Optical interconnect for integrated circuits

Abstract

This thesis presents the research pertaining to the development of essential components of optical interconnect between dies in a package, involving both guided and free-space propagation of light. In order to pursue such an objective, it required the development of a simpler approach to the design of planar silica lens pairs; develop the technology for fabricating such lens pairs, and modeling the critical factors, like alignment non-idealities, that affect the optical loss of such a scheme involving both guided and free-space propagation.

A methodology based on the ABCD matrix method has been developed to design and evaluates the performance of a planar silica lens pair system for a prescribed (‘ideal’) free-space propagation distance. The optical loss of a designed system under various fabrication and experimental imperfections has been calculated and verified against the simulation results obtained from the commercial beam propagation method (BPM) software, BPM_CAD by Optiwave.

A two-level optical system comprising of a planar silica lens pair and a pair of 45° micromirror, which is equivalent to a chip to chip optical interconnects in a 3D integrated system, has been theoretically analysed for optical loss due to micromirrors deviation from the ideal 45° and an angular tilt between the two levels.

For the implementation of the planar silica lens pair, a hollow cathode PECVD system was used to deposit low stress thick graded index silica film on silicon wafer from a mixture of O2/SiH4/CF4 gases. Technique of depositing low stress thick fluorine doped silica film was developed and films up to 38 µm thickness with very low compressive stress (16 Mpa) were deposited on silicon substrate. Lens front-face curvature was defined by vertical deep oxide etch using a state of art STS–ICP Advanced Oxide Etch (AOE) system.

The planar silica lens pair designed for 200 and 500 µm of ‘ideal’ free-space propagation distance were fabricated and optically tested. A successful implementation of such a scheme, involving guided and free-space optical propagation has been demonstrated for the first time.

Practical demonstration and optical characterization of in-plane chip to chip optical interconnects has been performed, however, integration of 45° micromirror and practical demonstration of stacked-die optical interconnect based on planar silica lens pair has been left for future work.