Concepts and Design of Novel Integrated Photonic Devices based on

Silicon-organic Hybrid Technology

Patrick Steglich

1,2

, Mauro Casalboni

and Sigurd Schrader

Faculty of Engineering and Natural Sciences, University of Applied Sciences Wildau, Wildau, Germany

Department of Industrial Engineering, University of Rome ”Tor Vergata”, Rome, Italy

1 RESEARCH PROBLEM

This doctoral thesis is about the development of inte-

grated photonic devices through the implementation

of electro-optical organic materials in a CMOS-like

production line. Slot-waveguide structures are the key

element in order to integrate organic materials into sil-

icon photonics. For that reason, slot-waveguides have

been employed in order to develop high-speed modu-

lators for telecommunication interconnects (Weimann

et al., 2014).

The major advantage of slot-waveguides is the fact

that the guided light is conﬁned in between two sili-

con rails (Almeida et al., 2004). For that reason the

light is forced to interact directly with the surrounding

material. Figure 1 shows a detailed cross-sectional

Figure 1: Cross-sectional view of a SOI strip-waveguide

(left) and a SOI slot-waveguide (right). The pictures below

show the optical ﬁeld amplitude distribution. Both waveg-

uide structures are on top of a buried oxide (BOX) substrate.

The slot-waveguide structure shows enhanced ﬁeld inten-

sity in the slot interior.

view and compares the guided optical ﬁeld in a strip-

waveguide to a slot-waveguide.

The reason for this high conﬁnement in-between the

silicon rails is the large-index contrast of the high-

index silicon and the low-index surrounding material.

At the interface the normal electric ﬁeld, which is ac-

cording to Figure 1 the E

ﬁeld, undergoes a large dis-

continuity. This results in a ﬁeld enhancement in the

low-index region which is proportional to the ratio of

the dielectric constant of the surrounding material to

that of silicon.

The high conﬁnement inside the slot is of special

beneﬁt for electro-optical applications. The so called

silicon-organic hybrid (SOH) technology uses organic

materials with exceptional high linear electro-optical

coefﬁcients as surrounding material (Leuthold et al.,

2013). Current electro-optic modulators are based on

semiconductors like silicon. In silicon photonics, fun-

damental speed limitations are related to carrier injec-

tion and removal (Vivien and Pavesi, 2013). There-

fore, parametric processes are impaired by nonpara-

metric processes like two-photon absorption and be-

come to the main speed limiting factor. Additionally,

silicon has a lack of linear electro-optical coefﬁcients.

All this can be overcome by using organic materials

with nonlinear optical properties as active material.

The most challenging issue of SOH technol-

ogy based slot-waveguides is the compatibility with

common complementary-metal-oxide-semiconductor

(CMOS) fabrication processes since such integrated

photonic devices need a high integration rate and a

cost efﬁcient mass production environment.

For that reason, one focus of this thesis lies on

design trade-offs and on the development of an ap-

proach in order to improve silicon-on-insulator (SOI)

slot-waveguide structures for a CMOS-like produc-

tion environment.

Beside the theoretical investigations there are sev-

eral critical issues during the fabrication process. The

slot width is limited due to common lithography re-

strictions and has a high side-wall roughness after

etching which leads to optical losses. After the Back-

End-Of-Line (BEOL) which includes the metal con-

tacts, as it can be seen in Figure 2, it is necessary to

open the slot-waveguide structures in order to deposit

the organic material inside the slot interior. This step

is critical because a combination of dry and wet etch-

ing is necessary. Nevertheless, it is necessary in order

to maintain CMOS compatibility before the deposi-

Steglich P., Casalboni M. and Schrader S..

Concepts and Design of Novel Integrated Photonic Devices based on Silicon-organic Hybrid Technology.

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

Figure 2: Cross section of two full implemented slot-waveguides with stripload and electrode connection (not to scale). The

slot-waveguide structure has to be opened in order to ﬁll-out the slot with a nonlinear optical cladding. The slot-waveguide

rails are connected by tungsten vias to metal contacts. In this conﬁguration, both slot-waveguides can be used as phase shifter

for a Mach-Zehnder modulator.

tion of organic material.

