TeCHnICAL dIGesT Laser Materials processing · TeCHnICAL dIGesT Laser Materials processing
Transcript of TeCHnICAL dIGesT Laser Materials processing · TeCHnICAL dIGesT Laser Materials processing
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TeCHnICAL dIGesT
Laser Materials processingLasers continue to play an expanding
role in materials processing, with new
applications developing all the time in
fields as diverse as microelectronics and
food packaging. Typically, non-contact
laser processing offers greater speed,
flexibility and lower cost than traditional
manufacturing methods, while frequently
delivering superior results. What follows is
a selection of articles that exemplify these
advantages and explores the diverse range
of laser technologies being employed to
achieve them.
2 Diverse materials expand applications for established, reliable CO2 technology
9 Picosecond laser enables new high-tech devices
20 Smart optics improve surface treatment
25 Slice and dice: Laser micromachining for consumer electronics
Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST
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Diverse materials expand applications for established, reliable CO2 technology
by AnDrew HeLD
THe LonG-wAve InfrAred light emitted by a CO2 laser is unique
among industrial lasers in that it can be used to process almost any
material, both metals and non-metals. This inherent advantage has
made the CO2 laser the industry “workhorse” for over 35 years, yet
to be displaced by any other laser type. Today, this well-established technology
continues to find use in new applications because they involve materials that
can only be processed with a long-wave infrared laser. This article presents some
specific examples of cutting-edge applications that now depend on fully sealed,
RF excited, slab CO2 lasers in the 30–1,000 W power range.
display glass cutting
Virtually all types of flat-panel displays (FPDs) currently in production utilize
thin sheet glass in their construction. Manufacturers of smartphones and other
handheld devices are using increasingly thinner glass for these devices in order to
minimize weight. However, this thin glass must still withstand rough handling,
from being dropped to being pressed upon (for touch screens).
Unfortunately, mechanical glass cutting doesn’t work well with substrates under
1 mm thick. It can produce microcracks, create debris, and leave significant
mechanical stress in the finished edge, making it easier to break. All these problems
necessitate further post processing, which takes time and increases production cost.
CO2 laser-based glass cutting is a non-contact process that completely eliminates
the problem of microcracking and chipping. Also, laser cutting produces
essentially no residual stress in the glass, resulting in higher edge strength. This
is critical, because regardless of where the breaking force is applied, the crack
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Diverse materials expand applications for established, reliable CO2 technology
initiates at the glass defect, often
found at the edges. Consequently,
laser-cut glass can withstand two
to three times as much force as
mechanically cut glass.
In one technique, called laser scribing
(FIGURE 1), a CO2 laser beam is
focused on to the surface of the glass,
which is being translated to create a
continuous cut. The 10.6 µm light is
strongly absorbed by glass, causing
localized, rapid heating. A jet of liquid
or air is then used to quickly cool
the glass, and the resulting thermal
shock produces a continuous crack
in the glass that is typically about
100 µm deep. The glass then passes under a mechanical roller or chopper bar
that imparts enough force to propagate the crack through the entire substrate
thickness and break it. This break is free of debris and perpendicular to the
surface. Typical laser sources for this process are the Coherent Diamond E400,
K250 and GEM100, providing 400, 250, and 100 Watts respectively at a wavelength
of 10.6 µm.
fpd film cutting
Another important trend in the FPD market is toward brighter, higher-resolution
displays at an ever lower cost point. A key technology in achieving this is the
use of advanced polarization films. Specifically, in LCD-based displays, the
liquid crystal material is sandwiched between a pair of orthogonally oriented
polarization films, and the display contrast ratio, viewing angle, resolution, and
brightness are all ultimately limited by the quality of these polarizers. Once
again, traditional mechanical (knife) cutting of polarization films presents
several limitations, and CO2 laser cutting is emerging as a key enabler in lowering
production costs and improving device quality.
The major drawback of mechanical cutting of polarization films is that it
necessitates significant post-processing. Specifically, this is required to achieve
FIGURE 1: Schematic illustration of laser scribing.
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Diverse materials expand applications for established, reliable CO2 technology
the desired edge quality, as well as to remove particulates created by the cutting
process that can subsequently contaminate the LCD. Post-processing includes
stacking the cut films, and then cleaning and polishing of the cut edges.
Another limitation of mechanical cutting is that it reduces process utilization. In
particular, polarization films are typically cut down from large rolls into smaller,
rectangular shapes having curved corners. This curved corner shape cannot be
produced with a series of straight line cuts running the length and width of the
roll, precluding the use of conventional slitters, for example. Instead the pattern
of each shape must be cut out individually, resulting in a small amount of unused
material left between each cut pattern.
CO2 laser cutting addresses both these concerns. It delivers better edge quality, and
does not produce significant contaminating particulates. The use of scanning optics
enables curved cuts without stopped motion of the roll. Laser cutting also produces
a smaller kerf width than mechanical methods, and better overall cutting precision
over the entire width of the roll. This enables cutting patterns to be nested much
closer together, which decreases wastage of the expensive polarization films.
Coherent offers Diamond E400i, K225i and G100i CO2 lasers with output at 9.4 µm
specifically for applications such as polarization film cutting. Many polymers,
including these polarizers, absorb more strongly at this wavelength than at 10.6
µm, making cutting at this shorter wavelength more efficient. Not only does this
enable cutting at lower powers (which lowers cost), but also avoids problems with
film curling.
Polarization films are actually
constructed of several individual
layers (FIGURE 2). Curling occurs
because of differences in laser
absorption (and subsequent
heating) between these layers.
However, virtually all the
materials used in polarization film
construction absorb strongly at 9.4
µm, thus avoiding this problem.
FIGURE 2: A polarization film can consist of several layers of material. The film may curl if these layers are heated at different rates.
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
Diverse materials expand applications for established, reliable CO2 technology
wire stripping
CO2 lasers have been used for some time for wire stripping because they can
deliver both high quality and greater flexibility than mechanical techniques.
