OrgaconTM
Conductive Transparent Films
Application sheet
Patterning Orgacon™ film by means of UV lithography.
Guidelines
[1] Introduction
In many applications, conductive layers need to be
patterned to perform certain functions or to provide
isolation. Orgacon™ film consists of a PEDOT/PSS (poly
(3,4) ethylenedioxythiophene / polystyrenesulfone acid)
layer coated onto a polymer substrate, such as PET. The
electrical conductivity of this layer can be destroyed by
exposing the layer to even a low concentration of a strong
oxidant for a short time, e.g. a 1 % concentration of sodium
hypochlorite (NaOCl
or bleach).
This technique is
remarkedly different from the patterning of ITO layers where
strong acids such as HBr are used to chemically etch away
the ITO.
The destruction of the conductivity of the PEDOT layer is
called ‘deactivation’ or ‘passivation’, because the patterning
technique does not remove the PEDOT layer from the
substrate, resulting in a smooth patterned surface and only
small differences in light transmission between conductive
and non-conductive areas.
Longer exposure times or higher oxidant concentrations will
eventually etch away the exposed PEDOT layer. Exposure to
other strong oxidizers will also etch away the layer, as
opposed to passivating it.
The principle of patterning by passivation is applicable by
means of different patterning methods. The selection of a
patterning method is dependent on the resolution of the
required pattern. For coarse patterns, such as the edge
isolation for electro-luminescent lamps, a screenprinting
paste has been developed, which does the job by simply
screenprinting the negative of the desired conductive
pattern with this deactivation paste (see Orgacon™ Strupas
application note).
Another method makes use of a
screenprintable resist followed by deactivation in a NaOCl
solution, after which the etch resist is removed, yielding the
desired conductive pattern (see application note ‘Orgacon™
film patterning by screenprinting mask’).
This application note explains how to use UV lithography to
pattern a PEDOT layer. While involving more steps than the
other methods, it offers also the finest patterning resolution.
[2] Patterning process
The UV lithography process generally consists of the
following steps:
•
Substrate selection and preparation
•
Resist selection and application
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•
•
•
•
Exposure
Resist development
Passivation or etching of the PEDOT layer
Resist stripping
Issues that have to be takencare of are imaging defects,
underetching, isolation resistance, and surface quality.
In the next sections we will expand on all of these process
steps, and address some of the issues involved.
2.1. Substrate selection and preparation
2.1.1. Substrate selection
Orgacon
is
commercially
offered
on
PET
(polyethyleneterephtalate) in thicknesses between 63 and
175 micron.
The film has an antistatic backlayer for
improved handling and dust shielding. The conductive
PEDOT layer is applied onto a special adhesion layer, which
will not be removed even when etching away the PEDOT
layer completely.
For experimental purposes, Agfa is offering PEDOT layers on
other substrates such as PEN (polyethylenenaphalate) or PC
(polycarbonate). In these case the layer buildup will not be
the same as with ORGACON™, potentially resulting in a
different passivation behaviour.
However, the basic
principles still apply.
2.1.2. Substrate preparation
Although the PEDOT layer is fairly robust, care should be
taken in handling the material. Scratches and dent marks
can adversely affect the patterning for high-resolution
patterning.
The PEDOT layer can be cleaned with DI water or
isopropanol to remove superficial dirt.
2.2. Resist selection and application
Many photoresists are available with very different
properties. For PEDOT patterning, a positive photoresist
from Clariant1 has been selected: AZ111 XFS (hereafter
called AZ111). The resist has been specifically selected
because it offers a good adhesion on many surfaces, has a
low bake temperature, and is compliant with the flexible
substrate (which might be repeatedly bent during handling).
1
http://www.clariant.com
The patterning procedure has been optimised for usage with
AZ111, properties of which are listed in Table 1.
Solid Content (%)
Viscosity (cSt at 25C)
Absorptivity (I/g*cm) at 375 nm
Solvent
Max water content (%)
19.3
25.2
0.65
Methoxypropylacetate
0.5
Table 1: Selected properties of AZ111 XFS (source:Clariant)
The resist is normally applied by spincoating. A suitable
chuck is needed to hold down the piece of substrate. Care
should be taken that the resist is applied to the conductive
side of the substrate. Optimal resist thickness is 1 µm (dry
layer).
2.3. Exposure
It has been found that a baking step prior to exposure will
improve the etching. It is therefore advised to include a pre
exposure bake step at 110°C for 3 minutes.
The absorption peak of the AZ111 resist is around 380 nm.
