ultra-low crosstalk, cmos compatible waveguide crossings for densely integrated photonic...

Ultra-low crosstalk, CMOS compatible waveguide crossings for densely integrated

photonic interconnection networks

Adam M. Jones,1,2,* Christopher T. DeRose,1 Anthony L. Lentine,1 Douglas C. Trotter,1 Andrew L. Starbuck,1 and Robert A. Norwood2

1Applied Photonic Microsystems, Sandia National Laboratories,Albuquerque, NM,87123, USA 2College of Optical Sciences, University of Arizona,Tucson, AZ, 85721,USA

*[email protected]

Abstract: We explore the design space for optimizing CMOS compatible waveguide crossings on a silicon photonics platform. This paper presents

simulated and experimental excess loss and crosstalk suppression data for

vertically integrated silicon nitride over silicon-on-insulator waveguide

crossings. Experimental results show crosstalk suppression exceeding 49/-

44 dB with simulation results as low as 65/-60 dB for the TE/TM mode in a waveguide crossing with a 410 nm vertical gap.

2013 Optical Society of America

OCIS codes: (130.0130) Integrated optics devices; (200.4650) Optical interconnects.

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#185599 - $15.00 USD Received 19 Feb 2013; revised 25 Mar 2013; accepted 8 Apr 2013; published 9 May 2013(C) 2013 OSA 20 May 2013 | Vol. 21, No. 10 | DOI:10.1364/OE.21.012002 | OPTICS EXPRESS 12002

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30. R.D. Kekatpure, Applied Photonic Microsystems Group, Sandia National Laboratories, 1515 Eubank SE, Albuquerque, N.M. 87123, A. Jones, D.C. Trotter, A. Starbuck, C. DeRose, and A.L. Lentine are preparing a

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1. Introduction

Device density in photonic interconnection networks continues to grow at a rapid pace as the

bandwidth limitation of electrically connected networks makes photonics economically viable

in long and medium-haul applications such as tele- and data communications [1,2]. This

increase in device density and corresponding increase in on-chip network complexity

naturally leads to a significant increase in the number of optical waveguide crossings required

to perform the desired signal routing [3,4]. Waveguide crossings reduce system performance

in two significant ways. First, the signal is attenuated at each crossing due to scattering from

the intersection. This leads to increased system loss and differential path loss based on the

number of crossings traversed, both of which negatively impact the power budget of the

optical system. If enough crossings are present in the system, this factor alone leads to the

requirement for optical amplifiers and signal grooming components which ultimately increase

system cost and complexity. Second, as the optical signal scatters from the intersection,

power is coupled into the waveguide being crossed leading to unintended effects on the

optical power distribution. For a large number of waveguide crossings, this crosstalk can

cause a substantial unintended signal power in a given waveguide leading to a power penalty

and increased communication errors. An example schematic of the routing portion of a 16

port Banyan switch is shown in Fig. 1.


Fig. 1. Routing portion of a 16 port non-blocking Banyan switch matrix with NOS crossings at

various angles; for N ports, the maximum number of waveguide crossings is given by N/2 + 1.

Because the scattered power scales directly with index contrast, excess loss and crosstalk

suppression are especially poignant issues in high-index-contrast systems. For example, in

silicon-on-insulator (SOI) waveguides, the core-cladding refractive index contrast is near 2.0

in the near infrared. Several authors have made strides in optimizing in-plane waveguide

crossing performance via modification of the waveguide dimensions in the region near the

crossing. Approaches include the use of photonic crystals [5,6], subwavelength diffraction

gratings [7,8], resonant cavities [911], wavefront matching [12,13], multimode interference regions [1416], mode transformation [1720] and others [2123] with the best broadband, fabrication tolerant results reported by Bogaerts, et.al [17]. with demonstrated crossing excess

loss as low as 0.16 with 40 dB crosstalk suppression at 1550 nm. Bock, et. al. [7] demonstrated superior broadband loss performance in subwavelength grating waveguides

with < 0.1 dB per crossing reported. These structures, however, may require advanced lithography nodes due to the small feature size and relatively large aspect ratio.

