Apparatus and method for controlling the performance of a droplet generator

Abstract

To provide a method and apparatus for controlling formation of droplets used to generate EUV radiation.SOLUTION: A droplet source 92 includes: a fluid exiting a nozzle 98; and a sub-system having an electro-actuatable element producing a disturbance in the fluid. The droplet source produces a stream that breaks down into droplets that in turn coalesce into larger droplets as they progress towards an irradiation region. The electro-actuatable element is driven by a control signal having a sine wave component and a square wave component. Various parameters such as a phase difference between the sine wave component and the square wave component are measured and controlled to minimize formation of noncoalesced satellite droplets in the stream.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] Cross-reference of related applications

[0001] This application claims priority to U.S. Patent Application No. 62 / 810,768 filed on 26 February 2019, U.S. Patent Application No. 62 / 928,429 filed on 31 October 2019, and U.S. Patent Application No. 62 / 959,275 filed on 10 January 2020, all of which are incorporated herein by reference in their entirety.

[0002]

[0002] This application relates to extreme ultraviolet ("EUV") light sources and methods of operating them. These light sources provide EUV light by creating a plasma from a source material or a target material. In one application, EUV light is collected and used in a photolithography process to fabricate semiconductor integrated circuits. [Background technology]

[0003]

[0003] To expose a resist-coated substrate such as a silicon wafer and fabricate extremely small features on the substrate, a beam of EUV light with a patterned design may be used. EUV light (sometimes called soft X-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5 nm to about 100 nm. One particular wavelength of interest for photolithography is around 13.5 nm.

[0004]

[0004] Methods for generating EUV light include, but are not limited to, converting a source material into a plasma state having chemical elements that emit lines in the EUV region. These elements may include, but are not limited to, xenon, lithium, and tin.

[0005]

[0005] In one such method, often called laser-produced plasma (LPP), the desired plasma can be generated by irradiating a source material, for example in the form of droplets, streams, or wires, with a laser beam. In another method, often called discharge-produced plasma (DPP), the required plasma can be generated by positioning a source material having a suitable emission line between a pair of electrodes and causing a discharge between the electrodes.

[0006]

[0006] One technique for generating droplets involves melting a target material, such as tin, sometimes referred to as a source material, and then forcing the molten source material through a relatively small-diameter orifice, such as one with a diameter of about 0.1 μm to about 30 μm, under high pressure, thereby generating a layered fluid jet with a velocity in the range of about 30 m / s to about 200 m / s. Under most conditions, instabilities that naturally occur in the flow leaving the orifice, such as thermal noise or vortex emission, will cause the flow to split into droplets. These droplets have varying velocities and combine with each other during flight to form larger droplets.

[0007]

[0007] In the EUV generation process considered here, it is desirable to control the splitting / combining process. For example, to synchronize the droplets with the optical pulses of the LPP drive laser, a repetitive disturbance with an amplitude exceeding the amplitude of random noise can be applied to the continuous layered fluid jet diverging from the orifice. By applying a disturbance with the same frequency (or its harmonics) as the number of repetitions of the pulsed laser, the droplets are synchronized with the laser pulses. For example, the disturbance can be applied to the flow by coupling an electronically actuated element (such as a piezoelectric material) to the flow and driving the electronically actuated element with a periodic waveform. In one embodiment, the electronically actuated element expands and contracts in diameter (by several nanometers). This dimensional change is mechanically coupled to a capillary, which expands and contracts in diameter accordingly. This volumetric displacement generates sound waves and elastic waves in the capillary terminating at the orifice. The target material within the orifice is then periodically accelerated by sound waves, eventually producing widely spaced droplets at the frequency of the drive laser, in a frequency range far below the natural Rayleigh splitting frequency of the fluid microjet. The natural splitting frequency of the fluid jet is in the range of approximately 3 to 15 MHz, while the expected drive laser operation is in the range of approximately 50 to 160 kHz. This means that, in order to obtain the desired final droplets, many small microdroplets must be merged into a periodic droplet flow consisting of droplets much larger than the diameter of the orifice.

[0008]

[0008] As used herein, the term “electronically actuated element” and its derivatives mean a material or structure, including but not limited to piezoelectric materials, electrostrictive materials, and magnetostrictive materials, that undergoes a dimensional change when exposed to a voltage, electric field, magnetic field, or a combination thereof. Apparatuses and methods using electronically actuated elements to control droplet flow are disclosed, for example, in U.S. Patent Application Publication No. 2009 / 0014668A1, entitled “Laser Produced Plasma EUV Light Source Having a Droplet Stream Produced Using a Modulated Disturbance Wave,” published on January 15, 2009, and in U.S. Patent No. 8,513,629, entitled “Droplet Generator with Actuator Induced Nozzle Cleaning,” issued on August 20, 2013. Both documents are incorporated herein by reference in their entirety.

[0009]

[0009] Thus, the challenge of the droplet generator is to place a droplet of appropriate size at the principal focus of a focusing mirror used to collect EUV radiation, where the droplet is used as a target material for generating EUV radiation. The droplet must reach the principal focus at a repeatable position and timing within a specific spatial and temporal stability criterion, i.e., within an acceptable margin. The droplet must also reach at a given frequency and velocity. Furthermore, the droplet must be completely coalesced. That is, the droplet must be monodisperse (of uniform size) and reach at a given driving frequency.

[0010]

[0010] For example, the droplet flow must not contain "satellite" droplets, i.e., smaller droplets of the target material that could not coalesce to form the main droplet. Meeting these criteria is complicated by the fact that, for small orifices and high pressures, it will be necessary to merge many microdroplets using a driving mechanism of an electronically actuated element. The operating window is usually very small, making the system sensitive to performance variations such as performance changes over time. For example, if the performance of the droplet generator changes, it may produce droplets that do not coalesce completely by the time they reach the main focus. Eventually, the performance of the droplet generator will degrade to the point where it must be taken offline for maintenance or replacement.

[0011]

[0011] One method for controlling the coalescence is to impart a hybrid waveform to the molten target material exiting the nozzle. A hybrid waveform is a periodic piezoelectrically excited waveform that can be used to control and optimize the coalescence process in various droplet generators in various systems operating at various power levels, such as 250W. See, for example, international patent application PCT / EP2019 / 050100, filed on 3 January 2019, entitled "Apparatus for and Method of Controlling Coalescence of Droplets in a Droplet Stream," which is incorporated herein by reference in its entirety.

[0012]

[0012] The generation and coalescence of droplets must be controllable in a manner that allows for the optimization of these processes. [Overview of the Initiative]

[0013]

[0013] Below, a brief overview of one or more embodiments is provided to give a basic understanding of those embodiments. This overview is not intended to be a comprehensive overview of all possible embodiments, nor to identify key or significant elements of all embodiments, nor to describe in detail the scope of any or all embodiments. The sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed descriptions to be found later.

[0014]

[0014] According to one aspect of one embodiment, a device is disclosed comprising: a target material dispenser having a nozzle and adapted to provide a flow of target material that splits into first droplets after exiting the nozzle; an electronically operable element mechanically coupled to the target material dispenser and arranged to induce velocity perturbations in the flow based on an applied waveform, the velocity perturbations causing the first droplets to merge in one or more stages to ultimately form second droplets larger than the first droplets within a certain merging distance from the nozzle; and a waveform generator electrically coupled to the electronically operable element and adapted to generate an applied waveform, the waveform having a sinusoidal component and a square component, the sinusoidal component having amplitude, the square component having a phase difference with the sinusoidal component, and the amplitude and phase difference being selected to minimize the merging distance while avoiding abrupt changes in the merging distance. The electronically operable element may be a piezoelectric element.

