Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L383–L385
Part 2, No. 4B, 15 April 2001c2001
The Japan Society of Applied Physics

Propagation Characteristics of Ultrahigh- Optical Waveguideon Silicon-on-Insulator Substrate
Atsushi S AKAI, Go H ARA and Toshihiko BABA
Division of Electrical and Computer Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan(Received January 22, 2001; accepted for publication February 19, 2001)

A submicron rectangular waveguide with a relative refractive index difference  of 45% was fabricated on a silicon-on-insulator
substrate, and its propagation characteristics were evaluated using the Fabry-Perot resonance method. The propagation loss wasof the order of 10 cm−1 , and was dominated by light scattering at rough interfaces. However, a large modal effective index ofmore than 4.5 and a low bend loss of less than 1 dB at a 0.5-µm-radius bend were also observed. These results suggest thepotential of an ultrasmall and high-density lightwave circuit, which accepts the relatively large propagation loss.
KEYWORDS: SOI, optical waveguide, integrated photonics, lightwave circuit

An optical waveguide with strong optical confinement allows dense and sophisticated optical wiring in lightwave circuits. Photonic crystal waveguides1, 2) are attracting considerable attention for this reason. On the other hand, awaveguide with a high relative refractive index difference ≡ (n 21 −n 22 )/2n 21 between the core (index n 1 ) and claddings(index n 2 ) is another candidate for achieving the same degree of strong optical confinement, although its principle ishighly conventional. For this purpose, a commercially available silicon-on-insulator (SOI) substrate is a promising platform. It provides a crystal-quality high-index (n 1 ∼ 3.5) Si
film on low-index (n 2 ∼ 1.45) SiO2 , which can be used forboth electronic circuits and waveguides at optical fiber communication wavelengths. Thus far, rib-type waveguides on
SOI substrates with a low propagation loss of 0.1 cm−1 order
have been demonstrated.3, 4) However, the bend radius wasof mm order due to the essentially weak optical confinementof this type of structure. For an optical confinement strong
enough to realize micron-size bends, the Si core must becompletely etched down and exposed to low-index claddings.
Such a waveguide with a rectangular Si core has also been reported.5) For such a high- waveguide, however, the scattering loss at rough sidewalls dominates the propagation loss.6)
The relation between the scattering loss and the waveguidestructure which satisfies the single-mode condition is an important issue to be investigated for further advance of this typeof waveguide. In this study, we fabricated such SOI rectangular waveguides, employing electron beam (EB) lithography
and inductively coupled plasma (ICP) etching to reduce thesidewall roughness. We measured the propagation loss andthe effective index (sometimes called the group index) of theguided mode using the Fabry-Perot resonance method. We
also fabricated and evaluated a sharp bend, which is a key
component for the miniaturization of lightwave circuits.
The SOI substrate used in this study had a 0.32-µm-thick
p-Si layer bonded on a 1-µm-thick SiO2 layer. The fluctuation in the Si thickness was less than 20 nm inside a 7×7 mm2cleaved piece. Since the resistivity of the Si layer was relatively high (14 ·cm), the free carrier absorption loss of light
was negligible compared with the scattering loss at roughsidewalls as described below. For the EB lithography, a highresolution positive resist, ZEP 520, Nippon Zeon, Inc., wasused. After the liftoff process to transfer the pattern to a metalmask, the Si layer was completely etched down into a rectangular shape by ICP etching with CF4 and Xe gases. Inthe measurement, transverse electric (TE) polarized light of
λ ∼ 1.55 µm from a tunable laser diode was focused on thecleaved end of the waveguide. The top view of a 1-µm-wide
waveguide and a near-field pattern (NFP) of light are shown inFig. 1. According to the 3-dimensional finite difference time
domain (FDTD) simulation, waveguide widths of 0.23 µm
and 0.51 µm result in cut off conditions of the TE-like polarized 0th- and 1st-order modes at λ = 1.55 µm. The circular
light spot observed in Fig. 1 indicates the quasi-single-mode
propagation caused by the scattering loss discrimination ofhigher-order modes.
For the Fabry-Perot resonance method, we formed two
cleaved facets as input and output ends of the waveguide. For
a constant power of the input light, the light output was detected directly by a sharpened single-mode fiber. The transmission spectrum was measured with changing λ, as shownin Fig. 2. The propagation loss coefficient α and the effective
index n eff are given by√
1
Imax /Imin − 11·√(1)
α = − lnLRImax /Imin + 1∂n eqλ2= n eq − λ,(2)2Lλ∂λwhere L is the waveguide length, R is the facet modal reflectivity, Imax and Imin are the peak and bottom intensities of eachresonance, respectively, λ is the space between neighboringn eff =Fig. 1. Scanning electron micrograph (upper) of waveguide top and NFPof TE-like polarized light output at λ = 1.55 µm with white lines drawn
for better understanding (lower).

