Hybrid energy control in EUV lithography systems

Abstract

A system and method for controlling an extreme ultraviolet (EUV) output power in an EUV radiation source in which a control loop controls at least one system variable that affects a drive laser power when the drive laser generates a laser pulse that converts a portion of a target material in an irradiation zone, another control loop controls a hit / miss emission pattern that determines which laser pulses will strike a portion of the target material in the irradiation zone and which will not strike a portion of the target material in the irradiation zone.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application 63 / 405,533, filed on September 12, 2022, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a system for generating extreme ultraviolet (EUV) radiation.Such a system typically generates EUV radiation in pulses and requires control over the energy of the pulses. Background Art

[0004] EUV radiation, eg, electromagnetic radiation having a wavelength of about 50 nm or less (sometimes referred to as soft x-rays) and including radiation having a wavelength of about 13.5 nm, can be used in lithography processes to create extremely small features in substrates such as silicon wafers.

[0005] Some methods for generating EUV radiation include converting a target material (also referred to as a source material) from a liquid state to a plasma state. Preferably, the target material includes at least one element, such as xenon, lithium, or tin, having one or more emission lines in the EUV range. In one such method, a laser plasma ("LPP") is generated by irradiating a corresponding mass of a target material having a desired emission line element using individual drive laser beam pulses.

[0006] Thus, one LPP technique involves generating a stream of droplets of a target material and irradiating at least some of the droplets with pulses of laser radiation. In more theoretical terms, an LPP source generates 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), thereby creating a highly ionized plasma with an electron temperature of tens of eV.

[0007] According to one example, by using a 10.6 μm wavelength CO 2 The LPP EUV lithography source generates the required 13.5nm radiation by focusing a laser beam onto a tin droplet target to create a highly ionized plasma. These ions radiate EUV photons isotropically. A temperature-controlled graded multilayer coated normal-incidence mirror (collector) collects these photons and focuses them to an intermediate point, from where they are relayed to the scanner optics and ultimately to the wafer. Debris mitigation technology, based on the flow of hydrogen buffer gas, protects the collector from the plasma. High-energy ions, fast neutral particles, and residual source element particles are mitigated to maintain the reflectivity of the collector mirror and enable this component to have a long service life.

[0008] There are systems where multiple pulses can be applied to a droplet, for example, a main conversion pulse is preceded by one or more conditioning pulses that change the geometry and / or distribution of the target material in the droplet. Thus, after the conditioning pulses are applied, the droplet may take on a different form, such as a disk or mist of the target material. Thus, the term "droplet" is used broadly herein to refer to any shape or distribution of the target material prior to conversion.

[0009] The energy delivered to the wafer by the EUV radiation beam per unit area during the exposure time (or a particular number of EUV radiation beam pulses) is called the dose or exposure energy (e.g., in Joules). The formation of microelectronic features on the wafer depends on the appropriate dose reaching the wafer ("target dose"). If too little energy reaches the wafer during the exposure time (the dose is too low and less than the target dose), the radiation-sensitive materials of the wafer will not be activated and microelectronic features will not be formed on the wafer or the microelectronic features will be incompletely formed on the wafer. If too much energy reaches the wafer during the exposure time (the dose is too high and greater than the target dose), the radiation-sensitive materials of the wafer may be exposed outside the boundaries of the slit pattern image used to expose the wafer, and microelectronic features are incorrectly formed on the wafer. Dose errors may be caused by variations in the energy in the EUV radiation beam, and these variations may come from, for example, noise in the radiation source and / or interference inside or outside the radiation source.

[0010] In other words, one technical challenge of using laser pulses to generate EUV radiation is the need to control the amount, or "dose," of EUV light energy that is applied to a particular item being processed, such as a semiconductor wafer. For example, as part of a manufacturing process, a specified amount of EUV radiation energy may be required to complete some task on a semiconductor wafer, such as curing a layer of photoresist. In order to obtain consistent results on different wafers, it is best to apply the same amount of EUV radiation energy to each wafer, and to do so as accurately as possible. For example, a system for controlling the amount of EUV radiation energy applied is disclosed in U.S. Patent No. 9,693,440, entitled "EUV LPP Source with Improved Dose Control by Tracking Dose over Specified Window," issued on June 27, 2017.

[0011] All patent applications, patents, and printed publications cited herein are incorporated by reference in their entirety, except for any definitions, subject matter disclaimers, or disclaimers, unless the incorporated material is inconsistent with the explicit disclosure herein, in which case the language of the disclosure controls.

[0012] Meeting this challenge is complicated because the power of each driving laser pulse can vary. Since the EUV energy released when a laser pulse hits a droplet varies with the power of the laser pulse, the EUV light energy generated by the conversion of any given droplet can also vary.

[0013] One way to achieve dose control is to select a desired dose target for each EUV pulse packet or group of pulses. A burst can be such a group of EUV pulses. The EUV energy generated by each laser pulse hitting the corresponding droplet is measured. The total cumulative dose for each group of pulses is then calculated by adding the EUV energy from each droplet in a series of droplets (starting with the first droplet in the packet). Once the dose target for a packet is reached, the remaining pulses in the packet are "skipped" or "missed", that is, the drive laser pulses are generated but are not allowed to hit any droplets. Skipping a droplet is typically achieved by firing the laser at a location outside the irradiation zone where the droplet is located, or by firing the laser when the droplet will not be there when the laser pulse arrives at the irradiation location.

[0014] Another way of dose control that does not involve skipping droplets is to control the pulse energy of each laser pulse, thereby adjusting the duration of the pulse or the pulse amplitude from the laser master oscillator. This is achieved by adjusting the RF excitation energy of the laser's RF excitation signal. Controlling the energy of individual EUV pulses by controlling the energy of the drive laser pulses can be achieved in a variety of ways, including changing the amplitude and / or pulse width of the RF excitation signal.

