Rectangular photonic crystal nanobeam cavities in bulk diamond

At a Glance

Metadata	Details
Publication Date	2017-07-10
Journal	Applied Physics Letters
Authors	Sara Mouradian, Noel H Wan, Tim Schröder, Dirk Englund
Institutions	University of Copenhagen, Massachusetts Institute of Technology
Citations	98
Analysis	Full AI Review Included

Rectangular Photonic Crystal Nanobeam Cavities in Bulk Diamond

(Analysis of arXiv:1704.07918v1 for 6CCVD)

Executive Summary

This research demonstrates a highly consistent and scalable fabrication methodology for high-quality photonic crystal nanobeam cavities (PCNCs) engineered into bulk Single Crystal Diamond (SCD). The results are critical for advancing solid-state quantum electrodynamics (QED) and quantum computing based on diamond Nitrogen Vacancy (NV) centers.

Core Application: Efficient coupling of the NV center Zero Phonon Line (ZPL, 637 nm) emission using the Purcell effect for enhanced entanglement rates in quantum information systems (QIS).
Material Success: Successful fabrication in high-purity, low-nitrogen bulk SCD (less than 1 ppm N concentration).
Achieved Performance: Measured optical Quality (Q) factors consistently exceeded 14,000 (spectrometer-limited resolution).
Simulation Benchmark: Cavity designs showed radiation-limited simulated Q factors greater than $1 \times 10^6$.
Fabrication Consistency: Demonstrated high cavity yield and deterministic resonance trends across a full parameter sweep on the diamond chip.
Methodology: Utilizes electron-beam lithography, low-stress SiN hard masks, and a precise quasi-isotropic high-temperature (200 °C) undercut etching process to create free-standing rectangular nanobeam structures.

Technical Specifications

The following hard specifications were achieved or utilized in the fabrication and measurement of the PCNCs:

Parameter	Value	Unit	Context
Material Substrate	$3 \times 3 \times 0.3$	mm	Single Crystal Diamond (SCD), {100} top face
Nitrogen Purity	< 1	ppm	N Defect Density (Native NV density $\approx$ 1 ppb)
Target NV ZPL Wavelength	637	nm	Required coherence window for QIS
Measured Optical Q Factor (Minimum)	14,000	Unitless	Instrument-limited resolution
Measured Optical Q Factor (PLE)	14,700	Unitless	Lower bound on Cavity B mode
Simulated Optical Q Factor	$> 1 \times 10^6$	Unitless	Radiation-limited for TE mode
Nanobeam Height (H)	230	nm	Final structural dimension
Nanobeam Width (W)	250	nm	Final structural dimension
Hole Radius (r)	58	nm	Periodic structure dimension
Base Lattice Spacing (a)	192	nm	Periodic structure dimension
Etch Selectivity (SiN:Diamond)	$\approx 30:1$	Unitless	Anisotropic oxygen etch
Measurement Temperature	4	K	Cryogenic measurement conditions (PL/PLE spectroscopy)

Key Methodologies

The complex fabrication relies on precision etching and multi-layer masking to achieve free-standing structures in bulk SCD. The steps are summarized below:

Hard Mask Deposition: Plasma-Enhanced CVD (PECVD) application of 180 nm low-stress Silicon Nitride (SiN).
Pattern Transfer: Electron-Beam Lithography (EBL) followed by a CF₄ Plasma RIE step to define the nanobeam structure into the SiN hard mask.
Anisotropic Diamond Etch (Initial): Deep vertical etch using Inductively Coupled Oxygen Plasma (ICP).
- Parameters: 0.15 Pa pressure, 500 W ICP power, 240 W RF power. Etch depth was 2.5 times the final desired height.
Conformal Protection Layer: Atomic Layer Deposition (ALD) of 20 nm Aluminum Oxide (Al₂O₃) to protect the nanobeam side walls.
Selective Top Mask Removal: CF₄ RIE to remove Al₂O₃ from the top surface, exposing the SiN mask underneath for the subsequent deep etch.
Second Anisotropic Diamond Etch (Deepening): Further oxygen etch using the same parameters as step 3, removing an additional 1 µm of material.
Quasi-Isotropic Undercut Etch (Critical Step): High-temperature, high-pressure etch to release the structure and define the final beam height (H = 230 nm).
- Parameters: 200 °C, 3 Pa pressure, 900 W ICP power (No RF forward bias).
Mask Strip: Removal of residual SiN and Al₂O₃ using 49% Hydrofluoric Acid (HF).

