Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
  • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
  • G03F7/70—Microphotolithographic exposure; Apparatus therefor
  • G03F7/70008—Production of exposure light, i.e. light sources
  • G03F7/70033—Production of exposure light, i.e. light sources by plasma extreme ultraviolet [EUV] sources
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
  • B05B17/00—Apparatus for spraying or atomising liquids or other fluent materials, not covered by the preceding groups
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05G—X-RAY TECHNIQUE
  • H05G2/00—Apparatus or processes specially adapted for producing X-rays, not involving X-ray tubes, e.g. involving generation of a plasma
  • H05G2/001—Production of X-ray radiation generated from plasma
  • H05G2/002—Supply of the plasma generating material
  • H05G2/0027—Arrangements for controlling the supply; Arrangements for measurements
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05H—PLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
  • H05H1/00—Generating plasma; Handling plasma
  • H05H1/24—Generating plasma

Definitions

• the present invention relates generally to laser technology for photolithography, and, more particularly., to optimization of extreme ultraviolet (EUV) iight production.
• EUV extreme ultraviolet
• EUV Extreme ultraviolet
• soft x-rays electromagnetic radiation having wavelengths of between 10 and 110 nm
• EUV lithography is generally considered to include EUV light at wavelengths in the range of 10 - 14 o e, and is used to produce extremely small features (e.g.,. sub-32 nm features) in substrates such as silicon wafers.
• Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements (e.g... xenon, lithium., tin, indium, antimony, tellurium, aluminum, etc.) with one or mare emission line(s) in the EUV range.
• the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired Sine-emitting element., with a laser bea at an irradiation site.
• the line-emitting element may be in pure form or alloy form (e.g., an alloy that is a liquid at desired temperatures), or may b mixed or dispersed with another material such as a liquid. Delivering this target material and the laser beam simultaneously to a desired irradiation site (e.g., a primary focal spot) within an LPP EUV source plasma chamber for plasma initiation presents certain timing and control problems.
• a desired irradiation site e.g., a primary focal spot
• the laser beam it is necessary for the laser beam to be focused on a position through which the target material will pass and timed so as to intersect the target material when it passes through that, position in order to hit the target properly to obtain a good plasma, and thus, good EUV light,
• a droplet generator heats the target material and extrudes the heated target material as droplets which travel along an x-axis- of the primar focal spot to intersect the laser beam traveling along a z-axis of the primary focal spot, Ideally, the droplets are targeted to pass through the primary focal spot.
• EUV light output is maximized, f63
• plasma formed from preceding droplets within a burst interferes with trajectories of succeeding droplets within the burst, pushing the droplets out of the x-axi of the primary focal spot.
• the droplets are displaced ("pushed-out") along the y- and/or z-axes away from the primary focal spot when hit by the laser beam.
• This push-out ramps up rapidly (e.g., in about 15-20 ms) and can be quite large (e.g., 120 ⁇ » displacement from the primary focal spot).
• the large and rapid nature of the push-out is especially problematic during continuous mode firing of the EUV system because re-alignment of droplets to the primary focal spot cannot be achieved before the laser tires again and iases a succeeding droplet outside the primary focal spot.
• the effect of the push-out is that plasma generated from succeeding droplets is not focused in th primary focal spot of the collector, and, consequently, EUV light output is not optimized.
• [i j in another embodiment is presented a system for pre-compensatlon of push-out from a primary focal spot of target material droplets during burst-firing of an extreme ultraviolet laser light source
• a droplet generator comprising; a droplet generator; a sensor; one or more axis controller; , one or more actuator to position the droplet generator; wherein the sensor Senses one or more droplet during a burst, the one or more droplet delivered from the droplet generator to a target position at which the one or more droplet is to be !ased; and the one or more axis controller: calculates an axial position of each of the one or more droplet; estimates an open-loop droplet position for each of the one or more sensed droplet in the burst calculates, after the burst has ended., a pre-co pensatio correction based on the open-loop droplet position of the one or more droplet in the burst; calculates an updated target position with the pre-eompensatlan
• F iG, 1 is a schematic illustrating some of the components of a typical LPP EUV system.
