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Title:
MONITORING LIGHT EMISSIONS
Document Type and Number:
WIPO Patent Application WO/2020/086901
Kind Code:
A1
Abstract:
Provided is a system that includes a vacuum chamber with an interior region that is configured to receive a target and a light beam. The target material emits extreme ultraviolet (EUV) light when in a plasma state. The system also includes a detection system configured to image the interior region by detecting light emission from atoms, ions, or molecules in the interior region and producing a representation of a spatial distribution of the light emission in the interior region. A control system is coupled to the detection system. The control system is configured to analyze the representation of the spatial distribution to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region, and determine whether to adjust a property of the light beam and/or a property of the vacuum chamber based on the spatial distribution of the light emission.

Inventors:
BROWN DANIEL JOHN WILLIAM (US)
RAFAC ROBERT JAY (US)
TAO YEZHENG (US)
FOMENKOV IGOR VLADIMIROVICH (US)
STEWART JOHN (US)
Application Number:
PCT/US2019/057944
Publication Date:
April 30, 2020
Filing Date:
October 24, 2019
Export Citation:
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Assignee:
ASML NETHERLANDS BV (NL)
International Classes:
H05G2/00
Foreign References:
US20140264091A12014-09-18
US20170280545A12017-09-28
Attorney, Agent or Firm:
MARCELLI, Mark (US)
Download PDF:
Claims:
WHAT IS CLAIMED IS:

1. A system comprising:

a vacuum chamber comprising an interior region, wherein the interior region is configured to receive a target and a light beam, the target comprises target material, and the target material emits extreme ultraviolet (EUV) light when in a plasma state;

a detection system configured to image the interior region, the detection system configured to detect light emission from atoms, ions, or molecules in the interior region and to produce a representation of a spatial distribution of the light emission in the interior region; and

a control system coupled to the detection system, the control system configured to: analyze the representation of the spatial distribution of the light emission to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region; and

determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the spatial distribution of the light emission.

2. The system of claim 1, wherein the light emission comprises fluorescence.

3. The system of claim 2, wherein the fluorescence comprises laser-induced

fluorescence.

4. The system of claim 1, further comprising one or more spectral filters configured to be positioned relative to the detection system, the spectral filters being configured to only allow some wavelengths to reach the detection system.

5. The system of claim 4, wherein each of the one or more spectral filters is configured to transmit light having a wavelength in one of a plurality of emission lines of the target material.

6. The system of claim 5, wherein at least one of the one or more spectral filters is configured to transmit a wavelength in a visible light range.

7. The system of claim 4, wherein the vacuum chamber is further configured to contain a gas in the interior region, and the spectral filter is configured to transmit light having a wavelength at an emission line of the gas.

8. The system of claim 1, wherein the control system is configured to receive a plurality of representations of the interior region, each of the plurality of representations is associated with a different time, and the control system being configured to analyze the representation of the interior region comprises the control system being configured to analyze each of the plurality of representations to determine the spatial distribution of the light emission in the interior region at each of the different times.

9. The system of claim 8, wherein the light emission in the interior region results from an energy event in the interior region, and the different times are all times that occur after the energy event.

10. The system of claim 9, wherein the energy event comprises an interaction between the light beam and the target, and the light emission is an emission from: (a) the target material, (b) a plasma formed from the interaction between the light beam and the target material, and/or (c) debris formed from the interaction of the light beam and the target.

11. The system of claim 1, wherein the control system is configured to receive an extended exposure representation of the interior region, the extended exposure representation of the interior region comprising an average of the spatial distribution of the emission in the interior region over a temporal period.

12. The system of claim 9, wherein the vacuum chamber is further configured to contain a gas in the interior region, the energy event comprises an interaction that adds energy to the gas, and the light emission is an emission from the gas.

13. The system of claim 12, wherein the interaction that adds energy to the gas comprises (a) an interaction between the light beam and the gas, (b) an interaction between the gas and a plasma formed from an interaction between the light beam and the target, and/or (c) an interaction between ions and the gas.

14. The system of claim 1, wherein the control system being configured to analyze the representation to determine a spatial distribution of the light emission in the interior region comprises the control system being configured to estimate a shape and/or a spatial distribution of intensity of the light emission.

15. The system of claim 1, further comprising:

a first spectral filter configured to transmit light having a wavelength in a first wavelength band; and

a second spectral filter configured to transmit light having a wavelength in a second wavelength band, and wherein

the control system being configured to analyze the representation comprises: the control system being configured to estimate an amount of light emission in the first wavelength band and to estimate an amount of light emission in the second wavelength band, and the control system is further configured to estimate an ionization fraction of the target material based on comparing the estimated amount of light emission at the first wavelength band and the estimated amount of light emission at the second wavelength band.

16. The system of claim 15, wherein the control system determines whether to adjust at least one property of the light beam based on the estimated ionization fraction.

17. The system of claim 16, wherein the control system determines whether to adjust a pointing direction of the light beam based on the estimated ionization fraction.

18. The system of claim 1, wherein the light beam comprises a main pulse light beam comprising an energy sufficient to convert at least some of the target material to a plasma that emits EUV light.

19. The system of claim 1, wherein the light beam comprises a pre-pulse light beam.

20. The system of claim 4, wherein the control system being configured to analyze the representation further comprises the control system being configured to compare the spatial distributions of fluorescence in the interior region at least at two the different times to estimate a velocity of ions in the interior region, and to compare the estimated velocity to a velocity specification, and the control system is configured to determine whether to adjust a pressure of the gas based on the comparison of the estimated velocity to the velocity specification.

21. The system of claim 1, wherein the representation of the spatial distribution comprises a representation of a two-dimensional representation.

22. The system of claim 1, wherein the light beam comprises a pulsed light beam, and the control system being configured to adjust at least one property of the light beam comprises the control system being configured to adjust at least one property of a later-occurring pulse of the pulsed light beam.

23. An EUV light source comprising:

a vacuum chamber configured to: contain a gas in an interior region and to receive a target and a light beam, the target comprising target material that emits extreme ultraviolet (EUV) light in a plasma state;

a monitor comprising at least one sensor, the at least one sensor being configured to detect emissions from the gas in the interior region and to produce an indication of the detected emissions; and

a control system coupled to the monitor, the control system configured to:

analyze the indication of detected emissions; and

determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the analysis.

24. The EUV light source of claim 23, wherein the monitor comprises a detection system configured to image a portion of the interior region and to produce a representation of a spatial distribution of detected emissions in the portion.

25. The EUV light source of claim 24, wherein the control system is configured to receive a plurality of representations over a period of time, each representation indicating a spatial distribution of detected emissions in the portion at a different time in the period of time, and the control system is configured to determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on two or more of the plurality of representations.

26. The EUV light source of claim 23, wherein the gas comprises hydrogen, and the detected emission comprises an H-alpha (H-a) and/or an H-beta (H-b) emission from the hydrogen.

27. The EUV light source of claim 24, further comprising:

a first spectral filter configured to transmit a first band of wavelengths; and a second spectral filter configured to transmit a second band of wavelengths, wherein, in operational use, the first spectral filter and the second spectral filter are between the portion and the detection system; and

the control system being configured to analyze the detected emissions comprises the control system being configured to compare a representation of emissions that are transmitted by the first spectral filter to a representation of emissions that are transmitted by the second spectral filter; and the determination of whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber is based on the comparison.

28. The EUV light source of claim 24, further comprising a pressure controller coupled to interior of the vacuum chamber, the pressure controller being configured to change a pressure of the gas in the interior of the vacuum chamber, and wherein the control system is coupled to the pressure controller.

29. A method of controlling an EUV light source, the method comprising:

providing a target to a target region in a vacuum chamber, the vacuum chamber containing a gas in an interior region;

causing an interaction between a light beam and the target in the target region;

detecting light emission from atoms, ions, and/or molecules in the interior region of the vacuum chamber, the light emission being a response to an energy event in the vacuum chamber, the energy event comprising an event that adds energy to the target and/or the gas; analyzing the detected light emission to determine a spatial distribution of light emission in the interior region; and

determining whether to adjust a property of the light beam and/or the gas based on the analysis.

Description:
MONITORING LIGHT EMISSIONS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Application No. 62/751,267 which was filed on October 26, 2018 and titled MONITORING LIGHT EMISSIONS, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] This disclosure relates to monitoring light emissions. The light emissions may be emissions of light that occur in a vacuum chamber of an extreme ultraviolet (EUV) light source.

BACKGROUND

[0003] Extreme ultraviolet (“EUV”) light, for example, electromagnetic radiation having wavelengths of 100 nanometers (nm) or less (also sometimes referred to as soft x-rays), and including light at a wavelength of, for example, 20 nm or less, between 5 and 20 nm, or between 13 and 14 nm, may be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers, by initiating polymerization in a resist layer.

[0004] Methods to produce EUV light include, but are not necessarily limited to, converting a material that includes an element, for example, xenon, lithium, or tin, with an emission line in the EUV range in a plasma state. In one such method, often termed laser produced plasma (“LPP”), the required plasma may be produced by irradiating a target material, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that may be referred to as a drive laser. For this process, the plasma is typically produced in a sealed vessel, for example, a vacuum chamber, and monitored using various types of metrology equipment.

SUMMARY

[0005] In one general aspect, a system includes a vacuum chamber including an interior region, the interior region is configured to receive a target and a light beam, the target includes target material that emits extreme ultraviolet (EUV) light when in a plasma state; a detection system configured to image the interior region, the detection system configured to detect light emission from atoms, ions, or molecules in the interior region and to produce a representation of a spatial distribution of the light emission in the interior region; and a control system coupled to the detection system, the control system configured to: analyze the representation of the spatial distribution of the light emission to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region; and determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the spatial distribution of the light emission.