One particular advantage of the mid-infrared output of the CO2 laser is that it
is readily absorbed by virtually all insulation materials, but highly reflected by
most conductors. Thus, the CO2 laser beam can easily cut through any insulation
type without damage to the conductor, and this process is highly controllable
and repeatable, even over a large range of laser powers. This enables the system
to remove insulation from many
types of wire and cable: single-core
wire, twin leads, shielded twisted
pairs, multiconductor cables, shielded
and screened wire and cable, ribbon
cable, coaxial cable, and complex 2D-
and 3D-shaped conductors such as
coils. Laser wire stripping can also be
accomplished with a fairly high degree
of accuracy (typically about ±0.003
in.), and cutting parameters do not
vary over time due to tool wear. This
technique is becoming increasingly
utilized to process the extremely fine
wires being used in highly compact
mobile electronic devices.
FIGURE 3 depicts one way in
which laser wire stripping can be
implemented. In this case, wire is
spool-fed into the system where its motion is controlled by belt drives. Stripping is
performed using two 30 W CO2 lasers, located on opposite sides of the wire from
each other.
Each of the laser beams is swept over the wire insulation using dual
galvanometer-mounted mirrors. Both lasers first make single cuts along the axis
of the wire. Then, each laser makes cuts perpendicular to the axis of the wire
at both ends of the first cut. Because there are two lasers coming from opposite
FIGURE 3: The four steps of the laser stripping and mechanical cutting process.
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Diverse materials expand applications for established, reliable CO2 technology
sides, this cuts the insulation virtually all the way around the circumference of
the wire. Next, the wire is moved to a cutting station, and a mechanical blade
cuts in the center of the stripped area.
One manufacturer of automated stripping workstations utilizes the Coherent
GEM 30. It determined that 30 W of output is optimal for this application, because
lower power might be insufficient for very high feed rates and higher power would
add unnecessary cost. The Coherent GEM 30 was chosen in particular because
it provides an ideal combination of small footprint, low cost-of-ownership, and
highly focusable beam with a smooth profile resulting in superior quality cuts
with clean edges.
food packaging
The CO2 laser is also finding use in cutting, slitting, scoring, and perforating many
of the newer materials used in food packaging. This includes cartons and pouches
made of several different plastics, foils, paper, or a laminated combination of
these materials. One key advantage of CO2 laser processing is that its high speed
is compatible with the rapid pace of existing production lines. Also, it is a flexible
digital process in which cutting parameters can be quickly changed through
software rather than hard tooling. This is increasingly important in the food
industry, where product cycles are short and manufacturers must often process
short runs, with quick switches between film types and packaging formats.
Once again, the availability of CO2 lasers with alternative output wavelengths is
useful for the flexible food packaging market. Because the films used in these
applications are very thin, they only absorb a small percentage of the incident
laser power. Since these polymers typically have an infrared absorption spectrum
consisting of numerous sharp peaks, small shifts in laser wavelength can have a
dramatic impact on absorption efficiency. Thus, optimizing laser wavelength can
substantially increase the processing speed for a given laser power level. For example,
the commonly used packaging material biaxially oriented polypropylene (BOPP)
absorbs much more strongly at 10.2 µm than at 10.6 µm, resulting in nearly a four
times increase in cutting speed at the shorter wavelength for 75 µm thick film.
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Diverse materials expand applications for established, reliable CO2 technology
Flexible packaging materials are often processed in a roll-to-roll format — that
is, the original material is unrolled and processed while it moves (at speeds of up
to 300 meters/minute), and then taken up by another roller. Rolls are typically
between 1-2 m wide. These materials can usually be processed with less than 100
W of laser power. The compact size of sealed CO2 lasers with output in the 30–100
W range enables several units to be arrayed across the width of the roll so that
multiple points can be processed simultaneously. Scanning optics are used to cut
complex patterns.
The power of each of the lasers can be independently and actively controlled
as needed to enable
precise depth control,
and to compensate on
the fly for variations
in material thickness.
This level of control is
particularly necessary
when processing
materials for “easy
opening” package
applications, in which
a material is partially
cut through in order to
facilitate opening by
the consumer. However,
it is critical that the
material not be cut completely through, as this would allow oxygen in and thus
shorten the shelf life of the food product.
Conclusion
While CO2 laser technology has been available for many years, it continues to
service an ever-growing range of cutting edge applications. This is primarily due
to three factors. First, the long-wave infrared output of CO2 lasers proves to be
a particularly good match for processing a wide range of materials, especially
those used in high technology products. Second, the development of a new
generation of compact, sealed CO2 lasers has resulted in sources that deliver
FIGURE 4: Perforating thin films is one of the key processes in food packaging that utilize compact CO2 lasers. (Photo courtesy of LasX Industries)
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Diverse materials expand applications for established, reliable CO2 technology
an unprecedented level of lifetime and reliability, together with low cost of
ownership and minimal required maintenance. Finally, the high quality beam
output and pulsing characteristics of these sealed CO2 lasers facilitates the
production of high precision features and enables excellent process control.
Dr. AnDrew HeLD is director of marketing at Coherent Inc.
[email protected] free: (800) 527-3786 phone: (408) 764-4983
Superior Reliability & Performance
Benelux +31 (30) 280 6060China +86 (10) 8215 3600France +33 (0)1 8038 1000Germany +49 (6071) 968 333
Italy +39 (02) 31 03 951Japan +81 (3) 5635 8700Korea +82 (2) 460 7900UK +44 (1353) 658 833
Reliable CO2 Lasers Give You the Cutting Edge.
Proven Laser Technology Handles Today’s Most Demanding Processing Tasks.
Coherent’s Diamond CO2 lasers, with their unique combination of long-wave IR output, high reliability and low cost of ownership, are able to process a wide range of materials at market enabling prices.
