Three-dimensional structured on-chip stacked zone plates for nanoscale X-ray imaging with high efficiency

Fresnel zone plates are the key optical elements for nanoscale focusing of X-ray beams with high spatial resolution. Conventional zone plates manufactured by planar nanotechnology processes are limited by the achievable aspect ratios of their zone structures. Additionally, ultra-high resolution X-ray optics with high efficiency requires three-dimensional （3-D） shaped tilted zones. The combination of high spatial resolution and high diffraction efficiency is a fundamental problem in X-ray optics. Based on electrodynamical simulations, we find that the optimized zone plate profile for volume diffraction is given by zone structures with radially increasing tilt angles and decreasing zone heights. On-chip stacking permits the realization of such advanced 3-D profiles without significant loss of the maximum theoretical efficiency. We developed triple layer on-chip stacked zone plates with an overlay accuracy of sub-2 nm which fulfills the nanofabrication requirements. Efficiency measurements of on-chip stacked zone plates show significantly increased values compared to conventional zone plates.