Low loss silicon waveguides for application of optical interconnects

Abstract

We present low loss shallow-ridge silicon waveguides with an average propagation loss of 0.274 dB/cm in the C-band, which can find applications in chip-level optical interconnects.

Full text

Low Loss Silicon Waveguides for Application of Optical
Interconnects
Po Dong1*, Wei Qian1, Shirong Liao1, Hong Liang1, Cheng-Chih Kung1, Ning-Ning Feng1, Roshanak
Shafiiha1, Joan Fong1, Dazeng Feng1, Ashok V. Krishnamoorthy2, and Mehdi Asghari1
1Kotura Inc., 2630 Corporate Place, Monterey Park, CA 91754, USA
2Sun Labs, Oracle, 9515 Towne Centre Dr., San Diego, CA 92121, USA
*E-mail address: pdong@kotura.com
Abstract: We present low loss shallow-ridge silicon waveguides with an average propagation loss
of 0.274 dB/cm in the C-band, which can find applications in chip-level optical interconnects.
1. Introduction
Optical interconnect technology is perceived as a key solution to solving major performance limitations in high
performance computing caused by bus bandwidth bottleneck, latency and power consumption issues [1-4]. Routing
silicon waveguides in optical interconnects may require a waveguide length of a few tens of centimeter for chip to
chip and macro-chip communications [5]. This requires that the waveguides have a propagation loss on the order of
0.1 dB/cm. However, the lowest propagation loss reported for silicon waveguides with a geometry of ~0.5 µm width
and ~0.25 µm height to date at λ=1550 nm is 1-2 dB/cm [6, 7]. Recently, several groups have reported shallow-ridge
or thin silicon waveguides with losses of 0.3 – 1.0 dB/cm by a selective oxidation fabrication technique [8-12]. With
this technique, however, it may be difficult to control the critical dimensions of fabricated devices since the thermal
oxidation rate would be affected by device density, hard mask thickness, and the cross section of the waveguides.
Here, we report low loss silicon ridge waveguides fabricated on SOI substrates with a top silicon thickness of 0.25
µm by CMOS-standard optical lithography and dry etching processes. These waveguides have an average
propagation loss of 0.274 dB/cm over the C-band for the fundamental quasi-TE mode.
2. Waveguide Design and Fabrication
The propagation loss of silicon waveguides arises mainly due to light scattering from the etched sidewalls.
Minimizing the optical field overlap with etched interfaces can effectively reduce the waveguide propagation loss.
Increasing waveguide width and decreasing etch depth can both realize this purpose. Here, we design a shallow-
ridge waveguide. The waveguide width and height is 2 µm and 0.25 µm respectively, and the etch depth to form the
ridge waveguide is 0.05 µm. In order to accurately measure waveguide losses of the order of 0.1 dB/cm with the
accuracy less than 0.01 dB/cm, we designed waveguides with spiral shaped waveguides with lengths ranging from a
few centimeters to 64 centimeters. The minimum bending radius in these spirals is 300 µm. The waveguide loss can
be extracted by measuring the insertion losses for waveguides with different lengths on the same chip.
The above silicon waveguides were fabricated using Soitec 6” SOI wafers with a 0.25 µm thick silicon layer and a
3 µm thick BOX layer. Waveguide pattern were defined by a Nikon Deep UV 0.25um Scanner. The pattern was
transferred to the oxide hard mask using a CHF3/O2 chemistry. We then etched the silicon layer using an HBr-based
silicon etch recipe. Both the oxide and silicon etch recipes were optimized to reduce sidewall roughness. A 1.2 µm
thick oxide was then deposited on the wafers as a cladding layer. The fabrication processes are all CMOS standard.
3. Measurement results
Fig. 1(a) shows a top-view optical microscopy picture of a fabricated waveguide with a total length of 64 cm, with a
spiral area of 6 mm by 3 mm. In Fig. 1(b), we show the insertion losses measured for different waveguide lengths as
a function of wavelength. The insertion losses are normalized to the power measured from direct fiber to fiber
coupling, and include both coupling loss from lensed fibers and waveguide propagation loss. From the insertion loss
at a particular wavelength for different waveguide lengths, we can extract the waveguide propagation loss using
linear fitting. Fig. 1(c) presents the waveguide loss spectrum in the entire C-band, demonstrating an average loss of
0.274 dB/cm and a standard derivation of 0.008 dB/cm. This verifies that the waveguide loss is uniform over the C-
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band. The insertion loss variation, shown in Fig. 1(b), is about 1 dB for 32 cm long waveguides. This variation may
come from multimode behavior.
(a)
4. Conclusion
Fig. 1. (a) A top-view optical microscope image of a 64-cm long spiral waveguide. The waveguide has a total
propagation loss about 17.5 dB. (b) Insertion loss spectrum for waveguides with different lengths on the same chip.
The insertion loss is normalized to fiber-to-fiber power. (c) Waveguide propagation loss as a function of wavelength in
the C-band.
(c)
(b)
In conclusion, we have demonstrated low loss shallow-ridge silicon waveguides with an average propagation loss of
0.274 dB/cm over the C-band. Standard lithography and dry etching are employed to fabricate these waveguides
from silicon-on-insulator wafers with a top silicon thickness of 0.25 µm. This waveguide can act as routing
waveguides in optical interconnect systems.
Acknowledgements: The authors acknowledge funding of this work by DARPA MTO office under UNIC program
supervised by Dr. Jagdeep Shah (contract agreement with SUN Microsystems HR0011-08-9-0001).
The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as
representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of
Defense.
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