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Scripta Materialia 77 (2014) 13–16
www.elsevier.com/locate/scriptamat
Laser lift-off transfer printing of patterned GaN light-emitting
diodes from sapphire to flexible substrates using a Cr/Au laser
blocking layer
Jaeyi Chun,a Youngkyu Hwang,b Yong-Seok Choi,b Jae-Joon Kim,a Tak Jeong,c
Jong Hyeob Baek,c Heung Cho Kob and Seong-Ju Parka,b,⇑
a
Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
c
LED Device Research Center, Korea Photonics Technology Institute, Gwangju 500-779, Republic of Korea
b
Received 20 December 2013; revised 3 January 2014; accepted 3 January 2014
Available online 9 January 2014
We develop a method to directly transfer the array of GaN-based light-emitting diodes (LEDs) from sapphire onto flexible
substrates by a laser lift-off (LLO) process. Cr/Au layers are employed as a laser blocking layer to protect the supporting polymer
layers from the laser beam and sharply separate the LEDs from the sapphire during the LLO process. This method dramatically
increases the transfer yield of patterned LEDs up to 95% by decreasing the laser-induced damage in the supporting polymer layers
and LEDs.
Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: GaN-based light-emitting diodes; Transfer printing; Laser lift-off; Flexible display
GaN-based light-emitting diodes (LEDs) have
been widely used for high-performance solid-state lighting systems due to their high internal and external quantum efficiencies, low power consumption and long-term
stability [1–3]. Recent research has demonstrated a
methodology to fabricate flexible LEDs by patterning
the GaN layer grown on a silicon or sapphire substrate
into printable formats and transferring the patterns onto
unconventional flexible polymer substrates [4–9]. This
technology enables the realization of next-generation
deformable displays and biomedical/implantable optoelectronic systems [10–12].
Maintaining the spatial alignment and orientation of
the GaN LED array is crucial in the transfer printing
process. In particular, high-quality GaN LEDs grown
epitaxially on a sapphire substrate require careful transfer onto flexible target substrates without misalignment
because the GaN LEDs should be repetitively transferred to and from dissimilar supporting substrates after
⇑ Corresponding
author at: School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju
500-712, Republic of Korea. Tel.: +82 62 715 2309; fax: +82 62 715
2304; e-mail: [email protected]
the array of GaN LEDs is separated from the sapphire
substrate by the laser lift-off (LLO) process [6–8]. However, direct transfer printing by adhering a flexible substrate onto the GaN LEDs and then removing the
sapphire substrate can provide an effective pathway to
reduce complex process steps and preserve the original
layout of the GaN LED array. Furthermore, direct
transfer printing could be developed into a cost-effective
mass production method for flexible devices with extremely small spacing or with a complicated layout. To directly transfer an array of GaN LEDs fabricated on
sapphire to a flexible substrate, polymer-based adhesives
should be used to adhere the GaN LEDs onto the flexible target substrate. However, employing a polymerbased supporting layer or adhesive introduces another
problem associated with the LLO process. The intense
laser beam penetrating between the GaN LED chips
could induce considerable stress on the polymer-based
layers, thereby generating unwanted damage, such as
cracks in the polymer-based supporting or adhesive layers. Therefore, a new strategy should be developed to
protect the polymer layers or glue from the intense laser
beam and ensure a high yield of transfer printing of
arrays of GaN LEDs.
1359-6462/$ - see front matter Ó 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.scriptamat.2014.01.005
14
J. Chun et al. / Scripta Materialia 77 (2014) 13–16
Here, we report on a method to directly transfer GaN
LEDs from sapphire substrates to flexible substrates
with polymer glue and supporting layers. We introduce
a laser blocking layer (LBL) to prevent unwanted penetration of the laser beam in the region between the GaN
LED chips and enhance the sharp separation of the
LEDs and supporting polymer layers from the sapphire
substrate. To demonstrate the efficacy of this method,
we show the successful transfer of 22 23 GaN LED arrays onto a flexible poly(ethylene terephthalate) (PET)
film with an adhesive of conductive epoxy and the electroluminescence (EL) emission from the GaN LEDs in
deformed configurations.
