We present a method for the optimization of the illumination in soft x-ray (SXR) full-field microscopes. The method
consists of imaging a single periodic grating with a period large compared to the wavelength of the illumination and
obtaining its Fourier spectrum in two orthogonal directions. The analysis of the cut-off frequency along the two
perpendicular directions allows the identification of angled illumination, which can be corrected in-situ by using the
Fourier analysis iteratively. The ability to characterize the illumination conditions and aberrations in the EUV/SXR
microscopes with a fast and simple analysis is critical to achieve the best quality images with the highest spatial
We describe recent advances in the demonstration of table-top full field microscopes that use soft x-ray lasers for illumination. We have achieved wavelength resolution and single shot exposure operation with a very compact 46.9 nm microscope based on a desk-top size capillary discharge laser. This λ=46.9 nm microscope has been used to captured full field images of a variety of nanostructure systems and surfaces. In a separate development we have demonstrated a zone plate microscope that uses λ=13.2 nm laser illumination to image absorption defects in a extreme ultraviolet lithography (EUVL) mask in the same geometry used in a 4x demagnification EUVL stepper. Characterization of the microscope's transfer function shows it can resolve 55 nm half period patterns. With these capabilities, the λ=13.2 nm microscope is well suited for evaluation of pattern and defect printability of EUVL masks for the 22 nm node.
We present results on a table-top microscope that uses an EUV stepper geometry to capture full-field images with a halfpitch
spatial resolution of 55 nm. This microscope uses a 13.2 nm wavelength table-top laser for illumination and
acquires images of reflective masks with exposures of 20 seconds. These experiments open the path to the realization of
high resolution table-top imaging systems for actinic defect characterization.
Phase sensitive x-ray microscopy techniques are important in the study of samples that exhibit phase contrast. One way
to detect these phase effects is to optically implement the radial Hilbert transform by using spiral zone plates (SZPs),
resulting in the imaging of the amplitude and phase gradient in a sample. This is similar to differential interference
contrast imaging in light microscopy. Soft x-ray microscopy using a SZP as a single element objective lens was
demonstrated through the imaging of a 1 μm circular aperture at a wavelength of 2.73 nm. A regular zone plate, a
charge 1 SZP, and a charge 2 SZP were fabricated on a silicon nitride membrane using electron beam lithography. The
negative e-beam resist hydrogen silsesquioxane (HSQ) was used for patterning, and the zone plates were electroplated
with nickel. These zone plates were then used as the imaging optic in a soft x-ray microscopy setup.
We have acquired images with sub-38 nm spatial resolution using a tabletop extreme ultraviolet (EUV) imaging system operating at a wavelength of 13.2 nm, which is within the bandwidth of Mo/Si lithography mirrors This zone plate-based, full-field microscope has the power to render images in only several seconds with up to a 10,000 μm<sup>2</sup> field of view. The ability to acquire such high-resolution images using a compact EUV plasma laser source opens many possibilities for nanotechnology, including in-house actinic inspection of EUV lithography mask blanks.
The production of defect-free mask blanks, and the development of techniques for inspecting and qualifying EUV mask blanks, remains a key challenge for EUV lithography. In order to ensure a reliable supply of defect-free mask blanks, it is necessary to develop techniques to reliably and accurately detect defects on un-patterned mask blanks. These inspection tools must be able to accurately detect all critical defects whilst simultaneously having the minimum possible false-positive detection rate.
There continues to be improvement in high-speed non-actinic mask blank inspection tools, and it is anticipated that these tools can and will be used by industry to qualify EUV mask blanks. However, the outstanding question remains one of validating that non-actinic inspection techniques are capable of detecting all printable EUV defects.
To qualify the performance of non-actinic inspection tools, a unique dual-mode EUV mask inspection system has been installed at the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratory. In high-speed inspection mode, whole mask blanks are scanned for defects using 13.5-nm wavelength light to identify and map all locations on the mask that scatter a significant amount of EUV light. In imaging, or defect review mode, a zone plate is placed in the reflected beam path to image a region of interest onto a CCD detector with an effective resolution on the mask of 100-nm or better. Combining the capabilities of the two inspection tools into one system provides the unique capability to determine the coordinates of native defects that can be used to compare actinic defect inspection with visible light defect inspection tools under commercial development, and to provide data for comparing scattering models for EUV mask defects.
To qualify the performance of non-actinic inspection tools, a novel EUV mask inspection system has been installed at the Advanced Light Source (ALS) synchrotron facility at Lawrence Berkeley National Laboratory. Similar to the older generation actinic mask inspection tool1, the new system can operate in scanning mode, when mask blanks are scanned for defects using 13.5-nm in-band radiation to identify and map all locations on the mask that scatter a significant amount of EUV light. By modifying and optimizing beamline optics (11.3.2 at ALS) and replacing K-B focusing mirrors with a high quality Schwarzschild illuminator, the new system achieves an order of magnitude improvement on in-band EUV flux density at the mask, enabling faster scanning speed and higher sensitivity to smaller defects. Moreover, the system can also operate in imaging mode, when it becomes a zone-plate-based full-field EUV microscope with spatial resolution better than 100 nm. The microscope utilizes an off-axis setup, making it possible to obtain bright field images over a field-of-view of 5x5 um<sup>2</sup>.
We present a simple setup for obtaining high resolution, sub-micron images using high harmonic generation (HHG) in a hollow-core waveguide as a light source. We demonstrate imaging with illumination at a wavelength of 30 nm using an all-reflective, double-multilayer mirror setup and a CCD camera as a recording device. For the magnifications of up to 50x used here, the all-reflective setup has advantages over zone plate microscopes because of the much larger working distances that allow for imaging of plasmas. This setup has also a throughput that is higher by at least a factor of three compared to zone-plate microscopes, and presents the additional advantage of preserving the temporal pulse width of the harmonics because diffractive optics are not used. This work demonstrates the feasibility of high-spatial-resolution, time-resolved, EUV imaging of plasmas and other objects using a tabletop compact light source.
Soft x-ray lasers have become practical light sources for applications such as interferometric diagnosis of plasmas, the measurement of optical constants, and the characterization of EUV optics. For many potential applications involving light-matter interaction and nonlinear physics, the achievable light intensity is a critical parameter. High intensity could be realized by focusing high pulse energy light with good beam quality. Recent progress at Colorado State University has realized milli-joule level pulse energy at 46.9 nm from a capillary discharge-pumped tabletop soft x-ray laser. Direct measurements of the spatial coherence of the laser beam using a two-pinhole interference method have shown very high spatial coherence of the beam. These results imply the possibility of achieving very high intensity in the soft x-ray region. In this paper we report the results of a focusing experiment conducted with the laser mentioned above and designed to measure the focusability of the beam. The spatial profile of the focused beam is measured with knife-edge scanning technique and characterized with an M-squared factor. The results suggest that 10<SUP>13</SUP> W/cm<SUP>2</SUP> intensity is achievable with modestly tight focusing.