NL2022007A

NL2022007A - Regeneration of a debris flux measurement system in a vacuum vessel

Info

Publication number: NL2022007A
Application number: NL2022007A
Authority: NL
Inventors: Ma Yue; Vladimirovna Dziomkina Nina; Tom Stewart John Iv; Li Tianqi
Original assignee: Asml Netherlands Bv
Priority date: 2017-12-15
Filing date: 2018-11-16
Publication date: 2019-06-21
Also published as: CN111480071A; KR20200096777A; JP2021507204A; JP7320505B2; KR102697196B1; WO2019115144A1

Abstract

An apparatus includes: a vessel; a target delivery system that directs a target toward an interaction region in the vessel, the target including target matter that emits extreme ultraviolet light When in a plasma state; and a metrology apparatus. The metrology apparatus includes a measurement system having a measurement surface configured to measure a flux of target matter; and a regeneration tool configured to regenerate the measurement system. Regeneration includes: preventing the measurement surface from becoming saturated, and/or de—saturating the measurement surface if it has become saturated.

Description

REGENERATION OF A DEBRIS FLUX MEASUREMENT SYSTEM

IN A VACUUM VESSEL

TECHNICAL FIELD

[0001] The disclosed subject matter relates to a system and method for regenerating a measurement system that measures an amount or flux of debris produced within a chamber of an extreme ultraviolet light source.

BACKGROUND

[0002] Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, for example, silicon wafers.

[0003] Methods to produce EUV light include, but are not necessarily limited to, converting a material that has 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 can 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. 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

[0004] In some general aspects, an apparatus includes: a vessel; a target delivery system that directs a target toward an interaction region in the vessel, the target including target matter that emits extreme ultraviolet light when in a plasma state; and a metrology apparatus. The metrology apparatus includes: a measurement system having a measurement surface configured to measure a flux of target matter; and a regeneration tool configured to regenerate the measurement system. Regeneration includes: preventing the measurement surface from becoming saturated, and/or de-saturating the measurement surface if it has become saturated.

[0005] Implementations can include one or more of the following features. For example, the metrology apparatus can include a control apparatus in communication with the measurement system and the regeneration tool. The control apparatus can be configured to activate the regeneration tool based on an output from the measurement system.

[0006] The measurement surface can be configured to interact with the target matter. The interaction between the target matter and the measurement surface produces a measurement signal. The measurement system can also include a measurement controller configured to receive the measurement signal and calculate the flux of the target matter across the measurement surface.

[0007] The metrology apparatus can include a crystal microbalance. The crystal microbalance can be a quartz crystal microbalance.

[0008] The vessel can define a cavity, and the vessel cavity can be held at a pressure below atmospheric pressure.

[0009] The interaction region can receive an amplified light beam, and the target can be converted into the plasma that emits extreme ultraviolet light when the target interacts with the amplified light beam.

[00010] The apparatus can also include an optical element that includes an optical element surface within the vessel. The metrology apparatus can be positioned relative to the optical element surface. The optical element can be an optical collector in which the optical element surface interacts with least some emitted extreme ultraviolet light when the target is converted to the plasma.

[00011] The regeneration tool can be configured to regenerate the measurement system without removing the metrology apparatus from the vessel. The regeneration tool can include a cleaning tool positioned to interact with the measurement system and configured to remove target matter that has been deposited on the measurement surface upon instruction by the measurement controller. The cleaning tool can include a free radical production unit that is configured to produce free radicals adjacent the measurement surface. The free radicals can chemically react with the deposited target matter to form a new chemical that is released from the measurement surface. The free radical production unit can include a wire filament adjacent the measurement surface and a power source that supplies a current to the wire filament. The wire filament can have a shape that matches a shape of the measurement surface. The free radical production unit can include a plasma generator that generates a plasma material in a plasma state adjacent the measurement surface, the plasma material including the free radicals. The free radicals can be free radicals of hydrogen produced from hydrogen molecules native within the vessel. The target matter on the measurement surface can include tin, such that the chemical that is released from the measurement surface includes tin hydride.

[00012] The apparatus can also include a removal apparatus configured to remove the released new chemical from the vessel. The removal apparatus can include a gas port that is in fluid communication with the interior of the vessel, and the released new chemical is transported from the interior of the vessel through the gas port.

[00013] The regeneration tool can be configured to remove the target matter from the measurement surface and in the presence of hydrogen in the vessel and without a reaction that requires oxygen.

[00014] In other general aspects, a method includes: supplying a target inside a cavity of a vessel; measuring a flux of the target matter over a measurement surface within the vessel cavity; and regenerating the measurement surface. The target includes matter that emits extreme ultraviolet light when it is converted into a plasma. Regenerating includes at least one of: preventing the measurement surface from becoming saturated and/or de-saturating the measurement surface if it has become saturated.

[00015] Implementations can include one or more of the following features. For example, the method can also include activating regeneration of the measurement surface based on the measured flux of the target matter over the measurement surface.

[00016] The flux of the target matter can be measured by interacting the target matter with the measurement surface so that the target matter is deposited on the measurement surface. [00017] The target can be supplied inside the vessel cavity by directing a plurality of targets toward an interaction region in the vacuum vessel. The interaction region also receives an amplified light beam such that the interaction between the target and amplified light beam in the interaction region converts the target into the plasma that emits the extreme ultraviolet light.

[00018] The measurement surface can be regenerated by removing deposited target matter from the measurement surface without removing the measurement surface from the vessel. The deposited target matter can be removed from the measurement surface by producing free radicals of an element adjacent the measurement surface, the produced free radicals chemically reacting with the deposited target matter to form a new chemical that is released from the measurement surface. The deposited target matter can include tin, the element can be hydrogen, the free radicals can be hydrogen radicals, and the new chemical can be tin hydride. The element adjacent the measurement surface can be native to the vessel cavity. The deposited target matter can be removed by removing the deposited target matter without the presence of oxygen. The method can include removing the released new chemical from the vessel cavity.

[00019] The flux of the target matter can be measured by measuring the flux of the target matter at a time during which the deposited target matter is not being removed from the measurement surface.

[00020] The deposited target matter can be removed from the measurement surface to thereby prevent the measurement surface from reaching its saturation limit.

[00021] The method can also include maintaining a cavity defined by the vessel at a pressure below atmospheric pressure. The method can further include estimating an amount of extreme ultraviolet emitted when the target matter is converted into plasma based on the measured flux. The method can further include estimating an amount of target matter deposited on a surface within the vessel cavity based on the measured flux.

[00022] hi other general aspects, an extreme ultraviolet light source includes: an optical source configured to produce an amplified light beam; a vessel that defines a cavity and configured to receive the amplified light beam at an interaction region in the cavity; a target delivery system configured to produce a target that travels along a target path toward the interaction region; and a metrology apparatus. The cavity is configured to be held at a pressure below atmospheric pressure. The target includes target matter that emits extreme ultraviolet light in a plasma state. The metrology apparatus includes: a measurement system having a measurement surface configured to measure a flux of target matter; and a regeneration tool configured to regenerate the measurement system. Regeneration includes: preventing the measurement surface from saturating; and/or de-saturating the measurement surface if it becomes saturated.

