Extreme Ultraviolet Optics Research Articles

Examining the influence of W thickness on the Si-on-W Interface: A comparative metrology analysis

W/Si thin-film multilayer structures are used in various applications such as X-ray, neutron, and extreme ultraviolet optics. The interfaces between the films play such a fundamental role in the performance of these structures that a sub-nanometer and non-destructive characterization of such interfaces is necessary, albeit challenging. In this study, we investigate the interface Si-on-W and the effect of W thickness on such an interface using low energy ion scattering (LEIS), X-ray reflectivity (XRR), and transmission electron microscopy (TEM). We extract the Si-to-W error-function-like compositional change to quantitatively compare the effective interface width measured by the different techniques. We demonstrate that, in the case of Si-on-W, the effective interface width measured by LEIS and XRR agrees with the values extracted from TEM analysis within a 0.1 nm error margin. Noting that TEM is a destructive method, these results exemplify the value of LEIS and XRR as analysis techniques for resolving thin film interfaces. Surprisingly, all techniques employed in the study show that a structure with 20 nm of W has a sharper Si-on-W interface compared to a structure with 4 nm of W, which we interpret as the effect of both the correlated roughness from the substrate − present in the case of 4 nm W − and the larger crystals − present in the W film in the case of 20 nm W- resulting in less intermixing. This directly shows the value of extracting exact interface widths for the analysis and understanding of thin film growth in multilayer systems.

Ultrasmooth Ti/Al multilayer with a Ti seed layer for EUV applications

Al-base multilayers have attracted much interest in the extreme ultraviolet (EUV) optics field, but high roughness of this multilayer due to the Al film is still a big concern. Here, a strategy of the seed layer was proposed to reduce the surface roughness and intermixing layer thickness of the Al-base multilayer. Ti film is not only a seed layer, but also an absorption layer in this novel multilayer. An optimized Ti/Al multilayer film structure was designed to work at 21.1 nm, while investigating the use of Ti as a seed layer to reduce the roughness and enhance the peak reflectivity. The experimental results showed that the Ti seed layer effectively reduced the surface roughness and intermixing layer thickness and improved the reflectance. At 21.1 nm, the peak reflectance reached 39.6%, with a bandwidth of only 1.0 nm and an RMS roughness of 0.17 nm. Ti/Al multilayer also exhibits good stability. This multilayer has potential application in high-precision optics, such as corona detection, which requires extreme low light scattering of multilayer mirror.

Recent developments in photoresists for extreme-ultraviolet lithography

This report describes recent developments and current needs in the field of high-resolution photopolymers and photomolecules briefly describing prior generation lithographic patterning materials. It subsequently concentrates on recent advances in both inorganic and polymeric materials for extreme ultraviolet (EUV) lithography. It is also part of a series of papers written in celebration of the centenary of the Polymeric Materials: Science and Engineering (PMSE) division of the American Chemical Society (ACS). PMSE has long been home to polymer chemists who have made important contributions to advances in semiconductor manufacturing as a result of PMSE's focus on polymer coatings research. While EUV lithography has been an area of research for several decades, only within the last 5 years has the combination of new light sources, EUV optics, tool advances and resist discoveries come together to transition this area from the patterning method of the future to a leading-edge manufacturing technology. (148 words).

Technical and personal remembrances of David A. Shirley in studies of surface magnetism, photoelectron spectroscopy, EUV lithography, and hydrogen storage

This article describes the influence of Professor David A. Shirley on the research science of one of his Ph.D. students in the diverse areas of surface magnetism, x-ray photoelectron spectroscopy (XPS), spin-resolved XPS (SRXPS), extreme ultraviolet (EUV) lithography, and hydrogen storage materials science. Examples are given from the author's work on Cr(001) surface magnetism, XPS, and SRXPS studies of multiplet-splitting in core-level photoemission from Fe. In addition, Dave's influence in understanding the radiation-induced deposition of carbon on EUV optics is described, along with the use of XPS in deciphering how hydrogen storage materials are modified by repeated hydrogen adsorption/desorption cycling. The current status of these particular topics is briefly summarized. These technical remembrances are combined with some fond personal stories about Dave, in recognition of his passing on March 29, 2021.

