UV lasers fuel precision micromachining - Spectra

t e c h n o l o g y
r e p o r t
UV lasers fuel precision
micromachining
HIGH POWER, SHORT PULSE WIDTH, AND HIGHER
a)
b)
REPETITION RATE YIELD HIGHER SPEED AND QUALITY
RAJESH PATEL, JAMES BOVATSEK, AND ASHWINI TAMHANKAR
M
obile devices such as
and
short
smartphones and tablets
pulse widths, FIGURE 2. Views of a scribe created using
single-pulse TimeShift technology. The
are evolving at a rapid pace.
vaporizes
As the devices are getting
material quickly, technology yielded scribe depths of 20µm
smaller, faster, lighter, and
reducing HAZ (a) and 25µm (b), respectively.
cheaper, they are becoming
and charring.
increasingly capable yet
The small focused beam spot enables machining smaller
more complex to manufacture, requiring miniaturization
features with higher precision. Higher power, higher pulse
and precision manufacturing of components. For key
repetition frequency (PRF), pulse shaping, and pulse splitting
components such as semiconductor chips, microelectronics
capabilities all can contribute to higher micromachining
packages, touch-screen displays, and printed circuit boards
throughput. And consistent, higher pulse-to-pulse stability
(PCBs), the industry continues to face challenges to drive
ensures process repeatability and helps achieve higher
up manufacturing yield and throughput while lowering cost.
process yield.
As a result, laser processes
Traditional UV Q-switched,
have increasingly been
diode-pumped solid-state
Depth vs. speed, silicon cutting
applied to advance mobile- Depth (µm)
(DPSS) lasers have performed
device manufacturing. As the
reasonably well in fulfilling
90
increasingly complex devices
sophisticated manufacturing
80
require more and more
requirements, but they have
60W Quasar
with TimeShift
70
sophisticated manufacturing
limitations in achieving higher
processes, advances in laser
speeds and maintaining higher
60
40W Quasar
with TimeShift
sources are also needed.
micromachining quality. A
50
40W Quasar
L a s e r s wi th s h o r te r
common
approach to increas40
wavelength,
shor ter
ing the processing speed is
30
pulse width, and low M 2
by increasing the laser’s PRF
20
(beam quality) enhance
while holding other process
10
micromachining processes
parameters fixed. However,
100 150 200 250 300 350 400 450 500 550 600
by creating a tightly focused
for a typical Q-switched DPSS
Speed (mm/s)
spot and minimizing heatlaser, this is not possible. For
affected zone (HAZ). High
these lasers, average power
FIGURE 1. Scribe depth vs. speed for silicon, illustrating
energy absorption, particularly the process optimization benefit possible using TimeShift
and pulse energy decrease
at ultraviolet (UV) wavelengths
quite rapidly as PRF increases.
technology.
Reprinted with revisions to format, from the November/December 2014 edition of INDUSTRIAL LASER SOLUTIONS
Copyright 2014 by PennWell Corporation
Also, laser pulse width and pulse-to-pulse energy fluctuations
debris on the top surface, despite scribing a 25-percent greater
tend to increase significantly at higher PRF.
depth than that achieved using single pulses.
Recognizing the need for new laser technology to overcome
these limitations, Spectra-Physics developed Quasar, a UV
Scribing alumina ceramic
hybrid fiber laser with a unique combination of high power and
Alumina (Al2O3) ceramic is used widely for microelectronic
short pulse width at high PRF. Introduced in 2013 at a 40W power
packaging due to its high dielectric property coupled with high
level (250kHz, 355nm wavelength), it has been scaled in 2014
strength, corrosion resistance, stability, and relatively low cost.
