Application Example

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Spinplasmonics (a special branch of Plasmonics) is inter-ested in the study of the interaction of electromagnetic waves and free electrons in metallic materials. Specifi-cally, spinplasmonics studies the properties of  plasmon polaritons on ferromagnetic materials. The knowledge obtained can be used to physically interconnect of elec-tronics and photonics for high-frequency data transmis-sion. The application of nanoscale spinplasmonic struc-tures as information transmission devices can increase the speed of information transfer in  the future, because

these devices can work at the plasmon frequency. This frequency is comparable to the frequencies of visible light (1015 Hz). The creation of an experimental spinplas-monic structure is described in this paper. First, EBL (Elec-tron Beam Lithography) and lift-off methods are used to prepare the ferromagnetic structure and the FIB device (Focused Ion Beam) was used for milling. The created structure was then studied using an NSOM device (Near-field Scanning Optical Microscopy) [1].

EBL and FIB in Spinplasmonics

PrefaceThe creation and modification of thin-film (ferro)magnetic materials has an  important role in the development of nanoelectronics, especially spin-plasmonics. Spinplasmonic struc-tures are often prepared using an electron beam lithography method in com-bination with the sputter deposition of layered structures (a composition of feromagnetic and non-magnetic me-tallic or dielectric thin layers). The sizes of these structures are typically in the micrometer range and their thickness-es tens of  nanometers. The focused ion beam (FIB) enables final structure modifications to be made. The pre-pared structures were used for experi-mental studies of the optical properties of surface plasmon polaritons with and

without the influence of an external magnetic field. An  NSOM device with a He-Ne laser was used to excite the plasmons and observe their subse-quent propagation along the creat-ed structures. The results obtained will be used in  subsequent studies of structures with different shapes from the same or  similar materials. More complicated structures (com-posed of multiple parts) can also be created.

f Fig. 1: Thin film composition (with layer

thicknesses), shape and description of

spinplasmonic structure

Design and Creation of Spinplasmonic StructureAll spinplasmonic structures consist of three basic parts (see Fig. 1) which were designed to be suitable for the NSOM measurements described be-low. The first part - the interference arms - are used to conduct light from the milled grid (second part) to the rec-

tangular interference area (third part). The interference arms perpendicular-ly converge on this area. A right angle was chosen to obtain the maximal dif-ference in the polarization of the light from the two arms. In this case, the di-rection of the external magnetic field is

perpendicular to the first arm (no light polarization change) and parallel to second arm (maximal light polarization change) or vice versa. The shape and size of the interference area of all spin-plasmonic structures were selected due to the measurement capability of

Application Example EBL and FIB in Spinplasmonics

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Instrumentation The LYRA3 (Fig. 2) is a useful com-bination of electron and ion sources and optical columns attached to a sin-gle chamber. Its SEM and FIB are fully PC-controlled and FIB control is com-pletely integrated into the SEM soft-ware. Part of this control software is the sophisticated DrawBeam patterning module (see Fig. 3), which is designed for controlling several lithographic processes (EBL, FIB milling and etch-ing, electron etching and electron or ion-beam-induced deposition). Many structures can be drawn and created with DrawBeam, it can also be used to set basic parameters that influence the exposition, milling and etching pro-cesses. A bitmap or other image for-

mat can be used as the pattern. The option of using several drawing layers for the same or for different lithograph-ic processes in  one project is a great advantage of DrawBeam. The user can simply overlay processes and sub-sequently switch between them and select the process to implement. The specific case (which relates to this ap-plication example – the combination of EBL and FIB milling) is shown in Fig. 3.

the NSOM device. The milled grid (par-allel lines) acts as a light input (laser source) to the interference arms from below.Two lithographic methods and several depositions were used to create the spinplasmonic structures. First, a Ti/Au double layer was gradually sputter-de-posited onto a quartz glass substrate.

An ultrathin Ti layer was used for bet-ter attachment of the subsequent gold (60  nm) layer on the substrate. After that, the EBL method was used for the creation of spinplasmonic Co/Au struc-tures (see Fig. 1). FIB milling was then used as the second lithographic meth-od when the grid was created. This grid consisted of three parallel lines 1.5 μm

apart, ~35 μm in length and 0.6 μm in width; see Fig. 4b). The depth of the lines was about 0.2 μm. This is more than the thickness of the depos-ited multilayer for maximal transmittance of the light during NSOM measurement - the sample was lit from below.

f Fig. 2: The LYRA3 FIB-FESEM is a power-

ful combina-tion of SEM and FIB for de-

manding users. It is based on a high-re-

solution Schottky FESEM column and

a high performance FIB column.

c Fig. 3: DrawBeam is a sophisticated patterning module designed for controlling several lithographic processes. EBL and FIB milling in

different layers were used for the step-by-step creation of the spinplasmonic structures in this case. The parameters of the electron ex-

position or FIB milling are displayed in the DrawBeam Process panel.

