Implementation of a Photonic Page Buffer Based on GaAs MQW Modulators Bonded Directly over Active Silicon VLSI Circuits

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Abstract
The tremendous progress in high performance Very-Large Scale Integrated circuit (VLSI) technology has made possible the incorporation of several million transistors onto a single silicon chip with on-chip clock rates of 200 MegaHertz (MHz). By the end of decade, the integration density for silicon Complementary Metal Oxide Semiconductor (CMOS) is expected to be over 20 million transistors and the projected on-chip clock rate is 500 MHz. This enormous bandwidth that will be available for computation and switching on a silicon integrated circuit will create a huge bottleneck for Input and Output (I/O) to the VLSI circuit. Technologies that are being developed at AT&T Bell Laboratories, now exist for attaching GaAs Multiple Quantum Well (MQW) photodetectors and light-modulators onto a prefabricated silicon integrated circuit using a well-established hybrid flip-chip bonding technique followed by substrate removal of the GaAs chip to allow surface-normal operation of the optical modulators at 850nm [1]. From a systems point of view, the demands made of optoelectronic integration method are (i) that the silicon integrated circuit be state-of-the-art, (ii) the circuit be unaffected by the integration process, (iii) that the design and optimization of the circuit proceed independently of the placement and bonding to the optical I/O. The first two goals have been achieved in reference 1, and this technique has been effectively applied to simple switching nodes for a smart-pixel based photonic switch in reference 2. In this paper we further achieve the third goal by demonstrating for the first time that modulators can be bonded directly above active submicron CMOS transistors (figure 1), and by applying the technique to the demonstration of a high-density 2Kbit first-in first-out (Fifo) page buffer circuit.

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