CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/292,150, filed December 21, 2021, titled TARGET SUPPLY CONTROL APPARATUS AND METHOD IN AN EXTREME ULTRAVIOLET LIGHT SOURCE, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The disclosed subject matter relates to an apparatus and method for tuning characteristics of targets delivered to a target space of a laser produced plasma extreme ultraviolet light source.

BACKGROUND

[0003] Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element with an emission line in the EUV range, for example, xenon, lithium, or tin, into a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

[0005] In some general aspects, an apparatus includes: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to the target material in the target material dispenser and configured to induce velocity perturbations in the stream of targets based on a target control signal; a detection apparatus configured to observe the targets in the stream at a point in the stream at which at least some of the targets have partly coalesced but not all of the targets have fully coalesced, and to generate a target detection signal based on the observation; a controller configured to receive the target detection signal and generate a waveform control signal based at least in part on the target detection signal; and a waveform generator in communication with the actuatable element and the controller. The waveform generator is configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal, the target control signal comprising a hybrid waveform including a superposition of a plurality of periodic waveforms.

[0006] Implementations can include one or more of the following features. For example, actuatable element can include an electro-actuatable element. The electro- actuatable element can include a piezoelectric element that is mechanically coupled to a capillary tube of the target material dispenser, the target material passing through the capillary tube.

[0007] The detection apparatus can be configured to detect targets in the stream having at least two distinct sizes. The detection apparatus can be configured to detect at least a spatial offset between at least two of the targets in the stream.

[0008] The controller generating the waveform control signal based at least in part on the target detection signal can include the controller adjusting one or more parameters of the waveform control signal. The controller adjusting one or more parameters of the waveform control signal can occur while the plasma generating system generates plasma from the targets. The controller adjusting one or more parameters of the waveform control signal can occur while the plasma generating system produces extreme ultraviolet light that is supplied to a substrate at a photolithography exposure apparatus.

[0009] The controller being configured to generate the waveform control signal based at least in part on the target detection signal can include the controller analyzing asymmetry of peaks associated with targets that have not coalesced relative to peaks associated with targets that have partly coalesced in the target detection signal. The asymmetry of the peaks can determine a coalescence length of the target stream. The controller being configured to generate the waveform control signal based at least in part on the target detection signal can include the controller adjusting one or more of an amplitude and a phase of at least one of the periodic waveforms of the target control signal.

[0010] Each periodic waveform can have a distinct frequency. The detection apparatus can include an illumination source and a light sensitive sensor arranged relative to the target stream.

[0011] In other general aspects, an apparatus includes: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to target material in the target material dispenser and configured to induce velocity perturbations in the target stream based on a target control signal; a detection apparatus configured to observe the targets in the stream, and to generate a target detection signal based on the observation; a controller configured to receive the target detection signal, analyze both a spacing and a sizing of peaks in the target detection signal, and generate a waveform control signal including updating parameters of the waveform control signal relating to one or more coalescence lengths in the target stream based at least in part on the analysis; and a waveform generator in communication with the actuatable element and the controller. The waveform generator is configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal. [0012] Implementations can include one or more of the following features. For example, target control signal can include a hybrid waveform including a superposition of a plurality of periodic waveforms.

[0013] The actuatable element can include a piezoelectric element that is mechanically coupled to a capillary tube of the target material dispenser, the target material passing through the capillary tube. [0014] The controller being configured to generate the waveform control signal including updating the parameters relating to one or more coalescence lengths can occur while the plasma generating system generates plasma from the targets. The controller can be configured to generate the waveform control signal by updating the parameters relating to one or more coalescence lengths by analyzing asymmetry of peaks associated with smaller targets relative to a peak associated with a larger target. The controller can be configured to generate the waveform control signal including updating the parameters relating to one or more coalescence lengths by adjusting one or more of an amplitude and a phase of at least one of the periodic waveforms of the target control signal.

[0015] The detection apparatus can include an illumination source and a light sensitive sensor arranged relative to the target stream.

[0016] In other general aspects, an apparatus includes: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to target material in the target material dispenser and configured to induce velocity perturbations in the target stream based on an amplitude of a target control signal; and a waveform generator in communication with the actuatable element. The waveform generator is configured to supply the target control signal to the actuatable element, the target control signal comprising a hybrid waveform including a superposition of a first periodic waveform, a second periodic waveform, and a third periodic waveform.

[0017] Implementations can include one or more of the following features. For example, the actuatable element can include an electro-actuatable element that is mechanically coupled to the target material in the target material dispenser. The waveform generator can be electrically coupled to the electro- actuatable element. The electro-actuatable element can include a piezoelectric element that is mechanically coupled to a capillary tube through which the target material is passed.

[0018] The apparatus can also include a detection apparatus configured to observe targets in the stream, one or more having not coalesced, one or more having once coalesced, and one or more having twice coalesced, and to generate a target detection signal based on the observation. The detection apparatus can detect targets in the stream having at least three distinct sizes.

[0019] The apparatus can also include a controller configured to adjust one or more parameters of a waveform control signal based on the generated target detection signal, the waveform generator being configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal. The controller can adjust one or more parameters of the waveform control signal while the plasma generating system generates plasma from the targets. The controller being configured to adjust the one or more parameters of the waveform control signal can include the controller analyzing asymmetry of peaks associated with the one or more targets that have not coalesced and/or have once coalesced relative to a peak associated with a target that has twice coalesced. The asymmetry of the peaks associated with the one or more targets that have not coalesced can determine a first coalescence length of the target stream and the asymmetry of the peaks associated with the one or more targets that have once coalesced can determine a second coalescence length of the target stream. The controller being configured to adjust one or more parameters of the waveform control signal based on the target detection signal can include the controller adjusting a first coalescence length based on a first set of properties of the target detection signal and adjusting a second coalescence length that is distinct from the first coalescence length based on a second set of properties of the target detection signal that are distinct from the first set of properties of the target detection signal. The controller being configured to adjust one or more parameters of the waveform control signal can include the controller adjusting one or more of an amplitude and a phase of at least two of the periodic waveforms of the target control signal.

[0020] Each periodic waveform can have a distinct frequency.

[0021] In other general aspects, a method includes: providing a stream of targets made of target material to an irradiation site; observing targets in the target stream at a point in the stream of targets at which at least some of the targets have partly coalesced but not all of the targets have fully coalesced; generating a target detection signal based on the observation; generating a target control signal based at least in part on the target detection signal, the generated target control signal comprising a hybrid waveform including a superposition of a plurality of periodic waveforms; and inducing velocity perturbations in the stream of targets based on the target control signal.

[0022] Implementations can include one or more of the following features. For example, the target control signal can be generated by adjusting one or more parameters of the target control signal. The one or more parameters of the target control signal can be adjusted while the targets are being provided to the irradiation site.

[0023] The target control signal can be generated by analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal. The asymmetry of the peaks can determine a coalescence length of the target stream. The asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal can be analyzed by analyzing a difference in spacing or time between two partly coalesced targets and analyzing a relative position between a partly coalesced target and a half-way position between two consecutive partly coalesced targets.

[0024] The target control signal can be generated by adjusting one or more of an amplitude and a relative phase of the periodic waveforms of the target control signal. The targets in the target stream can be observed by observing a light produced from an interaction between an illumination beam and the targets in the target stream.

[0025] In other general aspects, a method includes: providing a stream of targets made of target material to an irradiation site for a plasma generating system; observing targets in the target stream; generating a target detection signal based on the observation; analyzing both a spacing and a sizing of peaks in the target detection signal; generating a target control signal including updating parameters relating to one or more coalescence lengths in the target stream based at least in part on the analysis; and inducing velocity perturbations in the target stream based on the target control signal.

[0026] Implementations can include one or more of the following features. For example, all of the following — both the spacing and the sizing of the peaks in the target detection signal can be analyzed, the target control signal can be generated, and the velocity perturbations can be induced — can occur during an inline tuning mode while an extreme ultraviolet light beam is produced at the plasma generating system and is supplied to a substrate at a photolithography exposure apparatus. During the inline tuning mode, generating the target control signal including updating parameters relating to one or more coalescence lengths in the target stream can include inducing velocity perturbations in the target stream that cause the targets to at least partly coalesce but not fully coalesce prior to observing targets in the target stream.

[0027] All of the following — both the spacing and the sizing of the peaks in the target detection signal can be analyzed, the target control signal can be generated, and the velocity perturbations in the target stream can be induced - can occur during an offline tuning mode in between exposing a substrate with an extreme ultraviolet light beam that is produced at the plasma generating system. During the offline tuning mode, generating the target control signal including updating parameters relating to one or more coalescence lengths in the target stream can include inducing velocity perturbations in the target stream that cause the targets to at least partly coalesce or fully coalesce prior to observing targets in the target stream.

[0028] In other general aspects, a method includes: providing a stream of targets made of target material to an irradiation site for a plasma generating system; generating a target control signal comprising a hybrid waveform including a superposition of a first periodic waveform, a second periodic waveform, and a third periodic waveform; and inducing velocity perturbations in the target stream based on the target control signal.

[0029] Implementations can include one or more of the following features. For example, velocity perturbations in the target stream can be induced by setting one or more coalescence lengths in the target stream. The velocity perturbations in the target stream can be induced during an inline tuning mode while an extreme ultraviolet light beam is produced at the plasma generating system and is supplied to a substrate at a photolithography exposure apparatus. During the inline tuning mode, inducing velocity perturbations in the target stream can be based on an observation of targets in the stream, and the induction of velocity perturbations can cause the targets to at least partly coalesce but not fully coalesce prior to observing the targets in the target stream.

[0030] The velocity perturbations in the target stream can be induced during an offline tuning mode in between exposing a substrate with an extreme ultraviolet light beam that is produced at the plasma generating system. During the offline tuning mode, inducing velocity perturbations in the target stream can be based on an observation of targets in the stream, and the induction of velocity perturbations can cause the targets to at least partly coalesce or fully coalesce prior to observing the targets in the target stream.

[0031] The target control signal can be generated by analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal. The asymmetry of the peaks can determine a coalescence length of the target stream.

[0032] The asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal can be analyzed by: analyzing a difference in spacing or time between two partly coalesced targets and analyzing a relative position between a partly coalesced target and a half-way position between two consecutive partly coalesced targets; and analyzing a difference in spacing or time between a target that has not coalesced and an adjacent target that has partly coalesced.

[0033] The apparatus is able to confirm proper operation of the target material dispenser by determining whether targets are fully coalesced by the time they reach the irradiation region. This can be accomplished with an optical feedback system that can identify whether or not a certain electrical waveform supplied to the target material dispenser results in coalesced targets at the irradiation region.

