Laser systems for target metrology and modification in EUV sources

Abstract

A system and method for generating EUV radiation is disclosed in which a laser is used to irradiate a target material without changing the target material and then irradiate the target material to change the target material in a multi-stage process, wherein an irradiation stage is used to determine the timing of the shots at one or more irradiation stages.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 736,012, filed on September 25, 2018, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to light sources that produce extreme ultraviolet light by excitation of target materials, and in particular to measurement (eg, detection) and alteration (eg, preparation and / or ionization) of target materials in such sources. Background Art

[0004] Extreme ultraviolet ("EUV") light (eg, electromagnetic radiation with a wavelength of approximately 50 nm or less (sometimes also called soft X-rays), including light with a wavelength of approximately 13 nm) is used in photolithography processes to create extremely small features in substrates (eg, silicon wafers).

[0005] Methods for generating EUV light include, but are not limited to, changing the physical state of a target material to a plasma state. The target material includes an element having emission lines in the EUV range, such as xenon, lithium, or tin. In one such method, the desired plasma, often referred to as laser produced plasma ("LPP"), is produced by irradiating the target material (e.g., in the form of droplets, streams, or clusters of the target material) with an amplified light beam, which may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed container (e.g., a vacuum chamber) and monitored using various types of metrology equipment.

[0006] CO2 amplifiers and lasers that output an amplified beam at a wavelength of about 10,600 nm can offer certain advantages as drive lasers for irradiating target materials in LPP processes. This is particularly true for certain target materials, such as those containing tin. For example, one advantage is the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.

[0007] In an EUV light source, EUV can be generated by a two-step process, in which a droplet of a target material on the way to an irradiation site is first struck by a pre-pulse, which conditions the droplet for a subsequent phase change at the irradiation site. In this case, the conditioning may include changing the shape of the droplet, for example, flattening the droplet, or changing the distribution of the droplets, for example, causing some droplets to be at least partially dispersed in the form of a mist. For example, a pre-pulse strikes the droplet to modify the distribution of the target material, and a main pulse strikes the target to convert it into an EUV emitting plasma. In some systems, the pre-pulse and the main pulse are provided by the same laser, while in other systems, the pre-pulse and the main pulse are provided by two separate lasers.

[0008] It is important to "target" the fast-moving droplets to within a few microns to achieve efficient and debris-minimized operation of the light source. In some systems, reflected light from a pre-pulse or main pulse is analyzed, either of which is a working pulse, rather than a pulse used only for measurement, i.e., a measurement pulse. For example, U.S. Patent No. 7,372,056, issued May 13, 2008, entitled "LPP EUV Plasma Source Material Target Delivery System," which is incorporated herein in its entirety, discloses the use of a droplet detection radiation source and a droplet radiation detector that detects droplet detection radiation reflected from a droplet of target material.

[0009] U.S. Patent No. 8,158,960, entitled “Laser Produced Plasma EUV Light Source”, issued on April 17, 2012, which is incorporated herein in its entirety, discloses the use of a droplet position detection system that may include one or more droplet imagers that provide an output indicative of one or more droplet positions, for example, relative to an irradiation area. The imager(s) may provide the output to a droplet position detection feedback system that may, for example, calculate droplet positions and trajectories, from which droplet errors may be calculated. The droplet errors may then be provided as inputs to a controller that may, for example, provide position, direction, and / or timing correction signals to the system to control source timing circuitry and / or control beam position and shaping systems, for example, to change the position and / or focused power of light pulses delivered to the irradiation area.

[0010] U.S. Patent No. 8,653,491, entitled “System, Method and Apparatus For Aligning And Synchronizing Target Material For Optimum Extreme Ultraviolet Light Output,” issued on February 18, 2014 (the entirety of which is incorporated herein), discloses irradiating a first portion of multiple portions of a target material with a driving laser, and detecting light reflected from the first portion of the target material to determine the position of the first portion of the target material.

[0011] U.S. Patent No. 9,241,395, entitled “System and Method For Controlling Droplet Timing in an LPP EUV Light Source”, issued on January 19, 2016 (the entirety of which is incorporated herein), discloses a droplet illumination module that generates two laser curtains to detect droplets. The droplet detection module detects each droplet as it passes through the second curtain, determines when the source laser should generate a pulse so that the pulse arrives at the illumination site at the same time as the droplet, and sends a signal to the source laser to fire at the correct time.

[0012] U.S. Patent No. 9,497,840, entitled “System and Method for Creating and Utilizing Dual Laser Curtains from a Single Laser in an LPP EUV Light Source,” issued on November 15, 2016 (which patent is incorporated herein in its entirety), discloses the use of two laser curtains and a sensor that detects the position of droplets of target material as they pass through the curtains.

[0013] Therefore, in some systems, a pre-pulse laser or heating (vaporization) laser reflected from a droplet is used to locate the droplet in space by collecting the reflected light and imaging it on a sensor. The successful implementation of this system involves several technical challenges. For example, if the droplet is not yet in the laser beam, there is little or no reflection. Therefore, the system can only work best when it is effectively redundant in the sense that the laser has already hit the droplet. Moreover, for some types of pre-pulses, there is little or no reflection even if the pre-pulse hits the droplet, because the pre-pulse plasma effectively absorbs it, so the signal-to-noise ratio may become too small to be of practical use. In addition, when the same pulse is used to illuminate the droplet for position measurement and extended target, the position of the droplet can only be determined after the laser is fired, so any errors in the pulse cannot be corrected.

