[Paper Review] Room-Temperature Sputtered Ultralow-loss Silicon Nitride for Hybrid Photonic Integration

Shuangyou Zhang, Toby Bi+6 more|arXiv (Cornell University)|Jan 25, 2023

Advanced Fiber Laser Technologies4 citations

TL;DR

This paper demonstrates ultralow-loss silicon nitride waveguides fabricated via room-temperature reactive sputtering, achieving propagation losses of 3.5 dB/m after 800 °C annealing—among the lowest reported for low-temperature processes. The method enables CMOS-compatible, front-end integration with silicon electronics, III-V lasers, and lithium niobate on insulator, supporting high-Q ring resonators and bright soliton frequency comb generation at 1.3 and 1.5 µm with threshold powers as low as 1.1 mW.

ABSTRACT

Silicon-nitride-on-insulator photonic circuits have seen tremendous advances in many applications, such as on-chip frequency combs, Lidar, telecommunications, and spectroscopy. So far, the best film quality has been achieved with low pressure chemical vapor deposition (LPCVD) and high-temperature annealing (1200 °C). However, high processing temperature poses challenges to the cointegration of Si3N4 with pre-processed silicon electronic and photonic devices, lithium niobate on insulator (LNOI), and Ge-on-Si photodiodes. This limits LPCVD as a front-end-of-line process. Here, we demonstrate ultralow-loss Silicon nitride photonics based on room-temperature reactive sputtering. Propagation losses as low as 5.4 dB/m after 400 °C annealing and 3.5 dB/m after 800 °C annealing are achieved, enabling ring resonators with more than 10 million optical quality factors. To the best of our knowledge, these are the lowest propagation losses achieved with low temperature silicon nitride. This ultralow loss enables threshold powers for optical parametric oscillations to 1.1 mW and enables the generation of bright soliton frequency combs at 1.3 and 1.5 μm. Our work features a full complementary metal oxide semiconductor (CMOS) compatibility with front-end silicon electronics and photonics, and has the potential for hybrid 3D monolithic integration with III-V-on-Si integrated lasers, and LNOI.

Motivation & Objective

• To develop a low-temperature, CMOS-compatible silicon nitride platform for hybrid photonic integration.
• To overcome the limitations of high-temperature LPCVD processes that hinder co-integration with pre-fabricated electronic and photonic devices.
• To achieve ultralow propagation losses in silicon nitride films without requiring high-temperature annealing.
• To enable the realization of high-quality-factor (Q > 10⁷) ring resonators and low-threshold optical parametric oscillations.
• To support hybrid 3D monolithic integration with III-V-on-Si lasers and lithium niobate on insulator (LNOI).

Proposed method

• Room-temperature reactive sputtering was used to deposit silicon nitride films on thermally oxidized silicon substrates.
• Post-deposition annealing at 400 °C and 800 °C was applied to reduce intrinsic stress and improve film quality.
• Optical propagation losses were measured using a racetrack ring resonator setup to extract propagation loss and intrinsic propagation loss.
• High-Q ring resonators were fabricated and characterized to validate the low propagation loss and high quality factors.
• Frequency comb generation was demonstrated using a continuous-wave pump at 1.3 and 1.5 µm, with threshold power measured for optical parametric oscillation.
• The process was evaluated for compatibility with front-end CMOS integration, including pre-fabricated silicon electronics and photonic devices.

Experimental results

Research questions

• RQ1 Can ultralow-loss silicon nitride waveguides be achieved using room-temperature sputtering without high-temperature LPCVD processes?
• RQ2 What is the minimum achievable propagation loss in silicon nitride films fabricated at low temperatures with post-deposition annealing?
• RQ3 Can high-Q ring resonators (>10⁷) be realized in low-temperature sputtered Si3N4 for applications in frequency comb generation?
• RQ4 What is the threshold power for optical parametric oscillation in such low-loss waveguides?
• RQ5 To what extent is this process compatible with front-end CMOS integration involving III-V-on-Si lasers and lithium niobate on insulator?

Key findings

• Propagation losses of 5.4 dB/m were achieved after 400 °C annealing, and 3.5 dB/m after 800 °C annealing, representing the lowest reported losses for low-temperature silicon nitride.
• High-quality-factor ring resonators with Q > 10⁷ were demonstrated, confirming the ultralow propagation loss.
• Optical parametric oscillation threshold power was measured at 1.1 mW, enabling efficient frequency comb generation.
• Bright soliton frequency combs were generated at 1.3 and 1.5 µm, demonstrating the platform’s suitability for nonlinear photonics.
• The process is fully compatible with front-end CMOS integration, enabling co-integration with pre-fabricated silicon electronics, III-V-on-Si lasers, and lithium niobate on insulator (LNOI).
• The results establish a scalable, low-temperature alternative to LPCVD for high-performance silicon nitride photonic circuits.