Apparatus and method for extending the life of a targeted material delivery system

Abstract

A system for generating EUV radiation is disclosed in which current flowing through a target material in an orifice 320 of a nozzle in a drop generator is controlled by providing alternate lower impedance paths for the current and / or by limiting high frequency components of a drive signal applied to the drop generator.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 752,116, filed on October 29, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to apparatus and methods for plasma generation of extreme ultraviolet ("EUV") radiation, the plasma being produced by discharge or laser ablation of a target material in a container. In such applications, optical elements are used, for example, to collect and direct the radiation for use in semiconductor lithography and inspection. Background Art

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

[0005] Methods for generating EUV radiation include converting a target material into a plasma state. The target material preferably includes at least one element, such as xenon, lithium or tin, and has one or more emission lines in the EUV portion of the electromagnetic spectrum. The target material can be a solid, liquid or gas. One technique involves generating a stream of droplets of the target material and irradiating at least some of the droplets with one or more pulses of laser radiation. Such a source generates EUV radiation by coupling laser energy into a target material having at least one EUV emitting element, thereby producing a highly ionized plasma with an electron temperature of tens of eV.

[0006] One technique for generating droplets involves melting a target material, such as tin, and then forcing it through a relatively small diameter orifice (such as an orifice having a diameter of about 0.5 μm to about 30 μm) under high pressure to produce a droplet stream with a droplet velocity ranging from about 30 m / s to about 150 m / s. Under most conditions, instabilities in the stream exiting the orifice cause the stream to break into droplets in a process known as Rayleigh breakup. These droplets may have varying velocities and may combine with each other to aggregate into larger droplets.

[0007] In the EUV generation process considered here, it is desirable to control the decomposition / aggregation process. For example, in order to synchronize the droplets with the light pulses of the driving laser, a repetitive disturbance with an amplitude exceeding that of random noise can be applied to the continuous stream. By applying the disturbance at the same frequency as the repetition frequency of the pulsed laser (or its higher harmonics), the droplets can be synchronized with the laser pulses. For example, the disturbance can be applied to the stream by coupling an electrically actuatable element (such as a piezoelectric material) to the stream and driving the electrically actuatable element with a periodic waveform. In one embodiment, the diameter of the electrically actuatable element will contract and expand (in nanometers). This change in size is mechanically coupled to a structure that defines a cavity, such as a tube or capillary that undergoes a corresponding contraction and expansion of the diameter. The diameter of the column of target material (e.g., molten tin) within the cavity also contracts and expands (and also expands and contracts in length) to induce velocity disturbances in the stream at the nozzle outlet.

[0008] As used herein, the term "electrically actuatable element" and its derivatives mean a material or structure that undergoes a dimensional change when subjected to a voltage, an electric field, a magnetic field, or a combination thereof, including but not limited to piezoelectric materials, electrostrictive materials, and magnetostrictive materials. Apparatus and methods for controlling droplet streams using electrically actuatable elements 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," published on August 20, 2013, the entire contents of which are incorporated herein by reference in their entirety.

[0009] The task of the droplet generator is therefore to place droplets of the appropriate size at the primary focus that will be used to generate EUV. The droplets must arrive at the primary focus within certain spatial and temporal stability criteria, that is, the position and timing are repeatable within acceptable margins. They must also arrive at a given frequency and speed. In addition, the droplets must be fully aggregated, which means that the droplets must be monodisperse (uniform in size) and reach a given drive frequency. For example, the droplet stream should be free of coaxial "satellite" droplets, that is, smaller droplets of the target material that fail to aggregate into a main droplet. Meeting these criteria is complicated by the fact that droplet generator performance varies over time. For example, when the performance of a droplet generator varies, droplets may be generated that are not fully aggregated when the droplets arrive at the primary focus. Eventually, the performance of the droplet generator will degrade to the point where the droplet generator must be taken offline for maintenance or replacement.

[0010] Another failure mode of such droplet generators is a gradual drift in the droplet stream angle. Such drift leads to instability in EUV source operation and, in some instances, to loss of droplets when the angle becomes too large and droplets begin to pinch the exit orifice of the droplet generator. Such drift tends to be unidirectional and may grow until the droplet generator steering system is out of range to correct the droplet position or until a droplet is pinched by the exit orifice. Such loss of droplets may lead to droplet generator replacement, affecting the availability of the entire system.