DESCRIPTION OF DRAWINGS

[0034] Fig. 1 is a schematic diagram of an apparatus that is arranged relative to a chamber of an extreme ultraviolet (EUV) light source, the apparatus including a target material dispenser, an actuatable element coupled to target material within the target material dispenser, a detection apparatus, a controller, and a waveform generator;

[0035] Fig. 2 A is a schematic diagram of an implementation of the apparatus of Fig. 1, showing details of the detection apparatus, the actuatable element, and the target material dispenser;

[0036] Fig. 2B is a schematic diagram showing a closer view of targets produced in the apparatus of Fig. 2A;

[0037] Fig. 2C is a schematic diagram illustrating coalescence of targets as a function of time and distance from the target material dispenser;

[0038] Fig. 3 is a block diagram of an implementation of the controller of Fig. 1 ; [0039] Fig. 4 A is a graph of an example of a composite waveform that can be supplied to the actuatable element, the composite waveform including a fundamental waveform that is a sine wave and a higher frequency waveform that is a square waveform;

[0040] Fig. 4B is a graph of an example of a composite waveform that can be supplied to the actuatable element, the composite waveform including a fundamental waveform that is a square wave and a higher frequency waveform that is a sine wave;

[0041] Fig. 4C is a graph of an example of a composite waveform that can be supplied to the actuatable element, the composite waveform including a fundamental waveform that is a sine wave and a higher frequency waveform that is also a sine wave;

[0042] Fig. 4D is a graph of the example of the composite waveform of Fig. 4 A, in which a phase between the fundamental waveform (the sine wave) and the higher frequency waveform (the square waveform) is varied;

[0043] Fig. 4E is a graph of an example of a composite waveform that can be supplied to the actuatable element, the composite waveform including a fundamental waveform that is a sine wave, a first higher frequency waveform that is a sine wave, and a second higher frequency waveform that is a square waveform;

[0044] Fig. 5A is a flow chart of a procedure performed to generate or control a desired stream of targets produced by the apparatuses of Figs. 1 and 2 A;

[0045] Fig. 5B is a flow chart of a procedure performed to generate a target control signal for use by the actuatable element based on a target detection signal supplied from the detection apparatus;

[0046] Fig. 6A is a graph of an example of a coalescence process and position distribution of targets moving along the -X direction at different distances from the end of the target material dispenser;

[0047] Fig. 6B is a graph of an example of a coalescence process and position distribution of targets moving along the -X direction at different distances from the end of the target material dispenser showing the output (the target detection signal) from the detection apparatus taken at different distances from the target material dispenser;

[0048] Fig. 7A is an example of a graph of a size of the targets versus spacing between the targets at a particular distance from the target material dispenser;

[0049] Fig. 7B is the corresponding output (the target detection signal) from the detection apparatus that produces the graph of Fig. 7A;

[0050] Fig. 8 A is an example of a graph of a size of the targets versus spacing between the targets at a particular distance from the target material dispenser;

[0051] Fig. 8B is the corresponding output (the target detection signal) from the detection apparatus that produces the graph of Fig. 8A; and

[0052] Fig. 9 is a schematic diagram of the apparatus of Fig. 1 implemented in an extreme ultraviolet (EUV) light source that produces an EUV light beam for use by an output device. DESCRIPTION

[0053] Referring to Fig. 1, an apparatus 100 includes a target material dispenser 105 that delivers a stream of targets 111 to an irradiation region 118 within a chamber 120 of an extreme ultraviolet (EUV) light source, the chamber 120 being defined by one or more walls 122. The targets 111 generally travel along a -X direction of the chamber 120, however, it is also possible for the targets 111 to have motion along a direction perpendicular to the -X direction (such as along the YZ plane of the chamber 120). The distance along the -X direction is measured from the exit of the target material dispenser 105.

[0054] The apparatus 100 includes a waveform generator 140 configured to generate and provide a target control signal 142 to an actuatable element 125 that is coupled to target material 106 within the target material dispenser 105. The target control signal 142 is a hybrid waveform that includes a superposition of a plurality of periodic waveforms. The actuatable element 125 is coupled to the target material 106 either by being in direct contact with the target material 106 or indirectly by way of another component that is directly in contact with the target material 106. The targets 111 are made up of the target material 106. The actuatable element 125 induces velocity perturbations into the target stream 110 based on the target control signal 142. The waveform generator 140 operates under the control of a controller 135 at least partially on the basis of data (a target detection signal) 132 from a detection apparatus 130. The detection apparatus 130 is configured to observe the targets 111 in the target stream 110 and to generate the target detection signal 132 based on the observation. The controller 135 receives this target detection signal 132 and generates a waveform control signal 137 based at least in part on the target detection signal 132. The waveform control signal 137 includes instructions for the waveform generator 140 regarding how to create the target control signal 142. [0055] The targets 111 in the stream can be made up of targets of various states of coalescence. The coalescence can arise from the above-mentioned velocity perturbations induced into the target stream by actuatable element 125. Because of the velocity perturbations, neighboring targets can travel at different speeds. And, as they travel, faster targets can merge with slower targets ahead of them. For example, when the target material dispenser 105 is operating as intended, a stream of pre-coalesced targets 112 are emitted, and the pre-coalesced targets 112 combine or coalesce with each other and form partially coalesced targets 114. The partially coalesced targets 114 combine or coalesce with each other and form a stream of fully coalesced targets 116. The velocity perturbations can be periodic in time, so that the pattern of coalescence can result in the fully coalesced targets 116 being evenly spaced after some sufficient distance from the material dispenser 105. The fully coalesced targets 116 are formed and then travel a further distance to reach the irradiation region 118. For example, the irradiation region 118 can be a distance DI 18 from an exit 107 of the target material dispenser 105. The targets 111 should be fully coalesced before an impact region that extends from the irradiation region 118, such impact region corresponding to the region where the flow or movement of the targets 111 can be impacted by plasma and other fluid flow that occurs at the irradiation region 118. This impact region can extend toward the target material dispenser 105 along the X direction a distance DI. In some implementations, the distance DI 18 is about 700-1000 mm while the distance DI along which the impact region extends is about 100-200 mm, and thus the targets 111 should be fully coalesced targets 116 about 400-500 mm from the exit of the target material dispenser 105.

[0056] When each fully coalesced target 116 reaches the irradiation region 118, as discussed below, a light pulse illuminates the fully coalesced target 116 that is in the irradiation region 118. The detection apparatus 130 is configured to detect and/or image targets 111 at a point in the stream 110 at which pre-coalesced targets 112 and/or partially coalesced targets 114 have formed, but not all of the targets 111 have fully coalesced to form the fully coalesced targets 116.

[0057] By detecting some targets 111 that have partly coalesced (the targets 112 or 114) but also detecting the targets 111 before all of them have fully coalesced (into the targets 116), the apparatus 100 is able to glean information about partial coalescence within the target stream 110 and because this information is obtained, the apparatus 100 is able to adjust or control the point or location at which the targets 111 partly or fully coalesce. Additionally, the apparatus 100 observes the targets 111 as they are coalescing, which permits the observation and detection of the pre-coalesced targets 112 and the partly coalesced targets 114 and before they have fully coalesced at a point at which the flow of the targets 111 is less likely to be disrupted due to other fluid dynamics within the chamber 120, thus making it easier to detect or image the targets 111. In some implementations, the detection apparatus 130 is configured to observe targets 111 at a distance from the exit 107 of the target material dispenser 105 that is about 5% to about 60% of the total distance from the exit 107 of the target material dispenser 105 to the irradiation region 118 (the distance or length being measured along the -X direction). For example, for an implementation in which the irradiation region 118 is placed about 600-700 mm from the exit 107 of the target material dispenser 105 (along the -X direction), the detection apparatus 130 can be configured to observe the targets 111 between about 50 mm and about 400 mm after their exit from the target material dispenser 105.

[0058] In various implementations, the apparatus 100 is able to perform the control of the location at which the targets 111 partly or fully coalesce without relying on a measurement of a transfer function of the target material dispenser 105. Moreover, the apparatus 100 can be configured to perform the partial and/or full coalescence control while the extreme ultraviolet (EUV) light is being produced at the irradiation region 118 due to the interaction between the light pulse and the target 116 at the irradiation region 118. In essence, the detection apparatus 130 detects, observes, senses, or images targets of different sizes such as the pre-coalesced targets 112 and the partially coalesced targets 114. Therefore, the controller 135 analyzes not only the spacing (such as a distance or a spatial offset between two of the targets 111 such as adjacent targets 111) or location of the targets 111 that are detected by the detection apparatus 130 but also the size of the targets 111 by analyzing the size of peaks within the target detection signal 132. Because targets of different coalescence states are imaged by the detection apparatus 130 and analyzed by the controller 135, it is further possible to more finely tune the target control signal 142 and adjust or control more than one coalescence length. Namely, the apparatus 100 is able to control different coalescence lengths by configuring a target control signal 142 that is a hybrid waveform that includes a superposition of three periodic waveforms (instead of just two periodic waveforms). The coalescence length is the length along the -X direction relative to the output of the target material dispenser 105 at which a coalescence occurs. A partial coalescence length is that length or distance at which the partial coalescence occurs while the full coalescence length is that length or distance at which the full coalescence occurs.

[0059] With these improvements, the apparatus 100 performs the partial and/or full coalescence control with less sensitivity to noise in the detection apparatus 130. The apparatus 100 may also be configured to make adjustments to the target control signal 142 without having to halt the production of EUV light and without having to perform a tuning calibration for the coalescence lengths of the targets 111. The apparatus 100 is able to track the coalescence lengths as the dynamics of the target material dispenser 105 are varied while reducing or eliminating pre- or partly coalesced targets (referred to as satellites or satellite targets) at the irradiation region 118.