[00023] Implementations can include one or more of the following features. For example, the measurement surface can be configured to interact with target matter, in which the interaction between the target matter and the measurement surface produces a measurement signal; and the measurement system can also include a measurement controller that receives the measurement signal and calculates a flux of target matter across the measurement surface. The regeneration tool can include a cleaning tool positioned to interact with the measurement system. The cleaning tool can be configured to regenerate the measurement system by removing target matter that has been deposited on the measurement surface.

[00024] The extreme ultraviolet light source can also include an optical collector that collects at least some of the emitted extreme ultraviolet light for use by an external lithography apparatus.

[00025] In other general aspects, a metrology system is used in an extreme ultraviolet light source. The metrology system includes: a metrology apparatus configured to measure a flux of target matter across a measurement surface within the vessel; and a regeneration tool coupled to the metrology apparatus. The metrology apparatus includes: a measurement system having the measurement surface configured to interact with the target matter, in which the interaction between the target matter and the measurement surface produces a measurement signal; and a measurement controller configured to receive the measurement signal and calculate the flux of the target matter across the measurement surface based on the received measurement signal. The regeneration tool is configured to regenerate the measurement system. Regeneration includes: preventing the measurement surface from becoming saturated; and/or de-saturating the measurement surface if it has become saturated. The regeneration tool includes a cleaning tool positioned to interact with the measurement surface and to remove target matter that has been deposited on the measurement surface upon instruction from the measurement controller. [00026] In other general aspects, an apparatus includes: a vessel; a means for delivering a target toward an interaction region in the vessel, the target including target matter that emits extreme ultraviolet light when in a plasma state; and a metrology apparatus. The metrology apparatus includes: a means for measure a flux of target matter across a measurement surface within the vessel; and a means for regenerating the measurement surface. The means for regenerating includes: a means for preventing the measurement surface from becoming saturated; and/or a means for de-saturating the measurement surface if it has become saturated.

DRAWING DESCRIPTION

[00027] Fig. 1 is a block diagram of an apparatus that includes a self-regenerating metrology apparatus within a cavity defined by a vessel; [00028] Fig. 2 is a side cross sectional view and a zoomed-in view taken at A of an implementation of the metrology apparatus of Fig. 1; [00029] Fig. 3A is a perspective view of an implementation of the metrology apparatus of Figs. 1 and 2, in which the metrology apparatus is designed with a measurement system that includes a crystal microbalance having a measurement surface and a free radical regeneration tool that includes a wire filament adjacent the measurement surface; [00030] Fig. 3B is a block diagram of the metrology apparatus of Fig. 3 A; [00031] Fig. 3C is a perspective view of the wire filament adjacent the measurement surface of the crystal microbalance of Figs. 3A and 3B; [00032] Fig. 3D is a side cross sectional view of the wire filament adjacent the measurements surface of the crystal microbalance of Figs. 3A-3C; [00033] Fig. 4 is an implementation of an extreme ultraviolet (EUV) light source in which a self-regenerating metrology apparatus, such as the apparatuses of Figs. 1-3D, can be implemented within the EUV light source; [00034] Fig. 5A is a rear perspective view of an optical element that is an optical collector, in which the self-regenerating metrology apparatus of Figs. 1-4 can be adjacent the optical collector; [00035] Fig. 5B is a front perspective view of the optical element of Fig. 5A; [00036] Fig. 5C is a side cross sectional view of the optical element of Fig. 5A; [00037] Fig. 5D is a plan view of the optical element of Fig. 5A; [00038] Fig, 6 is a block diagram of procedure for regenerating a measurement surface; [00039] Fig. 7 is a schematic diagram showing a side cross sectional view of a measurement surface during the procedure of Fig. 6; [00040] Fig. 8 is a graph of a deposition thickness of a coating on a measurement surface versus time, depicting an application of the procedure of Fig. 6 and the metrology apparatus of Figs. 3A-3D; [00041] Fig. 9 is a graph of a removal rate in arbitrary units versus a distance between the wire filament of Figs. 3A-3D and the measurement surface of Figs. 3A-3D for different values of standard liter per minute; [00042] Fig. 10 is a block diagram of a lithography apparatus that receives the output of the EUV light source of Fig. 4; and [00043] Fig. 11 is a block diagram of a lithography apparatus that receives the output of the EUV light source of Fig. 4.

DESCRIPTION

[00044] Referring to Fig. 1, an apparatus 100 includes a self-regenerating metrology apparatus 105 within a cavity 118 defined by a vessel 120. The metrology apparatus 105 includes a measurement system 110 that has a measurement surface 112 configured to measure a flux of a target matter 125. The flux of the target matter 125 is the mass of the target matter 125 that crosses an area over a particular amount of time. Moreover, because the density of the target matter 125 may be known, it is possible to determine or estimate the flux of the target matter 125 by determining a thickness of the target matter 125 that is deposited on the measurement surface 112.

[00045] Over time, the target matter 125 builds up as a coating 127 on the measurement surface 112, which causes the measurement surface 112 to become saturated. The measurement surface 112 is saturated when it can no longer produce any useful information about the flux of the target matter 125. The saturation limit on the measurement surface 112 is related to the saturation thickness of the coating 127 of the target matter 125, and this saturation thickness can be relatively small when compared to the saturation thickness of nearby materials within the vessel 120 and thus the measurement surface 112 approaches its saturation limit well before the nearby materials in the vessel 120 need to be cleaned, repaired, or replaced due to being coated with the target matter 125. Thus, it becomes inefficient to have to replace the measurement system 110 each time the measurement surface 112 becomes saturated. To this end, the metrology apparatus 105 includes a regeneration tool 115 that is configured to regenerate the measurement system 110. At some moments, regeneration of the measurement system 110 can involve preventing the measurement surface 112 from becoming saturated. In other moments, such as when the measurement surface 112 has already become saturated, regeneration of the measurement system 110 involves de-saturating the measurement surface 112.

[00046] The regeneration tool 115 can be configured to operate (that is, remove the target matter 125 that coats the measurement surface 112) even though the regeneration tool 115 is exposed to molecular hydrogen, which can be present in the cavity 118. Moreover, the regeneration tool 115 can be configured to operate without the use of or presence of oxygen; that is, oxygen is not needed or required in order for the regeneration tool 115 to operate or perform any functions.

