Coherent Diffraction ImagingCopyright: Forschungszentrum Jülich | Jan Bußmann
Coherent Diffraction Imaging (CDI) is a lens-less imaging technique  exhibiting wavelength-limited spatial resolution not impacted by the quality of optical elements. Instead of mapping each point of the sample onto a detector as it is done by conventional microscopy, in CDI, a diffraction pattern produced by the sample is recorded. Optical elements of a conventional microscope are replaced by phase retrieval algorithms that allow for numerical reconstruction of the sample profile from the recorded pattern yielding both amplitude and phase information. This method is particularly useful with EUV (5-50 nm wavelengths) and soft x-ray (1-5 nm wavelengths) radiation. In addition to high spatial resolution, high elemental contrast may also be achieved due to significant differences in optical properties of materials in the mentioned spectral ranges [2-5].
Application of this method since its introduction was associated with coherent light-sources. In EUV and soft x-ray spectral ranges these are mainly synchrotrons and free electron laser facilities. Some success in small-scale laboratory environment has been achieved utilizing high harmonics of fs-lasers and with soft X-ray lasers.
Our research group aims to make CDI accessible to broad research community by employing simpler light sources. Discharge plasma based EUV sources are promising candidates for that. These sources have been a subject of intense investigation targeting EUV lithography [6-7]. They are compact, scalable and failsafe instruments with low maintenance cost and complexity, extending the availability of EUV light to a broad field applications.
Setup and Future ProjectsCopyright: Forschungszentrum Jülich | Jan Bußmann
The experimental apparatus consists of a gas discharge plasma EUV source developed by the Fraunhofer Institute for Laser technology, a multilayer mirror for narrowing spectral bandwidth of the radiation illuminating structured gold-coated silicon nitride membranes acting as samples (see figure 2). The samples are fabricated at the Helmholtz Nanofacility, Forschungszentrum Jülich. These samples are designed to test the method`s concept and to optimize setup and numerical algorithms.
The coherent diffraction imaging concept with the plasma based sources has been demonstrated already by our group . Future projects aim at gaining information about material properties of the sample and its thickness by tuning the illumination wavelength using phase gratings.
Furthermore, Ptychography – an extension of CDI  enables to overcome the CDI´s oversampling requirement and allows increasing the Field-of-View. The ptychography algorithms also allow faster reconstruction of the sample image and of the incident beam profile. Previously, CDI and ptychography have been used with thin membranes and in the transmission mode. One of the future goals is realizing these techniques in a reflective mode that could extend their feasibility for imaging nanostructures of thick samples. Reflective measurements in combination with scatterometry are of high interest for the application in e.g. mask inspection for EUV lithography.
 Miao, J. et al.: ‘Extending the methodology of X-ray crystallography to allow imaging of micrometre-sized non-crystalline specimens’, Nature, 400, 342-344, 1999
 Miao, J. et al. ‘Coherent X-ray Diffraction Imaging’, IEEE J. Sel. Top. Quant. Electron., 18, 399-410, 2012
 Spence, J.C.H.: ‘Diffractive (Lensless) imaging’ in Science of Microscopy, Springer, 2007
 Nugent, K.: ‘Coherent methods in the X-ray sciences’, Adv. Phys., 59, 1-99, 2010
 Chapman, H. N., and Nugent, K. A.: ‘Coherent lensless X-ray imaging’, Nat. Photon., 4, 833-839, 2010
 Bergmann, K. et al.: ‘Highly repetitive, extreme-ultraviolet radiation source based on a gas-discharge plasma‘, Appl. Opt., 38, 5413-5417, 1999
 Benk, M. and Bergmann, K.: ‘Brilliance scaling of discharge sources for extreme-ultraviolet and soft x-ray radiation for metrology applications’, J. Micro/Nanolith. MEMS MOEMS, 11, 021106, 2012
 Juschkin, L. et al.: ‘Tabletop coherent diffraction imaging with a discharge plasma EUV source’, Proc. SPIE, 8849, 88490Y, 2013
 Rodenburg, J. et al.: 'Hard-X-Ray Lensless Imaging of Extended Objects', Physical Review Letters, 98, 2007
|Jan Bussmann M. Sc.|
|+49 2461 6196469|
|Maciej Grochowicz B. Sc.|
|+49 2461 615010|
|Michal Odstrcil M.Sc.|
Ph.d. Student, EXTATIC
|+49 2461 6196466|
|Dr.Denis Rudolf Dipl.-Phys.|
|+49 2461 6196469|