[0015] Both approaches have potential advantages and disadvantages using conventional techniques. For example, the CO is activated by controlling the duty cycle of a pulse width modulated (PWM) RF excitation signal. 2 The driving laser power is characterized by a slow response time of approximately 0.2 milliseconds. This affects the maximum achievable bandwidth of the control loop employing this control method. In contrast, methods that implement on-droplet or off-droplet firing are characterized by fast response speed, as the command to switch droplet firing on and off can produce the desired result essentially immediately. However, the reliance on hit / miss droplet actuation is combined with the introduction of an additional periodic disturbance in the control loop that affects the stability of on-droplet EUV generation and, therefore, the dose. Relative to a system in which all shots are fired onto the droplet, the hit / miss firing pattern adds an additional error component to the delivered dose.

[0016] Therefore, being able to mitigate and limit the negative impact of hit / miss control would improve energy control. It is against this background that the need for the present invention arises. Summary of the invention

[0017] The following briefly summarizes one or more embodiments in order to provide a basic understanding of these and other possible embodiments. The present content is not an extensive overview of all contemplated embodiments, is not intended to identify the key or critical elements of all embodiments, and is not intended to set limitations on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to a more detailed description presented later.

[0018] According to one aspect of the embodiment, a system is disclosed 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 pattern. The combined action of the two control loops provides responsive and robust energy control.

[0019] According to another aspect of the embodiment, a system for modulating the power output of multiple extreme ultraviolet (EUV) pulsed radiation sources is disclosed, the radiation source including a droplet generator and a laser, the droplet generator being suitable for generating multiple droplets of a target material, and the laser being arranged to generate multiple conversion pulses to irradiate an irradiation area through which individual droplets pass, the system comprising: an EUV pulse energy control loop, which is suitable for maintaining the energy of individual EUV pulses among the multiple EUV pulses within a defined energy value range; and an emission mode control loop, which is suitable for determining which individual droplets among the droplets are irradiated by the conversion pulses.

[0020] The energy of the conversion 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 include a module for controlling the electrical characteristic of the RF excitation signal. The electrical characteristic of the RF excitation signal may include an amplitude of the RF excitation signal. The electrical characteristic of the RF excitation signal may include a pulse width of the RF excitation signal.

[0021] The emission mode control loop may include a trigger signal module for controlling the timing of applying a trigger signal to the laser, and when the trigger signal is applied to the laser, the trigger signal causes the laser to emit a conversion pulse. The emission mode control loop may include a steering module for steering the conversion pulse emitted by the laser, and when the emission mode control loop command hits, the steering module causes the conversion pulse to hit the droplets in the irradiation area, and when the emission mode control loop command misses, the steering module causes the conversion pulse to miss the droplets in the irradiation area.

[0022] The system may further include a plasma sensor arranged to sense a degree of instability in the plasma generated by conversion of the one or more droplets, wherein the emission mode control loop is responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma beyond (i.e., exceeds) a predetermined threshold.

[0023] The EUV pulse energy control loop may be adapted to be responsive to the shot mode control loop such that if the shot mode control loop commands a miss for the next shot, the EUV pulse energy control loop reduces the energy in the RF excitation signal applied to the laser for the next shot. The EUV pulse energy control loop may receive a commanded target value for the EUV average energy and use the commanded target value to set an actual target value for the 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 1.

[0024] According to another aspect of the embodiment, a system for controlling the average power output of a source of multiple extreme ultraviolet (EUV) pulsed radiation is disclosed, the radiation source including a droplet generator, a laser, and an EUV energy measurement sensor, the droplet generator being suitable for generating multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to irradiate an irradiation zone through which individual droplets pass, and the EUV energy measurement sensor being used to measure the output energy per droplet generated by irradiating the individual droplets and generate an output energy per droplet measurement signal, the system comprising: a first control subsystem responsive to the output power per droplet measurement signal and adapted to maintain the energy of individual EUV pulses within a defined range by controlling at least one system variable affecting the laser power; and a second control subsystem responsive to the output power per droplet measurement signal and adapted to determine which individual conversion pulses will be used to irradiate corresponding individual droplets in the irradiation zone by controlling the timing of the conversion pulses.

[0025] The at least one system variable affecting the laser power may include the amplitude of the RF excitation signal. The at least one system variable affecting the laser power may include the pulse width of the RF excitation signal.

[0026] The second control subsystem may include a trigger signal module for controlling the timing of applying a trigger signal to the laser, which trigger signal causes the laser to emit a switching pulse when the trigger signal is applied to the laser.

[0027] The second control subsystem may include a steering module for steering the switching pulse emitted by the laser. When the emission mode control loop command hits, the steering module causes the switching pulse to hit the droplets in the irradiation area. When the emission mode control loop command misses, the steering module causes the switching pulse to miss the droplets in the irradiation area.

[0028] The system may further include a plasma sensor arranged to sense a degree of instability in the plasma generated by conversion of the one or more droplets, wherein the second control subsystem is responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma above a predetermined threshold.

[0029] The first control subsystem may be adapted to be responsive to the second control subsystem such that if the second control subsystem commands a miss for the next shot, the first control subsystem reduces the energy in the RF excitation signal applied to the laser for the next shot.

[0030] The first control subsystem may receive a command target value for EUV average energy and use the command target value to set an actual target value for EUV average energy greater than the command target value. The first control subsystem may maintain a ratio of the command target value to the actual target value at a fixed value greater than 1.

[0031] According to another aspect of one embodiment, a method of controlling the average power output of multiple extreme ultraviolet (EUV) pulsed radiation sources is disclosed, the sources including a droplet generator and a laser, the droplet generator being suitable for generating multiple droplets of a target material, and the laser being arranged to generate multiple conversion pulses to irradiate an irradiation area through which individual droplets pass, the method comprising: using a first control subsystem to maintain the energy of individual EUV pulses within a defined energy value range; and using a second control subsystem to determine which individual conversion pulses of the conversion pulses are to be used to irradiate corresponding individual droplets in the irradiation area.

