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Broadband supercontinuum generation in

nitrogen-rich silicon nitride waveguides using

a 300 mm industrial platform

CHRISTIAN LAFFORGUE,

1,†,

*SYLVAIN GUERBER,

1,2,†

JOAN MANEL RAMIREZ,

GUILLAUME MARCAUD,

CARLOS ALONSO-RAMOS,

XAVIER LE ROUX,

DELPHINE MARRIS-MORINI,

ERIC CASSAN,

CHARLES BAUDOT,

FRÉDÉRIC BOEUF,

SÉBASTIEN CREMER,

STÉPHANE MONFRAY,

AND LAURENT VIVIEN

Centre for Nanoscience and Nanotechnology (C2N), CNRS, Université Paris-Sud, Université Paris-Saclay, UMR 9001,

91405 Orsay Cedex, France

Technologie R&D, STMicroelectronics, SAS, 850 rue Jean Monnet, 38920 Crolles, France

III-V lab, a joint venture from Nokia Bell Labs, Thales and CEA, 1 Avenue Augustin Fresnel, 91767 Palaiseau Cedex, France

*Corresponding author: christian.lafforgue@c2n.upsaclay.fr

Received 2 October 2019; revised 30 December 2019; accepted 5 January 2020; posted 6 January 2020 (Doc. ID 379555);

published 27 February 2020

We report supercontinuum generation in nitrogen-rich (N-rich) silicon nitride waveguides fabricated through

back-end complementary-metal-oxide-semiconductor (CMOS)-compatible processes on a 300 mm platform.

By pumping in the anomalous dispersion regime at a wavelength of 1200 nm, two-octave spanning spectra cover-

ing the visible and near-infrared ranges, including the O band, were obtained. Numerical calculations showed that

the nonlinear index of N-rich silicon nitride is within the same order of magnitude as that of stoichiometric silicon

nitride, despite the lower silicon content. N-rich silicon nitride then appears to be a promising candidate for

nonlinear devices compatible with back-end CMOS processes.

https://doi.org/10.1364/PRJ.379555

1. INTRODUCTION

For many years, nonlinear optics has been unlocking new func-

tionalities in optical communications (imaging or sensing, for

example). Among these functionalities, we can cite electro-

optic modulation through the Pockels effect, parametric ampli-

fication, or frequency conversion. There is a particular interest

in applications involving frequency conversion. In this context,

third-order nonlinear effects are of great concern, especially

supercontinuum generation (SCG).

The latter has been widely studied in photonic crystal fibers

[1], leading to advances in optical coherence tomography [2],

precise measurement of optical frequencies [3], sensing and

microscopy [4], to name a few. Recently, efforts have been

made to develop SCG on-chip. High nonlinearities have been

achieved, e.g., in chalcogenide glasses [5] or III–V materials

[6,7]. However, these materials are not appropriate for large

scale and low-cost production of compact electronics and

photonics devices due to their lack of complementary-metal-

oxide-semiconductor (CMOS) compatibility. This obstacle can

obviously be overcome with silicon photonics. Silicon has a

high nonlinear index, and interesting SCG results have been

demonstrated in the past few years [8–14]. However, the large

two-photon absorption (TPA) in the near-infrared wavelength

range is a major drawback for nonlinear photonics using silicon

[15]. On the other hand, a silicon nitride (SiN

) platform is

compatible with silicon technology and offers a nonlinear index

10 times higher than that of silicon dioxide with negligible TPA

in the near-infrared wavelength range [16]. Hence, in the last

decade, multiple studies have demonstrated wide SCG in SiN

waveguides featuring broadband spectra [17–29]. Applications

of supercontinuum in SiN

have been shown with, for example,

an f -to-2f interferometer for carrier envelope offset frequency

detection [21,24,30–32], or mid-infrared generation of disper-

sive waves (DWs) for gas spectroscopy [28]. However, most

of the reported SiN

devices were fabricated using high tem-

perature processes such as low-pressure chemical vapor depo-

sition (LPCVD) or annealing steps to avoid cracks in the films

and to achieve ultralow linear losses. Such high temperature

fabrication steps are not compatible with back-end CMOS

processes, hindering the integration of SiN

active devices on

electronic–photonic-integrated circuits for large-scale and low-

cost production. Plasma-enhanced chemical vapor deposition

(PECVD) addresses this issue since it is a low temperature dep-

osition method (<500°C) widely used to deposit SiN

films

in CMOS foundries. Nonetheless, this deposition method in-

volves the use of precursor gas such as silane, resulting in N–H

dangling bonds in the SiN

film, known to be responsible for

352

Vol. 8, No. 3 / March 2020 / Photonics Research

Research Article

strong absorption at wavelengths around 1.5 μm. Recently, re-

sults have been published indicating the possibility of drasti-

cally reducing the linear losses at 1550 nm by employing

deuterated SiN

to shift the absorption band due to N–H

bonds from 1.5 μmto2μm, allowing low loss in the C band

without needing high temperature processes [33]. Wang et al.

also established a back-end CMOS-compatible process to fab-

ricate silicon-rich nitride waveguides [34] through bandgap en-

gineering. Despite showing good nonlinear properties and low

TPA at 1550 nm wavelength, the linear losses are still high

(10 dB/cm) because of Si–H and N–H bonds as a result of

the film deposition method. Other work has been done on an

annealing-free process using stoichiometric SiN

based on an

ultralow deposition rate in LPCVD [35]. Several deposition

steps with different orientation of the wafer are operated to

avoid cracks by spreading the tensile stress in diverse directions.

