The Fraunhofer Institute for Reliability and Microintegration (IZM) carried out a multi‑partner project funded under the ECSEL M01‑M42 programme (grant FKZ 16ESE0352) from 1 May 2019 to 31 October 2022, with a six‑month extension. The consortium comprised Fraunhofer IZM, Fraunhofer IMS, the Norwegian company IDEAS, the South‑Korean subcontractor NNFC, the French laser manufacturer ALMAE, the Austrian company BESI‑AT, and the University of South Norway. The work was organised into five main work packages: WP2 defined specifications, WP3 produced design concepts, WP4 developed process flows, and WP5 implemented packaging and integration. Three application areas were targeted: low‑cost thermal imaging systems (UC2), passive fibre alignment for single‑mode transceivers (UC3), and a heart‑monitoring system (UC4).

In the thermal‑imaging use case, a monolithic silicon cap was engineered with a 300 µm roof thickness and a total cap thickness of 650 µm, while the surrounding silicon frame measured 700 µm. The design ensured a 500 µm clearance to the pad ring and incorporated a 10 µm gold‑based sealing ring to enable AuSn soldering. An anti‑reflective coating was applied only to the outer side of the capping parts to preserve optical performance, and a getter was integrated inside the package. Wafer‑level hermetic sealing was achieved under a vacuum of 10⁻¹ mbar, with a yield exceeding 95 %. Bond tests using a flat germanium lid on a silicon frame revealed cracks and partial failures, leading to the adoption of a full‑silicon packaging approach that proved more reliable for wafer‑level bonding.

For the passive fibre‑alignment application, a 3‑D silicon bench with four 1.6 mm‑pitch channels was fabricated. Optical simulations performed by DustPhotonics guided the geometry of the bench, including recesses and V‑grooves for a cylindrical lens, an isolator, and a single‑mode fibre. Two flip‑chip bonding strategies were evaluated: liquid‑solder assisted self‑alignment with mechanical stops, and high‑precision thermocompression bonding using electroplated gold bumps. The latter, performed on a flip‑chip bonder at IZM, achieved a bonding precision of 0.5 µm. Two variants of the external‑modulation laser (EML) were co‑designed with ALMAE: a straight‑beam device and a 7°‑tilted device to mitigate back‑reflection. Component‑level fiducials were fabricated at different heights, and a semi‑additive lithography process was introduced to overcome challenges in plating deep recesses, improving the completeness of the fiducials.

In the heart‑monitoring use case, high‑density polyimide flex substrates were produced and populated with electronic components using flip‑chip and surface‑mount techniques. The substrates were then encapsulated in thermoplastic polyurethane (TPU) to create a flexible, robust system. The integration of the flex substrate with functional parts was completed, although progress was slowed by COVID‑19 restrictions that limited in‑person process development.

Overall, the project delivered a comprehensive process chain from design to wafer‑level packaging for silicon‑based photonic and MEMS devices. The technical achievements include a robust silicon cap design, high‑yield hermetic sealing, sub‑micrometre flip‑chip alignment, and successful integration of flexible electronics. The collaboration among German, Norwegian, South‑Korean, French, and Austrian partners, supported by the ECSEL programme, enabled the translation of these technologies into low‑cost, high‑performance imaging and communication systems.