[0032] According to another aspect of an embodiment, a method for controlling the average power output of a source of multiple extreme ultraviolet (EUV) pulsed radiation is disclosed, the source including a droplet generator, a laser, and an on-droplet EUV energy measurement sensor, the droplet generator being suitable for generating multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to irradiate an irradiation zone through which individual droplets pass, and the on-droplet EUV energy measurement sensor being used to measure the output energy per droplet generated by irradiating the individual droplets and to generate an output energy per droplet measurement signal, the laser output power of the laser being at least partially determined by a radio frequency (RF) excitation signal applied to the laser, the method comprising: maintaining the energy of the individual EUV pulses within a defined range by controlling at least one electrical characteristic of the RF excitation signal; and determining which individual conversion pulses of the conversion pulses will be used to irradiate corresponding individual droplets in the irradiation zone.

[0033] The electrical characteristic of the RF excitation signal may include an amplitude of the RF excitation signal.The electrical characteristic of the RF excitation signal may include a pulse width of the RF excitation signal.

[0034] Determining which individual ones of the conversion pulses are used to irradiate corresponding individual droplets in the irradiation zone may include controlling the timing of applying a trigger signal to a laser that, when applied to the laser, causes the laser to emit the conversion pulses. Determining which individual ones of the conversion pulses are used to irradiate corresponding individual droplets in the irradiation zone may include commanding a steering module to cause the conversion pulses to strike a droplet in the irradiation zone to cause a hit and to cause the conversion pulses to miss a droplet in the irradiation zone to cause a miss.

[0035] The method may also include sensing a degree of instability in the plasma generated by the conversion of one or more droplets, and determining which individual ones of the conversion pulses to use to irradiate corresponding individual droplets in the irradiation zone may be based at least in part on the degree of instability to avoid emission modes that increase the degree of instability in the plasma to exceed a predetermined threshold.

[0036] Maintaining the energy of the individual EUV pulses within the defined range may include reducing the energy in the RF excitation signal applied to the laser for the next shot if the next shot misses. Maintaining the energy of the individual EUV pulses within the defined range may include setting an actual target value for the EUV average energy greater than a commanded target value for the EUV average energy. Setting the actual target value for the EUV average energy greater than the commanded target value for the EUV average energy may include setting a ratio of the commanded target value to the actual target value to a fixed value greater than 1.

[0037] Further embodiments, features, and advantages of the present invention, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate illustrative embodiments and, together with the description, further serve to explain the principles of the embodiments and to enable one skilled in the relevant art(s) to make and use the embodiments.

[0039] Figure 1 is a partially schematic functional block diagram, not to scale, of the overall broad concept of an EUV lithography system.

[0040] Figure 2 is a timing diagram illustrating some of the principles of a hit / miss fire pattern.

[0041] Figure 3 is a partially schematic functional block diagram, not to scale, of an overall broad concept of an energy control system for an EUV lithography system according to an aspect of one embodiment.

[0042] Figure 4is a partially schematic functional block diagram, not to scale, of the general concept of an energy control system for an EUV lithography system according to an aspect of one embodiment.

[0043] Figure 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.

[0044] More features and advantages of the disclosed subject matter, as well as the structure and operation of various embodiments of the disclosed subject matter will be described in detail below with reference to the accompanying drawings. The embodiments presented in this article are for illustrative purposes only. Based on the teachings presented in this article, other embodiments will be apparent to those skilled in the relevant art. DETAILED DESCRIPTION

[0045] Various embodiments are now described with reference to the accompanying drawings, wherein the same reference numerals are used to refer to the same elements throughout. In the following description, for the purpose of explanation, many specific details are set forth to facilitate a thorough understanding of one or more embodiments. However, in some or all cases, it is apparent that any embodiment described below can be practiced without adopting the specific design details described below. In other cases, well-known structures and devices are shown in block diagram form to facilitate description of one or more embodiments.

[0046] However, before describing such embodiments in more detail, it is helpful to describe an example environment in which embodiments of the present invention may be implemented. Figure 1 , which shows a schematic diagram of an exemplary EUV radiation source (e.g., a laser-generated 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 CO 2 A laser source, generating a 10.6 μm or 1 μm radiation beam 112. A pulsed gas discharge CO 2 The laser source 122 may have a DC or RF excitation operating at high power and at a high pulse repetition rate. In this example, a pulsed gas discharge CO 2 The laser source 122 has RF excitation applied by an RF excitation source 123 under the control of an EUV source controller system 160 , as described in more detail below.

[0047] The EUV radiation source 100 also includes a target delivery system 124 for delivering a target material. In this example, the target material is a liquid, but it can also be a solid or a gas. The liquid is in the form of droplets, but it can be a continuous stream. In the following description, tin is used as a non-limiting example of a target material, but it should be understood that other materials can be used instead of tin or as a supplement to tin.

[0048] In the depicted system, a target material delivery system 124 introduces droplets 114 of a target material into the interior of a vacuum chamber 126 having chamber walls 127. The vacuum chamber 126 includes an irradiation zone 128 in which the target material can be irradiated to produce a plasma. It should be noted that, as used herein, an irradiation zone is an area in which irradiation of the target material may or is expected to occur, and is an irradiation zone even when irradiation does not actually occur. The EUV light source may also include a beam steering system 132, which also operates under the control of an EUV light source controller system 160, as described in more detail below.

[0049] In the system shown, the components are arranged so that the droplet 114 travels substantially horizontally. The direction from the laser source 122 toward the irradiation zone 128 (i.e., the nominal propagation direction of the beam 112) can be used as the Z-axis. The path taken by the droplet 114 from the target material delivery system 124 to the irradiation zone 128 can be used as the X-axis. Therefore, Figure 1 The view is perpendicular to the XZ plane. The orientation of the EUV radiation source 100 can be rotated relative to gravity. Although a system is described in which the droplets 114 travel substantially horizontally, one of ordinary skill in the art will appreciate that other arrangements can be used in which the droplets travel vertically or at an angle between 90° (horizontal) and 0° (vertical) relative to gravity.