[00047] The target matter 125 is produced within the vessel 120 as follows. The apparatus 100 includes a target delivery system 140 that directs a stream 142 of targets 145 toward an interaction region 150 in the vessel 120. The target 145 includes target matter 125 that emits extreme ultraviolet (EUV) light 155 when it is converted into a plasma material 160 (which is also referred to as a light-emitting plasma material 160). However, some of the target matter 125 is not fully converted into the plasma material 160 in the interaction region 150, or some of the plasma material 160 reverts back into target matter 125. Because of this, the target matter 125 that remains (which is either not converted into the plasma material 160 or reverts back) can travel throughout the cavity 118 of the vessel 120 and coat various objects such as walls or optical elements within the cavity 118 of the vessel 120. The measurement system 110 is provided at a suitable position or at a plurality of positions within the vessel 120 to determine the flux of the target matter 125 that travels through that portion of the interior of the vessel 120 at which the measurement system 110 is placed. While only one measurement system 110 is shown in Fig. 1, as discussed below, the cavity 118 of the vessel 120 can be outfitted with a plurality of measurement systems 110 at various locations, depending on the particular information that needs to be obtained relating to the flux of the target matter 125. Moreover, one or more of these measurement systems 110 can be incorporated in a metrology apparatus 105 that includes the regeneration tool 115.

[00048] The presence of the leftover or remaining target matter 125 in the vessel 120 is debris in the form of particles, vapor residue, or pieces of matter that are present in the target 145. This debris can accumulate on surfaces of objects in the vessel 120. For example, if the target 145 includes molten metal of tin, then tin particles can accumulate on (or coat) one or more optic surfaces or walls within the vessel 120. The debris that forms on the surfaces and also the coating 127 that forms on the measurement surface 112 can include vapor residue, ions, particles, and/or clusters of matter formed from the target matter 125. The presence of the debris from the target matter 125 within the vessel 120 can reduce the performance of the surfaces within the vessel 120, and also reduce the overall efficiency of the measurement system 110. [00049] The target delivery system 140 delivers, controls, and directs the targets 145 in a stream 142 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 145 can be any material that emits EUV light when in a plasma state. For example, the target 145 can include water, tin, lithium, and/or xenon. The target 145 can be a target mixture that includes the target matter 125 and impurities such as non-target particles.

[00050] The target matter 125 is the substance that, when in a plasma state (the plasma material 160), has an emission line in the EUV range. The target matter 125 can 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 matter 125 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 matter can be the element tin, which can be used as pure tin (Sn); as a tin compound, for example, SnBr4, SnBr2, SnH4; 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 145 includes only the target matter.

[00051] The cavity 118 within the vessel 120 can be held at vacuum, that is, at a pressure below atmospheric pressure. For example, the cavity 118 can be held at a low pressure of between about 0.5 Torr (T) to about 1.5 T (for example, at 1 T), which is the pressure selected for generation of the EUV light 155. The metrology apparatus 105 is therefore configured to operate in the vacuum environment within the cavity 118 of the vessel 120, which means it is designed to work in a vacuum (such as at 1 T). Moreover, the metrology apparatus 105 is designed to enable its use without having to change the design or operation of the vessel 120. Thus, the metrology apparatus 105 is configured to operate in the environment in which EUV light 155 is produced most efficiently.

[00052] Referring also to Fig. 2, in some implementations, a metrology apparatus 205 includes, as the regeneration tool 115, a free radical regeneration tool 215 that is adjacent the measurement surface 212 of the measurement system 210. The free radical regeneration tool 215 is a cleaning tool that is positioned to interact with the measurement system 210. The free radical regeneration tool 215 is configured to remove the target matter 125 that has built up as a coating 227 on the measurement surface 212 of the measurement system 210. The free radical regeneration tool 215 includes a free radical production unit that is configured to produce free radicals 216 adjacent the measurement surface 212, and these free radicals 216 chemically react with the deposited target matter 125 of the coating 227 to form a new chemical 228 that is released from the measurement surface 212. For example, the new chemical 228 can be in a gaseous state, and thus becomes released from the measurement surface 212. The new chemical 228, which is in the gaseous state, can then be pumped out of the vessel 120.

[00053] A free radical 216 is an atom, molecule, or ion that has an unpaired valence electron or an open electron shell, and therefore may be seen as having a dangling covalent bond. The dangling bonds can make free radicals highly chemically reactive, that is, a free radical can react readily with other substances. Because of their reactive nature, free radicals 216 can be used to remove a substance (such as the deposited target matter 125) from an object such as the measurement surface 212. The free radicals 216 can remove the deposited target matter 125 by, for example, etching, reacting with, and/or combusting the target matter 125.

[00054] The free radicals 216 can be created in any suitable manner. For example, the free radicals 216 can be formed by breaking up larger molecules 230 present in (or native to) the vessel 120 near the measurement system 210 or the free radical regeneration tool 215. Larger molecules 230 present in the vessel 120 and near the measurement system 210 can be broken up by any process that puts enough energy into these larger molecules, such as, for example, ionizing radiation, heat, electrical discharges, electrolysis, and chemical reactions. The formation of the free radicals therefore involves supplying enough energy to the larger molecules 230 to break a bond (generally covalent) between the atoms of the larger molecules.

[00055] As another example, the free radicals 216 can be formed at a location that is remote from the measurement system 210 and then they could be delivered to the measurement surface 212. Thus, the free radicals 216 can be formed outside of the vessel 120 and then transported into the vessel 120.

[00056] In other implementations, free radical regeneration tool 215 can be a capacitively coupled plasma (CCP) apparatus. In the CCP apparatus, two metal electrodes are separated by a small distance and are driven by a power supply (such as a radio-frequency (RF) power supply). When an electric field is generated between electrodes, the atoms of the larger molecules 230 are ionized and release electrons. The electrons in the gas are accelerated by the RF field and ionize the gas directly or indirectly by collisions, producing secondary electrons. Ultimately, a plasma is created when the electric field is strong enough.

[00057] In some implementations, as discussed above, the target 145 includes tin (Sn), and in these implementations, the target matter 125 deposited on the measurement surface 212 includes tin particles. As discussed above, the vessel 120 is a controlled environment, and one of the larger molecules 230 that can be present and permitted within the vessel 120 is molecular hydrogen (Hi). In this case, the free radical regeneration tool 215 creates the free radicals 216 from the molecular hydrogen that is native or present within the vessel 120. A free radical 216 of hydrogen is a single hydrogen element (H*). This chemical process can be represented by the following chemical formula: H2(g) θ 2 H* (g), where g indicates that the chemical is in the gaseous state.

[00058] Specifically, the generated free radicals of hydrogen H bond with the tin particles (Sn) on the measurement surface 212 and form the new chemical 228, which is called tin hydride (SnH4), which is released from the measurement surface 212. This chemical process is represented by the following chemical formula: 4 H (g) + Sn(s) <-» SnHrfg), where s indicates that the chemical is in the solid state.

[00059] In this way, the coating 227 (formed from the target matter 125) can be etched off or removed from the measurement surface 212 at a rate of at least 1 nanometers per min over the entire measurement surface 212, and not just the regions closest to the free radical regeneration tool 215. This is because the free radicals 216 are created at the location adjacent the measurement surface 212, as opposed to being created remote from the measurement surface 212 and then transported to the measurement surface 212. This is important because the hydrogen radicals H* are short lived and will tend to recombine to reform molecular hydrogen. The design of the free radical regeneration tool 215 enables the formation of the hydrogen radicals I I* as close as possible to the measurement surface 212 to thereby enable more of the hydrogen radicals H* to combine with the tin particles before they have a chance to recombine with each other to reform molecular hydrogen, and this permits the measurement system 210 to be regenerated without having to remove the metrology apparatus 205 from the vessel 120.