Total exposure energy should be in the range of 360 mJ/cm 2
(f1 µm layer thcikness). Depending on pattern resolution,
either a glass mask or a PET mask can be used. In either
case, good exposure practices should be used (collimated
light), to ensure adequate pattern transfer to the resist.
AZ111 does not normally require a post exposure bake to
increase development quality. However, it has been found
that at least one thermal baking step during resist
processing improves the residual resistance of thin lines. A
45 s post exposure bake at 110°C (preferably on hotplate) is
therefore recommended.
2.4. Development
The standard developing bath for AZ111 is AZ303. This
developer will adversely influence the resistance of the
PEDOT layer: a sixfold increase in surface resistance has
been observed when exposing a PEDOT layer to AZ303 for
15 min. This effect might contribute to higher resistance
values in thin lines.
Recommended development conditions are 40 s in a 20%
solution of AZ303 developer.
As mentioned before, executing at least one thermal step is
beneficial for the resistance of fine lines. This step can
either be a post exposure bake (see above), or a post bake
after development, such as 60s at 110°C (hotplate).
resistance of untreated PET, should be reachable. Using a
higher concentration of NaOCl and a longer exposure time,
one can attain 1E2 Ohm/sq (with etching of the PEDOT layer
instead of merely passivating).
Using even stronger
oxidizers, 1E14 Ohm/sq can finally be reached.
Of all of the tested passivation agents, NaOCl offers the best
combination of speed and ease of use. It is therefore
recommended to passivate PEDOT layers for most
applications that do not need the highest isolation
resistance.
2.5.1. NaOCl as a passivation agent
A concentration of NaOCl 1% is recommended.
Commercially available NaOCl has a typical concentration of
5-14%. Therefore care should be taken in preparing the
correct passivation solution from commercially available
NaOCl. It has also been observed that prepared passivation
solutions have a limited shelf life, so it is advisable to always
work with newly prepared solutions.
An exposure time of 5 s will passivate the exposed PEDOT
layers to a sheet resistance of 1E9 Ohm/sq. A very slight
color change from blue to yellowish can be observed under
reflected light conditions.
It is recommended that exposure time is kept as short as
possible, because it has been found that patterned edges
can exhibit some ‘underetching’, i.e. the line widths of the
protected area are affected by the passivation solution.
The passivation should therefore immediately followed by a
thorough rinse in DI water.
The surface condition after passivation has been briefly
studied. It was found that the layer is only minimally
affected with a thickness decrease of less than 100 nm.
Longer exposure times or higher NaOCl concentrations will
etch away the PEDOT layer, rather than just passivating it.
However, the maximum surface resistance that can be
reached is still in the order of 1E9 Ohm/sq, so there is no
advantage.
[2.5.2] K 2Cr2O7 + HNO3 as a passivation agent
At maximum concetrations, the isolation resistance achieved
was 1E11 Ohm/sq.
The passivated areas become
transparent after patterning, which indicates that the PEDOT
layer is etched away rather than passivated.
Because of the toxicity of dichromate, it is not recommended
as a passivation agent.
2.5. Passivation of the PEDOT layer
[2.5.3] KMnO 4 + H2S04 as a passivation agent
After resist development, the substrate is ready for
deactivation of the exposed PEDOT areas. This can be
accomplished by exposing the PEDOT layer to strong
oxidizing agents. The following oxidizers have been tested:
•
NaOCl (bleach)
•
K2Cr2O7 (dichromate) + HNO3
•
KMnO4 (permanganate) + H2S04
The composition of this solution is 15 g KMnO4 + 200 ml
H2S04 + 1 l DI water. At ambient temperatures, the isolation
resistance will be 1E9 Ohm/sq after an exposure time of 10
s. As with dichromate, the passivated layers will become
transparent, which is an indication of etching the PEDOT
layer rather than passivating it. The short exposure time is
preferred to minimise the effect of underetching.
Depending on concentration, exposure time and oxidizing
agent, the PEDOT layer will be passivated or completely
etched away. A low concentration of NaOCl and a short
exposure time will passivate the PEDOT layer, yielding an
isolation resistance (the sheet resistance of passivated
areas) of 1E9 Ohm/sq. Theoretically 1E15 Ohm/sq, the sheet
By heating the solution to 60°C, higher isolation resistances
of 1E13 can be reached.
