www.laser-journal.de
UV Laser Processing for
Semiconductor Devices
Highly flexible laser-assisted fabrication for gallium nitride based devices
Olaf Krüger and Richard Grundmüller
The application of reliable laser
sources is well-established in several
fields of industry including automotive, electronics, and medical manufacturing on macro, micro, and even
nanometer scales [1]. In modern
semiconductor technology, silicon is
the dominating material. Further materials like gallium nitride (GaN) and
silicon carbide (SiC) allow for higher
operating frequencies and power levels, i. e., high-speed electronics and
high-power applications. However,
their chemical and physical properties
do not only allow extending the limits
of silicon-based device performance,
but also call for alternative methods of
processing.
At Ferdinand-Braun-Institut, LeibnizInstitut für Höchstfrequenztechnik
(FBH), high-performance devices using GaN-based epitaxial layers are
fabricated on SiC and sapphire substrates. Both substrates are very hard
and chemically stable, and are thus
difficult to pattern by means of classical mechanical or chemical techniques.
Hence, laser-assisted technologies to
fabricate vias (vertical interconnect
access) through SiC as well as laserassisted die separation of GaN-based
lasers and light emitting diodes (LEDs)
on sapphire substrate have been developed at FBH and are used in device fabrication.
Versatile semiconductor device
micromachining
To meet the special needs of semiconductor device fabrication, a turn-key
industrial grade laser workstation (ILS
500 Air, InnoLas GmbH) was customized and placed in an industry-compatible clean room with a two to four inch
26 Laser Technik Journal
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b
Fig. 1 Scanning electron micrographs of KrF laser-produced surfaces in photoresist
AZ4562 (Clariant) on silicon using orthogonal mask-dragging techniques: (a) blazed grating
(top) and pyramid-like structures (bottom), (b) pyramid-like structures. (Source: FBH)
process line for III-V semiconductor
devices.
Due to its technical maturity and
high reliability, a frequency-tripled,
diode-pumped solid-state (DPSS) laser
was chosen for the laser workstation.
The UV laser emitting at 355 nm provides an average power of 4.5 W and
a pulse length of < 30 ns at pulse repetition frequencies of up to 100 kHz,
allowing a high processing speed. Previous tests had proven that such laser
type ensures reliable operation as well as
high-quality and precise micromachining of SiC for the given application. This
laser was integrated into a class-1 lasersafe micromachining workstation.
The laser beam travels through a
beam attenuator, an aperture module,
a beam expansion telescope, a λ/4 plate,
and a galvanometer scanner and focusing head. The galvanometer scanner is
used to move the laser beam in the XY
plane across the workpiece’s surface.
The beam is focused to a spot size of
~15 µm, using a telecentric F-theta objective with a focal length of 56 mm.
A second type of laser extends the
basic set-up of the laser workstation,
an excimer laser delivering nanosecond
pulses with a maximum pulse repetition
frequency of 200 Hz. Depending on the
excimer gas in use, argon fluoride (ArF)
or krypton fluoride (KrF), the wavelength is 193 nm or 248 nm, respectively.
These two additional excimer wavelengths in the UV range significantly
extend the variability and potential applicability of the laser workstation. In
addition, flexible UV illumination optics (Optec s. a., Belgium) allow for high
demagnification illumination (15×)
through a pinhole mask for focal point
application as well as for homogenized
low demagnification illumination (5×)
for mask projection techniques. The
mask stage holds 5" masks (typically
chromium on fused silica) and the associated optical system is best suited for
direct processing of polymers such as
photoresists. The micromachining tool
allows us to synchronously move wafer
and mask stages and thereby to fabricate
blazed gratings and pyramid-like structures on polymer surfaces as depicted in
Fig. 1 for photoresist AZ4562 (Clariant).
Such 3D structures can be transferred
into semiconductor surfaces by either
wet or dry chemical etching, e. g. for
micro-optical applications such as antireflection layers and beam shaping elements for laser diodes.
The laser workstation is equipped
with an air-bearing XY stage enabling
a positioning accuracy of better than
1 µm. Four cameras and a pattern rec-
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Micro Material Processing
ognition system allow for automated
beam alignment. Two of the cameras
are placed above the stage level and allow for front-to-front alignment, i. e.
the alignment marks are located on the
surface to be laser processed. In case the
wafer should be processed on the opposite side (i. e., on the wafer’s backside),
the sample will be flipped on an openframe vacuum chuck, i. e. marks on the
wafer’s front will be face down and can
be recognized by the other pair of cameras located below the wafer stage.
