128Chapter 3AThe demonstration of >205-W dose-controlled EUV power on a researchplatform represents a significant milestone for the EUVL industry.Thecommonly understood entry point for HVM is >125 wph,which,with theoptical throughput of the NXE:3400B scanner,results in a required sourcepower of >205 W at IF.Based on learning from the >205-W demonstrationon a research platform at the beginning of 2016,progress in EUV powerscaling in 2016 and 2017 focused on securing the performance of the shippingconfiguration of 125 wph with an integrated 205-W EUV source andNXE:3400B scanner.This result was achieved in July 2017,where scannerthroughput of >126 wph was demonstrated on a complete NXE:3400Bscanner with the EUV source operating at ~207 W.Additionally,focus in 2017 was to demonstrate >250-W EUV power on astand-alone source in the shipping configuration.The main significance of250-W EUV power is two-fold:(1)to ensure that lifetime performance of theEUV source always enables >125 wph at customer sites and (2)to givecustomers flexibility in configuration of the NXE:3400B to include pellicles ormembranes designed to significantly improve mask and wafer defectivity,butwhich reduce optical throughput.In the second quarter of 2017,250 W was achieved in the shippingconfiguration EUV source to be paired with the NXE:3400B scanner.Thisrequired the complete integration of years of R&D,illustrating that all of thekey elements for laser-plasma interaction are well controlled and generallywell understood.Figure 3A.19 shows the 250-W demonstration,which meets2602552502452400500100015002000250030003500Time [sec]21.50.5500100015002000250030003500Time [sec]Figure 3A.19 Stand-alone stable operation of the ASML-Cymer LPP source in theshipping configuration at 250 W for one hour,meeting all dose specifications for powerstability at the wafer level(reprinted from Ref.13 with permission from M.Lercel).EUV Sources for High-Volume Manufacturing129350300250010020030040050060070080090010001100time [sec]Figure 3A.20 Demonstration of the open-loop power capability of an EUV source,showingstable 300-W operation with typical customer exposure settings.The power differencebetween the open-loop power,shown here,and 250-W closed-loop power represents thepower overhead,indicating significant power overhead for stable power delivery at 250 W(reprinted from Ref.13 with permission from M.Lercel).all requirements on EUV power stability for dose specifications at thewafer level.Additionally,Fig.3A.20 shows the open-loop 300-W averageEUV power at IF,demonstrating very good power overhead at the 250-Woperating point.With improvements to the EUV source discussed,the power targets forHVM have been achieved for the first time on a product-configuration EUVsource.This progress in EUV power scaling continues the rapid trendestablished over the past several years,shown in Fig.3A.21.2752503100 NOMO (shipped)3100 MOPA(research)2253100 MOPA+PP(research)3300 MOPA+PP(shipped)2003400 MOPA+PP(research)3400 MOPA+PP(Shipping 2017)17515012510075502500■20082009201020112012201320142015201620172018YearFigure 3A.21 Historical EUV power-scaling trend using LPP sources.All data points showthe delivered,dose-controlled EUV power at IF and must meet dose specifications forenergy stability at the wafer level for at least one hour to qualify for representation on this plot(reprinted from Ref.13 with permission from M.Lercel).130Chapter 3AThe previous sections outlined just some of the performance improvementsthat have been leveraged for EUV power scaling.The EUV power-scalingtrajectory has fundamentally relied on improvements to laser-to-plasmaenergy coupling and has been,in this way,just as much about scaling CE,leading to many simultaneous benefits for source operation.As the energybalance within the plasma shifts ever more toward EUV radiation,thereremains less energy for Sn debris kinetics,providing the potential forsignificant improvement to the lifetime of plasma-facing surfaces.3A.3.2 Tin target deliveryOne of the key technologies of an EUV LPP source consists of the generationof extremely small and stable tin (Sn)droplets at high frequency that are usedas the fuel for producing the plasma and EUV light.The droplet generatorprovides a constant stream of liquid tin droplets to the focal point of thecollector where the CO2 laser pulse is used to create the light-emitting plasma.The main requirement for the droplet generator is to deliver small droplettargets of identical size and spacing at the repetition rate of the laser pulses.These droplets are typically <30 um in diameter and are generated at a speedof roughly 70 to 100 m/s with inter-droplet timing stability better than 0.2%ofthe period.When irradiated by a COz laser,each tin droplet is evaporated,ionized,and heated to the optimal temperature at which the plasma producesEUV photons most efficiently.Droplets with high temporal and spatialstability have been consistently produced over a month of operation time.13Continuous improvement of the performance of droplet generators in terms ofreliability and availability,and continuing the reduction of the droplet size areboth paramount in supporting the future requirements of EUV sources.Sndroplets may be generated at a repetition rate of 50 kHz without interruption,which is the same repetition rate as the laser.The droplets themselves may begenerated with a reservoir where tin is heated above its melting temperature(231.9 C)and is then pushed through a filter and a nozzle by a pressurizedgas,as depicted in Fig.3A.22.The pressure applied and the size of the nozzledetermine the size and velocity of the droplets.A modulator on the nozzle ofthe droplet generator tunes the delivery rate of the droplets.One of the key advantages of the LPP EUV technology is that there is nodirect contact of EUV plasma with materials,which makes thermalmanagement of the source more practical than in other EUV sourceconfigurations.This advantage is achieved mainly through the use of mass-limited targets for generation of light-emitting plasma,namely,tin droplets.The main function of the droplet generator is to deliver droplets of moltentin of a required size at a repetition rate of ~50 kHz to the focal spot of theEUV collecting optic,where the droplets are irradiated by the drive laserpulses.In order to enable stable operation of the EUV source,droplets shouldhave exactly the same size and should arrive consistently in the same location