JP7241027B2 - Laser-produced plasma source - Google Patents

Description

Cross-reference to related applications
[0001] This application claims priority from European Application No. 17170322.6 filed May 10, 2017, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a seed laser for a laser-produced plasma source, eg for a lithographic or metrology apparatus.

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such cases, a patterning device, alternatively called a mask or reticle, can be used to generate the circuit patterns to be formed on the individual layers of the IC. This pattern can be transferred onto a target portion (eg comprising part of, one, or several dies) on a substrate (eg a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

[0004] The semiconductor industry continues to seek the development of lithographic techniques capable of printing the ever-decreasing dimensions of integrated circuits. Extreme ultraviolet (“EUV”) light (sometimes referred to as soft x-rays) is electromagnetic radiation generally having wavelengths between 10 and 120 nanometers (nm), with shorter wavelengths being used in the future. defined as what is expected to be Currently, EUV lithography generally involves EUV light with wavelengths in the range of 10 to 14 nm, and is used to produce very small features, for example features below 32 nm, on substrates such as silicon wafers.

[0005] At the time of this writing, the most commercially successful method of producing EUV light is one or more elements with one or more emission lines in the EUV region, such as xenon, lithium, tin, indium. , antimony, tellurium, aluminum, etc., to generate plasma. In one such method, often referred to as laser-produced plasma (“LPP”), the required plasma is a target material such as a droplet, stream, or cluster of material having the desired line-emitting element. , can be produced by irradiating with a laser beam at the irradiation site. The line-emitting element may be in pure form or in an alloyed form, eg an alloy that is liquid at the desired temperature, or may be mixed or dispersed with another material, such as a liquid.

[0006] In some prior art LPP systems, each droplet in the droplet stream is illuminated by a separate laser pulse to form a plasma from each droplet. Alternatively, some prior art systems have been disclosed in which each droplet is sequentially illuminated by two or more pulses of light. Optionally, each droplet is exposed to a so-called "pre-pulse" to heat, expand, vaporize, vaporize and/or ionize the target material and/or generate a weak plasma, followed by a so-called "main pulse". may generate an intense plasma and convert most or all of the material affected by the pre-pulse to plasma, thereby producing EUV emission. It will be appreciated that more than one pre-pulse can be used, more than one main pulse can be used, and that the functions of the pre-pulse and the main pulse may overlap to some extent.

[0007] Since the EUV output power in an LPP system generally corresponds to the drive laser power that irradiates the target material, sometimes a relatively low power oscillator or "seed laser" and a It may also be considered desirable to employ a configuration that includes one or more amplifiers of . Using a large amplifier allows the use of a low power and stable seed laser while still providing the relatively high power pulses used in the LPP process.

[0008] US Patent Application Publication No. 2014/0203194, incorporated herein by reference, describes a laser source for LPP systems in which the main pulse used to generate the plasma is preceded by a "pedestal." described. The pedestal has a beam intensity that begins approximately 400 ns before the main pulse and rises from 1 to 10% of the peak intensity of the main pulse. The presence of the pedestal is recognized to increase the EUV output power of the LPP source. In US2014/0203194, the pedestal is controlled using an optical shutter (eg comprising a Pockels cell and two polarizers) and/or a saturable absorber. Controlling pedestal energy is difficult given the high pulse repetition rates desired to achieve high power supplies.

[0009] The present invention seeks to provide an improved laser-produced plasma source, for example for use with lithographic device manufacturing processes and/or metrology equipment.

SUMMARY OF THE INVENTION In a first aspect, the present invention provides a seed laser module for a laser-produced plasma source, the seed laser module comprising a pulsed laser source configured to emit pulses of source radiation at a pulse repetition rate. a subsystem configured to provide an electrical signal; and an electro-optic coupled to the subsystem configured to receive a source radiation pulse and to emit a shaped radiation pulse under control of the electrical signal. a modulator, wherein the electrical signal comprises a gating pulse with a pulse repetition rate in phase with the source radiation pulse and one or more secondary pulses between successive ones of the gating pulses. .

[0010] In a second aspect, the present invention provides a seed laser module for a laser-produced plasma source, the seed laser module configured to emit pulses of source radiation at a pulse repetition rate. a laser source, a subsystem configured to provide an electrical signal, and coupled to the subsystem configured to receive a source radiation pulse and to emit a shaped radiation pulse under control of the electrical signal. and an acoustic device configured to apply an acoustic signal to the electro-optic crystal.

