Objective Currently, the selection of a working wavelength around 6.X nm is considered a leading candidate for optical lithography of the next generation, known as the beyond extreme ultraviolet (BEUV) lithography. The selection of the 6.X nm wavelength can be determined by two key considerations. First, this wavelength lies near the boron K-absorption edge, enabling high reflectivity in boron-containing multilayer mirrors. Specifically, theoretical models predict that La/B and La/B4C multilayer structures can achieve up to 75% reflectivity at normal incidence, comparable to the performance of conventional Mo/Si optics at 13.5 nm. Second, experimental studies have demonstrated that the reflectivity of La/B4C multilayer mirrors is more than 40%, hinting towards their potential in practical applications. The periodic thickness of La/B4C multilayers for the BEUV band is about 3.4 nm, which is around half that of the Mo/Si multilayers. The number of periods required in La/B4C multilayers is about 300, about five times greater than that of Mo/Si multilayers. These differences render the La/B4C multilayers much more challenging in terms of deposition processes and interface optimization techniques. Consequently, more researchers have been carrying out interface studies of La/B4C multilayers. Previous studies have indicated that the interface width of B4C-on-La is more than twice that of La-on-B4C. It is suggested that reducing the interface width of B4C-on-La is the key factor in enhancing reflectivity. The interface width of B4C-on-La is reduced from 1.5 nm to 1.2 nm by using LaN instead of La with nitrogen reactive sputtering. The LaN/B4C multilayers achieve a reflectivity of 58.1% at a central wavelength of 6.65 nm, which is 7% higher than that of La/B4C multilayers, a step forward for the BEUV multilayers. However, the degradation of the LaN layer in the La/B4C-based multilayers remains unsolved currently, hindering its further applications. In this work, a method of inserting an ultra-thin carbon interfacial barrier layer at the B4C-on-La interface is introduced to improve the interface quality of La/B4C multilayers and enhance their reflectivity. The optimized La/C/B4C multilayers achieve a reflectance of 60% at 6.65 nm under an incident angle of 12.5 degrees. This interface engineering strategy provides significant advances and further guidance for the development of 6.X nm and/or X-ray multilayers, fulfilling the requirements for BEUV multilayers used in the lithography of the next generation at the operating wavelength around 6.X nm. Methods The La/B4C multilayer samples are deposited on silicon wafer substrates by using pulsed direct current (DC) magnetron sputtering. The substrate roughness is around 0.15 nm, and the periodic thickness of all samples is about 3.42 nm. The interface structures are characterized by using X-ray reflectivity (XRR) and high-resolution transmission electron microscope (HRTEM) images. The EUV reflectance spectra are measured at the National Synchrotron Radiation Laboratory (NSRL). Furthermore, the measured reflectivity of the La/B4C and La/C/B4C multilayers with 100 periods in the 6.5-6.7 nm band is compared with the calculated results using the XRR fitting parameters. Results and Discussions According to the XRR results shown in Fig. 1 and Fig. 2, the measured periodic thickness of all samples is about 3.42 nm. The results indicate that the La/C/B4C interface structure exhibits the best performance, as confirmed by both XRR and HRTEM analyses (Fig. 2 & Fig. 4). The BEUV reflectivity of the La/C/B4C multilayer with 100 periods is about 25%, which is 7% higher than that of the La/B4C multilayer. This is attributed to the carbon layer preventing the direct contact of La with B4C. The theoretical calculations using the XRR fitting parameters are in good agreement with the measured results of reflectivity curves, as shown in Fig. 3. Therefore, the width of the transition region between the two primary materials in the multilayers should not exceed the thickness of the inserted carbon layer. This reduction in the transition region width substantially compensates for the increased absorption caused by the addition of the (extra) carbon in the La/C/B4C multilayers, eventually leading to the overall increase of their BEUV reflectivity. Conclusions A high-reflectivity BEUV mirror is successfully prepared by inserting a single interfacial barrier layer of 0.2 nm carbon at the B4C-on-La interface. The La/C/B4C multilayer mirror (with 250 periods) is measured at the NSRL and achieves a reflectance of 60.0% at 6.65 nm under an incident angle of 12.5 degrees (Fig. 5). This result addresses the gap in the high-reflectivity La/B4C multilayers in Chinese research and plays a significant role in advancing the application of BEUV multilayer mirrors for the lithography of the next generation. It substantially expands the potential applications of high-reflectivity mirrors in areas such as BEUV lithography and other X-ray scientific facilities. Next, our research will focus on the reflectivity enhancement, large-diameter mirror preparation techniques, stability evaluations, and validations for further practical applications.

