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UV and IR Laser-Patterning for High-Density Thin-Film Neural Interfaces

Conference Paper

UV and IR Laser-Patterning for High-Density Thin-Film Neural Interfaces

Abstract and Figures

Our limited understanding of the nervous system forms a bottleneck which impedes the effective treatment of neurological disorders. In order to improve patient outcomes it is highly desirable to interact with the nervous tissue at the resolution of individual cells. As neurons number in the billions and transmit signals electrically, high-density, cellular-resolution microelectrode arrays will be a useful tool for both treatment and research. This paper investigates the advantages and versatility of laser- patterning technologies for the development of such high-density microelectrode arrays in flexible polymer substrates. In particular, it aims to elucidate the mechanisms involved in laser patterning of thin polymers on top of thin metal layers. For this comparative study, a pulsed picosecond laser (Schmoll Picodrill) with two separate wavelengths (1064 nm (infrared (IR)) and 355 nm (ultraviolet (UV))) was used. A 5 μm thick electroplated layer of gold (Au) was used to form the microelectrodes. Laser- patterning was investigated to expose the Au electrodes when encapsulated by two different thermoplastic polymers: thermoplastic polyurethane (TPU), and Parylene-C, with thicknesses of maximum 25 μm. The electrode diameter and the distance between electrodes were reduced down to 35 μm and 30 μm, respectively. The structures were evaluated using optical microscopy and white light interferometry and the results indicated that both laser wavelengths can be successfully used to create high-density microelectrode arrays in polymer substrates. However, due to the lower absorption coefficient of metals in the IR spectrum, a higher uniformity of the exposed Au layer was observed when IR-based lasers were used. This paper provides more insight into the mechanisms involved in laser-patterning of thin film polymers and demonstrates that it can be a reliable and cost-effective method for the rapid prototyping of thin-film neural interfaces.
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UV and IR Laser-Patterning for High-Density Thin-
Film Neural Interfaces
Andrada I. Velea1,2, Student Member, IEEE, Joshua Wilson2, Anna Pak1,2, Manuel Seckel3, Sven Schmidt3, Stefan
Kosmider3, Nasim Bakhshaee1, Wouter A. Serdijn1, Fellow, IEEE, and Vasiliki Giagka1,2, Member, IEEE
1Bioelectronics Section, Department of Microelectronics, Delft University of Technology, Delft, The Netherlands,
2Technologies for Bioelectronics Group, Department of System Integration and Interconnection Technologies, Fraunhofer IZM,
Berlin, Germany,
3Embedding and Substrate Technologies Group, Department of System Integration and Interconnection Technologies, Fraunhofer
IZM, Berlin, Germany
Corresponding authors e-mails: a.velea-1@tudelft.nl, v.giagka@tudelft.nl
AbstractOur limited understanding of the nervous system
forms a bottleneck which impedes the effective treatment of
neurological disorders. In order to improve patient outcomes it is
highly desirable to interact with the nervous tissue at the
resolution of individual cells. As neurons number in the billions
and transmit signals electrically, high-density, cellular-resolution
microelectrode arrays will be a useful tool for both treatment and
research.
This paper investigates the advantages and versatility of laser-
patterning technologies for the development of such high-density
microelectrode arrays in flexible polymer substrates. In
particular, it aims to elucidate the mechanisms involved in laser
patterning of thin polymers on top of thin metal layers. For this
comparative study, a pulsed picosecond laser (Schmoll Picodrill)
with two separate wavelengths (1064 nm (infrared (IR)) and 355
nm (ultraviolet (UV))) was used. A 5 μm thick electroplated layer
of gold (Au) was used to form the microelectrodes. Laser-
patterning was investigated to expose the Au electrodes when
encapsulated by two different thermoplastic polymers:
thermoplastic polyurethane (TPU), and Parylene-C, with
thicknesses of maximum 25 μm. The electrode diameter and the
distance between electrodes were reduced down to 35 μm and 30
μm, respectively. The structures were evaluated using optical
microscopy and white light interferometry and the results
indicated that both laser wavelengths can be successfully used to
create high-density microelectrode arrays in polymer substrates.
