Photonic Damascene Process for Integrated High-Q Microresonator Based Nonlinear Photonics (1511.05716v1)

Abstract: High confinement, integrated silicon nitride (SiN) waveguides have recently emerged as attractive platform for on-chip nonlinear optical devices. The fabrication of high-Q SiN microresonators with anomalous group velocity dispersion (GVD) has enabled broadband nonlinear optical frequency comb generation. Such frequency combs have been successfully applied in coherent communication and ultrashort pulse generation. However, the reliable fabrication of high confinement waveguides from stoichiometric, high stress SiN remains challenging. Here we present a novel photonic Damascene fabrication process enabling the use of substrate topography for stress control and thin film crack prevention. With close to unity sample yield we fabricate microresonators with $1.35\,\mu\mathrm{m}$ thick waveguides and optical Q factors of $3.7\times10^{6}$ and demonstrate single temporal dissipative Kerr soliton (DKS) based coherent optical frequency comb generation. Our newly developed process is interesting also for other material platforms, photonic integration and mid infrared Kerr comb generation.

• The paper introduces a novel photonic Damascene process to fabricate high-Q SiN microresonators with near-unity yield by pre-patterning substrates to reduce film stress and prevent cracks.
• It achieves exceptional optical quality factors of 3.7 million, on par with traditional subtractive methods, validating its effectiveness in integrated nonlinear photonics.
• The method enables broadband Kerr soliton frequency comb generation with a 6.6 THz 3 dB bandwidth, promising advances for telecommunications, metrology, and spectroscopy.

Overview of the Photonic Damascene Process for Integrated High-Q Microresonator-Based Nonlinear Photonics

The paper presents the development of a novel photonic Damascene process aimed at fabricating high-quality (high-Q) silicon nitride (SiN) microresonators for nonlinear photonic applications. This process addresses significant challenges involved in the production of high confinement waveguides, with an emphasis on improving manufacturing reliability and device performance.

Key Contributions and Observations

The authors highlight the challenges in fabricating thick and high-stress SiN waveguides using traditional subtractive processes. These issues include high film stress leading to cracking, undesirable sidewall profiles, and reduced fabrication yield. To counter these challenges, the authors introduce the photonic Damascene process, inspired by microprocessor interconnect patterning techniques, which involves pre-patterning the substrate before the deposition of SiN.

Through this innovative approach, they achieve several noteworthy results:

• High Fabrication Yield: The introduction of a substrate pre-patterning technique drastically reduces film stress, preventing crack formation in stoichiometric SiN films and achieving a close to unity sample yield for waveguides with dimensions previously unattainable.
• Exceptional Quality Factors: A significant achievement highlighted in the paper is the realization of microresonators with optical Q factors of $3.7 \times 10^6$. This value is on par with state-of-the-art results from subtractive processes, thereby underlining the effectiveness of the proposed method.
• Broadband Comb Generation: Successfully demonstrating single temporal dissipative Kerr soliton (DKS) based frequency comb generation with a 3 dB bandwidth of 6.6 THz, the work showcases the utility of the fabricated devices in generating coherent frequency combs.

Practical and Theoretical Implications

This development in photonic fabrication has significant implications for the field of integrated photonics:

• Photonic Integration: The presented fabrication process, characterized by minimized production errors and improved waveguide integrity, paves the way for integrating SiN-based waveguides with other photonic components such as optoelectronic devices and novel 2D materials.
• Nonlinear Photonic Applications: The high Q factors and efficient frequency comb generation are critical for nonlinear photonic applications, impacting fields such as telecommunications, precision metrology, and spectroscopy.
• Design Flexibility: The capability to fabricate large waveguide cross-sections is instrumental in tailoring dispersion characteristics, essential for applications in different spectral regions, including the mid-infrared.

Future Developments

The implications of this fabrication approach extend to both immediate practical applications and longer-term theoretical developments. In photonic circuit design, the adaptability of the photonic Damascene process may lead to new architectures that leverage the stress control mechanism. Moreover, as demand increases for photonic integration in higher-frequency regimes, further refinement of this technique could enable the concurrent use of emerging materials and devices within a unified fabrication framework.

In conclusion, the proposed photonic Damascene process represents a significant advancement in the field of nonlinear photonics by merging innovative material engineering with traditional CMOS-compatible techniques. Its contributions towards the reliable production of high-Q SiN microstructures set a precedent for subsequent research and development within integrated photonic systems.