JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012
35
Effect of Self-Assembled Monolayer (SAM) on the
Oxide Semiconductor Thin Film Transistor
Seung-Hwan Cho, Yong-Uk Lee, Jeong-Soo Lee, Kang-Moon Jo, Bo Sung Kim, Hyang-Shik Kong,
Jang-Yeon Kwon, and Min-Koo Han
Abstract—In this paper, we proposed the self-assembled monolayer (SAM) as a protection layer against plasma and chemically
induced damages to the back interface of an oxide semiconductor during the deposition of the passivation layer. When a
thin-film transistor (TFT) is passivated with plasma-enhanced
chemical-vapor deposition (PECVD) SiO and solution-based
materials, the back interface of the oxide semiconductor could be
exposed to plasma and chemically induced damages, respectively.
We employed SAMs on the back surface of the oxide semiconductor prior to the passivation process to suppress such damage.
The hydrophobic Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation in mobility and subthreshold slope (SS)
due to ion bombardment during plasma treatment. The hydrophobic CH3 -SAM (octyltriethoxysilane) successfully blocked
chemically induced damage due to solution-based passivation.
Index
Terms—Back
interface,
oxide
semiconductor,
self-assembled monolayer (SAM), solution-based passivation.
I. INTRODUCTION
O
XIDE SEMICONDUCTOR thin-film transistors (TFTs)
have attracted considerable attention for large size
displays, because the oxide semiconductors, such as IGZO
[1]–[3] and ZTO [4]–[6], exhibit high carrier mobility, even in
an amorphous state and have good uniformity. However, oxide
semiconductor TFTs are very sensitive to environments such
as moisture and oxygen. It has been reported that the gaseous
molecules are strongly associated with characteristics of oxide
transistors [7]–[9]. The oxide semiconductor should be passivated with a dense passivation material, such as SiO to block
these molecules. SiO deposited by plasma-enhanced chemical-vapor deposition (PECVD) is widely used in passivation
materials of oxide semiconductors. In addition, solution-based
Manuscript received April 29, 2011; revised August 17, 2011; accepted
September 19, 2011. Date of publication November 14, 2011; date of current
version January 04, 2012.
S.-H. Cho is with the Department of Electrical Engineering and Computer
Science, Seoul National University, Seoul 151-744, Korea, and also with the
R&D Center, LCD department, Samsung Electronics, Gyeonggi-do 446-577,
Korea. (e-mail:[email protected]).
Y.-U. Lee, J.-S. Lee, and M.-K. Han are with the Department of Electrical
Engineering and Computer Science, Seoul National University, Seoul 151-744,
Korea (e-mail: [email protected]; [email protected]; [email protected].
kr).
K.-M. Jo, B. S. Kim, and H.-S. Kong are with the R&D Center, LCD department, Samsung Electronics, Gyeonggi-do 446-577, Korea. (e-mail:[email protected]; [email protected]; [email protected]).
J.-Y. Kwon is with the School of Integrated Technology, Yonsei University,
Incheon 406-840, Korea (e-mail: [email protected]).
Color versions of one or more of the figures are available online at http://
ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JDT.2011.2169936
passivation materials are under development for the printing
process. However, TFT characteristics can be altered due to
plasma damage and chemically induced damage caused by
the passivation mentioned above. The oxide semiconductor is
exposed to various plasma, including hydrogen, nitrogen, and
oxygen radicals, during SiO passivation by PECVD. It is well
known that plasma can degrade TFT characteristics due to ion
bombardment. The IGZO film becomes highly conductive by
the reduction of oxygen due to the incorporation of hydrogen
into the oxide semiconductor [10]. In addition, when the oxide
semiconductor is exposed to organic solvents, the adsorbed
solvent molecules with high polarity on the back interface of
the oxide semiconductor induce carriers in the back surface of
the channel and thus threshold voltage is shifted to the negative
direction [11]. Therefore, the passivation on the back interface
of the oxide semiconductor should be considered to protect
the back channel from plasma damage and chemically induced
damage because the reliability of the TFT is critical to the back
channel.
