Laser Lift-off Process

7.
An introduction to
Laser Lift-Off (LLO) technology
1
Cross-Section of VLEDs
[Source: SemiLED Cop. Ltd.]
2
1

Wafer bonding Technology
3
Transient-liquid-phase bonding
4
2

Bonding and transfer process
5
X-ray diffraction of transferred GaN
6
3

Pattern transfer by LLO
7
Laser Lift-off technology
8
4

[William S. Wong, Michael Kneissl, David W. Treat, Mark Teepe,
Naoko Miyashita, and Noble M. Johnson Electronic Materials Laboratory
XEROX Palo Alto Research Center E-mail:wswong@parc.xerox.com]
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5

11
Schematic view of the laser liftoff (LLO) process
High intensity laser pulses enter
the sample via the sapphire
substrate and thermally
decompose a thin GaN-layer at
the substrate interface.
The shock waves resulting from
the explosive production of
nitrogen gas during each laser
pulse are damped by placing the
GaN-sample into sapphire
powder.
A hot plate can be used to raise
the substrate temperature during
the process, in order to relieve
some of the accumulated thermal
strain.
12
6

A simulated temperature profile as a
function of time and depth
By solving the one- imensional heat equation, the extent of interaction at the
GaN/sapphire interface with a pulsed UV-laser is found to be highly localized to
the thin film/substrate interface.
For a single 38 ns,
600 mJ/cm 2 incident pulse from a
KrF laser
The numerical solution to the onedimensional
heat equation assumes
a semiinfinite slab in the sapphire
and GaN region.
Laser-induced heating
14
7

Enhancing microsystem functionality
through materials integration
15
Approach to integration
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8

Approach to integration
17
Free-standing InGaN LEDs
18
9

Cross sectional SEM micrograph of a
transferred GaN film onto a Si substrate
GaN on Si by Pd-In bonding and
laser lift-off (LLO)
GaN LED on Si
200 μm
Pd-In metal bilayers were used as bonding material which formed the
compound PdIn3 after the low-temperature bonding process.
Cleaving the Si substrate was performed to make the cleavage facets on
the GaN.
InGaN LEDs on Si Substrates
20
10

LLO integration process
21
Cleaved mirror facets
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11

Continuous wave InGaN LDs on Cu
23
Continuous wave InGaN LDs on Cu
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12

CW laser diodes on diamond
25
Improved output performance
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13

Laser Lift-off Process
27
Laser lift-off process
E g : E 0 > E 1 > hν> E 2 > E 3
hν
Sapphire
Buffer Layer
N-GaN
E c
E 0 E 1 E 2 E 3
E v
Undoped GaN
脈 衝 雷 射 被 吸 收 處
AlN
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14

Laser lift-off (LLO) technique
Thermal decomposition of an interfacial layer induced by
pulse irradiation of KrF (248nm) excimer laser
Only interfacial layer of GaN reach high temperature
Laser beam
Sapphire(Al 2 O 3 )
Buffer
layer
M.Q.W.
p-GaN
Sapphire(Al 2 O 3 )
n-GaN
(E g ≈ 9eV)
(E g =3.4eV)
GaN
(900ºC~1000ºC) C)
Δ
n-GaN
Ga + ½ N 2
Ni
Ni
29
The excimer laser system
Light intensity
Time
The schematic diagram of KrF ecximer laser
Laser medium
Wavelength
Maximum pulse energy
Maximum laser fluence
Full-Width-Half-Magnitude
Pulse repetition rate
KrF
248 nm
380~400 mJ/pulse
2000 mJ/cm 2
30 ns
1~100 Hz
Excimer Laser Micro-Machining System
(Excitech PS-2000)
35
15