Diamond CO2 lasers offer:• Power ranges from 30 Watt to 1000 Watt• Application specific wavelengths from 9 to 11 microns• Outstanding beam quality• Maintenance free and ultra-compact size
Whether you’re cutting glass for smartphone displays, stripping fine wires, or precision slitting plastic for easy-open food packages, Coherent Diamond CO2 lasers give you the superior performance and reliability that your cutting edge application demands.
To learn more, visit our website at www.Coherent.com/Ads (keyword: Reliable) or call sales at 800-527-3786.
Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST
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Picosecond laser enables new high-tech devices
by COLIn MOOrHOuSe, Coherent Inc.
THe deMAnd To reduce the size, weight, and material cost of
leading-edge devices has resulted in a requirement for precision
micromachining to improve product development in several industries.
Examples include making smaller and more powerful smartphones
with brighter displays, reducing the cost and increasing the efficiency of solar
cells, and machining the latest bio-absorbable medical stents. The unrelenting
pace of innovation in high-tech industries has led to ultrafast (picosecond)
industrial lasers becoming important tools for applications requiring high
precision. These lasers’ unique operating regime (megawatts of peak power)
enables clean cutting and patterning of sensitive materials and thin films used in
a number of novel devices as well as micromachining of wide bandgap, “difficult”
materials such as glass. In several instances, the picosecond laser is replacing
multi-step photolithography with a single-step direct-write laser process; in other
cases it supplants traditional cutting/drilling processes because it eliminates
costly post-processing cleaning steps, such as stent manufacturing. With a choice
of near-IR, green, or ultraviolet output, these lasers can micromachine almost any
material bringing new technologies to market successfully.
FIGURE 1. Schematic of a basic OLED structure.
Picosecond laser enables new high-tech devices
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
patterning oLeds
Organic, flexible electronic devices have many intrinsic advantages including
their light weight, thin dimensions, and transparency. In particular, flexible
organic light-emitting diodes (OLEDs) have tremendous potential for displays,
since they consume less power than other displays that use backlighting, as well
as for general lighting. An OLED is formed by an organic emissive layer, light
emitting polymer (LEP), sandwiched between an anode and a cathode layer.
OLEDs can be manufactured on reel-to-reel production lines, using screen/
inkjet printing for patterned deposition of device circuitry 1, 2. However, these
technologies are limited in terms of the minimum achievable feature size,
and the ability to maintain thickness uniformity within each layer. Large-area
uniformity has been demonstrated using full-area deposition techniques such as
spin coating, and laser patterning of these layers is extremely attractive for roll-
to-roll processing.
Ultraviolet (355 nm) nanosecond lasers, which have proven themselves in
numerous other electronics micromachining applications, have two limitations
for this type of patterning. First, they produce some molten material or debris,
which cannot be tolerated due to the approximately 100 nm thickness of the
layers. Second, it is extremely difficult to prevent damage to the underlying layers
due to the thermal penetration depth of the nanosecond laser pulse being greater
than the thickness of the layers.3 In contrast, picosecond lasers can readily deliver
debris-free patterning and selective depth control, because material removal
occurs before the material can respond to the acoustic/thermal stress.4 However,
to achieve selective removal of thin films, it is critical to maintain the laser
fluence close to the laser ablation threshold, Fth. This quantity can be estimated
by making a curve fit to a plot of the single-shot laser-ablated crater diameter
against the pulse energy.5
As an example, a LEP layer has been laser scribed from a SiO2 barrier (this
material was provided by the Holst Centre as part of the fast2light project6),
where a fluence of 1.6*Fth is required to give the desired quality shown in
FIGure 2. The superior pulse-to-pulse stability (<2%) of Coherent’s fiber-based
Talisker laser is particularly advantageous for fine control of the pulse energy to
make consistent scribes, and there is no melt or debris on the surface without any
Picosecond laser enables new high-tech devices
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post-laser cleaning. The high laser repetition rate of 200 kHz allows a fast scribing
speed of 6 m/s, scalable to 15 m/s using the ultraviolet 500 kHz Talisker 500.
silicon solar cells
Solar cell manufacturers are currently striving to reach cost parity with grid power.
There are two ways to move toward this goal: reduce cell cost, and improve the cell
light-conversion efficiency. The majority of solar cells are currently based on silicon
wafers, and the use of thinner <e;200 μm wafers is helping reduce material costs.
One way to increase the efficiency of these crystalline silicon (c-Si) solar cells is to
use a backside passivation layer to reduce recombination losses. To make electrical
contacts to the cell, through-holes must be drilled through the passivation layer,
typically formed from silicon dioxide/silicon nitride — this must be accomplished
with minimal damage to the underlying crystalline silicon to maintain the cell
performance and ensure good ohmic contact for current collection. Picosecond
lasers offer a lower-cost, direct-write alternative to photolithography for this task,
eliminating the cost of vast quantities of chemicals. Specifically, studies have
found that 355 nm picosecond pulses offer selective removal of the dielectric layers
with little or no damage to the underlying silicon.7 Furthermore, comparisons
demonstrate that the ablation threshold fluence for UV picosecond lasers is lower
than that of IR femtosecond lasers.8 These results have been verified in FIGure 3 with a laser ablation threshold, Fth value of 0.02 J/cm2 observed for 355 nm, 10
ps pulses, compared to 0.08 J/cm2 for 1030 nm, 500 fs pulses. The lower threshold
FIGURE 2. Scribes in LEP material using 1 μJ, 355nm pulses (fluence of 1.6**Fth) at a scribe speed of 6 m/s at 200 kHz.
Picosecond laser enables new high-tech devices
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
value enables patterning at lower energy and hence reduced damage to the
underlying silicon.