[0050] The EUV radiation source 100 may also include an EUV light source controller system 160 and a laser emission control system 165 that 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 an absolute or relative position of a target droplet (e.g., relative to the irradiation zone 128) and provides this output to a target position detection feedback system 162.

[0051] like Figure 1 As shown in , 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 illumination zone 128. For example, such adjustment can be achieved by repositioning the point at which the droplet generator 192 releases the target droplet 114. For example, the droplet release point can be repositioned by tilting the droplet generator 192 or translating the droplet generator 192 laterally. The droplet generator 192 extends into the chamber 126 and is preferably supplied with target material from the outside. A gas source (not shown) places the target material in the pressurized droplet generator 192. Droplets 114 that pass through the illumination zone 128 without being transformed continue to flow to the target material container 134.

[0052] Continue to refer Figure 1, the radiation source 100 may also include one or more optical elements. In the following description, the 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 an oblong ellipsoid, having a central aperture 135 to allow the laser radiation 112 to pass through and reach the irradiation area 128. The collector 130 has a first focus at the irradiation area 128 and a second focus at a so-called intermediate point 140 (also referred to as intermediate focus 140), where EUV radiation can be output from the EUV radiation source 100 and input to, for example, an integrated circuit lithography scanner 150.

[0053] 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 optical box or POB, which is configured to project a pattern imparted by a pattern forming device 154 (e.g., a mask) to a radiation beam onto a target portion (e.g., including one or more dies) of a substrate 152. The projection system 150 includes a slit filter 157. In this example, the slit 157 is rectangular and shapes the beam into an elongated rectangular beam. This shaped 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 to ultimately be manufactured into an integrated circuit device.

[0054] The detector 180 detects EUV energy at or near the surface of the wafer 152 and provides a signal indicative of the detected energy to the EUV light source controller system 160. As discussed in more detail below, the signal can be a series of energy values ​​measured by the detector 180, each energy value representing the amount of EUV energy at the substrate 152 at a particular time (e.g., the energy at the substrate 152 transmitted by a particular light pulse or series of light pulses). U.S. Patent No. 9,939,732, entitled "Controller for an Optical System," issued on April 10, 2018, discloses additional examples of arrangements such as those just described.

[0055] To summarize some key points, the detector 180 measures the energy at or near the surface of the substrate 152. The EUV light source controller system 160 receives the measurement results and can control the power of the (multiple) pulses of the laser 122 by managing the RF excitation signal applied to the laser 122 by the RF excitation source 123, thereby controlling the amount of power (e.g., instantaneous or average) at the substrate 152. In addition, by changing the timing of the trigger signal generated by the laser emission control system 165 or by controlling the steering of the beam 112 by the beam steering system 132, the EUV light source controller system 160 can control which droplets 114 are illuminated by the pulses from the laser 122.

[0056] As mentioned above, controlling the power at the wafer by managing the drive laser power, either by controlling the RF excitation signal itself or by arranging the system so that a controlled number of pulses itself do not irradiate droplets, has disadvantages. According to one aspect of an embodiment, these disadvantages are avoided or at least reduced by employing a hybrid energy control system for an LPP EUV source, the system using both a first energy control loop and a second hit / miss control loop, the first energy control loop being arranged to control the energy of individual EUV pulses to a desired level via available activation components, e.g., controlling the RF input to the drive laser (e.g., amplitude or pulse width or both), while the second hit / miss control loop ensures stability of the average EUV energy within a range around a defined power level via active control of the hit / miss shot pattern.

[0057] According to another aspect of one embodiment, the hit / miss droplet firing pattern can be adaptable and in particular controllable to avoid using a harmful hit / miss droplet firing pattern. An advantage of such a system having the ability to adapt the hit / miss droplet firing pattern is that the laser firing pattern can be adapted to different slit filters having different cutoff frequencies and frequency responses. Such a system can reduce or even eliminate periodic disturbances in the plasma generation process compared to a system lacking an adaptable firing pattern.

[0058] Figure 2 is the time window T f FIG. 1 is a graph of an emission pattern over time. Time in arbitrary units is plotted on the x-axis, and EUV energy in arbitrary units is plotted on the y-axis. Thus, the graph shows EUV energy as a function of time or EUV(t). Each vertical line indicates an EUV pulse where a laser pulse from the drive laser hits and converts a droplet of target material. Each X indicates an EUV pulse where a laser pulse from the drive laser does not hit (i.e., misses) a droplet of target material. The emission pattern is less than T. f The pattern starts at time t when an EUV pulse or hit occurs. k The next (n1–1) driving laser pulses hit, and then at time t k+n1 The n1th pulse of is missed. Then, another two time intervals T follow in the same pattern. At time t k +T f It is this firing pattern that can be adapted by adjusting it to avoid firing patterns that may, for example, easily cause disturbances in plasma generation.

[0059] In addition, according to one aspect of the embodiment, in combination Figure 2 The described system can be realized by, for example,f The time window T is adjusted by ending with a miss droplet instead of a hit droplet f The accumulated EUV energy on the window T f The expected cumulative EUV energy is reached or expected to be reached.

[0060] Figure 3 is a functional block diagram of an implementation of a hybrid energy control system according to aspects of one embodiment. Figure 3 In the embodiment, the driving laser 122 is provided with an excitation pulse by the RF excitation circuit 123. The energy characteristics (such as amplitude and pulse width) of the pulse from the RF excitation circuit 123 partially determine the energy generated by the laser 122 in the main conversion pulse 133. The main conversion pulse 133 from the laser 122 passes through the beam steering module 132 and is provided to the irradiation area 128 as the main conversion pulse 133. The characteristics of the pulse from the RF excitation circuit 123 are determined by the EUV pulse energy control module 310 based on the measurement signal from the EUV energy measurement module 180, and the EUV pulse energy control module 310 can be an EUV light source controller system 160 ( Figure 1 ). According to an aspect of one embodiment, the generated EUV energy is determined on a droplet-by-droplet or per-droplet basis. Thus, the elements just described together constitute elements of a first control loop 330 for controlling EUV energy on a droplet.

