High-yield, wafer-scale fabrication of ultralow-loss, dispersion-engineered silicon nitride photonic circuits

Abstract

Low-loss photonic integrated circuits and microresonators have enabled a wide range of applications, such as narrow-linewidth lasers and chip-scale frequency combs. To translate these into a widespread technology, attaining ultralow optical losses with established foundry manufacturing is critical. Recent advances in integrated Si3N4 photonics have shown that ultralow-loss, dispersion-engineered microresonators with quality factors Q > 10 × 106 can be attained at die-level throughput. Yet, current fabrication techniques do not have sufficiently high yield and performance for existing and emerging applications, such as integrated travelling-wave parametric amplifiers that require meter-long photonic circuits. Here we demonstrate a fabrication technology that meets all requirements on wafer-level yield, performance and length scale. Photonic microresonators with a mean Q factor exceeding 30 × 106, corresponding to 1.0 dB m−1 optical loss, are obtained over full 4-inch wafers, as determined from a statistical analysis of tens of thousands of optical resonances, and confirmed via cavity ringdown with 19 ns photon storage time. The process operates over large areas with high yield, enabling 1-meter-long spiral waveguides with 2.4 dB m−1 loss in dies of only 5 × 5 mm2 size. Using a response measurement self-calibrated via the Kerr nonlinearity, we reveal that the intrinsic absorption-limited Q factor of our Si3N4 microresonators can exceed 2 × 108. This absorption loss is sufficiently low such that the Kerr nonlinearity dominates the microresonator’s response even in the audio frequency band. Transferring this Si3N4 technology to commercial foundries can significantly improve the performance and capabilities of integrated photonics.

Integrated photonics enables continuous-beam electron phase modulation

“…The microresonators used in this study are made from Si 3 N 4 , a platform that is CMOS compatible with radiation hardness capability 62 and ultra-low propagation losses 56 , 57 . They also allow for dispersion engineering and efficient fibre-to-chip coupling.…”

“…Si 3 N 4 -based integrated photonic circuits have been driving major progress in nonlinear optical devices, in particular soliton microcombs 63 that are already used in numerous system-level applications ranging from coherent telecommunication 64 to astrophysical spectrometer calibration 65 , 66 . Here, the designed microresonators were fabricated using the photonic Damascene process 56 , 57 . A 4-inch silicon substrate with 4-μm-thick thermal wet SiO 2 cladding was used.…”

“…Stoichiometric Si 3 N 4 film was deposited on the patterned substrate via low-pressure chemical vapour deposition (LPCVD), to fill the waveguide preforms and to form the waveguide cores. Afterwards, etchback and chemical-mechanical polishing 56 were used to planarize the substrate and remove excess Si 3 N 4 . The entire substrate was further annealed at 1,200 °C to remove the residual hydrogen content in Si 3 N 4 , to reduce hydrogen-induced absorption loss in the Si 3 N 4 waveguides.…”

“…To facilitate high interaction strengths, we use photonic chip-based Si 3 N 4 microresonators, a platform with many important features, including radiation hardness, high power handling 55 , extremely low propagation losses (below 1 dB m −1 at 1,550 nm) 56 and flexibility, to engineer dispersion for phase matching. The chip was fabricated using the photonic Damascene process 56 , 57 ( Methods ) without top oxide cladding to allow for efficient free-electron–light interactions with the evanescent field. The ring microresonator of 20 μm radius is coupled to a bus waveguide and placed close to a triangular edge of the chip to minimize undesired electron–substrate interactions (Fig.…”

Suppression of avoided resonance crossing in microresonators

“…Microresonator-based Kerr frequency combs [1] have garnered a great deal of interest in the past decade, driven by various integrated photonic applications [2][3][4][5] that require an energyefficient, small-footprint, broadband, and coherent optical source on a chip. In the aim of improving the energy efficiency of frequency comb generation, a wide range of nonlinear photonic material platforms (e.g., Si 3 N 4 [6,7], SiC [8], AlN [9], LiNbO 3 [10], GaP [11], AlGaAs [12,13]) have been investigated in search of a higher Kerr nonlinearity, and fabrication technology improvements [14,15] have enabled higher quality factors (low loss) of the microresonators. The waveguide dimension of the microresonator is usually carefully designed concerning the dispersion and loss performance.…”