Ultraviolet-Patternable Polymer Insulator for Organic Thin-Film

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					                                                                                                                SS10213 150 Total pages 5
Japanese Journal of Applied Physics 50 (2011) 04DK23                                                                          REGULAR PAPER
DOI: 10.1143/JJAP.50.04DK23


Ultraviolet-Patternable Polymer Insulator for Organic Thin-Film Transistors
on Flexible Substrates
Chung-Ming Wu, Shui-Hsiang SuÃ, Hong-Tai Wang, Meiso Yokoyama, and Shen-Li Fu
Department of Electronic Engineering, I-Shou University, Kaohsiung County 840, Taiwan
Received September 27, 2010; accepted December 2, 2010; published online April 20, 2011

    In this work, we describe the fabrication of pentacene-based organic thin-film transistors (OTFTs) on a flexible substrate using a UV-patternable
    polymer material, mr-UVCur06, as the gate insulator. The device structure is poly(ethylene terephthalate) (PET)/indium–tin oxide (ITO)/mr-
    UVCur06/pentacene/Au (source/drain). In addition to its solution-processable capability, mr-UVCur06 is directly patternable by UV light in a low-
    temperature process. The OTFT has an on–off ratio that approaches 105 , and its pattern resolution can reach 5 m. Additionally, UV/ozone post-
    treatment of the patterned mr-UVCur06 can illuminate the organic contaminants from its surface and significantly improve the performance of
    OTFTs. Moreover, the effect of UV/ozone post-treatment on the polymer dielectric is confirmed using a Fourier transform infrared (FT-IR)
    spectrometer. Owing to its highly desired characteristics such as photopatternability and low-temperature process, mr-UVCur06 is feasible for
    low-cost, large-area flexible device applications. # 2011 The Japan Society of Applied Physics




1. Introduction                                                                       substrate                               substrate
Organic thin-film transistors (OTFTs) have attracted con-
siderable interest in developing commercial electronic                              gate insulator                           mr-UVCur06
                                                                                      substrate                               substrate
products such as smart cards, sensors, radio frequency
identification tags (RFID), electronic paper (e-paper) for-                                     photoresist coating
mats, and flat panel displays (FPDs).1–4) In addition to their                        photoresist
                                                                                    gate insulator                                        alignment and exposure
potential of low manufacturing costs and a low process                                substrate
temperature, OTFTs can serve as the driving circuits of                                        alignment and exposure
a display pixel so as to facilitate the reduction of optical
energy loss.5,6) Therefore, molecular and polymer materials
                                                                                     photoresist
have been extensively adopted for OTFT applications.7–9)                            gate insulator                           mr-UVCur06
Consequently, OTFT performance has significantly im-                                   substrate                               substrate
proved over the last decade, and is already comparable with                                    development
that of amorphous silicon thin-film transistors (-Si TFTs).10)
   However, OTFT-based devices are fabricated mainly with                           gate insulator
                                                                                      substrate                                            development
inorganic dielectric materials, such as SiO2 , HfO2 , and
                                                                                               etching
Ta2 O5 .11–13) Inorganic dielectric materials that serve as a
gate dielectric have significantly improved in electrical
performance. Despite their highly attractive characteristics                          substrate
such as a high dielectric constant, satisfactory heat and                                      striping
chemical resistance, high breakdown voltage, and long-term
stability, inorganic dielectric materials are infeasible for                          substrate                                substrate
flexible device applications since the film deposition may
                                                                                        (a)                                      (b)
damage a flexible substrate and the vacuum equipment is
prohibitively expensive. Applying OTFTs in such flexible
                                                                              Fig. 1. (Color online) Comparison of (a) conventional lithography and
commercial electronic products mentioned above requires a                     (b) photopatternable processes.
high-performance and feasible process. Therefore, several
polymeric dielectric materials, including poly(vinyl alcohol)
(PVA), poly(methyl methacrylate) (PMMA), benzocyclo-                          with the conventional photolithography process, the photo
butene (BCB), and polyimide (PI), have been widely                            patternable process [Fig. 1(b)] requires only three steps.
investigated owing to their potential low manufacturing cost                  Fabricating OTFTs on a flexible substrate for commercial
and large area applicability.14–17)                                           electronic applications requires a low-temperature gate
   Moreover, patterning of the gate insulator is an essential                 insulator process to not only prevent damage to the flexible
requirement. Photolithography followed by etching and                         substrate, but also form a high resolution pattern of a gate
photoresist removal is the conventional and most reliable                     insulator without lithography to reduce the complexity of
pattering process to access the gate electrode. Figure 1(a)                   fabricating OTFTs. However, defining the patterns of these
illustrates the photolithography processes that are commonly                  polymeric dielectric materials in OTFTs has many obstacles
used in manufacturing semiconductor devices, an approach                      for flexible device applications.
that requires at least six steps to define the pattern of a gate                  In this work, we present a low-temperature UV-pattern-
insulator. Obviously, such a conventional photolithography                    able polymer, i.e., mr-UVCur06, for use as a gate insulator
process is both complex and relatively expensive. In contrast                 in OTFTs fabricated on a flexible polyethylene terephthalate
                                                                              (PET) substrate. The leakage current, pattern resolution, and
Ã
 E-mail address: shsu@isu.edu.tw                                              electrical features are characterized for OTFT applications.
                                                                      04DK23-1                               # 2011 The Japan Society of Applied Physics
Jpn. J. Appl. Phys. 50 (2011) 04DK23                                                                                                                           C.-M. Wu et al.




