Catalyst-Free [2,3]-Sigmatropic Rearrangement Reactions of Photochemically Generated Ammonium Ylides

Article abstract of DOI:10.1055/s-0037-1610732

The rearrangement reaction of ammonium ylides furnishes valuable α,α-disubstituted amino esters. In this work, we describe the visible-light photolysis reaction of aryldiazoacetates in the presence of tertiary amines that react via a free ammonium ylide i

Full text of DOI:10.1055/s-0037-1610732

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 7, JULY 2019
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Oxide Electronics Transferred on Stiff-Stripe/
PDMS Substrate for High-Resolution
Stretchable Displays
Xiuling Li, Md. Mehedi Hasan , Hyo-Min Kim, and Jin Jang , Member, IEEE
Abstract— We report flexible oxide semiconductor-based materials, the mass production of these devices and electronics
electronics transferred on polydimethylsiloxane (PDMS)
elastomer substrate in the form of stiff stripes for stretch-
able displays. Stiff stripes with optimized geometries are
placed between PDMS substrate and oxide electronics
to provide strong adhesion and mechanical protection.
needs further study [10]. Current displays generally employ
inorganic materials such as amorphous oxide semiconductors
(AOSs) or silicon-based alternatives, which are brittle with
fracture strains of less than 2% [11], [12]. This needs the
Amorphous indium–gallium–zinc oxide (a-IGZO) thin-film implementation of strain relieves through mechanical designs
transistors (TFTs) and gate driver circuits on polyimide sub-
strate are cut into identical stripe shape and transferredonto
the stiff region. The turn-on voltage (VON) and mobility of the
fabricated a-IGZO TFTs vary within 0.4 V and 7.4%, respec-
tively, after 50% stretching and relaxation for 1000 cycles.
and maintaining high compatibility with mature display fabri-
cation processes.
Mechanical structures such as wrinkling/buckling and stiff
patches have been widely explored to dissipate the induced
The circuits on the stiff-stripe/elastomer substrate can be strain during large deformation. The wrinkled/buckled struc-
repeatedly stretched to 50% without mechanical and elec-
trical degradations. The results can be used for practi-
cal stretchable displays with high resolution over a large
area.
tures formed by the relaxation of prestretched substrates
after complete device fabrication have been widely employed
in numerous stretchable sensors and energy devices [13].
However, such structures introduce large tensile/compressive
strains through micrometer-scale bending radii, associated
with complex and nonlinear forming physics [14]–[16].
In addition, corrugation on the surface is less desirable for
displays [17]. Stiff patches (Young’s modulus E in GPa range)
provide mechanical protections on flat surfaces when the
elastomer substrate is macroscopically stretched. The brittle
inorganic components with high electrical performance can
be stretched to ꢀ10% [7]. The use of stiff patches, therefore,
has been considered as one of the most promising methods to
achieve stretchable displays.
Numerous studies have alternatively reported on stiff
patches either embedded in, directly deposited, or trans-
ferred on the elastomer substrate. Materials forming the stiff
region vary from polyimide (PI, EPI = 2.54 GPa) [18]–[21]
polyethylene terephthalate (PET, EPET = 2–2.7 GPa) [22],
diamond-like carbon (DLC, EDLC > 200 GPa) [23], and
SU-8 (ESU−8 = 4 GPa) [24] to complex multilayered struc-
Index Terms— Amorphous indium–gallium–zinc oxide
(a-IGZO) thin-film transistor (TFT), circuits, polydimethyl-
siloxane (PDMS), stiff stripes, stretchable.
I. INTRODUCTION
ISPLAY devices that offer mechanical flexibility and
stretchability can enable intimate integration into daily
D
life such as smart textiles and healthcare monitors, where
heavy and rigid origins are no longer qualified [1], [2].
