=Paper=
{{Paper
|id=Vol-1490/paper11
|storemode=property
|title=The research of the properties of thin films of molybdenum to form the contact masks for diffractive optics elements
|pdfUrl=https://ceur-ws.org/Vol-1490/paper11.pdf
|volume=Vol-1490
}}
==The research of the properties of thin films of molybdenum to form the contact masks for diffractive optics elements==
https://ceur-ws.org/Vol-1490/paper11.pdf
Computer Optics and Nanophotonics
The research of the properties of thin films of
molybdenum to form the contact masks for diffractive
optics elements
Poletaev S.D.
Image Processing Systems Institute, Russian Academy of Sciences,
Samara State Aerospace University
Abstract. Researched the parameters of the microstructures, obtained by laser
termochemical space recording in films of molybdenum with a thickness of 17,
35 and 70 nm, deposited on a glass and quartz substrates. Graphs of the spatial
resolution of the microstructures as a function of the laser power are plotted for
different substrate materials. It is shown that a higher spatial resolution of the
microstructures can be achieved in the molybdenum films with a 17 nm
thickness.
Keywords: microstructure, laser ablation, thermal recording of a molybdenum
film, glass and quartz substrates
Citation: Poletaev S.D. The research of the properties of thin films of
molybdenum to form the contact masks for diffractive optics elements.
Proceedings of Information Technology and Nanotechnology (ITNT-2015),
CEUR Workshop Proceedings, 2015; 1490: 90-96. DOI: 10.18287/1613-0073-
2015-1490-90-96
Introduction
It seems promising to use in various fields of science and technology of diffractive
optical elements (DOE), which are plates formed on their surface microstructure [1,
2]. The most important stage of manufacture of the microreliefs is get contact mask
resistant to plasma-chemical processes necessary to transmit the calculated
microstructure of the DOE in the substrate. Previously this had been circulated
lithographic (wet) technology [3–8].
At the present time to reduce the dimensions of the microstructures, the widely
used method of formation of topological drawing directly in the source layer of the
material of the contact mask without the use of photoresists. It is based on local
processing of thin films of chromium focused laser radiation [9, 10], under the
influence of which is thermochemical conversion of the surface layer of the work
material.
The disadvantage of this technology is the low resolution of about 0.8 μm.
Therefore urgency to the task of developing techniques that allow to overcome this
barrier. This result is possible through search and application of materials with
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contrasting characteristics to selectively use a maximum of activating radiation. In
this respect, a well-known series of works, where instead of chrome offers a variety of
alternative materials, such as silicon, indium phosphide and the oxides of various
metals [11–13]. Unlike standard technologies they propose to form a microstructure
by evaporation (ablation) of material.
In [14] have demonstrated the possibility of ablation of molybdenum films with a
thickness of about 0.5 μm picosecond laser beam with a wavelength of 1064 nm,
deposited on a sublayer of silicon nitride thickness of about 140 nm. The grounds
were glass substrate of a thickness of 3 mm.
Based on [14], we proposed an approach based on the ablation areas of the film of
molybdenum exposed to laser radiation [15].
The purpose of this work is to study the feature of the contact mask, obtained by
laser ablation of films of molybdenum, depending on the thickness of the films, which
will produce the optimum conditions of carrying out the process.
1. Methods and materials
The base served as an optically smooth substrate made of glass and fused quartz
brand KV of size 50×50 mm, thickness 3 mm. Film of molybdenum with a thickness
of 17, 35 and 70 nm were deposited by magnetron sputtering method on the "Caroline
D-12A" under the following conditions: the power of the magnetron is 700 W, the
temperature of the substrate is 200°C, a pressure of argon to 2.0∙10-1 Pa. The time of
deposition was determined by the finite thickness of the films and ranged from 2 to 8
min.
Patterns in the films formed on the laser writing station CLWS-200 [16–18].
Recording was conducted under the following conditions: operating wavelength of
the laser radiation is 488 nm; maximum power supplied to the recording head
100 mW; record structure – concentric rings with a pitch of 3 μm and an outer radius
of 3 mm; the magnitude of the power for each ring decreased from 100 % at the point
of greatest radius to 0 in the centre with a step of 0.5 %. The speed of rotation of the
sample – about 10 s-1. the Specified parameters of the process corresponded to the
maximum power density of laser radiation Emax = 2∙107 W/cm2. The effect of laser
radiation led to local evaporation thin film of molybdenum over the entire thickness.