[0060] In general, the control of the coalescence process of the targets 111 involves controlling the targets 111 such that they coalesce sufficiently before reaching the irradiation region 118 and have a frequency corresponding to the pulse rate of the light pulses that are used to irradiate the targets at the irradiation region 118. The target control signal 142 can be made up of separate periodic waveforms, thus permitting the phase and frequency of each waveform to be individually adjusted or tuned. The target control signal 142 can be defined as a voltage or a current signal. In one particular implementation, the target control signal 142 is made up of two periodic waveforms, that is, a first periodic waveform at a fundamental frequency that is substantially equal to the pulse rate of the light pulse that illuminates the irradiation region 118, and a second periodic waveform at a frequency that is greater than the fundamental frequency. For example, if the light pulses arrive at the irradiation region 118 at a rate of 50 kiloHertz (kHz) then the fundamental frequency is 50 kHz, and the second periodic waveform can have a frequency of 500 kHz. The second periodic waveform having the 500 kHz frequency can generate the stable partially coalesced targets 114, and the process of merging or coalescing these targets 114 into the fully coalesced targets 116 can be controlled by way of the amplitude of the first periodic waveform and the phase of the second periodic waveform. As the dynamics of the target material dispenser 105 drift over time, these particular parameters of the first and second periodic waveforms need to be adjusted or recalibrated to ensure the coalescence length is stable. [0061] The target material 106 that makes up the targets 111 is a material that produces EUV light when converted to a plasma by the light pulse at the irradiation region 118. In this way, components of the apparatus 100 operate as a plasma generating system, in which light (such as the light pulses at the irradiation region 118) interacts with material (such as the targets 111) to form a plasma. The target material 106 can include, but is not limited to, tin, lithium, xenon, or combinations of these elements. The targets 111 can be in the form of liquid droplets and/or solid particles contained within liquid droplets. For example, the element tin can be used as pure tin; as a tin compound such as SnBr4, SnBrj, SnFU; or as a tin alloy, such as tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination of these alloys. Depending on the target material 106 used, the targets 111 can be presented to the irradiation region 118 at any suitable temperature including room temperature or near room temperature (for tin alloys or SnBri), at an elevated temperature (for pure tin), or at temperatures below room temperature (for SnFU).

[0062] Referring to Fig. 2 A, an implementation 200 of the apparatus 100 is shown and includes an implementation 205 of the target material dispenser 105. The target material dispenser 205 includes a reservoir 208 holding the target material 206, such as molten tin, under pressure P. The target material dispenser 205 also includes a tube 209 such as a capillary tube that is in fluid communication with the interior of the reservoir 208 such that the target material 206 is forced to flow from the reservoir 208 under pressure P through the capillary tube 209, thus creating a continuous stream of pre-coalesced targets 112 exiting an orifice of the tube 209.

[0063] An implementation 225 of the actuatable element 125 is also shown in Fig. 2 A. The actuatable element 225 is coupled to the tube 209. For example, the actuatable element 225 can be contacting the tube 209 to deflect or squeeze the tube 209 and disturb the stream 110. In some implementations, the actuatable element 225 can contact one portion or one side of the tube 209. In other implementations, the actuatable element 225 can have a ring-shape or a cylindrical tube-shape and positioned to surround the circumference of the tube 209. The actuatable element 225 can be made up of a plurality of actuatable elements to selectively squeeze and release the tube 209 at respective frequencies (determined by the various waveforms of the target control signal 242). The actuatable element 225 can be an electro-actuatable element such as a piezoelectric element that is mechanically communicating with the capillary tube 209.

[0064] Fig. 2A also shows an implementation 230 of the detection apparatus 130 placed at a distance D230 in the -X direction from the exit of the tube 209. The detection apparatus 230 includes at least one illumination light source 231 configured to emit a diagnostic light beam 233 that is directed toward the trajectory of targets 211 in a target stream 210 such that when the target 211 passes across the diagnostic light beam 233, diagnostic light 234 is produced. The target stream 210 is shown at a snapshot in time, and the configuration that is shown in Fig. 2 A is not limiting and is not to scale, and is only meant to describe how the targets 211 generally travel and coalesce. The targets 211 can coalesce in other manners not shown in Fig. 2A. In some implementations, the diagnostic light beam 233 has a center wavelength in the near infrared region. For example, the diagnostic light 234 that is produced can be a portion of the diagnostic light beam that is reflected from, scattered from, or passes through the target 211. The detection apparatus 230 also includes at least one light sensitive sensor 229 configured to sense the diagnostic light 234. The illumination light source 231 can include one or more lasers and the light sensitive sensor 229 can be a photodiode or a camera. The detection apparatus 230 can also include one or more optic lenses, mirrors, or other optic devices for directing and shaping one or more of the diagnostic light beam 233 and the diagnostic light 234. Additionally, in other implementations, the light sensitive sensor 229 can include one or more phototransistors, photo-resistors, and/or the like.

[0065] The detection apparatus 230 is configured to place the diagnostic light beam 233 within a target region 227, such that the distance D230 is within the target region 227. In this arrangement, the at least one light sensitive sensor 229 detects or images the targets 211 at a point in the target stream 210 at which at least some of the targets 211 have partly coalesced but not all of the targets 211 have fully coalesced. In other words, the target region 227 is that region in the target stream 210 at which at least some of the targets 211 have partly coalesced but not all of the targets 211 have fully coalesced. The target detection region 227 is also a region, or the only region, where the diagnostic light beam 233 probes targets 211 and also where interactions between the targets 211 and the diagnostic light beam 233 are observed or detected. The properties of the diagnostic light 234 depend on how the diagnostic light beam 233 interacts with the target 211 (for example, how the diagnostic light beam

233 partly or completely bounces, continues to travel without change, diffracts, refracts, changes angle, scatters, and/or the like from the target 211). The properties of the diagnostic light 234 also depend on a size of the target 211 and the composition of the target material 206 that produces the target 211. The light sensitive sensor 229 receives and/or detects the diagnostic light 234, and generates the target detection signal 132.

[0066] The light sensitive sensor 229 responds to the diagnostic light 234 by generating different light response signals. The light response signals can be voltages or currents in a circuit that are caused by different changes in the electromagnetic properties of the light sensitive sensor 229, the different changes depending on the amplitude, frequency, duration and other properties of the diagnostic light 234. This light response signal can be processed, normalized, or left raw (as-is) and sent as the target detection signal 132 by the detection apparatus 230 to the controller 135.

[0067] In some implementations, the illumination source 231 and the light sensitive sensor 229 are arranged outside of the chamber 120, and windows in the chamber wall 122 and optics are arranged to allow the diagnostic light beam 233 to be transmitted into the chamber 120 and the diagnostic light

234 to be transmitted out of the chamber 120. In other configurations, some or all of the components of the detection apparatus 230 can be arranged partially or completely inside the chamber 120.

[0068] A closer view of the targets 211 in Fig. 2A is shown in Fig. 2B. Fig. 2B shows the snapshot in time of the stream of targets 210. Fig. 2B also shows the stream of targets 210 starting as a jet 2 lOj . Smaller-sized pre-coalesced targets 212 can be formed from the jet 2 lOj due to the Rayleigh Plateau instability. More pre-coalesced targets 212 are generally found closer to the start of the stream of targets 210 at -X = 0. Coalescence occurs while the targets 211 are moving along the -X direction and thus the partially coalesced targets 214 are, generally (but not always), farther along the -X direction than their respective smaller-sized pre-coalesced targets 212. Likewise, the fully coalesced targets 216 are, generally (but not always), farther along the -X direction than their respective smaller-sized pre-coalesced targets 212 and partially coalesced targets 214. In this particular example, Fig. 2B shows that after the target detection region 227, along the -X direction but before a distance D240, there are still targets 211 that are not fully coalesced, namely, one pre-coalesced target 212 and one partially coalesced target 214, which will later coalesce into one fully coalesced target 216 before or substantially at the distance D240. The distance D240 is the point of main coalescence, which is the point in the -X direction at which any target 211 should be a fully coalesced target, as further discussed below. More fully coalesced targets 216 are found the closer to the distance D240 along the stream of targets 210, and after the distance D240 all targets 211 are fully coalesced targets 216. In other words, the distribution of sizes of targets 211 along the -X direction in the stream of targets 210 starts with more pre-coalesced targets 212 closer to -X = 0, more fully coalesced targets 216 closer to -X = D240, and more partially coalesced targets 214 somewhere between -X = 0 and -X = D240, generally, around or closer to -X = Djso- Therefore, all targets 211 should coalesce into fully coalesced targets 216 so that only fully coalesced targets 216 can be found beyond the distance D240 in the -X direction and at the irradiation region 118.

[0069] Not all pre-coalesced targets 212 have the exact same size, but they can have substantially the same size. Likewise, not all partially coalesced targets 214 have the exact same size, but they can have substantially the same size. And again, not all fully coalesced targets 216 have the same size, but they can have substantially the same size. As further illustrated in Fig. 2C, as targets 211 travel generally along the -X direction toward the irradiation region 118, at a point in time tl that is later than time tO, some of the smaller-sized pre-coalesced targets 212 coalesce into partially coalesced targets 214, which explains the higher distribution of partially coalesced targets 214 and the decreased distribution of pre-coalesced targets 212 as the targets 211 move closer to the distance D240 in the -X direction. Also, at the point in time tl, some of the smaller-sized pre-coalesced targets 212 coalesce in one event into a fully coalesced target 216 while some have yet to coalesce. Similarly, at the point in time tl, the partially coalesced targets 214 coalesce with smaller-sized pre-coalesce targets 212 (not shown), coalesce with each other, and/or coalesce with fully coalesced targets 216 (not shown), forming larger partially coalesced targets 214 (not shown) and/or fully coalesced targets 216, which explains the higher distribution of fully coalesced targets 216 as the targets 211 move closer to and move beyond the distance Dz4o- Likewise, at a time t2 that is later than time tl, some of the pre-coalesced targets 212 are about to coalesce with some of the fully-coalesced targets 216, some of the partially coalesced targets 214 coalesce with each other to form fully coalesced targets 216, and some of the partially coalesced targets 214 coalesce with pre-coalesced targets 212 (not shown) to form larger partially coalesced targets 214 or fully coalesced targets 216. Given their different sizes, it is possible for two, three, or more pre-coalesced targets 212 to coalesce into partially coalesced targets 214. Moreover, it is possible for the pre-coalesced targets 212 to coalesce in a single event into a fully coalesced target 216 (instead of forming partially coalesced targets 214). In other words, in the coalescence process there can be any type of combinations and permutations of coalescence between two or more precoalesced targets 212, partially coalesced targets 214, and/or fully coalesced targets 216. The coalescence process results in at least one pre-coalesced target 212, partially coalesced target 214, or fully coalesced target 216, the resulting target 211 having a different size at each occurrence of the coalescence process based on the number and the sizes of each of the targets 211 that are part of the coalescence process. The diagnostic light beam 233 is arranged relative to the trajectory so that it can sense or detect aspects relating to the targets 211 as they travel along the trajectory toward the irradiation region 118, and therefore the detection apparatus 230 can detect or observe aspects relating to any stage of coalescence of the targets 211 (including the stages before coalescence into partial coalescence and/or full coalescence).