Figure 1 presents a schematic illustration of the fabrication of printable formats from a GaN LED layer
grown on a sapphire substrate and the transfer printing
of GaN LEDs onto a PET substrate by the LLO process. The GaN LED film, with a total thickness of
3.5 lm on the sapphire substrate consisted of n-GaN
(3.2 lm), five pairs of InGaN/GaN (3 nm/7 nm) multiple quantum wells (MQWs) and p-GaN (250 nm). The
process began with patterning an Ni masking layer on
the top of the GaN LED layer and etching the unprotected GaN region by inductively coupled plasma reactive ion etching to produce patterned LED pixels
(lateral size of each pixel: 300 lm 300 lm) (Fig. 1a).
A 1 lm thick SiO2 film deposited by plasma-enhanced
chemical vapor deposition was used to passivate the entire area of the GaN LEDs, including the sidewalls of
the GaN LED chips. After the patterning of photoresist
(PR) by conventional photolithography, etching of the
unprotected SiO2 region in a buffered oxide etchant
(BOE) generated the via-holes for metal contacts. Electron-beam (e-beam) evaporation of Ni/Au (5 nm/
10 nm) and lifting off of the residual PR followed by
annealing of the sample at 500 °C for 1 min provide pohmic contacts (Fig. 1b). Next, a Cr/Au (50 nm/
Figure 1. (a–f) Schematic illustration of the fabrication steps to
transfer GaN LEDs onto a PET film.
150 nm) layer was deposited as an LBL over the entire
area by e-beam evaporation (Fig. 1c). The LBL should
prevent the penetration of laser beams, thus protecting
the upper organic epoxy layers from unwanted laser
irradiation, and could be ablated by the excimer laser
during the LLO process. Next, the sidewalls of the
GaN LEDs were coated and patterned with SU-8 to
smooth and improve the adhesion of a glue layer
(Fig. 1d). After contacting a PET film coated with conductive epoxy (ITW Chemtronics, CW2400) to the sample for bonding, it was cured at 90 °C for 90 min
(Fig. 1e). The back side of the sapphire substrate was
then scanned with a KrF excimer laser beam (kmax:
248 nm, 750 mJ cm 2, beam spot: 400 lm 400 lm) to
separate the GaN LED patterns and transfer them onto
the PET substrate (Fig. 1f). Finally, the sample was
dipped in diluted HCl, Cr/Au etchants and BOE to remove any residual Ga, Cr, Au and SiO2 on the surface
of the separated GaN LED chips.
Figure 2 illustrates the advantage of using an LBL for
the direct transfer printing of patterned GaN LEDs on
the PET substrate. During the LLO process, irradiation
of the GaN LED region with the excimer laser induces
photothermal decomposition of GaN into Ga droplets
and gaseous N2, thus causing interfacial fracture between the patterned GaN LEDs and the sapphire substrate (Region A of Fig. 2a) [13–15]. On the other
hand, the intense laser beam is prevented from passing
through the GaN-free region between the GaN LED
chips (Region B of Fig. 2a) by the LBL (yellow layer
in Fig. 2a) due to the very low transmittance of the
Cr/Au layer at a laser wavelength of 248 nm (Fig. 2b).
Figure 2. (a) Schematic illustration of the LLO process to transfer
GaN LEDs onto a PET film by employing a Cr/Au LBL. (b) Optical
transmittance spectra of sapphire, Cr (50 nm)/sapphire and Cr
(50 nm)/Au (150 nm)/sapphire. The dashed line is the wavelength of
the KrF excimer laser, which is 248 nm. (c) Cross-sectional SEM image
of the GaN LED transferred onto a silicon substrate. The silicon
substrate was used to cleave the sample easily. (d) Magnified image of
the boxed area of (c). (e) Schematic illustration of the LLO process to
transfer GaN LEDs onto a PET film without using an LBL. (f)
Transfer yield and representative photographs of the GaN LEDs
transferred onto the PET film by using the LLO process with and
without an LBL.
