Edwin Jan Klein - Universiteit Twente

Chapter 5
5.1 Introduction
The material system in which most of the resonators presented in this thesis have been
fabricated is the silicon nitride/silicon oxide system (Si3N4/SiO2) [130]. The silicon
nitride is used to fabricate the core of the waveguides while silicon oxide (e.g.
thermal, TEOS or PECVD) is used in both the substrate and cladding layers. Both
materials are highly compatible with CMOS technologies and can therefore benefit
from all the advances made in these technologies.
Silicon nitride was chosen due to its high refractive index of ≈1.98 at 1550 nm which,
combined with silicon oxide as substrate or cladding layer, allows for potentially very
small resonators, down to a radius of approximately 15 µm. In addition, the material
is optically transparent from 400 nm to 2 um which makes it suitable for a wide range
of applications and allows for low-loss high index waveguides with reported losses
lower than 0.1 dB/cm [131]. Silicon nitride can be deposited in multiple layers of
which the thickness can be controlled to within a few nanometers. Also, because the
silicon nitride precursor gasses can be chosen to react to form stoichiometric Si3N4,
the process is very stable concerning the refractive index. This opposed to for instance
the SiliconOxiNitride process (SiON) where the ratios of the various precursors need
to be carefully controlled during layer deposition to obtain a uniform refractive index,
and from deposition to deposition for reproducibility. The use of silicon nitride,
however, does place a constraint on the thickness of the silicon nitride layer. Due to
tensile stress within the material it will crack and become unusable. The maximum
safe deposition thickness of a silicon nitride layer is therefore no more than ≈340 nm.
This thickness, however, allows for resonators as small as 15 µm which is more than
sufficient for many applications.
In total, three different fabrication processes have been used to realize different types
of silicon nitride based devices. The characteristics of these processes have been
summarized in Table 5.1. As shown, the process used to fabricate laterally coupled
resonators is the simplest while the process that uses projection lithography to
fabricate vertically coupled resonators is the most complex. The differences between
these processes, however, are not limited to the processing itself but also have an
effect on a higher level: in the layout of the masks and design choices made for the
various devices. In the following sections the fabrication processes and their
repercussions in the design of devices will be discussed in more detail.
Table 5.1. Comparison of the fabrication processes used to create Si3N4 based
devices.
Lateral Vertical I Vertical II
#guiding layers 1 2 2
#metal layers Zero or 1 1 2 (Cr, Au)
Litho type Contact Contact Contact + Projection
# mask exposure
steps
1 3 6
Tapers Horizontal Horizontal Horizontal, Vertical
CMP No No Yes
#major
processing steps
7 (etching, deposition,
litho and anneal)
14 (etching, deposition,
litho and anneal)
25 (etching, deposition,
litho, CMP and anneal)
110