[0061] In addition, the timing of the pulses from the drive laser 122 is determined by the laser timing control circuit 165, which in turn is controlled by the hit / miss control module 320. The hit / miss control module 320 can also be part of the EUV light source controller system 160. The hit / miss control module 320 can also provide control signals to the beam control module 132. The hit / miss control module 320 determines the control signal based on the signal from the EUV energy measurement module 180. The hit / miss control module 320 controls whether the pulses from the drive laser 122 hit the droplets in the irradiation area 128 based on the average power of the pulse group (such as the burst). The elements just described constitute elements of the second control loop 330, which controls the average amount of EUV energy generated in the pulse time window.

[0062] The hit / miss control module 320 may also provide a signal to the EUV pulse energy control module 310 so that the EUV pulse energy control module 310 may modify the RF excitation energy (eg, down to 60% of operating power) depending on whether the next laser pulse is about to leave the droplet.

[0063] Figure 4 is a functional block diagram of an implementation of a hybrid energy control system according to aspects of one embodiment. Figure 4 In, letters have been used to designate certain signals as follows.

[0064] Signal A indicates the desired or target average EUV energy within a predefined time window. This signal is an input from a user or a control system.

[0065] Signal B indicates the desired on-droplet EUV, ie, the EUV energy at which each droplet is to be generated, determined by the hybrid controller settings module 410 based at least in part on the input signal A.

[0066] Signal C indicates the desired cumulative EUV energy within a predefined time window, determined by the hybrid controller settings module 410 based at least in part on the input signal A.

[0067] Signal D is a laser power command signal generated by the on-drop EUV controller 420 at least partially based on signal B. Signal D is applied to the RF excitation source 123 to determine the energy of the excitation pulse applied to the laser 122, for example, to determine the power of the drive pulse generated by the laser 122 by controlling the amplitude and / or pulse width of the excitation pulse.

[0068] Signal E is a laser timing command signal generated by the hit / miss controller 430 based at least in part on signal C. Signal E is applied to trigger and control the timing of the drive pulses of the laser 122. In particular, the timing can be adjusted to cause a given drive pulse to hit or miss a droplet of target material currently in the irradiation zone by causing the laser to fire too early or too late. Hit / miss control can also be accomplished by, for example, using beam steering to the system 132 ( Figure 3 ) is achieved by steering the pulse away from the irradiated area.

[0069] Signal F is the on-droplet EUV energy generated when a main laser pulse or a conversion laser pulse converts a droplet of target material. Signal F is measured and the measurement result is fed back to the EUV on-droplet controller 420 as signal H (measured on-droplet EUV) for use as an input to determine the characteristics of signal D, and to the hit / miss controller 430 to generate the cumulative EUV energy within the time window.

[0070] Signal G indicates that EUV is in the container 126 ( Figure 1 ) and is transmitted as input to the hit / miss controller 430 for determining which firing modes will cause excessive perturbations. The hit / miss controller 430 may also have a lookup table, the data of which indicates firing modes that are known a priori to be harmful, and the hit / miss controller 430 is configured so that such firing modes are not commanded.

[0071] exist Figure 4In the arrangement shown in , a signal indicating the desired average power within a predefined time window is provided as signal A to the hybrid energy controller setting module 410. The hybrid energy controller setting module 410 provides a signal B indicating the desired amount of EUV generated on the droplet. The hybrid energy controller setting module 410 also generates a signal C indicating the desired cumulative EUV energy generated within the predefined time window.

[0072] Signal B is provided to the EUV droplet on controller 420, which generates signal D to control the amount of power generated by the laser 122. Signal D controls the amount of power generated by the laser 122 by controlling at least one system variable that affects the laser power, such as the energy characteristics of the excitation signal applied to the laser 122 by the 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.

[0073] Signal C is provided to a hit / miss controller 430 which generates timing commands that cause switching pulses from the laser 122 to controllably hit or miss the irradiation area 128 ( Figure 1 ) in the droplets.

[0074] The hybrid energy controller setup module 410, the hit / miss controller 430, and the EUV droplet-on controller 420 together constitute the hybrid energy controller 450. The elements marked 460 in the dashed box constitute the facility in the sense of operational control, ie, the entire system to be controlled.

[0075] Inside the container 126, depending on the result of the application of the timing command E, in the irradiation area 128 ( Figure 1 ) in the plasma. In other words, the EUV pulse train is formulated as a function f(droplet, laser) of the emission (hit / miss) pattern and the conversion pulse power from the laser 122. The resulting on-droplet EUV energy F is measured, and a signal H is returned to the EUV droplet controller 420 to form a first control loop for controlling the on-droplet EUV energy generated for each droplet. The signal H can also be provided to the hit / miss controller 430 to generate the accumulated EUV energy within the time window. The resulting EUV pulse train passes through the slit filter 157 to form a dose 154 to process the substrate.

[0076] In addition, the presence of undesirable plasma instabilities can be detected, and a signal G indicative of these plasma instabilities can be applied to the hit / miss controller 430. The hit / miss controller 430 can use this information to avoid using harmful emission modes that may cause excessive plasma instabilities. The ability to adjust the hit / miss emission mode also provides the ability to select a hit / miss emission mode based on the spectral characteristics of the slit filter used.

[0077] Figure 5 is a flow chart describing a method of controlling EUV energy according to another aspect of an embodiment. Figure 5 In the control branch, in step S100, the desired average EUV power in the predefined time window is determined. This determination may be based on input data indicating the desired EUV energy. Then, in the upper control branch, in step S110, the EUV energy on the droplet required to achieve the desired average EUV power is determined. Then, in step S120, a command is generated to determine the characteristics of the RF excitation signal of the laser, thereby controlling the energy in the laser main pulse. This command is provided to the laser. As mentioned above, the characteristics may include one or both of the amplitude and pulse width of the RF excitation signal.

