JP5659015B2 - Radiation source - Google Patents

Description

  The present invention relates to a radiation source, a method for generating radiation and a lithographic apparatus including the radiation source.

  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 case, a patterning device, also referred to as a mask or a reticle, may be used to generate a circuit pattern formed on an individual layer of the IC. This pattern can be transferred onto a target portion (eg including part of, one, or more dies) on a substrate (eg a silicon wafer). Usually, the pattern is transferred by imaging on a radiation-sensitive metal compound (resist) layer provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers that irradiate each target portion by exposing the entire pattern onto the target portion at once, and simultaneously scanning the pattern in a certain direction (“scan” direction) with a radiation beam, A so-called scanner is included that irradiates each target portion by scanning the substrate parallel or antiparallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

  In order to be able to project smaller structures on the substrate than ever before, it has been proposed to use extreme ultraviolet radiation having a wavelength in the range of 10 nm to 20 nm, for example in the range of 13 nm to 14 nm. It has further been proposed that radiation having a wavelength of less than 10 nm, for example 6.7 nm or 6.8 nm may be used. In the context of lithography, sometimes wavelengths below 10 nm are referred to as “beyond EUV”.

Extreme ultraviolet and super EUV radiation can be generated using a plasma. The plasma can be, for example, by directing a laser at particles of a suitable material (eg, tin), or by any suitable gas (eg, Sn vapor, SnH ₄ , or Sn vapor and any gas having a small nuclear charge (eg, Can be generated by directing the laser in a stream of a mixture of H ₂ to Ar). The resulting plasma emits extreme ultraviolet (or super EUV radiation), which can be collected and focused into focus using a collector mirror.

  In addition to extreme ultraviolet (or super-EUV radiation), the plasma generates debris having the form of particles such as thermalized atoms, ions, nanoclusters and / or microparticles. Debris can damage the collector mirror (or other components). A buffer gas may be provided near the plasma. Particles generated by the plasma collide with buffer gas molecules, thereby losing energy. In this way, at least some of the particles can be decelerated sufficiently so that they do not reach the collector mirror. Therefore, the damage given to the condensing mirror can be reduced. However, even when buffer gas is used, some particles can still reach the collector mirror and damage the collector mirror.

  It would be desirable to improve the effectiveness of the buffer gas.

  According to a first aspect of the invention, a radiation source comprising a chamber and a supply of plasma generating material, wherein the radiation generating material interacts with the laser beam by which the plasma generating material introduced into the chamber is thereby emitted. A radiation source is provided that further includes a conduit configured to deliver a buffer gas to the chamber and having an outlet adjacent to the interaction point.

  According to a second aspect of the present invention, there is provided a method for generating radiation comprising introducing a plasma generating material into a chamber to generate a radiation emitting plasma and directing a laser beam at the plasma generating material, the laser A method is provided further comprising introducing a buffer gas into the chamber at a location adjacent to the point where the beam and the plasma generating material interact.

  According to a third aspect of the invention, a radiation source, an illumination system for adjusting the radiation, a support structure for supporting a patterning device that serves to impart a pattern to the cross section of the radiation beam, and a substrate are held A lithographic apparatus comprising a substrate table for performing a projection and a projection system for projecting a patterned radiation beam onto a target portion of the substrate, the radiation source including a chamber and a supply of plasma generating material, Has an interaction point where a plasma generating material introduced into the chamber interacts with the laser beam, thereby generating a radiation-emitting plasma, and the radiation source is configured to deliver a buffer gas into the chamber. And a lithographic apparatus further comprising a conduit having an outlet adjacent to the interaction point.

Several embodiments of the present invention are described below by way of example only and with reference to the accompanying schematic drawings. In these drawings, the same reference numerals indicate corresponding parts.
FIG. 1 depicts a lithographic apparatus according to one embodiment of the invention. FIG. 2a shows a radiation source according to an embodiment of the invention. FIG. 2b shows a radiation source according to an embodiment of the invention. FIG. 3 shows a radiation source according to another embodiment of the invention.

FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The device
A radiation beam B (e.g. an illumination system (illuminator) IL configured to condition EUV radiation or super EUV radiation; and
A support structure (eg mask table) configured to support the patterning device (eg mask) MA and coupled to a first positioner PM configured to accurately position the patterning device according to certain parameters MT)
A substrate table (eg a wafer table) configured to hold a substrate (eg resist-coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate according to certain parameters ) WT,
A projection system (eg a refractive projection lens system) configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (eg comprising one or more dies) of the substrate W; PS.

