Diamond-On-Insulator Process

• Diamond-on-insulator is a substrate platform that integrates single-crystal diamond films with insulating layers for quantum, electronic, and photonic applications.
• The process leverages direct bonding with optimized surface treatments, achieving robust C–O–Si bonds through controlled cleaning, plasma activation, and annealing.
• Optimized bonding protocols and precise strain mapping preserve quantum emitter integrity, enabling high-yield, scalable fabrication for advanced nanophotonic devices.

A fabrication-ready diamond-on-insulator (DOI) substrate consists of a thin crystalline diamond film, typically single-crystal and {100}-oriented, directly bonded to an electrically and optically insulating layer (usually silicon dioxide on silicon). DOI enables integration of diamond’s superior quantum, electronic, and photonic functionalities with scalable device architectures. Key drivers include: robust monolithic bonding, preservation of spin and optical performance of quantum emitters, scalability to wafer-level processes, and compatibility with back-end CMOS or photonic integration. Multiple direct-bonding and transfer techniques have been demonstrated and systematically optimized for high-yield, strong, and uniform DOI substrates suitable for advanced quantum photonic devices.

1. Substrate Preparation and Direct Bonding Protocols

The canonical DOI fabrication route employs hydrophilic direct bonding, in which both diamond and insulator surfaces are chemically functionalized to achieve strong adhesion via covalent C–O–Si bonds:

• Surface Preparation: Diamond substrates (typically ∼300 μm thick, {100}-cut single crystal) undergo Piranha cleaning (3:1 H₂SO₄ : H₂O₂ at 75 °C, 30–60 min), generating hydroxyl-terminated (C–OH) surfaces (Chen et al., 22 Jan 2025, Varveris et al., 31 Mar 2025). SiO₂/Si handle wafers are synthesized with 300 nm PECVD SiO₂, then activated in O₂ plasma (RF 1000 W, 200–400 sccm O₂, 5 min) to form Si–OH surface groups (Chen et al., 22 Jan 2025).
• Bond Assembly: A thin layer of DI water is trapped at the interface, and the diamond is placed atop the SiO₂ in cleanroom ambient (20 °C, 30–50% RH), providing initial hydrogen bonding and van der Waals adhesion (Chen et al., 22 Jan 2025, Varveris et al., 31 Mar 2025).
• Annealing: The stack is annealed at 200 °C for 24 h under N₂ or air. Dehydration reactions between C–OH and Si–OH yield C–O–Si bridge bonds, expelling water and producing a robust interface (Chen et al., 22 Jan 2025, Varveris et al., 31 Mar 2025).
• Process Controls: Surface roughness (R_q) of diamond is a critical factor: values of 4–5 nm maximize both hydroxyl functionalization and effective contact, with optimization by AFM and XPS; finer roughness (<1.5 nm) reduces bond yield and strength (Chen et al., 22 Jan 2025). Particulate control (<100/cm²) and post-bond void mapping (via IR or optical interferometry) are essential for uniformity.

An alternative pathway utilizes deterministic transfer and plasma-activated dry bonding of ultra-thin single-crystal diamond membranes (down to 10 nm) onto diverse substrates, including Si, SiO₂, fused silica, sapphire, and lithium niobate. Here, dry O₂ plasma at room temperature achieves high-hydrophilicity and sub-0.5 nm interface layers, followed by thermal annealing (75–550 °C, substrate/adhesive dependent) to finalize bonding (Guo et al., 2023).

2. Thermomechanical Considerations and Interfacial Strain

The DOI stack’s integrity depends strongly on the thermal and mechanical stress environment across the diamond–SiO₂ interface, governed by thermal expansion mismatch:

• Thermal Strain: On cooling from anneal temperature $T_a$ to room temperature $T_0$, the strain is $\epsilon_{th} = (\alpha_d-\alpha_{ox})\Delta T$, with diamond $\alpha_d\approx 2.3\times 10^{-6}\,\text{K}^{-1}$ and Si-dominated $$\alpha_{ox}\approx 2.6\times 10^{-6}\,\text{K}^{-1}$$ at 200 °C. For $\Delta T\approx 180~\text{K}$, typical strain magnitude is $-5.4\times10^{-5}$, inducing an interfacial stress $\sigma\approx 70\,\text{MPa}$ for diamond’s Young’s modulus $E\approx 1.05\,\text{TPa}$ and Poisson’s ratio $\nu\approx0.2$ (Varveris et al., 31 Mar 2025).
• Strain Mapping: Depth-resolved optically detected magnetic resonance (ODMR) using nitrogen–vacancy (NV) centers tracks both volumetric (axial) and shear (transverse) strain components, yielding interface shifts $\Delta f_v \approx 0.45$ MHz and $\Delta f_s \approx 0.71$ MHz (over the 300 μm diamond thickness) (Varveris et al., 31 Mar 2025).
• Device Performance: Strain can shift the energy levels of quantum emitters but, when process windows are followed, ODMR contrast and linewidth remain nearly unchanged, indicating minimal degradation for quantum photonic performance (Varveris et al., 31 Mar 2025).