[0006] Implementations may include one or more of the following features. The light emission may include fluorescence. The fluorescence may include laser-induced

fluorescence. The control system being configured to analyze the representation also may include the control system being configured to compare the spatial distributions of fluorescence in the interior region at least at two the different times to estimate a velocity of ions in the interior region, and to compare the estimated velocity to a velocity specification, and the control system may be configured to determine whether to adjust a pressure of the gas based on the comparison of the estimated velocity to the velocity specification.

[0007] The system also may include one or more spectral filters configured to be positioned relative to the detection system, the spectral filters being configured to only allow some wavelengths to reach the detection system. Each of the one or more spectral filters may be configured to transmit light having a wavelength in one of a plurality of emission lines of the target material. In some implementations, at least one of the one or more spectral filters is configured to transmit a wavelength in a visible light range. The vacuum chamber may be further configured to contain a gas in the interior region, and the spectral filter may be configured to transmit light having a wavelength at an emission line of the gas.

[0008] The control system may be configured to receive a plurality of representations of the interior region, each of the plurality of representations may be associated with a different time, and the control system being configured to analyze the representation of the interior region may include the control system being configured to analyze each of the plurality of representations to determine the spatial distribution of the light emission in the interior region at each of the different times. The light emission in the interior region may result from an energy event in the interior region, and the different times are all times that occur after the energy event. The energy event may include an interaction between the light beam and the target, and the light emission may be an emission from: (a) the target material, (b) a plasma formed from the interaction between the light beam and the target material, and/or (c) debris formed from the interaction of the light beam and the target. [0009] The control system may be configured to receive an extended exposure representation of the interior region, the extended exposure representation of the interior region including an average of the spatial distribution of the emission in the interior region over a temporal period. The vacuum chamber may be further configured to contain a gas in the interior region, the energy event may be an interaction that adds energy to the gas, and the light emission may be an emission from the gas. The interaction that adds energy to the gas may include (a) an interaction between the light beam and the gas, (b) an interaction between the gas and a plasma formed from an interaction between the light beam and the target, and/or (c) an interaction between ions and the gas.

[0010] The control system being configured to analyze the representation to determine a spatial distribution of the light emission in the interior region may include the control system being configured to estimate a shape and/or a spatial distribution of intensity of the light emission.

[0011] In some implementations, the system also includes a first spectral filter configured to transmit light having a wavelength in a first wavelength band; and a second spectral filter configured to transmit light having a wavelength in a second wavelength band, and the control system being configured to analyze the representation may include: the control system being configured to estimate an amount of light emission in the first wavelength band and to estimate an amount of light emission in the second wavelength band, and the control system may be further configured to estimate an ionization fraction of the target material based on comparing the estimated amount of light emission at the first wavelength band and the estimated amount of light emission at the second wavelength band. The control system may determine whether to adjust at least one property of the light beam based on the estimated ionization fraction. The control system may determine whether to adjust a pointing direction of the light beam based on the estimated ionization fraction.

[0012] The light beam may include a main pulse light beam having an energy sufficient to convert at least some of the target material to a plasma that emits EUV light.

[0013] The light beam may include a pre-pulse light beam.

[0014] The representation of the spatial distribution may include a representation of a two- dimensional representation.

[0015] The light beam may include a pulsed light beam, and the control system being configured to adjust at least one property of the light beam may include the control system being configured to adjust at least one property of a later-occurring pulse of the pulsed light beam. [0016] In another general aspect, an EUV light source includes a vacuum chamber configured to: contain a gas in an interior region and to receive a target and a light beam, the target including target material that emits extreme ultraviolet (EUV) light in a plasma state; a monitor including at least one sensor, the at least one sensor being configured to detect emissions from the gas in the interior region and to produce an indication of the detected emissions; and a control system coupled to the monitor, the control system configured to: analyze the indication of detected emissions; and determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the analysis.

[0017] Implementations may include one or more of the following features. The monitor may include a detection system configured to image a portion of the interior region and to produce a representation of a spatial distribution of detected emissions in the portion. The control system may be configured to receive a plurality of representations over a period of time, each representation indicating a spatial distribution of detected emissions in the portion at a different time in the period of time, and the control system may be configured to determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on two or more of the plurality of representations.

[0018] The gas may include hydrogen, and the detected emission may include an H-alpha (H- a) and/or an H-beta (H-b) emission from the hydrogen.

[0019] The EUV light source also may include a first spectral filter configured to transmit a first band of wavelengths; and a second spectral filter configured to transmit a second band of wavelengths, where, in operational use, the first spectral filter and the second spectral filter may be between the portion and the detection system; and the control system being configured to analyze the detected emissions may include the control system being configured to compare a representation of emissions that are transmitted by the first spectral filter to a representation of emissions that are transmitted by the second spectral filter; and the determination of whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber may be based on the comparison.

[0020] The EUV light source also may include a pressure controller coupled to interior of the vacuum chamber, the pressure controller being configured to change a pressure of the gas in the interior of the vacuum chamber, and the control system may be coupled to the pressure controller.

[0021] In another general aspect, a method of controlling an EUV light source includes providing a target to a target region in a vacuum chamber, the vacuum chamber containing a gas in an interior region; causing an interaction between a light beam and the target in the target region; detecting light emission from atoms, ions, and/or molecules in the interior region of the vacuum chamber, the light emission being a response to an energy event in the vacuum chamber, the energy event including an event that adds energy to the target and/or the gas; analyzing the detected light emission to determine a spatial distribution of light emission in the interior region;

and determining whether to adjust a property of the light beam and/or the gas based on the analysis.

[0022] Implementations of any of the techniques described above may include an EUV light source, a system, a method, a process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below.

Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not necessarily to scale. On the contrary, the dimensions of the various features may be arbitrarily expanded or reduced for clarity.

Like numerals denote like features throughout the specification and drawings.

[0024] FIG. 1A is a block diagram of an example of an extreme ultraviolet (EUV) light source.

[0025] FIG. 1B is an example of a temporal profile of a optical pulse.

[0026] FIG. 2 is a block diagram of another example of an extreme ultraviolet (EUV) light source.

[0027] FIG. 3 is a flow chart of an example of a process for controlling an EUV light source.

[0028] FIGS. 4A-4D are example images of laser-induced fluorescence.

[0029] FIG. 5 is example data related to analyzing light emissions from a gas.

[0030] FIGS. 6A and 6B are block diagrams of an example of a lithographic apparatus.

[0031] FIG. 7 is a block diagram of an example of an EUV light source.

DETAILED DESCRIPTION

[0032] Techniques for controlling an extreme ultraviolet (EUV) lithography system and/or an EUV source based on analysis of emissions of light that occur within a vacuum chamber of the EUV source are disclosed. [0033] Referring to FIG. 1A, a block diagram of an extreme ultraviolet (EUV) light source 100 is shown. The EUV light source 100 includes a sensor system 130 and a control system 150. The sensor system 130 monitors emissions of light that occur inside a vacuum chamber 109 and provides information about the emissions to the control system 150. The emissions are analyzed by the control system 150, which is configured to make adjustments to one or more components of the EUV light source 100 based on the analysis of the emissions. The emissions may be emissions from a plasma 196, a gas 122, target material in a target 121r, and/or debris 195. Monitoring emissions in the vacuum chamber 109 allows determination and control of a variety of parameters of the EUV source 100 that affect the performance of the EUV source. For example, information from the sensor system 130 may be used to determine a portion or fraction of a target material (or fuel) that is ionized by a plasma- generation event and/or to determine an amount of energy deposited into a gas 122 that is in the vacuum chamber 109. Knowledge of such parameters allows the control system 150 to improve the performance of the EUV light source 100.

[0034] The monitored emissions are light emitted from one or more substances in the vacuum chamber 109. The substances include or are atoms, molecules, and/or ions. The emissions may be any kind of emission that involves light emerging from the substance. For example, the emissions may be optical emissions that occur as a result of atoms being excited by a high-temperature source. In another example, the emissions may be fluorescence from an atom, molecule, or ion. Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.

[0035] Moreover, the emissions may be laser-induced fluorescence. Laser-induced fluorescence is a process by which an atom, ion, or molecule absorbs laser light and an electron of the substance is excited to a higher energy level. After the excitation, the electron decays to a lower energy level and the atom, ion, or molecule emits light. This emitted light is laser-induced fluorescence. The laser-induced fluorescence may be generated by irradiating the substance with an optical beam 106 (which may be a laser) and/or by irradiating the substance with a laser beam 115 generated by a probe laser 108. Any laser that is suitable to excite the substances in the manner of interest may be used as the probe laser 108. For example, the probe laser 108 may be a laser (such as an optical parametric oscillator or other type of tunable laser) that is capable of being tuned to produce one of several different wavelengths.

[0036] The wavelength of a particular emission is determined by the properties of the substance and the amount of energy that is used to excite the substance. Moreover, a particular substance may produce emissions of more than one wavelength. For example, hydrogen gas emits light at a wavelength of 658.28 nanometers (nm) when a hydrogen electron transitions from the third lowest energy level to the second lowest energy level. This emission is called H-alpha (H-a) emission. However, hydrogen gas also emits light of other wavelengths. For example, hydrogen gas emits light at a wavelength of 486.14 nm when a hydrogen electron transitions from the fourth lowest energy level to the second lowest energy level. This emission is called H-beta (H-b) emission. Hydrogen gas also has other emission lines. The wavelength emitted by the hydrogen gas depends on the amount of excitation energy, which determines the energy level to which an electron is excited from the ground state. Similarly, other substances that may be present in the vacuum chamber 109 emit light of particular wavelengths depending on their respective physical properties and the energy with which the substance is excited.

[0037] By analyzing these emissions, the control system 150 is able to monitor conditions within the vacuum chamber 109 and adjust the environment in the vacuum chamber 109 accordingly. In particular, the control system 150 is configured to analyze and adjust one or more properties of a subsequent (later-occurring) pulse of an optical beam 106 and/or one or more properties of the vacuum chamber 109 based on the monitored emissions. The properties of a subsequent pulse of the optical beam 106 that may be adjusted include, for example, a size (for example, beam waist at a plasma formation location 123), an average and/or maximum energy, a temporal duration, and/or a position relative to the plasma formation location 123. The properties of the vacuum chamber 109 that may be adjusted include, for example, a pressure of a gas 122, a temperature of the gas 122, a flow rate of the gas 122, a flow direction of the gas 122, the size of the target 121r, and/or a spacing of targets in the stream of targets 121.

