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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

¼ 3.476), silica (n

SiO

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

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