defect-mediated resonance shift of silicon-on-insulator racetrack resonators

Defect-mediated resonance shift of silicon-on-

insulator racetrack resonators

J. J. Ackert,1,* J. K. Doylend,

1 D. F. Logan,

1 P. E. Jessop,

2 R. Vafaei,

3 and L.

Chrostowski3 and A. P. Knights

1

1Department of Engineering Physics, McMaster University, Hamilton, Ontario, L8S 4L7, Canada 2Department of Physics and Computer Science, Wilfrid Laurier University, Waterloo, Ontario, N2L 3C5, Canada

3Department of Electrical and Computer Engineering, University of British Columbia,

Vancouver,V6T 1Z4, British Columbia, Canada

*[email protected]

Abstract: We present a study on the effects of inert ion implantation of

Silicon-On-Insulator (SOI) racetrack resonators. Selective ion implantation

was used to create deep-level defects within a portion of the resonator. The

resonant wavelength and round-trip loss were deduced for a range of

sequential post-implantation annealing temperatures from 100 to 300 °C. As

the devices were annealed there was a concomitant change in the resonance

wavelength, consistent with an increase in refractive index following

implantation and recovery toward the pre-implanted value. A total shift in

resonance wavelength of ~2.9 nm was achieved, equivalent to a 0.02

increase in refractive index. The excess loss upon implantation increased to

301 dB/cm and was reduced to 35 dB/cm following thermal annealing. In

addition to providing valuable data for those incorporating defects within

resonant structures, we suggest that these results present a method for

permanent tuning (or trimming) of ring resonator characteristics.

©2011 Optical Society of America

OSIC codes: (130.3120) Integrated optics devices; (250.5300) Photonic integrated circuits;

(230.5750) Resonators.

References and Links

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#144602 - $15.00 USD Received 24 Mar 2011; revised 15 May 2011; accepted 27 May 2011; published 6 Jun 2011(C) 2011 OSA 20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 11969

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1. Introduction

Silicon photonics continues to attract a great deal of attention as a likely candidate technology

for the implementation of optical interconnections [1]. Together with its compatibility with

standard Complimentary-Metal-Oxide-Semiconductor (CMOS) processing methods, the

scalability of silicon has a potential for terabit data transmission when deployed inside a

Wavelength Division Multiplexing (WDM) optical communication system paradigm. To this

end, the SOI ring (or race-track) resonator is a key building block offering the ability to

modulate, route and detect individual channels.

The deliberate introduction of ion implantation induced defects into silicon waveguides

has been demonstrated to expand the optical functionality of silicon-based optical integrated

circuits, for example through the modification of carrier lifetime [2] and the fabrication of

waveguide photodetectors with sensitivity to sub-band wavelengths [3].

This report presents quantitative data on the impact of low concentrations of deep-levels

on the real part of the refractive index of silicon waveguides for wavelengths around 1550

nm, obtained from the study of the effects of inert ion implantation on race-track resonators.

The data presented is of general interest to those wishing to incorporate defects into ring

resonators, such as in the case of the resonant detectors, recently reported by Doylend et al.

[4] Logan et al. [5] and Preston et al. [6].

We also note the potential for defect implantation as a means of device trimming. One of

the major issues facing the implementation of ring resonator structures is fabrication

tolerance. These devices rely on evanescent field coupling, which strongly depends on the

waveguide width and separation from the ring. Small variations that arise in fabrication

prevent a resonant structure achieving its design specification within accepted tolerance. In

this case, active tuning, often through application of the thermo-optic effect, is required to

modify the resonance. This can be accomplished by local heating of the ring with a resistive

metal strip [7]. Another method, limited to rib waveguides, is to use a p-i-n diode structure

over the waveguide [8,9]. Applying a bias changes the concentration of free carriers in the

ring, thus altering the refractive index [10]. These methods yield adequate tuning but

introduce added complexity to the device. For large devices containing multiple rings [11]

active tuning becomes less attractive as each device requires its own independent tuner and

control circuit. The tuners consume chip area and power, reducing the advantages gained by

using resonant structures over other geometries such as Mach-Zender Interferometers (MZI)

[8].


We demonstrate that the resonance location of silicon race-tracks can be shifted by

introducing deep-levels within the silicon bandgap via ion implantation and subsequent

annealing. This represents a method to passively, and permanently tune a resonant structure.

