RIKEN Review No. 43 (January, 2002): Focused on 2nd International Symposium on Laser Precision Microfabrication (LPM2001)

History and future prospects of excimer lasertechnology

Dirk Basting, Klaus Pippert, and Uwe Stamm

Lambda Physik AG, Germany

We give a review about the historical development and various technological achievements in the field ofexcimer lasers since their discovery in 1970. The first commercial excimer laser model in the world, the EMG500 from Lambda Physik was operating at maximum repetition rates of only 20Hz, whereas today multi kiloHzexcimer lasers as the NovaLine lasers are available. In 2000 Lambda Physik introduced the most powerfulcommercial excimer laser, the Lambda Steel with 300W output power having extremely high stability. Overthe years various technological achievements like the magnetic switch control, the NovaTube laser chambertechnology, the NovaPowerSwitch solid-state pulsed power modules and the HaloSafe fluorine and chlorinegenerators could be realized which make the use of excimer lasers simple and user friendly. Recently, thedevelopment of Lambda Physik’s DuraTube technology gave a strong push towards the development of highpower 157 nm laser technology for microlithography and laser based micro-machining of “difficult” materialsas fused silica or teflon. Current and future technological developments are discussed which will strengthenthe position of excimer lasers in all industrial areas.

1. Introduction

The revolutionary progress in semiconductor, communica-tion, and information industries based on electronic andphotonic technologies demands for the development and en-hancement of various microfabrication techniques to supportmicro- and nano-technologies. The trend for “smaller sizeand higher speed” is evident in integrated microchips forcomputers, micro-optics, and micro-electro-mechanical sys-tems (MEMS). Biotechnology and medicine require micro-and nano-technological approaches as well.

For micro- and nano-technologies, laser micromachining iscurrently used in a large number of R&D and industrial ap-plications. The range of applications to which laser meth-ods are applied is continuously expanding, supported alsoby the development of novel processing techniques. Overthe last decades the excimer laser has obtained the key po-sition among lasers in various sectors of micromachining.Excimer lasers have developed into powerful manufacturingtools mainly because of two reasons: (i) The short wave-lengths of the excimer laser offers excellent quality of ma-chining and a great versatility in features which can be pro-duced. (ii) The progress in basic excimer laser technologyhas made the excimer lasers to reliable machines suitable forthe industrial environment.

There are certainly not many types of lasers which have foundsuch broad markets as the excimer laser (see Fig. 1,1)) Overthe last years the main growth results from increasing indus-trial use followed by medical applications while new sales intoR&D applications stay nearly constant. Today the largestknown industrial applications of excimer lasers are (i) basedon micromaching of different materials as polymers, ceramicsand glasses, applied for example in the production of ink jetcartridges by drilling the nozzles, (ii) excimer laser radiationis being used for changing the structure and properties ofmaterials as oxides, silicon or glass in bulk or thin films, asapplied for the production of active matrix LCD monitors,

Fig. 1. The development of total worldwide market of excimer lasersbetween 1993 and 2000.

fiber Bragg gratings in telecommunication, and high temper-ature superconducting films, (iii) employing the excimer laseras “short wavelength light bulb” in optical microlithographyfor the production of computer chips with critical dimensionsbelow 0.25µm (the largest homogeneous market for excimerlasers). The largest application of excimer lasers for medicaluse is in refractive laser surgery. By means of intense excimerpulses at 193 nm the surface of the human cornea is reshapedto change its refractive power and thus to correct for shortor long sightedness.

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All the widespread applications of excimer lasers in microma-chining and medicine are based on the early use of excimerlasers leading to the discovery of the ablation of materialsunder intense illumination with ultraviolet laser pulses by R.Srinivasan.2)

In the present paper we give a review about the developmentof excimer lasers and their technology and discuss currentand future trends in excimer laser technology.

2. The invention of excimer lasers

Excimer lasers are gas lasers that emit pulses of light witha duration of 10 ns to several 10 ns in the ultraviolet (UV)spectral range. They are the most powerful lasers in theUV. While a lot of different excimer laser transitions havebeen used to generate light pulses at various wavelengths be-tween 126 nm and about 660 nm (see Fig. 2, upper diagram),the most commonly used excimer lasers are krypton fluoride(KrF, 248 nm), argon fluoride (ArF, 193 nm) and xenon chlo-ride (XeCl, 308 nm). Recently also the very short wavelengthof the fluorine laser (F2, 157 nm) experiences increasing in-terest and applications.3)

The name excimer comes from excited dimer. The first ex-perimental evidence of excimer lasing was obtained by N. G.Basov et al. in 1970. They did use a high current electronbeam to excite liquid Xe.4) The Xe2∗ excimer laser emittedaround 172 nm. With this experimental proof the excimerlasers were invented as a new class of lasers.

