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Silicon nitride (SiN) is a promising candidate material for becoming a standard high-performance solution for integrated
biophotonics applications in the visible spectrum. As a key feature, its compatibility with the complementary-oxidemetal-
semiconductor (CMOS) technology permits cost reduction at large manufacturing volumes that is particularly
advantageous for manufacturing consumables. In this work, we show that the back-end deposition of a thin SiN film
enables the large light-cladding interaction desirable for biosensing applications while the refractive index contrast of the
technology (Δn ≈ 0.5) also enables a considerable level of integration with reduced waveguide bend radii. Design and experimental validation also show that several advantages are derived from the moderate SiN/SiO2 refractive index contrast, such as lower scattering losses in interconnection waveguides and relaxed tolerances to fabrication
imperfections as compared to higher refractive index contrast material systems. As a drawback, a moderate refractive
index contrast also makes the implementation of compact grating couplers more challenging, due to the fact that only a
relatively weak scattering strength can be achieved. Thereby, the beam diffracted by the grating tends to be rather large
and consequently exhibit stringent angular alignment tolerances. Here, we experimentally demonstrate how a proper
design of the bottom and top cladding oxide thicknesses allows reduction of the full-width at half maximum (FWHM)
and alleviates this problem. Additionally, the inclusion of a CMOS-compatible AlCu/TiN bottom reflector further
decreases the FWHM and increases the coupling efficiency. Finally, we show that focusing grating designs greatly
reduce the device footprint without penalizing the device metrics.
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