                                                                                Leakage Current Density (A/cm2 )
          (a)                Photoinitiator                                                                         1E-4
                                  hν
                                                                                                                    1E-5
                                                                                                                    1E-6

         (b)                                                                                                        1E-7
                                                                                                                    1E-8
                                                                                                                    1E-9
                                                                                                                   1E-10
                                                                                                                   1E-11

Fig. 2. (Color online) (a) Curing reaction of acrylates initiated by UV                                            1E-12
                                                                                                                           -50 -40 -30 -20 -10   0   10   20   30   40   50
light. (b) Schematic cross section of a pentacene-base OTFT with mr-
UVCur06 as an insulator.                                                                                                              Applied Voltage (V)

                                                                          Fig. 3. Leakage current of the ITO/mr-UVCur06 (490 nm)/Al structure
                                                                          determined at a frequency of 100 kHz by applying the voltage in a step of
                                                                          1 V from À50 to 50 V.
2. Experimental Methods
The UV-patternable polymer, mr-UVCur06, which is
usually used in nanoimprint techniques, was supplied by                   respectively. The thickness of the evaporation layers was
Micro Resist Technology. The photocurable system of                       controlled by an oscillating quartz monitor (Sycon STM-
mr-UVCur06 consists of two main polymer classes:                          100) and further calibrated by a -step surface profiler (AS-
acrylates (free-radical polymerization) and epoxy com-                    IQ series). Furthermore, leakage currents of the photo-
pounds (cationic polymerization). Figure 2(a) schematically               patternable films were characterized by preparing a metal–
depicts the free-radical polymerization of (meth)acrylates                insulator–metal (MIM) capacitor structure by sandwiching
initiated by UV exposure (acrylates R¼H, methacrylates                    the mr-UVCur06 materials between the glass/ITO and the
R¼CH3 , Ri various functional groups), while (b) shows                    Al electrode. Finally, the electrical characteristics of OTFTs
the OTFT configuration examined in this work. The device                   were determined using a Keithley 2400 programmable
structure is PET/indium–tin oxide (ITO)/mr-UVCur06/                       voltage-current source system. All the measurements were
pentacene/Au (source/drain). Prior to dielectric layer                    taken in an air atmosphere.
coating, ITO-coated PET substrates with a sheet resistance
of 20 
/square were solvent cleansed in an ultrasonic bath                3. Results and Discussion
in acetone and isopropyl alcohol then in deionized water.                 An MIM structure with mr-UVCur06 as the dielectric is
Next, the adhesion between the PET/ITO substrate and                      fabricated to investigate the gate leakage current. The
mr-UVCur06 was increased by treating the PET/ITO                          leakage current with the ITO/mr-UVCur06 (490 nm)/Al
substrate by UV ozone treatment for 10 min after wet                      structure was determined at a frequency of 100 kHz by
cleaning.                                                                 applying the voltage in a step of 1 V from À50 to 50 V.
   The gate insulator layer, mr-UVCur06, was coated on the                According to Fig. 3, the leakage current density was close
substrates by spin-coating, followed by soft baking at 100  C            to 10À8 A/cm2 with the minimum one lower than 10À10
in an oven to eliminate the residual solvent. The mr-                     A/cm2 . The relative dielectric constant of the mr-UVCur06
UVCur06 was then irradiated with the assistance of a                      (490 nm) was obtained with the MIM structures, which were
photomask on a contact aligner by a 365 nm UV light source                found to be 1.81 at the same frequency described above.
with an exposure dose of 2400 mJ/cm2 . Post baking was                    Evaluation of the leakage current and capacitance indicated
performed at 100  C in an oven. Next, the patterned gate                 that the mr-UVCur06 is applicable to OTFTs as a gate
insulator was obtained by selecting acetone as the develop-               insulator. The patterning results were subsequently inspected
ing solution to remove mr-UVCur06 in the area that was                    by scanning electron microscopy (SEM). According to
not exposed to UV light. Additionally, the exposed gate                   Fig. 4, the line and space of mr-UVCur06 are both well
insulator layer was developed by dipping into acetone for                 defined at 20 and 5 m, respectively. This finding demon-
60 s at room temperature, followed by rinsing in deionized                strates that the gate insulator can be well defined by a
water. Following the development process, annealing was                   photopatternable process. Furthermore, the process tempera-
performed at 100  C for 1 h to cure mr-UVCur06. Moreover,                ture of mr-UVCur06 only requires annealing at 100  C for
organic contaminants caused by acetone during photopat-                   1 h. From the aspects of a low process temperature,
terning were prevented using UV/ozone post-treatment of                   throughput, and high resolution, the UV-patternable poly-
the patterned gate insulator. An organic active layer and                 mer, mr-UVCur06, appears to be promising for flexible
source/drain electrode materials were then deposited                      device applications.
sequentially through a shadow mask by thermal evaporation.                   Figures 5(a) and 5(b) show the output (IDS versus VDS )
                                      
