Crystallographic Orientation Dependent Reactive Ion Etching in Single Crystal Diamond

At a Glance

Metadata	Details
Publication Date	2018-01-24
Journal	Advanced Materials
Authors	Ling Xie, Tony Zhou, Rainer Stöhr, Amir Yacoby
Institutions	University of Stuttgart, Harvard University
Citations	53
Analysis	Full AI Review Included

Technical Analysis: Crystallographic Orientation Dependent RIE in Single Crystal Diamond

6CCVD Technical Memorandum | Date: 2024-05-31 Subject: High-Fidelity Anisotropic Dry Etching for Quantum Photonics and NV Center Applications

Executive Summary

This paper presents a groundbreaking technique for achieving crystallographic orientation-dependent dry etching in Single Crystal Diamond (SCD) using controlled Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE). This capability is critical for manufacturing advanced devices in nanophotonics and quantum computing.

Core Breakthrough: The first demonstration of crystal direction dependent Reactive Ion Etching (RIE) in monocrystalline diamond, enabling highly anisotropic sculpturing analogous to wet etching techniques in silicon (Si-KOH).
Mechanism Control: Anisotropic etching is achieved by precisely tuning the RF substrate power (DC bias), ensuring the kinetic energy of incident ions remains below a critical threshold (≤ 60 eV), thus limiting the etch rate via chemical reaction barriers rather than physical bombardment.
Achieved Faceting: Selective revelation of low-index crystal planes, including {111}, {332}, {221}, and {331}, by modulating the substrate power between 0 W and 40 W.
Quantum Application: Direct application demonstrated through the fabrication of monolithic diamond nanopillars for Nitrogen Vacancy (NV) center magnetometry.
Performance Metrics: Nanopillars achieved half-tapering angles up to 21°—the highest reported angle—resulting in demonstrably higher photon collection efficiency and significantly enhanced mechanical strength.
Material Requirement: Requires high-quality, electronic- grade, monocrystalline (100) diamond with initial surface roughness (Ra) polished to less than 1 nm.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Type	Monocrystalline (100)	N/A	Electronic Grade Diamond
Initial Roughness (Ra)	< 1	nm	Polishing requirement for low-defect processing
Anisotropy Threshold	≤ 60	eV	Ion kinetic energy required for crystallographic etching
Critical Substrate Power	≤ 40	W	RF power corresponding to the 60 eV kinetic energy threshold
Max Tapering Angle	21	degrees	Half apex angle achieved in monolithic nanopillars
Etch Depth (Typical)	2 - 3	µm	Depth achieved per sample test
Achieved Facets (Low Bias)	{111}, {332}, {221}, {331}	Miller Indices	Selectively revealed planes corresponding to 0 W to 40 W bias
Nanopillar Diameter (Top)	350	nm	Dimensions optimized for NV center waveguiding
Nanopillar Length	1.5	µm	Final length of waveguide structures

Key Methodologies

The study utilized an ICP-RIE system to control the physical and chemical components of the etching process on (100) SCD substrates.

I. Diamond Substrate Preparation

Material Source: Electronic grade, 4x4x0.5 mm³ monocrystalline diamond from Element Six, polished by Delaware Diamond Knifes.
Cleaning: Boiling mixture of equal parts sulfuric acid, nitric acid, and perchloric acid (Piranha-like clean) to remove organics and oxygen terminate the surface.
Strain Relief: Subsequent RIE using Ar/Cl and O₂ plasma to remove a few µm of the top surface layer (high concentration of defects/dislocations from polishing), ensuring high surface smoothness.

II. Lithography and Masking

Adhesion Layer: 10 nm thick Titanium (Ti) layer deposited via evaporation to promote adhesion of the mask.
Mask Material: Three layers of Flowable Oxide (FOX-16, Dow Corning) spin-coated (3000 RPM for 45s, 100 °C bake for 10 min) to achieve a 1 µm thick layer.
Patterning: Electron beam lithography (100 keV energy, 54 µC per cm² dosage) followed by development in 25 wt. % TMAH.
RIE Mask Definition: Ar/Cl RIE used to etch the exposed 10 nm Ti layer, exposing the bare diamond surface for the main O₂ etch.
Mask Removal: Post-RIE immersion in HF solution to remove residual Ti and FOX mask.

III. Anisotropic ICP-RIE Etching Recipe

Etching experiments were conducted in a Plasma-Therm Versaline ICP-RIE system under precisely controlled oxygen plasma conditions:

Process Gas: O₂ (Oxygen)
Gas Flow Rate: 40 sccm
Chamber Pressure: 10 mTorr
ICP Source Power (Chemical Component): 900 W (Fixed)
Substrate Temperature: 10 °C (Fixed)
RF Substrate Power (Bias/Kinetic Component): Varied from 0 W to 120 W.
- Anisotropic Regime: 0 W to 40 W (DC bias 9 V to 60 V).
- Isotropic Regime: > 60 W.