Selection and deposition of organic materials with

large linear electro-optical coefﬁcients is another crit-

ical task. State of the art SOH modulators using

a novel class of so-called monolithic electro-optic

materials in order to yield high in-device electro-

optic coefﬁcients by avoiding dipole-dipole interac-

tions (Dalton et al., 2010; Palmer et al., 2014).

2 OUTLINE OF OBJECTIVES

The main objective of the thesis is the implementa-

tion of electro-optical organic materials in a CMOS-

like production environment in order to develop inte-

grated photonic devices such as novel electro-optical

modulators based on slot-waveguide structures. Con-

sequently, the scope of this thesis is the conception,

design, and modelling of passive and active photonic

components like strip-to-slot mode-converter, grating

coupler, slot-waveguide structures and electrical con-

tacts. Novel design concepts such as partially slotted

ring resonators and Fabry-Perot-Interferometer with

slotted cavity will be developed. Besides theoretical

investigations, device fabrication and evaluation as

well as deposition of electro-optical organic materi-

als are objectives of the thesis. The organic materials

need to have high thermal, photochemical and long-

term orientational stability as well as a minimized

self-aggregation to avoid scattering losses. Further-

more, large electro-optical coefﬁcients are necessary

in order to reduce the drive voltage of the modulators.

For this reason one part of the thesis is to ﬁnd ap-

propriate organic materials and deposition methods.

This implies the characterisation of linear and nonlin-

ear optical properties. In order to deposit the organic

materials on the slot-waveguide structure a new ap-

proach will be developed which includes an etching

procedure to open the slot-waveguide structure from

the backside of the SOI wafer.

3 STATE OF THE ART

The Karlsruhe Institute of Technology reported in

2013 and 2014 several silicon-organic hybrid modula-

tors based on a Mach-Zehnder interferometer operat-

ing at 10 Gbit/s, 12.5 Gbit/s, 40 Gbit/s, 84 Gbit/s,

and 112 Gbit/s (Palmer et al., 2013b; Korn et al.,

2013; Leuthold et al., 2013; Palmer et al., 2014; Korn

et al., 2014). They have demonstrated advanced mod-

ulation formats such as 16QAM (Quadrature Am-

plitude Modulation) as well as OOK (On-Off Key-

ing), BPSK (Binary Phase Shift Keying) and 8-ASK

(Amplitude Shift Keying) signals (Korn et al., 2013;

Palmer et al., 2013c). This is possible due to the fact

that organic materials have less free-carrier dispersion

which normally leads to an intrinsic coupling of am-

plitude and phase.

A silicon-polymer hybrid slot waveguide ring-

resonator modulator with a 6 dB bandwidth of 1 GHz,

a device tunability of 12.7 pm/V, and a Q-factor of

5000 were fabricated with 193 nm optical lithography

(Gould et al., 2011).

Palmer et al. published in 2013 strip-to-slot mode-

converter with record-low losses of about 0.02 dB and

negligible reﬂections between 1480 nm and 1580 nm

(Palmer et al., 2013a). Yang Liu et al. developed in

2011 a so called strip to strip-loaded slot mode con-

verter with losses around 0.81 dB (Liu et al., 2011).

PHOTOPTICS2015-DoctoralConsortium

The deposition of organic materials is typically

done by spin-coating but vapour deposition is more

suitable. Spin-coating is critical because the organic

material has to ﬁll-out the interior of the slot homo-

geneously. With spin-coating, it is not guaranteed

that the slot will be ﬁlled due to the centrifugal force

and due to the fact that the polymer system is liquid.

With its viscosity and surface tension it precludes the

polymer system from ﬁlling out the whole interior of

the slot homogeneously. Commercially available and

reliable organic materials such as M3 (commercial-

ized by GigOptix Inc.) have been successfully used

for several slot-waveguide based electro-optical mod-

ulators like in (Palmer et al., 2013b; Leuthold et al.,

2013; Korn et al., 2013; Palmer et al., 2013c). Special

methods have been developed in order to ﬁll-out the

interior of sub-micrometer slot-waveguide structures

with such solid crystals (Korn et al., 2014). Further-

more, multi-chromophore dendritic molecules, guest-

host and side-chain polymersystems have been used

as electro-optical cladding (Palmer et al., 2014).