Extremely bendable or foldable displays (bending radii in mil-
limeter scales) have been achieved using ultrathin substrates,
durable functioning layers, or tuning of the neutral mechanical
plane [3]–[6]. However, stretchability is more demanding than
flexibility. One of the key challenges is to tolerate much higher
levels of strains (typically >10%) without fracture or signifi-
cant degradation in electrical functionalities [7]–[9]. Although
there have been remarkable results on intrinsically stretchable
Manuscript received April 1, 2019; accepted May 15, 2019. Date of tures with mechanical gradients for optimizing adhesion and
publication June 5, 2019; date of current version June 19, 2019. This
strain distribution [7], [11], [23]. In recent works, electronic
devices are transferred onto stiff islands and are intercon-
work was supported in part by the Ministry of Trade, Industry and Energy
(MOTIE) under Grant 10052044 and in part by Korea Display Research
Corporation (KDRC) through the program for the Development of Future nected by stretchable conductors [17]–[25]. In addition to
Device Technology for the Display Industry. The review of this paper was
arranged by Editor X. Guo. (Corresponding author: Jin Jang.)
The authors are with the Department of Information Display, Advanced
avoid the thermal/chemical stability issues from direct fab-
rication on elastomers, transfer technique can employ high-
Display Research Center, Kyung Hee University, Seoul 130-701, South performance electrical devices with the effort of additional
Korea (e-mail: jjang@khu.ac.kr).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
transfer steps from conventional substrates [1], [17], [25].
However, this technique can be possible by transferring a
Digital Object Identifier 10.1109/TED.2019.2918160
single device or pixel onto each island-shaped stiff region.
0018-9383 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 7, JULY 2019
The yield of transfer processes becomes challenging for large-
area and high-pixel-density displays.
Here, we propose a concept involving the transfer of oxide
thin-film transistor (TFT)-based flexible electronics onto stiff
stripes on polydimethylsiloxane (PDMS) elastomer substrate.
In contrast to the previous reports, each stiff platform in a
stripe shape is proposed to support a small functional display
which can be integrated with high-performance TFT-based
gate driver and active pixels, and thus the transfer process is
possible with high fabrication yield. The stiff stripes, made of
commercially available adhesive tape, are cut into optimized
geometries and are placed between electronics and PDMS
substrate. The function of the stiff stripes includes providing
strong adhesion and mechanical protection of the brittle elec-
tronics. As a proof-of-concept study, oxide TFTs and circuits
(inverter and gate driver) on the flexible PI substrate are cut
into identical stripe shape and transferred onto the stiff region
to test stretchability and robustness. As the representative of
oxide TFTs, amorphous indium–gallium–zinc oxide (a-IGZO)
TFT was chosen herein since it has been considered the most
favorable device for display applications due to its relatively
high field-effect mobility (>10 cm2/V·s), excellent electrical
stability, superior scalability, and low-temperature process
[26]. The fabricated a-IGZO TFTs and gate driver circuits
are exposed to 50% repeated stretching without significant
degradation in mechanical and electrical properties.
Fig. 1. Preparation of stiff stripes on the PDMS substrate. (a) Schematic
and photograph showing the preparation of the stretchable PDMS
substrate. Inset: highly transparent and stretchable PDMS after being
peeled off from the glass carrier. (b) Transfer of stiff stripes onto the
PDMS substrate using the pick-and-place method. Stripe patterns are put
underneath PDMS to guide the transfer. (c) Stiff-stripe/PDMS substrate
after the complete process.
II. EXPERIMENT
used as a sacrificial layer to reduce the peel strength [3].
Fig. 1(b) shows the transfer of stripe-shaped adhesive tapes
onto the PDMS substrate using the pick-and-place method.
The stripes with identical width were transferred onto the
PDMS substrate with fixed spacing. Drawn stripe patterns
were placed underneath the PDMS substrate to guide for accu-
rate transfer. Fig. 1(c) shows the stiff-stripe/PDMS substrate
after the complete transfer process.
The a-IGZO TFTs and circuits were initially fabricated on a
flexible PI substrate. The stripe-shaped electronics membranes
were formed by using high-resolution laser cutting. Two-times
transfer process was performed to make the stretchable elec-
tronics. The stiff stripes were first transferred (first transfer)
onto PDMS elastomer substrate. Then, the flexible electronics
with identical stripe shape were transferred (second transfer)
onto the stiff region. The preparation process is described in
the following.