However, due to the lower absorption coefficient of metals in the
IR spectrum, a higher uniformity of the exposed Au layer was
observed when IR-based lasers were used. This paper provides
more insight into the mechanisms involved in laser-patterning of
thin film polymers and demonstrates that it can be a reliable and
cost-effective method for the rapid prototyping of thin-film neural
interfaces.
Keywordshigh-density microelectrode arrays, thin-film laser
drilling, polyurethane, Parylene-C.
I. INTRODUCTION
In the past few decades, neuromodulation has been widely
investigated as a means to develop tailored approaches for
treating various neurological disorders. However, the degree of
personalisation that can currently be achieved is hampered by
the limited understanding of the mechanisms that govern our
nervous system. To overcome this, it is of paramount importance
to increase the resolution at which we interact with the nervous
tissue.
To this end, high-density microelectrode arrays are needed.
In particular, flexible, polymer-based arrays that seamlessly
interface with the neural tissue have the potential for chronic use
[1]. To achieve high resolution patterns, photolithography is the
method of choice. However, when the electrodes have to be
exposed, if photolithography is employed, the designs of the
photomasks must be fixed and changes require the fabrication of
new masks, making it an expensive approach for prototyping.
Alternatively, the use of laser systems, which are single step
processes and can create arbitrary patterns for rapid prototyping,
can be used [2].
The aim of this study is to go beyond the state of the art and
make use of the versatility provided by laser-patterning
technologies to develop miniaturised, polymer-based high-
density electrode arrays, using thin noble metals as conductors
within thin and flexible encapsulation layers. To this end, this
paper focuses on investigating the use of laser-patterning alone
for biocompatible polymers that can be used in such neural
interfaces.
More than a decade ago, it was shown that 18 μm thick
platinum (Pt) electrodes with diameters of 50 μm and
encapsulated in 20 μm thick silicone rubber could be patterned
using a nanosecond laser system, with a 532 nm wavelength [3].
More recently, structures comprising 6, 25 μm thick
platinum/iridium (PtIr) electrodes with a diameter of 80 μm, 300
μm pitch between them and encapsulated in 10 μm of Parylene-
C were successfully patterned, using a picosecond laser, with a
355 nm wavelength [4]. However, for flexible and conformal
high-density arrays, smaller diameters and thinner metal layers
are of interest.
Recently, an attempt to develop such thin neural interfaces
combined femtosecond laser-patterning (with a 342 nm
wavelength) with conventional lithography (plasma etching of
polymers). In [5], electrodes of 40 μm in diameter and 500 μm
*This work is part of the Moore4Medical project funded by the ECSEL
Joint Undertaking under grant number H2020-ECSEL-2019-IA-876190.
pitch, using nm thick metal layers and a polyimide insulation
layer of 10 μm or 15 μm, were developed.
In this paper, for the development of thin-film, polymer-
based microelectrode arrays, as well as for investigating the
limitations imposed by such technologies, a pulsed picosecond
laser (Schmoll Picodrill) with two separate wavelengths (1064
nm and 355 nm) was used. The minimum beam diameter is 15
μm for the ultraviolet (UV) beam path and 40 μm for the
infrared (IR) one. For the test structures, resembling high-
density electrode arrays, a 5 μm thick electroplated layer of gold
(Au), encapsulated in thin film polymers, was used. For the
encapsulation, two different thermoplastic polymers were
investigated: thermoplastic polyurethane (TPU) and Parylene-C,
with a thickness of maximum 25 μm. The targeted electrode
diameter as well as the distance between electrodes were
reduced down to 35 μm and 30 μm, respectively. For
comparative reasons, thicker polymers (100 μm) as well as
larger diameters (700 μm) were also included in the study.
Since this technology aims to ease the fabrication process of
dense microelectrode arrays, it is of paramount importance to
ensure high quality for both the metal and encapsulation layers.
The different wavelengths were selected in order to compare the
performance of fundamentally different ablation mechanisms.
IR beams generally cause thermal-induced degradation of
polymers whereas UV beams have sufficient energy to induce a
chemical reaction, causing the polymeric bonds to break [6]. In
the IR spectrum, due to the melting of thermoplastic materials,
a reflow phenomenon followed by a partial redeposition of the
material can occur. Although the UV beams might be more
beneficial for polymers, the absorption coefficient for metals is
much larger in the UV spectrum, whereas in IR, the absorption
coefficient for Au is almost zero (Fig. 1) [7].
