Orgacon

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Orgacon®
Conductive Transparent Films
Application sheet
Patterning Orgacon® film
by UV photo-lithography
[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
conductivity of this layer can be destroyed by exposing it
to low concentration of a strong oxidant for a short time,
e.g. a 1 % concentration of sodium hypochlorite (NaOCl or
bleach). This technique is remarkably 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’, 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 oxidisers
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 EL lamps, a screenprinting paste has been
developed, which works by simply screen-printing the
negative of the desired conductive pattern with this
deactivation paste (see Orgacon® STRUPAS application
note). Another method makes use of a screen-printable
mask resist followed by deactivation in a NaOCl solution,
after which the resist is removed, yielding the desired
conductive pattern (see application note ‘Orgacon® film
patterning by screenprinting mask’).
Yet, issues remain when 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® film is available on PET in thickness, 63 to
175µm. The film has an antistatic backlayer for improved
handling and dust shielding, and a conductive PEDOT
layer onto a special adhesion layer, on the other side,. The
adhesion layer will stay even after etching of PEDOT layer.
2.1.2. Substrate preparation
Although 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. Therefore, the PEDOT layer can be cleaned
with DI water or isopropanol to remove superficial dirt.
2.2. Resist selection and coating
Many photoresists are available with very different
properties. For patterning, we recommend to use positive
photoresist from Clariant1 or similar: AZ111 XFS (or
“AZ111”). AZ111 XPS resist was selected specifically
because of its good adhesion to many surfaces, curability
at low temperature. The patterning procedure is developed
in combination with AZ111 – see Table 1.
Solid Content (%)
Viscosity (cSt at 25C)
Absorptivity (I/g*cm) at 375 nm
Solvent
Max water content (%)
This application sheet explains how to use UV lithography
to pattern PEDOT layer, which offers the finest patterning
resolution.
19.3
25.2
0.65
Methoxypropylacetate
0.5
Table 1: Selected properties of AZ111 XFS (source: Clariant)
[2] Patterning process
The UV lithography process generally consists of the
following steps:
•
Substrate selection and preparation
•
Resist selection and application
•
Exposure
•
Resist development
•
Passivation or etching of the PEDOT layer
•
Resist stripping
Last updated: 07/2005
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The resist is normally coated by spincoating (see fig 1). A
suitable clip/clamp is needed to hold down the piece of
substrate. The suitable resist thickness is 1 µm (dry layer).
Fig 1: Spin-coating resist
AZ72
PED
1
http://www.clariant.com
PE
2.3. Exposure
Baking process, prior to exposure, can improve etching.
Therefore, we advise to include pre-exposure baking step
at 110°C/3 min.
Out of all deactivation agents as listed above, 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 require high isolation
resistance.
2.5.1. NaOCl deactivation agent
Fig 2: UV exposure (1 min at 7000 µW/cm2)
The absorption peak of the AZ111 resist is around 380 nm.
Total exposure energy should be in the range of 360
mJ/cm2 (1µm layer thickness). The choice of mask, either
glass mask or PET mask, depends on the resolution. In
both cases, collimated light exposure should be used for
good imaging to the resist (see fig 2).
AZ111 does not normally require post exposure baking
process to increase development quality. However, it has
been also learned that one thermal baking step during
resist processing improves the residual resistance of thin
lines. 45sec post baking at 110°C (preferably on hotplate)
is recommended.
2.4. Development
We recommend to use NaOCl concentration 1%. Available
NaOCl is concentrated typically at 5-14%. Therefore,
please take time to prepare NaOCl with the correct
concentration. Once NaOCl deactivation is prepared, it
has limited shelf life, so we advise to prepare it
immediately before printing.
An exposure time of 5 sec will deactivate the non-masked
PEDOT layer and reduce its surface resistance to 1E9
Ohm/sq.
Very slight colour change, from blue to
yellowish, can be observed under reflected light
conditions. The thickness of the deactivated area
decreases by <100 nm.
We recommend to keep exposure time as short as
possible, because it patterned edges can exhibit some
‘under-etching’, i.e. line widths of the protected area are
affected by deactivation. Therefore, after the deactivation
the substrate should be thoroughly rinsed in DI water (see
fig 5).
NaClO 1%
The standard developer for AZ111 is AZ303. This will
adversely influence the resistance of the PEDOT layer: a
sixfold increase in surface resistance has been observed
when exposing PEDOT layer to AZ303 for 15 min. This
effect might contribute to higher resistance values in thin
lines. Recommended development conditions are 40sec
in 20% AZ303 developer (see fig 3).
Fig 4: Deactivation of non-masked PEDOT layer
Fig 3: Development of mask-resist (40 sec)
As mentioned before, one thermal step can improve the
resistance of fine lines. This step can either be a post
exposure baking (see above), or post baking after
development, such as at 110°C/60sec (hotplate).
