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	Karlsruhe Institute of Technology, Tohoku University
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Hard X-ray Compound Refractive Lens Arrays

Executive Summary

This research demonstrates the successful fabrication and characterization of a high-performance 2D array of Compound Refractive Lenses (CRLs) for hard X-ray microscopy. The findings validate a methodology for achieving sub-pixel resolution and a large field of view (FoV) in scanning transmission X-ray microscopy (STXM).

Application: Sub-pixel resolution scanning transmission hard X-ray microscopy (STXM) for optically thick samples.
Core Achievement: Development of a 34 x 34 multi-lens array generating 1156 point foci, enabling a large 3.5 mm² FoV.
Resolution: Achieved sub-pixel resolution with small average focal spot sizes (2.10 µm x 3.55 µm FWHM) at 34 keV.
Fabrication Method: Deep X-ray Lithography (LIGA process) utilizing radiation-stable SU-8 polymer resist, achieving sidewall roughness of approximately 20 nm.
Performance Metric: Demonstrated an average Spectral Intensity Enhancement (SIE) of 16 and 66% transmission, significantly reducing scanning time compared to conventional single-lens STXM.
Future Direction: The study explicitly discusses the need for higher energy optics (>80 keV) using materials like Nickel, requiring superior material stability and precision.
6CCVD Value Proposition: 6CCVD offers ultra-low roughness Single Crystal Diamond (SCD) substrates (Ra < 1 nm), providing superior thermal and radiation stability necessary to extend this high-precision optics research to higher photon energies and higher flux environments.

Technical Specifications

The following hard data points were extracted from the characterization of the 2D multi-lens array at the Diamond Light Source (B16 beamline).

Parameter	Value	Unit	Context
Operating Energy	34	keV	Monochromatic X-ray illumination
Energy Resolution	10^-4	$\Delta E/E$	Si (311) channel-cut monochromator
Array Configuration	34 x 34	Lenses	2D multi-lens array (1156 foci)
Field of View (FoV)	3.5	mm²	Total imaging area
Design Focal Length ($f_{v,h}$)	359	mm	Calculated optimal focal distance
Measured Focal Length ($f_{exp,v,h}$)	362 $\pm$ 4	mm	Experimental result
Vertical Focal Spot Size (FWHM)	2.10 $\pm$ 0.81	µm	Average measured FWHM
Horizontal Focal Spot Size (FWHM)	3.55 $\pm$ 0.62	µm	Average measured FWHM
Physical Aperture ($A_{ph}$)	55 x 55	µm	Matches detector pixel pitch
Average Transmission ($T_{avg}$)	66	%	Measured via SIE for 55 µm x 55 µm reference area
Average SIE (Gain)	16	-	Spectral Intensity Enhancement (1.8 µm x 1.8 µm reference area)
Resist Sidewall Roughness	~20	nm	Achieved via Deep X-ray Lithography
Total Scan Time (Estimated)	~50	min	For a complete 3.5 megapixel raster scan

Key Methodologies

The 2D CRL array was fabricated using high-precision lithography and mechanical assembly, and characterized using synchrotron radiation.

Material Selection: SU-8 epoxy-based negative resist (radiation stable up to 2 MJ/cm³) was used on a 525 µm silicon substrate.
Mask Preparation: An X-ray absorber working mask was created using 20 µm gold absorbers on a 2.5 µm titanium membrane.
Deep X-ray Lithography (LIGA): The SU-8 resist was exposed to X-rays (dose: 19,845 mA*min/cm³) to create 1D line focus lenses with parallel, smooth sidewalls (roughness ~20 nm).
Post-Processing: Post-exposure bake (66 °C for 20 h), followed by development (PGMEA for 2 h) and rinsing (isopropanol).
2D Assembly: To achieve a point focus, the 1D line focus lenses were cut by a diamond wafer saw, rotated 90° around the optical axis, and mounted in an interdigitated configuration.
X-ray Characterization: The array was tested at the Diamond Light Source (B16) using 34 keV X-rays generated by a Si (311) channel-cut monochromator.
Detection: Point foci were measured using a high-resolution indirect detector system (LuAG:Eu scintillator, 10x objective, CCD camera) resulting in an effective pixel size of 0.9 µm x 0.9 µm.
Data Analysis: Focal distance and spot size (FWHM) were determined by scanning the detector along the optical axis and fitting the foci using the 2D Gaussian method.

