Medusa 84 SiH - A novel high selectivity electron beam resist for diamond quantum technologies

At a Glance

Metadata	Details
Publication Date	2025-06-13
Journal	MRS Communications
Authors	Oliver Opaluch, Sebastian Westrich, Nimba Oshnik, Philipp Fuchs, Jan Fait
Analysis	Full AI Review Included

Technical Documentation & Analysis: Medusa 84 SiH Resist for Diamond Quantum Technologies

Executive Summary

This documentation analyzes the successful implementation of the novel electron beam resist, Medusa 84 SiH, as a high-performance substitute for Flowable Oxide (FOx) in the nanofabrication of single crystal diamond (SCD) quantum devices.

Application Focus: Fabrication of high-aspect-ratio diamond nanopillars containing shallow Nitrogen Vacancy (NV) centers, critical for quantum sensing and integrated photonics.
Material Compatibility: Medusa 84 SiH (an HSQ variant) demonstrated excellent etch resistance and compatibility with high-purity, electronic-grade SCD substrates.
High Selectivity: A diamond etching selectivity of 11 to 12 (Diamond:Resist) was achieved using O₂ + Ar ICP-RIE plasma, comparable to industry-standard FOx.
Quantum Coherence Preservation: Spin properties (T₂ coherence time ≈ 24 µs and T₁ spin lifetime ≈ 430 µs) were negligibly affected by the processing, confirming the structures are viable for quantum sensing.
High Fabrication Yield: Utilizing a 25 nm silicon adhesion layer, fabrication yields reached up to 96%, demonstrating robust process transferability.
Photonic Enhancement: Nanopillars achieved a 7.0 ± 0.9 increase in photon collection efficiency compared to bulk diamond, validating their effectiveness as photonic structures.
Surface Quality: The process maintained excellent surface quality, achieving a post-etch roughness (R_q,After) of 0.8 nm ± 0.2 nm on well-polished SCD.

Technical Specifications

The following hard data points were extracted from the research detailing the material properties and process outcomes for diamond nanostructuring.

Parameter	Value	Unit	Context
Diamond Material Grade	Electronic Grade SCD	N/A	(100)-oriented, [N]_s < 5 ppb, [B] < 1 ppb
Initial Surface Roughness (R_q,Before)	0.6 ± 0.5	nm	Measured via AFM (5 µm x 5 µm)
Post-Etch Surface Roughness (R_q,After)	0.8 ± 0.2	nm	Maintained surface quality after ICP-RIE
NV Center Implantation Depth	9.3 ± 3.6	nm	Predicted by SRIM simulation (6 keV N⁺, 7° angle)
Diamond Etch Selectivity (Diamond:Resist)	11 to 12	N/A	Achieved using O₂ + Ar “Pillar Etch” ICP-RIE
Adhesion Layer Thickness	25	nm	Electron beam evaporated Silicon (Si)
Nanopillar Diameter (Nominal)	180 - 240	nm	Used for NV center spin characteristic examination
Nanopillar Height (Max)	915 ± 4	nm	Achieved during 8 min “Pillar Etch” test
Resist Thickness (Medusa 84 SiH)	130 to 300	nm	Dependent on spin coating recipe
Fabrication Yield (with Si interlayer)	Up to 96	%	Primarily constrained by edge beads and adhesion failure
NV Center Coherence Time (T₂)	≈ 24	µs	Comparable before and after processing
Photon Collection Enhancement	7.0 ± 0.9	N/A	Relative increase compared to bulk diamond

Key Methodologies

The nanofabrication process utilized a sequence of high-precision steps, demonstrating the stringent material requirements for diamond quantum device manufacturing.