[0070] Referring to Fig. 3, an implementation 335 of the controller 135 is shown. As discussed above, the controller 335 is configured to receive the target detection signal 132 from the detection apparatus 130 (or 230). The target detection signal 132 is, using the example of Fig. 2A, a voltage signal related to a current produced from the detected light at the photodiode (the light sensitive sensor 229) of the detection apparatus 130 (or 230). In other implementations in which the light sensitive sensor 229 includes a camera, then the target detection signal 132 corresponds to an output from the camera. Generally, the controller 335 analyzes the data within the target detection signal 132. [0071] The controller 335 can determine whether the target detection signal 132 is substantially similar to a desired form and/or contains data or information that corresponds to a desired operational state. The desired form or shape of the target detection signal 132 is a signal shape that corresponds or substantially corresponds to the desired operational state. The desired operational state is a state of the apparatus 100 (or the like) in which the targets 111 are flowing in a desired manner (for example, in a way and/or rate that continuously induces or causes only fully coalesced targets reach the irradiation region 118). In an implementation, the controller 335 determines, from the target detection signal 132, the current operational state, that is, whether and when coalescence occurs and how the targets 111 are behaving as they travel along the trajectory generally along the -X direction toward the irradiation region 118. The controller 335 also knows which target control signal 142 supplied to the actuatable element 125 resulted in the target detection signal 132 currently being analyzed. Because of this, the controller 335 can make a determination about how to modify the waveform control signal 137 supplied to the waveform generator 140, to enable the waveform generator 140 to thereby modify the target control signal 142 supplied to the actuatable element 125 to improve the characteristics of the targets 111. The controller 335 can continue the analysis of the target control signal 142 supplied to the actuatable element 125 and the current operational state to change or transform any operational state into the desired operational state and maintain the desired operational state. Similarly, the controller 335 can be configured to restore or initiate the desired operational state.

[0072] To this end, the controller 335 includes a signal processing module 336 that analyzes the target detection signal 132. For example, if the light sensitive sensor 229 is a photodiode, then the signal processing module 336 is configured to analyze a set of time stamps corresponding to how and when the targets 111 interact with the diagnostic light beam 233 as the target 111 travels along its trajectory toward the irradiation region 118. The signal processing module 336 can be configured to determine other properties of the target detection signal 132. For example, the signal processing module 336 can determine an amplitude of the peaks within the target detection signal 132, including determining whenever an amplitude is greater than one or more threshold values. In analyzing the peaks of the target detection signal 132, information about a size (such as a diameter or an area) of the target 111 can be gleaned. The signal processing module 336 can also analyze start and end times at which peaks within the target detection signal 132 cross one or more threshold values and determine (or predict) various coalescence lengths and spatial offsets between targets 111. The signal processing module 336 can determine how to modify one or more phases and amplitudes of the target control signal 142 based on the analysis of peaks crossing threshold values and/or the determinations of coalescence lengths and/or spatial offsets. If the signal processing module 336 determines that adjustments to the target control signal 142 are needed (based on the analysis of the target detection signal 132), then the module 336 (via an output signal) sends or modifies the waveform control signal 137 to the waveform generator 140. As discussed below, the module 336 can modify the waveform control signal 137 by adjusting or updating one or more parameters of the waveform control signal 137. The parameters of the waveform control signal 137 may include, but are not limited to, frequency, form, bias, amplitude, and phase of one or more frequency components of a waveform, phase differences between components, and, according to any implemented communication protocol, information, instructions, data, and/or symbols. Moreover, the controller 335 (or its internal components) is configured to adjust the one or more parameters of the waveform control signal 137 while the plasma generating system generates plasma from the targets 211 (for example, while the plasma generating system produces extreme ultraviolet light that is supplied to a substrate at a photolithography exposure apparatus).

[0073] The controller 335 can include or have access to one or more programmable processors 337a and/or one or more computer program products 337b tangibly embodied in a machine-readable storage device for execution by a programmable processor. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processor receives instructions and data from memory 338. The memory 338 can be read-only memory and/or random-access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; nonlocal memory from servers or remote computers with a wired or wireless connection to the controller 135; and CD-ROM disks. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits).

[0074] The signal processing module 336 can include its own digital electronic circuitry, computer hardware, firmware, and software as well as dedicated memory, input and output devices, programmable processors, and computer program products. Likewise, the module 336 can access and use the memory 338, one or more input devices 339 (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.), one or more output devices 341 (such as speakers and monitors), one or more programmable processors 337a, and/or one or more computer program products 337b. [0075] Although the controller 335 is shown as a separate and complete unit, it is possible for each of its components and modules to be separate units, or for the controller 335 to be incorporated into other components. The controller 335 can include other modules not shown such as dedicated memory, input/output devices, processors, and computer program products, not shown in Fig. 3.

Moreover, the waveform generator 140 can be implemented within the controller 335 even though it is shown as a separate component. For example, the controller 335 and/or the waveform generator 140 can also interface with other components of the EUV light source (described below). For example, after the tuning mode is completed, the apparatus 100 (by way of the controller 135/335) can inform the EUV light source and begin operating under steady-state operation (if for some reason, the tuning mode was an offline tuning mode).

[0076] As another example, the controller 135/335 can receive information from an output device (such as output device 981 shown in Fig. 9, which can be a photolithography exposure apparatus) that receives the EUV light that is produced at the irradiation region 118 to process substrates (wafers). For example, the controller 135/335 can be privy to the processing stage or status of the photolithography exposure apparatus and can coordinate different tuning modes based on this information.

[0077] As a further example, the controller 135/335 can communicate with other metrology apparatuses that probe the target stream 110/210. For example, one or more metrology apparatuses are positioned near the irradiation region 118 and are configured to detect the properties (such as timing, size, shape) of the targets 111/211 in the stream near the irradiation region 118. The information from these metrology apparatuses can be analyzed by the controller 135/335 to determine whether satellites exist at the irradiation region 118, and the controller 135/335 (by way of the signal processing module 336) can make adjustments to various properties relating to the waveform control signal 137 to thereby adjust the target control signal 142. [0078] The waveform generator 140 generates the target control signal 142. The waveform generator 140 can be separate from the controller 135 (as shown in Fig. 3), integrated with the controller 135, or integrated within the actuatable element 125. The waveform generator 140 operates and generates the target control signal 142 under the control of the controller 135, at least partially on the basis of the waveform control signal 137 and/or the target detection signal 132. The waveform generator 140 provides the target control signal 142 to the actuatable element 125, which induces a velocity perturbation into the target stream 110, thus affecting the targets 111 in the interacting region 118. The waveform generator 140 can be an arbitrary signal generator that is capable of generating electrical waveforms over a wide range of signals including producing a hybrid waveform that includes a plurality of periodic waveforms, any one or more of which can be a sine wave, a square wave, a ramp or triangular wave, a pulse wave, a gaussian pulse wave, or an arbitrary wave. The controller 335 can be programmable, which means that the controller 335 is configured to be provided with coded instructions for the automatic performance of the task to cause the waveform generator 140 to generate the target control signal 142 for the actuatable element 125. As stated above, the target control signal 142 is periodic in time and it is made up of separate periodic waveforms, thus permitting the phase and frequency of each waveform to be individually adjusted or tuned by the waveform generator 140.

[0079] In one implementation, as discussed above, the target control signal 142 is a superposition of two periodic waveforms, that is, a first periodic waveform at a fundamental frequency that is substantially equal to the pulse rate of the light pulse that illuminates the irradiation region 118, and a second periodic waveform at a frequency that is greater than the fundamental frequency. Any arbitrary periodic waveforms can be used.

[0080] In one example, as shown in Fig. 4A, the fundamental waveform is a sine wave and the higher frequency waveform is an added small-amplitude square waveform (shown magnified). The two waveforms are superposed to obtain the composite waveform 442cA of the target control signal 142. The composite waveform 442cA is a voltage signal V(t) applied to the actuation element 125 as a function of time and is defined as follows in this example: where Ao is the amplitude of the fundamental waveform, fo is the frequency of the fundamental waveform, is the phase of the fundamental waveform, As is the amplitude of the square waveform, foNo is the frequency of the higher frequency waveform, and No is an integer greater than 0. In this example, the higher frequency (of the square waveform) is about ten times the fundamental frequency. As an example noted above, the fundamental frequency can be about 50 kHz and the higher frequency of the square waveform can be about 500 kHz. [0081] Other periodic waveforms can be used for the arbitrary periodic waveforms, such as, for example, sine waves, square waves, triangle waves, ramps, and other pulse shapes. Figs. 4B and 4C show two other possible implementations of the target control signal 142 having a first periodic waveform at a fundamental frequency equal to the pulse rate of the light pulse that illuminates the irradiation region 118, and a second periodic waveform at a frequency that is a multiple greater than the fundamental frequency. In Fig. 4B, a composite waveform 442cB of the target control signal 142 includes first periodic waveform that is a square wave at the fundamental frequency and a second periodic waveform that is a sine wave at a frequency that is greater than the fundamental frequency. In Fig. 4C, a composite waveform 442cC of the target control signal 142 includes a first periodic waveform that is a sine wave at the fundamental frequency plus a second periodic waveform that is a sine wave at a frequency that is greater than the fundamental frequency.

[0082] Referring to Fig. 4D, the phase between the first waveform (the sine waveform) and the higher frequency waveform (the square waveform) of the composite waveform 442cA can be varied. The figure depicts five examples of composite waveform 442cAi, 442cAii, 442cAiii, 442cAiv, 442cAv. As shown in Fig. 4D, this phase of each of the five examples is varied with respect to the neighboring examples. Such adjustments in the phase can be used to influence the position of one or more pre-coalesced targets 112, partially coalesced targets 114, and/or fully coalesced targets 116 relative to other targets 111 in a stream of targets 210.