B. Fabrication of Stretchable a-IGZO TFTs
Fig. 2(a)–(d) shows the key steps to prepare a-IGZO TFTs
on the stiff-stripe/PDMS substrate. First, flexible a-IGZO
A. Preparation of Stiff Stripes on PDMS Substrate
The preparation of stiff stripes on a PDMS substrate was TFTs on the PI substrate were fabricated [Fig. 2(a)]. Second,
started with a double-sided adhesive tape (Kapton DuPont) the TFT membrane was cut into a stripe shape using a green
that was cut into 30-mm-long stripes with widths varying from laser [Fig. 2(b)]. Third, the stripe-shaped TFT membranes
1 to 3 mm. The thickness, peel adhesion, and E of the adhesive were subsequently transferred onto the stiff region on the
tape are ∼150 μm (25-μm-thick stiff layer sandwiched by PDMS substrate [Fig. 2(c) and (d)]. To maintain the high
∼60-μm-thick adhesive layers on both sides), 120 N/25 mm, yield of the transfer process, wrinkling- or curling-free oxide
and 2–2.5 GPa, respectively. Fig. 1 shows the schematic (left) TFTs on PI are of great importance. The flexible devices
and photographs (right) of the preparation process. PDMS were initially fabricated on PI/glass substrate and subsequently
was chosen as a stretchable substrate due to its excellent detached from the glass carrier. For the detachment, a very thin
stretchability, high transparency, and good compatibility with CNT/GO layer was deposited between PI and glass carrier
human skins [27]. PDMS solution was prepared by mixing in order to reduce the peel strength [3]. The process of
a silicone gel and a cross-linker with a weight ratio of 10:1 ∼1.5-μm-thick PI substrate was optimized. The gas barrier
(Sylgard 184; Dow Corning). After depositing a mixture of layer on the top of PI is a stack of alternate SiO2 and
carbon nanotubes (CNTs)/graphene oxide (GO) on a carrier SiNx layers (a total of five layers starting with SiO2) with
glass, the PDMS substrate was prepared by spin coating, a thickness of 25 nm each deposited by plasma-enhanced
thermally cured at 150 °C for 15 min, and then peeled off chemical vapor deposition (PECVD). The detailed fabrication
from the glass carrier [Fig. 1(a)]. The very thin CNT/GO was process of a-IGZO TFT was described in [3]. TFT employs

LI et al.: OXIDE ELECTRONICS TRANSFERRED ON STIFF-STRIPE/PDMS SUBSTRATE
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Fig. 2. Fabrication of stretchable a-IGZO TFTs. (a) Schematic of flexible a-IGZO TFTs on the PI substrate. The flexible devices were fabricated on
PI/glass substrate and subsequently detached from the glass carrier. TFTs employ a BCE configuration (right). (b) Schematic of laser cutting to form
stripe-shaped TFT membranes. (c) Transfer process of TFT stripes onto the prepared stiff-stripe/PDMS substrate. (d) Illustration of the stretchable
sample after the complete fabrication process. (e) Photograph of the fabricated flexible a-IGZO TFTs free of wrinkling and curling after detachment.
(f) Photograph of the green laser cutting system used to form stripe-shaped TFT membrane. (g) Optical image showing the laser cutting edge
(width <1 µm) remained on the TFT stripes. (h) Photograph of a-IGZO TFT membranes on stiff-stripe/PDMS substrate after the complete transfer
process. The nonstretchable stripe region covers an area of 2.5 mm × 30 mm, whereas the PDMS spacing is 1.0 mm × 30 mm.
−1
the conventional back-channel-etched (BCE) structure with a (d log (IDS)/dVGS
)
of the range 10 pA ≤ IDS ≤ 100 pA,
20-nm-thick a-IGZO channel layer [Fig. 2(a)]. The bottom with VDS = 0.1 V. Turn-on voltage (VON) was taken as the gate
gate and source/drain electrodes are 100-nm-thick Mo by sput- voltage (VGS) at which IDS starts to monotonically increase
tering. The gate insulator and the passivation layer are PECVD with VDS = 0.1 V.
SiO2 with a thickness of 250 and 300 nm, respectively.
