# Optical Filters Utilizing Ion Implanted Bragg Gratings  in SOI Waveguides

Authors: M. P. Bulk; A. P. Knights; P. E. Jessop; P. Waugh; R. Loiacono; G. Z. Mashanovich; G. T. Reed; R. M. Gwilliam
Journal: Advances in Optical Technologies
 (2008)

---

## Abstract

The refractive index modulation associated with the implantation
of oxygen or silicon into waveguides formed in silicon-on-insulator (SOI)
has been investigated to determine the feasibility of producing planar, implantation
induced Bragg grating optical filters. A two-dimensional coupled mode theory-based
simulation suggests that relatively short grating lengths, on the order of a thousand microns,
can exhibit sufficient wavelength suppression, of>10 dB, using the implantation technique. Fabricated planar implanted slab-guided SOI waveguides demonstrated  an extinction of −10 dB for TE modes and −6 dB for TM modes for the case of oxygen implantation. Extinctions of −5 dB and −2 dB have been demonstrated with silicon implantation.


---

## Body

## 1. Introduction

The preliminary work of Soref et al. and the
subsequent study by Pogossian et al.
demonstrated the ability to produce single mode optical waveguides in silicon
at telecommunication wavelengths with cross-sectional dimensions compatible
with standard photolithography [1, 2]. Immediate interest pursued as this structure was a suitable candidate
for low-loss, high-index-contrast
waveguides capable of routing optical signals at the wafer scale. The integration of optical components on a
silicon platform also had the added benefits of low-cost large-scale
manufacturability utilizing existing CMOS facilities and the possibility of
amalgamating electrical and optical functionality on a single wafer.Consequently,
the miniaturized counterparts of many bulk optical components have been
realized in silicon. These include
detectors, couplers, filters, and sensors [3–6]. It is of particular interest to control the
propagation of individual optical wave-fronts in such devices. One method to achieve wavelength dispersion
exploits the unique behavior of optical waves propagating in a medium
containing a periodically modulated refractive index. Indeed, this approach has found extensive use
in such areas as photonic crystals, distributed feedback (DFB) lasers and optical
spectroscopy. In one-dimension, the perturbation
is referred to as a grating and serves as a robust and flexible tool in
photonics technology. Depending on the
desired outcome, gratings can be implemented to acts as couplers, reflectors, mode
converters, optical filters among others [7].In the case of silicon-on-insulator (SOI) waveguides,
a surface relief structure provides the most straightforward method for
fabrication of an integrated Bragg grating. Such devices are inherently passive,
although electrically tuneable gratings based on the plasma dispersion effect
have been developed [8] along with those which are thermooptically tuned [9].
The Bragg grating itself is typically formed through a lithographic patterning
technique and an etching sequence, although, relatively recently, focused ion
beam milling has been used to realize higher-order mode conversion and
filtering in SOI [10]. Perhaps the benchmark for performance of gratings in SOI
waveguides remains the work of Murphy et al. [11]. There, surface relief
gratings with a period of 223 nm were formed in 3 μm high SOI rib waveguides via interference
photolithography and plasma etching to a depth of 150 nm. The authors
demonstrated an optical transmission dip of 8 dB at the resonant wavelength of
1543 nm, for the fundamental TE mode. The device had a transmission loss of
nearly 10 dB which the authors attributed to sidewall scattering. The biggest
drawback of the fabrication method leading to surface relief is the inherent
deviation from a planar surface, limiting the ability of further
processing.This is particularly
problematic in nanophotonics, where the required thin processing layers are
quickly skewed by alterations of wafer topography.An
alternative method which we propose here involves the well-established ion
implantation process to introduce dopants and/or structural modifications to
the crystalline lattice. By selectively
introducing ions using a masking process, the refractive index can be periodically
modified to produce a Bragg grating. The
implantation process can alter the refractive index through either compound
formation, as in the case of oxygen implantation, or chemically inert structural
damage as achieved by self-irradiation [12, 13]. Apart from small volumetric changes, the
result is a Bragg grating retaining a nearly planar surface. This makes it a very interesting candidate
for photonic applications possessing surface sensitive topography, such as
wafer-bonding techniques required for three-dimensional photonic circuits. Therefore, this paper investigates the formation
of Bragg gratings via implantation of oxygen and silicon into SOI. To the
authors’ knowledge, this preliminary work is the first report of integrated
Bragg gratings in SOI waveguides formed via ion implantation.


