Micro-scale metallization of high aspect

Microelectronic Engineering 77 (2005) 116–124
www.elsevier.com/locate/mee
Micro-scale metallization of high aspect-ratio Cu and Au
lines on flexible polyimide substrate by electroplating using
SU-8 photoresist mask
S.H. Cho, S.H. Kim, J.G. Lee, N.-E. Lee
*
Department of Materials Engineering and Center for Advanced Plasma Surface Technology, Sungkyunkwan University,
300 Chunchun-dong, Suwon, Gyunggi-do 440-746, Republic of Korea
Received 10 July 2004; accepted 14 September 2004
Available online 19 October 2004
Abstract
In order to fabricate flexible microelectronic devices, fabrication of metallization lines and metal electrodes on the
flexible substrate is essential. Cu lines are often used as interconnect lines in electronic devices and Au as microelectrodes in organic transistors and bioelectronics devices due to its good electrochemical stability and biocompatibility.
For minimizing the size of device, the realization of metallization lines and microelectrodes with the scale of a few
micrometers on the flexible substrate is very important. In this work, micro-scale metallization lines of Cu and Au were
fabricated on the flexible polyimide (PI) substrate by electroplating using the patterned mask of a negative-tone SU-8
photoresist. Surface of PI substrate was treated by O2 inductively coupled plasma for improvement of the adhesion
strength between Cr layer and PI and in situ sputter-deposition of 100-nm thick Cu seed layers on the sputter-deposited
50-nm thick Cr adhesion layer was followed. Electroplating of high aspect-ratio Cu and Au lines using a sulfuric acid
and a noncyanide solution with the patterned SU-8 mask, respectively, removal of SU-8, and selective wet etch of Cr
adhesion and Cu seed layers were carried out. Micro-scale Au electrode lines were successfully fabricated on the PI substrate. Micro-scale gap-filled Cu lines with spin-coated polyimide on the PI substrate with the thickness of 6–12 lm and
the aspect ratio of 1–3 were successfully fabricated.
2004 Elsevier B.V. All rights reserved.
Keywords: Cu metallization; Au electrode; Electroplating; Polyimide; Gap-filling; Flexible electronics
1. Introduction
*
Corresponding author. Tel.: +823 129 07398; fax: +823 129
07410.
E-mail address: nelee@skku.edu (N.-E. Lee).
Polymer for microelectronic applications has
attracted a great deal of concern in the past few
years because polymers can be applied to the man-
0167-9317/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2004.09.007
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
ufacturing of various flexible and low-cost electronic and display devices [1–20]. There have been
extensive research activities on flexible electronics
based on polymeric materials [1–21]. Flexible
TFT (thin film transistor) [1–11], flexible flat panel
displays including organic light emitting diodes
(OLED) [12,13] and liquid-crystal displays
(LCD) [14], sensors [15–17], microelectromechanical systems (MEMS) [18], photovoltaic devices
[19,20] and FPCB (flexible printed circuit board)
[21] based on all polymeric materials or partial
employment of polymeric materials have been
developed due to low-cost and ease of fabrication.
It is expected that flexible microelectromechanical
(MEMS) and semiconductor devices as well as
flexible displays can be fabricated on flexible substrate for many applications in the future.
Cu metallization on flexible substrate has been
frequently used in FPCB and advanced packaging
technology. In this case, a subtractive fabrication
method for Cu lines with the scale of several tens
of micrometers has usually been used [21]. In this
method, wet etching of Cu from the Cu foil or thin
films on flexile substrate delineates Cu line patterns. For the fabrication of Cu lines with the
width of several micrometers and a high aspect-ratio, however, an additive fabrication or build-up
method that utilizes electroplating of Cu on patterned photoresist mask is expected to be required
for the precise dimensional control of fine patterns
[22]. For the fabrication of smaller devices or systems on flexible substrates in the near future,
therefore, micro-scale metallization on flexible
substrates should be developed.
Evaporated Au on organic materials using a
shadow mask has usually used as electrodes in organic TFT for ohmic contact formation [1–11]
and in bioelectronic devices [23] because of its
good biocompatibility and electrochemical stability. In this case, adhesion of Au layers on the polymer substrate is not enough for required
flexibility due to the poor bonding of Au to the
polymeric materials [24]. Adhesion improvement
between Au and polymer substrate is required
for the micro-scale Au metallization. Also, fabrication of the high aspect-ratio patterns is difficult
in case of using a shadow mask for pattern
formation.