[0011] Therefore, there is a need to extend the lifetime of such droplet generators in order to increase the availability of the system. Summary of the invention

[0012] An overview of one or more embodiments is presented below in order to provide a basic understanding of the embodiments. This overview is not an exhaustive overview of all contemplated embodiments, and is not intended to identify key or important elements of all embodiments, nor is it intended to set limitations on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to a more detailed description that will be presented later.

[0013] A system for generating EUV radiation is disclosed in which current flowing through a target material in an orifice of a nozzle in a droplet generator is controlled by providing alternate lower impedance paths for the current and / or by limiting high frequency components of a drive signal applied to the droplet generator.

[0014] According to one aspect of an embodiment, an apparatus for generating EUV radiation is disclosed, the apparatus comprising: a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, an electrical connector connected to the electrically actuatable element, the electrical connector being arranged to control an amount of current flowing through the target material at the orifice. The electrical connector connected to the electrically actuatable element may be arranged to provide a low impedance path between the electrically actuatable element and ground, the low impedance path not passing through the target material at the orifice. The structure defining the cavity may include a cylindrical tube, and the electrically actuatable element includes a cylindrical piezoelectric element arranged around the cylindrical tube and having an inner surface connected to ground via a low impedance path. The target material dispenser may also include a conductive coating around at least a portion of the structure defining the cavity. The conductive coating may have a resistivity of less than about 1E-06 Ohm-m. The conductive coating may be confined to a region of the structure defining the orifice. The electrically actuatable element may be positioned around a first axial portion of the cavity that does not have the conductive coating. The conductive coating may be connected to ground via a low impedance path. The device may also include an insulating coating on top of the conductive coating. The drive signal generator may be electrically coupled to the electrically actuatable element via an RF coaxial cable directly terminated at the electrically actuatable element.

[0015] According to another aspect of an embodiment, a device for generating EUV radiation is disclosed, the device including a target material dispenser, the target material dispenser including a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a highest frequency component of the drive signal is limited to a value in the range of about 3.5 MHz to about 7 MHz.

[0016] According to another aspect of an embodiment, a device for generating EUV radiation is disclosed, the device comprising a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a minimum rise / fall time of the drive signal is in the range of about 50 ns to about 100 ns.

[0017] According to another aspect of an embodiment, a device for generating EUV radiation is disclosed, the device comprising a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the orifice.

[0018] According to another aspect of the embodiment, an apparatus for generating EUV radiation is disclosed, the apparatus comprising a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive the target material, the orifice being arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, wherein the drive signal comprises a substantially constant DC bias. The bias may be negative. The bias may be positive. If the drive waveform consists of a plurality of pulses that are positive, the bias may be negative; and if the drive waveform consists of a plurality of pulses that are negative, the bias may be positive.

[0019] According to another aspect of the embodiment, an apparatus for generating a target material dispenser is disclosed, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a highest frequency component of the drive signal is limited to a value in a range of about 3.5 MHz to about 7 MHz; and an electrical connector connected to the electrically actuatable element, the electrical connector being arranged to control an amount of current flowing through the target material at the orifice.

[0020] According to another aspect of an embodiment, a device for generating EUV radiation is disclosed, the device comprising a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal, an electrical connector connected to the electrically actuatable element, the electrical connector being arranged and parameters of the drive signal being selected to control an amount of current flowing through the target material at the orifice.

[0021] According to another aspect of an embodiment, a method for dispensing a target material in an apparatus for generating EUV radiation is disclosed, the method comprising the steps of: providing a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive the target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element, the electrically actuatable element being mechanically coupled to the cavity and being arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and supplying a drive signal to the electrically actuatable element for supplying the drive signal, wherein the drive signal comprises a substantially constant DC bias.

[0022] According to another aspect of an embodiment, a method for dispensing a target material in an apparatus for generating EUV radiation is disclosed, the method comprising the steps of: providing a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive the target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element, the electrically actuatable element being mechanically coupled to the cavity and being arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and supplying a drive signal to the electrically actuatable element for supplying the drive signal, wherein a minimum rise / fall time of the drive signal is in the range of about 50ns to about 100ns.

[0023] According to another aspect of an embodiment, a method for dispensing a target material in an apparatus for generating EUV radiation is disclosed, the method comprising the steps of: providing a target material dispenser, the target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive the target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element, the electrically actuatable element being mechanically coupled to the cavity and being arranged to induce a velocity disturbance in the stream of droplets based on a drive signal; and supplying a drive signal to the electrically actuatable element for supplying the drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the orifice.