In the last decade integrated photonic sensors

based on slot-waveguide ring-resonators have also

been proposed (Dell’Olio and Passaro, 2007) and de-

veloped (Barrios et al., 2007). In case of label-free

bio-sensors it has been shown that the sensitivity of

slot-waveguides is more than three times higher com-

pared to conventional silicon strip-waveguides (Claes

et al., 2009).

There are several publications about ﬁeld conﬁnement

factors of slot-waveguide structures. These structures

consist of vertical silicon rails (Robinson et al., 2008)

or multiple nanolayers (Feng et al., 2006). However,

none of them consider SOI slot-waveguides with typ-

ical geometrical dimensions for CMOS-like produc-

tion processes.

4 METHODOLOGY

4.1 Simulation of Slot-waveguides

For the calculation of waveguide eigenmodes we em-

ploying a commercial full-vectorial 2D ﬁnite element

method (FEM) based mode solver from COMSOL

Multiphysics

. Doing this we sweep several param-

eters like the silicon rail width and slot width whereas

the height is ﬁxed to 220 nm. Assuming a wave-

length of 1550 nm, the refractive index for the silicon

is n

= 3.48 and for the BOX substrate n

box

= 1.444

(Palik, 1997; Tsang et al., 2002). The refractive index

of the surrounding material n

is variable because

it can be air, gas, ﬂuid or an optical nonlinear mate-

rial, depending on the application. In the following

we will use n

clad

as cladding refractive index instead

of n

because our simulation will use an organic

cladding material. For our simulations we consider

a total domain of D

tot

= 3 µm

which is illustrated in

Figure 1.

In order to yield the mode ﬁeld distribution and

effective refractive index, the refractive index distri-

bution n(x,y) for the structure shown in Figure 1 need

to be declared to calculate eigenvalues and eigenfunc-

tions of the wave equation

∇ × (∇ × E)− k

E = 0, (1)

where k

is the wave number in free space. By do-

ing this we get the optical ﬁeld distribution for the

quasi-TE and quasi-TM mode as shown in Figure 3.

In the following we will neglect the quasi-TM mode

because it is over two times of magnitude smaller than

the quasi-TE mode.

Figure 3: Optical ﬁeld distribution for the quasi-TE and

quasi-TM mode of a SOI slot-waveguide.

4.1.1 Field Conﬁnement Factor of

Slot-waveguides

In order to design, develop and improve slot-

waveguide structures for applications in the ﬁeld of

biophotonic or high-speed modulators it is necessary

to calculate characteristic values which describe the

conﬁnement and therefore the interaction of light with

the surrounding material. One ﬁgure of merit of how

well the guided modal ﬁeld is conﬁned in a certain re-

gion is the so-called ﬁeld conﬁnement factor.

The ﬁeld conﬁnement factor is usually deﬁned as

the ratio of the time averaged power ﬂow in the do-

main of interest (D

int

) to the time averaged power ﬂow

inside the total domain (D

tot

)

Γ =

Z Z

int

Re{[E × H

∗

] · e

} dxdy

Z Z

tot

Re{[E × H

∗

] · e

} dxdy

. (2)

E and H are the electric and magnetic ﬁeld vectors,

respectively, and e

is the unit vectors in z direction.

There are three different cases in order to choose

ConceptsandDesignofNovelIntegratedPhotonicDevicesbasedonSilicon-organicHybridTechnology

the domain of interest. In case of common strip-

waveguides the domain of interest is equal to the core

region, D

int

core

. In contrast to that, for bio-sensing

applications the region of the cover medium is consid-

ered to be the domain of interest, D

int

cover

, which

is valid for strip- and slot-waveguides as well. Con-

sidering slot-waveguides for electro-optical modula-

tors the domain of interest is equal to the slot region,

int

slot

. All possible domains of interest are illus-

trated in Figure 4.

Figure 4: Domains of interest: core D

core

, cladding D

clad

and slot D

slot

regions are highlighted in green.