FIGure 4 shows the exposed silicon surface following selective removal of a 100
nm SiO2 passivation layer from silicon using 355 nm picosecond pulses, at an
optimal fluence of ~1.5*Fth. Importantly, there are no melt features in the silicon as
can be found with nanosecond laser processing, thus eliminating the requirement
for post-laser chemical etch treatments. In addition, no sub-surface damage is
visible — some small particles in the center remain from the picosecond laser
ablated dielectric layer, but these can be removed easily by air. The high pulse
repetition rate of the Talisker laser allows contact holes to be drilled at rates of
200,000/second or more. Inclusion of this process within c-Si solar fabrication
allows efficiency gains of ~0.5-0.7%, which represents a significant advantage for
solar cell manufacturers.
FIGURE 3. Comparison of laser ablation threshold for 355 nm ps and infrared fs pulses for removal of a thin silicon nitride layer from silicon.
Picosecond laser enables new high-tech devices
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Thin-film solar cells
Despite improvements in
various c-Si architectures,
many in the solar industry
believe that thin-film solar
designs are the long-term
future, and represent the
approach that is most
likely to achieve grid
parity. This is because of the smaller amount of semiconductor material needed
in these designs, plus the potential for low-cost mass production via reel-to-
reel processing of flex substrates. Materials such as CuInSe2 (CIS) and CuInGaS2
(CIGS) exhibit wide optical absorption across the visible spectrum, making them
attractive as efficient photovoltaic materials in these devices. A typical thin-
film solar cell structure is shown in FIGure 5, where the high light absorption
means that the active layer is only ~1 μm thick, greatly reducing material costs
compared to c-Si solar cells.
To make a solar cell from this structure, P1, P2 and P3 scribes are required as
indicated in FIGURE 5 (and have been described numerous times in ILS magazine).
P1 scribes serve to isolate the back contacts of adjacent cells by patterning the
back-contact metal, which is usually molybdenum (Mo). P2 scribes open contacts
from the frontside to the backside of adjacent cells in order to connect the cells
in series maximizing the voltage output. P3 scribes are required to isolate the
front contacts of the cells by cutting through the transparent conductive oxide
(TCO) layer and CIGS down to the Mo back contact. P3 scribes are currently made
mechanically with mechanical “pins,” but these scribes are prone to leaving
residual material and can cause chipping of the layers, and the mechanical
pins also require frequent recalibration/maintenance. In addition, when the
supporting substrate is metal or plastic, mechanical scribing can damage these
layers, making a laser process more attractive.
FIGURE 4. Removal of a thin film silicon dioxide from silicon using 355 nm ps pulses at a fluence of ~0.11 J/cm2.
Picosecond laser enables new high-tech devices
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One of the main difficulties with laser ablation of CIGS has been that on melting,
it undergoes a phase transition from semiconductor to metal. Hence, any molten
CIGS in the scribe may potentially short-circuit the cell, reducing efficiency.
However, using a picosecond laser, together with careful control of the laser
fluence, it is now possible to produce high-quality clean scribes as shown in
FIGure 6(a) using 1064 nm pulses. An improved, extremely smooth, mirror-like
Mo surface is obtained using 532 nm pulses as in FIGure 3(b). This is most likely
due to the reduced plasma generated and shorter optical penetration depth at
532 nm. In both cases, complete removal of the CIGS from the scribe has been
demonstrated, with no molten material or electrical contact between the TCO
edges and molybdenum back-contact layer, resulting in high efficiency solar cells.
bioabsorbable stent cutting
Over half a million stents per year are used as permanent implants in the US
alone to prop open blood vessels. One complication of early, all-metal stents
was restenosis, where plaques form on the stent, re-blocking the opened blood
vessel. In response, stent manufacturers developed a second generation of stents,
coated with a bioabsorbable plastic containing an anti-restenosis drug. As the
coating dissolves over months, this drug slowly elutes on-site. However, there are
still potential long-term, post-operative complications (POCs) with these stents,
including a heightened risk of stroke due to clots forming as blood flows over the
stent surface. This issue can be eliminated with stents made completely from
bioabsorbable materials, which disappear completely after providing a support
framework over the critical months after vessel opening.
The polymers used in these stents must be strong enough to withstand stresses
within the body, and there are already many types of bioabsorbable materials
that meet this criterion.9 Extremely high precision (<20 μm) is obviously required
FIGURE 5.
Schematic of a typical CIGS structure and P1, P2, P3 scribes.
Picosecond laser enables new high-tech devices
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to manufacture the stent structure and deliver the requisite smooth surface
finish, making the laser an obvious choice for this task. Also, traditional metal
stent manufacturing techniques such as electrical discharge machining (EDM)
are not applicable to polymeric materials. However, polymers typically have low
thermal conductivity and low melting points, making it extremely challenging for
a laser to cut them precisely, and any remaining molten material usually results
in the loss of bioabsorption properties. The bioabsorbable materials also tend to
be highly transparent, limiting energy coupling of the laser light.
The improved optical absorption available at deep UV wavelength means that
excimer lasers have been a frequent choice for micromachining polymers. An
alternative approach is to use UV (355 nm) picosecond pulses focused to a small
spot size (~10 μm, which facilitates higher optical absorption in a small localized
area. Since polymers are good thermal insulators, the laser repetition rate must be
carefully limited (<100 kHz) and a high scan speed used to maintain low pulse
overlap, to produce high quality, fine features. FIGure 7 shows high-quality cuts
have been made in polylactide (PLA) material using 355 nm picosecond pulses.
Laser drilling of transparent materials
The strength, chemical inertness, and high transparency of glass are the reasons
for its wide use in many consumer electronics, such as touch screens, handheld
devices (tablets, smart phones, etc.), and flat-panel TVs. Also, future devices
FIGURE 6. Scribes in CIGS material made using 532 nm and 1064 nm picosecond pulses.