Low-Energy Plasma Source for Clean Vacuum Environments: EUV Lithography and Optical Mirrors Cleaning

Plasma cleaning of extreme ultra-violet (EUV) optics for the semiconductor industry requires atomic-level precision. Low-energy ions and neutrals can be highly beneficial for this purpose. However, ion energies in many industrial capacitively or inductively coupled plasmas may be too high for atomic precision processing so that ions can cause sputtering and re-deposition of materials and produce vacuum system contamination. We discuss two sources that create low-energy ions: a capacitively coupled (CCP) plasma source operating at several tens of MHz and an electron beam gas ionization source. The goal here is to minimize the contamination by limiting the ion impact energy to a few tens of electron-volts. Ion energy and flux measurements on a grounded surface are characterized with a compact retarding field ion spectrometer. Plasma-induced contamination is quantified using X-ray photo-electron spectroscopy (XPS). Low ion energy plasma sources introducing little surface contamination may be interesting for cleaning and accelerated testing in EUV lithography (EUVL)-related research and for cleaning of front-end optical mirrors in fusion reactor diagnostics.

Design of efficient broadband extreme ultraviolet multilayer mirrors using a two-parametric merit function

Broadband reflection is a basic need in extreme ultraviolet (EUV) optics, especially for the control of a broadband source such as attosecond pulse. In this paper, we propose a design method of broadband EUV multilayer mirrors based on genetic algorithm with a two-parametric merit function. Compared with a traditional merit function, which generally employs the root-mean-square deviation of the calculated reflectivity from the target reflectivity, the proposed two-parametric merit function consisting of mean value and mean square deviation of the reflectivity can realize higher mean value and lower mean square deviation of reflectivity in the desired band, without setting the target reflectivity in advance. Using this two-parametric merit function, a broadband EUV mirror with 20.35% mean value and 0.5% mean square deviation of reflectivity is realized in the wavelength range from 60 eV to 80 eV by optimizing an aperiodic Mo/Si multilayer. In addition, the broadband EUV mirrors for different bands are designed, and their performances of broadband reflection are analyzed and compared. Our research provides a way of designing an efficient broadband multilayer with absorbing materials, and could improve the study on the control of broadband EUV sources.

Synthesis and structural analysis of Mo/B periodical multilayer X-ray mirrors for beyond extreme ultraviolet optics

The synthesis and structural analysis of Mo/B periodical multilayer X-ray mirrors (PMMs) for beyond extreme ultraviolet (BEUV) optics was performed. The PMMs were deposited by a combination of pulsed DC and radio frequency (RF) magnetron sputtering. The structure was analyzed by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and grazing incidence X-ray reflectometry. The formation of 0.35 nm-thick interlayers comprised of a mixture of molybdenum borides was observed at the Mo/B interfaces. Furthermore, a low interface roughness of 0.3–0.4 nm was reported. The temperature of the substrate increased due to the increase in the sputtering power. This resulted in an increase in the thickness of the interlayers and the interface roughness; subsequently, the optical properties of the PMM deteriorated. Theoretical calculations were performed based on the real structure of the PMM to predict the reflectivity at a working wavelength of 6.7 nm. The reflectivity of approximately 53% was two times higher than that of the conventional B4C-based BEUV mirrors. Based on our study results, it can be concluded that the as-synthesized PMMs will perform better than the traditional mirrors and can be effectively used for the development of the next generation of BEUV lithography.

High numerical aperture Hartmann wave front sensor for extreme ultraviolet spectral range.

We present a novel, to the best of our knowledge, Hartmann wave front sensor for extreme ultraviolet (EUV) spectral range with a numerical aperture (NA) of 0.15. The sensor has been calibrated using an EUV radiation source based on gas high harmonic generation. The calibration, together with simulation results, shows an accuracy beyond λ/39 root mean square (rms) at λ=32nm. The sensor is suitable for wave front measurement in the 10 nm to 45 nm spectral regime. This compact wave front sensor is high-vacuum compatible and designed for in situ operations, allowing wide applications for up-to-date EUV sources or high-NA EUV optics.

Divergence Control of High-Harmonic Generation

We show that the divergence of extreme ultraviolet pulses from high harmonic generation, which is directly linked to the shape and size of the refocused beam, can be controlled by the relative delay between the fundamental and its intense orthogonally polarized second harmonic in two-color high-harmonic generation. We find that the divergence is minimized close to the delays were the number of emitted photons is maximized. These findings are rationalized as suppression and enhancement of long and short electron trajectories as function of the phase of the two-color laser field, respectively. The orthogonally polarized second harmonic introduces a lateral momentum component that can select one trajectory whereas it deflects the other. At the same time, the second harmonic profoundly influences the tunnel ionization process that initiates high-harmonic generation, which provides another trajectory gate. Our scheme for controlling the divergence facilitates larger numerical apertures for extreme ultraviolet microscopy. In addition, the associated reduction of the size of the focus is beneficial for extreme ultraviolet nonlinear optics and spectroscopy, as well as imaging and metrology of embedded structures.

