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Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview

TL;DR: An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this article, which can be combined to design complex functional circuits, but more important are manufactured in a single monolithic flow to create a compact photonic integrated circuit.
Abstract: An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks

Summary (6 min read)

Introduction

  • This paper reports on numerous different applications of different types of silicon nitride waveguides, each with its specific and unique optical confinement and transmission properties.
  • With the development of several different, standardized, building blocks that can be monolithically combined in the manufacturing process flow, complex functions can be obtained that can serve application domains such as high-speed and highcapacity optical communication, microwave photonics and medical diagnostics, to mention a few.

II. TRIPLEX WAVEGUIDES; MODELING

  • TriPleX waveguides are a family of waveguide geometries that is based on an alternating layer stack consisting of two materials: Si3N4 (silicon nitride) and SiO2 (silicon dioxide) that have refractive indices of about 1.98 and 1.45, resp., at 1.55 μm wavelength.
  • Most commonly used substrate is singlecrystal silicon, for some applications fused silica glass substrates have been used too (e.g., if a transparent chip in the visible light region is required).
  • Table I demonstrates cross-sections and SEM photographs of four different waveguide geometries that will be described in more details in Sections II-A to II-D. An overview of the manufacturing process can be found in Section II-E.

C. Symmetric Double-Stripe or SDS

  • The symmetric double-stripe (SDS) geometry consists of two stripes of Si3N4 of the same thickness at the top on top of each other, separated by an intermediate SiO2 layer.
  • The optimized geometry that is also offered in the current TriPleX IR MPW runs and has been used in many optical beam-forming networks (OBFN) devices [11], that is discussed more elaborately in Section VI-C.
  • A minimum bending radius of 100 μm is feasible, based on the criterion that bend losses should be below 0.01 dB/cm [14].

D. Asymmetric Double-Stripe or ADS

  • The asymmetric double-stripe (ADS) has a similar waveguide geometry to the SDS: the main difference is that the thickness of the upper Si3N4 stripe is different than the lower Si3N4 stripe, = SiO2 , = Si substrate, = background material (e.g., air).
  • See Fig. 2 for a schematic cross-section, while also the thickness of the intermediate SiO2 layer is significantly smaller: instead of 500 nm as for the SDS geometry, a thickness of 100 nm is used for the ADS geometry.
  • Please note that the group index is not the same as for the SDS waveguide: the ADS waveguide has a slightly larger group index of 1.77.
  • Propagation loss values on ADS waveguides are reproducibly measured to show similar or better values compared to SDS waveguides, so <0.1 dB/cm Table II gives an overview of the most important properties of the different TriPleX geometries discussed in this Section.

E. Manufacturing Process of the Basic TriPleX Geometries

  • The general process flow is illustrated schematically in Table III for the production of the different basic TriPleX geometries.
  • The left-hand column shows the production process for the box shell, the right-hand column is related to the other geometries, the double-stripe SDS and ADS.
  • The current scheme contains as many batch processes as possible, using equipment that is present in most CMOS foundries: this makes production of TriPleX waveguides suitable for mass production.
  • Typical deposition temperatures are between 300 °C and 400 °C.
  • The stress in PECVD SiO2 layers (Table III, step 10) is much less than in LPCVD layers, allowing for much thicker layers.

A. Routing Building Blocks

  • The most elementary basic building blocks are routing building blocks, that are based on regular straight waveguide building blocks (either with constant width or with varying width for lateral tapers), and on polar bend building blocks (based on bends with constant radius of curvature).
  • Based on these building blocks, more advanced routing building blocks can be defined, such as S-shaped bends.

B. Spot-Size Converters

  • SSCs modify the MFD of a waveguide either by using a vertical taper, as discussed in Section II-C, and/or by changing the waveguide width using a lateral taper.
  • The vertical taper can be designed to locally and adiabatically change the thickness of the guiding layer(s) such that discontinuities in MFD are avoided, to minimize the excess loss of the SSC, at the cost of a length of typically several hundreds of μm.
  • The SSC can be used to minimize coupling loss to e.g., a glass fiber, where the end-facet waveguide typically has a 90° angle, as the effective index of TriPleX waveguides are quite close to that of standard glass fibers for telecom).
  • Another application is coupling to a waveguide from another integrated optical chip such as light sources manufactured in InP technology.
  • Because of the much larger effective index of waveguides in other technologies such as InP, SOI, the SSC coupling loss can be decreased by placing it under a suitable angle different than 90°, more can be found in Section VI-A.

D. Phase Modulators

  • The phase modulator that is standardly used for the examples in this paper is based on thermo-optical tuning by means of heaters on top of the top cladding above a waveguide.
  • These are manufactured by defining structured metal patterns based on a chromium layer and a gold layer.
  • By combining e.g., two Y-junctions and two straight waveguides, one with a phase modulator and the other without, one can define a tunable coupler based on a Mach-Zehnder interferometer (MZI).
  • Thermo-optic actuators suffer from power dissipation, in the order of 300 mW per π phase-shift per modulator, and thermal cross-talk from neighboring heaters requires a distance of at least 250 μm to the nearest neighboring waveguide.
  • For these reasons, alternative types of phase modulators have been developed, as is mentioned in the first paragraph of Section III, namely stress-optic modulators (see the next section).