Laser lift-off (LLO) process
Laser fluence : 850 mJ/cm 2
Laser pulse time : 38 ns
E
1.24
=
λ
1.24
0.248
g
= =
C
5 eV
Sapphire
Sapphire(Al 2 O 3 )
400 μm
Buffer layer
(u-GaN)
M.Q.W.
p-GaN
n-GaN
GaN
(900ºC~1000ºC)
∆
Ga + ½ N 2
Oxidized Ni/Au
& Ti/Al/Ti/Au
Ni substrate
80 μm
[W. S. Wong, et al., APL,
pp. 599-601, 1998]
36
KrF
Copper mask: 1cm×1 cm
Laser beam
A block size after laser beam through
z
the lens (10×): 1 mm×1 mm
Reactive energy density:
10 倍 聚 焦 鏡
y
850-1000 mJ/cm 2
• Scanning velocity:
能 量 強
500-800 mm/min
Z=0 mm
Pattern LLO 加 工 區 ( 聚 焦 面 )
5 mm
x
Z= -3.74 mm
能 量 弱
整 面 LLO 加 工 區
GaN wafer
投 影 片
金 屬 加 工 平 台
-3.74 mm,0.1 mm,
wafer2D
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16

-
0.5 mm
Start
0.5 mm
0.5 mm
Laser beam size
1 mm
2-inch wafer
1 mm
End
: Overlap for neighboring laser pulse
39
-
(overlap)
y 軸 移 動 0.3 mm 打 一 發
0.3 mm
Sample
X 軸 移 動 0.3 mm 打 一 發
Laser beam size
1 mm
1 mm
雷 射 移 動 的 方 向
40
17

Patterned LLO technology
[Patent: I287309(ROC)]
Laser Beam
Mask
Patterned
Laser Beam
300×300, 230×230 1000×1000
S. J. Wang et al., Appl. Phys. Letts, vol. 87, 2005
360×360, 90 ×90 unit: μm 2
S. L. Chen et al., IEEE Photon. Technol. Lett, vol. 19,
41
2007
unit: μm 2
Die size: 300 × 300 μm 2
Die size: 1000 × 1000 μm 2
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48
The luminous efficiency and total output power of dichromatic
white LEDs with injection currents from 5 to 50 mA
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50
53
21

Substrates for III-nitride heteroepitaxy: a brief survey
Standard vapor phase epitaxy methods for GaN growth (HVPE,
MOCVD),
high growth temperatures (>1000 °C)
high concentration of ammonia and hydrogen
considerably reduce the choice of possible substrates.
For device production on an industrial scale, the substrate has to fulfill further
criteria such as minimum size (2”), atomically flat surfaces, and availability in
large quantities at an acceptable price.
Sapphire was and still is the most common substrate for the deposition of
GaN-based light-emitting diodes (LEDs), because of its reasonably low cost
and wide availability, and despite the fact that it has a large lattice constant
and thermal expansion coefficient mismatch with respect to GaN.
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57
22

Thermally induced decomposition
of GaN and the corresponding flux of nitrogen
from the GaN surface using a heating rate of 0.3 K/s
The flux of nitrogen molecules,
Φ(N2), leaving the crystal surface in
vacuum shows an exponential
increase with temperature above
830 °C, which can be parametrized
as:
61
Effect of a single shot from a Nd:YAG-laser on the
optical appearance of a GaN layer on sapphire
Effect of a single shot from a
Nd:YAG-laser on the optical
appearance of a GaN layer on
sapphire. For absorbed energy
densities below 200 mJ/cm2, no
visible effects can be observed.
For energy densities above the
sublimation threshold of approx. 250
mJ/cm2, the entire surface of the
GaN epilayer is transformed into
metallic Ga, giving rise to the dark
colour in the upper half of the
specimen pictures.
Close to the sublimation threshold,
inhomogeneities of the intensity
profile of the laser beam are clearly
visible in the irradiation patterns.
62
23