The tuning is accompanied by excess loss, but in many cases this trade-off may be within

device tolerance limits. These first results on defect mediated tuning suggest further

investigation to explore the limits of this technique, particularly in conjunction with selective

annealing as can be achieved via focussed laser activation [12].

2. Device description and experimental procedure

2.1 Formation of the waveguide structures

The structures used in this experiment were double-bus SOI racetrack resonators, consisting

of 30 µm radius curves and either 15 or 40 µm coupling regions. Fabrication was carried out

with the passive SOI platform of IMEC in Leuven, Belgium, and facilitated via CMC

Microsystems. The SOI wafers were comprised of a 220 nm thick top layer of silicon over a 2

µm buried oxide, and waveguides were patterned with 193 nm deep UV lithography [13].

Waveguide width was varied during fabrication by adjusting the photo-exposure; those

used in this study were estimated to be approximately 450 nm wide. Coupling in and out of

the waveguides was achieved through shallow-etch integrated grating couplers [14].

2.2 Ion implantation

Ion implantation was done at relatively high energy so that the implant species would

penetrate through to the oxide, leaving only structural, point defects in the waveguide layer.

Photolithography was used to define implantation windows as to avoid the coupling regions.

By avoiding the couplers we maintain the same coupling coefficients, which allows the

change in loss and the Quality factor (Q) from the implant to be observed. Figure 1 (a) shows

a scanning electron microscope (SEM) image of a racetrack resonator, while Fig. 1 (b) shows

the photomask layout of the implant windows.

Multiple chips were implanted with varying implant dosage below the amorphization

threshold [15]. The ions implanted were either boron at 350 keV or silicon at 700 keV,

energies sufficient for the ions to reside in the oxide layer. Subsequently in this work each

chip will be referred to in a manner which encompasses the implantation procedure it

underwent in terms of implant dosage, implant species, and a letter to signify variations in

resonator length; for example device A/1.75E14-Si refers to a chip with an implant dosage of

1.75x1014

cm2

, with silicon ions (at 700 keV). The prefix „A‟ designates a resonator

circumference of 218 µm, while „B‟ indicates 268 µm.

2.3 Characterization

After the implantation, cleaning the masking resist involved chip immersion in a H2SO4 +

H2O2 or „piranha etch‟ solution. The heat from this reaction formed the baseline annealing

temperature of 100 °C. Once implanted and cleaned, each resonator was annealed in

sequential steps of 25 °C, to a maximum of 300 °C. Measurements between annealing steps

were taken using a New Focus Model 6427 external cavity tunable diode laser, with a cleaved

polarization maintaining single mode fiber to couple light to the chip. A single mode fiber

was also used to couple light from the chip, where a Newport 818-1S1 power meter took

readings. Total coupling loss in and out of the device is estimated to be 11 dB. Repeated

measurements showed a maximum temperature drift of ± 0.1 nm, this error is small relative to

the total resonance shift observed.


Fig. 1. (a) SEM image of a racetrack resonator (b) The ion implantation photomask overlaid on

the racetrack design. The implantation windows avoid the couplers and bus waveguides. (SEM image taken at the Canadian Center for Electron Microscopy)

3. Results

3.1 Experimental results

Ion implantation and subsequent annealing has two pronounced effects on the resonator

characteristics. The first is a resonance shift observed after implantation, which decreases in

magnitude as the ring is annealed. The resonance wavelength thus progressively moves to

lower wavelengths after exposure to higher annealing temperatures. The second effect is that

the Q of the ring is reduced after implantation. This is unsurprising given the previously

reported increase in optical loss with increasing concentration of ion implantation defects in a

silicon waveguide [16]. As the annealing temperature is increased we observed the Q increase

towards the pre-implanted value.

An example of optical spectra for a single chip is shown in Fig. 2 where the transmitted

power of device A/3E14-Boron after various post-implantation annealing temperatures is

plotted.

Fig. 2. Resonance shifting for device A/3E14-Boron. The trend of this result is representative of other devices, including those that received silicon. As the annealing temperature is

increased the resonance peaks shift to lower wavelengths and the Q-factor increases.

