# Low-loss 200-mm diameter SiN platform including WI-SNSPD for quantum photonics applications from 900 to 940 nm

Valentin Brisson, Quentin Wilmart, Jonathan Faugier-Tovar, Sylvain Guerber, Fabien Laulagnet, Elisa Vermande, Elisa Collin, Stéphane Brision, Nicolas Dunoyer, Laura Boutafa, et al.

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Valentin Brisson, Quentin Wilmart, Jonathan Faugier-Tovar, Sylvain Guerber, Fabien Laulagnet, et al. Low-loss 200-mm diameter SiN platform including WI-SNSPD for quantum photonics applications from 900 to 940 nm. 2025 IEEE Silicon Photonics Conference (SiPhotonics), Apr 2025, London, France. pp.1-2, 10.1109/SiPhotonics64386.2025.10985532. in2p3-05191636

HAL Id: in2p3-05191636

https://in2p3.hal.science/in2p3-05191636v1

Submitted on 29 Jul 2025

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# Low-loss 200-mm diameter SiN platform including WI-SNSPD for quantum photonics applications from 900 to 940 nm

Valentin Brisson<fnref n="1" />, Quentin Wilmart<fnref n="1" />, Jonathan Faugier-Tovar<fnref n="1" />, Sylvain Guerber<fnref n="1" />, Fabien Laulagnet<fnref n="1" />, Elisa Vermande<fnref n="1" />, Elisa Collin<fnref n="1" />, Stéphane Brision<fnref n="1" />, Nicolas Dunoyer<fnref n="1" />, Laura Boutafa<fnref n="1" />, Olivier Castany<fnref n="1" />, Kévin Roux<fnref n="1" />, Emanuel Peinke<fnref n="1" />, Joël Bleuse<fnref n="2" />, Jean-Michel Gérard<fnref n="2" />, and Ségolène Olivier<fnref n="1" />

We demonstrate a SiN platform dedicated to quantum photonics around 900 to 940 nm. The platform is designed to operate with available deterministic single photon sources. Waveguides and passive photonic devices highlight low propagation and insertion losses. Highly performing waveguide-integrated SNSPD allow for efficient on-chip single photon detection.

**Keywords—silicon nitride, single photon detectors, integrated photonics, quantum photonics.**

We present a SiN photonic platform for wavelengths from around 900 to 940 nm, optimised for quantum photonic applications, and fabricated in the 200 mm CMOS line of CEA-Leti's pre-industrialization clean room facility. A stoichiometric LPCVD Si₃N₄ layer serves as waveguide layer. It is transparent at the wavelengths of interest, enables low-loss photon propagation around 900 nm, and has proven high performance at 1550 nm [1]. The 900-nm wavelength range was chosen for the availability of highest quality deterministic single photon sources, namely quantum dots of the III-V material group (e.g. InAs) integrated in microcavities. They have outstanding purity and indistinguishability [2], making them an excellent resource for quantum applications like fault-tolerant quantum computing [3-4], as this platform could enable.

For quantum photonic applications, minimizing optical losses is a strong requirement. The presented photonic platform is optimised to have low-loss single and multimode light propagation, bends, directional couplers, phase shifters and coupling to optical fibres. Single photon detection happens with waveguide-integrated superconducting nanowire single photon detectors (WI-SNSPD). As illustrated in figure 1, the platform consists of a 300 nm thick SiN layer for light propagation, routing and manipulation, thermo-optical phase shifters with insulating trenches, a SNSPD layer for efficient single photon detection, and deep trenches (facet) for edge coupling.

<figure><img src="image_2.png" /><figcaption>Figure 1: Cross-section of the SiN platform for $\lambda = 900$ to 940 nm photonics.</figcaption></figure>

The 300 nm thick SiN layer guides photons throughout fabricated photonic structures, with reasonably small propagation and insertion losses. Multimode waveguide propagation losses are 0.5 dB/cm (see figure 2); for single mode waveguides, they are 1.2 dB/cm. The platform allows for compact waveguide bends. For radii $r \ge 40$ µm the bend losses are smaller than 0.05 dB/90°. Passive photonic structures, like directional couplers, can be compact and low-loss. For light manipulation, resistive heaters can apply a pure phase shift to the guided photons. Insulating trenches have been developed to increase the phase shifter's efficiency [5]. We measured 19 mW/π, instead of about 100 mW/π without trenches, and phase shifter insertion losses much smaller than 0.1 dB.

