Silicon Nitride in Silicon Photonics By DANIEL J. BLUMENTHAL , Fellow IEEE,RENÉ HEIDEMAN,DOUWE GEUZEBROEK,ARNE LEINSE, AND CHRIS ROELOFFZEN | ABSTRACT The silicon nitride (Si3N4) planar waveguide plat- history of low-loss Si3N4 waveguide technology and a survey of form has enabled a broad class of low-loss planar-integrated worldwide research in a variety of device and applications as devices and chip-scale solutions that benefit from trans- well as the status of Si3N4 foundries. parency over a wide wavelength range (400–2350 nm) and KEYWORDS | Biophotonics; lasers; microwave photonics; fabrication using wafer-scale processes. As a complimentary optical device fabrication; optical fiber devices; optical fil- platform to silicon-on-insulator (SOI) and III–V photonics, Si N 3 4 ters; optical resonators; optical sensors; optical waveguides; waveguide technology opens up a new generation of system- photonic-integrated circuits; quantum entanglement; silicon on-chip applications not achievable with the other platforms nitride; silicon photonics; spectroscopy alone. The availability of low-loss waveguides (<1dB/m)that can handle high optical power can be engineered for linear and nonlinear optical functions, and that support a variety of pas- I. INTRODUCTION sive and active building blocks opens new avenues for system- Photonic-integrated circuits (PICs) are poised to enable an on-chip implementations. As signal bandwidth and data rates increasing number of applications including data commu- continue to increase, the optical circuit functions and com- nications and telecommunications [1], [2], biosensing [3], plexity made possible with Si3N4 has expanded the practical positioning and navigation [4], low noise microwave application of optical signal processing functions that can synthesizers [5], spectroscopy [6], radio-frequency (RF) reduce energy consumption, size and cost over today’s digital signal processing [7], quantum communication [8], and electronic solutions. Researchers have been able to push the atomic clocks [9]. Emerging system-on-chip applications performance photonic-integrated components beyond other are driving the demand for PICs that operate over an integrated platforms, including ultrahigh Q resonators, optical unprecedented optical bandwidth range, from the visi- filters, highly coherent lasers, optical signal processing cir- ble wavelength (∼400 nm) out to beyond the infrared cuits, nonlinear optical devices, frequency comb generators, (>2.3 μm), and deliver performance previously only and biophotonic system-on-chip. This review paper covers the achievable with bulk optic technologies. In this paper, we review the history and state of the art in silicon nitride (Si3N4) photonics, a third inte- Manuscript received February 19, 2018; revised July 10, 2018; accepted July 11, 2018. This work was supported in part by Defense Advanced Research Project gration platform that is complimentary in characteris- Agency (DARPA) Microsystems Technology Office (MTO) Space and Naval tics and performance to the silicon-on-insulator (SOI) Warfare Systems Center Pacific (SSC Pacific) (DoD-N W911NF-04-9-0001 iPhoD HR0011-09-C-01233, EPHI HR0011-12-C-0006, iWOG HR0011-14-C-0111, photonics and group III–V photonics platforms and is PRIGM:AIMS N66001- 16-C-4017) and Keysight Technologies. The views and compatible with foundry-scale processes. Si3N4 PIC tech- conclusions contained in this document are those of the authors and should not be interpreted as representing official policies of the Defense Advanced nology offers low optical attenuation, from the visible to Research Projects Agency or the U.S. Government or KeySight Technologies. beyond the infrared, a range not accessible with the other (Corresponding author: Daniel J. Blumenthal.) platforms. Today, SOI offers large volume photonic inte- D. J. Blumenthal is with the Department of Electrical and Computer Engineering, University of California–Santa Barbara, Santa Barbara, CA 93106 gration through traditional complementary metal–oxide– USA (e-mail: [email protected]). semiconductor (CMOS) infrastructure. Traditional SOI R. Heideman, D. Geuzebroek, A. Leinse,andC. Roeloffzen are with LioniX International BV, 7521 AN Enschede, The Netherlands (e-mail: photonics employs high-contrast waveguides formed from [email protected]; [email protected]; a silicon core surrounded by oxide cladding, keeping the [email protected]; [email protected]). light tightly confined to the core [1]. These strongly con- Digital Object Identifier 10.1109/JPROC.2018.2861576 fining waveguides lead to very compact photonic circuits 0018-9219 c 