X-ray reflective coatings made of Pt, Al₂O₃/Pt, and Al₂O₃/Ni/Pt produced by atomic layer deposition

Abstract

The X-ray reflectance of Pt-based coatings deposited by atomic layer deposition (ALD) has been measured in support of assessing the feasibility of using this deposition method for the production of X-ray mirror coatings that can achieve high X-ray reflectance without causing unacceptably large degradation of mirror figure as a result of coating stress-driven substrate deformation. Specifically, reflectance measurements of single-layer Pt, Al 2 O 3 /Pt bilayer, and Al 2 O 3 /Ni/Pt trilayer coatings grown by ALD on flat Si substrates were made using synchrotron radiation at X-ray energies in the range from 0.35 to 10 keV, revealing that the reflectance of the bilayers and trilayers is superior to that of single-layer sputtered Ir below 3.5 keV. Single-layer Pt and Al 2 O 3 /Pt bilayer coatings produced using thermal ALD were also deposited onto both the front and back surfaces of thin, figured, sub-arcsecond-quality X-ray telescope mirror segments made of polished, single-crystal Si, without discernible degradation of surface figure. These results, along with the successful implementation of batch-coating of two such mirrors simultaneously with high-reflectance coatings grown by thermal ALD, demonstrate the viability of employing stress-balanced, double-sided ALD coatings to mitigate substrate deformation resulting from film stress in high-reflectance coatings. This approach may thus enable the mass production of high-performance, sub-arcsecond X-ray telescope mirrors.