WO2023041306A1

WO2023041306A1 - Apparatus and method for actively heating a substrate in an euv light source

Info

Publication number: WO2023041306A1
Application number: PCT/EP2022/073709
Authority: WO
Inventors: Niels BRAAKSMA; Tianqi Li; Mehmet Altug YAVUZ
Original assignee: Asml Netherlands B.V.
Priority date: 2021-09-15
Filing date: 2022-08-25
Publication date: 2023-03-23
Also published as: TW202318112A; CN117999857A

Abstract

A protection apparatus (100) includes a buffer generator and a heating apparatus. The buffer generator is configured to interact a buffer with a surface of a substrate (130) that is positioned inside a chamber of an extreme ultraviolet (EUV) light source. The heating apparatus is in thermal communication with the substrate and is configured to increase a temperature of the substrate and the substrate surface.

Description

APPARATUS AND METHOD FOR ACTIVELY HEATING A SUBSTRATE IN AN EUV LIGHT SOURCE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 63/244,550, filed September 15, 2021, titled APPARATUS AND METHOD FOR ACTIVELY HEATING A SUBSTRATE IN AN EUV LIGHT SOURCE, which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

[0002] The disclosed subject matter relates to a protection apparatus and method for heating a substrate associated with a buffer fluid within a chamber of an extreme ultraviolet (EUV) light source.

BACKGROUND

[0003] 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.

[0004] 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 that can 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.

[0005] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, such as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC being formed. This pattern can be transferred onto a target portion (for example, including part of, one, or several dies) on a substrate (for example, a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (for example, resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Traditional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the target portions parallel or anti- parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0006] Extreme ultraviolet (EUV) light, for example, electromagnetic radiation having wavelengths of around 50 nanometers (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 or with a lithographic apparatus to produce extremely small features in substrates, for example, silicon wafers. Methods to produce EUV light include, but are not necessarily limited to, converting a material that has an element, for example, xenon (Xe), lithium (Li), terbium (Tb), gadolinium (Gd), or tin (Sn), with an emission line in the EUV range to a plasma state. For example, in one such method called laser produced plasma (LPP), the plasma can be produced by irradiating a target material, which is interchangeably referred to as fuel in the context of LPP sources, for example, in the form of a droplet, plate, tape, stream, or cluster of material, with an amplified light beam that can 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

[0007] In some general aspects, a protection apparatus includes a buffer generator and a heating apparatus. The protection apparatus can protect a buffer flow guide in an extreme ultraviolet (EUV) light source and provide for improved flow stability of a buffer gas within a chamber of the EUV light source. The buffer generator is configured to interact a buffer with a surface of a substrate (a substrate surface) that is positioned inside a chamber of the extreme ultraviolet (EUV) light source. The heating apparatus is in thermal communication with the substrate and is configured to increase a temperature of the substrate and the substrate surface.

[0008] Implementations can include one or more of the following features. For example, the heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate. The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

[0009] The substrate can be a buffer flow guide positioned adjacent to an optical element that interacts with light within the chamber of the EUV light source. The optical element can be an EUV collector mirror and the buffer flow guide can be positioned at an opening of the EUV collector mirror. The buffer flow guide can include a conically-shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror. The optical element can interact with light within the EUV light source chamber. The optical element can be a mirror configured to reflect EUV light or a window configured to pass EUV light. [0010] The protection apparatus can further include a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the substrate and the substrate surface. The protection apparatus can also include a temperature metrology device configured to measure a temperature of the substrate. The control apparatus can be in communication with the temperature metrology device such that it actively maintains the temperature of the substrate and the substrate surface by comparing the measured temperature to a target temperature. The temperature metrology device can include one or more of a thermocouple, an infrared camera, a thermometer, or a laser reading.

[0011] The buffer generator can include a buffer flow generator and the buffer can include a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across the substrate surface. The buffer fluid can be a gas that includes hydrogen. The buffer flow generator can pass the buffer fluid across the substrate surface by advectively transporting the buffer fluid across the substrate surface. The buffer flow generator can pass the buffer fluid across the substrate surface by reducing an amount of target material debris deposited on the substrate surface. And, the heating apparatus can be configured to maintain the effectiveness of the buffer flow generator in reducing the amount of target material debris that is deposited on the substrate surface.

[0012] The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates. The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value greater than 600 °C. The heating apparatus can include a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the substrate.

[0013] The protection apparatus can also include an insulating device associated with the substrate and configured to thermally insulate the substrate from other components within the EUV light source chamber.

[0014] In other general aspects, a protection apparatus includes: a buffer generator configured to interact a buffer with a surface of an optical element positioned inside a chamber of an extreme ultraviolet (EUV) light source and configured to interact with EUV light; a buffer guide positioned adjacent to the optical element and configured to guide the buffer relative to the optical element; and a heating apparatus in thermal communication with the buffer guide and configured to increase a temperature of the buffer guide.

[0015] Implementations can include one or more of the following features. For example, the heating apparatus can be configured to increase the temperature of the buffer guide to a value at which any temperature gradient at any location of the buffer guide remains below 10% of an average temperature of the buffer guide. The heating apparatus can be configured to increase the temperature of the buffer guide to a value that is greater than a melting point of a target material that travels through the EUV light source chamber. [0016] The optical element can be a collector mirror configured to collect EUV light. The buffer guide can include a conically-shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror.

[0017] The protection apparatus can further include a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the buffer guide. The protection apparatus can also include a temperature metrology device configured to measure a temperature of the buffer guide. The control apparatus can be in communication with the temperature metrology device such that it actively maintains the temperature of the buffer guide by comparing the measured temperature to a target temperature.

[0018] The buffer generator can include a buffer flow generator and the buffer can include a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across a surface of the buffer guide. The buffer fluid can be a gas that includes hydrogen. The heating apparatus can be configured to increase the temperature of the buffer guide to a value that is greater than a temperature at which at least some of the buffer fluid dissociates. The buffer flow generator can pass the buffer fluid across the surface of the optical element by advectively transporting the buffer fluid across the surface of the optical element. The buffer flow generator can pass the buffer fluid across the surface of the buffer guide by advectively transporting the buffer fluid across the surface of the buffer guide. The buffer guide can be made of aluminum, tungsten, or molybdenum.

[0019] The heating apparatus can be configured to increase the temperature of the buffer flow guide to a value greater than 600 °C. The heating apparatus can include a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the buffer guide. [0020] The protection apparatus can include an insulating device associated with the buffer guide and configured to thermally insulate the buffer guide from other components within the EUV light source chamber.

[0021] In other general aspects, a method is performed for protecting a substrate positioned inside a chamber of an extreme ultraviolet (EUV) light source. The method includes: passing a buffer fluid across a surface of the substrate; and actively increasing a temperature of the substrate and the substrate surface.

[0022] Implementations can include one or more of the following features. For example, actively increasing the temperature of the substrate and the substrate surface can include increasing the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate. Actively increasing the temperature of the substrate and the substrate surface can include increasing the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber. Actively increasing the temperature of the substrate and the substrate surface can include increasing the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

[0023] The buffer fluid can be passed across the substrate surface by advectively transporting the buffer fluid across the substrate surface. The buffer fluid can be passed across the substrate surface by reducing an amount of target material deposited on the substrate surface; and the method can also include maintaining the effectiveness of the buffer flow generator in reducing the amount of target material that is deposited on the substrate surface.

