Eindhoven, The Netherlands, June 11-13, 2008
Trimming of silicon ring resonator by electron beam
J. Schrauwen, D. Van Thourhout, R. Baets
Photonics Research Group, Department of Information Technology, Ghent University - IMEC, Gent,
Abstract - We present a technique to trim the resonance of silicon ring resonators. The
cladding oxide is compacted by electron beam bombardment, causing strain in the silicon
lattice, which leads to a 5 nm resonance shift.
Silicon-on-insulator (SOI) is gaining interest as preferable material system for future
ultra-compact integrated photonic components. The main advantages of this material
system are firstly the high refractive index contrast between silicon (core) and oxide or
air (cladding) enabling small bend radii and dense integration, and secondly mature fab-
rication facilities thanks to the electronics industry. One of the applications aimed at
by the telecom industry is optical filtering for wavelength (de)multiplexing. SOI is the
ideal platform to make these filters compact and low-cost. Several geometries, such as
ring resonators  and photonic band gap materials  have good filtering characteristics.
which is a serial fabrication technique and therefore unattractive for mass fabrication. In
previous work we have demonstrated that these filters can also be fabricated in a parallel
way, with 248 nm or 193 nm Deep-UV lithography (DUV) in a standard Complemen-
tary Metal Oxide Semiconductor (CMOS) facility . However, variations of the critical
dimensions of devices fabricated by optical lithography are inevitable. These variations
can be caused by wafer non-uniformity, by e.g. varying layer thicknesses on wafer edges,
or by non-uniformity within one chip, mainly caused by lithography imperfections near
mask edges. A way to assess critical dimension variations in a photonic circuit is to
evaluate the resonance wavelength shift of identically designed ring resonators, dispersed
over a wafer. These resonators are fabricated with Q-factors of about 104, or a 3 dB
bandwidth of 0.15 nm. In practice the resonance wavelength shifts exceed 1 nm, which is
unacceptable for many applications. The most common solution for this is active thermal
tuning , however, when many resonators have to be integrated on a single chip, this
would lead to high power consumption and important device complexity. Another ap-
proach to circumvent these process variations is trimming of the devices after fabrication.
In this paper we present a technique to locally and independently trim the resonances of
ring resonators on a silicon chip. This allows for complete compensation of resonance
wavelength variations on silicon photonic integrated components.
The resonance wavelength of a ring resonator is trimmed by changing the optical path
length of the resonator, in our case by varying the effective index of the guided mode and
ECIO ‘08 Eindhoven
Figure 1: Overview of the experiment: the right ring is trimmed by electron beam com-
paction; the left one is kept original as a reference to exclude temperature or ambient
Figure 2: Left: The resonance wavelength of a ring is red shifted to equate that of the
reference ring. One can notice a slight decrease of the Q factor. Right: Cross-section of
the 220 nm thick silicon ring resonator. The 2 keV electrons penetrate 70 nm into silicon
and oxide, and lead to volume compaction only in the oxide. This effect generates a
tensile strain in the silicon, parallel to the substrate. The effect of silicon strain dominates
the refractive index change. In the bottom drawing the first principal strain obtained from
a finite element simulation was overlaid.
not the length of the ring. An increase in effective index causes a red shift of the resonance
wavelength. This was demonstrated in several (low to medium index contrast) material
systems such as silica glass  and SiN/SiON . In SOI, the preferable material system
for future industrial deployment, the core material is silicon, which can not be compacted
by either UV or electrons. Only the SiO2cladding is susceptible to compaction. Due to
the imaging capabilities and the ease to precisely control the irradiation dose and energy
we have used an electron beam (from an FEI Nova 600 scanning ion/electron microscope)
to compact the SiO2cladding layer. The resonance frequency of silicon ring resonators
is extremely sensitive to changes of temperature and of the surrounding medium; rings
are therefore attractive as sensors. However, in this experiment we want to exclude all
external factors and investigate the resonance shifts caused only by electron beam irra-
diation. Therefore we have fabricated a sample with two rings, with different resonance
wavelengths, serially connected to the same waveguide, as depicted in Figure 1. Only one
of these rings is irradiated by imaging it with a scanning electron beam. The transmission
spectrum features two superimposed ring spectra. By evaluating only relative peak shifts
between the two ring spectra the external influences are excluded.
