Excimer Lasers—40 Never Looked Better

Excimer lasers—40 never looked better

LUDOLF HERBST

Despite its middle-aged standing small features at extremely 20 Hz—now, numerous excimer lasers in the laser community, the excimer high precision, based on the support multi-kilohertz repetition rates laser is a high-power source of UV fact that optical resolution (see Fig. 1). Just as important, laser pro- scales down directly with ducers have also substantially improved and DUV light with no equal in wavelength because of dif- the service characteristics and total cost certain critical applications. fraction. High pulse energy of ownership of excimer lasers to keep The first commercial excimer laser was combined with rapid repetition rate en- them competitive with other laser and introduced 40 years ago by Lambda ables high process throughput and re- non-laser technologies. Physik (now Coherent; Santa Clara, duces takt time (the total time it takes CA). Interestingly, its developers— to produce a single unit of a product). Vision correction Bernd Steyer and Dirk Basting—were While solid-state UV laser technolo- Every year, more than a million people both chemists whose main goal was gies have advanced tremendously over worldwide undergo LASIK surgery to to develop a light source for photo- the past 40 years, no new technology achieve perfect vision—dramatically chemistry and dye laser pumping. As has arisen to challenge the excimer la- improving the quality of life for count- soon as excimer lasers entered the mar- ser in delivering this particular combi- less individuals (see Fig. 2). ket, Lambda Physik began to investi- nation of characteristics. Introduced in 1989, LASIK was the gate other possible uses for a powerful From a practical standpoint, excimer first major non-scientific application source of short ultraviolet (UV) light. lasers have expanded their relevance in for excimer lasers and still remains the While most of the original applica- the market because of extensive efforts largest single excimer laser application tions for excimers can largely be relegat- by manufacturers to improve their out- in terms of unit volume. What start- ed to history, many others have evolved. put characteristics and tailor them to the ed with crude experiments on pig eyes It is fair to say that few other laser tech- needs of specific applications. For ex- has now evolved to more than 10,000 nologies have had a greater impact on ample, the first commercial excimer la- high-precision, compact, tabletop la- our daily life than excimers. Laser- ser, the Lambda Physik EMG 500, op- sers deployed worldwide at eye clinics assisted in situ keratomileusis (LASIK), erated at a maximum repetition rate of and LASIK centers. photolithography, and display In the LASIK procedure, ex- production are three primary cimer laser pulses at 193 nm are applications that illustrate the used to ablate material from unique properties of the exci- the human cornea to reshape mer laser that continue to en- it, thus changing its refractive sure its legacy as a key enabling power and allowing correction technology. for short- or long-sightedness and astigmatism. Unique output, To perform LASIK, a thin, unique benefits hinged flap is surgically lifted Excimer lasers offer a unique combination of UV wave- FIGURE 1. The first commercial length output together with excimer laser was the Lambda high pulse energy—attributes Physik EMG 500, which produced a that are key to their expand- pulse energy of 220 mJ at 248 nm, ing use. The short wavelength with a repetition rate up to 20 Hz. enables the production of very