• FIG, 2 is a diagram depicting EUV system components involved in optimization of EUV light production according to one embodiment
• FIG. 3a Is a schematic illustrating droplet orientation whe the driver laser is off.
• FIG. 3b is a schematic Illustrating droplet orientation when the drive laser is first pulsed.
• FIG, 3c is a schematic Illustrating a push out of droplets along the z-axis. observed when the drive laser is pulsed in a continuous mode.
• FIG, 3d is a schematic illustrating droplet-to-droplet feedback-controlled realignment of droplets to compensate for the push-out observed when the drive laser is pulsed in a continuous mode.
• FIG. 4 depicts droplet position along the z-axis as a function of time before, during, and after laser firing,
• FIG, 6 is a block diagram depicting EUV system components Involved In adaptive pre-compensatiort for droplet push-out according to one embodiment
• [201 R ' G. 7 s a flowchart. of a method to compensate for droplet pash-out according to one embodiment.
• FIG, 8 illustrates droplet position over time during a laser-firing burst with and without droplet-to-droplet feedback to re-afign droplets to a primary focal spot according to one embodiment
• FIG, 9 shows droplet position over time for simulated data during adaptive learning to pre-com ensate for droplet push-out according to one embodiment
• FIG. 10 shows droplet position error over time fo simulated data during adaptive learning to pre-compensate for droplet push-out according to one
• a semiconductor wafer is divided into multiple di s, each of which is to have the same type of Integrated circuit fabricated thereon. Therefore, the dies on the wafer need to be exposed to equivalent amounts of EUV light. To meet this requirement, the laser is fired at the same operation point for every exposure. Thus, the generated push- outs are similar in size. Although the push-outs have a repetitive character in a single operation point, the size of the push outs can differ across operating points and across EUV systems..
• Embodiments of a system and method described herein ma ke use of this repetitive character of the push-outs to adaptively pre-eompensate f or droplet push-out by learnin how big the droplet push-out is and adjusting a droplet generator between bursts in anticipation of the push-out.
• the EUV system repositions the droplet generator to deliver droplets In a succeeding burst to a target position that is offset from the primary focal spot based on the magnitude of the push- ⁇ observed in the previous burst.
• FIG, 1 illustrates some of the components of a typical LPP EUV system 100
• a drive laser 101 such as a C0 2 laser, produces a laser beam 102 that passes through a beam delivery system 103 and through focusing optics 104, Focusing optics 104 have a primary focal spot 105 at an irradiation site within an LPP EUV source plasma chamber 110.
• a droplet generator 106 produces and ejects droplets 107 of an appropriate target material that, when hit by laser beam 102 at the irradiation site, produce plasma that emits EUV light.
• An elliptical collector 108 focuses the EUV light from the plasma at an Intermediate focus 109 for del ivering the produced EUV light to, e.g., a lithography system.
• Intermediate focus 109 will typically be within a scanner (not shown) contain ing boats of wafers that are to be exposed to the EUV light* with a portion of the boat containing wafers currently being irradiated being located at intermediate focus 109.
• One type of LPP EUV light source may use a C0 2 laser and a zinc selenide ' ⁇ 2nSe) lens with an anil-reflective coating and a clear aperture of about 8 to 8 Inches.
• Drive laser 101 is fired in a pulsating manner in order to hit discrete droplets 107 separately. Although every sequence -of pulses comprises a burst, drive laser 101 can be fired In different burs modes, In a stroboscope mode ⁇ I.e., a mode with short bursts), the length of the bursts are limited to 1 ms t whereas in a continuous mode (i.e., a mode with Song bursts), the expected burst length is 3-4 seconds for each die. [28] When firing drive laser 101 in stroboseopic mode, EUV system 100 maintains droplets 107 on-target reasonably well using closed-loop (droplet-to-droplet) feedback.
• closed-loop droplet-to-droplet
• a closed-loop (droplet-to-droplet) feedback control system (''dro let-to -dro let feedback system”) has been used historically to keep droplets 107 targeted on primary focal spot 105 during pulse firing of drive laser IGl
• the d oplet-to-droplet feedback system comprises a line laser in combination with a sensor ⁇ e,g,, a narrow field (NF) camera) that measures droplet position along the y- and/or z-axis as droplet 107 is about to be lased.
• EUV system 100 uses the measured droplet position to command actuators (e.g., piezoelectric ("PZT") actuators) to re-align droplet generator 106 so that successive droplets (pushed out of primary focal spot 105 by plasma generated from preceding droplets 107) are re-aligned to b delivered to primary focal spot 105,
• actuators e.g., piezoelectric ("PZT" actuators
• PZT piezoelectric
• coarse movement actuators e.g., stepper motors
• coarse movement actuators are non-preferred for realigning- droplets 107 on-target.
• fine movement actuators e.g., PZT actuators
• the closed-loop (droplet-to-droplet) feedback system takes longer to correct a push-otil disturbance (e.g.,. approximately .4 ms) than is desirable.
• FIG. 2 A magnified schematic of EUV system components involved in optimization of EUV light production according to one embodiment is shown in FIG, 2,
• Laser beam 102 is delivered throug elliptical collector 108 to primary focal spot 105, Positioning of p imary focal spot 105 along the y » and z- axes is determined by focusing optics 104, to wit, a final focus lens (not shown) and a final focus steering mirror (not shown), as described in United States Patent Application No. 13/549,261 Frihauf et a!.), hereby incorporated by reference in its entirety herein.
• Energy output from the LPP EUV system varies based on how well laser beam 102 can be focused and can maintain focus over time on droplets 107 generated by droplet generator 106.
• Optimal energy Is output from EUV system 100 if the droplets are positioned in primary focal spot 105 when h it by laser beam 102. Such positioning of the droplets allows elliptical collector 10.8 to collect a maximum amount of EUV fight from the generated plasma for delivery to, e.g., a lithography system.
• a sensor 2G1 senses the droplets as they pass through a laser curtain during travei to primary focai spot 105 and provides droplet-to-droplet feedback to EUV system 100, which droplet-to-droplet feedback is used to adjust droplet generator 106 to re-align droplets 107 to primary focal spot 105 (i.e., "on-target"), 3i] How droplet position along the z-axis changes during laser firing in a continuous burst mode wili now be described with reference to FiGs. 3a,. 3b, 3c, 3d, and 4.
• NF narrow field
• FiGs, 3a, 3b, 3c, and 3d illustrate schematically the orientation of droplets 107, respectively, before, at initiation of, during, and after laser burst firing in a continuous burst mode
• FIG. 4 is a graph depicting droplet position along the z-axis over time with primary focal spot 105 indicated by a solid line at a z-axls position of 0. Droplets 107 iased while at primary focai spot 105 generate plasma within the focal spot of elliptical collector 108.
• Arrows 401a, 401b, 401c, and 401d indicate points in time that RGS, 3a, 3b, 3c, and 3d, respectively, occur, 132]
• droplets 107 ejected from droplet generator 106 to droplet catcher 301 are oriented in a straight line along the x-axis of primary focal spot .105.
• droplets 107 pass through primary focal spot 105,
• droplet push-out has bean controlled historically through a droplet -to-dropiet feedback system which adjusts droplet generator 106 after push-out has occurred to start moving succeeding droplets 107 back to primary focal spot 105.