[0011] In a third aspect, the present invention provides a drive laser device for a laser-produced plasma source, the drive laser device comprising a seed laser module as described above and a shaped pulse of radiation to amplify and drive an amplifier configured to form a pulse of radiation.

[0012] In a fourth aspect, the present invention provides a drive laser device as described above and a target material delivery system configured to deliver a target material to a target location and irradiate it with a drive radiation pulse to form a plasma. and a radiation collector configured to collect radiation emitted by the plasma.

[0013] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings.

1 is a simplified schematic diagram of some of the components of an embodiment of an LPP EUV light source; FIG. 1 is a simplified schematic diagram of some of the components of a seed laser module that may be used in an LPP EUV system; FIG. FIG. 4A is a simplified schematic diagram of some of the components of another seed laser module that may be used in an LPP EUV system; Fig. 3 is a graph of main pulse intensity versus time, showing the pedestal portion in enlarged inset; 1 is a simplified schematic diagram of some of the components of a seed laser module having two electro-optic devices, according to one embodiment of the present invention; FIG. FIG. 4 is a diagram illustrating the timing of pulses in one embodiment of the present invention; FIG. 4 is a simplified schematic diagram of some of the components of another seed laser module having two electro-optic devices, according to one embodiment of the present invention; FIG. 4 is a simplified schematic diagram of some of the components of another seed laser module having an electro-optic device, according to one embodiment of the present invention; 9 is a simplified schematic diagram of the seed laser module of FIG. 8 when it is in an off state; FIG. FIG. 4 is a simplified schematic diagram of some of the components of another seed laser module having two electro-optic devices, according to one embodiment of the present invention;

[0014] FIG. 1 is a simplified schematic diagram of some of the components of one embodiment of an LPP EUV light source 10. As shown in FIG. As illustrated in FIG. 1, the EUV light source 10 includes a laser source 12 that generates a beam of laser pulses and directs the beam from the laser source 12 along one or more beam paths. It is delivered into plasma chamber 14 to illuminate each target, such as a droplet, at irradiation site 16 .

[0015] As also illustrated in FIG. , where the droplets interact with one or more laser pulses, ultimately creating a plasma and generating EUV radiation. Various target material delivery systems have been presented in the prior art and their relative advantages will be apparent to those skilled in the art.

[0016] As noted above, the target material is an EUV emitting element, which may include, but is not necessarily limited to, materials including tin, lithium, xenon, or combinations thereof. The target material may take the form of liquid droplets, mist, and/or solid particles contained within the liquid droplets. For example, elemental tin can be used as a target material, as pure tin, as a tin compound such as SnBr ₄ , SnBr ₂ , SnH ₄ , as a tin alloy such as a tin-gallium alloy, a tin-indium alloy, or a tin-indium-gallium alloy. or a combination thereof. Depending on the material used, the target material may include at or near room temperature (e.g. tin alloy or _SnBr4 ), above room temperature (e.g. pure tin), or below room temperature (e.g. _SnH4 ) at irradiation site 16. Can be presented at various temperatures. In some cases, these compounds may be relatively volatile, such as _SnBr4 . Similar alloys and compounds of EUV emitting elements other than tin and the relative advantages of such materials and the materials described above will be apparent to those skilled in the art.

[0017] Returning to Figure 1, the EUV light source 10 includes an optical element 18, such as a near-normal incidence collector mirror having a reflective surface in the shape of a prolate ellipsoid (ie, an ellipse that rotates about its major axis). This optical element 18 thus has a first focus in or near the irradiation site 16 and a second focus in a so-called intermediate region 20, where the EUV light is emitted from the EUV light source 10 and input to a device that utilizes EUV light, such as an integrated circuit lithography tool (not shown), also referred to as a "scanner." As shown in FIG. 1, optical element 18 is formed with an aperture for allowing the laser light pulses generated by laser source 12 to pass through and reach irradiation site 16 .

[0018] The optical element 18 must have a surface suitable for collecting EUV light and directing it to the intermediate region 20 for subsequent delivery to a device that utilizes EUV light. For example, the optical element 18 has a graded multilayer coating with alternating layers of molybdenum and silicon and possibly one or more high temperature diffusion barrier layers, smoothing layers, capping layers, and/or etch stop layers. can.