E. Stress-Optic Actuator

  • To drastically decrease the power dissipation, ultra-lowpower stress-optic phase actuators are implemented in the TriPleX platform for visible light [18] as well as for the telecommunication C-band [21].
  • When the PZT on top of the waveguide is exposed to an electric field, the PZT expands in the direction along the electric field while simultaneously contracting in the two other directions resulting in stress in the waveguide structure.
  • The stress-optic actuators were fabricated using a symmetric double stripe TriPleX geometry (see Section II-C).
  • The PZT layer with a thickness 2 μm was grown using pulsed laser deposition allowing for the growth of high-quality layers on large wafer sizes and at commercial throughput [22].
  • Finally, platinum top electrodes with a thickness of 100 nm were deposited.

A. Ring Resonator Filter

  • An important functionality that can be realized with low loss TriPleX waveguide is optical filtering via micro-ring resonators ROELOFFZEN et al.: LOW-LOSS SI3 N4 TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321 (MRRs).
  • The ring waveguide behaves thus as a wavelength selective optical cavity that stores, enhances and drops out only resonant light, while all other light is transmitted to the output.
  • In order to increase the cavity life-time and Qfactor, the internal losses of the cavity, of which propagation loss is a main contribution, need to be substantially reduced.
  • The authors calculated the threshold power [28] for which they used the Q-factor given in Fig. 5 and the resonator length of 3400 μm.
  • Because the confinement is only moderate with a mode field area of about 10μm2 , the gain is mainly due to the lower nonlinear index of the SiO2 cladding [29], and the mode volume is relatively large.

C. Ring Resonator Based Delay Lines

  • Tunable delay lines are essential building blocks for implementing optical signal processing functions on chips.
  • In particular, microwave photonic applications like optical beamforming.

D. AWG Based Wavelength Independent Switch

  • Output shows three outputs instead of one, because of higher order spatial modes.
  • Fig. 9. Switching between 10 different output ports of the AWG, while measuring the output power of all 10 ports simultaneously.
  • As example, in Fig. 8(a) zero-order AWG design is shown, containing 3 inputs, 103 arms and 80 output waveguides without tuning elements with red light coupled into the waveguide.
  • This building block can be designed for many required output numbers.

A. Switched Delay Line

  • Switched delay lines are commonly used in OBFNs, signal processers, signal routers and buffering [35].
  • The schematic layout is shown in Fig. 10.
  • The optical cross talk, being the light travelling through an unwanted delay path interfering with the wanted delay path, is twice the amount of optical suppression of a tunable coupler.
  • The absolute difference per delay, which the authors call the delay error, is smaller than ±0.5 ps, as is shown in Fig. 12, where each dot indicates a separate measurement.
  • As is clear from these measurements, lowloss TriPleX is very suitable for fabricating switched delay lines.

C. High-Granularity WDM Filter

  • TriPleX circuits also promise a significant impact on the next generation elastic optical communication networks, where technologies enabling high spectral efficiency and fine granularity transmissions are of significant use to increase the network capacity and flexibility.
  • One reason for this is that the low-loss feature of the waveguide supports designs of filter structures with long delay paths, e.g., multiple cm, and therefore enables spectral processing with resolution finer than 1 GHz, which in contrast is challenging to implement using free-space optics.
  • One interesting function is a Nyquist-filtering (de)interleaver [38] implemented using a ring resonator-assisted Mach-Zehnder interferometer circuit that comprises an asymmetric MZI with each arm coupled with a ring resonator as shown in Fig. 14.
  • The design presented here employs a total of seven tuning elements, i.e., two tunable couplers at the input and output of the MZI, two tunable couplers of the two ring resonators, and three tunable phase shifters in the ring loops and MZI delay line.
  • Using this combination, a WDM superchannel ROADM was enabled, which supports a sub-channel spacing of 12.5 GHz, a factor of four smaller than the current 50 GHz DWDM grid.

D. High-Order Ring Resonators

  • Another interesting application of high-order ring resonators is input and output multiplexers (IMUXs and OMUXs) of satellite transponders in the field of satellite communications.
  • Currently, the implementation of these functions relies on bulky electronics.
  • As an important milestone of the waveguide technology, this filter shows an excellent combination of frequency selectivity, power rejection, and insertion loss when using the longest waveguides that were ever applied in such filters, i.e., each ring loop having an optical length of 21 cm.
  • A single pass through all resonators thus corresponds to about 1.7 m, and taking into account that multiple roundtrips are performed in the.

