Fabrication of high-aspect-ratio sub-30 nm zone plates by a zone-deepening process for hard X-ray microscopy

X-ray microscopy based on diffractive lenses with 10 - 30 nm resolution has been reported for decades. With the increasing demand for efficient and reproducible zone plates in hard X-ray microscopy, there remains a need for fabrication approaches that combine high precision with process simplicity. A single-sided, EBL-based method is presented, providing a rapid and reliable route to produce high-efficiency lenses. This paper reports a zone deepening technique by anisotropic dry etching into a deposited carbon layer (DCL) masked by patterned hydrogen silsesquioxane (HSQ) through electron beam lithography (EBL) to achieve ultra-high aspect ratios (zone-height/zone-width) of 30 nm zone plates. As a demonstration, zone plates with 30 nm outermost zone width and a high aspect ratio up to 43:1 were successfully fabricated. Simulations based on the discrete Hankel transform predict a 13% theoretical efficiency at 8 keV, which is approximately twice that of conventional EBL-based single-sided zone plates. Optical characterization of the fabricated zone plates confirms 11.6% efficiency and 30 nm resolution, with full-field transmission X-ray microscopy (TXM) imaging performed at BL18B for the resolution test, and efficiency measured at BL15U1A, both at the Shanghai Synchrotron Radiation Facility (SSRF). This novel process is expected to be applicable to zone plates with even narrower outermost zone widths and higher aspect ratios. Numerical simulations for such zone plates with 10 nm resolution predict above 10% efficiency at 10 keV. The method combines process simplicity and high structural precision within a standard EBL workflow, offering compatibility with existing zone doubling techniques for fabricating advanced hard X-ray optics and enabling future high-resolution, high-efficiency imaging applications.