J. Chun et al. / Scripta Materialia 77 (2014) 13–16
Figure 2c and d shows that the conductive epoxy is
protected by an LBL and that the LEDs are separated
from the sapphire substrate. The inner polymer layer
and SiO2 passivation layer are also detached from the
sapphire substrate after the LLO process. The effective
protection of the inner polymer layers from the intense
laser beam by the LBL can provide more freedom in
choosing the flexible organic materials, such as conductive epoxy and PET, for various device applications. In
addition, the ablation of the LBL metal layer by the laser beam would also enhance the sharp separation even
in Region B, to give a high yield of transfer printing
onto the PET substrate. In contrast, without the LBL,
as shown in Figure 2e, the intense laser beam damaged
the conductive epoxy layer and even caused the GaN
LEDs to fall apart from both the sapphire substrate
and the PET film. As a result, there is a large difference
in the transfer yield of GaN LEDs with and without an
LBL, as presented in Figure 2f. Introducing an LBL enables the GaN LED pixels and PET film to remain
strongly adhered, resulting in a high transfer yield up
to 95% with the correct arrangement (see the upper
two photographs of the inset in Fig. 2f). Otherwise,
the transfer yield is very low because a large number
of GaN LED pixels are lost from the PET substrate
(see the lower photograph of the inset in Fig. 2f).
To understand the microscopic process of the sharp
separation of the GaN-free region upon exposure to a
laser beam, the surface of the separated sapphire substrate after the LLO process was examined by optical
microscopy (OM), scanning electron microscopy
(SEM) and energy-dispersive X-ray spectrometry
(EDX). The contrasts in the OM image (Fig. 3a) indicate the regions corresponding to the GaN LED square
patterns that had already been lifted off and the remaining area of the GaN-free region. Next, EDX peaks of Al
(1.48 keV), Si (1.74 keV) and Au (2.12 keV) which come
from the sapphire, SiO2 passivation layer and LBL,
respectively, were measured from the GaN LED region
(Zone-A), the GaN-free region (Zone-B) and the edge of
Figure 3. (a) OM image of the sapphire donor substrate after the LLO
process. (b) SEM image of the selected area of (a). (c) The relative
atomic proportions of Au, Si and Al from EDX data in the selected
area of (b). (d) Magnified SEM image of Zone-B in (b). (e) The relative
atomic proportion of Au, Si and Al from EDX data in the selected
area of (d).
15
the laser beam spot in the GaN-free region (Zone-C) in
Figure 3b. The atomic composition of 98.3% Al and
1.7% Si in Zone-A, as shown in Figure 3c, indicates that
the separation of the GaN LEDs mainly occurs at the
interface between the GaN and the sapphire substrate.
The increased atomic composition of Au in Zone-C
compared with that in Zone-B is presumably related to
the faster resolidification caused by an edge effect in
the laser-exposed area [16,17]. The droplet-like surface
morphology in Zone-B, shown in Figure 3d, originates
from the vaporization of the Cr/Au LBL metals by the
laser beam, which has also been observed by other
groups after laser ablation of a Cr or Au surface
[16,17]. As shown in Figure 3e, the different atomic ratio
of Al/Si/Au of 0%/11.6%/88.4% in the droplet region
(Zone-B1 in Fig. 3d) compared with 2.3%/94.6%/3.1%
in the remainder (Zone-B2 in Fig. 3d) also indicates that
the droplets are formed from the LBL metal by the intense laser beam. The EDX analysis and SEM image
in Figure 2c strongly indicate that the LBL provides a
sharp delamination plane without significant damage
on the inner polymer layers during the LLO process.