This study demonstrated a frequency comb generation by

pumping a waveguide at 1550 nm wavelength. These methods

can be back-end compatible solutions for nonlinear photonics

in the C band, but still no advanc es have been shown to reduce

linear losses in the O band (1260–1360 nm) with similar proc-

esses. In this work, we focus on SCG within this range in the

frame of the STMicroelectronics industrial platform, aiming

at data communications application at 1.31 μm wavelength.

In this case, it is necessary to reduce the effective index of

the SiN

waveguide for optimal fiber-to-chip coupling. The

use of a nitrogen-rich (N-rich) SiN

is favorable in this case

since the refractive index of SiN

decreases when increasing the

amount of nitrogen [36], and it can be deposited with a low-

temperature PECVD technique. Moreover, the N–H bonds do

not affect the propagation loss in the O band, and it even has

been reported that linear losses at 1.31 μm are lower for N-rich

SiN

(<1dB∕cm) than for stoichiometric SiN

[36]. This is

the approach we chose in this study, as it permits us to obtain

low linear-loss films through a simple back end of line-compat-

ible process appropriate for the STMicroelectronics platform.

Here we report a two-octave spanning SCG covering the visible

range and the O band in N-rich SiN

waveguides fabricated on

a 300 mm platform at STMicroelectronics. Despite the non-

linear index of N-rich SiN

being unknown, we estimated the

nonlinear coefficient of our device by fitting curves obtained

through numerical simulations to the experimental data. It ap-

pears that the nonlinear coefficient of our N-rich SiN

wave-

guides is of the same order of magnitude as the one predicted

for a stoichiometric Si

waveguide with the same dimensions

(γ ≃ 1W

−1

· m

−1

). Since many SCG applications rely on co-

herence, we investigated the latter through numerical simula-

tion by calculating the first-order degree of mutual coherence.

It shows that for waveguides slightly longer than the soliton

fission length, the supercontinuum spectrum exhibits a high

coherence over more than one octave. Therefore, N-rich

SiN

waveguides appear to be a promising platform to develop

large-scale nonlinear photonics.

2. SAMPLE FABRICATION

The waveguides were fabricated in STMicroelectronics facilities

on a 300 mm platform. First, a thermal oxidation of a silicon

substrate is performed to obtain a 600 nm-thick silicon dioxide

(SiO

) layer. Two tetraethyl orthosilicate (TEOS) deposition

steps are carried out to form a total of 2 μm-thick SiO

layer.

Then a 600 nm-thick N-rich SiN

film is deposited with low-

temperature PECVD (480°C, <5 Torr with SiH

, NH

, and

gas). Finally, a deep-UV lithography step followed by an

etching to define the waveguide was carried out. Since the non-

linear coefficient of N-rich SiN

is unknown and expected to be

lower than the nonlinear coefficient of stoichiometric SiN

,we

fabricated long waveguides to have an important nonlinear in-

teraction length. Thus, the initial design consists in a spiral con-

stituted of two different widths: a narrow-enough section to be

single-mode in the bending regions to prevent important bend-

ing loss; and a larger multimode section to reduce the overlap

between the fundamental mode and the sidewalls in order to

diminish the linear propagation loss in the straight regions. We

performed numerical simulations using a mode solver to find

suitable waveguide widths for the single-mode and multimode

regions for wavelengths lower than 1300 nm, since at higher

wavelengths, the linear losses of N-rich SiN

increase [36].

For a 700 nm width, the waveguide is single-mode in the

near-infrared wavelength range (1000–1300 nm) while confin-

ing the light well. This value is then the one we used for the

single-mode regions. For widths larger than 1000 nm, the

waveguide is multimode in the near-infrared wavelength range.

Furthermore, the multimode section is engineered to optimize

the anomalous dispersion, as reported in the next section.

Transverse-electric (TE) polarization will be used because the

mode simulations show a lower modal effective area A

eff

than

the transverse-magnetic (TM) polarization (A

eff

 0.58 μm

in TE, A

eff

 0.72 μm

in TM for a 1200 nm-wide waveguide

at 1200 nm wavelength), which is an advantage for SCG as it

Fig. 1. (a) Schematic view of the waveguide section; (b) SEM view

of spiral waveguide; (c) and (d) TE mode profile at 1200 nm wave-

length for a 700 nm-wide waveguide and a 1200 nm-wide waveguide,

respectively; (e) schematic view of the final design for the straight

waveguide (top view).

Research Article

Vol. 8, No. 3 / March 2020 / Photonics Research 353

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