[0015]

[0015] According to another aspect of one embodiment, a device is disclosed comprising: a target material dispenser having a nozzle and adapted to provide a flow of target material that splits into first droplets after exiting the nozzle; an electronically operable element mechanically coupled to the target material dispenser and arranged to induce velocity perturbations in the flow based on an applied waveform, the velocity perturbations causing the first droplets to merge in one or more stages to form second droplets larger than the first droplets within a certain merging distance from the nozzle, the second droplets being spaced apart such that the second droplets pass a fixed point at an intersection; an intersection detector arranged to determine the intersection of the second droplets and to generate an intersection signal; and a waveform generator electrically coupled to the electronically operable element and adapted to generate an applied waveform, and adapted to generate a waveform applied at least in part based on the intersection signal.

[0016]

[0016] According to another aspect of one embodiment, a method is disclosed comprising the steps of: providing a flow of target material using a target material dispenser, wherein the target material dispenser comprises an electronically actuated element arranged to induce velocity perturbations in the flow based on a droplet control signal; determining whether the flow contains satellite droplets and generating a satellite detection signal indicating whether the flow contains satellite droplets; generating a waveform based at least in part on the satellite detection signal; and supplying the waveform to the target material dispenser. The method may further comprise the step of determining the crossing interval of the flow and generating a crossing interval signal, wherein the step of generating the waveform comprises generating the waveform based at least in part on the crossing interval signal.

[0017] According to another aspect of an embodiment, a method for determining the transfer function of a nozzle of a target material dispenser is disclosed. The method includes dispensing a flow of EUV target material from the target material dispenser; applying a waveform to an electronically actuatable element arranged to induce a velocity perturbation in the flow in response to a control signal; determining a minimum value of the amplitude of the sine wave component of the waveform in which the flow does not include satellites; determining the dependence of the combined length on the phase difference between the sine wave component and the rectangular wave component of the control signal and determining the jump boundary phase difference at which the discontinuity of the dependence occurs; determining the slope of the dependence of the jump boundary phase on the minimum value; determining a drag coefficient based on the slope; and determining the transfer function at the frequency of the sine wave component based on the minimum value and the drag coefficient.

[0018] According to another aspect of an embodiment, a method for optimizing the combined behavior of a flow of EUV target material from a target material dispenser is disclosed. The target material dispenser includes an electronically actuatable element arranged to induce a velocity perturbation in the flow in response to an applied control signal. The method includes determining a minimum value of the amplitude of the sine wave component of the control signal in which the flow does not include satellites; determining the dependence of the combined length on the phase difference between the sine wave component and the rectangular wave component of the control signal and determining the jump boundary phase difference at which the discontinuity of the dependence occurs; determining the slope of the dependence of the jump boundary phase on the minimum value; determining a drag coefficient based on the slope; determining a phase delay designed based on the drag coefficient; and determining an optimal phase difference as the difference between the jump boundary phase difference and the designed phase delay.

[0019] According to another aspect of an embodiment, a method for optimizing the coalescence behavior of the flow of EUV target material from a target material dispenser is disclosed. The target material dispenser includes an electronically actuatable element arranged to induce a velocity perturbation in the flow in response to an applied signal having a sine wave component and a square wave component. The method includes determining the width L of the maximum range of adjacent values of the phase difference between the sine wave component and the square wave component for which the flow contains no satellites. n Determining the width L2 of the maximum range of adjacent values of the phase difference between the sine wave component and the square wave component for which the flow contains satellites, and determining a value S as a statistical measure of the variation in the flow intersection interval within the range having the width L. n m m where ry is a statistical measure of the stability of the flow in the y direction and rz m is a statistical measure of the stability of the flow in the z direction, and determining a value YZStability as a statistical measure of the vector [ry m , rz m .

[0020] where W1, W2, W3, W4 are some positive real numbers, determining a cost function

Number

[0020]

[0021] According to another aspect of one embodiment, a method is disclosed comprising: providing a flow of target material that splits into first droplets after exiting a nozzle using a target material dispenser having a nozzle; using an electronically operable element mechanically coupled to the target material dispenser to induce a velocity perturbation in the flow based on an applied waveform, the velocity perturbation causing the first droplets to merge in one or more stages to ultimately form a second droplet larger than the first droplet within a certain merge distance from the nozzle; and using a waveform generator electrically coupled to the electronically operable element to generate an applied waveform, the waveform having a sinusoidal component and a square component, the sinusoidal component having amplitude, the square component having a phase difference with respect to the sinusoidal component, and the amplitude and phase difference being selected to minimize the merge distance while avoiding abrupt changes in the merge distance.

[0021]

[0022] According to another aspect of one embodiment, a method for operating a target material dispenser in an EUV radiation source is disclosed, comprising the steps of: generating a waveform having a sinusoidal component and a square component, wherein the sinusoidal component has amplitude and the square component has a phase difference with respect to the sinusoidal component; applying the waveform to an electronically operable element mechanically coupled to a target material dispenser having a nozzle for providing a flow of target material, wherein the flow of target material, after leaving the nozzle, splits into first droplets, which then merge in one or more steps to form second droplets larger than the first droplets within a certain merging distance from the nozzle; scanning a plurality of phase differences for a plurality of amplitudes to identify jump boundary combinations of amplitude and phase difference that cause abrupt changes in merging distance, and generating jump boundary curves; and using the combinations of amplitude and phase difference at least partially based on the jump boundary curves during operation of the EUV radiation source.

[0022]

[0023] According to another aspect of one embodiment, a method is disclosed comprising: releasing a flow of initial droplets of a first size from a droplet generator under the control of an electrical signal, wherein the flow of initial droplets merges at least once and travels a merged length to become a flow of final droplets of a second size larger than the first size, and the electrical signal having a first periodic component and a second periodic component that is phase-shifted from the first periodic component by a phase difference; operating the droplet generator with a phase difference of a value that does not include satellite droplets smaller than the second size in the flow of final droplets; and changing the value of the phase difference phase until satellite droplets are generated in the flow of final droplets in order to detect a jump boundary in the functional dependence of the merged length on the value of the phase difference. Operating the droplet generator with a phase difference value such that the final droplet flow does not contain satellite droplets smaller than the second size may involve operating the droplet generator with a phase difference value such that satellite droplets are expected to occur in the final droplet flow, and varying the phase difference value until satellite droplets occur in the final droplet flow in order to detect a jump boundary in the functional dependence of the combined length on the phase difference value may involve increasing the phase difference value until satellite droplets occur in the final droplet flow in order to detect a jump boundary in the functional dependence of the combined length on the phase difference value. The first periodic component may have a first frequency, and the second periodic component may have a second frequency that is an integer multiple of one of the first frequencies. One of the first and second periodic components may be a sine wave, and the other of the first and second periodic components may be a square wave.

[0023]

[0024] According to another aspect of one embodiment, a method for optimizing the combined flow behavior of an EUV target material from a target material dispenser is disclosed, the target material dispenser comprising an electronically actuated element arranged to induce velocity perturbations in the flow in response to an applied signal having a sinusoidal component and a square component, the method comprising: determining a first number range of adjacent values ​​of the phase difference between the sinusoidal component and the square component of the flow that does not contain satellites; determining a second number range of adjacent values ​​of the phase difference between the sinusoidal component and the square component of the flow that contains satellites; and determining the combined flow behavior of the EUV target material that is acceptable when the first number and the second number are equal to 1.

