Current trends in electronic industry, such as Internet of Things (IoT) and Cloud Computing call for high interconnect
bandwidth, increased number of active devices and high IO count. Hence the integration of on silicon optical waveguides
becomes an alternative approach to cope with the performance demands. The application and fabrication of horizontal
(planar) and vertical (Through Silicon Vias - TSVs) optical waveguides are discussed here. Coupling elements are used to
connect both waveguide structures. Two micro-structuring technologies for integration of coupling elements are
investigated: μ-mirror fabrication by nanoimprint (i) and dicing technique (ii).
Nanoimprint technology creates highly precise horizontal waveguides with polymer (refractive index nC = 1.56 at 650 nm)
as core. The waveguide ends in reflecting facets aligned to the optical TSVs. To achieve Total Internal Reflection (TIR),
SiO2 (nCl = 1.46) is used as cladding. TSVs (diameter 20-40μm in 200-380μm interposer) are realized by BOSCH process<sup>1</sup>,
oxidation and SU-8 filling techniques. To carry out the imprint, first a silicon structure is etched using a special plasma
etching process. A polymer stamp is then created from the silicon template. Using this polymer stamp, SU-8 is imprinted
aligned to vertical TSVs over Si surface.Waveguide dicing is presented as a second technology to create coupling elements
on polymer waveguides. The reflecting mirror is created by 45° V-shaped dicing blade.
The goal of this work is to develop coupling elements to aid 3D optical interconnect network on silicon interposer, to
facilitate the realization of the emerging technologies for the upcoming years.
Optical connectivity has the potential to outperform copper-based TSVs in terms of bandwidth at the cost of more complexity due to the required electro-optical and opto-electrical conversion. The continuously increasing demand for higher bandwidth pushes the breakeven point for a profitable operation to shorter distances. To integrate an optical communication network in a 3D-chip-stack optical through-silicon vertical VIAs (TSV) are required. While the necessary effort for the electrical/optical and vice versa conversion makes it hard to envision an on-chip optical interconnect, a chip-to-chip optical link appears practicable. In general, the interposer offers the potential advantage to realize electro-optical transceivers on affordable expense by specific, but not necessarily CMOS technology. We investigated the realization and characterization of optical interconnects as a polymer based waveguide in high aspect ratio (HAR) TSVs proved on waferlevel.<p> </p> To guide the optical field inside a TSV as optical-waveguide or fiber, its core has to have a higher refractive index than the surrounding material. Comparing different material / technology options it turned out that thermal grown silicon dioxide (SiO<sub>2</sub>) is a perfect candidate for the cladding (n<sub>SiO2</sub> = 1.4525 at 850 nm). In combination with SiO<sub>2</sub> as the adjacent polymer layer, the negative resist SU-8 is very well suited as waveguide material (n<sub>SU-8</sub> = 1.56) for the core. Here, we present the fabrication of an optical polymer based multimode waveguide in TSVs proved on waferlevel using SU-8 as core and SiO<sub>2</sub> as cladding. The process resulted in a defect-free filling of waveguide TSVs with SU-8 core and SiO<sub>2</sub> cladding up to aspect ratio (AR) 20:1 and losses less than 3 dB.
In future, computing platforms will invoke massive parallelism by using a huge number of processing elements. These elements need broadband interconnects to communicate with each other. Following More-than-Moore concepts, soon large numbers of processors will be arranged in 3D chip-stacks. This trend to stack multiple dies produces a demand for high-speed intraconnects (within the 3D stack) which enable an efficient operation. Besides wireless electronic solutions (inductive or capacitive as well as using antennas), optical connectivity is an option for bit rates up to the Tbit/s range, too. We investigated different candidates for optical TSVs. For optical transmission via optical Through-Silicon-Vias, we were able to demonstrate negligible losses and dispersion.