Castaing, M.

Chambonneau, M.

Fedorov, V. Y.

Février, S.

Gaponov, D.

Grojo, D.

Hideur, Ammar

Lavoute, L.

Utéza, O.

English

International audienceBased on the development of a µJ-class Thulium-doped fiber laser operating in the picosecond regime at 1970-nm wavelength, we introduce a solution for three-dimensional (3D) laser writing technologies inside silicon (Si). We reveal that the nonlinear effects preventing from bulk modification in Si with femtosecond pulses persist in the picosecond regime. However, these are strongly reduced which makes possible to derive conditions for a demonstration of data inscription and reading deep into a Si wafer

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Three-dimensional writing inside silicon using a 2-µm picosecond fiber laser

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Three-dimensional writing inside silicon using a 2-µm
picosecond fiber laser
M. Chambonneau, L. Lavoute, D. Gaponov, M. Castaing, V. Y. Fedorov,
Ammar Hideur, S. Février, O. Utéza, D. Grojo
To cite this version:
M. Chambonneau, L. Lavoute, D. Gaponov, M. Castaing, V. Y. Fedorov, et al.. Three-dimensional
writing inside silicon using a 2-µm picosecond fiber laser. Proceedings of ICALEO 2019, 2019, Pa-
per#142. ￿hal-02323573￿
THREE-DIMENSIONAL WRITING INSIDE SILICON  
USING A 2-µM PICOSECOND FIBER LASER 
Paper # Micro 1404 
 
M. Chambonneau1, L. Lavoute2, D. Gaponov2, M. Castaing2,V. Y. Fedorov3,4, A. Hideur5, S. Février6, 
S.Tzortzakis3,7,8, O. Utéza1, and D. Grojo1 
 
1 Aix-Marseille Univ., CNRS, LP3 UMR 7341, 13009 Marseille, France 
2 Novae, ZI du Moulin Cheyroux, 87700 Aixe-sur-Vienne, France 
3 Science Program, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar 
4 P. N. Lebedev Physical Institute of the Russian Academy of Sciences 
5 CNRS, INSA, Univ. Rouen Normandie, CORIA UMR 6614, 76801 Rouen, France 
6 Univ. Limoges, CNRS, XLIM UMR 7252, 87000 Limoges, France 
7 Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology—Hellas 
(FORTH), P.O. Box 1527, GR-71110 Heraklion, Greece. 
8 Materials Science and Technology Department, University of Crete, 71003 Heraklion, Greece. 
 
 
 