Picosecond laser enables new high-tech devices
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are anticipated to rely
more heavily on glass
to provide structural
rigidity and to create
features holes need to
be drilled through the
glass. However, a glass
substrates get thinner
to support smaller
and lighter devices, traditional glass-drilling techniques struggle to maintain
the quality demanded — even established industrial lasers such as carbon
dioxide (CO2) and nanosecond lasers have difficulty drilling these holes without
significant microcracking and chipping. It is critical that edge cracking and
residual edge stress are avoided in touchscreens where the panels almost always
break from the edge, even when stress is applied to the center.
The high peak power of picosecond lasers can avoid this problem, and since
glass is essentially transparent to visible wavelengths, 1064 nm offers the
highest average processing power for maximum removal rates whereas the UV
wavelength is better suited for thinner glass panels. As examples, FIGureS 8 and 9 show high-quality 1 mm-diameter holes drilled in 1 mm-thick D236T glass
and quartz using 1064 nm picosecond pulses. The microchips on the edges are
typically only a few microns — and more crucially, there are no microcracks on
the edges or sidewalls, which prevents fractures, making this process ideal for
handheld devices.
summary
Picosecond lasers are now industrially proven, robust tools, offering sturdy and
reliable performance suitable for a wide variety of industrial applications, such
as the examples described here. The combination of short picosecond pulses,
excellent pulse-to-pulse stability, and high repetition rates deliver precision cuts
FIGURE 7. Cuts through 200 μm PLLA bioabsorbable tube using 10 μJ 355nm picosecond pulses.
Picosecond laser enables new high-tech devices
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and patterns with
sharp, clean edges with
no requirement for
post-process cleaning.
Additionally, their
fine control of laser
energy and pulse-to-
pulse stability enables
blind holes, cuts, and
scribes to be created
with unprecedented
accuracy, even in
delicate materials
such as polymers. Moreover, the wavelength flexibility (1064 nm, 532 nm and
355 nm) and excellent beam quality (M2<1.3) of the Talisker Ultra picosecond
laser platform can
be utilized for laser
processes requiring
extremely tightly
focused laser beams
with high peak
power.10,11,12 The
Talisker 500 shares
a similar platform
design with a single-
output wavelength for
optimal performance
FIGURE 8. 1 mm-diameter hole drilled in 1 mm-thick D236T glass using 14 W of 1064 nm picosecond pulses.
FIGURE 9. 1 mm-diameter hole drilled in 1-mm thick quartz using 14 W of 1064 nm picosecond pulses.
Picosecond laser enables new high-tech devices
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
in production environments which is particularly advantageous for precise, small
feature sizes in glass, which is gaining market share in emerging applications as
alternatives to existing laser and non-laser processes.
references1. T.R. Hebner et al., “Ink-jet printing of doped polymers for organic light emitting
devices,” Applied Physics Letters 72(5), 519-521 (1998).
2. G.E. Jabbour et al., “Screen printing for the fabrication of organic light emitting devices,” IEEE Sel.Top. Quantum Electron. 7(5), 769-773 (2001).
3. A.F. Lasagni et al., “Direct laser interference patterning of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS) thin films,” Applied Surface Science, vol. 22, pp. 9186-9192, 2009.
4. D.M. Karnakis et al., “Ultrafast Laser Patterning of OLEDs on Flexible Substrate for Solid-state Lighting,” Journal of Laser Micro/Nanoengineering, vol. 4, pp. 218-223, 2009.
5. P.T. Mannion et al., “The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air,” Applied Surface Science 233(1-4), pp. 275-287, 2004.
6. www.fast2light.org
7. V. Rana and Z. Zhang, “Selective Removal of Dielectric Layers using Picosecond UV Pulses,” Proceedings of SPIE, vol. 7193, pp. 719321, 2009.
8. M. Bähr et al., “Ablation of dielectrics without substrate damage using ultrashort pulse laser systems,” Proceedings 25th EUPVSEC, 2010.
9. P. Törmala et al., “Bioabsorbable polymers: materials technology and surgical applications,” Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, vol. 212, pp. 101-111, 1998.
10. Coherent white paper: “Flat panel display defect repair using high peak-power picosecond lasers.”
11. Coherent white paper: “Micromachining of sapphire wafers for LED production using picosecond lasers.”
12. Coherent white paper: “Micromachining of glass using a fiber-based, high average power picosecond laser.”
COLIn MOOrHOuSe is with Coherent Inc.
Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST
20
Smart optics improve surface treatment
Higher output power and customized beam shapes substantially improve heat treating and cladding
by KeITH PArKer
LAsers HAve been effectively employed for several years for surface
treatment applications of heat treating and cladding of metal surfaces.
Recent advances in direct-diode laser design now deliver substantially
better results for these applications. In particular, higher laser output
power, combined with more flexible beam-shaping optics, have resulted in
significantly increased product throughput. Higher laser efficiencies also
minimize operating costs. This article reviews the key characteristics of current
high-power direct-diode lasers and optimized beam-shape optics, and presents
quantitative test results that verify the process improvements that these lasers
can deliver.
background
Cladding and heat treating (hardening) are processes used on metal parts to
improve their surface properties, such as resistance to mechanical wear or
corrosion. Cladding involves melting a material, usually supplied in powder or
wire form, onto a part to create a metallurgically bonded surface layer having
a different composition than the base material. In contrast, hardening entails
quickly heating the surface of the base material and then rapidly cooling it in
order to change its crystalline form, making it substantially harder (but more
brittle) than the base material.
Lasers provide a very effective means for producing the highly localized heating
necessary to accomplish both cladding and heat treating. High-power laser diode
systems in particular are well-matched to the needs of this application for several
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reasons. First, their near-infrared wavelength is relatively well absorbed by most
metals. Also, the conversion of electricity into laser light is inherently more
efficient with semiconductors than for any other laser medium. For example,
a diode laser system might have a total conversion efficiency of 50% (electrical
input to light delivered at the workpiece), while even an efficient fiber laser
converts only 35% of its input into light. Finally, the naturally rectangular-shaped
output beam from a diode laser array is well matched to the needs of large-area
applications. In particular, the rectangular cross-section of the diode laser array
output can be easily re-imaged to a line of laser light. This line is then scanned
across the surface in a direction perpendicular to its long axis to enable rapid
processing of large areas. This eliminates the need encountered with other lasers
to employ an optical system to transform (with some inevitable losses) a small
round beam into a large, rectangular shape.
