Ultrahigh-Resolution Soft-X-Ray Microscopy with Zone Plates in High Orders of Diffraction

We present an x-ray optical approach to overcome the current limitations in spatial resolution of x-ray microscopes. Our new BESSY full-field x-ray microscope operates with an energy resolution up to $E/\Delta E={10}^4$. We demonstrate that under these conditions it is possible to employ high orders of diffraction for imaging. Using the third order of diffraction of a zone plate objective with 25 nm outermost zone width, 14 nm lines and spaces of a multilayer test structure were clearly resolved. We believe that high-order imaging paves the way towards sub-10-nm real space x-ray imaging.

Figure 1

X-ray optical setup of the full-field x-ray microscope for third-order imaging. The capillary condenser generates the required hollow cone illumination. The image formed by the third order of diffraction of the zone plate is recorded by the CCD detector. Radiation from other zone plate diffraction orders (as indicated for the zeroth, first and negative first order) forms ring patterns in the image plane due to the hollow cone object illumination.

Figure 2

Calculated diffraction efficiency ${\eta }_3(d{r}_n,E=700\mathrm{eV})$ of a gold zone plate with line to space ratio $1:1$, operating in the order $m=3$. (a) Calculation for zone structures parallel to the optical axis. (b) Zone structures tilted with respect to the optical axis to fulfil the Bragg condition.

Figure 3

Left: X-ray microscope image of $\mathrm{Cr}/{\mathrm{B}}_4\mathrm{C}$ multilayer structures with 4 different periods recorded in the third order of diffraction. The values of the half pitch of the structures are 20.6, 17.5, 14.3, and 11.0 nm. The image is flat field corrected.

Figure 4

Top: Sections of x-ray micrographs of an advanced copper interconnect IC stack imaged at 0.9 keV photon energy. Image A was obtained in the first order of diffraction with an energy resolution of $E/\Delta E=4300$ and an exposure time of 5.2 s. Image B was obtained in third order of diffraction, energy resolution $E/\Delta E=1400$, exposure time 26 s. Abbreviations: $V=\mathrm{via}$, $B=\mathrm{\text{barrier layer}}$, $C=\mathrm{\text{interconnect structure}}$, $L=\mathrm{\text{low}}\mathrm{\text{−}}k$ dielectrics. Bottom: Power spectrum of image B. The cutoff frequency corresponds to 12 nm feature size.