Nanoscale piezoelectric surface modulation for adaptive extreme ultraviolet and soft x-ray optics.

Extreme ultraviolet and soft x-ray wavelengths have ever-increasing applications in photolithography, imaging, and spectroscopy. Adaptive schemes for wavefront correction at such a short wavelength range have recently gained much attention. In this Letter we report, to the best of our knowledge, the first demonstration of a functional actuator based on piezoelectric thin films. We introduce a new approach that allows producing a gradually varying surface deformation. White light interferometery is used to show the level of control in generating arbitrary surface profiles at the nanoscale.

Single-walled carbon nanotube membranes for optical applications in the extreme ultraviolet range

In this paper, we explore the possibility of using free-standing thin films from single-walled carbon nanotube (SWCNT) material in optics of the extreme ultraviolet (EUV) range. Test samples were fabricated using an aerosol chemical vapor deposition method. Synchrotron radiation was used to record the transmittance spectra of samples in the EUV range. The measured transmittance for a film 40 nm thick almost monotonously increases from 76% at a wavelength of 20 nm–99% at a wavelength of 1 nm. The measured stress-strain curve for the test samples shows that the SWCNT-based thin films have rather high ductility as opposite to fragile films made of conventional solid state materials. We use numerical simulations to demonstrate that the film strain occurs mainly by straightening and sliding of the nanotubes past each other without forming of strain localization responsible for fragile behavior. The combination of high radiation transmittance and unique mechanical properties makes the SWCNT-based thin films very promising for use in the EUV optics. In particular, such films can be used to protect delicate optical elements for EUV lithography from their contamination with debris particles.

Extreme UV secondary electron yield measurements of Ru, Sn, and Hf oxide thin films

Background: The secondary electron yield (SEY) of materials is important for topics as nanoparticle photoresists and extreme ultraviolet (EUV) optics contamination. Aim: Experimentally measure SEY and secondary electron energy distributions for Ru, Sn, and Hf oxide. Approach: The SEY and energy distribution resulting from 65 to 112 eV EUV radiation are measured for thin-film oxides or films with native oxide. Results: The total SEY can be explained by EUV absorption in the topmost nanometer of (native) oxide of the investigated materials. Conclusions: Although the relative SEY of Ru and Sn is well-explained by the difference in EUV absorption properties, the SEY of HfO2 is almost a factor 2 higher than expected. Based on the energy distribution of secondary electrons, this may be related to a lower barrier for secondary electron emission.

Predicting radiation-induced carbon contamination of EUV optics

Predictions are made for the radiation-induced carbon contamination threat to ruthenium-coated extreme ultraviolet (EUV) optics for a range of incident EUV intensities, exposure pressures and types of hydrocarbon. A calculational philosophy is developed that acknowledges the ruthenium capping layer may have adsorbed oxygen on it and that the carbon contamination film is partially hydrogenated. The calculations incorporate the Nitta Multisite Adsorption framework, which accounts for the configurational adsorption difficulty encountered by the adsorption of large molecules on surfaces. Contributions from “out-of-band” radiation are included, both in the direct photon-induced dissociation of hydrocarbon molecules and in the out-of-band production of secondary electrons. For the hydrocarbon molecules, n-tetradecane, n-dodecane, n-decane, and benzene, for a range of EUV powers and hydrocarbon pressures, predictions are made for carbon thicknesses, the overall carbon deposition rates, and the relative amounts of contamination produced by primary photon excitation, secondary electrons, and out-of-band radiation. The comparison is made to relevant prior experiments. The model, with no adjustable parameters, provides a good account of prior experiments on n-tetradecane, n-decane, and benzene over the pressure ranges examined by the experiments (∼1 × 10−10 to ∼1 × 10−7 Torr) and over the EUV intensity range 0.001–100 mW/mm2. The level of agreement is within a factor of ∼4 or better, which is consistent with expectations based on the experimental uncertainties. Comparison with prior data for n-decane indicates that the carbon deposit produced by the EUV-induced dissociation of hydrocarbons is substantially hydrogenated. Out-of-band radiation accounts for ∼9%–12% of the overall optic contamination. Secondary electrons account for ∼2% of the overall optic contamination. The results show that the dominant mechanistic cause of the EUV carbon contamination is primary photon absorption by the adsorbed hydrocarbon molecule. The removal of carbon or hydrogen by electron stimulated desorption due to secondary electrons or photon stimulated desorption by primary EUV absorption can be safely ignored as negligible compared to the EUV-induced carbon deposition rate. The results allow comparison with past experiments, provide a framework for conducting future experiments, and predict contamination threats relevant for practical EUV lithography tool operation. The calculations also clarify the underlying physical phenomena at work in the EUV carbon contamination problem.

