Page 318 - Introduction to Information Optics

6.2. Polymer Waveguides 303

driving the lasers and the output electrical signal from the photodetector can
be realized using conventional vias common to all electrical interconnects at
the board level. It is clear from the schematic shown in Fig. 6.1 that the
achievement of planarization helps the stacking of other electrical interconnec-
tion layers on top of the optical layer while the full embedding of the
transmitters (lasers) and receivers (detectors) makes the packaging 100%
compatible with microelectronic packaging on the surface of the PC board. In
this case, only electrical signals exist on the surface of the PC board and
therefore significantly ease optoelectronic packaging problems. The fully em-
bedded structure makes the insertion of optoelectronic components into
microelectronic systems much more realistic, considering the fact that the
major stumbling block for implementing optical interconnections onto high-
performance microelectronics is packaging incompatibility.
The research findings of the necessary building blocks for the architecture
shown in Fig. 6.1, polyimide-based waveguides, waveguide couplers, high-speed
thin-film transmitters using VCSELs, and thin-film receivers operating at
850 nm are described sequentially in this chapter. A board-level 1-48 clock
signal distribution for the Cray-T-90 supercomputer board is demonstrated
with a speed of 6 Gbit/sec. Finally, a polymer-based optical bus structure is
illustrated that has a full compatibility with existing board-level IEEE stan-
dardized buses such as VME bus and FutureBus.

6.2. POLYMER WAVEGUIDES

6.2.1. POLYMER MATERIALS FOR WAVEGUIDE FABRICATION

To provide system integration using guided-wave optical interconnections,
polymer-based material has many exclusive advantages. It can be spin-coated
on a myriad of substrates with a relatively great interconnection distance.
Many organic polymers are attractive microelectronic and optoelectronic
materials with potential applications as interlayer dielectric, protective over-
coat, a-ray shielding, optical interconnects, or even conductive electrical
interconnects. In addition to their ease of processing, they possess favorable
electrical and mechanical properties such as high resistivity, low dielectric
constant, light weight, and flexibility. Table 6.1 compares the advantages of
polymer-based devices with that of inorganic materials such as GaAs and
LiNbO 3.
The required interconnection distance makes polymeric material, especially
polyimides, the best candidate for guided-wave applications. Inorganic ma-
terials such as LiNbO 3, GaAs, and InP are good for small-size optical circuits
(—cm). However, board-level optical interconnects require interconnection