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JLMN-Journal of Laser Micro/Nanoengineering Vol. 7, No. 1, 2012
73
Hybrid Laser-Plasma Micro-Structuring of Fused Silica Based on
Surface Reduction by a Low-Temperature Atmospheric Pressure Plasma
Stephan BRÜCKNER*, Jennifer HOFFMEISTER**, Jürgen IHLEMANN***, Christoph GERHARD*, Stephan WIENEKE**
and Wolfgang VIÖL**
*Clausthal University of Technology, Institute of Energy Research and Physical Technologies, Leib-
nizstraße 4, 38678 Clausthal-Zellerfeld, Germany
**HAWK - University of Applied Sciences and Arts, Laboratory of Laser and Plasma Technologies,
Vo n -Ossietzky-Straße 99, 37085 Göttingen, Germany
***Laser Laboratory Göttingen e.V., Hans-Adolf-Krebs-Weg 1, 37077 Göttingen, Germany
E-mail: vioel@hawk-hhg.de
In this contribution, we report on a novel hybrid laser-plasma method for material processing
applications. This method is based on the combination of both an ArF excimer laser (λ = 193 nm)
and a low-temperature atmospheric pressure plasma jet source for the chemical reduction of glass
surfaces. Here, a hydrogen-containing plasma gas was applied. Due to the layer of silicon suboxide
that is generated in this vein, the absorption of the incoming machining laser beam is significantly
increased after 15 minutes of plasma-treatment. Several machining experiments in terms of front-
side ablation were performed on fused silica. Here, both pure and plasma-treated surfaces were ab-
lated using single laser pulses with a pulse duration of 20 ns. By introducing the presented hybrid
technique, the ablation threshold for micro-structuring was reduced significantly by a factor of 4.6
whereas the peak-to-valley height Rz of the machined area was decreased by a factor of 2.3. Further,
back-side ablation using the presented method was considered. By a terminal tempering process, the
initial transmission characteristics of fused silica can be reconstituted.
Keywords: hybrid laser-plasma technology, atmospheric pressure plasma, fused silica, silicon sub-
oxide, micro-structuring
1. Introduction
Due to its specific properties, fused silica is a well-
established and suitable optical medium for the production
of a variety of optical components such as UV-transparent
optics, semiconductor devices and integrated micro-optical
elements. Regarding laser based methods for the manufac-
ture of such components, material removal of fused silica
and glasses in general can be achieved by several tech-
niques such as laser ablation using UV-, IR- or NIR-laser
radiation [1-3], or laser induced backside wet etching [4,5].
Further, laser induced etching techniques introducing UV-
absorbing films such as toluene [6] and carbon [7] or laser
induced plasma-assisted ablation [8] can be applied. Be-
yond, an indirect processing method of fused silica surfaces
consists of the vacuum deposition and laser structuring of
UV-absorbing silicon suboxide layers (SiOX) and their sub-
sequent oxidation to SiO2 [9]. Also, since fused silica con-
sists of pure silicon dioxide (SiO2), its surface can be di-
rectly reduced to silicon suboxide (SiOX, where 1<X<2) or
silicon monoxide (SiO) by applying hydrogenous gases at
high temperatures [10].
The presented hybrid laser-plasma removal method is
based on the surface reduction of fused silica by a low-
temperature atmospheric pressure plasma and a subsequent
laser ablation. To our best knowledge, this is the first work
on surface processing of fused silica using such a plasma
source. In contrast to the pulsed laser deposition met hod
(PLD) [11] or the deposition of metastable silicon suboxide
by vacuum evaporation [9], the introduced plasma gener-
ates atomic hydrogen from the used forming gas which
directly generates a suboxidal layer onto the fused silica
substrate. This effect allows a significant decrease in re-
quired energy for laser ablation of fused silica.
2. Experimental setup and experimentation
For the plasma-treatment of the investigated 2 mm-
thick fused silica samples, a low-temperature potential-free
atmospheric pressure plasma jet “kinpen 09” from neoplas
tools GmbH was applied. The plasma source was directed
perpendicular onto the sample’s surface. During the plas-
ma-treatment, the samples were moved by a xy-linear stage.