[0014] In other systems, a secondary light source is used in addition to the pre-pulse laser to illuminate the droplet, and the camera is positioned to image the illuminated droplet. This avoids at least some of the challenges described above, but introduces a different challenge, namely, the measurement determines the position of the droplet only relative to the camera and the secondary laser, not directly relative to the pre-pulse or heating laser beam itself. Therefore, an additional step is required to relate the reference frame of the measurement system to the reference frame of the pre-pulse or heating laser. In practice, this can be very difficult.

[0015] Therefore, there is a need for a droplet position detection system that can avoid these challenges. Summary of the invention

[0016] A simplified overview of one or more embodiments is given below to provide a basic understanding of the embodiments. This overview is not an exhaustive overview of all contemplated embodiments, is not intended to identify key or important elements of all embodiments, and is not intended to set limitations on the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to a more detailed description that will be presented later.

[0017] According to one aspect of the embodiments, an apparatus and method for positioning a droplet of a target material in flight so that the droplet can be more accurately struck by a laser is disclosed.

[0018] According to another aspect of the embodiments, an apparatus and method are disclosed that combine the functionality of an independent light source with that of a pre-pulsed or heated laser by modulating the characteristics of the pre-pulsed or heated radiation source in time. This retains the advantages of both systems described above while avoiding many of the disadvantages. For example, using a single light source instead of two separate light sources involves only a single coordinate reference system, rather than a conversion between two reference systems that must be calibrated and matched.

[0019] According to another aspect of the embodiment, the hybrid light source can be configured to also emit long pulses or quasi-CW radiation to illuminate the droplets, so that in addition to providing short pulses for forming the target, the hybrid light source can also be used to locate the droplets. Quasi-continuous wave (quasi-cw) operation of the laser means that its pump light source is only turned on during specific time intervals, which are short enough to reduce thermal effects, but still long enough to keep the laser process close to its steady state, that is, the laser is optically in a continuous wave operation state. The irradiance of the long pulse or quasi-CW radiation is set too low to ionize the droplet or target surface, but is sufficient to provide a good reflection signal over a longer period of time. Once the droplets are located using long pulses or quasi-CW radiation, the remaining energy stored in the laser can be consumed in short pulses to pre-pulse the droplets and / or convert the target material in the droplets into plasma.

[0020] According to another aspect of the embodiment, long weak pulses may be scanned until the return signal is maximized, and then short strong pulses may be steered and / or fired based on the maximum return signal to best hit the target.

[0021] According to another aspect of the embodiment, the focus of the long weak pulse can be modified so that the beam width is relatively large during position sensing (ie, the measurement period). This can provide a larger detection or "capture" volume for the position sensing system.

[0022] Further embodiments, features, and advantages of the subject matter of the present disclosure, 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

[0023] Figure 1 is a schematic, non-scale illustration of an overall broad concept for a laser produced plasma EUV radiation source system according to one aspect of the present invention.

[0024] Figure 2 yes Figure 1 Schematic, non-scale view of a portion of a system.

[0025] Figure 3A and Figure 3B It is shown that Figure 1 and Figure 2 Diagram of some of the aiming principles in the system shown.

[0026] Figure 4 is a non-scale schematic diagram of an EUV system according to an aspect of the embodiments.

[0027] Figure 5A is a graph of irradiance as a function of emission time from a hybrid light source system according to one aspect of an embodiment.

[0028] Figure 5B is a graph of irradiance as a function of emission time from a hybrid light source system according to another aspect of the embodiments.

[0029] Figure 6 is a non-scale schematic diagram of an EUV system according to another aspect of the embodiments.

[0030] Figure 7 is a flow chart illustrating an operating mode of an EUV system according to aspects of an embodiment.

[0031] Figure 8 is a flow chart illustrating an operating mode of an EUV system according to aspects of an embodiment.

[0032] Fig. 9 is a flow chart illustrating an operating mode of an EUV system according to aspects of an embodiment.

[0033] Fig.10 is a flow chart illustrating an operating mode of an EUV system according to aspects of an embodiment.

[0034] Other features and advantages of the present invention, as well as 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 proposed herein for illustrative purposes only. Based on the teachings contained herein, other embodiments will be clear to those skilled in the relevant art. DETAILED DESCRIPTION

[0035] Various embodiments are now described with reference to the accompanying drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for the purpose of explanation, many specific details are set forth to facilitate a thorough understanding of one or more embodiments. However, it may be clear in some or all cases that any of the embodiments described below can be practiced without adopting the specific design details described below.

[0036] First reference Figure 1 , Figure 1 A schematic diagram of an exemplary EUV radiation source (e.g., a laser-generated plasma EUV radiation source 10) according to an aspect of an embodiment of the present invention is shown. As shown, the EUV radiation source 10 may include a pulsed or continuous laser source 22, which may be, for example, a pulsed gas discharge CO2 laser source, which produces a radiation beam 12 having a wavelength typically below 20 μm, such as in the range of about 10.6 μm or about 0.5 μm or less. The pulsed gas discharge CO2 laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

[0037] The EUV radiation source 10 also includes a target delivery system 24 that delivers target material in the form of liquid 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 target material into the interior of the vacuum chamber 26 to an irradiation region 28, in which 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 directed 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 is actually occurring. The EUV light source may also include a beam focusing and steering system 32, as will be described below in conjunction with Figure 2 Explained in more detail.

[0038] In the system shown, the components are arranged so that the droplet 14 travels substantially horizontally. The direction from the laser source 22 toward the irradiation area 28, i.e., the nominal propagation direction of the beam 12, can be regarded as the Z-axis. The path of the droplet 14 from the target material delivery system 24 to the irradiation area 28 can be regarded as the X-axis. Figure 1 The view of FIG. 1 is thus perpendicular to the XZ plane. Moreover, although a system is shown in which droplets 14 travel substantially horizontally, one of ordinary skill in the art will appreciate that other arrangements may be used in which droplets travel perpendicular to gravity or at an angle between 90 degrees (horizontal) and 0 degrees (vertical), inclusive, relative to gravity.

