SELECTIVE LASER ABLATION OF DIELECTRIC LAYERS
S. A. G. D. Correiaa,*, J. Lossena, M. Walda, K. Neckermannb, M. Bährb
a
Ersol Solar Energy AG,
Wilhelm-Wolff-Str. 23, D-99099 Erfurt, Germany
b
Solar Zentrum Erfurt – CiS Institut für Mikrosensorik GmbH,
Konrad-Zuse-Straße 14, 99099 Erfurt, Germany
ABSTRACT: In this work we have investigated the local opening of dielectric layers on silicon with a laser. We
have observed the influence of the laser wavelength and energy on the effectiveness of the selective removal of SiNx
and SiOx layers. The induced laser damage was quantified by lifetime measurements. Further we have investigated
the influence of the laser wavelength and pulse energy on the doping profile of an underlying phosphorous emitter.
The ablations were performed with nanosecond lasers with wavelengths of 355 nm, 532nm and 1064 nm
respectively.
Keywords: selective laser ablation, laser assisted decomposition, laser assisted evaporation.
1
INTRODUCTION
In this work we investigate how successfully
nanosecond pulse laser energy can be used to remove a
dielectric layer from a silicon substrate using 355 nm,
532 nm and 1064 nm wavelength lasers.
That process becomes challenging for SiNx and SiOx
dielectric layers because their optical absorption
coefficients are lower than that of the underlying silicon
substrate for the mentioned wavelengths.
Previous works from A. Grohe et. al [1] and P.
Engelhart et. al [2], provided some insights on the
subject of local opening of dielectric layers in Si using
nanosecond lasers with 355 nm, 532 nm, 1064 nm
wavelengths.
Our intent in this work is to gain a better insight
about the basic phenomena and factors influencing this
process.
1.1 Laser Mater Interaction
Laser-matter interaction depends on several physical
parameters such as, wavelength, pulse energy and pulse
duration (τp), besides the thermodynamic properties of
the material. Important to consider in this work are the
effect of the laser wavelength, energy and pulse duration.
The laser interaction with matter can be described in
different time scales. When a laser beam is absorbed by a
material the existing free electrons will receive energy
from the photon electro-magnetic field and oscillate. That
energy will be then transmitted to other electrons and
later to the lattice. If the laser pulse duration is smaller
than the electron cooling time (τp <τe), the resulting ions
will receive the electron energy fast enough so that their
lattice bounds will be broken with virtually no heat
transfer. This happens when τp is in the fs regime. When
the τp >~1ns>> τl, where τl is lattice heating time, the
electron and lattice are able to reach thermal equilibrium.
This leads to a heat diffusion dominated energy loss
mechanism. This way the material melts and evaporates.
The picosecond regime can be treated as an intermediate
regime between the nanosecond and femtosecond regime.
At last, when τp >> 1ms, the process can be completely
modelled by classical heat transfer [3].
The thermal penetration depth of a laser pulse is
given by,
LD = κ .τ p
were κ is the material thermal diffusivity. LD can be used
to calculate the material heat affected zone.
The material absorption coefficient, α will determine
how deep a photon beam from a certain wavelength will
penetrate in the material until it is amount completely
absorbed. The optical penetration depth, taken from the
Lambert-Beer law will be given by α-1.
Figure 1 shows the optical absorption curves of C-Si,
Si3N4 and SiO2 [4]. From this graphic the differences in
the absorption coefficients from the three materials are
evident. It is, however, known from the literature that the
absorption coefficient of SiNx can be different from zero
above the 355 nm wavelengths [1] [5]. In this work we
assume that the optical absorption of the SiNx and SiOx
layers here used are negligible when compared to the
optical absorption of Si. This assumption is confirmed by
[5] in the case of SiNx.
Figure 1: Optical Absorption coefficients for Si, Si3N4
and SiO2.
2
METHODOLOGY
We have created test structures of approx. 10mm x
5mm on differently prepared 156mm x 156mm p-type
Cz-wafers with 10-17Ωcm base resistivity. These wafers
where pre-processed in 8 different groups accordingly to
Table 1.
Table 1 – Experimental Groups.
The opened structures were investigated by optical
and electron microscopy. Subsequently the influence of
the laser energy on the remaining silicon substrate was
investigated by local life time measurements using the
microwave photo conductance decay method (µWPCD).
The effects of the laser energy on the phosphorous
diffused emitter profiles and the presence of N2 and H2
were investigated by Secondary Ion Mass Spectrometry
(SIMS).
3
The wafers from group 1, 3, 5 and 7 were coated with
Plasma Enhanced Chemical Vapour Deposited SiNx,
while the wafers from group 2, 4, 6 and 8 were coated
with Atmospheric Plasma Chemical Vapour Deposited
SiOx.
