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

Available online at www.sciencedirect.com 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 © Published by Elsevier B.V B.V This is an open access article under the CC BY-NC-ND license ©2016 2016The TheAuthors Authors Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/) 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 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: goldenberg@inp.nsk.su 1875-3892 © 2016 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of SFR-2016 doi:10.1016/j.phpro.2016.11.029 166 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 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 = 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 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 mm thickness less than 30% It is sufficient to consider dose distribution uniform enough Fig 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 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 Calculation was at typical electron current 100 mA in the storage ring Fig Brass X-mask, general view Fig 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 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 (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 168 B.G Goldenberg et al / Physics Procedia 84 (2016) 165 – 169 Table Lasers parameters Laser Wavelength, nm Pulse duration, ns Repetition rate, kHz Pulse energy, mJ Spot size, Pm Power density, GW/cm2 1064 10 10÷50 0.23 10 30 532 10 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 mm X-ray 2D masks with openings of 100, 260 and 300 Pm in diameter with 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 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) 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 (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 - 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 and 3) Diffraction pattern is shown in Fig (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 B.G Goldenberg et al / Physics Procedia 84 (2016) 165 – 169 169 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 References Acuna, G., Heucke, S F., Kuchler, F., Chen, H.-T., Taylor, A J., Kersting, R., 2008 Surface plasmons in terahertz metamaterials Optics Express 16, 18745-18751 Agafonov, A., Volodkin, B., Volotovsky, S., Kaveev, A., Knyazev B., Kropotov, G., Tykmakov, K., Pavelyev, V., Tsygankova, E., Tsypishka, D., Choporova, Yu., 2014 Optical elements for focusing of terahertz laser radiation in a given two-dimensional domain Optical Memory and Neural Networks (Information Optics), No 3, 185–190 Gerasimov, V., Knyazev, B., Nikitin, A., Zhizhin, G., 2015 Experimental investigations into capability of terahertz surface plasmons to bridge macroscopic air gaps Optics Express 23, 33448-33459 Goldenberg, B.G., Lemzyakov, A.G., Nazmov, V.P., Pindyurin V.F., 2016 Multifunctional X-ray lithography station at VEPP-3 This Proceedings Goloshevsky, N., Aleshin, A., Baev, S., Bessmeltsev, V., Smirnov, K., 2008 Precision laser system based on complementary scanning principle for dielectric materials microprocessing Proc SPIE 6985, Fundamentals of Laser Assisted Micro- and Nanotechnologies, 69850M-1 – 69850M-9 Knyazev, B., Choporova, Yu., Mitkov, M., Pavelyev, V., Volodkin, B., 2015 Generation of terahertz surface plasmon polaritons using nondiffractive Bessel beams with orbital angular momentum Phys Rev Letters 115, Art 163901, pp Knyazev, B., Cherkassky, V., Choporova, Yu., Gerasimov, V., Vlasenko., M., Dem’yanenko, M., Esaev, D., 2011 Real-time imaging using a high-power monochromatic terahertz source: comparative description of imaging techniques with examples of application J Infrared Milli Terahz Waves 32, 1207–1222 Knyazev, B., Cherkassky, V., Choporova, Yu., Gerasimov, V., Vlasenko, M., 2010 The Talbot effect in the terahertz spectral range 35th Int Conf on Infrared, Millimeter and Terahertz Waves, Rome, Italy, www.irmmw-thz2010.org/ Kulipanov, G.N., Bagryanskaya, E.G., Chesnokov, E.N., Choporova, Y.Yu., Gerasimov, V.V., Getmanov, Ya.V., Kiselev, S.L., Knyazev, B.A., Kubarev, V.V., Peltek, S.E., Popik, V.M., Salikova, T.V., Scheglov, M.A., Seredniakov, S.S., Shevchenko, O.A., Skrinsky, A.N., Veber, S.L., Vinokurov, N.A., 2015 Novosibirsk free electron laser—facility description and recent experiments IEEE Transactions on Terahertz Science and Technology 5, 798-809 Levichev, E.B Status and perspetives of VEPP-4 complex (in Russian) Particles and Nuclei Letters, XIII, 7, 2016 Michel, P, Klopf, M., Kovalev, S., Liedke, M., Beyer, R., Bemmerer, Schramm, U., 2016 ELBE Center for high-power radiation sources Journal of large-scale research facilities 2, A39, pp, http://jlsrf.org/index.php/lsf/article/view/58 Otsuji, T., Shur, M., 2014 Terahertz Plasmonics IEEE Microwave Magazine 15, 43-50 Redo-Sanchez, A., Laman, N., Schulkin, B., Tongue, T., 2013 Review of terahertz technology readiness assessment and applications J Infrared Milli Terahz Waves 34, 500–518 Saile, V., 2009 Introduction: LIGA and Its Applications WILEY-VCH Verlag GmbH & Co KGaA V of Advanced Micro and Nanosystems pp 1–10  ... 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. .. behind one of the metallized PMMA gratings (Fig 5) (a) (b) Fig (a) Self- imaging of a metallized PMMA grating recorded with the microbolometer matrix in the Talbot plane ( z 31.5 mm) Width of the. .. 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