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 within a cone-shaped sheet to an annular beam dump.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. application Ser. No. 14/852,777 entitled “EXTREME ULTRAVIOLET SOURCE WITH MAGNETIC CUSP PLASMA CONTROL”, filed Sep. 14, 2015, which is a divisional of U.S. patent application Ser. No. 14/317,280 entitled “EXTREME ULTRAVIOLET SOURCE WITH MAGNETIC CUSP PLASMA CONTROL”, filed on Jun. 27, 2014, now U.S. Pat. No. 9,155,178, which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosed technology relates to the production of extreme ultraviolet (EUV) light especially at 13.5 nm for lithography of semiconductor chips. Specifically it describes configurations of the laser-produced-plasma (LPP) light source type that have increased plasma heat removal for scaling to ultimate power. 
     BACKGROUND 
     There is a need for more powerful sources of extreme ultraviolet (EUV) light at 13.5 nm in order to increase the throughput of semiconductor patterning via the process of EUV Lithography. Many different source designs have been proposed and tested (see historical summary for background [1]) including the highly efficient (up to 30%) direct discharge (DPP) lithium approach [2,3,4,5,6,7] and also laser-plasma (LPP) irradiation of tin—containing [8] or pure tin droplets [9,10,11]. Laser irradiation of tin droplets has been the subject of intensive recent development [12,13], particularly in the pre-pulse variant [11], which has a demonstrated efficiency of 4% and a theoretical efficiency of up to 6%. 
     In both lithium DPP and tin LPP approaches it is necessary to keep metal atoms from condensing on the collection mirror that faces the EUV-emitting plasma. Also, in the tin LPP approach, but not with lithium DPP, there are fast ions ranging up to 5 keV that have to be stopped otherwise the collection mirror suffers sputter erosion. The design of a successful EUV source based on a metal vapor must strictly protect against deposition on the collector of even 1 nm of metal in days and weeks of operation, and this factor provides the most critical constraint on all of the physics that can occur in a high power source. In the case of lithium, extremely thorough metal vapor containment is provided via a buffer gas heat pipe [2]. However, the heat pipe containment technology cannot be extended to tin sources because the heat pipe temperature would have to be 1300° C. to provide the equivalent tin vapor pressure versus 750° C. for lithium. This vastly higher working temperature renders the heat pipe approach essentially unworkable for tin whereas it is very practicable for lithium. 
     Harilal et al. [14,15] have performed a series of studies on the use with a tin LPP source of either a magnetic field, a buffer gas, or a combination of these to slow down fast ions and protect the collection optic. Many magnetic field configurations have been discussed [16-29], with and without a buffer gas, to trap and exhaust tin ions. Methods have been proposed [30,31] to further ionize tin atoms so that they may be controlled by an applied magnetic field. The symmetrical magnetic mirror trap [18] has a long axial exhaust path for tin ions and if this path has a shallow gradient of magnetic field, can suffer from a build-up of plasma density as successive tin droplets are irradiated. Two things begin to go wrong: 1) there is an EUV absorption cross section of 2×10 −17  cm 2  for tin atoms that causes increasing EUV absorption loss as the plasma density builds, and 2) the mirror magnetic trap is unstable [14] to lateral plasma loss, which can expose the collection optic to tin atoms. Refinements of the mirror trap have been described [20,23] in which an asymmetry is introduced so that plasma flow is toward a weaker magnetic field at one end of the mirror configuration. This also can be combined with an electric field [20] to aid plasma extraction at the end with lower magnetic field. However, only a relatively constricted path is available for plasma exhaust toward one end of such a trap configuration, implying a limited heat removal capacity. Other magnetic configurations [27,29] have been designed to protect the collection optic, but these rely on gas cooling, and do not provide a specific path for plasma flow toward a large area plasma beam dump. Accordingly, the power scaling of such configurations is limited due to lack of heat removal. 
     Buffer gases have been discussed [15,32,33] to reduce ion energy and protect the collection optic. One of the main buffer gases used has been hydrogen [13,33] but as plasma power increases there is an increasing fraction of molecular hydrogen dissociation that can lead to vacuum pumping and handling problems of reactive hydrogen radicals. Coolant gases with more favorable properties, in that they do not react chemically, are argon and helium. These gases have higher EUV absorption than hydrogen [15], so they may only be used at lower density. However, argon has substantial stopping power for fast tin ions [15], and is particularly effective when a magnetic field is combined with a gas buffer to lengthen the path of tin ions via curvature in the field. 