[0024] Further embodiments, features, and advantages of the present invention as well as the structure and operation of various embodiments are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the methods and systems of embodiments of the present invention by way of example and not limitation. Together with the detailed description, the accompanying drawings are also used to explain the principles of the methods and systems presented herein and enable those skilled in the art to make and use the methods and systems presented herein. In the accompanying drawings, the same reference numerals indicate the same or functionally similar elements.

[0026] Figure 1 is a schematic, non-scale illustration of the overall broad concept of a system for a laser produced plasma EUV radiation source according to one aspect of the present invention.

[0027] Figure 2 yes Figure 1 Schematic, non-scale view of a portion of a system.

[0028] Figure 3A is a diagram of a droplet generator nozzle assembly according to one aspect of an embodiment.

[0029] Figure 3Bis a diagram of a droplet generator nozzle assembly according to another aspect of an embodiment.

[0030] Figure 4A Depicts the excitation waveform of the piezoelectric element in the droplet generator nozzle assembly, Figure 4B The resulting current in the piezoelectric element is plotted, and Figure 4C Depicted is the resulting simulated current through an orifice in a droplet generator nozzle assembly according to an aspect of the present invention.

[0031] Other features and advantages of the present invention and the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Based on the teachings contained herein, other embodiments will be apparent to those skilled in the relevant art. DETAILED DESCRIPTION

[0032] Various embodiments are now described with reference to the accompanying drawings, wherein like reference numerals are used to refer to like elements throughout the text. In the following description, for the purpose of explanation, many specific details are set forth to facilitate a thorough understanding of one or more embodiments. However, it may be apparent in some or all instances that any of the embodiments described below may be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in the form of block diagrams to facilitate the description of one or more embodiments.

[0033] However, before describing such embodiments in more detail, an example environment in which embodiments of the present invention may be implemented is exemplarily presented. In the following description and in the claims, terms such as "upper", "lower", "top", "bottom", "vertical", "horizontal", etc. may be used. These terms are intended to show only relative orientations, not any orientation relative to gravity.

[0034] First reference Figure 1 , shows a schematic diagram of an exemplary EUV radiation source (e.g., a laser-generated plasma EUV radiation source 20) according to one embodiment of the present invention. As shown, the EUV radiation source 20 may include a pulsed or continuous laser source 22, which may be, for example, a pulsed gas discharge CO2 laser source that produces the radiation beam 12. The pulsed gas discharge CO2 laser source may have a DC or RF excitation operating at high power and high pulse repetition frequency.

[0035] The EUV radiation source 20 also includes a target delivery system 24 for delivering the target material in the form of droplets or a continuous liquid stream. In this example, the target material is a liquid, but it can also be a solid or a gas. The target material can be made of tin or a tin compound, although other materials can be used. In the depicted system, the target material delivery system 24 introduces droplets 14 of the target material into the interior of the vacuum chamber 26 to an irradiation region 28, where the target material can be irradiated to produce a plasma. In some cases, an electric charge is placed on the target material to allow the target material to be diverted toward or away from the irradiation region 28. It should be noted that, as used herein, an irradiation region is an area where irradiation of the target material can occur, and is an irradiation region even at times when no irradiation actually occurs.

[0036] The EUV radiation source 20 may also include an EUV light source controller system 60, which may also include a laser excitation control system 65. The EUV radiation source 20 may also include a detector such as a target position detection system, which may include one or more droplet imagers 70 that generate an output indicating an absolute or relative position of a target droplet (e.g., relative to the irradiation area 28) and provide the output to the target position detection feedback system 62.

[0037] The target position detection feedback system 62 can use the output of the droplet imager 70 to calculate the target position and trajectory, from which the target error can be calculated. The target error can be calculated droplet by droplet, or averaged, or in some other way. The target error can then be provided as an input to the light source controller 60. In response, the light source controller 60 can generate a control signal, such as a laser position, direction, or timing correction signal.

[0038] like Figure 1 As shown in , the target material delivery system 24 may include a target delivery control system 90. In response to a signal (e.g., the target error described above or some amount derived from the target error provided by the system controller 60), the target delivery control system 90 is operable to adjust the path of the target droplet 14 through the irradiation area. This can be achieved, for example, by repositioning the point at which the target delivery mechanism 92 releases the target droplet 14. For example, the droplet release point is repositioned by tilting the target delivery mechanism 92 or by offsetting the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably supplied with a target material and a gas source from the outside to place the target material under pressure in the target delivery mechanism 92.

