Page 363 - System on Package_ Miniaturization of the Entire System

Integrated Chip-to-Chip Optoelectr onic SOP 335

160 mW per channel. Therefore, a single optical channel dissipates 280 mW, or about
2.5 times the power dissipated for a single equalized copper differential channel.
However, while the differential copper channel can compensate for transmission line
losses up to 0.75 m on FR4 [28], the only distance limitation for the optical channel is
that dictated by the optical absorption of the polymer used to form the waveguide
channel, and distances of many meters can easily be optically linked and at bit rates that
are higher than 10 Gb/s. Depending on the application, the optical alternative may be
preferred. In the case of flexible, interboard interconnects, the optical alternative may
offer much higher channel density and flexibility over impedance-matched coaxial
cable or individual optical fibers or even optical fiber-base transponders [25].
6.4.4 Reliability

1310-nm EELs and 850-nm VCSELs
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Failure in a population of lasers or chips is measured as failures in time (FIT) over 10 of
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device service hours. Thus a FIT of 10 signifies 10 devices have failed in 10 hours. The
rate of failures need not be monotonic. There are numerous reports regarding the
reliability of AlGaInAs-based transmitter lasers operating at 1310 nm [34–35]. Generally
the failure rate increases monotonically with time of use [36].
In [34], the mean-time-to-failure, or median life, is estimated at 82,000 hours
(9.4 years) at 85°C, which is significantly less than Telcordia standards but sufficient for
board-level integration of less than roughly 100 lasers. Wearout failure rates at 40°C after
5, 10, and 20 years of service can be calculated using the same lognormal model. A FIT
value of 11—that is 11 failures per billion device hours—is calculated for lasers with 5 years
of service. Corresponding FIT values after 10 and 20 years are 29 and 60, respectively.
These FIT numbers show that the lasers have sufficient long-term reliability for use in the
applications described here. The reliability, allied with the attractive price of FP lasers,
makes these devices competitive in short-haul 10-Gb/s communications.
The failure rate for 850 nm, 2.5-Gb/s VCSELs is more complicated and displays a
rapid increase in FITs after 5 years of service. To date, 10-Gb/s VCSELs have a lower
yield, higher failure rates, and a higher price than 10-Gb/s, 1310-nm EELS [37].
CMOS Processors
In comparison, CMOS processors display the typical bathtub failure rate distribution
in time. In the infant mortality region where manufacturing defects can cause rapid
failure during burn-in, the goal is to reach 200 FITs during the first 3 months. Ideally
the failure rate should remain at 200 FITs for up to 7 years of service. Thereafter the
onset of the wearout region depends on diffusion processes in older CMOS technology
and on gate oxide hot electron wearout in later, thin oxide technologies, starting at the
100-nm node [38].

Optical Polymers
Reliability data on polymer waveguides has not risen to the same level as that of lasers.
This is clearly because the polymer materials that have been developed for lightwave
circuits (see Section VI), generically polycarbonates, acrylates, polyimides, olefins,
polymethylmethacrylates (PMMAs), polycyanurates, siloxanes, organically modified
ceramics (ORMOCERs), and BCB, have not had the field exposure and testing necessary
to assess their reliability for high-end server application wherein component reliability
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is measured in terms of 10 hours of service. PMMA-based fibers are in service in the