Patent Application: US-201514852777-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 symmetrical cusp magnetic field configuration with a low magnetic field barrier to radial motion . plasma overflows in a full range of radial directions and is conducted by radial field lines to a large area annular array of beam dumps .

Description:
herein the corresponding like elements of different realizations of the invention are labeled similarly across the drawing set , and will not always be listed in their entirety . we describe the underlying magnetic field configuration in its first , symmetric , embodiment with reference to fig1 . the basic cusp configuration of the present invention comprises four circular coils divided into two sets : coils 10 and 30 in the upper half , and coils 20 and 40 in the lower half . in fig1 the coils are shown in cross section . there is a vertical axis 1 of rotational symmetry . 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 . in the symmetrical cusp equal and opposite currents flow in coils 10 and 20 and they have the same number of turns in their windings . they therefore generate equal and opposite magnetic fields that cancel to zero at central point 60 . additional field shaping is performed by coils 30 and 40 . coil 30 carries a current in the same direction as coil 10 , and coil 40 an equal current to coil 30 but in the opposed direction . the final cusp field , indicated by magnetic field lines 50 , has a disc shape around its vertical symmetry axis . this shape is designed to channel a radial plasma flow into an annular plasma beam dump as described below . more detail on the central region of the cusp is given in fig2 . in that figure coils 10 and 20 correspond to those labeled 10 and 20 in fig1 . the magnetic field variation along lines ab and cd of fig2 is shown qualitatively in fig3 where x represents distance along the labeled lines . the field within coil 10 or coil 20 has a central value b 0 lying on axis 1 between points c and d . this value b 0 exceeds the central value b m half way between a and b . when the cusp axial field exceeds its radial field in this manner , then plasma leakage dominates at the circle of positions defined by all possible locations of the center of line ab around rotation axis 1 . plasma outflow from this locus then follows radial field lines toward the gap between coils 30 and 40 and enters the annular plasma beam dump . with the above description of the cusp field in place , we show in fig4 the disposition of several further elements of the euv source . the outline of a vacuum chamber 70 is shown , where chamber 70 may surround all of the coil elements , or part of the set of coils . 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 ) 75 that propagate through the center of coil 10 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 27π steradians [ 11 ]. laser light that is not absorbed or scattered by a droplet is captured in beam dump 80 attached to coil 20 . 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 fig4 , 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 different wavelength to the main pulse laser . in addition to magnetic capture of ionized tin in the cusp field , there is also a flowing buffer gas to sweep neutral tin atoms toward the plasma dump , as discussed below . in fig5 we show one embodiment of the invention in which a symmetrical magnetic cusp field guides the plasma ( vertically shaded ) from interaction region 60 toward plasma beam dumps 140 arranged azimuthally around chamber 70 . for clarity in fig5 , the droplet generator and droplet capture device are not shown , but instead we show the majority configuration which is an annular plasma beam dump 140 leading into vacuum pumps 150 . smaller items such as the droplet generator and laser beams for droplet measurement are interspersed between the larger plasma dump elements and may be protected from the plasma heat and particle flux by local field - shaping coils or magnetic elements . in operation , this embodiment has a stream of argon atoms entering for example through the gap between coil 10 and collection optic 110 , to establish an argon atom density of approximately 2 × 10 15 atoms cm − 3 in front of collection optic 110 . a stream of droplets is directed toward region 60 and irradiated by one or more laser pulses to generate euv light . plasma ions from the interaction can have an energy up to 5 kev [ 14 ] and are slowed down by collisions with argon atoms at the same time as they are directed in curved paths by the cusp field , with the result that a thermalized plasma , more than 99 . 9 % argon and less than 0 . 1 % tin ions , accumulates in the cusp central region . after a short period of operation ( less than 10 − 3 sec ) the accumulated thermal plasma density , and by implication its pressure , exceeds the pressure of the containment field b m at the waist of the cusp ( discussed above in relation to fig2 and 3 ). plasma then flows toward beam dumps 140 guided by the outer cusp magnetic field . the presence of a plasma flow causes neutral argon atoms to be entrained in the flow , and pumped effectively into beam dumps 140 and vacuum pumps 150 . the plasma is more than 99 . 9 % argon when tin droplet size of 20 micron diameter is used at a repetition frequency of 100 khz . these parameters correspond to 1 . 5 × 10 19 tin atoms per second in the flow , once it has reached steady state . the argon flow at a density of 10 15 cm − 3 , velocity of 1 × 10 5 cm sec − 1 and in a plasma cross - sectional area of 1000 cm 2 is 10 23 argon atoms per second , exceeding the tin flow by 6 , 600 times . it can be seen that the plasma cooling is dominated by argon , with a very minor tin component within the flow . a further embodiment of the invention is shown in fig6 in which two collection optical elements 110 and 160 are deployed , one on either side of the radial plasma flow . each of 110 and 160 is a surface generated by rotation around vertical symmetry axis 1 . they achieve euv reflectivity of , on average , approximately 50 % by means of graded mo — si multilayer stacks . each is protected from neutral tin atoms by a flow of clean argon that enters at positions 200 , and ultimately is pumped away via plasma beam dumps 140 and vacuum pumps 150 . the large solid angle of the combined collectors will improve source power in circumstances where source size is sufficiently small to be matched to the etendue of the stepper . we describe the underlying magnetic field configuration in its second major , near - symmetric , embodiment with reference to fig7 . this configuration comprises four circular coils divided into two sets : coils 10 and 30 in the upper half , and coils 20 and 40 in the lower half in fig7 the coils are shown in cross section . there is a vertical axis 1 of rotational symmetry . 