However, many of the highest performance waveguide crossings reported in the literature

are resonant in nature and therefore suffer from narrow optical bandwidths and strict

fabrication tolerances. As an alternative, vertically separated waveguides present a design

tolerant option for achieving low loss, low crosstalk waveguide crossings. This approach has

been discussed in the literature [4,24] with vertical transitions [25], vertically coupled

resonators [26], waveguide crossings in low index contrast systems [27] and active optical

materials [28] demonstrated for specific use-cases.

In this paper, we present a numerical and experimental study of the performance of silicon

nitride over SOI vertical waveguide crossings for photonic interconnection networks. We

have chosen a silicon nitride over SOI system because both materials are: 1) present in most

modern CMOS lines enabling direct integration with current electronics fabrication processes

and economies of scale, 2) exhibit low loss across the S, C, and L optical telecommunication

transmission bands given proper processing conditions and waveguide geometries, and 3) are

capable of low loss, small radii bends due to the large index contrast of both waveguide

systems. The use of silicon nitride as a secondary optical layer is particularly advantageous in


that propagation losses as low as 2.9 dB/m have been reported [29]. In addition, proper

tapering of the silicon nitride layer at the end-facets of the chip can yield fiber-to-waveguide

coupling losses as low as 0.5 dB [30] for standard single mode fiber, thereby enhancing the

commercial viability of the presented technology via reduced packaging cost.

2. Device geometry

The optical system analyzed here is a four port nitride-over-silicon (NOS) waveguide

crossing as shown in Fig. 2. Silicon is used as the primary optical routing layer with vertical

transitions into and out of silicon nitride in the crossing region. Adiabatic vertical waveguide

transitions [25] 100 m in length were used, yielding low-loss transitions between optical layers. Five m radii bends are used to produce minimal bending loss and a perpendicular crossing geometry. The silicon waveguides were 400 nm wide and 230 nm thick while the

silicon nitride waveguides were 250 nm thick with a 1.2 m width. These geometries were chosen to ensure single mode operation while minimizing propagation loss in each layer.

Fig. 2. Schematic of the crossing (a), cross-sectional view of the crossing region perpendicular

to the silicon nitride (b) and silicon (c) waveguides, and top-down schematic view of the waveguide crossing (d).

All fabrication was carried out using a silicon photonic process developed within the

CMOS foundry at Sandia National Laboratories MESA facility. The silicon waveguides were fabricated via optical lithography followed by dry etching of the silicon device layer. A

subsequent reduction of sidewall roughness via thermal oxidations, which consumed 20 nm,

yielded a final device layer thickness of 230 nm. PECVD oxide was then deposited followed

by CMP to planarize the structure leaving the desired thickness oxide as a vertical buffer

between the silicon and silicon nitride waveguides. After CMP, silicon nitride was deposited

via CVD followed by a second lithography process and subsequent dry etch to create the

silicon nitride waveguides. Finally, PECVD oxide was deposited to passivate and protect the

structure while also serving as an upper cladding.

The thickness of the buffer oxide between waveguide layers was critical in determining

device performance. Variance limitations in the CMP process resulted in a vertical gap


variability of 100 nm from the targeted thickness. In order to ensure the validity of the

resulting data, a Rudolf AutoEL III ellipsometer with a vertical thickness resolution of 3

was used to measure the thickness of the buffer oxide. Utilizing this technique, error in the

analysis due to variability in the CMP process was significantly reduced; however, variation

in the flatness of the planarized surface and differential performance of the process over

structures of different size resulted in a vertical measurement error of 50 nm.

Advances in CMP technology including reticle modification, endpoint monitoring, and

multi-zone polishing heads have enabled process standard deviations of less than 5 nm over a

set of 5000 wafers [31]. This level of process control will ultimately enable performance

repeatability rivaling that of lithographically defined structures and supports the validity of

using this approach in novel systems and commercial products alike.

3. Crosstalk calculation

Crosstalk in a passive four port optical system, as depicted in Fig. 2, is defined as the ratio of

the output power in the cross state to the sum of the signals in the cross and bar states.

Defining the power coupling coefficient from input port i to output port j as kij, the crossing-induced crosstalk for input ports 1 and 2 may be calculated as

13 13 14 241 213 14 14 23 24 23

.XT XT

and (1)

For systems of interest, the power in the cross state is sufficiently low that the crosstalk is

approximately the ratio of the power in the cross state to the power in the bar state; this leads

to the set of simplified relations displayed in Eq. (1).