On each wafer there were created 7 x 3 structures
named marks. Each mark corresponded to a specific
pulse fluence (J/cm2). This way, seven marks were
opened at each of the three different wavelengths as
represented in Figure 2.
Figure 2: Organization of the structures opened at
different energies and wavelengths. The structures are
name as ‘mark #’. The marks on each column were
opened using the same wavelength and the marks on each
line were opened with approximately the same fluences.
RESULTS AND DISCUSSION
3.1 Visual analysis
The pictures from figure 4 and figure 5 show the
differences in results obtained among shiny etched
wafers with different coatings.
In the pictures one can see inhomogeneities in the
structures opened with the 532 nm laser. This
inhomogeinities were caused by the laser pulse energy
variation due to an internal software error in our laser’s
automation control. This way the pulse energy becomes
slightly bigger than the programmed value when the laser
travels at lower speeds during its acceleration or
deceleration. The real pulse energy only matches the
programmed one when the beam is passing in the middle
of the mark. The influence of that overlapping error is
then bigger and better identifiable at lower fluences.
The quality of the opening on each structure was
attributed considering the percentage of coating free
surface and the presence of any visually identifiable
damage. An extreme melting or the existence of grooves
in the surface is qualified as damage. The best opening
for all the SiNx coated wafers was observed in mark 4,
for the 355 nm laser, mark 3 for the 532 nm laser and
mark 2 for the 1064 nm wavelength laser respectively.
In this experiment we have used 1064 nm wavelength
pulses of approximately 200 ns of duration. The pulses of
the 355 nm and 532 nm wavelength lasers had durations
between 10 ns and 35 ns.
The following graphic shows the experimental
parameters used in the experiment.
Figure 4: Picture of the structures opened on a wafer
from group 5. The number on the side of each structure
corresponds to its mark number. From the left to the right
the best opened structures correspond to mark 2, mark 3
and mark 4. The structures from middle column are
inhomogeneous due to an undesired pulse energy
variation.
Figure 3: Graphic of the fluences used to open the
structures at different wavelengths. The error for the
fluences used on mark 1 and 2 is about 400 mJ/cm2. The
error for the fluences used on marks 3, 4, 5, 6 and 7 is
about 50 mJ/cm2.
The best openings for the SiOx coated wafers were
observed in mark 3, mark 2 and again mark 2 for the 355
nm, 532 nm and 1064 nm wavelengths, respectively.
This shows that the SiOx layer required more energy to
opened when using the same laser wavelength.
from Figure 7. The picture shows the structures opened
on a textured wafer with SiNx coating. We attribute this
effect to the reduced reflectance of the texture surface.
Figure 5: Picture of the structures opened on a shiny
etched SiOx coated wafer. The numbers on the side of
each structure indicate its mark number. From the left to
the right the best opened marks are mark 2, mark 2 and
mark 3.
The results also show, as expected, that it requires
less energy to open the structures when using shorter
wavelengths. A smaller optical absorption depth will be
responsible for a higher maximum surface temperature.
The effect of the pulse overlapping should also not be
omitted. This parameter will control the percentage of
opened surface if the used pulse energy is big enough to
cause any damage in the Si/dielectric layer. This means
that with a bigger pulse overlapping it would be possible
to open mark 5, mark 4 and mark 3 in the SiNx coated
wafers for example.
Figure 6 shows a differential interference contrast
microscope picture of mark 4 (at 355 nm) from the SiNx
coated shiny etched wafer shown in figure 4. The vertical
stripes visible on the surface were caused by the effect
overlapping of the laser beam, after each passage of the
laser beam and by the pulse Gaussian spatial energy
distribution. In this picture it can also be seen that the
surface morphology on mark 4 is almost similar to the
morphology of the adjacent SiNx coated region. The
channels seen on the surface of mark 4 are an indication
that a melting process has occurred.
Figure 7: Picture of a textured wafer with SiNx coating
showing that the surface texture leads to a slightly better
opening process due to higher light absorption.
Figure 8 shows the pyramids from the surface of
mark 4 on the wafer from figure 7 for the 355 nm laser.
The rounding of the pyramid tops due to melting is
clearly visible on that figure.
Figure 8: Scanning electron microscope picture of the
surface of mark 4 of a SiNx coated wafer after 355 nm
laser energy absorption. The surface melting is evident.
Figure 9 shows that the deformation of the pyramids
on the surface of mark 5 of the same wafer is smaller but
still visible.
Figure 6: Microscope picture showing the surface
morphology of mark 4 opened with a 355nm wavelength
laser on the wafer from picture 4.