Periodic Mo/Si/C multilayers were fabricated on inclined substrates using magnetron sputtering deposition, either without applying a substrate bias voltage or with various bias voltages applied. Their surface and interface roughness, surface power spectral densities, multilayer structures, grazing-incidence X-ray reflectivity, and extreme ultraviolet reflectivity (EUVR) were characterized. The layer thickness and density were determined by fitting the reflectance spectrum. Overall, the multilayers deposited under a substrate bias voltage demonstrated lower surface and interface roughness and higher layer density, both of which contributed positively to the EUVR. However, applying a substrate bias voltage also increased interface diffusion, particularly at bias voltages exceeding -50 V, which adversely affected the EUVR. By selecting an appropriate substrate bias voltage, the formation of ripples in multilayers deposited on inclined substrates can be mitigated, resulting in reduced surface and interface roughness, increased layer density, and ultimately higher EUVR. A reflectivity of 67.2 % was achieved on a substrate with an inclination angle of 39 degrees using a 60-period Mo/Si/C multilayer structure.

Freestanding Zr filters are important devices for improving spectral purity in the extreme ultraviolet range of 7-20 nm, and their irradiation resistance directly determines their life and efficiency. We prepared multilayered Zr/B4C and Zr/Si filters using magnetron sputtering. Their transmittance reached a maximum of 23% (lambda =13.5 nm). Microwatt-radiation-induced structural changes in the filters were investigated at the metrology beamline (BL08B) of the National Synchrotron Radiation Laboratory. The aging of the Zr filters was measured and analyzed. The experimental results revealed that the damage was noticeable on the irradiated filter surfaces with different states, suggesting that the main factors causing the degradation of the filters were oxidation and carbon contamination at the surfaces. Furthermore, the thermal stability of the Zr filters was studied by annealing, and the heat accumulation during the damage process was estimated using finite-element numerical simulations and X-ray photoelectron spectroscopy measurements. Silicide formation at the Zr-Si-O system interfaces was found to be key to enhancing the stability of the filters.