[0024] The method can further include thermally insulating the substrate from other components within the EUV light source chamber.

[0025] The method can also include actively maintaining the temperature of the substrate and the substrate surface to a target temperature or a target temperature function. The method can further include measuring a temperature of the substrate, wherein actively maintaining the temperature of the substrate and the substrate surface comprises comparing the measured temperature to the target temperature or the target temperature function.

DRAWING DESCRIPTION

[0026] Fig. 1A is a schematic diagram of a protection apparatus arranged and configured relative to a substrate positioned within a chamber of an extreme ultraviolet (EUV) light source;

[0027] Fig. IB is a schematic diagram of the protection apparatus of Fig. 1 A, including a buffer flow generator that is configured to interact a buffer with a surface of the substrate and a heating apparatus in thermal communication with the substrate and configured to increase a default nominal temperature of the substrate;

[0028] Fig. 2 is a schematic diagram of the protection apparatus of Fig. IB, including a control apparatus in communication with the heating apparatus and configured to adjust the heating apparatus;

[0029] Fig. 3 A is a schematic diagram of an implementation of the protection apparatus of Figs. 1 A and IB that is arranged relative to a buffer flow guide, which is an implementation of the substrate, the buffer flow guide being positioned adjacent an EUV collector mirror;

[0030] Fig. 3B is a perspective view of the protection apparatus, the buffer flow guide, and the EUV collector mirror of Fig. 3A;

[0031] Fig. 3C is a plan view of the protection apparatus, the buffer flow guide, and the EUV collector mirror of Fig. 3A;

[0032] Figs. 4A and 4B are rear and front perspective views of the EUV collector mirror of Figs. 3A- 3C;

[0033] Fig. 4C is a side cross-sectional view of the EUV collector mirror of Figs. 4A and 4B;

[0034] Fig. 4D is a plan view of the EUV collector mirror of Figs. 4 A and 4B;

[0035] Fig. 5A is a perspective view of the buffer flow guide of Figs. 3A-3C; [0036] Fig. 5B is a side cross-sectional view of the buffer flow guide of Fig. 5A;

[0037] Figs. 6A-6D are side cross-sectional views of the buffer flow guide of Figs. 5A and 5B, showing different implementations of the heating apparatus;

[0038] Fig. 7 is a schematic diagram of a protection apparatus in an implementation of an EUV light source;

[0039] Fig. 8 is a side cross-sectional view of the EUV collector mirror, the buffer flow guide, the protection apparatus of Fig. 3A and an insulating device placed between the buffer flow guide and the EUV collector mirror to prevent the EUV collector mirror from being heated when the buffer flow guide is heated;

[0040] Fig. 9 is a flow chart of a procedure performed by the protection apparatus of Figs. 1A, IB, 2, 3A-3C, and 7;

[0041] Fig. 10 is a schematic diagram of an EUV light source that supplies EUV light to a photolithography exposure apparatus, the protection apparatus being implemented within the EUV light source; and

[0042] Fig. 11 is a schematic diagram of an implementation of the photolithography exposure apparatus of Fig. 10 in more detail, including the EUV light source, an illumination system IL, and a projection system PS.

DESCRIPTION

[0043] Referring to Figs. 1 A and IB, a protection apparatus 100 is arranged and configured relative to a substrate 130 positioned within a chamber 140 defined by a vessel 145 of an extreme ultraviolet (EUV) light source. The protection apparatus 100 is configured to protect the substrate 130 from debris that is produced during the production of EUV light, as discussed below. An example of an EUV light source is shown in Fig. 7. The EUV light source includes a stream 150 of targets 151 directed toward an interacting region 141 within the chamber 140. Each target 151 includes target matter that emits EUV light 152 when it is converted into a plasma 153. In some implementations, the target 151 is converted to the plasma 153 by irradiating the target 151 with a pulse of an amplified light beam (not shown in Fig. 1 A, but discussed below with reference to Fig. 7). Some of the light from the amplified light beam can be scattered from the target 151 as scattered light radiation 154. In the process of creating the plasma 153 and EUV light 152, unwanted debris 110 is produced. The debris 110 can be quite hot because of the amount of energy carried in each amplified light beam pulse.

[0044] The debris 110 is mostly produced from ionized target matter and also at least partially produced from leftover or remaining target matter in the chamber 140. The leftover or remaining target matter can be target matter that is not converted into the plasma 153 in the interacting region 141 and/or the leftover or remaining target matter can be produced from plasma 153 that reverts back into target matter (without producing the EUV light 152). The process of generating the EUV light 152 relies on converting the target matter in many targets 151 into plasma 153, and thus a large amount of ionized, or remaining target matter can be produced in the process. Different phases of the target matter tend to deposit on surfaces of various objects inside the chamber 140. The ionized target matter is highly energetic and quickly travels through the chamber 140. As the ionized debris interacts with buffer gas (discussed below), this energy is transferred into the buffer gas, and after sufficient interaction, the ionized target matter becomes hot debris 110, which is carried through the chamber 140 by the buffer gas. And, through diffusive transport, the hot debris 110 can coat various surfaces of objects such as walls, optical elements, and components (and the substrate 130). The debris 110 that forms on the substrate surface 131 can include vapor residue, ions, particles, and/or clusters of matter formed from the target matter.

[0045] The debris 110 can severely impair the performance of the EUV light source by blocking the EUV light 152 or by contaminating the objects within the chamber 140. The debris 110 can therefore form a coating on the substrate surface 131 that effectively blocks the surface 131. For example, if the surface 131 is an optical surface that is meant to interact with light in the chamber 140, its efficiency will drop as it becomes coated with debris 110. As another example, if the surface 131 is a non-optical surface (that does not interact with light), then the debris 110 coating the surface 131 can cause other serious problems for other objects nearby within the chamber 141. The debris 110 can cause the surface 131 and the substrate 130 to heat up, which can lead to the debris 110 being ejected from the surface 131 and onto other objects within the chamber 140. The debris 110 can cause other problems that lead to a reduction in the production of EUV light 152. For example, the debris 110 can be flaked off, dropped off, spit off, or dripped off the surface 131. In summary, the presence of such debris 110 can reduce the performance of the surfaces within the chamber 140 and reduce the overall efficiency of the EUV light source and production of EUV light 152. As discussed below, if the target 151 includes molten metal of tin, then tin particles, clusters of tin, tin residue, or tin ions can accumulate on (or coat) one or more structures within the chamber 140.

[0046] During operation of the EUV light source, the substrate 130 and a surface 131 of the substrate that faces the interacting region 141, is potentially exposed to EUV light 152, scattered light radiation 154, and debris 110. As shown in Fig. IB, in order to protect the substrate 130 and the substrate surface 131 from the debris 110 that is produced during the production of EUV light 152, the protection apparatus 100 includes a buffer generator 115 that is configured to interact a buffer 105 (which can be a gas) with the substrate surface 131. In some implementations, as discussed herein, the buffer generator 115 is a buffer flow generator and the buffer 105 is a buffer fluid such as a buffer gas. In these implementations the buffer flow generator 115 is configured to flow the buffer fluid 105 (which can be in the form of one or more streams of buffer fluid) across the substrate surface 131. The buffer fluid 105 can be any suitable gas such as molecular hydrogen. The buffer fluid 105 performs two tasks. First, the components (such as molecules or atoms) of the buffer fluid 105 repeatedly collide with debris 110 causing the debris to decelerate and become thermalized (that is, they reach thermal equilibrium). Second, the buffer fluid 105 pushes or transports the debris 110 (which can be thermalized) away from the substrate surface 131 using advective transport or advection. Thus, it is the bulk motion of the buffer fluid 105 that transports this debris 110 away from the substrate surface 131.

