A new material platform of Si photonics for implementing architecture of dense wavelength division multiplexing on Si bulk wafer

Abstract

A new materials group to implement dense wavelength division multiplexing (DWDM) in Si photonics is proposed. A large thermo-optic (TO) coefficient of Si malfunctions multiplexer/demultiplexer (MUX/DEMUX) on a chip under thermal fluctuation, and thus DWDM implementation, has been one of the most challenging targets in Si photonics. The present study specifies an optical materials group for DWDM by a systematic survey of their TO coefficients and refractive indices. The group is classified as mid-index contrast optics (MiDex) materials, and non-stoichiometric silicon nitride (SiNx) is chosen to demonstrate its significant thermal stability. The TO coefficient of non-stoichiometric SiNx is precisely measured in the temperature range 24–76 °C using the SiNx rings prepared by two methods: chemical vapor deposition (CVD) and physical vapor deposition (PVD). The CVD-SiNx ring reveals nearly the same TO coefficient reported for stoichiometric CVD-Si3N4, while the value for the PVD-SiNx ring is slightly higher. Both SiNx rings lock their resonance frequencies within 100 GHz in this temperature range. Since CVD-SiNx needs a high temperature annealing to reduce N–H bond absorption, it is concluded that PVD-SiNx is suited as a MiDex material introduced in the CMOS back-end-of-line. Further stabilization is required, considering the crosstalk between two channels; a ‘silicone’ polymer is employed to compensate for the temperature fluctuation using its negative TO coefficient, called athermalization. This demonstrates that the resonance of these SiNx rings is locked within 50 GHz at the same temperature range in the wavelength range 1460–1620 nm (the so-called S, C, and L bands in optical fiber communication networks). A further survey on the MiDex materials strongly suggests that Al2O3, Ga2O3 Ta2O5, HfO2 and their alloys should provide even more stable platforms for DWDM implementation in MiDex photonics. It is discussed that the MiDex photonics will find various applications such as medical and environmental sensing and in-vehicle data-communication.

Analysis of Optical Integration between Si3N4 Waveguide and a Ge-Based Optical Modulator Using a Lateral Amorphous GeSi Taper at the Telecommunication Wavelength of 1.55 µm

“…Regarding the possibility of practical fabrication for 1 µm thick SiN, it was proposed by Ref. 23 that physical vapor deposition (PVD) had the potential to achieve a low-loss SiN material with maximum thickness beyond that normally obtainable with chemical vapor deposition (CVD) due to its lower built-in stress. Performance variation with respect to the change in taper tip width is discussed in the next section.…”

“…In these regards, Si 3 N 4 waveguide could be regarded as a promising alternative suitable for wavelength division multiplexing implementation in photonic integrated circuits on bulk Si substrate [19][20][21][22][23]. Due to the relatively-low thermo-optics coefficient of Si 3 N 4 and a moderate refractive index contrast between Si 3 N 4 and SiO 2 , Si 3 N 4 was proposed to be advantageous for wavelength multiplexing in terms of polarization independence, phase error due to fabrication imperfections, temperature stability, and wideband operation for telecommunications applications [23][24][25]. Additionally, a Si 3 N 4 refractive index~2 is sufficiently high to realize compact photonic circuits with SiO 2 cladding (n~1.45) [20,23].…”

“…Due to the relatively-low thermo-optics coefficient of Si 3 N 4 and a moderate refractive index contrast between Si 3 N 4 and SiO 2 , Si 3 N 4 was proposed to be advantageous for wavelength multiplexing in terms of polarization independence, phase error due to fabrication imperfections, temperature stability, and wideband operation for telecommunications applications [23][24][25]. Additionally, a Si 3 N 4 refractive index~2 is sufficiently high to realize compact photonic circuits with SiO 2 cladding (n~1.45) [20,23]. Moreover, silicon nitride-based waveguides can possibly be deposited directly onto bulk Si substrate at a relatively low temperature [23,26,27].…”

“…Additionally, a Si 3 N 4 refractive index~2 is sufficiently high to realize compact photonic circuits with SiO 2 cladding (n~1.45) [20,23]. Moreover, silicon nitride-based waveguides can possibly be deposited directly onto bulk Si substrate at a relatively low temperature [23,26,27]. Nevertheless, comparing to SOI waveguides, efficient optical coupling between Si 3 N 4 waveguide and Ge-on-Si photonic components could be considered significantly more challenging due to an even larger refractive index difference between Si 3 N 4 and Ge.…”

Recent Progress on Ge/SiGe Quantum Well Optical Modulators, Detectors, and Emitters for Optical Interconnects

“…For optical interconnect applications, a compatible optical modulator should be able to meet several requirements, including: (1) a sufficiently-high extinction ratio (ER) (e.g., >5 dB ER [27]); (2) a relatively-low insertion loss (IL) (e.g., <3 dB IL [27]); (3) a complementary metal-oxide semiconductor (CMOS) drivable voltage (e.g., ≤2 V [32]); (4) a compact footprint and low-energy dissipation (e.g., <100 fJ/bit [5]); (5) CMOS-compatible materials and high-volume manufacturing compatibility; (6) a large 3-dB bandwidth for high-speed modulation (e.g., 14.3 GHz for on-chip clock in 2022 [5]); and (7) a relatively-large optical bandwidth (e.g., 10-20 nm [33]) for possible wavelength division multiplexing operation and a good temperature stability [34,35]. Several Si-compatible optical modulators have been proposed and investigated in order to demonstrate an optical modulator that can be manufactured by, and monolithically-integrated with, Si-microelectronics [33].…”