[0038] Various components of the EUV source 100 are discussed prior to discussing the control system 150 in more detail.

[0039] The EUV source 100 also includes a target supply system 110 that emits a stream 121 of targets. The target supply system 110 includes a target formation apparatus 117, which defines an orifice 119 that is fluidly coupled to a reservoir 118. In operational use, the target material is in a flowable state (for example, the target material is molten and at a temperature that is above its melting point) and the reservoir 118 is pressurized to a pressure P. The pressure P is greater than the pressure in the vacuum chamber 109. Thus, in operational use, target material flows through the orifice 119 and into the vacuum chamber 109 to form the stream of targets 121. In the example of FIG. 1A, the stream of targets 121 travels from the orifice 119 to the plasma formation location 123 generally in the x direction, with the target 121r (which is one of the targets in the stream 121) being at the plasma formation location 123 at the time depicted in FIG. 1A.

[0040] The targets in the stream of targets 121 may be droplets of target material. The target material may be any material that emits EUV light when in a plasma state. For example, the target material may include water, tin, lithium, and/or xenon. The target material may be a target mixture that includes a target substance and impurities such as non- target particles.

The target substance is the substance that, when in a plasma state, has an emission line in the EUV range. The target substance may be, for example, a droplet of liquid or molten metal, a portion of a liquid stream, solid particles or clusters, solid particles contained within liquid droplets, a foam of target material, or solid particles contained within a portion of a liquid stream. The target substance can be, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the target substance can be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr 4 , SnBn, SnH 4 ; as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. Moreover, in the situation in which there are no impurities, the target material includes only the target substance.

[0041] During operation of the EUV source 100, the plasma 196 is formed by interacting the optical beam 106 with the target 121r at the plasma formation location 123. Plasma includes fine or small particles collectively called plasma particles. The plasma particles may be, for example, vaporized, atomized, and/or ionized particles of the fuel, and the monitored emissions may include emissions from any of these substances. The interaction of a light beam with target material, where the light beam has an energy sufficient to convert at least some of the target material to a plasma, is referred to as a plasma-generating event. Each plasma-generating event also generally produces debris (for example, fragments or pieces of target material that are not converted to the plasma 196) and the monitored emissions may include emissions from the debris. Thus, during operation of the EUV source 100, the plasma 196 and the debris 195 are present in the chamber 109 after a plasma-generating event.

[0042] The EUV source 100 also includes a light-generation module 105, which generates the optical beam 106. The light-generation module 105 may be, for example, a carbon dioxide (CO2) laser or a solid state laser. The light-generation module 105 may include various other components that are not shown in FIG. 1A, such as pre-amplifiers, power amplifiers, optical elements (such as mirrors) used to direct light, and beam combiners. In some implementations, the light-generation module 105 includes more than one optical source and may include more than one laser and may include different types of lasers. FIG. 2 shows an example of a light generation module 205 that includes more than one optical source.

[0043] The optical beam 106 may be a train of pulses each of which is separated from the nearest pulse in time. FIG. 1B shows an example of a temporal profile (optical power as a function of time) of a pulse 104 within the train. The pulse 104 is an example of one of the pulses that may be part of the optical beam 106. The pulse 104 has a peak power 103 and a finite temporal duration 102. In the example of FIG. 1B, the pulse duration 102 is the time during which the pulse 104 has a non-zero power. The time for the pulse 104 to increase from zero to the peak power 103 is the rise time of the pulse. In other implementations, the pulse duration 102 and/or the rise time may be based on other metrics. For example, the pulse duration 102 may be less than the time during which the pulse 104 has a non-zero power, such as the full-width at half maximum (FWHM) of the pulse 104. Similarly, the rise time may be measured between two values other than zero optical power and the peak optical power 103.

[0044] In the example shown, the power of the pulse 104 increases from zero power to the peak power 103 monotonically and decreases from the peak power 103 to zero

monotonically. Other temporal profiles are possible. For example, the power of a pulse may increase from zero to the peak power non-monotonically. A pulse may have more than one peak energy point. Moreover, the pulses in the train of pulses that make up the optical beam 106 may have different temporal profiles.

[0045] The optical beam 106 is directed to the vacuum chamber 109 on an optical path 107 by a beam delivery system 111 that includes one or more optical components 112. The optical components 112 may include any components that are able to interact with the optical beam 106. The components 112 also may include devices that are able to form and/or shape the pulse 104. For example, the optical components 112 may include passive optical devices such as mirrors, lenses, and/or prisms, and any associated mechanical mounting devices and/or electronic drivers. These components may steer and/or focus the optical beam 106. Additionally, the optical components 112 may include components that modify one or more properties of the optical beam 106. For example, the optical components 112 may include active optical devices, such as acousto-optic modulators and/or electro-optic modulators, capable of changing the temporal profile of the optical beam 106 to form the pulse 104. [0046] The pulse 104 leaves the beam delivery system 111 and enters the vacuum chamber 109. The pulse 104 passes through an aperture 113 of an optical element 114 to reach the plasma formation location 123. An interaction between the pulse 104 and the target material in the target 121r produces the plasma 196 that emits light 197. The light 197 includes light with wavelengths that correspond to the emission lines of the target material in the target 121r.

[0047] The light 197 includes EUV light 198 and out-of-band light. Out-of-band light is light at wavelengths not in the EUV light range. For example, the target material may include tin. In these implementations, the light 197 includes the EUV light 198 and also includes out-of-band light such as deep ultraviolet (DUV), visible, near infrared (NIR), mid wavelength infrared (MWIR), and/or long- wavelength infrared (LWIR) light. The EUV light 198 may include light having a wavelength of, for example, 5 nanometers (nm), 5nm-20nm, l0nm-l20nm, or less than 50nm. The DUV light may include light having wavelengths between about l20nm-300nm, the visible light can include light having wavelengths between about 390nm-750nm, the NIR light may include light having wavelengths between about 750nm-2500nm, the MWIR light may have a wavelength between about 3000nm-5000nm, and the LWIR light may have a wavelength between about 8000nm-l2000nm.

[0048] The optical element 114 has a reflective surface 116 that is positioned to receive at least some of the light 197. The reflective surface 116 has a coating that reflects the EUV light 198 but does not reflect out-of-band components of the light 197 or reflects only a nominal amount of the out-of-band components of the light 197. In this way, the reflective surface 116 directs only the EUV light 198 to the lithography apparatus 199.

[0049] The EUV source 100 also includes a gas management system 140 that supplies the gas 122 to the vacuum chamber 109. The gas 122 may be, for example, hydrogen or oxygen. The gas management system 140 may include pumps, valves, and other components used in the management of gas. The gas management system 140 is configured to control various properties of the gas 122 that is supplied to the vacuum chamber such as, for example, temperature, pressure, and/or flow rate. For example, the gas management system 140 may supply the gas 122 at a flow rate that is sufficient to move debris (such as the debris 195) in a controlled fashion and/or control the temperature and/or pressure of the gas 122 to influence aspects of plasma production.

[0050] The EUV light source 100 also includes the sensor system 130, which provides a signal 157 that includes data related to monitored emissions to the control system 150. As noted above, the monitored emissions may include emissions from the plasma 196, emissions from the gas 122, and/or emissions from the debris 195. The sensor system 130 includes a sensor module 134 that includes one or more sensors 135. The sensor 135 is any detector or sensor capable of detecting or sensing light having the wavelengths of emissions of interest. Thus, in the example of FIG. 1 A, the sensor 135 may be a sensor capable of detecting emissions from the plasma 196, a sensor that is capable of detecting one or more wavelengths that may be emitted from the gas 122, and/or a sensor that is capable of sensing wavelengths of light emitted from the debris 195.

[0051] In some implementations, the sensor 135 is capable of producing data that includes spatial information about the emissions. For example, the sensor 135 may be a two- dimensional array of sensors, with each sensor being configured to sense light emitted from a particular portion of the vacuum chamber 109. Each sensor is fixed and has a known location relative to the portion of the vacuum chamber 109 the sensor monitors, thus, the relative location of the sensed emissions may also be determined. In these implementations, the spatial information shows how the emissions are distributed in the vacuum chamber 109.

The data from the sensor 135 may be used to form a two-dimensional spatial representation (such as an image) of the vacuum chamber 109 (or a portion of the vacuum chamber 109), with the image showing the relative locations of the monitored emissions within the vacuum chamber 109.

[0052] Moreover, the sensor 135 may be capable of producing many two-dimensional spatial representations of the monitored emissions in the vacuum chamber 109 over a period of time. For example, the sensor 135 may be a video sensor that captures frames (images) that are collected at a frame rate determined by the video sensor. In these implementations, each frame is a representation of the emissions in the vacuum chamber 109 at a different time. In another example, the sensor is a camera with an exposure mechanism that allows the sensor to sense emissions over a finite period of time. In these implementations, the data produced by the sensor 135 represents the time-average of emissions in the vacuum chamber 109. The sensor module 134 may include more than one sensor. In these implementations, the sensors

135 are positioned at different locations relative to a particular region of the vacuum chamber 109 such that the data produced by the sensors 135 may be used to generate a three- dimensional spatial representation of the monitored emissions.

[0053] Furthermore, the sensor system 130 also may include a spectral filter module 137.

The spectral filter module 137 includes one or more spectral filters 136. The spectral filters

136 allow control of which specific wavelength or wavelengths are sensed by the sensors 135. In this way, particular emissions may be separated from the total emissions in the vacuum chamber such that only emissions of interest are monitored. When included in the sensor system 130, the spectral filters 136 are positioned on an optical path between the sensor 135 and a monitored portion of the interior of the vacuum chamber 109.