[0039] The EUV radiation source 10 may also include an EUV light source controller system 60, which may also include a laser emission control system 65, and a beam steering system 32. The EUV radiation source 10 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 position or a 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.

[0040] 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, on an average, or on some other basis. 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, and provide the control signal to the laser beam steering system 32. The laser beam steering system 32 can use the control signal to change the position and / or focus power of the laser beam focus within the chamber 26. The laser beam steering system 32 can also use the control signal to change the geometry of the interaction of the light beam 12 and the droplet 14. For example, the light beam 12 can be made to impact the droplet 14 eccentrically or at an angle of incidence other than a direct head-on impact.

[0041] like Figure 1 As shown, the target material delivery system 24 may include a target delivery control system 90. The target delivery control system 90 is operable in response to a signal (e.g., the target error described above, or some quantity derived from the target error provided by the system controller 60) to adjust the path of the target droplet 14 through the irradiation area 28. This may be accomplished, for example, by repositioning the point at which the target delivery mechanism 92 releases the target droplet 14. For example, the droplet release point may be repositioned by tilting the target delivery mechanism 92 or by moving the target delivery mechanism 92. The target delivery mechanism 92 extends into the chamber 26 and is preferably supplied with a target material and gas source from the outside to place the target material under pressure in the target delivery mechanism 92.

[0042] continue 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 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 (e.g., 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 shape of an oblong ellipse, with a central hole allowing the laser radiation 12 to pass through and reach the irradiation area 28. The collector 30 may be, for example, an ellipse with 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 can be output from the EUV radiation source 10 and input into, for example, an integrated circuit lithography scanner or stepper 50, which uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner using a reticle or mask 54. The silicon wafer workpiece 52 is then further processed in a known manner to obtain integrated circuit devices.

[0043] Figure 1 The arrangement also includes a temperature sensor 34, such as a thermocouple located within the chamber 26, to measure the local temperature of the gas within the chamber 26, ie the temperature at the sensor. Figure 1 One temperature sensor is shown, but additional temperature sensors may obviously be used. The temperature sensor 34 generates a signal indicative of the measured temperature and provides this as an additional input to the controller 60. The controller 60 bases the control signal it provides to the beam steering system 32 at least in part on the temperature signal.

[0044] Continue to Figure 2, it can be seen that the beam steering system 32 can include one or more steering mirrors 32a, 32b and 32c. Although three mirrors are shown, it should be understood that more than three or as few as one steering mirror can be used to steer the beam. In addition, although mirrors are shown, it should be understood that other optical devices such as prisms can be used, and one or more steering optics can be located inside the chamber 26 and exposed to plasma generated debris. See, for example, U.S. Patent No. 7,598,509, entitled "Laser Produced Plasma EUV Light Source," issued on October 6, 2009, the entire contents of which are incorporated herein by reference. For the illustrated embodiment, each steering mirror 32a, 32b and 32c can be mounted on a corresponding tilt actuator 36a, 36b and 36c, which can enable each steering mirror 32a, 32b and 32c to be independently moved in either or both of two dimensions.

[0045] As mentioned above, usually, for a reference coordinate system such as Figure 3A As shown, Z is the direction of propagation of the laser beam 12 and is also the direction from the collector 30 to the irradiation site 110 and the EUV intermediate focus. X is in the droplet propagation plane. Y is orthogonal to the XZ plane. To make it a right-handed coordinate system, the trajectory of the droplet 14 is set to the -X direction.

[0046] The main components of the aiming error are Figure 3B ΔX and ΔY are shown. These errors typically need to be kept less than about 5 um. Errors in the Z direction are less critical because the Rayleigh length of the laser focus is relatively long, so errors of 100 μm or worse can be tolerated.

[0047] Assuming a constant droplet velocity, the X position error ΔX is primarily a result of timing errors, i.e., the timing of the laser shots. Timing error correction can be accomplished fairly well by detecting the time at which a droplet crosses the laser curtain 115 in the illumination area near the illumination site 110. Since the laser curtain 115 is provided by a separate laser source and can therefore be always on, this measurement can be performed even when the laser is operating. Moreover, since the curtain is made wider in the YZ plane, the measurement performed using the laser curtain 115 is relatively tolerant to misalignment in the Y, Z directions. However, there is still a need to improve the determination of ΔX.

[0048] Determining the Y error ΔY is more challenging. Reflections of the pre-pulse of the light beam 12 from the droplet 14 can be used for all the required Y corrections and some of the required X corrections. In principle, to measure the position of the droplet Y, a separate illuminator could be used, such as a laser curtain 115 for the arrival time. This would require the use of a high frame rate imaging 2D detector (camera) to resolve the position, while the arrival time would only require a non-imaging scattered light detector. Therefore, there is also a need to improve the determination of ΔY.

[0049] The improved system will increase the capture range in space, i.e., "see" the droplet / target when the working pulse is very misaligned with the droplet / target. This is particularly a problem in the Y direction, because the independent X timing system already ensures relatively small misalignments along this direction. It is also possible to obtain advance information on the position of the droplet in space at a certain time before the next laser pulse is emitted, so that the error in the X direction can be predicted and corrected by adjusting the timing of the next pulse. This correction can also be made for Y, but usually correcting for Y requires moving at least one steering mirror, which is slower than changing the time delay. The improved system also uses a single laser for (one or more) working pulses and measurement pulses. The improved system will also provide a setup option, i.e. "offline" aiming. This provides the following advantages: the measurement pulse will not significantly interfere with the droplet and therefore will not generate debris. For example, if a pre-pulse is used to position the droplet at the pre-pulse focus, it will generate too much debris and cannot be used in a source equipped with a collecting mirror.