Deposition was initiated with a 600-A-thick pentacene layer               and transfer (IDS versus VGS ) characteristic curves of OTFTs
                                            
as an organic active layer, then a 1200-A-thick Au layer                  with a patterned and unpatterned gate insulator, mr-
as the source and drain electrodes at a pressure below                    UVCur06. The field-effect mobility () was estimated from
                                                    
5 Â 10À6 Torr and an evaporation rate of 1–2 A/s. The                     the plot of the square root of the drain current (IDS 1=2 ) and
channel length and width of the device were 50 and 500 m,                gate voltage (VGS ) in the saturation regime:
                                                                     04DK23-2                                                     # 2011 The Japan Society of Applied Physics

                                                                                                                                                                         SS10213
Jpn. J. Appl. Phys. 50 (2011) 04DK23                                                                                                          C.-M. Wu et al.

                                                                                      -12
                                                                                                  unpatterned
                                                                                      -10         patterned

                                                                                       -8

                                                                                       -6




                                                                          IDS ( μA)
                                                                                       -4

                                                                                       -2

                                                                                       0

                                                                                       2
                                                                                             0            -20          -40       -60          -80
                                                                                                                     VDS (V)
Fig. 4. SEM cross-sectional image of the mr-UVCur06 patterned at a line                                                (a)
and space of 20 and 5 m, respectively.

                                                                                                                                       unpatterned
                                                                                                                                       patterned     0.0016
                                                                                  1E-6
                         W
                  IDS ¼     Â Ci ðVG À VT Þ2 ;              ð1Þ
                        2L                                                                                                                           0.0012




