Rapid prototyping on SOI 220 nm of integrated optical structures with polymer cladding within the QUANTEP collaboration

Introduction: QUANTEP develops quantum-optics devices using silicon photonics. Foundry runs highlighted the need for in-house prototyping for fast iterations, often before final oxide encapsulation and before full optical testing is available. We present a SOI 220 nm workflow where a reversible polymer overcladding acts as a non-permanent photonic environment to accelerate early fabrication of nanowaveguides and grating couplers, compatible with post-processing such as metallization and local rework.

Methods: Patterns were defined by electron-beam lithography (EBL) on SOI 220 nm and transferred by reactive ion etching (RIE). In this first iteration, an Al hard mask improved pattern transfer; in parallel, the flow migrated toward high-etch-resistance resists (e.g., CSAR-class) to reduce complexity in subsequent runs. The design set included full-etch and partial-etch grating coupler (GC) variants: full-etch structures were obtained by RIE, while partial-etch regions were implemented locally by ion beam (FIB) milling and quantified by atomic force microscopy (AFM). Selected chips were coated with approximately 2 um polymethyl methacrylate (PMMA) to enable waveguide confinement; EBL-defined windows preserved access for alignment, metallization, and local rework while keeping polymer coverage on photonic areas. Etch depths and film thicknesses were obtained from AFM and tilted-stage SEM.

Results and Conclusions: SEM confirms fabrication of nanowaveguides and GC structures. AFM validates a partial-etch depth of 65 +/- 6 nm versus a 70 nm target and verifies PMMA thickness. The workflow provides a SOI 220 nm prototyping route combining RIE transfer, FIB partial-etch, and reversible polymer overcladding. While not replacing final SiO2 encapsulation, PMMA increases processing flexibility and enables access windows, metallization, and local rework that would be more complex with oxide cladding.