E. Programmable Photonic Integrated Circuits for General Purpose Optical Processor Chip

  • Integrated optical signal processors promise a wide range of applications in several different fields including optical communication networks, microwave photonics, optical sensing, biophotonics, and quantum optics.
  • In practice, this paradigm may have issues of cost and flexibility, which are determining factors from a commercial perspective.
  • A radically different approach in contrast to ASPICs is so-called programmable optical chips [40], i.e., universal signal processors integrated on optical chips.
  • In principle, when provided with a sufficiently large network scale, one can program those Mach-Zehnder couplers to implement an arbitrary interferometric circuit topology for signal processing purposes, e.g., FIR and IIR filters, with full control of circuit parameters (amplitude and phase of each optical path in the circuit).
  • As the network size scales up, waveguide loss plays an increasingly significant role for the device performance.

F. All-Optimized Integrated MWP Notch Filter

  • Microwave photonics (MWP) is an emerging technology of which photonic technologies are used to generate, distribute, process, and measure RF and microwave signals [42].
  • Recently, there is a strong paradigm shift towards the incorporation of photonic circuits in MWP leading to integrated MWP devices and systems [43].
  • The waveguide geometry used in this work was the symmetric double-stripe waveguide (Section II-C) with loss of 0.1 dB/cm and ring bend radius of 125 microns.
  • This leads to a ring filter with FSR of 25 GHz and quality factor of >900,000 allowing high resolution filtering of 150 MHz.
  • The measured RF gain, noise figure and dynamic range of the filter in the range of 1–12 GHz is shown in Fig. 17(c).

G. Planar Waveguide Based Sensor

  • The monitoring of the wavelength of light is crucial for many applications such as in optical communications [46], wavelength-division-multiplexing [47], specifically for realtime wavelength control and tuning of on-chip hybrid lasers [48].
  • An advantage of using MRR is the possibility to realize wavelength meters via high-Q resonators that can offer a smaller footprint and a much higher resolution as compared to other integrated optic approaches such as, e.g., Mach-Zehnder interferometers.
  • The named estimation range, however, would not be sufficient for many applications, e.g., monitoring on-chip hybrid lasers across the entire C-band that comprises tens of nanometers.
  • It can be clearly seen that the FSR has become much wider compared to previous work, reaching a value of 43 nm.
  • The wavelength meter was first calibrated with 1001 different known input wavelengths, by.

B. Tunable Hybrid Lasers With High Coherence

  • Enabled by the low-loss properties and the mature tapering options that offer high chip-to-chip coupling efficiency, TriPleX is a highly suitable waveguide platform to realize external feedback circuits with long optical cavity lengths for bandwidth narrowing of diode lasers.
  • As a result, such hybrid lasers are able to provide laser light with a much higher spectral purity as compared to typical DFB and DBR lasers, i.e., single-mode oscillation with narrow spectral linewidth.
  • In the experimental realization, the laser comprises an InP reflective semiconductor optical amplifier (RSOA) coupled to the external TriPleX waveguide circuit.
  • The RSOA usually possess a HR coated back-facet to reduce cavity losses, and a low reflectivity front facet to impose lasing via the feedback from the external waveguide resonator circuit.
  • Here the circuits are used as a highly frequency selective feedback mirror for imposing single-frequency operation at a narrow spectral linewidth.

C. Binary Tree Optical Beamforming Network

  • Phased array antenna systems can be found in multiple applications as radar systems, radio astronomy and communication systems like the upcoming 5G mobile networks systems.
  • Optical beamforming networks (OBFN)s can be used to control the beam shape and direction.
  • The use of cascaded ring ROELOFFZEN et al.: LOW-LOSS SI3 N4 TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321 resonators has two benefits [55].
  • First, for a certain target delay value the maximum delay bandwidth increases approximately linear with the number of ring resonators.
  • The manufactured OBFN has ring resonators with an FSR of 14 GHz.

VII. OUTLOOK

  • The activities in the past decade have provided great insights in the possibilities and the capabilities of the TriPleX platform.
  • Challenges and solutions evolve out of combined expertise of multidisciplinary teams.

