Gratings, one of the most important energy dispersive devices, are the fundamental building blocks for the majority of optical and optoelectronic systems. The grating period is the key parameter that limits the dispersion and resolution of the system. With the rapid development of large X-ray science facilities, gratings with periodicities below 50 nm are in urgent need for the development of ultrahigh-resolution X-ray spectroscopy. However, the wafer-scale fabrication of nanogratings through conventional patterning methods is difficult. Herein, we report a maskless and high-throughput method to generate wafer-scale, multilayer gratings with period in the sub-50 nm range. They are fabricated by a vacancy epitaxy process and coated with X-ray multilayers, which demonstrate extremely large angular dispersion at approximately 90 eV and 270 eV. The developed new method has great potential to produce ultrahigh line density multilayer gratings that can pave the way to cutting edge high-resolution spectroscopy and other X-ray applications.

V/Sc multilayer is experimentally demonstrated for the first time as a high reflectance mirror for the soft X-ray water window region. It primarily works at above the Sc-L edge (λ = 3.11 nm) under near normal incidence while a second peak appears at above the V-L edge (λ = 2.42 nm) under grazing incidence. The V/Sc multilayer fabricated with a d-spacing of 1.59 nm and 30 bilayers has a smaller interface width (σ = 0.27 and 0.32 nm) than the conventional used Cr/Sc (σ = 0.28 and 0.47 nm). For V/Sc multilayer with 30 bilayers, the introduction of B₄C barrier layers has little improvement on the interface structure. As the number of bilayers increasing to 400, the growth morphology and microstructure of the V/Sc layers evolves with slightly increased crystallization. Nevertheless, the surface roughness remains to be 0.25 nm. A maximum soft X-ray reflectance of 18.4% is measured at λ = 3.129 nm at 9° off-normal incidence using the 400-bilayers V/Sc multilayer. According to the fitted model, an s-polarization reflectance of 5.2% can also be expected at λ = 2.425 nm under 40° incidence. Based on the promising experimental results, further improvement of the reflectance can be achieved by using a more stable deposition system, exploring different interface engineering methods and so on.

<!-- *** Custom HTML *** --> Novel mask absorber designs are catching the attention of the EUVL community due to their ability to mitigate mask 3D effects. Material selection is part of such an optimization. We propose several candidates as novel EUV lithography mask absorbers, namely TaTeN, Ru–Ta, and Pt–Mo alloys. The choice of these materials is based on their theoretical performance evaluated by EUV imaging simulation based on their complex refractive index N ( λ ) = n ( λ ) + ik ( λ ) , where the optical constants n and k relate to the phase velocity and the absorption of electromagnetic radiation with a wavelength λ, respectively. The materials are deposited as thin films on Si substrate with an additional Ru layer to mimic the cap of the multilayer mirror on the real mask. The experimental n and k values are determined by analyzing EUV reflectivity data obtained using a 13.5-nm synchrotron EUV radiation. The imaging simulation presented consists of calculating several imaging metrics including non-telecentricity, normalized image log-slope, and threshold-to-size for specific use cases using the novel absorber. It also compares the proposed materials with the reference TaBN absorber. TaTeN shows higher absorption than TaBN and refraction closer to 1, which improves phase matching for a high k absorber. The refractive index of Ru–Ta and Pt–Mo alloys exhibits a large difference from that of air and provides the required phase shift of attenuated phase shift masks. The characterizations of these materials target the requirements of an EUVL mask: durability for mask cleaning, mask lifetime, and etchability for mask patterning. The stability is first tested against several standard mask cleaning solutions by a beaker test for up to 24 h. The samples are also exposed to hydrogen plasma to imitate the working environment in an EUV scanner. Concerning material patterning, chemical reactive ion etch is applied for preliminary tests. A proper etch recipe is found for TaTeN with a good etch rate (about 60 nm / min) and good selectivity to the Ru underlayer (Ru etch is negligible).

BackgroundA plausible approach for mitigating the mask 3-D (M3D) effects observed in extreme ultraviolet (EUV) lithography is to replace the existing mask absorber with alternative materials. Absorbers with a high EUV extinction coefficient k allow for lower best focus variation (BFV) through pitch and reduced telecentricity errors (TCEs).AimWe aim to evaluate Ta-Co alloys as potential high-k mask absorbers from material suitability and imaging standpoints.ApproachWe study the film morphology, surface composition, and stability of Ta-Co alloys in mask cleaning solutions and a hydrogen environment as present in the EUV scanner to assess the material suitability from an experimental aspect. Optical constants for three selected compositions, viz., TaCo, Ta2Co, and TaCo3, were determined from EUV angle-dependent reflectivity measurements. Next, utilizing rigorous simulation software, the imaging performance of Ta-Co alloys is evaluated and compared with the reference absorber. The recommended absorber thickness for Ta-Co alloy absorbers is based upon normalized image log slope (NILS) enhancement, threshold to size, and balancing of diffraction order amplitudes. A 10 nm line and space pattern with a pitch of 20 nm and 14 nm square contact holes with a pitch of 28 nm are used for the simulation study using high numerical aperture 0.55 EUV lithography process settings. The primary imaging metrics for through pitch evaluation include NILS, TCE, and BFV.ResultsThe Ta-Co alloys exhibit a higher EUV extinction coefficient k compared with the currently used Ta-based absorber. TaCo and Ta2Co demonstrate smooth surfaces and are stable in a hydrogen environment and in mask-cleaning solutions.ConclusionTa-Co alloys allow for a reduction in M3D effects at a lower absorber thickness compared with a 60 nm Ta-based reference absorber.

The Kossel effect is the diffraction by a periodically structured medium, of the characteristic X-ray radiation emitted by the atoms of the medium. We show that multilayers designed for X-ray optics applications are convenient periodic systems to use in order to produce the Kossel effect, modulating the intensity emitted by the sample in a narrow angular range defined by the Bragg angle. We also show that excitation can be done by using photons (X-rays), electrons or protons (or charged particles), under near normal or grazing incident geometries, which makes the method relatively easy to implement. The main constraint comes from the angular resolution necessary for the detection of the emitted radiation. This leads to small solid angles of detection and long acquisition times to collect data with sufficient statistical significance. Provided this difficulty is overcome, the comparison or fit of the experimental Kossel curves, i.e., the angular distributions of the intensity of an emitted radiation of one of the element of the periodic stack, with the simulated curves enables getting information on the depth distribution of the elements throughout the multilayer. Thus the same kind of information obtained from the more widespread method of X-ray standing wave induced fluorescence used to characterize stacks of nanometer period, can be obtained using the Kossel effect.

Stimulated emission is a fundamental process in nature that deserves to be investigated and understood in the extreme ultra-violet (EUV) and x-ray regimes. Today, this is definitely possible through high energy density free electron laser (FEL) beams. In this context, we give evidence for soft-x-ray stimulated emission from a magnesium oxide solid target pumped by EUV FEL pulses formed in the regime of travelling-wave amplified spontaneous emission in backward geometry. Our results combine two effects separately reported in previous works: emission in a privileged direction and existence of a material-dependent threshold for the stimulated emission. We develop a novel theoretical framework, based on coupled rate and transport equations taking into account the solid-density plasma state of the target. Our model accounts for both observed mechanisms that are the privileged direction for the stimulated emission of the Mg L_2,3 characteristic emission and the pumping threshold.