     SUMMARY 
     It is an object of the present technology to provide a symmetric cusp magnetic field within the EUV source to allow a higher power to be handled than in prior art. The symmetric cusp field is characterized by having equal opposed inner coils that establish strong opposed axial magnetic fields and a zero field point at the mid-position between them. Off axis, the radial magnetic field is weaker than the axial magnetic fields, so that plasma leakage occurs radially toward an annular beam dump location. Outer coils maintain a guiding field for plasma to deliver it to the annular beam dump. Several features of this geometry allow high power handling:
         1) There is stable plasma containment at the center of the cusp;   2) There is controlled plasma outflow at the equatorial magnetic field minimum of the cusp;   3) The plasma outflow is guided by the radial magnetic field onto an annular plasma beam dump that can have a large area, maximizing the power that can be handled.       

     This design incorporates an inflow of buffer gas, preferably argon, that serves the following purposes:
         1) Sufficient buffer gas density (approximately 50 mTorr, if argon) degrades the energy of tin ions from the laser-plasma interaction, until they are thermalized at low energy (approximately 1 eV) within the cusp trap;   2) Fresh buffer gas flows past the collection mirror surface to sweep away neutral tin atoms that otherwise would pass through the magnetic field without deflection and deposit on the mirror;   3) The buffer gas within the cusp trap dilutes the tin density via continual replenishment to prevent tin buildup and consequent EUV absorption;   4) The buffer gas plasma outflow from the cusp carries both the tin ions and the vast majority of process heat down pre-determined magnetic field flow lines onto the plasma beam dump. In this it is aided by the large heat capacity of metastable and ionic buffer gas species;   5) Radiation from the trapped buffer gas plasma can provide additional heat loss, this time to the chamber walls and collection optic. Resonance radiation can create buffer gas metastables throughout the chamber that can Penning ionize neutral tin atoms, aiding their collection via the magnetic field;   6) The plasma outflow contributes a powerful vacuum pump action with a well-defined direction toward the plasma beam dump.       

     Accordingly we propose an extreme ultraviolet light source comprising: a chamber; a source of droplet targets; one or more lasers focused onto the droplets in an interaction region; a flowing buffer gas; one or more reflective collector elements to redirect extreme ultraviolet light to a point on the common collector optical axis which is an exit port of the chamber; an annular array of plasma beam dumps disposed around the collector optical axis; a magnetic field provided by two sets of opposed, symmetrical field coils that carry equal but oppositely directed currents to create a symmetrical magnetic cusp, wherein the laser-plasma interaction takes place at or near the central zero magnetic field point of the cusp and heat is removed via radial plasma flow in a 360 degree angle range perpendicular to the optical axis toward the annular array of plasma beam dumps. 
     It is a further object of this invention to provide a near-symmetric cusp field for the capture and subsequent guiding toward an annular plasma beam dump of the tin ions and buffer gas ions from a laser-plasma interaction region. We define a “near-symmetric” cusp field as one in which the opposed axial magnetic fields may not be equal, but they both exceed the maximum radial magnetic field, implying that plasma out-flow will not be axial, but will be wholly radial. In the near-symmetric case the zero magnetic field point of the cusp lies between the axial coils and is closer to one of them. 
     Accordingly we propose an extreme ultraviolet light source comprising: a chamber; a source of droplet targets; one or more lasers focused onto the droplets in an interaction region; a flowing buffer gas; one or more reflective collector elements to redirect extreme ultraviolet light to a point on the common collector optical axis which is an exit port of the chamber; an annular array of plasma beam dumps disposed around the collector optical axis; a magnetic field provided by two sets of opposed, near-symmetrical field coils that carry oppositely directed currents to create a near-symmetrical magnetic cusp, wherein the laser-plasma interaction takes place at or near the zero magnetic field point of the cusp and heat is removed via radial plasma flow in a 360 degree angle range perpendicular to the optical axis toward the annular array of plasma beam dumps. 
     According to embodiments, an extreme ultraviolet light source comprises: a chamber; a source of droplet targets; one or more lasers focused onto the droplets in an interaction region; a flowing buffer gas; a reflective collector element to redirect extreme ultraviolet light to a point on the collector optical axis which is an exit port of the chamber; an annular beam dump disposed around the collector optical axis; a magnetic field provided by two sets of opposed magnetic field generators that create an asymmetrical magnetic cusp with magnetic well conforming to the inside shape of the collector, wherein the laser-plasma interaction takes place at or near the zero magnetic field point of the cusp and heat and target material particles are removed to a beam dump via magnetically guided plasma flow in a cone-shaped plasma sheet with cone axis parallel to the optical axis. 