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IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 4, JULY/AUGUST 2018 4400321
Low-Loss Si
3
N
4
TriPleX Optical Waveguides:
Technology and Applications Overview
Chris G. H. Roeloffzen , Marcel Hoekman , Edwin J. Klein, Lennart S. Wevers, Roelof Bernardus Timens ,
Denys Marchenko
, Dimitri Geskus, Ronald Dekker, Andrea Alippi, Robert Grootjans, Albert van Rees ,
Ruud M. Oldenbeuving, J
¨
orn P. Epping, Ren
´
e G. Heideman, Kerstin W
¨
orhoff, Arne Leinse, Douwe Geuzebroek
,
Erik Schreuder, Paulus W. L. van Dijk, Ilka Visscher
, Caterina Taddei , Member, IEEE, Youwen Fan ,
Caterina Taballione, Yang Liu
, David Marpaung , Leimeng Zhuang , Meryem Benelajla, and Klaus-J. Boller
(Invited Paper)
Abstract—An overview of the most recent developments and im-
provements to the low-loss TriPleX Si
3
N
4
waveguide technology is
presented in this paper. The TriPleX platform provides a suite of
waveguide geometries (box, double stripe, symmetric single stripe,
and asymmetric double stripe) that can be combined to design com-
plex functional circuits, but more important are manufactured in
a single monolithic process flow to create a compact photonic inte-
grated circuit. All functionalities of the integrated circuit are con-
structed using standard basic building blocks, namely straight and
bent waveguides, splitters/combiners and couplers, spot size con-
verters, and phase tuning elements. The basic functionalities that
have been realized are: ring resonators and Mach–Zehnder inter-
ferometer filters, tunable delay elements, and waveguide switches.
Combination of these basic functionalities evolves into more com-
plex functions such as higher order filters, beamforming networks,
Manuscript received September 29, 2017; revised January 10, 2018; accepted
January 10, 2018. Date of publication January 15, 2018; date of current version
February 14, 2018. (Corresponding author: Chris G. H. Roeloffzen.)
C. G. H. Roeloffzen, M. Hoekman, E. J. Klein, L. S. Wevers, R. B. Timens,
D. Marchenko, D. Geskus, R. Dekker, A. Alippi, R. Grootjans, A. van Rees,
R. M. Oldenbeuving, J. P. Epping, R. G. Heideman, K. W
¨
orhoff, A. Leinse,
D. Geuzebroek, E. Schreuder, P. W. L. van Dijk, and I. Visscher are with the Li-
oniX International BV, Enschede AL 7500, The Netherlands (e-mail: c.g.h.
roeloffzen@lionix-int.com; m.Hoekman@Lionix-int.com; e.j.klein@lionix-
int.com; l.Wevers@Lionix-int.com; r.b.timens@lionix-int.com; d.Marchenko@
Lionix-int.com; d.Geskus@Lionix-int.com; r.Dekker@Lionix-int.com; A.
Alippi@lionix-int.com; r.grootjans@lionix-int.com; albertvanrees@gmail.
com; r.m.oldenbeuving@lionix-int.com; j.p.Epping@Lionix-int.com; r.g.
Heideman@Lionix-int.com; k.Worhoff@Lionix-int.com; a.leinse@lionix-int.
com; d.h.geuzebroek@lionix-int.com; f.Schreuder@Lionix-int.com; p.w.l.
vandijk@lionix-int.com; i.visscher@lionix-int.com).
C. Taddei and Y. Fan are with the LioniX International BV, Enschede AL
7500, The Netherlands, and also with the University of Twente, Laser Physics
and Nonlinear Optics Group, MESA+Research Institute for Nanotechnol-
ogy, Enschede AE 7500, The Netherlands (e-mail: C.Taddei@lionix-int.com;
y.fan@utwente.nl).
C. Taballione, M. Benelajla, and K.-J. Boller are with the University of
Twente, Laser Physics and Nonlinear Optics Group, MESA+Research In-
stitute for Nanotechnology, Enschede AE 7500, The Netherlands (e-mail:
c.taballione@utwente.nl; meriembenelajla@gmail.com; k.j.boller@utwente.
nl).
Y. Liu and D. Marpaung are with the Centre for Ultrahigh Bandwidth Devices
for Optical Systems, University of Sydney, NSW 2006, Australia (e-mail:
yliu3472@uni.sydney.edu.au; david.marpaung@sydney.edu.au).
L. Zhuang is with the Electro-Photonics Laboratory, Electrical and Com-
puter Systems Engineering, Monash University, Clayton, VIC 3800, Australia
(e-mail: leimeng.zhuang@monash.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTQE.2018.2793945
and fully programmable architectures. Introduction of the active
InP chip platform in a combination with the TriPleX will intro-
duce light generation, modulation, and detection to the low-loss
platform. This hybrid integration strategy enables fabrication of
tunable lasers, fully integrated filters, and optical beamforming
networks.
Index Terms—Beam steering, integrated optics, lasers, optical
communication, optical filters, optical waveguides.
I. INTRODUCTION
S
ILICON photonics has reached significant interest in the
last 15 years [1] for applications in datacom and telecom-
munications.
The manufacturing of silicon photonics circuitry shows great
similarity with manufacturing process flows of microelectronics
circuits and requires lithography equipment to accurately pattern
the planar silicon wires to guide the light. The transparency
of silicon waveguides for wavelengths between 1100 nm and
1550 nm is compatible with the use of optical fibers for optical
signal transport. Silicon nitride, especially high-quality Si
3
N
4
,
extends the usable spectral window towards 405 nm [2]. The
high index contrast of silicon nitride with respect to silicon oxide
makes it a perfect candidate for production of compact photonic
circuitry for both infrared and visible light applications. This
paper reports on numerous different applications of different
types of silicon nitride waveguides, each with its specific and
unique optical confinement and transmission properties.
With the development of several different, standardized,
building blocks that can be monolithically combined in the man-
ufacturing process flow, complex functions can be obtained that
can serve application domains such as high-speed and high-
capacity optical communication, microwave photonics and med-
ical diagnostics, to mention a few.
Moreover, the development of several tool kits, product de-
velopment tool kits (PDKs), for circuitry design, mask layout
and performance simulation of these photonic devices, has im-
proved the access to this field of integrated photonics. The
user-friendliness and availability of the right tools supports the
foundries that provide access to manufacturing line in the form
1077-260X © 2018 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution
requires IEEE permission. See http://www.ieee.org/publications
standards/publications/rights/index.html for more information.