  Illumination systems include refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, to induce, shape, or control radiation Various types of optical components can be included.

  The support structure supports the patterning device, such as by supporting the weight of the patterning device. The support structure holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as whether or not the patterning device is held in a vacuum environment. The support structure can hold the patterning device using mechanical, vacuum, electrostatic or other clamping techniques. The support structure may be, for example, a frame or table that can be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

  As used herein, the term “patterning device” is broadly interpreted to refer to any device that can be used to provide a pattern in a cross-section of a radiation beam so as to create a pattern in a target portion of a substrate. Should be. It should be noted that the pattern imparted to the radiation beam may not exactly match the desired pattern in the target portion of the substrate, for example if the pattern includes phase shift features or so-called assist features. . Typically, the pattern applied to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

  Examples of patterning devices include masks and programmable mirror arrays. Masks are well known in lithography, and generally in EUV or super EUV, the lithographic apparatus is reflective. One example of a programmable mirror array uses a matrix array of small mirrors, and each small mirror can be individually tilted to reflect the incoming radiation beam in various directions. The tilted mirror patterns the radiation beam reflected by the mirror matrix.

  As used herein, the term “projection system” should be interpreted broadly to encompass all types of projection systems. Usually, in an EUV or super EUV lithographic apparatus, the optical element is reflective. However, other types of optical elements may be used. The optical element may be in a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”.

  As shown herein, the apparatus is of a reflective type (eg, employing a reflective mask).

  The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In such “multi-stage” machines, additional tables can be used in parallel, or one or more tables are used for exposure while a preliminary process is performed on one or more tables. You can also.

  Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The radiation source and the lithographic apparatus may be separate components. In such a case, the radiation source is not considered to form part of the lithographic apparatus, and the radiation beam is directed from the radiation source SO to the illuminator IL, eg, a suitable guiding mirror and / or beam extractor. Sent using a beam delivery system BD that includes a panda. In other cases the source may be an integral part of the lithographic apparatus. Radiation source SO and illuminator IL may be referred to as a radiation system along with a beam delivery system if necessary.

  The illuminator IL may include an adjuster that adjusts the angular intensity distribution of the radiation beam. In general, at least the outer and / or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the illuminator pupil plane can be adjusted. In addition, the illuminator IL may include various other components such as integrators and capacitors. If the radiation beam B is adjusted using an illuminator, a desired uniformity and intensity distribution can be provided in the cross section of the radiation beam.

  The radiation beam B is incident on the patterning device (eg, mask MA), which is held on the support structure (eg, mask table MT), and is patterned by the patterning device. After being reflected by the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. The substrate table is used, for example, to position various target portions C in the path of the radiation beam B using the second positioner PW and the position sensor IF2 (eg, interferometer device, linear encoder, or capacitive sensor). The WT can be moved accurately. Similarly, using the first positioner PM and another position sensor IF1 to accurately position the mask MA with respect to the path of the radiation beam B, eg after mechanical removal of the mask from the mask library or during a scan. You can also. In general, the movement of the mask table MT can be achieved by using a long stroke module (coarse positioning) and a short stroke module (fine positioning) that form part of the first positioner PM. Similarly, movement of the substrate table WT can also be achieved using a long stroke module and a short stroke module that form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1 and M2 and substrate alignment marks P1 and P2. In the illustration, the substrate alignment mark occupies a dedicated target portion, but the substrate alignment mark can also be placed in the space between the target portion (these are known as scribe line alignment marks). Similarly, if a plurality of dies are provided on the mask MA, the mask alignment mark may be placed between the dies.

The example apparatus can be used in at least one of the modes described below.
1. In step mode, the entire pattern applied to the radiation beam is projected onto the target portion C at once (ie, a single static exposure) while the mask table MT and substrate table WT remain essentially stationary. Thereafter, the substrate table WT is moved in the X and / or Y direction so that another target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged during a single static exposure.
2. In scan mode, the mask table MT and substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (ie, a single dynamic exposure). The speed and direction of the substrate table WT relative to the mask table MT can be determined by the (reduction) magnification factor and image reversal characteristics of the projection system PS. In the scan mode, the maximum size of the exposure field limits the width of the target portion during single dynamic exposure (non-scan direction), while the length of the scan operation determines the height of the target portion (scan direction). Determined.
3. In another mode, while holding the programmable patterning device, the mask table MT remains essentially stationary and the substrate table WT is moved or scanned while the pattern applied to the radiation beam is moved to the target portion. Project onto C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device can also be used after each movement of the substrate table WT or between successive radiation pulses during a scan as needed. Updated. 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 described above.

