Laser system for source material conditioning in an EUV light source

Abstract

An apparatus and method are disclosed in which off-droplet measurement of pre-pulse energy is used for pulse energy control instead of on-droplet measurement. The pre-pulse energy is measured during the off-droplet non-exposure period and controlled to a pre-pulse energy setpoint. The pre-pulse energy can then be open loop controlled to the pre-pulse energy setpoint during the on-droplet period. This effectively decouples the EUV dose control loop from the pre-pulse energy control loop and avoids the negative effects of coupling such loops, e.g., loss of part of the dose adjustment range available to the dose controller.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to U.S. Application No. 62 / 887,160, filed August 15, 2019, entitled “LASER SYSTEM FOR SOURCE MATERIAL CONDITIONING IN AN EUV SOURCE,” which is incorporated by reference herein in its entirety. TECHNICAL FIELD

[0003] This application relates to light sources that generate extreme ultraviolet light by excitation of a source material, and in particular to systems that use one or more laser pulses to prepare and excite EUV source material. BACKGROUND

[0004] Extreme ultraviolet (“EUV”) light (e.g., electromagnetic radiation having a wavelength of about 50 nm or less, sometimes also referred to as soft x-rays, and including light having a wavelength of about 13 nm) is used in photolithography processes to produce extremely small features on substrates (e.g., silicon wafers).

[0005] Methods for generating EUV light include, but are not limited to, changing the physical state of a source material to a plasma state. Source materials include elements that emit lines in the EUV range, such as xenon, lithium, or tin. In one such method, commonly referred to as laser produced plasma (“LPP”), the desired plasma is created by irradiating a source material (e.g., in the form of a droplet, stream, or cluster of the source material) with an amplified light beam, which can be referred to as a drive laser. For this process, the plasma is typically created in a sealed vessel (e.g., a vacuum chamber) and monitored using various types of metrology equipment.

[0006] CO2amplifiers and lasers, which output an amplified light beam having a wavelength of about 10,600 nm, can present certain advantages as drive lasers for irradiating source materials in LPP processes. This can be particularly true for certain source materials, such as tin-containing materials. For example, one advantage can be the ability to produce a relatively high conversion efficiency between drive laser input power and output EUV power.

[0007] In EUV light sources, EUV can be produced in a multi-step process in which droplets of source material are first impacted by one or more pulses en route to an irradiation location, which pulse(s) primarily condition the droplet, in its original form or a modified form, for subsequent phase conversion at the irradiation location. Conditioning in this context can include changing the shape of the droplet, e.g., flattening the droplet, or redistributing the droplet material, e.g., at least partially dispersing some of the droplet material into a mist. For example, one or more pulses can impact a droplet to modify the distribution of source material, and then a subsequent pulse can impact the modified droplet to transform it into a plasma that emits EUV. In some systems, these pulses are provided by the same laser, while in other systems, these pulses are provided by separate lasers. These conditioning pulses are sometimes referred to as "pre-pulses" because they are temporally earlier than the pulse(s) that transform the droplet.

[0008] In some EUV systems, the energy of these one or more pre-pulses ("PP energy") exhibits a tendency to drift. The PP energy also exhibits a tendency to jump after the recharge of the laser that is used to generate the pulses. These slow and fast fluctuations can alter the droplet modification process, and ultimately modify the EUV power. For example, a 10% to 20% drift / jump in the PP energy can result in a 1% to 4% loss of EUV power.

[0009] There is a need to provide systems and methods that can provide greater PP energy stability. SUMMARY

[0010] The following presents a summary of one or more embodiments in order to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments, or to delineate 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 the more detailed description that is presented later.

[0011] According to an aspect of embodiments, an apparatus and method are disclosed in which the PP energy outside the droplet is measured and controlled to drive the PP energy outside the droplet to a PP energy outside the droplet setpoint. The PP energy thus acquired is then used during the on-droplet period. This effectively decouples the EUV dose control loop from the PP energy control loop, and avoids the negative effects of coupling such loops, e.g., loss of part of the dose adjustment range available to the dose controller. In other words, the PP energy loop is only effective for off-droplet irradiation, while the dose control loop is only effective for on-droplet irradiation.