[0039] Further details regarding various droplet dispenser configurations and their relative advantages can be found, for example, in U.S. Pat. No. 7,872,245, entitled “Systems and Methods for Target Material Delivery in a Laser Produced Plasma EUV Light Source,” issued on January 18, 2011, U.S. Pat. No. 7,405,416, entitled “Method and Apparatus For EUV Plasma Source Target Delivery,” issued on July 29, 2008, and U.S. Pat. No. 7,372,056, entitled “LPP EUV Plasma Source Material Target Delivery System,” issued on May 13, 2008, the entire contents of which are incorporated herein by reference in their entirety.

[0040] continue Figure 1 , the radiation source 20 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 the discussion is also applicable to other optical elements. The collector 30 may be, for example, a normal incidence reflector implemented as an MLM with an additional thin barrier layer of, for example, B4C, ZrC, Si3N4 or C deposited at each interface to effectively prevent thermally induced interlayer diffusion. Other substrate materials, such as aluminum (Al) or silicon (Si), may also be used. The collector 30 may be in the form of a prolate spheroid having a central hole to allow the laser radiation 12 to pass through and reach the irradiation area 28. The collector 30 may be, for example, in the shape of an ellipsoid having a first focus at the irradiation area 28 and a second focus at a so-called intermediate point 40 (also referred to as intermediate focus 40), where EUV radiation may be output from the EUV radiation source 10 and input to, for example, an integrated circuit lithography scanner 50, which uses the radiation, for example, to process a silicon wafer workpiece 52 using a reticle or mask 54 in a known manner. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain integrated circuit devices.

[0041] Figure 2The droplet generation system is illustrated in more detail. The target material delivery system 90 delivers droplets to the irradiation site / primary focus 28 within the chamber 26. The drive signal generator 230 provides a drive waveform to an electrically actuatable element in the droplet generator 90, which causes a velocity disturbance in the droplet stream. The drive waveform may include a single sine wave, a combination of several sine waves with different frequencies, or a combination of sine waves and pulse waves. By carefully selecting the parameters of the drive waveform, a velocity disturbance can be applied to the molten tin nozzle, resulting in the formation of droplets necessary for the normal operation of the EUV light source at a frequency of 40-100kHz at a typical distance of 5-20cm from the droplet generation system. The drive signal generator 230 operates under the control of the controller 250 based at least in part on data from the data processing module 252. The data processing module 252 receives data from one or more detectors. In the example shown, the detector includes a camera 254 and a photodiode 256. The droplets are illuminated by one or more lasers 258. In this typical arrangement, the detector detects / images droplets at points in the flow where accumulation is expected to occur.Also, the detector and laser are arranged outside the vacuum chamber 26 and observe the flow through windows in the walls of the vacuum chamber 26.

[0042] The target material delivery system 90 may include a reservoir storing a fluid under pressure (e.g., molten tin). The reservoir is in fluid communication with a cavity that terminates at a nozzle having an orifice that allows the pressurized fluid in the reservoir to flow through the orifice, thereby forming a continuous stream that subsequently breaks up into a plurality of micro-droplets that then coalesce into larger droplets.

[0043] Such an arrangement in Figure 3A is shown in Figure 3A , the structure defining the cavity 300 is in the form of a tube or capillary 310. The capillary 310 terminates in a nozzle having an orifice 320. The column of molten target material in the cavity 300 is under pressure and is discharged from the orifice 320 in the form of a stream and is broken into droplets 330. As mentioned above, the velocity disturbance in the column of target material in the cavity 300 is caused by an electrically actuatable element 340, which in the example shown is a cylinder. The electrically actuatable element 340 can be, for example, a piezoelectric element. In the configuration shown, the electrically actuatable element 340 has an electrode 350 on its outer diameter and an electrode 360 ​​on its inner diameter. The electrode 350 is connected to the drive signal source 230 through a connector 410. The connector 410 can be an RF coaxial cable (e.g., with a 50 ohm nominal impedance) that terminates at the outer electrode 350. The electrode 360 ​​is connected to a ground potential through a connector 420. The drive signal source 230 applies a drive signal to the element 340 , thereby causing a change in the dimension of the electrically-actuable element 340 that is mechanically coupled to the target material in the cavity 300 .

