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°. 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.

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.

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.

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° (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.