The purpose of the paper is to report that SAM successfully
suppresses physical and chemical damages on the back interface of the oxide semiconductor as a protection layer. We have
deposited SAM on the back surface of the oxide semiconductor
prior to passivation. SAMs are easily formed by the chemisorption of hydrophilic head groups on a substrate packed closely to
each other by van der Waals interactions [12]. SAM can be applied widely in a passivation layer, since a close-packed SAM is
resistant to chemical and physical damages. In this paper, we investigated SAM as a protection layer against physical and chemical damages caused by passivation on the back interface of an
oxide semiconductor.
II. EXPERIMMENTAL
We fabricated bottom gate a-GIZO TFTs, as shown in Fig. 1.
200 nm Mo metal is deposited on a glass substrate as a gate
electrode. SiO (300 nm) was deposited by PECVD at 370 C as
a gate insulator. A 40 nm ITO metal is used as drain and source
electrodes. The a-IGZO active layer was formed by sputtering.
Each layer is patterned by photolithography and etching. Then,
we deposited two kinds of functionalized SAMs (3-chloropropyltriethoxysilane (Cl-), octyltriethoxysilane (CH -)) on the
oxide TFT by the solution method prior to passivation [13].
We employed various widely used materials, such as PMMA,
SiO , and SiN , as passivation layers. PMMA is spin-coated
by the solution method and annealed at 160 C for 20 min SiO
with N O treatment is deposited at 150 C and SiN without
1551-319X/$26.00 © 2011 IEEE
36
JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012
Fig. 1. Cross section of fabricated a- IGZO TFT.
Fig. 2. V
(10 pA) shift of TFT with various SAMs before and after SAM
V
(10
pA
pA (after SAM treatment) – V
treatment ( V
pA) (before SAM treatment).
1
(10 ) =
(10 )
pretreatment is deposited at 180 C by PECVD, respectively,
to 2000 Å film thickness.
III. RESULTS AND DISCUSSION
A. Transfer Characteristics of a-IGZO TFT With SAM
Treatment
After SAM treatment, we observed the different turn-on
) (
at
pA) shift of oxide TFTs
voltage (
with two different SAM treatments, as shown in Fig. 2
(10 pA) (after SAM treatment) –
(10 pA)
(
in Cl-SAM treated TFT was
(before SAM treatment)). The
shifted to the negative direction, while that for in CH -SAM
treated TFT varied in the range of
V to 0.4 V.
This phenomenon is due to SAM having a dipole moment according to a functional group [14]–[17]. The dipole moment is
negative in the Cl-functional group [16], while it is positive in
the CH -functional group [17] (in this paper, the positive direction in the dipole moment is same with that of electric field toward the back interface of the oxide semiconductor). Thus, each
SAM can generate the built-in local electric field that accumulates or depletes electrons on the oxide semiconductor. Cl-SAM
with negative dipole moment creates a local electric field normal
to the semiconductor [16]. This electric field accumulates electrons near the back interface of the oxide semiconductor. Therefore, Cl-SAM with a negative dipole moment forms a high conductive back channel which requires more negative gate bias to
deplete the active channel layer. In contrast, CH -SAM with a
positive dipole moment generates the opposite build-in electric
field to Cl-SAM with a negative dipole moment [16]. The magshift in CH -SAM treated-TFT was relatively
nitude of the
small compared to Cl-SAM treated-TFT. In addition, the direcshift did not indicate any significant meaning, as
tion of the
Fig. 3. Transfer characteristics of a-IGZO TFT as a function of applied gate
bias stress time for the (a) TFT(A) with Cl-SAM. (b) TFT(B) without SAM.
shown in Fig. 2. It may be attributed to the fact that the number
of holes are much less than that of electrons in n-type oxide
semiconductor such as IGZO and the built-in electric field due
to CH -SAM is not large enough to induce carriers [17]. In that
case, the electric field due to CH -SAM with a positive dipole
moment could not sufficiently induce holes near the back interface of the oxide semiconductor. Therefore, the small number
of induced holes near the back interface can scarcely bend the
band. On the other hand, the electric field due to Cl-SAM with a
negative dipole moment sufficiently accumulates electrons near
the back channel and thus can easily bend the band. This agrees
shift in TFT with CH -SAM treatment
fairly well with the
being negligible.