[0019] Those skilled in the art will appreciate that optical elements other than prolate ellipsoidal mirrors may be used as optical element 18. FIG. For example, optical element 18 may alternatively be a parabola that rotates about its longitudinal axis, or may be configured to deliver a beam having an annular cross-section to an intermediate position. In other embodiments, optical element 18 may utilize coatings and layers other than or additional to those described herein. A person skilled in the art will be able to select an appropriate shape and configuration for optical element 18 in a particular situation.

[0020] As shown in FIG. The EUV light source 10 may also include a beam conditioning unit 24 that is placed between the laser source 12 and the focusing unit 22 to expand, steer and/or shape the laser beam, and /or have one or more optical elements for shaping the laser pulse. Various focusing units and beam adjusting units are known in the art and can be appropriately selected by one skilled in the art.

[0021] As noted above, in some cases, laser source 12 includes one or more seed lasers and one or more amplifiers. The seed laser produces laser pulses which are then amplified into a laser beam that irradiates the target material at irradiation site 16 to form a plasma that produces EUV radiation.

[0022] Those skilled in the art will appreciate that many types of seed lasers can be used to generate the pre-pulses and main pulses. For example, a conventional dual chamber transversal flow laser source may be used in what is customarily known as a "master oscillator power amplifier" ("MOPA") configuration. Alternatively, the power amplifier may comprise a fast axial laser. A single laser source may generate both the pre-pulse and the main pulse. Alternatively, separate seed lasers may be used to generate the pre-pulse (PP) and main pulse in what is commonly known as a MOPA+PP laser.

[0023] One type of seed laser commonly used in some embodiments of EUV systems is a _CO2 laser, although other embodiments may use YAG (Yttrium Aluminum Garnet) lasers. If there are two seed lasers, they may be of different types. However, a YAG laser, for example, would require a different amplifier or amplifier chain than that used with a _CO2 laser. Those skilled in the art will recognize that there are types of lasers other than _CO2 lasers and YAG lasers, and configurations other than MOPA lasers and MOPA+PP lasers, and which types and configurations of lasers are suitable for the desired application. will be able to determine whether

[0024] Returning to FIG. 1 , EUV energy detector 28 detects the amount of EUV power generated in plasma chamber 14 . EUV energy detector 28 comprises a sensor in plasma chamber 14 , for example an EUV side sensor positioned at 90° to the laser beam, or a sensor in a scanner that measures the energy passing through intermediate focus 20 . EUV energy detectors, for example comprising photodiodes, are generally known to those skilled in the art. As is familiar to those skilled in the art, the EUV energy generated by the collision of the droplet with the laser pulse is calculated by integrating the EUV power signal provided by the EUV energy detector 28 over the period the droplet is illuminated. be done.

[0025] EUV controller 29 is configured to determine the intensity of a subsequent laser pulse based on the amount of EUV generated by one or more previous pulses. The EUV controller obtains a measure of the amount of EUV generated from the previous pulse via EUV energy detector 28 . EUV controller 29 determines the target intensity of subsequent laser pulses using an algorithm described below. The target intensity is based on the determined stability of the plasma sustained between laser pulses in plasma chamber 14 . The more stable the plasma, the higher the intensity of subsequent laser pulses can be, up to known limits. If the plasma is unstable or unstable, the EUV controller 29 can reduce the intensity of subsequent laser pulses.

[0026] The EUV controller 29 can be implemented in a variety of ways known to those skilled in the art, including access to memory capable of storing executable instructions to implement the functions of the modules described. including, but not limited to, a computing device comprising a processor having a A computing device may have one or more input and output components, including components that communicate with other computing devices over a network (eg, the Internet) or other form of communication. The EUV controller 29 comprises one or more modules embodied in arithmetic logic such as software or executable code.

A pulse actuator (not shown) actuates the laser source 12 to aim the irradiation site 16 and fire a laser pulse. Actuators may be electrical, mechanical and/or optical components and are commonly known to those skilled in the art.

[0028] As mentioned above, the laser source 12 may comprise a seed laser module and one or more amplifier stages. An example of a seed laser module 30 is illustrated in FIG. Seed laser module 30 includes two seed lasers, a pre-pulse seed laser 32 and a main pulse seed laser 34 . Those skilled in the art will appreciate that when such an embodiment involving two seed lasers is used, the target material is first blasted by one or more pulses from the pre-pulse seed laser 32 and then by the main pulse seed laser 34 . It will be appreciated that the illumination may be by one or more pulses from .

