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PhD Defence Rogier van den Bos

hydrogen induced blister formation in Mo/Si multilayer structures 

Rogier van den Bos is a PhD Student in the Industrial Focus Group XUV Optics. His supervisor is Professor Fred Bijkerk from the Faculty of Science and Technology.

Blister formation due to incident particles is well explored in single materials. Yet, layered materials may show a different behavior under exposure. A frequently used heterogeneous structure is the so-called multilayer mirror which is used in optical systems of synchrotrons, telescopes and free electron lasers. When using light in the soft X-ray and XUV wavelength range, conventional lens systems can’t be used as the light at these short wavelengths is highly absorbing and has a low refractive power for all materials. Therefore multilayer mirrors are used instead, to focus and reflect light at soft X-ray and XUV wavelengths.

For the fabrication of multilayer mirrors, molybdenum and silicon are a commonly used material combination due to their optical constants. Making a Mo/Si bi-layer stack by depositing subsequent Mo and Si layers with thicknesses precisely matched to the wavelength of the light, a highly reflective Bragg mirror structure 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. In some cases of usage, the multilayer sees a background gas environment, for instance hydrogen. These gases may penetrate and form nanometer sized surface blisters.

To be able to predict the hydrogen exposure conditions that lead to blister formation, the root cause of blister formation needs to be known. Blisters can be formed by pressure building up in microscopic voids near the surface and/or be the result of the release of compressive stress in the layered system. For the Mo/Si multilayer system under investigation the root cause of the blister formation was one of the questions that were investigated.   

To study the blister formation in the multilayer system, a model description of the growth and stability of blisters by elastic deformation within the multilayers was developed. The model describes how stabilized blisters can be formed in nanometer thick Mo/Si multilayers under the building up of hydrogen pressure. The model considers the trapping of molecular hydrogen under a circular blister cap, causing the blister cap to deflect elastically outward until a stable blister is formed. In the model, the energy to elastically deform the blister cap is balanced by the work done by the expanding hydrogen gas and release of compressive internal stress in the multilayer. From the model, the adhesion energy of the blister cap with the remaining underlying multilayer, the required internal pressure, and the critical H-dose for blister formation can be calculated when the elastic constants of the blister cap material are known.

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. Expected blister sizes from the blister model were compared with experimentally observed blister sizes that where formed in Mo/Si multilayer. A good agreement was found between model calculations and experimental results. One of the observations that could be explained by the model was a significant increase in the blister number density when the ion energy was increased above a certain ion energy threshold. The increase in blister number density showed a correlation with the ion penetration depth. When hydrogen ions have enough energy to directly penetrate to the depth of delamination, a strong increase in blister number density was observed. It was also observed that increasing ion flux, leads to a slight decrease in average blister size. This was predicted by the blister model as smaller blister sizes require an increase in the blisters' internal pressure. Above agreement between model and experiment suggested that the root cause of the blister formation in the Mo/Si multilayer was mainly caused by hydrogen pressure building up in voids near surface.

To confirm the minor impact of intrinsic stress in multilayer on blister formation, a method was developed to experimentally verify the contribution of compressive intrinsic stresses on the blister formation. This was done by varying the thickness of the Si layer. Increasing the thickness of Si was found to introduce a higher average compressive stress in the multilayer film. Measurements of the average film stress before and after hydrogen exposure did not reveal a strong correlation between stress relaxation and the observation of surface blisters. Also the observed small blister sizes couldn’t be explained by taking only intrinsic stresses into account. We could therefore conclude that observed blisters in the Mo/Si multilayer were mainly caused by the buildup of hydrogen pressure.