[0078] While steps S110 and S120 are being performed, in the lower control branch in step S130, the accumulated EUV energy on the droplet required to achieve the desired average EUV power is determined. A command is then generated in step S140 to determine the hit / miss firing pattern of the laser, which in turn will control the total energy of the laser main pulse within a predefined time window. This command is provided to a timing circuit that controls the timing of the laser switching pulses. This command may alternatively or additionally be provided to a beam steering module that controllably steers the main switching pulses toward or away from the irradiation zone.

[0079] In step S150, the laser fires with the commanded RF excitation and in accordance with the commanded hit / miss firing pattern, i.e., a hit or miss for a given pulse. When this occurs, in step S160, the EUV energy generated by the conversion pulse is measured and fed back for use in steps S110 and 130. In addition, according to an aspect of one embodiment, in step S170, instabilities in the plasma that may be exacerbated by certain hit / miss trigger patterns are measured, and this information is also fed back to step S140 to be used in determining which hit / miss trigger pattern to use.

[0080] Additionally, as indicated, in step S180, information regarding the characteristics of the slit filter (such as its spectral response or identified emission modes that are incompatible with the slit filter) may be provided to determine which emission mode to use in step S140.

[0081] exist Figure 5 In the example of , an interaction between step S120 and step S140 is also provided, that is, if it is determined in step S140 that the next shot will miss, then in step S120, it can be determined to reduce the RF excitation energy provided to the laser to, for example, about 60%. In addition, the system can be configured so that only the results of the laser shots (hits) on the droplets contribute to the measurement of the average and / or cumulative and / or per-droplet EUV energy output.

[0082] Therefore, according to one aspect of one embodiment, the hybrid controller includes two control loops. One of the control loops (the hit / miss shot pattern control loop) operates to ensure stability of the average EUV energy that controls the hit / miss shot pattern. The other control loop (the laser pulse power control loop) operates to control the energy of the individual EUV pulses to a desired level by controlling the laser power. The system also has the ability to adjust the hit / miss shot pattern to avoid harmful hit / miss shot patterns. The hit / miss shot pattern control loop calculates the output of the hit / miss droplet command to minimize dose error.

[0083] According to another aspect of one embodiment, when the hit / miss shot mode control loop commands a miss on the current droplet, the laser pulse power control loop can be disabled. This results in a zero error input for laser shots (misses) outside the droplet. This will drive the energy of laser shots (hits) on the droplet only to the desired value.

[0084] According to another aspect of one embodiment, the actual target value of the average power of the laser pulse power control loop can be set higher than the command target value of the average power of the laser pulse power control loop. For example, the actual target value of the laser pulse power control loop can be set higher than the command target value of the laser pulse power control loop at a fixed ratio. In other words, the ratio of the actual target value to the command target value will be maintained at a fixed value greater than 1. This will provide some room for adjusting the hit / miss pattern to avoid harmful hit / miss patterns.

[0085] The present disclosure is carried out by means of functional building blocks, which illustrate the realization of specified functions and the mutual relationship between these functions. For ease of description, the boundaries of these functional building blocks are arbitrarily defined in this article. The boundary of substitution can be defined, as long as the specified functions are properly performed and their mutual relationship is maintained.

[0086] The above description includes examples of one or more embodiments. Of course, it is not possible to describe all conceivable combinations of components or methods for describing the aforementioned embodiments, but those of ordinary skill in the art will recognize that many further combinations and arrangements of various embodiments are possible. Therefore, the described embodiments are intended to cover all such changes, modifications, and variations that belong to the spirit and scope of the appended claims. In addition, to the extent that the term "includes" is used in the specific embodiments or claims, such term is intended to be inclusive in a manner similar to the term "comprising" because "comprising" should be interpreted when used as a transitional word in the claims. In addition, although the elements of the aspects and / or embodiments may be described or stated in the singular, the plural form may be considered unless explicitly stated or clearly limited to the singular from the context. In addition, all or part of any aspect and / or embodiment may be used together with all or part of any other aspect and / or embodiment, unless otherwise stated.

[0087] The implementations may be further described using the following terms.

[0088] 1. A system for modulating the power output of a source of a plurality of extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator and a laser, the droplet generator being adapted to generate a plurality of droplets of a target material, and the laser being arranged to generate a plurality of conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, the system comprising:

[0089] An EUV pulse energy control loop adapted to keep the energy of individual pulses of the plurality of EUV pulses within a defined energy value range; and

[0090] A firing mode control loop is adapted to determine which individual ones of the droplets are illuminated by at least one associated one of the transition pulses.

[0091] 2. The system of clause 1, wherein the energy of a conversion pulse in the plurality of conversion pulses depends at least in part on an electrical characteristic of an RF excitation signal applied to the laser, and wherein the EUV pulse energy control loop comprises a module for controlling the electrical characteristic of the RF excitation signal.

[0092] 3. The system of clause 2, wherein the electrical characteristic of the RF excitation signal comprises an amplitude of the RF excitation signal.

[0093] 4. The system of clause 2, wherein the electrical characteristic of the RF excitation signal comprises a pulse width of the RF excitation signal.

[0094] 5. The system of clause 1, wherein the emission mode control loop comprises a trigger signal module for controlling the timing of applying a trigger signal to the laser, wherein when the trigger signal is applied to the laser, the trigger signal causes the laser to emit a switching pulse.

[0095] 6. A system according to claim 1, wherein the emission mode control loop includes a steering module, which is used to steer the conversion pulse emitted by the laser, and when the emission mode control loop command hits, the steering module causes the conversion pulse to hit a droplet among the multiple droplets in the irradiation area, and when the emission mode control loop command misses, the steering module causes the conversion pulse to miss the droplets in the irradiation area.

[0096] 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 emission mode control loop is responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma above a predetermined threshold.

[0097] 8. The system of clause 1, wherein the EUV pulse energy control loop is adapted to be responsive to the shot mode control loop such that if the shot mode control loop commands that the next shot will be a miss, the EUV pulse energy control loop reduces the energy in the RF excitation signal applied to the laser for the next shot.