II. METHODS
A. Laser technology
The laser used for these investigations, comparable with
what has previously been reported, is a pulsed picosecond laser
(Schmoll Picodrill) with a frequency range from 200 kHz up to
1 MHz.
Fig. 1. Absorption coefficient of different metals. Image adjusted from [7].
Fig. 2. Diameter variation for a picosecond laser depending on the
intensity of the beam. Figure adapted from [8].
Fig. 3. Fabrication flow used for the development of the test structures.
a) Concept
b) Actual design
Fig. 4. Test structures used for laser-patterning (a-concept and b-actual
design). The diameters of the metal as well as the diameters of the opening
in the polymer layer were varied between 45 800 µm and 35 700 µm
respectively. The distance between electrodes was also varied from 30
120 µm.
The laser uses two separate beam paths. The IR beam has a
maximum power of 50 W and a minimum beam diameter of ~40
μm, when focused. The second, a UV beam, has a maximum
power of 14 W and a minimum beam diameter of ~15 μm, when
focused. Depending on the intensity of the laser beam, different
results, such as redeposition of material or melted areas can be
observed on the surface of the material under test. Fig. 2
illustrates such an example for a metal surface subjected to laser-
patterning [8].
For the Schmoll Picodrill laser, two different drilling modes
are available: "punched" and "hatched". With the "punched"
mode, the pulsed laser directs its energy to a fixed location and
thus, small vias with the diameter of the laser beam can be
created. The "hatched" option, on the other hand, is used when
larger openings, with diameters > 30 μm are desired. In this case,
the pulsed laser moves and directs its energy over a predefined
area.
B. Materials
For the microelectrode arrays to induce as little damage as
possible to the tissue, it is of great importance to have thin,
flexible and conformal materials. Therefore, this paper presents
the use of polymers for encapsulation purposes and thin Au
layers for the development of the electrodes. Recent
publications, employing similar laser-patterning technologies
use metal layers with a minimum thickness of 18 µm and
encapsulation layers down to 10 μm [3].
In this work, the targeted metal is a 5 μm thick electroplated
Au layer while for the encapsulation, two different thermoplastic
materials are used: a thermoplastic polyurethane type (Platilon
4201AU, from Covestro), with a thickness of 25 μm and a
Parylene-C, deposited using Parylene-C dimers from Special
Coating Systems with a maximum thickness of 20 μm.
C. Fabrication of test structures
The fabrication of samples comprised of several steps as
illustrated in Fig. 3.
Since the conductive Au layer is deposited by means of
electroplating, a thin (70 μm) copper (Cu) substrate is needed to
ensure the transfer of metal ions, during the process. The Cu
substrate was roughened by chemical etching and dried in an
oven at 55 °C for 30 minutes. Next, a layer of dry negative
photoresist (RD-1225) was applied on top of Cu, patterned,
using laser direct imaging (LDI), and developed.
Later, Au was electroplated on the defined areas and the
photoresist mask was finally removed. Before encapsulation, the
structures had to be transferred on a rigid carrier. To this end,
epoxy-glass sheets (FR4) were used and laminated, using an
intermediate TPU layer, on top of the electroplated structures.
As Cu does not fulfil the biocompatibility requirements for
implantable devices, the backside Cu layer was etched in copper
chloride (CuCl2), hydrogen chloride (HCl) and hydrogen
peroxide (H2O2) at 49 °C and later rinsed in deionized water.
For the encapsulation, two different processes were used.
One consisted of using thermocompression to laminate the TPU
material on top of the Au structures, while the other was a
chemical vapour deposition process (CVD) for the Parylene-C
material.
The TPU lamination was performed at 190 °C and 10 bar
while the Parylene-C CVD process consisted in the deposition
of Parylene monomers onto the surface of interest, at 25 °C and
0.05 Torr.
TABLE 1: Parameters used during the laser-patterning method.