The morphology and elemental composition of the surface of the nanostructures
were studied using scanning electron microscope (SEM) Hitachi TM3030 with
integrated EDS spectrometer.
2. Analysis of the results
In Fig. 1 shows the dependence of the width of the lanes from the laser output
power for a radius record of 3 mm. Dependence obtained for quartz and glass
substrates. The width of the recorded tracks in the first place, depends linearly on the
laser power. When they are the same width for quartz substrates requires
approximately 20% more power, due to the difference in the thermophysical
properties of quartz and glass. Quartz, having a larger thermal conductivity, assigns
laser energy from the field of film. Film thickness of 17 nm show higher spatial
resolution, 2500 mm-1, against 1500 mm-1 for films with a thickness of 35 nm. The
permissions that are close to the maximum was reached in the power density in the
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range (0,7...1,2)∙107 W/cm2. Note that this is significantly less than the theoretical
values obtained, for instance, in [11].
a)
b)
Fig. 1. – The dependence of the line width of the nanostructure on the power of laser radiation
for films with a thickness of 17 nm (a) and for films with a thickness of 35 nm (b)
Presented on Fig. 2 pictures of SAM films with a thickness of 35 nm, exposed to
the laser beam, allow the edges of the tracks to watch the products of destruction as
representing a zone of thermal influence. Most likely, their emergence with the
removal of a significant portion of the substance from the area of reactive ablation
recoil pressure PA-world jet [19]. As shown earlier, their height does not exceed the
thickness of the film [15].
For films with a thickness of 17 nm, the formation of degradation products is not
typical (Fig. 3). This can be explained by the fact that the energy of laser radiation is
enough for a quick and complete evaporation of the material at the point of impact.
For these two cases are well for-markedly boundary of the critical power at which the
evaporation of the metal stops. Also around tracks observed subtle trace of a width
equal to the diameter of the laser spot. Probably, this area needs to impose restrictions
on the minimum recording period. For films with a thickness of 70 nm of the
radiation energy is insufficient for the formation of tracks even in areas with a
minimum radius of recording (Fig. 4). It is important to note the following. For
systems with circular scanning absorbed by the material energy (dose) of laser
radiation at a constant power density varies with the radius record, since the radius of
the record specifies an effective time of exposure. The decrease of power density
from the edge to the center with simultaneous increase of exposure time to have an
opposite effect on the magnitude of the radiation energy. However, in our case a
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decrease in the size of the track as you approach the center, that allows claim about
the decrease of the absorbed energy.
Fig. 2. – The SEM picture of the surface nanostructures on film of molybdenum with a
thickness of 35 nm
a)
b)
Fig. 3. – The SEM picture of the surface nanostructures on film of molybdenum with a
thickness of 17 nm: the shape and the characteristic sizes of tracks (a) General view (b)
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Conclusion
The study of microstructures of metallic patterns of the DOE showed that the best
spatial resolution and minimum defects in the figure of the mask is achieved at a film
thickness of molybdenum 17 nm.
Minimum attainable period of record limit width trace of the laser spot and is in
our case 0.8 μm. Spatial resolution will be limited to this value and can be increased
by reducing the wavelength and, correspondingly, the diameter of the laser spot.
Given the fact that the selectivity of the plasma etching films of molybdenum
relative to quartz can reach several hundred, the result opens the way for the creation
of structures with submicron resolution, in particular, short-focus of the DOE, and
also allows you to create patterns with a minimum number of manufacturing
operations. In addition, as described in the article, the research is also relevant for the
development of certain laser technology [20 – 22].
Fig. 4. – The SEM picture of the surface nanostructures on film of molybdenum with a
thickness of 70 nm
Acknowledgements
The work was supported by RFBR (grant No. 14-07-00177a). The authors Express
gratitude to the company "INTERLAB" (Russia, Moscow) for their help in the study
of samples on the microscope Hitachi TM.
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