The same surface morphology was observed on the
best marks opened with the 532 nm and 1064 nm
wavelength laser in the shiny etched SiNx coated wafers.
Similar results, with some small differences, were
observed for the best marks on the shiny etched SiOx
coated wafers.
The influence of the texture can be seen in the wafer
Figure 9: Scanning electron microscope picture of the
surface of mark 5 of a SiNx coated wafer after 355 nm
laser energy absorption. The surface melting is still
detectable.
Figure 10 shows that it is difficult to identify signs of
melting on the surface of mark 6, although some signs of
the dielectric opening are macroscopically visible, as for
the case of figure 6 for example.
Figure 10: Scanning electron microscope picture of the
surface of mark 6 of a SiNx coated wafer after 355 nm
laser energy absorption. The surface melting is not easily
detectable.
3.2 Effective Lifetime Analysis
The analysis of the effective minority lifetime, here
named simply lifetime, is used to quantify the laser
damage created on each structure.
Figure 11 shows the lifetime map of a shiny etched
SiNx coated wafer with emitter. The reader should pay
attention to the fact that the rectangles drawn around
each mark in this figure don’t coincide with the structure
area and have only a demarking purpose.
The lifetime map shows that the lifetime in general
decreases for high applied fluences in the case of the 355
nm and 532 nm laser opened structures and for the first
mark opeed at 1064 nm. Surprisingly the lifetime of the
structures opened with the 1064 nm and 355 nm lasers
show an increase in lifetime, in relation to the wafer
average value, when lower fluences are applied. Further
experiments showed that this phenomenon also can occur
when 532 nm wavelength pulses are used.
Figure 11: Lifetime map of a Shiny etched wafer with a
SiNx coating and emitter. The lifetime decreases for high
applied fluences. For the 1064 nm and 355 nm marks the
lifetime increases for low applied fluences.
The following graphics show the average lifetime
plotted for each mark for wafers from tree different
groups. This lifetime value was determined by averaging
the lifetime values inside the rectangle defined by each
mark.
In all graphics mark 8 corresponds to a reference
lifetime. This lifetime was determined by averaging the
lifetime in an adjacent rectangle with the same
dimensions as the rectangle defined by all the laser
structures together. The lifetimes measured in the
structures with the optimal opening are identified with a
round dot.
The graphic from figure 12 and figure 13 show the
effect caused by the presence of an emitter.
Their analysis shows that the lifetime of the best
openings is smaller than the reference value in the case of
the 355 nm and 532 nm wavelength lasers. The lifetimes
decrease about 2.5 µs (-8.3%) for the first case and about
9 µs (-28%) for the second case. The lifetime in the
structure opened with the 1064 nm laser increases in
about 5 µs (+14%).
In the wafer with no emitter all the lifetimes in the
best structure decrease abruptly in relation to the average
lifetime. However the increase of lifetime for low
fluences is still observed for the IR laser.
Figure 12: Graphic of the effective lifetime measured at
each mark for a shiny etched, SiNx coated wafer with
emitter. The big circles identify the best opened
structures for each wavelength.
Figure 13: Graphic of the effective lifetime measured at
each mark for a shiny etched, SiNx coated wafer with no
emitter. The big circles identify the best opened
structures for each wavelength.
The graphic from figure 14 can be used to compare
the differences in lifetime variation between a SiNx
coated wafer and a SiOx coated wafer. Both wafers have
an emitter. The graphic shows two different things. In
first place the lifetime in the structures opened by the UV
and green lasers varies the same way as for the the SiNx
coated wafers, i.e., the lifetime decreases strongly for
higher fluences and it tends to increase above the
reference value for smaller fluences. The second
information that can be extracted from the graphic is that,
for the 1064 nm laser, the lifetime minimum occurs at a
value below the maximum applied energy.
Figure 14: Effective lifetime measured at each mark for
a shiny etched, SiOx coated wafer with emitter. The big
circles identify the best opened structures for each
wavelength.
3.3 SIMS Analysis
SIMS showed to be an useful technique to investigate
the depth of the laser damage. The following
measurements were performed on the SiNx coated
wafers.
We were interested to learn about the relationship
between the observed changes and the absorbed laser
energies and wavelengths and to analyze the changes
occurred in the emitter doping profiles of the best
obtained structures.
The results show that the measured profiles in the
opened structures were not dramatically changed when
low fluences were applied. For higher fluences the
phosphor surface concentration is reduced by about one
order of magnitude for the case of the 355 nm and 532
nm lasers and more than that for the case of the 1064 nm
laser.
We have then compared the depth at which a specific
concentration was detected with the depth at which that
same concentration was measured in the reference
profile. We classified that difference as a ‘displacement’.