     The present technology thereby integrates, synergistically, an advantageous magnetic field configuration with an effective buffer gas. Consequently, it is anticipated that application of this invention will extend the process power (i.e. the absorbed laser power) to the range of 30 kW and above, generating a usable EUV beam at the exit port of 150 W, or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the symmetrical cusp magnetic field configuration by itself. The configuration has a vertical axis of rotational symmetry. 
         FIG. 2  shows magnetic field lines near the center of the symmetrical cusp configuration of  FIG. 1 . 
         FIG. 3  illustrates the strength of the magnetic field along directions defined with reference to  FIG. 2 . 
         FIG. 4  shows the symmetrical cusp magnetic field coils in relation to the collection optic, the droplet generator, the droplet capture unit, the incident laser beam and the laser beam dump. 
         FIG. 5  shows the symmetrical cusp magnetic field coils guiding radial plasma flow toward the annular array of plasma beam dumps and vacuum pumps. 
         FIG. 6  shows a realization of the invention in which there are two collection optical elements. The figure has a vertical axis of rotational symmetry. 
         FIG. 7  illustrates the near-symmetrical cusp magnetic field configuration by itself. The configuration has a vertical axis of rotational symmetry. 
         FIG. 8  shows magnetic field lines near the center of the near-symmetrical cusp configuration of  FIG. 7 . 
         FIG. 9  illustrates the strength of the magnetic field along directions defined with reference to  FIG. 8 . 
         FIG. 10  shows a realization of the invention in which the cusp field is near-symmetric, having its lowest field barrier in the radial direction. 
         FIG. 11  illustrates certain system elements including a plurality of lasers and a buffer gas return flow loop. 
         FIG. 12  shows a cross section in the plane that is perpendicular to the optical axis and contains the interaction region, with additional system elements including the annular array of plasma beam dumps, the vacuum pumps and droplet injection and diagnostics ports. 
         FIG. 13A  illustrates a simple cusp formed by opposed permanent magnets. 
         FIG. 13B  shows a cusp field generator comprised of a combination of coils and a yoke of soft magnetic material. 
         FIG. 14  shows a modified cusp field geometry that supports a cone-shaped plasma sheet and high collector angle. 
     
    
    
     DETAILED 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  FIG. 1 . 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  FIG. 1  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  FIG. 2 . In that figure coils  10  and  20  correspond to those labeled  10  and  20  in  FIG. 1 . The magnetic field variation along lines AB and CD of  FIG. 2  is shown qualitatively in  FIG. 3  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  FIG. 4  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 2π 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  FIG. 4 , 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  FIG. 5  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  FIG. 5 , 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  FIGS. 2 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  cmsec −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  FIG. 6  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  FIG. 7 . 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  FIG. 7  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  FIG. 8 . In that figure coils  10  and  20  correspond to those labeled  10  and  20  in  FIG. 7 . The magnetic field variation along lines AB, CD and EF of  FIG. 8  is shown qualitatively in  FIG. 9  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  FIG. 10  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  FIG. 10 , 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  FIG. 12 . 
     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  FIGS. 7 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  FIG. 11 . 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  FIG. 12 . This figure depicts a cross section of the system in a plane perpendicular to axis of symmetry  1  that is illustrated for example in  FIG. 11 . 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-systems may access interaction region  60  via ports  290  that are shielded from the ion flux by field-shaping elements  300 . 
     Devices that generate a suitable cusp magnetic field are a) combinations of current-carrying coils, examples of which are described herein, b) permanent magnets, and c) current-carrying coils that induce magnetism in shaped yokes of soft magnetic material. Each of these may be incorporated separately, or together in any combination, to form a “magnetic field generator”. Examples of purely current-carrying generators are given above. Examples of the latter two types of generator will now be discussed. 
       FIG. 13A  shows two cylindrical permanent magnets,  510  and  520 , mounted coaxially on axis of rotational symmetry  1 . These each have a central empty cylinder and are opposed to each other so that, for example, the “north pole” of magnet  510  is opposite the “north pole” of magnet  520 . Magnetic field lines  525  define a cusp field with a magnetic null (i.e. B=0) at location  535 . Permanent magnets are of limited field strength that is borderline for control of the plasma necessary to brake fast tin ions emanating from the EUV generating laser-target interaction. If they are used, their zero power consumption is beneficial toward overall source electrical efficiency. 