Objective Mo/Si multilayer films exhibit the highest measured reflectivity in the extreme ultraviolet (EUV) region, and their combination with an EUV light source enables EUV lithography. In practical applications of EUV light sources, Mo/Si multilayer mirrors are always curved and have large diameters. The angle of the incident light constantly changes along the curved surface. To match the multilayer peak reflectivity with the angle of the incident light, the period thickness of the Mo/Si multilayer films must be distributed in a transverse gradient along the surface to ensure high EUV reflectivity. Simultaneously, given that the mirror is close to the light source, the multilayer films must operate in an environment with a high thermal load. Higher temperatures can increase the formation of silicide at the multilayer interface, causing the optical performance to decrease. Therefore, Mo/Si multilayer films for EUV light sources also have high thermal stability requirements. To address these issues, we use a shadow mask to correct the periodic thicknesses of the multilayer films at different positions on a curved substrate with a diameter of 300 mm. Carbon is selected as the diffusion barrier material to investigate the influence of C-barrier layer on the thermal stability of Mo/Si multilayer. Methods In this study, two sets of Mo/Si multilayer films are deposited via direct current(DC) magnetron sputtering onto super-polished silicon wafers, and the thickness control and thermal stability of the Mo/Si multilayer films are investigated separately. For the study of thickness control, the target period thickness is from 6.96 nm to 7.31 nm, and the ratio of Mo layer thickness to period thickness is approximately 0.40. As the multilayer thickness on a large curved substrate cannot be measured directly, we prepare a substituted substrate to estimate the multilayer thickness at selected points on the surface. The shadow mask technique is used to adjust the periodic thickness of the multilayer films at different positions on the entire mirror. C is selected as the barrier material for the thermal stability study. Mo/Si, Mo/Si/C, and Mo/C/Si/C multilayer films are annealed at 300 degrees C for 2 h. By observing the X-ray reflectivity (XRR) and EUV reflectivity before and after annealing, the effect of the C barrier layer on the thermal stability of Mo/Si multilayer films is investigated. Results and Discussions In the study involving film thickness control, the XRR measurement results show that the samples at different positions exhibit similar layer structures (Fig.4). Atom force microscope (AFM) tests are performed on Mo/Si multilayer films deposited at four different positions on the entire mirror. The surface roughness values of the four samples are 0.128,0.123,0.124, and 0.118 nm. The morphologies of the four samples are similar (Fig.5). Using the shadow mask, the deviation of the period thickness on the 300-mm diameter curved substrate is controlled within +/- 0.45% of the expected period thickness (Fig.6).In the study involving thermal stability, after annealing at 300 degrees C for 2h, the period thickness of Mo/Si multilayer films changes from 6.99 nm to 6.69 nm, the period thickness of Mo/Si/C multilayer films changes from 6.96 nm to 6.91 nm, and the period thickness of Mo/C/Si/C multilayer films is almost same before and after the annealing, which changes from 6.97 nm to 7.00 nm. The C barrier layer can effectively mitigate the interdiffusion at the interface of the Mo and Si layers, which improves the thermal stability of the multilayer films (Fig.7). The EUV reflectivity of Mo/Si multilayer films decreases from 64.4% to 55.4% after annealing at 300 degrees C for 2 h, and the central wavelength has a shift of 0.51 nm. The EUV reflectivity of the Mo/Si/C multilayer films decreases from 66.4% to 59.6% after annealing, and the center wavelength shifts by 0.11 nm. The reflectivity of Mo/C/Si/C multilayer films decreases by 1.8% after annealing, and central wavelength shifts by 0.02 nm (Fig.8 and Table 2). Furthermore, the EUV reflectivity results show that the C barrier inserted at both interfaces of the Mo/Si multilayer films can significantly improve thermal stability. Conclusions Initially, graded Mo/Si multilayer films are deposited on a large-diameter curved substrate using a shadow mask.Compared with the designed period thickness, the deviation in the period thickness at different positions on the entire substrate is controlled within +/- 0.45%. The layer structure and surface roughness of the Mo/Si multilayer films are almost identical at different positions. This study provides useful guidance for the fabrication of large curved multilayer mirrors for EUV light sources. Next, the thermal stability of the Mo/Si multilayer films is investigated after inserting a C-barrier layer. The results show that the thermal stability of the Mo/C/Si/C multilayer films is optimal, and that of the Mo/Si multilayer films is the worst. The Mo/C/Si/C multilayer films exhibit only 1.8% reflectivity loss after annealing at 300 degrees C for 2 h, and the center wavelength and bandwidth do not change. The higher reflectivity and multistability of Mo/Si multilayer films for EUV sources are currently under investigation

Multilayers (MLs) are a good choice for high-harmonic generation (HHG) sources for selecting single-wavelength radiation. MLs working around 30.4 nm with a significantly reduced bandwidth based on high Bragg order reflection have been designed and fabricated. The narrowband MLs were characterized using grazing incidence x-ray reflectometry, extreme ultraviolet reflectivity, and intrinsic stress measurements. The results indicate that Mg/SiC MLs designed with 3rd Bragg order have narrowest bandwidth of 0.71 nm (full width half maximum) with a spectral resolution (lambda/Delta lambda) of 42, and reflectance of 30% under near normal incidence geometry. Based on these results, the simulation of narrowband MLs working at large incident angles demonstrates that 3rd Bragg order ML has a better spectral selectivity compared with 1st Bragg order ML for HHG sources.