In real systems, the crossing cannot be directly probed and factors such as coupling and

propagation loss must be taken into account when attempting to extract the power coupling

coefficients. The externally measured (port-to-port) crosstalk for input ports 1 and 2 may be

written as

13 1 3 1 3 13 0 3 3 13114 1 4 1 4 14 0 4 4 14

.P L L P L

EXTP L L P L

(2)

2 4 2 4 24 024 4 4 24223 2 3 2 3 23 0 3 3 23

.L L PP L

EXTP L L P L

(3)

where i is the fiber-to-waveguide power coupling efficiency for port i, Li is the

propagation loss from port i to the waveguide crossing, and P0 is the input power. If the path and coupling losses for all ports are equal, the above relations reduce to the crossing-induced

crosstalk; however, factors such as alignment error and variability in fabrication still

introduce error in attempting to directly calculate the crosstalk for a given structure.

By taking the product of the externally calculated crosstalk values, the path and coupling

losses cancel leaving the product of the crossing-induced crosstalk, given by:

3 3 13 134 4 24 241 2 1 24 4 14 3 3 23 14 23

.L L

EXT EXT XT XTL L

(4)

The square root of Eq. (4) yields the geometric mean of the crosstalk which may be

rigorously compared to the data with no assumptions or approximations applied to the result.

For systems in which the crossing waveguides have similar mode volumes and propagation

constants, the crosstalk values for both input ports converge and the results from the above

treatment reduce to those produced by simplified formulations of coupled mode theory

(CMT) [32].


While CMT treatments have been developed [32], these results cannot be used directly in

this case, even if scattering loss is accounted for. Although reciprocity dictates that each

,i j jik k i j , it does not require the transmission coefficients for input ports 1 and 2 ( 14k and

23k here) to be equal. Due to reduced confinement of the silicon nitride mode in NOS

crossings, the magnitude of the interaction of the optical mode in silicon nitride with the

crossing silicon waveguide is far stronger than the inverse case. In addition, the magnitude of

the field coupled to radiation modes caused by interaction with an index perturbation is

directly proportional to the index contrast of the perturbation. The index contrast of the SOI

waveguide is approximately a factor of four larger than its silicon nitride counterpart. Thus,

significantly increased scattering of the optical modes in the silicon nitride waveguide is

expected. These factors combine to invalidate the assumptions made in typical coupled mode

theory treatments and thus indicate the need for a rigorous treatment of the crossing region

interaction for this system.

4 FDTD simulation results

4.1 Modal field calculations

The devices tested were simulated in three dimensions using the commercially available finite

difference time domain (FDTD) software FDTD Solutions (Lumerical Inc.) [33]. The

simulations were performed at the highest density grid setting using a conformal mesh to

enhance the speed and accuracy of the result. The optical modes of the isolated silicon and

nitride waveguides are presented in Fig. 3 for the fundamental TE and TM modes. Figure 4

shows the TE and TM modes in a cross-section through the crossing region perpendicular to

the silicon waveguide as shown in Fig. 2(c) for structures with various vertical gaps between

the silicon and silicon nitride waveguides. Analogous mode simulations for cross-sections

perpendicular to the silicon nitride waveguides (as in Fig. 2(b)) are presented in Fig. 5.

Fig. 3. Simulated TE (a,b) and TM (c,d) normalized modal field amplitudes for the isolated silicon (a,c) and silicon nitride (b,d) waveguides. The silicon waveguides are 230 X 400 nm

while the silicon nitride waveguides are 250 X 1200 nm (waveguide borders represented by

dark lines).


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Fig. 4. TE (a-c) and TM (d-f) normalized modal field amplitudes within the crossing region

perpendicular to the silicon waveguide (refer to Fig. 2(c)) for waveguide crossings with 0 (a,d),

150 (b,e), and 400 nm (c,f) vertical gaps. The silicon and silicon nitride waveguide booundaries are represented by dark lines.