       FIG. 13B  shows in cross section two opposed current carrying coils  550  and  560  mounted coaxially on axis of rotational symmetry  1 . The direction of current flow, labeled by an X for current entering the page and a dot for current coming out of the page, is opposed in coils  550  and  560 . Surrounding these coils is a coaxial cylindrical yoke  540  of highly permeable material, for example soft iron. When coils  550  and  560  are energized, a magnetic flux  525  is induced across the gaps in the yoke, to generate a cusp field with its magnetic null at location  535 . By variation of the positions of various coils, the currents within them, the geometry of the yoke and its relative permeability, an infinite variety of cusp field designs may be produced. 
     A further embodiment of the invention, that provides improved extreme ultraviolet light collection efficiency, is illustrated in  FIG. 14 . In  FIG. 14  the chamber is not shown and the magnetic field generating coils are shown in cross section. Their annular geometry is defined via vertical axis  1  of rotational symmetry. With reference to  FIG. 14 , a cusp field is generated by opposed magnetic field generators  330  and  360  above, and  340  and  350  below, drawn as current-carrying coils. 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. The term “magnetic field generator” refers not only to current carrying coils but also to permanent magnets or soft magnetic material arranged as yokes powered by current-carrying coils, either individually or in appropriate combination. The laser-plasma interaction region  60  is located on rotational symmetry axis  1  at or close to the magnetic null point of the cusp. Extreme ultraviolet radiation from position  60  propagates as typical ray  420  to a distant location on symmetry axis  1  that is an exit port of the chamber. On its path it undergoes a single reflection at the surface of collection optical element  400  that carries a multi-layer optical coating designed to reflect EUV light. Element  400  is a truncated ellipsoid of revolution about symmetry axis  1 . It may have a central hole to accommodate magnetic generator  350  and to allow buffer gas entry at least at locations  200 . Because of its forward-projecting aspect, ellipsoidal optical element  400  may subtend at location  60  a collection solid angle far in excess of 2π steradians. 
     In operation, the buffer gas is ionized to plasma by the exhaust energy of the laser-plasma interaction at position  60  and this plasma, shown in vertical shading, is trapped within magnetic well  500  shown in cross section by vertical stripe shading, with overflow into cone-shaped plasma sheet  440 . The magnetic well is a volume defined by a closed surface of constant magnetic field that has a lesser value of magnetic field at all points within that volume. Plasma overflows from containment in a magnetic well via the points of least containment field. In this case the overflow locus is a circle lying on the cone-shaped plasma sheet. The magnetic field design is asymmetrical and is such that the plasma exhaust, carrying buffer gas and target material atoms and ions, overflows into and is guided in, cone-shaped sheet  440  past the forward-projecting edge of optical element  400  toward annular beam dump  140 . The shape of magnetic well  500  can be distorted to conform to the inner shape of collector  400  via relatively stronger field generation at magnetic field generators  340  and  360 , and weaker generation at generators  330  and  350 . Here the term “conform” is used in a loose sense to indicate that the magnetic well is smaller than the collector surface but has the same general shape where they are closest to each other. It may be advantageous to have the target material droplet stream (not shown) enter via a hole in collector  400 , and to have unused droplets exit through a second hole in the collector. Also, droplet position monitoring may require additional small holes in the collector. 
     In a further embodiment of the invention the flowing buffer gas may comprise a mixture of two or more gases taken from the set hydrogen, helium and argon. The use of a mixture enables additional performance beyond use of a single species. For example, a dominant argon buffer can supply the fundamental plasma braking effect [34] while a small addition of hydrogen can provide tin scavenging off a collector optic to maintain its high reflectivity [35,36,37]. In prior work [12] there has been 100% hydrogen usage for reasons to do with its better stopping power [38] as a neutral gas, for fast tin ions, than for example argon. This comparison is made after the relative densities of the two gases have been adjusted to give constant EUV optical transmission. When plasma electrons are the dominant braking agent [34], the nature of the ions in the plasma is not of primary importance and the advantages of relegating hydrogen to a minority species are many: 
     a. Although injected as molecular hydrogen (H 2 ), the source plasma conditions at (approximately) density 10 15  electrons cm −3  and temperature 2 eV cause rapid dissociation of H 2  into H atoms. These can re-combine to H 2  on surfaces with release of heat, or they can participate in chemical reactions to form hydrides such as stannane (SnH 4 ). Because dissociation is on such a large scale, it becomes difficult to predict the heat load on any part of the surface in contact with the exhaust flow.