[0029] Seed laser module 30 is shown as having a "folded" arrangement rather than a straight arrangement of components. In practice, such an arrangement is common to limit module size. To accomplish this, the beams produced by the laser pulses of the pre-pulse seed laser 32 and the main pulse seed laser 34 are directed onto desired beam paths by a plurality of optical components 36 . Depending on the particular configuration desired, optical component 36 can be a lens, filter, prism, mirror, grating, or any element that can be used to direct a beam in a desired direction. In some cases, optical component 36 may also serve other functions, such as changing the polarization of the passing beam.

[0030] As known to those skilled in the art, seed lasers include optical components such as output couplers, polarizers, rear mirrors, gratings, acousto-optic modulation (AOM) switches, and the like. Among these optical components, seed lasers 32 and 34 include Q-switched AOMs used in giant pulse shaping and used to generate pulsed output beams. As is known in the art, a Q-switched AOM controls the timing of pulse emission from the seed laser.

[0031] In the embodiment of Figure 2, the beam from each seed laser first passes through an electro-optic modulator 38 (EOM). The EOM 38 is used as a pulse shaping unit with the seed laser to trim the pulses produced by the seed laser into pulses with shorter widths and faster rise and/or fall times. Shorter pulse widths and relatively fast rise and/or fall times will increase EUV output and light source efficiency. This is because the interaction time between the pulse and the target is short and the unwanted portion of the pulse does not use up the amplifier gain. Although two separate pulse shaping units (EOMs 38) are shown, alternatively a common pulse shaping unit may be used to trim both the pre-pulse seed and the main pulse seed.

[0032] In some embodiments described herein, the EUV controller 29 can control the intensity of the laser pulses by adjusting the timing of the EOM 38 relative to the Q-switched AOM in the seed laser. . In one embodiment, the timing is adjusted by changing the delay of the Q-switch AOM trigger or EOM trigger relative to the laser firing trigger. The laser fire trigger is based on when the droplet arrives at the detection line laser in the plasma chamber.

[0033] In other embodiments, EUV controller 29 may control the intensity of the laser pulses by varying the voltage applied to the EOM crystals of EOM 38. FIG. One material that can be used for such crystals is cadmium telluride (CdTe), but there are other materials that are used in EOMs. Light can pass through the EOM 38 when a high voltage HV (approximately 5,000 volts, or 5 kilovolts or 5 kV) is applied to the crystal. When no voltage is applied to EOM 38 , laser pulses do not pass through EOM 38 . To control the strength of the passing pulse, the high voltage HV is adjusted according to the desired strength. Therefore, the high voltage HV applied to the crystal is increased to generate a higher intensity laser pulse, and the high voltage HV applied to the crystal is decreased to generate a lower intensity laser pulse. .

[0034] Returning to FIG. 2, the beam from the seed laser next passes through acousto-optic modulators (AOMs) 40 and 42 . As will be explained below, AOMs 40 and 42 act as "switches" or "shutters" to prevent laser pulse reflections from the target material from reaching sensitive components of the system, such as the EOM and seed laser. Like, it works to deflect. Thus, AOMs 40 and 42 prevent reflections from damaging sensitive components. In the embodiment shown here, the beam from each seed laser passes through two AOMs. However, in some embodiments, the beam from each seed laser may pass through only a single AOM on each pass.

[0035] In some embodiments, AOMs 40 and 42 may be manipulated to adjust the intensity of the laser pulse. AOMs 40 and 42 are opened and closed by applying radio frequency (RF) power to transducers coupled to crystals within the AOMs. The intensity of the laser pulse is proportional to the amount of RF power applied. Thus, more RF power is applied to increase the intensity of the laser pulse, and less RF power is applied to decrease the intensity of the laser pulse.

[0036] After passing through AOMs 40 and 42, the two beams are "combined" by beam combiner 44. As shown in FIG. Since the pulses from each seed laser are generated at different times, this effectively means that two temporally separated beams are placed on a common beam path 46 for further processing and use. means.

[0037] After being placed on the common beam path, the beam from one of the seed lasers (again, only one at a time) passes through a beam delay unit 48 as is known in the art. . The beam is then directed through preamplifier 50 and then beam expander 52 . Following this, the beam passes through a thin film polarizer 54 and is directed onwards by an optical component 56 . This optical component is also the element that directs the beam to the next stage in the LPP EUV system and may also serve other functions. The beam generally travels from optical component 56 to one or more optical amplifiers and other components, as is known in the art.