[0012] According to one aspect of one embodiment, an apparatus for generating extreme ultraviolet radiation using a source material is disclosed, the apparatus configured to operate in an exposure mode in which the source material is pulsed irradiated and a non-exposure mode in which the source material is not irradiated, the apparatus comprising: a radiation source configured to generate at least one off-droplet pulse during the non-exposure mode and a plurality of on-droplet pulses during the exposure mode; and an energy controller configured to perform an energy measurement of the at least one off-droplet pulse during the non-exposure mode and to drive a pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one off-droplet pulse, the radiation source further configured to generate the on-droplet pulses during the exposure mode with an energy at the pulse energy setpoint. The pulses can be pre-pulses and the energy controller can be a pre-pulse energy controller. The pre-pulse energy controller can be configured to perform an energy measurement of a plurality of off-droplet pre-pulses. The apparatus can further comprise a dose controller configured to control a size of an energy dose delivered to the source material. A control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

[0013] According to another aspect of embodiments, a method of controlling an apparatus for generating extreme ultraviolet radiation using a source material is disclosed, the apparatus having an exposure period in which the source material is pulsed irradiated and a non-exposure period in which the source material is not irradiated, the method comprising: generating an off-droplet pulse during the non-exposure period; controlling a size of an energy of the off-droplet pulse to be at a pulse energy setpoint; and generating an on-droplet pulse during the exposure period while controlling an energy of the on-droplet pulse to be at the pulse energy setpoint. The pulses can be pre-pulses and the energy controller can be a pre-pulse energy controller. Generating the on-droplet pre-pulse during the exposure period while controlling the energy of the on-droplet pre-pulse can comprise performing an open loop control of a pre-pulse energy of the on-droplet pre-pulse. Controlling the size of the energy can comprise calibrating the pre-pulse energy controller based at least in part on a measured size of the energy of the at least one off-droplet pre-pulse. The method can further comprise controlling a size of an energy dose delivered to the source material.

[0014] According to another aspect of embodiments, there is disclosed an apparatus for generating extreme ultraviolet radiation, the apparatus comprising a source of droplets of source material; a laser source of pulsed laser radiation, the laser source configured to operate in an exposure mode in which the source material is irradiated by laser pulses and a non-exposure mode in which the source material is not irradiated, the laser source configured to generate at least one off-droplet pulse during the non-exposure mode and a plurality of on-droplet pulses during the exposure mode; and an energy controller configured to perform an energy measurement of the at least one off-droplet pulse during the non-exposure mode and to drive a pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one off-droplet pulse, the radiation source further configured to generate the on-droplet pulses during the exposure mode with an energy at the pulse energy setpoint. The pulse can be a pre-pulse and the energy controller can be a pre-pulse energy controller. The pre-pulse energy controller can be configured to perform an energy measurement of a plurality of off-droplet pre-pulses. The apparatus can further comprise a dose controller configured to control a size of an energy dose delivered to the source material. A control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

[0015] According to another aspect of embodiments, there is disclosed a method of generating extreme ultraviolet radiation using a source material, the method comprising: generating droplets of the source material; generating off-droplet laser pulses that do not impinge any of the droplets of the source material; controlling a size of energy of the off-droplet laser pulses to be at a pulse energy setpoint; and generating on-droplet pulses that impinge the droplets of the source material while controlling an energy of the on-droplet pulses to be at the pulse energy setpoint. The pulse can be a pre-pulse and the energy controller can be a pre-pulse energy controller. Generating the on-droplet pre-pulse can comprise performing open loop control of a pre-pulse energy of the on-droplet pre-pulse. Controlling the size of energy can comprise calibrating the pre-pulse energy controller based at least in part on a measured size of energy of the at least one off-droplet pre-pulse. The method can further comprise controlling a size of an energy dose delivered to the source material.

[0016] Further embodiments, features, and advantages of the disclosed subject matter, as well as structures and operations of various embodiments, are described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0017] The accompanying drawings, which are incorporated herein and constitute a part of the specification, illustrate concepts set forth in the disclosure and, together with the description, further serve to explain the principles of the disclosure and to enable a person skilled in the relevant art to make and use the disclosure.

