Introduction
Waveguide resonators are crucial components in photonic integrated circuits (PICs), enabling functionalities such as modulation and tunable filtering. Traditionally, fabricating these resonators has relied on techniques like photolithography, which requires dedicated nanofabrication facilities and is relatively costly, especially at smaller resolution scales. Nanoimprint lithography (NIL) offers a cost-effective and efficient alternative for patterning nanophotonic devices with resolutions down to just a few nanometers.
In this article, we will explore the fabrication of high-quality (Q) silicon nitride (SiN) waveguide resonators using a combination of thermal and UV nanoimprint lithography techniques. The resulting devices exhibit intrinsic Q factors up to the order of 105 with nearly-zero waveguide dispersion and high extinction ratios.
Fabrication Process
The process begins with the fabrication of a main mold using conventional i-line stepper lithography. This one-time use mold contains the design for waveguide resonators with a 100 μm radius, 0.4 μm gap between bus and resonator waveguides, 1 μm bus waveguide width, and 3 μm resonator waveguide width (for single mode and high-Q). The waveguide height is 700 nm.
This main mold is then used to imprint patterns onto a soft, liquid Topas® mold material spin-coated on a silicon substrate via thermal nanoimprint at 140°C and 6 bar pressure.
The patterned Topas mold has an inverse structure of the designed waveguides and serves as a stamp for fabricating the final integrated waveguide devices with nanometer resolution.
To create the waveguide resonators, a silicon substrate is first oxidized to form a 4 μm silicon dioxide (SiO2) layer to prevent light leakage. A 700 nm SiN film is then deposited using LPCVD. Next, a UV nanoimprint resist is spin-coated at 3000 rpm for a ~500 nm thickness.
The Topas stamp is then imprinted onto the resist under UV exposure at 4 bar for 6 minutes, transferring the waveguide patterns.
Reactive ion etching is used to etch back the SiN layer, forming a 240 nm slab waveguide with nearly zero dispersion.
Device Characterization
To characterize the fabricated devices, a 1550 nm tunable laser is used for transmission measurements with lensed fibers for input/output coupling in the TE mode.
The transmission spectrum in Figure 4 shows an intrinsic Q factor around 1.5 x 105 with extinction ratios up to 30 dB, indicating the resonator is operating at near critical coupling.
To demonstrate tunability, microheaters are fabricated on top of the waveguide with a 2 μm SiO2 cladding. Varying the applied voltage shifts the cavity resonance via the thermal-optic effect.
By analyzing the free-spectral range shifts, the waveguide dispersion is determined to be around -35 ps/nm/km, as shown in Figure 5b.
Conclusion
In this article, we demonstrated the fabrication of high-performance SiN waveguide resonators using a combination of thermal and UV nanoimprint lithography. The NIL technique allows for cost-effective manufacturing of nanophotonic devices with resolutions rivaling expensive lithography tools.
The fabricated resonators exhibit Q factors up to 1.5 x 105, 30 dB extinction ratios, nearly zero dispersion, and resonance tunability via integrated microheaters. These results highlight the potential of nanoimprint lithography for realizing high-quality integrated photonic components for applications like communications, sensing, and quantum photonics.
Reference
[1] P.-H. Wang, H.-Y. Zheng, Y.-H. Liu, and C.-M. Wang, "High-Q Silicon Nitride Waveguide Resonators by Nanoimprint Lithography," IEEE Journal of Selected Topics in Quantum Electronics, vol. 30, no. 6, pp. 4100308, Nov. 2023, doi: 10.1109/JSTQE.2023.3338800
Comments