# Multilayer integration platform for dense optical phased arrays

Sarra Salhi, Jean-René Coudevylle, Eric Cassan, Carlos Alonso Ramos, Daniele Melati

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Sarra Salhi, Jean-René Coudevylle, Eric Cassan, Carlos Alonso Ramos, Daniele Melati. Multilayer integration platform for dense optical phased arrays. 2025 Photonics North Conference, May 2025, Ottawa, Canada. 10.1109/PN66844.2025.11097134. hal-05136614

HAL Id: hal-05136614

https://hal.science/hal-05136614v1

Submitted on 30 Jun 2025

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# Multilayer integration platform for dense optical phased arrays

S. Salhi¹*, J.R Coudevylle¹, E. Cassan¹, C. Alonso-Ramos¹ and D. Melati¹

¹Centre de Nanosciences et de Nanotechnologies, Université Paris-Saclay, CNRS, 91120 Palaiseau, France

*Corresponding author: sarra.salhi@universite-paris-saclay.fr

**Abstract:**

In this paper, we present the development of a multilayer silicon-silicon nitride platform for dense optical phased arrays. We demonstrate the viability of this platform achieving low propagation losses of 1.3 dB/cm and 2.55 dB/cm for single mode waveguides realized in the silicon and silicon nitride layers, respectively.

On-chip optical phased arrays (OPAs) provide a promising alternative to bulky mechanical systems for optical beam generation and steering, enabling rapid beam control with reduced device sizes and improved efficiency for applications such as LiDAR and free-space optical communications, [1][2]. Dense OPA configurations – including many closely spaced antenna elements – are of particular interest because the reduced space between the antennas helps suppressing unwanted sidelobes, increasing the beam steering range [3]. Antenna density in planar integration technologies is however limited by the need to include routing waveguides to feed the antennas. The possibility to realize a multilayer architecture with two guiding layers, as conceptually shown in Fig. 1(a), would provide additional flexibility for the designers to conceive denser optical phased arrays with improved overall performance. The two guiding layers could be used to place routing waveguides above or below the antennas, simplifying circuit layout, or to interleave two antenna arrays to halve antenna distance. In addition, combining silicon and silicon nitride layers would give simultaneously access to high power handling, efficient phase shifters and micron-scale antennas, hence representing a promising platform for scalable and dense OPA systems. Here, we present our recent work in the development of a silicon-silicon nitride multilayer platform for dense optical phased array integration, demonstrating successful planar layer stacking and low propagation losses in both guiding layers.

In order to realize the platform schematically shown in Fig. 1(b,c), the fabrication begins with a silicon-on-insulator wafer with a silicon thickness of 220 nm. Silicon waveguides and devices are defined via electron-beam lithography, followed by inductively coupled plasma etching. Planarization is then carried out in two steps using flowable oxide (FOx) resist: a diluted FOx solution (1:2 ratio) is spin-coated onto the sample, baked at 150 °C for 3 min, and subsequently annealed at 600 °C for 1h in a nitrogen atmosphere. This initial layer provides a smooth base layer and allows to reliably fill gaps in the silicon layer up to 75 nm. A second, thicker FOx layer (using a FOx dilution of 2:1) is deposited to achieve an overall thickness of approximately 350 nm above the waveguide structure, as shown in Fig. 1(e).
<figure><img src="image_2.png" /><figcaption>Fig 1.(a) : Sketch of a dense optical phased array realized in a multilayer silicon-silicon nitride integration platform , (b) 3D overview and (c) cross-sectional view of the multilayer platform with silicon and silicon nitride waveguides. Annealed FOx is used as buffer layer. (d) Flatness of the annealed FOx layer after the planarization process (e) SEM image of the silicon waveguides after FOx annealing, showing an excellent filling of waveguide gaps up to 75 nm. (f) Experimentally measured propagation loss of the silicon and silicon nitride waveguides.</figcaption></figure>

This layer is also annealed in a nitrogen furnace at 600 °C for 1h, converting the FOx into a layer having similar
properties as SiO2 and yielding a planarized surface with about 7 nm thickness fluctuation, as demonstrated by
the atomic force microscope measurement reported in Fig. 1(d). After the planarization steps, a 400 nm silicon
nitride layer is deposited via plasma-enhanced chemical vapor deposition (PECVD). Waveguide and devices are
again defined with electron beam lithography and inductively coupled plasma etching. This stack provides the
fundamental guiding layers for the subsequent fabrication of the different devices of an integrated OPA system.
Propagation losses are hence of primary importance. Fig. 1(f) reports on their characterization using a cut-back
technique for waveguide realized in both the silicon and silicon nitride layers. Low losses of approximately
1.3 dB/cm for silicon waveguides and 2.5 dB/cm for silicon nitride waveguides were measured, demonstrating
the viability of these platforms for optical phased array (OPA) applications and beyond.

References

1. Poulton, C. V., et al. “Coherent solid-state LiDAR with silicon photonic optical phased arrays.” *Optics Letters*, 42(20), 4091–4094 (2017)

2. Ji, X., et al. “Breaking the delay–bandwidth limit in a photonic architecture.” *Nature*, 558, 411–414 (2018)

3. Sun, J., Timurdogan, E., Yaacobi, A., Hosseini, E. S., & Watts, M. R. “Large-scale nanophotonic phased array.” *Nature* 493, 195–199 (2013)