A dielectric Kinoform zone plate with 15 nm resolution and high efficiency for soft X-ray optics

Abstract

X-ray microscope as an important nanoprobing tool is expected to play a powerful role in nano-inspections of materials. Despite the fast advances of high resolution focusing/imaging reported, the diffraction efficiency of existing binary zone plates as the lenses in the microscope is mostly around 5% in practice and rapidly goes down to 1-2% when the resolution approaches 10 nm, failing its applications in advanced scientific research. Zone plates with Kinoform profile are supposed to be high efficient, little progress for achieving both high resolution and high efficiency has been reported. The conflict between the resolution and the efficiency in X ray optics has become a long-lasting bottleneck in the further development of X-ray microscope. Based on our earlier success in developing high efficiency Kinoform zone plates by greyscale electron beam lithography, we proposed, in this work, a new zone plate structure by combing a dielectric Kinoform zone plate with an atomic layer deposited HfO2 zone plate to achieve high resolution focusing/imaging with high efficiency. Beam propagation method was applied in designing and optimizing Kinoform shapes of the zones, greyscale and/or binary electron beam lithography was carried out for generating both 3D Kinoform as well as rectangular zones with aspect ratio of 23/1 in dielectric resist, and finally high-quality atomic layer deposition of HfO2 was conducted to form the 15-nm wide outermost zone. Optical characterizations by an in-house developed soft X ray microscope demonstrated 15-nm resolution focusing/imaging with over 7% efficiency, which is the highest with such a resolution as far as our awareness. The origin of such an improvement behind the proposed novel lenses is interpreted by our BMP calculation results and the comparisons with existing performances by other reports in literature was discussed.