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Strategies for Wave-guides on CMOS

OIC Strategies for Global Interconnect Upon CMOS

Dr Terry V Clapp, Dr Jon V DeGroot jnr & Dr Ann Norris

February 2004

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Intent

• I will show what is driving the requirement to consider optical communications at the chip level.

• I will show what challenges the CMOS platform offers.

• I will offer some materials, process and waveguide propositions for these applications.

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The Interconnect Environment

Local2

IntermediateUp to 4

GlobalUp to 5

Pre-metal dielectric

Tungstenplugs

Copper damasceneNucleation and barrier

Etch stop and dielectric cap

Passivation

VIA

Interconnect level & number of layers

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Design Window

• The design window appears to favour dielectrics as the preferred optical material.– Spin-on glassy dielectrics would give a low-k

and low temperature option.– Polymers either spun-on &/or printed might be

a preferable process option in view of topology and the need for electrical connections.• Polysiloxanes and perfluoropolymers both have a

demonstrated process compatibility and acceptance for deployment in CMOS foundries.

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PWG/OIC Efforts• Dow Corning working in collaboration with Gemfire and

others to develop low cost OIC fabrication process as well as prototypes via photo-patterning.

Multi-mode 100 x 100 micron

Early attempt at single-mode6 x 5 micron nominal dimensions

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UV-Vis Spectrum of DC OE4100through 1 cm pathlength

0

1

2

3

4

5

200 400 600 800 1000

wavelength, nm

Ab

so

rpti

on

Absorption <0.02dB/cm at 850 nm

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Spot Size / Confinement

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Relative versus Absolute size• Say 75um pitch for the global

interconnect, – at high bandwidth cross-talk is

likely to be an issue,– very high speed modulation

will become critical in terms of routing and power.

• Optical @<20um pitch for wave-guides, if,– you work in the visible– you can provide sources

and receivers… in fact CW lasers off-chip with on-chip modulators seems a better architecture.

• For square SM buried rib wave-guides,– the core-cladding index

difference determines mode confinement up to the diffraction limit,

– the higher the difference,• the more difficult it is to make

the guide low loss,• the tighter the bends you can

have,

– at shorter wavelengths,• the core diameter is smaller,• the cladding thickness can be

reduced.

• Are mirrors vs bends the answer?

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Conclusions

• It seems clear that the optical penetration in the communications infrastructure will penetrate right down to the chip level.

• The demands of that environment seem to favour the adoption of technology in the visible wavebands.

• Materials are available but much research needs to be done to establish the boundary design rules & demonstrate the technology.

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Acknowledgement

The contribution of my colleagues at Dow Corning is gratefully acknowledged