[0024]

[0025] Other embodiments, features, and advantages of the present invention, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. [Brief explanation of the drawing]

[0025]

[0026] The accompanying drawings incorporated herein and forming part thereof illustrate the methods and systems of embodiments of the present invention as examples, not as limitations. The drawings, together with the detailed description, further serve to illustrate the principles of the methods and systems presented herein and to enable those skilled in the art to construct and use these methods and systems. In the drawings, the same reference numerals represent identical or functionally similar elements.

[0026] [Figure 1]

[0027] This is a simplified schematic diagram of an EUV light source connected to an exposure device. [Figure 2]

[0028] This is a schematic diagram of the droplet generation subsystem for an EUV light source. [Figure 3]

[0029] This diagram illustrates a technique for connecting one or more electronically actuated elements to a fluid to create disturbance in the flow exiting an orifice. [Figure 4]

[0030] This diagram shows the state of fusion in a droplet flow. [Figure 5A]

[0031] The components of a synthetic hybrid waveform that can be used according to one aspect of one embodiment are shown. [Figure 5B]

[0031] The components of a composite hybrid waveform that can be used according to one aspect of one embodiment are shown. [Figure 6]

[0032] This is a diagram illustrating satellite formation as a function of phase difference according to one embodiment. [Figure 7]

[0033] This figure shows the satellite formation behavior as a function of the intersection spacing and the ratio of the length of the region having satellites to the region not having satellites, according to one aspect of one embodiment. [Figure 8]

[0034] This diagram shows the dependence of the combined length on the phase difference according to one embodiment of the present invention. [Figure 9]

[0035] This is a map of the combined length as a function of sinusoidal amplitude and rectangular phase according to one embodiment of the present invention. [Figure 10]

[0036] This graph shows the effect of drag force magnitude on the relationship between (1) the product of the nozzle transfer function and the sinusoidal amplitude and (2) the rectangular phase, according to one aspect of one embodiment. [Figure 11]

[0037] This graph shows the effect of time on the relationship between (1) the product of the nozzle transfer function and the sinusoidal amplitude and (2) the rectangular phase, according to one aspect of one embodiment. [Figure 12]

[0038] This figure shows the configuration of the satellite-free region of the operation as a function of the rectangular phase and breaking sine amplitude according to one aspect of one embodiment. [Figure 13]

[0039] This figure shows the relationship between the slope of the jump boundary and the drag coefficient according to one embodiment of the case. [Figure 14]

[0040] This figure shows the relationship between the drag coefficient, nozzle transfer function, and breaking sinusoidal amplitude according to one embodiment of the theory. [Figure 15]

[0041] This flowchart shows the process of inferring the presence or absence of satellite droplets using jump boundary data according to one aspect of one embodiment. [Figure 16]

[0042] This figure shows a specific practice in the coordinate system used to explain EUV emission generation according to one aspect of one embodiment. [Figure 17]

[0043] This figure shows the relationship between the positions of droplets according to one embodiment of the product. [Figure 18]

[0044] This graph shows the relationship between the rectangular phase and the droplet position when the droplet position is related to the nozzle transfer function, according to one aspect of one embodiment. [Figure 19]

[0045] This flowchart shows the process for determining the transfer function of a droplet generator according to one embodiment of the invention. [Figure 20]

[0046] This figure shows the relationship between the positions of droplets according to one embodiment of the product. [Figure 21]

[0047] This figure shows the relationship between the positions of droplets according to one embodiment of the product.

[0027]

[0048] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, will be described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein. [Modes for carrying out the invention]

[0028]

[0049] Next, various embodiments will be described with reference to the drawings. Throughout this text, the same reference numerals are used to refer to the same elements. In the following description, many specific details are given for illustrative purposes to facilitate a complete understanding of one or more embodiments. However, it will be apparent that in some or all cases any of the embodiments described below can be implemented without adopting the specific design details described below. In other cases, well-known structures and devices are shown in block diagram form to facilitate the description of one or more embodiments. Below, a brief overview of one or more embodiments is presented to provide a basic understanding of those embodiments. This overview is not intended to be a comprehensive overview of all possible embodiments, nor is it intended to identify key or significant elements of all embodiments, nor to describe in detail the scope of any or all embodiments.

[0029]

[0050] However, before describing such embodiments in more detail, it is useful to present exemplary environments in which embodiments of the present invention may be carried out. In the following description and claims, the terms “up,” “down,” “top,” “bottom,” “vertical,” “horizontal,” and similar terms may be used. These terms are intended to indicate only relative orientation and not orientation relative to gravity.

[0030]

[0051] Referring first to Figure 1, a schematic diagram of an exemplary EUV radiation source according to one embodiment of the present invention, for example, a laser-generated plasma EUV radiation source 10, is shown. As shown, the EUV radiation source 10 may include a pulsed laser source or a continuous laser source 22, which may be a pulsed gas discharge CO2 laser source that generates a beam 12 of radiation with wavelengths generally less than 20 μm, for example, in the range of about 10.6 μm or up to about 0.5 μm. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

[0031]

[0052] The EUV radiation source 10 also includes a target delivery system 24 that delivers the source material in the form of liquid droplets or a continuous liquid stream. In this example, the source material is a liquid, but it may also be a solid or a gas. The source material may consist of tin or a tin compound, but other materials may be used. In the illustrated system, the source material delivery system 24 introduces droplets 14 of the source material into an irradiation area 28 inside a vacuum chamber 26, where the source material can be irradiated to generate plasma. As used herein, the irradiation area is the area where irradiation of the source material can occur, and is an irradiation area even when irradiation is not actually occurring. The EUV light source may also include a beam focusing and control system 32, as will be described in more detail below in relation to Figure 2.

[0032]

[0053] In the illustrated system, the components are arranged so that the droplet 14 moves substantially horizontally. The direction from the laser source 22 to the irradiation area 28, i.e., the nominal direction of beam 12 propagation, can be considered the Z-axis. The path that the droplet 14 takes from the source material delivery system 24 to the irradiation area 28 can be considered the X-axis. Therefore, the viewpoint in Figure 1 is perpendicular to the XZ plane. Although a system in which the droplet 14 moves substantially horizontally is illustrated, it will be understood by those skilled in the art that other arrangements can be used in which the droplet moves vertically or at an angle including 90 degrees and 0 degrees (perpendicular) between 90 degrees (horizontal) and 0 degrees (vertical) relative to gravity.

[0033]

[0054] The EUV radiation source 10 may also include an EUV light source controller system 60, and in addition to the beam maneuvering system 32, it may also include a laser emission control system 65. The EUV radiation source 10 may also include a detector such as a droplet position detection system, which may include one or more droplet imagers 70 that generate an output representing the absolute or relative position of a droplet with respect to the irradiation area 28 and provide this output to the target position detection feedback system 62.