[0044] As also shown, the capillary 310 is coated with a conductive coating 370, which may be, for example, chromium. Moreover, the conductive coating 370 may be covered by an insulating coating 380. The insulating coating 380 may be arranged to cover only a portion of the conductive coating 370, such as in a region axially coextensive with the electrically actuatable element 340. The purpose of the insulating coating 380 is to provide an insulating layer between the PZT electrode 360 ​​and the conductive coating 370. The electrically actuatable element 340 is bonded to the conductive coating or to the insulating coating (if present) by the adhesive material forming the bonding layer 390. The end of the droplet generator may be enclosed in a droplet generator cage 400. The purpose of the conductive coating 370 is to protect the target material leaving the orifice 320 from the electrostatic field generated by the uncompensated surface charge on the capillary, so that the droplets are not charged, repel each other and do not aggregate. The conductive coating 370 preferably has a resistivity of no more than about 1E-06 Ohm-m. The conductive coating may be grounded to the tin flow through the orifice. In addition to the ground path at the orifice, the conductive coating 370 may also have a dedicated connection to the grounded droplet generator housing 450.

[0045] As mentioned, there may be a tendency for the droplet stream 330 to drift laterally, so that eventually the stream clamps the edge of the outlet hole 430 in the droplet generator cage 400. One mechanism that appears to cause the droplet stream to drift is the formation of SnOx particles in the nozzle orifice. The formation of SnOx particles in the nozzle orifice is promoted by a current having an RF component (referred to herein as an RF signal) flowing through the nozzle orifice, either by electrolysis, electrophoresis, or a thermal mechanism such as Joule heating.

[0046] One source of the RF current flowing through the nozzle orifice appears to be the current flowing through the conductive coating on the nozzle and continuing to the molten tin in the nozzle. One reason for the presence of this RF current is that for high frequency components, the impedance of the parasitic capacitance between the electrically actuatable element in the form of a piezoelectric tube around the capillary and the conductive coating formed by the bonding layer (and the insulating layer, if present) is smaller than for low frequency components, while the main inductive impedance of the connection 420 is larger. Therefore, a larger part of the return current is directed to the lower impedance path, that is, through the parasitic capacitance and through the tin inside the nozzle.

[0047] Therefore, it is desirable to reduce the RF current flowing through the nozzle by reducing the parasitic inductance on the return (ground) connection 420. One means of reducing this inductance is to provide a very short connection 420 to the inner electrode 360. For example, the physical length of this return path for previous implementations can be in the range of about 50 cm to about 100 cm. This can correspond to a parasitic inductance in the range of about 0.5 μH to about 2 μH. In various implementations of the droplet generator, shorter connections (such as 10 cm or less) can reduce the parasitic inductance.

[0048] More specifically, the electrode 360 ​​can be grounded by providing an electrical connection to a droplet generator cage 400 that is mounted on the nozzle and is itself grounded. In this case, the length of the electrical path to ground is reduced to about 3 cm, and the parasitic inductance associated with this connection is reduced to about 35 nH. The electrode 360 ​​can also be grounded by providing a short wire connection to other grounded elements, such as the heater block of the droplet generator. The inductance of the ground connection 420 can also be achieved by grounding the internal electrode 360 ​​to the metal housing of the droplet generator.

[0049] Therefore, there are multiple paths through which current can flow in and around the droplet generator assembly. From the perspective of the key functionality of the droplet generator, these different paths are essentially equivalent. However, some of these paths cause undesirable current to flow through the tin in the nozzle orifice, thereby promoting the formation of SnOx particles that hinder the flow of tin. Therefore, the goal is to cause more current to flow through paths that do not involve tin in the nozzle orifice. As described above, one measure to achieve this goal may be to reduce the inductance of some other ground paths. Another way to achieve this goal is to control the frequency of the drive signal. To the extent that the impedance of the path through the tin in the nozzle orifice is primarily capacitive, while the impedance of other paths is primarily inductive, then taking measures to limit the high-frequency components in the drive signal will tend to cause its path through the nozzle to have a higher impedance.