[0038] Various tunable seed lasers suitable for use as both pre-pulse seed lasers and main-pulse seed lasers are known in the art. For example, in one embodiment, the seed laser may be a _CO2 laser with a sealed gas fill containing _CO2 at sub-atmospheric pressure, e.g., 0.05 to 0.2 atmospheres, pumped by a radio frequency discharge. good too. In some embodiments, a grating may be used to help define the optical cavity of the seed laser, and the grating may be rotated to align the seed laser with a selected line of rotation. .

[0039] FIG. 3 is a simplified block diagram of one embodiment of a seed pulse generation system 60. As shown in FIG. Similar to seed laser module 30, seed pulse generation system 60 generates a seed pulse, shapes the seed pulse, and amplifies the seed pulse. However, the seed pulse generation system 60 includes two preamplifiers 74 and 84 instead of the single preamplifier 50 of the seed laser module 30 of FIG. The addition of the second preamplifier and the additional gain provided by the second preamplifier causes the power amplifier located ahead of the seed pulse generation system 60 to self-laser and forward laser pulses. ) modulation and gain-stripping of the preamplifiers 74 and 84 of the seed pulse generation system 60 may be increased. The resulting self-lasing in the power amplifier has been observed as wide-width pulses lasting several microseconds. To mitigate these effects of adding a second preamplifier, seed pulse generation system 60 of FIG. 3 uses the seed laser of FIG. It includes additional isolation steps placed between the elements of the module 30 . The isolation stage of seed pulse generation system 60 may be added to or implemented within seed laser module 30 of FIG.

[0040] Seed laser 62, although shown as a single unit in FIG. Again, seed pulse generation system 60 may also include more than one seed laser 62 . EOM 64 shapes the pulses as described above in connection with EOM 38 of FIG.

A first isolation stage 66 is positioned between the EOM 64 and the first preamplifier 74 . First isolation stage 66 comprises first AOM 68 , delay element 70 and second AOM 72 . The delay element 70 also has an optical arrangement that folds the beam. The first isolation stage 66 operates similar to AOMs 40 and 42 and delay line 41 of FIG. 2 to deflect laser pulse reflections from the target material from reaching seed laser 62 . As further detailed herein, the isolation stage 66 provides better isolation from the amplified pulse that passed through the first preamplifier 74 .

[0042] To amplify the seed pulse generated by seed laser 62, the seed pulse passes through two or more preamplifiers, rather than just one preamplifier as shown in FIG. By using two or more preamplifiers, the seed pulse can be amplified in stages, which has many advantages. Using separate amplifiers with smaller individual gains prevents self-lasing of the amplifiers. Another advantage provided by the use of an isolation stage with multiple preamplifiers is that the gain is so high that the reflected light, even 1% after being deflected by 99%, still damages the seed laser 62. The difference is that the reflected light can be diverted mid-amplification before it is strong enough for .

The first preamplifier 74 is followed by a second isolation stage 76 comprising a first AOM 78 , a delay element 80 and a second AOM 82 . A second isolation stage 76 can deflect reflected light originating from portions of the LPP EUV system other than the first isolation stage. Since the second preamplifier 84 follows the second isolation stage 76 of the pulse traveling to the irradiation site, all of the reflected light reaching the second isolation stage 76 is also filtered by the second preamplifier 84. will be amplified.

[0044] Although not shown, a further isolation stage may follow the second preamplifier 84 before the beam is directed to further elements of the LPP EUV generation system. Such additional isolation stages can divert reflected light coming from additional components of the LPP EUV system before the reflected light is amplified by the second preamplifier 84 .

[0045] Experiments have shown that the amount of desirable EUV radiation produced is enhanced by the presence of a small amount of additional radiation that excites the target material into a plasma just prior to the main pulse of radiation. . This additional radiation is called a pedestal. An example of a main pulse structure including a pedestal is shown in FIG. 4, which is a graph of radiant power versus time at the point of irradiation of the target material. The main pulse peaks at a time index of 1 μs in this example and has a peak power of 15 MW. The pedestal Pd, visible only in the enlarged inset, has a maximum width of about 400 ns and a power of about 0.03 MW (ie less than 1% of the peak power of the main pulse). The pedestal profile will have a slow rise (or ramp-up) in power, possibly with a plateau just before the peak of the main pulse.

