Hot-Wire Chemical Vapour Deposition (HWCVD) hydrogenated amorphous silicon (a-Si:H) compact 3D slope waveguide interconnect for vertical coupling in multilayer silicon photonics platform

Abstract

This research project investigates three-dimensional (3D) waveguide interconnects for vertical integration in multilayer silicon photonics (SiP). Multilayer configuration permits the freedom for stacking up a large number of optical components on a single chip to obtain a dense circuit footprint. However, the challenge in realizing multilayer technology is making 3D vertical optical vias through depositable material that is compatible with a complementary metal-oxide-semiconductor (CMOS) fabrication line. The main requirement for a 3D vertical via is a compact design, preferably using a high index waveguide material, for efficient utilization of silicon real estate.
An interlayer slope waveguide coupler which directly couples light from one signal plane to another is proposed. The design of the interlayer coupler was inspired by electrical interconnects in CMOS circuitry, with device structure analogous to an S-bend waveguide. The proposed 3D coupler is compact to allow for high-density integration within a small footprint area. The structure comprises a waveguide placed at a low-level plane and another waveguide at an upper-level plane, connected by a waveguide on a slope. The slope with bending geometry is employed to allow gradual transition of light propagation at the slope interfaces. This avoids large scattering losses due to abrupt change at the slope junctions. The inclination angle of the slope determines the characteristics of the device. The interlayer slope waveguide is characterised in terms of loss in dB per slope. Similar to any waveguide bend, minimal slope loss can be achieved at a large bend radius which effectively means a small inclination slope angle. Thus it is necessary to work at the smallest possible inclination slope angle, while maintaining compactness.
The novelty in this work is the fabrication of a slope structure, designed to be used as a platform for transporting the optical signal directly from one plane to another. Silicon dioxide (SiO2) deposited by plasma-enhanced chemical vapour deposition (PECVD) at 350ºC was used as the slope platform. The slope structure was obtained by wet etching the PECVD SiO₂ in buffered hydrofluoric acid, NH₄:HF (7:1), using S1813 as the optical lithography resist. By varying the parameters during the optical lithography process, four slope angles were obtained: 11.8º, 16.7º, 20.8º and 25.3º. Subsequently, a hot-wire chemical vapour deposition (HWCVD) tool was used to deposit hydrogenated amorphous silicon (a-Si:H) film as the guiding material. Two sets of waveguide were fabricated, having dimensions of (i) 400 nm (w) by 400 nm (h), and (ii) 600 nm (w) by 400 nm (h). From the measurements, the lowest loss equal to 0.17 dB/slope was obtained from the 11.8º slope angle for 600 nm (w) by 400 nm (h) waveguide dimensions. The highest loss equal to 0.47 dB/slope was obtained from the 25.3º slope angle for 400 nm (w) by 400 nm (h) waveguide dimensions. The increase in loss is apparent to be originated from the significant mode-mismatch, which in turn was caused by a high effective index difference and a substantial change in the direction of propagation through the bend in the higher slope angle.
As part of the research on making the interlayer slope waveguide coupler, an experiment was conducted to measure crosstalk between two waveguides placed orthogonally to each other. The two waveguides were separated by a PECVD SiO₂ cladding layer of varying different thickness. The aim of this experiment was to investigate the minimum cladding thickness required to isolate two waveguides placed orthogonally on top of each other, and to demonstrate the ability of the interlayer slope waveguide to function in a multilevel optical network. In the measurement, crosstalk isolation of 22 dB was achieved from a thickness of 200 nm for waveguide dimensions of 400 nm (w) by 400 nm (h). Furthermore, waveguide dimensions of 1000 nm (w) by 400 nm (h) were isolated by 21 dB with 50 nm waveguide separation. The measured results conform to the theory and simulated results, in which smaller waveguides require higher isolation than larger waveguides. All measurements were obtained for a wavelength of 1550 nm with transverse electric (TE) mode polarization.
In demonstrating the interlayer slope waveguide to function as an actual waveguide coupler in a multi-level platform, a device called a fly-over slope waveguide was designed and fabricated. The development of this device is still at a preliminary stage due to time constraints. The first finding from this experiment was that crosstalk isolation with an average value of 45 dB was achieved when the separation between the two crossing waveguides was larger than 10 μm in the x-direction and 1 μm in the z-direction. Second, coupling increased to 18 dB when the underlying waveguide was placed 0 μm in the x-direction and 290 nm closer to the slope interface in the z-direction. These results conformed to the previous crosstalk experiment, in which minimum cladding thickness required to isolate two crossing waveguides was 200 nm for waveguide dimensions of 400 nm (w) by 400 nm (h). Therefore, the proposed interlayer slope waveguide can be used as a 3D vertical optical via for a multilayer SiP platform.

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ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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