Average (W) Average (W)
hatched mode 35 µm diameter 0.5W every 4th pulse 3 loops 0.375 0.5W every 4th pulse 4 loops 0.5
hatched mode 40 µm diameter 0.5W every 4th pulse 3 loops 0.375 0.7W every 2nd pulse 1 loop 0.35
hatched mode 45 µm diameter 1W every 4th pulse 2 loops 0.5 1W/0.4W every 2nd pulse 1 loop each 0.7
hatched mode 50 µm diameter 1W every 4th pulse 2 loops 0.5 1W/0.4W every 2nd pulse 1 loop each 0.7
hatched mode 90 µm diameter 1W every 4th pulse 2 loops 0.5 1W/0.4W every 2nd pulse 1 loop each 0.7
punched mode 0.5 W every 4th pulse 10 loops 1.25 0.5 W every 4th pulse 9 loops 1.12
Average (W) Average (W)
hatched mode 35 µm diameter 1.5W every 4th pulse 2 loops 0.75 1.5W every 2nd/ 1W every 4th pulse 1 loop each 1
hatched mode 40 µm diameter 1.5W every 4th pulse 2 loops 0.75 1.5W every 2nd/ 1.3W every 4th pulse 1 loop each 1
hatched mode 45 µm diameter 1.5W every 4th pulse 2 loops 0.75 1.5W every 2nd/ 1.3W every 4th pulse 1 loop each 1
hatched mode 50 µm diameter 1.5W every 4th pulse 2 loops 0.75 1.5W every 2nd/ 1.5W every 4th pulse 1 loop each 1.1
hatched mode 90 µm diameter 1.5W every 4th pulse 2 loops 0.75 1.5W every 2nd/ 1.5W every 2nd/ 1.5W every 4th pulse 1 loop each 1.8
punched mode 3W every 4th pulse 4 loops 3 2.5 W every second pulse 2 loops 2.5
Diameter
Power
Parylene-C (UV laser)
Parylene-C (IR laser)
Fig. 5. Average power used for laser-patterning of TPU and Parylene-C.
0
0.5
1
1.5
2
2.5
3
3.5
hatched mode
35 µm
diameter
hatched mode
40 µm
diameter
hatched mode
45 µm
diameter
hatched mode
50 µm
diameter
hatched mode
90 µm
diameter
punched mode
Average power (W) used to create openings in TPU and Parylene-
C using a picosecond laser
TPU (UV laser) Parylene-C (UV laser) TPU (IR laser) Parylene-C (IR laser)
D. Design of test structures
The test structures used in this work (Fig. 4), mimic different
design topologies used in the development of flexible
microelectrode arrays. The Au electrode diameters were varied
from 45 μm to 800 μm in steps of 5 μm, with the exception of
the two largest dimensions (110 µm and 800 µm). Similarly, the
diameters of the openings in the polymeric layer were varied
from 35 μm to 700 μm and the distance between electrodes, was
reduced down to 30 μm.
E. Characterisation methods
In order to qualitatively evaluate the patterned structures and
the surface of the metal underneath, two characterisation
methods were used in combination: optical microscopy coupled
with white light interferometry.
III. RESULTS AND DISCUSSION
The UV and IR lasers were both used to pattern arrays of
circles with various diameters and edge-to-edge spacings, in
both TPU and Parylene-C. The most important parameters are
specified in Table 1. As illustrated, irrespective of the
wavelength used, higher average power levels (calculated by (1)
and up to 0.2 W higher for the UV laser as well as up to 1.05 W
higher for the IR laser) were needed to create openings in the
Parylene-C layer. This is a result of the fundamental differences
between the two polymers used. Although both are considered
thermoplastic materials and have similar densities, 1.289 g/cm3
for Parylene-C [9] and 1.15 g/cm3 for TPU [10], some important
differences can be underlined, which are of great importance
when using UV or IR lasers.
The first one is the melting point of the two materials, an
aspect of paramount importance when using IR-based lasers.
Parylene-C has a melting point of about 290 °C [9], whereas
TPU has a melting point between 155 and 185 °C [10]. Even
more important is the chemical composition of the two
materials, especially when using UV lasers, for which the
ablation mechanism is chemical rather than thermal.
