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Picosecond lasers come of age for micromachining PROVIDES USERS WITH PRECISION, THROUGHPUT, AND CoO

MAGNUS BENGTSSON, DIRK MĂœLLER, AND BERNHARD KLIMT

I

t has been known for some time that lasers with have pulsewidths in the tens of nanoseconds range and which pulsewidths in the picosecond regime can offer remove material via a photothermal interaction (see FIGURE 1). signifcant advantages over longer pulse lasers Here, the focused laser beam acts as a spatially confned, for a wide variety of industrial microprocessing intense heat source. Targeted material is heated up rapidly, tasks. In particular, these benefts include virtueventually causing it to be vaporized (essentially boiled away). ally no heat affected zone, as well as the The advantage of this Photothermal Interaction ability to work with an extremely broad approach is that it enables Nanosecond range of materials, even those that are transparrapid removal of relatively large pulsewidth Recast ent in the visible and near-infrared. However, early amounts of target material (parlaser beam material picosecond lasers did not possess the necessary ticularly considering the multiSurface debris reliability, cost of ownership (CoO) and other prackHz repetition rates at which tical characteristics required to make them feaQ-switched lasers typically sible for use in many production environments. A operate). Furthermore, nanoHeat-affected Microcracks zone new generation of industrial picosecond lasers has second laser technology is well now emerged that provides the qualities needed to established, and these sources Photoablation deliver on the promise of this technology. This article are highly reliable and have Picosecond explores the principal differences between nanosecattractive CoO characteristics. pulsewidth laser beam ond and picosecond laser processing, reviews the However, for the most demandbasic architecture of currently available picosecond ing tasks, the size of the HAZ sources, and explores how they are being successand the frequent presence of Minimal Min No signfcant nfcant fully used in industry today. some recast material around heat-affected heat-a microcracks ks or zone surface debris ebris the laser-produced feature, or Picosecond vs. nanosecond delamination of surface coatFIGURE 1. Comparison of The goal of micromachining is spatial selectivity, that is, ings, can present a limitation. photothermal interaction vs. the production of micron-scale features, such as holes The second mechanism for photoablation. and grooves, while avoiding peripheral thermal damlaser material removal is based age to surrounding material. In other words, the goal on photoablation (FIGURE 1). is precise, clean cuts with minimal heat affected zone (HAZ). Here, laser photons directly break the bonds holding the tarThere are two basic mechanisms by which a laser can preget material together, essentially atomizing it. This is a relatively cision drill, scribe, or cut a material. Many traditional applicold process leading to less HAZ. Plus, it is very clean, leavcations rely on infrared and visible Q-switched lasers, which ing no recast material, with minimal need for post-processing. www.industrial-lasers.com