4400321 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 4, JULY/AUGUST 2018
of multi-project wafer (MPW) service, where industrial and aca-
demic customers share the cost in a rapid prototyping shuttle run.
A big challenge for both silicon photonics and silicon nitride
waveguides, is the integration of active sources such as lasers
and amplifiers. Although a breakthrough was reported by In-
tel in 2005 [3], where they presented their continuous silicon
laser, alternatives to interface and integrate lasers with the pas-
sive silicon platform are still investigated. This paper presents a
novel approach of a planar integration strategy of InP and Si
3
N
4
photonic circuits. It highlights the most important interface and
assembly challenges, which have successfully been overcome,
and resulted in a new world-record-performance for a narrow-
linewidth hybrid laser. This hybrid approach is the foundation
for the design and fabrication of more complex devices and mod-
ules that are built from hyperfunctional InP and Si
3
N
4
chips.
The requirement to develop a means and capability to fabricate
these products in an economical way in medium to high vol-
ume is evident and drives the need for process optimization,
automation and standardization of processes and workflows.
The structure of the paper is organized as follows. Chapter
II describes the properties of silicon nitride (Si
3
N
4
) waveguide
structures, particularly TriPleX geometries and their fabrication
processes. The descriptions of basic building blocks, straight
and bent waveguides, splitters/combiners and couplers, spot
size converters and phase tuning elements, are given in
Chapter III. Chapter IV describes basic functionalities, such as
ring resonator, Mach-Zehnder interferometer filters, delays and
switches. These functionalities are combinations of the basic
building blocks. More complex functionalities, with higher
level of integration, are described in Chapter V. Combination
of passive and active functions, using hybrid integration of InP
and TriPleX, is described in Chapter VI. The paper ends with
Chapter VII giving a prospect of these technologies for future
applications.
II. T
RIPLEXWAVEGUIDES;MODELING
TriPleX waveguides are a family of waveguide geometries
that is based on an alternating layer stack consisting of two
materials: Si
3
N
4
(silicon nitride) and SiO
2
(silicon dioxide)
that have refractive indices of about 1.98 and 1.45, resp.,at
1.55 μm wavelength. Most commonly used substrate is single-
crystal silicon, for some applications fused silica glass substrates
have been used too (e.g., if a transparent chip in the visible
light region is required). Table I demonstrates cross-sections
and SEM photographs of four different waveguide geometries
that will be described in more details in Sections II-A to II-D.
An overview of the manufacturing process can be found in
Section II-E.
A. Box Shell
The box shell geometry consists of a SiO
2
box-shaped core
surrounded by a Si
3
N
4
shell. Two geometries have been defined:
a low-index-contrast (LIC) variant, which allows for low-loss
coupling of light to or from a glass fiber, and a high-index-
contrast (HIC) variant, which allows for compact device layouts
because of a small bending radius. The LIC geometry optimized
for telecom applications at 1.55 μm wavelength consists of a
TAB LE I
S
CHEMATIC LAYOUT OF TRIPLEXGEOMETRIES AND SEM IMAGES OF
REALIZED STRUCTURES THAT ALLOW FOR LOW OPTICAL PROPAGATION LOSS:
B
OX SHELL (A), SINGLE-STRIPE (B), SYMMETRIC DOUBLE-STRIPE (C), AND
ASYMMETRIC DOUBLE-STRIPE (D) [4]
Depending on the layer thicknesses LIC and HIC versions of A-to-D exist.
The deposition techniques are discussed in Section II-E.
1 × 1 μm
2
SiO
2
core surrounded by a 50 nm thick Si
3
N
4
shell [5], while the HIC geometry has a 0.5 × 0.5 μm
2
core sur-
rounded by a 170 nm shell [6]. A noteworthy property of the box
shell geometry is that optical birefringence caused by material
anisotropy can be compensated for by geometrical anisotropy
[7]. The bending radius and mode-field diameters (MFDs) are:
R = 500 μm, MFD
x×y
= 3.6 × 3.6 μm for the LIC geometry
and R = 150 μm, MFD
x×y
= 1.4 × 1.4 μm for the HIC geom-
etry. If a mode-field diameter is specified in this article, it refers
to the diameter where the optical power has dropped to 1/e² of
the maximum intensity.
B. Single-Stripe
The single-stripe geometry consists of a single Si
3
N
4
stripe
and is the most interesting geometry for achieving ultra-low-loss
propagation on-chip. This can be realized by choosing the stripe
thickness in t he range of several tens of nm, and by optimizing
the stripe width [8]. However, the low propagation loss is at the
cost of a large bending radius in the order of thousands of μm.
Propagation losses as low as 0.03 dB/cm have been achieved at
1.55 μm wavelength, using a bending radius of R = 2000 μm,
see also Section V-D. A further reduction of these propagation
losses has been achieved by replacing the PECVD SiO
2
layer in
the top cladding by a bonded thermally grown SiO
2
film [9]. A
single-mode single-stripe of 50 nm thickness and 5.3 μm width

