=Paper=
{{Paper
|id=Vol-1490/paper10
|storemode=property
|title=Laser ablation of thin films of molybdenum for the fabrication of contact masks elements of diffractive optics with high resolution
|pdfUrl=https://ceur-ws.org/Vol-1490/paper10.pdf
|volume=Vol-1490
}}
==Laser ablation of thin films of molybdenum for the fabrication of contact masks elements of diffractive optics with high resolution==
https://ceur-ws.org/Vol-1490/paper10.pdf
Computer Optics and Nanophotonics
Laser ablation of thin films of molybdenum for the
fabrication of contact masks elements of diffractive optics
with high resolution
Poletaev S.D.
Image Processing Systems Institute, Russian Academy of Sciences,
Samara State Aerospace University
Abstract. Considered the task of reducing the thickness of the contact lines of
the pattern masks used in the formation of the microrelief of diffractive optical
elements (DOE) and produced by laser ablation of thin films of refractory
metals. For contact mask of DOEs on molybdenum films with thickness of 40 nm
using a laser ablation patterns recorded with elements of the picture width 0.25–
0.3 µm. This is approximately 3 times smaller than the characteristic dimensions,
obtained by thermochemical recording chromium films of the same thickness in
the standard process. Reactive ion etching in an inductively coupled plasma
through a mask was formed micro-relief height up to 300 nm in a quartz substrate.
We have shown promising applications of thin films of molybdenum as a metallic
mask in the formation of microrelief of DOEs.
Keywords: diffractive microrelief, metallic mask, laser ablation,
thermochemical recording, film molybdenum, reactive ion etching
Citation: Poletaev S.D. Laser ablation of thin films of molybdenum for the
fabrication of contact masks elements of diffractive optics with high resolution.
Proceedings of Information Technology and Nanotechnology (ITNT-2015),
CEUR Workshop Proceedings, 2015; 1490: 82-89. DOI: 10.18287/1613-0073-
2015-1490-82-89
Introduction
Thermochemical laser writing [1, 2], the contact masks plays a decisive role for a
wide range of [3-9] The methods of forming the microrelief diffractive optical
elements. Currently, the widely used form metalized microrelief mask thin films of
metals [1-2, 5], in which during exposure to laser radiation is focused thermochemical
conversion of the surface layer of the working material. The starting material is
widely used chromium [1-2, 5]. The sequence of formation of micro-relief in a quartz
substrate, in this case, following [5, 7]:
─ Chromium plating film of a given thickness on a substrate;
─ The formation of topological pattern of the future impact of the laser element in the
film;
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Information Technology and Nanotechnology (ITNT-2015)
Computer Optics and Nanophotonics Poletaev S.D. Laser ablation of thin films of…
─ The creation of metallic liquid etching mask film of chromium areas not exposed
to laser radiation;
─ Plasma etching the substrate through the resulting metallic mask (the formation of
microrelief in the substrate).
The disadvantage of this technology is pretty low resolution. Standard achievable
feature size structures in this case - the order of the wavelength, i.e. about 0.8 μm
[10]. In this regard, the actual task is the development of technological methods for
creating elements with high spatial resolution.
On the basis of the above-described process sequence, for example, in [11], there
has been an element size of 0.5 μm on the structure of the chromium films 50 nm
thick inflicted thermal vacuum process substrates of optical glass.
Patent [12] describes how to increase the resolution of the method of laser
thermochemical oxidation film of titanium thickness of 3 - 60 nm, deposited on the
glass substrate.
A characteristic feature of the studies described in [1-11], is that the resistance to
the subsequent chemical resistance increases for portions of film exposed to the laser
radiation. In contrast to [1-11], we propose an approach based on evaporation
(ablation) portions of the film exposed to laser radiation.
The purpose of this paper is the experimental investigation of the possibility of
further increasing the spatial resolution diffraction microrelief formed by using the
contact masks using laser recording. It is proposed to achieve this total rejection of
liquid chemical processes of lithography through the use of new materials and other
physical effects of producing binary microstructures.
1. Problem statement and proposed approach
In [13] have demonstrated the possibility of ablation of molybdenum films
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. Ablation of the films of molybdenum with a thickness of about 0.5
μm was carried out by laser beam with a maximum energy flux density of 260
W/cm2, and it was suggested that the molybdenum is removed from the substrate
surface without chemical transformations. In our case, a contact mask on the basis of
thin films of molybdenum was used for forming a diffraction microrelief in the
following sequence of operations:
─ sputtering thin films of molybdenum on a substrate;
─ the formation of metal mask element, the influence of laser radiation on the film of
molybdenum;
─ reactive-ion etching in inductively coupled plasma substrate through a metallic
mask (formation of microrelief in the substrate).
