A low loss silicon nitride nanophotonic waveguide platform by reactive sputtering
Creators: name: FRIGG, Andreas
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)
classificationCode: 401806
subject: Nanomanufacturing

description: Photonic integrated circuits (PICs) underpin energy efficient, small form factor devices for telecom, datacom, sensing, LIDAR or quantum computing applications while leveraging existing complementary metal oxide (CMOS) fabrication foundries where thousands of PICs can be fabricated on 200 to 300 mm wafers. Silicon photonics was the first PICs platform based on Silicon-on-Insulator (SOI) wafers, which use a silicon waveguide core for electro/optical integration. Nowadays the field of research in PICs has diversified to include other material stacks that can be processed on wafer-scale in a CMOS-environment. Most noteworthy is the emergence of Silicon Nitride (SiN), which offers lower propagation losses and a broader transparency window for applications spanning visible to mid-infrared wavelengths. The highest quality and lowest loss SiN waveguides are deposited by low pressure chemical vapour deposition (LPCVD) and high-temperature annealing (1200°C). However, the high processing temperatures and the resulting high tensile stress limits the co-integration with sensitive substrates and CMOS structures (e.g. metals) as the high temperatures may cause diffusion and crack formations. These limitations restrict LPCVD as front-end-of-line process (FEOL).
 
However, for many applications it is attractive to add a SiN layer, with a low temperature deposition method to previously processed substrates as it opens opportunities for back-end-of-line (BEOL) integration of optical waveguides with temperature sensitive platforms such as processed CMOS silicon electronics or lithium niobate on insulator (LNOI). So far, the most common low-temperature deposition technique of SiN is plasma enhanced chemical vapour deposition (PECVD), however such SiN thin-films suffer from significant losses (~2 dB/cm) at typical communication wavelengths (C-band) due to unwanted hydrogen bonds. These losses prevent the use of PECVD SiN for applications such as low loss wavelength division multiplexing (WDM) filters in the C-band and high-Q SiN microring resonators for optical frequency comb generation.
 
The high optical waveguide losses of PECVD SiN are the main motivation to explore and investigate a low temperature, low loss SiN waveguide platform based on reactive sputtering. Reactive sputtering is a promising BEOL deposition method for waveguide core layers and might offer lower propagation losses compared to PECVD as the formation of hydrogen bonds is less likely during the deposition process. Furthermore, reactive sputtering might allow the deposition of thick (&gt; 0.7 µm) SiN layers with low loss and low stress, which can be used to create anomalous dispersive waveguides required for frequency comb generation. 

Hence, the aim of this thesis is to explore if reactive sputtering can be used as a deposition method to fabricate low loss, low stress SiN waveguides which are fully CMOS-compatible for BEOL applications. The main contributions in this thesis are:
 
1) Investigation and development of reactive sputtered SiN thin-films with low optical losses, low stress, room temperature deposition and investigation of post-annealing at modest temperature of 200°C and 400°C. Thin-film losses of 0.25 dB/cm were achieved at a modest stress level of -25 MPa. 
2) Fabrication and characterization of medium confinement waveguides using 400 nm thick reactive sputtered thin films. Demonstration of waveguide propagation losses of 1.1 dB/cm (Qint=3.0 × 105) for single mode dimensions and losses of 0.6 dB/cm (Qint=5.81 × 105) for multimode waveguides for a wavelength of 1580 nm. 
3) Simulation, fabrication and characterization of anomalous dispersive, high confine-ment waveguides using the 850 nm thick reactive sputtered thin films. Demonstration of microring resonator with losses as low as 0.54 dB/cm (Qint=6.6 × 105) at 1580 nm, which enabled the generation of a natively mode-spaced (NMS) frequency comb with an estimated on-chip pump power of 850 mW.
4) Deposition of high-quality SiN films on the temperature sensitive LNOI platform to form optically loaded rib waveguide as an alternative fabrication method for optical waveguide compared to direct etching of lithium niobate thin film. This approach offers the advantages of reasonably high index contrast for compact waveguides, low propagation loss and wide transparency window, while also harnessing the mature CMOS-compatible fabrication techniques of SiN.

These demonstrations show that sputtered SiN thin-films are an attractive method to fabricate low loss, low stress SiN waveguides which are fully CMOS-compatible for BEOL applications. This makes this deposition attractive for (1) Complex multi-level photonic integration on pre-processed CMOS wafers or chips with active, temperature sensitive optoelectronics devices such as photodiodes, p-n junction silicon modulators or heaters [1, 2]. Possible applications are optical coherence tomography (OCT) [3], CMOS imagers [4] and telecommunication (2) Nonlinear applications such as frequency comb generation for spectroscopy, metrology, sensing and wavelength multiplexed transceivers in telecommunication [5] (3) straightforward monolithical integration of SiN waveguides with temperature sensitive substrates such as thin-film lithium niobate on silicon (LNOI) and AlGaAs-on-insulator as an optical loading material or routing layer [6, 7].