System and method to adaptively pre-compensate for target material push-out to optimize extreme ultraviolet light production

Energy output from a laser-produced plasma (LPP) extreme ultraviolet light (EUV) system varies based on how well the laser beam is focused on droplets of target material to generate plasma at a primary focal spot. Maintaining droplets at the primary focal spot during burst firing is difficult because generated plasma from preceding droplets push succeeding droplets out of the primary focal spot. Current droplet-to-droplet feedback control to re-align droplets to the primary focal spot is relatively slow. The system and method described herein adaptively pre-compensate for droplet push-out by directing droplets to a target position that is offset from the primary focal spot such that when a droplet is lased, the droplet is pushed by the laser beam into the primary focal spot to generate plasma. Over time, the EUV system learns to maintain real-time alignment of droplet position so plasma is generated consistently within the primary focal spot.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to laser technology for photolithography, and, more particularly, to optimization of extreme ultraviolet (EUV) light production.

2. Description of the Prior Art

The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be 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 nm, and is used to produce extremely small features (e.g., sub-32 nm features) in substrates such as silicon wafers. These systems must be highly reliable and provide cost-effective throughput and reasonable process latitude.

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 more emission line(s) in the EUV range. In one such method, often termed laser-produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam 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 be 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. Specifically, 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 primary 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. When the laser beam hits the droplets at the primary focal spot, EUV light output is maximized.

When the laser fires, however, plasma formed from preceding droplets within a burst interferes with trajectories of succeeding droplets within the burst, pushing the droplets out of the x-axis of the primary focal spot. The result is that 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 μm 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 fires again and lases a succeeding droplet outside the primary focal spot. Thus, the effect of the push-out is that plasma generated from succeeding droplets is not focused in the primary focal spot of the collector, and, consequently, EUV light output is not optimized.

Current methods to compensate for droplet push-out rely on droplet-to-droplet feedback control of the droplet generator to re-align droplets in the primary focal spot after the push-out has occurred. Such droplet-to-droplet feedback control is not ideal, however, because of the relatively long time necessary to re-align droplets relative to the speed at which the droplets travel. For example, when the laser is firing in a continuous mode, the droplet-to-droplet feedback after plasma from a first droplet causes a push-out disturbance is too slow to completely re-align a next droplet to the primary focal spot target before that next droplet is hit by the laser beam.

What is needed therefore is an improved way to accurately re-position the droplets of target material more rapidly so the laser beam strikes the droplets within the focal spot of the laser beam.

SUMMARY

In one embodiment is presented a method of pre-compensating for push-out from a primary focal spot of target material droplets during burst-firing of an extreme ultraviolet laser light source comprising: sensing one or more droplet during a burst, the one or more droplet delivered from a droplet generator to a target position at which the one or more droplet is to be lased; calculating an axial position for each of the one or more sensed droplet in the burst; estimating an open-loop droplet position for each of the one or more droplet in the burst by subtracting droplet-to-droplet feedback for each of the one or more droplet in the burst from the calculated axial position for each of the one or more droplet in the burst; calculating after the burst has ended a pre-compensation correction based on the open loop droplet position from the one or more droplets in the burst; calculating an updated target position with the pre-compensation correction; and commanding one or more actuator to reposition a droplet generator to deliver, during a succeeding burst, droplets of target material to the updated target position.

In another embodiment is presented a system for pre-compensation of push-out from a primary focal spot of target material droplets during burst-firing of an extreme ultraviolet laser light source 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 lased; 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-compensation 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-compensation correction; and commands one or more actuator to reposition the droplet generator to deliver, during a succeeding burst, droplets of target material to the updated target position.