[0018] Figure 1 is a schematic, non-to-scale view of the general, broad concept of a laser-produced plasma EUV radiation source system according to an aspect of embodiments.

[0019] Figure 2 is a schematic, non-to-scale view of a portion of a system according to an aspect of an embodiment. Figure 1 is a schematic, non-to-scale view of a portion of a system according to an aspect of an embodiment.

[0020] Figure 3 is a non-to-scale schematic view of a drive pulse system according to an aspect of an embodiment.

[0021] Figure 4 is a flowchart showing an operational mode of a pulse delivery system according to an aspect of an embodiment.

[0022] Additional features and advantages of the present application will be better understood from the following detailed description of illustrative embodiments of the present application that results in the accompanying drawings. The present application is not limited to the particular embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Other embodiments will result upon practicing the teachings contained herein. DETAILED DESCRIPTION

[0023] Various embodiments are now described with reference to the drawings. While specific configurations are described in detail below, various equivalent configurations known to those skilled in the art are also considered. In describing various embodiments, specific terminology is employed for the sake of clarity. However, the specific

[0024] Reference is first made to Figure 1 , Figure 1 A schematic diagram of an exemplary EUV radiation source, e.g., a laser-produced plasma EUV radiation source 10, according to an aspect of an embodiment of the present application is shown. As shown, the EUV radiation source 10 can include a pulsed or continuous laser source 22, e.g., a pulsed gas discharge CO2laser source, which produces a beam of radiation 12 at a wavelength generally below 20 μιη, e.g., in a range from about 10.6 μιη to about 0.5 μιη or less. The pulsed gas discharge CO2laser source can have DC or RF excitation operating at high power and high pulse repetition rate.

[0025] The EUV radiation source 10 also includes a source material delivery system 24 for delivering source material in the form of droplets or a continuous liquid stream. In this example, the source material is a liquid, but can also be a solid or a gas. The source material can be made of tin or a tin compound, but other materials can also be used. In the depicted system, the source material delivery system 24 introduces droplets 14 of source material into the interior of the vacuum chamber 26 to an irradiation region 28 where the source material is irradiated to produce a plasma. It should be noted that, as used herein, the irradiation region is the region in which irradiation of the source material can occur, and is the irradiation region even when no irradiation is actually occurring. The EUV light source can also include a beam focusing and steering system 32, which will be described below in connection with Figure 2 This will be explained in more detail.

[0026] In the depicted system, the direction from the laser source 22 toward the irradiation region 28 (i.e., the nominal propagation direction of the beam 12) can be taken as the Z-axis. The path of the droplets 14 from the source material delivery system 24 to the irradiation region 28 can be taken as the -X-axis. Figure 1 The view of the system is thus perpendicular to the XZ-plane. Furthermore, the system can be configured so that the droplets 14 travel as depicted, although one of ordinary skill in the art will understand that other arrangements can be used in which the droplets travel horizontally or at some angle relative to gravity between 90 degrees (horizontal) and 0 degrees (vertical), inclusive.

[0027] The EUV radiation source 10 can also include an EUV light source controller system 60, which can also include a laser shot control system 65 and the beam steering system 32. The EUV radiation source 10 can also include a detector, such as a droplet position detection system, which can include one or more droplet imagers 70 that generate an output indicative of the absolute or relative position of the droplets, e.g., relative to the irradiation region 28, and provide that output to a position detection feedback system 62.

[0028] The droplet position detection feedback system 62 can use the output of the droplet imagers 70 to calculate droplet position and trajectory, from which a droplet position error can be calculated. The droplet position error can be calculated on a droplet-by-droplet basis, or averaged, or calculated on some other basis. The droplet position error can then be provided as input to the light source controller 60. In response, the light source controller 60 can generate a control signal, such as a laser position, direction, or timing correction signal, and provide that 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 focal power of the laser beam focal point 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 beam 12 with the droplet 14. For example, the beam 12 can be offset from center or impinge on the droplet 14 at a different angle of incidence than direct normal incidence.