The effect of implantation dose on the resonance shift and propagation loss is summarised

in Fig. 3. In Fig. 3(a), we show a comparison of resonance shift versus annealing temperature

for three nominally identical resonators (from three different chips) which received different

implant dosage. There is a clear increase in the resonance shift with increasing implantation

dose, indicating a direct relationship between the real component of the refractive index (RI)

and the concentration of deep levels introduced by the implantation. The shift in RI is positive

after ion implantation, with a progressive recovery toward the value for unimplanted


waveguides as annealing is increased. This is discussed in detail in section 3.2 Modelling the

RI shift.

To extract the propagation loss from measured data such as that shown in Fig. 2, the

resonance spectra were fitted with the analytical through-port transmission function for

double bus resonators with identical couplers, Eq. (1):

2 2 2 2

2 4 2

2 cos

1 2 cos

in

out

P t A t At

P A t At

(1)

where Pin is input power, Pout is output power, t is the transmission coefficient, γ is the

insertion loss, A = exp(αL) with L as the resonator length, α is the loss coefficient, and δ =

2π/(λneffL) is the phase. After fitting the results, it was found that the transmission coefficients

remained within the fitting error (as expected given that the coupling region was masked

during implantation), but the loss decreased with annealing. Figure 3(b) shows the extracted

values for excess propagation loss as a function of annealing temperature, for the same

devices as Fig. 3 (a). It is instructive to compare the loss values for the lowest temperature

anneal (which is likely to be representative of the loss in the as-implanted condition) to values

extracted from the empirical model proposed by Foster et al. [16]. The usefulness of the

Foster model is that it allows the comparison of expected loss for a wide range of

implantation conditions (i.e. variations in ion species, ion dose, and ion energy). For the

silicon implanted samples the values of expected loss would be 23, 96 and 152 dBcm1

for

increasing ion dose. These compare to measured values (shown in Fig. 3b) of 71, 239 and

301dBcm1

. The values measured here are thus approximately twice those which might be

expected from the empirical model proposed by Foster et al. We feel that this discrepancy is

not wholly unexpected given the different experimental conditions used by Foster et al. in

their work, from which the empirical model was developed. We also note that the Foster

model has previously underestimated experimental optical loss [16].

As the devices are annealed the loss trends downward, in a manner consistent with the

shift in resonance. The response of resonance wavelength and propagation loss to annealing is

similar. This is not surprising given that these properties represent the real and complex

components of the refractive index respectively, related via the Kramer-Kronig relations. The

precipitous decrease in both properties at temperatures > 200°C suggest that the deep-level

responsible for the initial post implantation change is the silicon divacancy, previously

observed to be the primary optically active defect formed after relatively low dose inert ion

implantation in a silicon waveguide [17].

Fig. 3. Resonance shift (relative to the implanted state), as a function of annealing: (a) three identical devices, were implanted with different doses of silicon at 700 keV; (b) the total

optical loss of the race-tracks. Note: Marker size is indicative of the uncertainty.


The increase in propagation loss may also be represented by the Q-factor of the resonator.

This was extracted by curve fitting and plotted in Fig. 4 for two representative devices. An

increase in blue shift of the resonance peak (compared with the as-implanted device annealed

at 100°C) corresponds to an increasing Q as the defects are removed.

Fig. 4. Quality Factor and resonance shift (relative to the implanted state) versus annealing temperature for resonator (a) A/1.5E14-Si and (b) B/1.25E12-Si

3.2 Modelling the RI shift

The change of the effective index of the SOI waveguide, versus the change in the silicon

index, is plotted at a representative wavelength of 1563 nm in Fig. 5 (a). The slope is dneff /

dnSi = 1.177. This quantity is also known as the effective index susceptibility, previously used

to describe the effective index changes in SOI waveguides due to the core index changes (due

to temperature) or cladding changes [18]. Figure 5 (b) shows the mode profile is increasingly

localized in the centre of the waveguide as index increases. This is similar to the effect of

modal dispersion and gives rise to an increase in the effective index that is surprisingly 17.7%

larger than the change in the silicon index itself.

Fig. 5. a) The change in the waveguide effective index as a function of the change in the silicon

index of refraction for a 450 x 220 nm waveguide at 1563 nm; b) The mode profile in the in-plane direction, at the centre of the waveguide (at a height of 110 nm), for different silicon

indices. nSi = 3.48 (red) and 3.47 (blue), nSiO2 = 1.444. 450 x 220 nm waveguide. This

waveguide has an ng (1563 nm) = 4.55.