ROELOFFZEN et al.: LOW-LOSS SI
3
N
4
TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321
Fig. 1. Schematic cross-section of the standard TriPleX SDS waveguide for
the TE
00
mode at 1.55 μm wavelength: on the left-hand side is the HIC
waveguide with stripe thicknesses t
g 1
= t
g 2
= 170 nm; on the right-hand side
is the LIC waveguide with stripe thicknesses t
g 1
= t
g 2
= 35 nm; same for both
HIC and LIC: intermediate layer thickness t
int
= 0.5 μm, a waveguide width
of w = 1.2 μm, and an etching angle of α
g
= 82°. Typical layer thicknesses for
the SiO
2
bottom cladding t
b
and for the SiO
2
top cladding t
c
are 8 or 15 μm.
Color legend:
= Si
3
N
4
, = SiO
2
, = Si substrate, = background
material (e.g., air).
processed in this way has shown a loss of 0.007 dB/cm [10],
while the best result of 0.001 dB/cm obtained so far was on
single-stripe geometries at 1.58 μm wavelength: one of 40 nm
thickness and 13 μm width, and another of 50 nm thickness and
6.5 μm width [9].
C. Symmetric Double-Stripe or SDS
The symmetric double-stripe (SDS) geometry consists of two
stripes of Si
3
N
4
of the same thickness at the top on top of each
other, separated by an intermediate SiO
2
layer. The optimized
geometry that is also offered in the current TriPleX IR MPW
runs and has been used in many optical beam-forming networks
(OBFN) devices [11], that is discussed more elaborately in Sec-
tion VI-C. A schematic cross-section of the HIC SDS can be
found in Fig. 1. The propagation loss of the HIC SDS wave-
guide is below 0.1 dB/cm, and MFD
x×y
= 1.6 × 1.7 μm. The
effective index and group index for the TE
00
mode at 1.55 μm
wavelength are 1.535 and 1.72, resp. A minimum bending radius
of 100 μm is feasible, based on the criterion that bend losses
should be below 0.01 dB/cm [14].
By vertically tapering the thickness of both Si
3
N
4
layers
from 170 nm down to 35 nm near the end-facets, while the
intermediate SiO
2
layer thickness is kept constant at 500 nm,
allows for a spot-size converter (SSC) for low-loss fiber-to-chip
coupling to standard telecom glass fibers with a MFD of 10 μm:
fiber-to-chip coupling losses from SMF28 single-mode fiber to
the LIC SDS waveguide below 0.5 dB/facet have been achieved.
D. Asymmetric Double-Stripe or ADS
The asymmetric double-stripe (ADS) has a similar waveguide
geometry to the SDS: the main difference is that the thickness of
the upper Si
3
N
4
stripe is different than the lower Si
3
N
4
stripe,
Fig. 2. Schematic cross-section of the standard TriPleX ADS waveguide for
the TE
00
mode at 1.55 μm wavelength: on the left-hand side is the HIC
waveguide, with bottom stripe thickness t
g 1
= 75 nm, intermediate layer thick-
ness t
int
= 100 nm, top stripe thickness t
g 2
= 175 nm, a waveguide width of
w = 1.1 μm, and an etching angle of α
g
= 82°; on the right-hand side is the
LIC waveguide, made by locally removing the top Si
3
N
4
layer after deposition
of the ADS layer stack. Typical layer thicknesses for the SiO
2
bottom cladding
t
b
and for the SiO
2
top cladding t
c
are 8 or 15 μm. Color legend: = Si
3
N
4
,
= SiO
2
, = Si substrate, = background material (e.g., air).
see Fig. 2 for a schematic cross-section, while also the thickness
of the intermediate SiO
2
layer is significantly smaller: instead
of 500 nm as for the SDS geometry, a thickness of 100 nm
is used for the ADS geometry. The lower Si
3
N
4
layer of the
ADS geometry allows for optimization of coupling to exter-
nal components such as fibers and/or active materials without
adjusting its thickness, while the upper Si
3
N
4
stripe thickness
can be optimized for a target bending radius and is removed
completely (tapered down to zero thickness) elsewhere in the
circuit. The thickness for the upper Si
3
N
4
stripe of 175 nm has
been chosen to give the HIC ADS geometry the same effective
index of 1.535, the same minimum bending radius of 100 μm
as the HIC SDS geometry. Please note that the group index is
not the same as for the SDS waveguide: the ADS waveguide
has a slightly larger group index of 1.77. The mode-field size
is MFD
x×y
= 1.5 × 1.2 μm which is also different than for
the SDS waveguide (especially in vertical direction the ADS
mode field is more compact). Compared to the SDS geometry,
the ADS geometry requires less fabrication steps, going with
higher yield and even better optical characteristics. Propagation
loss values on ADS waveguides are reproducibly measured to
show similar or better values compared to SDS waveguides, so
<0.1 dB/cm The ADS propagation losses are expected to be
reduced further to values close to 0.01 dB/cm.
The complete removal of the thick top layer of Si
3
N
4
allows
for a “natural” transition from the LIC ADS to the HIC ADS
waveguides, which -in turn- makes i t possible to combine the
potentially ultra-low-loss properties of the single-stripe geome-
try with the small bending radii of the HIC ADS geometry, by
placing a spot-size converter in between (for more discussion
on this topic, see Section III-C).
Table II gives an overview of the most important properties of
the different TriPleX geometries discussed in this Section. The