## 2. Ion Implanted Grating Design and Considerations

Some authors
have already pointed out the possibility of fabricating isolated buried oxide
regions through a high dose oxygen implantation [14] in order to create hybrid
SOI/silicon wafers for CMOS applications in a manner similar to the buried
oxide formed in the separation by IMplantation of OXygen (SIMOX) process [12]. Such
formation is generally conducted using doses around 1018 ions/cm2,
with energies of 150–200 keV and at high temperatures (65°C) in order to
prevent amorphization [15]. Traditional SIMOX processes need high dose implants
in order to obtain a stoichiometric oxide. In the case of grating fabrication,
high dose is still a desirable process parameter, whereas the implant energy
will be lower as the oxide layer needs not to be buried but close to the wafer
surface. For this reason, implantation energy can be lowered to 20–30 keV.
Following the implantation process, the substrate generally contains a series
of extended defects. Depending on the implant condition these defects may
include stacking faults, dislocation loops, amorphous clusters, oxygen
precipitates, and columnar defects [16]. In general, SIMOX process requires
postimplant annealing treatment up to 1300°C. The postimplant anneal is
essential in order to finalize oxide formation, as it stimulates oxygen
diffusion and SiO2 formation via Ostwald Ripening [17], while repairing
lattice damage. By this mechanism, small faulty zones and precipitates tend to
coalesce into bigger aggregations in order to achieve a more favorable energy
configuration. Although the structures reported here have not received such a
high-temperature anneal, it is likely that isolated precipitates of SiO2 are formed during the high temperature implantation stage.In contrast, the
low temperature self-irradiation of silicon results in the production and
accumulation of point defects, defect complexes, and locally amorphous regions
until it becomes thermodynamically favorable for the lattice to undergo a
transition from the crystalline to amorphous state [18]. Of particular interest to optical devices is
the ensuing increase of the index of refraction as a result of damage
accumulation. The increase continues
until the amorphous transition, at which point the refractive index saturates
to a value approximately 0.27 higher (for a wavelength of around 1550 nm) than
that of crystalline silicon [19]. It should be noted that although propagation
losses tend to be high, amorphous silicon (a–Si)
has demonstrated guiding capabilities. Therefore, if applied periodically, a–Si
serves as a unique candidate for grating structures.In order to
achieve a first-order Bragg grating in silicon, the Bragg condition,(1)Λ=λ2Neff,
requires a period
(Λ) of 222 nm, assuming a wavelength (λ) of 1550 nm and an effective index (Neff) of approximately
3.48. This poses a fabrication challenge
since the lateral damage/straggle must remain below 55 nm in order to achieve
adequate modulation. Since the
implantation profiles depend on the implantation energy, dose, temperature,
implantation angle, and ion mass, the lateral profile can be manipulated by
varying these parameters.The implantation
profiles can be construed using ion implantation simulators. In the case of the
oxygen implantation, in order to achieve a uniform dopant implantation to a
depth of 70 nm and retain adequate lateral modulation, it was determined that a
dual implant process was required. The
first implant utilized ion energy of 20 keV with a dose of 1.6× 1017 ions/cm2. A subsequent surface
implant was carried out at 10 keV with a dose of 0.8 × 1017 ions/cm2.For silicon
self-irradiation, and implanting at a temperature of 77 K to minimize dynamic
annealing, a dose of approximately 1015 ions/cm-2 is
required to induce amorphization [18]. The
total damage to the lattice is characterized by the number of displacements per
unit volume of the crystal. When the
defect contributions are higher than a critical value, the substrate can be approximated
as amorphous. Although reported values
vary, lattice displacement concentrations in the range of 10% to 25% of the
silicon density appear to satisfy this condition [20, 21].Assuming an
impenetrable masking layer, it was determined that to provide suitable lateral
modulation the implant energy should remain below approximately 60 keV. The 60 keV implant limit also determines the
extent of the grating depth. Using
similar analysis, a modulated index is expected to exist to a depth of
approximately 150 nm. Since the degree
of coupling from the forwards to the backwards modes depends directly on the
overlap integral of the mode with the grating region, it is important to
quantify the suitability of the depth limitation.A
two-dimensional coupled mode theory (CMT) approach assuming TE modes in a slab SOI
waveguide, with an upper-cladding of air, was applied to approximate the
strength of the grating [22]. Assuming
an implantation dose capable of rendering the silicon amorphous and taking the
depth of the implant to be fixed at 100 nm, the degree of modal overlap can be
altered by varying the slab height. This
is analogous to changing the silicon over-layer height. Figure 1 depicts the maximum reflected power
for different grating lengths while varying the slab height, assuming a
lossless medium for simplicity. From the
figure, it is apparent that over 90% of the power is expected to be reflected
at the resonant wavelength for slab heights on the order of 2 μm and a grating length of approximately 1500 μm. Similar analysis was performed assuming a 70-nm deep grating consisting
of a periodic oxide (as opposed to amorphous silicon) and grating lengths of
1000 μm were found to result in approximately 35 dB
reflections.Figure 1
Coupled mode theory prediction of peak
reflected power as a function of the silicon slab waveguide height for grating
lengths of 500, 1000, and 1500μm. Simulations are performed with regard to amorphous
silicon induced via self-implantation. The depth of amorphous silicon is taken
to be 100 nm.


