Process design for the manufacturing of soft X-ray gratings in single-crystal diamond by high-energy heavy-ion irradiation

At a Glance

Metadata	Details
Publication Date	2022-10-19
Journal	The European Physical Journal Plus
Authors	Y. Zamora Garcia, Michael Martin, M.D. Ynsa, V. Torres‐Costa, Miguel L. Crespillo
Institutions	Universidad Autónoma de Madrid, Consejo Superior de Investigaciones Científicas
Citations	12
Analysis	Full AI Review Included

Executive Summary

This paper validates a novel manufacturing process for high-performance optical gratings using highly focused Swift Heavy-Ion Irradiation (SHII) on single-crystal diamond substrates.

Core Value Proposition: SHII creates precise surface topography (swelling) by inducing buried structural damage, enabling the fabrication of UV/soft X-ray gratings on diamond.
Substrate Advantage: Single-crystal diamond offers superior thermal conductivity and radiation hardness, addressing the increasing heat load challenges in modern synchrotron and Free Electron Laser (FEL) facilities.
Target Geometry: The process successfully replicated a simplified blazed grating geometry inspired by the xLEG grating of the ALBA LOREA beamline (20 µm period, 0.0625° blaze angle).
Process Validation: Preliminary proof-of-concept experiments using 9 MeV ¹²C⁺³ ions successfully manufactured 12 reproducible blazed lines, validating the fluence calculation methodology.
Key Challenge (Smoothing): The elastic response of the diamond material contributes significantly to pattern smoothing, limiting the achievable line density. The current method is deemed feasible for 20 µm period gratings with minimal efficiency loss.
Upscaling Bottleneck: Total irradiation time for a full, millimeter-length grating is estimated to be several tens of hours, necessitating exploration of heavier ion species (e.g., Silicon or Gold) to reduce required fluence and manufacturing time.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Single-crystal diamond	N/A	Type IIa, (100) orientation
Substrate Impurities (N, B)	< 1 ppm (N), 50 ppb (B)	N/A	Optical-grade quality
Ion Species	¹²C⁺³	N/A	Swift Heavy-Ion Irradiation (SHII)
Ion Energy	9	MeV	Used for implantation
Beam Current	~200	pA	Used during scanning
Beam Size (FWHM)	~5 x 2	µm²	Focused beam dimensions
Target Grating Period	20	µm	50 lines/mm (Simplified geometry)
Target Blaze Angle (Exp.)	0.0625	°	Ultra-shallow angle for proof-of-concept
Experimental Swelling	15 to 25	nm	Measured surface topography (Fig. 9)
Fluence Range (Calculated)	10¹⁴ to 3 x 10¹⁵	ions/cm²	Required to achieve target swelling
Pattern Smoothing (Model)	3.7	µm	Gaussian sigma used to model elastic response
LOREA Heat Load (Max)	1	W	Thermal load on the reference grating stage
LOREA Power Density (Max)	10	mW/mm²	Power density on the reference grating stage
Diamond Density (ρ_d)	3515	kg/m³	Material parameter
Diamond Young’s Modulus (E_d)	1220	GPa	Material parameter

Key Methodologies

The manufacturing process relies on calculating the required ion fluence profile (Φ(x)) necessary to generate a specific target swelling profile (h(x)) via the structural damage mechanism.

Target Profile Definition:
- A blazed geometry (linear swelling profile, h(x) = A + Bx) was chosen for the proof-of-concept, mimicking the dispersive direction of a grating.
- The minimum swelling (h(0) = A) was set to correspond to a non-zero minimum fluence (10¹⁴ ions/cm²) to avoid infinite beam velocity during scanning.
Fluence Profile Calculation (Numerical Inversion):
- The swelling equation (Eq. 2), which relates swelling h(x) to fluence Φ(x) and nuclear energy density deposition S_n(z), was numerically inverted.
- The material constants (a and b) and the S_n(z) profile (obtained via SRIM simulation for 9 MeV ¹²C ions) were fixed parameters.
- The numerical solution yielded the required fluence profile Φ(x) necessary to achieve the target linear swelling h(x).
Irradiation and Scanning:
- Irradiation was performed using the CMAM microbeam line in frontal geometry.
- The beam was scanned pixel-by-pixel using custom software. Scanning steps were much smaller than the 20 µm grating period to ensure a quasi-continuous fluence deposition profile.
- The calculated fluence Φ(x) was converted into discrete beam charge values (Q) for each pixel, based on the grating width (w), length (l), ion charge state (q), and number of scan lines (N).
Characterization and Modeling:
- The resulting surface topography was measured using Atomic Force Microscopy (AFM) in non-contact mode.
- The measured profile was compared to the ideal target profile, showing smoothing due to the non-zero beam size and the elastic response of the diamond.
- The elastic effect was modeled by convoluting the ideal swelling profile with a Gaussian function (sigma 3.7 µm) and validated using Finite Element (FE) simulations (COMSOL Multiphysics), showing excellent agreement with AFM data.

Commercial Applications

This technology is critical for developing next-generation optical components that must withstand extreme operational environments, primarily driven by advancements in light source technology.

Synchrotron and FEL Facilities:
- Manufacturing high-efficiency, radiation-hard gratings for beamline monochromators (e.g., UV and soft X-ray regimes).
- Addressing the increasing thermal load demands from high-brightness, high-yield light sources where traditional silicon substrates fail.
High-Power Laser Optics:
- Creating diffractive optical elements (DOEs) and gratings for high-power laser systems where thermal management is paramount.
Extreme Environment Sensing/Optics:
- Utilizing the radiation hardness of diamond for optical components deployed in environments subject to high particle flux or intense radiation fields.
Advanced Diamond Manufacturing (Related to 6ccvd.com focus):
- The process provides a method for precise, localized modification of single-crystal diamond, potentially applicable to creating micro-optical structures, waveguides, or quantum defects (though not the focus of this paper, the SHII technique is foundational).

Tech Support

Original Source

DOI: https://doi.org/10.1140/epjp/s13360-022-03358-3

References

1997 - Gratings, mirrors and slits: beamline design for soft X-ray synchrotron radiation sources