Mirror stability is crucial for their applications, especially in the far ultraviolet (FUV) where Al mirrors protected with fluoride are the preferred choice. This study evaluates the effect of storage conditions on the performance of Al/LiF/MgF2 mirrors and compares them with previous investigations of Al/LiF/eMgF(2) mirrors. This study focuses on FUV reflectivity, structural characteristics, and the long-term stability of mirrors stored in various environments. Coatings were deposited on the super-polished silicon wafers using thermal evaporation. Three sets of mirrors were stored in the environment with 20% relative humidity (RH), 40% RH, and 80% RH, respectively. Additionally, three other sets were stored in vacuum, nitrogen (N-2), and oxygen (O-2), respectively, in sealed bags kept in a 40% RH environment. Mirrors stored in a 20% RH environment demonstrated the best performance, exhibiting the smallest decline in reflectivity and a relatively stable surface morphology. Conversely, mirrors stored in 80% RH experienced a significant reduction in reflectivity, accompanied by the most pronounced deterioration in surface morphology. It was concluded that the chemical changes and high roughness of the films contributed to the decrease in the reflectivity.

The Mo/Si multilayer mirror has been widely used in EUV astronomy, lithography, microscopy and other fields because of its high reflectivity at the wavelength around 13.5 nm. During the fabrication of Mo/Si multilayers on large, curved mirrors, shadow mask was a common method to precisely control the period thickness distribution. To investigate the effect of shadow mask on the microstructure of Mo/Si multilayers, we deposited a set of Mo/Si multilayers with and without the shadow mask on a curved substrate with aperture of 200 mm by direct current (DC) magnetron sputtering in this work. Grazing incidence X-ray reflectivity (GIXR), diffuse scattering, atomic force microscope (AFM) and X-ray diffraction (XRD) were used to characterize the multilayer structure and the EUV reflectivity were measured at the National Synchrotron Radiation Laboratory (NSRL) in China. By comparing the results, we found that the layer microstructure including interface width, surface roughness, layer crystallization and the reflectivity were barely affected by the mask and a high accuracy of the layer thickness gradient can be achieved.

To investigate the thermal stability of Mo/Si multilayers with different initial crystallinities of Mo layers, two kinds of Mo/Si multilayers were deposited by DC magnetron sputtering and annealed at 300°C and 400°C. The period thickness compactions of multilayers with crystalized and quasi-amorphous Mo layers were 0.15 nm and 0.30 nm at 300°C, respectively, and the stronger the crystallinity, the lower the extreme ultraviolet reflectivity loss. At 400°C, the period thickness compactions of multilayers with crystalized and quasi-amorphous Mo layers were 1.25 nm and 1.04 nm, respectively. It was shown that multilayers with a crystalized Mo layer had better thermal stability at 300°C but were less stable at 400°C than multilayers with a quasi-amorphous Mo layer. These changes in stability at 300°C and 400°C were due to the significant transition of the crystalline structure. The transition of the crystal structure leads to increased surface roughness, more interdiffusion, and compound formation.

A laboratory-based reflectometer designed for characterizing the reflectivity of optical coatings in 30- to 200-nm wavelength range was recently developed at IPOE. An RF-produced gas-discharge light source is applied to generate characteristic lines. The light source is mounted on a grazing incident monochromator with a 146-deg deviation angle between the incident and diffracted arms. By precisely adjusting the toroidal grating inside the monochromator chamber, monochromatic lights are acquired through the exit slit. A collimator mirror and two sets of collimation slits with 2 mm x 2 mm dimension are utilized for reducing the divergence of the beam incident on the sample. A high-precision six-axis translation stage, which allows a heavy sample with a maximum diameter of 100 mm, is used to control positions of the samples and the detector. A chopper disk used both for incident light intensity monitor and signal modulation is placed with an incidence angle of 70 deg relative to the incident light beam. The configuration, adjustment process, and test results of the reflectometer are presented in detail. The experimental reflectivity results for Al/LiF/MgF2 film obtained from our laboratory and BESSY-II Synchrotron as well as Hefei Synchrotron Light Source are given and compared for demonstrating the reliability of the system. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License.

Quasi-periodic nanostructures induced by ion bombardment (IB) on solid surfaces are characterized by small periods (10-100 nm) and large areas. Quasi-periodic nanoripple structures with the transverse feature size of around 100 nm and the gradually significant transverse periodicity and longitudinal continuity were fabricated on antireflection coatings by Argon-IB. To improve the characterization area, the morphological characteristics of the self-organized nanoripples were characterized by using extreme ultraviolet (EUV) scatterometry. The results show that in terms of samples, their transverse and longitudinal morphological features obtained by the in-plane and conical mode of the EUV scatterometry are in agreement with those obtained by atomic force microscope. These results demonstrate that the proposed method is feasible to characterize the basic morphological characteristics of quasi-periodic nanoripples and can provide a basis for subsequent quantitative analysis. In addition, the characterization area of self-organized nanoripple structures has reached an order of the mm(2) by EUV synchrotron radiation, and the characterization range of the Metrology Beamline of Hefei Light Source is extended to self-organized nanostructures, which can provide a reference for future studies on the scattering characterization of EUV lithography masks.