                                                                                                                                                              IDS1/2 (A)1/2
where  denotes the field-effect mobility, L and W are the                          1E-7

channel length and width, respectively, Ci denotes the                     IDS (A)
                                                                                                                                                     0.0008

insulator capacitance per unit area, and VT refers to the
                                                                                  1E-8
threshold voltage. The VT of the device was determined                                                                                               0.0004

according to the plot of IDS 1=2 and VGS by extrapolating the
measured data to IDS ¼ 0. When VGS was swept from +60                             1E-9
                                                                                                                                                     0.0000

to À60 V and VDS was set at À50 V,  and ION =IOFF of                                       -60           -40          -20        0            20

the OTFT were 0.116 cm2 VÀ1 sÀ1 and 3:5 Â 103 , respec-                                                              VGS (V)
tively, with mr-UVCur06 as a gate insulator but unpatterned                                                            (b)
by UV-light exposure. However, after patterning the gate
insulator,  and ION =IOFF of the OTFT became 0.08                         Fig. 5. (Color online) (a) and (b) show the output (IDS versus VDS ) and
cm2 VÀ1 sÀ1 and 3:8 Â 102 , respectively, which are worse                  transfer (IDS versus VGS ) characteristic curves of OTFTs with a patterned
than those of the unpatterned one. This phenomenon                         and unpatterned gate insulator, mr-UVCur06, respectively.
suggests that developing the UV-patternable polymer, mr-
UVCur06, by acetone leaves organic contaminants on the
patterned gate insulator surface during photopatterning.                   The optimal on/off ratio of OTFT appears for the patterned
   An attempt was made to prevent the organic contaminants                 gate insulator with UV/ozone post-treatment for 60 s, which
from degrading the performance of OTFTs, which was                         is attributed to the lowest off-state current among them. This
caused by development by acetone during photopatterning.                   finding suggests that the organic contaminants were removed
UV/ozone post-treatment was subsequently applied to the                    by the UV/ozone post-treatment for 60 s. The obvious shift
patterned gate insulator. Figures 6(a) and 6(b) show the                   of the threshold voltage from negative to positive took place
output (IDS versus VDS ) and transfer (IDS versus VGS )                    in these devices with UV/ozone post-treatment. According
characteristic curves, respectively, of the OTFTs with a                   to a previous study, UV light increases the negative fixed
patterned gate insulator post-treated by UV/ozone for 0,                   charge at the interface of the pentacene/patterned gate
30, 60, and 180 s. At a VGS value of À60 V, the saturated                  insulator.18)
currents of OTFTs with a patterned gate insulator treated by                  Moreover, the effect of UV/ozone post-treatment on the
UV/ozone for 30 and 60 s, respectively, were 27 and 35 A.                 patterned gate insulator was confirmed using the Fourier
The saturated currents of OTFTs with a patterned gate                      transform infrared (FT-IR) spectrometer (Digilab FTS
insulator increase with the exposure time of UV/ozone. The                 3000MX). Figure 7 shows the FT-IR absorption spectra.
saturated currents of OTFTs with a patterned gate insulator                The bands can be attributed to the following: the bands
treated by UV/ozone for 120 s is almost the same as that of                appearing at 1500 and 1710 cmÀ1 are attributed to the carbon
the device treated for 60 s. (This result is not shown in this             (C–C) groups and RCOOH groups, respectively. According
work.) However, the saturated current was only 12 A for                   to Fig. 7, the C–C peak intensity almost does not change,
the device with a patterned gate insulator treated by UV/                  and the intensity of the RCOOH peak decreases with an
ozone for 180 s. Figure 6(b) shows the transfer character-                 increase in the UV/ozone post-treatment time. However, the
istics of the IDS –VGS and the square root of IDS –VGS curve at            RCOOH peak intensity increases again when the UV/ozone
a fixed VDS of À50 V. The , ION =IOFF , and VT of the OTFT                 post-treatment time reaches 180 s. The variations of the
with a patterned gate insulator treated by UV/ozone for 30 s               RCOOH peak intensity suggest that organic contaminants
were 0.234 cm2 VÀ1 sÀ1 , 1:08 Â 103 , and 21 V; those of the               can be removed by a treatment time of up to 60 s, followed
device treated for 60 s were 0.34 cm2 VÀ1 sÀ1 , 5:5 Â 104 , and            by further exposure to UV/ozone and possibly the genera-
2.5 V; and in addition, those of the device treated for 180 s              tion of dissociation on the surface of the patterned gate
were 0.09 cm2 VÀ1 sÀ1 , 5:3 Â 103 , and 16 V, respectively.                insulator. UV irradiation on organic polymers for an excess
                                                                   04DK23-3                                     # 2011 The Japan Society of Applied Physics

                                                                                                                                                     SS10213
Jpn. J. Appl. Phys. 50 (2011) 04DK23                                                                                                                                            C.-M. Wu et al.


