TeCHnICAL dIGesT Laser Materials processing · TeCHnICAL dIGesT Laser Materials processing

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SPONSORED BY: TECHNICAL DIGEST Laser Materials Processing Lasers 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

Transcript of TeCHnICAL dIGesT Laser Materials processing · TeCHnICAL dIGesT Laser Materials processing

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sponsored by:

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

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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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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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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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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.

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[email protected] free: (800) 527-3786 phone: (408) 764-4983

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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.

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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.

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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

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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 &lte;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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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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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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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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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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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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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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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