manufacturing of self bearing microstructures of the pseudo metallic type for diffraction experiments in the terahertz range

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ScienceDirect
Physics Procedia 84 (2016) 165 – 169

International Conference "Synchrotron and Free electron laser Radiation: generation and
application", SFR-2016, 4-8 July 2016, Novosibirsk, Russia

Manufacturing of self-bearing microstructures of the pseudometallic type for diffraction experiments in the terahertz range
B.G.Goldenberga*, B.A.Knyazeva,b, A.N.Gentseleva, A.G.Lemzyakova, S.G.Baevc
a

Budker Institute of Nuclear Physics SB RAS, Novosibirsk, 630090, Russia
b
Novosibirsk State University, Novosibirsk, 630090, Russia
c
Institute of Automation and Electrometry Siberian Branch of the Russian Academy of Sciences 630090, Russia

Abstract

Specific features of the LIGA-technology methods elaborated at the Siberian Synchrotron and Terahertz Radiation Centre
(SSTRC, BINP SB RAS) and fabrication of terahertz filters and optical elements based on high-aspect self-bearing
microstructures of the pseudo-metallic type are described. The essence of the method consists in deep X-ray lithographic
patterning of an organic glass (PMMA) substrate followed by covering its entire surface with a thin layer of metal (silver). The
structures produced are using in the experiments at the Novosibirsk free electron laser.
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B.V. B.V.
This is an open access article under the CC BY-NC-ND license
©2016

2016The
TheAuthors.
Authors.
Published
by Elsevier
( />Peer-review under responsibility of the organizing committee of SFR-2016.
Peer-review under responsibility of the organizing committee of SFR-2016.
Keywords: Deep X-ray lithography, X-ray masks, microstructures of the pseudo-metallic type, free electron laser, teraherttz radiation

1. Motivation
Terahertz science and technology are the fields rapidly developing during past three decades (see, e. g., RedoSanchez et al., 2013). One of the mainstreams in the terahertz range is imaging applications (see Knyazev et al.,
2011, and references in it), which are very important for medicine, radioscopy, and security. Another field of activity
in this range is terahertz plasmonics, in particular, the study of surface plasmon polaritons (Gerasimov et al., 2015;

* Corresponding author. Tel.: +7-383-329-4697; fax: +7-383-330-71-63.
E-mail address:

1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
( />Peer-review under responsibility of the organizing committee of SFR-2016.
doi:10.1016/j.phpro.2016.11.029


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B.G. Goldenberg et al. / Physics Procedia 84 (2016) 165 – 169

Knyazev et al., 2015), which can be used in integrated optics devices (Otsuji and Shur, 2014). Since characteristic
wavelengths in the region of interest spans a broad spectrum from 30 μm to 0.3 mm, for both the fields, fabrication
of different kinds of diffractive elements with reliefs from tens μm to one mm is necessary. An example of such
structure in plasmonics is described by Acuna et al. (2008).

Most of studies in the terahertz range have been performed using broadband radiation sources based on the
application of femtosecond laser pulses. The appearance of high power monochromatic terahertz sources, FELBE
(Michel et al., 2016) and NovoFEL (Kulipanov et al., 2015) free electron lasers, opened new experimental
opportunities and had required fabrication of diffractive optical elements for laser beam manipulation (Agafonov et
al., 2014). One of the classical optics effects is the Talbot effect, which is for a long time known in visible range, but
was observed for the first time in the terahertz range by Knyazev et al. (2010). Large wavelength of terahertz
radiation (Novosibirsk free electron laser wavelength was 130 or 141 μm) makes it possible to perform experiments
on the propagation of monochromatic terahertz radiation through periodic structures with openings and slits which
size is close to wavelength. Such experiments were performed recently at the Novosibirsk free electron laser
(NovoFEL). Results of the experiments will be published elsewhere.
In this paper we describe a technique, which were applied to the fabrication of periodic structures for the study of
the Talbot effect using transmission 1D and 2D gratings. Since the scalar diffraction theory does not formally valid
for the calculation of diffraction patterns if the slits/openings have dimensions close to the wavelength, it is of
interest to compare results of experiments performed using both dielectric and metallized gratings with different
aspect ratio. For this reason we have fabricated the gratings using both thin and thick plates and films.
2. Manufacturing of optical elements for NovoFEL
We used a deep X-ray lithography for the fabrication of such structures. The exposure was carried out at the
"LIGA" station of the VEPP-3 electron storage ring (Goldenberg et al., 2016; Levichev, 2016). Typical electron
energy E = 2 GeV; magnetic field at the emission point B = 2.0 T. Spectral distribution of the VEPP-3 SR has a
wide range of 0.2 to 9 Å. Aluminum foil 115 um thickness was used to suppress low energy part of spectra.
Resulting spectra provide difference of absorbed dose at PMMA resist layer 1 mm thickness less than 30%. It is
sufficient to consider dose distribution uniform enough.

Fig. 1. Spectral distributions of power absorption in PMMA resist calculated for aluminium spectral filters and brass X-mask. (1) beamline beryllium windows 500 μm thick and aluminum foil 115 μm thick ("Be500Al155" line), (2) - beryllium windows 500 μm thick and
aluminum foil 115 μm thick and brass 50 mm ("Be500Al155 Brass50" line).

