Overcoming Si3N4 film stress limitations for High Quality factor ring resonators (1306.2994v1)

Abstract: Silicon nitride (Si3N4) ring resonators are critical for a variety of photonic devices. However the intrinsically high film stress of silicon nitride has limited both the optical confinement and quality factor (Q) of ring resonators. We show that stress in Si3N4 films can be overcome by introducing mechanical trenches for isolating photonic devices from propagating cracks. We demonstrate a Si3N4 ring resonator with an intrinsic quality factor of 7 million, corresponding to a propagation loss of 4.2 dB/m. This is the highest quality factor reported to date for high confinement Si3N4 ring resonators in the 1550 nm wavelength range.

• The paper demonstrates a trenching strategy that mitigates intrinsic Si3N4 film stress, enabling fabrication of 910 nm resonators with an unprecedented 7M intrinsic Q.
• It details a multi-step fabrication process, including electron beam lithography and high-temperature annealing, to effectively minimize crack propagation.
• This advancement enhances optical confinement and reduces propagation loss to 4.2 dB/m, paving the way for improved on-chip routing, frequency combs, and high-precision sensing.

Overcoming Film Stress Limitations in Silicon Nitride Ring Resonators

The paper "Overcoming Si N Film Stress Limitations for High Quality Factor Ring Resonators" presents a significant advancement in the field of photonics by addressing the challenges posed by the intrinsic stress in silicon nitride ($\text{Si}_3\text{N}_4$) films used in ring resonators. Silicon nitride ring resonators are integral components for on-chip optical routing, frequency combs, and high-precision sensing; however, their widespread adoption has been hindered by the high film stress that limits the thickness and thus the performance of these devices.

The authors' contribution lies in their novel approach to mitigate the film stress limitations that have historically constrained the optical confinement and quality factor ($Q$) of $\text{Si}_3\text{N}_4$ resonators. They introduce the concept of mechanical trenches that are strategically positioned to isolate photonic devices from cracks that might propagate due to stress. This method effectively contains the spread of cracks, thereby enabling the fabrication of thicker films without the risk of catastrophic failure.

The experiments demonstrated in the paper lead to the fabrication of $\text{Si}_3\text{N}_4$ ring resonators with an unprecedented intrinsic quality factor of 7 million, surpassing previous records for high confinement $\text{Si}_3\text{N}_4$ resonators within the 1550 nm wavelength range. This exceptional quality factor, which corresponds to an ultra-low propagation loss of 4.2 dB/m, is achieved through the deposition of a 910 nm thick $\text{Si}_3\text{N}_4$ film, a considerable improvement over the maximum 400 nm thickness previously limited by stress-induced cracking.

To achieve these results, the authors employed a multi-step fabrication process involving crucial steps such as trench definition, electron beam lithography, and high-temperature annealing. By utilizing trenches, they provided a strategic defense against crack propagation during the handling and processing of wafers. Additionally, post-fabrication annealing at high temperatures further mitigated intrinsic stresses, allowing for the stability and high performance of the resulting resonators.

The paper elucidates the optical mode profiles in waveguides of varying thicknesses, exemplifying how increased thickness reduces mode overlap with waveguide boundaries, thereby minimizing scattering losses. This is contrasted with existing approaches that either settle for thin films with delocalized modes or employ high-temperature deposition techniques to overcome stress limitations, both of which have significant drawbacks.

In terms of implications, the authors' method provides a new path to high-performance photonic devices, enabling advancements in low-loss optical routing, nonlinear optics with low power thresholds, and high-sensitivity sensors. By overcoming the traditional stress limitations of $\text{Si}_3\text{N}_4$ deposition, this work opens up a previously unexplored design space for integrated optics and MEMS devices.

Looking forward, the authors' stress mitigation strategy could inspire further innovations in the fabrication of photonic devices, potentially affecting a wide range of applications, from telecommunications to biosensing. The demonstrated potential for high $Q$ devices in challenging environments suggests that $\text{Si}_3\text{N}_4$ could continue to be a material of choice for photonic circuits that demand high performance and integration capabilities. Future research may focus on refining these techniques, exploring alternative trenching methods such as photolithographically defined features, and integrating these innovations into complex photonic systems.