Nanoscale diffusion, compound formation and phase transitions in Mo/Si multilayer structures

This thesis addresses the physical and chemical phenomena in Mo/Si multilayer structures with and without B4C diffusion barrier layers at the interfaces, which are applied as extreme ultraviolet (EUV) / soft X-ray optics. Since interdiffusion and interlayer formation limit both the performance and the lifetime of the optics, the interlayers and their evolution in time have been investigated in detail. To this end, a procedure was developed to utilize the depth-resolved information in a LEIS spectrum in order to non-destructively study interdiffusion in ultrathin films with sub-nanometer resolution. Using this method, it was found that the diffusion in Mo/B4C/Si layered structures initially obeys Fick’s second law. However, after a certain time, the diffusion rate instantaneously increased by up to one order of magnitude. The cause of this acceleration was found to be the amorphous-to-nanocrystalline transition of the MoSi2 interface. Besides diffusion and crystallization, the chemical processes upon annealing of Mo/B4C/Si layered structures have been identified. Mo/B/Si and Mo/C/Si samples were studied as reference systems. Initially, predominantly MoBxCy (resp. MoBx, MoCx) formed, plus small amounts of SiBxCy (resp. SiBx, SiCx). Subsequently, MoSi2 formed, while the already formed MoBxCy (resp. MoBx, MoCx) diffused further into the Mo layer. Stable silicides, through which Si diffused to form MoSi2 in the second stage, only formed in the samples with C and B4C interlayers. Furthermore, it was investigated whether the interlayer thickness can be reduced by depositing the multilayer mirror at a low temperature. Even after warming up to room temperature, the interlayers that formed upon cryogenic deposition were found to be nearly 60 % thinner than after room temperature deposition. Finally, since rough interfaces between the layers of a multilayer mirror are equivalent to thick interlayers, it is important to control the roughness during the deposition. The development of roughness is usually mitigated through ion bombardment of the Si layers. Therefore, in order to gain understanding, the roughness evolution of Si surfaces upon Ar ion erosion was studied in real-time. It was demonstrated that ion treatment can cause roughening as well as smoothening, depending on the initial roughness of the substrate

Understanding solid state diffusion in multilayered structures on a picometer lengthscale

Multilayered Extreme UV mirrors present unprecedented fundamental questions to solid state diffusion, requiring understanding of diffusion phenomena on lengthscales of only picometers. Using x-ray diffraction applied in situ during thermal annealing, we have investigated diffusion processes well below the Tammann temperature. Resolving picometer structural changes in Mo/Si multilayers reveals diffusion limited compound interface growth, exhibiting Arrhenius-like diffusion behaviour with a reduced activation energy connected to diffusion in the nano-crystalline layers and interfaces. These results are relevant for controlling diffusion processes on a picometer length scale, with potential spin-off to many other thin film applications

Roughness evolution of Si surfaces upon Ar ion erosion

We studied the roughness evolution of Si surfaces upon Ar ion erosion in real time. Following the theory of surface kinetic roughening, a model proposed by Majaniemi was used to obtain the value of the dynamic scaling exponent β from our data. The model was found to explain both the observed roughening and the smoothening of the surfaces. The values of the scaling exponents α and β, important for establishing a universal model for ion erosion of (Si) surfaces, have been determined. The value of β proved to increase with decreasing ion energy, while the static scaling exponent α was found to be ion energy independen

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

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 diffusio

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.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin