Patent Application: US-201514943132-A

Abstract:
a laser - produced plasma extreme ultraviolet source has a buffer gas to slow ions down and thermalize them in a low - temperature plasma . the plasma is initially trapped in a mirror magnetic field configuration with a low magnetic field barrier to axial motion . plasma overflows axially at each end of the mirror into magnetic cusps and is conducted by radial magnetic field lines to annular beam dumps disposed around the waist of each cusp .

Description:
we describe the magnetic field configuration with reference to fig1 . the laser / plasma interaction occurs at central position 60 . the laser beams 20 that are necessary to expand and heat incoming droplets may be delivered along the axis of chamber 70 , shown as a dashed outline . chamber 70 has rotational symmetry around symmetry axis 1 . for times when droplets are not present , or the target is missed , there is a beam dump 80 for the laser beams . in this drawing the tin droplet stream and catcher for unused droplets are not shown . they may be positioned is several ways , one of which will be shown in fig5 . also symmetrical around axis 1 is the euv collection mirror 110 which has a central hole to admit the laser beams . a typical ray of euv light 120 leaves the interaction position 60 , reflects off mirror 110 and proceeds to the chamber exit point on axis 1 , a position referred to as the “ intermediate focus ” between the source optic and the stepper illuminator optic . the magnetic field configuration in fig1 has rotation symmetry around axis 2 that runs perpendicular to axis 1 . it comprises a central , approximately parallel set of field lines generated by the aligned currents in coils 30 and 40 . within the cross section of each winding the direction of current flow is shown by a dot for current coming out of the page and an x for current flowing into the page . outboard of coils 30 and 40 lie coils 50 and 60 , respectively , that carry currents opposed to those in 30 and 40 in order to create magnetic null points at each end , these null positions being the center of two magnetic cusps . the radial cusp fields , perpendicular to axis 2 , intersect beam dumps 140 that are cylindrical and axially aligned on axis 2 . in this manner , the exhaust particles and heat from interaction point 60 are directed by the magnetic field onto lines around the inside of beam dumps 140 , to spread the particle and heat load over a large area on each . the field at the center of coils 50 and 60 is higher than elsewhere in the configuration , causing a blocking action . more detail on the central region of the particle catcher cusps is given in fig3 . in that figure coils 40 and 60 correspond to those labeled 40 and 60 in fig1 . the magnetic field variation along lines ab , ac and bd of fig3 is shown qualitatively in fig4 where x represents distance along the labeled lines . the field within coil 60 has a central value b b lying on axis 2 between points b and d . this is a high blocking field that shunts plasma particles back toward the cusp central null points . field b b exceeds the central value b m at the mirror exit half way between a and c . in turn the value b m exceeds value b w at the cusp waist between a and b . when the cusp axial fields b b and b m both exceed its radial field b w in this manner , then radial plasma leakage dominates at the circle of positions defined by all possible locations of the center of line ab around rotation axis 2 . plasma outflow from this locus then follows radial field lines toward the inside of cylindrical plasma beam dump 140 . a further embodiment of the invention is shown in fig2 . this is functionally the same magnetic configuration as in fig1 with the difference that field coils 50 and 60 are substituted by a single coil 160 that creates a high magnetic field in yoke 150 of high permeability material . the cusp fields are generated by field lines emanating from the end surfaces of yoke 150 . this embodiment reduces the number of superconducting coils from 4 to 3 , and also gives much better access for the vacuum manifolds that are shown in fig5 . with the above description of the mirror and cusp fields in place , we show in fig5 the disposition of several further elements of the euv source . the outline of a vacuum chamber 70 is shown . axis of rotational symmetry 1 defines the symmetry axis of chamber 70 . set into the wall of chamber 70 is droplet source 85 that delivers a stream of material in approximately 20 micron diameter droplets at a high velocity ( order of 200 msec − 1 ) toward interaction location 60 . droplets that are not used are captured in droplet collector 95 at the opposite side of the chamber . entering on the chamber axis is a laser beam ( or beams ) 20 that propagate through a hole in the center of collection mirror 110 toward interaction region 60 , where laser energy is absorbed by a droplet and highly ionized species emit 13 . 5 nm euv light . for example , the co 2 laser at 10 . 6 micron wavelength has been found to be effective [ 11 ] with tin droplets for conversion to euv energy , with 4 % conversion demonstrated into 2 % bandwidth light centered at 13 . 5 nm in 2π steradians [ 11 ]. laser light that is not absorbed or scattered by a droplet is captured in beam dump 80 . euv light emitted from region 60 is reflected by collection optic 110 to propagate as typical ray 120 toward the chamber exit port for euv . collection optic 110 has rotational symmetry around axis 1 . the chamber is shown truncated at the bottom in fig5 , but it continues until reaching the apex of the cone defined by converging walls 70 and rotation axis 1 . at that position , known as the “ intermediate focus ” or if , the beam of euv light is transferred from chamber 70 via a port into the vacuum of the stepper machine . in prior work [ 11 ] the laser has been applied as two separate pulses , a pre - pulse and a main pulse , where the pre - pulse evaporates and ionizes the tin droplet and the main pulse heats this plasma ball to create the high ionization states that yield euv photons . when the pre - pulse is a picosecond laser pulse it ionizes very effectively [ 12 ] and creates a uniform pre - plasma to be heated by the main pulse , which is of the order of 10 - 20 nsec duration . complete ionization via the pre - pulse is a very important step toward capture of ( neutral ) tin atoms which , if not ionized , will not be trapped by the magnetic field and could coat the collection optic . the pre - pulse laser may be of shorter wavelength than the main pulse laser in order to couple the laser - induced shock better into the tin droplet . the buffer gas ( chosen from the list hydrogen , helium or argon ) may be introduced at location 10 and then flow through the central hole in the mirror . alternatively it may be introduced at another location , or several locations in the wall of chamber 70 . its main function is to moderate the energy of exhaust tin ions leaving interaction region 60 at energies up to 5 kev . these ions are trapped by the magnetic field lines , but need to have frequent collisions in order to lose energy . the plasma density without added buffer gas would be too low to moderate tin ion energies before they reached the beam dumps , so that a high sheath voltage would exist at collectors 140 and damaging ion impact energies would occur . the equation governing this system is given in [ 34 ]. only a modest buffer density , roughly in the range 1 pa to 20 pa is sufficient to greatly reduce tin ion impact energies . this buffer density can help to catch tin atoms and prevent them reaching the collector , but as the buffer gas becomes ionized its greater role is to provide a sufficient electron density to ionize these neutral tin atoms and put them again under control of the magnetic guide field . with reference to fig5 , the exhaust gases are pumped by vacuum manifold 200 and pass through vacuum pump and cleaning / processing unit 220 before being returned via line 240 to re - enter chamber 70 . a second vacuum manifold behind the opposite cusp is not shown for reasons of space . 1 . “ euv sources for lithography ” ed v . bakshi , spie press , bellinghaven , wash . 2005 . 7 . m . mcgeoch proc . sematech intl . euv lithography symp ., toyama , japan 28 th oct . 2013 8 . m . richardson et al ., j . vac . sci . tech . 22 , 785 ( 2004 ) 9 . y . shimada et al ., appl . phys . lett . 86 , 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