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 . in the near - symmetrical cusp opposite but unequal currents flow in coils 10 and 20 when it is considered , for example that they have the same number of turns in their windings . they therefore generate unequal and opposite magnetic fields that cancel to zero at point 60 , which is no longer exactly centered between coils 10 and 20 . additional field shaping is performed by coils 30 and 40 . coil 30 carries a current in the same direction as coil 10 , and coil 40 a current not equal to that in coil 30 in the opposed direction . the final cusp field , indicated by magnetic field lines 50 , has a disc shape around its vertical symmetry axis . this shape is designed to channel a radial plasma flow as described below . more detail on the central region of the cusp is given in fig8 . in that figure coils 10 and 20 correspond to those labeled 10 and 20 in fig7 . the magnetic field variation along lines ab , cd and ef of fig8 is shown qualitatively in fig9 where x represents distance along the labeled lines . the field within coil 10 has value b 0 lying on axis 1 between points e and f , and the field within coil 20 has value b 1 on axis 1 between points c and d . values b 0 and b 1 both exceed the lowest radial magnetic field b m between a and b . when the cusp axial fields both exceed its radial field in this manner , then plasma leakage dominates at the circle of positions defined by all possible locations of the lowest field point on line ab around rotation axis 1 . plasma outflow from this locus then follows radial field lines toward ( and between ) coils 30 and 40 . one embodiment of the near - symmetrical cusp system is illustrated in fig1 in which magnetic field lines guide the plasma ( vertically shaded ) from interaction region 60 toward plasma beam dumps 140 arranged azimuthally around chamber 70 . the laser - plasma interaction takes place at or near to the null magnetic field point 60 which is now closer to coil 20 than to coil 10 for the case illustrated in which coil 10 generates a higher field than coil 20 . for clarity in fig1 , the droplet generator and droplet capture device are not shown , but instead we show the majority configuration which is an annular plasma beam dump 140 leading into vacuum pumps 150 . smaller items such as the droplet generator and laser beams for droplet measurement are interspersed between the larger plasma dump elements and may be protected from the plasma heat and particle flux by local field - shaping coils or magnetic elements , discussed below in relation to fig1 . a buffer gas chosen from the set hydrogen , helium and argon is flowed through the chamber at a density sufficient to slow down fast ions from the laser - plasma interaction , but not absorb more than 50 % of the extreme ultraviolet light as it passes from the plasma region to an exit port of the chamber . absorption coefficients for these gases are discussed in [ 15 ]. an argon buffer is preferred for the reasons discussed , and typically may be provided in the density range between 1 × 10 15 and 4 × 10 15 atoms cm − 3 . in operation , this embodiment has a stream of argon atoms 200 entering for example through the gap between coil 10 and collection optic 110 , to establish an argon atom density of approximately 2 × 10 15 atoms cm − 3 in front of collection optic 110 . a stream of droplets is directed toward region 60 and irradiated by one or more laser pulses to generate euv light . plasma ions from the interaction can have an energy up to 5 kev [ 14 ] and are slowed down by collisions with argon atoms at the same time as they are directed in curved paths by the cusp field , with the result that a thermalized plasma , more than 99 . 9 % argon and less than 0 . 1 % tin ions , accumulates in the cusp central region . after a short period of operation ( less than 10 − 3 sec ) the accumulated thermal plasma density , and by implication its pressure , exceeds the pressure of the containment field b m at the waist of the cusp ( discussed above in relation to fig7 and 8 ). plasma then flows toward beam dumps 140 guided by the outer cusp magnetic field . in order to contain the argon plasma density of approximately 10 15 atoms / ions cm − 3 at a temperature of 1 . 5 ev , the minimum cusp confinement magnetic field has a value in the range 0 . 01 - 1 . 0 tesla . in a preferred configuration the minimum cusp confinement magnetic field has a value in the range 50 mt to 200 mt . the presence of a plasma flow causes neutral argon atoms to be entrained in the flow , and pumped effectively into beam dumps 140 and vacuum pumps 150 . the plasma is more than 99 . 9 % argon when tin droplet size of 20 micron diameter is used at a repetition frequency of 100 khz as discussed above . system elements of the above embodiments are drawn in fig1 . in general a plurality of laser systems 220 , 240 etc . are directed via a lens or lenses toward the interaction region 60 within chamber 70 . the buffer gas that is exhausted via beam dumps 140 and vacuum pumps 150 is cleaned and pressurized in gas reservoirs 210 . as needed , gas is flowed via tubes 200 to be re - injected into the chamber at a typical location between coil 10 and collection optic 110 . additional system elements of the above embodiments are drawn in fig1 . this figure depicts a cross section of the system in a plane perpendicular to axis of symmetry 1 that is illustrated for example in fig1 . this plane includes the interaction location 60 . lines of magnetic force b run radially in this view . the flux lines are guided into beam dumps 140 and vacuum pumps 150 by elements 300 that may either be small antiparallel field coils , magnetic shield material , or a combination thereof . the “ annular beam dump ” is in practice divided into a plurality of elements 140 that are arranged in the plane perpendicular to the axis of symmetry that contains position 60 . this division is for two main reasons : a ) vacuum pump flanges are usually round , so they cannot be positioned without gaps so as to pump at all locations around a continuous annular beam dump ; and b ) there has to be access in this plane for the droplet stream and for optical systems that detect droplet position . all of these sub - 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