Fig. 5. TE (a-c) and TM (d-f) normalized modal field amplitudes within the crossing region

perpendicular to the silicon nitride (refer to Fig. 2(b)) for waveguide crossings with 0 (a,d),

150 (b,e), and 400 (c,f) nm vertical gaps. The silicon and silicon nitride waveguide boundaries are represented by dark lines.

Due to high confinement, the TE mode in the silicon guide experiences little modification

as the vertical gap decreases, retaining its fundamental shape with minor modification of the

exponential decay in the vertical direction. Qualitatively, the expected magnitude of the field

scattered by the waveguide crossing is proportional to the level of disturbance experienced by

the optical mode. Because of this, we expect the TE mode in silicon to demonstrate very low

crosstalk and excess loss values. Conversely, the TM field shifts significantly and begins to

strongly overlap the silicon nitride waveguide as the two waveguides are brought into

proximity. This is expected due to the reduced confinement of the TM optical mode in the

silicon waveguide. It is further expected that the crosstalk and excess loss values for the TM

optical mode will be larger than those of the TE mode.

In the nitride waveguide, the response is somewhat counterintuitive; the crossing exhibits

superior performance for the TM mode. Field confinement within the high index regions is


reduced for the TM field with peaks at the upper and lower boundaries of the high index

region, much like isolated silicon and nitride waveguides. This reduces the magnitude of the

field in the crossing waveguide, which ultimately leads to improved crosstalk. The large size

of the TM mode also leads to a reduction in excess loss as the response to the index

perturbation is effectively reduced. This phenomenon is well demonstrated in structures such

as those developed by Bogaerts [17] and others [14,15].

4.2 E-field propagation

Images of the E-field propagating through the silicon and silicon nitride waveguides are

presented in Figs. 6 and 7 respectively. These images show the differential in magnitude of

the field coupled to radiation modes upon interaction with the coupling region for each input.

Also of import is the reflected field, which is made visible by the interference pattern to the

left of the interaction region. While difficult to see for the silicon input, the response from the

silicon nitride input creates a clear standing wave pattern.

Fig. 6. Normalized field amplitude propagating through the silicon waveguide with 0 (a,c) and

400 nm (b,d) gaps for TE (a,b) and TM (c,d) polarizations. The rectangle above the silicon waveguide represents the nitride waveguide borders.

Fig. 7. Normalized field amplitude propagating through the nitride waveguide with 0 (a,c) and 400 nm (b,c) gaps for TE (a,b) and TM (c,d) polarizations. The silicon waveguide is shown in

black below the nitride waveguide.


4.3 Crosstalk and excess loss simulations

Crosstalk and excess loss simulations were performed using mode monitors in FDTD

Solutions [33]. These monitors capture the electric field passing through a given region and

numerically evaluate the overlap integral with the modal field of the waveguide thus

eliminating errors due to stray fields present in the simulation window. Using this technique,

the excess loss and crosstalk values were predictively modeled for a wide range of vertical

gaps and signal wavelengths. The results of these simulations for optical inputs in the silicon

(port 1) and silicon nitride (port 2) waveguides are presented in Figs. 8 and 9 respectively.

The following graphs contain a good deal of data and are organized as follows. The

simulated response of the system over the 1500 to 1600 nm wavelength range is represented

as a set of filled patches with the boundaries at 1500 and 1600 nm represented as dashed and

solid lines, respectively. Points shown along each line are simulation data points with the line

between points generated by spline interpolation. In all cases, the system varied

approximately linearly with wavelength. Each experimental data point is represented by a

circle showing the response at 1550 nm with error bars used to show the response of the

system within the 1500 to 1600 nm wavelength range. The filled patches surrounding each

data point indicate measurement error of both throughput and interlayer thickness. Excluding

the TM response of the silicon nitride input for buffer thicknesses less than 550 nm, the

excess loss of the crossing was directly proportional to wavelength. The inset in Fig. 8(b)

shows the crossing point for this behavior with a number of simulations performed around

this point to ensure the validity of the simulation data.

Fig. 8. Excess loss as a function of gap for silicon (a) and silicon nitride (b) input ports over

the 1500 to 1600 nm wavelength range for TE and TM polarizations.

Fig. 9. Crosstalk vs. gap for silicon and silicon nitride input ports over the 1500 to 1600 nm

wavelength range for TE (a) and TM (b) polarizations.