[00060] Referring to Figs. 3A and 3B, an example of a metrology apparatus 305 is shown. The metrology apparatus 305 is designed with a free radical regeneration tool 315 that includes a wire filament 365 adjacent a measurement surface 312 of a measurement system 310 and a power source 370 that supplies a current to the wire filament 365. The wire filament 365 should be made of a material that has a high melting point, at least high enough so that it can withstand temperatures that are high enough to provide enough heat to break the bonds of the nearby larger molecules. For example, in some implementations, the current that flows through the wire filament 365 can raise the temperature of the wire filament 365 to over 1000° C. Moreover, the wire filament 365 should be non-chemically reactive with the larger molecules or other components within the vessel 120. Additionally, the wire filament 365 can be made of a material that is a catalyst to the chemical reaction noted above, HiCg) θ 2 H* (g), where g indicates that the chemical is in the gaseous state. In this way, the wire filament 365 can be any substance that speeds up this chemical reaction, but is not consumed by the chemical reaction. For example, the material of the wire filament 365 can be tungsten (W), rhenium (Re), or an alloy of one or more of W and Re. Lastly, the wire filament 365 should be robust and have a high tensile strength to withstand temperature fluctuations during use. For example, the wire filament 365 can be made of tungsten or rhenium.

[00061] With reference also to Figs. 3C and 3D, the wire filament 365 can be energized by the current from the power source 370 to heat up to a temperature that is high enough to cause any native hydrogen molecules 330 within the vessel 120 and adjacent the wire filament 365 to energize to the point at which the atoms within the molecules 330 break apart into the free radicals. The wire filament 365 has a shape that conforms to or is complementary to the shape of the measurement surface 312 to more efficiently permit interaction between the free radicals 216 and the coating 327 on the measurement surface 312.

[00062] In some implementations, the measurement system 310 includes a crystal niicrobalance, such as a quartz crystal microbalance. A crystal microbalance is shown in Fig. 3A. The crystal microbalance is a device that outputs a measurement signal that can be used to determine the flux of the target matter 125 impinging upon the measurement surface 312. The amount of mass deposited on the measurement surface 312 is correlated to a change in one or more resonance frequencies associated with the measurement surface 312. Thus, by measuring the change in the one or more resonance frequencies, it is possible to determine how much mass has been deposited on the measurement surface 312. The crystal microbalance includes a crystal such as a quartz crystal and a set of electrodes that provide an alternating potential to the faces of the crystal to cause the crystal to oscillate at the one or more resonance frequencies. The measurement surface 312 can correspond to one of the faces of the crystal.

[00063] The measurement surface 312 is held in an adapter or flange 375 that is made of a non-reactive material, and this flange 375 is mounted to a housing 377, which can be water cooled. The measurement surface 312 can include a coating (not visible) of a thin layer of free radical resistant material such as Zirconium nitride (ZrN). In this implementation, the free radicals 216 that are produced would react with the target matter 125 deposited as the coating 327 on the measurement surface 312 but would not react with the ZrN coating so that the ZrN remains intact even though the free radical regeneration tool 315 is in operation.

[00064] If the target matter 125 is tin and the crystal microbalance is a quartz crystal microbalance, then the saturation limit of the measurement surface 312 is about 8 micrometers (pm). The saturation limit is the maximum thickness of a coating 327 formed from the deposited target matter 125; above this saturation limit, the measurement system 110 is unable to accurately measure the flux of the target matter 125. By contrast, other elements within the vessel 120 can withstand a deposited coating of the target matter 125 that has a thickness that is on the order of several thousand times thick as 8 pm. The free radical regeneration tool 315 is capable of removing the coating 327 without the need to open up the vessel 120 and without the need to stop the operation of other components within the vessel 120.

[00065] Referring to Fig. 4, a self-regenerating metrology apparatus 405, such as the apparatuses 105, 205, 305, can be implemented within an EUV light source 400 in which the vessel 120 is an EUV vacuum chamber 420, as discussed next. The EUV light source 400 includes the target delivery system 440 that directs the stream 442 of targets 445 toward an interaction region 480 in the EUV chamber 420. The interaction region 480 receives an amplified light beam 481. As discussed above, the target 445 includes matter that emits EUV light when it is in a plasma state. An interaction between the matter within the target 445 and the amplified light beam 481 at the interaction region 480 converts some of the matter in the target 445 into the plasma material 460. The plasma material 460 emits EUV light 455. The plasma material 460 has an element with an emission line in the EUV wavelength range. The created plasma material 460 has certain characteristics that depend on the composition of the target 445. These characteristics include the wavelength of the EUV light 455 produced by the plasma material 460.

[00066] The plasma material 460 can be considered to be a highly ionized plasma with electron temperatures of several tens of electron volts (eV). Higher energy EUV light 455 can be generated with other fuel materials (other kinds of targets 445), for example, terbium (Tb) and gadolinium (Gd). The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma material 460, and then collected by an optical element 482. [00067] Referring also to Figs. 5A-5D, the optical element 482 can be an optical collector in which a surface 483 interacts with at least some of the emitted EUV light 455. The surface 483 of the optical collector 482 can be a reflective surface that is positioned to receive at least a portion of the EUV light 455 and to direct this collected EUV light 484 for use outside the EUV light source 400 (shown in Fig. 4). The reflective surface 483 directs the collected EUV light 484 to a secondary focal plane, where this EUV light 484 is then captured for use by a tool 485 (such as a lithography apparatus) outside the EUV light source 400. Exemplary lithography apparatuses 1000, 1100 are discussed with reference to respective Figs. 10 and 11.

[00068] The reflective surface 483 can be configured to reflect light in the EUV wavelength range but absorb or diffuse or block light outside the EUV wavelength range. The optical collector 482 also includes an aperture 590 that permits the amplified light beam 481 to pass through the optical collector 482 toward the interaction region 480. The optical collector 482 can be, for example, an ellipsoidal mirror that has a primary focus at the interaction region 480 and a secondary focus at the secondary focal plane. This means that a plane section (such as plane section C-C) is in the shape of an ellipses or a circle. Thus, the plane section C-C cuts through the reflective surface 483, and it is formed from a portion of an ellipse. A plan view of the optical collector 482 shows that the edge of the reflective surface 483 forms a circular shape. [00069] Although the optical collector 482 shown herein is a single curved mirror, it can take other forms. For example, the optical collector 482 can be a Schwarzschild collector having two radiation collecting surfaces. In an implementation, the optical collector 482 is a grazing incidence collector that includes a plurality of substantially cylindrical reflectors nested within one another.

