Monday March 4th 2019, Mr Arash Edrisi, a former PDEng student in the Industrial Focus Group XUV Optics, has successfully defended his PDEng thesis, entitled 'Alternative EUV absorptive materials and novel architectures for EUV reticle in 7nm node technology scanner'. He obtained his degree as PDEng Design Process Technology. His supervisor was Prof.dr. F. Bijkerk from the Faculty of Science and Technology.
Arash Edrisi has a master degree in Chemistry from the University of Eastern Finland (UEF). In the XUV group, Arash focused on the engineering of an alternative absorber layer structure, which is capable of being used in next-gen EUV photolithographic equipment. To reduce the printable feature size, next-gen lithographic tools will be equipped with increased numerical aperture optics, which inevitably lead to imaging aberrations due to increased shadowing of mask features ("Mask-3D" effects). In order to overcome such increased shadowing, the height of absorber features on the mask should be strongly reduced, requiring more absorptive materials. As part of the Horizon 2020 "SeNaTe" program, Arash investigated several highly absorbent material candidates and found their as-deposited structure to be rough and polycrystalline, which is unacceptable for the application.
In order to engineer the layer structure to have low roughness and repress crystallization during growth, Arash designed several structures. For Ni films, a structure was developed where the film was doped with boron during growth. As a function of dopant concentration, the crystallinity is repressed, with a fully amorphous growth at dopant levels above 25%. In this way, the crystallinity could be reduced to acceptable levels, while maintaining a high Ni content, essential to the absorption efficiency of the layer. For Ag, a material that was shown to exhibit island growth with textured crystallinity and high roughness, Arash developed a Ge adhesion layer. Combining Ag with Ag into a Ag/Ge multilayered structure, he was able to demonstrate a smooth Ag based absorber structure where crystalline growth was strongly suppressed due to the fact that the Ag crystallites did not extend into the Ge layers, essentially "resetting" nucleation at each Ag layer in the multilayer structure, and thereby limiting crystallite size to the (few nanometer) thickness of the Ag layers.
The impact of his work is found in the active further development of such high absorptive materials in subsequent H2020 projects. Arash is currently pursuing his scientific career in the United States.
More information on this project is available from Dr. R.W.E. van de Kruijs, r.w.e.vandekruijs@utwente.nl