In the optimum marks, it can be seen that the best
structure obtained with the 355 nm laser (mark 4) has an
average displacement of 200 nm while the best structure
obtained with the 532 nm laser (mark 3) has a
displacement of about 170 nm. The best structure
obtained with the 1064 nm laser (mark 2) is very deeply
diffused when compared with the profiles opened with
shorter wavelengths.
Two distinct factors should be considered when
trying to understand the diffusion process occurred with
the 1064 nm laser. First, the optical absorption depth
affects on the maximum surface temperature, which can
be seen when comparing the results obtained for mark 1
between the 355 nm laser and the 532 nm laser. For the
355 nm laser mark 1 the phosphorus was partly
evaporated while for the 1064 nm laser it was not.
Secondly much longer laser pulse durations will cause a
deeper heat diffusion. This can be observed when
comparing the short wavelength lasers (tens of ns of
duration) with the 1064 nm wavelength laser (hundreds
of nanoseconds duration) for the same fluences.
Figure 15: SIMS profiles of phosphorus after ~200 ns
Infra-red laser pulses absorption in a SiNx coated wafer.
The best opened structure was mark 2.
Figure 16: SIMS profiles of phosphorus after >35 ns 532
nm laser pulses absorption in a SiNx coated wafer. The
best opened structure was mark 3.
Figure 17: SIMS profiles of phosphorus after ~10 ns 355
nm laser pulses absorption in a SiNx coated wafer. The
best opened structure was mark 4.
The profiles show in figure 18 and figure 19 indicate
the nitrogen and hydrogen distributions for mark 4, mark
5 and mark 6 and for the 355 nm laser and SiNx coated
wafers.
In figure 18 we see that the concentration of N2
inside the sample increases strongly for mark 4. This
distribution appears to be related to the Si melting
process. Mark 4 was the mark on which the Si melting
became more clearly visible.
Figure 18: Nitrogen distribution after 355 nm laser
pulses absorption in a SiNx coated wafer. The best
opened structure was mark 4.
The H2 concentration also increases with applied
laser energy. This redistribution process might explain in
part the increase of lifetime for lower fluences.
The removal of the SiNx will occur when enough
energy was absorbed from Si to allow the material to
reach a temperature T>1877.9 °C, approximately. At this
point the partial pressure of the N2 in the SiNx reaches
one atmosphere [7]. This leads to the dielectric
decomposition.
In the case of the SiOx , the material will melt at about
1600°C and form a glass phase. Its evaporation and
subsequent removal should occur at temperatures above
2230°C.
This way we believe that we can explain why the
SiOx required more energy to be removed from the
surface of the wafer and show that there is a process
window where the removal of the dielectric layers can
occur before the evaporation of the Si.
We could say in this case that we are not in presence
of a dielectric laser ablation process, but, more correctly
in the presence of a laser induced thermal
decomposition/evaporation process.
4
CONCLUSION
3.4 Discussion
To explain the results we had a look on the
thermodynamic properties of Si as a semiconductor and
the SiNx and SiOx, both ceramics. Table 2 resumes those
properties [6] [7] [8].
In this work we have successfully realized the
opening of SiOx and SiNx dielectric layers on Si wafers
for 355 nm, 532 nm and 1064 nm wavelength
nanosecond lasers. The effect the laser energy,
wavelength and duration on the material lifetime and
emitter profile was determined. We have shown that it is
possible to locally open dielectric layers of SiOx and
SiNx at the mentioned wavelengths with minimal heat
damage and dopant redistribution in the Si. We have also
observed that the laser melting of the Si is partly
responsible for the redistribution of the elements existing
on the surface of the material.
The observations done in this work made us assume
the presence of a laser induced heat transfer process. In
this process the thermal energy generated in the Si causes
the subsequent decomposition or melting and evaporation
of the dielectric coatings.
Table 2 – Experimental Groups.
5
Figure 19: Hydrogen distribution after 355 nm laser
pulses absorption in a SiNx coated wafer. The best
opened structure was mark 4.
AKNOWLEDGEMENTS
We would like to thank, in first place Dr. Karsten
Mayer, from Ersol AG, for is help in the numerical
simulations of heat transfer.
We would also like to thank Dr. Ines Dani from the
Fraunhofer IWS in Dresden, for the SiOx coating work.
6
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Based on the data from table 2 we explain the local
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laser absorption in the dielectrics is negligible, we are
able to state that the Si behaves as heat source after
having absorbed the laser energy. The maximum
temperature reached on the Si surface and the depth of
the generated temperature field will depend on the optical
absorption coefficient, laser pulse duration and energy,
besides the material thermal properties.
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