In case of low-index-contrast waveguides, Equa-

tion 2 can be simplify using the linear relationship

between the electric and magnetic ﬁelds

Z Z

Re{[E × H

∗

] · e

} dxdy =

ωµ

Z Z

|E|

dxdy,

(3)

which leads to

Γ =

Z Z

int

|E|

dxdy

Z Z

tot

|E|

dxdy

. (4)

However, for high-index-contrast waveguides and

especially for slot-waveguides this linear relation-

ship does not apply since they must satisfy different

boundary conditions (Robinson et al., 2008). Because

of that, in this work all conﬁnement factors are calcu-

lated according to Equation 2.

4.1.2 Effective Nonlinear Area of

Slot-Waveguides

A ﬁgure of merit of how well the waveguide geome-

try supports the nonlinear interaction is the so called

effective nonlinear area (Koos et al., 2007). The

smaller the effective nonlinear area provided by the

waveguide structure the higher the nonlinear interac-

tion which is important for electro-optical modula-

tors.

For the analysis of low-index-contrast systems, it is

usually assumed that the gradient of the dielectric

constant is approximately zero in the entire cross sec-

tion. But this approximation is not valid for high-

index-contrast material systems. Therefore, Koos et

al. derived the effective nonlinear area for high-index-

contrast waveguides in 2007 by using the slowly vary-

ing envelope approximation (Koos et al., 2007). The

effective nonlinear area results then from the nonlin-

ear Schr

odinger equation

e f f

clad



Z Z

tot

Re{[E × H

∗

] · e

} dxdy



Z Z

int



dxdy

, (5)

with the free-space wave impedance Z

/ε

≈

377 Ω. In our case is the domain of interest equal

to the cladding domain, D

int

= D

slot

. In case of low-

index-contrast material systems it can be assumed that

core

≈ n

clad

≈ n

box

≈ n

int

, and the longitudinal ﬁeld

becomes negligible (Koos et al., 2007). Furthermore,

the transverse components of the electrical ﬁeld E

and the magnetic ﬁeld H can be approximated by a

scalar function F with the condition E ≈ F · e

and

H ≈ (n

int

)F · e

where e

and e

are the unit vec-

tors in x and y direction, respectively (Koos et al.,

2007). Further it can be stated that D

int

= D

tot

if the

nonlinearity is homogeneous in D

tot

. Now Equation

5 becomes simpliﬁed to

e f f



Z Z

tot



dxdy



Z Z

int



dxdy

, (6)

which is similar to the common deﬁnition of an effec-

tive area (Agrawal, 2012).

4.2 Concept Development and CMOS

Implementation

An appropriate approach to implement nonlinear or-

ganic materials in silicon based fabrication environ-

ment is the use of striploaded slot-waveguides. A

detailed cross sectional view of two parallel im-

plemented slot-waveguides is depicted in Figure 2.

Through a doped silicon stripload and tungsten vias,

the slot-waveguides are directly connected to the

metal electrodes. The tungsten vias are connected

PHOTOPTICS2015-DoctoralConsortium

with the silicon stripload through a thin silicide layer

for better connectivity and conductivity.

A SOI slot-waveguide typically consists of two

silicon rails with a ﬁxed height of 220 nm due to com-

mon CMOS-like production restrictions. As it can be

seen in Figure 1 both silicon rails are located on top

of a buried oxide (BOX) substrate and are separated

from each other by a slot width s and have a rail width

The SOH modulators will be fabricated in a

0.13 µm SiGe BiCMOS production line at the Insti-

tute of High-Performance Microelectronics (IHP) in

Frankfurt(Oder), Germany. In order to transfer the

new concepts into a CMOS production routine, it is

necessary to develop a design with 2D layers. Each

layer deﬁnes a production step such as etching or de-

position at a certain area. The design will be done

with the CAD software TexEDA



. Every single layer

has to be deﬁned with careful attention on the produc-

tion restrictions and design rules for the fabrication

process.

Figure 5 shows the concept of a SOH slot-

waveguide ring resonator without silicon striploads.

The ring is partially slotted in order to obtain higher

Q-factors.

Figure 5: a) Strip-to-slot mode-converter. b) SOH slot-

waveguide ring resonator. c) Optical ﬁeld distribution of

the SOH slot-waveguide ring resonator modulator.