[0047] The advective transport of the debris 110 away from the substrate surface 131 should happen sufficiently rapidly to prevent the debris 110 from depositing on the substrate surface 131 by way of diffusive transport. Diffusive transport is the movement of the debris 110 from a region of higher concentration, that is, closer to the target 151 in the interacting region 141, to a region of lower concentration, that is, away from the interacting region 141. This diffusive transport is therefore driven by a gradient in concentration of the debris 110. Additionally, energy from the scattered light radiation 154, the EUV light 152, and even the debris 110 can heat up the buffer fluid 105 and the substrate surface 131, and this causes the flow or flows of the buffer fluid 105 to reorder and potentially impair efficiency in the advective transport. The unwanted reordering of the flow field of the buffer fluid 105 can introduce effects such as flow recirculation that adversely leads to increased contamination on the substrate surface 131 and other surfaces adjacent to or near to the substrate surface 131 (not shown in Figs. 1A and IB). And, transient effects due to this increased contamination can even be introduced at the interacting region 141, thus causing a drop in efficiency in production of EUV light 152. On top of these issues, the substrate 130 can be positioned at a location within the chamber 140 where it is a challenge to adequately cool the substrate 130 to potentially offset the heating up of the substrate surface 131.

[0048] The reordering of the advective transport of the debris 110 away from the substrate surface 131 occurs due to the process of generating the plasma 153. Plasma 153 is produced or generated in a periodic manner since each pulse of the amplified light beam interacts with one target 151 in the interacting region 141 for a finite period of time. Thus, the heat loads applied to the substrate surface 131 from the debris 110, the EUV light 152, and the scattered light radiation 154 fluctuate from close to none to some maximum power (for example, shortly after the target 151 has interacted with a pulse of the amplified light beam within the interacting region 141). The temperature therefore fluctuates quite a bit at the substrate surface 131. For example, the temperature can deviate from a nominal temperature Td by up to +50 °C. Thus, if the nominal temperature Td is 20 °C, then the temperature can fluctuate from 20 °C to 70 °C. This means that the temperature fluctuates from the nominal temperature Td to a value that is 250 % greater than the nominal temperature Td.

[0049] To this end, the protection apparatus 100 is further designed to protect the substrate 130 from such rapid degradation due to temperature fluctuations in the presence of the buffer fluid 105 that is passed across the substrate surface 131. Specifically, the protection apparatus 100 includes a heating apparatus 120 in thermal communication with the substrate 130. The heating apparatus 120 is configured to increase the nominal temperature Td of the substrate 130 and the substrate surface 131 to a new nominal temperature Td’. The nominal temperature is increased to a level or value at which the temperature fluctuations noted above are less significant or have a much smaller impact on operation of the EUV light source and production of EUV light 152. Using the example above, the heating apparatus 120 can raise the temperature to a new nominal temperature Td’ of 800 °C. In this example, the temperature of the substrate surface 131 fluctuates between 800 °C and 850 °C.

Therefore, the temperature fluctuates from the new nominal temperature Td’ to a value that is 6.25% greater than the new nominal temperature Td’. Thus, by increasing the nominal temperature Td to a new nominal temperature Td’, the temperature fluctuations are neutralized because the size of the fluctuations is much smaller than the new and increased nominal temperature Td’.

[0050] There are several benefits associated with actively heating the substrate 130 and the substrate surface 131. First, active heating technology within the heating apparatus 120 is generally easier to engineer and implement than active cooling technology. Second, as noted above, the relative temperature fluctuation caused by the radiative heating is significantly reduced, thus allowing for engineering of the buffer fluid 105 and the buffer flow generator 115 that accounts for the prescribed radiative heating.

[0051] Third, depending on the material used in the buffer fluid 105, if the new nominal temperature Td’ is greater than the dissociation temperature of the buffer fluid 105, then radicals can be produced from the buffer fluid 105, and these radicals can enhance the cleaning process on the substrate surface 131. For example, if the buffer fluid 105 includes hydrogen gas or if hydrogen gas is present within the chamber 140, the target matter includes tin (Sn), and the new nominal temperature Td’ is greater than the dissociation temperature of hydrogen, then generated free radicals of molecular hydrogen H2 can bond with tin particles, as discussed next. A simple free radical of hydrogen is a single hydrogen element with an unpaired valence electron (H*). The chemical process that transpires due to the dissociation of hydrogen is represented by the following chemical formula:

H2 (g) <-> 2 H* (g), where g indicates that the chemical is in the gaseous state.

The generated free radicals H* of the hydrogen bond with the tin particles in the debris 110 and form a new chemical, which is called tin hydride (SnH i), which is then released from the substrate surface 131. This chemical process is represented by the following chemical formula:

4 H* (g) + Sn (s) <-> SnH i (g), where s indicates that the chemical is in the solid state.

Another chemical process produces SnH i (g) from 3 H* (g) and 1 Sn (s) and the SnH i (g) can further combine with 1 H* (g) to produce SnH i (g). There are many other free radicals and ions that can be formed from molecular hydrogen H2 due to the heating from the heating apparatus 120. For example, Deuteron H2⁺ and Triton H3⁺ can also react with tin and form gaseous tin hydrides, although they may not be as dominant as tin hydride SnH4.

[0052] Fourth, a sufficiently hot substrate surface 131 can prevent debris 110 from depositing on the substrate surface 131 (independent of the advective flow of buffer fluid 105), thereby reducing the risk of contamination of nearby objects through cleaning and redeposition processes. [0053] The protection apparatus 100 is configured to operate (that is, prevent debris 110 from sticking to the substrate surface 131) by heating the substrate 130 and substrate surface 131 while ensuring that any elements adjacent to or near the substrate are minimally affected by the temperature changes that happen at the substrate 130 and substrate surface 131.

[0054] The protection apparatus 100 can be configured to operate (that is remove the debris 110 from the substrate surface 130) even though it is exposed to molecular hydrogen, which is present in the chamber 140. Moreover, the protection apparatus 100 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 protection apparatus 100 to operate and/or perform its functions. The protection apparatus 100 is designed to prevent the debris 110 from depositing on the substrate surface 131 without requiring the removal of the substrate 130 from the chamber 140. Secondarily, the protection apparatus 100 also enhances the naturally occurring removal process whereby the free radicals of hydrogen H* (g) chemically react with or bond with the tin particles in the debris 110 and form the new chemical tin hydride (SnH i). The operation of the substrate 130 within the chamber 140 that contributes to the production of the EUV light 152 and/or maintain the operation of the chamber 140 need not be halted in order to clean the substrate surface 131. Thus, the operation of the EUV light source does not need to be halted or shut down in order for the protection apparatus 100 to clean the substrate surface 131.

[0055] In some implementations, the chamber 140 is maintained at a vacuum, that is, at a pressure below atmospheric pressure. For example, the chamber 140 can be at a low pressure of between about 0.5 Torr (T) to about 1.5 T (for example, at 1 T). A particular pressure may be suitable for the most efficient generation of EUV light 152. The protection apparatus 100 is configured to operate in the environment of the chamber 140, and thus, if the chamber 140 is maintained at 1 T then the protection apparatus 100 is able to operate at that pressure. In other implementations, the chamber 140 is maintained at atmospheric pressure.