The experiment was performed in situ, inside the vacuum chamber of a scanning elec-
Eindhoven, The Netherlands, June 11-13, 2008
tron microscope, by providing it with vacuum fiber feedthroughs. The optical input and
output signals are transported by single mode fibers, glued (with UV-curable glue) in a
near-vertical position above grating couplers. The optical circuit, with grating couplers,
waveguides, tapering sections, and ring resonators, was fabricated by DUV lithography
in a CMOS pilot line . Light was generated by a super luminescent LED with center
wavelength at 1530 nm and detected by a spectrum analyzer with a resolution of 60 pm.
The graph in Figure 2 shows part of the experiment. The left transmission dip - belonging
to the right ring from Figure 1 - is trimmed to equate the resonance of the other ring.
In this graph a resonance wavelength red shift of about 3 nm is shown. This part of the
experiment was performed with a 0.84 nA beam, by scanning it over the ring resonator
for 400 s. Figure 2 shows a slight decrease of the Q factor. The original position of the
resonance was around 1523.9 nm, so in the complete experiment a total resonance shift
of 4.91 nm was obtained.
Two distinct physical processes cause the effective index change of the mode in the sil-
icon ring resonator, as depicted in Figure 2. The first is a larger refractive index of the
oxide cladding due to volume compaction; the second is the stress this oxide compaction
induces in the silicon lattice. In our experiment we have worked with 2 keV electrons,
which have a penetration depth of about 70 nm in Si and SiO2(this was calculated with
Monte Carlo simulations, and confirmed by ). The silicon ring is 220 nm thick, on top
of a 2 µm oxide layer; therefore the electrons can not penetrate the silicon. The mode
overlap with the compacted oxide is lower than 1.5%, as was calculated with a mode
expansion tool. From [7, 8] we have estimated the maximum amount of refractive index
change lower than 3% (i.e. for a compaction of about 10%), with a total irradiation dose
of 2.8 x 1023keV/cm3(the total dose in our experiment). This can not lead to more than
0.5 nm shift in resonance wavelength. We can thus conclude that the largest fraction of
the observed resonance wavelength shift is caused by strain in the silicon lattice. Finite el-
ements simulations were used to evaluate this effect, as is shown in Figure 2. The overlay
picture illustrates the deformed mesh (with a scale factor of 2) and the first principal strain
in the case of a 10% compacted oxide layer with a thickness of 70 nm. Since a complete
study was beyond the scope of this work, we have chosen to estimate the influence of
compaction induced stress on the effective index of the supported modes without detailed
simulations of the optical mode profile in the strained lattice. We have therefore calcu-
lated the average silicon strain in the dominant direction: perpendicular to the waveguide
propagation direction and in the plane of the substrate surface. The resonance wavelength
shift was calculated by using only the p11component of the silicon elasto-optical tensor.
This shift is calculated for varying oxide compaction rates and for different compacted
layer thicknesses. The results of this simulation support the fact that tensile strain in the
silicon waveguide can account for the observed resonance wavelength shift of more than
5 nm. Although all experiments in this report were performed with an electron beam,
they can in principle be repeated with UV since the penetration depth at wavelengths be-
tween 200 nm and 400 nm is sufficiently low to create compaction induced strain. Ring
resonators in other semiconductor materials can be compacted in a similar way, as well as
other kinds of cavities, such as photonic crystal cavities. It can be argued that this method
is too slow for mass fabrication purposes. However, we believe that it can be accelerated
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by using higher beam currents. Furthermore, realistic shifts will not often exceed 1 nm.
This makes that a typical trim will be performed in seconds. Specifically in combination
with vertical grating couplers and in situ readout, this technique is suited for rapid and
automatic trimming of devices before packaging and on wafer scale.
We report on the trimming of a silicon ring resonator by electron beam irradiation. The
oxide cladding is subject to volume compaction, causing tensile strain in the silicon lat-
tice. Both effects generate an increase in refractive index, generating a red shift in reso-
nance wavelength. The dominant effect is the tensile strain in silicon. In our experiment
we have measured a maximum resonance wavelength red shift of 5 nm, which would be
sufficient to compensate for variations on wafer scale and on chip scale due to optical
This work was partly supported by the European Union through the Network of Excel-
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