Development of hard masks for reactive ion beam angled etching of diamond
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-03-01 |
| Journal | Optics Express |
| Authors | Cleaven Chia, Bartholomeus Machielse, Amirhassan Shams-Ansari, Marko LonÄar |
| Institutions | Paris Centre for Quantum Technologies, Harvard University Press |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research focuses on optimizing hard masks for Reactive Ion Beam Angled Etching (RIBAE) of bulk diamond, specifically targeting high-performance quantum photonic devices operating at telecommunication wavelengths (~1550 nm).
- Problem Addressed: Extended etch times required for larger telecom-wavelength devices lead to severe mask erosion and redeposition, resulting in rough sidewalls and degraded optical quality (low Q-factors).
- Optimal Mask Solution: A thick Hydrogen Silesquioxane (HSQ) layer combined with a thin (1 nm) amorphous Alumina (Al2O3) adhesion layer (HSQ-alumina) provided the best etch profile and optical performance for photonic crystals.
- Performance Benchmark (Photonic Crystals): The HSQ-alumina mask achieved an intrinsic optical Q of 250,000 at 1520.0 nm, nearly an order of magnitude improvement over the PMMA/Nb mask (39,700 intrinsic Q).
- Performance Benchmark (Racetrack Resonators): The HSQ-Niobium (Nb) mask achieved the highest Q-factor overall: 776,000 intrinsic Q at 1621.0 nm, demonstrating superior smoothness compared to the HSQ-Ti mask (286,000 total Q).
- Key Mechanism: The amorphous nature of the Alumina underlayer prevents grain structure transfer (micromasking) into the diamond, ensuring smooth sidewalls, while the thinness of the layer eliminates mask redeposition.
- RIBAE Advantage: RIBAE offers superior etch uniformity and yield over large areas (up to 4-inch diameter) compared to traditional Faraday cage angled etching.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Refractive Index (n) | 2.4 | - | Material property |
| Diamond Bandgap | 5.5 | eV | Material property |
| Target Wavelength | ~1550 | nm | Telecommunication range |
| Best Intrinsic Q (Racetrack) | 776,000 | - | HSQ-Nb mask, RIBAE |
| Best Intrinsic Q (Photonic Crystal) | 250,000 | - | HSQ-Alumina mask, RIBAE |
| HSQ Mask Thickness | 1 | ”m | Spin-coated layer thickness |
| Alumina Adhesion Layer Thickness | 1 | nm | Atomic Layer Deposition (ALD) |
| RIBAE Recipe (High Power) Beam Voltage | 200 | V | Used for HSQ-Ti, HSQ-Nb |
| RIBAE Recipe (Low Power) Beam Voltage | 150 | V | Used for HSQ-Alumina |
| RIBAE Stage Angle (α) | 45 | ° | Angle relative to ion beam |
| Diamond Lateral Etch Rate (200 V recipe) | ~3.8 | nm/min | Using HSQ-Nb mask |
| Diamond Lateral Etch Rate (150 V recipe) | ~2.3 | nm/min | Using HSQ-Alumina mask |
| Vertical Etch (ICP-RIE) Bias Power | 100 | W | Oxygen plasma etch |
| Vertical Etch (ICP-RIE) ICP Power | 700 | W | Oxygen plasma etch |
| Vertical Etch Pressure | 10 | mTorr | Oxygen plasma etch |
| Nb Sputtering Thickness | ~200 | nm | Used for HSQ-Nb mask stack |
Key Methodologies
Section titled âKey MethodologiesâThe fabrication process involves four primary steps: mask definition, vertical etching, angled etching (RIBAE), and mask removal.
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Substrate Preparation and Cleaning:
- Diamond substrates were cleaned sequentially using 49% Hydrofluoric Acid (HF), Piranha solution (H2SO4:H2O2, 3:1 ratio), and ultrasonic agitation in acetone/methanol.
