A process optimization study on diamond-on-insulator (DOI) substrates shows how plasma etching can be tuned to thin bonded single-crystal diamond membranes into photonic-grade films suitable for quantum photonics manufacturing. The work also introduces a color-based thickness evaluation method that lets teams estimate diamond thickness from ordinary optical microscope images with nanometer-scale resolution.
Why diamond-on-insulator remains attractive for quantum photonics
Diamond color-center platforms are drawing strong interest because they can support initialization, manipulation, entanglement, and readout of individual qubits with high fidelity. For photonics professionals, the appeal is not just the quantum emitter itself, but the possibility of integrating these centers with waveguides, resonators, and other on-chip optical structures.
The main manufacturing constraint is scale. Heteroepitaxial single-crystal diamond is still limited in size, and photonic-grade DOI substrates are not yet as straightforward to produce as more mature silicon photonics materials. That makes it difficult to translate promising device demonstrations into processes compatible with larger-volume fabrication workflows.
Plasma etching as a thinning route for bonded diamond membranes
The study develops an inductively coupled plasma reactive-ion etch (ICP-RIE) recipe for thinning direct-bonded (100) single-crystal diamond membranes into large-area thin-film DOI substrates. The key objective is to remove material while preserving the integrity of the bond interface and maintaining the surface quality required for nanophotonic devices.
According to the authors, the optimized etch provides enough micromasking and surface control to support film formation without resorting to more complex transfer schemes. That matters for production because it reduces process steps and avoids methods such as under-etching or pedestal fabrication, both of which can add variability and complicate downstream integration.
In the reported result, a 10 μm diamond plate bonded to SiO2/Si was thinned to a DOI substrate with diamond thickness at or below 300 nm. The film is approximately 300 nm thick across a 0.5 × 0.5 mm2 area, with surface roughness below 0.5 nm, while the bonding interface remains intact.
Free-standing chiplets fabricated with standard lithography
One of the more practical aspects of the work is that the resulting DOI substrate supports fabrication of photonic chiplets using a standard two-step lithography flow. For teams building diamond photonic components, compatibility with established lithography is a meaningful step toward manufacturable device platforms.
The authors demonstrate free-standing photonic chiplets formed directly on the thinned DOI film. That is notable because it suggests a route to device-scale integration without introducing specialized thin-film transfer steps or elaborate structural scaffolding. In industrial terms, the fewer unusual process modules required, the easier it becomes to qualify the flow and repeat it across lots.
For integrated quantum photonics, this kind of substrate platform could help bridge the gap between high-performance diamond devices and scalable manufacturing practices used in mainstream microfabrication.
Colorimetry offers a fast thickness check from microscope images
Beyond the etch process, the paper also presents a colorimetric study of diamond visibility on SiO2. The goal is to quantify how apparent color changes with thickness in common color spaces and use that relationship to infer film thickness from standard optical microscope images.
The method is designed as a practical validation tool. Instead of relying only on white-light interferometry, the authors show that automated thickness extrapolation from microscope images can achieve 5 nm resolution and agrees well with WLI measurements. For labs and production lines, that could provide a faster, lower-friction way to screen samples and monitor process drift.
- Optimized ICP-RIE thins bonded diamond membranes into photonic-grade DOI films.
- Reported film thickness is around 300 nm over 0.5 × 0.5 mm2.
- Surface roughness is below 0.5 nm, with the bond interface preserved.
- Standard two-step lithography is used to fabricate photonic chiplets.
- Colorimetry enables thickness estimation from optical microscope images with 5 nm resolution.
Implications for scalable diamond nanophotonics
For the photonics industry, the study is important because it combines two enabling tools: a substrate fabrication route that looks compatible with established processing, and a quick optical method for thickness evaluation. Together, these developments address both manufacturing and metrology constraints, which are often the bottlenecks in moving advanced photonic materials toward repeatable production.
If these techniques continue to hold up across larger areas and more device designs, they could support more scalable fabrication of diamond nanophotonic components for quantum networks, distributed sensing, and modular quantum computing architectures.
Source: arXiv preprint