DETAILED DESCRIPTION OF THE INVENTION

A semiconductor wafer is divided into multiple dies, 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 make use of this repetitive character of the push-outs to adaptively pre-compensate for droplet push-out by learning how big the droplet push-out is and adjusting a droplet generator between bursts in anticipation of the push-out. Specifically, 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-out observed in the previous burst. As a droplet (delivered by the repositioned droplet generator) is hit by the laser beam, a ramp-up of the push-out of that droplet begins, thereby driving the droplet into the primary focal spot to generate plasma that is, in turn, focused by an elliptical collector onto an intermediate focus before being passed to or used by, e.g., a lithography system. Over time, the EUV system learns to maintain droplet position on-target. This adaptive nature of the pre-compensation is important in order to avoid long calibration procedures for a static target adaptation.

FIG. 1illustrates some of the components of a typical LPP EUV system100. A drive laser101, such as a CO2laser, produces a laser beam102that passes through a beam delivery system103and through focusing optics104. Focusing optics104have a primary focal spot105at an irradiation site within an LPP EUV source plasma chamber110. A droplet generator106produces and ejects droplets107of an appropriate target material that, when hit by laser beam102at the irradiation site, produce plasma that emits EUV light. An elliptical collector108focuses the EUV light from the plasma at an intermediate focus109for delivering the produced EUV light to, e.g., a lithography system. Intermediate focus109will typically be within a scanner (not shown) containing 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 focus109. In some embodiments, there may be multiple drive lasers101, with beams that all converge on focusing optics104. One type of LPP EUV light source may use a CO2laser and a zinc selenide (ZnSe) lens with an anti-reflective coating and a clear aperture of about 6 to 8 inches.

Drive laser101is fired in a pulsating manner in order to hit discrete droplets107separately. Although every sequence of pulses comprises a burst, drive laser101can be fired in different burst modes. In a stroboscopic mode (i.e., a mode with short bursts), the length of the bursts are limited to 1 ms, whereas in a continuous mode (i.e., a mode with long bursts), the expected burst length is 3-4 seconds for each die.

When firing drive laser101in stroboscopic mode, EUV system100maintains droplets107on-target reasonably well using closed-loop (droplet-to-droplet) feedback. To achieve higher EUV light power output, however, drive laser101is more often run in the continuous mode (long bursts) in which plasma is created during longer intervals of time. During these longer continuous bursts of firing, droplets107interact with the plasma and, as a result, are pushed out away from primary focal spot105. This displacement, or “push-out”, negatively impacts EUV light production because the plasma generated is no longer concentrated within the primary focal spot so maximal EUV light cannot be collected by elliptical collector108and refocused for downstream use.

A closed-loop (droplet-to-droplet) feedback control system (“droplet-to-droplet feedback system”) has been used historically to keep droplets107targeted on primary focal spot105during pulse firing of drive laser101. The droplet-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 droplet107is about to be lased. EUV system100uses the measured droplet position to command actuators (e.g., piezoelectric (“PZT”) actuators) to re-align droplet generator106so that successive droplets (pushed out of primary focal spot105by plasma generated from preceding droplets107) are re-aligned to be delivered to primary focal spot105. One disadvantage of this droplet-to-droplet feedback system is that it operates without taking into consideration repeated errors in position across bursts (i.e., inter-burst target errors), which limits the actuators that can be used to re-align droplets on-target. Specifically, because coarse movement actuators (e.g., stepper motors) introduce into the EUV system vibration which will push droplets107further off-target, coarse movement actuators are non-preferred for re-aligning droplets107on-target. Using only fine movement actuators (e.g., PZT actuators) avoids the introduction of vibration, but limits how rapidly droplets can be re-aligned to target as well as the range over which droplets can be repositioned. Consequently, the closed-loop (droplet-to-droplet) feedback system takes longer to correct a push-out disturbance (e.g., approximately 0.4 ms) than is desirable.