[0098] 9. The system of clause 1, wherein the EUV pulse energy control loop receives a command target value for EUV average energy and uses the command target value to set an actual target value for EUV average energy that is greater than the command target value.

[0099] 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 1.

[0100] 11. A system for controlling the average power output of a source of multiple extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator, a laser, and an EUV energy measurement sensor, the droplet generator being adapted to generate multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to illuminate an illumination zone through which individual droplets of the droplets pass, and the EUV energy measurement sensor being adapted to measure the output energy per droplet generated by illuminating the individual droplets and to generate an output energy per droplet measurement signal, the system comprising:

[0101] a first control subsystem responsive to the per-droplet output power measurement signal and adapted to maintain individual pulse energies of the EUV pulses within a defined range by controlling at least one system variable affecting the power of the laser; and

[0102] A second control subsystem is responsive to the per-drop output power measurement signal and is adapted to determine which individual ones of the transition pulses are to be used to illuminate corresponding individual ones of the droplets in the illumination zone by controlling the timing of the transition pulses.

[0103] 12. The system of clause 11, wherein the at least one system variable affecting the power of the laser comprises an amplitude of an RF excitation signal.

[0104] 13. The system of clause 11, wherein the at least one system variable affecting the power of the laser comprises a pulse width of an RF excitation signal.

[0105] 14. The system of clause 11, wherein the second control subsystem comprises a trigger signal module for controlling the timing of applying a trigger signal to the laser, the trigger signal causing the laser to emit a switching pulse when the trigger signal is applied to the laser.

[0106] 15. A system according to claim 11, wherein the second control subsystem includes a steering module, which is used to steer a conversion pulse among the multiple conversion pulses emitted by the laser, and when the emission mode control loop command hits, the steering module causes the conversion pulse to hit the droplets in the irradiation area, and when the emission mode control loop command misses, the steering module causes the conversion pulse to miss the droplets in the irradiation area.

[0107] 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 also responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma above a predetermined threshold.

[0108] 17. A system according to clause 11, wherein the first control subsystem is adapted to be responsive to the second control subsystem such that if the second control subsystem commands that the next shot will be a miss, the first control subsystem reduces the energy in the RF excitation signal applied to the laser for the next shot.

[0109] 18. The system of clause 11, wherein the first control subsystem receives a command target value for EUV average energy and uses the command target value to set an actual target value for EUV average energy that is greater than the command target value.

[0110] 19. The system of clause 18, wherein the first control subsystem maintains a ratio of the command target value to the actual target value at a fixed value greater than 1.

[0111] 20. A method of controlling a source of a plurality of extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator and a laser, the droplet generator being adapted to generate a plurality of droplets of a target material, and the laser being arranged to generate a plurality of conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, the method comprising:

[0112] maintaining the energy of individual ones of the EUV pulses within a defined energy value range using a first control subsystem; and

[0113] A second control subsystem is used to determine which individual ones of the transition pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone.

[0114] 21. A method of controlling a source of multiple extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator, a laser, and an on-droplet EUV energy measurement sensor, the droplet generator being adapted to generate multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, and the on-droplet EUV energy measurement sensor being adapted to measure a per-droplet output power generated by illuminating the individual droplets and to generate a per-droplet output power measurement signal, the laser output power of the laser being at least partially determined by a radio frequency (RF) excitation signal applied to the laser, the method comprising:

[0115] maintaining the 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

[0116] It is determined which individual ones of the conversion pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone.

[0117] 22. The method of clause 21, wherein the electrical characteristic of the RF excitation signal comprises an amplitude of the RF excitation signal.

[0118] 23. The method of clause 21, wherein the electrical characteristic of the RF excitation signal comprises a pulse width of the RF excitation signal.

[0119] 24. A method according to clause 21, wherein determining which individual ones of the conversion pulses will be used to irradiate corresponding individual ones of the droplets in the irradiation area includes: controlling the timing of applying a trigger signal to the laser, when the trigger signal is applied to the laser, the trigger signal causes the laser to emit a conversion pulse.

[0120] 25. A method according to clause 21, wherein determining which individual ones of the conversion pulses will be used to irradiate corresponding individual ones of the droplets in the irradiation area includes: commanding a steering module to cause the conversion pulses to impact the droplets in the irradiation area to cause a hit and to cause the conversion pulses to miss the droplets in the irradiation area to cause a miss.

[0121] 26. The method of clause 21 further comprises sensing a degree of instability in the plasma generated by conversion of one or more of the droplets, wherein determining which individual of the conversion pulses will be used to irradiate corresponding individual droplets of the droplets in the irradiation zone is based at least in part on the degree of instability to avoid an emission pattern that increases the degree of instability in the plasma to exceed a predetermined threshold.

[0122] 27. The method of clause 21, wherein maintaining the energy of individual ones of the EUV pulses within the 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.

[0123] 28. The method of clause 21, wherein maintaining the energy of individual ones of the EUV pulses within the defined range comprises setting an actual target value of the EUV average energy to be greater than a commanded target value of the EUV average energy.

[0124] 29. The method of clause 28, wherein setting the actual target value of EUV average energy to be greater than the commanded target value of EUV average energy comprises setting a ratio of the commanded target value to the actual target value to a fixed value greater than 1.

[0125] The implementations described above and other implementations are within the scope of the following claims.

Claims

1. A system for modulating the power output of a source of a plurality of extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator and a laser, the droplet generator being adapted to generate a plurality of droplets of a target material, and the laser being arranged to generate a plurality of conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, the system comprising: an EUV pulse energy control loop adapted to maintain the energy of individual pulses of the plurality of EUV pulses within a defined energy value range; as well as A firing mode control loop is adapted to determine which individual ones of the droplets are illuminated by at least one associated one of the transition pulses.

