CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/405,533 which was filed on 12 September 2022 and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to systems for the production of extreme ultraviolet (EUV) radiation. Such systems typically generate EUV radiation in pulses and require that the energy of the pulses be controlled.

BACKGROUND

[0003] EUV radiation, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays) and including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.

[0004] Some methods for generating EUV radiation include converting a target material (also called a source material) from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium, or tin, with one or more emission lines in the EUV range. In one such method, laser produced plasma (“LPP”) is generated by using individual drive laser beam pulses to irradiate respective masses of a target material having the required line-emitting element.

[0005] Thus, one LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with laser radiation pulses. In more theoretical terms, LPP sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10’ s of e Vs.

[0006] According to one example, an LPP EUV lithography light source generates the required 13.5 nm radiation by focusing a 10.6 pm wavelength CO2 laser beam onto tin droplet targets creating highly ionized plasmas. EUV photons are radiated isotropically by these ions. Photons are collected with a temperature-controlled graded multilayer coated normal-incidence mirror (collector) and focused to an intermediate point from where they are relayed to the scanner optics and ultimately to the wafer. The collector is protected from the plasma by debris mitigation technology based on flows of a hydrogen buffer gas. High-energy ions, fast neutrals, and residual source element particles are mitigated to maintain the reflectivity of the collector mirror and enable a long lifetime for this component.

[0007] There are systems which can apply multiple pulses to a droplet, e.g., a main converting pulse preceded by one or more conditioning pulses that alter the geometry and/or the distribution of the target material in the droplet. Thus, after application of the conditioning pulses, the droplet may assume a different form such as a disk or mist of target material. The term “droplet” is accordingly used herein broadly to refer to any shape or distribution of the target material just prior to conversion.

[0008] The amount of energy delivered to a wafer by the EUV radiation beam per unit area over an exposure time (or a particular number of pulses of the EUV radiation beam) is referred to as the dose or the exposure energy (for example, in units of Joules). The formation of the micro-electronic features on the wafer depends on the proper dose (a “target dose”) reaching the wafer. If too little energy reaches the wafer over the exposure time (the dose is too low and is less than the target dose), the radiationsensitive material of the wafer is not activated and the micro-electronic features are not formed or are incompletely formed on the wafer. If too much energy reaches the wafer over the exposure time (the dose is too high and is greater than the target dose), the radiation-sensitive material of the wafer can be exposed outside of the bounds of the image of the slit pattern used to expose the wafer and the microelectronic features are improperly formed on the wafer. The dose error can be caused by variations in the amount of energy in the EUV radiation beam, and these variations can arise from, for example, noise in the radiation source and/or disturbances that are internal or external to the source.

[0009] In other words, one technical challenge in using pulses of laser light to generate EUV radiation is the need to control the amount, or “dose,” of EUV light energy being applied to a particular item being treated, such as a semiconductor wafer. For example, a specified amount of EUV radiation energy may be required to accomplish some task, such as curing a layer of photoresist, on a semiconductor wafer as part of the manufacturing process. In order to obtain consistent results across different wafers, it will be desirable to apply the same amount of EUV radiation energy to each wafer, to as great a degree of accuracy as practicable. Systems for controlling the amount of EUV radiation energy that is applied are disclosed in, for example, U.S. Patent No. 9,693,440, issued June 27, 2017, and titled “EUV LPP Source with Improved Dose Control by Tracking Dose over Specified Window.”

[0010] All patent applications, patents, and printed publications cited herein are incorporated herein by reference in their entireties, except for any definitions, subject matter disclaimers or disavowals, and except to the extent that the incorporated material is inconsistent with the express disclosure herein, in which case the language in this disclosure controls.

[0011] Meeting this challenge is complicated by the fact that the power in each drive laser pulse may vary. Since the amount of EUV energy released when a laser pulse hits a droplet varies with the power in the laser pulse, the EUV light energy generated by conversion of any given droplet may also vary.

[0012] One way to effect dose control is to select a desired dose target for each packet or grouping of EUV pulses. A burst may be such a grouping of EUV pulses. The EUV energy that is generated by each laser pulse hitting a corresponding droplet is measured. A total accumulated dose for each grouping of pulses is then calculated by adding the EUV energy from each droplet over the series of droplets, starting with the first droplet in the packet. Once the dose target for the packet is achieved, the rest of the pulses in that packet are “skipped” or “missed,” i.e., the drive laser pulses are generated but are not permitted to hit any droplets. Skipping a droplet is typically accomplished either by firing the laser at a location other than the irradiation region at which the droplet is located, or by firing the laser at a time when a droplet will not be at the irradiation site when the laser pulse arrives there.

[0013] Another way to effect dose control that does not involve skipping droplets is to control the pulse energy of each laser pulse, adjusting either the duration of the pulse or the magnitude of the pulse from the master oscillator of the laser. This is accomplished by adjusting the RF excitation energy of the RF excitation signal of the laser. Control of the energy of individual EUV pulses via control of the energy of the drive laser pulses can be accomplished in a number of ways including changing the amplitude and/or pulse width of the RF excitation signal.

