Aluminum nitride on insulator: Material and processing optimization for integrated photonic applications

. Thin film aluminum nitride on insulator (AlNOI) has gained attention as a promising material platform for integrated photonic circuits (PICs) due to its ability to operate over a wide spectral range covering the ultra-violet to mid-infrared regions, while enabling a broad range of passive photonic functionalities. This study aims to optimize sputtered AlNOI films for PICs, with an emphasis on the spectroscopic ellipsometry study over a range from 0.19 µm to 25 µm. Furthermore, we discuss our approach for fabricating AlNOI PICs components, with a particular focus on optimizing the etching process to attain smooth sidewall waveguides.


Introduction
Over the past decades, there has been a growing demand for miniaturized solutions in fields such as sensors, data communications, and telecommunications, driving an increase in the diversity of photonic integrated circuits (PICs).This diversification of applications also requires more various and flexible material platforms.In this regard, different materials including aluminum nitride (AlN) have been investigated for integrated photonic applications.Due to its large bandgap of 6.2 eV, AlN has one of the widest reported transparency windows among photonic materials, reaching from the ultraviolet (UV) up to the mid-infrared (mid-IR), making it a promising material for a wide range of photonic applications [1].Furthermore, with a refractive index of 2.1 at a wavelength of 1.55 µm, single mode propagation can be realized in less than 900 nm wide waveguides.Hence, a high integration density of photonic components can be achieved [1,2].In addition to its good linear optical properties, AlN supports both 2 nd and 3 rd order opticalnonlinearities due to its non-centrosymmetric crystal structure, allowing for functionalities such as second harmonic generation [3], third harmonic generation [4], and electro-optic modulation [2].The usability of AlN for non-linear optical applications is further improved by its capability of high optical power handling due to the absence of two-photon absorption [1], a low thermo-optic coefficient and enhanced heat dissipation induced by high thermal conductivity [5].
Additionally to its optical properties, AlN is a promising candidate for merging integrated photonics with other functionalities, such as acoustics (high velocity for bulk and surface acoustic waves [6]) or microelectromechanical systems (MEMS) (piezoelectricity [1,2]) within one material platform, offering a variety of applications, for example reconfigurable and programmable integrated photonics [7].
This work focuses on optimizing optical properties of sputtered AlN thin films grown on SiO2 insulator (AlNOI).Moreover, we present our fabrication strategy for AlNOI PICs, with a specific emphasis on achieving smooth side-wall waveguides, which is crucial for lowloss propagation of the optical signal.

AlN thin film characterization
For the fabrication of integrated photonic devices, AlN films were sputtered (Spider 600, Pfeiffer) on top of a SiO2 layer on 4-inch Si-wafers with different recipes (see [8] for more details) with a targeted thickness of 600 nm.The real and imaginary part of the refractive index of the AlN layer were retrieved via spectroscopic ellipsometry measurement (SE-2000 combined with an IRSE extension for full UV to mid-IR range characterization, Semilab) and are depicted in Fig. 1 (a).By modelling the spectroscopic ellipsometry data, a refractive index of 2.06 was retrieved for telecom wavelength (1.55 µm).Furthermore, a low root mean square (RMS) surface roughness of 1.10 nm was measured by atomic force microscopy (AFM) (Park NX20, Park Systems) in noncontact mode within an area of 2 µm × 2 µm in the wafer centre (Fig. 1 (b)).In addition, x-ray diffraction (XRD) (XPert MRD XL, Malvern Panalytical) rocking curve measurements of the (002) AlN peak were performed to characterize the crystal quality of the thin film.A fullwidth at half-maximum (FWHM) of 1.9 ° indicates that the thin film is highly c-axis oriented.

PIC fabrication
Using the acquired optical properties of AlN, various AlNOI PIC components such as waveguides, grating couplers, directional couplers, and ring resonators for a targeted wavelength of 1.55 µm were designed using a commercial software (Ansys' Lumerical).For more details on the simulations, please refer to our previous work [8].
To fabricate the designed AlNOI PICs, a 300 nm thick SiO2 layer was deposited via PECVD (PlasmaPro 100 PECVD, Oxford) on top of the optimized AlN layer, which was used as hard mask during the AlN etching process.The SiO2 hard mask was structured using deep UV lithography (ASML PAS 5500/350C), a process which is easily scalable for high volume production, followed by a dry etching process (PlasmaPro 100 ICP Etch system, Oxford).The pattern was then transferred onto the AlN layer with another dry etching step (PlasmaPro 100 ICP Etch system, Oxford), followed by a short step of Ar bombardment for polishing of the sidewalls.A SEM image (Thermo Scientific Heliios G4) of a fabricated directional coupler before the removal of the SiO2 hard mask is depicted in Fig. 2 (a) together with a zoom into the coupling region of the two waveguides in Fig. 2 (b).The tilted-view SEM image shown in Fig. 2 (c) reveals the smooth sidewalls of the fabricated AlNOI waveguides.The slanted sidewalls of the waveguides are attributed to the receding hard mask during the AlN etching process and are expected to be improved by optimizing the etching process of the hard mask.

Conclusion
We have presented a spectroscopic ellipsometry study of AlN films over a wide spectral range (0.19 µm -25 µm) and optimized AlNOI platform for PICs.In addition, we have demonstrated the fabrication of AlNOI-based PICs targeted at telecom wavelength (1.5 µm).Our results demonstrate that the developed AlNOI is a promising material platform for PIC applications spanning from the UV to the mid-IR wavelengths.This work has been supported by Silicon Austria Labs (SAL), owned by the Republic of Austria, the Styrian Business Promotion Agency (SFG), the federal state of Carinthia, the Upper Austrian Research (UAR), and the Austrian Association for the Elec-tric and Elec-tronics Industry (FEEI).The authors acknowledge the support of the EPFL Center of MicroNanoTechnology (CMi).

Fig. 1 .
Fig. 1.Characterization of sputtered AlN on SiO2.(a) Complex refractive index (real part n imaginary part k) of AlN retrieved from spectroscopic ellipsometry measurement in the wavelength range of 0.19 µm -25 µm.(b) AFM measurement (2 µm × 2 µm) of the AlN surface with an RMS of 1.10 nm.

Fig. 2 .
Fig. 2. SEM image of a fabricated directional coupler (a) with a zoom into the coupling region highlighting the 85 nm wide coupling gap between the two waveguides (b) and a tilted view of the waveguides (c).