We design, fabricate, and demonstrate a silicon nitride (Si3N4) multilayer platform optimized for low-loss and compact multilayer photonic integrated circuits. The designed platform, with 200 nm thick waveguide core and 700 nm interlayer gap, is compatible for active thermal tuning and applicable to realizing compact photonic devices such as arrayed waveguide gratings (AWGs). We achieve ultra-low loss vertical couplers with 0.01 dB coupling loss, multilayer crossing loss of 0.167 dB at 90° crossing angle, 50 μm bending radius, 100 × 2 μm2 footprint, lateral misalignment tolerance up to 400 nm, and less than −52 dB interlayer crosstalk at 1550 nm wavelength. Based on the designed platform, we demonstrate a 27 × 32 × 2 multilayer star coupler.
© 2015 Optical Society of America
Recent developments in photonic integrated circuits (PICs)  are attracting strong interest due to their advances of building scalable, high-throughput, and low-cost on-chip optical communication system. Silicon nitride (Si3N4) based PICs are becoming an outstanding solution because of relatively low optical propagation losses . The multilayer structure is promising for future low-loss, small-footprint, and high-density PICs.
Achieving low interlayer vertical coupling loss and waveguide crossing loss is essential for realizing practical Si3N4 multilayer PICs. The interlayer couplers in multilayer platform utilized various methods including directional couplers [3, 4], grating couplers , reflector mirrors , and inverse tapers couplers .However, typical reported values for vertical coupling losses are ~1 dB for grating coupled [5, 6] configurations, or reasonably low 0.2 dB for inverse taper couplers with waveguides accompanying large bending radius of 9 mm . On the other hand, typical reported values for multilayer crossing losses are 0.4-0.6 dB , or reasonably low crossing loss of 0.254 dB when using a relatively wide waveguide width of 15 μm . Hence, the trade-off between vertical coupling and multilayer crossing loss for various interlayer distances is an important consideration in designing the waveguide for each layer. For devices with fewer multilayer crossings, shorter interlayer gap helps improve interlayer coupling but introduces large excess losses at crossing. For devices with many crossings, a larger interlayer gap increases coupling losses but reduces total insertion loss. Therefore, optimization of multilayer platform design for low total losses should be based on requirements for specific applications.
In this paper, we report a Si3N4/SiO2 multilayer platform design with core thickness of 200 nm and an interlayer gap of 700 nm optimized for compact multilayer photonic integrated circuits involving devices such as arrayed waveguide gratings (AWGs) with smaller than a 3 × 1 mm2 die. Based on this design, no measurable excess bending loss is reported with 50 μm bending radius. We achieve 0.01 dB coupling loss from proposed vertical coupler with a 100 × 2 μm2 footprint and 0.167 dB multilayer crossing losses at 90° crossing angle. Further, we demonstrate a 27 × 32 × 2 multilayer star coupler, which is useful for future 3-D photonic integrated devices.
2. Waveguide design
To design a practical platform of Si3N4/SiO2, there are several factors that should be considered, such as device footprint, loss budget, and functional compatibility. For this paper, we aim to optimize a platform for compact and low loss AWGs within 2.2 × 0.7 mm2. Therefore, the bending radius needs to be reasonably small.
Figure 1(a) illustrates the cross section of proposed waveguide, where Si3N4 is the waveguide core and SiO2 is the cladding. In Fig. 1(b), the blue curve shows simulated confinement factors with varying waveguide core thicknesses and the red curve shows corresponding bending radii. As the waveguide core thickness reduces below 100 nm, the mode is less confined in waveguide core, and the minimum bending radius required for < 0.1 dB loss/90° bending increases rapidly, leading to large device footprint. However, thick (>400 nm) silicon nitride waveguide cores increase the possibility of stress-induced cracks. Therefore, this paper investigates 200 nm Si3N4 core thickness that yields 50 μm bending radius.
Another consideration for the selection of waveguide core thickness is the requirement for thermal tuning for devices such as AWGs. Thermo-Optical Coefficient (TOC) of Si3N4 is approximately 2.5 times that of SiO2 . Waveguides with small confinement factors will have a smaller fraction of optical mode confined in Si3N4 and require a larger heater power to achieve the same amount of phase shift. Furthermore, thinner optical cores also require thicker overcladding, which makes the heater less effective in heating the waveguide core. For these reasons, thinner silicon nitride waveguide cores require higher electrical power to achieve a given amount of optical phase shift. Figure 1(c) shows simulation plots of the optical phase change vs. electrical power of the heater of 1000 μm length and 2 μm width shown for three different overcladding thicknesses (2 µm, 5 µm, 8 µm). The insets show simulated heat distribution for the silicon nitride waveguide with core dimensions 2 μm × 200 nm.
3. Simulation and optimization of Si3N4 platform
Multilayer platform PICs occupies a smaller footprint compared to a single layer PICs of equivalent functionality. As Fig. 2(a) illustrates, it is important to achieve low loss vertical couplers that interconnect between the layers while keeping the crosstalk and scattering loss minimal at the crossings.
Figures 2(b)-2(d) simulate vertical coupling transmission with varying taper lengths, interlayer misalignments, and interlayer gaps, respectively. In Fig. 2(b), the interlayer gap is 200 nm, and the vertical transmission decreases when taper lengths are less than 30 μm. In Fig. 2(c), the taper length and the interlayer gap are 100 μm and 200 nm, respectively. In this case, lateral misalignments up to 1.2 μm still supports strong vertical coupling, and this design is applied to a process requiring strong fabrication tolerance. Figure 2(d) illustrates that the vertical transmission starts to decrease when the interlayer gap increases above 800 nm for the taper length of 100 μm.