3. Interface Chemistry and Characterization Metrics

Robust chemical characterization is vital to quantifying interface quality and predicting mechanical and quantum device performance:

• X-ray Photoelectron Spectroscopy (XPS): XPS surveys of diamond surfaces after Piranha and O₂ plasma reveal the progressive increase of C–OH and C–O–C content (up to $\sim$15 at.%) with treatment time and surface roughness. This higher hydroxylation directly correlates with increased C–O–Si bond density after annealing, and thus with interfacial shear strength (with optimal $\tau_{max}=9.6\,\text{MPa}$ for $R_q=4.48\,\text{nm}$, $t_p=60\,\text{min}$) (Chen et al., 22 Jan 2025).
• AFM and HRTEM: Atomically flat surfaces ($R_q\leq0.3\,\text{nm}$) and sub-nm interfacial regions (down to 0.5 nm) are routinely achieved in transferred-membrane approaches; high-resolution imaging confirms atomically sharp, defect-free bonding with preserved crystallinity (Guo et al., 2023).
• Photoluminescence and Interferometry: Wide-field PL mapping reveals interference fringes in unbonded areas, providing a quantitative map of local air gaps and interface voids. Post-bond, a target of $<0.1\,\%$ unbonded area is set for high uniformity (Varveris et al., 31 Mar 2025).

4. Process Optimization, Yield, and Scalability

DOI process optimization addresses parameter windows for maximum bond strength, yield, and scalability:

• Shear Strength and Yield: Shear test protocols define yield as the ratio of successful bonds to total attempts. Under optimal roughness and plasma/piranha conditions, yields of $\sim$90% and $\tau_{\text{max}}=9.6\,\text{MPa}$ are attained for 4 mm $\times$ 4 mm area junctions (Chen et al., 22 Jan 2025). In membrane transfer processes, yield exceeds 95% when surface roughness $R_q\leq0.3\,\text{nm}$ and plasma parameters are tightly controlled (Guo et al., 2023).
• Process Scaling: Use of PECVD for SiO₂ growth and compatibility with wafer-scale surface processing facilitate CMOS-aligned scaling. All-wet and all-dry functionalization routes are compatible with wafer-level or die-level integration platforms (Chen et al., 22 Jan 2025, Guo et al., 2023).
• Critical Parameters: Table of processing variables:

Variable	Optimized Value	Comments
Diamond $R_q$	4–5 nm (bonding)	For Piranha-based, robust interface (Chen et al., 22 Jan 2025)
Plasma Power	1000 W/200–400 sccm	O₂ (SiO₂), short O₂ descum (diamond)
Annealing	200 °C/24 h (N₂/air)	Strong C–O–Si formation
Membrane $R_q$	≤0.3 nm (transfer)	Ensures sub-nm, atomically sharp interface (Guo et al., 2023)

5. Preservation of Quantum Emitter Properties

A central requirement is the preservation of diamond's quantum and optical properties within the DOI structure:

• NV Center Coherence: Spin-echo $T_2$ times for bonded, thin ($\sim$150 nm) $^{15}$N-implanted diamond membranes on SiO₂ reach $T_2=632\pm21\,\mu\text{s}$, comparable to bulk values, underlining minimal interface-induced decoherence (Guo et al., 2023).
• ODMR Contrast and Linewidth: Minimal decrease in ODMR signal contrast ($\sim$0.36%) and linewidth narrowing ($\sim$0.38 MHz) after bonding, supporting emitter integrity (Varveris et al., 31 Mar 2025).
• Defect Levels: Interface XPS and HRTEM confirm negligible residue or contamination; plasma activation removes sp² carbon, further reducing decoherence pathways (Guo et al., 2023).

6. Nanophotonic Device Integration and Applications

DOI enables fabrication of diverse nanophotonic circuits with high efficiency and cooperative quantum functionalities:

• TiO₂-on-Diamond Photonic Platform: Templated atomic layer deposition (ALD) of TiO₂ onto diamond membranes, defined by e-beam lithography, yields resonators with Q-factors of $2\times10^4$–$3\times10^4$ on glass and $Q\sim4.0\times10^3$ on 50 nm diamond. Cooperativity and Purcell factors ($F_P\sim175–270$) enable efficient spin-photon interfacing (Butcher et al., 2020, Guo et al., 2023).
• Directly-Etched Cavity Modes: Hard-mask RIE on 280 nm diamond membranes produces ring resonators with Q up to $2.2\times10^4$ (Guo et al., 2023).
• System Integration: DOI supports electronics, grating couplers, and detectors on planar dielectric layers, and integration with TIRF microscopy for bio-interfacing, owing to high index contrast and evanescent field localization (Guo et al., 2023).

7. Limitations, Challenges, and Future Directions

Remaining challenges include scaling to larger diamond areas, further optimizing interface uniformity, and increasing throughput:

• Scaling: Uniform surface roughness and functionalization over cm- or wafer-scale substrates remain active areas of process development (Chen et al., 22 Jan 2025).
• Annealing Protocols: Investigation into faster, higher-temperature anneals (e.g., up to 300 °C) aims to shorten processing cycles while retaining yield and bond strength (Chen et al., 22 Jan 2025). For transferred membranes, maximizing mechanical stability against thermomechanical stress is required, especially at thicknesses $<$50 nm (Guo et al., 2023).
• Integration: Incorporation of thin-film diamond membranes (10 nm–1 μm) is essential for next-generation high-index-contrast photonic and quantum circuits on CMOS-compatible platforms (Guo et al., 2023, Butcher et al., 2020).
• Defect Mitigation: Reduction of interfacial voids and particulate contamination—through higher ISO certification, megasonic rinsing, and in situ bubble venting—remains a critical process control (Varveris et al., 31 Mar 2025).

Scalable DOI fabrication underpins advancing applications in on-chip quantum networks, hybrid quantum transducers, nanophotonics, high-voltage electronics, and integrated quantum biosensing (Guo et al., 2023, Varveris et al., 31 Mar 2025, Butcher et al., 2020, Chen et al., 22 Jan 2025).