to 60W (200–300kHz), increasing both
In a typical manufacturing scenario, a
its average power and pulse energy. At
large-size alumina sheet having multiple
Depth Average depth vs. fluence
(µm)
at 500mm/s, 200kHz
the same time, its minimum pulse width
modules has to be separated or singulated
2×10ns, 10ns sep
has been decreased from 5 to 2ns and its
into individual modules at the end of the
7
6
maximum PRF increased from 500kHz to
processing cycle. In a common technique
TimeShift
78%
5
3.5MHz. These output characteristics give
for singulation known as “scribe and break,”
technology
4
engineers access to new regimes of laser
a deep scribe in substrate is created using
3
1×20ns pulse
process parameter space.
a laser and the substrate is then separated
2
1
In this article, results are presented
by using mechanical force. A UV laser with
0
from applying this combination of high
high power can provide a clean, precise
100
150
200
0
50
Fluence (J/cm2)
UV power at high PRF, independently
way of creating scribes at a high speed.
adjustable pulse width, and advanced
Similar to silicon scribing, we have
FIGURE 3. Scribe depth vs. fluence for
pulse manipulation capabilities to
demonstrated that the Quasar laser can
alumina, illustrating the throughput
micromachining of various microelectronic
be used to create scribes in alumina at
benefit of TimeShift technology.
materials, including silicon (applications in
a higher speed with a minimal thermal
chip manufacturing), alumina (application in
effects using higher power and TimeShift
microelectronics packaging manufacturing), glass (applications
technology. FIGURE 3 shows the clear advantage of using doublein touch-panel display manufacturing), and copper (applications
pulse burst micromachining over single-pulse machining. By
in PCB and microelectronic packaging manufacturing).
splitting the energy available in a single 20ns pulse into two subpulses, an increase in ablation depth of up to 78 percent can be
Silicon dicing in semiconductor fabrication
achieved. Also, FIGURE 4 shows that double-pulse burst mode
Laser dicing of silicon wafers is an alternative to conventional
creates same depth scribe using 40 percent less energy than
dicing with a precision saw. As wafers have become thinner and
the single pulse and has less loose debris on the top surface.
lasers have become more powerful, advantages over saw-based
dicing increase dramatically. Achieving higher dicing speed
Glass cutting in flat-panel display
and good cut quality are very important to compete against
In the display manufacturing process, touch-screen and LCD
conventional saw processes.
modules require both straight cuts for singulating pieces of glass
We have demonstrated scribes at high scribing speeds
and curved cuts for creating features such as corners, holes, and
with minimal thermal damage to the material on ~100µm-thick,
slots. As glass substrates used in consumer electronics displays
polished, single-crystal silicon wafers using Quasar lasers.
continue to become thinner and stronger (through chemical or
In FIGURE 1, the curve (a) for single 25ns pulses at 200kHz
thermal treatment), laser glass machining tools are showing great
establishes the basic trend that as scribe speed increases, scribe
potential for providing high-quality cuts and high throughput
depth decreases. By taking advantage of higher power at higher
while reducing yield losses associated with the conventional
repetition rate and TimeShift technology,
which allows a wide range of softwarea)
b)
settable pulse energies and pulse widths,
we observed an almost-3X increase in the
speed over a single 25ns pulse scribing
condition for a 50µm-deep scribe.
FIGURE 2 shows debris and HAZ for
50µm
50µm
scribes carried out using the same energy
in a single pulse and using TimeShift to
create a burst of pulses at 500mm/s and
FIGURE 4. Comparison of alumina scribing quality using TimeShift technology. The top
200kHz. Scribes using this technology view (a) of the scribe used the single-pulse mode at 170µJ/pulse, while the same view (b)
resulted in high ablation quality with less
using the double-pulse mode enabled 101µJ/pulse. Scribe depth is 4µm in both cases.
t e c h n o l o g y
mechanical scribe-and-cleave process.
