Multilayer Fresnel Zone Plates (ML-FZPs)
ML-FZPs are very interesting for hard X-ray focusing as they have essentially limitless aspect ratios. The use of ALD lets us fabricate a very precise multilayer stack following the zone plate law (Fig. 2). Then, this stack is sliced to the desired thickness and polished using FIB micromachining and micromanipulation to deliver an ML-FZP. In addition to the freely selectable aspect ratio, these optics have a second distinct advantage over the conventional lithographic FZPs, they can be made to have a very small Δr thanks to the precision of the ALD. We have fabricated and demonstrated ML-FZPs with half-pitch resolutions down to 16 nm and efficiencies of up to >10 % in the soft X-ray energies.
Fig2: a) An SEM image of an ML-FZP on a TEM grid sliced and polished using a FIB. b) An SEM image showing the ALD layers of the ML-FZP.