The far ultraviolet (FUV) spectrum is critical for the observation of phenomena in space, among various other applications. Hence, the aim of our study was to investigate the effect of the humidity of Al mirrors coated with LiF and enhanced MgF2 within the FUV spectrum. The samples were first coated with Al and LiF films on a super-polished silicon wafer at room temperature, heated to 220 degrees C, and then coated with an enhanced MgF2 film. All materials were deposited by thermal evaporation. The samples were stored in environments with 20% relative humidity (RH), 40% RH, 80% RH, and 90% RH. The reflectivity remained stable when the environment was 20% RH, which was the optimal storage environment. The reflectivity of the Al / LiF / eMgF(2) mirrors stored in high RH decreased over time, the roughness increased over time. Furthermore, the decrease rate of reflectivity and increase rate of roughness decreased over time. The degradation of reflectivity can be attributed to the presence of oxygen in environments with a high RH. (c) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)

多层膜反射镜是X射线波段和极紫外波段的重要光学部件.碳化硼作为常见的反射膜材料,其薄膜成分及光学常数计算的准确性对反射镜的反射性能具有明显影响.本研究使用直流磁控溅射技术制备碳化硼薄膜,利用X射线光电子能谱(XPS)、X射线全反射(XRR)、原子力显微镜(AFM)和同步辐射光源等对试样进行了表征,利用改进的拟合函数拟合了基底和薄膜的反射率曲线.结果表明,非晶碳化硼薄膜的元素化学状态相同,其基本成分包含碳化硼和含氧碳化硼;在5～45 nm波段,薄膜B/C摩尔比为4.23时,反射性能最好,同时其基底与薄膜的电子密度差值最大,试样反射性能变化与试样电子密度差值变化基本一致;与原始拟合函数相比,改进的拟合函数提高了薄膜光学常数计算的准确性.

Fe Ⅻ line (19.5 nm) is one of important targets of imaging detection in solar spectrum, and 19.5 nm multilayer mirrors were employed to reflect 19.5 nm line and suppress sidelobe. 19.5 nm multilayer mirrors with a bandwidth of 1.0 nm are in demand in Solar X-EUV imager installed in FengYun Ⅲ satellite. Thickness ratios of Mo layer of 0.15 and 0.11, respectively, are utilized in multilayer mirrors at normal and 45° incidence. The influence of roughness/diffusion on reflectance is discussed, and profile changes of substrate after deposition are also analyzed.

The stability of Cr/C multilayer during irradiation or thermal annealing was investigated using grazing incidence X-ray reflectivity measurement, X-ray photoelectron spectroscopy, X-ray diffraction analysis, small-angle X-ray scattering analysis, and soft X-ray reflectivity measurement. One sample was irradiated with a white beam of synchrotron radiation and five other samples were annealed at various temperatures. The 18-h irradiation treatment caused local surface contaminants but did not affect the buried stacks. The annealing treatment resulted in increased reflectivity at approximately 1.2 keV, and the multilayer remained stable for temperature up to 700 °C. Thus, the Cr/C multilayers exhibited excellent stability during irradiation and thermal treatments and can be used for the mirrors and multilayer gratings of third-generation synchrotron radiation systems.

A fabrication method comprising near-field holography (NFH) with an electron beam lithography (EBL)-written phase mask was developed to fabricate soft X-ray varied-line-spacing gratings (VLSGs). An EBL-written phase mask with an area of 52 mm × 30 mm and a central line density greater than 3000 lines mm-1 was used. The introduction of the EBL-written phase mask substantially simplified the NFH optics for pattern transfer. The characterization of the groove density distribution and diffraction efficiency of the fabricated VLSGs indicates that the EBL-NFH method is feasible and promising for achieving high-accuracy groove density distributions with corresponding image properties. Vertical stray light is suppressed in the soft X-ray spectral range.