Fig. 2. Lasing wavelengths of the different excimers (upper dia-gram). Schematic potential diagram of the KrF rare gas mono-halide (lower diagram)

For the “important” excimer lasers of today the name ex-cimer laser is used only by convention since here excitedcomplexes (exciplexes) of rare gas monohalides rather thanexcited dimers form the active medium. Also exciplexes existwith some stability only in the excited state. The exciplexesare formed by Ar, Kr, or Xe with F or Cl. The most impor-tant are ArF, KrF, XeCl and XeF.

The general principle of the excimer laser transitions is shownin the lower part of Fig. 2 for the example of KrF. The upperlaser level is an ionically bound charge transfer state of the2P rare gas positive ion (Kr+) and the 1S halogen negativeion (F−). In difference to many other laser types, the upperlaser level is populated by a three-body collision involvinghere Kr+, F−, and a third collision partner (called buffergas, for example Ne or He). While there is a minimum inthe potential energy curve in the upper state it is still ratherunstable. The excited KrF∗ molecule decays after severalnanoseconds via emission of a photon into Kr and F. Theseform the ground state which is covalently bonded and consistsof separate Kr and F atoms for large internuclear separations.The components Kr and F are then available for anotherexcitation cycle.

Looking back into the mid 70’s of the 20th century it is ex-tremely interesting to note that the (exciplex) excimer laserhas been discovered basically within the short time frameof one year after investigation of fluorescence spectra of itsactive molecules, the rare gas monohalides.

Fluorescence spectra of other rare gas halides were almostat the same time, i.e. in 1974 under investigation by sev-eral groups at University of Cambridge, Cambridge, UK,5) atKansas State University, Kansas, USA,6) and at the Avco Ev-erett Research Laboratory, Everett, Massachusetts, USA.7)

The first laser action of exciplexes was then reportedin 1975, again almost simultaneously by several researchteams at Naval Research Laboratory, Washington, USA,8)

Northrop Research and Technology Center, Hawthorne,USA,9) at Avco Everett Research Laboratory, Everett, Mas-sachusetts, USA,10–12) and at Sandia Laboratories, Albu-querque, USA.13) At the end of 1975 excimer lasers with allimportant UV wavelengths had been demonstrated experi-mentally. In addition to fundamental investigation of the ex-cimer laser and its properties itself, scientists started to builtup excimer lasers which could be used in potential researchapplications.

3. Early commercial excimer lasers

It was not easy at all to develop excimer lasers as commercialdevices manufacturable in a (small) industrial environmentsoon after the reported laboratory results. Nevertheless as-tonishing fast, Lambda Physik did develop and manufactureits first commercial excimer laser–the EMG 500–which wasintroduced into the market in 1977, only 2 years after theinvention of exciplex excimer lasers. In Fig. 3 a photographof the first commercial excimer laser, the LAMBDA Physikmodel EMG 500 is shown. The laser could be operated atvarious wavelengths like 193 nm, 222 nm, 248 nm, 282 nm,308 nm, 337 nm (as nitrogen laser), 351 nm, 427 nm (as ni-trogen laser), and 713 nm with a repetition rate selectablebetween 0.05Hz (i.e. 1 pulse in 20 seconds) and 20Hz. The

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Fig. 3. The first commercial excimer laser EMG 500. Pulse energyof 220mJ @ 248 nm, repetition rate between 0.05Hz and 20Hz.

Fig. 4. Excimer laser EMG 103 MSC without cover showing the maincomponents including high-voltage circuit, pulsed power moduleswith thyratron, laser chamber and cooling circuit.

laser was made for basic research applications of the new pow-erful UV light pulses with until that time unachievable peakpower of up to 10 MW! Energy at 248 nm exceeded 220mJwhich was almost two orders of magnitude higher than theenergy obtained from the most powerful predecessor of theexcimer laser–the UV nitrogen laser. The gas lifetime wasabout 60000 shots, thyratron and electrode lifetime about 10million shots. In 1977 about 5 laser were built and shippedto the first customers of excimer lasers in the world.