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

  FIG. 2 schematically shows a radiation source SO according to an embodiment of the invention. FIG. 2a shows a cross section of the radiation source SO as seen from one side, and FIG. 2b shows a cross section of the radiation source as seen from above.

  The radiation source SO includes a chamber 1. The chamber 1 is defined by a wall 2 and a collecting mirror 3. The condensing mirror 3 has a reflecting surface that reflects extreme ultraviolet wavelengths.

  The source 4 is configured to supply droplets of material (eg, tin) to the chamber 1. The collector 5 is arranged at the bottom of the chamber 1 under the supply 4 and is configured to collect the material that has passed through the chamber 1.

  The collector mirror 3 is configured to focus the radiation at the focal point FP, from which the radiation can travel to the illuminator IL of the lithographic apparatus (see FIG. 1). The laser 6 is used to generate a radiation beam 7 that is directed through the aperture 8 into the chamber 1. The aperture 8 includes, for example, a window that transmits the wavelength of the laser beam 7. The beam dump 9 is positioned in the chamber 1 and positioned so that any portion of the laser beam 7 that does not interact with the material provided by the material source 4 is incident on (and is absorbed by) the beam dump. Is done. The gas cooler 10 extends from the side wall of the chamber to the chamber 1.

  The buffer gas supply source extends from the side wall of the chamber to the chamber 1 and has an outlet 12 for delivering the buffer gas in the vicinity of the interaction point 13 where the laser beam 7 is incident on the material supplied from the material supply source 4. A conduit 11 having

  During use, the chamber 1 is filled with a suitable buffer gas (eg hydrogen). The laser 6 generates a laser beam 7 that passes through the aperture 8 in the condenser mirror 3 and enters the chamber 1. The material source 4 produces droplets of material that fall down through the chamber 1 into the collector 5. When the material droplet passes through the interaction point 13, the interaction between the laser beam 7 and the material droplet converts at least a portion of the material into a plasma. The plasma emits extreme ultraviolet light, which is collected by the collector mirror 3 and focused on the focal point FP. Extreme ultraviolet light travels from the focal point FP to the illuminator IL (see FIG. 1) of the lithographic apparatus.

  The portion of the droplet of material that does not interact with the laser beam 7 continues to fall through the chamber 1 and is collected by the collector 5.

  The plasma generated by the interaction of the laser beam 7 with the droplets of material may contain particles that damage the collector mirror 3. The buffer gas in the chamber 1 is intended to decelerate the particles so that they do not reach the collecting mirror 3. However, the intensity of the interaction between the laser beam 7 and the tin particles at the interaction point 13 is pushed so that the buffer gas is heated away from the interaction point when the laser beam interacts with a droplet of material. That's right. This results in the buffer gas in the region around the interaction point having a higher temperature and lower density.

  In a conventional extreme ultraviolet radiation source (buffer gas is introduced from the side wall of the chamber), the heated buffer gas is moved away from the area around the interaction point 13 (the heated buffer gas is Some time passes (for example, moving towards the gas cooler 10). The time taken for the heated buffer gas to move away from the region around the interaction point 13 is, for example, about several tens of milliseconds. The delivery time of successive material droplets to the interaction point 13 is much shorter, for example 10-20 microseconds. This means that the heated buffer gas can continue to be present in the region around the interaction point 13 during the generation of a continuous pulse of EUV radiation.

  The area around the interaction point 13 occupied by the heated buffer gas may contain a significant proportion of the volume between the interaction point 13 and the collector mirror 3. The heated buffer gas in this region has a lower density than the unheated gas, so that there is less interaction between the plasma particles and the buffer gas. As a result, there is a high possibility that the particles reach the condenser mirror 3. When this occurs, the condensing mirror 3 may be damaged.

  There are further influences that can have an effect on the problems mentioned above. Many of the fast ions generated at the interaction point 13 move in the direction of the collector mirror 3. When these fast ions are stopped by the buffer gas, the ions transfer their momentum to the buffer gas, thereby causing the buffer gas to flow toward the collector mirror 3. This further reduces the density of the buffer gas in the region around the interaction point 13.

  The above problems are solved by the conduit 11 shown in FIG. 2 or its severity is reduced. Conduit 11 has an outlet 12 located adjacent to interaction point 13, thereby delivering unheated buffer gas to interaction point 13. Thus, instead of the unheated buffer gas flowing into that region only after the heated buffer gas has moved away from the region around the interaction point 13, the outlet 12 of the conduit 11 is heated. No buffer gas is carried directly and immediately into the area around the interaction point 13. As a result, until the next droplet of material reaches the interaction point 13, a newly delivered buffer gas is present in the region around the interaction point 13.