The spectral distribution of dose rate absorbed in the PMMA resist with beamline beryllium windows 500 μm
thick and aluminum foil 115 μm thick is illustrated in Figure 1, the curve 1. The dose distribution after brass foil 50

B.G. Goldenberg et al. / Physics Procedia 84 (2016) 165 – 169

167

μm thickness is illustrated in Figure 1, the curve 2. Calculation was at typical electron current 100 mA in the storage
ring.

Fig. 2. Brass X-mask, general view.

Fig. 3. SEM photo of X-mask pattern.

To perform the X-ray lithography the X-ray masks are needed. Usually high-contrast pattern of X-ray mask for
deep lithography is created by galvanic deposition of heavy metals (for example 20 um of gold) on the X-ray
transparent bearing membrane. To produce the pattern photolithography or soft X-ray lithography are commonly
used (Saile, 2009).

Fig. 4. SEM photo of silver coating PMMA mesh fragment.

Since a critical dimension of the resulting structures is about tens microns, we decided using the laser cutting
technology of a metal foil to produce X-ray masks. It allows us to eliminate the bearing membrane and to exclude
most of process steps. In this work, we examined the results of experiments on two different laser systems based on
solid-state pulsed lasers listed in Table 1 (see Goloshevsky, 2008). Brass foils 50 Pm thickness were used as X-ray
absorber material. Calculated contrast of brass X-ray mask is about 16. It is sufficient for working with PMMA
resist. The best results from the viewpoint of the cut edge roughness were obtained with a brass foil patterned with
the 1064-nm laser that provided the average roughness about 10 Pm.


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B.G. Goldenberg et al. / Physics Procedia 84 (2016) 165 – 169

Table 1. Lasers parameters.
Laser

Wavelength,
nm

Pulse
duration, ns

Repetition
rate, kHz

Pulse energy,
mJ

Spot size,
Pm

Power density,
GW/cm2

1

1064

10

10÷50

0.23

10

30

2

532

10

2

1

15

55

A number of X-ray masks with working area 50 mm in diameter were fabricated. The patterns were arrays of slits
of 130 or 300 Pm wide with a period of 1 mm. X-ray 2D masks with openings of 100, 260 and 300 Pm in diameter
with 1 mm period were also fabricated.
PMMA sheets were exposed at the “LIGA” station. Microstructures were developed in the well-known GGdeveloper at a room temperature with and without ultrasonic support.
Manufactured brass X-ray masks were used to produce the number of self-bearing polymer structures by deep Xray lithography on 0.5 and 1 mm thick PMMA sheets and 90-Pm thick polypropylene (PET) sheets. Some of the
PMMA structures were coated with silver. Coating was produced by means of DC magnetron sputtering. The DC
power was 100 W and the flow rate of the Ar was 20 sccm. The polymer structures coated with the metal interact
with electromagnetic radiation like the bulk metal, because the skin layer depth (several tens nm) is much less than
the silver thickness (about a micrometer).
3. Testing of the gratings at the NovoFEL
The fabricated gratings were applied to study the Talbot effect in the terahertz range. In this spectral range both

metal covered and uncovered PMMA plates were opaque to the terahertz radiation, and they represented the
amplitude gratings, albeit the boundary conditions, apparently, were different for metal and dielectric slits/openings.
Since polypropylene is highly transparent to terahertz radiation, the gratings made of PET were the phase optical
elements. These experiments are now in progress, and their results will be published elsewhere. Here we present, as
an example, images recorded with an uncooled microbolometer matrix (Knyazev et al., 2011) at a Talbot plane
behind one of the metallized PMMA gratings (Fig. 5).

(a)

(b)

Fig. 5. (a) Self-imaging of a metallized PMMA grating recorded with the microbolometer matrix in the Talbot plane ( z 31.5 mm). Width of the
slits - 300 Pm, period - 1 mm, plate thickness – 480 Pm, radiation wavelength -130 Pm. Size of the frame is 16.36 u 12.24 mm. (b) Intensity
distribution along a slit image: the red line – experiment; the black line – simulation.

It is seen, that the self-imaging is observed even for the slit dimension equal to about two wavelengths. The
image demonstrates also an important role of diffraction in the experiments with terahertz radiation.
Electromagnetic wave passing the grating diffracts on a technological bridge (see Figs. 2 and 3). Diffraction pattern
is shown in Fig. 5 (b). It reasonably agrees with the diffraction pattern calculated for an infinite strip having the
same width as the bridge. The difference between the patterns may be caused by radiation diffraction on opposite
ends of the slots.


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4. Acknowledgement
The work was performed using the equipment belonging to the Siberian Synchrotron and Terahertz Radiation
Center (SSTRC). It was supported by the Russian Science Foundation, project 14-50-00080. Experiments on study

transmission of THz radiation through the grating, which are now in progress, are supported by the Russian
Foundation of Basic Research, grant 15-02-06444. B. K. thanks D. Vershinin for the assistance in the Talbot
experiments.

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