[00070] Referring again to Fig. 4, the EUV light source 400 includes an optical system 486 that produces the amplified light beam 481 due to a population inversion within a gain medium or mediums. The optical system 486 can include an optical source that produces a light beam, and a beam delivery system that steers and modifies the light beam and also focuses the light beam to the interaction region 480. The optical source within the optical system 486 includes one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses that form the amplified light beam 481, and in some cases, one or more pre-pulses that form a precursor amplified light beam (not shown). 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 optical system 486 produces the amplified light beam 481 due to the population inversion in the gain media of the amplifiers even if there is no laser cavity. Moreover, the optical system 486 can produce the amplified light beam 481 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the optical system 486. The term “amplified light beam” therefore encompasses one or more of: light from the optical system 486 that is merely amplified but not necessarily a coherent laser oscillation and light from the optical system 486 that is amplified and is also a coherent laser oscillation.

[00071] The optical amplifiers used in the optical system 486 can include as a gain medium a gas that includes carbon dioxide (CO2) and can amplify light at a wavelength of between about 9100 and 11000 nanometers (nm), and for example, at about 10600 nm, at a grain greater than or equal to 100. Suitable amplifiers and lasers for use in the optical system 486 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, 10 kW or higher and a high pulse repetition rate, for example, 40 kHz or more.

[00072] The EUV light source 400 also includes a control apparatus 487 in communication with one or more controllable components or systems of the EUV light source 400. The control apparatus 487 is in communication with the optical system 486 and the target delivery system 440. The target delivery system 440 can be operable in response to signals from one or more modules within the control apparatus 487. For example, the control apparatus 487 can send a signal to the target delivery system 440 to modify the release point of the targets 445 to correct for errors in the targets 445 arriving at the desired interaction region 480. The optical system 486 can be operable in response to signals from one or more modules within the control apparatus 487. The various modules of the control apparatus 487 can be free-standing modules in that data between the modules is not transferred from module to module. Or, one or more of the modules within the control apparatus 487 can communicate with each other. The modules within the control apparatus 487 can be co-located or separated from each other physically.

[00073] For example, the module that controls the target delivery system 440 can be colocated with the target delivery system 440 while a module that controls the optical system 486 can be co-located with the optical system 486.

[00074] The metrology apparatus 405 includes a control apparatus 488 that is in communication with the measurement system 410 and the regeneration tool 415 of the metrology apparatus 405. The control apparatus 488 is configured to receive an output from the measurement system 410, analyze the output as needed, and perform actions such as send data to the control apparatus 487 or activate the regeneration tool 415 based on the analysis. The control apparatus 488 therefore can include a measurement controller in communication with the measurement system 410 and configured to receive the measurement signal from the measurement system 410 and to calculate the flux of the target matter 425 across the measurement surface of the measurement system 410. The control apparatus 488 can provide the signal to activate or actuate the regeneration tool 415. For example, the control apparatus 488 can provide a signal to the power source 370 of the metrology apparatus 305 to thereby supply the current to the wire filament 365.

[00075] The EUV system 400 also includes a removal or exhaust apparatus 489 configured to remove the released chemical 428 from the EUV chamber 420 as well as other gaseous byproducts that can form within the EUV chamber 420. As discussed above, the released chemical 428 is formed from the interaction of free radicals 416 (which are produced from the larger molecules 430 by the regeneration tool 415) with the target matter 425 that has deposited on the measurement surface of the measurement system 410. The removal apparatus 489 can be a pump that removes the released chemical 428 from the EUV chamber 420. The removal apparatus 489 can include a gas port that is in fluid communication with the interior or cavity 418 of the EUV chamber 420 such that the released new chemical 428 is transported from the cavity 418 through the gas port and out of the EUV chamber 420. For example, once the chemical 428 is formed, it is released, and because the chemical 428 can be volatile, it is sucked to the removal apparatus 489, which removes the released chemical 428 from the EUV chamber 420.

[00076] Other components of the EUV light source 400 that arc not shown include, for example, detectors for measuring parameters associated with the produced EUV light 455. Detectors can be used to measure energy or energy distribution of the amplified light beam 481.

Detectors can be used to measure an angular distribution of the intensity of the EUV light 455. Detectors can measure errors in the timing or focus of the pulses of the amplified light beam 481. Output from these detectors can be provided to the control apparatus 487, which can include modules that analyze the output and adjust aspects of other components of the EUV light source 400 such as the optical system 486 and the target delivery system 440.

[00077] In summary, an amplified light beam 481 is produced by the optical system 486 and directed along a beam path to irradiate the target 445 at the interaction region 480 to convert the material within the target 445 into plasma that emits light in the EUV wavelength range. The amplified light beam 481 operates at a particular wavelength (the source wavelength) that is determined based on the design and properties of the optical system 486.

[00078] While only one metrology apparatus 405 is shown in EUV chamber 420 of Fig. 4, it is possible to configure a plurality of metrology apparatuses 405 throughout the EUV chamber 420. Other possible locations for the metrology apparatus 405 are marked by the cross icons 495 shown in Fig. 4. For example, the metrology apparatus 405 can be positioned next to any optical element that includes a surface that could potentially interact with the target matter 125 and thus potentially become coated with the debris during operation of the EUV light source 400. Thus, one or more metrology apparatuses 405 can be positioned next to the optical collector 482 such as near a rim of the optical collector 482; next to a cone placed between a wall of the EUV chamber 420 and the optical collector 482; and/or near the target delivery system 440, near an exhaust unit (such as the removal apparatus 489).

[00079] The measurement system 310 is any device that can measure properties of the coating 327 that forms on the measurement surface 312 and then enable an analysis to determine the thickness of such coating 327 and therefore a flux of the target matter 125. In other implementations, the measurement system 310 is designed as a refractometer, an ellipsometer; and/or a 4-point probe.

[00080] As discussed above, in the implementation of Figs. 3A-3D, the metrology apparatus 305 is designed with a free radical regeneration tool 315 that includes the wire filament 365 adjacent the measurement surface 312. There are other ways to design a free radical regeneration tool 315. In other implementations, the free radical regeneration tool 315 can include a plasma generator that enables the production or generation of a material in a plasma state (a plasma material) at a location that is local to or adjacent to the measurement surface 312 from a material already present and native (a native material such as the larger molecules 330) within the vessel 120. A material (such as the larger molecules 330) is native or present within the vessel 120 if it exists within the vessel 120 without needing to be transported into the vessel 120 from outside the vessel 120. The plasma material includes the free radicals 216 that chemically react with the target matter 125 that has been deposited as the coating 327 on the measurement surface 312, as discussed above. In addition to the free radicals, the plasma material can include other components that do not react with the target matter 125, such as ions formed from the native material, electrons produced from the native material, and chemically neutral items. The free radical regeneration tool 315 is able to remove more of the target matter 125 (that is deposited as the coating 327) as the number of free radicals present in the plasma material is increased. To put this another way, the higher the density of free radicals within the plasma material, the higher the rate of debris removal.