• the droplet-to-droplet feedback system waits for the push-ou and then fights the disturbance by adjusting droplet generator 105 with actuators (e.g., PZT actuators) to re-align successive droplets 107 to target (i.e., primary focal spot 105). Because the droplet-to-clroplet feedback process does not begin until after push-out has occurred, however, error between an actual position of droplet 107 and primary focal spot 105 can be very large.
• actuators e.g., PZT actuators
• droplets 107 should be ⁇ and should stay) within ⁇ 5 ⁇ im of primary focal spot 105.
• the drop!et-to-droplet feedback process requires substantial time (e.g., approximately 0.4 seconds) after the start of s burst to reposition droplets 107 back o «- target.
• the embodiments discussed herein take advantage between bursts of the push-out disturbance and thereby reduce the time necessary for droplet-to-droplet feedback systems to re-align droplets during a burst to a correct z-axis position to overcome the push-out disturbance. Minimizing the droplet-to-droplet feedback also allows EUV system 100 to rely on PZT actuators to reposition droplets 107 and to avoid having to use stepper motors (which introduce vibration Into the EUV system and thereby interfere with successful repositioning of droplet 107 on-tsrgeti
• FIG. 5 in contrast to known systems and methods of producing EUV light, embodiments of the system and method described herein re-align droplets during the inter-burst interval (i.e., between bursts to a target position S02 which is displaced away from primary focal spot 105 such that laser beam 102 strikes droplet 107 at target position 502, As ' laser beam 102 strikes droplet 107 at target position 502, droplet 107 is pushed-out, but the push-out phenomenon pushes droplet 107 into (rather than out of) primary focal spot 105 as plasma 302 is generated, Thus plasma 302 is generated at primary focal spot 105— that is, in the focal spot of elliptical collector 108— and produced EUV light is collected and focused by elliptical: collector 108 at Intermediate focus 109,
• target position 502 can, but need not coincide with primary focal spot 105.
• FIG. 38J A block diagram providing an overview of a pre-compensation control loop used to adaptiveiy adjust droplet target position according to one embodiment is presented in FIG. 6.
• Droplet generator 106 ejects droplets 107 along an x-axis to a target (x ⁇ , y-> z- ⁇ position at which droplets 10? are to be !ased as discussed with respect to FIG, 1,
• One or more sensor 201 inside LPP EUV source chamber 110 senses one or more axial position (e.g., along a y-axis, along a z-axis, or along both axes) of ejected droplet 107. The sensed axial position of droplet 10?
• y-axis controller 603Y and/or the sensed position of droplet 107 along the 2-axis is passed to z-axis controller 603z,
• Axial controllers 6Q3Y and/or 6032 determiners) an updated target position for droplet 107 that pre-compensates for anticipated droplet push-out (as discussed in greater detail elsewhere herein).
• Axial controllers 603Y and/or 603z then output(s) commands to, respectively, y-axis actuators .604v and/or z-axis actuators 604z ⁇ e,g>, stepper motors and/or PZTs) to adjust droplet generator 106 such that succeeding droplets 107 are delivered to the pre-compehsated axial target position 502,
• a pre-compensation control loop is used to adaptiveiy adjust droplet position.
• axial controllers 603.Y and/or 603z determine ⁇ ) an updated droplet position for droplet 107 that pre-compensates for anticipated droplet push-out (as discussed in greater detail elsewhere herein).
• sensor 201 senses droplet 107 and sends data about droplet 107 to axial controllers 6G3Y and/or 6Q3z. in one embodiment, sensing of droplet position is triggered by a start of a burst and is terminated by cessation of the burst.
• the field of view of sensor 201 determines sampling-frequency (e.g., reducing the field of view of sensor 201 allows for a increased frame rate) of droplets,
• a position of the sensed droplet along the y- and/or z-axis is calculated.
• sensor 201 measures an axial position of droplet 107 (in pixels) and determines a vertical centroid and a horizontal centroid for droplet 107.
• Axial controllers then perform coordinate transformations on the vertical and horizontal centroids.