5 EXPECTED OUTCOME

5.1 Theoretical Investigations

Theoretical investigations are mainly done in order

to ﬁnd optimized geometrical parameters for slot-

waveguide structures. Figures of merit such as

ﬁeld conﬁnement factors and effective nonlinear area

are calculated and published (Steglich et al., 2014;

Steglich et al., 2015a; Steglich et al., 2015b). Fur-

ther simulations will concentrate on design improve-

ments of strip-to-slot mode-converter in order to de-

crease optical losses.

5.2 Device Fabrication

First SOH modulator concepts are transferred to

an appropriate design and fabricated at the IHP

in a 0.13 µm SiGe BiCMOS production line us-

ing 200 mm SOI wafers and 248 nm DUV lithog-

raphy. For instance, SOH slot-waveguide ring res-

onator modulators with a circumferences of 69.3 µm

and 95.5 µm have been fabricated. The transmis-

sion spectrum is shown in Figure 6. Measured

Q-factor, Full-Width-Half-Maximum (FWHM) and

Free-Spectrum-Range (FSR) of three fabricated mi-

cro slot-waveguide ring resonators are listed in Ta-

ble 1. The stated slot widths are not yet conﬁrmed

by the results of a focus ion beam and may there-

fore slightly differ from the expected value. We ex-

pect a Q-factor of about 29000 which is almost six

times higher compared to state of the art SOH slot-

waveguide ring resonator modulators (Gould et al.,

2011). Future work will concentrate on the deposi-

tion of organic materials from the vacuum instead of

spin-coating. The advantage is the possibility to ﬁll-

out trenches with high aspect ratios at the backside of

SOI wafers. This is necessary to open slot-waveguide

structures without the destruction of the BEOL and

in order to deposit organic materials without covering

the metal electrodes.

Figure 6: Transmission of SOH slot-waveguide ring res-

onator resonators with different slot widths fabricated with

a 248 nm DUV lithography.

ConceptsandDesignofNovelIntegratedPhotonicDevicesbasedonSilicon-organicHybridTechnology

Table 1: Characteristics of the fabricated SOH slot-waveguide ring resonator resonators.

circumference [µm] slot width [nm] center wavelength [nm] FWHM [nm] FSR [nm] Q-factor

69.3 130 1598.03 0.31465 6.7 5079

69.3 150 1586.5 0.47424 7 3345

95.5 130 1597.05 0.05497 3.3 29054

95.5 110 1573.58 0,01567 3.4 100420

6 STAGE OF THE RESEARCH

Three year of research will be needed. The ﬁrst year

has been completed successfully. This period in-

cluded a ﬁrst concept development and optimization

through numerical simulations as well as a ﬁrst de-

sign realization. Figure of merits such as ﬁeld con-

ﬁnement factors and nonlinear effective areas of slot-

waveguide structures are calculated and presented at

the EOSAM 2014 in Berlin as an oral presentation

(Steglich et al., 2014). Further simulation results

and SOI slot-waveguide design trade-offs will be pre-

sented at the PHOTOPTICS 2015 in Berlin as FULL

paper with an oral presentation (Steglich et al., 2015a)

and at the CLEO/Europe 2015 in Munich (Steglich

et al., 2015b). Beside theoretical investigations, ﬁrst

electro-optical modulators have been fabricated with

a SiGe BiCMOS pilot line at the IHP. We expect to

demonstrate our ﬁrst measurements on SOH modula-

tor during the next few months.

ACKNOWLEDGEMENTS

The authors would like to thank the German Federal

Ministry of Education and Research (BMBF) for the

ﬁnancial support under contract no. 03FH086PX2,

the University of Applied Sciences Wildau (THWi),

Germany, and the Ministry of Science, Technology

and Culture of the federal state Brandenburg, Ger-

many, for ﬁnancial support. The authors would also

like to thank Fr. Nanni, P. Prosposito, F. De Matteis,

and R. De Angelis from the University of Rome Tor

Vergata, Italy, St. Meister and A. Al-Saadi from the

Technical University Berlin, Germany, and L. Zim-

mermann, D. Knoll, D. Stolarek, J. Katzer, St. Lis-

chke, H. Silz, B. Tillack and W. Mehr from the In-

stitute of High-Performance Microelectronics (IHP),

Germany, for their encouragement and support in the

framework of the Joint-Lab IHP-THWi.

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