Tin, The Enabler-Hydrogen Diffusion into Ruthenium.

Hydrogen interaction with ruthenium is of particular importance for the ruthenium-capped multilayer reflectors used in extreme ultraviolet (EUV) lithography. Hydrogen causes blistering, which leads to a loss of reflectivity. This problem is aggravated by tin. This study aims to uncover the mechanism via which tin affects the hydrogen uptake, with a view to mitigation. We report here the results of a study of hydrogen interaction with the ruthenium surface in the presence of tin using Density Functional Theory and charge density analyses. Our calculations show a significant drop in the energy barrier to hydrogen penetration when a tin atom or a tin hydride molecule (SnHx) is adsorbed on the ruthenium surface; the barrier has been found to drop in all tested cases with tin, from 1.06 eV to as low as 0.28 eV in the case of stannane (SnH4). Analyses show that, due to charge transfer from the less electronegative tin to hydrogen and ruthenium, charge accumulates around the diffusing hydrogen atom and near the ruthenium surface atoms. The reduced atomic volume of hydrogen, together with the effect of electron–electron repulsion from the ruthenium surface charge, facilitates subsurface penetration. Understanding the nature of tin’s influence on hydrogen penetration will guide efforts to mitigate blistering damage of EUV optics. It also holds great interest for applications where hydrogen penetration is desirable, such as hydrogen storage.

A precision polishing method for Wolter-I type optical mandrel

In the application of the 13.5-nm extreme ultraviolet (EUV) collector optics of the discharge produced plasma (DPP) source, the Wolter-I structure based on the X-ray grazing incidence optical design is usually adopted. The requirement of the surface roughness of this collector optics is demanding, which should reach to several nanometers. Currently, the mandrel replication is the main fabrication technology of this collector optics. Obviously, the requirement of the surface roughness of the mandrel used for the replication process should reach to the same level of the collector optics. However, the fabrication of the Wolter-I mandrel with high surface quality requirement still faces great challenge. In this paper, we propose a method that applies an elastic spherical tool to polish the surface of the Wolter-I mandrel, in which the shape of the elastic sphere can be changed relatively easily to accommodate the aspherical surface of the Wolter-I mandrel. A mathematical model regarding the movement trace of the polishing tool was established. Based on this model, the contact force between the polishing sphere and the aspherical surface can be controlled to point from the center of the polishing sphere in the normal direction to the polishing point. The influence of the polishing parameters on the material removal thicknesses is simulated and analyzed. The polishing experiments of ellipsoid and hyperboloid are carried out considering the difference of the diameter of each radial section on the Wolter-I mandrel surface. The proposed method is proved to reduce the surface roughness of the mandrel coated with chemically deposited nickel-phosphorous (NiP) alloy after diamond turning effectively. The root mean square (RMS) roughness of the mandrel surface after polishing can be reduced to 1.56 nm. It indicates that this method is suitable for further polishing of nickel-plated Wolter-I mandrel after ultra-precision turning. As a result, the manufacturing precision and efficiency of the Wolter-I mandrels can be improved by using the proposed polishing approach in this study, and the manufacturing cost can be reduced accordingly.

Spectral tailoring of nanoscale EUV and soft x-ray multilayer optics

Extreme ultraviolet and soft X-ray (XUV) multilayer optics have experienced significant development over the past few years, particularly on controlling the spectral characteristics of light for advanced applications like EUV photolithography, space observation, and accelerator- or lab-based XUV experiments. Both planar and three dimensional multilayer structures have been developed to tailor the spectral response in a wide wavelength range. For the planar multilayer optics, different layered schemes are explored. Stacks of periodic multilayers and capping layers are demonstrated to achieve multi-channel reflection or suppression of the reflective properties. Aperiodic multilayer structures enable broadband reflection both in angles and wavelengths, with the possibility of polarization control. The broad wavelength band multilayer is also used to shape attosecond pulses for the study of ultrafast phenomena. Narrowband multilayer monochromators are delivered to bridge the resolution gap between crystals and regular multilayers. High spectral purity multilayers with innovated anti-reflection structures are shown to select spectrally clean XUV radiation from broadband X-ray sources, especially the plasma sources for EUV lithography. Significant progress is also made in the three dimensional multilayer optics, i.e., combining micro- and nanostructures with multilayers, in order to provide new freedom to tune the spectral response. Several kinds of multilayer gratings, including multilayer coated gratings, sliced multilayer gratings, and lamellar multilayer gratings are being pursued for high resolution and high efficiency XUV spectrometers/monochromators, with their advantages and disadvantages, respectively. Multilayer diffraction optics are also developed for spectral purity enhancement. New structures like gratings, zone plates, and pyramids that obtain full suppression of the unwanted radiation and high XUV reflectance are reviewed. Based on the present achievement of the spectral tailoring multilayer optics, the remaining challenges and opportunities for future researches are discussed.