  This newly delivered buffer gas is not heated and is therefore more dense than the heated buffer gas. Therefore, the buffer gas is more effective. Accordingly, embodiments of the present invention provide improved protection of the collector mirror 3 against particles generated during plasma formation. This therefore allows the collector mirror 3 to have a longer life before cleaning and / or replacement compared to the other cases.

  The buffer gas may be delivered at a high speed (for example, 100 to 2000 m / s). This offers the advantage of pushing the heated buffer gas away quickly from the area around the interaction point 13. The buffer gas may be delivered with a supersonic gas jet directed at or near the interaction point 13. The supersonic gas jet has the advantage that the density of the buffer gas in the jet may be substantially greater than the average density of the buffer gas in the chamber, so that the buffer gas adjacent to the interaction point 13 and Provides increased interaction with fast ions.

  Since conduit 11 introduces buffer gas into chamber 1, one or more holes (not shown) may be used to pump the buffer gas out of chamber 1, thereby allowing buffer gas in the chamber to flow out. Adjust the pressure. The gas cooler 10 adjusts the temperature of the buffer gas.

  The conduit 11 is provided in a selected arrangement such that extreme ultraviolet light obscured by the conduit 11 is obscured by other elements of the device if the conduit 11 is not present. Accordingly, the conduit 11 is placed in front of the gas cooler 10, which obscures EUV radiation regardless of whether the conduit 11 is present. The conduit 11 is displaced perpendicular to the laser beam 7 so that the laser beam does not enter the conduit 11 but instead passes through it next to the beam dump 9.

  As described above, the outlet of the conduit 11 is adjacent to the interaction point 13. The outlet of the conduit 11 may be within the outer boundary of the region where the heated buffer gas is continuously present during operation of the EUV source when no buffer gas is supplied through the conduit 11.

  The distance between the outlet 12 of the conduit 11 and the interaction point 13 may be selected by considering the following. That is, the closer the outlet 12 is to the interaction point 13, the more effective the delivery of the heated buffer gas to the area around the interaction point 13. However, the closer the outlet 12 is to the interaction point 13, the more likely the conduit 11 will undergo sputtering of ions to the conduit. In one example, the outlet 12 is within 15 cm from the interaction point and within 10 cm from the interaction point. The outlet may be 3 cm or more from the interaction point. The distance between the interaction point 13 and the collecting mirror 3 may be 20 cm.

  The rate at which the buffer gas is provided through the outlet 12 may be sufficient to substantially remove the heated buffer gas from the area around the interaction point 13. The velocity may be sufficient to achieve this before the next laser and material droplet interaction. To achieve this, the rate at which buffer gas should be provided through outlet 12 is determined by the volume of buffer gas that is heated by the interaction between the laser and the material droplet, and the interaction between the laser and the material droplet. It can be calculated based on the frequency that occurs (ie, the frequency of the EUV source).

  Another embodiment of the present invention is schematically illustrated in FIG. FIG. 3 shows the radiation source SO viewed from one side. Most of the elements of the radiation source SO shown in FIG. 3 correspond to those shown in FIG. 2 and will not be described again here. However, the conduit 11 of FIG. 2 is not present in FIG. Instead, the conduit 21 passes through the aperture 8 in the collector mirror 3 and travels parallel to the laser beam 7. The conduit 21 is provided with an outlet 22 adjacent to the interaction point 13. Conduit 21 is used to introduce buffer gas in the vicinity of interaction point 13 in a manner equivalent to that described above for FIG. Conduit 21 may obscure some EUV radiation generated by the plasma in chamber 1, while the amount of EUV radiation concealed is relatively small (eg, only its cross-section rather than the length of the conduit). Positioned to obscure EUV radiation). The distance between the outlet 22 and the interaction point 13 may be selected using the criteria further described above for FIG.

  The advantage of the embodiment shown in FIG. 3 is that the flow of buffer gas provided by the conduit leaves the collector mirror 3 rather than toward the collector mirror 3 (so that the heated buffer gas is removed from the collector mirror 3). To help you push away).

  A modified version of the embodiment shown in FIG. 3 may include two tubes, one inside the other. The laser beam may be configured to pass along an inner tube of the two tubes, and the buffer gas may be configured to pass along a channel formed between the two tubes. . In this case, in order to allow the laser beam to travel unimpeded from the laser to the interaction point, the corner shown in FIG. 3 may not be present in the inner tube of the two tubes.