[0014] Both methods have potential advantages and shortcomings using conventional techniques. For example, actuation of the CO2 drive laser power via control of the duty cycle of a Pulse Width Modulated (PWM) RF excitation signal is characterized by a slow response time that is of the order of 0.2 milliseconds. This affects the maximum attainable bandwidth of a control loop employing such a control method. In contrast, a method implementing on-off droplet firing is characterized by a fast response as the command to fire on-droplet or off-droplet can produce desired results essentially instantaneously. However, reliance on hit/miss droplet actuation is coupled with the introduction of additional periodic disturbances into the control loop that impact the stability of the on-droplet EUV generation, and so affects dose as the result. The hit/miss firing pattern adds an additional error component to the delivered dose relative to systems in which all shots are fired on droplets.

[0015] Being able to mitigate and limit the negative effects of hit/miss control would thus improve energy control. It is in this context that the need for the present invention arises.

SUMMARY

[0016] The following presents a succinct summary of one or more embodiments in order to provide a basic understanding of those and other possible embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor set limits on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a streamlined form as a prelude to the more detailed description that is presented later.

[0017] According to one aspect of an embodiment there is disclosed a system in which at least two control loops cooperate to improve energy control. One of the control loops controls the energy of the RF excitation signal applied to the laser. The other control loop affects the hit/miss firing patterns. Combined action of these two control loops provides for energy control that is responsive and robust.

[0018] According to another aspect of an embodiment there is disclosed a system for modulating power output of a source of a plurality of extreme ultraviolet (EUV) pulses of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material and a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, the system comprising an EUV pulse energy control loop adapted to maintain an energy of individual ones of the plurality of EUV pulses within a defined range, and a firing pattern control loop adapted to determine which individual ones of the droplets are irradiated by converting pulses.

[0019] The energy of a converting pulse may depend at least in part on an electrical characteristic of an RF excitation signal applied to the laser, and the EUV pulse energy control loop may comprise a module for controlling the electrical characteristic of the RF excitation signal. The electrical characteristic of the RF excitation signal may comprise an amplitude of the RF excitation signal. The electrical characteristic of the RF excitation signal may comprise a pulse width of the RF excitation signal.

[0020] The firing pattern control loop may comprise a trigger signal module for controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser. The firing pattern control loop may comprise a steering module for steering a conversion pulse emitted by the laser, the steering module causing the conversion pulse to strike a droplet in the irradiation region when the firing pattern control loop commands a hit and the steering module causing the conversion pulse to miss a droplet in the irradiation region when the firing pattern control loop commands a miss.

[0021] The system may further comprise a plasma sensor arranged to sense a degree of instability in a plasma generated by conversion of one or more droplets wherein the firing pattern control loop is responsive to the plasma sensor to avoid commanding firing patterns that increase a degree of instability in the plasma in excess of (i.e., past) a predetermined threshold.

[0022] The EUV pulse energy control loop may be adapted to be responsive to the firing pattern control loop in such a way that the EUV pulse energy control loop reduces the energy in the RF excitation signal applied to the laser for a next shot if the firing pattern control loop commands the next shot to be a miss. The EUV pulse energy control loop may receive a commanded target value for EUV average energy and use the commanded target value to set an actual target value for EUV average energy greater than the commanded target value. The EUV pulse energy control loop may maintain a ratio of the commanded target value to the actual target value at a fixed value greater than one.

[0023] According to another aspect of an embodiment there is disclosed a system for controlling an average power output of a source of a plurality of pulses of extreme ultraviolet (EUV) of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material, a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, and an EUV energy measurement sensor for measuring a per-droplet output energy generated by irradiation of individual droplets and producing a per-droplet output energy measurement signal, the system comprising a first control subsystem responsive to the per-droplet output power measurement signal and the adapted to maintain an energy of individual ones of the EUV pulses within a defined range by controlling at least one system variable impacting the power of the laser and a second control subsystem responsive to the per-droplet output power measurement signal and adapted to determine which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region by controlling timing of the converting pulses.

[0024] The at least one system variable impacting the power of the laser may comprise an amplitude of an RF excitation signal. The at least one system variable impacting the power of the laser may comprise a pulse width of an RF excitation signal.

[0025] The second control subsystem may comprise a trigger signal module for controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser.

[0026] The second control subsystem may comprise a steering module for steering a conversion pulse emitted by the laser, the steering module causing the conversion pulse to strike a droplet in the irradiation region when the firing pattern control loop commands a hit and the steering module causing the conversion pulse to miss a droplet in the irradiation region when the firing pattern control loop commands a miss.