Ion-beam polishing of fused silica substrates for imaging soft x-ray and extreme ultraviolet optics.

We have studied the surface treatment of polished fused silica by neutralized Ar ions with energy of 500-1500 eV and incidence angles of 0-90°. We found the following regularities: for samples that passed the standard procedure of deep polishing (initial effective roughness σ(eff)∼0.5 nm), the effective roughness decreases to the ultrasmooth level (i.e., σ(eff)∼0.25 nm in the range of spatial frequencies q∈[4.9×10(-2)-63] μm(-1)). The effect begins to be noticeable at the material removal of 150 nm and reaches saturation at depths of removal greater than 1 μm. For supersmooth samples (σ(eff)<0.3 nm), the effective roughness keeps the initial level at material removal down to tens of micrometers. The optimal ion energy range is 800-1300 eV (maximum smoothing effect); at higher energy some surface roughness degradation is observed. All the smoothing effects are observed at the incidence angle range θ(in)=0-35°. Increasing the ion energy above 1300 eV increases the etching rate by up to 4 μm per hour (J(ion)=0.8 mA/cm2), which allows for deep aspherization of sized substrates. The technique allows for manufacturing the optical elements for extreme ultraviolet and soft x-ray wavelength ranges with a numerical aperture of up to 0.6.

Variation in phase defect size on extreme ultraviolet mask before and after reflective multilayer coating

It is very difficult to predict how multilayer defects known as “phase defects” impact on wafer printed image when embedded in an extreme ultraviolet (EUV) mask. Therefore, researchers have reported many techniques to analyze and characterize phase defects using scanning probe microscopes (SPMs) and phase defect inspection tools that employ deep ultraviolet or EUV optics. To characterize the phase defects using SPM or other inspection tools, preparing and employing a programmed phase defect mask is a practical way to address the task because the locations, sizes, and quantity of phase defects can be defined to fit experiments. For this study, a programmed phase defect mask was prepared to investigate the size uniformity of the programmed phase defects. The designed phase defects were holes 40-, 50-, 70-, or 80-nm-wide and 4.5-nm-deep. Using a SPM, the phase defects were measured for their depths and widths before and after coating with the multilayer. As a result, variations in the measured depths and widths were much smaller than the defect-to-defect variations, reflecting measurement repeatability. In addition, the variations in the depths and widths of the phase defects after multilayer coating were larger than before coating. It was also found that a given group of phase defects exhibited significant variations in depth and width after multilayer coating, even if they were the same prior to coating. This result indicated that it is difficult to estimate precoating phase defect sizes based on measurements after the multilayer is coated.

Interferometric broadband Fourier spectroscopy with a partially coherent gas-discharge extreme ultraviolet light source

Extreme ultraviolet (EUV) spectroscopy is a powerful tool for studying fundamental processes in plasmas as well as for spectral characterization of EUV light sources and EUV optics. However, a simultaneous measurement covering a broadband spectral range is difficult to realize. Here, we propose a method for interferometric broadband Fourier spectroscopy connecting soft x ray and visible spectral ranges with moderate spectral resolution. We present an analytical model to recover the spectrum from a double-slit interferogram. We apply our model for spectral characterization of a partially coherent gas-discharge EUV light source operated with different gases in the spectral range between 10 and 110 nm wavelengths. Our approach allows a simple and fast broadband spectroscopy with fully or partially spatially coherent light sources, for instance, to characterize out-of-band radiation in EUV lithography applications.

Phosphorus-based compounds for EUV multilayer optics materials

We have evaluated the prospects of phosphorus-based compounds in extreme ultraviolet multilayer optics. Boron phosphide (BP) is suggested to be used as a spacer material in reflective multilayer optics operating just above the L-photoabsorption edge of P (λ ≈9.2 nm). Mo, Ag, Ru, Rh, and Pd were considered for applications as reflector materials. Our calculations for multilayer structures with perfect interfaces show that the Pd/BP material combination suggests the highest reflectivity values, exceeding 70% within the 9.2 – 10.0 nm spectral range. We also discuss the potential of fabrication of BP-based multilayer structures for optical applications in the extreme ultraviolet range