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High-Q-factor Al2O3 micro-trench cavities integrated with silicon nitride waveguides on silicon

Zhan Su, Nanxi Li, Henry C. Frankis, E. Salih Magden, Thomas N. Adam, Gerald Leake, Douglas Coolbaugh, Jonathan D. B. Bradley, and Michael R. Watts

Open Access Open Access

Abstract

We report on the design and performance of high-Q integrated optical micro-trench cavities on silicon. The microcavities are co-integrated with silicon nitride bus waveguides and fabricated using wafer-scale silicon-photonics-compatible processing steps. The amorphous aluminum oxide resonator material is deposited via sputtering in a single straightforward post-processing step. We examine the theoretical and experimental optical properties of the aluminum oxide micro-trench cavities for different bend radii, film thicknesses and near-infrared wavelengths and demonstrate experimental Q factors of > 106. We propose that this high-Q micro-trench cavity design can be applied to incorporate a wide variety of novel microcavity materials, including rare-earth-doped films for microlasers, into wafer-scale silicon photonics platforms.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (7)
Fig. 1
Fig. 1 Aluminum oxide micro-trench cavity fabrication process: (i) PECVD deposition and (ii) patterning of a 200-nm-thick Si3N4 film on a 6-µm-thick PECVD SiO2 lower cladding layer on silicon; (iii) deposition and planarization of a PECVD SiO2 layer to a height of 100 nm above the first Si3N4 layer and deposition of a second 200-nm-thick Si3N4 film; (iv) patterning of the second Si3N4 film; (v) deposition and planarization of a ~5-µm-thick SiO2 top cladding; (vi) definition of the micro-trench into the SiO2 cladding using RIE and the second Si3N4 layer as an etch stop followed by deposition of a 100-nm-thick SiO2 layer; and (vii) deposition of a ~1-µm thick Al2O3 layer by reactive sputtering. Steps i–vi are completed in a silicon foundry, while step vii is carried out as a post-processing step.

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Fig. 2
Fig. 2 (a) Schematic of a micro-trench cavity. (b) Cross-sectional SEM image taken along the dotted line in (a). The Pt coating is used as a protective layer during the FIB cutting of the chip and is not a part of the cavity design. (c) Magnified image of the cross-section (marked by the red rectangle in (b)) showing the trench angle, silicon nitride waveguide and gap.

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Fig. 3
Fig. 3 (a) Measured refractive index of the amorphous Al2O3 film vs. wavelength. (b) Ex-field of the TE-like fundamental mode of the micro-trench cavity. (c) Ey-field of the TM-like fundamental mode of the micro-trench cavity. (d) Effective indices of the TE-like mode for different wavelengths and film thickness. (e) Effective indices of the TM-like mode for different wavelengths and film thickness.

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Fig. 4
Fig. 4 Mode intensity overlap with Al2O3 for the (a) TE-like mode at 980nm, (b) TM-like mode at 980nm, (c) TE-like mode at 1550nm, and (d) TM-like mode at 1550nm for different microcavity radii and Al2O3 film thickness.

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Fig. 5
Fig. 5 (a) Measured refractive indices of the PECVD Si3N4 film vs. wavelength. (b) Ex-field of the TE-like mode and (c) Ey-field of the TM-like mode of the double nitride waveguide. (d) Effective indices of the TE-like mode and (e) effective indices of the TM-like mode for different wavelengths and waveguide widths.

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Fig. 6
Fig. 6 Transmission spectra and Q factors measured for micro-trench cavities with a 1.16-µm-thick Al2O3 film. (a) (b) Sample transmission spectra for a 150-µm-radius cavity measured at 1480 nm and 1610 nm wavelength with TE-polarized inputs. (c) Measured (points) and calculated (lines) intrinsic Q factors for different radii and wavelength ranges for TE-polarized modes. The calculated Q factors were determined using a finite-difference modesolver. (d) (e) Sample transmission spectra for a 150-µm-radius cavity measured at 1480 nm and 1610 nm wavelength with TM-polarized inputs. (f) Measured (points) and calculated (lines) intrinsic Q factors for different radii and wavelength ranges for TM-polarized modes.

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Fig. 7
Fig. 7 Transmission spectra and Q factors measured for micro-trench cavities with a 1.58-µm-thick Al2O3 film. (a) (b) Sample transmission spectra for a 150-µm-radius cavity measured at 1480 nm and 1610 nm wavelength with TE-polarized inputs. (c) Measured (points) and calculated (lines) intrinsic Q factors for different radii and wavelength ranges for TE-polarized modes. The calculated Q factors were determined using a finite-difference modesolver. (d) (e) Sample transmission spectra for a 150-µm-radius cavity measured at 1480 nm and 1610 nm wavelength with TM-polarized inputs. (f) Measured (points) and calculated (lines) intrinsic Q factors for different radii and wavelength ranges for TM-polarized modes.

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Equations (2)

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n eff = m λ 2 π R ,
γ = ∫ Al 2 O 3 I d A ∫ ∞ I d A ,
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