Abstract 
Based on the development of a µJ-class Thulium-doped 
fiber laser operating in the picosecond regime at 1970-
nm wavelength, we introduce a solution for three-
dimensional (3D) laser writing technologies inside 
silicon (Si). We reveal that the nonlinear effects 
preventing from bulk modification in Si with 
femtosecond pulses persist in the picosecond regime. 
However, these are strongly reduced which makes 
possible to derive conditions for a demonstration of data 
inscription and reading deep into a Si wafer.   
Introduction 
First experiments using femtosecond lasers in the 
prospect of a 3D laser writing technology applicable to 
silicon (Si) have revealed strong nonlinear effects 
affecting energy delivery. These include highly efficient 
multiphoton absorption and important plasma effects far 
before reaching the focal region, which prevent from 
localizing enough energy density in the bulk of Si to 
induce any permanent material change [1-3]. Here, we 
introduce a solution based on the development of a µJ-
class Thulium-doped fiber laser operating in the 
picosecond regime. Supported by nonlinear propagation 
simulations and energy flow analyses, we reveal that 
nonlinear propagation effects persist in the investigated 
regime near critical power conditions. However, the 
conditions for reliable writing deep into Si wafers are 
accessible. Moreover, we anticipate a direct benefit of 
the Thulium-doped fiber laser technology used in these 
experiments with the possibility for bulk processing of 
even narrower band gap materials (e.g. Germanium). 
This represents a new challenge in this rapidly growing 
field of research after the recent demonstrations of the 
first in-chip functional microsystems fabricated inside 
Si by laser writing [4-6]. 
Results and discussions 
To target a pulse energy level consistent with the 
application of bulk machining in silicon we rely on the 
development of a Thulium-doped fiber system 
delivering 2-ps pulses at 1970 nm. While table-top 
systems deliver outstanding performances on the order 
of hundreds of μJ [7], monolithic all-fiber systems are 
usually limited to sub-μJ energy level [8] [9]. We have 
developed a 5 μJ-class ultrafast thulium-doped fiber 
system. The chirped pulse amplification approach was 
applied to the seed pulses generated in a dissipative 
soliton oscillator [10]. This concept allows all-fiber 
integration, high seed average power, adjustable 
spectrum to fit dispersive transmission band 
components, and pre-stretched pulse. The power 
amplifier stage features 25-micron core diameter active 
fibers instead of rod-type photonic crystal fibers. In 
order to boost the pulse energy above the μJ level, the 
nonlinear threshold, imposed by the active fiber core 
size, was increased by stretching the seed pulse to 550 
ps by means of a chirped fiber Bragg grating stretcher. 
The system also includes pigtailed acousto-optical 
modulator for repetition rate control. Finally, the output 
pulse was compressed using chirped volume Bragg 
grating. The linear amplification regime was observed 
up to an output average power of 500 mW at a repetition 
rate of 100 kHz.  
To investigate the damage response of bulk silicon, we 
carried out measurements using a conventional 
machining experiment. Linearly polarized pulses are 
focused with NA=0.85 at different depths in an intrinsic 
Si wafer (<100>-orientation, double side polished) 
using an asphere mounted on a motorized stage (along 
the optical axis Z). XY positioning is performed by 
motorized motion of the Si wafer. The theoretical spot 
diameter is about 1.5 µm (FWHM) according to 
calculations. We employ dark-field microscopy for 
detecting the light scattered by any produced damage in 
the bulk. In practice, the sample is illuminated at grazing 
angle with white-light and a microscope unit equipped 
with an InGaAs camera is installed for in situ 
observation. Irradiations are systemically repeated 10 
times for each tested condition so that we can make a 
statistical analysis of the damage initiation and discuss 
the results in terms of damage probability.  
The figure 1 illustrates the unusual damage response 
resulting from irradiation of Si with 10000 pulses. It 
shows the measured damage probability as a function of 
the energy of the 2-ps pulses. The corresponding 
experiments have been repeated after placing the 
geometrical focus at two different depths: 100 and 150 
µm. Each measurement results from irradiations 
separated by 50 µm to avoid any side effects from 
previous irradiations. For both depths, we first note a 
transition from 0% to 100% damage probability for a 
pulse energy of  250 nJ thus showing that the damage 
threshold can be exceeded in contrary to femtosecond 
laser experiments for similar focusing conditions. In 
practice, as soon as the transition region near threshold 
is exceeded, we observe highly repeatable 
modifications. However, a surprising feature occurs at 
higher energy (around 450 nJ and shown by an arrow in 
Fig. 1) where the damage probability drops down to 
zero indicating a reduction of the delivered fluence 
below the damage threshold. It is also interesting to note 
that by increasing the energy, it remains possible to 
overcome this reduction and retrieve the 100% damage 
probability with apparently less localized modifications 
as more important scattering by the damage sites is 
detected. 
To understand this unusual damage response, we turned 
to nonlinear propagation simulations. All the details on 
the simulations method were reported in Ref. [11]. The 
results in the picosecond regime and comparisons to 
experiment are available in Ref. [12]. In brief, our 
simulations account for Kerr refraction, nonlinear 
ionization and the associated plasmas effects. It was 
originally developed to quantify the intensity clamping 
process generally experienced by propagating intense 
pulses and being severely pronounced when ultrashort 
pulses are tightly focused in bulk Si [3]. In this previous 
work, the comparisons with measurements showed that 
the essential contributions to the intensity clamping are 
correctly accounted in the model [3]. We have identified 
the progressive depletion of the pulse by pre-focal 
nonlinear absorption, plasma screening and defocusing 
as contributing factors to the clamping. However, we 
also found the importance of Kerr refraction for a fully 
quantitative agreement. Interestingly, without 
introducing any additional physical contribution in the 
simulations, we found a small delocalization and re-
localization of the laser energy flux above clamping 
conditions in the picosecond regime [12]. This can be 
directly associated to the non-monotonic damage 
probability which is found above clamping. Kerr 
focusing being the only nonlinear process localizing the 
laser energy, it is natural to expect that it is the one 
competing with the previously mentioned effects 
clamping the intensity to cause the modulation of the 
laser pulse energy density.  
With the knowledge of this peculiar damage response 
shown in figure 1 we derived the conditions to achieve 
well-localized modifications in two distinct planes. The 
energy of the 2 ps pulses is first set slightly above the 
modification threshold but below the delocalization 
energy range (320 nJ per pulse focused with a numerical 
aperture NA=0.85) for reliable digital writing of binary 
data at a depth of 150 µm below the surface of a Si 
Figure 1. Damage probability measurements with 
10000 pulses focused below the surface of Si. Two 
focusing depths are tested: 100 and 150 µm. Damage 
occurrence is established using dark field IR 
microscopy. Several transitions between 0% to 100% 
damage probability evidence a non-monotonic change 
of the delivered fluence due to the nonlinear effects. 
The absence of damage in an energy range above 
threshold is attributed to nonlinear delocalization of the 
radiation. 
wafer. Data of a second information are subsequently 
inscribed on the surface using the same procedure at 
reduced pulse energy for pixels defined by clean 
ablation (confirmed by SEM observations). As shown 
in figure 2, the data in the two planes are clearly 
distinguished using the infrared dark-field microscope 
provided that the depth of field is short enough using a 
high magnification microscope objective. This 
demonstrates the possibility for multi-layer writing and 
reading information inside silicon wafers by picosecond 
lasers. 
 Summary and conclusion 
Bulk damage test experiments in crystalline silicon have 
revealed that the energy flux of 2-µm pulses is strongly 
affected by nonlinear effects. However, in contrary to 
the femtosecond regime, the modification threshold can 
be exceeded when 0.85 NA focusing is used. This 
enabled us to successfully inscribe data inside a Si wafer 
and, more importantly this indicated that 
microfabrication applications can be addressed on the 
basis of the rapidly developing Thulium-doped fiber 
laser technologies. 
 
Acknowledgements 
This research has been performed in the frame of the 
International Associated Laboratory “MINOS” and has 
received funding from the European Research Council 
(ERC) under the European Union’s Horizon 2020 
research and innovation program (Grant Agreement No. 
724480). 
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Figure 2. Bulk-surface two-plane data inscription by 
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Three-dimensional writing inside silicon using a 2-µm picosecond fiber laser

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