Process flow ensures
com­patibility
To be compatible with standard semiconductor device fabrication that typically defines fabrication steps and modules as process layers, the laser micro
processing step was introduced and
specified as a process layer in the device
layout. To start with, CAD layout data
of the devices are imported into a CAD
software package preparing the process
code for the laser micromachining tool.
With this program called Graffy (Durst
CAD Consulting GmbH) various data
formats including GDSII and DXF can
be imported. The software package allows quick modifications and adjustments in the layout as well as the attachment of technology parameters for laser
treatment. All information about positions and shapes of alignment marks
and about the laser pattern to be created
(e. g. vias and scribing lines) are taken
from the CAD layout. Data are then fed
into the Graffy postprocessor that automatically generates the program code
for laser machining. The CAD system
offers high flexibility for pattern generation, which is particularly interesting
for rapid prototyping.
To precisely position the laser spot
on the surface with respect to existing
structures on the wafer, a vision system
including automated pattern recognition is used to measure the actual wafer
position on the stage. In a first step, shift
and rotation of the wafer is determined
by means of two (pre-) alignment marks
on the wafer. In a second step, the fine
alignment is performed using proper
marks or patterns in or near the field
to be processed. Controlling the movement of the stage and of the laser spot
by the galvanometer scanner allows for
step-and-repeat as well as write-on-the-
a
b
Fig. 2 (a) Optical micrograph of a sapphire target hit by a single laser shot. The diameter
of the gold aiming point is 20 µm. The distance between two circles is 10 µm. The sample
was laser processed on the back using front-to-backside alignment. The deviation on the
target structure shown is X-0.4 µm and Y+0.4 µm (center-to-center). (b) Optical micrograph
of a transistor detail with a 7.5 µm wide laser scribe isolating a gold drain connection
between two adjacent transistor devices. (Source: FBH)
fly processing modes. Such modes are
also realized in state-of-the-art litho­
graphy tools for semiconductor device
fabrication including electron beam
lithography systems and optical projection lithography tools (wafer steppers
and wafer scanners).
The alignment precision of the center of the laser spot with respect to existing structures is better than ± 1 µm, i. e.
less than 10 % of the DPSS laser beam
diameter. This accuracy can also be
achieved when the alignment marks are
located on the reverse side of the wafer,
i.e. on the side opposite to the one processed by the laser (front-to-backside
alignment). The positioning accuracy
can be verified using laser target structures patterned on silicon or sapphire by
means of standard contact lithography.
Different target structures (mesh size
and target dimensions) were realized
by 2 µm wide gold lines. Nonius-like
structures were also processed to allow direct measurement of the overlay
precision of scribe lines or indentations
created by the laser using a plain optical
microscope. A typical result after laser
processing is shown in Fig. 2a for backside processing. A single laser shot was
deposited onto the target pattern on the
wafer’s backside. Target structures allow
to easily and quickly estimate the positioning accuracy by optical microscopy.
The example shown in Fig. 2a demonstrates a beam positioning accuracy of
better than ± 1 µm for front-to-backside
alignment. In the case shown here, the
DPSS laser was positioned by moving
the XY stage. The positioning accuracy
is nearly the same when using the highspeed galvanometer beam positioner
over a 10 × 10 mm² target field. Fig. 2b
shows another example for the high
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
beam positioning accuracy on transistor scale. Here, a 7.5 µm wide laser
scribe was placed in between two gold
air-bridges to isolate a drain connection
between adjacent transistors.
Laser-assisted via processing
High-power AlGaN/GaN high electron
mobility transistors (HEMTs) for RF
applications are processed on semi-insulating SiC substrates that are typically
~400 µm thick. To boost the transistor
performance and to simplify their assembly and chip mounting, a vertical
interconnect access with low inductivity between the source contact on the
front and the ground electrode on the
backside is required. Due to its hardness and chemical inertness patterning
of SiC is laborious and time-consuming. Currently, only advanced inductively coupled plasma (ICP) etching
can provide significant etching rates of
SiC of ~1 µm/min and etching of holes
through 400 µm thick SiC is impossible. Laser radiation, however, offers a
Fig. 3 Cross-section optical
micrograph of a through-wafer
via drilled by a DPSS UV laser.
The laser hits from the top
(backside of the wafer), the
laser exit is on the bottom
(wafer’s front with source pad
of transistor).The via’s sidewall
is completely covered by a
metal layer, approx. 5 µm thick.