Laser advances
In the past, most high-power diode laser cladding has been performed with
systems having output power in the 1–4 kW range. One significant advance in diode
laser technology has been to push this output level to 8 kW through a number of
incremental improvements. These include the use of more efficient power supplies
which are specifically configured to act as constant current (rather than voltage)
sources, as well as advances in the semiconductor materials and cooling methods.
The result is the ability to derive greater output power than previously available
from a given form factor device. Most importantly, this power has been achieved
while retaining the inherent high efficiency of the semiconductor laser source.
A significant practical advance for large-area cladding and heat treating
applications has been the development of modular beam-shaping optics that
enable the beam dimensions at the work piece to be precisely matched to
the process requirements. For example, Coherent’s flexible implementation is
accomplished by a set of optics that can be rapidly changed, literally without
the use of any tools. Two different sets of optics are employed: one to change
the length of the beam, and another to alter the beam width. Thus, the nominal
output from the Coherent laser (the 8 kW output model) is 1 mm wide by 12 mm
long. Two different beam optics modules can change this length to either 24 mm
or 6 mm. A separate set of optics enable expansion of the 1 mm width to 3 mm,
6 mm, 8 mm, or 12 mm. These can be combined to achieve a total of 15 different
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beam shapes. Furthermore, all these optics are configured to maintain a constant
working distance (from laser to focal plane), so that changing the beam shape
does not necessitate any other setup changes.
Another significant improvement in high-power diode laser implementation is
the use of closed-loop pyrometer control. Specifically, an optical pyrometer is
used to measure the temperature of the work piece within the laser interaction
zone. For the aforementioned unit, this is accomplished by sampling two
separate wavelengths to enable absolute, rather than simply relative temperature
measurement. The pyrometer signal is then used as a feedback to control laser
driver current so as to maintain a constant temperature on the workpiece.
The ability to hold a constant process temperature delivers more consistent
results and maximizes system efficiency. In particular, it enables the system to
automatically compensate for changes in part thickness (which affects the ability
of the workpiece to conduct heat
away from the interaction zone),
variations in the bulk material, or
changes in the relative speed of
the beam with respect to the part.
The latter is particularly important
because relative beam speed
usually changes when part motion
goes through a 90° or 180° turn.
In the past, determining the exact
parameters needed to maintain
constant temperature during such
a turn usually required lengthy
advance experimentation, and
limited the ability to accommodate
later alterations to processing
geometry. All this setup time is
eliminated by pyrometer control
since constant temperature is
maintained on the fly while
flexibility is maximized.
24 mm
a)
b)
23 mm
20 mm
20.4 mm
0.6 mm
FIGURE 1. Results of high-power diode laser cladding on a 12 mm thick, 1018 Low Carbon Steel substrate, using Hoganas Cladding Power 1559-40-60% (60% WC) and an argon cover gas.
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Improved process results
A study examined the effects of laser power and line beam size on effective
throughput for both cladding and heat treating, which produced surprising
results. In the first experiment (FIGURE 1), the time required to clad a 1 m2 area
continuously with a constant thickness of material, utilizing various different
laser powers and line beam sizes, was measured. Measuring the time required
to clad a given area with a certain thickness, rather than just linear processing
speed, is important because this incorporates all the process factors that come
into play, and is ultimately the metric of concern to users.
As expected, the higher-power system required the shortest amount of time to
clad the full 1 m2 area (TABLE 1). Specifically, at the highest power (7 kW and
a 3 × 24 mm line beam with matching, co-axial cladding nozzle), it took 104
minutes to clad 1 m2. In contrast, processing the same area with 4 kW and a
1 × 12 mm line beam took 250 minutes. Clad quality was similar at both powers.
The unexpected aspect of the result is that, although the power increased by only
a factor of 1.75 (4 kW to 7 kW), throughput increased by a factor of 2.5. This is 40%
higher than expected from a linear power-to-throughput relationship. The key
element in achieving this non-linear increase is altering the line beam width in
order to make maximal use of the increased laser power.
A similar experiment was performed for large-area heat treating. The laser was
operated at 7 kW, and optics were used to transform the length and width of the
line beam to a final rectangular shape of 8 × 24 mm at the workpiece (FIGURE 2).
To achieve a hardness of 61 HRC, a travel speed of 27 mm/s was chosen with the
7 kW system. This resulted in a heat-treated surface width of 20.4 mm, and a
maximum case depth of 0.6 mm.
HigHLigHt Laser ModeL
Power(kw)
BeaM diMensions
(MM)
CLad widtH at 1 MM tHiCkness
(MM)
effeCtive CLad widtH(MM)
nuMBer of Passes to Cover
1M2
tiMe/Pass(seC)
totaL ProCessing tiMe
(Min)
4000L 4 1 × 1 2 12 8 125 120 250
8000d 7 3 × 24 23 20 50 125 104
TABLE 1. Summary of results for high-power cladding
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When operating at 4 kW, a 8 × 12
mm rectangular beam was utilized,
with a travel speed of 17 mm/sec in
order to achieve the same hardness
within the same case depth. In this
instance, the heat-treated surface
width was only 13 mm, with a
maximum case depth of 0.7 mm.
With a 1.6× faster travel speed, and a
1.6× wider heat treatment profile, the
time needed to heat treat 1 m2 area
is 2.6× shorter at 7 kW than at 4 kW (TABLE 2). Interestingly, this shows the same
non-linear relationship between laser power and throughput as for the large area
cladding experiment — namely, a power increase of only 1.75× resulted in a 2.6×
faster throughput.