117
In this paper, micro-scale Cu and Au metallization processes on flexible polyimide (PI) substrate
were investigated. Fabrication in this experiment
is based on a LIGA-like process [25] utilizing UV
(ultra-violet) lithography and electroplating of
Cu and Au lines. For electroplating of Cu and
Au, Cr adhesion and Cu seed layers were deposited in sequence after O2 inductively coupled plasma treatment of the PI substrate for the
improvement in the adhesion strength of the Cr
adhesion layer to the polyimide substrate. Electroplating of Cu and Au on patterned SU-8 mask was
carried out on patterned SU-8 mask for the formation of Cu and Au lines with the scale of a few
micrometers. Also, gap-filling process of the patterned Cu lines was performed by spin coating of
a polyimide solution. The results obtained in this
experiment can be applied to the fabrication of
flexible microelectronic devices.
2. Experiment
Surface of polyimide film was treated by O2
inductively coupled plasma (ICP) before the sputter deposition of Cr/Cu layers for the improvement
of adhesion of Cr layer to the PI substrate. Hightemperature PMDA-ODA polyimide (PI) films
(Du Pont; Kapton) with the thickness of 125
lm and squared shape of 2 cm · 2 cm were used
as substrate and processed without the carrier.
For plasma surface treatment of the polyimide
films, a modified commercial 8-in. inductively coupled plasma (ICP) etcher having a 3.5-turn spiral
copper coil on the top of chamber separated by a
1-cm thick quartz window and a turbo pump
backed by a rotary pump was used in this experiment. A top RF power of 13.56 MHz was applied
to the top electrode coil to induce ICP. Bottom
electrode power of 13.56 MHz was applied to the
substrate holder to induce self-bias voltage (Vdc)
to the sample and in turn ion-bombardment.
Surface treatments were carried out at the O2
flow of 300 sccm and the fixed top RF power of
40 W and bottom powers of 0, 60 and 125 W with
the DC self-bias voltage of 0, 130 and 280 V for
35 s. The surface roughness and contact angle of
the PI film surface treated by O2 ICP were
118
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
measured by atomic force microscopy (AFM) and
contact angle measurement for the surface property measurement. Next, 50-nm thick Cr adhesion
and 100-nm thick Cu seed layers were deposited in
sequence by in situ sputter deposition at the Ar
flow of 20 sccm, the chamber pressure of 2 mTorr,
and the RF (13.56 MHz) power of 100 W (0.3 W/
cm2). Resistivity of electroplated copper film with
no pattern was calculated from the sheet resistance
values obtained from a 4-point probe measurement. The thickness of electroplated Cu and Au
was measured by an a-step profile meter.
A contact anglemeter (SEO 300X by Surface &
Electro-Optics Corporation) was used for contact
angle measurements. Deionized (DI) water was
used as a liquid probe for contact angle measurements. After water was dropped onto the surface
of the plasma-treated PI films, the contact angle
of water on the substrates was immediately measured by means of the advanced angle method. All
experiments were carried out at 25 ± 0.1 C.
For the evaluation of adhesion strengths of the
Cr layer on the plasma-treated PI surfaces, Cu
electroplating with the thickness of 20 lm was performed on the 150-nm thick Cu/Cr seed/adhesion
layers deposited on the strip of PI with the width
of 0.5 cm and the length of 3 cm. Adhesion
strength of the Cr/Cu films on the flexible substrates (Cu/Cr/PI systems) was measured using Micro-tester system by LLOYD Instruments
(AMETEK) and the method to deduce peel
strength values from the T-peel tests [26] was used
following the procedure described in the previous
literature [27].
For the formation of mask used in the electroplating of Cu and Au, a negative photoresist,
SU-8 2010 [28], with the thickness of about 10–
12 lm was coated on the surface of the Cu/Cr/PI
substrate by a spin coater. The coated substrate
was placed for about 1 h on the flat board in the
clean room for the exclusion of bubbles. Electroplating of Cu line was carried out in the sulfuric
acid bath with the electric current density of the
12 mA/cm2 and at the bath temperature of 25
C. Electroplating of the Au lines was carried
out in the noncyanide solution [29] with the electric current density of 5 mA/cm2 and at the temperature of 60 C. Removal of SU-8 was carried
out in acetone and then by chemical dry etching
using a remote microwave plasma source with
CF4/O2/Ar process gases [30]. The layers of Cr
and Cu were selectively etched using a wet etching
solution.
PI 2560 gap-filler supplied by HD MicroSystems was spin-coated for the gap-filling of Cu lines
with the line and spacing of @4–12 lm. After spin
coating of the gap-filler, the samples were slowly
cured at 250 C on the hot plate and the polyimide
layer with the thickness of @20 lm was formed.
