[Paper Review] Tailoring Broadband Kerr Soliton Microcombs via Post-Fabrication Tuning of the Geometric Dispersion

Grégory Moille, Daron Westly+2 more|arXiv (Cornell University)|Sep 13, 2021

Advanced Fiber Laser Technologies39 references28 citations

TL;DR

This paper demonstrates a post-fabrication dry-etching technique to fine-tune the thickness of silicon nitride (Si3N4) microresonators at the wafer level with sub-10 nm precision, enabling precise control over geometric dispersion and dispersive wave (DW) positions in broadband Kerr soliton microcombs. The method preserves high optical quality factors and allows for chip-level trimming and multi-project wafer integration, achieving a 40 THz tuning range of DWs with 73 GHz/nm sensitivity.

ABSTRACT

Geometric dispersion in integrated microresonators plays a major role in nonlinear optics applications, especially at short wavelengths, to compensate the natural material normal dispersion. Tailoring of geometric confinement allows for anomalous dispersion, which in particular enables the formation of microcombs which can be tuned into the dissipative Kerr soliton (DKS) regime. Due to processes like soliton-induced dispersive wave generation, broadband DKS combs are particularly sensitive to higher-order dispersion, which in turn is sensitive to the ring dimensions at the nanometer-level. For microrings exhibiting a rectangular cross section, the ring width and thickness are the two main control parameters to achieve the targeted dispersion. The former can be easily varied through parameter variation within the lithography mask, yet the latter is defined by the film thickness during growth of the starting material stack, and can show a significant variation (few percent of the total thickness) over a single wafer. In this letter, we demonstrate that controlled dry-etching allows for fine tuning of the device layer (silicon nitride) thickness at the wafer level, allowing multi-project wafers targeting different wavelength bands, and post-fabrication trimming in air-clad ring devices. We demonstrate that such dry etching does not significantly affect either the silicon nitride surface roughness or the optical quality of the devices, thereby enabling fine tuning of the dispersion and the spectral shape of the resulting DKS states.

Motivation & Objective

• To address the challenge of natural thickness variation in Si3N4 films across wafers, which limits consistent microcomb performance.
• To enable post-fabrication tuning of geometric dispersion in air-clad Si3N4 microresonators for precise control over dispersive wave (DW) spectral positions.
• To develop a wafer-scale, selective dry-etching process that maintains high optical quality factor (Q) and surface smoothness.
• To demonstrate chip-level trimming for dispersion engineering in dissipative Kerr soliton (DKS) microcombs, supporting multi-project wafer runs.

Proposed method

• A selective dry-etching process using reactive ion etching (RIE) with 45 W RF power and 20 s duration achieves thickness reduction with ~4 nm per step resolution.
• An air-clad microresonator design with oxide cladding at waveguide facets enables post-fabrication access to the Si3N4 layer while maintaining low insertion loss for fiber coupling.
• Optical characterization and real-time feedback during trimming allow for precise control of dispersion and DKS state evolution.
• Theoretical modeling using the Lugiato-Lefever equation with adjusted dispersion profiles validates experimental results and confirms minimal impact on ring width.
• Wafer-scale selective masking enables multi-project fabrication with different thicknesses across a single wafer, accommodating diverse spectral band requirements.
• Surface roughness and Q-factor measurements confirm no significant degradation after etching, ensuring optical performance is preserved.

Experimental results

Research questions

• RQ1 Can post-fabrication dry etching enable sub-10 nm thickness tuning of Si3N4 microresonators without degrading optical quality factors?
• RQ2 To what extent can thickness tuning control the spectral position of dispersive waves (DWs) in broadband dissipative Kerr soliton (DKS) microcombs?
• RQ3 Can this trimming technique compensate for natural thickness variations across a 100 mm Si3N4 wafer to improve yield of devices with target dispersion?
• RQ4 How does the combination of air-clad microresonators and oxide-clad waveguide facets enable both post-fabrication tuning and low insertion loss?
• RQ5 Can the tuning range of DWs be predicted and controlled using integrated dispersion models?

Key findings

• The dry-etching process achieves a thickness tuning resolution of 4 nm per step, enabling precise control over geometric dispersion in Si3N4 microresonators.
• Dispersive wave (DW) positions in DKS microcombs are tuned at a rate of approximately -73 GHz per nm of thickness reduction, with experimental and simulated spectra showing close agreement.
• The optical quality factor (Q) remains high and unchanged after trimming, with no significant increase in surface roughness observed.
• The technique enables a total DW tuning range of 40 THz across a 30 nm thickness variation, demonstrating broad spectral control in microcombs.
• The method supports multi-project wafer fabrication by allowing different thicknesses to be selectively etched across a single wafer, enabling diverse device projects on one chip.
• Theoretical simulations using the Lugiato-Lefever equation with adjusted dispersion profiles closely match experimental results, confirming that the etching process is predominantly physical and does not alter the ring width.