[0050] Figure 3B Shows Figure 3A An alternative arrangement of in which the conductive coating 370 is modified and the connections 350, 360 to the electrically actuatable element 340 are arranged closer to the end of the orifice 320 of the capillary 310. Figure 3B In the configuration, a portion of the capillary 310 is free of the conductive coating, for example, the capillary is masked during an axisymmetric Cr sputtering process to remove the small gap capacitance between the internal piezoelectric electrode and the conductive coating 370, while maintaining a conductive path between the front surface of the capillary 310 and the orifice 320 tin to prevent the microdroplets from being charged and making free-flight aggregation difficult. Figure 3BIn the embodiment of the present invention, the connections 350, 360 to the electrically-actuable element 340 are repositioned, i.e., flipped so that the internal electrode 360 ​​is wrapped around the forward-facing surface of the electrically-actuable element 340 (as opposed to the rearward-facing surface) to suppress known electromagnetic fields that can cause micro-droplets to be charged and make free-flight aggregation difficult.

[0051] Therefore, another way to mitigate droplet drift is to reduce the high frequency content of the modulation signal. For example, this can be accomplished by increasing the rise and fall times of the pulse wave component of the modulation signal to avoid sharp transitions of the high frequency Fourier component or by limiting the maximum frequency of the sine wave when the drive signal does not contain a pulse wave component. Therefore, for example, for this purpose, it is desired that the rise and / or fall time of the pulses in the drive signal is in the range of about 50ns to about 100ns, and the sine wave frequency is limited to the range of about 3.5MHz to about 7MHz to mitigate the drift effect. Again, this is because at lower frequencies, the impedance of the connector 420 is lower, while the impedance of the path including the parasitic capacitance of the piezoelectric element and the tin in the nozzle is significantly higher. Therefore, the magnitude of the RF current flowing through the tin in the nozzle is reduced. Moreover, the magnitude of the drive signal can be reduced. However, as noted above, the usability of these techniques may be limited by concerns that they may reduce droplet aggregation efficiency.

[0052] The advantage of this method of mitigating the drift of the droplet stream is that it does not require changes to the droplet generator hardware. However, its disadvantage is that it reduces the range of choices for the frequency components of the signal that can be used to achieve optimal focusing of the droplets. The drive signal is usually optimized to obtain the shortest possible focusing, and high-frequency components often lead to this result. Increasing the energy of the high-frequency components of the excitation waveform also increases droplet timing stability.

[0053] Figure 4A , Figure 4B and Figure 4C An example of a simulated current / voltage waveform during droplet generator operation is shown, where a pulse signal is used for tin jet modulation. Figure 4A shows an input voltage waveform of a drive signal for an electrically actuatable element (in this case a piezoelectric element), and Figure 4B The resulting input current is shown. Figure 4C The resulting orifice current is shown. As can be seen, voltage / current spikes are generated during this operation. Parasitic current spikes propagating through the molten tin in the nozzle promote the formation of SnOx. These are called Figure 4C As described above, these current spikes stimulate the formation of SnOx. Therefore, the problem of SnOx formation in the nozzle orifice can be alleviated by controlling (ie, reducing, including eliminating) these current spikes.

[0054] As mentioned above, one measure to control these parasitic current spikes is to provide a low impedance electrical connection to the internal electrode of the electrically actuatable element. A long wire (about 0.5 m to about 1 m) typically runs through the entire body of the droplet generator, reaching an RF type connector (e.g., a bayonet nut connector (BNC)) at the end of the wire. The inductance of the wire is about 0.5 μH, which can cause voltage / current spikes when a fast rising drive signal (rise time of about 10 ns to about 20 ns) is sent through it. Using a short wire (less than about 10 cm) to provide another low inductance electrical connection from the electrically actuatable element to ground helps reduce these spikes and eliminate drift problems.

[0055] When the current spikes that cause SnOx formation when using a pulse modulated signal waveform have a specific polarity, biasing the drive signal to DC of the opposite polarity can be used to prevent these spikes from obtaining values ​​that would otherwise contribute to the formation of SnOx. For example, when the current spikes are positive, a negative bias of about -2V to about -10V can be applied, and vice versa.

[0056] The present invention has been described above by means of functional building blocks that illustrate the implementation of specific functions and their relationships. For ease of description, the boundaries of these functional building blocks are arbitrarily defined in this article. Alternative boundaries can be defined as long as the specific functions and their relationships are properly performed.