[0083] In another implementation, as mentioned above, the target control signal 142 is a superposition of three periodic waveforms, that is, a first periodic waveform at a fundamental frequency that is substantially equal to the pulse rate of the light pulse that illuminates the irradiation region 118, a second periodic waveform at a frequency that is greater than the fundamental frequency, and a third periodic waveform at a frequency that is also greater than the fundamental frequency. Any arbitrary periodic waveforms can be used. In one example, a composite waveform 442cE is shown in Fig. 4E, in which the fundamental waveform is a sine wave, the second periodic waveform is a sine wave, and the third periodic waveform is a square waveform. The three waveforms are superposed to obtain the composite waveform 442cE of the target control signal 142. The composite waveform is a voltage signal V(t) applied to the actuation element 125 as a function of time and is defined as follows in this example: where Ao is the amplitude of the fundamental waveform, fo is the frequency of the fundamental waveform, is the phase of the fundamental waveform, Ai is the amplitude of the second sine wave, fi is the frequency of the second sine wave, 0i is the phase of the second sine wave, As is the amplitude of the square waveform, foNoNi is the frequency of the higher frequency square waveform, No and Ni are positive integers. In one example, such as shown in Fig. 4E, the frequency fi (of the second sine wave) is ten times the fundamental frequency fo and the frequency foNoNi (of the square waveform) is forty times the fundamental frequency fo. In this particular example, No = 10 and Ni = 4. For example, if the fundamental frequency fo is 50 kHz, then the frequency f\ of the second sine wave is 500 kHz and the frequency foNoNi of the square waveform is 2000 kHz. The value of the frequency foNoNi can be selected based on a voltage that has higher transfer function amplitude. The controller 135 can instruct (via the waveform control signal 137) the waveform generator 140 to adjust the values of Ao and to control the location and time at which the final coalescence (to form the fully coalesced targets 116) occurs. Additionally, the controller 135 can instruct (via the waveform control signal 137) the waveform generator 140 to adjust the values of Ai and to control the location and time at which sub-coalescence (to form the partially coalesced targets 114) occurs.

[0084] Referring to Fig. 5A, a procedure 550 is performed to generate or control a desired stream of targets (such as the streams of targets 110 or 210). The procedure 550 can be performed at least partly by components of the apparatus 100, 200. During the procedure 550, the stream of targets 110 is provided to the irradiation site 118 of the plasma generating system (551). For example, and with reference to Fig. 2 A, the target material dispenser 205 is configured to emit the targets 211 along the - X direction toward the irradiation site 118. The target material 206, in a fluid state, being under the force of the pressure P (as well as other possible forces such as gravity), flows from the interior of the reservoir 208 and through the opening of the tube 209 to form the stream 210. The trajectory of the targets 211 that are formed from the target material 206 generally extends along the -X direction, although it is possible for the trajectory of the targets 211 to include components along the plane perpendicular to the -X direction (that is, Y and Z components).

[0085] The targets 211 in the target stream 210 are observed (552). In some implementations, the detection apparatus 230 observes the targets 211 in the stream of targets 210 at a point in the stream of targets 210 at which at least some of the targets 211 have partly coalesced but not all of the targets 211 have fully coalesced. For example, as shown in Fig. 2 A, the targets 211 can be probed at the distance D230 in the -X direction from the exit of the tube 209. In the implementation of Fig. 2A, the illumination light source 231 emits the diagnostic light beam 233 toward the trajectory of targets 211 within the target region 227 in the target stream 210 such that when the target 211 interacts with the diagnostic light beam 233, the diagnostic light 234 is produced. The light sensitive sensor 229 detects, receives, or senses the diagnostic light 234. In this way, the targets 211 are observed.

[0086] In the implementations noted, in order for the detection apparatus 230 to observe the targets 211 in the stream 210 at this point in the stream 210 (at which at least some of the targets 211 have partly coalesced but not all of the targets 211 have fully coalesced), the target control signal 142 produces a composite waveform that controls the operation of the actuatable element 125. For example, the target control signal 142 produces a composite waveform that enables the stream 210 to behave in a manner that leads to the targets 211 partly coalescing and also some fully coalescing and doing so within the detection region 227 (Fig. 2A). In other implementations, such as during a time when EUV light is not being produced at the irradiation region 118 or when the output device (such as the output device 981 of Fig. 9) is not receiving EUV light and is not processing substrates, the apparatus 100 can be configured to operate the actuatable element 125 so that partial coalescence (or full coalescence) has not occurred prior to the targets 111/211 reaching the detection region 227. For example, such a configuration could be useful during a full parameter scan during calibration of the target material dispenser 105 and the actuatable element 125.

[0087] For example, Fig. 6A shows a graph 660A of a particular coalescence process and position distribution of targets moving in the -X direction (such as targets 211) at different distances from the end of the tube (such as tube 209), which is also referred to as the nozzle. The bigger dots on Fig. 6A represent fully coalesced targets (such as fully coalesced target 216). The smaller dots represent precoalesced targets (such as pre-coalesced target 212). The medium-sized dots represent partially coalesced targets (such as partially coalesced target 212). Note that the graph 660A does not have a time axis; instead, the vertical axis indicates the distance (for example, in millimeters) from the tube 209, and the horizontal axis indicates the spacing between targets 211 (target or droplet spacing) at a particular distance from the tube 209 in the -X direction. Fig. 6A shows the data from Fig. 2C in a manner that uses distance as opposed to time. For example, at a distance close to 0 millimeters from the tube 209, such as at 5 millimeters (labeled as 661 A in Fig. 6 A), at any point in time during operation of the target material dispenser 205, that there are generally many pre-coalesced targets (such as pre-coalesced targets 212). On the other hand, at 125 millimeters from the tube 209 (labeled as 662A in Fig. 6A), at any point in time during operation of the target material dispenser 205, there exists a fully coalesced target and partially coalesced targets (such as partially coalesced target 214) slightly positioned closer to the tube 209 and slightly positioned farther away from the tube 209. In this example, main or full coalescence occurs at a distance 663A from the tube 209 and subcoalescence (where partially coalesced targets 214 are formed) occurs at a distance 664 A from the tube 209. Targets are observed at a distance Desox at which at least some of the targets 211 have partly coalesced but not all of the targets 211 have fully coalesced. In this particular example, Desox is at 25 millimeters from tube 209 in the -X direction.

[0088] Referring again to Fig. 5A, the target detection signal 132 is generated based on the observation or sensing of targets 211 at step 552 (553). For example, in the apparatus 200 of Fig. 2A, the detection apparatus 230 generates the target detection signal 132 based on the observation of the diagnostic light 234 by the light sensitive sensor 229, and the detection apparatus 230 sends the target detection signal 132 to the controller 135. In one implementation, the detection apparatus 230 generates the target detection signal 132 by generating a light response to the diagnostic light 234, and processing the light response or not changing light response (leaving the light response raw or as-is). [0089] Next, the target control signal 142 is generated based on the target detection signal 132 (554). For example, with reference to the process 554B in Fig. 5B, in some implementations, the controller 135 receives the target detection signal 132 (from the detection apparatus 130) and generates the waveform control signal 137 by processing the information or data in the target detection signal 132 (555B), and the waveform generator 140 generates the target control signal 142 based on the waveform control signal 137 (556B). With reference to the implementation of Fig. 3, the signal processing module 336 (within the controller 335) analyzes the target detection signal 132 at 555B. The signal processing module 336 analyzes variations or deviations in the sizes and distances between targets 211. For example, the signal processing module 336 analyzes both the spacing and the sizing of the peaks in the target detection signal 132 based at least on the values of time stamps that correspond to peaks of intensity in the target detection signal 132. Moreover, the signal processing module 336 determines, based at least on the variation and deviation analysis, whether adjustments to the target control signal 142 are needed and sends or modifies the waveform control signal 137 according to the needed or desired adjustments to the target control signal 142 to maintain or modify the stream of targets 210 (and targets 211) in the desired or appropriate form for more efficient plasma generation. Additionally, the controller 335 (by way of the signal processing module 336) adjusts, based at least on the variation and deviation analysis, one or more parameters of the waveform control signal 137 as the plasma generating system is generating plasma from the targets 211. As discussed above, the waveform control signal 137 is a hybrid composite waveform including a superposition of a plurality of periodic waveforms.

[0090] To assist in explaining how the signal processing module 336 analyzes the target detection signal 132 (for example, using peak analysis), Fig. 6B shows a graph 660B of a particular coalescence process and position distribution of targets moving in the -X direction (such as targets 211) at different distances from the end of the tube (such as tube 209). In this process, main coalescence (where all targets 211 have fully coalesced) occurs at a distance 663B that is about 380 millimeters in the -X direction from the output of the tube 209. And, sub-coalescence (where the targets 211 have partly coalesced) occurs at a distance 664B that is about 50 mm in the -X direction from the output of the tube 209. At a distance 661B that is relatively close to the output of the tube 209 (such as at about 25 mm from the tube 209), there are generally many pre-coalesced targets (such as pre-coalesced targets 212). A graph 661BG shows an output S132 of the light sensitive sensor 229 (that is, the target detection signal 132) versus time for diagnostic light 234 that is produced from an interaction between targets 211 and a diagnostic light beam 233 that is located at the distance 661B. The graph 661BG shows an example with multiple similar small peaks at different points in time. On the other hand, at about 100 mm from the tube 209 (labeled as 662B in Fig. 6B), at any point in time during operation of the target material dispenser 205, there exists a fully coalesced target and partially coalesced targets (such as partially coalesced target 214) slightly positioned closer to the tube 209 and slightly positioned farther away from the tube 209. A graph 662BG shows an output of the light sensitive sensor 229 (that is, the target detection signal 132) versus time for diagnostic light 234 that is produced from an interaction between targets 221 and a diagnostic light beam 233 that is located at the distance 662B. The graph 662BG shows an example with multiple peaks at different points in time, with some of the peaks larger than others. At the distance 66 IB, it is evident from the graph 661BG, that only pre-coalesced targets 212) are sensed when analyzing the size of the peaks in the graph 661BG. All of the peaks in the graph 661BG correspond to pre-coalesced targets 212, which means that none of the targets 211 have engaged in coalescence at the distance 661B. On the other hand, at the distance 662B, it is evident from the graph 662BG that pre-coalesced targets 212 and partially coalesced targets 214 are sensed when analyzing the size of the peaks in the graph 662BG. The larger peaks 665B represent partially coalesced targets 214 and the smaller peaks 666B represent pre-coalesced targets 212.

[0091] As noted above, the detection apparatus 230 is configured to observe the targets 211 in the target stream 210 and is configured to observe some targets that have partly coalesced (such as the targets 214) but not all of the targets 211 have fully coalesced. Thus, the diagnostic light beam 233 should be placed at a distance that is greater than the distance 661B in order to be able to observe some targets that have partly coalesced. In particular, the diagnostic light beam 233 should be placed at a distance greater than 664B, which, as noted above, is the location at which sub-coalescence (where the targets 211 have partly coalesced) first occurs but before the distance 663B (at which point all targets 211 have fully coalesced). Thus, it is acceptable for the diagnostic light beam 233 to be placed at the distance 662B.

