Nanofabrication of Zone Plate Optics for Compact Soft X-Ray Microscopy

Abstract: This Thesis describes the development of the KTH/Stockholm nanofabrication process for diffractive soft x-ray optics. The effort is motivated by the need for and requirement of specially designed diffractive optics and test objects for compact x-ray microscopy as well as optics for other applications such as phase imaging. The optics have been fabricated in-house, in the KTH Nanofabrication Laboratory.The nanofabrication process is based on electron-beam lithography in combination with reactive ion etching (RIE) and nickel electroplating. This process has successfully been used for the fabrication of micro zone plates, condenser zone plates, diffractive optical elements for differential-interference microscopy, and different test structures. Optics with electroplated feature sizes down to 25 nm have been fabricated with high aspect-ratios. Special consideration has been given the reproducibility and optimization of the process parameters. This is essential for improving the yield and quality of the fabricated optics. The work includes, e.g., improved e-beam writing strategies and controlled electroplating. Furthermore, a high diffraction efficiency is necessary for our applications, which are based on compact low-power sources. This requires the fabrication of optics with a high and uniform aspect ratio. For this purpose the electroplating process step has been improved with an in-situ light-transmission-based thickness control method for optimum mold filling, and pulse and pulse-reverse techniques for uniform plating.