The working distance of the plasma jet nozzle to the fused
silica surface was 1 mm. In order to initialise the reduction
process, forming gas 90/10 (consisting o f 90% nitrogen and
10% hydrogen) was used as working gas. The treatment
time at each point of the sample surface was varied in the
range from 0 to 15 minutes with a gas flow rate of 25 slm.
After 5 and 15 minutes, transmission spectra (including
reflexio n lo sses) were taken in order to verify the reduction
progress with respect to a pure fused silica sample.
The subsequent laser ablation was performed after a
maximum plasma-treatment duration of 15 minutes using
an ArF excimer laser “LPX 315” from Lambda Physik
(λ = 193 nm). By an optical setup, consisting of two con-
vex lenses (f1 = 750 mm, f2 = 100 mm), a mask imaging
was realised in order to image a diaphragm on the fused
silica sample’s surface as shown in figure 1.
DOI:10.2961/jlmn.2012.01.0014
JLMN-Journal of Laser Micro/Nanoengineering Vol. 7, No. 1, 2012
74
Fig. 1 Schematic and functional principle of the setup for plasma
pre-treatment and subsequent front-side ablation of fused silica.
The aperture of this diaphragm was 3 mm. The demag-
nification was about 15-times, leading to an irradiated spot
of about 200 µm in diameter on the front-side of the sample.
In addition to this standard front-side configuration, some
experiments in the back-side configuration (where the
beam passes through the sample and the image plane is on
the back-side [1]) have been carried out.
In order to investigate the influence of the plasma-
treatment on the machining properties of the fused silica
surface, the fluence threshold Φmin for substrate ablation of
both treated and untreated reference surfaces was deter-
mined by applying a series of single laser pulses with in-
creasing energy. The pulse duration t was 20 ns. Following,
ablation experiments were performed just above the partic-
ular ablation threshold.
3. Results and discussion
By applying the plasma-treatment, the transmission of
the investigated fused silica samples was reduced signifi-
cantly as shown in figure 2.
Fig. 2 Transmission spectra of a pure fused silica sample (solid
line) and after plasma-treatment for 5 min (dashed line) and 15
min (dotted line).
In comparison to a pure fused silica sample, the trans-
mission at the wavelength of the used laser of 193 nm was
reduced by a total percental value of 5.8% after 5 minutes
and 14.1% after 15 minutes plasma-treatment.
For front-side ablation of pure fused silica using a sin-
gle laser pulse with a pulse duration of t = 20 ns, an abla-
tion threshold of 6 J/cm² was determined. In comparison,
the plasma-treated surfaces show a significant decrease in
required fluence for ablation due to the increased absorb-
ance by means of the reducing-acting forming gas. Here,
front-side ablation was already achieved at a fluence of
1.3 J/cm². Hence, the ablation threshold was reduced by a
factor of 4.6 by applying the plasma-treatment. Further-
more, disturbing effects such as micro-cracks and melt are
avoided in this vein. The resulting geometry was measured
using a confocal scanning microscope “PLµ2300” from
Sensofar as shown in figure 3.
Fig. 3 Isometric projection (above) and cross-section (below) of
pure (left) and plasma-treated (right) fused silica surfaces after
single pulse front-side ablation near the particular ablation thresh-
old (6 J/cm² left, 1.8 J/cm² right).
In addition to the above-mentioned reduction of abla-
tion threshold, the peak-to-valley height Rz of the plasma-
treated machined area (Rz =34.7 nm) is reduced by a factor
of 2.3 with respect to pure laser ablated fused silica, where
Rz = 79.8 nm. As shown in figure 4, in the case of the
plasma-treated surface. the complete irradiated spot of
about 200 µm diameter is ablated with a smooth surface
and perfect contour accuracy according to the mask dia-
phragm. In contrast, for the untreated surface, the ablated
area looks quite porous and does not fill the complete irra-
diated spot.
Fig. 4 Microscope image of the pure (a) and plasma-treated (b)
fused silica sample after front-side ablation with a single pulse
near the particular ablation threshold (6 J/cm² left, 1.8 J/cm² right).