[0027] The system may further comprise a plasma sensor arranged to sense a degree of instability in a plasma generated by conversion of one or more droplets wherein second control subsystem is responsive to the plasma sensor to avoid commanding firing patterns that increase a degree of instability in the plasma past a predetermined threshold.

[0028] The first control subsystem may be adapted to be responsive to the second control subsystem in such a way that the first control subsystem reduces the energy in the RF excitation signal applied to the laser for a next shot if the second control subsystem commands the next shot to be a miss.

[0029] The first control subsystem may receive a commanded target value for EUV average energy and use the commanded target value to set an actual target value for EUV average energy greater than the commanded target value. The first control subsystem may maintain a ratio of the commanded target value to the actual target value at a fixed value greater than one.

[0030] According to another aspect of an embodiment there is disclosed a method of controlling an average power output of a source of a plurality of extreme ultraviolet (EUV) pulses of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material and a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, the method comprising using a first control subsystem to maintain an energy of individual ones of the EUV pulses within a defined range and using a second control subsystem determine which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region.

[0031] According to another aspect of an embodiment there is disclosed a method of controlling an average power output of a source of a plurality of pulses of extreme ultraviolet (EUV) of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material, a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, and an on-droplet EUV energy measurement sensor for measuring a per-droplet output power generated by irradiation of individual droplets and producing a per-droplet output power measurement signal, a laser output power of the laser being determined at least in part by a radio frequency (RF) excitation signal applied to the laser, the method comprising maintaining an energy of individual ones of the EUV pulses within a defined range by controlling at least one electrical characteristic of the RF excitation signal and determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region.

[0032] The electrical characteristic of the RF excitation signal may comprise an amplitude of the RF excitation signal. The electrical characteristic of the RF excitation signal may comprise a pulse width of the RF excitation signal.

[0033] Determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region may comprise controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser. Determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region may comprise commanding the steering module to cause the conversion pulse to strike a droplet in the irradiation region to effect a hit and causing the conversion pulse to miss a droplet in the irradiation region to effect a miss.

[0034] The method may further comprise sensing a degree of instability in a plasma generated by conversion of one or more droplets determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region may be based at least in part on the degree of instability to avoid firing patterns that increase the degree of instability in the plasma past a predetermined threshold.

[0035] Maintaining an energy of individual ones of the EUV pulses within a defined range may comprise reducing the energy in the RF excitation signal applied to the laser for a next shot if the next shot is a miss. Maintaining an energy of individual ones of the EUV pulses within a defined range may comprise setting an actual target value for EUV average energy greater than a commanded target value for EUV average energy. Setting an actual target value for EUV average energy greater than a commanded target value for EUV average energy may comprise setting a ratio of the commanded target value to the actual target value at a fixed value greater than one.

[0036] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the embodiments and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the relevant art(s) to make and use the embodiments. [0038] FIG. 1 is a not-to-scale partially schematic functional block diagram of an overall broad conception for an EUV lithography system.

[0039] FIG. 2 is a timing diagram illustrating certain principles of hit/miss firing patterns.

[0040] FIG. 3 is a not-to-scale partially schematic functional block diagram of an overall broad conception of an energy control system for an EUV lithography system according to an aspect of an embodiment.

[0041] FIG. 4 is a not-to-scale partially schematic functional block diagram of an overall broad conception of an energy control system for an EUV lithography system according to an aspect of an embodiment.

[0042] FIG. 5 is a flow chart of a method of modulating EUV output energy in an EUV lithography system according to an aspect of an embodiment.

[0043] Further features and advantages of the disclosed subject matter, as well as the structure and operation of various embodiments of the disclosed subject matter, are described in detail below with reference to the accompanying drawings. The embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings presented herein.

DETAILED DESCRIPTION

[0044] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments.

[0045] Before describing such embodiments in more detail, however, it is instructive to describe an example environment in which embodiments of the present invention may be implemented. With initial reference to FIG. 1 there is shown a schematic diagram of an exemplary EUV radiation source, e.g., a laser produced plasma EUV radiation source 100. As shown, the EUV radiation source 100 may include a pulsed laser source 122, which in this example is a pulsed gas discharge CO2 laser source producing a beam 112 of radiation at 10.6 pm or 1 pm. The pulsed gas discharge CO2 laser source 122 may have DC or RF excitation operating at high power and at a high pulse repetition rate. In this example, the pulsed gas discharge CO2 laser source 122 has RF excitation applied by an RF excitation source 123 under the control of an EUV light source controller system 160 as described more fully below.

[0046] The EUV radiation source 100 also includes a target delivery system 124 for delivering target material. In this example, the target material is liquid but it could also be a solid or a gas. The liquid is in the form of droplets but it could be a continuous liquid stream. Tin is used as a nonlimiting example of a target material in the description which follows with the understanding that other materials could be used instead of, or in addition to, tin.