III. RESULTS AND DISCUSSION
Fig. 2(e) shows a photograph of the flexible sample free of
wrinkling and curling after detachment. The flexible mem-
brane (a total thickness of ∼2 μm) was cut into a stripe
shape using a CO2 laser system [Fig. 2(f)]. The laser beam
(wavelength of 532 nm) can be positioned in tens of microm-
eters resolution offering fast, noncontact, and highly accurate
cutting. Fig. 2(g) shows an optical image of the laser cutting
edge on the TFT membrane. The width of the burning path
was less than 1 μm, indicating the capability of high resolution
by laser cutting. The stripe-shaped TFT membranes were
then transferred onto the stiff region on the PDMS substrate.
Fig. 2(h) shows a photograph of the stretchable oxide TFTs
after complete preparation steps.
A. Stretchability of Stiff-Stripe/PDMS Substrate
The thickness, optical transmittance in the visible range, and
stretchability of the fabricated PDMS substrate are ∼45 μm,
>95%, and ∼220% [Fig. 1(a) (inset)], respectively. The
stretchability of stiff-stripe/PDMS substrate was evaluated by
varying the width of stiff stripes. We define the stretch factor
α by W/S, where “W” and “S” represent the width of the
stiff stripe and soft spacing region [Fig. 3(a)], respectively.
W of the stiff stripe is varied from 1 to 3 mm, whereas S is
fixed at 1 mm. Fig. 3(b) shows a photograph of the prepared
sample with W = S = 1 mm (α of 1), under a relaxed state.
The stretched state of the sample is illustrated in Fig. 3(c). The
stretching strain (ε) is calculated from the total length change
of the entire substrate under relaxed and stretched states,
C. Measurement Set-Up and Characterization
TFTs were measured in dark and at room temperature i.e., the substrate that is initially 21-mm long is elongated
using an Agilent 4156C precision semiconductor parameter up to 31.5 mm, corresponding to elastic stretching of 50%.
analyzer. TFT gate driver circuits were characterized using Fig. 3(d) shows a photograph of the sample is stretched to 50%
an Agilent DSOX2012A oscilloscope with load resistance and using the stretch machine. The shape of the stripes remains
capacitance of 2 Mꢀ and 12 pF, respectively. TFTs and circuits unchanged, whereas the spacing region (PDMS) is elongated,
were stretched using a home-designed machine.
enabling the stretchability of the entire substrate. The array
For TFT characterization, the field-effect mobility (μFE
)
is assumed to be made of a series of n identical stripes and
was derived from transconductance gm = ∂ IDS/∂VGS, with n + 1 spacing. Since the rigid stripe region is not deformable
VDS
=
0.1 V. Subthreshold swing (SS) was taken as (Estripe = 2–2.5 GPa ꢀ EPDMS = 1.84 MPa), and that the

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 7, JULY 2019
is ∼115% from the average results of five tested samples
compared with the calculated value. The stretchability of the
substrate was further tested with varying α factors. Fig. 3(e)
shows stretchability of the stiff-stripe/PDMS substrate that
can be tuned from ∼115%, 75%, 60% to 48%, respectively,
by increasing W of stiff stripes from 1, 2, 2.5 to 3 mm, with
fixed S of 1 mm (α varies from 1 to 3). The experimen-
tally measured results are consistent with the calculated ones
using (3). Considering the stripes with small W require more
transfer times, while wider stripes limit the stretchability of the
entire substrate, W of the stripes was chosen to be 2.5 mm,
corresponding to α of 2.5, to guarantee the feasibility of
transfer and the overall stretchability.
B. Stretchable a-IGZO TFTs
The nonstretchable stripe containing functional electronics
covers an area of 2.5 mm × 30 mm, whereas the stretchable
spacing (PDMS) between the stripes is 1.0 mm × 30 mm
[Fig. 2(h)]. Electrical performance of the a-IGZO TFTs
was tracked during detachment, laser cut, and transfer
[Fig. 2(a)–(c)] onto the stiff-stripe/PDMS substrate. Negligible
changes were found in device performance. The channel
width (W) and length (L) are 20 and 11 μm, respectively.