Figure 4a shows 22 23 arrays of LEDs (dark gray)
fabricated by LBL-assisted LLO transfer printing onto a
PET film coated with conductive epoxy (silver). The
transfer yield of GaN LEDs from the sapphire substrate
to the PET film was 95%. The EL image obtained at
4 mA from a randomly selected LED pixel indicates that
laser-induced damage was not introduced in the device
during the LLO process (inset of Fig. 4a). The EL emission from the LEDs upon bending up to 13 mm in tensile mode and 26 mm in compressive mode confirms the
mechanical flexibility of the device under the various
bending configurations (Fig. 4b and c). In particular,
the successful EL emission upon folding the GaN-free
region by 90°, as shown in Figure 4d, also indicates that
the LBL protects the inner layers, such as the conductive
epoxy layer, during the LLO process. We also measured
the EL spectra of the GaN LEDs before and after transfer printing, as shown in Figure 4e. The GaN LEDs on a
sapphire substrate show an EL peak at 463 nm with
weak shoulders at an injection current of 4 mA. On
the other hand, the GaN LEDs on a PET substrate
showed an EL peak wavelength of 469 nm with more
interference fringes. The strong interference fringes are
associated with enhanced Fabry–Perot interference by
a reflector of LBL (yellow layer in the inset of Fig. 4e)
under the GaN LED chips [18]. The 6 nm red shift of
the EL peak is attributed to the increased junction temperature in the transferred GaN LEDs caused by the
poor heat dissipation due to the relatively low thermal
conductivities of the epoxy and polymer substrate compared to the sapphire [19,20]. This result is similar to the
previous reports on GaN LEDs transferred onto flexible
polymer substrates [6,7]. It is believed that the EL peak
shift due to low heat dissipation can be improved by
introducing nanoparticles to the polymer materials for
high thermal conductivity or designing micro/nanosize
textured reflectors as efficient heat sinks for LEDs
[21,22]. Figure 4f shows the light output power–current–voltage (L–I–V) characteristics of GaN LEDs on
a PET film. The device shows a typical rectifying I–V
curve and the linear increase in the light output power
16
J. Chun et al. / Scripta Materialia 77 (2014) 13–16
This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant No.
2008-0062606, CELA-NCRC) and the Industrial Strategic technology development program (10041878), Development of WPE 75% LED device process and standard
evaluation technology funded by the Ministry of Trade,
Industry & Energy (MOTIE/KEIT).
Figure 4. (a) Photograph of flexible 22 23 GaN LEDs on a PET film
through LLO transfer printing using a Cr/Au LBL. The inset of (a)
shows a zoomed optical microscope image of EL from a randomly
selected pixel of the GaN LEDs at an injection current of 4 mA. (b–d)
Photographs of EL from a randomly selected pixel of the GaN LEDs
at an injection current of 4 mA upon bending up to 13 mm in tensile
mode (b) and 26 mm in compressive mode (c), and upon folding 90°
against the edge of acryl cube (d). The insets of (b–d) show crosssectional photographs of the deformed configurations of the device. (e)
Representative normalized EL spectra at 4 mA for GaN LEDs before
and after transfer printing. The inset of (e) illustrates the configuration
of GaN LEDs on a PET film. (f) Representative light output L–I–V
characteristics of GaN LEDs on a PET film.
with increasing injection current, indicating that the
GaN LEDs are not significantly damaged by the LLO
process during the LBL-assisted transfer printing
process.
In conclusion, the synergetic effects of an LBL in protecting inner polymer layers from an intense laser beam
and inducing sharp separation of GaN LEDs and inner
polymer layers from a sapphire substrate enable the direct and precise transfer of GaN LED arrays by the
LLO process. The successful transfer printing of patterned GaN LEDs onto a PET film with a high transfer
yield of 95% and their EL emission upon bending in
both tensile and compressive modes demonstrate the
versatility of this method for various applications,
including next-generation deformable displays and other
optoelectronic systems.
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