Fabricated Cu and Au patterns were observed
using a scanning electron microscope (SEM) operating at the acceleration voltage of 7.5 kV.
3. Results and discussion
Fig. 1 shows 4.0 · 4.0 lm2 AFM image of the
surface of O2 plasma-treated PI films as a function
of the bottom power. The root-mean-squared
(RMS) roughness of the untreated PI film was
0.9 nm. However, the RMS roughness values of
the plasma-treated PI films at the bottom power
of 0, 60, and 125 W were increased to 3.6, 11.1,
and 23.9 nm, respectively. In this experiment, the
bottom power in the ICP system controls the bombardment energy of oxygen ions in the plasma and
as a result reactive etching of the polyimide surface
leads to the increased morphological surface
roughening. As seen in Fig. 1, increased bias
power roughens the surface more due to a faster
etch rate under increased bombarding energy of
oxygen ions. The reason for using the range of
low substrate bias power density is that excessive
treatments under high-energy ion-bombardment
can cause more chain scission leading to a weak
boundary layer that is detrimental to metal/polymer adhesion.
Fig. 2 shows the contact angle values of the
plasma-treated PI films obtained together with
the RMS roughness values obtained from the
AFM images in Fig. 1. While the contact angle
of untreated PI films was 73.8, contact angles of
the O2 plasma-treated PI films by the bottom
power of 0, 60, and 125 W were reduced to 0.
The complete wetting of DI water is caused by a
very large increase in surface roughness induced
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
119
Fig. 1. A 4.0 · 4.0-lm2 AFM image of the surface by O2 plasma-treated PI films as a function of the bottom power. (a) untreated (b)
top: 40 W (c) top: 40 W, bottom: 60 W (d) top: 40 W, bottom: 125 W [RMS roughness: (a) 0.861 nm; (b) 3.567 nm; (c) 11.06 nm; (d)
23.92 nm].
by an oxygen ion bombardment together with
modified chemical bonding states. The combined
results indicate that the obtained RMS roughness
is inversely proportional to the contact angle, as
observed in Fig. 2. The decrease of the contact angle is attributed possibly to the increase in the total
No treatment
70
25
60
20
50
40
15
30
10
20
10
5
0
0
120
RMS roughness(nm)
Contact angle(deg.)
130
-10
No treatment
Top: 40 W
Top: 40 W
bottom: 60 W
Top: 40 W
bottom: 125 W
Fig. 2. Contact angle and RMS roughness values as a function
of the bottom power.
Peel strength (gf/mm)
O2: 300 sccm
Time: 35 sec
80
surface energy of PI films due to the increase in
surface area induced by roughening as well as
due to the creation of new binding states by O2
plasma treatment [31,32].
The adhesion strength measurement was performed using T-peel test. Fig. 3 shows a relation
Top: 40 W
110
Top: 40 w, bottom: 60 W
100
Top: 40 W, bottom: 125 W
90
80
70
60
50
40
0
5
10
15
20
25
RMS roughness (nm)
Fig. 3. Peel strength measured as a function of RMS
roughness.
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
between RMS roughness and peel strength. The
values of peel strength were averaged at least over
five samples for the sample plasma treatment conditions. The error bars in Fig. 3 indicate the range
of the measured values for the same plasma-treatment condition. If surface roughening is a major
contributor to the improvement of adhesion, O2
plasma-treated PI films at the bottom power of
125 W will produce the largest improvement in
peel strength. As we were expecting, O2 plasmatreated PI film at the bottom power of 125 W
was found to have the highest peel strength, 126
gf/nm. Based on this observation, it is concluded
that increase in surface roughness plays a big role
in the enhancement of adhesion at the range of
large surface roughness.
The effect of roughened surface on the adhesion
strength of metals on the PI surface has been already investigated although correlated quantitative measurements of roughness, contact angle,
and peel strength have not been given in detail
[33–36]. Important observation from those previous works is that a grass-like surface textures with
a large surface area, as observed in Fig. 1, possibly
provide the bonding sites out of the horizontal because the PI chains oriented intrinsically parallel to
the substrate surface are disturbed [35,36]. Interestingly, surface roughness effect was not effective
in the range of small surface roughness even
though the roughness values were not quantified
[34]. The results in this study also confirm the effec-
tiveness of surface roughening on the significant
adhesion improvement at the surface roughness
value as large as @24 nm.
Fig. 4 shows the resistivity of electroplated Cu
with the thickness of 0.8–3.7 lm in our experimental conditions as a function of the Cu thickness.
For the purpose of resistivity measurement, the
Cu/Cr/PI substrate without patterning was used.