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A. S AKAI et al.

resonance peaks and n eq is the equivalent index of the guided
mode defined as the propagation constant divided by the wave
number. Compared with the conventional cut back method,6)
this method yields a more precise evaluation of such high-
waveguides, since the ratio Imax /Imin is independent of thecoupling condition of the input and output light. The reflectivity R was assumed to be 42%, which was calculated by
the FDTD method. In this experiment, we prepared samples
A and B with different channel widths. Average propagation
losses for these samples are 35 − 50 cm−1 (152 − 217 dB/cm)
and 12 − 42 cm−1 (52 − 182 dB/cm), respectively, as shownin the inset of Fig. 2. The difference between these samples isin the roughness amplitude σ of sidewalls, i.e., ∼70 nm and∼20 nm in samples A and B, respectively. In general, the scattering loss in a single-mode waveguide with rough interfaces
is proportional to σ 2 .7) The qualitative agreement between
this expected result and the experimental result indicates thatthe propagation loss is dominated by the scattering loss. Theoretically, the scattering loss is also proportional to 2.5 .7)
The waveguide in this study has the maximum  of 45% between Si and air, nearly 150 times the typical value, <0.5%,of silica-based waveguides. Considering this large differencein  and the typical propagation loss of 0.01 cm−1 order insilica waveguides, we can conclude that the loss evaluated inthis study is 1−2 orders of magnitude lower than the expectedvalue. This is clearly attributed to the low roughness obtainedby the use of EB lithography and ICP etching. The loss will
be further reduced to nearly 1 cm−1 , if the sidewall roughness can be reduced to less than 5 nm. Such a low roughness
seems to be feasible, considering that σ < 10 nm has already

70300

25050200401503010020Loss [dB/cm]

Loss [cm-1]

60

50

100000.2

0.4

0.6

0.8

1.0

1.2

Width [ m]4Light Output [arb. units]

been achieved for III-V compounds through the optimizationof etching conditions.8) However, a loss of less than 0.1 cm−1
cannot be expected without sidewall of atomic-level smoothness which is not possible with the present technology. Weshould instead discuss the miniaturization of lightwave circuits, which accept a relatively large loss.
Figure 3 shows dispersion curves of the 0th-order mode fora channel width of 0.50 µm, which is calculated by the FDTD
method. Due to the large  and corresponding large change
in n eq , the second term in eq. (2), representing the structuraldispersion, becomes extraordinarily large. Due to this effect,n eff can be larger than the material index of Si. From themeasured resonance shown in Fig. 2 and eq. (2), n eff of thefabricated waveguide is evaluated to be 4.3−4.6. This almost
agrees with the theoretical curve. This large value cannot becaused by the material dispersion, since λ = 1.55 µm is fully
separated from the electric band edge, 1.1 µm, of Si. Thislarge n eff indicates a low group velocity, which enhances theinteraction of the waveguide and the guided light. This is oneof the reasons for the large scattering loss. However, it is also
effective for shortening functional devices.
One of the most attractive properties of high- waveguides
is the low-loss sharp bend. We fabricated such bends simultaneously with the straight waveguides. The top view of a 1µm-wide waveguide with a 90-degree bend and NFP of light
output at λ = 1.55 µm are shown in Fig. 4. The quasi-single
mode was observed for the same reason as mentioned above.

32

10
1.550

1.555

1.560

1.565

1.570

Wavelength [ m]Fig. 2. Transmission spectrum of TE-like polarized light in a 283-µm-longand 0.7-µm-wide waveguide, and propagation loss evaluated for variouschannel widths. Squares and circles denote samples A and B, respectively.

Fig. 3. Calculated dispersion characteristics of equivalent index n eq andeffective index n eff of the TE-like polarized 0th-order mode. Thickness
and width of Si channel are assumed to be 0.32 µm and 0.5 µm, respectively. Experimental result of n eff for the same structural waveguide is alsoplotted.

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A. S AKAI et al.

L 385

The light radiation at the bend is negligible compared with theoutput light intensity, even though the bend radius measured
from the waveguide center is as small as 2 µm. We also simulated the in-plane electric field distribution in a 0.5-µm-wide
waveguide with such a bend, as shown in Fig. 4, which also
shows that the radiation at the bend is negligible. Figure 5
shows 90-degree-bend loss specified by measuring the total
loss of bent waveguides and subtracting the propagation loss
in straight waveguides. The channel width was 0.5 µm. The
average bend loss is less than 1 dB even though the radius is assmall as 0.5 µm. Due to the ∼20% fluctuation of the FabryPerot resonance, as shown in Fig. 2, the bend loss includes±3 dB error. Considering this error, the maximum loss could
be over 4 dB. However, the NFP and simulated field profile inFig. 4 indicate a much smaller value.
In conclusion, we demonstrated rectangular waveguides on
an SOI substrate with an ultrahigh- and submicron-size Sicore. The sidewall roughness was suppressed to less than
20 nm by the use of EB lithography and ICP etching. However the propagation loss was of 10 cm−1 order and dominated by the scattering loss at interfaces. We should discuss acompact lightwave circuit which accepts at least 1 cm−1 loss.
From this point of view, the large effective index of morethan 4 and low loss of less than 1 dB at the 0.5-µm-radiusbend, which were obtained in this study, are very attractive.
The bend radius of 0.5 µm is smaller than that in a silicabased waveguide by a factor of 200 − 500. When consideringthat the size and the flexibility of the lightwave circuit arestrongly restricted by a large bend radius, the small bend radius demonstrated in this study suggests the high potential ofsuch a fully compact circuit.

Fig. 4. Top view of a 1-µm-wide waveguide with a 90-degree bend (upper), NFP of TE-like polarized light at λ = 1.55 µm (middle), and
3D-FDTD-simulated electric field distribution in 0.5-µm-wide waveguide(lower).

86r

Loss [dB]420-2-40Fig. 5.

2468Radius r [ m]10Bend loss in 0.5-µm-wide waveguide at λ = 1.55 µm.

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