• y-axis controller 6G3Y converts the pixels of the vertical centroid to calculate a y-axis position (e.g., in pm)
• z-axis controller 603z converts the pixels of the horizontal centroid to calculate a z-axis position (e.g., in ⁇ ) of droplet 107.
• This transformation minimizes any effect of sensor tilt on the measured (y,?.) position of droplet 107.
• the axial controllers estimate axial (z- or y-) open-loop positions of sensed droplet 107.
• the axial open-loop positions are the z- and/or y- positSon(s) of droplet 107 if no droplet-to-droplet feedback had been applied to reposition droplet 107— that.
• the axis controllers estimate the axial open-loop estimates by subtracting any droplet-to-droplet feedback adjustment from the determined droplet y- and/or z- posiiion(s).
• y-axis controller 603y subtracts the droplet-to-droplet feedback adjustment applied to compensate for y-axis push-out of droplet 107 from the y-axis position determined in step ?02 to eakulate an estimated open-ioop y-axis position of sensed droplet 107.
• step 704 the axis controllers determine whether the burst has ended, if the burst has not ended, the process returns to step 701 and steps 701, 702, 703,. and 704 are performed for another droplet 107. That is, steps 701, 702, 703, and 704 are iterated for the droplets !ased during a burst,
• step 705 the axis controllers calculate a pre- ⁇ corapensation correction to be applied to determine a new target position.
• the pre-compensati ' on correction should he in the opposite direction to the calculated -OLa g
• the number of data points used to calculate the OL depends on the frame/second speed of sensor 201 when sensing droplet 107 and on the length of time over which the hurst occurred, in one embodiment, because sensor speed is not always consistent and sensor 201 may not always capture a good image, position data are iterated to -fill in gaps between data frame and make a "continuous" signal before data sampling at a predetermined frequency.
• y-axis controller 603Y calculates a pre-compensation correction by determining the deviation of the averaged estimated open-loop y-axis position of sensed droplet 107 in the previous burst f rom the y-axis position of the primary focal spot, and multiplying that y-axis deviation by s learning gain.
• z-axis controller 6032 calculates a pre-compensation correction by determining the deviatio of the averaged estimated open-loop z-axis position of sensed droplet 107 " m the previous burst from the z-axis position of the primary focal spot, and multiplying that z-axis deviation by a learning gain (which may be the same learning gain as that used to calculate the y-axis pre-cornpensation correction).
• the learning gain Is a tunable parameter that can range between 0 and 1, but is preferably about 0,1 or less. Determination of the learning gain necessitates a trade-off between rapid convergence on-target and susceptibility to variable push-out disturbances.
• a large learning gain (e.g., K-i), for instance, works well for a first iteration of the adaptive loop because, during the first burst, there is no previous information from which the system can learn. Or, if the open-loop displacement is actually known (rather than being estimated), then a learning gain of 1,0 is acceptable because the known magnitude of the droplet push-out would indicate how far to offset target position 502 from primary focal spot ICS.
• the updated target position continues to rely almost completely on the position of the droplets during the immediately preceding hurst—which may not always be correct, For instance, if the push-out during a first burst is quite large for some reason, most of the change in target positioning is determined from the open-loop average position of that burst if the learning gain is 1.0. The result will be that the target position 502 wiii be displaced a considerable distance away from primary focal spot 105.
• a smaller learning gain (e.g., K-0.1), on the other hand, is preferable when the open-loop displacement Is estimated (rather than actually measured) so as to avoid an. over-reliance on the estimated open-loop displacement when determining the target position,
• the adaptive pre-compensation loop "learns" over time to achieve a stable target position 502, if, however, a very small learning gain is chosen (e.g., K 0.1), target position 502 will barely change In response to data gathered from previous bursts and the system will take longer to learn an acceptable stable displacement from primary focal spot 105.
• a learning gain that changes over time is preferred.