[0056] The heating apparatus 120 can be configured to increase the temperature of the substrate 130 and the substrate surface 131 to a value (new nominal temperature Td’) at which any temperature gradient at any location of the substrate surface 131 remains below 10% of an average temperature of the substrate 130. The heating apparatus 120 can be configured to increase the temperature of the substrate 130 and the substrate surface 131 to a value (new nominal temperature Td’) that is greater than a melting point of the target matter that forms the target 151. As discussed above, the heating apparatus 120 can be configured to increase the temperature of the substrate 130 and the substrate surface 131 to a value (new nominal temperature Td’) that is greater than a temperature at which at least some of the buffer fluid 105 dissociates. In view of the above, in some implementations, the heating apparatus 120 is configured to increase the temperature of the substrate 130 and the substrate surface 131 to a value (new nominal temperature Td’) that is greater than 600 °C.

[0057] As discussed, the target 151 is made of target matter or material. The target 151 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 form of target material, or solid particles contained within a portion of a liquid stream. The target 151 can be any material that emits EUV light 152 when in the plasma state. That is, the target 151 is a substance that, when in the plasma state, has an emission line in the EUV range. For example, the target 151 can include water, tin, lithium, and/or xenon. The target 151 can be a target mixture that includes the target matter as well as impurities such as non-target particles (which do not contribute to the production of the EUV light 152). As an example, the target 151 can be the element tin, which can be used as pure tin (Sn); as a tin compound such as SnBr4, SnBr2, SnH i; as a tin alloy, such as tin-gallium alloys, tin-indium alloys, tin-indium- gallium alloys; or any combination of these alloys. In the situation in which there are no impurities, the target 151 includes only the target matter.

[0058] In some implementations, as discussed below with reference to Figs. 3A-3C, the substrate 130 is a buffer flow guide that is positioned adjacent to an optical element that interacts with light (such as the EUV light 152 or other light) within the chamber 140. In these implementations, the optical element can be a collector mirror for capturing and redirecting the EUV light 152. The buffer flow guide can be positioned at an opening of the EUV collector mirror. The optical element can be a mirror configured to reflect the EUV light 152, but not necessarily the EUV collector mirror. The optical element can be a window configured to pass EUV light 152.

[0059] In some implementations, the heating apparatus 120 includes a resistive heating element in thermal communication with the substrate 130. The resistive heating element can be a resistive wire, a ceramic or a semiconductor, a thick film heater, a conducting silicone rubber material, or a composite material. In other implementations, the heating apparatus 120 includes a light source that directs a light beam toward the substrate 130 to heat the substrate 130. In still other implementations, the heating device 120 includes a thermoelectric device in thermal communication with the substrate 130. In other implementations, the heating apparatus 120 includes a friction device or a magnetic device. [0060] In sum, the heating apparatus 120 maintains the effectiveness of the buffer flow generator 115 in reducing the amount of debris 110 that is deposited on the substrate surface 131, despite the temperature fluctuations noted above.

[0061] Referring to Fig. 2, an implementation 200 of the protection apparatus 100 includes a control apparatus 225 that is in communication with the heating apparatus 120. The control apparatus 225 is configured to adjust the heating apparatus 120 to actively maintain the new nominal temperature Td’ of the substrate 130 and/or the substrate surface 131. To this end, the protection apparatus 200 also includes a temperature metrology device 226 that communicates with the control apparatus 225. The temperature metrology device 226 is configured to measure a temperature of the substrate 130 and/or the substrate surface 131 and to provide the measured temperature to the control apparatus 225. The temperature metrology device 226 can, for example, include a thermocouple device, an infrared camera, a thermometer, or a laser reading from a metrology port in the EUV chamber 140. In this way, the control apparatus 225 can actively maintain the new nominal temperature Td’ of the substrate 130 and/or the substrate surface 131 by, for example, comparing the measured temperature (from the temperature metrology device 226) to a target temperature Tt.

[0062] In some implementations, the control apparatus 225 includes memory that is accessible to one or more of the modules within the control apparatus. The memory is configured to store information output from each of these modules or information received from the temperature metrology device 226 for various use by other modules during operation of the control apparatus 225. The memory can be read-only memory and/or random-access memory and can provide a storage device suitable for tangibly embodying computer program instructions and data. The control apparatus 225 can include one or more input and/or output devices (such as a keyboard, touch-enabled devices, audio input devices as input and audio or video for output), and one or more processors. The control apparatus 225 can include a module for inputting information from the temperature metrology device 226, a module configured for processing the information from the temperature metrology device 226, and a module for outputting instructions to the heating apparatus 120. The control apparatus 225 can include other modules not described.

[0063] Communication between any of the modules and the memory within the control apparatus 225 or between the control apparatus 225 and the heating apparatus 120 and the temperature metrology device 226 can be by a direct or physical connection (for example, wired) or by a wireless connection so that information can be freely passed between the modules of the control apparatus 225, and between the control apparatus 225 and the heating apparatus and the temperature metrology device 226. Although the control apparatus 225 is represented as a box in which all of the components appear to be co-located, it is possible for the control apparatus 225 to be made up of components or modules that are physically remote from each other. Each of the modules can be a dedicated processing system for receiving data and analyzing data, or one or more of the modules can be combined into a single processing system. Each of the modules can include or have access to one or more programmable processors and can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. The modules can be implemented in any of digital electronic circuitry, computer hardware, firmware, or software.

[0064] Referring to Figs. 3A-3C, an implementation 300 of the protection apparatus 100 is arranged relative to a buffer flow guide 330, which is an implementation of the substrate 130. The buffer flow guide 330 is positioned adjacent an EUV collector mirror 332. The protection apparatus 300 includes a buffer generator 315 that is configured to interact a buffer 305 (which can be a gas) with a surface 331 of the buffer flow guide 330. The buffer generator 315 is also configured to interact the buffer 305 with an optical surface 333 of the EUV collector mirror 332. As mentioned above, the protection apparatus 300 is configured to operate (that is, prevent debris 310 from sticking to the surface 331) by heating the buffer flow guide 330 and the surface 331 while ensuring that the EUV collector mirror 332, which is adjacent to and near the buffer flow guide 330, is minimally affected by the temperature changes that happen at the buffer flow guide 330 and its surface 331. [0065] Referring to Figs. 4A-4D, the EUV collector mirror 332 includes the optical surface 333 that faces the interacting region 341. The EUV collector mirror 332 includes an opening 334; once installed in the chamber 140 and during EUV light source operation, the pulses of the amplified light beam are directed (along the -Y direction) through the opening 334 and toward the interacting region 341. The EUV collector mirror 332 can be, for example, an ellipsoidal mirror that has a primary focus at the interacting region 341 and a secondary or intermediate focus IF (shown in Fig. 7) at a secondary focal plane for use by an output device. 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 optical surface 333, and it is formed from a portion of an ellipse. In this way, the EUV light 152 that is produced in the interacting region 341 and that reflects from the optical surface 333 of the EUV collector mirror 332 is directed to the intermediate focus of the EUV collector mirror 332, as shown in Fig. 7. The plan view of the EUV collector mirror 333 (Fig. 4D) shows that the edge of the reflective optical surface 333 forms a circular shape.

