极紫外高透射率自支撑硅薄膜滤片

Freestanding Silicon Thin-Film Filters with High Transmission in Extreme Ultraviolet Range

自支撑薄膜滤片是极紫外波段重要的透射式光学元件。为获得13.5 nm极紫外高透射率滤片,本文选用硅(Si)作为膜层材料,通过脉冲直流磁控溅射在可溶性衬底沉积厚度为50 nm的Si单层膜,并成功制备了自支撑Si薄膜滤片样品。利用X射线反射、扫描电镜、同步辐射装置等测试手段分别对样品膜厚、形貌以及光学性能进行了表征,结果表明,50 nm厚的Si滤片在13.5 nm处透射率达到86.02%。进一步通过检测样品组分,结合IMD软件计算并分析了滤片氧化程度,解释了其在12.5~20 nm波段理论透射率与测量值之间存在差异的原因。该研究成果将极大地拓宽此类高透射率Si滤片在极紫外光学工程领域的应用前景。

Objective Freestanding thin-film filters are important transmissive optical elements for applications in extreme ultraviolet(EUV)bands.Silicon(Si)has the L2,3 absorption edge at the wavelength of 13 nm,providing high transmission atλ=13.5 nm.Therefore,it has been employed as one of the filtering materials in EUV lithography.Previously,Si is mostly adopted as an interlayer to form a multilayer structure with metallic materials,or attached to a nickel mesh to form a gridsupporting structure.However,till now there has been no thorough investigation on self-supporting thin-film filters conducted by sputtering a single-component Si material.To promote the application of Si-based freestanding filters in the EUV field and bridge such a gap in domestic research,we designed and fabricated a 50 nm-thin freestanding Si filter with high transmission at 13.5 nm.Methods The Si thin film was deposited on soluble or quartz substrates by pulsed direct current(DC)magnetron sputtering,and upon fabrication and gluing for encapsulation,a 50 nm-thickness filter sample with a flat surface is shown in Fig.1.Then,the film thickness and morphology were characterized by X-ray reflectivity(XRR)and field emission scanning electron microscopy(FE-SEM).The EUV transmission spectrum measurements were performed at the National Synchrotron Radiation Laboratory(NSRL).Furthermore,the difference between the theoretical and measured transmission values of the filter in the 12.5-20 nm band was further analyzed by X-ray photoelectron spectroscopy(XPS)and IMD software calculations.Results and Discussions According to the XRR fitting results shown in Table 1,the measured thickness of the thin film is 50.8 nm with a thin SiO2 layer of 1.9 nm.Figure 2(b)presents the cross-section SEM image of the Si filter,indicating the filter thickness around 50.26 nm is consistent with the XRR fitting results.Then,a sandwich model of"SiO2/Si/SiO2"was built in IMD to calculate the EUV transmission spectra.Figure 3 shows the measured transmission values and theoretical calculation("cal.1")values in the 10-20 nm band for the filter sample,demonstrating that the measured transmission value reaches 86.02%at 13.5 nm,with an obvious difference(ΔT%)between the two curves in the 12.5-20 nm region.To explain this phenomenon,we examined the sample's composition by XPS,as shown in Fig.4.A 5 nmthin SiOx is the majority compound at the surface,and there is a certain level of"bulk oxidation"according to the deep etching results in Fig.4(b).With such optimization of the sandwich model from"SiO2/Si/SiO2"to"SiOx/SiOy/SiOx"based on these XPS results,in Table 3 and Fig.5,theΔT%is decreased from 2.62%to 0.18%and the two curves coincide much better.Conclusions To obtain a highly transmissive EUV filter at 13.5 nm,we prepared a freestanding Si filter(50 nm-thin)with its transmission as high as 86.02%at 13.5 nm,combined with decent suppression in the deep ultraviolet(DUV)range.Meanwhile,the XPS results and the optimized IMD calculation model show that both the surface and bulk oxidation levels of the filters exert a significant influence on its EUV transmission,which is a direction that needs further research efforts.Our results will substantially expand the further applications of such ultra-thin Si filters to areas such as EUV lithography and other large-scale scientific facilities in short wavelengths.