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. PROCEEDINGS OF THE IEEE 1 Blumenthal et al.: Silicon Nitride in Silicon Photonics Fig. 1. Bend radii, propagation loss, and window of transparency for published Si3N4,SOI,andInPwaveguides. with small bend radii and moderate waveguide losses on an invited article by Munoz et al. [16] and several arti- the order of 0.1 dB/cm. The widely used group III–V cles in the recent IEEE JOURNAL ON SELECTED AREAS photonics material indium phosphide (InP) is a foundry IN QUANTUM ELECTRONICS (IEEE JSTQE) Special Issue scale process that provides waveguides with optical gain on Ultralow Loss Planar Waveguides and Their Appli- and efficient signal modulation in the telecommunications cations [17]. Early motivation to investigate Si3N4 pho- wavebands [10]. InP is used in many standalone PIC appli- tonic circuits was a platform that resided on a silicon cations and as a gain block for SOI PICs with a tradeoff in substrate, utilized compatible silicon processing technolo- higher waveguide losses (2 to 0.4 dB/cm [11]) and larger gies, and addressed applications at wavelengths where bend radii, as compared to SOI. silicon is absorbing [toward the short end of infrared (IR) Si3N4 PIC technology provides lower loss waveguides into the visible]. Today, a wealth of applications have and building blocks complimentary to SOI and III–V PICs. emerged that leverage this third wafer-scale platform to Si3N4 is traditionally used in standard CMOS processes complement the capabilities of SOI and III–V waveguide to insulate individual transistors, known as local oxi- technologies. dation of silicon (LOCOS), and is also used as gate material in ion-sensitive field-effect transistors (ISFETs) A. The Early Days purposes [12]. Optical waveguides employ a core layer of Early work focused on fabrication of thin film slab Si N embedded in a surrounding silicon dioxide (SiO ) 3 4 2 waveguides that guided 632-nm light [18], [19] and cladding material. The refractive index of the cladding employed Si N cores with a silicon dioxide lower cladding at 1.55-μm wavelength (1.98 for SiO )andcore(1.45 3 4 2 layer formed on a silicon substrate. These structures for Si N ) allows for designs that range from low- to 3 4 were formed by thermally oxidizing silicon to form a high-contrast waveguides with low propagation losses in lower waveguide cladding layer on top of the silicon the range of 0.3 dB/m to 1.0 dB/cm over the range substrate, followed by subsequent deposition of Si N from ∼400 to – 2350 nm. In general, the waveguide loss 3 4 as the waveguide core layer. As is the case today, the and minimum bend radius are design tradeoffs based on thermally grown lower cladding layer thickness must be desired performance, footprint and optical power density, sufficient (in the several micrometers to 15-μmrange)to and vary for each integration platform. Fig. 1 summarizes minimize absorption in the silicon substrate, and is depen- published data on waveguide propagation loss, minimum dent on the optical waveguide confinement. Thermally bend radius, and wavelength operating range for Si N , 3 4 oxidized silica remains one of the lowest loss oxide-based SOI, and InP waveguides [13]– [15]. cladding materials due to a low surface roughness from the original high-quality silicon substrate and the absence II. HISTORY OF Si N INTEGRATED 3 4 hydrogen in the material and growth process. The higher PHOTONICS index waveguide core is formed using deposition tech- Si3N4 waveguides have been of interest since the late niques like chemical vapor deposition (CVD) followed by 1970s with several historical summaries written, including patterning and etch steps to form channel waveguides. 2 PROCEEDINGS OF THE IEEE Blumenthal et al.: Silicon Nitride in Silicon Photonics Early devices employed an air upper cladding design, with hybrid-integrated InP, SOI, and silica waveguide chips. later advancements in upper cladding oxide deposition Optical packet buffers were designed to store packets as used to reduce losses dominated by surface roughness and is done in electronic routers. The buffers used InP 2 × 2 lithography induced waveguide scattering and material optical switches to direct packets in and out of silica on optical absorption. silicon waveguide delays [30], but the delay line and InP One of the first reported silicon nitride/silicon diox- to silica waveguide coupling losses limited storage to less ide (Si3N4/SiO2) waveguides was a single-mode channel than ten packet circulations. In the 2009 period, DARPA waveguide [20] with 1–2-dB/cm propagation loss. These established the iPHOD program to address on-chip losses early stage losses compared