4400321 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 4, JULY/AUGUST 2018
TAB LE II
O
VERVIEW OF THE MOST IMPORTANT PROPERTIES OF THE DIFFERENT TRIPLEXWAVEGUIDE GEOMETRIES
WG type Box shell Single-stripe Symmetric double-stripe Asymmetric double-stripe
Stack HIC BOX LIC BOX t
Si3N 4
=65 HIC SDS LIC SDS HIC ADS LIC ADS
(t
Si3N 4
/t
SiO2
/t
Si3N 4
) 170/500/170 50/1000/50 170/500/170 35/500/35 70/100/175 75/100/0
λ [nm] 1550 1550 1550 1550 1550 1550 1550
mode TE
00
TE
00
TE
00
TE
00
TE
00
TE
00
TE
00
w [μm] 0.84 1.1 4.2 1.2 1.0 1.1 0.8
w
SMB
[μm] 0.8 2.8 4.2 1.5 4.5 1.4 3.7
MFD
x
[μm] 1.4 3.6 4.7 1.6 10 1.5 10
MFD
y
[μm] 1.4 3.6 2.9 1.7 10 1.2 10
R
min
[μm] 150 500 2000 100 100
N
eff
1.555 1.455 1.459 1.535 1.447 1.535 1.446
N
g
1.76 1.49 1.50 1.72 1.46 1.77 1.46
α
prop
[dB/cm] 0.03 0.1 0.1
design wavelength λ and mode are given, the waveguide width
w. The parameter w
smb
is the waveguide width at the single-
mode boundary, MFD
x,y
are mode-field diameters where the
optical power has dropped to 1/e² of the maximum intensity,
R
min
is the minimum bend radius, where the bend losses are
below 0.01 dB/cm, N
eff
is the effective index, N
g
is the group
index, and α
prop
is the waveguide propagation loss.
E. Manufacturing Process of the Basic TriPleX Geometries
The general process flow is illustrated schematically in Ta-
ble III for the production of the different basic TriPleX ge-
ometries. The left-hand column shows the production process
for the box shell, the right-hand column is related to the other
geometries, the double-stripe SDS and ADS. The scheme for
single-stripe is nearly the same as for the double-stripe geome-
tries, except that step 4 and 5 can be skipped then.
The applied deposition techniques are the result of many
years of production optimization and characterization. The cur-
rent scheme contains as many batch processes as possible, using
equipment that is present in most CMOS foundries: this makes
production of TriPleX waveguides suitable for mass produc-
tion. At the same time the optical properties also have been
optimized. The highest quality SiO
2
layer results from wet ther-
mal oxidation of single-crystal silicon substrates to create an
SiO
2
layer (Table III, steps 1, 2), typically at temperatures equal
to or above 1000 °C. As discussed earlier in Section II-B, the
lowest reported propagation losses have been achieved using a
bonded thermally grown SiO
2
layer as top cladding.
Low-pressure chemical vapor deposition (LPCVD) is used
for the Si
3
N
4
layers and, where thermal oxidation is not ap-
plicable, also for the SiO
2
layer. For the latter process, the gas
tetraethylorthosilicate ( TEOS) is used. LPCVD deposition re-
sults in high quality layers, shows very good layer uniformity,
both in thickness and in refractive index, good step coverage
because of conformal growth, and the layer is deposited on both
sides of the substrate at the same time (Table III, step 3, 4, 5, 7,
and 9) [15], [16].
Drawback of this technology is the limited layer thickness:
for Si
3
N
4
, the critical layer thickness is in the order of 300 nm,
for SiO
2
the critical layer thickness is about 1500 nm. Above the
critical layer thickness, cracks will occur, caused by the inter-
nal stress due to differences in coefficient of thermal expansion
(CTE) between the deposited layer and the silicon substrate.
However, the stress of Si
3
N
4
films on Si is tensile, while SiO
2
films on Si are compressive: alternating stacks of oxide and
nitride therefore show reduced macroscopic stress values, and
consequently allow for more nitride incorporated in the wave-
guide. This forms the basis for the patented TriPleX technology.
As typical top cladding thicknesses of at least 8 μm cannot
easily be achieved if only LPCVD TEOS SiO
2
is being de-
posited, a part of the top cladding is deposited using another
deposition technique: plasma-enhanced chemical vapor depo-
sition (PECVD). Typical deposition temperatures are between
300 °C and 400 °C. This growth technique is directional, and as
the substrate has to lie down on a bottom electrode (for the gen-
eration of the plasma), this type of film is deposited on one side
of the substrate. The stress in PECVD SiO
2
layers (Table III,
step 10) is much less than in LPCVD layers, allowing for much
thicker layers.
Besides the optical quality and the surface roughness of the
used deposition techniques, another factor that is important for
low-loss propagation of light is the sidewall roughness or line
edge roughness (LER), which is determined by the details of
the photolithographic process and the etching technique that is
used for the waveguide definition (Table III, steps 6, 8) [4].
III. B
ASIC BUILDING BLOCKS (BASIC STRUCTURES &
F
UNCTIONALITIES)
TriPleX waveguide technology in principle is passive, as it
is not (yet) possible to monolithically integrate light sources,
electro-optical modulators, or detectors with competing spec-
ifications. However, to be able to tune optical properties of
integrated optical (IO) devices, thermo-optical tuning can be
achieved by means of resistive metal-based heaters on the top
cladding [17], [Section III-E], stress-optical tuning is possible
by integration of PZT piezoelectric devices on top of the top
cladding [18], [Section III-F], and electro-optical tuning can be
realized by locally defining sensing windows through the top
cladding until near the guiding layer that are filled with a liquid