Conclusion
The combination of higher output power with easily customized beam shapes
delivers a substantial improvement in results for large-area heat treating and
high-throughput cladding. In particular, this provides the ability to use longest
lines at which laser power is still sufficient to melt the clad material. This
translates directly into maximum throughput for a given power. Similarly,
for heat treating, laser power and line beam dimensions influence the rates
of heating and cooling. Thus, the ability to set these parameters enables the
resultant case depth and surface hardness properties to be precisely controlled.
Keith Parker is senior business development manager, direct diode and fiber laser
systems, at Coherent Inc.
HigHLigHt Laser ModeL
Power(kw)
BeaM diMensions(MM)
Case dePtH(MM)
nuMBer of Passes to Cover
1M2
tiMe/Pass(seC)
totaL ProCessing tiMe (Min)
4000L 4 1 × 1 2 0.7 90.1 58.8 89
8000d 7 3 × 24 0.6 55.9 37.0 35
TABLE 2. Summary of results for high-power heat treating.
24 mm
a)
b)
23 mm
20 mm
20.4 mm
0.6 mm
FIGURE 2. High-power diode laser heat treating steel produced 61 HRC to a maximum depth of 0.6 mm. Part distortion was minimal.
Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST
25
Slice and dice: Laser micromachining for consumer electronics
More efficient laptop screens, higher capacity flash memory sticks and faster computer processors all result from the replacement of mechanical cutting methods with laser micromachining
by vICTOr DAvID
over THe pAsT few years, notebook computer battery life has tripled;
the capacity of memory cards has increased while their cost has
declined; and computers, smart phones, and other digital devices
have become ever faster and more powerful. While many factors have
contributed to these improvements, the increased use of laser micromachining is
a common enabling theme. Consequently, the demand for laser micromachining
in the electronics industry has probably never been stronger.
bright Leds for long battery life
The use of efficient LEDs instead of inefficient cold cathode lamps as the backlight
source in liquid crystal displays has dramatically extended battery lifetime in
laptop computers and reduced energy consumption in televisions. As a result, the
LED industry is experiencing unprecedented growth.
LEDs used in flat panel displays are based on gallium nitride (GaN), which is
grown and patterned as thin (a few microns total) layers on a sapphire wafer.
Sapphire is ideal because it provides a lattice match for the GaN and is also
transparent. This is important because some of the light escapes the LED by
partially passing through the edge of the sapphire substrate. Sapphire is also
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
a fairly good thermal conductor, which
helps in heat sinking the LEDs. But
unfortunately, sapphire is a notoriously
difficult material to cut, second only to
diamond.
In practice, LEDs are patterned in bulk
on sapphire wafers measuring 2 inches
in diameter with a typical thickness of
about 100 microns. Thousands of LEDs
can be produced on each wafer because
the final LED chip may measure only
0.5 mm × 0.5 mm or even less. The LEDs
are then physically separated in a process
called singulation.
Traditionally, singulation was carried
out by scribing (partial cutting) with
a diamond saw wheel, followed by
physical snapping. But today, most LED
manufacturers have switched to laser
scribing, again followed by physical
snapping using a pressure edge (see
FIGURE 1). Here a focused, pulsed UV beam partially cuts through the sapphire.
Typically several passes are used to cut through approximately 30% of the wafer
thickness (see FIGURE 2). Conventional physical snapping follows.
Laser scribing has become the preferred method for several reasons. First, by
focusing the beam down to a spot size of a few microns or less, the laser scribe
can be much narrower than a saw cut and with significantly less edge damage
(cracking and chipping). This means that LED devices can be packed closer
together with narrower gaps, called streets. The high quality edge also eliminates
the need for post processing, which is impractical on such tiny devices. All this
translates into higher yields and therefore lower unit cost. In addition, tight
focusing enables fast scribing at lower laser powers, thus minimizing the cost of
implementing lasers.
266-nm laser 355-nm laser
Laser
SapphireSapphire
266-nm laser 355-nm laser
FIGURE 1. Bright LEDs are created on a thin sapphire wafer and then separated (singulated) by laser scribing followed by physical snapping with a pressure edge.
FIGURE 2. In LED singulation, a 266 nm (or 355 nm) pulse UV laser is used to scribe through approximately 30% of the total sapphire wafer thickness, followed by mechanical snapping.
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
What laser characteristics does scribing require? The most common laser
singulation method is front side (the device side) scribing using a 266 nm,
Q-switched DPSS laser. One of the most important laser parameters is beam
quality because a low M2 ensures good edge quality and allows minimum LED
separation. Basically, M2 is a number that describes how tightly a laser beam can
be focused; a perfect Gaussian beam has the theoretical minimum focused spot
size defined by M2 = 1. For all real lasers, usually M2 >1. (Many LED manufacturers
use the Coherent AVIA 266-3
principally because of its M2 <1.3
rating). Other key laser parameters
are reliability, pulse-to-pulse stability,
and an average power of at least 2.5
W to achieve target throughput rates.
Alternatively, a few manufacturers
scribe from the backside of the
sapphire using a 355 nm laser; this
wavelength produces some minor
debris so cutting from the backside
keeps this away from the LEDs
themselves. Here, beam quality is
even more important as sapphire is
quite transparent at 355 nm and can
only be machined at this wavelength
by using a high focused intensity to drive nonlinear absorption. Popular models
for this method are either the AVIA 355-5 or 355-7, again because both have an
M2 value of <1.3. In addition, a few LED manufacturers are investigating the
use of hybrid picosecond lasers such as the Coherent Talisker, where a 532 nm
wavelength should produce equivalent results to nanosecond pulses at 266 nm.
More memory in less space
The capacity of SD and microSD memory cards has been steadily increasing over
the past several years, yet the physical size and shape of these cards necessarily
remains the same. Plus, the unit cost per MB has dropped dramatically. The
two primary factors that have enabled this are greater circuit density through
advances in microlithography and the use of physically thinner wafers so that
more can be vertically stacked together within a given sized package.