[0054] The spectral filter 136 is any filter that is capable of allowing only some wavelengths, or a particular wavelength, to reach the sensor 135 while substantially preventing any other wavelength from reaching the sensor 135. The spectral filter 136 may be, for example, a spectral filter that only allows visible light to reach the sensor 135 or a spectral filter that only allows particular wavelengths within the visible spectrum to reach the sensor 135. The spectral filter 136 may separate wavelengths based on transmission, reflection, and/or absorption. For example, the spectral filter 136 may be a multi-layer dielectric stack that transmits wavelengths within a band of wavelengths while reflecting or absorbing all other wavelengths. In another example, the spectral filter 136 may be a dichroic mirror or a grating that reflect different wavelengths in different directions.

[0055] The spectral filter module 137 may include more than one spectral filter 136. For example, in some implementations, the sensor module 134 includes more than one sensor 135, and the spectral filter module 137 includes a spectral filter 136 for each of the sensors.

[0056] The EUV light source 100 also includes the control system 150, which uses information from the sensor system 130 to analyze the emissions in the vacuum chamber 109. The control system 150 also provides command signals 159, which are generated based on information about the emissions in the vacuum chamber 109, to the light-generation module 105, the target supply system 110, the gas management system 140, and/or the beam delivery system 111.

[0057] The control system 150 includes an analysis module 152. The analysis module 152 analyzes the information from the sensor system 130 and determines whether to make an adjustment to the optical beam 106 and/or the vacuum chamber 109 based on the analysis. The operation of the control system 150 and the analysis module 152 is discussed further with respect to FIG. 3. In the example of FIG. 1A, the analysis module 152 of the control system 150 is implemented using an electronic processor 154, an electronic storage 156, and an I/O interface 158. The electronic processor 154 includes one or more processors suitable for the execution of a computer program such as a general or special purpose microprocessor, and any one or more processors of any kind of digital computer. Generally, an electronic processor receives instructions and data from a read-only memory, a random access memory (RAM), or both. The electronic processor 154 may be any type of electronic processor. The electronic processor 154 executes the instructions that make up the analysis module 152. [0058] The electronic storage 156 may be volatile memory, such as RAM, or non-volatile memory. In some implementations, and the electronic storage 156 includes non-volatile and volatile portions or components. The electronic storage 156 may store data and information that is used in the operation of the control system 150. For example, the electronic storage 156 may store the instructions (for example, in the form of a computer program) that implement the analysis module 152. The analysis module 152 receives information from the sensor system 130 and also may receive information from the light-generation module 105, the gas management system 140, the supply system 110, and/or the beam delivery system 111.

[0059] The electronic storage 156 also may store instructions, perhaps as a computer program, that, when executed, cause the electronic processor 154 to communicate with components in the light-generation module 105, the gas management system 140, the beam delivery system 111, the supply system 110, and/or the sensor system 130. For example, the instructions may be instructions that cause the electronic processor 154 to provide the command signal 159 generated by the analysis module 152 to the light-generation module 105, the gas management system 140, the supply system 110, and/or the beam delivery system 111.

[0060] The command signal 159 is a signal that causes a component in the light- generation module 105 and/or the beam delivery system 111 to act in a manner that adjusts the optical beam 106 or a signal that causes the gas management system 140 to adjust a property of the gas 122. For example, the command signal 159 may be a signal that includes information sufficient to cause a valve and/or pump in the gas management system 140 to start operating, stop operating, or to continue operating but in a different manner. In another example, the command signal 159 is a signal that is capable of adjusting properties of the target supply system 110 that change the rate at which targets arrive at the plasma formation location 123. In this example, the command signal 159 may be a signal that includes information sufficient to cause the target formation apparatus 117 to vibrate at a different rate such that the size and/or rate of targets arriving at the plasma formation location 123 changes. In yet another example, the command signal 159 is a signal that operates on the light-generating module 105 and/or the beam delivery system 111 to change a property of the beam 106. For example, the command signal 159 may be a signal sufficient to cause a mirror in the beam delivery system 111 to move or a signal sufficient to adjust the operation of an electro-optic modulator in the beam delivery system 111. [0061] The I/O interface 158 is any kind of interface that allows the control system 150 to exchange data and signals with an operator, the light-generation module 105, one or more components of the light-generation module 105, the lithography apparatus 199, and/or an automated process running on another electronic device. For example, in some

implementations, the analysis module 152 may be programmed by an end-user to include analysis specific to the end-user. In these implementations, the analysis module may be programmed through the I/O interface 158. The I/O interface 158 may include one or more of a visual display, a keyboard, and a communications interface, such as a parallel port, a Universal Serial Bus (USB) connection, and/or any type of network interface, such as, for example, Ethernet. The I/O interface 158 also may allow communication without physical contact through, for example, an IEEE 802.11, Bluetooth, or a near- field communication (NFC) connection.

[0062] Referring to FIG. 2, a block diagram of an EUV light source 200 is shown. The EUV light source 200 is another example of an implementation of an EUV light source. The EUV light source 200 is the same as the EUV light source 100 (FIG. 1A), except the EUV light source 200 uses a light-generation module 205 that includes a first optical source 208_l, which emits a first optical beam 206_l, and a second optical source 208_2, which emits a second optical beam 206_2. A pulse 204_l is a pulse of the first optical beam 206_l, and a pulse 204_2 is a pulse of the second optical beam 206_2. The pulse 204_2 may be referred to as a“pre-pulse” optical beam, and the pulse 204_l may be referred to as a“main pulse” optical beam.

[0063] The EUV light source 200 includes the optical element 114, but for simplicity only the aperture 113 of the optical element 114 is shown in FIG. 2. The pulse 204_2 propagates along a beam path 207_2, passes through the aperture 113 of the optical element 114, and is delivered to an initial target region 224 via a beam delivery system 211_2. The initial target region 224 receives an initial target 22lp from the supply system 110. The initial target region 224 is displaced in the -x direction relative to the plasma formation location 123.

[0064] The pulse 204_2 interacts with the target 22 lp at the initial target region 224 to condition the target 22 lp and form the modified target 22 lm. The conditioning may enhance the ability of the target 22 lp to absorb the pulse 204_l. For example, although the EUV light-emitting plasma 196 is not generally produced at the initial target region 224, the interaction between the pulse 204_2 and the target 22lp may change the shape, volume, and/or size of the distribution of the target material in the initial target 22 lp and/or may reduce the density gradient of the target material along the direction of propagation of the main pulse 204_l . Moreover, the interaction between the pulse 204_2 and the initial target 22lp may produce a pre-plasma or a plasma that does not necessarily emit EUV light. The modified target 22 lm may be, for example, a disk-shaped distribution of target material that has a larger extent in the x-y plane than the target 22lp and a smaller extent along the z axis than the target 22lp. The modified target 22lm drifts to the plasma formation location 123 and is irradiated by the pulse 204_l to form the plasma 196.

[0065] In the implementation of FIG. 2, the control system 150 is coupled to the optical source 208_2 and the beam delivery system 211_2 such that the control system 150 may be used to control the properties of the second optical beam 206_2 (or a subsequent or later- occurring pulse of the optical beam 206_2). For example, the control system 150 may adjust the pulse energy of a later-occurring pulse of the optical beam 206_2, the location of a later- occurring pulse of the optical beam 206_2 relative to the expected location of the target 22 lp, and/or the duration of a later-occurring pulse of the optical beam 206_2. In this way, the control system 150 may be used to control the parameters of the conditioning of the initial target 22lp. The control system 150 is also coupled to the optical source 208_l and the beam delivery system 21 l_l and may be used to control the properties of the optical beam 206_l (or a pulse of the optical beam 206_l). Furthermore, the control system 150 is coupled to the gas management system 140 and is capable of adjusting one or more properties of the gas 122.

[0066] The optical sources 208_l and 208_2 may be, for example, two lasers. For example, the optical sources 208_l, 208_2 may be two carbon dioxide (CO2) lasers. In other implementations, the optical sources 208_l, 208_2 may be different types of lasers. For example, the optical source 208_2 may be a solid state laser, and the optical source 208_l may be a CO2 laser. In the example of FIG. 2, the first and second optical beams 206_l, 206_2 are pulsed. The first and second optical beams 206_l, 206_2 may have different wavelengths. For example, in implementations in which the optical sources 208_l, 208_2 include two CO2 lasers, the wavelength of the first optical beam 206_l may be about 10.26 micrometers (pm) and the wavelength of the second optical beam 206_2 may be between 10.18 pm and 10.26 pm. The wavelength of the second optical beam 206_2 may be about 10.59 pm. In these implementations, the optical beams 206_l, 206_2 are generated from different lines of the CO2 laser, resulting in the optical beams 206_l , 206_2 having different wavelengths even though both beams are generated from the same type of source.

[0067] The pulses 204_l and 204_2 have different energies and may have different durations. For example, the pre -pulse 204_2 may have a duration of at least 1 ns, for example, the pre-pulse may have a duration of 1-100 ns and a wavelength of 1 pm or 10 pm. In one example, the pre-pulse of radiation is a laser pulse that has energy of 15-60 mJ, a pulse duration of 20-70 nanoseconds (ns), and a wavelength of 1-10 micrometers (pm). In some examples, the pre-pulse may have a duration of less than 1 ns. For example, the pre-pulse may have a duration of 300 picoseconds (ps) or less, 100 ps or less, between 100-300 ps, or between 10-100 ps.

[0068] Each of the beam delivery systems 21 l_l and 211_2 is similar to the beam delivery system 111 (FIG. 1 A). In the example of FIG. 2, the first optical beam 206_l and the second optical beam 206_2 interact with separate beam delivery systems and travel on separate optical paths. However, in other implementations, the first optical beam 206_l and the second optical beam 206_2 may share all or part of the same optical path and also may share the same beam delivery system.