<figure><img src="image_3.png" /><figcaption>Figure 2: Propagation losses of multimode waveguides, measured on waveguide spirals of different lengths with grating couplers as in- and output (left). A 50-50 directional coupler (right).</figcaption></figure>

Efficient detection of single photons is possible with SNSPDs operating at cryogenic temperature [6,7,8]. The presented WI-SNSPD are superconducting NbN nanowires that are patterned on top of SiN waveguides in an evanescent coupling architecture. The NbN nanowires are 5 nm thick and 100 nm wide. In between SiN and NbN is a 10 nm thick AlN texturing layer (see figures 1 and 3a). For optimal operation, the WI-SNSPDs are cooled to 2.3 K [9]. In the experiments, single photons from a $\lambda = 915$ nm diode laser are injected in a SiN waveguide. When reaching an NbN nanowire they can break its superconductivity, generating a measurable voltage pulse. The photon flux reaching the WI-SNSPD is set to about $10^5$ photons per second (=$0.1$ MHz count rate). We measure the photon count rate as a function of the bias current $I_b$ and calculate detection efficiency. For a dark count rate of 100 Hz, the on-chip detection efficiency is ($72 \pm 4.3$) %, see figure 3b. The timing jitter is ($50.7 \pm 4$) ps. When increasing the incoming single photon flux, count rates up to 143 MHz were measured. Figure 3c shows the decay time $\tau$ of single detection events as a function of the device length $l$. Decay times are calculated by fitting the exponential decays as display the insets. We obtain $\tau/l = (0.056 \pm 0.003)$ ns/µm.

<footnote n="1">¹Univ. Grenoble-Alpes, CEA, LETI, F38000 Grenoble, France</footnote>

<footnote n="2">²Univ. Grenoble-Alpes, CEA, DRF, IRIG, PHELIQS, Grenoble, France</footnote>
The detector of figure 3b has a decay time of 2.17 ns ($l = 30$ µm).

<figure><img src="image_4.png" /><figcaption>Figure 3: (a) SEM image of a U-shaped WI-SNSPD. (b) On-Chip Detection Efficiency and Dark Count Rate of a single U-shaped WI-SNSPD with nanowire length of 2 × 30 µm. The 95% confidence interval is shown in light red. (c) Single event decay time for various WI-SNSPD of different device lengths.</figcaption></figure>

For efficient system integration, one needs to minimize the optical fibre-to-chip coupling losses [10], mostly resulting from a mismatch between the fibre's and PIC's modal fields in terms of diameter and wavefront. Edge coupler (EDC) devices were developed to reduce such losses by increasing the modal field diameter (MFD) of guided light so to match the MFD of optical fibres. For the EDC of figure 4, made in the 300 nm thick SiN layer and benefitting from locally removed silicon substrate, we measured a minimum of 2 dB losses per fibre-to-chip interface. This major contribution to the system losses motivates the development of a new optimised platform with significantly reduced coupling losses.

<figure><img src="image_5.png" /><figcaption>Figure 4: Suspended SiN edge coupler and measured mode profile.</figcaption></figure>

The cross-section of an EDC tip is a determining factor for its MFD, and typically confines the optical field stronger than desired, that means stronger than the optical fibre in use. A possibility to release this constraint it to reduce the

tip cross-section. As the tip width is typically given by the lithography resolution, the cross-section can be reduced by diminishing the waveguide layer thickness. For a future platform, we suggest to use an additional 100 nm thin SiN layer dedicated for edge coupling. An EDC in this layer can produce a MFD matching the MFD of the used optical fibres (e.g. HI 780 from Corning). Simulations predict fibre to chip coupling losses to be as low as 0.13 dB. An adiabatic transition between thin and thick SiN layers can be obtained via inversely shaped linear tapers. Simulated losses are 0.12 dB.

We showed a low-loss SiN quantum photonic platform for 900-940 nm operation with highly performing WI-SNSPD. A potential use-case could be a reconfigurable universal quantum processor, as developed by Quandela [4]. Taking into account the additional SiN layer, the total losses of a 20-mode quantum processor are expected to be beneath 2dB.

We would like to thank Quandela for valuable input and fruitful collaboration.

This work is funded by the Plan France 2030 through the OQuLus project (ANR-22-PETQ-0013), and by the EU Horizon programme (HORIZON-CL4-2022-QUANTUM) under grants Qu-PILOT n°101113983 and QU-TEST n°101113901.

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[8] M. A. Wolff, et al., "Broadband waveguide-integrated superconducting singlephoton detectors with high system detection efficiency", Applied Physics Letters 118, 154004 (2021).

[9] V. Brisson, et al., "Enhancement of the Superconducting Critical Temperature of NbN Ultra-thin Films in a CMOS-Compatible Silicon Nitride Photonic Platform on 200-mm-diameter Wafers", IOP Supercon. Sci. Tech. (under peer review).

[10] R. Marchetti, et al., "Coupling strategies for silicon photonics integrated chips [Invited]," Photon. Res. 7, 201-239 (2019).