Process-structure-property relationships in subtractive fabrication of silicon nitride microresonators for nonlinear photonics

This paper investigates the process-structure-property (P-S-P) relationships in the subtractive fabrication of silicon nitride (SiN) microresonators for nonlinear photonics. SiN is commonly used for integrated photonics due to its desirable properties, such as a wide transparency window, high nonlinear refractive index, and CMOS compatibility. The focus of the study is on two subtractive fabrication approaches using different etch masks: a polymer-based electron beam resist mask and a thin metallic mask.

The authors explore how the fabrication processes influence optical properties like optical loss and resonator dispersion, essential for generating nonlinear optical phenomena like frequency combs. The key findings include:

1. Fabrication Process Comparison: The polymer mask method results in devices with smoother waveguide sidewalls, leading to lower optical loss and higher quality factors (Q). In contrast, the metal mask process, which produces more vertical sidewalls, offers better dimensional control, but introduces higher sidewall roughness, leading to higher optical loss and lower Q factors.

2. Lithography and Etching Influence: The paper discusses how electron beam lithography (EBL) parameters such as beam current and shot pitch affect the optical performance of the resonators. It was observed that lower beam currents result in higher intrinsic quality factors and lower optical loss, particularly in polymer mask devices. However, in metal mask devices, these parameters had minimal impact.

3. Sidewall Roughness and Scattering Loss: The study links sidewall roughness to optical scattering loss using the Payne-Lacey model. The results show that the roughness of the sidewalls, which can be influenced by the etch-mask material, significantly contributes to the propagation loss.

4. Frequency Comb Generation: Devices fabricated using both polymer and metal masks were used to generate modulation instability (MI) combs and soliton combs. The devices fabricated with the metal mask process produced broader combs and dual dispersive waves (DWs), highlighting the importance of precise control over waveguide dimensions for efficient comb generation.

5. Impact of Geometry on Dispersion: The study also examines how waveguide geometry (such as width, height, and sidewall angle) affects resonator dispersion. The position of dispersive waves (DWs), which are critical for comb generation, is shown to be sensitive to slight changes in the resonator's geometry.

In conclusion, the paper emphasizes the trade-offs between dimensional accuracy and optical loss when selecting fabrication processes for SiN microresonators used in nonlinear photonics. It also underscores the importance of precise control over sidewall roughness and resonator geometry to optimize the performance of devices designed for applications like frequency comb generation.