## 3. Fabrication

The fabrication
process is depicted schematically in Figure2 for the cases of the oxygen implant
and the silicon implant. For the oxygen
implanted samples, a 100-nm layer of silicon nitride was deposited onto a 1.5 μm over-layer SOI wafer using LPCVD at 740°C. Electron-beam lithography then provided the
necessary resolution to produce the 222 nm gratings in the silicon nitride
layer to a length of 1000 μm. The
patterned silicon nitride layer then served as the implantation mask. The samples were subsequently implanted
following the dual implantation process formulated above at an elevated
temperature such that amorphization was prevented. Following implantation, the nitride mask was
removed.Fabrication process of planar SOI gratings for the case of (a) oxygen
implantation and (b) silicon implantation.
(a)(b)While
fabricating oxygen implanted samples, several processing issues have been
noticed that can negatively influence the device performance. First and
foremost, the necessity of high dose implantation associated with this is sputtering
and target erosion problems. In the current case, the nitride hardmask used for
pattern definition tends to sputter as the implantation advances. This means
that pattern definition becomes more and more uncertain by the termination of
the implantation. The sputtering yield is greatest on the mask sidewalls, as
the angle between the ion beam and the surface is not zero. Atomic force microscopy (AFM) measurements
on the implanted target also showed surface erosion of the order of 10 nm. This
means that the grating response could also be influenced by the surface
modification.The silicon
implanted samples were implemented on a 2.2-μm over-layer SOI wafer. The wafers were subject to a thermal
oxidation resulting in a 40-nm silicon dioxide layer. This layer serves as a buffer aiding the
removal of the photoresist following implantation. A 7000-μm grating pattern was produced holographically
in photoresist using a Lloyd’s mirror setup and a 325-nm HeCd laser. The photoresist layer then serves as the low-energy
implantation mask. After exposure and
development, scanning electron microscopy (SEM) images
determined the photoresist thickness to be 100 nm. Si implantation was carried out at energies
of 50 and 70 keV and a nominal dose of 1 × 1015 ions/cm2 at 77 K. Following implantation, the
photoresist mask was removed, followed by the oxide buffer layer.


