56-Gb/s 3D-integrated silicon optical transmitter based on micro-transfer printing.

We present an optical transmitter consisting of a SiGe BiCMOS driver amplifier heterogeneously integrated through micro-transfer printing onto a silicon photonic chip that contains a Mach-Zehnder modulator (MZM) and optical couplers. The driver has a footprint of only 200 µm × 300 µm, and features small bond pads of 20 µm × 20 µm. It was fabricated using a modified SiGe BiCMOS on SOI process to enable the release process prior to micro-transfer printing. The post-print metallization process to contact the EIC with the PIC consists of spin coating a layer of divinylsiloxane-bis-benzocyclobutene (DVS-BCB) to create a ramp to overcome the 20 µm step height, after which a gold metal layer is deposited to connect the driver to the MZM. To verify the operation of the optical transmitter, a high-speed measurement setup is used to capture eye diagrams. The 3D-integrated transmitter achieves 56 Gb∕s non-return to zero (NRZ) with a 5-tap feed-forward equalizer (FFE) and 28 GBd 4-level pulse amplitude modulation (PAM-4) with a 5-tap FFE and nonlinearity pre-compensation, both at a total transmitter power efficiency of 9.1 pJ∕bit. Integration through micro-transfer printing offers the possibility of greatly increasing scalability and fabrication throughput of short-reach optical transceivers for data centers.