Development of an Array of Compound Refractive Lenses for Sub-Pixel Resolution, Large Field of View, and Time-Saving in Scanning Hard X-ray Microscopy

At a Glance

Metadata	Details
Publication Date	2020-06-16
Journal	Applied Sciences
Authors	Talgat Mamyrbayev, Alexander Opolka, Alexey Ershov, Josephine Gutekunst, Pascal Meyer
Institutions	Tohoku University, Karlsruhe Institute of Technology
Citations	5
Analysis	Full AI Review Included

Executive Summary

This research details the development and characterization of a high-performance, two-dimensional (2D) array of Compound Refractive Lenses (CRLs) designed for hard X-ray microscopy applications requiring high spatial resolution and a large field of view (FoV).

Core Achievement: Fabrication of a 2D 34x34 multi-lens array (1156 point foci) using deep X-ray lithography (LIGA process) in radiation-stable SU-8 polymer.
Performance Metrics: Tested at 34 keV, the array achieved an average Spectral Intensity Enhancement (SIE) of 16 and an average transmission of 66%.
Resolution: Generated small point foci (FWHM) averaging 2.10 µm (Vertical) and 3.55 µm (Horizontal), enabling sub-pixel resolution scanning transmission X-ray microscopy (STXM).
Field of View (FoV): The array provides a large FoV of 3.5 mm², significantly larger than previous 2D arrays, suitable for studying relatively large samples.
Integration: The array is designed to match the pixel pitch (55 µm x 55 µm) of standard low-resolution photon-counting detectors (e.g., Medipix Merlin 3.0).
Future Scalability: The methodology is adaptable for fabricating Nickel CRL arrays for use at higher photon energies (above 80 keV, up to 100 keV), addressing the demand for high-energy focusing optics at bending magnet sources.

Technical Specifications

Parameter	Value	Unit	Context
Lens Array Configuration	34 x 34	elements	Total 1156 point foci
Field of View (FoV)	3.5	mm²	Total area covered by the array
Target Photon Energy (Test)	34	keV	Tested energy at Diamond Light Source B16
Designed Focal Length (f_v,h)	359	mm	Calculated design value
Measured Focal Length (f_exp,v,h)	362 ± 4	mm	Experimental result
Vertical Focal Spot Size (FWHM)	2.10 ± 0.81	µm	Measured average spot size
Horizontal Focal Spot Size (FWHM)	3.55 ± 0.62	µm	Measured average spot size
Spectral Intensity Enhancement (SIE)	16	dimensionless	Average focusing gain
Average Transmission (T_avg)	66	%	Measured via SIE
Individual CRL Aperture (A_ph)	55 x 55	µm	Matches detector pixel pitch
Detector Pixel Size (Medipix)	55 x 55	µm	Low resolution detector used for imaging
Refractive Surface Roughness	20	nm	Achieved via deep X-ray lithography
Total Array Length (L_CRL)	22.5	mm	Physical length along the optical axis
Radius of Curvature (R_v,h)	15.9	µm	Parabolic surface parameter
Refractive Index Decrement (δ)	2.43 x 10^-7	dimensionless	At 34 keV

Key Methodologies

The 2D multi-lens array was fabricated using deep X-ray lithography (a variant of the LIGA process) followed by mechanical assembly, and characterized using monochromatic synchrotron radiation.

Fabrication (Deep X-ray Lithography):
- Resist Material: SU-8 epoxy-based negative resist, applied in a 2 mm high layer onto a 525 µm silicon substrate.
- Mask: A working mask consisting of 20 µm thick gold absorbers on a 2.5 µm titanium membrane was used.
- Exposure: X-ray exposure was performed at a dose of 19,845 mA*min/cm².
- Baking and Development: Post-exposure bake was performed at 66 °C for 20 hours. Unexposed resist was removed using propylene glycol methyl ether acetate (PGMEA) for 2 hours, followed by rinsing in isopropanol.
Assembly (2D Array Formation):
- The lithography process initially yielded 1D line focus lenses.
- To create the 2D point focus array (SHS), the substrate was cut, and one set of 1D line focus lenses was rotated 90° around the optical axis and mounted in an interdigitated configuration.
X-ray Characterization:
- Beamline: Diamond Light Source B16 bending magnet test beamline.
- Energy: Monochromatic 34 keV X-rays (Energy resolution ΔE/E = 10^-4).
- Focal Spot Measurement: Point foci were measured using a high-resolution indirect detector system (5 µm LuAG:Eu scintillator coupled with a 10x objective and CCD camera, resulting in 0.9 µm effective pixel size).
- Data Analysis: Focal length and focal spot size (FWHM) were determined by fitting the foci using the 2D Gaussian method.
Sub-Pixel Resolution Imaging (STXM):
- Detector: Medipix Merlin 3.0 photon-counting detector (55 µm pixels).
- Scanning: The sample (a gold Siemens star test pattern) was raster-scanned using a piezo stage with 1 µm steps (sub-pixel shift) within the 55 µm pixel size.
- Reconstruction: Sub-pixel resolution images were reconstructed by combining the low-resolution images acquired at each sub-pixel step.

Commercial Applications

This technology provides a critical advancement in hard X-ray optics, enabling faster, higher-resolution imaging over larger areas, which is essential for non-destructive evaluation (NDE) and advanced materials research.

Non-Destructive Testing (NDT): High-energy X-rays (15 keV to 100 keV) allow for the study of optically thick objects, such as characterizing crack distributions and internal defects in metallic plates, composite materials, and industrial components.
Synchrotron Beamline Optics: The 2D multi-lens array serves as an attractive optical element for hard X-ray microscopy applications at synchrotron facilities, providing both large FoV and high spatial resolution simultaneously.
High-Speed Imaging: The multi-foci array enables rapid scanning transmission X-ray microscopy (STXM), significantly reducing the time required for a complete raster scan compared to conventional single-lens STXM, facilitating the study of dynamic processes.
Advanced Materials Research: Used for high-resolution structural analysis and characterization of complex materials, including polymers, ceramics, and high-density alloys, especially when high penetration depth is required.
Detector Technology Integration: The design is optimized for use with standard, relatively low-resolution pixel detectors (55 µm pitch), simplifying the overall system complexity and cost compared to systems requiring ultra-high-resolution detectors.

View Original Abstract

A two-dimensional array of compound refractive lenses (2D array of CRLs) designed for hard X-ray imaging with a 3.5 mm2 large field of view is presented. The array of CRLs consists of 2D polymer biconcave parabolic 34 × 34 multi-lenses fabricated via deep X-ray lithography. The developed refractive multi-lens array was applied for sub-pixel resolution scanning transmission X-ray microscopy; a raster scan with only 55 × 55 steps provides a 3.5 megapixel image. The optical element was experimentally characterized at the Diamond Light Source at 34 keV. An array of point foci with a 55 µm period and an average size of ca. 2.1 µm × 3.6 µm was achieved. In comparison with the conventional scanning transmission microscopy using one CRL, sub-pixel resolution scanning transmission hard X-ray microscopy enables a large field of view and short scanning time while keeping the high spatial resolution.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app10124132

References

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2019 - Time and Mechanism of Nanoparticle Functionalization by Macromolecular Ligands during Pulsed Laser Ablation in Liquids [Crossref]
2019 - Super-resolution scanning transmission X-ray imaging using single biconcave parabolic refractive lens array [Crossref]