Electrical resistivity was determined from the
measured sheet resistance and the film thickness
values of the electroplated copper film. For the
very thin layers with the thickness less than 1.5
lm, the effect of the layer thickness on measured
sheet resistance was corrected. The effect of 50nm thick Cr layer on measured sheet resistance value was neglected due to the small thickness and
higher resistivity. The sheet resistance was usually
measured at three different points and the averaged value was used to estimate the resistivity.
The thickness of the electroplated copper film
was increased linearly with increasing the electroplating time. With the increase of the electroplated
Cu thickness, resistivity was decreased. But at the
thickness larger than 1.5 lm, resistivity became
saturated at @1.8 · 10 6 X Æ cm that is close to
the resistivity of bulk copper @1.7 · 10 6 X Æ cm.
Other measurements showed that resistivity values
of Cu by chemical vapor deposition and sputtering
are @2 · 10 6 X/cm [37] and @ 3 · 10 6 X Æ cm [38],
respectively. Other reported resistivity values
obtained by electroplating are in the range of
4.0
40
Resistivity
Thickness
3.5
-7
Resistivity(10 Ω . cm)
30
3.0
2.5
20
2.0
10
1.5
Thickness (µm)
120
1.0
0
0.5
5
10
15
20
Electroplating time (min)
Fig. 4. Resistivity of unpatterned electroplated Cu films as a function of electroplating time.
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
Fig. 5. SEM image of patterned SU-8 mask.
1.8–1.9 · 10 6 X Æ cm [39], similarly to the value
obtained in this experiment.
Mask pattern with the scale of a few micrometers for Cu and Au electroplating was fabricated
using SU-8 negative-tone photoresist. SU-8 is
known to be stable in electroplating solutions.
Fig. 5 shows SEM micrograph of a patterned
SU-8 electroplating mask on the silicon substrate
121
with the various trench widths, 4–12 lm, optimized by varying the process parameters such as
spinning time, spinning speed, soft bake time,
exposure time, and development time [28]. Here,
Si substrate was used for the process optimization
and easy observation of fabricated patterns.
The same SU-8 patterning condition described
above was applied to the fabrication of SU-8 mask
on the plasma-treated PI substrates. Cu metallization lines on the patterned SU-8 mask with the
thickness of approximately 12 lm were electroplated by varying the electroplating time in the
copper sulfuric acid bath. Platinum (Pt) mesh
was used as an anode during electroplating. Au
electrodes on the patterned SU-8 mask were electroplated in the non-cyanide bath. Also, the Pt
mesh was used as the anode electrode for Au
electroplating.
After electroplating, SU-8 was successfully removed by a combined method of wet and dry
processes. For the complete dissolution of the
SU-8, a considerable number of solvents and
conditions have been tried [30]. Acetone is very
effective in cracking and crazing the cross-linked
Fig. 6. (a) SEM image after removal of the SU-8 photoresist, (b) quantitative chemical analysis results by EDS, (c) SEM image of the
Cu metallization lines after removal of the Cu/Cr layers, and (d) quantitative chemical analysis results by EDS after removal of seed
and adhesion layers.
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S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
SU-8 [30]. In this work, wet removal using acetone
was used first and then ashing of SU-8 using CF4/
O2/Ar microwave remote plasma was followed.
The SU-8 mask in the acetone was heated at 200
C on the hot plate for 10 min. SU-8 ashing in a
chemical dry etcher utilizes reactive oxygen radicals generated by a remote microwave plasma
source transported into the chamber and highly
energetic ions that can damage metallization lines
are minimized in the remote plasma.
Fig. 6(a) shows the SEM micrograph of electroplated Cu lines after removal of SU-8. The chemical analysis data in Fig. 6(b) obtained from the
Cr/Cu layers by energy dispersive spectroscopy
(EDS) indicate that carbon and nitrogen that are
main components of the SU-8 photoresist were
not found. This result indicates a successful removal of the SU-8 photoresist. The oxygen on
the surface of Cu seed layer, as seen in Fig. 6(b),
is possibly due to the oxidation of Cu during the
remote CF4/O2/Ar plasma ashing of SU-8
photoresist.
