Development of hard masks for reactive ion beam angled etching of diamond

At a Glance

Metadata	Details
Publication Date	2022-03-01
Journal	Optics Express
Authors	Cleaven Chia, Bartholomeus Machielse, Amirhassan Shams-Ansari, Marko Lončar
Institutions	Paris Centre for Quantum Technologies, Harvard University Press
Citations	21
Analysis	Full AI Review Included

Technical Documentation & Analysis: Hard Masks for RIBAE Diamond Etching

This document analyzes the research paper “Development of hard masks for reactive ion beam angled etching of diamond” to provide technical specifications and align the findings with 6CCVD’s advanced MPCVD diamond capabilities, focusing on quantum and telecommunication photonics applications.

Executive Summary

This research successfully addresses the critical challenge of mask erosion during Reactive Ion Beam Angled Etching (RIBAE) of bulk diamond substrates, a key technique for fabricating high-performance quantum photonic devices operating at telecommunication wavelengths.

RIBAE Validation: RIBAE is confirmed as a superior method to Faraday cage etching, offering improved undercut uniformity and scalability over large substrate areas.
Mask Optimization: Four mask stacks (HSQ-Ti, HSQ-Nb, PMMA/Nb, HSQ-alumina) were evaluated to minimize lateral erosion and mask redeposition, which degrade device quality (Q-factor).
Record Q-Factors: The HSQ-Nb mask stack enabled the fabrication of racetrack resonators achieving a total optical Q of 730,000 (intrinsic Q: 776,000) at 1621.0 nm, a significant improvement over previous HSQ-Ti results (286,000).
Telecommunication Photonic Crystals: The HSQ-alumina mask yielded the highest intrinsic optical Q of 250,000 for photonic crystals operating at 1520.0 nm, demonstrating the necessity of thick, amorphous masks for high-Q telecommunication devices.
Material Requirement: The success hinges on using high-quality, bulk Single Crystal Diamond (SCD) substrates, as the technique is designed for materials lacking thin-film-on-insulator platforms.
Future Direction: The findings emphasize the need for thick mask materials (> 1 µm) with low accumulated stress and high aspect ratio tolerance to fully suspend larger, telecommunication-wavelength devices.

Technical Specifications

The following hard data points were extracted from the experimental results, focusing on achieved performance and critical process parameters.

Parameter	Value	Unit	Context
Highest Total Optical Q	730,000	Dimensionless	Racetrack resonator, HSQ-Nb mask, 1621.0 nm
Highest Intrinsic Optical Q	776,000	Dimensionless	Racetrack resonator, HSQ-Nb mask, 1621.0 nm
Highest Intrinsic Optical Q (PC)	250,000	Dimensionless	Photonic crystal, HSQ-alumina mask, 1520.0 nm
Target Wavelength Range	~1550	nm	Telecommunication band devices
RIBAE Stage Angle ($\alpha$)	45	°	Fixed angle for angled etching
HSQ Mask Thickness	1	µm	Standard thickness used for all HSQ-based masks
Ti Adhesion Layer Thickness	40	nm	Electron beam evaporated layer (HSQ-Ti mask)
Nb Adhesion Layer Thickness	~200	nm	Sputtered layer (HSQ-Nb mask)
Alumina Adhesion Layer Thickness	1	nm	ALD layer (HSQ-alumina mask)
Lateral Etch Rate (High V)	3.8	nm/min	Diamond etching using 200 V beam (HSQ-Nb)
Lateral Etch Rate (Low V)	2.3	nm/min	Diamond etching using 150 V beam (HSQ-alumina)

Key Methodologies

The fabrication process relies on a four-step sequence: mask definition, vertical etch, angled etch (RIBAE), and mask removal. Key recipe parameters for the etching steps are summarized below.

1. Substrate Cleaning (Pre-Mask Definition)

Acid Treatment: 49% Hydrofluoric Acid (HF) for 5 minutes.
Piranha Clean: 96% Sulfuric Acid (H₂SO₄) and 30% Hydrogen Peroxide (H₂O₂) in 3:1 ratio for 5 minutes.
Rinse/Dry: Ultrasonic agitation in Acetone/Methanol, followed by Nitrogen gun drying.

2. Vertical Oxygen Etch (PlasmaTherm Versaline ICP-RIE)

This step transfers the mask pattern into the diamond substrate.