Overall, 10 a-IGZO TFTs on the stiff-stripe/PDMS substrate
before stretching exhibit μFE of 17.2 1.8 cm2/V·s, VON of
0
0.5 V, and SS of 0.29
0.05 V/dec. The mobility
of the a-IGZO TFTs herein is comparable with the reported
oxide-based devices and is much higher than those of a-Si:H
and organic TFTs [3], [10]. In addition, the excellent electrical
stability, VON uniformity, and the mechanical flexibility of
a-IGZO TFTs have been proven to be suitable for display
backplane applications [26].
Fig. 3.
Stretchability of stiff-stripe/PDMS substrate. (a) Schematic
and (b) photograph of one sample with stripe width (W) of 1 mm and
spacing (S) of 1 mm. (c) Schematic and (d) photograph of the sample
being stretched using a home-designed stretch machine. W of the stripe
region remains unchanged, whereas the spacing regions are elongated,
enabling the stretchability of the entire substrate. (e) Stretchability of the
stiff-stripe/PDMS substrate as a function of W of stripes, given a fixed S
of 1 mm. The results are averaged from five tested samples.
The stretchability of the a-IGZO TFTs was evaluated under
various stretch conditions and cyclic tests. Fig. 4(a) and (b)
shows the photographs of the a-IGZO TFTs under relaxed
(0%) and 50% stretching conditions, respectively. The two
ends of the sample are fixed in the stretch machine in such a
way that the elongation can be performed perpendicular to the
length direction of the stripes. Fig. 4(c) shows the evolution
of the TFT transfer characteristics being exposed to various
levels of strain from 0% (relaxed state), increased up to 50%,
and return back to 0% (rerelaxed state). The corresponding
evolutions of the extracted parameters are summarized in
Table I. Changes in VON, SS, and μFE are 0.4 V, 0.09 V/dec,
and <3.4%, respectively, during the stretching and relaxation
procedures. No significant changes in TFT performance are
found though the substrate is elongated by 1.5 times.
soft spacing (PDMS) region is elongated to S + ꢁS during
stretching [23]. The stretching strain ε over the entire substrate
can be described as
[nW + (n + 1)(S + ꢁS)] − [nW + (n + 1)S]
ε =
nW + (n + 1)S
ꢁS
ꢁS
∼
=
ꢀ
ꢁ
=
(1)
1
n+1
W + S
1 −
W + S
when n ꢀ 1
The strain in the spacing region (ε_spacing) is
ꢂ
ꢃ
It has been reported that the mechanical strain experienced
by oxide TFTs should stay well below ≈2% in order to
guarantee the electrical functionality and mechanical stabil-
ity [7], [12]. a-IGZO TFTs herein are placed on stiff stripes
transferred on the stretchable PDMS substrate. The electri-
cal and mechanical stability during stretching is supposed
ꢁS
W
S
ε_spacing
=
= ε 1 +
= ε(1 + α)
(2)
(3)
S
and
ε_spacing
ε =
.
1 + α
Given that α
=
W/S
=
1 and maximum ε_spacing to origin from the mechanical protection by using the stiff
(i.e., the stretchability of bare PDMS substrate) is ∼220%, stripes. Romeo and Lacour [24] previously reported that
the applied ε on the entire stiff-stripe/PDMS substrate can be the strain experienced by a-IGZO TFTs placed on SU-8
calculated as ∼110%. Experimentally measured stretchability (ESU−8 = 4 GPa) platform was close to 0% when the PDMS

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TABLE I
EVOLUTION OF TFT PERFORMANCE: TURN-ON VOLTAGE (V ),
ON
ON-CURRENT (I
), OFF-CURRENT (I
), ON-TO-OFF RATIO
OFF
ON
), SS, AND FILED-EFFECT MOBILITY (µ ) BEFORE
(I /I
ON OFF
FE
STRETCHING (RELAXED), BEING STRETCHED AT DIFFERENT
LEVELS OF STRAIN UP TO 50%, RETURN BACK TO RELAXED
(RERELAXED) STATE, AND UNDER REPEATED
STRETCHING TESTS AT 50% STRAIN
This appears to be due to the small strain in the stiff region
and the strong adhesion property of the tape [22], [28]. Note
that the stiff stripes used for mechanical protections herein are
low-cost and easily scalable for large-area applications.