[0047] In the system depicted the target material delivery system 124 introduces droplets 114 of the target material into the interior of a vacuum chamber 126 having a chamber wall 127. The vacuum chamber 126 includes an irradiation region 128 where the target material may be irradiated to produce plasma. It should be noted that as used herein an irradiation region is a region where target material irradiation may or is intended to occur and is an irradiation region even at times when no irradiation is actually occurring. The EUV light source may also include a beam steering system 132 which also operates under the control of the EUV light source controller system 160 as described more fully below. [0048] In the system shown, the components are arranged so that the droplets 114 travel substantially horizontally. The direction from the laser source 122 towards the irradiation region 128, that is, the nominal direction of propagation of the beam 112, may be taken as the Z axis. The path the droplets 114 take from the target material delivery system 124 to the irradiation region 128 may be taken as the X axis. The view of FIG. 1 is thus normal to the XZ plane. The orientation of the EUV radiation source 100 may be rotated with respect to gravity. While a system in which the droplets 114 travel substantially horizontally is depicted, it will be understood by one having ordinary skill in the art that other arrangements can be used in which the droplets travel vertically or at some angle with respect to gravity between and including 90° (horizontal) and 0° (vertical).

[0049] The EUV radiation source 100 may also include the EUV light source controller system 160 and a laser firing control system 165 which operates under the control of the EUV light source controller system 160. The EUV radiation source 100 may also include a detector such as a target position detection system 170 that generates an output indicative of the absolute or relative position of a target droplet, e.g., relative to the irradiation region 128, and provides this output to a target position detection feedback system 162.

[0050] As shown in FIG. 1, the target material delivery system 124 may include a target delivery control system 190. The target delivery control system 190 adjusts the path of the target droplets 114 through the irradiation region 128. This adjustment may be accomplished, for example, by repositioning the point at which a droplet generator 192 releases the target droplets 114. The droplet release point may be repositioned, for example, by tilting the droplet generator 192 or by laterally translating the droplet generator 192. The droplet generator 192 extends into the chamber 126 and is preferably externally supplied with target material. A gas source (not shown) places the target material in droplet generator 192 under pressure. Droplets 114 that pass through the irradiation region 128 without being transformed continue to a target material receptacle 134.

[0051] Continuing with FIG. 1, the radiation source 100 may also include one or more optical elements. In the following description, a collector 130 is used as an example of such an optical element. The collector 130 may be a normal incidence reflector, for example, implemented as a multilayer mirror. The collector 130 may be in the form of a prolate ellipsoid, with a central aperture 135 to allow the laser radiation 112 to pass through and reach the irradiation region 128. The collector 130 has a first focus at the irradiation region 128 and a second focus at a so-called intermediate point 140 (also called the intermediate focus 140) where the EUV radiation may be output from the EUV radiation source 100 and input to, e.g., an integrated circuit lithography scanner 150.

[0052] The integrated circuit lithography scanner 150 includes a projection system (e.g. a refractive or reflective projection lens system) 156, also referred to as a projection optics box or POB, configured to project a pattern imparted to the radiation beam by patterning device 154 (e.g., a mask) onto a target portion (e.g. comprising one or more dies) of a substrate 152. The projection system 150 includes a slit filter 157. In the example the slit 157 is rectangular and shapes the light beam into an elongated rectangular shaped light beam. This shaped light beam then passes through the mask 154 and propagates to the substrate 152 to provide a dose 153. After exposure the substrate 152 is then additionally processed in a known manner, ultimately to fabricate an integrated circuit device.

[0053] A detector 180 detects the EUV energy at or near the surface of the wafer 152 and supplies a signal indicative of the detected energy to the EUV light source controller system 160. As discussed in greater detail below, the signal may be a series of energy values measured by the detector 180, with each energy value representing an amount of EUV energy at the substrate 152 at a particular time (for example, the energy at the substrate 152 delivered by a particular pulse of light or a series of pulses of light). An additional example of an arrangement such as that just described is disclosed in U.S. Patent No. 9,939,732, issued April 10, 2018, and titled “Controller for an Optical System.”

[0054] To summarize some of the salient points, the detector 180 measures energy at or near the surface of the substrate 152. The EUV light source controller system 160 receives the results of the measurement and can control amount of power (e.g., instantaneous or average) at the substrate 152 through controlling the power of the pulse(s) of the laser 122 through managing the RF excitation signal applied to the laser 122 by RF excitation source 123. In addition, the EUV light source controller system 160 can control which droplets 114 are irradiated by a pulse from the laser 122 by changing the timing of a trigger signal generated by laser firing control system 165 or by controlling steering of the beam 112 by beam steering system 132.