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The high energy of ultraviolet (UV) photons means that in many energy of these pulses is then boosted in an amplifer to produce materials they can drive photoablation. Thus, Q-switched lasers the fnal output. with output in the UV remove material, at least to a certain extent, Most commercial picosecond products are based on one of the through photoablation. However, another way of achieving a purely following architectures: photoablative interaction with a material is through the use of • a fber laser oscillator followed by a fber or rod type amplifer, pulses in the picosecond domain or shorter. These ultrashort • a fber laser oscillator followed by a free space amplifer, and pulses allow for very high instantaneous peak powers (megawatts • a diode-pumped, solid-state oscillator followed by a free space and above), and their high fuence drives multiphoton absorpamplifer. tion, which excites electrons in the material and directly breaks The all-fber (oscillator and amplifer) approach has the advanatomic bonds (FIGURE 2). Moreover, tage of being relatively low cost and because the pulsewidth is shorter robust. A negative is that nonlinearia) 355-nm nanosecond laser than the thermal diffusion rate in the ties, scattering, and other effects in machined material, heat from residthe fber amplifer limit the maximum ual thermal effects is largely removed per pulse energy that can be attained with the atomized material before it to about 10 µJ (at a 10 ps pulsewidth). can spread as a HAZ. Thus, the only way of achieving high Besides producing effectively no average power is by increasing the HAZ, another major advantage of repetition rate. This poses practical ultrafast processing is that it is comdifficulties with the beam delivery patible with a very broad range of system because most beam defecmaterials, including several high bandtion mechanisms, such as galvanomgap materials (e.g., glass and certain eter scanners, are not fast enough to polymers) that have low linear, optical keep the individual pulses from overabsorption and hence are diffcult to lapping at the work surface. For examb) 355-nm picosecond laser machine with existing, commercially ple, with a 50 µm diameter focused available lasers. Specifcally, the techspot size and a pulse repetition rate nique is “wavelength agnostic,” that is, of 1 MHz, the scanner velocity of a nonlinear absorption can be induced 50 m/s scanner would be required to even if the material is normally transavoid pulse overlap. Only a few scanmissive at the laser wavelength. ners can deliver this rate of speed. Picosecond lasers are currently The result is that process throughput commercially available with outbecomes limited. put from the infrared to UV. GenerIn order to achieve the higher pulse ally speaking, UV picosecond lasers energies required for most applicadeliver the best results in terms of tions, a fber oscillator can be mated high precision and minimal HAZ. This with a free space amplifier. This is because they operate in the most approach, for example, is utilized in purely photoablative regime, and Coherent’s Talisker laser. Because of FIGURE 2. The same feature is produced in carbon can also be focused to the smallest fber reinforced polymer (CFRP) with a (a) 355-nm the relatively low energy output from spot sizes (due to diffraction). On the the fber seed, a regenerative amplinanosecond laser and a (b) 355-nm picosecond laser. other hand, infrared and visible pico- The shorter pulsewidth laser clearly produces a fer is used. In a regenerative amplifer, second lasers typically offer greater a pulse undergoes a large number of higher level of precision and less remelt. (Courtesy: output powers, thus enabling higher PhotoMachining) passes and can therefore be amplithroughput. fed very substantially. Regenerative amplifers also have the advantage of Picosecond laser architectures being compact and delivering good beam performance. Using While there is some diversity in the form and construction of comthis design, Talisker lasers can offer pulse energies of up to 180 mercially available, industrial, ultrafast lasers, they all utilize a cerµJ (at 1064 nm and 200 kHz). tain basic confguration. Specifcally, a passively mode-locked oscilThe third approach is to use a diode-pumped, solid-state oscillator is used to generate output at pulsewidths of about 10 ps or lator, which can produce higher pulse energies than a fber seed. shorter that are necessary to drive photoablation. However, most This is followed by a free-space amplifer, typically in either a mode-locked oscillators produce relatively low energy pulses (in the regenerative or multipass confguration. In fact, more than nanojoule range) at repetition rates in the tens of MHz. This repetione amplifer stage can be used to boost power to higher levtion rate is too high to be used with existing scanning technology, els. For example, the products Coherent Inc. obtained through so a pulse picker is used to select a fraction of these pulses. The their acquisition of Lumera Laser GmbH in 2012 are based on

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a Nd:YVO4 seed, followed by one or more transient amplifers, enabling the Lumera laser to reach pulse energies as high as 200 µJ (at 1064 nm). TABLE 1 summarizes the maximum average power that can be achieved with this fexible modular architecture when one, two, or three amplifer stages are used. A transient multipass amplifer is used in this product line because it confers several other advantages in this situation. In particular, when compared to a regenerative amplifer, a transient amplifer offers the ability to reach higher repetition rates, as well as greater fexibility in terms of adjusting repetition rate (from single pulse to 2 MHz in this case). Another extremely important beneft of the transient amplifer construction is that it supports “burst mode” operation in which

TABLE 1. Average power (max.) with fexible modular architecture and between 1 to 3 amplifers NUMBER OF AMPLIFICATION STAGES

MAXIMUM AVERAGE POWER (W)

