Page 353 - System on Package_ Miniaturization of the Entire System

Integrated Chip-to-Chip Optoelectr onic SOP 327

about 0.3 dB/cm between 1300 and 1600 nm, limited by the onset of O—H,
C—H, and C O stretch vibration overtones [10].
2. The glass transition temperature of core and cladding materials is desirable to
be above 300ºC, in which case polymer decomposition should occur by
sublimation above a temperature of 350ºC in order to maintain the waveguide
shape and compositional integrity over time, stress, and temperature.
3. A low bulk modulus is desirable in order to prevent cracking in flexible
waveguides; however, the polymer should have a flow rate that is slow
compared to time periods of years.
4. The monomer should be a liquid in the absence of solvents, and polymerization
should be initiated by a UV-activated catalyst. This is desirable from the point
of view of processing efficiency but is not essential.
5. A viscosity between 1000 and 10,000 cP may be desirable from the point of view
of a monomer dispensing on a substrate by spin coating or monomer extrusion to
obtain a 30- to 50-μm core and top cladding to adequately cover the core. A different,
lower viscosity polymer may be used as undercladding prior to core formation.
–4
6. The strain-induced birefringence of the core polymer should be less than ~10 ,
and the thermal optic coefficient should be less than ~10 /ºC.
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7. The coefficient of thermal expansion (CTE) of both the core and cladding
polymer should match that of the substrate, if possible, but should be no greater
than 150 ppm/ºC when the substrate is rigid and has a CTE of about 20 ppm/ºC.
Flexible substrates such as polyimides may be able to accommodate a larger
mismatch depending on the substrate thickness.
8. The difference in the index of refraction between the core and cladding should
be tunable over a large range without notable attenuation. A combination of
high contrast is desirable for high confinement and lower losses in short radius
turns. A practical range in the index of refraction of organic polymer waveguides
is between 1.47 and 1.55 at 1310 nm.
9. From the point of view of efficient processing, no adhesion promoter should be
necessary.
10. Other desirable attributes of the polymer waveguide are low moisture content
to minimize O—H attenuation, inexpensive base chemicals, long shelf life, and
environmental friendliness.
Numerous polymer materials have been investigated over the years for application as
low-loss optical waveguide material. With the exception of laminated glass waveguides [9],
all polymers thus far investigated have a generic near-IR absorption spectrum that can be
generically represented by the spectral absorption shown in Figure 6.3 in the wavelength
range of interest. While, in general, optical absorption in the best polymers is 10 greater (in
5
decibel units) than in the best optical fibers, the best reported organic optical polymers are
those with normalized absorption in the following ranges: 0.02- to 0.05-dB/cm loss in the
850- to 980-nm band, 0.2 to 0.3 dB/cm near 1310 nm, and 0.4 to 0.5 dB/cm near 1550 nm.
Broad categories of optically suitable polymers are polyimides, olefins [11],
polycarbonates [12], polymethylmethacrylates, polycyanurates, siloxanes, ORMOCERs,
benzocyclobutanes (BCB), and fluorinated versions of these. See, for example, [13]. For
a recent comprehensive review of polymer properties, see [10,14].