[0057] The foregoing description of specific embodiments so comprehensively discloses the general nature of the invention that others can easily modify and / or adapt to various applications for such specific embodiments without excessive experimentation by applying knowledge in the art, without departing from the general concept of the invention. Therefore, based on the teachings and guidance presented herein, such adaptations and modifications are intended to fall within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the words or terms herein are used for descriptive purposes rather than for limiting purposes, so that the terms or terms in this specification are to be interpreted by the technician based on the teachings and guidance. The breadth and scope of the present invention should not be limited by any of the exemplary embodiments described above, but should only be limited according to the attached claims and their equivalents.

[0058] Further aspects of the invention are set out in the following numbered clauses.

[0059] 1. An apparatus for generating EUV radiation, comprising:

[0060] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0061] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0062] a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal,

[0063] An electrical connection is connected to the electrically actuatable element, the electrical connection being arranged to control the amount of electrical current flowing through the target material at the orifice.

[0064] 2. Apparatus according to clause 1, wherein the electrical connection to the electrically actuatable element is arranged to provide a low impedance path between the electrically actuatable element and ground, the low impedance path not passing through the target material at the aperture.

[0065] 3. A device according to clause 1, wherein the structure defining the cavity comprises a cylindrical tube, and the electrically actuatable element comprises a cylindrical piezoelectric element arranged around the cylindrical tube and having an inner surface connected to ground via a low impedance path.

[0066] 4. A device according to clause 3, wherein the inner surface is connected to ground at a portion of the electrically actuatable element that is closest to the orifice.

[0067] 5. The apparatus of clause 1, wherein the targeted material dispenser further comprises a conductive coating surrounding at least a portion of the structure defining the cavity.

[0068] 6. A device according to clause 5, wherein the conductive coating has a resistivity of less than about 1E-06 Ohm-m.

[0069] 7. A device according to clause 5, wherein the conductive coating is confined to an area of ​​the structure that defines the aperture.

[0070] 8. The device according to clause 5, wherein the electrically actuatable element is positioned around a first axial portion of the cavity that is free of the conductive coating.

[0071] 9. The device of clause 5, wherein the conductive coating is connected to ground via a low impedance path.

[0072] 10. The device of clause 5, further comprising an insulating coating on the conductive coating.

[0073] 11. The apparatus of clause 1, wherein the drive signal generator is electrically coupled to the electrically actuatable element via an RF coaxial cable that terminates directly at the electrically actuatable element.

[0074] 12. An apparatus for generating EUV radiation, comprising:

[0075] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0076] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0077] A drive signal generator is electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a highest frequency component of the drive signal is limited to a value in the range of about 3.5 MHz to about 7 MHz.

[0078] 13. Apparatus according to clause 12, wherein the electrical connection to the electrically actuatable element is arranged to control the amount of electrical current flowing through the target material at the orifice.

[0079] 14. An apparatus for generating EUV radiation, comprising:

[0080] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0081] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0082] A drive signal generator is electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a minimum rise / fall time of the drive signal is in the range of about 50 ns to about 100 ns.

[0083] 15. An apparatus for generating EUV radiation, comprising:

[0084] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0085] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0086] A drive signal generator is electrically coupled to the electrically actuatable element for supplying a drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the aperture.

[0087] 16. Apparatus for generating EUV radiation, comprising:

[0088] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0089] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0090] A drive signal generator is electrically coupled to the electrically actuatable element for supplying a drive signal, wherein the drive signal comprises a substantially constant DC bias.

[0091] 17. The apparatus of clause 16, wherein the bias is negative.

[0092] 18. The apparatus of clause 16, wherein the bias is positive.

[0093] 19. The apparatus of clause 16, wherein if the drive waveform consists of pulses of positive polarity, the bias is negative, and if the drive waveform consists of pulses of multiple polarities, the bias is positive.

[0094] 20. An apparatus for generating EUV radiation, comprising:

[0095] a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0096] an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0097] a drive signal generator electrically coupled to the electrically actuatable element for supplying a drive signal,

[0098] An electrical connection to the electrically actuatable element is arranged and parameters of the drive signal are selected to control an amount of current flowing through the target material at the orifice.

[0099] 21. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of:

[0100] providing a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0101] providing an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0102] The electrically actuatable element is supplied with a drive signal for supplying the drive signal, wherein the drive signal comprises a substantially constant DC bias.

[0103] 22. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of:

[0104] providing a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0105] providing an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0106] The electrically actuatable element is supplied with a drive signal for supplying the drive signal, wherein a minimum rise / fall time of the drive signal is in the range of about 50 ns to about 100 ns.