Repeated stretching and relaxation were performed to test
the robustness of stretchable a-IGZO TFTs. Fig. 4(d) demon-
strates the evolution of the TFT transfer curves with varying
Fig. 4. Stretchable a-IGZO TFTs. Photographs of a-IGZO TFTs on stiff-
stripe/PDMS substrate under (a) relaxed (0%) and (b) 50% stretching
conditions using the stretch machine. Insets: optical images of a TFT
taken under relaxed and 50% stretching states, respectively. (c) Evolution
of the TFT transfer curves as a function of stretching strain. (d) Evolution
of the transfer curves with the variation of repeated cycles of 50%
stretching. (e) Turn-on voltage (V ) shift and (f) change in normalized
ON
field-effect mobility (µ ) of 10 TFTs measured after 50% stretching for
FE
stretch cycles. Changes in VON of
0.3 V, SS of
0.11 V,
1000 repeated cycles. The parameter distributions exhibit that V shifts
ON
1.2 cm2/V·s are found after 1000 stretching
within 0.4 V and µ varies by < 7.4%. TFTs are with a channel width
and μFE of
FE
of 20 µm and length of 11 µm.
cycles (Table I). Ten TFTs with similar initial performances
were tested and found fully functional after the repeated
stretch (50% strain, 1000 cycles). Fig. 4(e) and (f) shows the
variations in VON and normalized mobility of the 10 TFTs
are within 0.4 V and 7.4%, respectively. This once again
confirms the mechanical protection and strong adhesion by
using the stiff stripes. The TFTs currently can be repeatedly
stretched to 50% along one direction. An array of stiff-stripes
with soft PDMS spacing in both directions would enable the
stretchability in the transverse direction. The TFTs can be
stable and stretched to a maximum strain of ∼70% before
permanent disconnections occur in the soft spacing close to
the edge of the rigid stripe, where a peak strain is located from
FEM analysis and strain measurement [11], [24]. To further
improve the stretchability, stretchable substrate with lower
Young’s modulus [29], [30], larger soft spacing, and gradients
stiffness of the rigid platforms [11] would be considered.
substrate was stretched up to 20%. Na et al. [21] fabricated
a-IGZO TFTs on 6-μm-thick patterned PI (EPI = 2.54 GPa)
islands on the PDMS substrate and maintained stable opera-
tion at 50% stretching. Hong et al. [22] analyzed the strain
distribution using finite-element method (FEM) and found
that the inorganic TFTs and LEDs located on PET (EPET
=
2–2.7 GPa) islands experienced negligible strains when the
entire substrate was stretched up to 30%. Their fabricated
temperature sensor remained as stable functions when being
stretched to 50% [22]. The stripe region (Estripe = 2–2.5 GPa)
herein is >1000 times stiffer than the PDMS spacing region
(EPDMS = 1.84 MPa). The optical images shown in Fig. 4(a)
and (b) (insets) reveal that there are no changes in the TFT
under 50% stretching compared to that under relaxed states.
Therefore, the stretching of the entire substrate mainly results
in the elongation of the spacing region rather than in the
stiff stripes. The strain distribution on the stiff region is well
below the critical value, thus maintaining the stability of oxide
TFTs by experiencing minimum strains during mechanical
C. Stretchable a-IGZO TFT-Based Circuits
The a-IGZO TFT-based inverter and gate driver circuits
stretching. In addition, no crack or delamination was found were fabricated and characterized on the stiff-stripe/PDMS
from microscope images after the repeated stretching test. substrate [Fig. 2(h)]. Fig. 5(a) shows the circuit diagram (left)

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 66, NO. 7, JULY 2019
Fig. 5. Stretchable a-IGZO TFT-based circuits. (a) Circuit schematic (left) and optical image (right) of the fabricated inverter. (b) VTCs of the inverter
for various V ’s. (c) Block diagram of a single stage (top left), timing diagram (bottom left), and schematic (right) of a gate driver circuit. (d) Optical
DD
image of the fabricated gate driver circuits with 30-µm pitch. (e) Input signal (black) and the output waveform (blue) from the last stage of the
fabricated gate driver circuit before the stretch test. (f) Output waveform (red) of the last stage after the repeated stretch test. The supply voltage
was 15 V. Negligible changes are found after 50% stretching for 1000 cycles.
