally demonstrated. First, the technique is shown to completely clean a 300 mm diameter stainless steel dummy collector. Second, simulations and secondary ion mass spectroscopy depth profiles show that the technique does not erode the real multilayer mirrors. Finally, EUV reflectivity measurements demonstrate the ability of the technique to restore EUV reflectivity to Sn-coated multilayer mirrors. This technique has the potential to be used in conjunction with source operation, eliminating cleaning-related source downtime.

Interface characterization of Mo/Si multilayers

Complementary analysis techniques are applied in this work to study the interface structure of Mo/Si multilayers. The samples are characterized by grazing incident x-ray reflectivity, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and extreme ultraviolet reflectivity. The results indicate that the layer thickness is controlled well with small diffusion on the interface by forming MoSi2. Considering MoSi2 as the interface composition, simulating the result of our four-layer model fits well with the measured reflectivity curve at 13.5 nm.

Analysis of buried interfaces in multilayer mirrors using grazing incidence extreme ultraviolet reflectometry near resonance edges.

Accurate measurements of optical properties of multilayer (ML) mirrors and chemical compositions of interdiffusion layers are particularly challenging to date. In this work, an innovative and nondestructive experimental characterization method for multilayers is discussed. The method is based on extreme ultraviolet (EUV) reflectivity measurements performed on a wide grazing incidence angular range at an energy near the absorption resonance edge of low-Z elements in the ML components. This experimental method combined with the underlying physical phenomenon of abrupt changes of optical constants near EUV resonance edges enables us to characterize optical and structural properties of multilayers with high sensitivity. A major advantage of the method is to perform detailed quantitative analysis of buried interfaces of multilayer structures in a nondestructive and nonimaging setup. Coatings of Si/Mo multilayers on a Si substrate with period d=16.4 nm, number of bilayers N=25, and different capping structures are investigated. Stoichiometric compositions of Si-on-Mo and Mo-on-Si interface diffusion layers are derived. Effects of surface oxidation reactions and carbon contaminations on the optical constants of capping layers and the impact of neighboring atoms' interactions on optical responses of Si and Mo layers are discussed.