[0055] As mentioned, controlling power at the wafer through management of the drive laser power by controlling the RF excitation signal by itself or by arranging the system so that a controlled number of pulses do not irradiate droplets by itself has disadvantages. According to an aspect of an embodiment, these disadvantages are avoided or at least reduced by adopting a mixed energy control system for LPP EUV sources that employs simultaneous use of a first energy control loop arranged to control the energy of individual EUV pulses to a desired level via available actuation means, e.g., control of the RF input (e.g., amplitude or pulse width or both) to the drive laser, and a second hit/miss control loop which ensures stability of the average EUV energy in a range around a defined power level via active control of the hit/miss firing pattern.

[0056] According to another aspect of an embodiment, the hit/miss droplet firing pattern may be adaptable and in particular controllable to avoid the use of detrimental hit/miss droplet firing patterns. One advantage of such a system having the capability of adapting the hit/miss droplet firing patterns is that the laser firing pattern may be adaptable to different slit filters having different cutoff frequencies and frequency responses. Such a system can reduce and even set eliminate periodic disturbances during plasma generation more than a system lacking an adaptable firing pattern.

[0057] FIG. 2 is a graph of a firing pattern over a time window Tf. Time in arbitrary units is plotted on the x-axis and EUV energy in arbitrary units is plotted on the y-axis. The graph thus displays EUV energy as a function of time or EUV(t). Each vertical line indicates an EUV pulse in which a laser pulse from the drive laser hits and converts a droplet of target material. Each X indicates an EUV pulse in which the laser pulse from the drive laser does not hit, i.e., misses, a droplet of target material. The firing pattern repeats over a time interval T which is less than Tf. The pattern starts at a time tk at which there is an EUV pulse or a hit. There are hits for the next (nl - 1) drive laser pulses after which there is a miss at nlth pulse at time tk+ni- Then another two time intervals T ensue with the same pattern. An additional miss occurs at time tk+Tf. It is such a firing pattern that can be adapted by adjusting it to avoid a firing pattern that may, for example, be prone to causing disturbances in plasma generation.

[0058] Also according to an aspect of an embodiment, the system described in connection with FIG. 2 can adjust a cumulative EUV energy over the time window Tf by missing instead of hitting droplets during, e.g., at the end of the time window Tf when the desired cumulative EUV energy for the window Tf is attained to projected to be attained.

[0059] FIG. 3 is a functional block diagram of an implementation of a mixed energy control system in accordance with aspects of an embodiment. In FIG. 3 a drive laser 122 is provided with excitation pulses from an RF excitation circuit 123. The energy characteristics of the pulses from the RF excitation circuit 123, such as amplitude and pulse width, determine in part the energy produced by the laser 122 in a main conversion pulse 133. The main conversion pulse 133 from the laser 122 passes through a beam steering module 132 and is supplied as a main conversion pulse 133 to irradiation region 128. The characteristics of the pulse from the RF excitation circuit 123 are determined by a EUV pulse energy control module 310 which may be part of the EUV light source controller system 160 (FIG. 1) on the basis of a metrology signal from EUV energy metrology 180. In accordance with an aspect of the embodiment, the generated EUV energy is determined on an on-droplet (droplet-by-droplet or per- droplet) basis. The elements just described together thus constitute elements of a first control loop 330 which controls on-droplet EUV energy. [0060] Also, the timing of pulses from the drive laser 122 is determined by a laser timing control circuit 165 which is in turn under the control of a hit/miss control module 320. The hit/miss control module 320 may also be part of the EUV light source controller system 160. The hit/miss control module 320 also can supply a control signal to the beam steering module 132. The hit/miss control module 320 determines the control signal on the basis of a signal from the EUV energy metrology module 180. The hit/miss control module 320 controls whether the pulse from the drive laser 122 hits a droplet in the irradiation region 128 on the basis of an average power for a grouping of pulses such as a burst. The elements just described constitute elements of a second control loop 330 which controls average amount of generated EUV energy of in a time window of pulses.

[0061] This hit/miss control module 320 can also supply a signal to the EUV pulse energy control module 310 so that the EUV pulse energy control module 310 can alter the RF excitation energy (e.g., reduce to 60% of operating power) depending on whether a next laser pulse is going to be off droplet.

[0062] FIG. 4 is a functional block diagram of an implementation of a mixed energy control system in accordance with aspects of an embodiment. In FIG. 4 letters have been used to designate certain signals as follows.

[0063] The signal A is indicative of the desired or target average EUV energy over a predefined time window. This signal is an input from a user or a control system.

[0064] The signal B is indicative of the desired on-droplet EUV, that is, the EUV energy to be generated per droplet, as determined by a mixed controller setup module 410 based at least in part on input signal A.

[0065] The signal C is indicative of the desired cumulative EUV energy over the predefined time window, as determined by a mixed controller setup module 410 based at least in part on input signal A. [0066] The signal D is a laser power command signal generated by an EUV on droplet controller 420 based at least in part on the signal B. The signal D is applied to the RF excitation source 123 to determine the energy of the excitation pulse applied to the laser 122 either by, for example, controlling the amplitude and/or the pulse width of the excitation pulse to in turn determine the power of the drive pulse generated by the laser 122.