Here, we derive expressions for the change in the resonance condition as a function of the

change in index of refraction. First, consider a resonator with a uniform non-dispersive

medium, with a length L. For a mode index m, the mode condition is given by Eq. (2).


2

m

nLm

(2)

A change in index of refraction leads to a change in resonance wavelength, Eq. (3):

d L

dn m n

(3)

In a resonator with dispersion (material, waveguide), the mode condition is given by Eq.

(4):

2 eff

m

n Lm

(4)

and the free-spectral range is Eq. (5):

2

,eff

g eff

g

dnFSR n n

Ln d

(5)

To analyze the effect of the change in silicon index of refraction, we need to consider the

change in the mode profile that changes the index of refraction. Thus, we can write the

expression for effective index as, Eq. (6):

eff eff

eff g Si

Si

dn dnn n n

d dn

(6)

Using the mode condition (Eq. (4), we equate the mode at the initial wavelength to the

shifted wavelength, shown in Eq. (7).

00

0 0

eff effeffg Sig

Si

dn dndnn nn

d dnd

(7)

This allows us to find the wavelength shift, which simplifies to Eq. (8):

effSi

g Si

dnn

n dn

(8)

We note that in the case where the effective index change is equal to the silicon index

change, i.e., neglecting effective index susceptibility, the wavelength shift reduces to Eq. (9).

This relation was presented previously in [19]:

0

eff

g

n

n

(9)

In essence, the shift in resonance wavelength is determined by three factors: a) the shift in

the material index of refraction, b) the material and waveguide dispersion – a change in

wavelength causes a further change in effective index, and c) the change in material index

changes the mode profile resulting in an additional effective index change. It should be noted

that the same effect is found in the temperature dependence of waveguide-based resonators

[18,20].

Using Eq. (8) we can now calculate the maximum index shift for the implanted resonators

as a function of dose. The group index, ng = 4.545, was found from the resonator free-spectral

range, length and operating wavelength λ0 = 1563nm. From Fig. 3 the doses used were 1.25 x

1012

cm2

, 7.5 x 1013

cm2

and 1.75 x 1014

cm2

. The resonance wavelengths were shifted,

respectively, Δλ = 0.7 nm, 2.2 nm and 2.9 nm. With approximately one third of the waveguide


being implanted, this leads to increases in the silicon refractive index of ΔnSi = 0.005, 0.016

and 0.021 respectively.

3.3 Summary

The results described above provide an important insight into the impact of defects on the

characteristics of a ring resonator. The deliberate introduction of defects into a resonant

structure to date has been limited to the development of resonant enhanced detectors [4,5].

However, we suggest here that defect injection and selective annealing offers an intriguing

solution to post fabrication trimming of the response of resonators across a complete wafer.

Clearly, blanket ion implantation and traditional thermal annealing will simply result in a

systematic shift in the characteristics of all resonators on a single wafer. However, selective

laser-assisted annealing may provide local removal of defects on an areal scale of tens of

square microns [12]. This would then facilitate test and trimming at the wafer scale of each

resonator. The most obvious disadvantage of this approach is the increase in round-trip loss

with defect introduction. Future work will help elucidate the limitations of this trimming

technique.

4 . Conclusion

SOI double bus racetrack resonators have undergone selective ion implantation to introduce

deep level defects into the silicon waveguide. We have observed a shift in resonance of nearly

3 nm after ion implantation of silicon at an energy of 700 keV and a dose of 1.5x1014

cm2

.

The magnitude of the resonance shift corresponds to the excess loss within the resonator. This

is evidence for a refractive index increase of 0.021 associated with the concentration of

structural point defects within the silicon waveguide. This shift in the real part of the

refractive index, when combined with selective laser assisted annealing, presents a method for

permanent tuning of resonators on the wafer level scale.

Acknowledgements

The authors would like to thank Jack Hendriks at the University of Western Ontario for ion

implantation, Chris Brooks at McMaster University for photolithography assistance and Dan

Deptuck at CMC Microsystems for facilitating the device design and Greg Wojcik for useful

discussions. We also thank Lumerical Solutions Inc. for providing the mode solver MODE

solutions. The authors acknowledge the support of CMC Microsystems, the Canadian Institute

for Photonic Innovations and the Natural Sciences and Engineering Research Council of

Canada.


defect-mediated resonance shift of silicon-on-insulator racetrack resonators

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