JLMN-Journal of Laser Micro/Nanoengineering Vol. 7, No. 1, 2012
75
In comparison to pure laser ablation, a lower ablation
depth is achieved by the presented laser-plasma hybrid
method. However, this enables precise control of the abla-
tion profile by fine adjustment of the fluence.
Furthermore, a comparison of both front-side and back-
side ablation of plasma-treated fused silica surfaces was
carried out. Whereas single pulse back-side ablation of
untreated samples is not possible with this setup, because
ablation starts already at the front-side at the required high
fluence, for single-pulse back-sid e ablation of plasma-
treated samples at 1.3 J/cm², a higher removal rate was
observed. Figure 5 shows both an isometric projection and
cross-section of an ablated spot at the back-side of the
sample.
Fig. 5 Isometric projection (left) and cross-section (right) of a
plasma-treated fused silica surface after single pulse back-side
ablation at a fluence of 1.3 J/cm².
Compared to front-side ablation, where the depth of ab-
lat ion d was 45 nm, the depth of ablation was increased by
a factor of 3.9 in the case of single-pulse back-side ablation
at a fluence of 1.3 J/cm² (d = 175 nm). Comparable in-
creases were also found for higher fluences as shown by
the corresponding ablation depths in figure 6.
Fig. 6 Ablation depth vs. fluence for both front-side and back-
side single-pulse ablation of plasma-treated fused silica
In addition, for back-side ablat ion, the peak-to-valley
height Rz of 14.5 nm is 2.4 times lower in comparison to
single-pulse front-side ablation at a fluence of 1.3 J/cm².
Such significant differences in front- and back-side abla-
tion were already observed in previous work and could be
explained by the attenuation of the ablating laser pulse by
the plume of the removed material, which is only effective
for front side irradiation [1]. In the case of ablation at high-
er number of pulses, the ablation depth increased linearly.
When applying 35 laser pulses at 2.4 J/cm², an ablation
depth of approx. 11 µm was achieved, still featuring good
machining quality.
After the micro-structuring, the transmission can be in-
creased by a tempering process. For this purpose, the sam-
ple was tempered at 1000°C in air for inducing a re-
oxidisation of the plasma-treated layer. As shown in fig-
ure 7, the transmission at λ = 193 nm amounts to 83.8%
after 24 hours and 86.8% after 48 hours of tempering.
Fig. 7 Transmission spectra of a plasma-treated fused silica sam-
ple (dotted line) after 24 hours (dashed line) and 48 hours (solid
line) of tempering.
In comparison, a transmission of 88.4% at λ = 193 nm
was measured for the pure, untreated fused silica sample at
the beginning. Thus, almost the initial transmission charac-
teristics can be reconstituted in this vein.
4. Conclusion
In terms of front-side ablation, the presented hybrid la-
ser-plasma micro-structuring method allows a significant
decrease of required fluence for ablation of fused silica by
a factor of 4.6. In addition, a significant enhancement of
the contour accuracy of the imaged mask was observed.
Further, the resulting peak-to-valley height Rz of the ma-
chined area is reduced by a factor of 2.3. The depth of abla-
tion is reduced by 20% in the case of laser-plasma single-
pulse ablation.
The resulting surface roughness was furthermore re-
duced by applying back-side ablation. Here, a higher re-
moval rate was obtained additionally.
By a terminal tempering, the plasma-treated samples
can be re-oxidised, almost resulting in the initial transmis-
sion characteristics. Thus, this technique offers a novel and
economic alternative for the manufacture of high-precisio n
micro-structures in fused silica substrates. Further im-
provement could be achieved by introducing a laser beam
homogeniser to the presented optical setup.
Acknowledgments
The authors gratefully acknowledge the support by the
European Regional Development Funds (EFRE) and the
Workgroup Innovative Projects of Lower Saxony (AGiP) in
the frame of the Lower Saxony Innovation Network for
Plasma Technology (NIP). Further, the support by the Fed-
eral Ministry of Economics and Technology (BMWi) in the
frame of the research project PROKLAMO is gratefully
acknowledged.
JLMN-Journal of Laser Micro/Nanoengineering Vol. 7, No. 1, 2012
76
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(Received: November 22, 2011, Accepted: January 4, 2012)
