Abstract
In the first part of this article we experimentally show that contrast between the very thin layers of La and B enables close to theoretical reflectance. The reflectivity at 6.8 nm wavelength was measured from La/B multilayer mirrors with period thicknesses ranging from 3.5 to 7.2 nm at the appropriate angle for constructive interference. The difference between the measured reflectance and the reflectance calculated for a perfect multilayer structure decreases with increasing multilayer period. The reflectance of the multilayer with the largest period approaches the theoretical value, showing that the optical contrast between the very thin layers of these structures allows to experimentally access close to theoretical reflectance. In the second part of the article we discuss the structure of La/B and LaN/B multilayers. This set of multilayers is probed by hard X-rays (λ = 0.154 nm) and EUV radiation (λ = 6.8 nm). The structure is reconstructed based on a simultaneous fit of the grazing incidence hard X-ray reflectivity and the EUV reflectivity curves. The reflectivity analysis of the La/B and LaN/B multilayer mirrors shows that the lower reflectance of La/B mirrors compared to LaN/B mirrors can be explained by the presence of 5% of La atoms in the B layer and 63% of B in La layer. After multi-parametrical optimization of the LaN/B system, including the nitridation of La, the highest near normal incidence reflectivity of 57.3% at 6.6 nm wavelength has been measured from a multilayer mirror, containing 175 bi-layers. This is the highest value reported so far.
Highlights
One of the possible wavelengths for the generation extreme ultraviolet (EUV) lithography is around 6.8 nm [1,2,3,4]
Experiments with large period multilayers for off normal reflectance of 6.8 nm radiation have demonstrated that the material combination of La with B shows sufficient optical contrast to obtain almost theoretical reflectance values if the influence of interface roughness can be reduced
Analysis of short period La/B multilayer structures for normal incidence reflectance of 6.8 nm light has revealed that the multilayer has a low optical contrast that is most likely caused by intermixing between the layers
Summary
One of the possible wavelengths for the generation EUV lithography is around 6.8 nm [1,2,3,4]. The most commonly used multilayer material combinations for normal incidence reflectance at this wavelength, just above the boron Kα absorption edge, are Mo/B4C or La/B4C [1, 5]. Due to their good thermal stability, Mo/B4C multilayer mirrors are often considered for applications using high power soft X-ray light sources, for example X-ray free electron lasers [6]. It has been reported that nitridation of La in La/B4C multilayer mirrors increases the multilayer reflectivity [2] This increase is assumed to be caused by the nitrogen passivation of the lanthanum preventing or mitigating lanthanum-boride formation. We compare two ways of La nitridation: N-ion post treatment of the La layers and reactive magnetron sputtering of La using a N2 and Ar gas mixture
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