## 4. Results

Optical transmission
spectra were obtained utilizing a tunable laser operating in the range of 1525
to 1570 nm. For the oxygen implanted
samples, the signal was coupled to the waveguides using an objective lens. Light was then measured via a free-space power
meter at the output. Prior to the
objective lens, a beam splitter and half-wave plate were inserted to provide
discrimination between TE and TM modes. The silicon implants were measured in transmission in a similar fashion,
except for the use of a low-loss tapered optical fiber to butt couple light into
the waveguide.Figure3 shows
the TE mode of the “as-implanted" oxygen implanted sample, following the
removal of the nitride mask. The peak response occurs at approximately 1547 nm
for the TE polarized light and shows a discrimination of approximately 10 dB. This appears to be a promising
result, considering no post implantation annealing was preformed.Figure 3
Experimental TE response
of oxygen implant Bragg grating.Using CMT
analysis, the expected result on reflected power of a grating formed of SiO2 can be overlaid with the previous result to compare the expected and measured grating
response. It is also possible to approximate
the index of refraction responsible for the response in Figure 3 as a single
fit factor, by assuming that the oxygen implant achieves a depth of
approximately 70 nm. The result is shown
in Figure 4.Figure 4
Simulated response of oxygen grating
corresponding to experimental results.From Figure4,
it is shown that the expected transmission drop, assuming complete oxide synthesis,
is on the order of −55 dB, suggesting that the current results could be
dramatically improved. Further, we
approximate the index of refraction of the “as-implanted" region to lie at around
3.08. What is thus clear in this
preliminary work is that the high dose, high temperature oxygen implantation, does not form
continuous volumes of SiO2. In agreement with previous work [17],
this should only be expected following postimplant annealing at around 1300°C.
An effort to fabricate gratings with fully formed oxide is currently in
progress.The resultant
data for the 50 keV and 70 keV silicon implanted samples for TE light are shown
in Figures5 and 6. The 50 keV samples show a wavelength suppression of
approximately −5 dB at approximately 1530 nm with the oxide layer intact. Upon
removing the oxide layer, the response drops to an extinction of around −3 dB. SEM images determined that no residual
photoresist mask remained on the oxide and, therefore, suggested the damage inherent in the oxide
layer contributes to the grating response. For the case of the 70 keV implant
with the oxide removed, the sample exhibited a −5 dB suppression. The spectral response of the 50 keV response
is in agreement with the CMT model for the holographically defined 220 nm
gratings, however, a −3 dB suppression corresponds to an implanted index of only
3.493. This value is lower than expected
for completely amorphized silicon and is either a result of ion straggle
inflicting damage underneath the mask material reducing modulation or is a
result of incomplete amorphization. For
the 70 keV samples, the implant depth is approximately 170 nm, and the corresponding
index to produce the −5 dB extinction is 3.491.Figure 5
Experimental TE response of 50 keV silicon
implanted Bragg grating.Figure 6
Experimental TE response of 70 keV silicon
implanted Bragg grating.It is noted that for both the oxygen and silicon
implanted grating devices there exists a deviation from the anticipated
resonant wavelength of 1550 nm. This can be attributed to an error associated
with the lithographical fabrication. In the case of electron-beam lithography,
we might anticipate a deviation inΛ on
the order of a few nm’s, reflected in the small deviation from the resonance at
1550 nm of only ~3 nm, while in the holographic lithography process, a slight angular
misalignment of the Lloyd’s mirror setup of 1° leads to a change in
the Bragg wavelength of ~25 nm (a direct result of the dependence of Λ on
the angle between the two mirrors). The measured resonance of ~1530–1532 nm
reflects a misalignment of the Lloyd’s mirror setup of ~1–2°. For
this preliminary work, we find these results encouraging while recognizing that
control of the lithographical process is critical to achieve precise resonance
wavelengths.In an attempt to
improve the lateral modulation, a 300°C anneal for 10 minutes was
performed on the silicon implanted samples. This annealing process was aimed to remove primary defects such as