[0046] The exact width, profile and power level of the pedestal that yields optimum EUV output can be determined empirically, depending on the type of target material and/or the delivery technique of the target material to the target point. will change. The optimal parameters of the pedestal will vary over time and between source modules. The present invention seeks to provide an improved approach to controlling pedestals. In one embodiment, pedestal control is achievable with little or no additional hardware, particularly with no additional elements on the beam path.

[0047] Figure 5 illustrates the electro-optic modulator 38 for the main pulse in more detail. The seed laser 34 outputs primarily polarized radiation, eg horizontally polarized radiation. A first polarizer 381 is used to clean up the polarization of the seed laser to reject radiation with undesired polarization states, eg, perpendicular polarization. The horizontally polarized radiation is then incident on the first electro-optic crystal 382 . Electro-optic crystal 382 is a birefringent crystal with electrodes and is configured such that the polarization state of incident radiation is rotated by 90° when a suitable voltage is applied by high voltage source 386 . Thus, incident horizontally polarized radiation is converted to vertically polarized radiation and emitted as vertically polarized radiation. With no voltage applied, the polarization of the incident radiation is unaffected. The second polarizer 383 is oriented at 90° to the first polarizer 381 and thus passes radiation when a voltage is applied to the first electro-optic crystal 382 and blocks radiation when no voltage is applied. .

[0048] The second electro-optic crystal 384 is similar to the first electro-optic crystal 382 and likewise changes the polarization state of the radiation passing through it only when a suitable voltage is applied by the high voltage source 386. is configured to rotate the The third polarizer 385 is oriented at 90° to the second polarizer 383 and thus passes radiation when a voltage is applied to the second electro-optic crystal 382 and blocks radiation when no voltage is applied. . Thus, the use of two electro-optic crystals and three polarizers provides two isolation stages to reduce radiation leakage when no gating pulse is applied to the electro-optic crystals by high voltage source 386. be done. The first, second and third polarizers 381, 383, 385 may be thin film polarizers.

[0049] The timing and shape of the gating pulse applied to the electro-optic crystal determines the timing and shape of the pulse output by the seed laser module and then amplified. Seed laser 34 may be a pulsed laser so that the gating electrical pulse is synchronized to the radiation pulse output by seed laser 34 .

[0050] The inventors of the present invention have determined that an effective approach to creating a pedestal is to allow controlled leakage of radiation through the electro-optic modulator 38 .

[0051] According to one embodiment of the present invention, controlling the leakage of radiation from the seed laser 34 through the electro-optic modulator 38 includes an additional voltage pulse, referred to herein as a secondary pulse. , is achieved by applying it to the crystal of the electro-optic modulator between the gating pulses GP. This is shown in FIG.

[0052] The top graph of Figure 6 shows the radiant power output by the seed laser 34 as a function of time. The second graph shows a conventional gating pulse GP used to control the shape of the radiation pulse delivered to the amplification stage of the drive laser. The third line labeled A shows the first option according to one embodiment of the invention. In this first option, the frequency of the pulses output by the high voltage source is doubled such that secondary pulses SP are applied to the electro-optic crystal between gating pulses GP. Frequency doubling of the high voltage pulses can be achieved very easily, for example by doubling the frequency of the timing signal supplied to the high voltage source. It is also possible to increase the frequency of the gating pulse by a larger multiple, such as 3 or 4, to increase the pedestal.

[0053] Although no or relatively little radiation is being output by seed laser 34 upon application of secondary pulse SP to electro-optic crystals 382, 384, the secondary pulse is emitted on the surface of electro-optic crystals 382, 384. creates or builds up a residual charge at , which causes sufficient leakage of radiation to create a pedestal at the beginning of the seed pulse. The frequencies of the secondary and gating pulses are high, for example 50 kHz, so that all charges on the electro-optic crystal generated by the secondary pulse are discharged after the secondary pulse and before the next gating pulse. There is no time to spare.

[0054] A second option according to the first embodiment of the invention is illustrated in the fourth graph labeled B of FIG. In a second option, the amplitude A and/or width d of the secondary pulse SP are controlled to control the amount of residual charge on the electro-optic crystal and thus the size and/or shape of the pedestal. , controlled. It is also possible to control the timing of the secondary pulse SP with respect to the gating pulse GP.

