In this thesis blister formation in multilayer systems was central. ‘We studied how stabilized blisters are formed in nanometer thick molybdenum/silicium multilayers, under the building up of hydrogen pressure,’ Rogier van den Bos says. ‘A model description of the growth and stability of blisters by elastic deformation within the multilayers was developed.’
For the fabrication of multilayer mirrors, molybdenum and silicon are a commonly used material combination, due to their optical constants. By depositing subsequently Mo and Si layers, with thicknesses precisely matched to the wavelength of the light, highly reflective Bragg mirror structures can be created.
‘It is a complex and expensive task to manufacture these reflective multilayer optics, due to the required precision in stacking of the nanometer thick layers,’ Rogier says. ‘These mirror systems can be used in optical systems of synchrotrons, telescopes and free electron lasers. Clever cleaning strategies and exposure to background gas environments, may influence the durability of these systems. An accurate generic model on how gasses penetrate and form nanometer sized surface blisters, is of high value in these applications.’
Blisters can be formed by pressure building up in microscopic voids near the surface, or be the result of the release of compressive stress in the layered system. ‘In this thesis work, these two hypotheses are tested,’ Rogier says. ‘Though blister forming cannot be entirely attributed to one of them, far more insight is now gained on the conditions favorable for blister growth. Designers of multilayer systems can now benefit from these findings, better taking into account internal stress risks, reflective performance parameters, and consider clever choices of materials and process fabrication steps.’
To experimentally verify the model, blisters were created by exposing several Mo/Si multilayer test samples to various well controlled hydrogen doses, fluxes and ion energies. ‘We were able to conclude that observed blisters in the Mo/Si multilayer test samples were mainly caused by the buildup of hydrogen pressure,’ Rogier writes.
‘At these nanometer length scales, surprising effects occur, especially at interfaces. What is special also is the relevance for soft X-ray and XUV wavelength ranges. Conventional lens systems can’t be used, as the light at these short wavelengths is highly absorbing and all materials have a low refractive power.’
During his PhD project Rogier learnt to plan his research in an independent manner, and to build the experimental setups himself. ‘I found out that logistics and planning are crucial, in building these complicated setups,’ he says. ‘You cannot proceed if previous stages are not fully finished yet.’
‘Also I learnt to present my findings in an attractive manner, be it for academic colleagues or for interested people within industry. As where various fields of research involved, from material sciences to plasma physics, I had to find ways to express my findings using the right terminologies.’
Already during his PhD project, Rogier started working at ASML. ‘I finished my thesis report, working during evenings in my attic room,’ he says.
Rogier: ‘At ASML I now work for customer support, solving various kinds of customer related problems. Content wise the job is less demanding, but I have to use all kinds of soft, problem-solving skills, to find the right kind of solutions within foreseeable time-scales, thereby managing people’s expectations in a right manner.’
‘This is quite different from my PhD work, but just as exciting. For example, I learnt that the best technical solution not always coincides with the best economic solution. Perhaps one day I will return to research again, but for the moment this job suits me very well. Also the traveling aspect, visiting Asian, American and European sites, appeals to me, getting to know new cultures.’