6CCVD Solutions & Capabilities

The research highlights the critical need for materials with extreme stability, low roughness, and high transparency for hard X-ray optics, especially when scaling to higher energies (>80 keV) or higher flux. 6CCVD’s MPCVD diamond materials are ideally suited to meet and exceed these requirements, offering a path to next-generation X-ray optics.

Applicable Materials

While the current study used polymer (SU-8) and discussed Nickel (Ni), Optical Grade Single Crystal Diamond (SCD) from 6CCVD offers superior performance for hard X-ray applications:

Material Requirement	6CCVD Solution	Advantage over Polymer/Ni
Radiation Stability	Optical Grade SCD	Diamond is the most radiation-hard material, eliminating degradation seen in polymers (SU-8) under high flux.
Thermal Management	Optical Grade SCD	Highest known thermal conductivity, crucial for managing heat load from high-flux synchrotron beams.
Surface Precision	Optical Grade SCD	Guaranteed polishing to Ra < 1 nm (up to 20x smoother than the 20 nm achieved in SU-8), minimizing scattering and improving focus quality.
High Energy Transparency	Optical Grade SCD	Excellent transparency at hard X-ray energies (34 keV and above 80 keV), reducing absorption losses compared to polymers or metals like Ni.
Substrate/Window	Polycrystalline Diamond (PCD)	Available in large plates (up to 125 mm diameter) and custom thicknesses (0.1 µm to 500 µm) for use as high-stability optical windows or substrates.

Customization Potential

6CCVD’s manufacturing capabilities directly address the precision and dimensional requirements necessary for replicating or advancing this CRL array research.

Dimensional Control: The paper used a small array (1.87 mm x 1.87 mm). 6CCVD can supply large-area PCD substrates up to 125 mm for scaling up the FoV of future arrays.
Ultra-Precision Polishing: The achieved focal spot size is highly dependent on surface quality. 6CCVD provides SCD polishing to Ra < 1 nm and Inch-size PCD polishing to Ra < 5 nm, ensuring minimal wavefront distortion and superior focusing performance compared to the 20 nm roughness reported.
Metalization Services: The paper discussed the potential for Ni CRLs fabricated via electroplating (LIGA). 6CCVD offers in-house metalization capabilities (including Au, Pt, Ti, W, and Cu) for creating high-precision contacts or masks on diamond substrates, supporting advanced LIGA or etching processes.
Custom Thickness: We provide SCD and PCD plates in thicknesses ranging from 0.1 µm to 500 µm, allowing researchers to optimize the substrate thickness for specific X-ray energy absorption and mechanical stability requirements.

Engineering Support

6CCVD’s in-house PhD team specializes in the material science of diamond for extreme environments. We can assist researchers in material selection and specification for similar Hard X-ray Microscopy and High-Flux Optics projects, ensuring the chosen diamond material maximizes performance, stability, and longevity.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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

1996 - A compound refractive lens for focusing high-energy X-rays [Crossref]
2002 - Parabolic refractive X-ray lenses [Crossref]
1998 - Transmission and gain of singly and doubly focusing refractive X-ray lenses [Crossref]
2004 - Refractive microlens array for wave-front analysis in the medium to hard X-ray range [Crossref]
2008 - Spatial harmonic imaging of X-ray scattering—Initial results [Crossref]
2018 - Scalable, large area compound array refractive lens for hard X-rays [Crossref]
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]