[0067] The signal E is a laser timing command signal generated by a hit/miss controller 430 based at least in part on the signal C. The signal E is applied to trigger and control the timing of the drive pulses of the laser 122. In particular, the timing may be adjusted to cause a given drive pulse to hit or miss a droplet of target material then currently in the irradiation region by causing the laser to fire too early or too late. Hit/miss control can also be achieved by steering the pulses away from the irradiation region, for example, by using the beam steering system 132 (FIG. 3).

[0068] The signal F is the on-droplet EUV energy generated when a main or conversion laser pulse converts a droplet of target material. The signal F is measured and the results of the measurement fed back as the signal H (measured on-droplet EUV) to the EUV on-droplet controller 420 to be used as an input in determining the characteristics of the signal D as well as to the hit/miss controller 430 to generate a cumulative EUV energy over a time window.

[0069] The signal G is indicative of EUV generated disturbances in the vessel 126 (FIG. 1) and is conveyed as an input to the hit/miss controller 430 to be used to determine which firing patterns result in excessive disturbances. It is also possible that the hit/miss controller 430 could have a lookup table with data indicating firing patterns that are a priori known to be detrimental with the hit/miss controller 430 being configured so that it does not command the use of such firing patterns.

[0070] In the arrangement shown in FIG. 4, a signal indicative of a desired average power over a predefined time window is supplied as signal A to a mixed energy controller setup module 410. The mixed energy controller setup module 410 supplies a signal B which is indicative of the desired amount of EUV generated on-droplet. The mixed energy controller setup module 410 also develops a signal C which is indicative of a desired cumulative EUV energy generated over the predefined time window.

[0071] The signal B is supplied to an EUV on-droplet controller 420 which generates a signal D to control the amount of power generated by the laser 122. The signal D controls the amount of power generated by the laser 122 by controlling at least one system variable impacting the power of the laser such as the energy characteristics of an excitation signal applied to the laser 122 by an RF excitation circuit 123. The energy characteristics include the amplitude and pulse width of the RF excitation signal generated by the RF excitation circuit 123.

[0072] The signal C is supplied to the hit/miss controller 430 which generates a timing command which causes conversion pulses from the laser 122 to controllably hit or miss droplets in the irradiation region 128 (FIG. 1).

[0073] The mixed energy controller setup module 410, the hit/miss controller 430, and the EUV on- droplet controller 420 together constitute a mixed energy controller 450. The elements in the dashed box labeled 460 constitute the plant in the sense of operations control, that is, the overall system to be controlled.

[0074] Inside the vessel 126 a plasma is generated in the irradiation region 128 (FIG. 1) depending on the outcome of application of the timing command E. In other words, an EUV pulse train is developed as a function f(droplets, laser) of the firing (hit/miss) pattern and the power of the conversion pulse from the laser 122. The measured on-droplet EUV energy F is measured and the signal H returned to the EUV on-droplet controller 420 to constitute a first control loop which controls the on-droplet EUV energy generated by each droplet. The signal H may also be supplied to the hit/miss controller 430 to generate a cumulative EUV energy over a time window. The resulting EUV pulse train passes through the slit filter 157 to constitute a dose 154 to treat a substrate.

[0075] Also, the presence of undesirable plasma instabilities can be detected and a signal G indicative of those plasma instabilities can be applied to the hit/miss controller 430. The hit/miss controller 430 can use this information to avoid the use of detrimental firing patterns that may result in excessive plasma instabilities. The capability of adjusting the hit/miss firing pattern also provides a capability to select a hit/miss firing pattern according to the spectral characteristics of the slit filter being used. [0076] FIG. 5 is a flow chart describing a method of controlling EUV energy according to another aspect of an embodiment. In FIG. 5, in a step S100, a determination is made of a desired average EUV power for a predefined time window. This determination could be based on input data indicative of the desired EUV energy. Then, in an upper control branch, in a step SI 10, the on-droplet EUV energy needed to attain the desired average EUV power is determined. Then, in a step SI 20 a command is generated to determine the characteristics of the RF excitation signal for the laser, which in turn controls the energy in the main pulse of the laser. This command is supplied to the laser. As mentioned, the characteristics may include one or both of the amplitude and the pulse width of the RF excitation signal. [0077] Concurrently to the execution of the steps SI 10 and S120, in a lower control branch in a step S130 the on-droplet cumulative EUV energy needed to attain the desired average EUV power is determined. Then, in a step S140 a command is generated to determine the hit/miss firing pattern for the laser which will in turn control the total energy in the main pulses of the laser over the predefined time window. This command is supplied to a timing circuit which controls the timing of the conversion pulses of the laser. This command can alternatively or additionally be supplied to a beam steering module which controllably steers main conversion pulses towards or away from the irradiation region. [0078] In a step SI 50 the laser is fired with the commanded RF excitation and consistent with the commanded hit/miss firing pattern, i.e., hit or miss for a given pulse. While this occurs, in a step S160 the EUV energy generated by the conversion pulse is measured and fed back to be used in step SI 10 and step 130. Also, according to an aspect of an embodiment, in a step S170 instabilities in the plasma which may be exacerbated by certain hit/miss firing patterns are measured, and this information is also fed back to step S140 to be used in determining which hit/miss firing pattern to use.