As indicated in Fig. 2(a), light incident into a cross gets transmitted, reflected, scattered, or becomes crosstalk to other layers. As simulated in Fig. 3(a), there is a trade-off between interlayer coupling transmission and crossing transmission as gap distance varies. The larger gap helps to reduce crossing loss while smaller gap improves vertical transmission. The value of gap optimized for ultra-low vertical coupling loss is 700 nm, where crossing loss is reasonably low. Another trade-off is between interlayer crosstalk and reflection by varying the crossing angle. Large crossing angle keeps low crosstalk while small crossing angle prevents reflection. Overall, platform optimization of the interlayer gap is to achieve low total insertion loss, based on the relative quantity between vertical coupler and multilayer crossing; while optimization of crossing angle is to meet devices application.
4. Device fabrication
We fabricated the devices using ASMLTM PAS 5500 300 deep-UV lithography stepper technology on 6-inch Silicon wafers. Figures 4(a)-4(k) show the principle steps of the fabrication process, starting with the growth of Low-Temperature Oxide (LTO) by Low-Pressure Chemical Vapor Deposition (LPCVD), serving as a lower cladding layer. We use stoichiometric Si3N4 by LPCVD at 800 °C as waveguide layer and Inductively Coupled Plasma (ICP) Etching to define waveguides on both layers. Interlayer cladding is LTO planarized by Chemical Mechanical Polishing (CMP). The overcladding layer is planarized LTO, and the heater metal is 20 nm titanium and 200 nm platinum deposited by electron-beam evaporation.
Figures 4(l) and 4(m) are device photos of inverse taper tips and multilayer crossings. Figure 4(n) is the surface roughness measurement of planarized LTO by Atomic Force Microscopy (AFM). The RMS roughness is 1 nm.
5. Characterization of vertical coupler and multilayer crossing
We characterize the fabricated devices by using single mode lensed fibers as input and output. The insertion loss of the straight waveguide is 3.2 dB including the facet coupling losses and the propagation loss in the straight waveguide. The coupling loss between lensed fiber and the waveguide is 1.5 dB per facet. The measured straight waveguide propagation loss is 0.3 ± 0.1 dB/cm.
Figure 5(a) shows the schematic of lower layer waveguide core, where ten sections of inverse tapers and positive tapers are in series. Figure 5(b) shows the schematic of the Si3N4 lower layer core and the upper layer core overlaid on its top with a 700 nm gap to achieve light transmission by 20 stages of inter-layer coupling. The measured transmission is less than −60 dB and −3.4 dB for devices with single layer only (no vertical couplers) and with double layer, respectively, indicating strong coupling between layers. The plots in Figs. 5(c)-5(e) show the measured vertical transmission values (normalized to the waveguide transmission of the same length) of vertical couplers with varying overlapping length, interlayer lateral misalignment, and taper length. The measured excess loss of the 20 stages of cascaded vertical couplers was 0.2 dB (normalized to the waveguide transmission of the same length), indicating that the intrinsic loss of each vertical coupling is on the average 0.01 dB per coupling for the vertical coupler with taper length of 100 μm and overlapping length of 100 μm. Interlayer misalignment tolerance of the ASML stepper is less than 60 nm. The overlapping length is indicating the misalignment in propagating direction, where the tolerance is 15 μm for inverse taper with a length of 100 μm. The lateral misalignment tolerance of 400 nm is reasonable regarding taper tip width of 250 nm.
Figures 6(a)-6(c) illustrate measured transmission values for a waveguide crossing with a number of cascaded waveguides at another layer crossing at angles of 30°, 60°, and 90° with interlayer gap of 500 nm, 700 nm, and 900 nm, respectively. For a 700 nm interlayer gap, the measured crossing transmission is – 0.167 dB at crossing angle of 90°. The corresponding crosstalk is −52 dB, as shown in Fig. 6(d).
6. Multilayer star couplers
Figure 7(a) shows an example of multilayer PICs composed of multiple connected couplers. On this PIC, there are 27 input ports that feed into nine 3 × 2 couplers on layer 1 (lower layer). The two outputs from the 3 × 2 couplers feed two different 9 × 32 star couplers, one on layer 1, and the other on layer 2 via a vertical coupler. By utilizing multilayer platform, the footprint of this PIC is reduced from 80 mm2 to 40 mm2. In Fig. 7(b), the output transmission increases by 2 dB per port from #15 to #32, resulting in >25 dB variation from multilayer crossing with 300 nm interlayer gap. Figure 7(c) shows the 32 × 2 multiple output transmission with 800 nm interlayer gap. The corresponding variation is about 5 dB across all outputs, mainly resulting from the star coupler design.
We demonstrate a multilayer Si3N4 platform for compact 3D PIC with ultra-low interlayer coupling loss of 0.01 dB, low multilayer crossing loss of 0.167 dB at 90° crossing angle, compact bending radius of 50 μm, small interlayer crosstalk of −52 dB, strong fabrication tolerance of lateral and tangential misalignment up to 400nm and 15 μm. Our inverse taper vertical couplers also have a compact device footprint of 100 × 2 μm2, which is small compared to other vertical coupling solutions [3, 5, 6] and other inverse taper designs .
Devices were fabricated in Marvell Nanolab facility, University of California, Berkeley, and North California Nanotechnology Center, University of California, Davis.
This work was supported in part by DARPA DSO under the contracts HR0011-11-1-0005 and W911NF-12-1-0311, DARPA TTO under grant no. W31P4Q-13-1-006, and by National Science Foundation (NSF) under ECCS-1028729.
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