In these years the difficulties in developing commercial ex-cimer lasers were the same worldwide as various basic tech-nologies needed for excimer lasers were not or not sufficientlydeveloped. Figure 4 shows an Lambda Physik excimer lasermodel EMG 103 MSC without cover–an advanced excimerlaser model introduced in 1983. In the 70’s technologiesneeded in the excimer laser were not developed at all or inthe proper combination. These technologies are high voltageand pulsed power technology including thyratron switching,and the discharge and pre-ionization technology. The vacuumtechnology, the fluorine/chlorine resistive materials and their

mounting technology for the excimer laser chamber were notexisting. Technologies for the gas fill circuit, optics in contactwith corrosive gases, cooling of the powerful laser chamber aswell as the preparation of high purity laser gases was needed.

The reasons why Lambda Physik was so fast in commer-cializing the excimer laser are likely several. Pulsed trans-verse excitation gas laser discharge system had been devel-oped earlier at Lambda Physik for its UV nitrogen lasers.In 1975 several years of experience and data about the per-formance of the discharge system in nitrogen lasers at cus-tomer sites were available at Lambda Physik. During thedevelopment of the first excimer laser the experience fromthe nitrogen laser discharge system was considered and thetransverse discharge system was adapted to the requirementsof excimer lasers. Pre-ionization was developed in addi-tion to ensure uniform large volume discharge. Since bothfounders of Lambda Physik, Bernd Steyer and Dirk Bast-ing were chemists they were used to work also with aggres-sive chemicals. This made a major advantage during theselection of materials for and the design of the excimer lasertube. Another advantage for Lambda Physik was its loca-tion in Gottingen, Germany and the excellent contacts of thescientists both to the Max-Planck-Institutes and the Georg-August-University of Gottingen. As for example fluorine wasnot available on the market with the required purity andin sufficient quantities at that time, close contacts with andadvice and help from Prof. Oskar Glemser from UniversityGottingen, one of the worldwide experts in fluorine chemistry,did accelerate the development of excimer lasers at LambdaPhysik significantly. And as Lambda Physik had started itsbusiness with nitrogen lasers for dye laser pumping at Prof.Fritz Schafers laboratory at the Max-Planck Institute for bio-physical chemistry, and had earlier developed commercial dyelasers with complicated optics, also the knowledge in opticsand highly stable mounting technology was available whichcould be used for the excimer laser.

And last but not least–the driving force for the developmentof the first commercial excimer laser was the application–photochemistry and dye laser pumping, specialty of theLambda Physik founders. This all together made the fastdevelopment of the excimer laser at Lambda Physik possible–the effects of synergy and multidisciplinary know–how avail-able at Lambda and in the area of Gottingen.

Soon after the introduction of the first commercial excimerlaser EMG 500–a total number of about 20 lasers have beeninstalled worldwide–fast progress in several areas of excimerlaser technology was made. The development of automaticsynchronization of pre-ionization and main gas dischargeleading to higher energy stability, of gas processors for in-creased laser gas lifetime, and magnetic assist and switch con-trol for increased thyratron, discharge electrode, and gas life-time allowed Lambda Physik to introduce improved excimerlasers. The new excimer laser models were the EMG 100and 200 series as well as the EMG 100 and 200 MSC. Withthe MSC (magnetic switch control) technique lifetimes of themost expensive excimer laser components–thyratron switchand laser chamber–could be increased by one order of mag-nitude. Under typical operation conditions in the researchenvironment this meant operation of the new excimer lasersof several years without change of thyratron or laser chamber.Available UV laser power did exceed the 100W level for thefirst time. Also gas lifetime was increased by more than anorder of magnitude with the new excimer lasers–on Septem-

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ber 16, 1983 Lambda Physik scientists switched off an EMG103 MSC after 110 million shots of continuous operation witha single gas fill of XeCl–a world record of gas lifetime (a gaslifetime of 100 million shots is comparable to the gas lifetimerequirements for todays lithography lasers in semiconductorfabs).

The improvement in all areas of excimer laser technology andperformance and the demonstration of the capability of ex-cimer laser pulses to open new ways of fundamental investi-gation of laser-material interaction and its application2) ledto a rapid increase in the market for excimer lasers. In 1984Lambda Physik shipped more than 200 excimer laser all overthe world and employed about 80 scientists and technicians.