6CCVD Solutions & Capabilities

The complexity and precision required to replicate this high-Q PCNC fabrication demand premium material quality and tight dimensional control, core specialties of 6CCVD. Our MPCVD Single Crystal Diamond (SCD) capabilities directly address the foundational requirements of this research, offering significant advantages for scaling and future integration.

Applicable Materials

The research critically relies on High-Purity Single Crystal Diamond (SCD) with extremely low nitrogen concentration (less than 1 ppm) to ensure coherent NV centers.

6CCVD Material Specification	Relevance to Research	Advantage for Client
Optical Grade SCD	Required for coherent NV centers and low scattering loss in the PCNC.	6CCVD guarantees high-purity SCD with nitrogen concentrations comparable to or better than those used in this study.
Custom Thickness Control	The substrate used was 0.3 mm thick. Future designs may require thinner membranes or thicker supports.	We supply SCD plates/wafers in customized thicknesses from 0.1 µm to 500 µm, precisely matched to research requirements.
{100} Crystal Orientation	Specified orientation for optimal etch rates and resultant rectangular cross-sections.	6CCVD provides SCD material reliably oriented to {100}, {110}, or {111} planes as required by anisotropic etching recipes.

Customization Potential & Scalability

While the paper used small $3 \times 3$ mm samples, scaling up QIS systems requires larger, high-quality material.

Large-Area Substrates: 6CCVD can supply SCD substrates significantly larger than the $3 \times 3$ mm sample size used here, up to $20 \times 20$ mm plates. For highly scalable production approaches, our Polycrystalline Diamond (PCD) substrates can reach 125 mm wafer sizes with exceptional surface quality (Ra < 5 nm).
Precision Polishing: The success of the multi-step anisotropic and isotropic etching processes relies on an ultra-smooth initial diamond surface. 6CCVD provides state-of-the-art SCD polishing, achieving surface roughness Ra less than 1 nm, minimizing scattering loss in the final nanobeam devices.
Integrated QED Systems: Although this paper focuses purely on optical readout, future iterations mentioned (enabling mode conversion to ridge/channel waveguides and integration with other color centers) may require electrical contacts. 6CCVD offers custom metalization services (Au, Pt, Pd, Ti, W, Cu) applied directly to the diamond surface, ready for integrated quantum circuit assembly.

Engineering Support

This research highlights the complex intersection of advanced material science (MPCVD diamond growth) and high-precision nanofabrication (EBL, ICP etching).

Our in-house PhD team provides specialized engineering consultation to help researchers select the optimal diamond properties (purity, orientation, strain state, and surface termination) necessary to replicate or extend high-Purcell-factor QED projects utilizing NV, SiV, or GeV centers. We assist in translating fabrication specifications (like the 230 nm height or specific orientation requirements) into material design parameters.

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

View Original Abstract

We demonstrate the fabrication of photonic crystal nanobeam cavities with rectangular cross section into bulk diamond. In simulation, these cavities have an unloaded quality (Q) factor of over 1 × 106. Measured cavity resonances show fundamental modes with spectrometer-limited Q factors ≥14×103 within 1 nm of the nitrogen vacancy centers zero phonon line at 637 nm. We find high cavity yield across the full diamond chip with deterministic resonance trends across the fabricated parameter sweeps.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4992118