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Using permanganate leaves a brownish layer of MnO2 on the
passivated areas. This layer can easily be removed by
reduction in one of the following solutions (10 s exposure
time) :
•
•
3g Na2SO3 + 25 ml H2S04 + 1 l DI water
10g Fe2SO4 + 100 ml H2S04 + 1 l DI water
2.6. Resist stripping
The recommended stripper for AZ111 is AZ100. Exposure of
PEDOT conductive layers to this solvent will adversely affect
the conductivity. Therefore it is recommended to use
methoxypropanol for resist stripping, as this solvent will not
affect the conductivity.
Exposing the substrate for 60 s to methoxypropanol,
followed by a rinse step in DI water will remove the resist.
Shorter exposure times can leave a white residue on the
PEDOT layer, indicating that the resist is not completely
removed.
By UV light exposure (energy of 360 mJ/cm2), the stripping
time in methoxypropanol can be reduced to 30 s.
The methoxypropanol bath should be kept free from water
as much as possible, as the presence of water can affect the
stripping quality.
[3] Underetching and pattern resolution
Underetching is an unwanted effect that can occur during
patterning, resulting in the condition that lines in the pattern
have a smaller width than the corresponding lines in the
mask. Even if the line width is kept, it is still possible that
solvent used during processing adversely affect the
conductivity in the protected areas by laterlly attacking the
PEDOT layer at the unprotected side walls. This will affect
the resistance of the lines. A number of factors can
contribute to underetching :
•
Imaging
•
Influence of developpping and stripping solutions
•
Passivation chemistry
By using a high-quality photomask, adequate imaging
equipment (collimated light), and correct exposure
procedures, the imaging factor can be kept under control.
Short development times limit the influence of the
developper can be minimized. It is recommended to use
methoxypropanol as stripping agent, as it does not affect
the conductivity of the PEDOT.
NaOCl, which is the recommended passivation chemistry,
will diffuse under the resist pattern and decrease the
conductivity of the patterned lines.
It is therefore
recommended to limit the etch time to the absolute
minimum (which is 5 s) to reach an isolation resistance of
1E9 Ohhm/sq, followed immediately by a thorough rinse in
DI water.
The observed underetching is in the range of 10 µm. Due to
the effect of underetching, the minimum attainable line
width is limited. It has been shown that lines down to 20
micron can be formed, however at this width there is already
a profound effect on the resistance value, almost doubling
the original sheet resistance value. The resolution limit of
the patterning is the subject of further study. Figure 1
shows a 50 µm line pattern created by UV patterning.
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Fig.1 : line of 50om (x292).
[4] Visualisation of the pattern
When patterning substrates, it is often needed to align 2
patternd substrates to each other (e.g. as is the case for a
typical display device). Therefore a pattern typically will
contain reference marks or targets.
A characteristic of PEDOT patterning with NaOCl is the
limited difference in visual appearance of passivated and
non-passivated areas. To enhance the visibility of reference
marks, the area around the marks can be treated with a
coloring solution just before stripping. The effect of the
coloring solution will be to permanently change the color of
the passivated area, while the transparency of the nonpassivated area is preserved as it is protected by the resist.
The coloring solution is prepared by dissolving 2g of
methylene blue in 1 l of DI water. The area of the
reference marks should be exposed for approx. 60 s. This
can be done by putting a few drops of the colorant onto the
area of the marks, after which the substrate is quickly and
thoroughly rinsed in DI water.
[5] Conclusion
Finely structured patterns on PEDOT layers are feasible by
using UV lithography. An optimised procedure based on the
usage of AZ111 resist has been developed. Application of
this procedure will result in an isolation resistance of 1E9
Ohm/sq in the passivated areas. However, the optimised
procedure will exhibit underetching in the range of 10 µm,
limiting the minimum attainable resolution.
Step
Resist spincoating
Pre Exposure Bake
Exposure
Post Exposure Bake
Development
DI water rinse
Conditions
AZ111, 1 µm dry layer
thickness
110°C, 180 s
380 nm, 360 mJ/cm2
110°C, 45 s hotplate
AZ303 20%, 40s
40 s
Optional: Post Bake
Passivation
DI water rinse
110°C, 20 s hotplate
NaOCl 1%, 5s
40 s
Optional : Reference
mark coloration
Drying
Full-plane Exposure
Resist stripping
DI water rinse
Drying
Methylene blue 2g/l, 60 s
90°C, 60-180 s
380 nm, 360 mJ/cm2
Methoxypropanol, 30s
40s
90°C, 60-180s
Remarks
Layer thickness 1 µm
110°C is max T
Immediately
development
110°C is max T
after
Immediately
after
passivation
Optional, followed by
water rinse
Water-free