August 27, 2015

Femtosecond EUV tabletop microscope able to get 0.5 nanometer axial resolution and should reach 0.1 nanometer resolution

A new microscope system can image down to sub-nanometer resolution and will be able to reach 0.1 nanometer (angstrom) imaging of atoms. It works by boosting wavelength to ultra-high harmonics using an aperture reflection-mode microscope illuminated by a 30-nanometer source. Full-field images of 40-to-80 nanometer lateral resolution result an axial resolution of just six angstroms (0.6 nanometers) with an exposure time of about one minute. The team is also working toward making movies of functioning nano systems with a temporal resolution of 10 femtoseconds

The secret sauce in their process is using coherent EUV light, unlike that used in lithography, which is an omnidirectional flash. By using femto-second pulses of EUV lasers the researchers hope to not only image tiny objects, but also to adapt the technology to memory devices and medical applications.

A computer algorithm is required to reconstruct the image from the light scattered by the 10-femtosecond pulses from the EUV laser that scans over the object to be imaged. For the future they plan on downsizing the beams from 30-nanometers to one-nanometer, producing sub-angstrom resolution — the size of atoms.

The laser-like beams of EUV are produced from the 27th harmonic of a 40-nanometer source and even more to 1-nanaometer by reaching up to 5000th harmonic.

Tabletop EUV ptychography. (a) Schematic of the tabletop EUV microscope. (b) SEM of the sample with a scale bar is 10 um. (c) Representative diffraction pattern from the ptychographic scan. (d) Diffraction pattern from (c) after tilted plane correction.

Ultramicroscopy - High contrast 3D imaging of surfaces near the wavelength limit using tabletop EUV ptychography

This image used false colors to bring out the details of a photo synthesized from the University of Colorado's extreme ultra violet (EUV) femtosecond laser pulses.(Source: University of Colorado)


• High NA reflection ptychography with 40nm resolution (1.3λ) on a tabletop.
• Amplitude and phase contrast provides 3D surface mapping with composition sensitivity.
• 5 Angstrom axial resolution, validated with AFM.
• Higher contrast than standard SEM is achieved.
• Noninvasive technique with a long working distance over 3cm.
• Enables future nanoscale imaging with fs resolution.


Scanning electron microscopy and atomic force microscopy are well-established techniques for imaging surfaces with nanometer resolution. Here we demonstrate a complementary and powerful approach based on tabletop extreme-ultraviolet ptychography that enables quantitative full field imaging with higher contrast than other techniques, and with compositional and topographical information. Using a high numerical aperture reflection-mode microscope illuminated by a tabletop 30 nm high harmonic source, we retrieve high quality, high contrast, full field images with 40 nm by 80 nm lateral resolution (≈1.3λ), with a total exposure time of less than 1 min. Finally, quantitative phase information enables surface profilometry with ultra-high, 6 Å axial resolution. In the future, this work will enable dynamic imaging of functioning nanosystems with unprecedented combined spatial (less than 10 nm) and temporal (less than 10 fs) resolution, in thick opaque samples, with elemental, chemical and magnetic sensitivity.

Exposure time and not damaging like scanning electron scope

They have demonstrated tabletop HHG ptychographic coherent imaging of a surface with unprecedented fidelity, comparing favorably with well-established techniques such as SEM and AFM. They achieve lateral resolutions of 40 nm×80 nm horizontally, as well as sub-nanometer axial precision. EUV reflection provides a powerful imaging contrast mechanism; it has composition sensitivity unlike AFM and improved contrast compared to SEM. Additionally, ptychography CDI does not require the sample to be conductive as in conventional SEM. Instead of serial, point-by-point scanning, EUV ptychography employs a wide field of view at every scan position, significantly decreasing the time for scanning, and making source flux the only practical limit for high volume imaging. The increase of the imaging speed makes this method attractive for real applications involving large-area imaging, such as semiconductor inspection. In contrast to AFM, this microscopy provides a long working distance—in this work, only limited by sample-detector distance (31.66 mm). HHG ptychographic CDI also compares favorably with the SEM in terms of damage—the SEM often left surface contamination and charge after scanning.

The cumulative exposure time for this technique is quite comparable to an SEM (in our case, ~20× shorter than the total acquisition time of the SEM). However, the total acquisition time for the exposure was quite slow (~1 h) in this prototype instrument, primarily due to the slow readout of the CCD and settling time of the stages. Fast-readout CCD technology does exist and can bring readout times down significantly and continuous scanning modes are possible. Image reconstruction using a GPU currently takes an intermediate time (~2–60 min) depending on desired fidelity, but likely can be performed in near real-time with further optimization and upgraded hardware.

The resolution we achieve can be improved by using shorter HHG wavelengths, including the technologically important 13.5 nm. Additionally, by taking a second data set with the sample rotated at 90°, we can ensure a high resolution of 1.3λ in both x- and y-directions. Furthermore, substantial increases in imaging NA are possible using larger or multiple CCDs, or a CCD with a through-aperture that allows closer sample placement. Combined with the femtosecond pulse duration naturally associated with HHG sources, HHG CDI can combine ultrahigh spatial and temporal resolution, to probe the fastest dynamic processes relevant to function at the nanoscale.

SOURCES- EETimes, Ultramicroscopy, University of Colorado

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