 
b. It is desirable that tin or other target material be condensed and recycled. Reactions into stannane and other hydrides can occur on surfaces or in the chamber volume, leading to downstream deposition on cool surfaces and even decomposition on hot surfaces. Lack of specificity makes it difficult to define a tin recycling stream that is close to 100% accurate and effective.
 
c. Hydrogen is explosive when mixed with air [38] leading to the need for severe handling precautions that add additional complexity and cost to an EUV source running on hydrogen alone.
 
     Further realizations of the invention will be apparent to those skilled in the art and such additional embodiments are considered to be within the scope of the following claims. 
     REFERENCES 
     
         
         1. “EUV Sources for Lithography” Ed V. Bakshi, SPIE Press, Bellinghaven, Wash. 2005. 
         2. U.S. Pat. No. 7,479,646 McGeoch, Jan. 20, 2009 
         3. U.S. Pat. No. 8,269,199 McGeoch, Sep. 18, 2012 
         4. U.S. Pat. No. 8,440,988 McGeoch, May 14, 2013 
         5. U.S. Pat. No. 8,569,724 McGeoch, Oct. 29, 2013 
         6. U.S. Pat. No. 8,592,788 McGeoch, Nov. 26, 2013 
         7. M. McGeoch Proc. Sematech Intl. EUV Lithography Symp., Toyama, Japan 28 Oct. 2013 
         8. M. Richardson et al., J. Vac. Sci. Tech. 22, 785 (2004) 
         9. Y. Shimada et al., Appl. Phys. Lett. 86, 051501 (2005) 
         10. S. Fujioka et al., Phys. Rev. Lett. 95, 235004 (2005) 
         11. S. Fujioka et al., Appl. Phys. Lett. 92, 241502 (2008) 
         12. H. Mizoguchi et al, Proc SPIE 2014 
         13. D. Brandt et al., Proc SPIE 2014 
         14. S. S. Harilal et al., Phys. Rev. E69 026413 (2004) 
         15. S. S. Harilal et al., Appl. Phys. B86, 547-553 (2007) 
         16. U.S. Pat. No. 7,271,401 Imai et al., Sep. 18, 2007 
         17. U.S. Pat. No. 7,705,533 Komori et al., Apr. 27, 2010 
         18. U.S. Pat. No. 7,999,241 Nagai et al, Aug. 16, 2011 
         19. U.S. Pat. No. 8,143,606 Komori et al., Mar. 27, 2012 
         20. U.S. Pat. No. 8,492,738 Ueno et al., Jul. 23, 2013 
         21. U.S. Pat. No. 8,507,883 Endo et al., Aug. 13, 2013 
         22. U.S. Pat. No. 8,569,723 Nagai et al., Oct. 29, 2013 
         23. U.S. Pat. No. 8,586,953 Komori et al., Nov. 19, 2013 
         24. U.S. Pat. No. 8,586,954 Asayama et al., Nov. 19, 2013 
         25. U.S. Pat. No. 8,629,417 Nagai et al., Jan. 14, 2014 
         26. U.S. Pat. No. 8,710,475 Komori et al., Apr. 29, 2014 
         27. U.S. Pat. No. 8,519,366 Bykanov et al., Aug. 27, 2013 
         28. U.S. Pat Appl. 20140021376 Komori et al. date 
         29. U.S. Pat Appl. 20110170079 Banine et al, Jul. 14, 2011 
         30. U.S. Pat. No. 7,271,401 Imai et al., Sep. 18, 2007 
         31. U.S. Pat. No. 7,705,533 Komori et al., Apr. 27, 2010 
         32. U.S. Pat. No. 7,671,349 Bykanov et al., Mar. 2, 2010 
         33. U.S. Pat. No. 8,198,615 Bykanov et al., Jun. 12, 2012 
         34. McGeoch “Cusp plasma control for the tin LPP source” Proc. SPIE Advanced Lithography San Jose February 2016 
         35. U.S. Pat No. 7,462,850 Banine et al., Dec. 9, 2008 
         36. U.S. Pat No. 7,598,503 Van Herpen et al., Oct. 6, 2009 
         37. U.S. Pat No. 8,624,208 Nagai et al., Jan. 7, 2014 
         38. U.S. Pat No. 8,785,892 Ershov et al. Jul. 22, 2014