2. The system of claim 1 , wherein the energy of a conversion pulse in the plurality of conversion pulses depends at least in part on an electrical characteristic of an RF excitation signal applied to the laser, and wherein the EUV pulse energy control loop comprises a module for controlling the electrical characteristic of the RF excitation signal.

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

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

5. The system of claim 1 , wherein the emission mode control loop comprises a trigger signal module for controlling the timing of applying a trigger signal to the laser, wherein when the trigger signal is applied to the laser, the trigger signal causes the laser to emit a switching pulse.

6. The system of claim 1 , wherein the emission mode control loop comprises a steering module, the steering module being configured to steer a switching pulse emitted by the laser, wherein when the emission mode control loop commands a hit, the steering module causes the switching pulse to hit a droplet among the plurality of droplets in the irradiation area, and when the emission mode control loop commands a miss, the steering module causes the switching pulse to miss a droplet in the irradiation area.

7. The system of claim 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 emission mode control loop is responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma above a predetermined threshold.

8. The system of claim 1 , wherein the EUV pulse energy control loop is adapted to be responsive to the shot mode control loop such that if the shot mode control loop commands that the next shot will be a miss, the EUV pulse energy control loop reduces the energy in the RF excitation signal applied to the laser for the next shot. 9 . The system of claim 1 , wherein the EUV pulse energy control loop receives a command target value for EUV average energy and uses the command target value to set an actual target value for EUV average energy that is greater than the command target value. 10 . The system of claim 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 1.

11. A system for controlling the average power output of a source of multiple extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator, a laser, and an EUV energy measurement sensor, the droplet generator being adapted to generate multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to illuminate an illumination zone through which individual droplets of the droplets pass, and the EUV energy measurement sensor being adapted to measure the output energy per droplet generated by illuminating the individual droplets and to generate an output energy per droplet measurement signal, the system comprising: a first control subsystem responsive to the per-droplet output power measurement signal and adapted to maintain individual pulse energies of the EUV pulses within a defined range by controlling at least one system variable affecting the power of the laser; as well as A second control subsystem is responsive to the per-drop output power measurement signal and is adapted to determine which individual ones of the transition pulses are to be used to illuminate corresponding individual ones of the droplets in the illumination zone by controlling the timing of the transition pulses.

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

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

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

15. The system according to claim 11, wherein the second control subsystem comprises a steering module, the steering module being used to steer a conversion pulse among the plurality of conversion pulses emitted by the laser, and when the emission mode control loop command is hit, the steering module causes the conversion pulse to hit the droplets in the irradiation area, and when the emission mode control loop command is not hit, the steering module causes the conversion pulse to not hit the droplets in the irradiation area.

16. The system of claim 11, further comprising a plasma sensor arranged to sense a degree of instability in a plasma generated by the conversion of one or more droplets, wherein the second control subsystem is further responsive to the plasma sensor to avoid commanding an emission mode that increases the degree of instability in the plasma above a predetermined threshold.

17. The system of claim 11 wherein the first control subsystem is adapted to be responsive to the second control subsystem such that if the second control subsystem commands that the next shot will be a miss, the first control subsystem reduces the energy in the RF excitation signal applied to the laser for the next shot. 18 . The system of claim 11 , wherein the first control subsystem receives a command target value for EUV average energy and uses the command target value to set an actual target value for EUV average energy that is greater than the command target value.

19. The system of claim 18, wherein the first control subsystem maintains a ratio of the command target value to the actual target value at a fixed value greater than 1.

20. A method of controlling a source of a plurality of extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator and a laser, the droplet generator being adapted to generate a plurality of droplets of a target material, and the laser being arranged to generate a plurality of conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, the method comprising: maintaining the energy of individual EUV pulses of the EUV pulses within a defined energy value range using a first control subsystem; as well as A second control subsystem is used to determine which individual ones of the transition pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone.

21. A method of controlling a source of multiple extreme ultraviolet (EUV) pulsed radiation, the source comprising a droplet generator, a laser, and an on-droplet EUV energy measurement sensor, the droplet generator being adapted to generate multiple droplets of a target material, the laser being arranged to generate multiple conversion pulses to illuminate an illumination zone through which individual ones of the droplets pass, and the on-droplet EUV energy measurement sensor being adapted to measure a per-droplet output power generated by illuminating the individual droplets and to generate a per-droplet output power measurement signal, the laser output power of the laser being at least partially determined by a radio frequency (RF) excitation signal applied to the laser, the method comprising: maintaining the energy of individual EUV pulses of the EUV pulses within a defined range by controlling at least one electrical characteristic of the RF excitation signal; as well as It is determined which individual ones of the conversion pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone.

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

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

24. The method of claim 21 , wherein determining which individual ones of the conversion pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone comprises: The timing of applying a trigger signal to the laser is controlled, and when the trigger signal is applied to the laser, the trigger signal causes the laser to emit a switching pulse.

25. The method of claim 21 , wherein determining which individual ones of the conversion pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone comprises: A steering module is commanded to cause the transition pulse to strike a droplet in the impingement area to cause a hit and to cause the transition pulse to miss a droplet in the impingement area to cause a miss.

26. The method of claim 21 , further comprising sensing a degree of instability in the plasma generated by conversion of one or more of the droplets, wherein determining which individual ones of the conversion pulses are to be used to irradiate corresponding individual ones of the droplets in the irradiation zone is based at least in part on the degree of instability to avoid an emission pattern that increases the degree of instability in the plasma above a predetermined threshold.

27. The method of claim 21 , wherein maintaining the energy of individual ones of the EUV pulses within a defined range comprises: If the next shot is a miss, the energy in the RF excitation signal applied to the laser for the next shot is reduced.

28. The method of claim 21 , wherein maintaining the energy of individual ones of the EUV pulses within a defined range comprises: The actual target value of EUV average energy is set to be greater than the command target value of EUV average energy.

29. The method of claim 28, wherein setting the actual target value of EUV average energy to be greater than the command target value of EUV average energy comprises: The ratio of the command target value to the actual target value is set to a fixed value greater than 1.

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