7 nm Spatial Resolution in Soft X-ray Microscopy

Abstract

* Benedikt Rösner, benedikt.roesner@psi.ch X-ray microscopy and spectroscopy are powerful tools to gain valuable insight into many exciting, scientifically relevant samples. Development in microscopic methods has always aimed for achieving spatial resolutions as high as possible. Although highly confined X-ray beams with spot sizes well below 10 nm have been characterized in recent years using mirrors [1], Fresnel zone plate (FZP) lenses [2,3] or multilayer Laue lenses [4,5], X-ray imaging with single-digit nanometer resolution remains an extreme challenge.We report on unprecedented spatial resolution down to 7 nm achieved at two soft X-ray microscopes using photon energies between 700 eV and 850 eV. The experiments were performed at the scanning transmission X-ray microscopes (STXM) at the PolLux beamline at the Swiss Light Source [6] and the Hermes beamline at Synchrotron Soleil [7]. The scientific applications of these two instruments lie in the combination of microscopy and spectroscopy, where high spatial resolution provides a major benefit for scientific research.The key element in a STXM for achieving small spot sizes and thus high resolution is the Fresnel zone plate (FZP) commonly used as X-ray lens. An elegant method to fabricate high-resolution FZPs has been introduced by doubling the line density obtained from the lithography step utilizing atomic layer deposition [8,9]. This method has been applied to fabricate FZPs with zone widths down to 12 nm [8][9][10]. By further optimization of the nanofabrication process, we have fabricated FZPs with outermost zone widths well below 10 nm and heights between 60 nm and 80 nm [11]. The specifications for the lenses were chosen according to the illumination at the PolLux and Hermes beamlines [6,7], ensuring that the zone plate dimensions match the coherence length and energy bandwidth of each beamline. To achieve the highest possible spatial resolution, the experimental geometry and mechanical stability of the STXM endstations had to be optimized. Special attention has to be paid to the extremely short focal length, and to the mechanical stability of the sample stage. While the former challenge can be addressed by using a thin order-sorting aperture, the latter one has to be addressed by carefully optimizing the position control and feedback system. In this way, the maximum amplitude of sample displacement with respect to the zone plate has been brought to ± 3 nm perpendicular to the beam axis.In order to characterize the spatial resolution of our FZPs, we recorded images of high-resolution test patterns. The test patterns consist of a periodic repetitions of an iridium line, an HSQ line, a second iridium line, and a gap. The lateral dimensions of each line or gap of the investigated test samples are 10 nm, 9 nm, 8 nm, and 7 nm (Figure 1). We scanned the test objects in point-by-point mode with steps

Imaging three-dimensional magnetic systems with x-rays

“…For suitably thin systems, one could directly combine the reconstruction algorithms described in [122,121] with high spatial resolution soft X-ray imaging that is provided by current transmission X-ray microscopy and holographic techniques. In particular, with single digit nanometre spatial resolution soft X-ray magnetic imaging already having been reported with STXM [108], this combination of high spatial resolution soft X-ray imaging with the iterative magnetic tomography reconstruction algorithm offers a route to the determination of magnetic structures on length scales below 10 nm.…”

Biological laboratory x-ray microscopy

“…Contact x-ray microscopy is rarely used today, while TXM and STXM regularly demonstrate significant results in fields like cell biology, material science and environmental science [32,33]. The resolution achieved by these techniques currently approach the 10-nm level [34,35].…”

Additive Nano-Lithography with Focused Soft X-rays: Basics, Challenges, and Opportunities

“…Higher resolutions as well as smaller photon energies both lead to shorter distances of the optics and the sample. Thus, ultra-high resolution is so far limited to photon energies above 600 eV [45,77]. The focal length is also a limiting factor for the use of higher diffraction orders (m > 1) for improved spatial resolution.…”

“…Equation (1) shows that the spot size in STXM is mainly dependent on the outermost zone width of the applied Fresnel zone plate ∆r N . Since the zones also need to be placed precisely with high aspect ratios of the structures, resolution in X-ray microscopy is mainly an issue of nanofabrication [44,45]. It is very important that f is determined by the wavelength of the incident photons in nanometers λ and the square of the outermost zone width ∆r N (Equation (2)).…”

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