208Chapter 4BLaserTin FilmTin SupplyDiskNeutralsClean EUV-0000EUVTin SupplyLiquid Tin BathDiskCooling(circulated)Capacitor(a)Figure 4B.1 Conceptual diagrams of a LDP source configuration:(a)front view and(b)topview(reprinted from Ref.22).(Sn)baths,which are connected to the energy storage capacitors.While theelectrodes rotate through the Sn baths,the surfaces of the electrodes arecovered by liquid Sn.The capacitors are charged to a specified voltage priorto the discharge.A combined laser beam provided by two Nd:YAG lasers3.4then irradiates the cathode to create a weak Sn plasma.The Sn plasma bridgesthe anode and cathode,and consequently ignites the discharge.A pulsedcurrent of approximately 15 kA in peak and a few hundreds of nanoseconds inpulse duration flows into the plasma,causing the plasma density andtemperature increase,and the EUV emission to occur.The EUV emissionvolume depends on how the laser and discharge parameters are tuned:arelatively large volume is used to gain radiation energy (mJ,for lithographypurposes)and a small volume is used to enhance brightness (mJ/mm2/sr,formask inspection purposes).Pulse repetition rate is usually between a few kHzand a few tens of kHz.Some reports have shown the capability of operation at40-100kHz.3As mentioned above,the surfaces of the electrodes are always covered byliquid Sn.Such a feature eliminates the need for complex spatial and timingsynchronization between the target and laser irradiation,as a Sn-coveredelectrode acts as an unlimited fuel supply,making a LDP source a highlyrobust and stable system.As is commonly known,5 Sn is the most efficient material for radiationhaving a wavelength of 13.5 nm.The conversion efficiency from the inputenergy to the output energy is much higher than that for xenon (up to 1%).Liquid Sn also works as the electrical conductor.Electrical power isapplied to the electrodes through liquid Sn.Therefore,despite the rotatingelectrodes,there is no moving mechanical conductor in the LDP source,which potentially leads to the short lifetime.Liquid Sn in the Sn baths is always circulating.Liquid Sn is supplied tothe source head module from the Sn circulation modules,which areindependently connected to the anode and cathode.The Sn circulationmodule consists of a reservoir that contains molten Sn,pumps that circulateliquid Sn,heaters that melt Sn in the initial state,and a cooling system thatHigh-Brightness LDP Source for Mask Inspection209cools the temperature of liquid Sn.The thermal conductivity of liquid Sn is26 W/mK at 300C and much higher than that of water(0.6 W/mK at 20 C).Part of the heat (almost 50%)created by the discharge operation is absorbedby liquid Sn and transferred to the Sn circulation modules.By activating thecooling,the liquid Sn temperature is kept constant in the reservoirDebris mitigation is a subject of high importance on all Sn-fueled EUVsources since particles and metal vapors are emitted from the discharge regionand can cause degradation of the optical elements by deposition.Ions are alsoemitted from the discharge region and can sputter optical elements as well.In order to provide debris-free EUV photons,the LDP source employs a veryefficient debris mitigation system or debris shield.A conceptual diagram ofthe debris shield is shown in Fig.4B.1(b).The debris shield captures all ofthe particles and vapors,and captures or slows down most of the ions.Several components are used in the debris shield.One of them is Ar gas.Thelocal pressure inside the debris shield is maintained higher than in the otherregion to provide high overall optical transmission and to maximize debrismitigation performance.4B.2 LDP System ConfigurationThe LDP system comes with several cabinets,as shown in Fig.4B.2.The maincabinet (shown in the middle)is installed in the clean room and is directlyintegrated to the inspection metrology tool;the sub-cabinets,such as powerdistributor,control module,and chiller are installed in the sub-fab or grayroom.The total footprint depends on the cabinet layout.When the L-shapedlayout is employed (as in Fig.4B.2),the footprint is 7.4 x 4.4 m.The LDPFigure 4B.2 Overall cabinet layout example of a LDP system.210Chapter 4Bsystem is designed for up to 10-kHz steady-state operation and requires peakelectrical power up to 75 kVA.The number of sub-cabinets will be decreasedin the future by combining several functions that are currently distributed inseveral cabinets into one cabinet.Figure 4B.3 shows the main cabinet.The main body of the cabinetcontains high-voltage components,such as the source head module,capacitormodule,and Sn circulation module.All of the modules can be easily replacedat the time of maintenance.The vacuum chamber placed in front of the maincabinet is the interface to the inspection tool.The main cabinet has the size of2.4×2.0×2.0m3.Figure 4B.4 shows the metrology setup used to evaluate the system'sperformance in terms of brightness and EUV power.In order to calculatebrightness,a calibrated inband EUV energy monitor measures inband EUVradiation energy,and an inband EUV camera captures and analyzes theemission image.The monitor is placed after the debris shield to measure thedebris-free EUV power,whereas the camera is placed in front of the debrisshield (plasma side).The EUV energy monitor can be replaced with the other metrology tools,such as an EUV spectrometer,an EUV beam analyzer,or an out-of-band(OoB)energy monitor.This energy monitor consists of a Zr-coated Siphotodiode,a Mo/Si multilayer mirror,and a solid-angle-limited aperture.It is calibrated on the Xe-source-based calibration test bench with an energymonitor calibrated at the Physikalisch-Technische Bundesanstalt (PTB)synchrotron facility.Figure 4B.3 Illustration of the main cabinet of LDP source (reprinted from Ref.21).