[0050] Figure 4 1 is a non-scale schematic diagram of an EUV system 100 according to one aspect of an embodiment. Among other features, the EUV system 100 includes a radiation source 102 capable of generating a radiation beam 104. The radiation beam 104 propagates into a chamber 106 where it strikes a quantity of target material 14 at an irradiation location 110 in the irradiation region 28.

[0051] In the example shown, the target material 14 is initially in the form of a stream of droplets released by a target material dispenser 112, which in the example is a droplet generator. The target material droplets 14 can be ionized in this form by a main pulse. Alternatively, the target material 14 can be pre-treated for ionization with, for example, a pre-pulse that can change the geometric distribution of the target material 14. Therefore, it may be necessary to accurately hit the target material 14 with a pre-pulse to ensure that the target material 14 is in the desired shape (disk, cloud, etc.), and to accurately hit the target with a main pulse to promote efficient generation of EUV radiation. Here, either the pre-pulse or the main pulse is referred to as working radiation because they mainly cause a change in the shape and / or physical state of the target material 14.

[0052] As used herein, the term "irradiation site" is used to refer to a location 110 in the chamber 106 at which the target material 14 is struck by a heating pulse. The location 110 may coincide with the primary focus of the condenser 114. The term "irradiation region" is used herein to refer to the region 28 or 118 that includes the irradiation site 110, as well as the region of the target material irradiated by the pre-pulse (pre-pulse focusing) and / or the measurement pulse, i.e., the region encountered by the target material 14 before reaching the irradiation site 110 and that may also be irradiated by the radiation source 102.

[0053] Figure 4 The system also includes an optics module 124. The optics module includes a beam steering system and may be part of other system optics that can focus and steer the radiation beam 104 under the control of the control module 122. As described more fully below, the control module 122 may use the reflected metrology radiation to detect the location of the target material 14.

[0054] According to one aspect of the embodiment, the radiation source 102 is configured as a hybrid light source that emits not only working radiation but also measurement radiation. As described more fully below, this can be achieved, for example, by using a laser that is suitable for generating radiation of varying power in a time-division scheme, where one power level is used for measurement and another power level is used for working. Or, for example, this can be achieved by using an optical device that attenuates the beam at a specific time (time division) or divides the laser beam into beams of different powers that propagate at least partially along different paths (spatial division). The term "measurement radiation" is used herein to refer to radiation that is primarily used to determine the position of the target material 14 and whose irradiance or energy density is below a value that will significantly perturb the target material. "Significant perturbation" means that the measurement radiation does not cause a change in the physical state or geometric distribution of the droplets, for example, does not ablate the droplets, does not ionize the droplets, does not flatten the droplets, does not disperse the droplets, or does not cause the target material in the droplets to boil. The measurement radiation is used to find the target material 14 before the target material 14 reaches the position to be irradiated by the working radiation. Finding the position of the target material enables the system to determine the optimal steering of the working radiation.

[0055] refer to Figure 5A In a time-separated arrangement, the measurement radiation 150 may be a long pulse (compared to the duration of the working radiation 152) or a quasi-CW radiation for irradiating the target material 14. The measurement radiation 150 may be substantially continuous, such as Figure 5A As shown, it can also be one or more pulses, such as Figure 5BAs shown. Pulsed metrology radiation is particularly practical for certain kinds of laser sources, for example, a laser comprising an oscillator that emits a train of pulses at a frequency of about 100 MHz. If the duration of interval A is 10 μs, 1000 weak oscillator pulses can be scattered from the droplet during interval A and integrated, for example, on a camera. The laser can then emit an amplified pulse as the 1001st oscillator pulse to generate a strong pulse 152 for operational purposes, for example, as a pre-pulse for forming a target.

[0056] In the example shown, the measurement radiation 150 has a duration A, and the working radiation 152 has a duration B. During the interval A, the droplets are evaluated by the measurement radiation 150. When the system is used to perform an offline setup of the alignment of the system, such as during an initial stage or maintenance, the duration A lasts at least until the system has determined the position of the target material 14, and then, with appropriate steering based on the determined position, the radiation source 102 emits a shorter working radiation 152 to ionize the surface and pre-pulse the target material 14 and / or heat the target material 14 into a plasma. When the system is used to maintain appropriate steering when the source is online, the detection completed in the time interval A will be used to correct the steering in real time, but the system does not have to wait until the correct steering is achieved before irradiating the target material with the working radiation.

[0057] If the measurement radiation 150 is substantially continuous, its amplitude should remain below level C. The duration of the measurement interval A and the amplitude of level C are selected so that the droplet 14 is not significantly disturbed during the measurement interval A. This is a function of both the instantaneous amplitude of the measurement radiation applied during interval A and the duration of interval A. Essentially, the integral of the energy coupled into the droplet 14 over time should not exceed a predetermined amount that will significantly disturb the droplet 14. Level C should also not exceed an instantaneous ablation threshold. Likewise, for pulsed measurement radiation, the pulse should not cumulatively couple enough energy into the droplet 14 to bring the target material close to its boiling temperature, so the amplitude of the pulse and the duration of interval A are interdependent.

[0058] In other words, for the case of substantially continuous measurement radiation 150, level C is selected so that the substantially continuous measurement radiation 150 is below the amount that would couple enough energy into the droplet 14 over interval A to significantly disturb it. For the case of pulsed measurement radiation 150, the amplitude of a single pulse may exceed level C, but will again be selected to be below level D so that the cumulative amount of energy coupled into the droplet 14 by the pulses over interval A is insufficient to significantly disturb it. In either case, as implied above, the energy coupled into the droplet 14 should not at any time exceed the amount required to immediately disturb the droplet.