[0055] A third option according to the first embodiment of the invention is illustrated in the fifth graph labeled C of FIG. In a third option, multiple secondary pulses SP1, SP2 are provided between each successive pair of gating pulses GP. The amplitudes A1, A2 and/or widths d1, d2 of the secondary pulses SP1, SP2 control the amount of residual charge applied to the electro-optic crystal, and thus the size and/or shape of the pedestal. so that it is controlled. It is also possible to control the timing of the secondary pulses SP1, SP2 with respect to the gating pulse GP. More than two secondary pulses may be provided per gating pulse period.

[0056] A second embodiment of the invention is illustrated in FIG. Parts of the second embodiment that are identical to those of the first embodiment are indicated with the same reference numerals and are not further described for the sake of brevity. In a second embodiment, high voltage source 387 is configured to apply a DC bias voltage to electro-optic crystals 382 and 384 in addition to gating pulses. The DC bias voltage can be less than about 20% of the peak voltage of the gating pulse. For example, if the peak voltage of the gating pulse is about 5,000V or higher, the DC bias voltage can be less than about 1,000V. A DC bias on a particular one of the electro-optic crystals induces an electric field within that electro-optic crystal, which ultimately results in a charge distribution affecting the pedestal. Therefore, the DC bias voltage is controlled to optimize the output of EUV radiation.

[0057] A third embodiment of the invention is illustrated in Figures 8 and 9, which respectively show an electro-optic modulator using a single electro-optic crystal in an ON state and an OFF state. is shown. Parts of the third embodiment that are identical to the previous embodiment are indicated with the same reference numerals and will not be described further for the sake of brevity.

[0058] In the ON state shown in FIG. When a sufficient voltage is applied to the electro-optic crystal 382, for example by a gating pulse or a secondary pulse, the polarization state of the radiation is rotated to vertical so that the radiation passes through the second polarizer 383, which passes the vertical polarization state. pass.

[0059] In the off-state shown in FIG. As with the previous embodiment, a controlled amount of leakage of radiation sufficient to create a pedestal can be caused by residual charge on electro-optic crystal 382 . Residual charge may be induced by application of a secondary pulse as in the first embodiment or by application of a bias voltage as in the second embodiment.

[0060] In an electro-optic modulator with only one electro-optic crystal, a certain amount of radiation leaks through the second polarizer 383, which may not be 100% selective due to misalignment, for example. can. Such leakage contributes to pedestal formation and is considered in determining the controlled leakage created by the application of residual charge to the electro-optic crystal.

[0061] A fourth embodiment is illustrated in FIG. Parts of the fourth embodiment that are identical to the previous embodiment are indicated with the same reference numerals and will not be described further for the sake of brevity. In a fourth embodiment, an actuator 388 is provided to rotate one or more of the polarizers, eg the second polarizer 383 . By selectively rotating the polarizer 383, controlled radiation leakage can be achieved even when the electro-optic crystals 382, 384 are not energized. A similar effect can be obtained by rotating the first and third polarizers together. Actuators allow real-time control of the pedestal, but in some cases this may not be necessary and mechanical adjusters may be used instead. A controlled leakage of radiation can create the desired pedestal.

[0062] In a fifth embodiment, an electric field is created in the electro-optic crystal by applying a mechanical force to the electro-optic crystal. As is known, electro-optic crystals are also generally piezoelectric. Mechanical force may be applied by, for example, piezoelectric actuators or other forms of actuators. Desirably, the mechanical force has a frequency selected to induce resonant vibrations in the electro-optic crystal.

[0063] In embodiments of the invention, the control of radiation leakage to form the pedestal may be performed in feedback or feedforward mode or a combination thereof. Feedback control of radiation leakage is preferably based on measurements of the available EUV radiation power in the source module or lithographic apparatus.

[0064] In a further embodiment of the invention, there is provided a laser-produced plasma source comprising a seed laser module as identified above. The laser-produced plasma source further includes a sensor configured to provide a sensor signal characteristic of the predetermined wavelength of radiation collected by the radiation collector, the subsystem responsive to measurements from the sensor. is configured to provide an electrical signal under control of the sensor signal so as to control the seed laser module to adjust the intensity of the pedestal portion of the co-shaped radiation pulse. Alternatively, or in combination with a controlled electrical signal, the sensor signal may be used to control an acoustic device that determines the residual charge on the electro-optic crystal of the electro-optic modulator. Alternatively, or in combination with a controlled electrical signal or control of an acoustic device, the sensor signal is used to control adjusters configured to rotate one or more of the polarizers of the electro-optic modulator. may be

[0065] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0066] Certain embodiments provide a machine readable document configured to instruct the various devices illustrated in FIG. It may include a computer program containing one or more sequences of instructions. This computer program may be executed, for example, in the laser control unit LACU of FIG. 1 or the supervisory control system SCS or a combination of both. A data storage medium (eg semiconductor memory, magnetic disk or optical disk) storing such a computer program may also be provided.

