Three-step lithography to the fabrication of vertically
coupled micro-ring resonators in amorphous
silicon-on-insulator
Jun Cheng (程 俊)* and Nan Yan (严 楠)
School of Electromechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
*Corresponding author: 2120100260@bit.edu.cn
Received March 21, 2015; accepted June 8, 2015; posted online July 8, 2015
A simple method to fabricate vertically coupled micro-ring resonators in amorphous silicon-on-insulator is
created by a three-step lithography process. First, the linear loss at 1.55 μm of the a-Si:H film is calculated
to be 0.2 0.05 dB∕cm. Then, the bottom line waveguide of Su-8 with a flat top surface of 300 nm is created
by etching. The thickness of Su-8 can easily be controlled by the etching time. Finally, by opening the window
pattern and etching several layers, the first layer marks made by electron beam lithography are found with a
50 nm resolution, and the high quality of the micro-ring resonator is demonstrated.
OCIS codes: 220.0220, 230.4000.
doi: 10.3788/COL201513.082201.
Micro-ring resonators are a rapidly developing area of
research in photonic devices with a wide range of applica-
tions, including signal-processing filters, sensors, lasers,
modulators, switches, memory, and slow-light elements.
Generally speaking, micro-ring resonators represent
frequency-selective elements that can perform a variety
of functions, such as add/drop filtering, switching,
and modulating in wavelength division multiplexing sys-
tems
[1]
. The basic structure of micro-ring resonators con-
sists of a ring-shaped waveguide coupled to either one or
two straight optical waveguides. The ring and the bus
waveguide may be positioned in the same horizontal plane
(lateral coupling), or the ring can be located above the bus
waveguide (vertical coupling). Vertical coupling to the
resonator offers several advantages over lateral coupling:
1) First and foremost among these is that the ring/bus in-
teraction can be controlled to a fine degree, as the vertical
separation is obtained by a well-controlled deposition,
rather than by etching fine gaps
[2]
; 2) buried waveguides
suffer less from scattering loss and offer better input/
output coupling; 3) finally, if the waveguides are posi-
tioned directly below the ring, the interaction strength
becomes insensitive to small alignment deviations
[3,4]
. This
last fact is beneficial, as the optimum performance is
achieved when the degree of coupling of both of the bus
waveguides to the ring are identical. There are two kinds
of vertical coupling micro-ring resonators. In Fig.
1(a), the
add/drop bus is above the input/throughput bus and
the ring, which are on the same plane. The add/drop
bus transforms the micro-resonator into a wavelength-
selective filter that allows signals of certain wavelengths
to be transferred from one waveguide port (input) to an-
other (drop). In Fig.
1(b), the ring is above the input/
throughput bus, and is coupled to a single waveguide
(input/throughput).
People often use silica, silica-on-silicon, silicon, silicon-
on-insulator (SOI), silicon nitride, and oxynitride, poly-
mers, semiconductors, and crystalline materials such as
lithium niobate and calcium fluoride as materials to
fabricate micro-resonators
[5]
. In particular SOI substrates
are now widely used for making highly efficient photonic
integrated circuits
[6–9]
. However, to be complementary
metal oxide semiconductor back-end compatible, the
maximum temperature of the deposition process should
be restricted to 400°C. Among the three kinds of silicon
(amorphous, polycrystalline, or crystalline), only amor-
phous silicon of high quality, low loss, low temperature
(100°C–400°C), and that allows flexible multilayer stack-
ing by plasma-enhanced chemical vapor deposition
(PECVD) can be deposited
[10–12]
. It indicates that the
propagation loss of hydrogenated amorphous silicon
(a-Si:H) waveguides have approached that of their crystal-
line Si counterparts. In this Letter, we concentrate on the
fabrication of vertically coupled micro-ring resonators
in amorphous SOI materials, as depicted in Fig.
1(b).
An important feature of this structure is the higher
refractive index (n) contrast of the different materials with
respect to SOI technology: a-Si:H (n
a-Si
¼ 3.6), silicon
(n
Si
¼ 3.476), silica (n
SiO
2
¼ 1.444), and air (n
air
¼ 1).
This high index contrast allows for the design of small,
wavelength-scale nano-photonic structures.
Fig. 1. Vertically coupled micro-ring resonators. (a) Four-port
and (b) two-port configurations.
COL 13 (8), 082201(2015) CHINESE OPTICS LETTERS August 10, 2015
1671-7694/2015/082201(5) 082201-1 © 2015 Chinese Optics Letters
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