With maturing of the excimer laser technology new develop-ments came up at Lambda Physik–some of them found mar-kets and some of them did not. In 1984 an oscillator-amplifierconfiguration of excimer lasers with unstable resonator–theEMG 150–was commercialized. The principle of the EMG150 was adapted from solid-state lasers which had been op-erated with amplifiers over many years. The amplifier wasequipped with an unstable resonator which was injection-seeded by the low divergence beam from the oscillator. Forthe excimer radiation this meant orders of magnitude higherbrightness, lowest beam divergence and extremely good fo-cusability of the laser pulses at energy levels in the several100mJ range. With bandwidth-narrowing resonator config-urations in the oscillator excimer laser, the EMG 150 de-livered high power UV pulses with less than 3 pm spectralbandwidth–later used to investigate the ozone hole.14)

Two years later another excimer laser was commercialized–the EMG 105 i–the first 1 kHz repetition rate excimer laserin the world. Developed on request of a customer having aspecific application in mind, the EMG 105i delivered 20Waverage power at 1 kHz at 248 nm–more than 10 years be-fore the first 248 nm, 1 kHz lithography laser was installed.However, the development was to early. The application wasnot successful and other markets did not arise at that time.Lambda Physik built about 20 EMG 105i lasers and then dis-continued the product as no significant sales could be seen.

In the mid 80’s the progress in excimer laser parametersand technology as well as extensive results about innovativeapplications of excimer lasers from research laboratories allover the world fueled the first industrial applications of ex-cimers. In 1985 the company Karl Suss introduced an indus-trial lithography mask aligner system with a Lambda PhysikEMG laser. Somewhat later the largest industrial installationof excimer lasers in the world was built up at Siemens Nix-dorf Informationssysteme in Augsburg, Germany (see Fig. 5).The underlying process had been developed under leadershipof F. Bachmann at Siemens AG Munich. The installationwas including up to 20 excimer laser in production in 1990.The mid 80’s was the time when the idea of taking the nextchallenge in excimer laser development was born at LambdaPhysik–making excimer laser machines for use in an indus-trial manufacturing environment. Various companies all overthe world supported this idea as for example IBM, Siemens,Canon, Nikon, ASML, Zeiss and many others. The resultingproducts were the industrial excimer lasers LAMBDA 1000and LAMBDA 3000 for micromachining applications and theLAMBDA 248 L–the first industrial lithography laser in theworld. All these lasers were designed in the first attempt tomeet the maintenance requirements of the industry and did

Fig. 5. Large industrial excimer installation at Siemens Nixdorf In-formationssysteme, Augsburg, Germany, containing 9 EMG 1003iexcimer lasers in 1988.

include the latest excimer laser technology developed overmore than 10 years at Lambda Physik.

4. Excimer laser key technologies

4.1 High voltage pulsed power circuits–MSC and NovaPow-erSwitchThe “heart technology” of any excimer laser is its electricaldischarge circuit including high voltage switch, capacitors forenergy storage and transfer, pre-ionization arrangement andelectrodes. The first excimer lasers did use thyratrons asfast high voltage switches in rather conventional electricalcircuits. The pulsed power circuit must switch tens of kilo-volts on a nanosecond time scale. The thyratrons used as theswitching element in these power supplies represented a lim-iting factor for system lifetime especially if higher duty cycleof the laser operation was needed. A breakthrough in excimertechnology was accomplished in 1983 when Lambda Physik’sscientists invented the magnetic switch control (MSC) fortransferring the electrical energy efficiently into the laserchamber. MSC did prevent current reversal in the pulsedpower circuit and allowed optimum adaptation of the thyra-tron switch to other high voltage components. This resultedin an increase in lifetime of the most expensive componentsof the laser–thyratron and laser tube–by more than one orderof magnitude. Thousands of lasers have been installed withMSC technology with lifetimes of the thyratrons seen up to4 billion pulses at some of them.

The development of very high repetition rate excimer lasersin the mid and late 90’s however met the limit of theMSC technology. At 2 kHz repetition rate, for example athyratron would survive only 23 days or less. The key tosolve the lifetime issue was solved by completely eliminat-ing the thyratron and replace it by semiconductor switchesas for example thyristors. In 1997 the development of theNovaPowerSwitch–a pulsed power circuit with low voltagesemiconductor switch combined with magnetic isolator (MI)and pulse transformer to achieve the required discharge volt-ages could be introduced into the first industrial excimerlasers.15) The lifetime of a NovaPowerSwitch circuit is ba-sically unlimited–determined only by the aging of the semi-

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conductor components.