• a large learning gain that decreases over time allows the pre-compensation to converge quickly to a "best" target position that Is stable and relatively Insensitive to fluctuations in droplet position over time
• step 706 the axis controllers update target position 502 by adding the pre- compensation corrections calculated in step 705 to target position 502 of the previous burst to obtain an updated pre-compensated target.
• y -axis controller 6Q3Y calculates an updated pre- compensated target along the y-axis and z-axis controller 603z calculates an updated pre-compensation target along the 2-axls.
• this updated pre-compensation target is not primary focal spot 105, but Is a learned displacement that allows the push- out phenomenon to push the droplets back into primary focal spot 105 when a next burst begins.
• learning algorithms e.g., Least Mean Squared o Recursive Least Squares equations
• Pre-compensation based upon the previous burst alone may be susceptible to measurement noise.
• data obtained from some or ail previous bursts are used to caicuiate the pre-compensation correction befare using that pre-compensation correction to update the re-compensated target.
• the axis controllers command axial actuators to reposition d opiet generator 106 so that droplets 107 ejected from droplet generator 108 are In the updated pre-compensated target position when Iased
• y-axis controller 803Y sends a command to y-axis actuators controlling movement of droplet generator along the ⁇ -axis
• z-axis controller 603z sends a command to a z-axis actuators controlling movement of droplet generator along the z-axis such that droplets 107 ejected from droplet generator 106 are Iased as they pass through the updated (that is, e -compensated) (y,z) target.
• step 70S calculation of the pre-conipensatlon correction (step 70S), updating of the target position 502 (step 706), and commanding the axia l actuators (step 707 ⁇ occ during the inter-hurst interval.
• some accommodations are implemented during bursts to mitigate undesired effects on the pre-corn ensated target adaptation. For instance, after target adjustment., ramping (i.e., ramp-up and ramp-down) of the push-out phenomenon should be allowed to continue without droplet-to-droplet feedback control Instead, the push-out should reach the target by itself without any control action because laser beam 102 will push droplets into primary focal spot 105. So, during these ramping periods, the d let-to- dro let feedback system is made inoperative.
• One way to achieve this inoperabillty is to set error in droplet positioning to zero so that no droplet-to-dropiet feedback action is initiated during the ramping periods.
• An open- loop estimate 803 is determined for droplets 107, Once the burst has ended, an average of the open-loop estimates ⁇ OL avg ) is calculated and then multiplied by a learning gain ( ) to obtain a pre- compensation correction. The pre-compensated correction Is then added to the previous target position (primary focal spot 105 In this example) to obtain an updated target position S02), Re-alignment of droplet generator
• the axis controllers slowly learn how much d ro i et-to- droplet feedback, on average, is needed to re-align ⁇ droplets 10? to be on-target. Briefly, after target position 502 is adjusted to pre* compensate for the push-out disturbance in a first burst, the push-out disturbance for the succeeding (second) burst is reduced. After target position 502 is adjusted to pre- compensate for ths push-out disturbance in the second burst, the push-out disturbance for the succeeding ⁇ third ⁇ burst is further reduced, and so on. This learned pre- compensatio is. Illustrated with simulated data in FiGs, 9 and 10.
• FIG. 9 illustrates convergence of target pre-eompensat on over time for simii!ated data of a block wave push out with noise.
• the simulated data depict droplet position along the z-axis over time as drive laser 101 is fired in a continuous burst mode with a learning gain equal to 0.5. Bursts are indicated fay numbered columns, During burst 1 (when target position 502 is set to primary focal spot 105), a push-out 901 of approximately 25 ⁇ away from focal spot 105 is observed. The push-out is corrected by droplet-to-droplet feedback control which re-aiigns the droplet position (aibeit later during the burst) back to primary focal spot 105.
• an updated target position 502 (at approximately -12 ⁇ along the z- axis from primary focal spot 105) is determined (by z-axis controller 6Q3z),
• droplet 107 is lased during bursts, droplet 107 is pushed-out (approximately 25 ⁇ xm along the z-axis) from updated target position 502 to a position that is only about 10 ⁇ xm along the z-axis from primary focal spot 105.