Application ExampleEBL and FIB in Spinplasmonics

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Experimental ConditionsThe created structure was studied using the NSOM method. First, the structure topography was measured (see Fig.  5a)). Then the optical meas-urements were carried out when the sample was lit from below – transmis-sion mode (He-Ne laser; 633 nm) via the FIB milled grid. The first measure-ment was carried out without the influ-ence of  an  external magnetic field. In this case electromagnetic waves with TM (Transversal Magnetic) polarization spread along the Au sample surface (wavelength about 587 nm; see Fig. 5 c)) and along the interference arms

with the Au/Co/Au material depth profile (wavelength about 566 nm). The interference of these waves arises in the rectangular interference area of the spinplasmonic structures. The ac-quired images with the distribution of the electromagnetic field (see exam-ples on Fig. 5b) or 6a)) were processed using the Fourier transformation meth-od (FT) for obtaining interference im-ages between individual surface plas-mons (Fig. 5c) or 6c)).After that, the sample with spinplas-monic structures was inserted into a homogeneous magnetic field during

the NSOM measurements (the orienta-tion of this field is parallel to the plane of the spinplasmonic structures). These conditions caused a longitudinal Kerr effect [1], which changed the linear po-larization of the light on the arm (paral-lel to the magnetic field) to an elliptical polarization. This state generates a ro-tation of the resulting interference lines (see example on Fig. 6c)). This rotation depends on the intensity of external magnetic field used. We can also mod-ulate the spread of light in spinplas-monic structures this way.

e Fig. 4: SEM images of example spin-

plasmonic structure:

a) result of electron beam lithography

method,

b) structure with FIB milling lines (grid)

e Fig. 5: Example of unprocessed data

from NSOM measurement:

a) spinplasmonic structure topography

with height profile,

b) distribution of electromagnetic field

with position of structure marked;

NSOM data processed using Fourier

transformation: c) interference of sur-

face plasmon polaritons with the abo-

ve wavelengths (587 nm on Au surfa-

ce and 566 nm on Au/Co/Au surface)

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Application Example EBL and FIB in Spinplasmonics

e Fig. 6: Results of NSOM measurements:

a) unprocessed data with presence of

external homogenous magnetic field

6.4 mT,

b) orientation of magnetic field used and

of studied structure, direction of spre-

ad of TM waves,

c) processed data (FT) with interference

line orientation shown without (black)

and with (green) external magnetic

field

ConclusionThe experimental study of the proper-ties of surface plasmon polaritons on ferromagnetic material was succesfully completed. Spinplasmonic structures of specific shape (for NSOM measure-ment with external magnetic field) and size were created from a Ti/Au/Co/Au multilayer on a quartz glass substrate, first by using the EBL method and by  the FIB milling method in the sec-ond step. Only one unique device was used for both these lithographic meth-

ods – the TESCAN LYRA3 FIB-FESEM. This device contains a high-resolution Shottky FESEM column and high-per-formance FIB with ultra-high resolu-tion. The electron and ion beams were controlled by a single software mod-ule during the lithographic process. The spinplasmonic structures created were studied using the NSOM method, in which the sample was lit from below via the FIB milled grid. The light inten-sity of the plasmon interference lines

on the sample surface was measured by scanning the NSOM tip. Moreover, the rotation of  plasmon interference lines was examined, especially in terms of the effect of the external magnetic field. The results obtained will be ben-eficial in the field of spinplasmonics, which aims to change the basic princi-ple and increase the speed of informa-tion transfer in high-frequency devices (microprocesors, data memories, …).

AcknowledgmentWe wish to thank Petr Dvořák from the Institute of Physical Engineering FME BUT for providing the above informa-tion on his spinplasmonic research and measurement results.

� References[1] Dvořák P.: Study of properties surface plasmon polaritons on magnetic materials, Brno University of Technology, Faculty of Mechanical

Engineering, Institute of Physical Engineering, Master`s thesis, 2011.

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