A magnified schematic of EUV system components involved in optimization of EUV light production according to one embodiment is shown inFIG. 2. Laser beam102is delivered through elliptical collector108to primary focal spot105. Positioning of primary focal spot105along the y- and z-axes is determined by focusing optics104, to wit, a final focus lens (not shown) and a final focus steering mirror (not shown), as described in U.S. patent application Ser. No. 13/549,261 (Frihauf et al.), hereby incorporated by reference in its entirety herein. Energy output from the LPP EUV system varies based on how well laser beam102can be focused and can maintain focus over time on droplets107generated by droplet generator106. Optimal energy is output from EUV system100if the droplets are positioned in primary focal spot105when hit by laser beam102. Such positioning of the droplets allows elliptical collector108to collect a maximum amount of EUV light from the generated plasma for delivery to, e.g., a lithography system. A sensor201(e.g. narrow field (NF) camera) senses the droplets as they pass through a laser curtain during travel to primary focal spot105and provides droplet-to-droplet feedback to EUV system100, which droplet-to-droplet feedback is used to adjust droplet generator106to re-align droplets107to primary focal spot105(i.e., “on-target”).

How droplet position along the z-axis changes during laser firing in a continuous burst mode will now be described with reference toFIGS. 3a,3b,3c,3d, and4.FIGS. 3a,3b,3c, and3dillustrate schematically the orientation of droplets107, respectively, before, at initiation of, during, and after laser burst firing in a continuous burst mode.FIG. 4is a graph depicting droplet position along the z-axis over time with primary focal spot105indicated by a solid line at a z-axis position of 0. Droplets107lased while at primary focal spot105generate plasma within the focal spot of elliptical collector108. Arrows401a,401b,401c, and401dindicate points in time thatFIGS. 3a,3b,3c, and3d, respectively, occur.

Referring first toFIG. 3a, before drive laser101is fired, droplets107ejected from droplet generator106to droplet catcher301are oriented in a straight line along the x-axis of primary focal spot105. At the point in time shown by arrow401ainFIG. 4, droplets107pass through primary focal spot105.

Referring now toFIG. 3b, as the pulse of laser beam102hits a first droplet107at primary focal spot105, the target material of first droplet107is vaporized and a plasma302is generated at primary focal spot105. EUV light emitted from plasma302is collected by elliptical collector108and reflected onto intermediate focus109where it passes into or is used by, e.g., a lithography system. At the point in time shown by arrow401binFIG. 4, droplet107is at primary focal spot105.

Referring now toFIG. 3c, as plasma302is formed by irradiation of droplet107, surrounding droplets107are displaced (“pushed out”) from primary focal spot105of elliptical collector108. Thus, as laser beam102strikes a first droplet107, succeeding droplet107is pushed out along the z-axis from the z-axis coordinate of primary focal spot105. At the point in time shown by arrow401cinFIG. 4, droplet107is displaced (approximately 75 μm in this example) from primary focal spot105. Because of this push-out, plasma302is no longer produced at primary focal spot105and thus, produced EUV light is not focused by elliptical collector108at intermediate focus109(i.e., the produced EUV light is out of focus). If the push-out phenomenon is uncontrolled, plasma302generated from irradiation of additional successive droplets107can trigger additive z-axis push-out of successive droplets107. Consequently, EUV production is significantly disturbed—enough to ruin a die on a wafer during exposure—especially when drive laser101is fired in continuous mode. Although firing drive laser101at a fixed operation point generates push-outs of similar sizes, the push-outs drift as drive laser101is fired for longer time periods.

Referring now toFIG. 3d, droplet push-out has been controlled historically through a droplet-to-droplet feedback system which adjusts droplet generator106after push-out has occurred to start moving succeeding droplets107back to primary focal spot105. In essence, the droplet-to-droplet feedback system waits for the push-out, and then fights the disturbance by adjusting droplet generator106with actuators (e.g., PZT actuators) to re-align successive droplets107to target (i.e., primary focal spot105). Because the droplet-to-droplet feedback process does not begin until after push-out has occurred, however, error between an actual position of droplet107and primary focal spot105can be very large. Ideally, 10 ms after burst firing begins, droplets107should be (and should stay) within ±5 μm of primary focal spot105. As a result, as shown inFIG. 4, the droplet-to-droplet feedback process requires substantial time (e.g., approximately 0.4 seconds) after the start of a burst to reposition droplets107back on-target. Once droplet-to-droplet feedback processes have re-aligned droplets107to primary focal spot105(as shown by arrow4d), droplet-to-droplet feedback control signals can hold droplets107near primary focal spot105for the duration of the burst.

In contrast to current droplet-to-droplet feedback systems, 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 system100to rely on PZT actuators to reposition droplets107and to avoid having to use stepper motors (which introduce vibration into the EUV system and thereby interfere with successful repositioning of droplet107on-target).

Referring now toFIG. 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 position502which is displaced away from primary focal spot105such that laser beam102strikes droplet107at target position502. As laser beam102strikes droplet107at target position502, droplet107is pushed-out, but the push-out phenomenon pushes droplet107into (rather than out of) primary focal spot105as plasma302is generated. Thus plasma302is generated at primary focal spot105—that is, in the focal spot of elliptical collector108—and produced EUV light is collected and focused by elliptical collector108at intermediate focus109. One of skill in the art will recognize that, for a first firing burst in a continuous mode of operation, target position502can, but need not coincide with primary focal spot105.

A block diagram providing an overview of a pre-compensation control loop used to adaptively adjust droplet target position according to one embodiment is presented inFIG. 6. Droplet generator106ejects droplets107along an x-axis to a target (x-, y-, z-) position at which droplets107are to be lased as discussed with respect toFIG. 1. One or more sensor201inside LPP EUV source chamber110senses one or more axial position (e.g., along a y-axis, along a z-axis, or along both axes) of ejected droplet107. The sensed axial position of droplet107along the y-axis is passed to y-axis controller603yand/or the sensed position of droplet107along the z-axis is passed to z-axis controller603z. Axial controllers603yand/or603zdetermine(s) an updated target position for droplet107that pre-compensates for anticipated droplet push-out (as discussed in greater detail elsewhere herein). Axial controllers603yand/or603zthen output(s) commands to, respectively, y-axis actuators604yand/or z-axis actuators604z(e.g., stepper motors and/or PZTs) to adjust droplet generator106such that succeeding droplets107are delivered to the pre-compensated axial target position502.

In another embodiment, a pre-compensation control loop is used to adaptively adjust droplet position. In this embodiment, axial controllers603yand/or603zdetermine(s) an updated droplet position for droplet107that pre-compensates for anticipated droplet push-out (as discussed in greater detail elsewhere herein). Axial controllers603yand/or603zthen output(s) commands to, respectively, y-axis actuators604yand/or z-axis actuators604z(e.g., stepper motors and/or PZTs) to adjust droplet generator106such that succeeding droplets107are delivered to the pre-compensated axial droplet position.

Referring now toFIG. 7, a flowchart of one embodiment of a method for adaptively adjusting droplet target position to pre-compensate for droplet push-out is presented. During closed-loop laser operation (i.e., during a burst), the magnitude of the push-out disturbance cannot be measured directly. Instead, to effectively pre-compensate for droplet push-out, EUV system100measures the position of a droplet just before the droplet is lased and estimates the magnitude of the push-out. Thus, in step701, as droplet107passes through a laser curtain on its way to target position502, sensor201(e.g., an NF camera) senses droplet107and sends data about droplet107to axial controllers603yand/or603z. 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 sensor201determines sampling frequency (e.g., reducing the field of view of sensor201allows for an increased frame rate) of droplets.

In step702, a position of the sensed droplet along the y- and/or z-axis is calculated. To do this, sensor201measures an axial position of droplet107(in pixels) and determines a vertical centroid and a horizontal centroid for droplet107. Axial controllers then perform coordinate transformations on the vertical and horizontal centroids. Thus y-axis controller603yconverts the pixels of the vertical centroid to calculate a y-axis position (e.g., in μm), and z-axis controller603zconverts the pixels of the horizontal centroid to calculate a z-axis position (e.g. in μm) of droplet107. This transformation minimizes any effect of sensor tilt on the measured (y,z) position of droplet107.