Thermoplastic materials are linear polymers with molecular
structures comprising molecular bonds organised in a straight-
chain formation and thus relatively weak bonds [11]. The third
and equally important factor, is the glass transition temperature
(Tg) of the two materials, the point at which the reversible
transition of the amorphous component from/to a hard and brittle
condition to/from a viscous condition occurs. Therefore, the
higher the Tg, the less amorphous components the polymers
have, meaning that their composition exhibits a more crystalline
structure. For TPU, the nominal values of the Tg are between -
67 °C and 78.8 °C (according to the ASTM E1356 test method
[12]). On the other hand, Parylene-C has a Tg between 80 °C and
100 °C [13], much higher than for TPU, making it more
Fig. 6. Openings created in a 20 µm thick Parylene-C layer using an IR
laser beam in “punched” mode. The landing material was a 5 µm thick Au
layer.
Average Power = (Power/ No. Pulses) * No. Loops (1)
Fig. 7. Representation of the geometrical shape of the opening (top) and
its corresponding optical images (bottom).
Fig. 8. Rings defining the final opening diameter for the laser-patterning
in “hatched” mode.
Fig. 9. Openings of different diameters and distances between the
electrodes created in a 25 µm thick TPU layer using an IR laser beam in
“hatched” mode.
crystalline. This could, potentially have an influence on the
energy needed to break the bonds during a UV-based laser-
patterning technology.
Apart from the power values, which differ depending on the
material and size of the openings, the frequency at which the
laser actively delivers energy to the substrate is another
important aspect. However, for small openings, this does not
make a difference, although the overall average power
(calculated by (1)) is higher.
In Fig. 5, the average power needed for each type of material
using either the UV-based or IR-based lasers is shown for
different diameters and laser modes.
For the "punched" mode, given the results obtained, clear
conclusions cannot be made as the parameters used do not seem
to follow the same trend as for the "hatched" mode. Moreover,
the diameter of the opening could not be uniformly reproduced,
due to the fact that while using the "punched" mode there is no
control over the shape of the final opening (Fig. 6).
For the "hatched" mode, the variation observed was mainly
dependent on the sizes of the openings and structure of the
material. Since the complete opening of each structure as well as
the integrity of the metal underneath were the two key factors in
determining the suitability of such technology for developing
high-density neural interfaces, the evaluation consisted in
acquiring optical images and measuring the depth of the
openings using white light interferometry measurements.
Since the intensity of the laser is higher in the center of the
beam rather than at the edge, the top area of the opening has a
larger diameter compared to the diameter of the exposed Au
layer (Fig. 7). To compensate for this and in order to have the
desired dimension for the exposed area, the parameters used to
define the diameter of the hatched area were slightly larger than
the desired ones.
Moreover, when openings larger than the diameter of the
laser beam were desired, multiple rings, of different diameters,
had to be hatched (Fig. 8). The number of concentric rings
needed as well as the distance between them is strongly
dependent on the diameter of the beam as an overlap is needed
to ensure complete removal of the material. The energy required
for the inner rings, especially for Parylene-C, had to be adjusted
to ensure a uniform opening.
For both materials and both laser beams, different distances
between the openings have been established, ranging from 30
µm to 120 µm and the structures were successfully opened using
the same parameters as described in Table 1. An example of such
opened structures, having different distances between them is
illustrated in Fig. 9.
A. UV laser
UV lasers are known to induce a chemical reaction in the
material, which causes polymeric bonds to break, thus leading
to a rapid removal of polymer at desired locations. However,
depending on the type of polymer and their chemical bonds, the
results differ significantly.
Fig. 10. Openings created in a 25 µm thick TPU layer using a UV laser
beam in “hatched” mode. The landing material was a 5 µm thick Au layer.
Fig. 11. Openings created in a 20 µm thick Parylene-C layer using a UV
laser beam in “hatched” mode. The landing material was a 5 µm thick Au
layer.
Fig. 12. Opening created in a 100 µm thick TPU layer using a UV laser
beam in “hatched” mode. The landing material was a 5 µm thick Au layer.
Fig. 13. Damage occurred during the IR-based laser-patterning process
due to the different melting points of the layers comprising the structures
under test.