(Source: FBH)
Laser Technik Journal
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Fig. 4 Scanning electron micrograph of
a mounted AlGaN/GaN on SiC transistor
device with laser vias through the source
contact pads (5 × 8 × 500 µm L-band
power bar; Source: FBH)
Fig. 5 Scanning electron micrographs of cross-sections of a blind via hole after KrF
laser drilling terminating about 10 µm above the SiC/GaN interface (left) and after subsequent ICP etching of the SiC (middle). The plasma etching stops at the GaN epitaxial
layer. Optical micrograph of a cross-section of a blind via stopping at the source pad of a
transistor device (at the bottom, right).The via’s sidewall is completely covered by a metal
layer, approx. 5 µm thick. (Source: FBH)
unique one-step alternative to plasma
and wet-chemical etching of highly
resistant materials, which makes lithographical formation of etching masks
obsolete.
At FBH, a laser-assisted technology
for fabrication of two types of vias in
SiC, i. e. through-wafer vias and blind
vias, has been developed.
cal holes with a mean diameter of 50–
60 µm. Without optimizing the drilling
process for high throughput it takes
about 3 s to drill a 50 µm hole through
400 µm thick SiC.
The holes are drilled from the wafer’s backside through the transistor’s
source contact on the front. The drilling position on the backside is precisely
controlled using marks on the front
and the front-to-backside alignment
feature of the tool. After laser processing, the wafer is cleaned in buffered
solution of hydrofluoric acid to remove
debris [2]. The sidewalls are covered by
redeposited material (nanocrystalline
silicon, resolidified SiC) that forms a
smooth layer with a thickness between
0.5 µm and 4 µm at the laser exit and
entrance, respectively. Beneath the deposits, no evidence for laser-induced
micro cracks, extended defects, and
crystal damage was found [3, 4]. After
laser processing, the sidewalls of the
holes are metallized by depositing a
thin conducting seed layer, followed by
electroplating 5 µm of gold. Fig. 3 shows
the cross-section of a plated-through
via exhibiting a completely closed gold
layer. As confirmed by resistance measurements, a vertical electrical interconnect through the 400 µm thick SiC
is established. Depending on the via
diameter, the through via resistance
is 20 – 30 mΩ. Fig. 4 shows a mounted
transistor device consisting of five cells
with eight gate fingers each (gate width
500 µm). The source pads of this transistor are electrically connected to the
back; the holes drilled through by the
laser are clearly visible. Electrical measurements of the power performance of
Laser drilling enables throughwafer vias in 400 µm thick SiC
The DPSS laser beam providing a
Gaussian-like intensity profile is used to
directly drill one hole at a time. A typical pulse energy of 65 µJ/pulse at a laser
pulse repetition rate of 20 kHz is used
to create through-wafer micro holes
in 400 µm thick SiC. The laser beam is
moved on the sample to obtain coni-
Institute
Ferdinand-Braun-Institut, Leibniz-Institut für
Höchstfrequenztechnik (FBH)
Berlin, Germany
The FBH explores cutting-edge technologies
for innovative applications in microwaves and
optoelectronics. The institute is a competence
center for III-V compound semiconductors with
strong international reputation providing the full
range of capabilities, from design to fabrication
and device characterization. For customers in
industry and research, FBH develops high-value
products and services, including high-frequency
devices and circuits for communication and
sensor technology as well as high-power diode
lasers for materials processing, medical technology, and high-precision metrology.
www.fbh-berlin.de
28 Laser Technik Journal
5/2013 such AlGaN/GaN HEMT powerbars at
2 GHz confirmed full functionality [5].
Transfer, output and load-pull characteristics of the devices before and after
laser drilling revealed that the transistor’s DC and RF performances remain unchanged for gate-via distances
≥ 1 µm [6].
Laser-assisted processing of
blind vias in SiC
An alternative laser-assisted technology has been developed to fabricate
blind via holes for AlGaN/GaN HEMTs
on SiC [2]. In this case, the wafers are
thinned to a thickness of 100 µm and
covered by a protecting layer of indium
tin oxide (ITO) on their backside. For
laser drilling, the ArF excimer laser is
used providing a shaped beam that is
aligned and focused at the backside to
obtain the desired position and via cross
section on the sample. By percussion
drilling, i. e., without moving target or
beam, the hole is formed until the bottom of the hole is close to the interface
SiC/GaN (Fig. 5 left). Using a laser fluence of about 20 J/cm² per pulse and a
pulse repetition frequency of 50 Hz, 200
– 300 laser shots are required to achieve
a hole depth of 80 – 100 µm, which results in a drilling time of about 2 – 3 s
per hole.
