- The paper achieves record-low insertion loss (<0.002 dB) and near-zero crosstalk (<−62 dB) using an innovative 3D integration process.
- The paper employs CMP-enabled adiabatic transitions in Ta2O5-on-LNOI waveguides, eliminating intermediate layers and reducing fabrication complexity.
- The paper demonstrates scalable performance with 300 cascaded crossings, validating its potential for high-density photonic integrated circuits.
Problem Context and Motivation
Scaling photonic integrated circuits (PICs) to the level of very-large-scale photonic integration (VLSPI) demands interconnect technologies capable of supporting dense, low-loss, and low-crosstalk crossings. Conventional in-plane multimode interference (MMI) crossings, though fabrication-robust, typically offer insertion loss and crosstalk (∼0.1 dB, −50 dB) inadequate for hundreds or thousands of cascaded crossings. This aggregate penalty compromises the realization of broadband, high-density on-chip photonic functionalities in advanced platforms like thin-film lithium niobate (TFLN). Three-dimensional vertical crossing schemes, which spatially separate optical modes using multiple planes, have been proposed to alleviate these limitations but incur their own fundamental trade-off: increasing the vertical gap suppresses crosstalk and crossing loss yet degrades upward coupling efficiency, imposing constraints on device design and integration density.
Proposed Fabrication Approach
This work introduces a scalable and cost-effective process for fabricating 3D waveguide crossings utilizing Ta₂O₅-on-LNOI, exploiting chemical mechanical polishing (CMP) to achieve adiabatic, low-loss vertical waveguide transitions. The process involves:
- Patterning bottom-layer TFLN waveguides via PLACE (photolithography-assisted chemo-mechanical etching).
- Selective etching of the TFLN ends and subsequent 3 μm SiO₂ deposition/etching to define regions for vertical routing.
- Controlled Ti hardmask formation and SiO₂ dry etch (1.5 μm).
- CMP to round the SiO₂ interface, yielding tapered, adiabatic transitions for minimized scattering.
- Ta₂O₅ top-layer deposition (300 nm), waveguide patterning, and etching.
Crucially, the CMP-induced edge rounding ensures smooth refractive index variation across vertical bends, addressing losses associated with abrupt transitions in multilayer photonic platforms. No intermediate waveguide layer is required, reducing process complexity and cost.
Test structures consisting of 20 upper Ta₂O₅ waveguides (pitch 127 μm) each vertically coupling to, and crossing, 300 lower LN waveguides (10 μm pitch) were fabricated. Key performance metrics are:
- Insertion Loss: Measured loss per crossing is consistently below 0.002 dB across the wavelength range 1510–1630 nm.
- Crosstalk: Crosstalk at the crossing is less than −62 dB throughout the same band.
- Integration Density: Up to 300 cascaded crossings demonstrated with preserved performance, confirming the scalability and suitability for VLSPI.
These results represent an advancement both over legacy MMI-based crossings (typically >0.07 dB loss, −31 to −50 dB crosstalk) and alternative multilayer schemes (which often require multi-step coupling or intermediate layers and present more substantial process challenges).
Theoretical and Practical Implications
The process achieves its low-loss and low-crosstalk regime due to the adiabatic vertical coupling geometry and physical layer separation, mitigating both radiative loss and modal crosstalk. The Ta₂O₅-on-LNOI material configuration offers:
- CMOS Process Compatibility: All steps employ standard semiconductor foundry techniques, facilitating industrial adoption.
- Broadband Operation: The coupling structure exhibits wide tolerance in both operational wavelength and interlayer gap, extending practical device bandwidth and manufacturing robustness.
- Elimination of Intermediate Layers: By removing the need for a third, intermediate coupling layer, a major source of excess process complexity and transition loss is circumvented.
- Scalability: Experimental validation of 300 cascaded crossings with negligible aggregate penalty directly enables scalable neural network, switching, or signal distribution architectures in the photonic domain.
On the theoretical side, the technique demonstrates that the traditional trade-off between vertical gap (for crosstalk suppression) and coupling efficiency (for loss minimization) can be simultaneously addressed through precise adiabatic geometries enabled by CMP, suggesting broader application across hybrid photonic integration platforms.
Future Directions
This method paves the way for high-density photonic switching fabrics, on-chip optical interconnects, and deep integration of photonic neural networks on LNOI substrates. Potential areas for advancement include:
- Integration of active TFLN functionalities (modulators, switches, nonlinear elements) with multi-layer routing architectures.
- Waferscale implementation and automation for production-scale photonic processors.
- Fundamental exploration of adiabatic vertical coupling limits in other material systems or with advanced geometries for further loss and crosstalk reductions.
Conclusion
The paper presents a robust, industry-compatible method for fabricating 3D waveguide crossings on Ta₂O₅-on-LNOI platforms, achieving record-low insertion loss (<0.002 dB) and near-zero crosstalk (<−62 dB) over a broad spectral range, without relying on intermediate layers or complex process flows. This methodology addresses critical bottlenecks in the path toward practical VLSPI and is poised to accelerate the integration density and bandwidth of future photonic and hybrid quantum-classical information processing systems.