[0059] The duration A can be selected and then C can be selected. For example, it is often desirable to be able to locate the position of the droplet in the micrometer range, for example in the range of about 1 μm to about 5 μm. The speed of the droplet in a direction transverse to its flight direction (for example, in the Y direction) is about 0.2 m / s, so the longest interval A that does not smear the image of the droplet 14 is about 5 μm / 0.2 m / s, thus about 25 microseconds. With regard to continuous measurement radiation versus pulsed measurement radiation, if the repetition rate of the laser is about 50 kHz or higher (i.e., the period is about 20 microseconds or less) and the duration of the interval A is about the same, the measurement radiation in the interval A is substantially continuous. Therefore, the duration of the interval A can be in the range of about 100 ns to 20 μs. At the upper limit of this range, i.e., at about 20 μs, the measurement radiation will be substantially continuous.

[0060] The delay between determining the position of the target material and irradiating the droplet with the working radiation should be as short as possible, for example about 1 μs. As an upper limit, it is beneficial if the delay does not exceed the duration of one pulse (one shot) after the pulse repetition period of the plasma (about 20 μs).

[0061] A hybrid laser that can generate two different power levels can, for example, be constructed as a laser with a leaky Q switch. Q switching is a technique for obtaining high-energy short pulses from a laser by placing a variable attenuator (Q switch) inside the laser optical resonator to modulate the intracavity losses (thereby changing the Q factor of the laser resonator). The intracavity losses can be set so that the radiation power from the laser can be controlled. In a manner similar to lasers constructed for dual-pulse particle image velocimetry, the hybrid laser can also be constructed, for example, as a seed oscillator with one or more amplifiers.

[0062] The ablation threshold is expressed as radiation exposure or energy density (J / cm 2 ) instead of irradiance (W / cm 2 ), that is, the radiation energy received by the surface per unit area, which is about 1 J / cm for nanosecond pulses 2 , irradiance is the radiant energy received by a surface per unit area per unit time. Depending on the wavelength, the energy density of the working pre-pulse 152 is about 10 J / cm 2 To about 1000J / cm 2 , the wavelength may range, for example, from about 532 nm to 10,600 nm, and the duration may be, for example, from about 1 ps to about 100 ns. The operating heating pulse may be better specified in terms of irradiance, and for a wavelength of 10,600 nm and a duration of about 10 ns to about 100 ns, the irradiance is typically, for example, about 8 GW / cm 2 to about 15GW / cm 2 within the range.

[0063] Reference again Figure 4 , light from the measurement radiation 150 is reflected by the target material 14 as the target material 14 is on its way to the primary irradiation location 110. The beam splitter 116 redirects the reflected light 120 to the control module 122, which uses the redirected reflected light 120 to determine the position of the target material 14. The control module 122 uses the determined position to provide a control signal to the radiation source 102, which causes the radiation source 102 to switch from emitting the measurement radiation 150 to emitting the working radiation 152. Alternatively, the radiation source may automatically switch from emitting the measurement radiation 150 to emitting the working radiation 152 after a preset amount of time (A). The control module 122 also uses the reflected measurement radiation 120 to control the optical module 124 to steer the radiation beam 104.

[0064] According to another aspect of the embodiment, the control module 122 may scan the reflected metrology radiation 120 until the return signal is maximized, and then the control module 122 may cause the radiation source 102 to emit a short intense working radiation pulse at the maximum return signal to optimally hit the target material 14 .

[0065] According to another aspect of the embodiment, the optical module 124 may change the focus and / or direction of the radiation beam 104 during the emission of the measurement radiation 150 such that the beam width of the radiation beam 104 is larger during the emission of the measurement radiation 150. For example, during interval A, the average power may be selected to be approximately less than one tenth the magnitude of C, while during interval B, the average power may be selected to be approximately ten times greater than C. This results in an irradiance during interval B that is approximately one hundred times greater than the irradiance during interval A. In an example where the radiation source 102 is CW in nature, the area of ​​the beam must change by approximately one hundred times from interval A to interval B, and thus the beam width should change by approximately ten times. For example, during measurement phase A, the beam width would be approximately 1 mm, while during operating phase B, the beam width would be approximately 100 μm. The optical module 124 may be part of an overall optical system for the light source.

[0066] According to one aspect of the embodiment, the above system can be used for setup or alignment, wherein the operating pulses are not fired until the correct alignment is obtained. Once the correct alignment is achieved, the system can be used to maintain aiming accuracy during the remaining operation. Likewise, while the source is operating, it is usually necessary to fire in each cycle even if the aiming deviates from the optimal aiming. In other words, the laser must be fired even if the optimal aiming condition no longer holds. Therefore, the steering cannot be corrected for the next pulse that is closest, but the steering can be corrected for subsequent pulses.

[0067] like Figure 6As shown, in a spatially separated arrangement, the optical module 124 can also be additionally or alternatively configured to switch or split the light beam 104 between the two paths. Moreover, the optical module 124 can be configured to provide different beam widths or spot sizes for the two beams near the irradiation area 118. The optical module 124 can, for example, be an electro-optical device that is polarization-switched between states to select a path on a polarization beam splitter having different optical elements in the two paths. The different optical devices modify the near-field image or the far-field image of the radiation source 102 so that the beam width in the plane of the target material 14 is substantially different for the two paths, for example, resulting in an irradiance or energy density that is lower for one path than for the other. Figure 5A level C, while for the other path, the irradiance or energy density is alternately higher than Figure 5A The level C of the position sensing system. This allows a larger detection or "capture" volume to be provided for the position sensing system. In this embodiment, the radiation source 102 and the optical module 124 can be considered together as a hybrid laser. The measurement radiation and the working radiation can be generated simultaneously, but spatially separated, or generated sequentially and spatially separated, i.e., separated in space and time. This is in contrast to the measurement radiation and the working radiation being generated sequentially.

[0068] refer to Figure 7 According to aspects of the embodiments, a method for determining the position of a target material using a hybrid radiation source is disclosed. In step S300, the radiation source S is operated to irradiate the target material with measurement radiation. This can be done before generating working radiation in an alignment mode, or simultaneously with generating working radiation in an online aiming and holding mode. In step S302, a reflected portion of the measurement radiation is used to locate the target material. Once the position of the target material is determined, in step S304, the determined position is used to steer the working radiation from the source S to most accurately strike the target material.

[0069] refer to Figure 8According to another aspect of the embodiment, a method for determining the position of a target material using a hybrid radiation source is disclosed. In step S400, the radiation source S is operated to irradiate the target material with measurement radiation. This can be performed before generating the working radiation in the alignment mode, or simultaneously with generating the working radiation in the online aiming and holding mode. In step S402, the reflected portion of the measurement radiation is analyzed to determine the position of the target material by, for example, determining whether there is any reflected measurement radiation, or whether the amplitude of the reflected portion of the measurement signal exceeds a predetermined value or reaches a predetermined value or reaches its maximum value. In step S404, it is determined (e.g., by the control module 122 based on the reflected portion 120 of the measurement radiation) whether the position of the target material has been determined. If not, then in step S406, the source S continues to emit measurement radiation. If the determination in step S404 is affirmative, then in step S408, the working radiation from the source S is steered using the determined position to most accurately strike the target material.

[0070] Fig. 9 is a flow chart depicting a method for determining the position of a target material using a hybrid radiation source according to another aspect of an embodiment. In step S500, a radiation source S is operated to irradiate a target material with measurement radiation. This may be performed prior to generating working radiation in an alignment mode or may be performed simultaneously with generating working radiation in an online aiming and holding mode. In step S504, a determination is made (e.g., by the control module 122) as to whether a signal based on the amplitude of the reflected measurement radiation has reached a predetermined value (e.g., a maximum value). If not, then in step S506, the source S continues to emit measurement radiation. If the determination in step S504 is affirmative, then in step S508, the working radiation from the source S is steered using the determined position to most accurately strike the target material.

[0071] Fig.101 is a flow chart depicting a method for determining the location of a target material using a hybrid radiation source according to another aspect of the embodiment. In step S602, in step S500, the radiation source S is operated to irradiate the target material with measurement radiation. This can be done before generating the working radiation in the alignment mode, or simultaneously with generating the working radiation in the online aiming and holding mode. In step S604, the reflected portion of the measurement radiation is analyzed for a given condition that will indicate the location of the target material, for example, whether there is any reflected measurement radiation, or whether the amplitude of the reflected portion of the measurement signal exceeds a predetermined value or reaches a predetermined value or reaches a maximum value. In step S606, it is determined whether the analysis indicates that the given condition has been met. If not, then in step S608, the source S continues to emit measurement radiation. If the determination in step S606 is affirmative, then in step S610, the control module 122 uses the reflection to determine the target location. Then in step S612, the working radiation from the source S is steered using the determined position to most accurately hit the target material.

[0072] As described above, in the case where the hybrid laser emits measurement radiation and working radiation sequentially rather than simultaneously, the optical module 124 may be configured to make the beam width of the measurement radiation greater than the beam width of the working radiation.

[0073] The present disclosure is implemented with the aid 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 herein. Other boundaries may be defined as long as the specified functions and their relationships are properly performed. For example, control module functions may be divided between multiple systems, or at least partially performed by an overall control system.

[0074] The above description includes examples of one or more embodiments. Of course, it is not possible to describe every possible combination of components or methods for the purpose of describing the above-mentioned embodiments, but it will be appreciated by those skilled in the art that many other combinations and permutations of various embodiments are possible. Therefore, the described embodiments are intended to cover all such changes, modifications and variations that fall within the spirit and scope of the appended claims. In addition, to the extent that the term "includes" is used in the specific embodiments or claims, such terms are intended to be inclusive in a manner similar to that explained when the term "comprising" is used as a transitional term in the claims. In addition, although the elements of the described aspects and / or embodiments may be described or claimed in the singular, the plural form may be contemplated unless it is explicitly stated to be limited to the singular form. In addition, unless otherwise stated, all or part of any aspect and / or embodiment may be used together with all or part of any other aspect and / or embodiment.

[0075] Further aspects of the invention are set out in the following numbered clauses.

[0076] 1. A device comprising:

[0077] a laser system configured to generate measurement radiation and working radiation from a single laser source;

[0078] a first optical system arranged to transmit the measurement radiation along a first optical path to an illumination area;

[0079] a second optical system arranged to transmit reflected measurement radiation reflected by target material in said illumination area;

[0080] a measurement module arranged to receive the reflected measurement radiation, and the measurement module is configured to perform an analysis of the reflected measurement radiation and to generate a control signal based at least in part on the analysis; and

[0081] The steering module is arranged to receive the control signal, and the steering module is configured to steer the working radiation in response to the control signal, and the first optical system transmits the working radiation to an irradiation site in the irradiation area.

[0082] 2. The apparatus according to clause 1, wherein the laser system is further configured to generate only one of the measurement radiation and the working radiation at a given time.

[0083] 3. The apparatus according to clause 1, wherein the laser system is further configured to generate the measurement radiation and the working radiation from primary radiation from the single laser source.

[0084] 4. The apparatus according to clause 3, wherein the laser system is further configured to split the initial radiation into the measurement radiation and the working radiation.

[0085] 5. The apparatus of clause 1, wherein the control module generates the control signal a predetermined time after the control module determines that the reflected measurement radiation meets a predetermined criterion.