We have developed glass processing
techniques utilizing the laser-material
interaction effects created by the
TimeShift technology. In our patentpending process, tailoring of the individual
laser pulses reduces thermal loading and
the chipping or cracking it can cause in
the material. This has yielded good cut
quality at linear cutting speeds of over
1.5m/s in chemically strengthened glass
such as Corning Gorilla, Asahi Dragontail,
and Scott Xensation. Similar results have
also been obtained in soda lime glass,
advanced flexible glass such as Corning
Willow, and process development work for
machining sapphire is underway. FIGURE 5
shows results obtained in 0.7mm Gorilla
Glass having depth of the chemically
strengthened layer (DOL) of 40µm. It
shows clean-cut edges with minimal
chipping, and no visible micro-cracks.
FIGURE 5. Examples of straight line,
curvilinear, and hole cuts in 0.7mm Gorilla
Glass with DOL of 40µm, all obtained
utilizing the Quasar laser’s TimeShift
technology.
Copper cutting in advanced
packaging and interconnect
A typical flex circuit singulation application
involves clean and fast through-cutting of
thin (10–20µm) copper layers on a polymer
substrate. Also, via drilling in many PCB
constructions involves ablation of a
copper (Cu) layer of similar thickness. We
have investigated the potential effects
of a more subtle aspect of TimeShift
technology in these applications by
studying copper scribing using sub-pulse
(burst) processing to enhance the depth
a)
Depth (µm)
30
r e p o r t
b)
Depth (µm)
10×5ns pulses –
Variable pulse
separation
30
25
25
20
20
15
45µJ
15
10
20µJ
10
5
0
0
5
10
15
20
25
Pulse separation (ns)
30
5
0
0
5ns sub-pulses –
10ns pulse
separation
45µJ
20µJ
2
4
6
8
10
Number of 5ns sub-pulses
12
FIGURE 6. The effects of TimeShift features in copper scribing. Variation in material
removal rates with varying sub-pulse time separation (a), and varying number of subpulses (b). Total energy of each burst of sub-pulses was fixed at either 20 or 45µJ.
of grooves created in bulk Cu.
FIGURE 6A shows that 10 sub-pulses
separated by 10ns machined deeper
grooves than single pulse (0ns separation
case) of the same energy. Increasing
the pulse separation to 25ns, however,
resulted in lower material removal rates
than the single-pulse case. Effects such
as these can be easily isolated utilizing
the flexibility of TimeShift technology.
This can give the development engineer
insight into the laser-material interaction
mechanisms that may dominate the
machining results, permitting more rapid
and full process optimization for speed
and/or quality.
FIGURE 6B shows that for 5ns subpulse duration, dividing the total energy
in the pulse into a greater number of subpulses results in higher material removal
rates. Similar to the results shown
for silicon in FIGURE 1 and for alumina
in FIGURE 3, multiple sub-pulses also
tended to produce cleaner cut edges
with less debris.
pulse shape technology delivered by the
Quasar laser, significant advances in
micromachining can be achieved.
The processing benefits of the UV
laser have been demonstrated in several
common microelectronic materials used
in mass production, including silicon,
ceramics, glass, and copper. We have
shown that operating in new regimes of
process parameter space (higher power
at higher PRF), and utilizing the advanced
pulse splitting and shaping features, both
process speed and micromachining
quality can be simultaneously improved.
The results indicate that process recipe
development is straightforward using
this laser. With proper parameter
optimization, high quality and high
throughput can be achieved with this
new UV nanosecond pulsed laser source,
fueling the capabilities of today’s laser
micromachining processes to meet the
challenges of manufacturing tomorrow’s
consumer electronics products. ✺
ACKNOWLEDGEMENT
Summary
In manufacturing processes for mobile
consumer electronics devices, lasers are
routinely used to micro-machine a variety
of materials. We find that using the unique
combination of high UV power at higher
PRF, along with TimeShift programmable
Quasar is a registered trademark of
Spectra-Physics.
RAJESH PATEL ([email protected]) is
Director, Strategic Marketing & Applications, JAMES
BOVATSEK is Applications Lab Manager, and
ASHWINI TAMHANKAR is Senior Applications
Engineer, all at Spectra-Physics, Santa Clara, CA.

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