Crosstalk performance trends the same for both waveguide systems with a convergence of

the crosstalk values as the vertical gap increases. This result is important in that it validates

the use of the geometric mean in approximating the calculated crosstalk values for both

waveguide systems for sufficient gap sizes.

These simulations indicate that the crosstalk reaches exceptionally low values for

relatively modest vertical gap sizes. Specifically, 40 dB crosstalk suppression is achieved for TE and TM polarizations for both input waveguides with a vertical gap as low as 125 nm.

Using a gap size of 400 nm produces crosstalk suppression values exceeding 60 dB, leading to significantly superior crosstalk suppression when compared to in-plane approaches.

The excess loss for the silicon input is an order of magnitude lower than that of the silicon

nitride input. While the excess loss for the silicon input quickly approaches zero with excess

losses well below 0.1 dB for vertical gaps exceeding 400 nm, the excess loss for the silicon

nitride input is significant for small gap sizes increasing to maxima of 15 and 3.4 dB for TE

and TM polarizations, respectively, at a wavelength of 1550 nm. In order to achieve excess

losses below 0.1 dB, the vertical gap must be increased to over 1 m. At this gap size, the excess loss for the silicon input is negligible; reaching values below 0.003 and 0.03 dB for TE

and TM polarizations. The drawback of arbitrarily increasing the vertical gap between the

waveguides is that doing so may have unintended consequences in the design of multi-layer

optical networks such as an increase in the taper length required to efficiently transition

between layers, which ultimately affects the required on-chip footprint and the number of

crossings required to achieve superior system performance over single layer networks. In Fig.

10, we provide a comparison of vertical transition and crossing excess loss for the silicon

nitride input port. In examining this data, we see that as the vertical gap and taper lengths

change, the regime in which the excess loss is dominated by the crossing changes. This

behavior results in a design optimization process that must be carried out on a case-by-case

basis based on the crosstalk, excess loss, and on-chip area requirements of the system.

Fig. 10. Comparison of vertical transition (solid lines) and crossing loss (dashed green lines, number of crossings traversed in bold) for TE (a) and TM (b) modes. All crossing loss

measurements are for the silicon nitride input.

As an alternative to increasing the gap between optical layers, the excess loss of the

silicon nitride guide may be reduced via modification of the waveguide geometry in the

crossing region using one of several approaches in the literature [723]. This design and optimization is outside the scope of this paper and will be pursued in future work.

5. Experimental method and results

An Agilent 81600B tunable laser source was used to test the response of waveguide crossings

with silicon-to-nitride vertical gaps between zero and 400 nm with wavelengths ranging from


1500 to 1600 nm at a resolution of 10 pm. The polarization of the signal was controlled via a

Thorlabs FPC560 Three Paddle Polarization Controller that was used to reduce polarization

crosstalk to well below 30 dB. The optical measurement was performed in a fiber-to-fiber configuration using OZ Optics tapered and lensed fiber with a spot size of 2 m to enhance coupling efficiency to the on-chip waveguides. The signal was then detected using an Agilent

81635A Dual Optical Power Sensor with an intrinsic optical noise floor of 80 dBm. A schematic of the optical test setup is presented in Fig. 11.

Fig. 11. Schematic of the optical test setup.

Calculating the crosstalk requires accurate alignment of optical paths with signals near the

optical noise floor. This sort of alignment cannot be done using the standard technique (i.e.

translating each fiber until the maximum power is transmitted) as stray light often causes

local maxima in the transmitted power that do not correspond to optimal coupling to the on-

chip waveguide. Instead, we used a 10X objective with an optical axis normal to the surface

of the chip coupled to a Sensors Unlimited, Inc. InGaAs SWIR camera to image the crossing

region of the device. Light is preferentially scattered by the crossing region only when the

input signal is efficiently coupled to the waveguide. This method enables efficient coupling of

low-throughput optical paths with a high degree of confidence and repeatability yielding an

estimated alignment error of + \- 0.5 dB.