Gas Flow: 40 standard cubic centimeters per minute (sccm) O₂.
Pressure: 10 milliTorr.
Power: 100 W (Bias) and 700 W (ICP).

3. Reactive Ion Beam Angled Etching (RIBAE)

RIBAE uses an Intlvac Nanoquest Ion Beam Etching System with a Kaufman & Robinson ion beam source. The stage is tilted at $\alpha = 45^{\circ}$ with continuous rotation.

Recipe	Beam Voltage	Accelerator Voltage	Beam Current	ICP Power	Oxygen Flow
High Voltage (200 V)	200 V	26 V	100 mA	170 W	38 sccm
Low Voltage (150 V)	150 V	22 V	80 mA	130 W	30 sccm

4. Mask Removal (Post-RIBAE)

HSQ-Ti Mask: Immersion in 49% HF and 60% Nitric Acid (1:1 ratio) for 5 minutes, followed by Piranha solution for 5 minutes.
HSQ-Nb Mask: Mixture of Nitric Acid, Phosphoric Acid, and HF (1:1:1 ratio) to remove HSQ and Nb, followed by Piranha.
Final Step: Critical Point Drying (CPD) with carbon dioxide.

6CCVD Solutions & Capabilities

The successful fabrication of high-Q diamond quantum photonic devices via RIBAE is fundamentally dependent on the quality and preparation of the bulk diamond substrate and the precision of the masking layers. 6CCVD is uniquely positioned to supply the necessary materials and integrated services to replicate and advance this research.

Applicable Materials

The core requirement for this research is a high-purity, low-defect diamond substrate suitable for hosting color centers (like SiV$^{-}$) and minimizing optical loss.

Material Recommendation: Optical Grade Single Crystal Diamond (SCD)
- Purity: Essential for minimizing defects that cause optical scattering and loss, crucial for achieving Q-factors > 700,000.
- Thickness: 6CCVD supplies SCD plates up to 500 µm thick, and substrates up to 10 mm thick, providing the robust bulk material foundation required for RIBAE processing.
- Surface Quality: The paper highlights that mask roughness transfers to diamond sidewalls, increasing scattering loss. 6CCVD guarantees ultra-smooth polishing with Ra < 1 nm for SCD, ensuring the best possible starting surface for lithography and subsequent etching.

Customization Potential & Integrated Services

The complexity of the hard masks (HSQ-Ti, HSQ-Nb, HSQ-alumina) requires precise deposition of metallic and dielectric adhesion layers. 6CCVD offers comprehensive in-house capabilities to integrate these requirements directly onto the diamond substrate.

Research Requirement	6CCVD Capability	Value Proposition
Metallic Adhesion Layers (Ti, Nb)	Custom Metalization: 6CCVD offers in-house deposition of Ti, Pt, Pd, Au, W, and Cu. We can precisely deposit the required 40 nm Ti layer (or other refractory metals like W) via evaporation or sputtering, ensuring excellent adhesion and charge compensation.	Streamlined fabrication workflow by providing pre-metalized, ready-to-pattern diamond substrates, reducing customer processing steps.
Thick Mask Support	Substrate Stability: We provide substrates up to 10 mm thick, mitigating stress and delamination issues often encountered when depositing thick (> 1 µm) mask materials onto thin films.	Ensures mechanical stability during high-energy RIBAE processing and subsequent wet etching steps.
Large Area Uniformity	Custom Dimensions: While the paper used small samples, 6CCVD can supply SCD wafers up to 125 mm (PCD) or large SCD plates, supporting the RIBAE advantage of improved uniformity over large areas.	Enables scaling up of quantum photonic device fabrication and high-volume yield.
Surface Preparation	Advanced Polishing: Guaranteed SCD surface roughness of Ra < 1 nm. This is critical for minimizing optical scattering losses in high-Q devices, directly supporting the Q-factors achieved in this study.	Provides the lowest possible optical loss interface, essential for replicating and exceeding the 776,000 intrinsic Q results.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth and material science for quantum applications. We can assist researchers in optimizing material selection for similar Angled Etching and Quantum Photonic projects, particularly those targeting the telecommunication band (1550 nm) where mask erosion is most challenging. Our expertise ensures the chosen diamond substrate properties (e.g., nitrogen concentration, defect density) are perfectly matched to the intended color center (e.g., SiV$^{-}$, NV$^{-}$).

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

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.

Tech Support

Original Source

DOI: https://doi.org/10.1364/oe.452826