[0079] Also, as indicated, in a step SI 80 information about the characteristics of the slit filter, such as its spectral response or identified firing patterns that are incompatible with the slit filter, may be supplied to determine which firing pattern to use in step S140.

[0080] In the example of FIG. 5 there is also provision for interaction between the step S120 and the step S 140 in that in the step SI 20 it may be determined to reduce the RF excitation energy supplied to the laser, for example, to about 60%, if it is determined in step S140 that the next shot will be a miss. Also, the system may be configured such that only results for on-droplet laser shots (hits) contribute to measuring the average and/or cumulative and/or per-droplet EUV energy output.

[0081] Thus, according to an aspect of an embodiment, the mixed controller includes two control loops. One of the control loops, the hit/miss firing pattern control loop, operates to ensure stability of the average EUV energy controlling the hit/miss firing pattern. The other control loop, the laser pulse power control loop, operates to control the energy of individual EUV pulses to a desired level by controlling the laser power. The system also has the capability of adjusting the hit/miss firing pattern to avoid detrimental hit/miss firing patterns. The hit/miss firing pattern control loop calculates the output of a hit/miss droplet command to minimize dose error. [0082] According to another aspect of an embodiment, the laser pulse power control loop may be disabled when the hit/miss firing pattern control loop commands missing a current droplet. This results in a zero error input for off-droplet laser shots (misses). This will drive only the energy of on-droplet laser shots (hits) to a desired value.

[0083] According to another aspect of an embodiment, the actual target value for average power for the laser pulse power control loop may be set above the commanded target value for average power for the laser pulse power control loop. For example, the actual target value for the laser pulse power control loop may be set in a fixed ratio above the commanded target value for the laser pulse power control loop. In other words, a ratio of the actual target value and the commanded target value would be maintained at a fixed value greater than one. This would provide some latitude to adjust the hit/miss pattern to avoid detrimental hit/miss patterns.

[0084] The present disclosure is made with the aid of functional building blocks illustrating the implementation of specified functions and interrelationships between those functions. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions are appropriately performed and their interrelationships maintained.

[0085] The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. In addition, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated or clear from context. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.

[0086] The implementations can be further described using the following clauses.

1. A system for modulating power output of a source of a plurality of extreme ultraviolet (EUV) pulses of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material and a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, the system comprising: an EUV pulse energy control loop adapted to maintain an energy of individual ones of the plurality of EUV pulses within a defined range of energy values; and a firing pattern control loop adapted to determine which individual ones of the droplets are irradiated by at least an associated one of the converting pulses. 2. The system of clause 1 wherein an energy of a converting pulse in the plurality of converting pulses depends at least in part on an electrical characteristic of an RF excitation signal applied to the laser, and in which the EUV pulse energy control loop comprises a module for controlling the electrical characteristic of the RF excitation signal.

3. The system of clause 2 wherein the electrical characteristic of the RF excitation signal comprises an amplitude of the RF excitation signal.

4. The system of clause 2 wherein the electrical characteristic of the RF excitation signal comprises a pulse width of the RF excitation signal.

5. The system of clause 1 wherein the firing pattern control loop comprises a trigger signal module for controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser.

6. The system of clause 1 wherein the firing pattern control loop comprises a steering module for steering a conversion pulse emitted by the laser, the steering module causing the conversion pulse to strike a droplet of the plurality of droplets in the irradiation region when the firing pattern control loop commands a hit and the steering module causing the conversion pulse to miss the droplet in the irradiation region when the firing pattern control loop commands a miss.

7. The system of clause 1 further comprising a plasma sensor arranged to sense a degree of instability in a plasma generated by conversion of one or more droplets wherein the firing pattern control loop is responsive to the plasma sensor to avoid commanding firing patterns that increase a degree of instability in the plasma past a predetermined threshold.

8. The system of clause 1 wherein the EUV pulse energy control loop is adapted to be responsive to the firing pattern control loop in such a way that the EUV pulse energy control loop reduces the energy in an RF excitation signal applied to the laser for a next shot if the firing pattern control loop commands the next shot to be a miss.

9. The system of clause 1 wherein the EUV pulse energy control loop receives a commanded target value for EUV average energy and uses the commanded target value to set an actual target value for EUV average energy greater than the commanded target value.

10. The system of clause 9 wherein the EUV pulse energy control loop maintains a ratio of the commanded target value to the actual target value at a fixed value greater than one.

