R. W. E. van de Kruijs

Thermally enhanced interdiffusion in Mo∕Si multilayers

The formation and development of Mo-Si interfaces in Mo∕Si multilayers upon thermal annealing, including a transition to h‐MoSi2, have been investigated using high resolution transmission electron microscopy, x-ray reflectivity, and x-ray diffraction measurements. The silicide layers naturally formed at Mo-Si interfaces, i.e., just upon and after the deposition, are amorphous and have different thicknesses for as-deposited samples, with the Mo-on-Si interlayer being the largest. In addition, silicide growth at Mo-Si interfaces during annealing before the phase transformation predominantly takes place at the Mo-on-Si interface and a MoSi2 interface layer is formed. Diffusion continues until a thick MoSi2 layer is formed at the interface, at which point the interface crystallizes and diffusion speeds up, finally resulting in an abrupt intermixing and phase transition of the entire interface to h‐MoSi2. This model predicts an onset of the phase transition which does not depend primarily on the annealing temp...

Spectral-purity-enhancing layer for multilayer mirrors

We demonstrate, both theoretically and experimentally, that special spectral-purity-enhancing multilayer mirror systems can be designed and fabricated to substantially reduce the level of out-of-band radiation expected in an extreme ultraviolet lithographic tool. A first proof of principle of applying such spectral-purity-enhancement layers showed reduced out-of-band reflectance by a factor of five, while the in-band reflectance is only 4.5% (absolute) less than for a standard capped multilayer.

Enhanced diffusion upon amorphous-to-nanocrystalline phase transition in Mo/B4C/Si layered systems

The effect of an amorphous-to-nanocrystalline phase transition on the diffusion across an interface layer of subnanometer thickness has been investigated in real-time. The diffusion in the Mo/B4C/Si thin film structure studied was found to instantaneously enhance by an order of magnitude upon the formation of nanocrystals inducing the atomic-scale onset of grain boundary diffusion

Enhanced reflectance of interface engineered Mo/Si multilayers produced by thermal particle deposition

A new deposition technique that builds on the thermal particle characteristics typical for e-beam deposition is described. This technique applies magnetron sputtering in a special scheme where these characteristics of the e-beam deposition method are achieved. The method was used for interface engineering of Mo/Si multilayers, with different barrier layer materials being tested. Composition of the barrier layers formed was studied using XPS. Results are shown on the general example of a Mo/B4C/Si/B4C system. The ultra-thin reflectance enhancement B4C barriers can be deposited with low added stress, resulting in a multilayer stress as low as about -150 MPa. The best interface engineered multilayers reflect 70.5% at 13.3 nm and 70.15% at 13.5 nm. These results were achieved with 50 period multilayers terminated with a standard Si layer.

Stress mitigation in Mo/Si multilayers for EUV lithography

Although Mo/Si multilayers are now widely used in EUV lithography development programs, multilayer induced substrate stress continues to be a major issue. Standard stress values of -350 to -450 MPa, reported for Mo/Si systems produced by magnetron sputtering, induce an intolerable deformation of the surface figure of EUV optical components. Stress in e-beam deposited Mo/Si multilayers has not been reported before. At FOM Rijnhuizen, an extensive stress mitigation program has been carried out on multilayers produced by e-beam deposition and medium energy ion polishing. The stress in our standard, high reflectance Mo/Si multilayers is less than -200 MPa. Although e-beam deposition apparently halves the typical stress values obtained by sputter deposition, it is still above the allowable limit for the first lithographic system, the so-called Alpha Tool. To further reduce stress, the influence of the Mo fraction, the number of periods and the multilayer period or d-spacing has been investigated. Varying the Molybdenum fraction in e-beam deposited multilayers results in a similar dependency as reported for magnetron sputtered coatings, though at strongly reduced absolute values. Furthermore, variation of the d-spacing has a small influence on stress. The number of periods however, has no influence on the stress value in the range from 20 to 50 periods. Applying stress mitigation techniques based on adjustment of the Mo fraction, a high reflectance of above 69% at near normal incidence at 13.5 nm has been obtained for multilayers with a stress value of only -33 MPa. This has been achieved by using Mo and Si only. This stress value is sufficiently low to enable the first generations of EUVL optics.