[0066] As discussed below, in order to produce the EUV light 152, the pulses of the amplified light beam are directed along the -Y direction through the opening 334 of the EUV collector mirror 332 and toward the interacting region 341. The EUV collector mirror 332 is placed at a distance from the interacting region 341 to enable its primary focus to overlap the interacting region 341. Because of this, the reflective optical surface 333 of the EUV collector mirror 332 is close to the debris 110, and the optical surface 333 can become exposed to the debris 110. The buffer flow guide 330 is positioned within this opening 334 to provide some level of protection to the optical surface 333 and guide the flow of buffer fluid 305 from the center of the EUV collector mirror 332 (the opening 334) toward the primary focus (and the interacting region 341) and away from the optical surface 333. The buffer flow guide 330 is a relatively thin-walled piece of hardware that sticks out from the optical surface 333 toward the interacting region 341 along the -Y direction. Because of this, the buffer flow guide 330 can be challenging to adequately cool. On the other hand, the buffer flow guide 330 and its surface 331 become heated due to exposure to scattered light radiation 154, EUV light 152, and any other radiation generated when the plasma 353 is formed. This heating has a significant impact on the flow field in the vicinity of the buffer flow guide 330, and introduces effects such as flow recirculation that adversely can lead to increased contamination of debris 310 on the optical surface 333. Several factors relating to this heating introduce the transient effects noted above, and this complicates the design. First, the short distance (along the -Y direction) between the buffer flow guide 330 and the primary focus of the EUV collector mirror 332 mean that the transient effects are more likely to occur (since the buffer flow guide 330 and its surface 331 are more likely to become heated). Second, the nonsteady operation (in “bursts”) at which the EUV source operates for die exposure on the wafer (including dose control) can cause fluctuations in the heat load collector mirror 332 is exposed to. Thus, as discussed above, the protection apparatus 300 includes the heating apparatus 320 in thermal communication with the buffer flow guide 330, and configured to increase the temperature of the buffer flow guide 330 as well as the surface 331. The temperature of the buffer flow guide 330 and its surface 331 can be increased to a value (new nominal temperature Td’) at which any temperature gradient at any location of the surface 331 remains below 10% of an average temperature of the buffer flow guide 330. The temperature of the buffer flow guide 330 and its substrate surface 331 can be increased to a value (new nominal temperature Td’) that is greater than a melting point of the target matter (which can include tin) that forms the target 351. As discussed above, the temperature of the buffer flow guide 330 and its surface 331 can be increased to a value (new nominal temperature Td’) that is greater than a temperature at which at least some of the buffer fluid 305 dissociates. In view of these guides, in some implementations, the heating apparatus 320 is configured to increase the temperature of the buffer flow guide 330 and its surface 331 to a value (new nominal temperature Td’) that is greater than 250 °C, greater than 600 °C, greater than 800 °C, or greater than 1200 °C. [0067] Referring to Figs. 5A and 5B, the buffer flow guide 330 is a conically-shaped piece of solid material. The buffer flow guide 330 is made of a material that is non-chemically reactive to the target matter that forms the targets 351 and is also non-chemically reactive to the buffer fluid 305. In some implementations, such as if the target material includes tin and the buffer fluid 305 includes hydrogen, the buffer flow guide 330 is made of aluminum, tungsten, or molybdenum. The buffer flow guide 330 includes a central opening 335, and once the buffer flow guide 330 is installed in the opening 334 of the EUV collector mirror 332, the pulses of the amplified light beam (Fig. 7) can pass through the opening 335. The buffer flow guide 330 includes an outer conically-shaped wall 336 that faces the EUV collector mirror 332 when installed. The buffer flow guide 330 includes an annular rim 337 that faces the interacting region 341 and extends out toward the optical surface 333 when the buffer flow guide 330 is positioned within the opening 334 of the EUV collector mirror 332.

[0068] As discussed above, the heating apparatus 320 can be any suitable apparatus configured to heat up the substrate 130 and also the substrate surface 131. The heating apparatus 320 should have a geometry and placement that does not obstruct operation of the components of the EUV light source within the chamber 140. Thus, for example, the heating apparatus 320 should be remote from the opening 335 of the buffer flow guide 330 so as not to obstruct the pulses of the amplified light beam traveling along the -Y direction toward the interacting region 341.

[0069] Referring to Figs. 6A-6D, different implementations 620A, 620B, 620C, 620D of the heating apparatus 320 are shown. For example, the heating apparatus 620A is embedded within the bulk material of the buffer flow guide 330. Such a heating apparatus 620A can be a resistive heating element such as a resistive wire, a ceramic or a semiconductor material, a thick film heater, a conducting silicone rubber material, or a composite material, or a thermoelectric device. The heating apparatuses 620B and 620C are fixed to or formed around the outer conically-shaped wall 336 of the buffer flow guide 330. These heating apparatuses 620B, 620C should have a small enough profile that they do not interfere with the flow of the buffer fluid 305 between the buffer flow guide 330 and the EUV collector mirror 332. The heating apparatuses 620B and 620C can be a resistive heating element such as a resistive wire, a ceramic or a semiconductor material, a thick film heater, a conducting silicone rubber material, or a composite material, or a thermoelectric device. The heating apparatus 620D includes a light source 620D-1 that directs a light beam 620D-2 toward the buffer flow guide 330 to heat it to a new nominal temperature Td’ .

[0070] Referring to Fig. 7, a protection apparatus 700 is shown in an implementation of an EUV light source 760. In this implementation, the protection apparatus 700 is shown adjacent a generic substrate 730 that is near the EUV collector mirror 332 to thereby protect the substrate surface 731 from debris 710. While only one protection apparatus 700 is explicitly shown in the EUV chamber 740 of Fig. 7, it is possible to configure a plurality of protection apparatuses 700 throughout the EUV chamber 740. Other possible and exemplary (but not limiting) locations for the protection apparatus 700 are marked by the cross icons 768 shown in Fig. 7.

[0071] Other components of the EUV light source 760 are discussed next. The EUV light source 760 includes a target delivery system 761 that directs the stream 750 of targets 751 toward the interacting region 741 in the chamber 740. The interacting region 741 receives an amplified light beam 762, which can be a train of amplified light pulses. As discussed above, the target 751 includes matter that emits EUV light 752 when it is converted into the light-emitting plasma 753. An interaction between the matter within the target 751 and a pulse of the amplified light beam 762 at the interacting region 741 converts at least some of the matter in the target 751 into the light-emitting plasma 753, and that light-emitting plasma 753 emits the EUV light 752. The light-emitting plasma 753 has an element with an emission line in the EUV wavelength range. The light-emitting plasma 753 has certain characteristics that depend on the composition of the target 751. These characteristics include the wavelength of the EUV light 752 produced by the light-emitting plasma 753.

[0072] The light-emitting plasma 753 can be considered to be a highly ionized plasma with electron temperatures of several tens of electron volts (eV). Higher energy EUV light 752 can be generated with fuel materials (the target 751) other than tin such as, for example, terbium (Tb) and gadolinium (Gd). The energetic radiation (the EUV light 752) generated during de-excitation and recombination of the ions is emitted from the light- emitting plasma 753, and at least a portion of this EUV light 752 is collected by the EUV collector mirror 332. The optical surface 333 interacts with at least a portion of the emitted EUV light 752. The optical surface 333 is a reflective surface that is positioned to receive the portion of the emitting EUV light 752 and to direct this collected EUV light 752 for use outside the EUV light source 760. The reflective optical surface 333 directs the collected EUV light 752 to a secondary focal plane, where the EUV light 752 is then captured for use by a tool 763 (such as a photolithography exposure apparatus) outside the EUV light source 760. Exemplary lithography apparatuses are discussed with reference to Figs. 10 and 11.

