High-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-05-22 |
| Journal | Nano Letters |
| Authors | Amy Butcher, Xinghan Guo, Robert Shreiner, Nazar Delegan, Kai Hao |
| Institutions | University of Chicago, Argonne National Laboratory |
| Citations | 16 |
| Analysis | Full AI Review Included |
High-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2
Section titled âHigh-Q Nanophotonic Resonators on Diamond Membranes using Templated Atomic Layer Deposition of TiO2âExecutive Summary
Section titled âExecutive SummaryâThis research introduces a novel, scalable nanofabrication platform for integrating high-performance nanophotonic resonators with diamond membranes, critical for solid-state quantum networking.
- Core Innovation: Templated Atomic Layer Deposition (ALD) of amorphous Titanium Dioxide (TiO2) is used to construct resonators on top of diamond membranes, avoiding the conventional, damaging process of etching wavelength-scale features into the diamond.
- Performance Metrics: The platform achieved high Quality (Q) factors, reaching Q ~ 33,000 for microring resonators on fused silica and Q ~ 4,400 on 50 nm-thick diamond membranes.
- Quantum Interfacing Potential: The 1D photonic crystal cavities on diamond exhibit high simulated Purcell factors (FPurcell up to 175) and estimated cooperativity (1-10), enabling efficient spin-photon interfacing with color centers (e.g., SiV).
- Surface Quality and Yield: The ALD technique minimizes surface roughness, which is crucial for reducing scattering loss (proportional to Ï2/λ3) in the visible spectrum where many diamond color centers operate.
- Processing Flexibility: The fabrication process is highly reproducible and can be iterated multiple times on a single diamond sample (by mechanical removal of TiO2), facilitating precise alignment to individual color centers without damaging the underlying substrate.
- Material Versatility: The approach is flexible regarding both wavelength and substrate, making it suitable for integration with defects in silicon carbide, rare earth ions, or other material systems.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Refractive Index (TiO2) | n > 2.3 | N/A | Guiding material for visible light operation. |
| TiO2 Bandgap | ~3.3 | eV | Suitable for visible device operation. |
| Ring Resonator Q Factor (Fused Silica) | 33,260 | N/A | High-resolution resonance scan result. |
| 1D Cavity Q Factor (Fused Silica) | 19,640 | N/A | Measured Q for 25-fin device near 710 nm. |
| 1D Cavity Q Factor (Diamond Membrane) | 4,410 | N/A | Measured Q on 50 nm-thick single crystal diamond. |
| Simulated Purcell Factor (Max) | 175 | N/A | At cavity mode maximum (for Q = 4,400). |
| Simulated Purcell Factor (In Diamond) | 115 | N/A | Within the underlying diamond membrane. |
| Estimated Cooperativity | 1-10 | N/A | Based on comparisons with previous work. |
| Diamond Membrane Thickness | 50 | nm | Single crystal diamond used for quantum interfacing demonstration. |
| TiO2 ALD Deposition Temperature | 90 | °C | Ensures amorphous TiO2 phase and avoids PMMA reflow (Tg ~105 °C). |
| TiO2 ALD Deposition Rate | 0.6 | A/cycle | Typical deposition rate using TDMAT and water precursors. |
| TiO2 Annealing Temperature | 250 | °C | Post-fabrication anneal to reduce material optical loss. |
| 1D Cavity Wavelength Determinism (Std. Dev.) | 0.561 - 1.326 | nm | Standard deviation across ten 25-fin devices at three wavelengths (606-710 nm). |
Key Methodologies
Section titled âKey MethodologiesâThe fabrication relies on templated ALD, combining electron beam lithography (EBL) for template definition, low-temperature ALD for conformal filling, and selective etching for structure release.
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Template Patterning (EBL):
- PMMA (950K A4) resist is spin-coated (270-330 nm thickness).
- A 20 nm Au conduction layer is thermally evaporated to mitigate charging.
- EBL is performed at 100 kV with a dose of 1200 ”C/cm2.