[0029] As Figure 1 shown, the source material delivery system 24 can include a source material delivery control system 90. The source material delivery control system 90 can operate in response to a signal (e.g., the droplet position error described above, or some amount of the droplet position error provided by the system controller 60) to adjust the path of the source material through the irradiation region 28. This can be accomplished, for example, by repositioning the point at which the droplet 14 is released by the source material delivery mechanism 92. For example, the droplet release point can be repositioned by tilting the source delivery mechanism 92 or by moving the source delivery mechanism 92. The source material delivery mechanism 92 extends into the chamber 26 and is preferably externally supplied with source material, and the source material is connected to a gas source to place the source material under pressure in the source material delivery mechanism 92.

[0030] Continuing Figure 1 , the radiation source 10 can 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 also applies to other optical elements. The collector 30 can be a normal incidence reflector, for example, implemented as a Mo / Si multilayer mirror (MLM). The collector 30 can be in the form of an elongated ellipsoid with a central aperture to allow the laser radiation 12 to pass through and reach the irradiation region 28. The collector 30 can be, for example, in the shape of an ellipsoid having a first focal point at the irradiation region 28 and a second focal point at a so-called intermediate point 40 (also referred to as intermediate focus 40) at which the EUV radiation can be output from the EUV radiation source 10 and input to, for example, an integrated circuit lithography scanner or stepper 50, for example, which uses the radiation to process a silicon wafer workpiece 52 using a reticle or mask 54 in a known manner. The mask 54 can be transmissive or reflective. For EUV applications, the mask 54 is typically reflective. The silicon wafer workpiece 52 is then further processed in a known manner to obtain integrated circuit devices.

[0031] Continuing Figure 2As can be seen, 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 employed to control the light beam. Further, while mirrors are shown, it should be understood that other optical devices such as prisms can be used and that one or more steering optical devices can be positioned within the chamber 26. See, for example, U.S. Patent No. 7,598,509, entitled "Laser Produced Plasma EUV Light Source," issued October 6, 2009, the entire contents of which are incorporated herein by reference. For the illustrated embodiment, each of the steering mirrors 32a, 32b, and 32c can be mounted on respective tilt actuators 36a, 36b, and 36c, which can independently move each of the steering mirrors 32a, 32b, and 32c in one or two dimensions. The mirror 33 turns and focuses the light beam.

[0032] Figure 3 is a non-to-scale illustration of a drive pulse system according to an aspect of an embodiment. The drive pulse system is arranged to be able to provide a pulse to a droplet 14 of source material. Among other features, the drive pulse system includes a radiation source 22 that is able to generate a pulse 100. A beam steering system 32 relays the pulse 100 to a chamber 26 as a pulse 106, where the pulse 106 impinges on the droplet 14 of source material.

[0033] In the illustrated example, the source material 14 initially presents in the form of a droplet 108 in a stream of droplets released by a source delivery mechanism 92 Figure 1 ). If the source material has been subjected to one or more pulses, it can no longer be in the form of a droplet. For the sake of clarity, the source material is referred to herein as a droplet before it is subjected to any pulses, and the source material is referred to herein as a target after it is subjected to any pulses. According to an aspect of an embodiment, the pulse 106 is a first pulse that preconditions the source material, e.g., by changing the geometric distribution of the source material, e.g., from the droplet 108 to a conditioned target form 110, such as a disk, a cloud, etc. This conditioned target form 110 is then impinged upon by a second pulse 112 that further conditions the source material for phase conversion by a subsequent pulse.

[0034] Further, the term "pre-pulse" is sometimes used to describe a pulse whose primary purpose is to condition the target material, and the term "main pulse" is sometimes used to describe the final pulse whose primary purpose is to generate a plasma from the source material. However, in certain applications, the use of the pulses can not be so separate and distinct.

[0035] The two pulses can be generated by separate lasers or they can be generated by a single laser. For example,Figure 3 The system of the system can include a beam splitter 118 for splitting a portion of the energy of the light beam 106 into a light beam 106', so that the energy of the light beam 106 can be measured by a pre-pulse energy control system 120, which uses the measurement to optimize the pre-pulse energy. Figure 3 An RF control module 125 for controlling the dose is also shown in the.