[0073] The reflective optical surface 333 is configured to reflect light in the EUV wavelength range and can absorb, diffuse, or block light outside the EUV wavelength range. The EUV collector mirror 332 also includes the opening (or aperture) 334 that permits the pulses of the amplified light beam 762 to pass through the EUV collector mirror 332 toward the interacting region 741.

[0074] The EUV light source 760 includes an optical system 765 that produces the pulses of the amplified light beam 762 due to a population inversion within a gain medium or mediums of the optical system 765. The optical system 765 can include at least one 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 interacting region 741. The optical source within the optical system 765 includes one or more optical amplifiers, lasers, and/or lamps for providing one or more main pulses that form the amplified light beam 762. And, in some cases, the optical system 765 can also provide one or more pre-pulses that form a precursor amplifier light beam (not shown) that interacts with the target 751 prior to the interaction between the amplified light beam 762 and the target 751. Upon interaction with the pre-pulse, the target 751 forms a modified target. An example of a modified target (target 351) is shown in Fig. 3A. 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 765 produces the amplified light beam 762 due to population inversion in the gain media of the amplifiers even if there is no laser cavity. Moreover, the optical system 765 can produce the amplified light beam 762 that is a coherent laser beam if there is a laser cavity to provide enough feedback to the optical system 765. The term “amplified light beam” therefore encompasses one or more of: light from the optical system 765 that is merely amplified but not necessarily a coherent laser oscillation and light from the optical system 765 that is amplified and also a coherent laser oscillation. [0075] The optical amplifiers used in the optical system 765 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 gain greater than or equal to 100. Suitable amplifiers and lasers for use in the optical system 765 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.

[0076] The EUV light source 760 also includes a control apparatus 766 in communication with one or more controllable components or systems of the EUV light source 760. The control apparatus 766 is in communication with the optical system 765 and the target delivery system 761. Moreover, the control apparatus 766 can include or can communicate with the control apparatus 225 of the protection apparatus 200. The target delivery system 761 can be operable in response to signals from one or more modules within the control apparatus 766. For example, the control apparatus 766 can send a signal to the target delivery system 761 to modify the release point of the targets 751 to correct for errors in the targets 751 arriving at the interacting region 741. The optical system 765 can be operable in response to signal from one or more modules within the control apparatus 766. One or more of the modules within the control apparatus 766 can be co-located with each other. Or, one or more of the modules within the control apparatus 766 can be separated from each other physically. For example, the module that controls the target delivery system 761 can be co-located with the target delivery system 761 while a module that controls the optical system 765 can be co-located with the optical system 765.

[0077] The EUV system 760 can also include a removal or exhaust apparatus 767 configured to remove the debris 710 from the EUV chamber 740 as well as other gaseous byproducts that can form within the EUV chamber 740. The removal apparatus 767 can be a pump that removes the debris 710 from the EUV chamber 740. The removal apparatus 767 can include a gas port that is in fluid communication with the EUV chamber 740 such that the debris 710 is transported from the region near the substrate 130 toward and through the gas port and out of the EUV chamber 740.

[0078] Other components of the EUV light source 760 that are not shown, include, for example, detectors for measuring parameters associated with the produced EUV light 752. Detectors can be used to measure energy or energy distribution of the amplified light beam 762. Detectors can be used to measure an angular distribution of the intensity of the EUV light 752. Detectors can measure errors in the timing or focus of the pulses of the amplified light beam 762. Output from these detectors is provided to the control apparatus 766, which includes modules that analyze the output and adjust aspects of other components of the EUV light source 760 such as the optical system 765 and the target delivery system 761.

[0079] In summary, an amplified light beam 762 is produced by the optical system 765 and directed along a beam path to irradiate the target 751 at the interacting region 741 to convert material or matter within the target 751 into the plasma 753 that emits light in the EUV wavelength range. The amplified light beam 762 operates at a particular wavelength (the source wavelength) that is determined based on the design and properties of the optical system 765 as well as the properties of the target 751.

[0080] As mentioned above, other possible and exemplary (but not limiting) locations for the protection apparatus 700 are marked by the cross icons 768 shown in Fig. 7. For example, the protection apparatus 700 can be positioned next to any element that includes a surface that could potentially interact with the target matter or debris 710. Thus, one or more protection apparatuses 700 can be positioned next to the EUV collector mirror 332; next to the buffer flow guide 330 (as discussed above with reference to Figs. 3A-6D); next to any wall within the EUV chamber 740; or next to the target delivery system 761 or along the path between the target delivery system 761 and the interacting region 741. One or more protection apparatuses 700 can be positioned near the removal apparatus 767 to clean a surface of the port of the removal apparatus 767.

[0081] In some implementations, the protection apparatus 100 also includes an insulating device associated with the substrate 130 and configured to thermally insulate the substrate 130 from other components within the EUV chamber 140. For example, as shown in Fig. 8, an insulating device 801 can be placed between the buffer flow guide 330 and the EUV collector mirror 332 to prevent the EUV collector mirror 332 from being heated when the buffer flow guide 330 is heated.

[0082] Referring to Fig. 9, a procedure 970 is performed by the protection apparatus 100 (which can correspond to the protection apparatus 200 or the protection apparatus 300). A buffer fluid 105 is passed across the surface 131 of the substrate 130 (971) and a nominal temperature Td’ of the substrate 130 and the substrate surface 131 is actively increased (972). For example, the buffer flow generator 115 can direct and pass the buffer fluid 105 across the substrate surface 131 (971).

Moreover, the heating apparatus 120 can actively increase the temperature of the substrate 130 and the substrate surface 131 (972). This increase in temperature (due to the operation of the heating apparatus 120) occurs while the buffer fluid 105 is being passed across the substrate surface 131. [0083] The heating apparatus 120 can actively increase the temperature of the substrate 130 and the substrate surface 131 (972) by, for example, increasing the temperature of the substrate 130 and the substrate surface 131 to a value at which any temperature gradient at any location of the substrate surface 131 remains below 10% of an average temperature of the substrate 130. The heating apparatus 120 can actively increase the temperature of the substrate 130 and the substrate surface 131 (972) by, for example, increasing the temperature of the substrate 130 and the substrate surface 131 to a value that is greater than a melting point of the target matter within the target 151. The heating apparatus 120 can actively increase the temperature of the substrate 130 and the substrate surface 131 (972) by, for example, increasing the temperature of the substrate 130 and the substrate surface 131 to a value that is greater than a temperature at which at least some of the buffer fluid 105 dissociates.

[0084] The buffer fluid 105 can be passed across the substrate surface 131 by advectively transporting the buffer fluid 105 across the substrate surface 131.

[0085] The amount of debris 110 that is deposited on the substrate surface 131 is reduced by passing the buffer fluid 105 across the substrate surface 131. Moreover, the effectiveness of the buffer flow generator 115 in reducing the amount of debris 110 that is deposited on the substrate surface is maintained because the heating apparatus 120 actively increases the temperature of the substrate 130 and the substrate surface 131 (972), as discussed above.