[0034]

[0055] The droplet position detection feedback system 62 may also use the output of the droplet imager 70 to calculate the position and trajectory of the droplet, from which the droplet position error can be calculated. The droplet position error can be calculated for each droplet, on average, or based on other principles. The droplet position error can then be provided as input to the light source controller 60. In response, the light source controller 60 can generate control signals, such as laser position, direction, or timing correction signals, and provide these control signals to the laser beam steering system 32. The laser beam steering system 32 can use the control signals to change the location and / or focusing force of the laser beam focus within the chamber 26. The laser beam steering system 32 can also use the control signals to change the geometry of the interaction between the beam 12 and the droplet 14. For example, the beam 12 can be made to strike the droplet 14 off-center or at an incidence angle other than directly in front.

[0035]

[0056] As shown in Figure 1, the source material delivery system 24 may include a source material delivery control system 90. The source material delivery control system 90 is operable to adjust the path of the source material through the irradiation area 28 in response to a signal, for example, the droplet position error described above, or some amount determined from the droplet position error provided by the system controller 60. This can be achieved, for example, by repositioning the point at which the source material delivery mechanism 92 releases the droplet 14. The droplet release point can be repositioned, for example, by tilting or shifting the target delivery mechanism 92. The source material delivery mechanism 92 extends into the chamber 26 and is preferably supplied with source material from the outside and connected to a gas source to pressurize the source material in the source material delivery mechanism 92.

[0036]

[0057] Continuing to refer to Figure 1, the radiation source 10 may also include one or more optical elements. In the following discussion, the collector 30 is used as an example of such an optical element, but this discussion applies to other optical elements as well. The collector 30 may be a normal-incident reflector implemented as an MLM, for example, having additional thin barrier layers laminated at each interface to effectively block thermally induced interlayer diffusion, such as B4C, ZrC, Si3N4, or C. Other substrate materials, such as aluminum (Al) or silicon (Si), may also be used. The collector 30 may be in an elongated elliptical shape and has a central aperture to allow the laser radiation 12 to pass through and reach the irradiation area 28. The collector 30 may be ellipsoidal in shape, for example, having a first focal point in the irradiation area 28 and a second focal point (also called the intermediate focal point 40) at a so-called midpoint 40, and the EUV radiation can be output from the EUV radiation source 10 at its midpoint and can also be input to an integrated circuit lithography scanner or stepper 50, for example, which uses the radiation to process a silicon wafer workpiece 52 using known methods, such as using a reticle or mask 54. The mask 54 may be transparent or reflective. For EUV applications, the mask 54 is generally reflective. The silicon wafer workpiece 52 is then further processed using known methods to obtain an integrated circuit device.

[0037]

[0058] Figure 2 illustrates the droplet generation system in more detail. The source material delivery system 90 delivers droplets to the irradiation site / primary focus 48 in the chamber 26. A waveform generator 230 provides a driving waveform to an electronically actuated element in the droplet generator 90 that causes velocity perturbations in the droplet flow. The waveform generator 230 operates under the control of a controller 250, at least in part, based on data from a data processing module 252. The data processing module receives data from one or more detectors. In the illustrated example, the detectors include a camera 254 and a photodiode 256. The droplets are illuminated by one or more lasers 258. In this typical setup, the detectors detect / image droplets in the flow at points where coalescence is expected to have occurred.

[0038]

[0059] Figure 3 schematically illustrates the components of a simplified droplet source 92. As shown in the figure, the droplet source 92 may include a reservoir 94 that holds a fluid 96, such as molten tin, under pressure. As also shown, the reservoir 94 may be formed with a nozzle 98 that allows the pressurized fluid 96 to flow out of the reservoir 94 and subsequently split into multiple droplets, establishing a continuous flow. The waveform generated by the waveform generator 230 is used to drive an electronically actuated element 150 to generate droplets for EUV output. The electronically actuated element 150 generates disturbances in the fluid, which cause droplets with different initial velocities to coalesce, causing at least several adjacent droplet pairs to coalesce before reaching the irradiation area. The ratio of initial microdroplets to coalesced droplets can be any number within the range of, for example, about 10 to about 500.

[0039]

[0060] The entire droplet coalescence process may be considered as a sequence of multiple coalescence steps or regimes unfolding as a function of distance from the nozzle. This is illustrated in Figure 4. For example, in the first regime I, i.e., when the target material first leaves the orifice or nozzle, the target material is in the form of a velocity-perturbed layered fluid jet. In the second regime II, the fluid jet splits into a series of microdroplets with varying velocities. In the third regime III, measured by time of flight or distance from the nozzle, the microdroplets coalesce into intermediate-sized droplets with varying velocities relative to each other, referred to as sub-coalescence droplets. In the fourth regime IV, the sub-coalescence droplets coalesce into a droplet of the desired final size. The number of sub-coalescence steps is variable. The distance from the nozzle to the point where the droplets reach their final coalescence state is the coalescence distance or length L. Ideally, the droplet coalescence distance is as short as possible. Once droplets coalesce into larger droplets, they become less susceptible to the influence of source conditions such as hydrogen flow rate and ion bombardment.

[0040]

[0061] Therefore, the coalescence process when controlled by an excitation signal can be understood as having an initial partial coalescence or sub-coalescence regime that generates intermediate-sized droplets (higher frequency (typically 500 kHz) droplets) with an interval of approximately 2 μs, and a main coalescence in which the sub-coalescence droplets merge to form main droplets with an interval of approximately 20 μs (50 kHz). However, other intervals may be generated in other embodiments.

[0041]

[0062] Therefore, controlling the splitting / combining process involves controlling the droplets so that they are sufficiently combined before reaching the irradiation area and have a frequency corresponding to the pulse rate of the laser used to irradiate the combined droplets. A hybrid waveform may be supplied to an electronically actuated element to control the process by which Rayleigh-splitting microdroplets combine to form a fully combined droplet with a frequency corresponding to the laser pulse rate. The hybrid waveform can essentially consist of a combination of a first low-frequency periodic waveform and a second high-frequency periodic waveform. As an example, the hybrid waveform may consist of a low-frequency sine wave and a high-frequency square or block wave. However, it will be understood that the high-frequency periodic waveform does not necessarily have to be a square wave, and that the teachings herein relating to embodiments in which a square wave is used can be considered equally applicable to embodiments in which the second high-frequency periodic signal is not a square wave, to the extent that the context allows. Thus, the hybrid waveform signal may consist of a low-frequency sine wave (e.g., 50 kHz) shown in Figure 5A and a high-frequency block wave (e.g., 500 kHz) shown in Figure 5B. In the figure, the scales of the time axis and amplitude axis are arbitrary. Such a hybrid waveform can be characterized by reference to five adjustable parameters, including a) sinusoidal amplitude, b) square phase (i.e., the phase difference between the sine wave and the square wave), c) square amplitude, d) square uptime (duty cycle), and d) square frequency. The main merging process depends primarily on the sinusoidal amplitude and square phase, and may further depend on the other three adjustable parameters mentioned above.

[0042]

[0063] As already mentioned, if complete merging is not achieved, the droplet flow will contain smaller droplets, referred to as satellite droplets or microsatellites. The presence of satellite droplets can be detected by several methods, such as the use of a droplet detection module (DDM), crossover spacing, a droplet formation camera (DFC), or a combination thereof, or even through monitoring changes in the EUV signal. Systems and methods for monitoring droplet flow are disclosed, for example, in U.S. Patent No. 9,241,395, issued on January 19, 2016, entitled "System and Method for Controlling Droplet Timing in an LPP EUV Light Source." The entire contents of that patent are incorporated herein by reference. However, using monitoring equipment relatively far from the flow makes it difficult to directly observe satellites or measure the merging distance. It would be useful to have methods for inferring conditions such as the presence of satellites or the merging distance from parameters that can be more directly verified.