4.2 NovaTube and DuraTube technologyInternal corrosion has long been one of the most serious prob-lems limiting excimer laser performance and reliability. Overtime, the corrosive gases used in excimer lasers chemically re-act with the laser tube and its components. This process hasseveral negative effects on laser operation. The resulting con-taminants quench laser action, thereby reducing laser oper-ating efficiency, which can significantly limit the lifetime of agas fill. Reduced efficiency also negatively impacts laser beamquality and pulse energy stability. Equally important, corro-sion limits overall tube lifetime and increases the frequencyof routine optics component cleaning and replacement.

Another milestone in the excimer technology could beachieved by the development of the NovaTube technology in1994, an all metal/ceramic laser chamber construction thatsignificantly reduces corrosion (Fig. 6). Specifically, all in-sulators and high voltage feedthroughs in the laser chamberare made from corrosion resistant high-density ceramics. Themetal parts of the laser tube are of special carbon- and silicon-free alloys to avoid the generation of contaminants such asfor example SiF4 and CF4, which have a detrimental effectonto laser operation, even at low concentrations at the ppmlevel. After clean room assembling of the NovaTube a multi-step passivation process eliminates all traces of contaminantsincluding water from the tube components, ensuring that ex-posed surfaces will remain inert. NovaTube technology didenhance the gas lifetime of the excimer laser by more than 10times, even at the very short 193 nm wavelength. In 1997 No-vaTube was improved by implementing a soft pre-ionizationtechnique of type of a barrier-discharge corona giving furtherenhancement of both the gas and, even more important, laserchamber lifetime. However, also with NovaTube technologythe laser chamber lifetime is still dependent on the rated out-put power of the laser. As a rule of thumb is valid the lowerthe power the longer the chamber lifetime. In 1998 for thefirst time a chamber lifetime above 10 billion pulses could bedemonstrated for a 10W KrF NovaLine lithography laser op-erating at 1 kHz.16) And also the high power laser LAMBDA4308 with 200W output at 308 nm17) exhibits tube lifetimesin excess of 2 billion pulses–this is more than 6 months ofoperation in a typical production environment.

Extending NovaTube, in 1999 another laser tube technol-ogy was developed which matched the experience from No-

Fig. 6. Schematic of the NovaTube excimer laser chamber.

vaTube with the requirements of most efficient operation andlongest lifetime at the 157 nm wavelength of the F2 laser–the DuraTube technology. DuraTube, which today is used inall Lambda Physik 157 nm laser employs materials and pre-ionization arrangement specifically developed for F2 lasers.With DuraTube technology another world record in gas life-time at 157 nm could be achieved in 2000–a NovaLine F1020lithography laser did operate more than 1 billion pulses on asingle gas fill.18)

4.3 HaloSafe gas generatorsThe storage and handling requirements of the halogen gasesused in excimer laser operation make an additional invest-ment necessary to install excimer lasers–the gas installation.While installation requirements for the rare gases (Ar, Kr,Xe) and the buffer and flushing gases (Ne, He) are simple,the gas installation for the halogen gas supply according tothe usual safety regulations is more demanding. At installa-tions of several lasers, the cost of the halogen gas installationdivided by the number of lasers operated, seems reasonable.At single excimer laser installations however, the cost of thegas and safety installations may contribute to a significantportion of the total cost of the laser system. The final so-lution of this problem would be a sealed-off excimer laserswhich does nod need an external gas supply. A first step intothis direction was introduced in 1994–the HaloSafe halogengas generator system which is a sealed module, either built-inthe excimer laser or externally connected to the laser. Thetwo versions of the gas generator produce either HCl or F2

on demand, from solid, inert materials just before a new gasfill of the excimer laser is necessary.

As example we explain here the principle of the fluorine gener-ator: By heating of the complex salt K2NiF6.KF pure fluorineis generated according to the reaction

2K2NiF6.KF� K3NiF6 + F2

The fluorine generation occurs at temperatures above 300C, below 300 C fluorine is adsorbed again in the salt. Togenerate the necessary amount of fluorine for a single laserfill the HaloSafe generator is heated computer controlled. Assoon as enough fluorine is generated the amount is injectedinto the laser chamber and the HaloSafe reactor is cooleddown. The excessively generated fluorine in the reactor isadsorbed by the complex salt and no gaseous fluorine remainsin the reactor making the reactor a safe device.

