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 sponsored by:

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.

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Diverse materials expand applications for established, reliable CO2 technology

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Picosecond laser enables new hightech devices

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Smart optics improve surface treatment

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Slice and dice: Laser micromachining for consumer electronics

Diverse materials expand applications for established, reliable CO2 technology by Andrew Held

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he 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.

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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 Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

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 FIGURE 1: Schematic illustration of laser scribing. 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.

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The major drawback of mechanical cutting of polarization films is that it necessitates significant post-processing. Specifically, this is required to achieve Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

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.

FIGURE 2: A

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polarization film can consist of several layers of material. The film may curl if these layers are heated at different rates.

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.

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 2Dand 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: The

four steps of the laser stripping and mechanical cutting process.

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.

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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 Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

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 FIGURE 4: Perforating thin films is one of the key processes in food facilitate opening by packaging that utilize compact CO2 lasers. (Photo courtesy of LasX the consumer. However, Industries) 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

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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 Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

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.

8 Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

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.

[email protected] www.Coherent.com toll free: (800) 527-3786 phone: (408) 764-4983

Benelux +31 (30) 280 6060 China +86 (10) 8215 3600 France +33 (0)1 8038 1000 Germany +49 (6071) 968 333

Superior Reliability & Performance

Italy +39 (02) 31 03 951 Japan +81 (3) 5635 8700 Korea +82 (2) 460 7900 UK +44 (1353) 658 833

Picosecond laser enables new high-tech devices by Colin Moorhouse, Coherent Inc.

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he 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.

10 Industrial Laser Solutions for Manufacturing :: TECHNICAL DIGEST

Picosecond laser enables new high-tech devices

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 rollto-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

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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 (