Polished diamond X-ray lenses

At a Glance

Metadata	Details
Publication Date	2022-03-15
Journal	Journal of Synchrotron Radiation
Authors	Rafael Celestre, Sergey Antipov, Edgar Gomez, Thomas Zinn, Raymond Barrett
Institutions	European Synchrotron Radiation Facility, Euclid Techlabs (United States)
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: Polished Diamond X-ray Lenses

Executive Summary

This documentation analyzes the fabrication and performance of polished single-crystal diamond (SCD) Compound Refractive Lenses (CRLs) for high-energy X-ray optics, highlighting the critical role of advanced material processing in achieving superior performance compared to commercial Beryllium (Be) lenses.

Core Achievement: SCD X-ray lenses, produced via femtosecond laser ablation followed by mechanical polishing, achieved focusing performance comparable to industry-standard R=50 µm Be lenses.
Surface Quality Improvement: Post-polishing successfully reduced the surface roughness (Sa) from 300-500 nm (as-ablated) to approximately 20 nm, achieving optical transparency.
Superior Scattering Performance: Polished SCD lenses demonstrated a Small-Angle X-ray Scattering (SAXS) background signal nearly two orders of magnitude lower than both unpolished diamond and commercial O30-H Be lenses, confirming diamond’s advantage for low-noise, high-flux applications.
Figure Error: The polished SCD lenses exhibited excellent RMS figure errors (σ ~ 0.9 µm), slightly better than the equivalent commercial Be lenses (σ ~ 1.1 µm).
Material Advantage: Diamond maintains a superior refraction-to-absorption ratio (δ/µ) compared to Be at high X-ray energies (>30 keV), requiring fewer lenses in a stack and offering unmatched thermal stability for high-power synchrotron beamlines.
6CCVD Value Proposition: 6CCVD offers the high-purity SCD material and precision polishing capabilities (Ra < 1 nm) necessary to replicate and surpass these results, providing custom dimensions and engineering support for next-generation X-ray optics.

Technical Specifications

Parameter	Value	Unit	Context
Lens Geometry	Bi-concave 2D Paraboloid	N/A	Compound Refractive Lenses (CRLs)
Diamond Apex Radius (R)	100	µm	Target radius for SCD lenses
Be Apex Radius (R)	50	µm	Commercial reference lenses
SCD Crystal Thickness (L_c)	~500	µm	Raw material thickness used
SCD Web Thickness (t)	~20	µm	Minimum thickness at lens center
Physical Aperture (A_phys)	~440	µm	Geometric aperture of lenses
As-Ablated Roughness (Sa)	300 - 500	nm	Before post-processing
Polished Roughness (Sa)	~20	nm	After chemical-mechanical polishing
RMS Figure Error (σ) - Polished SCD	0.9 ± 0.2	µm	Measured via X-ray Speckle Vector Tracking (XSVT)
RMS Figure Error (σ) - Be Reference	1.1 ± 0.1	µm	Commercial R=50 µm Be lenses
SAXS Background Reduction	~100x	N/A	Polished SCD vs. O30-H Be (at q=0.045 nm^-1)
X-ray Energy Tested (Focusing)	10	keV	Beam caustics and wire scans
X-ray Energy Tested (Metrology)	17, 30	keV	XSVT measurements
Equivalent Stack Size (10 keV)	10x SCD vs. 11x Be	N/A	For similar focusing strength

Key Methodologies

The fabrication and characterization relied on advanced micro-machining and at-wavelength metrology techniques:

Raw Material: Low dislocation density HPHT (100)-oriented Single Crystal Diamond (SCD) plates were used, typically 500 µm thick.
Laser Ablation: Bi-concave paraboloid profiles were created using femtosecond laser micro-machining (515 nm green laser, 200 fs pulse duration). This method minimizes thermal damage but leaves a rough surface.
Mounting: Diamond plates were precisely pressed into 12 mm-diameter bronze support disks, ensuring compatibility with standard synchrotron CRL hardware (transfocators).
Post-Polishing (CMP): A specialized chemical-mechanical polishing (CMP) procedure was developed, utilizing a conformal needle polishing bit and 0.1 µm diamond slurry to reduce surface roughness and achieve optical transparency.
Visible-Light Metrology: Scanning confocal laser microscopy was used for initial inspection of geometric parameters (R, penetration depth).
At-Wavelength Metrology (XSVT): X-ray Speckle Vector Tracking (XSVT) was employed at 17 keV and 30 keV to accurately measure figure errors and alignment of the front and back refractive surfaces, overcoming limitations of visible-light inspection.
Performance Measurement: Focusing capabilities were quantified using 2D CCD detectors and high-resolution wire scans (200 µm tungsten wire) at 10 keV to determine the Full Width at Half Maximum (FWHM) beam size at the focal plane.
Scattering Analysis (SAXS): Small-Angle X-ray Scattering measurements (12.23 keV) were performed to compare the background signal generated by polished SCD, unpolished SCD, and commercial Be lenses.