[0069] Referring to FIG. 3, a flowchart of a process 300 is shown. The process 300 is an example of a process that may be performed by the control system 150.

[0070] Light emitted from a substance in the vacuum chamber 109 is detected (310). The emitted light is detected by the sensor 135. The substance may be an atom, ion, and/or molecule. The substance may be part of the gas 122, the plasma 196, and/or the debris 195. The light emitted from the substance may be fluorescence or laser-induced fluorescence. The emitted light is detected by the sensor 135. The sensor 135 produces data that indicates the characteristics of the emitted light. For example, the data may indicate the intensity of the emitted light. In some implementations, the data indicates the relative location of the emitted light in the vacuum chamber 109. In these implementations, the data may be used to form a two-dimensional representation, such as an image. Moreover, in some implementations, the sensor module 134 includes more than one sensor 135. The more than one sensor 135 may be positioned relative to a particular portion of the vacuum chamber 109. In these implementations, the data from the sensors 135 may be used together to form a stereoscopic representation that represents the spatial distribution of the light emissions in the vacuum chamber 109 in three dimensions.

[0071] In some implementations, the sensor 135 collects data over a relatively short period (for example 20 microseconds (ps) or shorter, such as a period of 10 nanoseconds (ns) or less) such that the detected emissions are associated with a single plasma-generating event. These implementations allow one or more components of the EUV light source 100 or 200 to be changed on a pulse-to-pulse basis. Moreover, monitoring over relatively short periods allows generation of fast time-resolved images, such as shown in FIGS. 4A-4D and 5. In other implementations, the sensor 135 collects data over a longer period such that the detected emissions are associated with more than one plasma-generating event.

[0072] An indication of the detected emissions is analyzed (320). The indication is data received via the signal 157 from the sensor system. The signal 157 includes information that describes the emissions, such as the intensity of the detected emissions. In some

implementations, the signal 157 includes location information about the detected emissions. For example, the signal 157 may include a read out of a two-dimensional array of the sensors 135, with the intensity of emissions detected by each sensor in the array being included in the signal 157. Based on such information, the control system 150 determines the relative location of the detected emissions.

[0073] As discussed above, in some implementations, the sensor system 130 includes the spectral filter module 137 and one or more spectral filters 136. In these implementations, the spectral filters 136 determine which wavelengths reach the sensor or sensors 135. For example, the spectral filters 136 may include a filter that is designed to only allow

wavelengths associated with H-a emissions to reach the sensor 150. In these

implementations, the signal 157 may include data that indicates that a particular signal 157 includes information related to detected H-a emissions.

[0074] Furthermore, the signal 157 may include data that relates to the conditions under which the emissions were generated. For example, the signal 157 may include information about the sensor, such as exposure time. In another example, the signal 157 may include information about the environment in the vacuum chamber. Examples of such environmental information include the temperature, pressure, and/or flow rate of the gas 122 and

information about the optical beam 106, such as pulse duration, pulse energy, and/or pulse wavelength.

[0075] The analysis module 152 of the control system 150 is capable of performing a variety of analyses on the indication of the detected emissions. The various analyses may be stored on the electronic storage 156 as, for example, computer programs that are executable by the electronic processor 154. Any type of analysis on the detected emissions may be performed. Specific examples of data and corresponding analysis of that data are discussed with respect to FIGS. 4A-4D and 5. Analysis other than the ones discussed in these examples may be performed by the analysis module 152.

[0076] Referring also to FIGS. 4A-4D, an example in which the emissions are laser-induced fluorescence of neutral atomic tin is shown. In this example, tin is used as the target material, and the neutral atomic tin may be tin debris and/or tin that is not converted into the plasma 196. In the example of FIGS. 4A-4D, the sensor 135 is a camera that images the plasma formation location 123 and produces a two-dimensional image of the vacuum chamber 109.

[0077] In the example of FIGS. 4A-4D, the sensor 135 is an intensified charge coupled device (ICCD) with an exposure time of about 10 nanoseconds (ns), the spectral filter 136 was placed between the sensor 135 and the plasma formation location 123, and the laser- induced fluorescence is formed by exciting neutral tin atoms with the laser beam 115 from the probe laser 108. In this example, the probe laser 108 is a tunable laser, and the laser beam 115 is a pulsed light beam with pulses that had a duration of a few nanoseconds (for example, 10 ns or less). Additionally, the probe laser 108 was tuned such that the laser beam 115 had a wavelength of 286.3 nm, which excites neutral atomic tin that is in the ground state. Some fraction or percentage of the neutral tin atoms decay via an electronic transition that emits light (laser-induced fluorescence) at 317.5 nm. The spectral filter 136 in this example is a band-pass filter centered at 317.5 nm. Additionally, FIGS. 4A-4D relate to a system that uses a pre-pulse and a main pulse. Thus, these figures are discussed with respect to FIG. 2.

[0078] FIG. 4A is an image 400A of the laser-induced fluorescence from neutral tin in the vacuum chamber 109 at 200 nanoseconds (ns) after the pre-pulse (the pulse 204_2 of FIG. 2) interacts with the initial target 22 lp (FIG. 2). FIG. 4B is an image 400B of the laser-induced fluorescence from neutral tin in the vacuum chamber 109 at 1900 ns after the pre-pulse interacts with the initial target 22 lp. FIG. 4C is an image 400C of the laser- induced fluorescence from neutral tin in the vacuum chamber 109 at 300 ns after the main pulse (the pulse 204_l of FIG. 2) interacts with the modified target 220m (FIG. 2). FIG. 4D is an image 400D of the laser-induced fluorescence from neutral tin in the vacuum chamber 109 at 900 ns after the main pulse interacts with the modified target 220m. Each pixel of each image 400A-400D represents an amount of laser-induced fluorescence in a particular region of the vacuum chamber 109. The interaction between the main pulse and the modified target 220m is a plasma-generating event.

[0079] The analysis module 152 determines the amount of tin ionized by the interactions by analyzing the images 400A-400D to determine the intensity of certain spectral lines and the relative intensity of these lines. For example, the intensity of emissions from neutral tin may be compared to the intensity of singly or doubly ionized tin to determine the fraction of atoms of target material that are ionized after the plasma-generating event. The intensity of emissions is proportional to the number of tin atoms, neutral tin atoms in the example of FIGS. 4A-4D. Thus, if the intensity of emissions from neutral tin atoms decreases at the same time that intensity from ion species increases, then this is evidence of changing ionization fraction.

[0080] Other features of the images 400A-400D may be analyzed. For example, the spatial distribution of the intensity may be analyzed to estimate the distance traveled by the neutral tin atoms and/or the velocity of the neutral tin atoms. For example, as seen in FIG. 4D, the distance traveled from the origin (the location of the interaction between the main pulse and the modified target 220m) and the elapsed time (900 ns after the interaction in this example) gives the velocity of those neutral tin atoms.

[0081] Furthermore, the orientation angle of the fluorescence is indicative of the angle or orientation of the modified target 220m relative to the direction of propagation of the main pulse and the orientation angle of the fluorescence changes as the orientation of the modified target 220m changes. Thus, the orientation of the modified target 220m also may be determined from images such the images 400A-400D.

[0082] The analysis module 152 also determines other information about the emissions from the images 400A-400D. For example, the analysis module 152 may apply a morphological operator to identify a ring structure 401 in the image 400C. The ring structure 401 expands in space as time since the plasma-generating event passes. The analysis module 152 also identifies the ring structure 401 in the image 400D. By comparing the spatial characteristics of the ring structure 401 in the image 400C to the ring structure 401 in the image 400D, the velocity of the tin atoms may be estimated. For example, the radius and/or the diameter of the ring structure 401 in the images 401C and 401D may be compared and used, with knowledge of the amount of time between the images 400C and 400D, to estimate the velocity of the tin atoms. Moreover, in some implementations, velocity of the tin atoms is determined from a single image. For example, velocity of the tin atoms may be determined from a single image when the time from the interaction of the main pulse and the modified target 220m is known for that single image. When the velocity of the tin atoms is determined from two or more images, changes in velocity of the tin atoms also may be determined.

[0083] Furthermore, the morphological operator may be used to determine an orientation of the ring structure 401. The orientation of the ring structure provides an indication of the orientation of the modified target 220m. For example, the major and minor axes of the ring structure 401 may be estimated after identifying the ring structure 401, and the orientation of the ring structure 401 may be estimated from the axes.

[0084] The images 400A-400D are provided as examples of data that the sensor system 130 may provide to the control system 150. Other types of laser-induced fluorescence may be monitored. For example, images showing laser-induced fluorescence of ions of target material formed during a plasma-generation event may be generated and provided to the control system 150. In another example, the emissions from the gas 122 are analyzed. The gas 122 may emit light due to for example, heat in the vacuum chamber 109 from the pulse 104 (or the pulse 204_l and/or the pulse 204_2 of FIG. 2) and/or ions moving in the gas 122, by the formation of the plasma 196, or by direct excitation by the probe laser 108. FIG. 5 shows an example related to analyzing the emissions from the gas 122 to determine the amount of energy deposited into the gas 122 as a result of a plasma-generation event.

[0085] In the example of FIG. 5, the gas 122 was hydrogen gas and the sensor 135 was a camera that produced two-dimensional images of the plasma formation location 123. The pulse of light converts at least some of the target material to plasma that emits EUV light. In the example of FIG. 5, the pulse energy was 860 milliJoules (mJ), the wavelength of the pulse was 10 pm, and the duration of the pulse was 10 ns. The target was a tin droplet that had a radius of about 50 pm. In this implementation, the spectral filter 136 was a band-pass filter with a narrow spectral band centered on the H-a emission wavelength and was placed between the plasma formation location 123 and the sensor 135. Thus, H-a emissions to reached the sensor 135 but light of other wavelengths was substantially prevented from reaching the sensor 135.