[0092] The signal processing module 336 analyzes the target detection signal 132 output from the detection apparatus 230 (when the diagnostic light beam 233 is placed at the distance 662B). For example, the signal processing module 336 determines both the size and timing of targets 211 based at least on the timing and sizes of the peaks 665B and/or peaks 666B. With the analysis of the target detection signal 132, the controller 335 determines whether the targets 211 flow and behave in their appropriate and desired form, for example, by comparing the timing and sizes of the peaks 665B and/or peaks 666B with specific parameters relating to the required timing and sizes of peaks. In some implementations, the controller 335 analyzes an asymmetry of peaks associated with targets 211 that have not coalesced relative to one or more peaks associated with targets 211 that has partly coalesced or fully coalesced in the target detection signal 132, and the asymmetry of the peaks can therefore determine a coalescence length of the target stream 210. For example, with reference to Fig. 6B, the controller 335 (and specifically the signal processing module 336) measures one or more asymmetries of the peaks 666B relative to the peaks 665B. The one or more asymmetries can determine a coalescence length of the target stream 210.

[0093] The controller 335 performs the analysis by analyzing one or more metrics relating to the target detection signal 132. For example, in some implementations, the controller 335 performs the analysis by analyzing a relative position between a partly coalesced target and a half-way position between two consecutive fully coalesced targets (referred to as the a metric or the b metric in the discussion below). In some implementations, the controller 335 performs the analysis by analyzing a difference in spacing or time between a target that has not coalesced and an adjacent target that has partly coalesced (referred to as the c metric below). Moreover, the controller 335 can use other metrics in the analysis. These various analyses are discussed below in greater detail with reference to Figs. 7A, 7B, 8A, and 8B and with reference to Equations 3-13.

[0094] Referring to Fig. 5B, once the controller 335 adjusts the one or more parameters of the waveform control signal 137 (or maintains the waveform control signal 137 if it determines that adjustments are not needed) (555B), then the waveform control signal 137 is provided to the waveform generator 140, which then generates the target control signal 142 based at least in part on the waveform control signal 137 (556B). In an implementation, the waveform generator 140 generates the target control signal 142 by adjusting one or more of an amplitude and a relative phase of one or more of the periodic waveforms of the target control signal 142. The perturbations in the stream 210 of targets 211 are induced based on the target control signal 142 (557). For example, the waveform generator 140 sends the target control signal 142 to the actuatable element 125, 225, and the actuatable element 125, 225 induces the velocity perturbations in the stream of targets 211 based on the target control signal 142. In this way, the actuatable element 125, 225 induces or causes one or more coalescence lengths in the target stream 210 and/or one or more deviation corrections to the coalescence lengths in the target stream 210. With the appropriate target control signal 142, the actuatable element 125, 225 induces perturbations to the stream of targets 210 that bring the stream of targets 210 and the targets 211 back from deviations to the correct form or distribution of distances and sizes at the appropriate detection point. Bringing and maintaining the stream of targets 210 and the targets 211 to the correct form or distribution of distances and sizes at the appropriate detection point results in main coalescence occurring at the desired distance, and specifically prior to the irradiation region 118 while reducing or eliminating satellites (that is, those targets such as 212, 214 that are not fully coalesced targets 216) at the irradiation region 118.

[0095] In some implementations, as discussed above with reference to Equation 1, the composite waveform used in the target control signal 142 is a superposition of two periodic waveforms. Figs. 7A and 7B show an example of an analysis relating to the asymmetry of peaks associated with targets 211 that have not coalesced relative to one or more peaks associated with targets 211 that have at least partly coalesced in the target detection signal 132 in which the target control signal 142 is given by Equation 1. Fig. 7A shows a graph 770 of a size of the targets 211 (in arbitrary units) versus a spacing between targets (in arbitrary units) at a particular distance from the tube 209 in the -X direction. For example, this distance is the location at which the diagnostic light beam 233 probes the targets 211. The targets 77 li, 77 lii are about twice as large as the targets 772i, 772ii, 772iii, 772iv, 772v. The targets 77 li, 77 lii are referred to as main targets and the targets 772i, 772ii, 772iii, 772iv, 772v are partially coalesced targets. A main target is a target that has partially coalesced and is located at a node. The node corresponds to the target and its moving location at which other targets that are closest to that node move toward to thereby coalesce with the main target. The main targets 77 li, 77 lii will eventually therefore become fully coalesced targets. Moreover, such main targets have a volume that is greater than the volume of the partially coalesced targets not at the nodes, and a volume that is greater than half the size of the volume of the fully coalesced targets that they will eventually become. Fig. 7B shows a graph 775 of the output S132 of the detection apparatus 230 (which is the target detection signal 132) versus time (in arbitrary units) at the distance or location that the diagnostic light beam 233 probes the targets 211 in Fig. 7A. In Fig. 7B, peaks 665Bi and 665Bii correspond to consecutive or adjacent main targets 77 li, 77 lii, and peaks 666Bi, 666Bii, 666Biii, 666Biv, 666Bv correspond to respective partially coalesced targets 772i, 772ii, 772iii, 772iv, 772v. The controller 335 (and specifically the signal processing module 336) analyzes the metrics (or parameters) a and b that are shown in Figs. 7A and 7B. The metric a is a distance in spacing (or time) between a particular partially coalesced target 772iii and an antinode 773, and the metric b is a distance in spacing (or time) between the next or adjacent partially coalesced target 772ii and the antinode 773. The antinode 773 is the middle between two consecutive main (and eventually fully coalesced) targets 77 li, 77 lii. The metrics a and b add up to a value that is the distance in spacing (or time) between adjacent partially coalesced targets 772ii and 772iii. Based on the discussion above, the partially coalesced targets 772i, 772ii are moving toward the main target 77 li while the partially coalesced targets 772ii, 772iv, 772v are moving toward the main target 77 lii.

[0096] The signal processing module 336 calculates adjustments to the composite waveform shown in Equation 1 based on the metrics a and b as follows: Equation [3] is the phase of the fundamental waveform, Ao is the amplitude of the fundamental waveform, a and b are the above discussed metrics, r^-b) and r^+b) are the references compared to the target detection signal 132 for values of (a-b) and (n+b), respectively, and K _a -b and K _a+ b are adjustable feedback gains that amplify the effect of the comparison between the references r^-b) and r^a+b) and the metrics «-b and a+b. In general, the value of the reference r< _a -b) can be zero for situations in which the chamber 120 is maintained at vacuum pressures, and the reference r( _a -b) can be nonzero for situations in which the chamber 120 is not at vacuum pressures (and there could be flow). Through the adjustments calculated by the signal processing module 336, a feedback loop is created between the detection of targets 111, 211 in the stream of targets 110, 210 and the vibrations induced by the actuatable element 125, 225 to minimize the changes in the stream of targets 110, 210 (the derivatives of phase and sinusoidal amplitude in Equations 3). When the stream of targets 110, 210 and the targets 111, 211 are continuously flowing as desired, there are no changes (or substantially no changes) in the target detection signal 132. Therefore, when equals 0 or approximates equals 0 or approximates 0, there are no changes (or substantially no changes) in the target detection signal 132 and none or minimal change is required in the velocity perturbations induced by the actuatable element 125, 225. Likewise, when the stream of targets 110, 210 and/or the targets 111, 211 change or deviate from the desired sizing, sequence, timing, and/or spacing, those deviations are reflected in the target detection signal 132, and the metrics n-h and a+b deviate from the respective references r^-b) and r^+b). The signal processing module 336 calculates changes in that correspond to the changes in the metrics a and b.

[0097] With the calculated deviations, the signal processing module 336 calculates the changes necessary to the waveform control signal 137 for the generation of a target control signal 142 that causes the actuatable element 125, 225 to induce vibrations that correct the stream of targets 110, 210 and/or the targets 111, 211 back to the desired sizing, sequence, timing, and/or spacing. Note that increasing one or more of the gains K _a -b and K _a+ b causes a larger change in respective values of and thus the correction or convergence from a deviated state to a desired state happens more rapidly. However, increasing K _a -b and K _a+ b amplifies any noise in the comparison of the references and the metrics, making the feedback more sensitive to noise. On the other hand reducing one or more of the gains K _a -b and K _a+ b causes a smaller change in respective values of and thus the correction or convergence toward the desired state is slower, but occurs with less sensitivity to noise. The gains K _a -b and K _a+ b and the rate of calculation iterations of the Equation 3 are balanced to reach and maintain (or substantially maintain) the stream of targets 110, 210 and/or the targets 111, 211 in the desired state.

[0098] In some implementations, as discussed above with reference to Equation 2, the composite waveform used in the target control signal 142 is a superposition of three periodic waveforms (a hybrid waveform of two sine waveforms plus a square wave). Figs. 8A and 8B show an example of an analysis relating to the asymmetry of peaks associated with targets 211 that have not coalesced relative to two or more peaks associated with targets 211 that have either partially coalesced or fully coalesced in the target detection signal 132, in which the target control signal 142 is given by Equation 2. Fig. 8A shows a graph 870 of a size of the targets 211 (in arbitrary units) versus a spacing between targets (in arbitrary units) at a particular distance from the tube 209 in the -X direction. For example, this distance is the location at which the diagnostic light beam 233 probes the targets 211. Targets 87 li, 87 lii are the largest targets in graph 870. On the other hand, targets 872i, 872ii, 872iii, 872iv, 872v, and 872vi are smaller than the targets 87 li, 87 lii; and targets 876i, 876ii, 876iii, and 876iv are the smallest targets. The targets 87 li, 87 lii are fully coalesced targets (or main targets that will become fully coalesced targets) that are located at nodes; the targets 872i, 872ii, 872iii, 872iv, 872v, and 872vi are partially coalesced targets; and the targets 876i, 876ii, 876iii, and 876iv are pre- coalesced targets. Fig. 8B shows a graph 875 of the output S132 of the detection apparatus 230 (which is the target detection signal 132) versus time (in arbitrary units) at the distance or location that the diagnostic light beam 233 probes the targets 211 in Fig. 8A. In Fig. 8B, peaks 865i and 865ii correspond to respective consecutive fully coalesced or main targets 87 li and 87 lii; peaks 874i, 874ii, 874iii, 874iv, 874v, and 874vi correspond to respective partially coalesced targets 872i, 872ii, 872iii, 872iv, 872v, and 872vi; and peaks 877i, 877ii, 877iii, and 877iv correspond to respective precoalesced targets 876i, 876ii, 876iii, and 876iv. The controller 335 (and specifically the signal processing module 336) analyzes metrics (or parameters) a, b, and c that are shown in Fig. 8B, with a and b also shown in Fig. 8A. The metric a is a distance in spacing (or time) between a particular partially coalesced target 872iii (corresponding to peak 874iii) and an antinode 873, and the metric b is a distance in spacing (or time) between the next or adjacent partially coalesced target 872iv (corresponding to peak 874iv) and the antinode 873. The antinode 873 is the middle between two consecutive main (or fully coalesced) targets 87 li and 87 lii. The metrics a and b add up to a value that is the distance in spacing (or time) between adjacent partially coalesced targets 872iii (corresponding to peak 874iii) and 872iv (corresponding to peak 874iv). The metric c is a distance in spacing (or time) between partially coalesced target 872iii (corresponding to peak 874iii) and precoalesced target 876iii (corresponding to peak 877iii). In other words, the metric c is the distance in spacing (or time) between the target 872iii (of the prior sequence of targets) and the target 876iv (of the continuing sequence of targets).