6CCVD Solutions & Capabilities

This research confirms the critical role of extremely high-quality SCD substrates and precise, multi-step nanofabrication in realizing complex quantum devices. 6CCVD is uniquely positioned to supply the foundational materials and engineering support required to replicate and extend this work into high-volume, production-ready research projects.

Applicable Materials

To achieve the Ra < 1 nm finish and electronic grade quality required for crystallographic etching and high-performance NV centers, 6CCVD recommends:

Optical Grade Single Crystal Diamond (SCD): Our standard SCD material provides the necessary low-defect density and superior mechanical properties used in this study. We guarantee an initial surface roughness of Ra < 1 nm for our standard high-polish SCD wafers, meeting the strain-relief-processed surface quality benchmark established by the authors.
Custom SCD Dimensions: We offer SCD wafers and plates in custom dimensions and thicknesses (0.1 µm - 500 µm) up to large inch-size capability (up to 125mm for PCD/later stage scale-up).
Boron-Doped Diamond (BDD) Extensibility: The principle of crystallographic etching is likely transferable to other diamond materials. For researchers exploring electrochemical or advanced thermal applications, we offer Boron-Doped Diamond (BDD) films where similar anisotropic dry etching techniques could be developed.

Customization Potential & Fabrication Support

The complexity of the experimental fabrication required precise material preparation, masking, and metal adhesion promoters (Ti). 6CCVD provides end-to-end material customization to streamline the researcher’s workflow:

Research Requirement	6CCVD Capability	Value Proposition for Replication
High-Purity (100) Diamond Substrates	MPCVD Growth & Precision Polishing (Ra < 1 nm)	Guarantees minimum defect accumulation and optimal surface quality crucial for subsequent RIE faceting.
Adhesion/Masking Layers (10 nm Ti)	Internal Metalization Services (Au, Pt, Pd, Ti, W, Cu)	Eliminate outsourcing steps. We provide custom Ti, Pt, or multilayer metal stacks directly deposited onto the SCD surface, ready for e-beam lithography masking.
Custom Footprint and Thickness	Plates/Wafers up to 125mm (PCD), SCD up to 500 µm	Provides the specific 4x4 mm³ cut size used, or allows scaling up experiments onto larger SCD or PCD platforms.
Precise Etch Control	SCD/PCD Thickness Tolerance	Our tight thickness control ensures process consistency, critical for realizing uniform 2 µm to 3 µm etch depths across large areas.

Engineering Support

Achieving crystallographic dependent etching requires balancing ion kinetics against chemical barriers—a highly sensitive process. 6CCVD’s in-house PhD material science team specializes in the properties of MPCVD diamond and can assist clients working on similar:

Nanophotonics applications (e.g., solid immersion lens (SIL) fabrication).
NV center quantum sensing/magnetometry projects.
Advanced MEMS/NEMS development utilizing high-strength, anisotropically etched diamond structures.

We provide consultation on material purity selection (e.g., nitrogen concentration optimization for NV centers) and material dimensions to ensure optimal integration with customer-specific RIE and lithography tooling.

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

View Original Abstract

Abstract Sculpturing desired shapes in single crystal diamond is ever more crucial in the realization of complex devices for nanophotonics, quantum computing, and quantum optics. The crystallographic orientation dependent wet etch of single crystalline silicon in potassium hydroxide (KOH) allows a range of shapes to be formed and has significant impacts on microelectromechanical systems (MEMS), atomic force microscopy (AFM), and microfluidics. Here, a crystal direction dependent dry etching principle in an inductively coupled plasma reactive ion etcher is presented, which selectively reveals desired crystal planes in monocrystalline diamond by controlling the etching conditions. Using this principle, monolithic diamond nanopillars for magnetometry using nitrogen vacancy centers are fabricated. In these nanopillars, a half‐tapering angle up to 21° is achieved, the highest angle reported in the literature, which leads to a high photon efficiency and high mechanical strength of the nanopillar. These results represent the first demonstration of a crystallographic orientation dependent reactive ion etching principle, which opens a new window for shaping specific nanostructures which is at the heart of nanotechnology. It is believed that this principle will prove to be valuable for the structuring and patterning of other single crystal materials as well.

Crystallographic Orientation Dependent Reactive Ion Etching in Single Crystal Diamond

At a Glance

Technical Analysis: Crystallographic Orientation Dependent RIE in Single Crystal Diamond

Executive Summary

Technical Specifications