[0043]

[0064] Sub-combination is a crucial part of the combination process using hybrid waveform excitation signals. This is because increasing the sub-combination length leads to interference between the main and sub-combination processes, increasing the overall combination length. The increased combination length increases the likelihood of satellite formation due to plasma pressure from the irradiation area.

[0044]

[0065] Furthermore, insufficient sub-combination increases the velocity jitter of the sub-combined droplets (which can be at a frequency of 500 kHz), which can lead to low-frequency satellites (where satellites are located next to a portion of the main droplet) or droplet timing errors. Satellites and timing errors can affect dose stability and collector lifetime in EUV systems.

[0045]

[0066] As a result, it is advantageous to first characterize the sub-combination process and then use that characterization as at least a partial basis for controlling the time-varying signals that generate pressure fluctuations in the droplet generator nozzle. This determination can be repeated, for example, to improve droplet coalescence (e.g., to reduce the rate of satellite droplet formation). When such determination is used to improve the hybrid waveform, the procedure may be called hybrid waveform optimization (HWO).

[0046]

[0067] As implied above, one of the main challenges in optimizing the parameters of the hybrid waveform excitation signal is to characterize the sub-combination process using low-frequency droplet metronome equipment, which would typically be available in an EUV system. In various systems, there may be provisions for image-based low-frequency satellite detection in the illumination area (e.g., the collector's primary focus). The sampling frequency (rate) of this signal may be less than 20 Hz, which is significantly lower than the main droplet frequency, which may be 50 kHz. There may also be provisions for determining the crossover interval, which is the timing between two droplets. The frequency (rate) of this signal may be the same as the main droplet frequency (e.g., 50 kHz). There may also be provisions for image-based position measurement of droplets in the y and x directions. The frequency (rate) of this signal may be 1 kHz, which is lower than the main droplet frequency.

[0047]

[0068] Generally, the signals from these measurements and detections only contain information about the state of the main droplet, and the metronidation equipment may not be available to directly measure sub-combination performance. In one aspect of one embodiment, the Specified herein discloses a system and method for quantifying sub-combination performance using the metronidation described above. This makes it possible to detect the presence of low-frequency satellites after tuning the hybrid waveform excitation signal, which cannot be directly observed by typical metronidation equipment. It also enables the optimization of sub-combination performance using measurements available from conventional metronidation equipment. Furthermore, it enables the characterization of "health metrics" to characterize the performance of droplet generator nozzles in the stable generation of sub-combination droplets. Finally, it enables the optimization of sub-combination to enhance the robustness of the tuning solution.

[0048]

[0069] According to one embodiment of one example, the HWO may be used to optimize parameters such as combinations of the five parameters of the hybrid waveform excitation signal described above. For example, in various embodiments of optimization, two of these parameters (rectangular amplitude and rectangular uptime) may be used to control the sub-combination process.

[0049]

[0070] In particular, phase scan data may be used to quantify sub-combination performance. When used here, a phase scan refers to scanning a rectangular phase parameter to determine a set of conditions for a specific value of the scanned phase. One condition is whether a satellite exists at that value of the rectangular phase. This is a Boolean "yes or no" decision and can be used to set a flag. Another condition to be detected may be the timing spread between two droplets, which is referred to as the crossover interval. A statistical measure of this spread can be determined, for example, an interval within 3 sigma, i.e., 3 standard deviations of the mean. This may also be referred to as the 3-sigma crossover interval, and s iIt is represented by. Another condition that can be detected is the 3-sigma of the y and z positions of the droplet stream. At each setting, the 3-sigma values of the y and z droplet locations are determined, ry i and rz i is represented by.

[0050]

[0071] This data can then be used to optimize the subassembly performance. First, looking at the presence or absence of satellites, generally, there are regions of adjacent rectangular phase settings that produce satellites. Adjacent means that the region is not interrupted by values that do not produce satellites. Note that this region can be determined by satellite detection using DFC. The result is shown in a representative example of FIG. 6, which is a plot of the 3-sigma crossing interval as a function of the rectangular phase. In FIG. 6, the values of the rectangular phase that do not produce satellites are indicated by ordinary points. The values of the rectangular phase that produce satellites are indicated by points surrounded by circles. The region where satellites occur (the phase range of adjacent phase values) can be referred to as a satellite island SI. The region without satellites can, similarly, be referred to as a satellite-free island SFI. According to one technique for analyzing this data, the width L1 of the rectangular phase of the widest satellite-free island is determined. The satellite-free island width in the case of a complete subassembly is L n is represented by. In other words, L n is the theoretical value of the island width assuming that the mating process starts with uniformly distributed droplets.

[0051]

[0072] Note that L n is a function of the product of TF (the nozzle transfer function at 50 kHz) and the sine Ampl (the amplitude of the sine wave component) and the droplet drag coefficient. When the measured island width is equal to L n there are uniformly distributed subassembly droplets.

[0052]

[0073] Next, the width of the widest satellite island is determined. This width is represented by L2. In the case of a complete subassembly, this quantity would be equal to 2π - L n

[0053] ​

[0074] In the next step, a statistical measure such as the p-norm of the crossover interval within the satellite-free region is determined, S m It is expressed as follows.

[0054]

[0075] Next, a statistical measure such as the 3-sigma p-norm of y-stability is determined as ry_m, and a statistical measure such as the 3-sigma p-norm of z-stability is determined as rz_m. Then, YZStability is given by the vector [ry m ,rz m It is determined as the weighted p-norm of ].

[0055]

[0076] Based on the above determination, the sub-coalescence quantifier can be defined as the value that minimizes the cost function, for example, as follows:

[0056]

[0077]

number

[0057]

[0078] Here, W1, W2, W3, and W4 are some positive real numbers.

[0058]

[0079] The above metrics can provide a useful estimate of the likelihood of plasma-induced satellite existence. In other words, the sub-coalition parameters can be set to the minimizer of the above cost function quantities. By setting the sub-coalition parameters to the minimizer of the above cost function quantities, the presence of satellites can be minimized, and therefore the sub-coalition performance is optimized.

[0059]

[0080] Another metric for sub-merging performance is that it may be considered unacceptable if multiple satellite-free regions are found. In other words, if sub-merging performance is acceptable, the dependence of the 3-sigma intersection spacing as a function of the rectangular phase should have one satellite-free region and one satellite region.

[0060]

[0081] As already mentioned, satellites appear when sub-combination performance collapses. Sub-combination is related to the high-frequency component of the nozzle transfer function. Therefore, the metrics related to sub-combination as described above provide feedback on that component of the nozzle transfer function.

[0061]

[0082] The above technologies / metrics also provide objective functions for the HWO optimization process. Furthermore, they provide metrics for quantifying droplet generator performance with respect to the sub-combination process.

[0062]

[0083] The technologies described herein can reduce the likelihood of generating plasma-induced satellites, increase the lifespan (robustness) of the adjustment solution as determined by HWO, and provide performance indicators based on sub-combination performance to assist in replacement decisions, i.e., decisions to replace the DG in a deactivated state.