and optical image (right) of an enhancement-load-type inverter. design and the operation of the gate driver can be found
The load and driving TFTs, respectively, have W of 60 and in our previous report [32]. L of all TFTs was fixed at
480 μm, whereas L is fixed at 10 μm. This gives an ideal gain 4 μm, whereas W values were optimized through SmartSpice
of 2.83 calculated from the square root of β ratio, the ratio of simulation.
channel W of driving to load TFTs [31]. The initial voltage
Fig. 5(d) shows an optical image of the fabricated gate driver
transfer characteristics (VTCs) of the inverter are indicated circuits. A single stage occupies an area of 30 μm × 720 μm
by black circular dots in Fig. 5(b). Good level conversions which makes it suitable for high-resolution and narrow-bezel
are obtained at various supply voltages (VDD’s). The gains displays. Fig. 5(e) shows the input and output waveform of
calculated from the slopes of VTCs are 2.20, 2.26, 2.32, the gate driver circuit as-transferred on the stiff-stripe/PDMS
respectively, for the VDD’s of 5, 7.5, and 10 V, which are substrate (relaxed state). The input signal has an amplitude
comparable to the ideal gain value. VTCs after 1000 stretch- of 15 V and a pulsewidth of 20 μs. The high output volt-
ing cycles (50% strain) are plotted together as shown in age (VH), rise time (Tr), and fall time (Tf ) are, respectively,
Fig. 5(b)(red solid lines). No significant changes are found ∼14.7 V, 0.62 μs, and 0.46 μs, which are comparable with
compared to that of the initial performance. The static gains the reported results in the literature [32]. Note that the gate
are 2.23, 2.28, and 2.35, respectively, with increasing VDD’s. driver circuits herein were fabricated on ultrathin (1.5 μm)
Fig. 5(c) shows the schematic circuit design (left top), PI substrate, and measured on the stiff-stripe/PDMS substrate.
timing diagram (left bottom), and block diagram (right) of Fig. 5(f) shows the circuit performance after 1000 stretching
a 16-stage gate driver based on a-IGZO TFTs. The circuit cycles (50% strain). No significant degradations are found
design employs nonoverlapping four-phase clock signals from the last stage output waveforms, giving the VH , Tr, and
(CLK1–CLK4) with a 25% duty cycle. A single stage of the Tf of 14.6 V, 0.78 μs, and 0.67 μs, respectively. No cracks are
gate driver consists of nine TFTs (T1–T9) and one capaci- found in the TFTs and metal lines after repeated test through
tor (C); Each stage of the circuits provides two separate output the microscopes. These results indicate that all 144 TFTs and
nodes, one for the next stage (“n” node) and the other one metal lines in the fabricated gate driver circuits are func-
(“Output” node) for driving the display panel. This design can tioning well during the repeated mechanical stretching. This
improve the yield of fabricated gate drivers by reducing the proves the feasibility of employing the high-performance oxide
process influences, such as contaminants or particles. Detailed TFT-based gate drivers for stretchable display applications.

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TABLE II
COMPARISON OF THIS PAPER WITH THE STATE-OF-THE-ART HYBRID STRETCHABLE TFTS USING STIFF PLATFORMS
Compared with the state-of-the-art hybrid stretchable TFTs on stretchable PDMS substrate. Commercially available adhe-
using stiff platforms (Table II), this approach employs com- sive and rigid tapes are employed for strong adhesion and
mercially available adhesive tapes that provide excellent mechanical protection of brittle electronics. The fabricated
adhesion and mechanical protections of brittle electronics a-IGZO TFTs and gate driver circuits can be repeatedly
following a simple transfer process. The design of stiff stripes stretched up to 50% without delamination and significant
is suitable for the integration of complicated electronic sys- performance degradation. This approach is low cost, easily
tems. Especially the number of external control lines can be scalable and reproducible and thus can be used for achieving
significantly reduced, whereas the flexibility and yield can be stretchable displays by taking advantage of the mature fabri-
further improved by the integration of TFT-based gate driver cation process and high-performance oxide devices.
circuits. This approach is practical by taking advantage of
the mature fabrication process and high-performance oxide
devices. A combination of high-performance integrated dis-
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