[00081] In some implementations, the free radical regeneration tool 315 is designed as an inductively-coupled plasma (ICP) tool, which includes, as the plasma generator, an electrical conductor placed adjacent the measurement surface 312. The electrical conductor is connected to the power source of the metrology apparatus 305, and is housed inside a dielectric tubing such as porcelain, ceramic, mica, polyethylene, glass, or quartz. In the ICP process, a time-varying electric current is flowed (from the power source) through the electrical conductor and the flow of the time-varying electric current produces a time-varying magnetic field adjacent this electrical conductor. And, the produced time-varying magnetic field induces an electric field or current at the location adjacent the measurement surface 312. The induced electric current is large enough to generate, from the native material within the vessel 120, the plasma material at the location adjacent the measurement surface 312.

[00082] In other implementations, the free radical regeneration tool 315 is designed as a heated capillary. The free radical regeneration tool 315 is not limited to the specific designs noted herein and can be any tool that produces free radicals.

[00083] Referring again to Fig. 2, because of the relatively small size of the measurement surface 212, the free radicals 216 that are produced by the free radical regeneration tool 215 (or 315) flow across the measurement surface 212 after being formed by the tool 215 by the act of diffusion. However, because the pressure in the vessel 120 could be relatively high (even though it is a vacuum, it may be a low vacuum), in some implementations in which the measurement surface 212 is larger or other factors reduce the amount of diffusion, it can be challenging for the free radicals to disperse across the measurement surface 212 without additional assistance. Accordingly, the metrology apparatus 205 can also include a gas flow mechanism that is configured to push or disperse the free radicals 216 across the entire surface of the measurement surface 212.

[00084] Referring to Fig. 6, a procedure 600 is performed by the apparatus 100. The target 145 is supplied inside the cavity 118 of the vessel 120 (605). The target 145 includes target matter 125 that emits EUV light 155 when it is converted into a plasma material 160. The flux of the target matter 125 is measured over the measurement surface 112 (for example, using the measurement system 110), which is within the vessel 120 (610). The measurement surface 112 is regenerated (615). Regeneration of the measurement surface 112 (615) can include preventing the measurement surface 112 from becoming saturated (615A). Regeneration of the measurement surface 112 (615) can include de-saturating the measurement surface 112 if it has become saturated (615B). Or, regeneration of the measurement surface 112 (615) can include both preventing the measurement surface 112 from becoming saturated (615A) and de-saturating the measurement surface 112 if it has become saturated (615B).

[00085] With reference to Fig. 4 as well, the target 445 can be supplied inside the cavity 418 (605) by directing the plurality or stream 442 of targets 445 toward the interaction region 480 in the EUV chamber 420. The interaction region 480 also receives the amplified light beam 481 such that the interaction between the target 445 and the amplified light beam 481 in the interaction region 480 converts the target 445 into the plasma material 460, which emits the EUV light 455.

[00086] The flux of the target matter 125 can be measured (610) by interacting the target matter 125 with the measurement surface 112 so that the target matter 125 is deposited on the measurement surface 112.

[00087] The regeneration of the measurement surface 112 (615) can be activated based on a flux that is measured over the measurement surface 112. Moreover, regeneration (615) can be performed and completed without removing the measurement surface 112 from the vessel 120. [00088] Referring also to Fig. 7, the measurement surface 312 is shown to illustrate how the regeneration of the measurement surface (615) is affected. The measurement surface 312 is regenerated (615) by removing the deposited target matter 125 from the measurement surface 312. The target matter 125 forms the coating 327 on the measurement surface 312 (716). The deposited target matter 125 (which forms the coating 327) is removed from the measurement surface 312 by producing the free radicals 216 (717). The free radicals 216 can be produced from the element or material such as the larger molecules 230 already present in the vessel 120 and adjacent to the measurement surface 312. Additionally, after the free radicals 216 are produced, then these free radicals 216 are chemically reacted with the deposited target matter 125 (which forms the coating 327) on the measurement surface 312 to form the new chemical 228, which is released from the measurement surface 312 (718). The deposited target matter 125 can be removed from the measurement surface 312 without the use of oxygen as a catalyst or an element to a reaction.

[00089] The procedure 600 can also include the step of removing the released chemical 228 from the vessel 120, for example, using the exhaust apparatus 489.

[00090] The flux of the target matter 125 can be measured 610 by measuring the flux of the target matter 125 at a time during which the deposited target matter 125 is not being removed from the measurement surface 112. Thus, the measurement of the flux of the target matter 125 can happen at a time that is distinct from the time that the measurement surface 112 is regenerated (615). The removal of the deposited target matter 125 from the measurement surface 112 (which is a part of regeneration 615) can prevent the measurement surface 112 from reaching its saturation limit. Moreover, the removal of the deposited target matter 125 should happen after the flux of the soon-to-be-removed target matter 125 has been measured by the measurement system 110 because if the target matter 125 is removed too quickly, then it would not be possible for the measurement system 110 to determine the flux of the target matter 125. [00091] The measured flux 610 can additionally be used by the control apparatus 487 to estimate an amount of EUV light 455 that is emitted from the plasma material 460. For example, the stability of the production of the EUV light 455 is generally correlated or related to the production of the target matter 125 (which is debris of tin, for example). A large fluctuation in the measured flux 610 of the target matter 125 can therefore indicate an unstable operation of the EUV light source 400. Moreover, the measured flux 610 can be used to further estimate an amount of target matter 125 that is deposited on a surface within the vessel 120, for example, a surface that is adjacent the metrology apparatus 105.

[00092] The metrology apparatus 105 can be placed at particular locations throughout the vessel 120, depending on what information is desired. For example, the measured flux 610 can be used to determine a malfunction of equipment that is adjacent to a particular metrology apparatus 105. As another example, the measured flux 610 can be used to measure a cleaning rate of a surface if the metrology apparatus 105 is placed next to a surface that is being cleaned. [00093] The measured flux 610 can be used to determine whether a flow field of transported target matter 125 is altered. In particular, the target matter 125 can be entrained in molecular hydrogen present in the vessel 120, and this molecular hydrogen is transported through the vessel 120 according to particular flow paths, and if the metrology apparatus 105 is placed near a particular flow field, it can then be used to determine whether that flow field is altered by measuring the flux 610 of the target matter 125.

[00094] Referring to Fig. 8, a graph 800 is shown that depicts an application of the procedure 600 and metrology apparatus 305 of Figs. 3A-3D. The graph 800 shows the deposition thickness 805 of the coating 327 versus time 810 (or a pulse accumulation equivalent). A saturation limit 815 of the measurement surface 312 is also depicted in the graph 800 as a dashed line. As discussed above, in the example in which the measurement system 310 is a quartz crystal microbalance, and the target matter 125 is tin, then the saturation limit 815 can be in a range of 5-15 pm. Initially, the metrology apparatus 305 functions in a measurement mode 820, and in this mode 820, the measurement system 310 is operating to measure the flux of the target matter 125 across the measurement surface 312. In the measurement mode 820, the free radical regeneration tool 315 is not operating and therefore the wire filament 365 is not supplied with current from the power source 370. During this measurement mode 820, the thickness 805 of the coating 327 is generally increasing. The slope of the graph 800 in this measurement mode 820 can be stored within memory of the measurement system 310 or of the control apparatus that receives the output from the measurement system 310 and used as a system performance monitor and process excursion protection. Moreover, the measurement system 310 operates to determine the deposition rate, the flux, or other properties of the target matter 125. As the thickness 805 of the coating 327 reaches the saturation limit 815, the metrology apparatus 305 switches to operating in regeneration mode 825. In regeneration mode 825, the measurement system 310 may or may not perform functions, but the wire filament 365 is energized by the current from the power source 370 and thus actively act to remove the target matter 125 that has deposited as the coating 327 on the measurement surface 312. This cycle of the measurement mode 820 and the regeneration mode 825 repeats, as needed, during operation of the apparatus 100. Moreover, the timing or frequency of the cycle can be selected depending on the desired data acquisition frequency of the measurement system 310.