[0036] In the shown system, the energy of the pulse 106 is typically measured "on-droplet", i.e. during the exposure period when the pulse hits the droplet, and the measurement is used as part of a closed loop control system for controlling the pre-pulse energy. The energy is measured by a pre-pulse energy control system 120, which provides a signal indicative of the measured energy to the controller 65. The controller 65 in turn uses the measured energy to control the energy of the pulse. When the controller is used in this way to control the pre-pulse energy at the exposure period, the dose stability, i.e. the pulse-to-pulse stability of the energy provided to the droplet, exhibits a tendency to worsen. The thermal (cold-to-hot) transient at start-up reduces the adjustable range (headroom) of the energy dose provided to the droplet in a pulsed fashion. The dose stability also suffers from low frequency disturbances. These effects can be at least partially due to the interaction between the pre-pulse energy control system 120, which uses the driving laser energy to actuate two mechanisms for pulse energy control, i.e. (1) the peak and integrated energy of the main pulse, and (2) the target size (controlled by controlling the PP energy), and the dose controller, RF control module 125. This type of PP energy control, i.e. using on-droplet PP energy measurement, reduces the available range of the target size that can be used for dose control.

[0037] Therefore, the optimization of the pre-pulse energy using the on-droplet pulse energy measurement undesirably interacts with other controls, especially with the dose control, which is influenced by the RF control module. However, if an off-droplet measurement of the pre-pulse energy is used for the pre-pulse energy optimization, these undesired interactions can be avoided. Therefore, the optimization of the pre-pulse energy takes place during the off-droplet pulse. The pre-pulse energy can then be controlled open loop during the on-droplet period. This effectively decouples the EUV dose control loop from the PP energy control loop.

[0038] Figure 4 is a flow chart showing a method of controlling the pre-pulse energy based on a measurement made during a non-exposure interval, i.e. an off-droplet interval when the pre-pulse is not used to irradiate any droplet. In step S100, which takes place during the off-droplet (non-exposure) interval, the pre-pulse energy is measured using a closed loop control method and controlled (driven) to a pre-pulse energy setpoint. In step S110, a droplet is generated. During step S120, the droplet is irradiated using a pulse having an energy at the pre-pulse energy setpoint.

[0039] The present disclosure is made with the help of functional building blocks that account for the implementation of specified functions and their relationships. For ease of description, the boundaries of these functional building blocks have been arbitrarily defined herein. Alternative boundaries can be defined as long as the specified functions and their relationships are properly performed. For example, the control module function can be divided among several systems or performed at least in part by the entire control system.

[0040] The above description includes examples of one or more embodiments. Of course, for the description of the above embodiments, it is not possible to describe every possible combination of components or methods, but one of ordinary skill in the art can recognize that many additional combinations and permutations of the various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims. Additionally, where the term "includes" is used in the detailed description or claims, the term is intended to be equivalent to the term "comprising" as explained in the specification, when used as an opening transitional phrase in a claim. Furthermore, although elements of the described aspects and / or embodiments can be described or claimed in singular form, plural forms can be intended, unless expressly otherwise stated. Also, all or a portion of any aspect and / or embodiment can be used with all or a portion of any other aspect and / or embodiment, unless otherwise stated.

[0041] Other aspects of the application are set out in the numbered clauses below.

[0042] 1. An apparatus for generating extreme ultraviolet radiation using a source material, the apparatus configured to operate in an exposure mode in which the source material is pulsed irradiated and a non-exposure mode in which the source material is not irradiated, the apparatus comprising:

[0043] a radiation source configured to generate at least one droplet-outer pulse during the non-exposure mode and a plurality of droplet-on pulses during the exposure mode; and

[0044] an energy controller configured to perform an energy measurement of the at least one droplet-outer pulse during the non-exposure mode and drive the pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one droplet-outer pulse, the radiation source further configured to generate the droplet-on pulses during the exposure mode with an energy at the pulse energy setpoint.

[0045] 2. The apparatus of clause 1, wherein the pulse is a pre-pulse and the energy controller is a pre-pulse energy controller.

[0046] 3. The apparatus of clause 2, wherein the pre-pulse energy controller is configured to perform energy measurements of a plurality of droplet-outer pre-pulses.