However, gas produced with the fluorine or HCl generatoris more expensive than that from the halogen bottle of highpurity gas suppliers. Calculating the total amount of moneyneeded to operate the excimer laser and taking into accountlocal safety regulations, this technology is specifically favor-able for medical and scientific installations.

4.4 Bandwidth-narrowing technologiesIn the second half of the 80’s a large push towards the edge oftechnological possibilities of excimer lasers came from theirintended use in semiconductor chip manufacturing and themoving target specifications for the laser parameters. Whileearly estimates did forecast the need of 248 nm excimer lasersfor microlithography at critical chip dimensions of 1.0µm,the actual start of production was at 0.25µm dimensions in1998. This shift in time was mainly caused by the immenseimprovement in resolution of Hg i-line microlithography step-pers with higher numerical aperture lenses and sophisticated

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Fig. 7. Narrow bandwidth resonator with optical elements as prismsand grating (left), laser tube with pulsed power module (center)and outcoupling mirror and beam diagnostics (right).

mask technology. For the excimer lasers as light sources theprogress in the established lithography technology resultedin the demand of narrower spectral laser bandwidth, higherspectral purity, significantly improved energy stability andhigher repetition rate. The required spectral characteristic ofthe excimer laser output is determined by the imaging lens ofthe deep UV scanner system which, at higher NA for higherresolution, tolerates only an ultra-narrow laser spectrum.

To generate narrow-bandwidth radiation from excimer lasersresonator the principles of the dye lasers invented inde-pendently by Fritz Schafer and Peter Sorokin were appliedby Lambda Physik researchers.19, 20) similar as in the earlyEMG 150 narrow-bandwidth oscillator. Early attempts ofbandwidth-narrowed lithography excimer lasers did actu-ally consider oscillator-amplifier configurations as that of theEMG 150. But it turned out that the cost of the laser sys-tem would be too high. The reliability needed for the usein semiconductor fabs seemed unlikely to be achievable. Oneof the main challenges for narrow-bandwidth excimer lasersfor lithography was therefore the development of UV opticalcomponents including coatings with the needed surface qual-ity and efficiency to achieve the required power levels froma single excimer oscillator. In the mid 80’s Lambda Physikscientists started to work closely together with specialists inmanufacturing of optical components for the UV range toaccomplish this goal. The result was the first lithography-type excimer laser emitting at 248 nm–the Lambda 248L. Ithad adopted exactly the dye laser resonator design principlesof Schafer and others which later have been applied also tothe ArF excimers (see Fig. 7). The cavities of lithographylasers include highly efficient line-narrowing elements such ashigh-resolution optical gratings and etalons allowing to selectbandwidths of the radiation of below 1pm in the early 90’s21)

and below 0.4 pm in 2000.22)

With the use of lithography lasers in chip manufacturingthe lifetime of the optical components under the high UVpower densities occurring in the excimer laser resonators be-came a major issue. In addition to continuously improvingthe UV radiation resistance of the optics, resonator config-urations were needed which reduce the power load on theoptical elements. As result of a joint R&D effort betweenLambda Physik and Matsushita Research Institute of Tech-nology in Japan, the lifetime of the optical components inhigh power UV narrow-bandwidth cavities could be signif-icantly increased employing the polarization coupled res-onator principle PCR.23) PCR significantly reduces the powerload on the critical resonator elements and increases their life-time.

5. Recent trends and progress in excimer laser technology

5.1 General requirements for excimer lasersSince 1977 the requirements from the various industrial, med-ical and scientific applications as well as technical possibili-ties in excimer development determine the trends of excimerlaser technology. Different applications have different require-ments. For example, for a R&D application in first line theparameters of the excimer laser must fit: energy, bandwidth,wavelength, energy stability or average power or repetitionrate or pulse duration. A new energy level, higher repetitionrate or another bandwidth from the excimer laser opens newresearch fields–with results researchers have in mind, but no-body may predict exactly. For industrial use the requiredparameters of the excimer laser usually have been fixed in aR&D phase (at least on a years basis). For use in productionit is assumed the required laser parameters are met. There-fore, of higher importance is throughput, cost of ownership(CoO) and maintenance intervals since these determine theprofitability of the specific application. Similar with medicalapplication–installation cost, operating cost, safety require-ment cost and maintenance cost have to be calculated care-fully against the number of treatments expected.