                                                                                                                   -50
        (a)                            VGS= 0 V          without UV/Ozone post-treament
                              -8                                                                                              VGS = 0 V     with UV/Ozone post-treament 30 s
                                       VGS = -10 V
                                                                                                                              VGS = -10 V
                                       VGS = -20 V                                                                 -40
                                                                                                                              VGS = -20 V
                              -6       VGS = -30 V
                                                                                                                              VGS = -30 V




                                                                                                     IDS ( μA)
                                       VGS = -40 V                                                                 -30
                 IDS ( μA)

                                                                                                                              VGS = -40 V
                              -4       VGS = -50 V
                                                                                                                              VGS = -50 V
                                       VGS = -60 V                                                                 -20        VGS = -60 V
                              -2
                                                                                                                   -10

                              0
                                                                                                                    0
                                   0           -20            -40         -60         -80                                0           -20         -40                      -60    -80
                                                             VDS (V)                                                                           VDS (V)
                             -50                                                                               -50
                                        VGS = 0 V       with UV/Ozone post-treament 60 s                                     VGS = 0 V with UV/Ozone post-treament 180 s
                                        VGS = -10 V                                                                          VGS = -10 V
                             -40                                                                               -40
                                        VGS = -20 V                                                                          VGS = -20 V
                                        VGS = -30 V                                                                          VGS = -30 V
                             -30                                                                               -30



                                                                                                   IDS ( μA)
              IDS (μA)




                                        VGS = -40 V                                                                          VGS = -40 V
                                        VGS = -50 V                                                                          VGS = -50 V
                             -20        VGS = -60 V                                                            -20
                                                                                                                             VGS = -60 V

                             -10                                                                               -10


                              0                                                                                     0
                                   0              -20         -40         -60         -80                                0           -20         -40                      -60    -80
                                                           VDS (V)                                                                             VDS (V)
                                                                                                                          0.005
        (b)                                                                             without UV/Ozone post-treament
                                                      1E-4
                                                                                        with UV/Ozone post-treament 30 s
                                                                                        with UV/Ozone post-treament 60 s
                                                      1E-5                              with UV/Ozone post-treament 180 s
                                                                                                                          0.004


                                                      1E-6
                                                                                                                                                 0.003
                                                                                                                                                         I DS1/2 (A1/2)



                                                      1E-7
                                           IDS (A)




                                                                                                                                                 0.002
                                                      1E-8

                                                      1E-9                                                                                       0.001


                                                     1E-10
                                                                                                                                                 0.000
                                                     1E-11
                                                                    -60         -40         -20                0         20           40
                                                                                             VGS (V)

Fig. 6. (Color online) (a) and (b) show the output (IDS versus VDS ) and transfer (IDS versus VGS ) characteristic curves of OTFTs with a patterned gate
insulator, which are post-treated by UV/ozone for 0, 30, 60, and 180 s.



time leads to dissociative excitation of C–C or C–H bonds in
the near-surface layer.19) Another study has also demon-                                          4. Conclusion
strated that the radiative excitation of polymethyl meth-                                         In this work, we have successfully fabricated and character-
acrylate films in air results in the formation of –COH as well                                     ized OTFTs using a UV-patternable polymer material, mr-
as –COOH functional end groups, which have been                                                   UVCur06, as the gate insulator. In addition to its solution-
identified recently as an electron trap by Torikai et al. and                                      processable capability, mr-UVCur06 is directly patternable
Chua et al.20,21)                                                                                 by UV light in a low-temperature process. By using mr-
                                                                                      04DK23-4                                      # 2011 The Japan Society of Applied Physics

                                                                                                                                                                                       SS10213
Jpn. J. Appl. Phys. 50 (2011) 04DK23                                                                                                                             C.-M. Wu et al.


                                                              3                                    this research under Contract Nos. NSC98-2218-E-214-001




                                                                     Absorbance (arb. unit)
                                                                                                   and 98-2221-E-214-003-MY3. The authors would also like
                                                              2
                                                                                                   to thank the MANALAB at ISU, Taiwan.


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                                                                                              04DK23-5                         # 2011 The Japan Society of Applied Physics

                                                                                                                                                                          SS10213