[0099] The signal processing module 336 calculates adjustments to the composite waveform shown in Equation 2 based on the metrics a, b, and c as follows: where fa is the phase of the first fundamental waveform; Ao is the amplitude of the fundamental waveform; Ai is the amplitude of the second waveform; a, b, and c are the above discussed metrics

(which are static) is a dynamic metric are the references compared to the metrics (a-b), (n+b), c, and respectively; and are adjustable feedback ^ gains that amplify the effect of the comparison between the references and the metrics. Through the adjustments calculated by the signal processing module 336, a feedback loop is created between the detection of targets 111, 211 in the stream of targets 110, 210 and the vibrations induced by the actuatable element 125, 225 to minimize the changes in the stream of targets 110, 210 (the derivatives of phase and sinusoidal amplitude in Equations 4 and 5). When the stream of targets 110, 210 and the targets 111, 211 are continuously flowing as desired, there are no changes (or substantially no changes) in the target detection signal 132. Therefore, when each an equal 0 or approximate 0, there are no changes (or substantially no changes) in the target detection signal 132 and none or minimal change is required in the velocity perturbations induced by the actuatable element 125, 225. Likewise, when the stream of targets 110, 210 and/or the targets 111, 211 change or deviate from the desired sizing, sequence, timing, and/or spacing, those deviations are reflected in the target detection signal 132, and the metrics deviate from the respective references

[0100] The signal processing module 336 calculates changes in that occur due to the changes in these metrics. With the calculated deviations, the signal processing module 336 calculates the changes necessary to the waveform control signal 137 for the generation of a target control signal 142 that causes the actuatable element 125, 225 to induce vibrations that correct the stream of targets 110, 210 and/or the targets 111 back to the desired sizing, sequence, timing, and/or spacing. Note that increasing the gains K results in greater changes since the i terms would be larg ^b er, and thus the correction or convergence from a ^b deviated state to a desired state faster. This results in a greater sensitivity to noise. On the other hand, red 1uci’ng t1he gai •ns causes sma 1l1ler c1hanges i ’n t 1he terms and/or and this makes convergence toward the desired state slower, but with less sensitivity to noise. The gains and the rate of calculation iterations of the Equations 4 and 5 are balanced to reach and maintain (or substantially maintain) the stream of targets 110, 210 and/or the targets 111, 211 in the desired state.

[0101] The values of Ao and can be changed to thereby control the location and time at which a final coalescence (that is, to form the fully coalesced targets 216) occurs. On the other hand, the values of i and can be changed to thereby control the location and time at which sub-coalescence (to form the partially coalesced targets 214) occurs.

[0102] While Equations 4 and 5 rely on comparisons between the respective references (the r values) and the values of it is alternatively possible to perform the comparisons based on other values that rely on a, b, and c. For example, one of the references can be compared to a value Moreover, it may be desired in some situations for the values of a and b to be equal to each other for optimum target production, while in other situations, optimum target production may require that a and b be distinct from each other. For example, in a vacuum environment of the chamber 120, it may be desirable for a and b to be equal to each other while in a non-vacuum environment of the chamber 120 it may be desirable for a and b to be distinct from each other.

[0103] As evident from Equation 4, static feedback is used for the main coalescence (at which the fully coalesced targets 216 are created). In static feedback, the inputs (metrics), a and b, are static metrics. On the other hand, with reference to Equation 5, both static and dynamic feedback are used for sub-coalescence (at which the partially coalesced targets 214 are created). In particular, while the input (metric) c is a static metric, the input is a dynamic metric (in that it changes).

[0104] Other types of feedback or combinations of feedback are possible to calculate the feedback

.. _ , . . . ad ^J iustments dt dt dt dt For examp rle, it is peossible to use only j static feedback for the feedback ad

^J justments dt and dt that control sub-coalescence.

[0105] In some implementations, dynamic feedback can be used for both the feedback adjustments that control main coalescence and the feedback adjustments that control sub- coalescence. Dynamic feedback uses a gradient of a metric as one of the inputs (for example, changes. The dynamic metrics can be introduced and the equations for dynamic feedback are updated as follows:

A phase-dithering process can be used to estimate the dynamic metrics. In the phase-dithering process, the feedback signals are measured at two different settings. For example, to measure two waveforms are set up with following parameters: and ai and az are considered as the feedback metrics for each setting. In this case, Equation [10]

Moreover, a similar approach can be used to estimate While the dynamic metric in this particular example is a derivative (such as it is possible to use other types of dynamic metrics such as extremum seeking.

[0106] The signal processing module 336 can use any combination (for example, dynamic and/or static) feedback for either or both main and sub-coalescence (that is, for the respective Ao, 0 terms and the Ai, <>i terms). The rationale for selecting dynamic or static feedback can be based on the location at which the targets 111 are probed (such as, for example, where the diagnostic light beam 233 interacts with the targets 211), the location at which sub-coalescence occurs, and the location at which main coalescence occurs. In the implementations noted in Equations 4 and 5, the targets 211 are probed at about 70 mm from the end of the tube 209, sub-coalescence occurs at about 100 mm from the end of the tube 209, and main coalescence occurs at about 400 mm from the end of the tube 209. If, for any systems, these distance requirements change, it is possible to use other combinations of dynamic and static feedback.

[0107] As evident from Equations 4 and 5, the feedback applied to the main coalescence (through the adjustments is decoupled from the feedback applied to the sub-coalescence (through the adjustments and “■)■ This is due in part to the fact that the feedback applied to the main coalescence relies on the metrics a and b while the feedback applied to the sub-coalescence relies on

, . . de the metrics c and — dip— • .

[0108] Referring to Fig. 9, the apparatus 100 is a part of an EUV light source 990 that supplies an EUV light beam 980 to an output device 981 (for example, to a substrate at a photolithography exposure apparatus). Specifically, the EUV light beam 980 is produced at the irradiation region 118 when a target 11 Ir in the region 118 is irradiated with an amplified light beam 982 (that is produced by a light source 983). The target 11 Ir is converted into a plasma state upon irradiation with the amplified light beam 882, and this plasma state has an emission line in the EUV range and thus produced EUV light 984. An EUV light collector 985 captures this EUV light 984 and directs it as the EUV light beam 980 to the output device 981. The EUV light source 990 can additionally include a tin management apparatus 991 positioned to adjust a flow of a buffer near the irradiation region 118 to reduce damage to components that are near the irradiation region 118. The coalescence lengths are configured such that the targets fully coalesce before reaching the irradiation region 118 and also at a distance that is away from any region that is impacted by the buffer flow. In some implementations, all of the steps or some of the steps in the procedure 550 are performed as a part of an inline tuning mode while an extreme ultraviolet (EUV) light beam 980 is produced at the plasma generating system and is supplied to an output device 981 (for example, to a substrate at a photolithography exposure apparatus).

[0109] Some aspects of the procedure 550 can be performed as a part of an offline tuning mode, that is, in between exposing the substrate with the EUV light beam 980 that is produced at the plasma generating system. Under certain conditions, such as between exposures of substrates at the photolithography exposure apparatus 981, it may be advantageous to generate satellites at the irradiation region 118 in order to obtain information about how the system is working. In this case, the main or sub-coalescence metrics (a, b, c) can be pushed to a location that is before the location at which the targets 111 are probed by the detection apparatus 130. To put it another way, the main coalescence location (as well as the sub-coalescence location) can be pushed to occur before the location D230. This can be done by using a different feedback control loop that adds a phase shift to the current estimate of an optimum setting, measures the feedback, and then updates the parameter, and then reverts the phase shift. And, this different feedback control loop is performed and completed in between exposures of the substrates and thus is “offline.” As an example, the different feedback control loop uses the following waveforms: and if ai and a ₂ are the feedback metrics, then the adjusted values (Af and fa') of Ao and fa are given by:

Similar logic can be used to control sub-coalescence by way of the values Ai and fa. In some implementations, the offline tuning mode can be performed for the sub-coalescence feedback control to update the values of Ai and fa, while the inline tuning mode can be performed for the main coalescence feedback control to update the values of Ao and fa. The risk of using this offline tuning mode is that during the offline tuning, main coalescence or sub-coalescence could exceed the target metrology. This means that neither the sub-coalescence nor the main coalescence is observed by the apparatus 100. However, after the update using this offline tuning, the target stream 110 is satellite- free at the irradiation region 118. In this way, drift in response of the target material dispenser 105 can be tracked, by performing occasional offline tuning. [0110] The embodiments can be further described using the following clauses:

1. An apparatus comprising: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to the target material in the target material dispenser and configured to induce velocity perturbations in the stream of targets based on a target control signal; a detection apparatus configured to observe the targets in the stream at a point in the stream at which at least some of the targets have partly coalesced but not all of the targets have fully coalesced, and to generate a target detection signal based on the observation; a controller configured to receive the target detection signal and generate a waveform control signal based at least in part on the target detection signal; and a waveform generator in communication with the actuatable element and the controller, and configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal, the target control signal comprising a hybrid waveform including a superposition of a plurality of periodic waveforms.

2. The apparatus of clause 1, wherein the actuatable element comprises an electro-actuatable element.

3. The apparatus of clause 2, wherein the electro- actuatable element comprises a piezoelectric element that is mechanically coupled to a capillary tube of the target material dispenser, the target material passing through the capillary tube.

4. The apparatus of clause 1, wherein the detection apparatus is configured to detect targets in the stream having at least two distinct sizes.

5. The apparatus of clause 1, wherein the detection apparatus is configured to detect at least a spatial offset between at least two of the targets in the stream.

6. The apparatus of clause 1, wherein the controller generating the waveform control signal based at least in part on the target detection signal comprises the controller adjusting one or more parameters of the waveform control signal.