Stress Relief Etch: Initial ICP-RIE (Oxford Instruments Plasmalab 100) was performed to remove polishing damage prior to nitrogen implantation.
Nitrogen Implantation: N⁺ ions implanted at 6 keV and a 7° angle (fluence 2 x 10¹¹ cm^-2) to create shallow NV centers.
Annealing & Cleaning: Diamond annealed at 800 °C under high vacuum (< 7.8 x 10^-7 mbar) for 2 hours, followed by Tri-Acid cleaning (HNO₃:HClO₄:H₂SO₄, 1:1:1) at 500 °C to remove non-diamond carbon.
Adhesion Layer Deposition: A 25 nm silicon layer was deposited via electron beam evaporation to promote Medusa 84 SiH adhesion.
Resist Application: Medusa 84 SiH (SX AR-N 8400) was spin-coated using a tailored two-step process (1500 rpm for 3s, then 4000 rpm for 30s) and soft-baked at 100 °C for 2 min.
EBL & Proximity Correction: Electron Beam Lithography (EBL) performed at 30 kV (Raith eLiNE system). A conductive resist (ESpacer 300Z) was applied to minimize electrical charging on the insulating SCD substrate.
Etching (Two Steps):
- Step 1 (Adhesion Layer Removal): Biased SF₆ plasma ICP-RIE to remove the 25 nm silicon interlayer.
- Step 2 (Pillar Etch): Oxygen-based O₂ + Ar “Pillar Etch” ICP-RIE (Sentech PTSA-ICP Plasma Etcher SI 500) to transfer the mask pattern into the diamond.
Mask Removal & Final Cleaning: Residual Medusa 84 SiH removed using Buffered Oxide Etching (BOE) and residual silicon removed using Potassium Hydroxide (KOH). Final Tri-Acid cleaning performed.

6CCVD Solutions & Capabilities

The research successfully demonstrates a robust nanofabrication pathway for diamond quantum devices, relying heavily on ultra-high purity, low-roughness SCD substrates. 6CCVD is uniquely positioned to supply and customize the foundational diamond materials required to replicate and scale this research.

Applicable Materials

To achieve the high coherence times and low surface roughness necessary for shallow NV center sensing, the researchers utilized high-purity electronic-grade SCD. 6CCVD offers direct equivalents and superior customization options:

Research Requirement	6CCVD Material Solution	Key Specification Match
High Purity, Electronic Grade Diamond	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen ([N] < 5 ppb) and boron content. Ideal for NV center creation via implantation.
Low Surface Roughness (R_q < 1 nm)	Precision Polished SCD Wafers	Standard polishing achieves Ra < 1 nm, ensuring minimal strain and high-quality surface for EBL and RIE.
Substrate for Etch Resistance Testing	Polycrystalline Diamond (PCD) Plates	Available up to 125 mm diameter, suitable for large-scale process development and high-volume RIE testing.
Alternative Sensing Platform	Boron-Doped Diamond (BDD)	Available for electrochemical sensing or alternative quantum defect studies (e.g., SiV, GeV).

Customization Potential

The paper highlights the challenges of working with small diamond samples (3.2 mm x 3.2 mm) and the necessity of an adhesion-promoting interlayer. 6CCVD’s in-house capabilities directly address these engineering needs:

Custom Dimensions and Substrates: 6CCVD provides custom laser cutting to match the exact small dimensions (e.g., 4.0 mm x 2.0 mm) used in the study, minimizing material waste and handling complexity. We also offer plates/wafers up to 125 mm (PCD) for scaling up production.
Advanced Metalization Services: The research required a 25 nm Si adhesion layer. 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu) which can be utilized to deposit alternative, highly robust hard masks or adhesion layers (e.g., Ti/Pt/Au stacks) that may offer even higher selectivity or better adhesion than the Si layer used.
Thickness Control: We supply SCD and PCD layers with precise thickness control from 0.1 µm up to 500 µm, and substrates up to 10 mm, enabling optimization of the initial material thickness for specific implantation depths and subsequent RIE processes.

Engineering Support

The successful transfer of the FOx process to Medusa 84 SiH required significant optimization of EBL doses, spin-coating recipes, and RIE parameters to manage charging and edge bead effects on insulating SCD.

6CCVD’s in-house PhD team specializes in MPCVD diamond growth and post-processing, offering expert consultation on:

Material Selection for Quantum Sensing: Guidance on selecting the optimal SCD grade, orientation ((100) vs. (111)), and nitrogen concentration for specific NV center applications.
RIE Process Compatibility: Assistance in defining material specifications compatible with high-density O₂ plasma etching, ensuring maximum selectivity and minimal surface damage.
Hard Mask Integration: Support for integrating custom hard masks (e.g., metal or oxide layers) for high-aspect-ratio structures, extending beyond the capabilities of spin-on resists like Medusa 84 SiH.

Call to Action

6CCVD provides the high-quality, customizable diamond substrates essential for advancing quantum technologies and high-selectivity nanofabrication. Leverage our expertise and superior material quality to accelerate your research.

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

Tech Support

Original Source

DOI: https://doi.org/10.1557/s43579-025-00757-2