=Paper=
{{Paper
|id=Vol-1638/Paper105
|storemode=property
|title=The efficiency of optical microstructures formed on molybdenum films
|pdfUrl=https://ceur-ws.org/Vol-1638/Paper105.pdf
|volume=Vol-1638
|authors=Sergey D. Poletayev,Sergey G. Volotovsky
}}
==The efficiency of optical microstructures formed on molybdenum films ==
https://ceur-ws.org/Vol-1638/Paper105.pdf
Data Science
THE EFFICIENCY OF OPTICAL
MICROSTRUCTURES FORMED ON MOLYBDENUM
FILMS
S.D. Poletayev1,2, S.G. Volotovsky1,2
1
Image Processing Systems Institute - Branch of the Federal Scientific Research Centre “Crys-
tallography and Photonics” of Russian Academy of Sciences, Samara, Russia
2
Samara National Research University, Samara, Russia
Abstract. We have considered influence of technological errors of formation of
the amplitude optical lattices made by method of a laser ablyation of thin films
of molybdenum on distribution of diffraction orders in a distant zone of diffrac-
tion. For this purpose we have digitized a profile of a site of the created diffrac-
tion structure with the subsequent transformation to amplitude or phase func-
tions of a transmission.
Keywords: diffractive microrelief, metallic mask, laser ablation, thermochemi-
cal recording, film molybdenum, reactive ion etching.
Citation: Poletaev SD, Volotovsky SG. The efficiency of optical microstruc-
tures formed on molybdenum films. CEUR Workshop Proceedings, 2016;
1638: 882-887. DOI: 10.18287/1613-0073-2016-1638-882-887
Introduction
Currently the ablation of metal films is widely used by laser radiation in such areas as
semiconductor production, photo-electric devices, diffraction lattices and other high-
technology appendices [1-9]. In this field researches is actual the task of forming of
the amplitude diffraction optical elements (DOE), and also the masking layers applied
to transfer of a microrelief in a substrate is actual. In this respect is perspective thin
films of molybdenum [6] as molybdenum has the following features are perspective:
– low temperatures of oxidation of metal, and oxide sublimation;
– high ratio of heat diffusivities metal/oxide.
In works [10-12] the possibility of forming of optical microstructures by a laser abla-
tion of thin films of molybdenum is shown. The effect of triple reducing the size of a
zone of an ablation has been found (in comparison with diameter of a focal spot).
However edges of tracks turned out uneven, around tracks heat-affected zone (HAZ)
caused by metal oxidation was created.
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The purpose of this work consists in identification of influence of technological errors
of production of lattices on efficiency of their work.
Modeling of a spatial range of focusing in a distant zone has been for this purpose
carried out.
Methods and materials
Microstructures were formed at station of the laser record CLWS-200 [13-14] in the
following sequence of technological operations:
– a dusting of a thin film of molybdenum on a substrate;
– formation of a test lattice by a local ablyation of a film under the influence of laser
radiation.
Optically smooth substrates from glass and fused quartz the size 50×50 of mm, 3 mm
thick formed the basis. Film of molybdenum with a thickness of 17 nm were deposit-
ed by magnetron sputtering method on the instruments «Caroline D-12A» (Russia,
Zelenograd) under the following conditions: a magnetron power - 700W, substrate
temperature - 200º C, the argon pressure – 0,2Pa. Deposition time was 2 min.
Laser recording was carried out under the following conditions: the working length of
the laser wavelength - 488 nm; the maximum power delivered to the recording head -
100 mW; record structure - concentric rings with step 3 µm and an outer radius of 3
mm; the amount of power to each ring is decreased from 100% at the point of largest
radius to the center 0 in increments of 0.5%. Sample rotation speed - 10 s-1. These
process parameters correspond to the maximum laser density of power Еmax = 20
MW/cm2. The impact of laser radiation led to local ablation of the molybdenum film.
Analysis of the results
The fragment of the analyzed lattice is given in fig. 1 [10]. The width of the tracks is
in the range of 250-400 nm.
The structure shown in fig. 1 through binarization can be converted to a binary dif-
fraction grating (Fig. 2b).
The resulting structure is an amplitude grating with different sized areas. In this case
the resulting amplitude is a lattice, but can easily be converted into a phase diffraction
grating. Consider the work of the resulting structure in both cases.
For clarity, in fig. 3 shows the profilogram a plot of the derived structure (fig. 3a),
taken in a direction perpendicular to the recording tracks, by using a profilometer
KLA-Tencor P16+. The profile is the same binary grating shown in Fig. 3b.
To calculate the intensity distribution in the far zone of diffraction of a plane light
wave in the considered two-dimensional structure used fast algorithms of direct calcu-
lation of the Fourier transform [15-17].
Calculation based on numerical integration, in contrast to the algorithms of the fast
Fourier transform, allows to correctly evaluate the physical characteristics of the gen-
erated fields. This is especially important in the analysis of the dispersive properties
of diffraction gratings [18, 19].
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Data Science Poletayev SD, Volotovsky SG. The efficiency…
Fig. 1. Image of the lattice analysed
a) b)
Fig. 2. Transformed an ideal (a) and (b) lattice (negative image)
а)
b)
Fig. 3. The profiles plot (a) real and (b) given to the binary appearance of the diffraction struc-
ture
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Data Science Poletayev SD, Volotovsky SG. The efficiency…
In fig. 4a shows the intensity distribution in order, in fig. 4b is the same distribution
together with the intensity distribution of an ideal amplitude grating (fig. 2a) with the
same period and width of the tracks.
The standard deviation of the two graphs in fig. 4b is 0.26, given the nature of the
distribution, is not a big mistake.
Was also simulated diffraction by a phase grating of this type. This digitized distribu-
tion shown in fig. 1, was transformed into the phase transmission function, the profile
of which is shown in fig. 5.
In fig. 6a shows the distribution of intensity in the far zone of diffraction obtained
after using the Fourier transform. In Fig. 6b shows the same distribution compared to
the distribution of ideal phase grating with the same period and width of the tracks.
Mean-square deviation of the two graphs in fig. 6b is 0.21, which, given the nature of
the distribution, is also not a big mistake, because in this case more important than an
exact match of the positions of the maximum.
a)
b)
Fig. 4. The intensity distribution orders in the far zone of diffraction from the amplitude struc-
ture, shown in fig. 3 (a), the same distribution of intensity on the order in the far zone of dif-
fraction from the amplitude structure, shown in fig. 2b) compared to the distribution from a
perfect lattice (dashed line) (b)
Fig. 5. Section transmissive phase functions obtained from the profilograms in fig. 2a
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Data Science Poletayev SD, Volotovsky SG. The efficiency…
a)
b)
Fig. 6. The intensity distribution orders in the far field diffraction phase structure shown in fig.
5a), the same intensity distribution over orders in the far zone of diffraction from the amplitude
structure, shown in fig. 5b) compared to the distribution from a perfect lattice (dashed line) (b)
Conclusion
The work experimentally shows the possibility of creating optical structures of sub-
micron resolution. Also by simulation it is shown the performance of the generated
structures both in amplitude and in phase. The magnitude of the standard deviations
for the distributions of intensity of 0.26 and 0.21, respectively, for amplitude and
phase gratings are not critical, assuming the structure of the resulting distributions
(there is almost perfect coincidence of the position of diffraction orders). Therefore,
demonstrated working technology for forming diffractive structures with a minimum
element size of 0.25 µm.
Acknowledgements
The work was funded RFBR grants № 14-07-00177, 16-29-09528, 16-29-11744.
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