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1711lfw_46 46 10/25/17 1:22 PM (by a femtosecond laser demands of the chip industry. As a result, or microkeratome) from oscillator/amplifier configurations with the outer surface of the high power (around 100 W) and outstand- cornea. The excimer ing performance characteristics are now laser beam is shaped the standard for this application. and projected using Active spectral narrowing (to much less fast-scanning mirrors, than 1 pm) and sophisticated dose and ablating corneal mate- linewidth control are widely utilized. And rial in the precise pat- while other technologies, such as extreme tern required to correct ultraviolet (EUV) lithography at 13 nm the individual patient’s will complement the excimer laser for the vision. The flap is then most critical layers at 10 nm, the future replaced, sealing and of the excimer laser still looks bright for protecting the front of photolithography applications. the eye. FIGURE 2. The LASIK procedure improves the quality of life for The precision of the more than a million people every single year. Display production 193 nm argon fluoride The two most common flat-panel display (ArF) excimer laser ablation process is (again, because of diffraction)—specifi- types for smartphones and other devic- essential for the predictability and safety cally, excimer lasers. es are active-matrix liquid-crystal dis- of the LASIK procedure. Plus, the short Both 248 and 193 nm lasers are used plays (AMLCDs) and active-matrix or- (nanosecond) pulse width and short wave- for photolithography. In particular, 193 ganic light-emitting diode (AMOLED) length remove corneal material in a rela- nm excimer lasers enable circuit pattern displays. Both of these use a backplane tively cold process called photoablation. features down to 10 nm, which is far be- consisting of a glass substrate on which low the diffraction limit. Achieving this a large number of thin-film transistors Photolithography necessitated the development of highly (TFTs) are patterned to form the actu- Excimer lasers are also essential to the fab- specialized excimers with grating-con- al pixel circuitry. The thin film is made rication of highly miniaturized integrated trolled line narrowing to minimize chro- of silicon (typically 50 nm thick) and ex- circuits (ICs). And, the availability of ev- matic aberrations in the imaging optics. posed using photolithography to yield the er-smaller, more powerful and, economi- A variety of other techniques, including desired circuit structures. cal microprocessors has in turn had a pro- immersion imaging, double or quadru- Large-scale chemical vapor deposi- found impact on modern society. ple exposures, and a range of clever op- tion (CVD) is used to create the amor- An IC itself consists of numerous elec- tical imaging methods, are used to pro- phous silicon (a-Si) layer. Converting this tronic components constructed on a single, duce even finer features. amorphous layer to polycrystalline sili- monolithic semiconductor wafer. The de- Over the last 25 years, companies in- con (poly-Si) improves the electron mo- tailed structure of these devices is built up cluding Cymer (San Diego, CA), an ASML bility, enabling small TFTs with excel- layer by layer in a process called photoli- company, and Gigaphoton (Oyama-shi, lent electrical characteristics that block thography, with the first step being to coat Japan) have made substantial advanc- less of the backlight, leading to brighter a semiconductor wafer with a light-sen- es in excimer technology tailored for li- displays that draw less power—particu- sitive photoresist. A reticle (mask) con- thography to keep pace with the relentless larly critical for small, high-resolution dis- taining the desired circuit pattern is il- plays. Moreover, the transition to OLED luminated with UV laser light and the Excimer laser technology comprised of emissive pixels mask pattern is projected onto the wa- line beam without a backlight sets high demands on fer surface, after which the exposed re- TFT performance. a-Si sist is developed and the wafer is chemi- Poly-Si The a-Si layer is transformed into po- cally etched to physically remove material ly-Si by heating it with the excimer laser from the exposed areas to produce the ac- in a process called excimer laser anneal- tual features. This process is then repeat- ing (ELA; see Fig. 3). Specifically, a pulsed Substrate ed as many as 30 or 40 times to produce excimer laser line beam is scanned over the entire circuit structure. Translation the a-Si film, which efficiently absorbs the The original photolithography light 308 nm excimer output. sources were mercury lamps, but the need FIGURE 3. A diagram shows the basic This high absorption, combined with to produce smaller features drove manu- elements of the excimer laser annealing (ELA) the high pulse energy of the excimer laser, facturers to shorter-wavelength sources process for display substrates. makes it possible to achieve near-complete

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melt of the thin silicon layer with each of the panel. This allows the entire panel pioneer for ELA. The success of this pulse. The high absorption of the sili- to be processed in one (or two, respective- application is based on considerable con also prevents the UV light from pen- ly) passes under the laser beam, which is advances in high-power excimer laser etrating significantly into the substrate, critical to achieving the necessary process technology and the UV optical system avoiding thermal stress and permitting utilization and high throughput. that provides the line beam homogene- the use of economical glass materials for For 20 years, Coherent (and, be- ity required for uniform annealing of the substrate. fore that, Lambda Physik) has been the ever-larger panels (see Fig. 4). Production oc- The processing capabilities of exci- curs on large glass mer lasers, together with ongoing im- panels, such as Gen provements in their performance, reli- 6 (1.5 × 1.8 m), that ability, and cost of ownership, continues are subsequently to make them a critical enabling tech- separated into nu- nology in many other industrial, medi- merous smaller dis- cal, and scientific processes. For exam- plays. In the ELA ple, excimer laser lift-off processes are system, the rectan- key to a new generation of flexible dis- gular output of the plays, and excimer-laser-produced fiber excimer laser is ho- Bragg gratings (FBGs) are vital to tele- mogenized and re- communications, sensing, and many fi- shaped into a long, ber laser designs. thin line typically Ludolf Herbst is product line manager at having a length that FIGURE 4. In the new Coherent LineBeam 1000/TwinVYPER system, Coherent LaserSystems, Göttingen, Germany; is equal to the width the output from four separate lasers is combined within the system to e-mail: [email protected]; www.co- (or half the width), produce a single line beam. herent.com.

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