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Laser microprinting of InO
x
active optical structures and
time resolved imaging of the transfer process
I. Zergioti
*
, D.G. Papazoglou, A. Karaiskou, N.A. Vainos
1
, C. Fotakis
Institute of Electronic Structure and Laser, Foundation for Research and Technology, Hellas, P.O. Box 1527, Heraklion 71110, Greece
Abstract
Recent advances of direct printing of compound microstructures by laser induced forward technique (LIFT) using a
femtosecond UV laser will be presented. LIFT is a technique enabling the direct controlled transfer of thin ®lm materials
between substrates. An ultrashort UV laser has been used to transfer compound material (e.g. InO
x
) onto glass substrates in order
to form optical diffractive structures. The LIFT process preserves the oxide's structural and physical properties. The use of laser-
based methods in the fabrication of optically activated microstructures initiates new possible applications in the area of
optoelectronics. Furthermore, time-resolved imaging of the plume produced by the femtosecond LIFT of InO
x
has been
performed using an image intensi®ed CCD (ICCD) camera in the time interval up to 2 ms after the laser pulse. The velocities of
the emitted particles were measured to 400 m/s.
#2002 Elsevier Science B.V. All rights reserved.
Keywords: Indium oxide; Optical microstructures; Fs laser microprinting; Laser induced forward transfer; Imaging
1. Introduction
The laser-induced forward transfer (LIFT) techni-
que [1,2] utilizes pulsed lasers to remove thin ®lm
material from a transparent support and deposit it onto
a suitable substrate. The thin ®lm is precoated on a
quartz plate (target) and is transferred using a single
laser pulse onto a receiving substrate placed parallel to
the target ®lm. The LIFT process has been success-
fully applied for the selective microdeposition of
metals [3±5] oxides [6] and diamonds [7] by several
groups during the last decade. In our previous work
[8], we have demonstrated the fabrication of high
resolution patterns of indium oxide diffractive struc-
tures by the ultraviolet femtosecond LIFT technique.
Indium oxide based thin-®lm compounds are cur-
rently ®nding several uses [9,10] in display, image
sensor and solar cell technology, owing to their unique
properties of high optical transparency and large con-
trollable electrical conductivity [11,12]. Their dynamic
optical properties also address innovative applications
in light modulation, optical switching [13] and holo-
graphic recording [14,15] and merit further detailed
fundamental and applied investigation.
The effectiveness of LIFT relies on de®ning the
critical parameters of laser and solid target interac-
tions. Its optimisation depends on the speci®c optical,
thermo-physical and mechanical properties of the
materials involved. The dynamics of the laser ablation
transfer technique using time resolved optical micro-
scopy has been studied by Sandy Lee et al. [16].A
pulsed Nd:YAG laser (1.064 mm, 250 ns) was used for
Applied Surface Science 197±198 (2002) 868±872
*
Corresponding author. Tel.: 30-81-391325;
fax: 30-81-391318.
E-mail address: zergioti@iesl.forth.gr (I. Zergioti).
1
Present address: NHRF-The National Hellenic Research
Foundation, 48, Vas. Constantinou Ave., Athens 11635, Greece.
0169-4332/02/$ ± see front matter #2002 Elsevier Science B.V. All rights reserved.
PII: S 0169-4332(02)00440-3
the ablation and the produced plume was observed
using a 25 ps probe pulse of a Nd:YAG-pumped dye
laser, which propagated nearly perpendicular to the
250 ns ablation pulse. They have observed velocities
of the metallic lift off ®lm up to 0.75 Mach under air
conditions. Recently, Young et al. [17] have investi-
gated the dynamics of a similar technique, ``Matrix
Assisted Pulsed Laser Evaporation Direct Write'',
using a Nd:YAG transfer laser (355 nm, 150 ns) with
ultra fast microscopy and they have measured the
plume velocity 0.2 km/s. Bullock et al. [18] have
studied the laser-induced back ablation of Aluminium
thin ®lms with picosecond laser pulses (Ti:sapphire,
1053 nm, 2±3.6 ps) under vacuum conditions, using a
shadowgraphic and interferometric system. Nakata
et al. [19] have investigated the LIFT process (dye
laser 440 nm, 9 ns) of metal ®lms by applying the
microscopic two-dimensional laser induced ¯uores-
cence method. They have measured the dynamic beha-
viour of gold atoms and emissive particles in vacuum
and in air, and they found velocities up to 2 km/s.
In this work we experimentally demonstrate that the
directly grown surface-relief dielectric InO
x
micro-
structures can be activated optically. We have applied
exclusively laser-based methods and produced, with-
out any additional processing, a thin-®lm target mate-
rial and transferred it selectively onto a receiver
substrate preserving the material's dynamic optical
properties. The designed grating form of the micro-
structures enables the demonstration of optical activa-
tion effects by means of photo induced diffraction
ef®ciency changes.
