Soft X-ray Science

CXRO investigates the interaction of soft x-rays with materials through advances in soft x-ray microscopy, spectroscopy, scattering, and magnetic imaging at the Advanced Light Source, probing nanoscale structure, chemistry, and spin dynamics.

Soft X-Ray Microscopy: XM-1

The XM-1 full-field soft x-ray microscope, designed and built by CXRO, has operated at ALS Beamline 6.1.2 since the mid-1990s. Using bending-magnet radiation between roughly 300 and 1300 eV, a condenser zone plate illuminates the sample and a CXRO-fabricated micro zone plate projects a magnified image onto a soft x-ray-sensitive CCD. With zone plates produced by CXRO nanofabrication, XM-1 achieved the first sub-15 nm resolution x-ray imaging¹ and now resolves features down to about 10 nm.

Because x-ray magnetic circular dichroism provides element-specific magnetic contrast, XM-1 is also a premier tool for nanomagnetism. With applied magnetic fields at the sample, it images magnetic domains, vortices, and skyrmions² in real space, revealing how spin textures form, move, and switch in magnetic nanostructures.

Diagram of the XM-1 soft x-ray microscope: ALS bending magnet source, plane mirror, condenser zone plate, pinhole, sample, micro zone plate, and soft x-ray sensitive CCD — A diagram of the soft x-ray microscope XM-1.

XM-1 magnetic-contrast images of skyrmions and magnetic domains in Pt/Co/Ir and Ta/CoFeB/MgO multilayers under varying magnetic fields — Imaging skyrmions with XM-1.

Soft X-Ray Reflectometry

CXRO built and operates the calibration and standards beamline 6.3.2 at the ALS, whose soft x-ray reflectometer is the world’s most precise instrument for measuring the reflectivity of EUV optics and for calibrating soft x-ray detectors.³

Tunable from roughly 1 to 40 nm in wavelength, the reflectometer measures the reflectivity of mirrors and multilayer coatings, the transmission of thin films, and the efficiency of gratings and detectors. It played a central role in the development of the Mo/Si multilayer coatings used in EUV lithography, and optics and detectors from laboratories and companies worldwide come to Beamline 6.3.2 for calibration, including the latest high-reflectance multilayers for the water window.

Wavelength precision: 0.007%
Wavelength uncertainty: 0.013%
Reflectance precision: 0.08%
Reflectance uncertainty: 0.08%
Spectral purity: 99.98%
Dynamic range: 10¹⁰

The soft x-ray reflectometer end station at ALS Beamline 6.3.2 — Soft x-ray reflectometer at ALS Beamline 6.3.2.

Resonant Scattering of EUV Photoresists

Resonant soft x-ray scattering (RSoXS) tunes the photon energy to the absorption edges of the elements in a sample, most often the carbon K-edge, turning subtle chemical differences into strong scattering contrast that conventional x-ray techniques cannot see. CXRO applies this chemical sensitivity to one of the central problems in EUV lithography: what the pattern looks like while it is still a latent image, chemically written into the resist but not yet developed.

Using critical-dimension RSoXS, CXRO researchers have reconstructed the three-dimensional latent image stored in EUV resists,⁴ revealing how photoacid diffusion, deprotection gradients, and developer chemistry shape the final profile at the nanometer scale. Following the same patterned field through exposure, post-exposure bake, and development shows how the buried chemical pattern evolves into the final structure, insight that is essential for understanding resolution limits and stochastic variability in EUV patterning.

RSoXS measurements of an MET5-patterned EUV photoresist: patterned field layout and scattering patterns after EUV exposure, post-exposure bake, and development — RSoXS of an MET5-patterned EUV photoresist: (a) patterned fields, with scattering measured (b) after EUV exposure, (c) after post-exposure bake, and (d) after development.