Hybrid Integration of InGaN Lasers in a Foundry-Fabricated Visible-Light Photonics Platform

TL;DR

This work demonstrates scalable integration of blue 450-nm InGaN laser diodes onto a foundry-fabricated visible-light Si photonics platform using passive-alignment flip-chip bonding, achieving a record on-chip power of 60.7 mW and on-chip wall-plug efficiency up to 7.8% while maintaining operation up to 80 °C. The approach relies on lithographically defined LD sockets, precise in-plane alignment, and a heat-assisted bonding process that enables independent placement of multiple lasers per chip. A proof-of-concept active PIC showcases laser-to-waveguide coupling, on-chip photodetection for power monitoring, and a thermo-optic switch for routing and variable attenuation, highlighting the platform’s potential for scalable arrays of visible-wavelength lasers. The study also analyzes alignment tolerances, bonding-induced variations, and epoxy underfill strategies, outlining clear paths to higher yield and broader applicability in AR/VR displays, quantum photonics, biosensing, and underwater communications.

Abstract

Visible-spectrum photonic integrated circuits (PICs) present compact and scalable solutions for emerging technologies including quantum computing, biosensing, and virtual/augmented reality. Realizing their full potential requires the development of scalable visible-light-source integration methods compatible with high-volume manufacturing and capable of delivering high optical coupling efficiencies. Here, we demonstrate passive-alignment flip-chip bonding of 450-nm InGaN laser diodes onto a foundry-fabricated visible-light silicon (Si) photonics platform with silicon nitride (SiN) waveguides, thermo-optic (TO) devices, and photodetectors. Hybrid laser integration is realized using a sub-micron-precision die bonder equipped with a vision alignment system and a heatable pickup tool, allowing independent placement of multiple lasers onto a single Si chip. Co-design of the lasers and Si photonics, with lithographically defined alignment marks and mechanical stoppers, enables precise postbonding alignment. Efficient optical coupling between lasers and the SiN waveguides is demonstrated, with a minimum measured coupling loss of 1.1 dB. We achieve a maximum on-chip optical power of 60.7 mW and an on-chip wall-plug efficiency of 7.8%, the highest reported for hybrid-integrated visible-spectrum lasers, to our knowledge. An active PIC is also shown, integrating a bonded laser, an on-chip photodetector for power monitoring, and a thermo-optic switch for optical routing and variable attenuation. Overall, this work highlights passive-alignment flip-chip bonding as a practical, high-performance approach for integrating lasers onto visible-spectrum PICs. We envision that continued refinement of this technique within our photonics platform will support increasingly complex PICs with integrated lasers spanning the visible spectrum.

Figures (15)

• Figure 1: Hybrid integration of InGaN laser diodes (LDs) in a visible-light Si photonics platform. (a) Cross-section of the photonics platform. PD: photodetector, M: metal, SiN: silicon nitride, TO switch: thermo-optic switch, TiN: titanium nitride, UBM: under-bump metallization. (b) (Top) Optical micrograph of an LD bonding socket on a Si photonic chip, and (bottom) confocal laser scanning micrograph of the socket with improved visibility of the SiN waveguide. (c) Scanning electron micrograph of the top surface of an InGaN LD before singulation. (d) Conceptual illustration of the flip-chip LD bonding method, with an LD close to the socket. The definition of the coordinate system and rotation axes is given.
• Figure 2: Flip-chip bonding technique. (a) Photograph of the die bonder with a vision alignment system, side-view microscope, and heatable pickup tool. Inset: overlaid view from the vision system of an LD and a bonding socket prior to bonding. (b) Micrograph of an LD held by the pickup tool approaching a Si photonic chip; the image was captured with a second side-view microscope not shown in (a). (c) Photograph of the pickup tool above a Si photonic chip. (d) Micrograph of three flip-chip bonded LDs on one Si photonic chip. (e) Micrograph of a bonded LD with emitted light coupled to a SiN waveguide.
• Figure 3: Simulated laser-to-waveguide coupling. (a) Optical mode profiles at the laser facet and SiN coupler facet (both inverse taper and taper design); $\lambda =$ 450 nm, transverse-electric (TE) polarization. Simulated in-plane misalignment tolerance of (b) inverse taper and (c) taper couplers. Insets: Illustrations of the LD-to-waveguide coupling (top-down view).
• Figure 4: Characterization of the flip-chip bonding process. (a) Confocal laser scanning micrograph of a representative flip-chip bonded LD (left) with an enlarged view of one set of alignment marks (right). Cross: Si photonic chip mark; squares: LD mark; both visible through the LD. Micrograph quality was limited by imaging through the $\approx$100-$\upmu$m-thick LD substrate. (b) On-chip optical power distribution of flip-chip bonded LDs (at 50-mA drive current) with inverse taper and taper laser-to-waveguide couplers; n=20 samples for each coupler design. Histograms of in-plane misalignment after flip-chip bonding along the (c) X- and (d) Y-axis; n=40, bin size=0.5 $\upmu$m, both coupler designs included.
• Figure 5: Characterization of a flip-chip-bonded laser diode showing the highest on-chip optical power achieved in this work. (a) L-I-V curves measured in continuous-wave (CW) mode. L: On-chip optical power; I: drive current; V: voltage drop. Device failure occurred at a drive current of 185 mA. (b) Optical spectra at different laser drive currents (normalized to the peak power at 80 mA current).