Following removal of SU-8 photoresist, a selective wet etching of the Cr adhesion and Cu seed
layers was carried out. During the etching of the
Cu seed layer, minimal etching of Cu lines cannot
be avoided. Cu seed layer was etched by nitric acid
(50%) mixed with water. The Cr adhesion layer
was dip-etched at 60 C with a solution containing
60 g/l of potassium permanganate and 200 g/l of
tri-basic sodium phosphate [8]. During Cr etching,
it was found that etching of Cu was negligible
from the SEM observation of Cu before and after
Cr etching. Fig. 6(c) shows Cu metallization lines
on the flexible PI substrate after the removal of
the Cu seed and the Cr adhesion layers. After
the removal of the Cu/Cr layers, roughness of Cu
lines was increased. Traces of Cu and Cr were minimal, judged form the EDS analysis data as shown
in Fig. 6(d). Observed manganese trace on the
etched surface is due to the use of potassium permanganate during Cr wet etching process and oxygen is from the PMDA-ODA polyimide (PI) film
(Du Pont; Kapton) in which 17.2 at.% of oxygen
exists [40]. This result indicates the Cu seed and Cr
adhesion layers were successfully etched on the
flexible substrate with the minimal attack on the
Cu lines.
For electroplating of Au lines, the same procedure of Cu/Cr layer deposition and SU-8 mask
patterning was performed. Fig. 7(a) shows the
SEM micrographs of the Au lines electroplated
in a noncyanide solution with the current density
of 5 mA/cm2 at the solution temperature of 60
C. The EDS analysis data in Fig. 7(b) obtained
from the substrate surface after removal of the
Cu/Cr (seed/adhesion) layers shows no residual
Cu and Cr on the PI substrate. This indicates the
complete removal of the Cu/Cr layers.
Gap-filling process for filling of the trenches between Cu lines was followed after formation of Cu
metallization lines. For this process, PI 2560 supplied by HD MicroSystems was used. For the
Fig. 7. (a) SEM image of the Au electrode lines after removal
of Cu/Cr layers and (b) quantitative chemical analysis results by
EDS on the polyimide surface after removal seed and adhesion
layers.
S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
removal of solvents and hardening of PI, curing of
coated layers was performed at the relatively low
curing temperature of 250 C since out-gassing
from the PI during curing at elevated temperature,
for example, at 400 C can occur. For the optimization of gap-filling process by the polyimide gapfiller, spinning speed of spin coater was found to
be the most critical parameter. For gap-filling of
the trench with the width of @4 lm and the aspect
ratio of @3, a two-step process during spin coating
was required. In our experimental conditions,
spinning with the speed of 500 rpm at the first step
and then with the speed of 2000–4000 rpm at the
second step was used to control the final thickness
of the PI. As seen in Fig. 8(a), at the condition of a
single-step process with the spinning speed of
2000–4000 rpm, trenches with the width of @4
lm and the aspect ratio of @1.5 were not filled
123
completely. As seen in Fig. 8(b), an optimized
gap-filling of the Cu lines with the trench width
of @4 lm and aspect ratio of @3 was successfully
obtained using a two-step spin coating. As shown
in Fig. 8(b), a globally planarized polyimide surface was obtained. For the device applications, a
multilevel metallization scheme can be possibly applied using a combination of polyimide patterning
and electroless plating of Cu vias.
4. Conclusion
Micro-scale Cu and Au metal lines on the flexible polyimide substrate were successfully fabricated by electroplating of Cu and Au using a
negative SU-8 photoresist mask. Cr adhesion and
Cu seed layers were deposited on O2 inductively
coupled plasma-treated polyimide substrate. The
results of the adhesion strength measurements
showed that the O2 plasma-treated PI film at the
bottom electrode power of 125 W has the highest
adhesion strength of 126 gf/nm. Photolithography
process of the SU-8 mask with the thickness of 8–
12 lm for Cu and Au electroplating was successfully developed. Electroplating of Cu and Au,
removal of SU-8 photo-resist and selective removal process of the Cr/Cu adhesion/seed layers
were developed for the fabrication of micro-scale
Cu and Au lines with the width as small as 4
lm. Gap-filling process of Cu metallization for filling the trenches with the width of @4 lm and the
aspect-ratio of @3 was also successfully applied.
Au electrodes with various pattern sizes applicable
to organic TFT and sensors were successfully fabricated using a non-cyanide electroplating. Cu
metallization process with the scale of a few
micrometers on the flexible polyimide substrate
can be used for the flexible electronic device fabrication as well as advanced flexible printed circuit
board technology.
Acknowledgements
Fig. 8. (a) SEM image after gap-filling process using a singlestep and (b) SEM image after gap-filling using a two-step
process.
This work was supported through the Center of
Excellency program by the Korea Science and
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S.H. Cho et al. / Microelectronic Engineering 77 (2005) 116–124
Engineering Foundation and Ministry and Science
and Technology (Grant No. R-11-2000-086-00000) and SAIT (Samsung Advanced Institute of
Technology).
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