- The Au layer is stripped, and the PMMA is developed in MIBK:IPA solution (1:3) at 7 °C.
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Pre-ALD Cleaning:
- A brief (3-second) oxygen plasma exposure is used in an ICP RIE tool (PlasmaTherm Apex).
- Recipe: 10 sccm O2, 50 W ICP power. This removes residual polymer adsorbates without damaging the substrate.
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Atomic Layer Deposition (ALD):
- Amorphous TiO2 is deposited using a commercial thermal ALD chamber.
- Precursors: Tetrakis(dimethylamido) titanium (TDMAT) and water.
- Temperature: 90 °C.
- Carrier Gas: 100 sccm N2 flow.
- The templates are significantly over-filled to aid in subsequent planarization.
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TiO2 Overfill Etch (ICP RIE):
- Excess TiO2 is removed using chlorine-based ICP RIE etching (PlasmaTherm Apex).
- Recipe: 150 W substrate bias, 400 W ICP power, 12 sccm Cl2, 8 sccm BCl3.
- Crucially, the etch stops on the PMMA resist, protecting the substrate and device sidewalls.
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Resist Stripping and Annealing:
- The remaining PMMA resist and etch residues are chemically stripped (Nanostrip, MicroChem) to reveal the templated TiO2 devices.
- The structures are annealed on a hot plate at 250 °C for two hours to reduce material optical loss.
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Diamond Membrane Preparation (Context):
- 500 nm-thick diamond membranes are generated via ion bombardment and CVD overgrowth.
- Membranes are adhered to a Si carrier chip using Hydrogen Silsesquioxane (HSQ) resist (annealed at 420 °C for 8 hours).
- The membranes are back-etched to the final 50 nm thickness using ICP etching (Ar/Cl2 plasma).
Commercial Applications
Section titled âCommercial ApplicationsâThis fabrication platform is highly relevant to emerging technologies requiring high-fidelity light-matter interaction and scalable integration of quantum components.
| Industry/Sector | Application/Product Relevance | Key Benefit |
|---|---|---|
| Quantum Computing & Networking | Integrated quantum networks, deterministic spin-photon interfacing, quantum repeaters. | Enables high cooperativity (FPurcell up to 175) and scalable, reproducible fabrication without damaging qubit coherence. |
| Integrated Photonics | High-Q spectral filters, on-chip waveguiding optics, general visible-frequency photonics. | Low optical loss and high Q factors (up to 33,000) achieved using a flexible, low-temperature ALD process. |
| Quantum Sensing | Label-free biosensing, high-sensitivity environmental monitoring using microcavities. | High Q factors (Q > 20,000) are essential for enhancing sensitivity in microcavity-based sensors (Ref. 35). |
| Solid-State Qubit Platforms | Interfacing with defects in materials beyond diamond (e.g., silicon carbide, rare earth ions). | The ALD platform is substrate-agnostic and wavelength-flexible, allowing integration with various quantum material systems. |
| Advanced Nanofabrication | High-aspect-ratio structuring and smooth sidewall fabrication for optical devices. | Templated ALD provides superior surface smoothness compared to conventional etching, minimizing scattering loss, especially in the visible spectrum. |
View Original Abstract
Integrating solid-state quantum emitters with nanophotonic resonators is essential for efficient spin-photon interfacing and optical networking applications. While diamond color centers have proven to be excellent candidates for emerging quantum technologies, their integration with optical resonators remains challenging. Conventional approaches based on etching resonators into diamond often negatively impact color center performance and offer low device yield. Here, we developed an integrated photonics platform based on templated atomic layer deposition of TiO<sub>2</sub> on diamond membranes. Our fabrication method yields high-performance nanophotonic devices while avoiding etching wavelength-scale features into diamond. Moreover, this technique generates highly reproducible optical resonances and can be iterated on individual diamond samples, a unique processing advantage. Our approach is suitable for a broad range of both wavelengths and substrates and can enable high-cooperativity interfacing between cavity photons and coherent defects in diamond or silicon carbide, rare earth ions, or other material systems.