[0047] 4. The apparatus of clause 1, 2, or 3, further comprising a dose controller configured to control a size of an energy dose delivered to the source material.

[0048] 5. The apparatus of clause 3, wherein a control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

[0049] 6. A method of controlling an apparatus for generating extreme ultraviolet radiation using a source material, the apparatus having an exposure period in which the source material is pulsed irradiated and a non-exposure period in which the source material is not irradiated, the method comprising:

[0050] generating a droplet-outer pulse during the non-exposure period;

[0051] controlling a size of energy of the droplet-outer pulse to be at a pulse energy setpoint; and

[0052] generating a droplet-on pulse during the exposure period while controlling an energy of the droplet-on pulse to be at the pulse energy setpoint.

[0053] 7. The method of clause 6, wherein the pulse is a pre-pulse and the energy controller is a pre-pulse energy controller.

[0054] 8. The method of clause 7, wherein generating the droplet-on pre-pulse during the exposure period while controlling an energy of the droplet-on pre-pulse comprises performing open loop control of pre-pulse energy of the droplet-on pre-pulse.

[0055] 9. The method of clause 7, further comprising measuring a size of energy of the at least one droplet-outer pre-pulse, and wherein controlling a size of energy comprises calibrating a pre-pulse energy controller based at least in part on the measured size of energy of the at least one droplet-outer pre-pulse.

[0056] 10. The method of clause 9, further comprising controlling a size of an energy dose delivered to the source material.

[0057] 11. An apparatus for generating extreme ultraviolet radiation, comprising:

[0058] a droplet source of source material;

[0059] A laser source of pulsed laser radiation, the laser source configured to operate in an exposure mode in which the source material is irradiated by laser pulses and a non-exposure mode in which the source material is not irradiated, the laser source configured to generate at least one off-droplet pulse during the non-exposure mode and a plurality of on-droplet pulses during the exposure mode; and

[0060] an energy controller configured to perform an energy measurement of the at least one off-droplet pulse during the non-exposure mode and to drive the pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one off-droplet pulse; the radiation source further configured to generate the on-droplet pulses with energy at the pulse energy setpoint during the exposure mode.

[0061] 12. The apparatus of clause 11, wherein the pulse is a pre-pulse and the energy controller is a pre-pulse energy controller.

[0062] 13. The apparatus of clause 12, wherein the pre-pulse energy controller is configured to perform energy measurements of a plurality of off-droplet pre-pulses.

[0063] 14. The apparatus of clause 11, 12, or 13, further comprising a dose controller configured to control a size of an energy dose delivered to the source material.

[0064] 15. The apparatus of clause 13, wherein a control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

[0065] 16. A method of generating extreme ultraviolet radiation using a source material, the method comprising:

[0066] generating a droplet of source material;

[0067] generating an off-droplet laser pulse that does not impact any droplet of source material;

[0068] controlling a size of energy of the off-droplet laser pulse to be at a pulse energy setpoint; and

[0069] generating an on-droplet pulse that impacts the droplet of source material while controlling energy of the on-droplet pulse to be at the pulse energy setpoint.

[0070] 17. The method of clause 16, wherein the pulse is a pre-pulse and the energy controller is a pre-pulse energy controller.

[0071] 18. The method of clause 17, wherein the generating the on-droplet pre-pulse comprises performing open loop control of a pre-pulse energy of the on-droplet pre-pulse.

[0072] 19. The method of clause 17, further comprising measuring an energy magnitude of the at least one off-droplet pre-pulse, and wherein controlling the energy magnitude comprises calibrating a pre-pulse energy controller based at least in part on the measured energy magnitude of the at least one off-droplet pre-pulse.

[0073] 20. The method of clause 19, further comprising controlling a magnitude of an energy dose delivered to the source material.

Claims

1. An apparatus for generating extreme ultraviolet radiation using a source material, the apparatus configured to operate in an exposure mode in which the source material is pulsed irradiated and a non-exposure mode in which the source material is not irradiated, the apparatus comprising: a radiation source configured to generate at least one off-droplet pulse during the non-exposure mode and a plurality of on-droplet pulses during the exposure mode; and an energy controller configured to perform an energy measurement of the at least one off-droplet pulse during the non-exposure mode and to drive a pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one off-droplet pulse, the radiation source further configured to generate the on-droplet pulses during the exposure mode with an energy at the pulse energy setpoint.