[0107] 23. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of:

[0108] providing a target material dispenser including structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material;

[0109] providing an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; and

[0110] A drive signal is supplied to the electrically actuatable element for supplying the drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the aperture.

[0111] Other implementations are within the scope of the following claims.

Claims

1. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, an electrical connection connected to the electrically actuatable element, the electrical connection being arranged to control an amount of electrical current flowing through the target material at the orifice; wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground through the electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator.

2. The apparatus of claim 1 , wherein the electrical connection to the electrically actuatable element is arranged to provide a low impedance path between the electrically actuatable element and ground, the low impedance path not passing through the target material at the orifice.

3. The device of claim 1 , wherein the structure defining the cavity comprises a cylindrical tube, and the electrically actuatable element comprises a cylindrical piezoelectric element disposed about the cylindrical tube and having an inner surface connected to ground via a low impedance path.

4. The device of claim 3, wherein the inner surface is connected to ground at a portion of the electrically actuatable element closest to the orifice.

5. The apparatus of claim 1, wherein the targeted material dispenser further comprises a conductive coating surrounding at least a portion of the structure defining the cavity.

6. The device of claim 5, wherein the conductive coating has a resistivity less than 1E-06 Ohm-m.

7. The device of claim 5, wherein the conductive coating is confined to a region of the structure defining and including the aperture.

8. The device of claim 5, wherein the electrically actuatable element is positioned around a first axial portion of the cavity that is free of a conductive coating.

9. The device of claim 5, wherein the conductive coating is connected to ground through a low impedance path.

10. The device of claim 5, further comprising an insulating coating on the conductive coating.

11. The apparatus of claim 1, wherein the drive signal generator is electrically coupled to the electrically actuatable element by terminating an RF coaxial cable directly at the electrically actuatable element.

12. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, wherein a highest frequency component of the drive signal is limited to a value in the range of 3.5 MHz to 7 MHz; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator.

13. Apparatus according to claim 12, wherein an electrical connection to the electrically actuatable element is arranged to control the amount of electrical current flowing through the target material at the orifice.

14. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, wherein a minimum rise / fall time of the drive signal is in the range of 50 ns to 100 ns; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator.

15. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the orifice; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator.

16. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, wherein the drive signal comprises a substantially constant DC bias; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator. The apparatus of claim 16 , wherein the bias is negative. The apparatus of claim 16 , wherein the bias is positive.

19. The apparatus according to claim 16, wherein if the driving waveform of the driving signal generator consists of pulses with positive polarity, the bias is negative; if the driving waveform consists of pulses with multiple polarities, the bias is positive.

20. An apparatus for generating EUV radiation, comprising: a target material dispenser comprising structure defining a cavity arranged to receive a target material and an orifice arranged to receive the target material from the cavity and deliver a stream of droplets of the target material; an electrically actuatable element mechanically coupled to the cavity and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as a drive signal generator electrically coupled to the electrically actuatable element for supplying the drive signal, an electrical connection connected to the electrically actuatable element, the electrical connection being arranged, and parameters of the drive signal being selected, to control an amount of current flowing through the target material at the orifice; wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground through the electrical connection, and wherein the outer electrode is electrically connected to the drive signal generator.

21. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of: providing a target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as supplying a drive signal to the electrically actuatable element by a drive signal source for supplying the drive signal, wherein the drive signal comprises a substantially constant DC bias; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal source.

22. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of: providing a target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as supplying a drive signal to the electrically actuatable element by a drive signal source for supplying the drive signal, wherein a minimum rise / fall time of the drive signal is in the range of 50 ns to 100 ns; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal source.

23. A method of dispensing a target material in an apparatus for generating EUV radiation, the method comprising the steps of: providing a target material dispenser comprising a structure defining a cavity and an orifice, the cavity being arranged to receive a target material, the orifice being arranged to receive the target material from the cavity and to deliver a stream of droplets of the target material; providing an electrically actuatable element mechanically coupled to the chamber and arranged to induce a velocity disturbance in the droplet stream based on a drive signal; as well as supplying a drive signal to the electrically actuatable element by a drive signal source for supplying the drive signal, wherein a maximum voltage of the drive signal is limited to limit the flow of current through the target material in the orifice; Wherein the electrically actuatable element has an outer electrode on its outer diameter and an inner electrode on its inner diameter, and wherein the inner electrode is connected to ground via an electrical connection, and wherein the outer electrode is electrically connected to the drive signal source.

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