di-interstitials and di-vacancies (which can be found at the end of the
implanted range in ion beam amorphized silicon) while retaining the amorphous
structure itself [18]. As a result,
amorphous pockets should remain and, in effect, be defined with a sharper
amorphous/crystalline boundary. The
results of the annealing on the transmission spectra are shown in Figures 5 and
6. It is seen that the grating response
is essentially quenched in both cases. This strongly suggests that the implantation conditions were not
sufficient to invoke complete amorphization. Hence, the grating comprised of simple point defects in an otherwise
crystalline structure. Therefore, the
low-temperature anneal was enough to eliminate the point defects and lower the
refractive index to near that of c-Si and consequently eliminating the Bragg
response. At the present time, further samples are being fabricated using a
higher implantation dose, and a report of the annealing will be made in the
future on the successful thermal processing of these grating structures.No attention was paid to minimizing the optical
propagation loss in these relatively crude slab waveguides. It is anticipated
that Fresnel loss, mode mismatch between the input beam and the waveguide, and
incomplete collection of the output would introduce a significant “coupling”
loss in this system of tens
of dB. Of some importance to these specific devices is the excess loss caused
by the introduction of implantation induced lattice damage to the SOI (as in
the case of the silicon implantation). This has been previously measured to be
as much as 1000 dBcm-1 for almost completely irradiated rib waveguide
structures [23]. Given the shallow depth of the implantation used in this study
(on the order of 100 nm) and thus the relatively small overlap of the mode with
the lattice damage, we expect an excess loss in the region of 10 dB. Although
this is undesirable, it is likely to be reduced significantly following low-temperature
annealing [19]. The balance between minimizing optical loss and producing a
strong interaction between the propagating mode and the grating will be dealt
with in some detail in a future publication.For this initial
study, grating fabrication is limited to slab waveguides, although they will
ultimately be of more practical importance when incorporated into rib
waveguides. Whereas rib waveguides can
be made effectively single mode by suitable choices of rib width and etch depth
[2], slab waveguides of the thicknesses used in this study are highly
multimode. The resonances shown in Figures
3 to 5 result from the coupling of light in the forward propagating fundamental
mode into the reverse propagating fundamental mode. One might also expect to see resonances due
to coupling from the forward propagating fundamental mode into higher-order
reverse propagating modes or from higher-order forward propagating modes into
reverse propagation of the same mode. The former type of higher-order resonance is observed in the case of
relief gratings on rib waveguides where the higher-order modes are leaky [11]. We also expect to see this when implanted gratings
are used in rib gratings. The higher-order
resonances occur at wavelengths that are a few tens of nm lower than that of
the fundamental resonance, corresponding to effective index differences of 1–2%
between low-order modes in these structures. In this work, however, we observed neither type of higher-order
resonance. We believe that the former
type will be much weaker than in the rib waveguide case because of more
complete mode orthogonality. The latter
case is absent because the input light couples predominantly into the
fundamental mode so that any individual higher-order mode has very little power
in it to lose to resonant coupling.


## 5. Conclusion

This report
demonstrated the viability of utilizing ion induced refractive index change in
the manufacture of an optical filter in silicon waveguides. Response curves typical of passive grating
filters were achieved, and extinction ratios of−10 dB and −7 dB were found for
oxygen implants and −5 dB and −2 dB for silicon implants for TE and TM polarized
light, respectively. From CMT analysis,
it appears that the current results have the potential to be improved upon
significantly. These implanted gratings
could serve a useful role in future three-dimensional integrated photonics
whose fabrication is contingent on surface sensitive bonding techniques.



---

*Source: 276165-2008-04-23.xml*