[0067] The breadth and scope of the present invention is not limited by any of the above-described exemplary embodiments, but is defined only by the following claims and equivalents thereof.

Claims (15)

A seed laser module for a laser-produced plasma source, comprising:
a pulsed laser source configured to emit pulses of source radiation at a pulse repetition rate;
a subsystem configured to provide an electrical signal;
an electro-optic modulator coupled to the subsystem and configured to receive the source pulse of radiation and to emit a shaped pulse of radiation under control of the electrical signal;
The seed laser module, wherein the electrical signal comprises gating pulses of the pulse repetition rate in phase with the source radiation pulses and one or more secondary pulses between successive ones of the gating pulses. 2. The seed laser module of claim 1, wherein said secondary pulses are 180[deg.] out of phase with said source radiation pulses. 2. The seed laser module of claim 1, wherein the secondary pulses have a frequency N times the pulse repetition rate. However, N is an integer of 2 or more. 3. The subsystem of claim 1, wherein the subsystem comprises a first source configured to supply the gating pulse and a second source configured to supply the one or more secondary pulses. 4. The seed laser module according to any one of 1 to 3. the amplitude of the one or more secondary pulses, the width of the one or more secondary pulses, and the one or more amplitudes for the associated one of the gating pulses. 5. The seed laser module of any one of claims 1-4, further comprising a pulse controller configured to control at least one of: a particular phase of one of the secondary pulses. the electro-optic modulator comprises an electro-optic crystal;
6. The seed laser module of any one of claims 1-5 , wherein the seed laser module further comprises an acoustic device configured to apply an acoustic signal to the electro-optic crystal. 7. The seed laser module of Claim 6 , wherein the acoustic device comprises a transducer mechanically coupled to the crystal. the electro-optic modulator comprises a polarizer;
8. The seed laser module of any one of claims 1-7 , wherein the seed laser module further comprises an adjuster configured to rotate the polarizer. The electro-optic modulator includes, in sequence from the pulsed laser source, a first polarizer, a first crystal having controllable birefringence, and a second polarizer oriented perpendicular to the first polarizer. , a second crystal with controllable birefringence and a third polarizer oriented parallel to the first polarizer;
9. The seed laser module of Claim 8 , wherein the adjuster is configured to rotate the second polarizer. The electro-optic modulator includes, in sequence from the pulsed laser source, a first polarizer, a first crystal having controllable birefringence, and a second polarizer oriented perpendicular to the first polarizer. , a second crystal with controllable birefringence and a third polarizer oriented parallel to the first polarizer;
9. The seed laser module of Claim 8 , wherein the adjuster is configured to rotate the first polarizer and the third polarizer. 11. The seed laser module of any one of claims 8-10 , wherein the adjuster comprises an actuator. A drive laser device for a laser-produced plasma source, comprising:
a seed laser module according to any one of claims 1 to 11 ;
an amplifier configured to amplify the shaped pulse of radiation to form a drive pulse of radiation;
A drive laser device, comprising: 13. The drive laser device of claim 12 , further comprising a pre-pulse seed laser module configured to output a pre-pulse seed pulse prior to output of the shaped pulse of radiation by the seed laser module. a drive laser device according to claim 12 or 13 ;
a target material delivery system configured to deliver target material to a target location and be irradiated by said driving radiation pulse to form a plasma;
a radiation collector configured to collect radiation emitted by said plasma;
A laser-produced plasma source. a sensor configured to provide a sensor signal characteristic of radiation of a predetermined wavelength collected by said radiation collector, said subsystem providing said electrical signal under control of said sensor signal; comprising a sensor;
a control device configured to control the seed laser module to adjust the intensity of the pedestal portion of the shaped radiation pulse in response to the sensor measurements;
15. The laser produced plasma source of claim 14 , further comprising:

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