The investigations conducted using the Schmoll Picodrill
laser have shown that for Parylene-C, the removal required
double the frequency compared to TPU, to open structures with
diameters ranging from 35 µm to 90 µm. This can be seen from
the parameters used in both cases, for all diameters, except from
the 35 µm one where the difference is not significant and in order
to understand the behaviour, in the future, more investigations
are needed.
Moreover, since the absorption of metals in the UV spectrum
is larger, some polymeric residues could, potentially, still be
found on the surface of the electrodes, as more energy or laser-
patterning loops would have damaged the Au layer underneath.
In order to prove this, inspection methods, such as X-Ray
Photoelectron Spectroscopy (XPS) that could evaluate the
chemical composition on the surface of the electrodes would be
required. However, these investigations were out of scope for
the current paper.
From the optical inspection of the patterned structures, dark
coloured areas (Fig. 10 and Fig. 11) on the surface of the Au
layer were observed after the removal of both materials. These
indicated the potential presence of polymeric residues which
could also be caused by the roughness of the metal layer. The
roughness of Au which can be seen also from the different
visible peaks on the exposed areas, is due to the intermediate Cu
layer on which Au was electroplated. Nevertheless, to remove
such particles, an oxygen plasma cleaning step could be included
in the process flow [5].
The misalignment observed for the openings created in
Parylene-C (Fig. 11) are caused by the fact that Parylene is
slightly opaque after deposition and thus the contrast between
Parylene and the Au layer underneath was not always strong
enough to be distinguished accurately by the laser camera.
For the sake of comparison, thicker TPU layers (100 µm)
with larger diameter openings (700 µm) were investigated. In
this case, 10 repetitions with a laser power of 1 W were needed
to remove the TPU layer. Since the exposed area is much larger,
the Au layer appears to be more uniform in the optical image
(Fig. 12). However, the surface roughness caused by the Cu
layer on top of which Au was electroplated, is still visible.
Fig. 14. Openings created in a 25 µm thick TPU layer using an IR laser
beam in “hatched” mode. The landing material was a 5 µm thick Au layer.
Fig. 15. Openings created in a 20 µm thick Parylene-C layer using an IR
laser beam in “hatched” mode. The landing material was a 5 µm thick Au
layer.
Fig. 16. Arrays comprising of 50 and 100 opened structures.
a) 25 µm thick TPU openings using the “punched” mode of the laser.
b) A 25 µm thick TPU opening with a diameter of 35 µm.
b) A 25 µm thick TPU opening with a diameter of 90 µm.
Fig. 17. White light interferometry measurements for TPU openings
created using the “punched” or the “hatched” modes of the picosecond
laser. The landing material was a 5 µm thick Au layer.
B. IR laser
IR-based laser-patterning induces a thermal ablation of
polymers. For Parylene-C, in all cases, higher average power
levels were needed and these could, potentially, be even higher
for a Parylene-C layer of 25 µm (the same thickness as the TPU
layer). Moreover, during this process, failure of some structures
was also observed. This was due to the stack of layers the
structures consisted of (FR4 - TPU - Au - Parylene-C). Since
the melting point for TPU is lower than for Parylene-C, the
bottom surface of the structures reached the melting
temperature faster than the top surface. Therefore, some of the
metal pads were either completely removed or tilted at a certain
angle, depending on the reflow of TPU over the entire surface
area (Fig. 13).
Nevertheless, diameters ranging from 35 µm to 90 µm were
opened using the "hatched" mode of the IR laser, leading to
exposed Au areas with an increased uniformity and less
residues compared to the UV-based laser (Fig. 14 and Fig. 15).
These results are a first indication that IR beams are more
suitable for laser-patterning of polymers when the integrity of
the thin metal layers underneath is required.
C. White light interferometry
White light interferometry measurements were used for
each of the opened structures to evaluate if the polymeric layer
was completely removed for the desired diameter of the
exposed Au layer. Fig. 16 illustrates arrays of opened structures
comprising of either 50 or 100 elements.
In Fig. 17 and Fig. 18, examples of such measurements are
illustrated for openings with diameters of 35 µm and 90 µm in
both Parylene-C and TPU using the "hatched" mode of the two
different laser beams. Moreover, the openings created using the
"punched" mode of the lasers were also measured although their
very small diameter varied from one opening to another.