Combined EUV reflectance and X-ray reflectivity data analysis of periodic multilayer structures

We present a way to analyze the chemical composition of periodical multilayer structures using the simultaneous analysis of grazing incidence hard X-Ray reflectivity (GIXR) and normal incidence extreme ultraviolet reflectance (EUVR). This allows to combine the high sensitivity of GIXR data to layer and interface thicknesses with the sensitivity of EUVR to the layer densities and atomic compositions. This method was applied to the reconstruction of the layered structure of a LaN/B multilayer mirror with 3.5 nm periodicity. We have compared profiles obtained by simultaneous EUVR and GIXR and GIXR-only data analysis, both reconstructed profiles result in a similar description of the layered structure. However, the simultaneous analysis of both EUVR and GIXR by a single algorithm lead to a ∼ 2x increased accuracy of the reconstructed layered model, or a more narrow range of solutions, as compared to the GIXR analysis only. It also explains the inherent difficulty of accurately predicting EUV reflectivity from a GIXR-only analysis.

Infrared antireflective filtering for extreme ultraviolet multilayer Bragg reflectors

An extreme ultraviolet multilayer mirror with an integrated spectral filter for the IR range is presented and experimentally evaluated. The system consists of an IR-transparent B4C/Si multilayer stack which is used both as EUV-reflective coating and as a phase shift layer of the resonant IR antireflective (AR) coating. The AR coating is optimized in our particular case to suppress CO2 laser radiation at a wavelength of 10.6 μm, and a suppression of more than two orders of magnitude is demonstrated. The method allows high suppression over a large angular acceptance range, relevant for application in lithography systems.

Infrared suppression by hybrid EUV multilayer-IR etalon structures

We have developed a multilayer mirror for extreme UV (EUV) radiation (13.5 nm), which has near-zero reflectance for IR line radiation (10.6 μm). The EUV reflecting multilayer is based on alternating B4C and Si layers. Substantial transparency of these materials with respect to the IR radiation allowed the integration of the multilayer coating in a resonant quarter-wave structure for 10.6 μm. Samples were manufactured using magnetron sputtering deposition technique and demonstrated suppression of the IR radiation by up to 3 orders of magnitude. The EUV peak reflectance amounts 45% at 13.5 nm, with a bandwidth at FWHM being 0.284 nm. Therefore such a mirror could replace conventional multilayer mirrors to suppress undesired spectral components in monochromatic imaging applications, including EUV photolithography.

Chemical interaction of B4C, B, and C with Mo/Si layered structures

To enhance the thermal stability, B4C diffusion barrier layers are often added to Mo/Si multilayer structures for extreme ultraviolet optics. Knowledge about the chemical interaction between B4C and Mo or Si, however is largely lacking. Therefore, the chemical processes during annealing up to 600?°C of a Mo/B4C/Si layered structure have been investigated in situ with hard x-ray photoelectron spectroscopy and ex situ with depth profiling x-ray photoelectron spectroscopy. Mo/B/Si and Mo/C/Si structures have also been analyzed as reference systems. The chemical processes in these systems have been identified, with two stages being distinguished. In the first stage, B and C diffuse and react predominantly with Mo. MoSix forms in the second stage. If the diffusion barrier consists of C or B4C, a compound forms that is stable up to the maximum probed temperature and annealing time. We suggest that the diffusion barrier function of B4C interlayers as reported in literature can be caused by the stability of the formed compound, rather than by the stability of B4C itself.

Emissivity of freestanding membranes with thin metal coatings

Freestanding silicon nitride membranes with thicknesses down to a few tens of nanometers find use as TEM windows or soft X-ray spectral purity filters. As the thickness of a membrane decreases, emissivity vanishes, which limits radiative heat emission and resistance to heat loads. We show that thin metal layers with thicknesses in the order of 1 nm enhance the emissivity of thin membranes by two to three orders of magnitude close to the theoretical limit of 0.5. This considerably increases thermal load capacity of membranes in vacuum environments. Our experimental results are in line with classical theory in which we adapt thickness dependent scattering terms in the Drude and Lorentzoscillators.