ROELOFFZEN et al.: LOW-LOSS SI
3
N
4
TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321
TABLE III
G
ENERIC PROCESS FLOW FOR THE FABRICATION OF THE BASIC TRIPLEXGEOMETRIES [4]
crystal, and by generation of an electric field across the liquid
crystal [19], [20].
In this section, an overview is given of the basic building
blocks that are also available through MPW runs [11], [12],
[13].
A. Routing Building Blocks
The most elementary basic building blocks are routing build-
ing blocks, that are based on regular straight waveguide build-
ing blocks (either with constant width or with varying width
for lateral tapers), and on polar bend building blocks (based
on bends with constant radius of curvature). Based on these
building blocks, more advanced routing building blocks can be
defined, such as S-shaped bends. Furthermore, transition losses
between different routing elements due to a lateral offset in the
position of maximum light intensity (caused by a difference in
radius of curvature) can be solved geometrically in two ways:
either by introducing a lateral offset at the junction between
the elements to avoid discontinuity in the position of maximum
light intensity, or by designing a different type of bend shape
that adiabatically changes the radius of curvature such as e.g., a
(co)sine-shaped bend.
B. Spot-Size Converters
SSCs modify the MFD of a waveguide either by using a
vertical taper, as discussed in Section II-C, and/or by changing
the waveguide width using a lateral taper. The vertical taper can
be designed to locally and adiabatically change the thickness
of the guiding layer(s) such that discontinuities in MFD are
avoided, to minimize the excess loss of the SSC, at the cost of
a length of typically several hundreds of μm. The SSC can be
used to minimize coupling loss to e.g., a glass fiber, where the
end-facet waveguide typically has a 90° angle, as the effective
index of TriPleX waveguides are quite close to that of standard
glass fibers for telecom). Another application is coupling to a
waveguide from another integrated optical chip such as light
sources manufactured in InP technology. Because of the much
larger effective index of waveguides in other technologies such
as InP, SOI, the SSC coupling loss can be decreased by placing
it under a suitable angle different than 90°, more can be found
in Section VI-A.
C. Splitters/Combiners and Couplers
To split light in a waveguide into two or more branches (or
to combine light from two waveguides into one branch), a Y-
junction is an interesting option, as the splitting (or combining)
ratio is theoretically 50% and wavelength- and polarization-
independent. However, to make the Y-junction splitting ratio
robust against technological variations, a certain minimum gap
between the two branches has to be taken into account. The extra
robustness is at the penalty of additional excess loss (sub-dB
level), which is also wavelength- and polarization-dependent.

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  • ...A wide variety of building blocks have been realized in the Si3N4 platform including bends, crossings, gain blocks, and directional couplers [52], [59]....

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "Low-loss si3n4 triplex optical waveguides: technology and applications overview" ?

An overview of the most recent developments and improvements to the low-loss TriPleX Si3 N4 waveguide technology is presented in this paper. The TriPleX platform provides a suite of waveguide geometries ( box, double stripe, symmetric single stripe, and asymmetric double stripe ) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit. Color versions of one or more of the figures in this paper are available online at http: //ieeexplore. Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform. This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks.