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Abstract
We report on high-quality tellurium oxide waveguides integrated on a low-loss silicon nitride wafer-scale platform. The waveguides consist of silicon nitride strip features, which are fabricated using a standard foundry process and a tellurium oxide coating layer that is deposited in a single post-processing step. We show that by adjusting the Si3N4 strip height and width and TeO2 layer thickness, a small mode area, small bend radius and high optical intensity overlap with the TeO2 can be obtained. We investigate transmission at 635, 980, 1310, 1550 and 2000 nm wavelengths in paperclip waveguide structures and obtain low propagation losses down to 0.6 dB/cm at 2000 nm. These results illustrate the potential for compact linear, nonlinear and active tellurite glass devices in silicon nitride photonic integrated circuits operating from the visible to mid-infrared.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Silicon nitride (Si3N4) has emerged as a platform of choice for low loss, compact and high performance photonic integrated circuits (PICs) for a wide variety of applications [1–4]. Silicon nitride waveguides have high refractive index contrast enabling compact waveguide bends [5], ultra-low losses throughout the visible and infrared [6–8], nanoscale feature resolution through mature processing techniques and allow for volume chip production through standard wafer-scale fabrication steps [9]. Because it is a silicon-foundry-compatible material, silicon nitride waveguides and devices can be included in 3D-integrated active silicon photonics platforms [10,11]. Silicon nitride is widely applied in passive devices such as filters, multiplexors and switching and routing components for communications systems [12]. More recently it has evolved as a high-performance medium for nonlinear optical devices [13,14], quantum microsystems [15], monolithic and hybrid on-chip lasers [16–20], and sensing and spectroscopic circuits [21–23]. Additional functionality can be added to silicon nitride waveguide platforms through post-processing. In particular, various active devices have been developed on silicon nitride, including by bonding of III-V semiconductor materials [16] and monolithic deposition of rare-earth-doped aluminum oxide thin films [17–20].
Tellurium oxide (TeO2) is a highly promising material for active monolithically integrated optical devices that has received recent attention [24–26]. Like Si3N4, TeO2 is transparent throughout the visible and near-infrared and into the mid-infrared with low dispersion. Waveguides defined in bulk glass [27–32] and thin films [33–36] have been realized with low losses, of 0.1 dB/cm at 1550 nm. TeO2 has a higher refractive index than Si3N4, thus also enabling highly compact waveguides and devices. It has a significantly higher nonlinearity [37] and higher Raman gain than Si3N4 [38], as well as negligible two-photon absorption at telecommunications wavelengths [35]. Meanwhile, unlike silicon nitride it is an excellent host for rare earth ions [39], exhibiting high rare earth solubilities and low quenching, large emission bandwidth and high gain, as well as low phonon energies (∼700–800 cm−1), potentially enabling radiative transitions that are inaccessible in other oxide rare-earth-host materials [40]. TeO2 also has a high acousto-optic figure of merit [41]. TeO2 films can be deposited at low temperature [35], ideal for post-processing onto a variety of photonic platforms. Post-processing of TeO2 onto an established low-loss waveguide platform such as silicon nitride could alleviate challenges with the material – particularly with developing standard wafer-scale and high-resolution etch recipes for the doped films [39] – and leverage the cost-effective production methods and standard photonic integrated circuit architectures available with silicon device fabrication technology [42]. Further, utilizing mature high-resolution fabrication methods could enable incorporating smaller TeO2 waveguide geometries, leading to more compact devices and smaller effective mode areas, higher intensities and more highly efficient all-optical devices [43]. Silicon nitride is particularly attractive because the waveguides are low loss compared to other volume production platforms such as silicon-on-insulator and enable small features and bending radius. TeO2 can potentially add enhanced functionality to silicon nitride PICs, including amplification and light emission in monolithic rare-earth-doped waveguides, higher nonlinearity, acousto-optic devices and Raman gain and emission. Selected optical properties of TeO2 and Si3N4 are compared in Table 1.
Table 1. Selected optical properties of Si3N4 and TeO2 (from [7,8,14,25,38,46–48])a
Here, we demonstrate a new waveguide platform based on TeO2-coated Si3N4 waveguides which combines the advantages of the two integrated optical materials. It does this by utilizing the wafer-scale, high-resolution and low-loss waveguides available with Si3N4 technology, while potentially bringing new and enhanced linear, nonlinear and active functionalities via tellurite glass thin films. We describe the TeO2 film and waveguide fabrication, waveguide design and optimization, and characterize losses in cut-back structures, showing < 1 dB/cm loss at 1310, 1550 and 2000 nm wavelengths.
2. Fabrication
2.1. TeO2 thin film deposition
We deposit tellurium oxide thin films using a radio frequency (RF) reactive sputtering process similar to those reported in [33,35]. A high purity (99.999%) 3-inch metallic tellurium target is sputtered using a magnetron sputtering gun in an argon/oxygen ambient. The substrate holder is rotated at 8 rpm at a distance of 15 cm from the target and left at ambient temperature. For the development of high-performance devices it is essential that the deposited film has low optical propagation loss. Nayak et al. [33], Pietralunga et al. [34] and Madden and Vu [35] found that stoichiometry is a dominant factor in loss optimization, with stoichiometric 2:1 oxygen to tellurium ratio (TeO2) films having the lowest optical losses. Metal rich films experience a significant increase in optical loss due to absorption from the excess metal atoms, while oxygen-rich films experience a gradual increase in optical loss with increasing oxygen content.
To optimize the TeO2 recipe, we carried out depositions on silicon, thermally oxidized silicon with 6-µm-thick SiO2 and glass substrates, all mounted to the sample holder by carbon tape. We set the argon flow rate to 12 sccm and RF sputtering power to 150 W for all depositions and adjusted the oxygen flow rate to control the oxygen content and optical properties of the deposited films. We obtained near-stoichiometric, confirmed by Rutherford back scattering measurements, and low-loss films at oxygen flow rates ranging from 7.5 to 8.3 sccm. The oxygen flow necessary to achieve low-loss films was found to rise slightly as the Te target was sputtered more. Additionally, some hysteresis between depositions was noted, where the oxygen flow rate required for stoichiometry shifts slightly if a previous deposition was at an oxygen flow rate that didn’t achieve stoichiometry. This is believed to be the result of the target itself changing its oxidation state. At these process parameters the deposition rate was found to be 22 and 25 nm/min, with a refractive index of 2.08 at a 1550 nm wavelength, measured by spectroscopic ellipsometry for the films deposited on silicon. The film loss was measured using prism coupling and found to be at the detectable limit of 0.1 dB/cm for the fundamental transverse electric (TE) mode at 1550 nm wavelength in 0.2-0.4 µm-thick TeO2 films on thermally oxidized silicon.
2.2. TeO2-coated Si3N4 waveguide fabrication
We fabricated silicon nitride strip waveguides on a 100-mm-diameter silicon wafer with an 8-µm-thick thermal oxide lower cladding layer using a standard wafer-scale foundry process [1,9]. We first deposited a 0.2-µm-thick low-pressure chemical vapor deposition (LPCVD) Si3N4 film. The Si3N4 layer thickness was chosen to avoid film cracking due to stress difference with the silicon substrate and achieve strong lateral optical confinement while still enabling high optical intensity overlap with the TeO2 coating layer to be deposited on top. Paperclip strip waveguides with widths of 1.2 µm, large bending radius (≥ 0.6 mm) and lengths varying from 2.45 to 3.93 cm were defined in the Si3N4 layer by stepper lithography and reactive ion etching. The wafer was annealed at > 1100 °C in N2 for several hours to drive out hydrogen from the Si3N4 waveguides and reduce absorption at wavelengths around 1.5 µm due to Si-H and N-H bonds. The wafer was then diced into individual chips and transferred from the foundry. We then deposited a 0.38-µm-thick TeO2 coating layer onto the individual chips. The film loss was measured to be ≤ 0.1 dB/cm at 1550 nm using the prism coupling method. End facets were prepared by focused ion beam (FIB) milling in the region of the waveguide at the edge of the chip. A 1-µm-thick fluoropolymer top cladding was then spin-coated onto the chips (Cytop, n = 1.39 at 1550 nm). Figure 1(a) summarizes the process flow for the preparation of chips.
Fig. 1 (a) Fabrication steps used to fabricate TeO2-coated Si3N4 waveguides: (i) growth of a 6-µm-thick thermal SiO2 layer on a 10-cm Si wafer, (ii) deposition of a 0.2-μm-thick LPCVD Si3N4 film, (iii) patterning of Si3N4 film using stepper lithography to form strip waveguides, (iv) dicing of wafer into chips, (v) deposition of a TeO2 layer by reactive sputtering, (vi) waveguide facet polishing using FIB milling (vi) spin-coating of fluoropolymer top-cladding. (b) SEM image of a smooth FIB-etched waveguide facet for improved fiber-chip coupling efficiency. (c) SEM cross-section image of an uncoated Si3N4 waveguide. (d) SEM cross-section image of a Si3N4 waveguide coated with a 0.3-µm-thick TeO2 film.
To increase the fiber-chip coupling efficiency and reduce the waveguide insertion loss, we used FIB milling to fabricate smooth waveguide end facets, as shown in Fig. 1(b). The FIB stage was tilted so that the beam was incident normal to the chip surface. A 30 kV beam with a current of 300 pA was used, ensuring that 240 seconds of milling was sufficient to remove the edge roughness completely. Figure 1(c) shows an SEM image of a Si3N4 waveguide without any layer on top after final processing. The composite TeO2-Si3N4 waveguide structure prior to top cladding deposition is shown in Fig. 1(d).
3. Calculated TeO2-coated Si3N4 waveguide properties
We investigated the theoretical properties of the TeO2-coated Si3N4 waveguides using a finite element method (FEM) modesolver. The waveguide structure used in calculations is shown in Fig. 2(a), indicating the silicon nitride film height, silicon nitride strip width, and tellurium oxide film height, . It is estimated that a layer thickness of half the TeO2 film height is deposited on the strip sidewalls due to the angled orientation of the sputtering gun relative to the substrate during TeO2 deposition. A sample optical mode profile, which shows the calculated electric field for the fundamental transverse electric (TE) polarized mode at a wavelength of 1.55 µm and for , and is displayed in Fig. 2(b). All calculations were carried out for TE polarization and are based on the material dispersion relations, which are plotted in Fig. 2(c) and were obtained from spectroscopic ellipsometry measurements for TeO2 and Cytop and references [44] and [45] for Si3N4 and SiO2, respectively. We initially investigated the cutoff strip width for single-mode behavior for and varying TeO2 film heights as shown in Fig. 2(d). Due to the asymmetry of the waveguide structure we found that the calculated first order mode is also typically a TE polarized mode. Therefore, for widths below the single mode condition the waveguide often supports only the fundamental TE polarized mode, and no additional TE or TM polarized modes. In Fig. 2(d) it can be seen that the cutoff width increases with wavelength and with increasing TeO2 film height. The cutoff Si3N4 strip width ranges from 0.5 to 2.0 µm for the wavelength and TeO2 film height ranging from 0.6 to 2.0 µm and 0.2 to 0.6 µm, respectively. The fabricated waveguides described in section 2.2 are determined to be single mode at wavelengths > ~1.3 µm.
Fig. 2 (a) Cross-section profile of the TeO2-coated Si3N4 waveguide structure showing the Si3N4 strip width and height, and , respectively, and TeO2 film height,. (b) Sample simulated electric field profile of the fundamental TE-polarized mode calculated for , and . (c) Refractive indices of the waveguide core and cladding materials at wavelengths from 0.6 to 2.0 μm. (d) Si3N4 waveguide width, , below which the waveguide will support a single TE mode at wavelengths from 0.6 to 2.0 μm and for and .
The calculated optical properties of the TeO2-coated Si3N4 waveguides are displayed in Fig. 3. The polarization was set to TE and the Si3N4 strip dimensions were set to 1.2 µm × 0.2 µm, which are the same as the fabricated waveguides for all the calculations. Figure 3(a) shows the simulated effective refractive index versus wavelength for different TeO2 film thicknesses. For thicker films the waveguide mode becomes more confined in the TeO2 coating, causing the effective index to rise. The effective index decreases at longer wavelengths due to an expanded optical mode, which overlaps more with the lower refractive index SiO2 and Cytop cladding layers. The percentage of the optical mode power confined in the TeO2 coating and Si3N4 strip layers is shown in Fig. 3(b). A large optical confinement in the TeO2 coating is desirable to take advantage of its material properties such as high non-linearity and light emission in rare-earth-doped films. The results show that for a 0.4-µm thick TeO2 coating and at a wavelength of 1.5 µm approximately 70% of mode power is confined in the coating, with < 20% confined to the nitride layer, and the rest of the optical power in the cladding materials. Thicker TeO2 coatings can increase the optical confinement up to almost 90% in the coating. However thicker tellurium oxide coatings in this waveguide structure have two drawbacks: a larger mode area and increased radiation loss in waveguide bends. Smaller mode areas result in increased optical power density, which is key towards the fabrication of efficient non-linear and rare earth light emitting/amplifying devices. Figure 3(c) shows the simulated effective mode area versus TeO2 film thickness. For example, waveguides with TeO2 film thicknesses of 0.2, 0.4 and 0.6 µm will have mode areas of 0.90, 1.20 and 1.75 µm2 at 1.5 µm, which are approximately 3, 4 and 6% of the mode area of a standard 9/125 single mode fiber (30 µm2). Figure 3(d) shows the minimum bending radius, selected as the bending radius below which radiation loss exceeds 0.01 dB/cm, which was simulated using a commercial eigenmode bend solver. The minimum bending radius is seen to be strongly wavelength and TeO2 film height dependent. The data shows that a waveguide with a 0.4 µm thick TeO2 coating can be bent to a radius of 0.2 mm with almost no increase in bending loss at a wavelength of 1.5 µm. Meanwhile, coating thicknesses of 0.2 and 0.6 give minimum bend radii of 0.07 and 0.59 mm, respectively at this wavelength. At shorter wavelengths the minimum bend radius tends to increase due to increased mode overlap with the slab-like TeO2 coating, while at longer wavelengths it increases due to mode expansion and reduced confinement to the waveguide core. The simulations show that for the selected silicon nitride strip dimensions an acceptable compromise between maximizing optical confinement in the TeO2 layer, while maintaining a compact mode size and tight bending radius, can be obtained. The fabricated TeO2-coated Si3N4 waveguides have a calculated effective index, effective mode area, confinement factor and minimum bend radius of 1.84, 1.20 µm2, 66%, and 0.18 mm, respectively, at 1550 nm.
Fig. 3 Calculated optical properties of the fundamental TE mode in TeO2-coated Si3N4 waveguides for Si3N4 strip dimensions of 1.2 µm × 0.2 µm, TeO2 coating thicknesses of 0.2, 0.4 and 0.6 μm, and wavelengths ranging from 0.6 μm to 2.0 μm. (a) Effective refractive index. (b) Optical intensity overlap with the Si3N4 strip and TeO2 coating. (c) Effective 1/e electric field mode area. (d) Minimum waveguide bend radius, defined as the radius below which radiation losses exceed 0.01 dB/cm.
4. Waveguide loss measurements
We measured the optical propagation loss in the fabricated paperclip TeO2-coated Si3N4 waveguides using a fiber coupling setup. Light from a 635 or 1310 nm diode laser or 980, 1550 or 2000 nm tunable laser was coupled to the chip. 635 nm light was coupled via a cleaved fiber, while the other wavelengths were coupled via tapered fibers with 2.5-µm spot diameter using xyz alignment stages. The input polarization was controlled by polarization paddles. Transmitted signal at the output was measured by an InGaAs photodiode integrating sphere for 980 and 1310 nm light, an InGaAs photodiode for 1550 nm light, and an extended InGaAs amplified photodiode for 2000 nm light. Figure 4(a) shows the TeO2 film loss vs. wavelength obtained by prism coupling measurements in a 0.38-µm-thick film on a thermally oxidized silicon substrate deposited at the same time as the paperclip structures. The loss is observed to increase from 0.1 ± 0.1 dB/cm at 1550 and 1310 nm to 0.5 ± 0.2 and 1.2 ± 0.3 dB/cm at 980 and 638 nm, respectively. The paperclip waveguide test structures are illustrated in Fig. 4(b), showing waveguide lengths varying from 2.45 to 3.93 cm, designed to fit on a 1-cm-long chip. The minimum bend radius for all waveguides was set at 0.6 mm, so as not to introduce noticeable bending radiation loss. Figure 4(c) (inset) shows a top view image of 635-nm light being edge coupled onto and travelling across a paperclip waveguide of 3.45-cm length. To measure the propagation loss at 635 nm, regular single mode 630-nm fibers were used rather than tapered fibers. Because of the larger mode mismatch and higher fiber-chip coupling losses (as compared to those measured using the tapered fibers at other wavelengths) and higher propagation loss at this wavelength, the loss for 635-nm light was characterized using top-down image analysis of the scattered light travelling through the waveguide. Image processing was used to determine the relative intensity of red light along the length of the waveguide, which was then fit with an exponential relationship to determine a waveguide loss of 8.4 ± 1.1 dB/cm, as shown in Fig. 4(c). The insertion loss measured at 980, 1310, 1550 and 2000 nm is shown in Fig. 4(d). The propagation loss was fitted using linear regression. The fits give y-intercepts ranging from 9.6 dB at 980 nm to 11.6 dB at 1550 nm, suggesting 5–6 dB of fiber-chip coupling loss at each facet, with approximately 3 dB attributed to mode overlap and Fresnel reflections. At 980 μm the fit gives a higher loss of 3.1 ± 0.3 dB/cm which can be attributed in part to the higher TeO2 film loss and the transition to the waveguide being strongly multimode at 980 nm. The fits at 1310, 1550 and 2000 nm give waveguide losses of 0.8 ± 0.3, 0.8 ± 0.3 and 0.6 ± 0.2 dB/cm.
Fig. 4 (a) Film loss measured at wavelengths of 638, 850, 980, 1310, and 1550 nm using the prism coupling method. Inset: 638 nm wavelength light streak in the film. (b) Schematic of the paper-clip structures used to determine the waveguide propagation loss via cut-back measurements. (c) Scattered intensity of red light versus propagation distance along waveguide, from a top view microscope image (inset), fit with an exponential relationship to measure 8.4 ± 1.1 dB/cm of waveguide loss. (d) Insertion loss of waveguides with lengths from 2.45 cm to 3.93 cm at 980, 1310, 1550, and 2000 nm wavelengths. Linear regression fitting was used to calculate propagation losses 3.1 ± 0.3, 0.8 ± 0.3, 0.8 ± 0.3, and 0.6 ± 0.2 dB/cm, respectively.
5. Discussion
These results demonstrate the design and fabrication of compact and low-loss tellurium-oxide-coated silicon nitride waveguides. The waveguide properties are compared to other reported integrated tellurite waveguides based on thin films in Table 2. We calculated the number of waveguide modes supported, TeO2 confinement factor, effective mode area, minimum bend radius, and effective refractive index using the geometries and refractive indices reported here and in [34,35,39] and a FEM modesolver. In the approach demonstrated here, structuring of the TeO2 is not required, avoiding complex post-processing steps and difficulties with non-standard TeO2 etching, which has been shown to be particularly challenging for doped films [39] and requires extra steps because of TeO2’s solubility in standard chemicals [34,49]. Instead, the TeO2 is deposited in a single straightforward low-temperature step onto silicon nitride waveguides, allowing for compatibility with standardized and production-level Si3N4 PIC platforms. The low losses and smaller dimensions available through mature and high-resolution silicon nitride processing enable flexible design of waveguide properties, such as the number of modes supported, mode size, minimum bend radius and relative confinement in the TeO2 layer. The waveguides presented here exhibit significantly smaller mode size and bend radius compared to previously reported TeO2 waveguides of ~1 µm2 and 0.18 mm, respectively for a wavelength of 1550 nm. A tradeoff required for this design is less optical confinement in the TeO2 film compared to etched TeO2 rib waveguides. The propagation losses measured here at 1550 nm are slightly higher than the losses reported in [35] for multimode waveguides, but are comparable, within the experimental error, to the losses for the single-mode Er-doped waveguides reported in [39]. The TeO2 film and waveguide propagation loss are seen to increase at lower wavelengths. We note that the TeO2 film loss at 638 nm is most highly sensitive to the oxygen flow rate and film stoichiometry. Nayak et al. demonstrated that the TeO2 film loss at 633 nm can be reduced from 2.20 to 0.26 dB/cm by heating at 200 °C [33]. We expect that the higher waveguide losses observed at 635 and 980 nm can be reduced by lower-loss TeO2 films, using single-mode waveguide designs at these wavelengths, and optimizing the waveguide geometry to minimize any scattering loss at the TeO2-Si3N4 interface and waveguide sidewall. Towards longer wavelengths, we observe a loss of 0.6 dB/cm at 2000 nm, which is promising for emerging mid-infrared devices. The results are similar to the propagation loss of 0.5 dB/cm extracted from the Q factor which we recently measured in TeO2-coated Si3N4 microring resonators at 1550 nm [36]. However, in this work we demonstrate more practical top-cladded waveguides as well as significantly lower fiber-chip coupling losses using FIB facet etching.
Table 2. Properties of waveguides fabricated using TeO2 thin films (λ = 1.5 µm, TE polarization)
In this work we have demonstrated a proof-of-concept of this composite tellurite-silicon nitride waveguide platform and there is significant room for future work. Using the same process flow, the TeO2 layer can be doped with rare earth elements including erbium, thulium and ytterbium by reactive co-sputtering [39,48] in order to build integrated amplifiers and lasers into silicon nitride PICs. The calculated waveguide properties presented here provide a means for optimizing the layout of devices operating at particular wavelengths (e.g. for different optical amplifier pump and signal wavelengths). For example, a larger Si3N4 strip size can be applied for regions requiring tight bends, whereas a smaller and thinner strip might be used for higher overlap with rare-earth-doped TeO2 coating for optical gain sections in amplifiers and lasers. The silicon nitride strip height and TeO2 coating thickness can also be adjusted in order to optimize the dispersion and effective nonlinearity, and potentially develop more highly efficient nonlinear optical devices. We can further reduce the fiber-chip coupling losses by using spot-size converters [9] and shadow masking during post processing TeO2 deposition to realize adiabatic transitions between TeO2-coated and Si3N4 passive waveguides [50,51]. Similar methods can also be applied for co-integration of active TeO2 waveguides with Si3N4 cavity structures for on-chip lasers and passive silicon nitride devices for active-passive PICs.
6. Conclusion
We have demonstrated low-loss TeO2 waveguides integrated on a silicon nitride photonics platform. The TeO2-coated Si3N4 waveguide structures are fabricated using standard wafer-scale Si3N4 technology, followed by TeO2 thin film deposition via reactive sputtering in a straightforward low temperature post-processing step. The low losses measured here and the potential for doping the TeO2, combined with large optical intensity overlap with the tellurium oxide layer, small mode size and small bending radius motivates future development of compact and highly efficient nonlinear and active tellurite glass devices within silicon nitride PICs.
Funding
Natural Sciences and Engineering Research Council of Canada (NSERC) (STPGP 494306, RGPIN-2017-06423, EGP 522094-18); Canadian Foundation for Innovation (CFI) (CFI Project #35548); Voucher for Innovation and Productivity (VIPI) (Program Project #30288).
Acknowledgments
We acknowledge Rafael Kleiman, and Shahram Ghanad-Tavakoli and Doris Stevanovic of the Centre for Emerging Device Technologies (CEDT) for their support with the reactive sputtering system, John Luxat and Zhilin Peng for assistance with FIB SEM in the Centre for Advanced Nuclear Systems (CANS) Post Irradiation Examination Hot Cell Facility, and the use of the Canadian Centre for Electron Microscopy (CCEM).
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