[00095] For a measurement system 310 that is a quartz crystal microbalance, and having a ZrN surface coating, and depending on the distance between the wire filament 365 and the measurement surface 312, the rate of removal of the tin from the measurement surface 312 can be as high as 4 nanometers (nm) per minute at a radial distance of about 20 millimeter (mm) from the circumference of the wire filament 365. The quartz crystal microbalance is small in size and the measurement surface 312 is well within 20 mm, and therefore the rate of removal of tin from a quartz crystal microbalance in these circumstances is greater than 4 nm/minute. Such a removal rate is much higher (for example, tens of time higher) than the deposition rate on a nearby critical surface, which is about 450 nm/gps, thus improving the time distribution of the regeneration of the measurement surface 112.

[00096] For example, Fig. 9 shows a graph 900 of the removal rate 905 in arbitrary units versus a distance between the wire filament 365 and the measurement surface 312 for different values of standard liter per minute (slm).

[00097] Referring to Fig. 10, in some implementations, the metrology apparatus 105 (or 205, 305,405) is implemented within an EUV light source 1000 that supplies EUV light 1084 to a lithography apparatus 1085. The lithography apparatus 1085 includes an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV light 1084); 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.

[00098] The illumination system IL can 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. 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 MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure MT can be a frame or a table, for example, which can be fixed or movable as required. The support structure MT can ensure that the patterning device is at a desired position, for example, with respect to the projection system PS.

[00099] The term “patterning device” should be broadly interpreted as referring to any device that can 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 can correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. The patterning device can 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. [000100] The projection system PS, like the illumination system IL, can 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 can be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[000101] As here depicted, the apparatus is of a reflective type (for example, employing a reflective mask).

[000102] The lithographic apparatus can be of a type having two (dual stage) or more substrate tables (and/or two or more patterning device tables). In such “multi-stage” machines, the additional tables can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other tables are being used for exposure.

[000103] The illuminator IL receives an extreme ultraviolet radiation beam (the EUV light 1175) from the EUV light source 1000. 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 can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required lineemitting element, with a laser beam. The EUV light source 1000 can be designed like the EUV light source 400. As discussed above, the resulting plasma emits output radiation, for example, EUV radiation, which is collected using the optical element 482 (or a radiation collector). [000104] 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 PSI 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 can be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[000105] The depicted apparatus could be used in at least one of the following modes: [000106] 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.

[000107] 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.

[000108] 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.

[000109] Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

[000110] Fig. 11 shows an implementation of the lithographic apparatus 1185 in more detail, including the EUV light source 1100, the illumination system IL, and the projection system PS. The EUV light source 1100 is constructed and arranged as discussed above when describing EUV light source 400.

[000111] The systems IL and PS are likewise contained within vacuum environments of their own. The intermediate focus (IF) of the EUV light source 1100 is arranged such that it is located at or near an aperture in an enclosing structure. The virtual source point IF is an image of the radiation emitting plasma (for example, the EUV light 484).

[000112] From the aperture at the intermediate focus IF, the radiation beam traverses the illumination system IL, which in this example includes a facetted field mirror device 1122 and a facetted pupil mirror device 1124. These devices form a so-called “fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 1121, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA (as shown by reference 1160). Upon reflection of the beam 1121 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 1126 is formed and the patterned beam 1126 is imaged by the projection system PS via reflective elements 1128, 1130 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.

[000113] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to EUV chamber 420. More elements than shown may generally be present in illumination system IL and projection system PS. Further, there can 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. 11.

[000114] Referring again to Fig. 4, the target delivery system 440 can include a droplet generator arranged within the EUV chamber 420, and arranged to fire a high frequency stream 442 of droplets toward the interaction region 480. In operation, the amplified light beam 481 is delivered in a synchronization with the operation of droplet generator, to deliver pulses of radiation to turn each droplet (each target 445) into the light-emitting plasma 460. The frequency of delivery of the droplets can be several kilohertz, for example 50 kHz.

[000115] In some implementations, the energy from the amplified light beam 481 is delivered in at least two pulses: namely, a pre pulse with limited energy is delivered to the droplet before it reaches the interaction region 480, in order to vaporize the fuel material into a small cloud, and then a main pulse of energy is delivered to the cloud at the interaction region 480, to generate the light-emitting plasma 460. A trap (which can be, for example, a receptacle) is provided on the opposite side of the EUV chamber 420, to capture fuel (that is, the target 445) that is not, for whatever reason, turned into plasma.

[000116] The droplet generator in the target delivery system 440 includes a reservoir that contains the fuel liquid (for example, molten tin) and a filter and a nozzle. The nozzle is configured to eject droplets of the fuel liquid toward the interaction region 480. The droplets of fuel liquid can be ejected from the nozzle by a combination of pressure within the reservoir and a vibration applied to the nozzle by a piezoelectric actuator (not shown).