[0086] Four two-dimensional images 500A-500D (of many more images taken) are shown in FIG. 5. Each of the images 500A-500D was obtained at a different time. Thus, the images 500A-500D are images of the relative intensity or amount of H-a emissions at the plasma formation location 123 at four different times.

[0087] The analysis module 152 is configured to analyze images such as 500A-500D to determine spatial characteristics of a shockwave or blast-wave 504. The blast-wave 504 is formed in the gas 122 by the plasma-generating event. The spatial characteristics may include, for example, the radius, diameter, orientation of the semi-major axis, the orientation of the major axis, the orientation of the minor axis, and/or circumference of the blast-wave 504. The analysis module 152 locates the blast-wave 504 in one or more of the images collected by the camera by applying morphological operators and imaging processing techniques to the images. For example, the general shape of the blast-wave 504 is known to be a circle, and the analysis module 152 may apply a morphological filter that detects circular objects within images to locate the blast-wave 504 in an image from the camera. In another example, the analysis module 152 may apply an edge detector that relies on the difference in intensity between emissions at the edge of the shockwave 504 and the background. [0088] Once the spatial characteristics of the blast-wave 504 have been estimated, the analysis module 152 applies the Taylor-Sedov equation to estimate an amount of energy (E) deposited into the gas 122. The Taylor-Sedov equation is:

r(t) = 1.0429 Equation

(1), where E is the energy deposited into the gas 122, r is the radius of the blast-wave, p 0 is the density of the gas 122, and t-t 0 is the time since the plasma-generating event. The radius (r) at a particular time (t) is estimated from an image of the plasma formation location 123 captured at the time (t). The analysis module 152 estimates the amount of energy deposited into the gas 122 using Equation 1 and the estimate of the radius of the blast- wave 504 at a particular time (t).

The analysis module 152 also may determine other information from the images 500A-500D. For example, FIG. 5 also includes a plot of relative total H-a emission as a function of time since the plasma-generation event. To generate the plot 501, the value of each pixel in an image collected by the camera at a particular time was summed and normalized. The results were plotted as a function of time. The images 500A-500D correspond to four of the points included on the plot 501.

[0089] The data shown in FIGS. 4A-4D and 5 are examples of the types of data that the sensor system 130 may provide to the control system 150 via the signal 157. However, the sensor system 130 may be configured to collect any other data about the emissions in the vacuum chamber 109, and the analysis module 152 also may be configured to analyze such data. For example, in some implementations, the plasma formation location 123 is monitored by more than one sensor 135, each of which has a spectral filter 136 corresponding to a particular emission line of the target material or the gas 122. In these implementations, each sensor 135 provides data that specifies the spatial distribution of one of the emission lines of the substance at the plasma generation location 123 under the same operating conditions.

The measured emissions from each sensor is compared to the measured emissions measured by the other sensors to determine properties of the environment in the vacuum chamber 109. For example, in the case of comparing different possible emissions from the target material, such a comparison results in an estimate of the portion of the target material that was ionized to form the plasma 196. [0090] Furthermore, the analysis module 152 may be configured to compare the spatial distribution of a certain type of emission at two different times after a plasma event. For example, in an implementation in which tin is used as the target material, the sensor 135 may be used with a filter 136 that only allows an emissions from ionized tin to reach the sensor 135. By comparing images of the emission of the ionized tin taken at different times, the analysis module 152 is able to estimate the velocity and/or direction of motion of the tin ions.

[0091] Accordingly, the analysis module 152 analyzes information and data from the sensor system 130.

[0092] In addition to analyzing the data provided from the sensor system 130, the control system 150 also determines whether to make adjustments to the EUV light source 100 or 200 based on the analysis (330). The adjustment to the EUV light source 100 or 200 may be an adjustment to any component of the EUV light source 100 or 200 and may include an adjustment to more than one component of the EUV light source 100 or 200. Whether an adjustment is made and the nature of the adjustment (if any) depends on the results of the analysis discussed with respect to (320).

[0093] The EUV light source 100 or 200 may be associated with various performance specifications, and the analysis of the emissions may be used to determine whether the EUV light source 100 or 200 is operating within one or more performance specifications.

Conversion efficiency (CE) is an example of a performance specification. The conversion efficiency is the ratio of the energy supplied to the EUV light source 100 or 200 that is converted into the EUV light. The CE depends on the ionization fraction (the portion of target material that is converted to ions). As discussed above, the analysis of the emissions may be used to estimate ionization fraction. To increase the ionization fraction, the duration and/or energy of the pulses in the optical beam 106 may be increased. Thus, if the CE is below the specified CE, the control system 150 may issue a command signal 159 to the light- generation module 105 to change the duration and/or intensity of the pulses in the optical beam 106.

[0094] In another example, the control system 150 may issue the command signal 159 to the light generation-module 205 (FIG. 2) such that properties of the pre-pulse 204_2 are changed. As discussed above, the pre-pulse 204_2 conditions the target by changing the shape and/or density of the target such that the modified target 22 lm (FIG. 2) is more favorable to plasma production. The light-generation module 205 may be adjusted such that the intensity and/or duration of the pre-pulse 204_2 are such that a later-produced modified target 21 lm has a lower density and/or a different shape. Moreover, in some implementations, the control system 150 issues the command signal to the beam steering system 21 l_l such that the position of the pre-pulse 204_2 relative to the initial target location 224 is changed.

Furthermore, the size of the targets in the stream 121 may be adjusted to reduce the ionization fraction. In these implementations, the command signal 159 is provided to the target supply system 110 to, for example, change the frequency of vibration of the target formation apparatus 117 such that the size of the targets in the stream 121 is reduced.

[0095] In another example, the analysis of the emissions produces an estimated ion velocity that is greater than a desired ion velocity. In this example, the control system 150 issues the command signal 159 to the gas management system 140. The gas management system 140 causes the pressure of the gas 122 to increase such that ions created in subsequent plasma generating events have a lower velocity. In yet another example, the analysis of the emissions shows a relatively high amount of tin atoms at a time relatively soon after the plasma-generating event. A relatively high amount of tin atoms shortly after the plasma generating event is an indication of excess debris in the vacuum chamber 109. The control system 150 may issue the command signal 159 to the gas management system 140 to increase the flow rate of the gas 122 and/or change the direction of the flow of the gas 122 to move the debris away from the optical element 114.

[0096] In yet another example, analysis of the emissions is used to produce an estimate of the amount of energy deposited into the gas 122. The estimated amount of energy is compared to a threshold and/or a specification (for example, a range of acceptable amounts of energy), and, if the estimated amount of energy is above the threshold and/or does not meet the threshold, the control system 150 may issue a command to the light generation module 205 to reduce the power of the pre-pulse 204_2. Reducing the power or the pre -pulse 204_2 generally reduces the amount of ions and/or pre -pulse plasma produced during the interaction between the pre-pulse 204_2 and the initial target 22 lp, and thereby reduces the energy deposited into the gas 122.

[0097] In some implementations, the control system 150 issues command signals 159 to more than one component or system of the EUV light source 100 or 200. For example, to increase the ionization fraction, the control system 150 may issue the command signal 159 to the light-generation module 105 or 205, the target supply system 110, and the gas management system 140. Moreover, under some conditions, all performance specifications are met and/or the EUV light source 100 is operating acceptably, and no adjustments are made.

[0098] After determining whether to adjust the EUV light source 100 or 200, the control system 150 determines whether to continue monitoring the vacuum chamber 109 (340). If the monitoring continues, the process 300 returns to (310). If the monitoring does not continue, the process 300 ends. Moreover, in some implementations, the process 300 runs continuously during operation of the EUV light source 100 or 200 such that the control system continuously monitors the EUV light source 100 or 200. In these implementations, the control system 150 does not determine whether to continue monitoring the vacuum chamber 109 and instead monitors the vacuum chamber 109 continuously and without interruption during operation of the EUV light source 100 or 200.

[0099] FIG. 6A is a block diagram of a lithographic apparatus 600 that includes a source collector module SO. The lithographic apparatus 600 includes:

• an illumination system (illuminator) IL configured to condition a

radiation beam B (for example, EUV radiation).

• a support structure (for example, a mask table) MT constructed to support a patterning device (for example, a mask or a reticle) MA and connected to a first positioner PM configured to accurately position the patterning device;

• a substrate table (for example, a wafer table) WT constructed to hold a substrate (for example, a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate; and

• a projection system (for example, a reflective projection system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (for example, including one or more dies) of the substrate W.

[0100] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0101] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure may use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0102] The term“patterning device” should be broadly interpreted as referring to any device that may be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. The pattern imparted to the radiation beam may correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0103] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase- shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

[0104] The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0105] In the example of FIGS. 6A and 6B, the apparatus is of a reflective type (for example, employing a reflective mask). The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0106] Referring to FIG. 6A, the illuminator IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, for example, xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma is produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in FIG. 6A, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, for example, EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module may be separate entities, for example when a carbon dioxide (CO2) laser is used to provide the laser beam for fuel excitation.

[0107] In such cases, the laser is not considered to form part of the lithographic apparatus and the radiation beam is passed from the laser to the source collector module with the aid of a beam delivery system including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

[0108] The illuminator IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as s-outer and s-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as facetted field and pupil mirror devices. The illuminator IL may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0109] The radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (for example, an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS 1 can be used to accurately position the patterning device (for example mask) MA with respect to the path of the radiation beam B. Patterning device (for example mask) MA and substrate W may be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0110] The depicted apparatus may be used in at least one of the following modes: 1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (that is, a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed.

2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (that is, a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (for example, mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS.

3. In another mode, the support structure (for example, mask table) MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[0111] FIG. 6B shows an implementation of the lithographic apparatus 600 in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. The source collector module SO is constructed and arranged such that a vacuum environment can be maintained in an enclosing structure 620 of the source collector module SO. The systems IL and PS are likewise contained within vacuum environments of their own. An EUV radiation emitting plasma 2 may be formed by a laser produced LPP plasma source. The function of source collector module SO is to deliver EUV radiation beam 20 from the plasma 2 such that it is focused in a virtual source point. The virtual source point is commonly referred to as the intermediate focus (IF), and the source collector module is arranged such that the intermediate focus IF is located at or near an aperture 621 in the enclosing structure 620. The virtual source point IF is an image of the radiation emitting plasma 2.