In step703, the axial controllers estimate axial (z- or y-) open-loop positions of sensed droplet107. Conceptually, the axial open-loop positions are the z- and/or y-position(s) of droplet107if no droplet-to-droplet feedback had been applied to reposition droplet107—that is, the droplet push-out along the (z- and/or y-axes) without droplet-to-droplet feedback control to re-align droplets107to primary focal spot105. 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-position(s). Thus, y-axis controller603ysubtracts the droplet-to-droplet feedback adjustment applied to compensate for y-axis push-out of droplet107from the y-axis position determined in step702to calculate an estimated open-loop y-axis position of sensed droplet107. Similarly, the z-axis controller603zsubtracts the droplet-to-droplet feedback adjustment applied to compensate for z-axis push-out of droplet107from the z-axis position determined in step702to calculate an estimated open-loop z-axis position of sensed droplet107.

In step704, the axis controllers determine whether the burst has ended. If the burst has not ended, the process returns to step701and steps701,702,703, and704are performed for another droplet107. That is, steps701,702,703, and714are iterated for the droplets lased during a burst.

If the burst has ended, then in step705, the axis controllers calculate a pre-compensation correction to be applied to determine a new target position. Mathematically, the pre-compensation correction is
K*(OLavg−Tb)
where K is a learning gain, OLavgis an average of the estimated open-loop axial position data calculated in steps703for the previous burst, and Tbis the axial position of primary focal spot105. The pre-compensation correction should be in the opposite direction to the calculated OLavg. The number of data points used to calculate the OLavgdepends on the frame/second speed of sensor201when sensing droplet107and on the length of time over which the burst occurred. In one embodiment, because sensor speed is not always consistent and sensor201may 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.

Thus, y-axis controller603ycalculates a pre-compensation correction by determining the deviation of the averaged estimated open-loop y-axis position of sensed droplet107in the previous burst from the y-axis position of the primary focal spot, and multiplying that y-axis deviation by a learning gain. Similarly, z-axis controller603zcalculates a pre-compensation correction by determining the deviation of the averaged estimated open-loop z-axis position of sensed droplet107in 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-compensation 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=1), 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 position502from primary focal spot105. If, however, the learning gain remains set to 1.0, the updated target position continues to rely almost completely on the position of the droplets during the immediately preceding burst—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 position502will be displaced a considerable distance away from primary focal spot105. If the push-out for a second (succeeding) burst is small, however, the droplets will have been displaced too far away from primary focal spot105so will not be in a correct target position to be pushed into focal spot105and less plasma will be produced. Once again, however, the system will over-rely on this previous position when calculating a new target position after the second burst and, consequently, determine that the target position should be adjusted only by a small amount. This process will keep the target position bouncing back and forth between extreme target positions (e.g., small displacement from primary focal spot105→large displacement from primary focal spot105→small displacement from primary focal spot105, etc.) leading to poor EUV output and instability.

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. With the lower learning gain, the adaptive pre-compensation loop “learns” over time to achieve a stable target position502. If, however, a very small learning gain is chosen (e.g., K=0.1), target position502will 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 spot105.

Ideally, then, a learning gain that changes over time is preferred. For example, 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.

In step706, the axis controllers update target position502by adding the pre-compensation corrections calculated in step705to target position502of the previous burst to obtain an updated pre-compensated target. Mathematically,
Ti+1=Ti−K*(OLavg−Tb)
where Tiis the pre-compensated target of the previous burst and Ti+1is the updated pre-compensated target. Thus, y-axis controller603ycalculates an updated pre-compensated target along the y-axis and z-axis controller603zcalculates an updated pre-compensation target along the z-axis. Importantly, this updated pre-compensation target is not primary focal spot105, but is a learned displacement that allows the push-out phenomenon to push the droplets back into primary focal spot105when a next burst begins. One of skill in the art will recognize that other learning algorithms (e.g., Least Mean Squared or Recursive Least Squares equations) can be used to update the pre-compensated target.

Pre-compensation based upon the previous burst alone may be susceptible to measurement noise. Thus, in one embodiment, data obtained from some or all previous bursts are used to calculate the pre-compensation correction before using that pre-compensation correction to update the pre-compensated target.

In step707, the axis controllers command axial actuators to reposition droplet generator106so that droplets107ejected from droplet generator106are in the updated pre-compensated target position when lased. Thus, y-axis controller603ysends a command to y-axis actuators controlling movement of droplet generator along the y-axis and/or z-axis controller603zsends a command to a z-axis actuators controlling movement of droplet generator along the z-axis such that droplets107ejected from droplet generator106are lased as they pass through the updated (that is, pre-compensated) (y,z) target. When lased, droplets107are pushed out of target position502and into primary focal spot105where light from generated plasma302is collected by elliptical mirror108and focused onto intermediate focus109. Thus, the ramp-up of the push-out itself, rather than active steering, moves droplet107to primary focal spot105, and can therefore limit how rapidly droplets can be moved on target.

In one embodiment, calculation of the pre-compensation correction (step705), updating of the target position502(step706), and commanding the axial actuators (step707) occur during the inter-burst interval.

In one embodiment, some accommodations are implemented during bursts to mitigate undesired effects on, the pre-compensated 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 beam102will push droplets into primary focal spot105. So, during these ramping periods, the droplet-to-droplet feedback system is made inoperative. One way to achieve this inoperability is to set error in droplet positioning to zero so that no droplet-to-droplet feedback action is initiated during the ramping periods.

Making droplet-to-droplet feedback inoperable during ramping can impact functionality of the open-loop droplet position estimate on which the target adaptation is based. Typically, the learning gain is set to 0.01, which means that the open-loop droplet position estimate relies almost exclusively on the applied droplet-to-droplet feedback to estimate the actual position of the droplet. The result of this reliance is that when the droplet-to-droplet feedback system remains inoperable and the droplet position changes quickly due to the push-out, the open-loop droplet position estimate is far from the actual droplet position. Therefore, the learning gain is set to 1 during the inoperability period, thereby making the open-loop droplet position estimate fully dependent on the position measurement.

The process ofFIG. 6is depicted graphically with exemplar data inFIG. 8which illustrates droplet position over time during a laser-firing burst with and without closed-loop (droplet-to-droplet) feedback. As seen in the figure, droplets targeted to primary focal spot105undergo a push-out801along the z-axis away from primary focal spot105during the burst, and are eventually re-aligned802to primary focal spot105(through droplet-to-droplet feedback control as discussed elsewhere herein). An open-loop estimate803is determined for droplets107. Once the burst has ended, an average of the open-loop estimates (OLavg) is calculated and then multiplied by a learning gain (K) to obtain a pre-compensation correction. The pre-compensated correction is then added to the previous target position (primary focal spot105in this example) to obtain an updated target position502). Re-alignment of droplet generator106to deliver droplets107to target position502pre-compensates for potential z-axis displacement of droplets107by positioning the droplets to be pushed into primary focal spot105when lased (so as to thereby optimize generated plasma).

As the method ofFIG. 7is repeated for subsequent bursts, the axis controllers slowly learn how much droplet-to-droplet feedback, on average, is needed to re-align droplets107to be on-target. Briefly, after target position502is 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 position502is adjusted to pre-compensate for the 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-compensation is illustrated with simulated data inFIGS. 9 and 10.