Taking the requirements of all excimer laser applications to-gether we see the main trends over the years as following:(i) continuous reduction in cost of ownership (CoO)–whichincludes easiest (or better no) maintenance, highest compo-nent lifetime and long exchange intervals, long gas lifetimewith a long-term trend to sealed-off operation of the excimerlaser (no gas installations are needed anymore, see NovaTube,DuraTube and HaloSafe), fully automated and software sup-ported operation of the excimer laser by the main computercontrolling the application result as well as remote controland diagnostic capability via intranet, internet or telecommu-nication, (ii) shorter wavelength–as needed for high resolutionlithography down to the 70 nm and may be to the 50 nm nodeas well as micromachining of difficult materials with highestresolution,3) (iii) higher repetition rate operation–as neededfor lithography to achieve the required illumination dose sta-bility and lithography lens lifetime, for fiber Bragg gratingmanufacturing, for mask inspection systems for chip manu-facturing, for micromachining with serial instead of parallelapproaches, for medical use in refractive surgery with thecustomized ablation, (iiii) higher stability of the laser param-eters as energy, beam profile, divergence, pulse duration, andpeak intensity–for all applications, (v) higher laser power–toachieve the required throughput and/or the required size ofthe area of material which can be machined simultaneously.

5.2 Cost of ownership considerations and shorter wave-length requirementsThe continuous reduction of the installation, operation aswell as maintenance cost is one of the main requirementsfor the use of excimer lasers in industry or medical environ-ment. To accomplish easy maintenance and low mean timeto repair (MTTR) numbers, Lambda Physik had developeda new series of industrial lasers in 1996–the NovaLine series(see Fig. 8). While engineered in a fully modular way en-suring shortest exchange intervals of consumables or failurecomponents, the NovaLine series takes advantage of using thesame modules as electrical supply components in the controlcabinet, high voltage power supplies, on-board computer, gashandling system, cooling circuit, energy detection and stabi-lization hardware as well as total system software for all the

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Fig. 8. Modular designed industrial lithography and micromachininglaser NovaLine with open covers showing the main laser modules.

different NovaLine models for various applications.

NovaLine is currently one of Lambda Physik’s main industriallaser lines for microlithography and microfabrication. One ofthe first Lambda Physik lasers for the use on the productionfloor of chip manufacturers, for example, was the NovaLine248-0.8. The extremely narrow spectral width of the laseroutput of less than 0.8 pm FWHM at average power levels of10W at 1 kHz was achieved using the newly invented polar-ization coupled resonator (PCR, see above). For ink-jet noz-zle drilling the NovaLine 100 is applied–a 100W KrF laser.

Cost of ownership calculations and projections are one ofthe driving forces in new developments of industrial excimerlasers at Lambda Physik. As one example representative forthe whole industry, we show in Fig. 9 the historical and pro-jected development of operating cost for 157 nm lasers forlithography. Specifically the projections of cost are influencedto a large extent by the changing specification requirementsof the semiconductor industry. For example, the ArF ex-cimer laser scanner technology which was earlier thought tobe needed at 0.18µm dimensions is currently in competitionwith the KrF technology for 0.13µm chip production. Costof ownership comparable to the 248 nm lithography waferscanners is required. First ArF excimer laser scanners usingthe NovaLine A1020 and A1010 had been installed at semi-conductor manufacturers for process development of 193 nmlithography earlier in 1999.

It is obvious that 157 nm lithography lasers must be compet-itive in CoO with 193 nm technology as otherwise the 157 nmlithography would not be profitable. Based on 20 years ofexperience with 157 nm lasers in a variety of applications,Lambda Physik had developed the worldwide first lithogra-phy F2 laser, the NovaLine F630, in 1998. These lasers to-gether with their successor NovaLine F1020 with DuraTubetechnology, a 10W single-line laser operating at 1 kHz, havebeen used immediately for material characterization, opticsand resist development as well as lithography scanner devel-opment. Laser discharge chamber, solid-state pulsed powermodule and laser resonator optics have been optimized forlaser emission with spectral bandwidth of about 0.6 pm. Ma-

Fig. 9. Development of CoO of 248 nm, 193 nm, and 157 nm lithog-raphy lasers.

jor programs in lifetime enhancement of the expensive mod-ules of 157 nm lasers did run and are running at LambdaPhysik to follow or undergo the projected CoO.24) Long termand durability tests of the 157 nm lithography laser systemhave been carried out comprising multi-ten billion exposuretests of individual components. Data from these tests as wellas data obtained at several installations reveal lifetimes ofthe laser chamber in excess of 3 billion pulses and optics life-times above 2 billion pulses. Further programs are ongoingto reduce CoO until the introduction of 157 nm lithographyinto chip production.