1064 nm

532 nm

Rapid

1

10

4.3

Hyper Rapid 25

2

25

15

Hyper Rapid 75

3

75

50

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Apollo Instruments

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the pulse picker is set to pass a string of consecutive pulses (typically up to 10) instead of just a single pulse. This entire pulse train is then amplifed by the subsequent stages. The advantage of burst mode is that, in some cases, it dramatically increases the ablation rate (the amount of material removed per unit time), for a given average laser power. For example, tests have shown a fve to 10 times increase in ablation rate when a string of fve pulses is sent through the amplifer instead of just single pulses (both at a 1 MHz repetition rate). The average power produced in each case is similar, and, in fact, the per pulse energy is lower in burst mode (since the amplifer gain is now spread over several pulses). However, the ablation rate does not seem to depend linearly upon pulse energy when the pulses are spaced so closely. The exact mechanism for the ablation rate is still being investigated, but there are some favored theories emerging. It is thought that when there are only 20 ns or so between pulses, the material doesn’t have time to relax and remains in a “preconditioned” state. This allows for subsequent pulses in the burst to achieve greater material removal despite their lower energy. Burst mode expands the capabilities and opens up the parameter space of ultrafast micromachining substantially. It has proven most useful with materials having free electrons, such as steel, tungsten carbide, and silicon. Conversely, it provides little or no advantage with dielectrics such as ceramics and glass.

LASER MICROMACHINING PhotoMachining, Inc. performs precision laser micromachining on a variety of different materials including plastics, metals, glass, ceramics, etc. Our eleven different types of lasers allow us wide fexibility to address many applications. We also design and manufacture custom laser machine tools.

PhotoMachining, Inc. Contact sales@photomachining.com Tel: 603-882-9944 Web site: www.photomachining.com

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very thin glass very well. In one embodiment of ultrafast laser glass Reliable, industrial picosecond lasers cutting, called flamentation cutting, the laser creates a string-like Most industrial users can only take advantage of the benefts of defect through the entire thickness of the substrate, thus allowing picosecond laser processing if it is delivered in the form of prodit to be easily separated. Filamentation cutting is advantageous ucts with cost, reliability, and ease-of-use characteristics that because it can work with thickare competitive with other laser, and Picosecond processing applications space nesses from 50 µm to over 3 mm even non-laser, based tools. Coherat 1064 nm, 532 nm and 355 nm and can even be applied to certain ent, for example, employs design and Wafer Wafer types of strengthened glass. construction practices to ensure this Embossing Scribing Scribing The picosecond laser has also occurs in their products. Drums been found useful for cutting thin The frst is a design in which there metals. For example, PhotoMais substantial headroom, meaning an chining Inc., Pelham, NH, a supassembly that will perform properly PhotoPhotoplier of high precision laser microeven if some individual components voltaics voltaics machining services and systems deviate signifcantly from their nomMicrofuidics to the medical, aerospace, disinal specifications. Furtherplay, microelectronics and other more, it guarantees that, even Increasing Ceramic Marking Micro- Photo- Micro-Via Drilling repetition industries, has used these sysif changes occur in the product fuidics voltaics Drilling rate tems in the production of mediover time (such as decreases in Sapphire cal implants. Typical materials mirror refectivity or mechanical Dicing for these industries are staincreep in optical mounts), that laser outless steel, Hastelloy, Nitinol, and put will stay in specifcation. Injector Glass/Sapphire Micro-Via Nozzle Drilling Drilling titanium. According to PhotoMaNext, lasers are assembled from Drilling chining CEO, Ronald Schaefcomponents that will avoid contaminaSapphire fer, “We’ve been able to achieve tion within the laser cavity, in a cleanGlass/Sapphire Medical Dicing Photovoltaics Cutting Devices extremely good edge quality withroom, and then hermetically sealed to out post-processing using a laser. prevent contaminants from entering. Increasing laser power With nanosecond lasers, we usuAttention is also paid to mechanially get some kind of melt on the cal design and the use of adjustable FIGURE 3. Most of the key applications being serviced cut edge which requires a submounts, which might move over time, by picosecond lasers and the wavelength, average power, sequent electropolishing step is minimized. Plus, every unit is shock- and repetition rate regimes in which they operate. to eliminate. We’re typically protested at over 100 g. If output power ducing features of about 25 µm in varies at all as a result of shock testwidth on these products, so we use the 355 nm output in order to ing, the unit is failed, and the cause of the problem is determined. achieve the necessary spot size with a 100 mm focal length lens.” Finally, these lasers are temperature controlled with a chiller to Another area where PhotoMachining has successfully employed 0.1°C to ensure operational stability. Plus, the pump diodes are picosecond laser processlocated in the power supply and fber-coupled into the ing is in structuring small, oscillator and amplifers so as to eliminate any thermal intricate parts made from loading from these sources. carbon fiber reinforced The resulting reliability of these systems has been polymer (CFRP). “These quite high. Customers have reported uptime of higher composites are made than 98%, even in 24/7 operating environments. of woven fibers,” notes Typical applications Schaeffer, “and when you FIGURE 3 summarizes most of the key applications curstart cutting them they can rently being serviced by picosecond lasers and indichange shape because in30 µm cates the wavelength, average power, and repetition ternal stresses are relieved. 200 µm rate regimes in which they operate. If you add any heat to the An interesting emerging application for picosecond process, then you’ve got lasers is the cutting of thin glass used for smartphone a problem. We haven’t displays. Laser processing offers advantages over found any nanosecond mechanical methods for thin-glass cutting because it laser that can deliver the eliminates some of the post-processing fnishing steps results that we need.” In FIGURE 4. Photomachining has successfully required for the latter. CO2 lasers have been success- machined 30 µm sized features in CFRP using a one particular application fully employed for thin-glass cutting, but are limited to for an analytical instrupicosecond laser operating at 355 nm. (Courtesy: cutting weakly strengthened glass and can’t manage ment, Photomachining cut PhotoMachining)