• dropiet-to-droplei feedback control re-aiigns the droplet position back to primary focal spot 105 more rapidly than in the preceding burst SOS.
• the push-outs become increasingly smaller in magnitude and are more rapidly adjusted during the succeeding bursts with dropiet-to-dropiet feedback so that droplets remain on-target, in this example, FUV system 100 has learned by burst 4 to pre- compensate target position 502 to a sufficient degree that droplets are pushed (with m in imal dropiet-to -dropiet feedback) to within a reasonable distance 904 of primary focal spot 105 to generate plasma.
• Error in droplet position along the z-axis over time for the simulated data of FIG. 9 is presented in FIG. 10.
• FUV system 100 has reduced the droplet position error 1004 by hurst 4 to within 5 ⁇ xw. of primary focal spot 105.
• plasma generated within 5 am of primary focal spot 105 optimises EUV light production.
• This embodiment is less expensive to implement, but is less robust in that disturbances within the EUV system can negatively impact performance (e.g., EUV output), for example, if droplet jump (i.e., random droplet movement as, e.g., when debris clogs a nozzle of the droplet generator thereby altering a trajectory of an ejected droplet ⁇ is experienced during an inter-burst interval, there is no way to steer the droplet generator back to position until EUV Is again being produced.
• droplets in a next burst can be greatly displaced from a desired position— and droplet-to-drop!et feedback may take a long time or even be unable to reposition droplets to the desired position.
• this embodiment allows bursts of droplets to settle over a short time (e.g., over 3-4 bursts) to a point where droplets are positioned within an acceptable distance from the primary focal spot.
• an inter-burst deadband is used instead of inter-burst target, pre-compensation or droplet-io-dropiet feedback control.
• the deadband is chosen as the region in which the laser beam hits the droplets, and therefore allows droplets to be pushed-o t onto the target at every dark-light transient (e.g., at the start of laser burst-firing).
• the deadband parameters for the droplet generator steering control loop are set such that when droplets are a large distance from the primary focal spo droplets are steered to the target with .droplet-to-dropiet feedback, whereas when the dropiets are within a close range ⁇ e.g., 20 pm ⁇ of the primary focal spot, droplets are not actively steered (droplet-to-droplet feedback Is inoperable)- Since droplets do not drift away during short periods ⁇ e.g., a few hundreds of msecs), the inter-burst deadband brings the droplets back onto target at a beginning of exposure more accurately than the droplet-to-dropiet feedback control, This embodiment is independent of the amplitude and direction of the push-out, and typically does not need to be calibrated.
• an inverse control signal can be used in a feedforward fashion to move actuators to maintain droplets on target instead of inter-burst target pre-compensation or droplet-to-droplet feedback control, in this embodiment, the axial droplet position Is determined ⁇ as described in step 702 with reference to FiG. 7), after which an inverse of that position is determined, A control signal for that inverse position is sent to actuators to reposition the droplet generator to deliver droplets to an inverse position. Because droplets are generated at a high rate, this embodiment is particularly effective if fast actuators are used to reposition the droplet generator, 62]
• the disclosed method and apparatus have been explained above with reference to several embodiments.
• the described method and apparatus can he implemented in numerous ways, including as a process, an apparatus, or a • system.
• the methods described herein may be implemented by program instructions for instructing a processor to perform such methods, and such instructions recorded on a computer-readable storage medium such as a hard disk drive, floppy disk, optical disc such as a compact disc (CD ⁇ or digital versatile disc (DVD), flash memory,, etc., or a computer network wherein the program instructions are sent over optical or electronic communication links.
• a computer-readable storage medium such as a hard disk drive, floppy disk, optical disc such as a compact disc (CD ⁇ or digital versatile disc (DVD), flash memory, etc.

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