[0112] From the aperture 621 at the intermediate focus IF, the radiation traverses the illumination system IL, which in this example includes a facetted field mirror device 22 and a facetted pupil mirror device 24. These devices form a so-called“fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 21, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 660). Upon reflection of the beam 21 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 26 is formed and the patterned beam 26 is imaged by the projection system PS via reflective elements 28, 30 onto a substrate W held by the substrate table WT. To expose a target portion C on substrate W, pulses of radiation are generated while substrate table WT and patterning device table MT perform synchronized movements to scan the pattern on patterning device MA through the slit of illumination.

[0113] Each system IL and PS is arranged within its own vacuum or near-vacuum

environment, defined by enclosing structures similar to enclosing structure 620. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there may be more mirrors present than those shown. For example there may be one to six additional reflective elements present in the illumination system IL and/or the projection system PS, besides those shown in FIG. 6B.

[0114] Considering source collector module SO in more detail, a laser energy source including a laser 623 is arranged to deposit laser energy 624 into a fuel that includes a target material. The target material may be any material that emits EUV radiation in a plasma state, such as xenon (Xe), tin (Sn), or lithium (Li). The plasma 2 is a highly ionized plasma with electron temperatures of several lO's of electron volts (eV). Higher energy EUV radiation may be generated with other fuel materials, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma, collected by a near-normal incidence collector 3 and focused on the aperture 621. The plasma 2 and the aperture 621 are located at first and second focal points of collector CO, respectively. [0115] Although the collector 3 shown in FIG. 6B is a single curved mirror, the collector may take other forms. For example, the collector may be a Schwarzschild collector having two radiation collecting surfaces. In an embodiment, the collector may be a grazing incidence collector which comprises a plurality of substantially cylindrical reflectors nested within one another.

[0116] To deliver the fuel, which, for example, is liquid tin, a droplet generator 626 is arranged within the enclosure 620, arranged to fire a high frequency stream 628 of droplets towards the desired location of plasma 2. The droplet generator 626 may be the target formation apparatus 216 and/or includes an adhesive such as the adhesive 234. In operation, laser energy 624 is delivered in a synchronism with the operation of droplet generator 626, to deliver impulses of radiation to turn each fuel droplet into a plasma 2. The frequency of delivery of droplets may be several kilohertz, for example 50 kHz. In practice, laser energy 624 is delivered in at least two pulses: a pre pulse with limited energy is delivered to the droplet before it reaches the plasma location, in order to vaporize the fuel material into a small cloud, and then a main pulse of laser energy 624 is delivered to the cloud at the desired location, to generate the plasma 2. A trap 630 is provided on the opposite side of the enclosing structure 620, to capture fuel that is not, for whatever reason, turned into plasma.

[0117] The droplet generator 626 comprises a reservoir 601 which contains the fuel liquid (for example, molten tin) and a filter 669 and a nozzle 602. The nozzle 602 is configured to eject droplets of the fuel liquid towards the plasma 2 formation location. The droplets of fuel liquid may be ejected from the nozzle 602 by a combination of pressure within the reservoir 601 and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[0118] As the skilled reader will know, reference axes X, Y, and Z may be defined for measuring and describing the geometry and behavior of the apparatus, its various

components, and the radiation beams 20, 21, 26. At each part of the apparatus, a local reference frame of X, Y and Z axes may be defined. In the example of FIG. 6B, the Z axis broadly coincides with the direction optical axis O at a given point in the system, and is generally normal to the plane of a patterning device (reticle) MA and normal to the plane of substrate W. In the source collector module, the X axis coincides broadly with the direction of fuel stream 628, while the Y axis is orthogonal to that, pointing out of the page as indicated in FIG 6B. On the other hand, in the vicinity of the support structure MT that holds the reticle MA, the X axis is generally transverse to a scanning direction aligned with the Y axis. For convenience, in this area of the schematic diagram FIG. 6B, the X axis points out of the page, again as marked. These designations are conventional in the art and will be adopted herein for convenience. In principle, any reference frame can be chosen to describe the apparatus and its behavior.

[0119] Numerous additional components used in the operation of the source collector module and the lithographic apparatus 600 as a whole are present in a typical apparatus, though not illustrated here. These include arrangements for reducing or mitigating the effects of contamination within the enclosed vacuum, for example to prevent deposits of fuel material damaging or impairing the performance of collector 3 and other optics. Other features present but not described in detail are all the sensors, controllers and actuators involved in controlling of the various components and sub-systems of the lithographic apparatus 600.

[0120] Referring to FIG. 7, an implementation of an LPP EUV light source 700 is shown.

The light source 700 may be used as the source collector module SO in the lithographic apparatus 600. Furthermore, the light-generation module 105 of FIG. 1 may be part of the drive laser 715. The drive laser 715 may be used as the laser 623 (FIG. 6B).

[0121] The LPP EUV light source 700 is formed by irradiating a target mixture 714 at a plasma formation location 705 with an amplified light beam 710 that travels along a beam path toward the target mixture 714. The target material discussed with respect to FIGS. 1A,

2, and 3, and the targets in the stream 121 of FIGS. 1A and 2 may be or include the target mixture 714. The plasma formation location 705 is within an interior 707 of a vacuum chamber 730. When the amplified light beam 710 strikes the target mixture 714, a target material within the target mixture 714 is converted into a plasma state that has an element with an emission line in the EUV range. The created plasma has certain characteristics that depend on the composition of the target material within the target mixture 714. These characteristics may include the wavelength of the EUV light produced by the plasma and the type and amount of debris released from the plasma.

[0122] The light source 700 also includes the supply system 725 that delivers, controls, and directs the target mixture 714 in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. The target mixture 714 includes the target material such as, for example, water, tin, lithium, xenon, or any material that, when converted to a plasma state, has an emission line in the EUV range. For example, the element tin may be used as pure tin (Sn); as a tin compound, for example, SnBr 4 , SnBn, SnFLq as a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any combination of these alloys. The target mixture 714 may also include impurities such as non-target particles. Thus, in the situation in which there are no impurities, the target mixture 714 is made up of only the target material. The target mixture 714 is delivered by the supply system 725 into the interior 707 of the chamber 730 and to the plasma formation location 705.

[0123] The light source 700 includes a drive laser system 715 that produces the amplified light beam 710 due to a population inversion within the gain medium or mediums of the laser system 715. The light source 700 includes a beam delivery system between the laser system 715 and the plasma formation location 705, the beam delivery system including a beam transport system 720 and a focus assembly 722. The beam transport system 720 receives the amplified light beam 710 from the laser system 715, and steers and modifies the amplified light beam 710 as needed and outputs the amplified light beam 710 to the focus assembly 722. The focus assembly 722 receives the amplified light beam 710 and focuses the beam 710 to the plasma formation location 705.

[0124] In some implementations, the laser system 715 may include one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses and, in some cases, one or more pre -pulses. Each optical amplifier includes a gain medium capable of optically amplifying the desired wavelength at a high gain, an excitation source, and internal optics. The optical amplifier may or may not have laser mirrors or other feedback devices that form a laser cavity. Thus, the laser system 715 produces an amplified light beam 710 due to the population inversion in the gain media of the laser amplifiers even if there is no laser cavity. Moreover, the laser system 715 may produce an amplified light beam 710 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the laser system 715. The term“amplified light beam” encompasses one or more of: light from the laser system 715 that is merely amplified but not necessarily a coherent laser oscillation and light from the laser system 715 that is amplified and is also a coherent laser oscillation.

[0125] The optical amplifiers in the laser system 715 may include as a gain medium a filling gas that includes CO2 and may amplify light at a wavelength of between about 9100 and about 11000 nm, and in particular, at about 10600 nm, at a gain greater than or equal to 800 times. Suitable amplifiers and lasers for use in the laser system 715 may include a pulsed laser device, for example, a pulsed, gas-discharge CO2 laser device producing radiation at about 9300 nm or about 10600 nm, for example, with DC or RF excitation, operating at relatively high power, for example, lOkW or higher and high pulse repetition rate, for example, 40 kHz or more. The pulse repetition rate may be, for example, 50 kHz. The optical amplifiers in the laser system 715 may also include a cooling system such as water that may be used when operating the laser system 715 at higher powers. [0126] The light source 700 includes a collector mirror 735 having an aperture 740 to allow the amplified light beam 710 to pass through and reach the plasma formation location 705. The collector mirror 735 may be, for example, an ellipsoidal mirror that has a primary focus at the plasma formation location 705 and a secondary focus at an intermediate location 745 (also called an intermediate focus) where the EUV light may be output from the light source 700 and may be input to, for example, an integrated circuit lithography tool (not shown). The light source 700 may also include an open-ended, hollow conical shroud 750 (for example, a gas cone) that tapers toward the plasma formation location 705 from the collector mirror 735 to reduce the amount of plasma-generated debris that enters the focus assembly 722 and/or the beam transport system 720 while allowing the amplified light beam 710 to reach the plasma formation location 705. For this purpose, a gas flow may be provided in the shroud that is directed toward the plasma formation location 705.

[0127] The light source 700 may also include a master controller 755 that is connected to a droplet position detection feedback system 756, a laser control system 757, and a beam control system 758. The light source 700 may include one or more target or droplet imagers 760 that provide an output indicative of the position of a droplet, for example, relative to the plasma formation location 705 and provide this output to the droplet position detection feedback system 756, which may, for example, compute a droplet position and trajectory from which a droplet position error may be computed either on a droplet by droplet basis or on average. The droplet position detection feedback system 756 thus provides the droplet position error as an input to the master controller 755. The master controller 755 may therefore provide a laser position, direction, and timing correction signal, for example, to the laser control system 757 that may be used, for example, to control the laser timing circuit and/or to the beam control system 758 to control an amplified light beam position and shaping of the beam transport system 720 to change the location and/or focal power of the beam focal spot within the chamber 730.

