meteral SCIENCE report

Below is an image from my research at NIST and I think you all are equipped to solve to identify the mechanism of deformation.  This is a cross-sectional electron image of an extreme ultra-violet (EUV) mask (used for lithography or printing or nanoscale electronic circuits by Intel).  The EUV mask is composed of a Ni capping layer followed by stacked layers of Molybdenum (Mo, dark layer) and Silicon (Si, light layer) as shown in (a), all on top of a Silicon substrate.

It was proposed to try to use the helium ion beam to cut or “machine” the EUV mask, however other unexpected phenomena occurred.  First, there is an observed shrinking of the EUV mask thickness (b,c), second, the layers become obscured in the top of the stack (c,d), and third there is an expansion and ultimately, the formation of bubbles (d).

• Pristine EUV mask with no ion beam exposure. (b)-(d) Increasing helium ion beam dose from 1×10¹⁶ ions/cm² to 1×10¹⁸ ions/cm².

Given that pure Mo has BCC structure (density=10.28 g/cm³) and pure Si has diamond cubic structure (density=2.33 g/cm³, see your book for more details on this structure), with the addition of small amount of energy it is possible to form MoSi₂ with body-centered tetragonal structure (where a =0.321 nm and c = 0.452 nm), as shown below.

Give an explanation of the problem and what you think is happening, then answer these questions.

Questions:

• What are the point coordinates for the Silicon atoms in the MoSi₂ system? Generate a model for this and the crystal structures of pure (elemental) Silicon and Molybdenum using VMSE.
• On average, what volume of space is occupied by each Mo and Si atom in their elemental crystal forms and how much space is occupied by a molecule of MoSi₂ (you should not use their atomic radii but instead work backwards from density and crystal structure)? Which is more dense?
• What could explain the phenomenon of initial shrinking and apparent “mixing” of the layers?
• What could explain the bubbles that form at high dose? Why do they not form initially at low dose?v