[0063]

[0084] Regarding the phase scan ensemble, the above metrics (more specifically L1 and S) mThe probability of plasma-induced satellites can be calculated, and it can be determined which phase scans will produce plasma-induced satellites. Phase scans with plasma-induced satellites and phase scans without plasma-induced satellites can be classified based on a linear combination of these two metrics. In Figure 7, phase scans with plasma-induced satellites are indicated by asterisks, and phase scans without plasma-induced satellites are indicated by white circles. In this way, it is possible to predict the probability of plasma-induced satellites based on phase scan data. This data can be obtained when there is no plasma during the adjustment of the droplet generator.

[0064]

[0085] The combined length decreases approximately linearly as the sinusoidal amplitude increases, based on both combined simulations and bench tests in vacuum. The minimum combined length is located at the center of the satellite-free region constructed by modifying the rectangular phase.

[0065]

[0086] The term "breaking sinusoidal amplitude" is used to refer to the minimum sinusoidal amplitude that can produce a satellite-free configuration. This is a function of the drag coefficient. It can be used to correct the transfer function to account for drag. Typically, the transfer function is determined based on the assumption that the effect of drag on droplets in an EUV vessel is negligible. In reality, the hydrogen flow rate in the vessel can cause droplets to experience a non-negligible amount of drag. Essentially, the drag is determined, and from that the breaking sinusoidal amplitude is determined. This leads to a corrected transfer function. The calculation of this transfer function may need to be corrected based on the drag coefficient.

[0066]

[0087] Furthermore, it has been found that the drag force arising from the hydrogen flow rate near the droplets and the stable container pressure can significantly affect the coalescence process. The drag force acting on the coalesced droplets requires special attention in the process of optimizing the hybrid waveform parameters. Estimation of the transfer function based on the breaking sinusoidal amplitude in the HWO procedure may not be accurate if the presence of drag is not taken into account. Also, when drag is present, the center of the satellite-free region (in the rectangular phase space) is not the minimum point of the coalescence length.

[0067]

[0088] Regarding drag, the droplets in the regime under consideration can be treated as spheres, each with a diameter d. Furthermore, in the regime under consideration, the Reynolds number is relatively small, and therefore the drag force FD of a gas flowing at a constant velocity through a sphere can be approximated as follows.

[0068]

[0089]

number

[0069]

[0090] One consequence of considering drag is that the location of the minimum point of the coalescing length becomes partially dependent on the vessel pressure. The coalescing length is a discontinuous function of the excitation signal parameters, and the minimum point of the coalescing length (close to the discontinuity) is not a robust operating point with respect to changes in the nozzle transfer function.

[0070]

[0091] As mentioned above, the HWO procedure can optimize the five parameters of the hybrid waveform excitation signal described above. In various embodiments of this procedure, two of these parameters (sine amplitude and square phase) may be used to control the main coalescence process, and the remaining parameter may be used to control the sub-coalescence. An optimization procedure that takes drag into account offers the potential for improved performance compared to an optimization that assumes drag is negligible. This is especially true when there is a hydrogen flow rate rather than negligible vessel pressure.

[0071]

[0092] As already mentioned, the convergence length is the minimum distance from the nozzle at which all the microdroplets merge, for example, to form a 50 kHz droplet. If the convergence length is less than the distance between the nozzle and the principal focus of the EUV collector, a satellite-free configuration is obtained. The ideal operating point would have a small convergence length to provide robustness against hydrogen flow and shock waves originating from the plasma.

[0072]

[0093] The functional dependence of the combined length on the rectangular phase exhibits a discontinuity when drag is present. The location of this discontinuity is one of the satellite-free region boundaries, referred to as a jump boundary, as shown in Figure 8. In a simplified version of the HWO procedure, the operating rectangular phase may simply be set to the center of the satellite-free region, which is not the minimum point of combined length when drag is present. However, there are advantages to using an HWO procedure that can determine an operating rectangular phase value that is robust to nozzle performance variations and can provide a smaller combined length. In various embodiments, this process can be performed using only a satellite detector target formation metrology (TFM) and DFC in the primary focus / irradiation region.

[0073]

[0094] In summary, for fixed sinusoidal amplitude, rectangular uptime, rectangular amplitude, and rectangular frequency, the combined length is a discontinuity function of the rectangular phase. That is, a plot of the combined length as a function of the rectangular phase will show discontinuities for several rectangular phase values. Immediately before the discontinuity, the combined length is at or near its minimum value, and at the discontinuity, the combined length is at or near its maximum value. The location of this discontinuity is referred to herein as a jump boundary.

[0074]

[0095] Another method for evaluating this phenomenon is to consider the rectangular phase that causes the discontinuity as a function of the amplitude of the sinusoidal component. This is shown in Figure 9. In Figure 9, the x-axis is the increasing sinusoidal amplitude, and the y-axis is the increasing rectangular phase. The gray shading, extending from dark to light, represents the change in combined length, with darker areas indicating shorter lengths and lighter areas indicating longer lengths. The resulting boundary between the area of ​​maximum combined length (lightest) and the area of ​​minimum combined length (darkest) defines a curve that shows the dependence of the jump boundary location as a function of the sinusoidal amplitude.

[0075]

[0096] The jump boundary curve in Figure 9 provides a tool for determining the position of jump boundaries with sinusoidal amplitudes of various values ​​from a measurement at just one position on the jump boundary curve. In other words, once the shape of a part of the curve is determined, the shape of other parts of the curve can be determined by extrapolation. The curve can be calibrated for various drag conditions by using a lookup table that shows the y-displacement of the curve for various drag conditions.

[0076]

[0097] The jump boundary curves depend on the rectangular uptime and rectangular amplitude, and can also change over time (phase drift). However, generally, the shape of the curves defined by this dependence remains approximately identical to the change in time or drag force; instead, the effects of these changes will be phase drift, shifting these curves along the y-axis. This is illustrated in Figures 10 and 11. Figure 10 shows three jump boundary curves. The one labeled Large F0 represents a curve with a relatively large drag force, the one labeled Small F0 represents a curve with a relatively small drag force, and the unmarked intermediate curve represents a drag force of an intermediate magnitude. As can be seen, these curves have essentially the same shape and are simply shifted perpendicularly to each other. The amount of shift indicates the magnitude of the drag force. However, Figure 11 shows that the shift in the curves can also be at least partially attributable to the passage of time.

[0077]

[0098] The drag force and transfer function can also be determined from the breaking sinusoidal amplitude and the slope of the curve that defines the dependence of the jump boundary on the sinusoidal amplitude. In Figure 12, the breaking sinusoidal amplitude is first determined by performing a recursive phase scan with various sinusoidal amplitudes. From this, a curve that defines the dependence of the jump boundary on the sinusoidal amplitude is established. Therefore, the jump boundary curve can also be used to determine the drag coefficient (Figure 13). The transfer function can be calculated based on the breaking sinusoidal amplitude and the drag coefficient (Figure 14). Since the transfer function is essentially the horizontal scale of the curve that shows the dependence of the jump curve on the sinusoidal amplitude, the jump boundary can be determined without determining the crossing interval.

[0078]

[0099] Once a jump boundary is determined, it is possible to select operating conditions that minimize the merged length by preventing it from approaching the jump boundary. The merged length obtained using this procedure is smaller than that obtained using other techniques, which is desirable because it provides a margin for flow rate and plasma disturbances. The above method also minimizes the possibility of generating plasma-induced satellites.

[0079]

[0100] Since the transfer function is simply a scale on the horizontal axis, the above provides a method for estimating the nozzle transfer function. This is also a method for optimizing the parameters of the hybrid waveform excitation signal. Furthermore, it provides a novel method for determining the vessel pressure and the drag coefficient within the vessel.