[0086] 6. Apparatus according to clause 5, wherein the predetermined criterion is the detected presence of the reflected measurement radiation.

[0087] 7. Apparatus according to clause 5, wherein the predetermined criterion is that the amplitude of the reflected measurement radiation exceeds a predetermined threshold.

[0088] 8. Apparatus according to clause 5, wherein the predetermined criterion is a timing of the reflected measurement radiation.

[0089] 9. The apparatus according to clause 5, wherein the predetermined criterion is that the reflected measurement radiation reaches a maximum value.

[0090] 10. The apparatus of clause 5, wherein the predetermined time is immediately after the control module determines that the reflected measurement radiation meets the predetermined criterion.

[0091] 11. The apparatus of clause 1, wherein the irradiance of the measurement radiation is below a value that would significantly disturb the target material.

[0092] 12. The apparatus of clause 1, wherein the first optical system and the second optical system share at least one optical element.

[0093] 13. The device according to clause 1, wherein the measurement radiation is a measurement pulse.

[0094] 14. The device according to clause 1, wherein the measurement radiation is quasi-CW laser radiation.

[0095] 15. The apparatus of clause 1, wherein the working radiation comprises a pre-pulse.

[0096] 16. The apparatus of clause 1, wherein the working radiation comprises an ionizing pulse.

[0097] 17. An apparatus according to claim 1, wherein the control module generates a focusing control signal in response to determining that the reflected measurement radiation meets a predetermined criterion, and the apparatus further comprises a focusing module located in the first optical path and having a first state and a second state, wherein in the first state, a first beam width of radiation in the irradiation area has a first value, and in the second state, a second beam width of radiation passing through the irradiation area has a second value.

[0098] 18. A radiation source having a laser, the laser being configured to have a first state in which the laser generates measurement radiation and a second state in which the laser generates working radiation, the radiation source being responsive to a control signal to transition from the first state to the second state.

[0099] 19. A radiation source having a laser, wherein the laser is configured to have a first state and a second state, wherein in the first state, the laser generates measurement radiation having an irradiance lower than a predetermined value, and in the second state, the laser generates working radiation having an irradiance higher than the predetermined value, and wherein the radiation source responds to a control signal to transition from the first state to the second state.

[0100] 20. The apparatus of clause 19, wherein the predetermined value is an amplitude required to ionize the target material into a plasma state.

[0101] 21. An EUV source, comprising:

[0102] a radiation source comprising a laser, the laser being configured to have a first state in which the laser generates measurement radiation and a second state in which the laser generates working radiation, the radiation source being responsive to a control signal to transition from the first state to the second state;

[0103] a first optical system arranged to transmit the measurement radiation along a first path to a first location;

[0104] a second optical system arranged to transmit reflected measurement radiation reflected by the target material; and

[0105] A control module is arranged to receive the reflected measurement radiation, and the control module is configured to analyze the reflected measurement radiation and to generate the control signal based on the reflected measurement radiation.

[0106] 22. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of:

[0107] causing the laser to emit a measurement pulse;

[0108] reflecting at least a portion of a reflected portion of the measurement pulse using the target material;

[0109] determining a position of the target material using the at least a portion of the reflected portion of the measurement pulse;

[0110] determining a timing of emission of an operating pulse by the laser based at least in part on the determined position of the target material; and

[0111] The radiation source is caused to emit the working pulse at the determined timing.

[0112] 23. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of:

[0113] causing the laser to emit a measurement pulse;

[0114] reflecting at least a portion of a reflected portion of the measurement pulse using the target material;

[0115] determining a timing of emission of an operating pulse by the laser based at least in part on whether the at least a portion of the reflected portion meets a predetermined criterion; and

[0116] The laser is caused to emit the working pulse at the determined timing.

[0117] 24. The method according to clause 23, wherein the predetermined criterion is the presence of any reflected measurement pulses.

[0118] 25. A method according to clause 23, wherein the predetermined criterion is whether the amplitude of the irradiance of the reflected part of the measurement pulse exceeds a predetermined value.

[0119] 26. A method according to clause 23, wherein the predetermined criterion is whether the amplitude of the irradiance of the reflected part of the measurement pulse has reached a maximum value.

[0120] 27. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of:

[0121] controlling a beam width of a focusing module to increase a beam width of a light beam passing through the focusing module;

[0122] causing the laser to transmit a measurement pulse through the focusing module;

[0123] reflecting at least a portion of a reflected portion of the measurement pulse using the target material;

[0124] determining a position of the target material using the at least a portion of the reflected portion of the measurement pulse;

[0125] determining a timing of emission of an operating pulse by the laser based at least in part on the determined position of the target material;

[0126] controlling a beam width of a focusing module to reduce a beam width of a light beam passing through the focusing module; and

[0127] The laser is caused to emit the working pulse through the focusing module at the determined timing.

Claims

1. A device comprising: a laser system configured to generate measurement radiation and working radiation from a single laser source; a first optical system arranged to transmit the measurement radiation along a first optical path to an illumination area; a second optical system arranged to transmit reflected measurement radiation reflected by target material in said illumination area; a measurement module arranged to receive the reflected measurement radiation and configured to perform an analysis of the reflected measurement radiation and to generate a control signal based at least in part on the analysis, wherein the analysis comprises determining that the reflected measurement radiation meets a predetermined criterion; and a steering module arranged to receive the control signal, and the steering module is configured to steer the working radiation in response to the control signal, the first optical system transmitting the working radiation to an irradiation site in the irradiation area; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, the amplitude of the reflected measurement radiation exceeding a predetermined threshold, or the reflected measurement radiation reaching a maximum value. 2 . The apparatus of claim 1 , wherein the laser system is further configured to generate only one of the measurement radiation and the working radiation at a given time.