[0086] The procedure 970 can also include a sub-process 970a that performs active temperature control of the nominal temperature Td’ of the substrate 130 and the substrate surface 131, as discussed next. The sub-process 970a can be performed by the control apparatus 225 and the temperature metrology device 226 of the protection apparatus 200. The sub-process 970a includes measuring a temperature of the substrate 130 (and/or the substrate surface 131) (973). For example, the temperature metrology device 226 measures the temperature of the substrate 130 and/or the substrate surface 131, and provides the measured temperature to the control apparatus 225. Next, the measured temperature is compared with a target temperature (or target temperature range) and it is determined whether the measured temperature is at the target temperature or is within range of the target temperature (974). For example, the control apparatus 225 can compare the measured temperature (from the temperature metrology device 226) to a target temperature Tt. If the measured temperature (from the temperature metrology device 226) is within range of the target temperature (974), then protection apparatus 200 maintains the temperature of the substrate 130 and/or substrate surface 131

(975). For example, the control apparatus 225 can instruct the heating apparatus 120 to stop heating the substrate 130 until the measured temperature is outside the range of the target temperature (974). At that point, when it is determined that the measured temperature is outside the range of the target temperature (974), then the protection apparatus 200 can adjust the temperature of the substrate 130

(976). For example, the control apparatus 225 can instruct the heating apparatus 120 to begin heating the substrate 130 (976).

[0087] Referring to Fig. 10, in some implementations, the protection apparatus 100 is implemented within an EUV light source 1060 that supplies EUV light 1052 to a photolithography exposure apparatus 1063. The photolithography exposure apparatus 1063 includes an illumination system (illuminator) IL configured to condition a radiation beam B (for example, EUV light 1052); 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 W (for example, a resist coated wafer) being patterned 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. [0088] 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.

[0089] 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.

[0090] 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.

[0091] As here depicted, the apparatus is of a reflective type (for example, employing a reflective mask).

[0092] The photolithography exposure apparatus 1063 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.

[0093] Referring also to Fig. 11, the illuminator IL receives an extreme ultraviolet radiation beam (the EUV light 1052) from the EUV light source 1060. 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 line-emitting element, with a laser beam. The EUV light source 1060 can be designed like the EUV light source 760. As discussed above, the resulting plasma emits output radiation, for example, EUV radiation, which is collected using the EUV collector mirror 332 (or a radiation collector).

[0094] 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 can be aligned using patterning device alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0095] The depicted apparatus could 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. [0096] Combinations and/or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0097] Fig. 11 shows an implementation of the photolithography exposure apparatus 1063 in more detail, including the EUV light source 1060, the illumination system IL, and the projection system PS. The EUV light source 1060 is constructed and arranged as discussed above when describing EUV light source 760.

[0098] The systems IL and PS are likewise contained within vacuum environments of their own. The intermediate focus (IF) of the EUV light source 1060 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 1052).

[0099] 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 1080 and a facetted pupil mirror device 1081. These devices form a so-called “fly’s eye” illuminator, which is arranged to provide a desired angular distribution of the radiation beam 1082 (which is formed from the EUV light 1052), at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam 1082 at the patterning device MA, held by the support structure (mask table) MT, a patterned beam 1083 is formed and the patterned beam 1083 is imaged by the projection system PS via reflective elements 1084, 1085 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. [0100] Each system IL and PS is arranged within its own vacuum or near-vacuum environment, defined by enclosing structures similar to EUV chamber 740. 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. [0101] As mentioned above, in some implementations, and with reference to Fig. 7, the energy from the amplified light beam 762 is delivered in at least two pulses: namely, a pre pulse with limited energy is delivered to the target 751 before it reaches the interacting region 741, 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 interacting region 741, to generate the light-emitting plasma 753.

[0102] The droplet generator in the target delivery system 761 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 (targets 751) of the fuel liquid toward the interacting region 741. The target 751 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).

[0103] The implementations and/or embodiments can be further described using the following clauses:

1. A protection apparatus for a substrate positioned inside a chamber of an extreme ultraviolet (EUV) light source, the protection apparatus comprising: a buffer generator configured to interact a buffer with a substrate surface of the substrate; and a heating apparatus in thermal communication with the substrate and configured to increase a temperature of the substrate and the substrate surface.

2. The protection apparatus of clause 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate.

3. The protection apparatus of clause 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

4. The protection apparatus of clause 1, wherein the substrate is a buffer flow guide positioned adjacent to an optical element that interacts with light within the chamber of the EUV light source.

5. The protection apparatus of clause 4, wherein the optical element is an EUV collector mirror and the buffer flow guide is positioned at an opening of the EUV collector mirror.

6. The protection apparatus of clause 5, wherein the buffer flow guide includes a conically- shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror.

7. The protection apparatus of clause 4, wherein the optical element interacts with light within the EUV light source chamber. 8. The protection apparatus of clause 7, wherein the optical element is a mirror configured to reflect EUV light or a window configured to pass EUV light.

9. The protection apparatus of clause 1, further comprising a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the substrate and the substrate surface.

10. The protection apparatus of clause 9, further comprising a temperature metrology device configured to measure a temperature of the substrate, wherein the control apparatus is in communication with the temperature metrology device such that it actively maintains the temperature of the substrate and the substrate surface by comparing the measured temperature to a target temperature.

11. The protection apparatus of clause 10, wherein the temperature metrology device comprises one or more of a thermocouple, an infrared camera, a thermometer, and a laser reading.

12. The protection apparatus of clause 1, wherein the buffer generator comprises a buffer flow generator and the buffer includes a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across the substrate surface.

13. The protection apparatus of clause 12, wherein the buffer fluid is a gas that includes hydrogen.

14. The protection apparatus of clause 13, wherein buffer flow generator being configured to pass the buffer fluid across the substrate surface comprises advectively transporting the buffer fluid across the substrate surface.

15. The protection apparatus of clause 13, wherein the buffer flow generator being configured to pass the buffer fluid across the substrate surface comprises reducing an amount of target material debris deposited on the substrate surface; and the heating apparatus is configured to maintain an effectiveness of the buffer flow generator in reducing the amount of target material debris that is deposited on the substrate surface.

16. The protection apparatus of clause 12, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

17. The protection apparatus of clause 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value greater than 600 °C.

18. The protection apparatus of clause 1, further comprising an insulating device associated with the substrate and configured to thermally insulate the substrate from other components within the EUV light source chamber.

19. The protection apparatus of clause 1, wherein the heating apparatus comprises a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the substrate.

20. A protection apparatus comprising: a buffer generator configured to interact a buffer with a surface of an optical element positioned inside a chamber of an extreme ultraviolet (EUV) light source and configured to interact with EUV light; a buffer guide positioned adjacent to the optical element and configured to guide the buffer relative to the optical element; and a heating apparatus in thermal communication with the buffer guide and configured to increase a temperature of the buffer guide.

21. The protection apparatus of clause 20, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value at which any temperature gradient at any location of the buffer guide remains below 10% of an average temperature of the buffer guide.

22. The protection apparatus of clause 20, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

23. The protection apparatus of clause 20, wherein the optical element is a collector mirror configured to collect EUV light.

24. The protection apparatus of clause 23, wherein the buffer guide includes a conically-shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror.

25. The protection apparatus of clause 20, further comprising a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the buffer guide.

26. The protection apparatus of clause 25, further comprising a temperature metrology device configured to measure a temperature of the buffer guide, wherein the control apparatus is in communication with the temperature metrology device such that the control apparatus actively maintains the temperature of the buffer guide by comparing the measured temperature to a target temperature.