5.3 Higher repetition rate requirementsThe requirement of higher repetition rate comes from severalareas in industry and medicine. For lithography, laser repe-tition rate is at 4 kHz for 193 nm production tools. Medicalapplication needs currently 200Hz with 500Hz expected and1 kHz may be needed. Fiber Bragg grating manufacturingrequires for certain processing repetition frequencies as highas possible. One of the demands fueling the development ofhigh repetition rate excimer lasers is the need of higher doseenergy stability in illumination and microfabrication applica-tions. For statistical reasons, it is better to have the totalenergy needed distributed among many pulses with low en-ergy instead of having the entire energy in a single pulse, forexample. High repetition rate is then necessary to achievethe required throughput.

Lambda Physik has lasers running at above 4 kHz (seeFig. 10) to meet the requirements of the future. Years ago,when starting the discussion about 193 nm lithography in1995, it was unbelievable this excimer laser output perfor-mance could be achieved.25) Today, we assume that 4 kHz157 nm and 6 kHz or even 8 kHz 193 nm lasers will be de-veloped in near future. And where the repetition rate limitreally is–we do not know!

5.4 Higher power and stability trendsApplications as polycrystalline-silicon thin film transistor(TFT) annealing, ink-jet nozzle drilling, printed circuit boarddrilling, and lithography have been the main driving appli-cations for higher average power and better pulse-to-pulseand long term energy stability for industrial excimer lasers.For example, in strategic alliance for the thin film transistor(TFT) LCD manufacturing Japan Steel Works (annealer),

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Fig. 10. High repetition rate performance of an 193 nm lithographytype laser with 120W output power at 4 kHz.

MicroLas (optics manufacturer) and Lambda Physik (lasermanufacturer) have developed the first 308 nm excimer laserannealing system several years ago. The 308 nm wavelengthhas been shown to be well matched to the annealing pro-cess and to give significant advantage with respect to theoptics lifetime in the annealing system. While the early sys-tems have been used for process development and small scalemanufacturing of active matrix LCDs, today TFT annealingsystems are on the production floor since 1998. The necessaryyield for high volume production sets stringent demands ontothe energy stability of the laser. Increasing sizes of the dis-plays require higher pulse energy and higher average outputpower.

In 2000, as successor of the TFT annealing laser LAMBDA4000,17) the next generation of industrial lasers for excimerlaser annealing, Lambda Physik’s LAMBDA STEEL was in-troduced to the market26) with remarkably improved energystability of better than 1% standard deviation at the highaverage power. The LAMBDA STEEL 1000 allows opera-tion at 1 J pulse energy–at a 300W level the only commercialexcimer laser available today.

The making of larger active matrix LCD displays will re-quire more power from the excimer laser in the future–withdemonstrated 1 kW excimer power in the research labora-tory27) Lambda Physik is taking this challenge.

6. Conclusions

In the more than 25 years after their discovery excimer lasershave been used increasingly in a variety of R&D, medicaland industrial applications. The key of the success of the ex-cimer laser is its unbeatable UV performance with power lev-els not available from any other laser source. Revolutionaryand evolutionary technological achievements have translatedinto significant improvements of the parameters, the perfor-mance, the ease of handling and the reliability of excimerlasers.

Today, there are about 7000 Lambda Physik excimer lasersinstalled worldwide. The number of installed excimer lasersis rapidly increasing as fields of applications in medicine andindustry are widened. For the year 2001 we expect the num-ber of excimer lasers needed worldwide to be about 2000–forscientific, medical and industrial use.

The trends in the development of excimer lasers consist in

continuous reduction of their cost of ownership as well as im-provement of their ease of use, better stability of all laserperformance parameters, higher repetition rate and higheraverage power. The trend to use shorter wavelengths to ob-tain higher resolution and produce smaller features in mi-crolithography and microfabrication will continue to accel-erate short wavelength excimer development over the com-ing years–giving new momentum to achieve laser technologicquantum leaps at 193 nm and 157 nm. In the long run itseems the excimer laser will experience healthy competitionin microlithography and micromachining using wavelengthsbeyond the shortest known excimer laser wavelength as re-cently research in extreme ultraviolet (EUV) sources makessignificant progress.28)

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