Optical interconnects for system in package by MEMS technology

Abstract

This dissertation presents a novel prototype optical interconnect system achieved by MEMS technology for stacked silicon dies. The overall approach is drawn from concepts on 3D-TSV metal interconnect and introducing an optical interconnect. Optical interconnect is a promising solution for future global interconnect inside a chip package consisting of a stack of multiple dies. The design allows an optical beam to propagate both in plane and out-of-plane, hence establishing optical communication between stacked dies.

The entire system includes a pair of 45 micromirrors with one in an upward-facing (front) position and the other in downward-facing (rear) position. Fiber grooves were fabricated on each dies for the purpose of optical testing. Passive alignment structures have been integrated in the system to achieve accurate horizontal alignment. The research work have also proposed a planar silica graded index lens integrated 3D optical interconnect system. A pair of planar silica graded index lens is designed to sit in front of the 45 micromirror pair to mitigate the free space propagation loss. Optical beam free space spot sizes from these planar lenses are commensurate with the micromirror dimensions.

To fabricate high quality ultra-smooth 45 micromirror on the (100) monocrystalline silicon surface by anisotropic wet etching, surfactant added TMAH etching behavior was first investigated. Non-ionic surfactants NCW-601A, Triton X-100, NCW-1002 with both high concentration (25%) and low concentration (10%) TMAH solutions was extensively studied for their etching properties on (100) and (110) silicon surfaces. Etch rate, selectivity and surface roughness were studied. Etchant temperature was also varied from 60C to 90C to see the effects on etching behaviour. This research is the first reported use of a new surfactant NCW-1002 in low concentration TMAH. Better selectivity and surface roughness of the (110) plane (i.e. micromirror surface) can be achieved with this surfactant added low concentration TMAH at 60C.

In view of the intrinsic partial isotropic etching behavior in the vicinity of the (100) plane for low concentration TMAH with high concentration surfactant, this work has established and demonstrated for the first time in published literature that only a limited portion of the micromirror is truly 45 and is located approximately at the middle of the slope. To reduce the top curved portion, two new techniques have been developed. One technique involves timed HF dip treatments during the etching process to remove the overhanging oxide, which has impeded the etching at the top part of the slope due to the diffusion-dependant effect. The other technique involves a maskless etching. The entire top curved slope can thus be straightened out. To avoid the bottom curved portion, another technique was developed to allow a deeper etch at the lower end of the micromirror to recess the non-45 lower portion of the slope. Both these techniques can nearly double the size of the truly 45 portion of the micromirror.

Based on the framework of a front upward-facing 45 micromirror, a processing technique was developed to fabricate a novel rear downward-facing 45 micromirror using electro-chemical TMAH release. With the assistance of passive alignment structures, optical test was conducted to confirm successful optical interconnect between stacked dies using optical fiber. The loss was measured to be 14.7dB, among which free space transmission loss is approximately 10dB. To reduce the free space propagation loss, a pair of planar silica graded index lens integrated optical interconnect system was then proposed. Fabrication of the planar silica graded index lens with front 45 micromirror is confirmed in this research work.