2. The apparatus of claim 1, wherein the at least one off-droplet pulse and the plurality of on-droplet pulses are pre-pulses and the energy controller is a pre-pulse energy controller.

3. The apparatus of claim 2, wherein the pre-pulse energy controller is configured to perform an energy measurement of a plurality of off-droplet pre-pulses.

4. The apparatus of claim 2, further comprising a dose controller configured to control a size of an energy dose delivered to the source material.

5. The apparatus of claim 4, wherein a control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

6. A method of controlling an apparatus for generating extreme ultraviolet radiation using a source material, the apparatus having an exposure period in which the source material is pulsed irradiated and a non-exposure period in which the source material is not irradiated, the method comprising: generating an off-droplet pulse during the non-exposure period; controlling a size of an energy of the off-droplet pulse to be at a pulse energy setpoint; and generating an on-droplet pulse during the exposure period while controlling an energy of the on-droplet pulse to be at the pulse energy setpoint.

7. The method of claim 6, wherein the off-droplet pulse and the on-droplet pulse are pre-pulses. performing open loop control of a pre-pulse energy of the on-droplet pre-pulse. calibrating a pre-pulse energy controller based at least in part on the measured size of the energy of the at least one off-droplet pre-pulse.

8. The method of claim 7, wherein generating an on-droplet pre-pulse during the exposure period while controlling an energy of the on-droplet pre-pulse comprises:

10. The method of claim 9, further comprising controlling a size of an energy dose delivered to the source material.

9. The method of claim 7, further comprising measuring an energy level of at least one pre-pulse outside the droplet, and wherein controlling the energy level comprises:

11. An apparatus for generating extreme ultraviolet radiation, comprising: a droplet source of a source material; a pulsed laser radiation source configured to operate in an exposure mode in which the source material is pulsed irradiated by laser radiation and a non-exposure mode in which the source material is not irradiated, the laser source configured to generate at least one off-droplet pulse during the non-exposure mode and a plurality of on-droplet pulses during the exposure mode; and a pre-pulse energy controller configured to perform an energy measurement of a plurality of off-droplet pre-pulses. ​ ​ an energy controller configured to perform an energy measurement of the at least one off-droplet pulse during the non-exposure mode and to drive a pulse energy to a pulse energy setpoint based at least in part on the energy measurement of the at least one off-droplet pulse; the laser source is further configured to generate the on-droplet pulses having an energy at the pulse energy setpoint during the exposure mode.

12. The apparatus of claim 11, wherein the at least one off-droplet pulse and the plurality of on-droplet pulses are pre-pulses and the energy controller is a pre-pulse energy controller.

13. The apparatus of claim 12, wherein the pre-pulse energy controller is configured to perform an energy measurement of a plurality of off-droplet pre-pulses.

14. The apparatus of claim 13, further comprising a dose controller configured to control a size of an energy dose delivered to the source material.

15. The apparatus of claim 14, wherein a control loop of the dose controller is decoupled from a control loop of the pre-pulse energy controller.

16. A method of generating extreme ultraviolet radiation using a source material, the method comprising: generating a droplet of source material; generating an off-droplet laser pulse that does not impinge on any droplet of source material; controlling a size of energy of the off-droplet laser pulse to be at a pulse energy setpoint; and generating an on-droplet pulse that impinges on the droplet of source material while controlling energy of the on-droplet pulse to be at the pulse energy setpoint.

17. The method of claim 16, wherein the off-droplet laser pulse and the on-droplet pulse are pre-pulses.

18. The method of claim 17, wherein generating an on-droplet pre-pulse comprises performing open loop control of a pre-pulse energy of the on-droplet pre-pulse.

19. The method of claim 17, further comprising measuring a size of energy of at least one off-droplet laser pre-pulse, and wherein controlling a size of energy comprises calibrating a pre-pulse energy controller based at least in part on the measured size of energy of the at least one off-droplet laser pre-pulse.

20. The method of claim 19, further comprising controlling a size of an energy dose delivered to the source material. ​

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