FIG. 9illustrates convergence of target pre-compensation over time for simulated data of a block wave push out with noise. The simulated data depict droplet position along the z-axis over time as drive laser101is fired in a continuous burst mode with a learning gain equal to 0.5. Bursts are indicated by numbered columns. During burst 1 (when target position502is set to primary focal spot105), a push-out901of approximately 25 μm away from focal spot105is observed. The push-out is corrected by droplet-to-droplet feedback control which re-aligns the droplet position (albeit later during the burst) back to primary focal spot105. During the inter-burst interval following burst 1, an updated target position502(at approximately −12 μm along the z-axis from primary focal spot105) is determined (by z-axis controller603z). When droplet107is lased during burst 2, droplet107is pushed-out (approximately 25 μm along the z-axis) from updated target position502to a position that is only about 10 μm along the z-axis from primary focal spot105. Because the push-out is smaller than in burst 1, droplet-to-droplet feedback control re-aligns the droplet position back to primary focal spot105more rapidly than in the preceding burst903. As this process iterates, the push-outs become increasingly smaller in magnitude and are more rapidly adjusted during the succeeding bursts with droplet-to-droplet feedback so that droplets remain on-target. In this example, EUV system100has learned by burst 4 to pre-compensate target position502to a sufficient degree that droplets are pushed (with minimal droplet-to-droplet feedback) to within a reasonable distance904of primary focal spot105to generate plasma. Error in droplet position along the z-axis over time for the simulated data ofFIG. 9is presented inFIG. 10. As shown, EUV system100has reduced the droplet position error1004by burst 4 to within 5 μm of primary focal spot105. As discussed above, plasma generated within 5 μm of primary focal spot105optimizes EUV light production.

In another simplified embodiment, neither inter-burst target pre-compensation nor droplet-to-droplet feedback control occurs between bursts. One way to achieve this inoperability is to (1) set error in droplet position to zero (i.e., turn off droplet-to-droplet feedback) during the entire inter-burst interval when the drive laser is not firing so that no droplet-to-droplet feedback action is initiated during inter-burst intervals and (2) turn off inter-burst target pre-compensation. In this embodiment, droplets are targeted towards the primary focal spot and coordination of droplet position with the primary focal spot is adjusted by the droplet-to-droplet feedback within a burst. 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. Thus, droplets in a next burst can be greatly displaced from a desired position—and droplet-to-droplet feedback may take a long time or even be unable to reposition droplets to the desired position. Nevertheless, if there are no disturbances within the laser system, 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.

In another simplified embodiment, an inter-burst deadband is used instead of inter-burst target pre-compensation or droplet-to-droplet feedback control. The deadband is chosen as the region in which the laser beam hits the droplets, and therefore allows droplets to be pushed-out 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 spot, droplets are steered to the target with droplet-to-droplet feedback, whereas when the droplets are within a close range (e.g., 20 μm) 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-droplet feedback control. This embodiment is independent of the amplitude and direction of the push-out, and typically does not need to be calibrated.

In still another embodiment, an inverse control signal can be used in a feed-forward 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 step702with reference toFIG. 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.

The disclosed method and apparatus have been explained above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in light of this disclosure. Certain aspects of the described method and apparatus may readily be implemented using configurations other than those described in the embodiments above, or in conjunction with elements other than those described above. For example, different algorithms and/or logic circuits, perhaps more complex than those described herein, may be used, as well as possibly different types of drive lasers and/or focus lenses. As another example, embodiments of the disclosed system and method have been described with reference to a laser firing in a continuous mode, although embodiments of the system and method herein can also be implemented in a laser firing in a stroboscopic mode.

Further, it should also be appreciated that the described method and apparatus can be 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. It should be noted that the order of the steps of the methods described herein may be altered and still be within the scope of the disclosure.

It is to be understood that the examples given are for illustrative purposes only and may be extended to other implementations and embodiments with different conventions and techniques. While a number of embodiments are described, there is no intent to limit the disclosure to the embodiment(s) disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents apparent to those familiar with the art.