In addition, we measured the bias stability of a-IGZO TFTs
to investigate SAM as a blocking layer from moisture. The experiments were performed at air condition (40% relative hushift in both TFT
midity). We observed similar positive
(A) with Cl-SAM and TFT (B) without SAM under positive
V,
V, from 0 s to
gate bias stress (
1000 s) (the result is not shown here). In contrast, the virgin
TFT (B) (without SAM treatment) exhibits large negative
shift, including a change of subthreshold slope (SS) compared to
TFT (A) with Cl-SAM treatment under negative gate bias stress
V,
V, from 0 s to 1000 s), as shown
(
in Fig. 3. It was previously reported that threshold voltage is
CHO et al.: EFFECT OF SAM ON OXIDE SEMICONDUCTOR TFT
Fig. 4. Schematic image of field-induced adsorbed H O molecules on the back
interface of oxide semiconductor. (a) virgin TFT. (b) TFT with SAM treatment.
37
Fig. 5. Change of mobility and subthreshold swing (SS) before and after SiOx
passivation (1SS = SS (after SAM treatment) – SS (before SAM treatment)).
TABLE I
ELECTRICAL CHARACTERISTICS OF OXIDE TFTS.
shifted in the negative direction and SS is degraded under negO molecules on
ative bias stress, because the field-induced
the back surface of the oxide semiconductor can act as electron
donors as well as acceptorlike deep traps [7]. However, the surface of the oxide semiconductor in Cl-SAM treated TFT is much
more hydrophobic than in virgin TFT due to the adsorbed SAM
on the back surface of the oxide semiconductor. Fig. 4 shows
the schematic image of field-induced adsorbed O molecules
on the back interface of oxide semiconductor in TFTs with and
shift
without SAM treatment, respectively. Larger negative
in virgin TFT was observed, since the number of field-induced
O molecules in SAM treated TFT is much fewer
adsorbed
than in the virgin TFT. Therefore, we expect the hydrophobic
Cl-SAM successfully blocks the adsorption of these field-induced O molecules.
B. Transfer Characteristics of a-IGZO With SiO or SiN
Passivation
First, we investigated SAM, as a protection layer that protects the active layer from plasma damage and hydrogen incorporation during the inorganic passivation process. After SAM
treatment, we employed the widely used SiO and SiN film
for mass production by the PECVD method on the oxide semiconductor. SiO with N O treatment was deposited at 150 C
and SiN without pretreatment was deposited at 180 C, respectively, to 2000 Å film thickness.
We observed the variation of mobility and subthreshold swing
before and after SiO deposition. Table I shows the change of
mobility and SS before and after SiO passivation. The mobility in SAM treated TFT decreased from 6.53 cm V s
to 5.24 cm V s, while that in virgin TFT decreased from
6.12 to 4.30cm V s as shown in Fig. 5. The change of SS in
Fig. 6. Change of mobility and subthreshold swing (SS) before and after the
CF plasma treatment (1SS = SS (after SAM treatment) – SS (before SAM
treatment)).
SAM treated TFT was 0.58 V/decade, while that in virgin TFT
was 1.1 V/decade. When the back interface of the oxide semiconductor is exposed to plasma, a damaged bond can be created
in the active layer by ion bombardment and it can act as a defect site [10]. Thus, mobility decreases due to damaged bond
and SS increases due to defect creation by ion bombardment
respectively during plasma treatment. In addition, the adsorbed
SAM, as a very thin organic layer on the oxide semiconductor,
is resistant to physical damage such as ion bombardment. We
treated CF plasma on TFTs after SAM deposition to further investigate the effect of plasma treatment on a-IGZO. This is quite
similar to the behavior of SiO -passivated TFT before and after
passivation, as shown in Fig. 6. Based on this fact, we can explain SAM may effectively suppress physical plasma damage,
such as ion bombardment and then the degradation in mobility
and SS of SAM-treated TFT is less than in virgin TFT.
In contrast, after SiN passivation, we observed all TFTs
1
A to
1.5
A at
V,
(from
V) were not modulated by gate voltage (always
on-state), even though they were treated with SAM. Similarly,
all TFTs became highly conductive after H plasma treatment
on a-IGZO (the result is not shown here). This is well known
as an incorporation of hydrogen provided from SiH and NH
into a-IGZO film [10]. This result indicates that the main cause
of high conductivity of a-IGZO during SiN passivation is the
oxygen reduction by hydrogen. SAM cannot effectively block
the incorporation of hydrogen into a-IGZO, in contrast to the
suppression of plasma damage, such as ion bombardment.
38
Fig. 7. Relationship between CA and V
PMMA deposition.
JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012
variation of TFT before and after
C. Transfer Characteristics of a-IGZO After Solution-Based
Passivation
The back interface of an oxide semiconductor is easily
exposed to various organic solvents, such as chlorobenzene,
hexane, and acetonitrile during solution-based passivation. In
this paper, we employed poly-methyl methacrylate (PMMA)
that is widely used in the solution-based passivation process.
PMMA was spin coated on TFTs and thermally annealed at
160 C for 20 min in air ambient. We measured the current
voltage characteristics of the TFTs under dark and air. After
shift was observed in all the
PMMA passivation, negative
TFTs and the magnitude of
shift in SAM-treated TFT
in CH -SAM treated
was much less than in virgin TFT.
TFT was shifted from 0.8 V to 6.6 V before and after pasin virgin TFT was shifted from 2.0 V to
sivation, while
12.6 V before and after passivation. This experimental result
indirectly indicates that the hydrophobic SAM on the surface of
an oxide semiconductor suppresses chemically induced change
by PMMA solution. The PMMA solvent is chlorobenzene. It
has been reported that gaseous molecules, including chlorobenzene, are absorbed or desorbed on the oxide semiconductor
and they change electric properties of the oxide semiconductor
TFT [7], [11]. They can easily accumulate electrons near the
back interface of the oxide semiconductor. Especially, in the
case of solvent molecules, there is a close relationship between
shift of TFT and dielectric constant of molecules [11].
When the adsorbed solvent molecules with higher dielectric
constant have higher polarity and higher electronegativity, they
induce carriers near the back interface of the oxide semiconductor compared to solvent molecules with lower dielectric
constant (lower polarity). It was suggested in the paper that
the adsorption of high-polar solvent molecules caused more
band bending near the back channel surface. Therefore, it was
considered that Chlorobenzene with high polarity, the solvent
shift in our
of PMMA solution, is the main cause for the
experiments. Chlorobenzene in the PMMA passivation layer
can remain after hardbake (160 C, 20 min).
We measured the contact angle (CA) of each TFT, since CA
represents the area density of deposited SAM on the surface, as
a criterion of hydrophobicity or hydrophilicity to confirm the
shift and SAM. If CA is larger, more
relationship between
of the surface is hydrophobic. The surface of SAM-treated TFT
Fig. 8. Relationship between CA and V
variation of TFT with negative bias
stress (at V
V, V
V for 1 h).
= 020
=1
with a large CA is more hydrophobic than is virgin TFT. We believe that SAM on TFT with higher CA is denser and more solid
than in the virgin TFT and blocks the adsorption of organic solvent, Chlorobenzene, considerably. Fig. 7 demonstrates the relationship between the CA of each TFT with various SAMs and
variations. TFT with higher CA caused
the corresponding
shift.
less negative
In addition, we investigated the stability of oxide semiconductor TFT under gate bias stress after PMMA formation, because the remaining solvent in PMMA is adsorbed due to the
. Fig. 8 shows
electric field. It can affect the behavior of
shifts for various TFTs with negative bias stress. Under
V,
V for 1 h),
negative bias stress (at
shift in SAM-treated TFT was less than in virgin TFT and
shift for each SAM-treated TFTs
there was a variation of
with different contact angle. This is quite similar to the
shift mechanism of the adsorption of chlorobenzene molecule
discussed above.
is shifted in a negative direction due
We observed that
to PMMA solvent and the remaining solvent in PMMA can affect characteristics of TFT, even though TFT was hardbaked
(160 C, 20 min) after PMMA formation. The chemically induced change can be suppressed by employing SAM on the back
surface of an oxide semiconductor.
IV. CONCLUSION
We investigated the effects of self-assembled monolayer
(SAM), as a protection layer of an oxide semiconductor
against plasma and chemically induced damages during the
deposition of the passivation layer. When TFT is passivated
with PECVD SiO and solution-based materials, plasma and
chemically induced damages on the back interface of the oxide
semiconductor cannot be avoided. However, the hydrophobic
Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation of TFT characteristics, such as mobility and SS, due
to damaged bonds and defect creation by ion bombardment
during plasma treatment. The hydrophobic CH -SAM (octyltriethoxysilane) blocked the adsorption of PMMA solvent
and thus suppressed chemically induced damage caused by
solution-based passivation. In addition, there is no additional
photolithographic process for SAM deposition and patterning.
