Novel Optical Bumps using Graded Index Couplers for Flip-Chip Photonic Packaging

A graded index optical coupler was designed to connect silicon oxynitride photonic waveguides on separate chips, or between a chip and an interposer, demonstrating a simulated coupling loss of 0.37 dB and 0.42 dB at 1550 nm and 1310 nm wavelength, respectively, for an 11 𝜇m vertical chip-to-chip gap. A robust 1-dB lateral and vertical alignment tolerance of ± 2.66 𝜇m and 2.45 𝜇m, respectively, at 1550 nm wavelength were also demonstrated, allowing for high speed, automated pick-and-place die bonders to be used for passive assembly of photonic chips. In addition, the same coupling design was simultaneously used for fiber-to-chip coupling, showing the potential of the coupler to act as a universal photonic packaging interface. When simulating coupling to a standard flat-faceted SMF-28 fiber at 1550 nm and 1310 nm wavelength,coupling losses of 0.62 dB and 0.46 dB, respectively, were observed. The fiber-to-chip coupling simulations also demonstrated wide 1-dB alignment tolerances of ± 2.88 𝜇m laterally and ± 2.58 𝜇m vertically at 1550 nm wavelength. The device was also ultra-broadband, demonstrating a simulated 1-dB wavelength tolerance of over 1400 nm for both fiber-to-chip and flip-chip optical coupling, showing low loss across nearly the entire near infrared spectrum. Alongside simulations, CMOS foundry compatible fabrication processes were established to deposit, pattern, and etch greater than 10 𝜇m thick silicon oxynitride graded index lenses on silicon and glass substrates. The simulation and fabrication results, combined with a relatively compact footprint allowing for pitches down to 20 𝜇m, may enable Petabyte per second optical I/O - a critical need for data center co-packaged optics based Ethernet switch systems within the next decade.