High reflectivity of mirrors is very important for many applications in the vacuum ultraviolet, such as for space observation, synchrotron radiation. This paper focuses on the substrate temperature's effect on the performance of Al mirrors when depositing the upper MgF2 layer. Al films are deposited on the substrates at room temperature by thermal evaporation, and a 5 nm MgF2 film is deposited on Al coating at room temperature immediately. Heating the substrate to various temperatures ranging from room temperature to 350 degrees C, then a 20 nm MgF2 film is deposited on the surface of Al/MgF2. The thickness of each layer is characterized using grazing incidence x-ray reflectivity. The reflectivity of sample is measured at the incident angle of 5 degrees in the wavelength range of 105 similar to 130 nm. The reflectivity of all samples fabricated at above room temperature is higher than the sample at room temperature below 115nm. The reflectivity of mirror at 350 degrees C temperature is lower than other mirrors, and the reflectivity of the samples at 300 degrees C and 200 degrees C is similar. There are more black dots on the surface of mirror at 350 degrees C than 300 degrees C, and no black dot on the surface of mirror at 200 degrees C. The measured results using surface profiler show that the black dots are small holes that increase the roughness of mirror and reduce the reflectivity. So the best temperature for depositing the upper MgF2 layer is in 200 similar to 300 degrees C to obtain high reflectivity of Al mirrors in vacuum ultraviolet.

Aluminum mirrors with high reflectivity have a wide range of applications in the vacuum ultraviolet, such as for space observation, synchrotron radiation. Due to the oxidation of Al has great effect on the reflectivity below the wavelength of 160 nm, Al-protected mirrors have experienced significant development over the past years, particularly like Al over-coated with LiF, MgF2. This paper focuses on Al mirror over-coated with LiF, MgF2 and LiF+MgF2 in the wavelength range of 105-130 nm. The mirrors are prepared by three steps thermal boat evaporation method. The reflectivity of mirrors in the wavelength range of 105 similar to 130 nm is measuring at the National Synchrotron Radiation Laboratory (NSRL) of China, and the structural properties have been analyzed using grazing incidence X-ray reflectometer. Compared the reflectivity of samples placed in the relative humidity of 40%, the reflectivity of Al + LiF mirrors has dropped significantly within 2 months, while the reflectivity of Al+MgF2 mirrors almost has no change within 30 months. The reflectivity of Al +LiF+MgF2 mirrors stored in the relative humidity of 40% performs well within 4 months and we will further study the time stability.

The random layer thickness variations can induce a great deformation of the experimental reflection of broadband extreme ultraviolet multilayer. In order to reduce this influence of random layer thickness fluctuations, the multiobjective genetic algorithm has been improved and used in the robust design of multilayer with a broad angular bandpass. The robust multilayer with a lower sensitivity to random thickness errors have been obtained and the corresponding multilayer mirrors were fabricated. The experimental results of robust Mo/Si multilayer with a wide angular band were presented and analyzed, and the advantage of robust multilayer design was demonstrated.

Narrowband Mg/SiC multilayer mirrors working around 30.4 nm with a significantly reduced bandwidth (FWHM ≈ 0.7 nm) basing on high order reflection have been designed and fabricated by direct-current (DC) magnetron sputtering.

极紫外(EUV)宽带多层膜的光谱性能对膜厚控制精度要求较高,仅由时间控制膜厚的镀膜系统难以满足其精度控制要求.本文提出了基于进化算法的宽带EUV多层膜离散化膜系设计方法,与传统膜系设计相比,离散化膜系所制备多层膜具有更为优良的EUV反射光谱性能.为验证离散化膜系设计在宽带EUV多层膜研制中的优越性,采用磁控溅射方法对具有离散化膜系的宽带多层膜反射镜进行了制备和测试.测试结果表明:研制的宽角度多层膜反射镜可实现入射角带宽为0°～17°,高于41％的反射率;研制的堆栈宽角度多层膜反射镜可实现入射角带宽为0°～18.5°,高于35％的反射率;研制的宽光谱多层膜反射镜可实现波长带宽为12.9～14.9 nm,高于21％的反射率.