27. The protection apparatus of clause 20, wherein the buffer generator comprises a buffer flow generator and the buffer includes a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across a surface of the buffer guide.

28. The protection apparatus of clause 27, wherein the buffer fluid is a gas that includes hydrogen.

29. The protection apparatus of clause 27, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

30. The protection apparatus of clause 27, wherein the buffer flow generator being configured to pass the buffer fluid across the surface of the buffer guide comprises advectively transporting the buffer fluid across the surface of the buffer guide.

31. The protection apparatus of clause 20, wherein the buffer guide comprises aluminum, tungsten, or molybdenum. 32. The protection apparatus of clause 20, wherein the heating apparatus is configured to increase the temperature of the buffer flow guide to a value greater than 600 °C.

33. The protection apparatus of clause 20, further comprising an insulating device associated with the buffer guide and configured to thermally insulate the buffer guide from other components within the EUV light source chamber.

34. The protection apparatus of clause 20, wherein the heating apparatus comprises a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the buffer guide.

35. A method for protecting a substrate positioned inside a chamber of an extreme ultraviolet (EUV) light source, the method comprising: passing a buffer fluid across a substrate surface of the substrate; and actively increasing a temperature of the substrate and the substrate surface.

36. The method of clause 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate.

37. The method of clause 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

38. The method of clause 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

39. The method of clause 35, wherein passing the buffer fluid across the substrate surface comprises advectively transporting the buffer fluid across the substrate surface.

40. The method of clause 35, further comprising thermally insulating the substrate from other components within the EUV light source chamber.

41. The method of clause 35, wherein passing the buffer fluid across the substrate surface comprises reducing an amount of target material deposited on the substrate surface; the method further comprising maintaining an effectiveness of the buffer flow generator in reducing the amount of target material that is deposited on the substrate surface.

42. The method of clause 35, further comprising actively maintaining the temperature of the substrate and the substrate surface to a target temperature or a target temperature function.

43. The method of clause 42, further comprising measuring a temperature of the substrate, wherein actively maintaining the temperature of the substrate and the substrate surface comprises comparing the measured temperature to the target temperature or the target temperature function.

[0104] Other implementations are within the scope of the following claims.

Claims

2. The protection apparatus of claim 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate.

3. The protection apparatus of claim 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

4. The protection apparatus of claim 1, wherein the substrate is a buffer flow guide positioned adjacent to an optical element that interacts with light within the chamber of the EUV light source.

5. The protection apparatus of claim 4, wherein the optical element is an EUV collector mirror and the buffer flow guide is positioned at an opening of the EUV collector mirror.

6. The protection apparatus of claim 5, wherein the buffer flow guide includes a conically- shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror.

7. The protection apparatus of claim 4, wherein the optical element interacts with light within the EUV light source chamber.

8. The protection apparatus of claim 7, wherein the optical element is a mirror configured to reflect EUV light or a window configured to pass EUV light.

9. The protection apparatus of claim 1, further comprising a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the substrate and the substrate surface.

10. The protection apparatus of claim 9, further comprising a temperature metrology device configured to measure a temperature of the substrate, wherein the control apparatus is in communication with the temperature metrology device such that it actively maintains the temperature of the substrate and the substrate surface by comparing the measured temperature to a target temperature.

11. The protection apparatus of claim 10, wherein the temperature metrology device comprises one or more of a thermocouple, an infrared camera, a thermometer, and a laser reading.

12. The protection apparatus of claim 1, wherein the buffer generator comprises a buffer flow generator and the buffer includes a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across the substrate surface.

13. The protection apparatus of claim 12, wherein the buffer fluid is a gas that includes hydrogen.

14. The protection apparatus of claim 13, wherein buffer flow generator being configured to pass the buffer fluid across the substrate surface comprises advectively transporting the buffer fluid across the substrate surface.

15. The protection apparatus of claim 13, wherein the buffer flow generator being configured to pass the buffer fluid across the substrate surface comprises reducing an amount of target material debris deposited on the substrate surface; and the heating apparatus is configured to maintain an effectiveness of the buffer flow generator in reducing the amount of target material debris that is deposited on the substrate surface.

16. The protection apparatus of claim 12, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

17. The protection apparatus of claim 1, wherein the heating apparatus is configured to increase the temperature of the substrate and the substrate surface to a value greater than 600 °C.

18. The protection apparatus of claim 1, further comprising an insulating device associated with the substrate and configured to thermally insulate the substrate from other components within the EUV light source chamber.

19. The protection apparatus of claim 1, wherein the heating apparatus comprises a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the substrate.

21. The protection apparatus of claim 20, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value at which any temperature gradient at any location of the buffer guide remains below 10% of an average temperature of the buffer guide.

22. The protection apparatus of claim 20, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

23. The protection apparatus of claim 20, wherein the optical element is a collector mirror configured to collect EUV light.

24. The protection apparatus of claim 23, wherein the buffer guide includes a conically- shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror.

25. The protection apparatus of claim 20, further comprising a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the buffer guide.

26. The protection apparatus of claim 25, further comprising a temperature metrology device configured to measure a temperature of the buffer guide, wherein the control apparatus is in communication with the temperature metrology device such that the control apparatus actively maintains the temperature of the buffer guide by comparing the measured temperature to a target temperature.

27. The protection apparatus of claim 20, wherein the buffer generator comprises a buffer flow generator and the buffer includes a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across a surface of the buffer guide.

28. The protection apparatus of claim 27, wherein the buffer fluid is a gas that includes hydrogen.

29. The protection apparatus of claim 27, wherein the heating apparatus is configured to increase the temperature of the buffer guide to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

30. The protection apparatus of claim 27, wherein the buffer flow generator being configured to pass the buffer fluid across the surface of the buffer guide comprises advectively transporting the buffer fluid across the surface of the buffer guide.

31. The protection apparatus of claim 20, wherein the buffer guide comprises aluminum, tungsten, or molybdenum.

32. The protection apparatus of claim 20, wherein the heating apparatus is configured to increase the temperature of the buffer flow guide to a value greater than 600 °C.

33. The protection apparatus of claim 20, further comprising an insulating device associated with the buffer guide and configured to thermally insulate the buffer guide from other components within the EUV light source chamber.

34. The protection apparatus of claim 20, wherein the heating apparatus comprises a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the buffer guide.

36. The method of claim 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate.

37. The method of claim 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber.

38. The method of claim 35, wherein actively increasing the temperature of the substrate and the substrate surface comprises increasing the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates.

39. The method of claim 35, wherein passing the buffer fluid across the substrate surface comprises advectively transporting the buffer fluid across the substrate surface.

40. The method of claim 35, further comprising thermally insulating the substrate from other components within the EUV light source chamber.

41. The method of claim 35, wherein passing the buffer fluid across the substrate surface comprises reducing an amount of target material deposited on the substrate surface; the method further comprising maintaining an effectiveness of the buffer flow generator in reducing the amount of target material that is deposited on the substrate surface.

42. The method of claim 35, further comprising actively maintaining the temperature of the substrate and the substrate surface to a target temperature or a target temperature function.

43. The method of claim 42, further comprising measuring a temperature of the substrate, wherein actively maintaining the temperature of the substrate and the substrate surface comprises comparing the measured temperature to the target temperature or the target temperature function.