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Mask Definition (Varies by Material):
- HSQ-Ti: 40 nm Ti (adhesion/charge compensation) evaporated, followed by 1 ”m HSQ spin-coating. Patterned via Electron Beam Lithography (EBL) and developed in TMAH. Ti etched using Ar/Cl2 ICP-RIE.
- HSQ-Nb: 200 nm Nb sputtered, followed by 1 ”m HSQ. Pattern transferred from HSQ to Nb using Ar/Cl2 etch.
- PMMA/Nb (Inversion): 800 nm PMMA spun, patterned via EBL (positive tone). Nb sputtered conformally, followed by grazing-incidence Argon ion beam milling (300 V beam, 20° stage angle) to planarize and remove excess Nb.
- HSQ-Alumina (Optimal): 1 nm Al2O3 deposited via Atomic Layer Deposition (ALD), followed by 1 ”m HSQ spin-coating. Patterned via EBL. Alumina removed using a short Ar/Cl2 etch prior to vertical diamond etch.
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Vertical Anisotropic Etching (Pattern Transfer):
- The mask pattern was transferred into the diamond using Oxygen (O2) ICP-RIE.
- Recipe: 40 sccm O2 flow, 10 mTorr pressure, 100 W bias power, 700 W ICP power.
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Reactive Ion Beam Angled Etching (RIBAE):
- Performed using an Intlvac Nanoquest Ion Beam Etching System with O2 ion beams. Continuous stage tilt and rotation were used to ensure symmetric cross-sections and uniformity.
- High Power Recipe (200 V): 200 V beam, 100 mA current, 170 W ICP, 38 sccm O2.
- Low Power Recipe (150 V, used for HSQ-Alumina): 150 V beam, 80 mA current, 130 W ICP, 30 sccm O2.
- Goal: Undercut the diamond to create suspended, free-standing structures with a triangular cross-section (target apex angle ~45°).
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Mask Removal and Cleaning:
- Substrates were immersed in a solution of 49% HF and 60% nitric acid (1:1 ratio), followed by Piranha solution.
- Final drying was performed using a critical point drying process with carbon dioxide.
Commercial Applications
Section titled âCommercial ApplicationsâThis technology is critical for advancing diamond-based quantum and classical photonics, especially where bulk substrates must be used due to the lack of thin-film-on-insulator platforms.
- Quantum Communication and Computing:
- Fabrication of high-Q photonic crystal nanobeams and waveguides integrated with color centers (e.g., negatively-charged silicon-vacancy center, SiV-).
- Creation of quantum memory nodes and memory-enhanced quantum communication devices operating at visible and telecom wavelengths.
- High-Performance Optical Components:
- Development of high-Q racetrack resonators (Q up to 776,000) for low-loss optical systems.
- Fabrication of diamond mirrors, gratings, and metasurface frequency converters.
- Nonlinear Optics:
- Realization of diamond devices leveraging strong third-order optical nonlinearity, such as frequency combs, Raman lasers, and supercontinuum sources.
- General Bulk Material Processing:
- The optimized mask techniques (especially the HSQ-alumina stack) can be adapted for nanofabrication in other bulk materials that are not available heteroepitaxially or as thin films-on-insulator, requiring high-aspect-ratio angled etching.
View Original Abstract
Diamond offers good optical properties and hosts bright color centers with long spin coherence times. Recent advances in angled-etching of diamond, specifically with reactive ion beam angled etching (RIBAE), have led to successful demonstration of quantum photonic devices operating at visible wavelengths. However, larger devices operating at telecommunication wavelengths have been difficult to fabricate due to the increased mask erosion, arising from the increased size of devices requiring longer etch times. We evaluated different mask materials for RIBAE of diamond photonic crystal nanobeams and waveguides, and how their thickness, selectivity, aspect ratio and sidewall smoothness affected the resultant etch profiles and optical performance. We found that a thick hydrogen silesquioxane (HSQ) layer on a thin alumina adhesion layer provided the best etch profile and optical performance. The techniques explored in this work can also be adapted to other bulk materials that are not available heteroepitaxially or as thin films-on-insulator.