Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Plasmonics: Novel On-Chip Interconnections
Abstract— We report antenna designs for surface plasmon polariton coupling to metal-insulator-metal waveguides. The
resulting plasmonic modes were tuned for maximum propagation length along the waveguide: for 1550 nm, we have observed
more than 40 micron propagation (along the length of the waveguide, +x direction) while for 532 nm this is below 10 micron.
Our results lay the foundations for on-chip coupler-waveguide-photodetector interconnect technology.
State-of-the-art on-chip and chip-to-chip technologies rely
on copper interconnects. Ever increasing data transfer rates
and Moore’s Law dictate smaller devices packed into the same
area [1]. Besides, International Technology Roadmap for
Semiconductor Industry reports project devices that process
and transmit data faster. Interconnections, not chips, have
become the limiting factor for the future of scaling. Copper
interconnects cannot meet the demanding data transfer rate
requirements and have significant bandwidth limitations due
to RC time delays, resistive losses, and frequency dependent
cross-talk at high modulation frequencies. In addition to these
limitations, electronic information transmission on a chip also
suffer from impedance mismatch. On the device side, there is
a high impedance and low capacitance, while transmission
lines have low impedance and high capacitance. This
mismatch limits the power transfer, even for the optimum
cases [2-4].
The incident light’s polarization significantly changes the
coupling efficiency. Because antennas couple SPP’s to the
waveguides in the near-field, the distance between the antenna
and the waveguide was kept smaller than 50 nm. The antenna
should not be connected to the waveguide either; otherwise
interference of plasmonic modes reduces propagation length.
Antenna and waveguide parameters were swept and the field
profiles were compared for metal layer thicknesses of 100,
150 and 200 nm. The longest propagation length has been
achieved for 200 nm, while above 200 nm, the propagation
length is no longer enhanced by changing the thickness.
Thinner arms (60nm) tend to propagate SPP’s over longer
distances. Reducing antenna gap enhanced local field between
the arms, but did not increase the propagation length.
Surface plasmon polaritons (SPP) are collective electron
oscillations along the interface of metal and a dielectric.
Because of the fast decaying fields both inside the metal and
the dielectric, the mode field profile is highly confined along
the interface. That is why; SPP’s can be a solution for
achieving high integration densities, due to the small
attenuation lengths, i.e. 14 nm as our simulations show. SPP’s
are proposed to be the data transfer medium for on-chip
clocking and signaling. The proposed geometry for
transferring SPP’s is a metal-insulator-metal (MIM)
waveguide structure where coupled SPP’s propagate along the
two metal-insulator interfaces over long distances.
In this work, we introduced a simple nanoantenna coupler
and a metal-insulator-metal waveguide for demonstrating the
possibility of high density and reliable interconnects. The
interconnect structure is as in Figure (a) and is placed on 500
nm thick thermal oxide. Metallic nanoposts in front of the
waveguide behave as an antenna, increasing the coupling of
optical field into the waveguide. The waveguide consists of
silver cladding and silicon oxide core. The structure is excited
by plane waves from the left, propagating to the right. The
simulations were repeated to optimize the structures over the
antenna arm lengths and widths, gaps, metal layer thicknesses,
and waveguide-to-antenna distances. The design was
optimized for the longest propagation distance for the
telecommunication wavelength of free space = 1550 nm. For
efficient coupling, (i) arm length and the cladding width have
been assumed to be equal, (ii) the arm gap and the core width
are taken as equal. Our simulations (not shown) indicated that
not making these two assumptions significantly reduced
coupling efficiency.
Figure: (a) Simulation volume, incident wave, nanoantenna and the waveguide
structure. (b) Top view of the interconnect. (c) E-field profile along the center of the
waveguide and the antenna (y=0 line), normalized with respect to incident field
(d) E-field enhancement profile along the substrate and the interconnect interface,
optimized for free space = 1550 nm (e) Normalized E-field intensity profile along the
substrate and the interconnect interface, for free space = 780 nm.
The propagation distances for = 532 nm, 780 nm,
1550nm were 3, 9, about 50 μm, respectively. Metal’s
absorption progressively increases for shorter wavelengths.
That is why; shorter wavelengths cannot propagate as long as
in the 1550 nm case.
The subwavelength nature of these interconnects enables
high integration density. We are going to investigate this
technique for noise immunity and broadband applications.
This work was supported by TUBITAK 108E163, 109E044,
EU FP7 PIOS.
[1] Moore, G. E., Electronics, Vol. 38, No. 8, April 19, 1965
[2] ITRS 2007 Edition,
http://www.itrs.net/Links/2007ITRS/2007_Chapters/2007_Interconnect.pdf
[3] Ali K. Okyay, PhD Thesis, Stanford University, September 2007
[4] Gramotnev, Bozhevolnyi; Nature Photonics 4, 83 - 91 (2010)
[5] L. Tang et. al., Nature Photonics 2, 226 - 229 (2008)
6th Nanoscience and Nanotechnology Conference, zmir, 2010 625