Experimentally determined decomposition depth caused by
a single pulse of a Nd:YAG laser in GaN at room
temperature as a function of the pulse intensity absorbed in
the thin GaN layer on sapphire.
The energy density of the laser shots
should be kept as close as possible
to the threshold value necessary for
GaN decomposition. Any additional
energy will lead to unwanted thermal
and mechanical stress which can
favour film spalling and peel-off after
sapphire removal.
The change in slope for pulse intensities exceeding 350 mJ/cm2 is probably
due to absorption or reflection of the laser light by the metallic Ga layer formed 63
during the process.
Dependence of the laser-induced etch depth of GaN
on the number of laser pulses with and without the
presence of HCl-vapor
The laser treatment was performed at 300 K with an absorbed pulse intensity of
290 mJ/cm2 and a pulse repetition rate of 10 Hz. From the slope of the straight
64
line an average etch rate of 330 nm/s can be deduced.
24

Comparison between the maximal etch rates for GaN
achieved by laser- nduced etching and conventional
reactive ion etching methods
ICP: inductively coupled plasma
MIE: magnetron ion etching
CAIBE: chemically assisted ion
beam etching
ECR: electron cyclotron resonance
RIE: reactive ion etching
Using laser-induced thermal decomposition of GaN, much higher etch rates in
the range of 1 μm/s can be realized.
65
Temporal and spatial variation of the temperature at the sapphire/GaN interface
(depth zero) during and after a Nd:YAG laser pulse (left figure, λ = 355 nm, τ = 6
ns, I = 300 mJ/cm2) and a KrF excimer laser pulse (right figure, λ = 248 nm, τ =
38 ns, I = 600 mJ/cm2).
The intensities of the two pulses were chosen such as to obtain the same
maximum temperature of 1100 K (sublimation temperature, cf. Fig. 2).
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Because of the much longer pulse duration in the case of the KrF laser, a
higher pulse energy of typically 600 mJ/cm2 is necessary to heat the GaN
above the sublimation threshold, whereas pulse energies of 300 mJ/cm2 are
sufficient in the case of the Nd:YAG laser.
Since optical thermalization and relaxation processes in GaN occur on a much
faster timescale of ps or a few ns, the temperature profiles generated by the
different laser pulses can be calculated quite easily in a one-dimensional
model based on the temporal shape of the laser pulse, the known absorption
coefficients of GaN, and the thermal properties ( heat capacitance and
conductivity) of GaN and sappire.
67
Photograph of a 275 μm thick free-standing GaN
film, after removal from the 2” sapphire substrate
The missing pieces at the wafer border actually had broken off right after the
HVPE growth.
68
26

Experimental results
Image of a HVPE-GaN sample with an area of about 1 cm 2 and a thickness of
30 μm fixed with epoxy resin to a glass holder after laser lift-off
The delaminated sapphire substrate is shown on the left.
69
Process sequence for the laser lift-off of 2” GaN
membranes
a) Laser lift-off of the GaN film covered with silicone
elastomer and mounted onto a support;
b) Sapphire removal after laser scanning;
c) Deposition of a ∼3 μm thick layer of thermoplastic
adhesive at 120 °C;
d) Peel-off of the silicone elastomer.
Subsequently, the GaN film can be easily isolated by dissolving the
thermoplastic adhesive in an acetone bath.
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27