In the present work we have also investigated the
LIFT process with ultra fast photography in order to
understand better the forward dynamic transfer of the
material onto the substrate.
2. Experiment
The experimental procedure of the fabrication of
the InO
x
grating is depicted in Fig. 1. We have
fabricated InO
x
thin ®lms on fused silica plates by
reactive pulsed laser deposition using pure indium and
oxygen as it is described more detailed in our previous
work [8]. These thin ®lms have been used as target
materials in the laser microprinting operation outlined
in Fig. 1. A hybrid distributed feedback dye laser/KrF
excimer laser, delivering 500 fs duration pulses of
10 mJ/pulse, at l248 nm was used. The InO
x
®lm is
selectively ablated and redeposited on a corning glass
(UV grade) receiver substrate positioned in close
proximity to the target material (5±10 mm). The max-
imum laser energy density on target was 500 mJ/cm
2
and a high-precision step and repeat operation was
applied to fabricate the grating microstructures [4].
The dynamics of the LIFT process was studied using
an image intensi®ed CCD (ICCD) camera as this is
depicted in Fig. 2.Athin®lmofInO
x
(200 nm in
thickness) was irradiated with the KrF excimer based
laser (500 fs, 248 nm) at experimental conditions simi-
lar to those applied for the InO
x
fabrication. The emitted
micro-plume under air conditions, was imaged onto the
two-dimensional ICCD camera sensor through UV
lenses. The spectral range of operation of the ICCD
detector is from 200 to 800 nm. The total plume emis-
sion was recorded in a single shot image operation. A
frame grabber was used to collect the ICCD image after
triggering a pulsed delay generator, which in turn
triggered the ICCD and the ®ring laser. The gate time
of the camera was 50 ns and the delay time between the
laser and the camera varied between 0 and 4.5 ms.
3. Results
Fig. 3 shows an AFM image of the InO
x
grating
microprinted on glass substrate. The grating lateral
dimensions are 5 mm100 mm. The structures were
analysed with X-ray diffractometry and the results
were in full agreement with our previous work [4].The
Fig. 1. Outline of the LIFT method for the InO
x
grating
microprinting.
I. Zergioti et al. / Applied Surface Science 197±198 (2002) 868±872 869
refractive index, n, and the extinction coef®cient, k,of
the oxide target materials have also been estimated by
thin-®lm matrix methods at n2:00:03 and k
0:005 0:002, using independent measurements at
l633 nm. The high error of kis attributed to surface
contaminations. Fig. 4 displays the time-resolved InO
x
plume emission in air obtained by an ICCD camera.
The applied laser ¯uence was 465 mJ/cm
2
, similar to
Fig. 2. Schematic diagram of the imaging set-up.
Fig. 3. AFM image of InO
x
microprinted grating on glass.
870 I. Zergioti et al. / Applied Surface Science 197±198 (2002) 868±872
Fig. 4. Time-resolved InO
x
plume emission images captured by an ICCD camera. The plume velocity is 0.4 km/s.
I. Zergioti et al. / Applied Surface Science 197±198 (2002) 868±872 871
those applied in the microprinting experiments of the
InO
x
grating fabrication. The spot size of the laser
beam on the target ®lm was 200 mm300 mm.
The images have been inversed (black corresponds
to bright) and the brightness has been normalized to
brightest region. A gate width of 50 ns was used for
image capture. The velocity of the emitted species was
measured up to 400 m/s. The emission intensity
reached peak value at around 200 ns and then decayed
slowly up to 5 ms. The use of the femtosecond laser
produced plumes much faster than those reported [17]
to be produced by nanosecond lasers. Nakata et al.
[19] have measured the emissive species velocities
using an ICCD under the air conditions to 230 m/s and
a pulsed dye laser (440 nm, 9 ns). Furthermore, in our
experiments the emissive plume is highly directive.
The lateral expansion is minimal leading to an emis-
sive plume divergence of the order of 48.
Further study of the emitted micro plume in both the
LIFT and the ablation mode is under way using sha-
dowgraphic imaging method. The preliminary results
show a very good agreement between the two methods.
4. Conclusions
In conclusion, optically activated InO
x
surface relief
microstructures have been developed through the
LIFT method. Optical activation of these structures
has been demonstrated by using near UV laser irradia-
tion. Time resolved plume emission images were
obtained and indicated that femtosecond laser pulses
produced plumes much faster than those reported by
using nanosecond laser pulses. The measured velo-
cities were up to 400 m/s.
Acknowledgements
The described work has been carried out through
projects within the ultraviolet laser facility (ULF),
which operates at FORTH, with support from the
EU Directorate Research, Programme Access to
Large Installation at the Foundation for Research
and Technology-Hellas. We would like to thank
Dr. D. Anglos for providing assistance with the ICCD
detector.
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