The small peaks observed at the outer ring of the openings
are due to the redeposition of polymers at the outside of the
lasered area. This has been observed for both types of materials,
irrespective of the wavelength used and it becomes more visible
on the 3D reconstructed images of the measured samples (Fig.
19).
D. Selected metals and encapsulation materials
This study investigated the use of TPU and Parylene-C as
encapsulation materials for Au microelectrodes. Hence, it is far
from complete. State-of-the-art neural interfaces make use of
other polymers as well (i.e. silicone rubber [14], polyimide [5])
or combinations of ceramics and polymers in encapsulation
stacks for packaging purposes ([15], [16], [17]). In addition, Pt
or PtIr, or even iridium oxide, PEDOT, and more recently
graphene microelectrodes are preferred instead of Au for neural
stimulation purposes due to the higher charge storage capacities
these materials exhibit. A more complete future study should
also include the effect of laser-patterning on these material
combinations. A similar behaviour is expected for noble metals,
however, for polymer-based electrodes it is expected that such
technology is not selective enough and thus, the surface
integrity of the electrodes might be compromised. On the other
hand, for graphene electrodes, depending on the wavelength
used, the technology could, potentially be successful. One of
the most commonly used methods to evaluate graphene,
without damaging it, is Raman Spectroscopy, which typically
uses wavelengths in the range of 500 650 nm and as shown in
[3], laser-patterning of polymers was possible using a 532 nm
wavelength. However, the power levels required to open the
a) 20 µm thick Parylene-C openings using the “punched” mode of the
laser.
b) A 20 µm thick Parylene-C opening with a diameter of 35 µm.
b) A 20 µm thick Parylene-C opening with a diameter of 90 µm.
Fig. 18. White light interferometry measurements for Parylene-C openings
created using the “punched” or the “hatched” modes of the picosecond
laser. The landing material was a 5 µm thick Au layer.
Fig. 19. Circular areas around the hatched or punched regions where
polymers have been redeposited. The openings have diameters of 35 µm
and 90 µm respectively. The surface roughness of the metal layer is also
visible on the reconstructed 3D image.
polymer-based encapsulation layers as well as the reduced
thickness of graphene, might pose serious limitations for such
technologies.
IV. CONCLUSIONS
This paper investigates the advantages and versatility of
laser-patterning technologies for the development of high-
density microelectrode arrays. In particular, it aims to shine
more light into the mechanisms involved in laser patterning of
thin polymers on top of thin metal layers. To this end, it presents
a comparative patterning study using two distinct laser
wavelengths, a UV and an IR beam.
A pulsed picosecond laser was used and diameters ranging
from 35 µm to 700 µm were successfully created using both the
UV and IR lasers. It has been shown that, by using the IR laser
beam, a much higher surface uniformity is achieved due to the
low absorption coefficient of Au in the IR spectrum. Moreover,
for the UV laser, a redeposition of particles or the presence of
residues was more pronounced, in comparison to the IR-based
laser-patterned structures. However, this could be overcome by
employing oxygen plasma cleaning steps at the end of the
process. The white light interferometry results showed that the
test structures were completely opened, regardless of the number
present on the arrays (50 or 100 structures). Neither did the
change in distance among the patterned structures present any
changes in the parameters used or the results obtained. To the
best of the authors’ knowledge, this is the first time that laser-
patterning alone was employed for structures comprising such
thin metal and encapsulation layers.
Laser-patterning can be a fast, cost-effective and efficient
method for the development of high-density, polymer
encapsulated microelectrode arrays. The comparative study
presented in this work can be useful to increase our
understanding and enable an increased adoption of this versatile
technology for high-density, thin neural interfaces.
ACKNOWLEDGMENTS
We acknowledge the staff members of Else Kooi Laboratory
(EKL) from Delft University of Technology (TU Delft), The
Netherlands for helping us with the Parylene-C deposition
process and the Embedding and Substrate Technologies Group
from Fraunhofer Institute for Reliability and Microintegration
IZM, Berlin, Germany for their implication and support offered
throughout the project. Special thanks are to be addressed to all
our collaborators from the Bioelectronics group at TU Delft.
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