In the subsequent process step, the
thin remaining SiC is dry-etched using
an inductively coupled plasma (ICP)
without an additional etching mask, i. e.
no lithographic fabrication of a mask is
needed. The selectivity of the ICP etching of SiC versus GaN is > 100 : 1, accordingly the etching process will prac-
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Micro Material Processing
High power UV Laser
Scribed trenches
Sapphire
(lnAl)GaN layer
Separated wafer pieces
Carrier tape
Scribed trenches
Blade
Fig. 6 Schematic of the laser scribing and breaking technology for die separation of
GaN-based LEDs and laser devices (not to scale; Source: FBH)
tically come to a halt at the GaN layer
(Fig. 5 middle). A second plasma etching
step removes the GaN epitaxial layer
by reactive ion etching (RIE), allowing for a clean stop at the metal layer of
the front contact. After wet-chemical
removal of the protecting ITO layer, a
5 µm thick gold layer is deposited on
the wafer’s backside and the side walls
of the holes (Fig. 5 right), thereby forming
an electrical connection between the
contact pads on the front and the metal
layer on the backside of the device. The
electrical resistance of a single via depends on its diameter (cross-section),
on the thickness of the gold layer at the
side wall as well as on the thickness of
the sample (length of the via). Values in
the range of 10 – 30 mΩ were measured.
In contrast to the through-wafer via
process described above, the blind via
process does not pierce the source contact metal at the wafer’s front, which can
be advantageous for further processing
(e. g., resist coating) and soldering.
Laser scribing for smooth die
separation
A critical step in processing high-efficiency UV as well as blue LEDs and lasers on sapphire substrates is die separation. Mechanical dicing can produce
chipping, cracking, and delamination
along the scribe cuts. Group III-nitride
materials crystallize in a (hexagonal)
wurtzite structure, making it particularly challenging to fabricate rectangular dies by scribing and breaking. Although sapphire exhibits a hexagonal
crystal too, the unit cells of a c-plane
sapphire substrate and the GaN-based
heterostructures are rotated by 30° with
respect to each other. This prevents a
common cleavage plane of substrate
and epitaxial layers. Laser-assisted techniques, such as laser cutting and scribing, are attractive methods to address
this issue. Consequently, laser scribing
of sapphire wafers has been subject of
numerous studies investigating laser
systems featuring various wavelengths
from the deep-UV to the UV range,
with various pulse durations, beam
shaping, and power levels. In our case,
the nanosecond-pulsed DPSS laser
beam is used to scribe the material, followed by breaking on a commercial tool
(Fig. 6).
Laser processing parameters have
to be optimized to avoid damage to the
epitaxial layers and to maximize yield
and throughput. Due to the large band
gap of sapphire of 9 – 10 eV, the sapphire substrate is optically transparent
in the wavelength range from ~200 nm
to ~5500 nm. Therefore, absorption
at 355 nm (~4 eV) as provided by the
frequency-tripled DPSS laser is rather
low. However, at sufficiently high energy density in the laser focus, radiation can be absorbed by multi-photon
absorption processes and ablation of
sapphire occurs. Accordingly, nanosecond-pulsed laser processing of sapphire
at 355 nm is rather inefficient and the
quality of the process is limited. However, at FBH a laser scribing technology
has been successfully developed that allows subsequent cleaving of InAlGaNbased laser [7] and LED chips.
Using the automated alignment
of the micromachining tool, the laser
beam is precisely guided on the backside of the sapphire substrate. The
scribe has to be carefully aligned to the
crystal plane of the sapphire (a-plane)
to avoid zigzag breaking of the epitaxial
layers and to obtain a minimum density
of terraces and particles on the cleavage
plane. The scanning speed of the laser
beam, the number of passes, and the position of the beam focus in the material
Company
InnoLas Systems GmbH
Krailling, Germany
Fig. 7 GaN-based LED device on sapphire substrate (L × W × H = 1000 µm × 600 µm ×
430 µm) mounted with its epitaxial side down on an aluminium nitride submount and TO
socket, i. e. the sapphire side is facing upwards. The die was separated by laser scribing
and subsequent breaking. (Source: FBH, Schurian)
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
InnoLas Systems GmbH produces laser systems for micro material processing, particularly
in the photovoltaic industry, for semiconductors,
electronics and precision engineering. InnoLas
Systems designs and manufactures machinery
solutions as stand-alone systems or for inline
integration, perfectly adapted to industrial production conditions. Worldwide service and support ensures smooth setup and reliable operation. InnoLas Systems utilizes innovations in
laser technology to produce highly-efficient and
reliable processing systems.
www.innolas.com
Laser Technik Journal
5/2013
29
www.laser-journal.de
are adjusted to avoid excessive mechanical and temperature stress on the wafer.
