Skip to content
6CCVD Research

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

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-06-13
JournalMRS Communications
AuthorsOliver Opaluch, Sebastian Westrich, Nimba Oshnik, Philipp Fuchs, Jan Fait
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This analysis focuses on Medusa 84 SiH, a novel Hydrogen Silsesquioxane (HSQ) electron beam resist developed by Allresist GmbH, positioned as a direct, high-performance replacement for the discontinued DuPont FOx 16.

  • Core Value Proposition: Medusa 84 SiH enables reliable, high-resolution nanostructuring of single-crystal diamond (SCD), critical for future diamond-based quantum technologies, addressing the supply gap left by FOx discontinuation.
  • High Selectivity: The resist demonstrates a diamond etching selectivity of 11 to 12 against high-bias O2 + Ar ICP-RIE plasma, comparable to previous cutting-edge HSQ resists, allowing for high aspect ratio structures.
  • Quantum Viability Confirmed: Nanopillars fabricated using this resist maintain excellent spin properties in shallow Nitrogen Vacancy (NV) centers, showing coherence times (T2) up to ~25 ”s both before and after processing.
  • High Fabrication Yield: Utilizing a 25 nm silicon adhesion-promoting interlayer, the process achieves fabrication yields up to 96% for dense nanopillar arrays, demonstrating robust adhesion and process stability.
  • Photonic Enhancement: The resulting diamond nanopillars function as effective waveguides, achieving a 7.0 ± 0.9 increase in photon collection efficiency compared to bulk NV centers.
  • Improved Safety Profile: The resist uses butyl acetate as a solvent, offering higher personnel safety compared to the methyl isobutyl ketone (MIBK) solvent used in the reference FOx product.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Resist TypeMedusa 84 SiH (SX AR-N 8400)N/AHSQ-based negative tone EBL resist
Etching Selectivity (Diamond)11 to 12N/AAgainst high-bias O2 + Ar ICP-RIE plasma
Fabrication YieldUp to 96%%Achieved with 25 nm Si adhesion layer
NV Center Coherence Time (T2)~24.50 ”s (After)”sSpin Echo measurement in nanopillars
NV Center Lifetime (T1)~404 ”s (After)”sSpin lifetime measurement
Photon Collection Enhancement7.0 ± 0.9N/ARelative to bulk NV center saturation count rate
Resist Contrast (C)3.60 ± 0.08N/AMeasured on silicon substrate
Base Dose (D100)104.1 ± 0.9”C/cm2Dose required for full exposure
Resist Thickness Range130 to 300nmAchieved via two-step spin coating
EBL Acceleration Voltage30kVRaith eLiNE system
EBL Beam Diameter< 2nmRaith eLiNE system
Nitrogen Implantation Depth9.3 ± 3.6nmSRIM prediction (6 keV, 7° angle)
Annealing Temperature800°CFor NV center formation
Annealing Vacuum< 7.8 x 10-7mbarCustom-built furnace
Etched Structure Height915 ± 4nmMarker structure (8 min etch)
Surface Roughness (Rq, After)0.8 ± 0.2nmEtched diamond surface (excluding etch pits)

Key Methodologies

Section titled “Key Methodologies”

The nanofabrication process for diamond nanopillars involves precise EBL, ICP-RIE etching, and subsequent NV center characterization.

  1. Diamond Preparation and NV Creation:

    • Stress Relief Etch: Initial ICP-RIE etch to remove polishing damage.
    • Implantation: Nitrogen ion implantation (6 keV, 7° angle) at a fluence of 2 x 1011 cm-2.
    • Annealing: 800 °C for 2 hours under high vacuum (< 7.8 x 10-7 mbar) to form NV centers.
    • Cleaning: Tri-Acid solution (HNO3:HClO4:H2SO4 - 1:1:1) at 500 °C for 1 hour.

  2. Resist Application and Lithography:

    • Adhesion Layer: 25 nm silicon layer deposited via e-beam evaporation (critical for Medusa 84 SiH adhesion).
    • Spin Coating (Medusa 84 SiH): Two-step process to minimize edge bead formation on small substrates.
      • Step 1: 1500 rpm (500 rpm/s acceleration) for 3 s.
      • Step 2: 4000 rpm (1000 rpm/s acceleration) for 30 s.
    • Soft Bake: 100 °C for 2 min.
    • Conductive Layer: ESpacer 300Z (or Electra 92) applied at 4000 rpm and soft baked at 80 °C for 90 s to minimize charging during EBL.
    • EBL: Performed at 30 kV (Raith eLiNE). Proximity correction (NanoPECS) applied using dose scaling (typical dose factors 14-16 for nanopillars).
    • Development: Stirring in AR 300-44 (2.38% TMAH) for 90 s, followed by ultrapure water and IPA rinse.

  3. Plasma Etching (ICP-RIE):

    • System: Sentech PTSA-ICP Plasma Etcher SI 500.
    • Silicon Interlayer Removal: Biased SF6 plasma etch.
    • Diamond Pillar Etch: Oxygen-based “Pillar Etch” process (O2 + Ar) to transfer the resist pattern into the diamond.

  4. Mask Removal and Final Cleaning:

    • Resist Removal: Buffered Oxide Etching (BOE) to remove Medusa 84 SiH mask.
    • Silicon Removal: Potassium Hydroxide (KOH) to remove residual silicon.
    • Final Cleaning: Tri-Acid cleaning followed by IPA stirring and N2 blow-drying.

Commercial Applications

Section titled “Commercial Applications”

The successful implementation of Medusa 84 SiH for diamond nanostructuring supports several high-tech industries and applications:

  • Quantum Sensing and Metrology:
    • Fabrication of high-coherence, shallow NV centers in diamond nanopillars for nanoscale magnetic field, temperature, and electric field sensing.
    • Enabling commercial single NV scanning probes (e.g., Qnami, QZabre) by providing a reliable mask material for tip fabrication.
  • Diamond Photonics:
    • Manufacturing photonic structures (nanopillars, waveguides) that enhance photon collection efficiency (7x increase demonstrated), crucial for scalable quantum networks and quantum repeaters.
  • Advanced Semiconductor Nanofabrication:
    • Serving as a high-resolution, high-resistance EBL mask for materials requiring harsh oxygen plasma etching, including Silicon-on-Insulator (SOI), GaAs/InGaAsP photonics, and Indium Tin Oxide (ITO) for lasers.
  • General High Aspect Ratio Etching:
    • Applicable in research and development requiring nanoscale patterning and high etch resistance against ICP-RIE processes, especially where the high selectivity of HSQ/SiO2-like masks is necessary.
  • Safer Manufacturing:
    • The use of butyl acetate solvent improves personnel safety in cleanroom environments compared to legacy HSQ products using MIBK.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

Reads Similar to This

Fluorine-Terminated Polycrystalline Diamond Solution-Gate Field-Effect Transistor Sensor with Smaller Amount of Unexpectedly Generated Fluorocarbon Film Fabricated by Fluorine Gas Treatment Photoelectrical detection of nitrogen-vacancy centers by utilizing diamond lateral p–i–n diodes Microwave-free magnetometry with nitrogen-vacancy centers in diamond