Elsevier

Optics & Laser Technology

Volume 112, 15 April 2019, Pages 299-306
Optics & Laser Technology

Full length article
[INVITED] Silicon nitride photonic integration for visible light applications

https://doi.org/10.1016/j.optlastec.2018.10.059Get rights and content

Highlights

  • SiN PICs are an promising platform to miniaturize optical biosensing systems.

  • Four foundries are offering open-access generic manufacturing.

  • Different methods for sensing where SiN PICs are used are presented.

  • PIX4life is presented as current driver of development of SiN PICs in the visible.

  • Systems on several applications mentioned: OCT, cytometry, sensors, light sources.

Abstract

In this article, we provide a review of fabrication platforms and state of the art developments in the area of silicon nitride photonic integrated circuits (PICs) for the visible wavelength range, mostly employed in bio-medical and chemical sensing applications. Additionally, we introduce PIX4life, the European pilot line for silicon nitride PICs for visible applications. PIX4life provides a unified framework for the development of such PICs, including design and software, fabrication, characterization and packaging.

Introduction

The integration of a complex optical system into a monolithic substrate brings many advantages. With the reduction of size and volume, the stability achieved by electronically controlling many degrees of freedom, and the possibilities to scale up manufacturing in high-yield lithography processes, many optical systems that were previously secluded to research laboratories, can now be ported to industrial and commercial applications.

Although electronic integrated circuits (ICs) have become mature and extremely inexpensive over the last decades, the development of photonic integrated circuits (PICs) has been restricted to research labs and are, only now, maturing to the point of commercial introduction. Many of the same processes and methods used in IC fabrication can be leveraged in creating PICs in a CMOS fabrication line [1]. However, the main progress of silicon (silicon-on-insulator, SOI) PIC integration during the last decade has been primarily driven by the requirements of optical data- and telecommunications, focusing on the telecommunication wavelengths in the 1.55 μm and 1.31 μm regions [2].

Alternatively, silicon nitride (Si3N4) in its more thermo-dynamically stable form, SiN for simplicity, has raised a lot of interest in the last few years as a material for PICs [3]. It is an amorphous CMOS-compatible material with a lower refractive index compared to Silicon, making it more tolerant to fabrication imperfections and to waveguide sidewall roughness, directly resulting in lower propagation losses. Finally, it can allow for monolithic co-integration with silicon photo-diodes and CMOS based electronic circuitry [4].

For (bio-) chemical and medical sensing applications, the key properties of silicon nitride are its chemical, mechanical and thermal stability, as well as it transparency range which covers from the visible (around 400 nm) to the mid-infrared [5]. This last feature is extremely useful since it allows absorption, as well as fluorescent-based measurement techniques, which will be unaffected by the material response of both SiN and its typical Silicon Dioxide (SiO2) for visible and infrared light [6], [7].

These characteristic properties have enabled SiN PICs to be employed for high-accuracy and high-sensitivity sensor devices and instruments. Many applications like super-resolution microscopy, light sources, labelled and label-free bio-sensors, cytometry, OCT, etc. are benefiting from SiN PICs already [8], [9], [10], [11], [12], [13].

This paper is structured as follows. Section 2 describes the main properties of SiN and the manufacturing process of SiN photonic circuits, detailing the four main foundries that nowadays offer generic processes and open-access to their fabrication runs. Section 3 describes the different techniques for the measurement of the refractive index (RI), in order to use SiN photonic structures as RI-based sensors. Section 4 reviews the state of the art in SiN biophotonic sensing applications. Finally, Section 5 describes the EU PIX4life pilot line and its application demonstrators, together with a summary in Section 6.

Section snippets

Silicon nitride platforms for manufacturing visible PICs

Silicon nitride is a high-melting-point solid (>1000 °C) that is relatively chemically inert, a good property for biochemical sensing [6]. The thermal conductivity of SiN is 30 W/mK and its Coefficient of Thermal Expansion (CTE) is 3.3 ×106/°C. Its refractive index for visible wavelengths ranges from 2.0688 at 480 nm to 2.0209 at 852 nm [5], [14], enabling more compact devices compared to SiO2. Moreover, it has no two-photon absorption above 600 nm.

Within the wafer fabrication of SiN layers, a

Assay sensing through refractive index changes

Light propagation is affected by the guiding medium and by any change that occurs to it. Several different physical processes may contribute to these changes, such as heat e.g. from an exothermic reaction, pressure, scattering, absorption (maybe with fluorescence emission), or other means that change the permittivity of the material stack, i.e., its refractive index.

In the case of direct measurements of a biological or chemical sample, the top cladding of SiN waveguides should be replaced by a

Bio-medical applications of visible light

In order to realize PIC-based sensors for biomedical applications, in most cases the evanescent tale of the light is required to be guided through the analyte. In other systems, the light is emitted or collected through photonic integrated circuits so that the phase and amplitude of the light and the delay introduced during its propagation can be measured. One example of this is the technique known as Optical Coherence Tomography (OCT) where broadband coherent light can be emitted and collected

PIX4life: the European Pilot line for visible PICs

The H2020 funded European pilot line PIX4life [63] is composed of a consortium having both companies and research institutions, and started operations in 2016 by providing an end-to-end SiN PIC development framework and supply chain that spans from design to packaged chip components, by aligning and extending existing European know-how in design tools, source and detector integration, and micro-fluidic assembly for visible biophotonics.

At the core of the pilot line lies the development of two

Summary

This review paper has described the silicon nitride material platform for building photonic waveguides and circuits, with a clear focus on visible operation wavelengths and biophotonic applications. Several foundries offering generic and open-access MPW runs have been reviewed, describing their capabilities and processes. The sensing capabilities of SiN have been explained, by miniaturization and extreme price and power with an emphasis on the measurement of the refractive index changes. A

Acknowledgements

This work was supported by the EU Horizon 2020 research and innovations programme, funded by the European Commission under the grant agreement No. 688516 (PIX4life).

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