If the scribe depth is too small, the sapphire does not break exactly along the
scribe and terraces with high density
are formed. If the groove is too deep,
UV radiation of the laser penetrates the
sapphire and is absorbed by the GaNbased epitaxial layer, which damages
or degrades the functional layer. The
opening of the grooves is about 20 µm
wide. The scribe depth is adjusted to the
thickness of the wafer. A scribe depth
of about 390 µm is suitable for breaking
440 µm thick sapphire substrates into
dies with a size of 850 µm × 700 µm using a commercial scriber/breaker tool
(Dynatex). Fig. 7 shows a mounted LED
chip singularized by laser scribing and
subsequent breaking. Smaller dies were
obtained from thinner wafers.
As previously published, the presented technology has also been applied to produce smooth facets for
high-performance laser diodes both on
un-thinned sapphire and on bulk GaN
substrates [7]. State-of-the-art power
levels and threshold current densities
have been achieved.
Summary
The feasibility of laser-assisted via fabrication for AlGaN/GaN HEMTs on SiC
as well as of laser scribing for die separation of GaN-based LEDs and lasers
on sapphire has been proven on device
level. A complete process flow from
CAD wafer layout to laser processing
has been established. For reliable laser
processing a high beam positioning
accuracy of ± 1 µm with respect to the
device pattern on the wafer was verified
by optical microscopy using test target
structures. The direct laser drilling and
scribing approaches offer highly flexible
ways to create patterns, e.g., for rapid
prototyping. It has been demonstrated
that direct laser micro processing is well
compatible with semiconductor device
fabrication. Thus, laser micro ablation
turned out to be a promising alternative to the combination of conventional
lithographic, plasma, and micro mechanical processing of hard and inert
materials.
DOI: 10.1002/latj.201300010
[1] M. C. Gower: Opt. Express 7 (2000) 2, 56
[2] O. Krüger et al.: UV laser drilling of SiC for
semiconductor device fabrication, J. Phys.:
Conf. Ser. 59 (2007) 740-744.
[3] T. Wernicke et al.: Appl. Surf. Sci. 253 (2007)
8008
[4] O. Krüger et al.: Appl. Phys. A 93 (2008) 1,
85
[5] O. Krüger et al.: IEEE Electron Dev. Lett. 27
(2006) 6, 425
[6] T. Wernicke et al.: Abstract Book WOCSDICE 2006. 30th Workshop on Compound
Semiconductor Devices and Integrated
Circuits. J. Stake, Ed., Göteborg, Sweden:
Chalmers University of Technology (2006)
173-175
[7] J. R. van Look et al.: IEEE Photon. Technol.
Lett. 22 (2010) 6, 416
Authors
Olaf Krüger received his Ph. D. in chemistry
from the Humboldt-Universität Berlin (HUB),
Germany, in 1993. Afterwards he stayed as
a scientist at the HUB. From 1995 to 1997
he was working at the California Institute
of Technology, USA. In 1997 he joined the
Leibniz-Institut für innovative Mikroelektronik
(IHP), Frankfurt/Oder, Germany. Since
2001 he has been with the Berlin-based
Ferdinand-Braun-Institut, Leibniz-Institut für
Höchstfrequenztechnik (FBH), where he is presently Head of the
Process Technology Department. His current research interests
include laser micromachining for semiconductor device fabrication
and novel techniques for device assembly.
Richard Grundmüller received his diploma
degree (Dipl.-Ing. FH) in mechanical engineering from Fachhochschule München in 1990. In
1995 he started his own business as founder
and CEO of InnoLas GmbH. Currently he is
CEO/President of InnoLas Systems GmbH.
Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany, Tel.: +49 (30) 6392-3205,
Fax +49 (30) 6392-2685, E-mail: olaf.krueger@fbh-berlin.de
InnoLas Systems GmbH, Robert-Stirling-Ring 2, 82152 Krailling, Germany, Tel.: +49 (89) 899 4828-1001, Fax +49 (89) 899 4828-1111;
E-mail: richard.grundmueller@innolas.com
30 Laser Technik Journal
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