Hi everyone,

i need help.

Thank you in advance

I’m applying for a Senior Research Engineer / Process Development position in integrated photonics fabrication (AlN platform on 8-inch silicon wafers, for quantum PIC applications). As part of the application, I have to propose a detailed fabrication process flow and a process control strategy (max. 15 slides). I have experience in physics and photonics, but I want to make sure my process-development approach and the level of detail are realistic from a semiconductor / photonics foundry point of view.

Below is a summary of the flow and focus I’m planning to present. I’d really appreciate feedback: Does this structure make sense? Are there important process or metrology aspects I’m missing, especially around CD control and void-free oxide cladding?

Draft description of my planned process-development approach

The assignment is to outline a fabrication process for an AlN-based photonic integrated circuit platform for quantum applications, starting from a bare 8-inch silicon wafer. The target structure includes a silicon substrate, a buried oxide (BOX) layer (~1.5–10 µm), an AlN core layer (~200–400 nm), lateral feature sizes on the order of 100–300 nm, an oxide upper cladding (~2–5 µm) and a metal layer on top. The hiring manager explicitly asked me to focus on the fabrication process and process control, not on detailed photonic design.

Because of that, I plan to structure the presentation around process modules and manufacturability, rather than optical design:

1. Overall strategy I split the flow into the main modules: starting wafer and base layers, lithography (mix of 248 nm DUV stepper + EBL), hard mask transfer, AlN etch, oxide cladding deposition and planarization, metallization, and in-line metrology and process control. For each module I want to highlight critical parameters, main failure mechanisms, and release criteria before the next step.
2. Starting wafer and base stack I assume an 8-inch Si wafer with a suitable BOX and an AlN core layer. I would explicitly call out that I want to qualify thickness and uniformity (BOX + AlN), surface roughness, defect density, wafer bow and stress state, because these will drive lithography, plasma etch behavior, and mechanical stability of the full stack.
3. Lithography strategy: 248 nm DUV + EBL The task mentions a mix-and-match approach using a 248 nm DUV stepper and EBL, so I plan to use DUV for higher-throughput, alignment-critical layers and EBL for very critical test structures, early process development and fine features. I intend to discuss CD control (critical dimension) in terms of CD bias, CD uniformity and overlay between DUV and EBL layers, because this directly impacts waveguide width, coupling structures and overall yield.
4. Resist stack and hard mask concept I would not describe the resist in isolation, but as part of the full transfer chain into a hard mask and then into AlN. The selection criteria I want to emphasize: resolution, line-edge roughness after develop, plasma stability, process window for CD control, and suitability for transferring stable patterns into an oxide or metal hard mask with enough etch selectivity to AlN.
5. AlN etch process I treat the AlN etch as the most critical module, since it defines the core geometry. My focus would be on achieving an anisotropic profile with controlled etch depth, sufficient selectivity to the hard mask, and minimal sidewall roughness and damage. I also want to mention how I would approach endpoint detection, polymer formation / redeposition, and post-etch cleaning, because these directly affect sidewall quality and loss.
6. Cleaning and inter-step conditioning I consider cleaning and surface conditioning steps as intentional parts of the flow, not as afterthoughts. After lithography and etch, resist residues, polymers, particles and potential surface damage can strongly impact oxide deposition, adhesion and defectivity, so I plan to highlight how I would integrate strip/clean sequences and how I would monitor their effectiveness.
7. Oxide cladding deposition and void-free gap fill For the upper oxide cladding I want to emphasize gap fill and void-free coverage of the structured AlN waveguides, especially between closely spaced features. My plan is to discuss candidate deposition approaches (e.g. PECVD, HDP-CVD, possibly in combination with ALD and etch-back/CMP) with the explicit goal of avoiding voids in narrow trenches and between waveguides, since voids can increase scattering losses and raise reliability concerns.
8. Planarization (CMP / etch-back) To prepare for metallization, I’d propose a planarization approach that targets a controlled topography and thickness for the cladding. I’d briefly describe how I would tune CMP or etch-back for planarity, thickness uniformity, acceptable dishing/erosion, and mechanical integrity of the stack.
9. Metallization I plan to present metallization as the last big integration module, with clear requirements on adhesion, conductivity, pattern fidelity and alignment to the photonic core. Depending on the topography, I would discuss pros and cons of lift-off vs etch-based patterning, and how to manage thermal budget, step coverage, line continuity and contact quality on the planarized oxide.
10. Key process risks and development logic I will explicitly list the main risks I see: thickness non-uniformity, CD drift and CD non-uniformity, overlay errors, mask erosion, plasma-induced roughness, defect creation, incomplete oxide gap fill (voids), planarization defects and metal failures. My proposed development sequence is to stabilize critical modules first, then study module interactions, and finally qualify the full integration flow with statistical robustness.
11. Process control and KPIs Since the assignment asks for consistent, reliable and repeatable results, I want to define concrete KPIs for each major step: film thickness and uniformity, CD and CD bias, overlay, etch depth, sidewall angle, surface and sidewall roughness, defect density, cladding thickness, void level, metal continuity and adhesion, sheet resistance, wafer bow and run-to-run / lot-to-lot stability. I’d explain how I would monitor these via in-line and end-of-line measurements.
12. Characterization and metrology concept I plan to outline a measurement plan that combines incoming inspection, in-line monitoring, intermediate release checks and end-of-line qualification. Depending on the step this would include film-thickness metrology (ellipsometry, reflectometry), profilometry, CD-SEM, AFM, material analysis, defect inspection, cross-section analysis and electrical tests, so that process changes can be correlated with quality metrics instead of only relying on final optical performance.
13. Qualification and SPC For the last technical part, I want to describe a qualification strategy based on DOE, split lots and explicit decision boundaries to identify the most important process parameters and derive robust process windows. Once modules are reasonably stable, I would define SPC-oriented control charts for the key metrics to detect drifts early and gradually move from development mode to a more stable, production-like process for the platform.
14. My motivation and fit Finally, I will briefly explain why I’m interested in developing this AlN platform for quantum PICs and how I see my role: focusing on building a robust, data-driven process flow that connects individual unit processes into a reliable technology platform, and collaborating closely with designers, process engineers and project managers.

What I’d like feedback on

I’d be grateful for any comments on:

• Does this module-based structure and level of detail sound reasonable for a real photonics / semiconductor fab environment?
• Are there any obvious gaps in my approach (for example around CD control methodology, void-free oxide gap fill, CMP strategy, or metrology I should explicitly call out)?
• If you were evaluating this as a hiring manager for a process-development role, what would you definitely want to see that I might be missing?

Thanks a lot in advance for any pointers or criticism – I want to make sure I’m thinking like a process development engineer here, not just as a photonics person who happens to know some fabrication.