[0080]

[0101] A statistical measure of the variability of the crossing interval, such as the 3-sigma value of the crossing interval, will increase near the jump boundary. This is an alternative method for locating the jump boundary based on the crossing interval without using satellite detector metrology.

[0081]

[0102] As already mentioned, quantifying these parameters allows for the characterization of satellite formation, jump boundaries, and coalescence length, even if the satellite detection metrology is far from the nozzle and these conditions cannot be directly observed.

[0082]

[0103] As shown in Figure 15, in one embodiment, flow conditions such as merge length or satellite conditions (presence or absence) can be inferred from jump boundary data. In the first step S10, a sinusoidal amplitude value is selected. In step S20, the rectangular phase is scanned with respect to the sinusoidal amplitude at that time, and the jump boundary is determined as a combination of sinusoidal amplitude and rectangular phase that shows a sudden increase in merge length, i.e., a discontinuity. In step S30, it is determined whether a rectangular phase scan has been performed for all desired values ​​of sinusoidal amplitude. If YES, in step S40, the flow conditions (in the figure, satellite conditions as an example) for a given combination of sinusoidal amplitude and rectangular phase are inferred using the jump boundary data during operation. Otherwise, in step S35, the sinusoidal amplitude is changed, and the process returns to step S10.

[0083]

[0104] As previously mentioned, when droplets are formed using a hybrid waveform, first, microdroplets, i.e., droplets initially formed from the splitting of the flow leaving the droplet generator nozzle, coalesce to form high-frequency (typically 500 kHz) droplets, which are referred to herein as sub-combined droplets. These sub-combined droplets then coalesce to form fully combined main-frequency (typically 50 kHz) droplets. Ideally, during operation, neither these microdroplets nor sub-combined droplets reach the irradiation site. If they do, any of these high-frequency sub-combined droplets that reach the primary focus are referred to herein as sub-combined satellite droplets. Any microdroplets that reach the primary focus are referred to as microdroplet satellites. Further techniques for optimizing the parameters of the hybrid waveform excitation signal utilize droplet and satellite position and size information.

[0084]

[0105] As already mentioned, generally speaking, in the reference coordinate system, as shown in the conceptual diagram of the EUV system in Figure 16, Z is the direction along which the laser beam 12 propagates, as well as the direction from the collector 30 to the irradiation site or principal focus 28 and the EUV intermediate focus. X is in the droplet propagation plane. Y is orthogonal to the XZ plane. To make this a right-handed coordinate system, the trajectory of the droplet flow 14 is assumed to be in the -X direction. The origin is the irradiation site 28. The presence of satellite droplets in the flow at the irradiation site 28 can be detected by one or a combination of several methods, for example, by using DDM or DFC to observe the flow at the irradiation site.

[0085]

[0106] As shown in Figure 17, each of the fully coalesced droplets 400 has a sinusoidal amplitude range in its vicinity, with a sub-coaled satellite droplet 410 displaced in the -X direction of flow propagation, indicated by an arrow, also referred to herein as the flow direction, from the fully coalesced droplet. The distance in the X direction between the droplet and the satellite, labeled "A" in the figure, is referred to as STDD (Satellite-to-Droplet Distance). STDD is a linear function of rectangular phase, as shown in Figure 18. In Figure 18, the line labeled 450 shows the simulation of the rectangular phase of the sinusoidal amplitude for STDD vs. a first value, the line labeled 460 shows the simulation of the rectangular phase of the sinusoidal amplitude for STDD vs. a second value, and the line labeled 470 shows the simulation of the rectangular phase of the sinusoidal amplitude for STDD vs. a third value. The slopes of these lines are measurements of the transfer function * sinusoidal amplitude and can therefore be used to determine the nozzle transfer function.

[0086]

[0107] The analytical formula that quantifies the relationship between STDD and rectangular phase, denoted by φ, is determined as follows:

number

[0087]

[0108] The nozzle transfer function is an important indicator of the operating state of a droplet generator, as it shows the amount of voltage the droplet generator requires to impose a given relative velocity on the droplets. This relative velocity determines how quickly the droplets coalesce. The transfer function can be used to guide replacement decisions, for example, to replace a droplet generator if it cannot generate a relative velocity sufficient at the maximum input voltage to achieve complete coalescence at an acceptable distance before reaching the irradiation area.

[0088]

[0109] As shown in the flowchart in Figure 19, according to one embodiment, a sinusoidal amplitude value is selected in step S50. In step S60, the functional dependence of STDD on the rectangular phase at the selected sinusoidal amplitude is determined. In step S70, the transfer function is determined from the slope of the functional dependence determined in step S60. In step S80, the droplet generator (DG in the figure) operates according to the determined transfer function, and this includes, but is not limited to, a possible evaluation of the droplet generator for maintenance or replacement according to the transfer function.

[0089]

[0110] There are several ways to detect sub-combined satellite droplets. Another method involves using an imager such as DFC to determine whether the size of all satellite droplets corresponds to a known size of the sub-combined droplet. Here and elsewhere, "correspond" means that the size of the satellite is closer to the size of the sub-combined droplet than the size of the fully combined droplet or microdroplet, or that the microdroplet is equal to the high-frequency droplet.

[0090]

[0111] Another method for detecting sub-combined satellite droplets is based on the fact that the coordinates of the satellite's position in the lateral (Z) direction are an indirect measurement of the droplet size. The hydrogen flow rate in the chamber separates the fully combined droplets from the smaller droplets in the lateral direction. Thus, the sub-combined satellite droplet is translated by a specific distance B in the Z direction from the main fully combined droplet, as shown in Figure 20 for given flow rate conditions in the chamber.

[0091]

[0112] The position and size information of droplets and satellites can also be used to measure the sub-combination length, that is, the distance from the nozzle exit to the point where microdroplets combine to form sub-combination droplets. One technical challenge in hybrid waveform tuning is that increasing the sinusoidal amplitude can cause interference between main and sub-combination. As the main combination length decreases, the sub-combination process is influenced by the strong velocity generated by the low-frequency (sine) portion of the signal. In other words, as shown in Figure 21, as the voltage of the low-frequency portion of the signal increases, microdroplet satellites will be observed. The sub-combination length can be determined by measuring the minimum sinusoidal amplitude value that generates microdroplet satellites. Therefore, the sub-combination length can be used as an objective function in the optimization process of sub-combination parameters (rectangular uptime, rectangular amplitude). The process also provides an upper limit on the sinusoidal amplitude value that can be used to optimize the sinusoidal amplitude. This can be used as a parameter to assess the operating state of the droplet generator.

[0092]

[0113] The present invention has been described above with the help of functional building blocks illustrating embodiments of specific functions and their relationships. The boundaries of these functional building blocks are arbitrarily defined herein for the sake of clarity. Alternative boundaries may be defined, provided that the specific functions and their relationships are adequately implemented.

[0093]

[0114] The above description relating to specific embodiments has sufficiently revealed the overall nature of the invention, and by applying knowledge of the art, such specific embodiments can be readily modified and / or adapted to various uses without excessive experimentation and without departing from the overall concept of the invention. Accordingly, such adaptations and modifications shall fall within the meaning and scope of the equivalents of the disclosed embodiments based on the teachings and guidance presented herein. Since the expressions or terms herein are for illustrative purposes only and not limitation, it should be understood that the terms or expressions herein should be interpreted by those skilled in the art in terms of teachings and guidance. The breadth and scope of the invention is not limited by any of the exemplary embodiments described above, but is defined solely by the following claims and equivalents.