The microrelief formed on substrates of fused quartz brand KV of size 50×50 mm,
thickness 3 mm and 14 class of surface cleanliness. Film of molybdenum was
deposited by magnetron sputtering method on the "Caroline D-12A" [14] a thickness
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Computer Optics and Nanophotonics Poletaev S.D. Laser ablation of thin films of…
of 40 nm. The formation of topological drawing of patterns in the molybdenum film
(metal mask) was performed on the laser writing station CLWS200 [5, 13] with the
following parameters: operating wavelength of the laser radiation is 488 nm; the
power supplied to the recording head, is about 100 mW; record structure – concentric
rings with a pitch of 3 μm and the outer radius of 2 mm; the magnitude of the power
for each ring was reduced from 100% to 0 from the maximum power in 0.5%
increments. On the outer rings with the capacity of about 80 to 40 mW, the laser
radiation would result in a localized evaporation of thin films of molybdenum for all
thickness down to the quartz substrate.
2. Analysis of the results
The results of the research profile of microstructur-ture formed in the molybdenum
film when exposed to a laser beam of various capacities, represented in Fig. 1.
Measurement of the profile of the microstructure was carried out on a scanning probe
microscope (SPM) “Solver-Pro”. On the profile visible area of complete removal of
molybdenum (complete ablation). The boundary of the critical power at which
ablation stops, well marked (the power decreases from left to right, Fig. 1a). On the
edges of the formed structures have shown that the characteristic outbursts, which can
be explained by the release of material during exposure to the beam.
In Fig. 2 shows the same image of microstructures, but obtained by scanning
electron microscope (SEM) “Supra 25”. The picture shows a clear band width 253-
256 nm (Fig. 2a, b). On these pictures it can be seen that the edges of the grooves are
damage to the film or the formation of the projecting profile, which is confirmed by
the data obtained with SPM.
The width of the line of the laser beam (the portions of the substrate, free from
films of molybdenum) is 220...300 nm (Fig. 1b) and depends on the magnitude of
power greater than that required for ablation, which is confirmed by Fig. 2b.
For the formation of diffractive microrelief was used for reactive-ion etching of
quartz substrates at the "Caroline PE-15" induction plasma excitation from the
generator of radio-frequency voltage of 13.56 MHz. Working chamber cylindrical
shape of the planar type. The etching was conducted in an environment hexafluoride
SF6 [15]. To stabilize the discharge in the plasma mixture was added in argon [16 –
18]. Power from the RF source is supplied to the inductor that is installed at the top
inside the chamber. Etching of the sample 1 was carried out in the following mode:
power inductor – 400 W; power stage – 200 W; the flow rate hexafluoride SF6 – 60
cm3/min; flow rate of argon Ar – 50 cm3/min; the pressure of a gas is 5.0∙10-1 Pa; the
etching time of 10 min.
The mode of etching of the sample 2 from the mode of etching of the sample 1
differs only in the time of etching, is accounted to him for 15 min.
After reactive-ion etching of the substrate remnants of the mask was removed.
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Information Technology and Nanotechnology (ITNT-2015)
Computer Optics and Nanophotonics Poletaev S.D. Laser ablation of thin films of…
a)
b)
Fig. 1. – SPM results profile molybdenum film after laser writing: the border at the beginning
of the burn process when the critical power (a), the recorded patterns with a line width of 220
nm (b)
The resulting SPM profile of the samples is shown in Fig. 3a, b. The images show
that the quality of the surface microrelief of the sample 1 is higher than sample 2,
which is probably due to the long time of etching, resulting in the masking film of the
sample 2 is completely degraded in the plasma, which led to the destruction of the
surface microrelief. In addition to increasing the rate of etching in these areas can be
explained by changes in the chemical composition of the masking layer during laser
recording, the more the height of the mask at the edges of the grooves is higher than
in other areas. On the submitted drawings the width of the lines for samples 1 and 2 –
294 and 353 nm, respectively.
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Information Technology and Nanotechnology (ITNT-2015)
Computer Optics and Nanophotonics Poletaev S.D. Laser ablation of thin films of…
a)
b)
Fig. 2. – SEM image of the sample after laser writing: the border at the beginning of the burn
process when reaching the critical power (a), the enlarged part of image (b)
Conclusion
In our experiments, the possibility of creating optical structures of submicron
resolution, including with elements smaller than the diffraction limit (0.25 μm), based
on the dry etching of quartz using a contact mask obtained by the method of laser
ablation of molybdenum film. Reduction of the characteristic dimensions of the
diffractive microrelief [19–22] to create a DOE with a smaller focal lengths, with a
larger aperture, or DOE, is designed to lower the working wavelength. Of course, the
proposed improvements are not suitable for everyone [23–25] technological
approaches, but can be effectively used for a wide range [3–11] methods for forming
diffractive microrelief. Further research is planned and on the way of formation and
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Computer Optics and Nanophotonics Poletaev S.D. Laser ablation of thin films of…
use of thinner films (25 nm or less), which should lead to a further increase in the
resolution of laser writing.
a)
b)
Fig. 3. – SPM microrelief formed by etching in inductively coupled plasmas: profile of sample
1 (a) profile of sample 2 (b); the line width of 294 nm and 353 nm, respectively
Acknowledgements
The work is executed at financial support of the Ministry of education and science
of the Russian Federation, the grant of the President of the Russian Federation for
support of leading scientific schools NSH-4128.2012.9, grant RFBR No. 14-07-
00177a.
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