Fabrication and optical characterization of thin two-dimensional Si₃N₄ waveguides

Abstract

In view of the integration within Si-based optical devices, LPCVD (low-pressure chemical vapor deposition) thin-film Si₃N₄ waveguides have been fabricated on a Si substrate within a CMOS fabrication pilot-line. Different structures (channel, rib and strip-loaded) were designed, fabricated and characterized both optically and structurally to optimize waveguide performances. Geometry, sidewall as well as layer roughness of the waveguides have been investigated by scanning electron microscopy and atomic force microscopy. Optical guided modes have been observed and propagation loss measurements at 632.8 and 780 nm have been performed by using the cut-back technique, the insertion loss technique and scattered light collection. The channel waveguides have shown propagation losses of about 0.1–0.2 dB/cm. Differences between geometries and lithographic processes have been discussed. Polarization dependence of propagation losses has been investigated too.

Optical guided modes have also been measured in the near-infrared range (at 1544 nm), where propagation losses are about 4.5–5 dB/cm, quite larger with respect to the visible, because of the poorer confinement factor of the optical modes.

Introduction

In recent years some concerns related to fundamental materials and processing aspects about the evolution of microelectronics have been raised [1]. For example, as the integration is progressing the length of the interconnects on a single chip is getting longer and more complex. Nowadays chips have total interconnection length per unit area of the chip of some 5 km/cm^² with a chip area of 450 mm². This results in device limitations in terms of speed, packaging and power dissipation [2]. A possible solution could be the use of optical interconnects [3]. Till now, various passive optical components have been developed for WDM-based photonic networks [4]; however, most of these are not compatible with silicon technology. Hybrid technology makes the system fabrication difficult and costly because different materials have to be assembled in a unique device. Silicon microphotonics, which merges photonics and silicon microelectronic components, might reduce the cost of production and improve the integrability of a device [5], [6]. It is predicted that optical interconnects will be used to connect computer boards in few years, while the use of optical interconnects within the chip will possibly be realized in 10–15 years from now [7].

The first component ubiquitous in silicon microphotonics is the optical waveguide, which is used to channel light signal in the system. A natural choice is to look for some of the commonly used dielectrics in microelectronics. Thin waveguides with Si₃N₄ core surrounded by SiO₂ cladding layers represent one of the best solutions to have large refractive index difference (Δn≅0.55), low scattering losses, no absorption losses and compatibility with Si technology. Various low-loss optical waveguides, produced by different techniques, have been reported in the last 30 years [8], [9], [10], [11], [12], [13], [14], [15], and some applications in optical devices have been already demonstrated [16], [17], [18]. However, it was shown that Si₃N₄ waveguides present remarkable losses due to the vibrational overtones of Si–H and N–H bonds, thus preventing competitive waveguides for the third telecom window [10], [14].

In this paper, we report on the systematic fabrication and characterization of low-loss Si₃N₄/SiO₂ waveguides produced within a CMOS fabrication pilot-line by low-pressure chemical vapor deposition (LPCVD). The use of LPCVD reduces interface roughness and consequently the related scattering losses. We investigated the optical properties of thin-film Si₃N₄ waveguides deposited on Si substrate as a function of the waveguide geometry and fabrication technology by using visible and infrared light. Three kinds of waveguides were designed, fabricated and characterized: rib, channel and strip-loaded.