7. The apparatus of clause 6, wherein the controller adjusting one or more parameters of the waveform control signal occurs while the plasma generating system generates plasma from the targets.

8. The apparatus of clause 7, wherein the controller adjusting one or more parameters of the waveform control signal occurs while the plasma generating system produces extreme ultraviolet light that is supplied to a substrate at a photolithography exposure apparatus.

9. The apparatus of clause 1, wherein the controller being configured to generate the waveform control signal based at least in part on the target detection signal comprises the controller analyzing asymmetry of peaks associated with targets that have not coalesced relative to peaks associated with targets that have partly coalesced in the target detection signal. 10. The apparatus of clause 9, wherein the asymmetry of the peaks determines a coalescence length of the target stream.

11. The apparatus of clause 9, wherein the controller being configured to generate the waveform control signal based at least in part on the target detection signal comprises the controller adjusting one or more of an amplitude and a phase of at least one of the periodic waveforms of the target control signal.

12. The apparatus of clause 1, wherein each periodic waveform has a distinct frequency.

13. The apparatus of clause 1, wherein the detection apparatus comprises an illumination source and a light sensitive sensor arranged relative to the target stream.

14. An apparatus comprising: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to target material in the target material dispenser and configured to induce velocity perturbations in the target stream based on a target control signal; a detection apparatus configured to observe the targets in the stream, and to generate a target detection signal based on the observation; a controller configured to receive the target detection signal, analyze both a spacing and a sizing of peaks in the target detection signal, and generate a waveform control signal including updating parameters of the waveform control signal relating to one or more coalescence lengths in the target stream based at least in part on the analysis; and a waveform generator in communication with the actuatable element and the controller, and configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal.

15. The apparatus of clause 14, wherein the target control signal comprises a hybrid waveform including a superposition of a plurality of periodic waveforms.

16. The apparatus of clause 14, wherein the actuatable element comprises a piezoelectric element that is mechanically coupled to a capillary tube of the target material dispenser, the target material passing through the capillary tube.

17. The apparatus of clause 14, wherein the controller is configured to generate the waveform control signal including updating the parameters relating to one or more coalescence lengths occurs while the plasma generating system generates plasma from the targets.

18. The apparatus of clause 14, wherein the controller is configured to generate the waveform control signal including updating the parameters relating to one or more coalescence lengths by analyzing asymmetry of peaks associated with smaller targets relative to a peak associated with a larger target.

19. The apparatus of clause 14, wherein the controller is configured to generate the waveform control signal including updating the parameters relating to one or more coalescence lengths by adjusting one or more of an amplitude and a phase of at least one of the periodic waveforms of the target control signal.

20. The apparatus of clause 14, wherein the detection apparatus comprises an illumination source and a light sensitive sensor arranged relative to the target stream.

21. An apparatus comprising: a target material dispenser configured to provide a stream of targets made of target material for a plasma generating system; an actuatable element coupled to target material in the target material dispenser and configured to induce velocity perturbations in the target stream based on an amplitude of a target control signal; and a waveform generator in communication with the actuatable element and configured to supply the target control signal to the actuatable element, the target control signal comprising a hybrid waveform including a superposition of a first periodic waveform, a second periodic waveform, and a third periodic waveform.

22. The apparatus of clause 21, wherein the actuatable element comprises an electro-actuatable element that is mechanically coupled to the target material in the target material dispenser.

23. The apparatus of clause 22, wherein the waveform generator is electrically coupled to the electro- actuatable element.

24. The apparatus of clause 22, wherein the electro-actuatable element comprises a piezoelectric element that is mechanically coupled to a capillary tube through which the target material is passed.

25. The apparatus of clause 21, further comprising a detection apparatus configured to observe targets in the stream, one or more having not coalesced, one or more having once coalesced, and one or more having twice coalesced, and to generate a target detection signal based on the observation.

26. The apparatus of clause 25, wherein the detection apparatus detects targets in the stream having at least three distinct sizes.

27. The apparatus of clause 25, further comprising a controller configured to adjust one or more parameters of a waveform control signal based on the generated target detection signal, the waveform generator being configured to supply the target control signal to the actuatable element based at least in part on the waveform control signal.

28. The apparatus of clause 27, wherein the controller adjusting one or more parameters of the waveform control signal occurs while the plasma generating system generates plasma from the targets.

29. The apparatus of clause 27, wherein the controller being configured to adjust the one or more parameters of the waveform control signal comprises the controller analyzing asymmetry of peaks associated with the one or more targets that have not coalesced and/or have once coalesced relative to a peak associated with a target that has twice coalesced.

30. The apparatus of clause 29, wherein the asymmetry of the peaks associated with the one or more targets that have not coalesced determines a first coalescence length of the target stream and the asymmetry of the peaks associated with the one or more targets that have once coalesced determines a second coalescence length of the target stream.

31. The apparatus of clause 27, wherein the controller being configured to adjust one or more parameters of the waveform control signal based on the target detection signal comprises the controller adjusting a first coalescence length based on a first set of properties of the target detection signal and adjusting a second coalescence length that is distinct from the first coalescence length based on a second set of properties of the target detection signal that are distinct from the first set of properties of the target detection signal.

32. The apparatus of clause 27, wherein the controller being configured to adjust one or more parameters of the waveform control signal comprises the controller adjusting one or more of an amplitude and a phase of at least two of the periodic waveforms of the target control signal.

33. The apparatus of clause 21, wherein each periodic waveform has a distinct frequency.

34. A method comprising: providing a stream of targets made of target material to an irradiation site; observing targets in the target stream at a point in the stream of targets at which at least some of the targets have partly coalesced but not all of the targets have fully coalesced; generating a target detection signal based on the observation; generating a target control signal based at least in part on the target detection signal, the generated target control signal comprising a hybrid waveform including a superposition of a plurality of periodic waveforms; and inducing velocity perturbations in the stream of targets based on the target control signal.

35. The method of clause 34, wherein generating the target control signal based at least in part on the target detection signal comprises adjusting one or more parameters of the target control signal.

36. The method of clause 35, wherein adjusting one or more parameters of the target control signal occurs while the targets are being provided to the irradiation site.

37. The method of clause 34, wherein generating the target control signal based at least in part of the target detection signal comprises analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal.

38. The method of clause 37, wherein the asymmetry of the peaks determines a coalescence length of the target stream.

39. The method of clause 37, wherein analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal comprises analyzing a difference in spacing or time between two partly coalesced targets and analyzing a relative position between a partly coalesced target and a half-way position between two consecutive partly coalesced targets. 40. The method of clause 34, wherein generating the target control signal based at least in part on the target detection signal comprises adjusting one or more of an amplitude and a relative phase of the periodic waveforms of the target control signal.

41. The method of clause 34, wherein observing the targets in the target stream comprises observing a light produced from an interaction between an illumination beam and the targets in the target stream.

42. A method comprising: providing a stream of targets made of target material to an irradiation site for a plasma generating system; observing targets in the target stream; generating a target detection signal based on the observation; analyzing both a spacing and a sizing of peaks in the target detection signal; generating a target control signal including updating parameters relating to one or more coalescence lengths in the target stream based at least in part on the analysis; and inducing velocity perturbations in the target stream based on the target control signal.

43. The method of clause 42, wherein analyzing both the spacing and the sizing of the peaks in the target detection signal, generating the target control signal based at least in part on the analysis, and inducing velocity perturbations in the target stream based on the target control signal occurs during an inline tuning mode while an extreme ultraviolet light beam is produced at the plasma generating system and is supplied to a substrate at a photolithography exposure apparatus.

44. The method of clause 43, wherein, during the inline tuning mode, generating the target control signal including updating parameters relating to one or more coalescence lengths in the target stream comprises inducing velocity perturbations in the target stream that cause the targets to at least partly coalesce but not fully coalesce prior to observing targets in the target stream.

45. The method of clause 42, wherein analyzing both the spacing and the sizing of the peaks in the target detection signal, generating the target control signal based at least in part on the analysis, and inducing velocity perturbations in the target stream based on the target control signal occurs during an offline tuning mode in between exposing a substrate with an extreme ultraviolet light beam that is produced at the plasma generating system.

46. The method of clause 45, wherein, during the offline tuning mode, generating the target control signal including updating parameters relating to one or more coalescence lengths in the target stream comprises inducing velocity perturbations in the target stream that cause the targets to at least partly coalesce or fully coalesce prior to observing targets in the target stream.

47. A method comprising: providing a stream of targets made of target material to an irradiation site for a plasma generating system; generating a target control signal comprising a hybrid waveform including a superposition of a first periodic waveform, a second periodic waveform, and a third periodic waveform; and inducing velocity perturbations in the target stream based on the target control signal.

48. The method of clause 47, wherein inducing velocity perturbations in the target stream based on the target control signal comprises setting one or more coalescence lengths in the target stream.

49. The method of clause 48, wherein inducing velocity perturbations in the target stream based on the target control signal occurs during an inline tuning mode while an extreme ultraviolet light beam is produced at the plasma generating system and is supplied to a substrate at a photolithography exposure apparatus.

50. The method of clause 49, wherein, during the inline tuning mode, inducing velocity perturbations in the target stream is based on an observation of targets in the stream, and the induction of velocity perturbations causes the targets to at least partly coalesce but not fully coalesce prior to observing the targets in the target stream.

51. The method of clause 48, wherein inducing velocity perturbations in the target stream based on the target control signal occurs during an offline tuning mode in between exposing a substrate with an extreme ultraviolet light beam that is produced at the plasma generating system.

52. The method of clause 51, wherein, during the offline tuning mode, inducing velocity perturbations in the target stream is based on an observation of targets in the stream, and the induction of velocity perturbations causes the targets to at least partly coalesce or fully coalesce prior to observing the targets in the target stream.

53. The method of clause 47, wherein generating the target control signal comprises analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal.

54. The method of clause 53, wherein the asymmetry of the peaks determines a coalescence length of the target stream.

55. The method of clause 53, wherein analyzing asymmetry of peaks associated with targets that have not coalesced relative to a peak associated with a target that has partly coalesced in the target detection signal comprises: analyzing a difference in spacing or time between two partly coalesced targets and analyzing a relative position between a partly coalesced target and a half-way position between two consecutive partly coalesced targets; and analyzing a difference in spacing or time between a target that has not coalesced and an adjacent target that has partly coalesced.

[0111] The breadth and scope of the protected subject matter should not be limited by any of the above-described clauses or embodiments but should be defined only in accordance with the following claims and their equivalents.