Process flow for bonding and transfer
71
Experimental results
Delaminated 3 μm thick 2” GaN film (not structured) wafer-bonded onto GaAs
(left) and the corresponding GaN-free sapphire substrate (right)
The transferred GaN film is essentially defect free, except for some peeledoff
areas at the wafer rim.
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Summary and conclusions
The present state-of-the-art concerning the laser-induced lift-off of both thin
and thick GaN films and heterostructures from sapphire substrates has been
reviewed.
The physical background of the laserinduced thermal decomposition of GaN by
short intense pulses of KrF excimer lasers and the third harmonic of Nd:YAG
lasers is discussed, and potential applications for rapid etching of GaN have
been outlined.
Of particular interest for future applications is the possibility to produce
freestanding GaN films by laser lift-off of heteroepitaxial layers from transparent
sapphire substrates.
Specific applications include the production of freestanding GaN
pseudosubstrates starting from thick HVPE grown epilayers, as well as the
delamination of thin GaN device heterostructures for the purpose of wafer
bonding onto foreign substrates or for flip-chip bonding in device technology.
73
Summary and conclusions
Using optimized processes, the defect free lift-off of entire 2” wafers can be
achieved for GaN film thicknesses ranging from 3 to 300 μm.
Separation of thin device heterostructures from their sapphire substrates opens
up new possibilities for the formation of electrical contacts, the extraction of
photons, and for thermal management.
Thick GaN films grown by HVPE can be removed from their sapphire
substrates to obtain freestanding GaN pseudosubstrates for the homoepitaxial
growth of high quality epilayers.
The required processing steps, including surface preparation after laser lift-off,
have been described and the structural and optoelectronic properties of
homoepitaxial GaN layers deposited on freestanding pseudosubstrates have
been investigated.
All results suggest that the laser lift-off method may become a major technique
in future III-nitride device technology.
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75
Schematic diagram of the YAG-LED and KrF-LED
transfer process
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For higher intensities, the absorbed photon energy leads to local heating of
the layer and causes the destruction of the GaN.
Only those devices with median energy densities approximately 800
mJ/cm2 for KrF laser and 200 mJ/cm2 for YAG laser have good yields 90%.
Under a reverse bias of −5 V, the leakage current of YAG-LED was 1.65103
nA, which was 10 000 times higher than that of the KrF-LED 0.17 nA.
These degradations were caused by the laser lift-off processes, which
generated the screw dislocations. The screw dislocation density penetrated
through MQW region of YAG-LED was 2.9109 cm−2, which was ten times
higher than that of the KrF-LED 3.75108 cm−2.
This is because the absorption coefficient of GaN at 248 nm for KrF-LED is
2105 cm−1, which is 3.33 times higher than that at 355 nm for YAG-LED.
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81
Process flow for bonding and transfer of InGaN SQW LED
from sapphire onto Si
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33

Starting material—prefabricated InxGa12xN SQW LED/sapphire and Si
supporting substrate; deposit Pd–In bilayer on InxGa12xN device layer and Pd
on receptor Si substrates;
(2) Bond InxGa12xN SQW LED/sapphire onto Si supporting substrate,
(3) KrF laser irradiation of the sapphire/InxGa12xN SQW LED/PdIn3 /Si
structure through the transparent sapphire substrate;
(3) Heat post laser-processed structure above melting point of Ga to release
sapphire substrate; and
(4) CAIBE etch to isolate device and deposition of n-contact metal. The inset
shows InxGa12xN SQW structure after LLO and metal contact definition.
83
Process flow for bonding and transfer of InGaN SQW LED
from sapphire onto Si
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34

Room-temperature dc I –V characteristics for a typical 250mm
3250mm InxGa12xN SQW LED on Si
Photograph of a functioning InxGa12xN SQW blue LED on Si.
The LED shows no dark-line features due to microcracking from either the
LLO process or the thermal expansion coefficient mismatch between GaN
and Si.
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Schematic diagram of the nano-roughed LED structure
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The schematic diagram of
laser etching process setup
87
SEM and AFM images of the Ni nano-mask on p-GaN
surface morphology of a nano-roughened LED sample
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Forward I–V curves and Room temperature EL spectra
of conventional and nano-roughened LEDs*
* with laser etching energy of 300 mJ/cm2 at a current of 20mA
89
[IEEE PTL Vol. 29, No. 3, 2008]
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[IEEE PTL Vol. 29, No. 3, 2008]
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[IEEE PTL Vol. 29, No. 3, 2008]
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I–V characteristics of chemically lifted off
vertical LED
[IEEE PTL Vol. 29, No. 3, 2008]
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Thank You!
2007/10/29
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