11. A system for controlling an average power output of a source of a plurality of pulses of extreme ultraviolet (EUV) of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material, a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, and an EUV energy measurement sensor for measuring a per-droplet output energy generated by irradiation of individual droplets and producing a per-droplet output energy measurement signal, the system comprising: a first control subsystem responsive to the per-droplet output power measurement signal and the adapted to maintain an energy of individual ones of the EUV pulses within a defined range by controlling at least one system variable impacting the power of the laser; and a second control subsystem responsive to the per-droplet output power measurement signal and adapted to determine which individual ones of the converting pulses are to be used to irradiate respective individual ones of the droplets in the irradiation region by controlling timing of the converting pulses.

12. The system of clause 11 wherein the at least one system variable impacting the power of the laser comprises an amplitude of an RF excitation signal.

13. The system of clause 11 wherein the at least one system variable impacting the power of the laser comprises a pulse width of an RF excitation signal.

14. The system of clause 11 wherein the second control subsystem comprises a trigger signal module for controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser.

15. The system of clause 11 wherein the second control subsystem comprises a steering module for steering a conversion pulse of the plurality of conversion pulses emitted by the laser, the steering module causing the conversion pulse to strike a droplet in the irradiation region when the firing pattern control loop commands a hit and the steering module causing the conversion pulse to miss a droplet in the irradiation region when the firing pattern control loop commands a miss.

16. The system of clause 11 further comprising a plasma sensor arranged to sense a degree of instability in a plasma generated by conversion of one or more droplets wherein the second control subsystem is further responsive to the plasma sensor to avoid commanding firing patterns that increase a degree of instability in the plasma past a predetermined threshold.

17. The system of clause 11 wherein the first control subsystem is adapted to be responsive to the second control subsystem in such a way that the first control subsystem reduces the energy in the RF excitation signal applied to the laser for a next shot if the second control subsystem commands the next shot to be a miss.

18. The system of clause 11 wherein the first control subsystem receives a commanded target value for EUV average energy and uses the commanded target value to set an actual target value for EUV average energy greater than the commanded target value.

19. The system of clause 18 wherein the first control subsystem maintains a ratio of the commanded target value to the actual target value at a fixed value greater than one.

20. A method of controlling a source of a plurality of extreme ultraviolet (EUV) pulses of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material and a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, the method comprising: using a first control subsystem to maintain an energy of individual ones of the EUV pulses within a defined range of energy values; and using a second control subsystem to determine which individual ones of the converting pulses are to be used to irradiate respective individual ones of the droplets in the irradiation region.

21. A method of controlling a source of a plurality of pulses of extreme ultraviolet (EUV) of radiation, the source including a droplet generator adapted to generate a plurality of droplets of a target material, a laser arranged to generate a plurality of converting pulses to irradiate an irradiation region through which individual ones of the droplets pass, and an on-droplet EUV energy measurement sensor for measuring a per-droplet output power generated by irradiation of individual droplets and producing a per-droplet output power measurement signal, a laser output power of the laser being determined at least in part by a radio frequency (RF) excitation signal applied to the laser, the method comprising: maintaining an energy of individual ones of the EUV pulses within a defined range by controlling at least one electrical characteristic of the RF excitation signal; and determining which individual ones of the converting pulses are to be used to irradiate respective individual ones of the droplets in the irradiation region.

22. The method of clause 21 wherein the electrical characteristic of the RF excitation signal comprises an amplitude of the RF excitation signal.

23. The method of clause 21 wherein the electrical characteristic of the RF excitation signal comprises a pulse width of the RF excitation signal.

24. The method of clause 21 wherein determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region comprises controlling a timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a conversion pulse when the trigger signal is applied to the laser.

25. The method of clause 21 wherein determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region comprises commanding a steering module to cause the conversion pulse to strike a droplet in the irradiation region to effect a hit and causing the conversion pulse to miss a droplet in the irradiation region to effect a miss.

26. The method of clause 21 further comprising sensing a degree of instability in a plasma generated by conversion of one or more of the droplets and wherein determining which individual ones of the converting pulses are used to irradiate respective individual ones of the droplets in the irradiation region is based at least in part on the degree of instability to avoid firing patterns that increase the degree of instability in the plasma past a predetermined threshold.

27. The method of clause 21 wherein maintaining an energy of individual ones of the EUV pulses within a defined range comprises reducing the energy in the RF excitation signal applied to the laser for a next shot if the next shot is a miss.

28. The method of clause 21 wherein maintaining an energy of individual ones of the EUV pulses within a defined range comprises setting an actual target value for EUV average energy greater than a commanded target value for EUV average energy. 29. The method of clause 28 wherein setting an actual target value for EUV average energy greater than a commanded target value for EUV average energy comprises setting a ratio of the commanded target value to the actual target value at a fixed value greater than one.

[0087] The above described implementations and other implementations are within the scope of the following claims.