3. The apparatus of claim 1, wherein the laser system is further configured to generate the measurement radiation and the working radiation based on primary radiation from the single laser source. 4 . The apparatus according to claim 3 , wherein the laser system is further configured to split the initial radiation into the measurement radiation and the working radiation.

5. The apparatus of claim 1, wherein the measurement module generates the control signal a predetermined time after the measurement module determines that the reflected measurement radiation meets the predetermined criterion. 6 . The apparatus of claim 5 , wherein the predetermined time is immediately after the measurement module determines that the reflected measurement radiation meets the predetermined criterion.

7. The apparatus of claim 1, wherein the measurement radiation has an irradiance below a value capable of significantly perturbing the target material.

8. The apparatus of claim 1, wherein the first optical system and the second optical system share at least one optical element.

9. The device according to claim 1, wherein the measurement radiation is a measurement pulse.

10. The device according to claim 1, wherein the measurement radiation is quasi-CW laser radiation.

11. The apparatus of claim 1, wherein the working radiation comprises a pre-pulse.

12. The apparatus of claim 1, wherein the working radiation comprises an ionizing pulse.

13. The device of claim 1 , wherein the measurement module generates a focusing control signal in response to determining that the reflected measurement radiation satisfies a predetermined criterion, and the device further comprises a focusing module located in the first optical path and having a first state and a second state, wherein in the first state a first beam width of radiation in the irradiation area has a first value, and in the second state a second beam width of radiation passing through the irradiation area has a second value.

14. A radiation source having a laser, the laser being configured to have a first state in which the laser generates measurement radiation and a second state in which the laser generates working radiation, the radiation source being responsive to a control signal to transition from the first state to the second state based at least in part on: the detected presence of reflected measurement radiation, whether the amplitude of the irradiance of a reflected portion of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected portion of the reflected measurement radiation reaches a maximum value.

15. A radiation source having a laser, the laser being configured to have a first state and a second state, wherein in the first state the laser generates measurement radiation having an irradiance below a predetermined value, and in the second state the laser generates working radiation having an irradiance above the predetermined value, the radiation source being responsive to a control signal to transition from the first state to the second state based at least in part on: the detected presence of reflected measurement radiation, whether the amplitude of the irradiance of a reflected portion of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected portion of the reflected measurement radiation reaches a maximum value.

16. The radiation source of claim 15, wherein the predetermined value is an amplitude required to ionize the target material into a plasma state.

17. An EUV source, comprising: a radiation source comprising a laser, the laser being configured to have a first state in which the laser generates measurement radiation and a second state in which the laser generates working radiation, the radiation source being responsive to a control signal to transition from the first state to the second state; a first optical system arranged to transmit said measurement radiation along a first path to a first location; a second optical system arranged to transmit reflected measurement radiation reflected by the target material; as well as a control module arranged to receive the reflected measurement radiation, and the control module is configured to analyze the reflected measurement radiation and to generate the control signal based on the reflected measurement radiation, wherein the analysis comprises determining that the reflected measurement radiation meets a predetermined criterion; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation reaches a maximum value.

18. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of: causing the laser to emit a measurement pulse; reflecting at least a portion of the measurement pulse using the target material; determining a position of the target material using the at least a portion of the reflected portion of the measurement pulse; determining, based at least in part on the determined position of the target material, timing of emission of an operating pulse by the laser and whether the at least a portion of the reflected portion meets a predetermined criterion; as well as causing the radiation source to emit the working pulse at the determined timing; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation reaches a maximum value.

19. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of: causing the laser to emit a measurement pulse; reflecting at least a portion of the measurement pulse using the target material; determining a timing for the laser to emit an operational pulse based at least in part on whether the at least a portion of the reflected portion of the measurement pulse meets a predetermined criterion; as well as causing the laser to emit the working pulse at the determined timing; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation reaches a maximum value.

20. A method of irradiating a target material using a radiation source comprising a laser, the method comprising the steps of: controlling a beam width of a focusing module to increase a beam width of a light beam passing through the focusing module; causing the laser to emit measurement pulses through the focusing module; reflecting at least a portion of the measurement pulse using the target material; determining a position of the target material using the at least a portion of the reflected portion of the measurement pulse; determining, based at least in part on the determined position of the target material, timing of emission of an operating pulse by the laser and whether the at least a portion of the reflected portion meets a predetermined criterion; controlling a beam width of a focusing module to reduce a beam width of a light beam passing through the focusing module; as well as causing the laser to emit the working pulse through the focusing module at the determined timing; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation exceeds a predetermined value, or whether the amplitude of the irradiance of the reflected part of the reflected measurement radiation reaches a maximum value.

21. An apparatus comprising: a first optical system arranged to transmit the measurement radiation along a first optical path to the illumination area; a second optical system arranged to transmit reflected measurement radiation reflected by target material in said illumination area; a measurement module arranged to receive the reflected measurement radiation and configured to perform an analysis of the reflected measurement radiation and to generate a control signal based at least in part on the analysis, wherein the analysis comprises determining that the reflected measurement radiation meets a predetermined criterion; and a steering module arranged to receive the control signal, and the steering module is configured to steer the working radiation in response to the control signal, the first optical system transmitting the working radiation to an irradiation site in the irradiation area; Wherein the predetermined criterion is selected from the following: the detected presence of the reflected measurement radiation, the amplitude of the reflected measurement radiation exceeding a predetermined threshold, or the reflected measurement radiation reaching a maximum value.

22. The apparatus of claim 21, wherein the measurement module generates the control signal a predetermined time after the measurement module determines that the reflected measurement radiation meets the predetermined criterion.

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