27.2 µm
FIGURE 3. As silicon wafers for memory chips get thinner, the maximum sawing speed gets successively slower. In contrast, maximum speed for laser cutting gets considerably faster.
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
At present, typical memory wafer thickness is currently 80 microns or less; 50
microns is considered cutting edge; and 20-micron wafers are being investigated
at the R&D level. For economies of scale, these wafers are up to 300 mm in
diameter. Since silicon is a crystalline material, a 300 mm x 50 micron wafer is
incredibly delicate and easily chipped or broken by mechanical contact. And, with
a typical post-process value of well over $100K, breakages must be avoided during
the singulation process.
Traditionally, singulation involved multiple passes with a diamond saw wheel.
But at 80 microns thickness, the saw must be slowed to an uneconomical rate
using low cut pressure to avoid chipping, cracking, and breaks (see FIGURE 3).
This has created tremendous opportunities for lasers. Many chip producers have
now switched to cutting with a Q-switched 355 nm DPSS laser. Like the saw,
laser cutting has to be done in multiple passes to minimize thermal damage,
which is removed by subsequent post-processing. For this reason, the single most
important laser parameter is a very high pulse repetition rate. Specifically, the
typical scan rate is 600 to 750 mm/sec in order to achieve an overall cut rate of
about 150 mm/sec with around five passes. Plus, this application needs very good
edge quality that requires 50% pulse-to-pulse spatial overlap. Coherent therefore
developed a very high repetition rate laser just for this thin wafer application
(the AVIA 355-23-250), which combines a 250 kHz pulse rate with power output
>8 W to deliver sufficient cutting power per pass. There is also growing interest
in process development using hybrid picosecond lasers since the shorter pulse
duration produces much less heat affected zone (HAZ), eliminating the need for
post-processing.
faster computers and phone applications
As integrated circuit features shrink, the insulating gaps between circuit
interconnects become narrower. Traditionally, the insulating material used in
these gaps is silicon oxide. But, higher circuit speeds require lower impedance
lines, which means using materials with a lower dielectric constant, i.e., higher
resistance. Thus, there is an interest in switching to so-called “low-κ materials,”
that is, materials with a lower dielectric constant (denoted κ).
Low-κ can be achieved by using traditional silicon oxide, but at lower porosity. In
addition, entirely new materials are being considered, again often with increased
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
porosity to increase the air content
and thereby further lower their
κ value. As with memory chips,
these fast processors are created as
thin epitaxial layer objects that are
densely packed on a large silicon
wafer. The problem here with
singulation is that low-κ materials are
all soft. Thus, traditional diamond
sawing can cause considerable
damage, including delamination, to
the circuits (see FIGURE 4). However,
these are thicker wafers than
memory devices, so laser sawing is not quite economically practical at this time.
As a result, a hybrid process is now becoming the preferred method. Specifically,
a 355 nm, Q-switched DPSS laser is used to cut through the soft epitaxial layers
to create crack stops. This is then followed by mechanical sawing through the
wafer itself. Two versions are
currently used as shown in
FIGURE 5. For wafers designed
with wide streets between
the individual circuits, the
laser may be used to make
narrow scribes down either
edge of each street, in a single
pass. With narrower streets,
several beams in parallel may
be used to make a single
scribe that is wide enough
to accommodate the saw
blade cut. The former is
more commonly used as
it requires less laser power for a given throughput, i.e. lower processing costs.
Key laser parameters here are beam quality and high repetition rate. A typical
laser for this application is the AVIA 355-23-250 which provides the requisite 30
Laser cuttingSaw cuttin
g
Increasing thickness
Maximumcuttingspeed
FIGURE 4. With so-called low-κ materials, mechanical sawing can cause major damage to the integrated circuits.
Low-κ layer
Silicon
Mechanical saw causes delamination
FIGURE 5. Chips using low-κ materials use laser scribing down the street between the chips. The laser scribes act as crack stops enabling high speed sawing with no damage to the circuitry.
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Industrial Laser Solutions for Manufacturing :: TeCHnICAL DIGeST
microjoules per pulse and M2 < 1.3. Moreover, it can deliver these specifications
at a repetition rate of 250 kHz, which supports 200 mm/sec scribe rates with 50%
pulse-to-pulse overlap.
Conclusion
In conclusion, the shrinking dimensions of electronic components, together with
a shift in materials, continue to make laser scribing an ever-more attractive and
economically viable process. Plus, laser manufacturers have worked to improve
the performance, reliability, and cost of ownership characteristics of their
products to even further broaden the range of tasks for which they are applicable.
vICTOr DAvID is senior product line manager with Coherent Inc.
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Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST
Coherent, Inc. is one of the world’s leading providers of lasers and laser-based solutions. Our products feature superior reliability and performance, and provide significant cost advantages for commercial and industrial customers competing in the most demanding markets. The unique characteristics of our product portfolio, combined with our history of innovation, provide a decided advantage to our scientific customers as they seek breakthroughs in research or in the development of cutting-edge applications.
Founded in 1966, we design, manufacture and market laser sources, laser tools and systems, accessories and components for a wide range of markets and applications. In addition to laser sources and tools, we also offer leading-edge beam forming and beam guidance systems as well as laser beam measurement and control equipment.
The capabilities of our products are exceptionally diverse. That’s why they are used in such a wide range of markets and applications: microelectronics, including semiconductor test and measurement, and advanced packaging; graphic arts and display; materials processing; instrumentation for biotechnology and medical imaging; production of flat panel displays and solar cells; and, of course, in advanced engineering, genetics, biology, chemistry, and physics.
For more details about Coherent’s Portfolio of Lasers for Materials Processing applications, refer to the links below.
LInks:
DIAMOND C02 Lasers
Talisker Family of Fiber-Based Lasers
HighLight Diode Laser Systems
AVIA Family of Pulsed Q-Switched Lasers
Applications Finder for Materials Processing