logo
ResearchBunny Logo
Introduction
4th-generation synchrotron sources, by creating extremely compact electron beam bunches, produce highly brilliant X-ray beams, potentially revolutionizing scientific research. Facilities like Max IV Laboratory in Sweden, with its multi-bend achromat-based storage ring (operational since 2015), SIRIUS, and ESRF-EBS (both operational since 2020), exemplify this advancement. These sources significantly improve the magnetic lattice structure, resulting in a dramatic reduction in electron beam transverse size and divergence. This leads to a substantial increase in X-ray beam brilliance (photons emitted per solid angle and per surface unit). Crucially, for an inherently incoherent source like a synchrotron, the coherence of the X-ray beam is directly proportional to the source size and inversely proportional to the measurement distance. The expected coherent X-ray flux increase, dependent on energy and source power, can reach a factor of 200 in the 6–10 keV range. This enhanced flux opens new avenues for coherent X-ray techniques, particularly impacting crystal microscopy. X-ray crystal microscopy, crucial in material science and related fields, combines the high penetration power of X-rays with 3D nanoscale spatial resolution of crystalline properties, offering capabilities complementary to electron beam diffraction methods. Coherent diffractive X-ray imaging elegantly circumvents the spatial resolution limits of X-ray optics by numerically inverting measured diffraction patterns. Applied to crystal microscopy (Bragg condition), this yields fine, high-contrast features (speckles) in the diffraction patterns, revealing lattice strain and tilt information. Finite-support based Bragg coherent diffraction imaging (BCDI) has achieved notable results, but it's limited to small (0.1-1 µm) samples and struggles with weak or inhomogeneous strain fields. Bragg ptychography, using a scanned illumination probe, offers an alternative, capable of imaging larger, inhomogeneous samples and weaker strain fields. However, its extreme sensitivity to instabilities and demanding setup requirements have hindered its broader application. This study explores the potential of 4th-generation synchrotron X-ray beams for Bragg ptychography crystal microscopy, comparing results with those from a 3rd-generation source.
Literature Review
The literature extensively discusses coherent diffractive X-ray imaging techniques and their applications in material science. Finite-support based Bragg coherent diffraction imaging (BCDI) has been shown to be a powerful technique for imaging crystalline samples. However, its limitations, particularly concerning sample size and sensitivity to weak or inhomogeneous strain fields, are well documented in the literature. Several studies highlight the challenges associated with BCDI, particularly in achieving high-quality reconstructions for complex samples. Meanwhile, Bragg ptychography has emerged as a promising alternative, offering advantages in terms of sample size and strain field sensitivity. While the potential of Bragg ptychography has been recognized, the practical implementation challenges due to sensitivity to experimental instabilities and the demands on the setup have been significant barriers, as evidenced by the limited number of successful experiments reported in the literature. This paper builds upon the existing literature by investigating the impact of the advanced capabilities of 4th-generation synchrotron radiation sources on overcoming these limitations inherent to Bragg ptychography.
Methodology
The Bragg ptychography experiment was conducted at the NanoMAX beamline of Max IV Laboratory, Sweden, using a lithographically-patterned crystalline Si-star test chart (180 nm thick, 2 µm diameter, 11 branches, 55-760 nm branch width). A 12 keV monochromatic beam, focused to ~80 nm, illuminated the sample (~6 × 10⁹ photons·s⁻¹). Data acquisition employed a continuous fly-scan mode along one axis and a step mode along another, at various angles. Each frame's exposure time was 33 ms, totaling ~30 minutes for the 3D dataset (~50,000 points). Radiation damage was monitored, and acquisition time was limited to 50% of the damage threshold. The experiment intentionally did not optimize against sample drift, leading to expected distortions. Initial Bragg ptychography inversion yielded degraded images. A novel 3D Bragg ptychography inversion scheme was developed, incorporating sample position refinement and accounting for the fly-scan mode. This scheme iteratively refines the sample position and corrects for scanning distortions during the reconstruction process. The 3D reconstruction provided amplitude (electron density) and phase (crystalline displacement field projection along [110]) maps. Spatial resolution was assessed using Fourier shell correlation (FSC) analysis, revealing anisotropic resolution (~23, 7, and 12 nm along x, y, z). The retrieved probe was analyzed using multi-mode decomposition and compared to a forward ptychography analysis. A numerical study simulating datasets with varying photon counts was performed to validate the effect of photon statistics on the reconstruction quality.
Key Findings
The new 3D Bragg ptychography inversion scheme successfully retrieved a high-quality image of the Si-star sample, revealing fine details previously unattainable with 3rd-generation sources and standard inversion methods. The amplitude map accurately depicted the star pattern's shape, including small defects confirmed by SEM. The phase map revealed inhomogeneous strain and lattice tilts, likely due to the patterning process, with radial phase variations following the star's branches. Analysis of the phase map yielded strain and lattice tilt maps. Positive strain indicated local stretching, while tilt maps showed symmetric rotations around axes corresponding to branch alignment. A strain gradient was observed at branch surfaces, with a strained layer at the Si pattern/wafer interface, consistent with previous observations. The 3D spatial resolution was anisotropically determined to be ~23, 7, and 12 nm along x, y, and z, respectively. The multi-mode probe decomposition showed high agreement between forward and Bragg ptychography results for the first mode. However, differences in mode power distribution were observed, potentially due to different photon statistics between forward and Bragg scattering. The numerical study confirmed that photon count statistics influenced the reconstruction accuracy. The study definitively shows the superiority of using a 4th-generation synchrotron source for Bragg ptychography, allowing for high-quality reconstructions despite significant scanning imperfections, and providing much improved data quality compared to 3rd-generation sources.
Discussion
This study demonstrates that the enhanced coherent flux of 4th-generation synchrotron sources drastically improves the capabilities of Bragg ptychography for crystalline imaging. The ability to obtain high-quality reconstructions even with significant scanning imperfections highlights the robustness and potential of this approach. The detailed 3D crystalline information extracted (strain, tilt) provides invaluable insights into material microstructure. The success of the novel 3D inversion scheme addresses a major limitation of Bragg ptychography, paving the way for broader application of this powerful technique. This advancement opens new possibilities for high-throughput studies of complex crystalline materials, enabling investigation of weak and highly inhomogeneous strain fields in extended samples. The findings significantly advance nanoscale crystallography and material characterization.
Conclusion
The research successfully demonstrated the significant advantages of using 4th-generation synchrotron sources for Bragg ptychography. A novel 3D inversion scheme enabled high-quality reconstructions despite suboptimal scanning conditions. This work opens new avenues for high-throughput studies of complex crystalline materials and provides valuable insights into nanoscale crystallography. Future work could explore the application of this technique to a wider range of materials and investigate further improvements in data acquisition and processing strategies to enhance the robustness and efficiency of Bragg ptychography. Development of improved sample stages to mitigate sample drift would be highly beneficial for routine use of this powerful technique.
Limitations
The study focused on a specific type of sample (lithographically patterned Si-star). While this allowed for a controlled evaluation, the generalizability to other materials and sample geometries requires further investigation. The current 3D inversion scheme relies on assumptions about the sample and its interaction with the X-ray beam; refinement of these assumptions could further improve the reconstruction accuracy. The impact of radiation damage, although monitored, remains a potential limiting factor for highly sensitive samples. Finally, the accessibility of 4th generation synchrotron sources might restrict the widespread adoption of this technique in the near term.
Listen, Learn & Level Up
Over 10,000 hours of research content in 25+ fields, available in 12+ languages.
No more digging through PDFs, just hit play and absorb the world's latest research in your language, on your time.
listen to research audio papers with researchbunny