2.5. Deactivating PEDOT layer
After resist development, the substrate is ready for
deactivation of the non-masked PEDOT areas, by exposing
the to strong oxidising agents. The following oxidisers
have been tested:
•
•
•
NaOCl (bleach)
K2Cr2O7 (dichromate) + HNO3
KMnO4 (permanganate) + H2S04
Depending on concentration, exposure time and oxidising
agent, the PEDOT layer will be deactivated or completely
etched away. Low concentration of NaOCl and a short
exposure time will deactivate the PEDOT layer, resulting
in the sheet resistance at 1E9 Ohm/sq. Theoretically you
can also reach 1E15 Ohm/sq, if using untreated PET. With
high concentration of NaOCl and longer exposure time,
you can reach 1E2 Ohm/sq (PEDOT layer is etched away).
The use of even stronger oxidisers enables you to reach
1E14 Ohm/sq.
Last updated: 07/2005
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Fig 5: Rinsing PEDOT layer in water
2.5.2 K2Cr2O7 + HNO3 deactivation agent
At maximum concentrations, this agent reduces surface
resistance of deactivated area to 1E11 Ohm/sq. The
deactivated areas become transparent after patterning, as
the PEDOT layer is etched away.
2.5.3 KMnO4 + H2S04 as a passivation agent
The composition of this solution is 15 g KMnO4 + 200 ml
H2S04 + 1L DI water. At ambient temperatures, t this agent
reduces surface resistance of deactivated area to 1E9
Ohm/sq after 10sec exposure. As with dichromate, the
deactivate areas become transparent, as the PEDOT layer
is etched away. The short exposure time is preferred, since
it minimises the effect of under-etching. By heating it to
60°C, you can expect the result o fhigher isolation
resistances at 1E13.
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
[4] Visualisation of the pattern
The recommended stripper for AZ111 is AZ100. Exposure
of PEDOT layer to this solvent will affect the conductivity.
Therefore it is recommended to use methoxypropanol for
resist stripping.
When patterning, it is often required to pattern 2 layers of
circuit lines on top of one another. Therefore a pattern
typically will contain reference marks or targets.
60sec methoxypropanol exposure on PEDOT layer,
followed by rinsing in DI water, will finally remove the
resist (see fig 6). Shorter exposure times can leave white
residue on the PEDOT layer, indicating that the resist is
not completely removed. Methoxypropanol bath should be
kept free from water as much as possible, as the presence
of water can affect the stripping quality.
PEDOT conductive
Fig 6: Methoxypropanol exposure
By UV light exposure (energy of 360 mJ/cm2), stripping
time in methoxypropanol can be reduced to 30 sec.
Under-etching is a undesired phenomena. This can occur if
patterned circuit lines have small gap than the
corresponding lines in the resist-mask. Even if the line
width is reproduced successfully, it is still possible that
solvent used during processing adversely affect the
conductivity in the protected areas by latterly attacking
the PEDOT layer at the unprotected sidewalls. This will
affect surface resistance of the circuit lines. A number of
factors can contribute to under-etching:
Imaging
Influence of developing and stripping solutions
Passivation chemistry
You can keep such undesired phenomena under control,
by using good quality photo-resist, appropriate imaging
equipment (collimated light), and correct exposure. Short
development times limit the influence of the developer. It
is recommended to use methoxypropanol as stripping
agent, as it does not affect the conductivity of the PEDOT.
NaOCl deactivation agent will diffuse
pattern and decrease the conductivity
lines. It is, therefore ,recommended to
time to its minimum (which is 5 sec)
resistance of 1E9 Ohm/sq, followed
thorough rinsing in DI water.
The colorant is prepared by dissolving 2g of methylene
blue in 1 L of DI water, and 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. The
reference mark area should be exposed for approx. 60sec.
[5] Conclusion
[3] Under-etching and pattern resolution
•
•
•
A characteristic of PEDOT patterning with NaOCl is small
difference in visual appearance of the deactivated and
non-deactivated areas. To enhance the visibility of
reference marks, the area around the marks is treated with
colorant just before stripping. The effect of the colorant
will be to permanently change the colour of the
deactivated area, while transparency of the nondeactivated area is preserved as it is protected by the
resist.
under the resist
of the patterned
limit the etching
to reach surface
immediately by
The observed under-etching is in a range of 10µm. Due to
the effect of under-etching, minimum reproducible line
width is limited to 20µm – although almost doubling the
original sheet resistance value. Figure 1 shows a 50µm
line pattern created by UV patterning.
Finely structured patterns on PEDOT layer are ready for UV
lithography. Agfa has developed the optimised process
using AZ111 resist. This will result in an isolation
resistance of 1E9 Ohm/sq in the deactivate areas.
However, the process may result in under-etching in a
range of 10 µm or finer line with.
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
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Immediately after
development
110°C is max T
Immediately after
passivation
Optional, followed by
water rinse
Water-free
For more info, please do not hesitate to contact the staff
at Agfa.
Advanced Materials
Speciality Industries
Agfa-Gevaert N.V.
Septestraat 27
B-2640-Mortsel
Belgium
HIROKO IINUMA
Tel. ++81 (0)3 5704 31 40
Fax ++81 (0)3 5704 30 89
e-mail: [email protected]
Last updated: 07/2005
Layer thickness 1 µm
110°C is max T
[6] MORE INFORMATION
LOUIS BOLLENS
Tel. ++32 (0)3 444 29 96
Fax ++32 (0)3 444 76 62
e-mail: [email protected]
Fig.1 : line of 50µm (x292).
Remarks
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