Abstract
We present a rigorous electrical and optical analysis for a metal-oxide-semiconductor
(MOS)–capacitor microring optical modulator on silicon-on-insulator
(SOI) with a low-mobility and high optical loss polysilicon gate that could
be fabricated by the complementary metal-oxide-semiconductor (CMOS)-compatible
solid-phase-crystallization (SPC) process. Critical coupling was designed
for the 25.5 $\mu$m radius rib-waveguide microring. Modulation speed, operating power
at 3.3 V operating voltage, and insertion loss (IL) were analyzed with respect
to the doping level of the SPC p-polysilicon gate and the n-crystalline silicon
channel. $4.6\times 10^{-2}{\hbox {pJ}}/{\mu {\hbox {m}}}^{2}$ operating power per switch
can be achieved with 74 GHz modulation speed for $3\times 10^{18}\ {\hbox {cm}}^{-3}$ doping
level in both the SPC p-polysilicon gate and the n-crystalline silicon channel.
For 40 GHz operation, 10–12 dB IL is achievable with the SPC polysilicon,
and 9 dB IL is achievable with the well-annealed polysilicon that is lossless.
Tolerance of the critical coupling to the gap width variation, temperature
drifting of the microring, and wavelength drifting of the light source were
analyzed and discussed extensively, and the extinction ratio (ER) was estimated
for various situations. The 90 nm CMOS fabrication specification, if applied
to the gap width variation of the microring, would leave a large margin in
the ER to other sources of the critical coupling deviation. We found that $\sim\pm 0.2^{\circ}{\rm C}$ temperature stability for the microring and the light source is
required for a minimum ER of 5 dB if temperatures of both elements are controlled
independently. Comparison between the MOS–capacitor modulators with
microring and with Mach–Zehnder types was analyzed and discussed. In
contrast to the Mach–Zehnder modulator, the modulation speed of the
microring can be pushed up by increasing the doping level up to $\sim$1$\times 10^{18}\ {\hbox {cm}}^{-3}$ without
significantly increasing the IL.
© 2009 IEEE
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