[0094]

[0115] Other aspects of the present invention are described in the following numbered clauses. 1. A target material dispenser having a nozzle and adapted to provide a flow of target material that splits into first droplets after exiting the nozzle, An electronically actuated element mechanically connected to a target material dispenser and arranged to induce velocity perturbations in the flow based on an applied waveform, wherein the velocity perturbations cause a first droplet to coalesce in one or more stages, ultimately forming a second droplet larger than the first droplet within a certain coalescing distance from the nozzle. A waveform generator adapted to generate a waveform to be electrically connected to and applied to an electronically actuated element, wherein the waveform has a sinusoidal component and a square wave component, the sinusoidal component has amplitude, the square wave component has a phase difference with respect to the sinusoidal component, and the amplitude and phase difference are selected to minimize the merged distance while avoiding abrupt changes in the merged distance. A device equipped with the following features. 2. The apparatus described in Clause 1, wherein the electronically actuated element is a piezoelectric element. 3. A target material dispenser having a nozzle and adapted to provide a flow of target material that splits into first droplets after exiting the nozzle, An electronically actuated element mechanically connected to a target material dispenser and arranged to induce velocity perturbations in the flow based on an applied waveform, wherein the velocity perturbations cause a first droplet to merge in one or more stages, ultimately forming a second droplet larger than the first droplet within a certain merging distance from the nozzle, and the second droplets are spaced apart so that they pass a fixed point at an intersecting interval. An intersecting interval detector is provided to determine the intersecting interval of a second droplet and to generate an intersecting interval signal. A waveform generator electrically coupled to an electronically actuated element, adapted to generate an applied waveform, and adapted to generate a waveform applied at least partially based on an intersecting signal, A device equipped with the following features. 4. Providing a flow of a target material using a target material dispenser, wherein the target material dispenser comprises an electronically actuated element arranged to induce velocity perturbations in the flow based on a droplet control signal. The system determines whether the flow contains satellite droplets and generates a satellite detection signal indicating whether the flow contains satellite droplets. To generate a waveform based at least partially on the satellite detection signal, The waveform is supplied to the target material dispenser, A method that includes [a certain feature]. 5. The method according to Clause 4, further comprising determining the crossing interval of flows and generating a crossing interval signal, wherein the step of generating a waveform comprises generating a waveform based at least in part on the crossing interval signal. 6. A method for determining the transfer function of a nozzle of a target material dispenser, Dispensing the flow of EUV target material from the target material dispenser, Applying a waveform to an electronically actuated element positioned to induce velocity perturbations in the flow in response to a control signal, The process involves determining the minimum amplitude of the sinusoidal component of the waveform that does not include satellites, and This involves determining the dependence of the combined length on the phase difference between the sinusoidal and square wave components of the control signal, and also determining the jump boundary phase difference where a discontinuity in the dependence occurs. Determining the slope of the dependence of the jump boundary phase on the minimum value, Determining the drag coefficient based on the slope, Determining the transfer function at the sinusoidal component frequency based on the minimum value and drag coefficient, A method that includes [a certain feature]. 7. A method for optimizing the combined flow behavior of an EUV target material from a target material dispenser, wherein the target material dispenser comprises an electronically actuated element arranged to induce velocity perturbations in the flow in response to an applied control signal, and the method is: The process involves determining the minimum amplitude of the sinusoidal component of the control signal that does not include satellite signals, This involves determining the dependence of the combined length on the phase difference between the sinusoidal and square wave components of the control signal, and also determining the jump boundary phase difference where a discontinuity in the dependence occurs. Determining the slope of the dependence of the jump boundary phase on the minimum value, Determining the drag coefficient based on the slope, Determining the phase delay designed based on the drag coefficient, The optimal phase difference is determined as the difference between the jump boundary phase difference and the designed phase delay, A method that includes [a certain feature]. 8. A method for optimizing the combined flow behavior of an EUV target material from a target material dispenser, wherein the target material dispenser comprises an electronically actuated element arranged to induce velocity perturbations in the flow in response to an applied signal having sinusoidal and square wave components, and the method is: The width L of the maximum range of adjacent values ​​of the phase difference between the sinusoidal component and the square wave component, which does not include satellites. n To determine, The flow determines the width L2 of the maximum range of adjacent values ​​of the phase difference between the sinusoidal component and the square wave component, including the satellite. Width L nThe value S as a statistical measure of the variability of flow crossing intervals within the range having m To determine, ry m rz is a statistical measure of flow stability in the y direction. m While is a statistical measure of flow stability in the z direction, the value YZStability is a vector [ry m ,rz m To judge as a statistical measure of ], When W1, W2, W3, and W4 are some positive real numbers, the cost function

number

Claims

1. A target material dispenser having a nozzle and adapted to provide a flow of target material that splits into first droplets after exiting the nozzle, An electronically actuated element mechanically connected to the target material dispenser and arranged to induce velocity perturbations in the flow based on an applied waveform, wherein the velocity perturbations cause the first droplets to merge in one or more stages to form second droplets larger than the first droplets within a certain merging distance from the nozzle, and the second droplets are spaced apart such that the second droplets pass through fixed points at crossing intervals. An intersection interval detector is arranged to determine the intersection interval of the second droplet and to generate an intersection interval signal. A waveform generator electrically connected to the electronically operable element and adapted to generate the applied waveform, and adapted to generate the applied waveform at least partially based on the crossover signal, wherein the applied waveform has a sinusoidal component and a square wave component, A device equipped with the following features.

2. The apparatus according to claim 1, wherein the electronically actuated element is a piezoelectric element.

3. The waveform generator is configured to process data based on at least a portion of it. The apparatus according to claim 1, further comprising a data processing module.

4. The apparatus according to claim 3, wherein the data processing module is configured to receive data from at least one detector.

5. The apparatus according to claim 4, wherein the at least one detector includes at least one of a camera and a photodiode.

6. The apparatus according to claim 1, wherein the target material dispenser further comprises a reservoir configured to hold the target material under pressure.

7. Providing a flow of target material using a target material dispenser, wherein the target material dispenser comprises an electronically actuated element arranged to induce velocity perturbations in the flow based on a droplet control signal, The system determines whether the flow contains satellite droplets and generates a satellite detection signal indicating whether the flow contains satellite droplets. The method of generating a waveform based at least partially on the satellite detection signal, wherein the waveform has a sinusoidal component and a square wave component. The waveform is supplied to the target material dispenser, A method that includes [a certain feature].

8. The method according to claim 7, further comprising determining the intersection interval of the flow and generating an intersection interval signal.

9. The method according to claim 8, wherein generating the waveform is performed by generating the waveform based at least partially on the crossover interval signal.

10. The method according to claim 7, wherein the electronically actuated element is a piezoelectric element.

11. Further comprising providing a data processing module configured to process data, The method according to claim 7, wherein generating the waveform is at least partially based on the data processed by the data processing module.

12. The method according to claim 11, wherein the data processing module receives data from at least one detector.

13. The method according to claim 12, wherein the at least one detector includes at least one of a camera and a photodiode.

14. The method according to claim 7, wherein the target material dispenser further comprises a reservoir configured to hold the target material under pressure.

Images