A 10GHz Hybrid OpticalElectrical Clock

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A 10GHz Hybrid Optical/Electrical Clock
Distribution Network for Gigascale Integration
Anthony V. Mule, Stephen M. Schultz Thomas K.
Gaylord , and James D. Meindl
Wednesday, Nov. 10th, 1999
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Outline

• 1) Proposed system concept
• System constraints for GHz ?P clock network
• Proposed solution Optical Clock Distribution
  Network (OCDN)
• System schematic / Optical properties
• Key component Focusing grating coupler
• 2) Components of optical power budget
• Minimum fanout of distribution
• Optical power required by receivers
• Optical power available for detection
• 3) Conclusions

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System Constraints

• Given
• 1) ITRS projections for 50nm generation
• Area of 750mm2 (ASIC)
• 1.4 billion transistors
• flocal 10GHz
• Vdd 0.55 V
• Maximum power density of 50 W/cm2
• 2) Architectural projections of single chip
  multiprocessor for generations beyond 100nm

Question How to clock the system?
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Proposed Solution
Optical Clock Distribution Network (OCDN)

• No on-chip photonic sources
• Monolithic silicon-based detection
• Board-level, guided-wave, global propagation
  of local clock frequency
• Surface-relief focusing grating couplers to
  overcome misalignment of flip-chip assembly

? Design for Manufacturability
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System Concept
GSI Chip
Detector
twiring
Package
tPackage
Waveguide
Focusing Grating Coupler
tg
tsolder
tw
Printed Wiring Board
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Key Optical System Properties
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Focusing Grating Couplers
Focal spot size under non-ideal wavelength,
spatial variations
Grating-to-chip optical path trace to
estimate total output coupling into preferential
order
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Optical Power Budget
Three components 1) Minimum fanout of
distribution to reach all nodes operating at
local clock frequency, flocal,and maximum
power density of 50 W/cm2 FOmin 2)
Incident optical power required by receivers
to operate at BER of 10E-15, bit rate of
flocal (Gb/s ) Prec 3) Amount of optical power
available at the output of an m-level H-tree
distribution Pout
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Minimum Fanout of Distribution FOmin
Given
Ngatesx106 (total) 177.5 Ngatesx103 (node)
106.3 Nodes, total 1024

a) Model for ave. wire length, Lavg
b)
c)
d) Asymptotic limit Cw .4CL
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Receiver Sensitivity1 Prec
BER 10-15RZ Bit Rate 10 Gb/s 50nm
Technology ? Prec 16.5?W
(1)
(2)
1 J.J. Morikuni et. al, Improvements to the
standard theory for photoreceiver noise. J.
Lightwave Tech., vol.12, pp.1174-1183, July 1994.
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Output Power Pout
a) b) c) d) e) f)
a) Y-junction TE b) Laser-waveguide CE c) Grating
CE d) Air/package TE e) Arc bending loss f)
Propagation loss
TE Transmission Efficiency CE Coupling
Efficiency
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Receiver Sensitivity1 (Prec) and Output Power
(Pout)
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Conclusions

• Approximately 1W of optical input power will
  be required to clock 10GHz system
• Main sources of optical loss
• Y-junction scattering loss
• Grating coupler loss
• Laser-waveguide coupling loss
• Overcome bandwidth limitations of global
  electrical interconnects at cost of high optical
  input power

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Related Research Efforts

• J.W. Goodman et. al. (Stanford), Optical
  interconnections for VLSI systems. Proceedings
  of the IEEE vol.72, no.7, p.850-66, 1984.
• L.C. Kimerling, et al. (MIT), Materials for
  monolithic silicon microphotonics. Materials
  and Devices for Silicon- Based Optoelectronics
  Symposium pp.45-56, 1998.
• R.T.Chen et.al (UT Austin), Optical clock
  distribution in supercomputers using
  polyimide-based waveguides. Proceedings of
  the SPIE - The International Society for
  Optical Engineering vol.3632 p.123-33, 1999.

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Focusing Grating Couplers