[0128] The supply system 725 includes a target material delivery control system 726 that is operable, in response to a signal from the master controller 755, for example, to modify the release point of the droplets as released by a target material supply apparatus 727 to correct for errors in the droplets arriving at the desired plasma formation location 705. The target material supply apparatus 727 includes a target formation apparatus that employs an adhesive such as the adhesive 234.

[0129] Additionally, the light source 700 may include light source detectors 765 and 770 that measures one or more EUV light parameters, including but not limited to, pulse energy, energy distribution as a function of wavelength, energy within a particular band of wavelengths, energy outside of a particular band of wavelengths, and angular distribution of EUV intensity and/or average power. The light source detector 765 generates a feedback signal for use by the master controller 755. The feedback signal may be, for example, indicative of the errors in parameters such as the timing and focus of the laser pulses to properly intercept the droplets in the right place and time for effective and efficient EUV light production.

[0130] The light source 700 may also include a guide laser 775 that may be used to align various sections of the light source 700 or to assist in steering the amplified light beam 710 to the plasma formation location 705. In connection with the guide laser 775, the light source 700 includes a metrology system 724 that is placed within the focus assembly 722 to sample a portion of light from the guide laser 775 and the amplified light beam 710. In other implementations, the metrology system 724 is placed within the beam transport system 720. The metrology system 724 may include an optical element that samples or re-directs a subset of the light, such optical element being made out of any material that may withstand the powers of the guide laser beam and the amplified light beam 710. A beam analysis system is formed from the metrology system 724 and the master controller 755 since the master controller 755 analyzes the sampled light from the guide laser 775 and uses this information to adjust components within the focus assembly 722 through the beam control system 758.

[0131] Thus, in summary, the light source 700 produces an amplified light beam 710 that is directed along the beam path to irradiate the target mixture 714 at the plasma formation location 705 to convert the target material within the mixture 714 into plasma that emits light in the EUV range. The amplified light beam 710 operates at a particular wavelength (that is also referred to as a drive laser wavelength) that is determined based on the design and properties of the laser system 715. Additionally, the amplified light beam 710 may be a laser beam when the target material provides enough feedback back into the laser system 715 to produce coherent laser light or if the drive laser system 715 includes suitable optical feedback to form a laser cavity.

[0132] The preceding merely illustrates the principles of embodiments of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various

arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

[0133] This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as“lower,”“upper,”“horizontal,” “vertical,”“above,”“below,”“up,”“down,” top” and“bottom” as well as derivatives thereof (e.g.,“horizontally,”“downwardly,”“upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. Terms concerning attachments, coupling and the like, such as“connected” and“interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise.

[0134] Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Other implementations are within the scope of the claims. The appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.

[0135] Implementations of the disclosure may further be described using the following clauses:

1. A system comprising:

a vacuum chamber comprising an interior region, wherein the interior region is configured to receive a target and a light beam, the target comprises target material, and the target material emits extreme ultraviolet (EUV) light when in a plasma state;

a detection system configured to image the interior region, the detection system configured to detect light emission from atoms, ions, or molecules in the interior region and to produce a representation of a spatial distribution of the light emission in the interior region; and a control system coupled to the detection system, the control system configured to: analyze the representation of the spatial distribution of the light emission to determine a spatial distribution of the light emission from atoms, ions, or molecules in the interior region; and

determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the spatial distribution of the light emission.

2. The system of clause 1 , wherein the light emission comprises fluorescence.

3. The system of clause 2, wherein the fluorescence comprises laser-induced

fluorescence.

4. The system of clause 1 , further comprising one or more spectral filters configured to be positioned relative to the detection system, the spectral filters being configured to only allow some wavelengths to reach the detection system.

5. The system of clause 4, wherein each of the one or more spectral filters is configured to transmit light having a wavelength in one of a plurality of emission lines of the target material.

6. The system of clause 5, wherein at least one of the one or more spectral filters is configured to transmit a wavelength in a visible light range.

7. The system of clause 4, wherein the vacuum chamber is further configured to contain a gas in the interior region, and the spectral filter is configured to transmit light having a wavelength at an emission line of the gas.

8. The system of clause 1, wherein the control system is configured to receive a plurality of representations of the interior region, each of the plurality of representations is associated with a different time, and the control system being configured to analyze the representation of the interior region comprises the control system being configured to analyze each of the plurality of representations to determine the spatial distribution of the light emission in the interior region at each of the different times.

9. The system of clause 8, wherein the light emission in the interior region results from an energy event in the interior region, and the different times are all times that occur after the energy event.

10. The system of clause 9, wherein the energy event comprises an interaction between the light beam and the target, and the light emission is an emission from: (a) the target material, (b) a plasma formed from the interaction between the light beam and the target material, and/or (c) debris formed from the interaction of the light beam and the target.

11. The system of clause 1 , wherein the control system is configured to receive an extended exposure representation of the interior region, the extended exposure representation of the interior region comprising an average of the spatial distribution of the emission in the interior region over a temporal period.

12. The system of clause 9, wherein the vacuum chamber is further configured to contain a gas in the interior region, the energy event comprises an interaction that adds energy to the gas, and the light emission is an emission from the gas.

13. The system of clause 12, wherein the interaction that adds energy to the gas comprises (a) an interaction between the light beam and the gas, (b) an interaction between the gas and a plasma formed from an interaction between the light beam and the target, and/or (c) an interaction between ions and the gas.

14. The system of clause 1, wherein the control system being configured to analyze the representation to determine a spatial distribution of the light emission in the interior region comprises the control system being configured to estimate a shape and/or a spatial distribution of intensity of the light emission.

15. The system of clause 1, further comprising:

a first spectral filter configured to transmit light having a wavelength in a first wavelength band; and

a second spectral filter configured to transmit light having a wavelength in a second wavelength band, and wherein the control system being configured to analyze the

representation comprises: the control system being configured to estimate an amount of light emission in the first wavelength band and to estimate an amount of light emission in the second wavelength band, and the control system is further configured to estimate an ionization fraction of the target material based on comparing the estimated amount of light emission at the first wavelength band and the estimated amount of light emission at the second wavelength band.

16. The system of clause 15, wherein the control system determines whether to adjust at least one property of the light beam based on the estimated ionization fraction.

17. The system of clause 16, wherein the control system determines whether to adjust a pointing direction of the light beam based on the estimated ionization fraction.

18. The system of clause 1, wherein the light beam comprises a main pulse light beam comprising an energy sufficient to convert at least some of the target material to a plasma that emits EUV light.

19. The system of clause 1, wherein the light beam comprises a pre-pulse light beam.

20. The system of clause 4, wherein the control system being configured to analyze the representation further comprises the control system being configured to compare the spatial distributions of fluorescence in the interior region at least at two the different times to estimate a velocity of ions in the interior region, and to compare the estimated velocity to a velocity specification, and the control system is configured to determine whether to adjust a pressure of the gas based on the comparison of the estimated velocity to the velocity specification.

21. The system of clause 1, wherein the representation of the spatial distribution comprises a representation of a two-dimensional representation.

22. The system of clause 1, wherein the light beam comprises a pulsed light beam, and the control system being configured to adjust at least one property of the light beam comprises the control system being configured to adjust at least one property of a later- occurring pulse of the pulsed light beam.

23. An EUV light source comprising:

a vacuum chamber configured to: contain a gas in an interior region and to receive a target and a light beam, the target comprising target material that emits extreme ultraviolet (EUV) light in a plasma state;

a monitor comprising at least one sensor, the at least one sensor being configured to detect emissions from the gas in the interior region and to produce an indication of the detected emissions; and

a control system coupled to the monitor, the control system configured to:

analyze the indication of detected emissions; and

determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on the analysis.

24. The EUV light source of clause 23, wherein the monitor comprises a detection system configured to image a portion of the interior region and to produce a representation of a spatial distribution of detected emissions in the portion.

25. The EUV light source of clause 24, wherein the control system is configured to receive a plurality of representations over a period of time, each representation indicating a spatial distribution of detected emissions in the portion at a different time in the period of time, and the control system is configured to determine whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber based on two or more of the plurality of representations.

26. The EUV light source of clause 23, wherein the gas comprises hydrogen, and the detected emission comprises an H-alpha (H-a) and/or an H-beta (H-b) emission from the hydrogen. 27. The EUV light source of clause 24, further comprising:

a first spectral filter configured to transmit a first band of wavelengths; and

a second spectral filter configured to transmit a second band of wavelengths, wherein, in operational use, the first spectral filter and the second spectral filter are between the portion and the detection system; and

the control system being configured to analyze the detected emissions comprises the control system being configured to compare a representation of emissions that are transmitted by the first spectral filter to a representation of emissions that are transmitted by the second spectral filter; and the determination of whether to adjust at least one property of the light beam and/or at least one property of the vacuum chamber is based on the comparison.

28. The EUV light source of clause 24, further comprising a pressure controller coupled to interior of the vacuum chamber, the pressure controller being configured to change a pressure of the gas in the interior of the vacuum chamber, and wherein the control system is coupled to the pressure controller.

29. A method of controlling an EUV light source, the method comprising:

providing a target to a target region in a vacuum chamber, the vacuum chamber containing a gas in an interior region;

causing an interaction between a light beam and the target in the target region;

detecting light emission from atoms, ions, and/or molecules in the interior region of the vacuum chamber, the light emission being a response to an energy event in the vacuum chamber, the energy event comprising an event that adds energy to the target and/or the gas; analyzing the detected light emission to determine a spatial distribution of light emission in the interior region; and

determining whether to adjust a property of the light beam and/or the gas based on the analysis.

[0136] Other implementations are within the scope of the claims.