6CCVD Solutions & Capabilities

This research validates the critical need for high-quality, polished Single Crystal Diamond (SCD) for advanced X-ray optics. 6CCVD is uniquely positioned to supply the materials and processing required to replicate and advance this technology.

Applicable Materials

To achieve the low scattering and high thermal performance demonstrated in this paper, 6CCVD recommends:

Optical Grade Single Crystal Diamond (SCD): Our MPCVD-grown SCD offers high purity and low dislocation density, essential for minimizing bulk scattering and maximizing the refraction-to-absorption ratio (δ/µ), especially crucial for high-energy applications (>30 keV).
Custom Substrates: We provide SCD plates matching or exceeding the 500 µm thickness used, with the capability to supply substrates up to 10 mm thick for specialized high-aperture or high-power applications.

Customization Potential

The fabrication of high-performance CRLs requires extreme precision in material dimensions, geometry, and surface finish. 6CCVD offers comprehensive services to meet these demands:

Requirement from Paper	6CCVD Capability	Technical Advantage
Raw Material Dimensions	Custom Plates/Wafers up to 125mm (PCD)	Supports scaling up lens aperture or stack size.
Thickness Control	SCD thickness from 0.1 µm to 500 µm	Precise control over L_c (crystal thickness) and web thickness (t) requirements.
Ultra-Smooth Finish	Polishing down to Ra < 1 nm (SCD)	Significantly surpasses the 20 nm Sa achieved in the paper, guaranteeing even lower SAXS background and superior focusing fidelity.
Geometric Machining	Custom Laser Cutting and Etching	Capable of creating the initial bi-concave paraboloid geometry, replicating the femtosecond ablation step.
Mounting Integration	Custom Metalization (Au, Pt, Pd, Ti, W, Cu)	We offer in-house metalization services for precision mounting onto standard 12 mm bronze/copper alloy disks or for integrating active alignment features.

Engineering Support

The paper notes that the choice of diamond grade (HPHT vs. CVD, nitrogen content) significantly impacts cost and performance. 6CCVD’s in-house PhD material science team specializes in optimizing diamond growth parameters to meet specific X-ray optics requirements.

We provide consultation on:

Material Selection: Assisting engineers in selecting the optimal MPCVD diamond grade (e.g., low-nitrogen SCD or high-purity PCD) to balance cost, thermal performance, and scattering characteristics for similar Compound Refractive Lens (CRL) projects.
Process Optimization: Advising on the necessary polishing specifications to ensure figure errors and surface roughness meet the stringent demands of at-wavelength metrology (XSVT) and high-resolution focusing.

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

View Original Abstract

High-quality bi-concave 2D focusing diamond X-ray lenses of apex-radius R = 100 µm produced via laser-ablation and improved via mechanical polishing are presented here. Both for polished and unpolished individual lenses and for stacks of ten lenses, the remaining figure errors determined using X-ray speckle tracking are shown and these results are compared with those of commercial R = 50 µm beryllium lenses that have similar focusing strength and physical aperture. For two stacks of ten diamond lenses (polished and unpolished) and a stack of eleven beryllium lenses, this paper presents measured 2D beam profiles out of focus and wire scans to obtain the beam size in the focal plane. These results are complemented with small-angle X-ray scattering (SAXS) measurements of a polished and an unpolished diamond lens. Again, this is compared with the SAXS of a beryllium lens. The polished X-ray lenses show similar figure errors to commercially available beryllium lenses. While the beam size in the focal plane is comparable to that of the beryllium lenses, the SAXS signal of the polished diamond lenses is considerably lower.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s1600577522001795