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TABLE 2. Cost and throughput for various picosecond laser processing tasks MATERIAL

FEATURE

1 mm thick ceramic

100 µm hole

50W @ 1064 nm

1 hole/sec

360 holes/$

Printed circuit board

70 µm diameter, 70 µm deep hole

25W @ 532 nm

1000 hole/sec

360,000 holes/$

200 µm thick silicon

150 µm diameter hole

50W @ 1064 nm

5 holes/sec

1500 holes/$

Photovoltaic solar cell

Ablation of SiO2-or SiN-layer on Si

15W @ 532 nm

200,000 holes/ sec

70M holes/$

ITO on glass

50 µm wide lines, 200 nm deep

50W @ 1064 nm

>5 m/sec

1500 m/$

Remove 200 nm thick metal coating

40W @ 1064 nm

15cm2/sec

4500cm2/$

Metal on glass

OUTPUT REQUIRED

30-µm-wide features in a 200-µm-thick CFRP part (FIGURE 4). Photomachining was able to consistently produce these features over the entire length of a 1-inchdiameter part without introducing signifcant distortion using a picosecond laser, again operating at 355 nm. In terms of costs, the feature quality

THROUGHPUT

COST

delivered by a picosecond laser, together with the ability to eliminate post-processing steps in many cases, make their operating rates competitive with many other laser and non-laser technologies. Consumables and operating costs are relatively small for this type of laser so CoO is dominated by the depreciation of the original purchase

price. TABLE 2 summarizes the cost (per feature) that can be achieved with picosecond lasers for a number of common microprocessing tasks. In conclusion, picosecond technology has now been productized so that practical sources are available with the cost, reliability, and operational characteristics necessary to make them useful in industrial settings. Taken together with lasers in the nanosecond and sub-nanosecond regime, this provides users with an extremely broad portfolio of capabilities in terms of precision, materials compatibility, throughput, and CoO. Successfully developing a new laserbased application is best accomplished by considering sources that cover this entire parameter space and partnering with someone who is experienced in laser microprocessing applications development. ✺ MAGNUS BENGTSSON is director of strategic marketing at Coherent Inc., DR. DIRK MÜLLER is director of product line management at Lumera Laser Technology, Coherent, and BERNHARD KLIMT heads business development at Lumera Laser Technology, Coherent.

32nd

Savethe Date Hyatt® Regency Miami | Miami, Florida USA

October 6 -10, 2013

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Picosecond lasers