Therefore, close-packed hydrophobic SAM can be a promising
buffer layer prior to passivation to achieve greater stability.
CHO et al.: EFFECT OF SAM ON OXIDE SEMICONDUCTOR TFT
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Seung-Hwan Cho received the B.S. degree in material science and engineering from Korea University,
Seoul, Korea, in 200, and is currently working toward
the M.S. degree in electrical engineering and computer science from Seoul National University, Seoul,
Korea.
He joined LCD department, Samsung Electronics,
Kyunggi-do, Korea, as an engineer in 2005. From
2005 to 2009, he had performed the researches
regarding the design and characterization of organic
TFTs and solution-based inorganic TFTs for LCD
display. His research interests are solution-based semiconductor and solution-based passivation.
39
Yong-Uk Lee received the B.S. and M.S. degrees
in electrical engineering and computer science from
Seoul National University, Seoul, Korea, in 2009
and 2011, respectively.
His current research interests are solution processed oxide thin-film transistors and driving circuits
for active matrix liquid crystal display (AMLCD)
and active matrix organic light-emitting diode
(AMOLED)
Jeong-Soo Lee received the B.S. degree in electrical
engineering from Yonsei University, Seoul, Korea, in
2008, and is currently working toward the Ph.D. degree with unified M.S. and Ph.D. degrees in electrical
engineering and computer science from Seoul National University, Seoul, Korea.
His current research interests are solution processed oxide thin-film transistors and driving circuits
for active matrix liquid crystal display (AMLCD)
and active matrix organic light-emitting diode
(AMOLED)
Kang-Moon Jo received the B.S. degrees in physics
from the Department of Physics, Konkuk University,
Seoul, Korea, in 2009.
He joined Samsung LCD R&D center in 2009. His
current research interests are physics of oxide semiconductor TFT and new display technology.
Bo Sung Kim received B.S. from Pusan National
University, M.S. from KAIST, and Ph.D. in chemistry from Seoul National University, in 1990, 1992,
and 2000, respectively.
He joined Samsung Electronics Co, Ltd. (SEC) in
2000. He has worked the advanced TFT process developments for high-end LCD applications with large
size, high resolution, and high aperture. Since 2006,
he is a principal engineer at LCD R&D center, SEC,
being in charge of the project of solution-processed
TFT with oxide semiconductors. His current research
interests are organic TFTs, printed electronics, and flexible electronics in display applications.
Hyang-Shik Kong received the B.S. degree in
physics from Seoul National University, Seoul,
Korea, in 1985 and the M.S. and Ph.D. degrees in
physics from Korea Advanced Institute Science and
Technology, Daejeon, Korea, in 1988 and 1996,
respectively.
He is a Vice President and a Leader of process
development Group with the LCD Research and
Development Center, SAMSUNG Electronics,
Gyeonggi-do, Korea, where he has been working
since 1991. His current research interests are
advanced TFT process developments, printed electronics, and printing technologies for active matrix liquid crystal display (AMLCD).
40
Jang-Yeon Kwon received the B.S. and M.S.
degrees in Metallugical Engineering from Seoul
National University, Seoul, Korea, in 1997 and 1999,
respectively. He received Ph.D. degree in material
science and engineering from Seoul National University in 2002.
He joined Samsung Advanced Institute of Technology (SAIT), Gyunggi-do, Korea, as a principal researcher, where he had performed the research regarding characteristics of Si and oxide semiconductor
thin film transistor. He is currently a professor with
the School of Integrated Technology, Yonsei University, Incheon, Korea, from
2011. His current interests include the novel oxide semiconductor materials and
devices for next generation active matrix display applications.
JOURNAL OF DISPLAY TECHNOLOGY, VOL. 8, NO. 1, JANUARY 2012
Min-Koo Han received the B.S. degree in electrical
engineering from Seoul National University, Seoul,
Korea, in 1971 and the Ph.D. degree in electrical engineering from The Johns Hopkins University, Baltimore, MD, in 1979.
From 1979 to 1984, he was an Assistant Professor
with the Department of Electrical and Computer
Engineering, State University of New York, Buffalo. Since 1984, he has been with Seoul National
University, Seoul, where he is currently a Professor
with the Department of Electrical Engineering. His
current research interests are amorphous and polycrystalline silicon, oxide
semiconductors, and power semiconductor devices.