[000117] Other aspects of the invention are set out as in the following numbered clauses. 1. An apparatus comprising: a vessel; a target delivery system that directs a target toward an interaction region in the vessel, the target comprising target matter that emits extreme ultraviolet light when in a plasma state; and a metrology apparatus comprising: a measurement system comprising a measurement surface configured to measure a flux of target matter; and a regeneration tool configured to regenerate the measurement system, wherein regeneration comprises: preventing the measurement surface from becoming saturated, and/or de-saturating the measurement surface if it has become saturated. 2. The apparatus of clause 1, wherein the metrology apparatus includes a control apparatus in communication with the measurement system and the regeneration tool, wherein the control apparatus is configured to activate the regeneration tool based on an output from the measurement system. 3. The apparatus of clause 1, wherein: the measurement surface is configured to interact with the target matter wherein the interaction between the target matter and the measurement surface produces a measurement signal; and the measurement system further comprises a measurement controller configured to receive the measurement signal and calculate the flux of the target matter across the measurement surface. 4. The apparatus of clause 1, wherein the metrology apparatus includes a crystal microbalance. 5. The apparatus of clause 4, wherein the crystal microbalance is a quartz crystal microbalance. 6. The apparatus of clause 1, wherein the vessel defines a cavity, and the vessel cavity is held at a pressure below atmospheric pressure. 7. The apparatus of clause I. wherein the interaction region receives an amplified light beam, and the target is converted into the plasma that emits extreme ultraviolet light when the target interacts with the amplified light beam. 8. The apparatus of clause 1, further comprising an optical element that includes an optical element surface within the vessel, wherein the metrology apparatus is positioned relative to the optical element surface. 9. The apparatus of clause 8, wherein the optical element is an optical collector in which the optical element surface interacts with least some emitted extreme ultraviolet light when the target is converted to the plasma. 10. The apparatus of clause 1, wherein the regeneration tool is configured to regenerate the measurement system without removing the metrology apparatus from the vessel. 11. The apparatus of clause 1, wherein the regeneration tool includes a cleaning tool positioned to interact with the measurement system and is configured to remove target matter that has been deposited on the measurement surface upon instruction by the measurement controller. 12. The apparatus of clause 11, wherein the cleaning tool comprises a free radical production unit that is configured to produce free radicals adjacent the measurement surface, wherein the free radicals chemically react with the deposited target matter to form a new chemical that is released from the measurement surface. 13. The apparatus of clause 12, wherein the free radical production unit comprises a wire filament adjacent the measurement surface and a power source that supplies a current to the wire filament. 14. The apparatus of clause 13, wherein the wire filament is a shape that matches a shape of the measurement surface. 15. The apparatus of clause 12, wherein the free radical production unit comprises a plasma generator that generates a plasma material in a plasma state adjacent the measurement surface, the plasma material including the free radicals. 16. The apparatus of clause 12, wherein the free radicals are free radicals of hydrogen produced from hydrogen molecules native within the vessel. 17. The apparatus of clause 16, wherein the target matter on the measurement surface includes tin, such that the chemical that is released from the measurement surface includes tin hydride. 18. The apparatus of clause 12, further comprising a removal apparatus configured to remove the released new chemical from the vessel. 19. The apparatus of clause 18, wherein the removal apparatus includes a gas port that is in fluid communication with the interior of the vessel, wherein the released new chemical is transported from the interior of the vessel through the gas port. 20. The apparatus of clause 1, wherein the regeneration tool is configured to remove the target matter from the measurement surface and in the presence of hydrogen in the vessel and without a reaction that requires oxygen. 21. A method comprising: supplying a target inside a cavity of a vessel, wherein the target comprises matter that emits extreme ultraviolet light when it is converted into a plasma; measuring a flux of the target matter over a measurement surface within the vessel cavity; and regenerating the measurement surface, wherein regenerating comprises at least one of: preventing the measurement surface from becoming saturated and/or de-saturating the measurement surface if it has become saturated. 22. The method of clause 2 k further comprising activating regeneration of the measurement surface based on the measured flux of the target matter over the measurement surface. 23. The method of clause 21, wherein measuring the flux of the target matter comprises interacting the target matter with the measurement surface so that the target matter is deposited on the measurement surface. 24. The method of clause 21, wherein supplying the target inside the vessel cavity comprises directing a plurality of targets toward an interaction region in the vacuum vessel, the interaction region also receiving an amplified light beam such that the interaction between the target and amplified light beam in the interaction region converts the target into the plasma that emits the extreme ultraviolet light. 25. The method of clause 21, wherein regenerating the measurement surface comprises removing deposited target matter from the measurement surface without removing the measurement surface from the vessel. 26. The method of clause 25, wherein removing the deposited target matter from the measurement surface comprises producing free radicals of an element adjacent the measurement surface, the produced free radicals chemically reacting with the deposited target matter to form a new chemical that is released from the measurement surface. 27. The method of clause 26, wherein the deposited target matter includes tin, the element is hydrogen, the free radicals are hydrogen radicals, and the new chemical is tin hydride. 28. The method of clause 26, wherein the element adjacent the measurement surface is native to the vessel cavity. 29. The method of clause 26, wherein removing the deposited target matter comprises removing the deposited target matter without the presence of oxygen. 30. The method of clause 26. further comprising removing the released new chemical from the vessel cavity. 31. The method of clause 25, wherein measuring the flux of the target matter comprises measuring the flux of the target matter at a time during which the deposited target matter is not being removed from the measurement surface. 32. The method of clause 25, wherein removing the deposited target matter from the measurement surface thereby prevents the measurement surface from reaching its saturation limit. 33. The method of clause 21, further comprising maintaining a cavity defined by the vessel at a pressure below atmospheric pressure. 34. The method of clause 21, further comprising estimating an amount of extreme ultraviolet emitted when the target matter is converted into plasma based on the measured flux. 35. The method of clause 21, further comprising estimating an amount of target matter deposited on a surface within the vessel cavity based on the measured flux. 36. An extreme ultraviolet light source comprising: an optical source configured to produce an amplified light beam; a vessel that defines a cavity, the vessel configured to receive the amplified light beam at an interaction region in the cavity and the cavity configured to be held at a pressure below atmospheric pressure; a target delivery system configured to produce a target that travels along a target path toward the interaction region, the target comprising target matter that emits extreme ultraviolet light in a plasma state; and a metrology apparatus comprising: a measurement system comprising a measurement surface configured to measure a flux of target matter; and a regeneration tool configured to regenerate the measurement system, wherein regeneration comprises: preventing the measurement surface from saturating; and/or de-saturating the measurement surface if it becomes saturated. 37. The extreme ultraviolet light source of clause 36, wherein the measurement surface is configured to interact with target matter, wherein the interaction between the target matter and the measurement surface produces a measurement signal; and the measurement system further comprises a measurement controller that receives the measurement signal and calculates a flux of target matter across the measurement surface. 38. The extreme ultraviolet light source of clause 37, wherein the regeneration tool comprises a cleaning tool positioned to interact with the measurement system, wherein the cleaning tool is configured to regenerate the measurement system by removing target matter that has been deposited on the measurement surface. 39. The extreme ultraviolet light source of clause 36, further comprising an optical collector that collects at least some of the emitted extreme ultraviolet light for use by an external lithography apparatus. 40. A metrology system for use in an extreme ultraviolet light source, the metrology system comprising: a metrology apparatus configured to measure a flux of target matter across a measurement surface within the vessel, the metrology apparatus comprising: a measurement system comprising the measurement surface configured to interact with the target matter, wherein the interaction between the target matter and the measurement surface produces a measurement signal; and a measurement controller configured to receive the measurement signal and calculate the flux of the target matter across the measurement surface based on the received measurement signal; and a regeneration tool coupled to the metrology apparatus and configured to regenerate the measurement system, wherein regeneration comprises: preventing the measurement surface from becoming saturated; and/or de-saturating the measurement surface if it has become saturated, wherein the regeneration tool comprises a cleaning tool positioned to interact with the measurement surface and to remove target matter that has been deposited on the measurement surface upon instruction from the measurement controller. 41. An apparatus comprising: a vessel; a means for delivering a target toward an interaction region in the vessel, the target comprising target matter that emits extreme ultraviolet light when in a plasma state; a metrology apparatus comprising: a means for measure a flux of target matter across a measurement surface within the vessel; and a means for regenerating the measurement surface, wherein the means for regenerating comprises: a means for preventing the measurement surface from becoming saturated; and/or a means for de-saturating the measurement surface if it has become saturated.

Claims

A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being capable of applying a pattern in a section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.