The noise floor of the crosstalk measurement is defined as the minimum external crosstalk

measurable with this setup. While the noise floor of the detector is 80 dBm, the noise floor for the crosstalk measurement is defined as the ratio of the minimum measureable cross state

power to the average transmitted power in the bar state given by

1 3 1 3 13 0

1 4 1 4 14 0

75 46 .

29 minTE

TE

min L L P dBmEXT dB

max L L P dBm

(5)

1 3 1 3 13 0

1 4 1 4 14 0

75 49 .

26 minTM

TM

min L L P dBmEXT dB

max L L P dBm

(6)

In our system, optical loss and the presence of stray light resulted in crosstalk noise floors of

46 and 49 dB for the TE and TM polarizations respectively.

Of the < 30 dB excess loss present in the system, around 12 dB is attributed to fiber-to-waveguide coupling loss at each end facet with < 0.1 dB predicted by 3D FDTD simulation

for each vertical transition. Propagation loss is estimated to be 1.4 and 4.0 dB/cm with

bending losses estimated to be < 0.1 and ~1.5 dB per turn for TE and TM, respectively.

As depicted in Fig. 9 the data agrees well with the predictions of the 3D FDTD

simulations for small vertical gaps. For gaps larger than 200 nm, the measurements are near

the noise floor and may exhibit lower crosstalk than reported here. Port 1 of the device with a

400 nm vertical gap exhibited 49.4 and 44.3 dB crosstalk at a wavelength of 1550 nm for TE and TM polarized inputs respectively. This represents the lowest crosstalk measured in a

high-index-contrast system to date. Simulation data suggests that crosstalk values as low as

65 dB for TE and 60 dB for TM polarized inputs may be expected for these structures with crosstalk continuing to decrease as the vertical separation of the guides is increased. For


vertical separations greater than 1 m, these structures are predicted to exceed 100 dB crosstalk suppression for both the TE and TM polarizations.

Excess loss measurements were performed using the same equipment and wavelength

range as above; however, alignment here was performed in the standard way using the power

meter to ensure optimal throughput. Loss of the silicon nitride input of structures with 0, 2,

and 4 NOS crossings was measured on a wafer 305 nm vertical gap. Crossing excess loss was

then calculated by taking the slope of the line connecting the three data sets. While alignment

error was less than 0.2 dB, small differences in the optical path length of each structure and

the lack of data points with a large number of waveguide crossings led to an overall

measurement error of 0.5 dB as reflected in the data in Fig. 8(b).

6. Conclusion

We have reported on the use of NOS crossings as low loss crossing structures exhibiting

exceptional crosstalk suppression and excellent uniformity across the S, C, and L optical

transmission bands. Experimental results show crosstalk values as low as 49.40 and 44.34 dB for a device with a vertical separation of 410 nm at a wavelength of 1550 nm with excess

losses predicted by high resolution 3D FDTD to be below 0.001 and 0.05 dB per crossing for

TE and TM polarizations respectively for the silicon input waveguide. Crosstalk values for

both inputs and polarizations are expected to exceed 60 dB in this geometry. The excess loss of the silicon nitride input is nearly 1 dB for a 400 nm vertical gap with an un-modified

crossing region. We expect this loss to be significantly mitigated via intelligent engineering of

the crossing region geometry and/or increasing the vertical gap between optical layers.

A typical planar SOI waveguide crossing [17] requires 36 m2 on-chip area and produces

excess loss and crosstalk values of 0.16 dB and 40 dB respectively for a single polarization. The structure reported here requires an on-chip area of 0.3 m2 with comparable loss

performance and < 90 dB crosstalk suppression for a 1 m vertical gap. Performance is expected to greatly exceed the current state-of-the-art once the crossing geometry is

optimized. It should be noted, however, that one cannot simply assume the device will exhibit

superior performance because the crossing waveguide systems are physically disjoint.

Intelligent design and optimization of the crossing is still required to achieve suitable results.

Our results suggest that NOS waveguide crossings can be used as compact, efficient,

easily integrable structures in densely integrated photonic interconnection networks.

Acknowledgments

Sandia National Laboratories is a multi-program lab managed and operated by Sandia Corp.,

a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of

Energys National Nuclear Security Administration under contract DE-AC04-94AL85000. R.A. Norwood acknowledges the support of the CIAN ERC under grant #EEC-0812072.


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