  Although conduit 11 and conduit 21 having different locations and configurations are shown in FIGS. 2 and 3, other conduit locations and configurations may be used. The conduit location and configuration preferably does not obscure any EUV radiation, which is not obscured by any other component of the radiation source SO if the conduit does not obscure. In some cases this may not be achieved or it may be preferable to provide the conduit at some location where the conduit actually obscures some EUV radiation. In this case, it is desirable to minimize the amount of EUV radiation obscured by the conduit as much as possible. The appropriate arrangement and configuration for the conduit depends on the particular configuration of the radiation source in which the conduit is provided. More than one conduit may be provided (eg, both of the conduits shown in FIGS. 2 and 3 may be provided in a single EUV source).

  Although the above description refers to using hydrogen as the buffer gas, other suitable gases may be used.

  Although the above description refers to a droplet of material as tin, other suitable materials may be used.

The present invention is not limited to radiation sources that use droplets of material. One embodiment of the present invention, for example, generates a plasma from a gas rather than from a droplet of material. Suitable gases include Sn vapor, SnH ₄ , or a mixture of Sn vapor and any gas having a small nuclear charge (eg, H ₂ to Ar). A droplet or gas of material may be considered as an example of a plasma generating material.

  The wavelength of EUV radiation mentioned in the above description is, for example, in the range of 10-20 nm, for example in the range of 13-14 nm.

  Although the above description of embodiments of the present invention relates to a radiation source that generates EUV radiation, the present invention may be incorporated into a “super EUV” radiation, ie a radiation source that generates radiation having a wavelength of less than 10 nm. Super EUV radiation has, for example, a wavelength of 6.7 nm or 6.8 nm. A radiation source that generates super EUV radiation may operate in the same manner as the radiation sources described above.

  In the above description, the term “unheated buffer gas” refers to the outlet 12 and after the interaction between the laser beam and the plasma generating material (and before the next interaction between the laser beam and the plasma generating material). The buffer gas delivered from the outlet 22 is intended.

  The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims (11)

A radiation source comprising a chamber, a collector mirror and a source of plasma generating material, wherein the radiation source interacts with a laser beam when the plasma generating material introduced into the chamber thereby generating a radiation emitting plasma Has an interaction point that
The radiation source further includes a conduit that delivers buffer gas to the chamber and has an outlet adjacent to the interaction point;
At least a portion of the conduit passes through an aperture in the collector mirror of the radiation source, and at least a portion of the conduit includes two tubes, one inside the other, the inner tube being The laser beam is configured to pass along the inner tube, and the channel between the two tubes is configured to allow the buffer gas to pass along the channel. Being
Radiation source.   The outlet is located within the outer boundary of the region where the existing buffer gas will be heated and continuously present during operation of the radiation source if no new buffer gas is supplied through the conduit. The radiation source according to claim 1.   3. Radiation source according to claim 1 or 2, wherein the outlet of the conduit is within 15 cm of the interaction point.   4. A radiation source according to claim 3, wherein the outlet of the conduit is within 10 cm of the interaction point.   The radiation source according to claim 1, wherein the outlet of the conduit is 3 cm or more from the interaction point. A method of generating radiation comprising introducing a plasma generating material into a chamber to generate a radiation emitting plasma and directing a laser beam toward the plasma generating material, wherein the laser beam and the plasma generating material interact with each other. further comprising introducing a buffer gas at a location adjacent to a point which acts on the chamber, the buffer gas is supplied from the outlet of the conduits adjacent to the interaction point, at least a portion of said conduit, The tube passes through an aperture in the collector mirror of the radiation source, and at least a portion of the conduit includes two tubes, one inside the other, and an inner tube, where the laser beam is the inner tube The channel between the two tubes is configured to pass through the buffer gas. And it is configured so as to be able to pass along the channel,
Method. The place where the buffer gas is introduced, the would buffer gas already present there continuously during operation of the radiation source is heated when a new buffer gas is not supplied through the conduit region The method of claim 6 , wherein the method is within the outer boundary. The method according to claim 6 or 7 , wherein the buffer gas is introduced at a speed of 100 m / s or more. The method according to any one of claims 6 to 8 , wherein the buffer gas is introduced at a speed of 2000 m / s or less. The rate at which the buffer gas is introduced substantially removes the heated buffer gas from the region around the interaction point prior to the next interaction between the laser beam and the plasma generating material. 10. A method according to any one of claims 6 to 9 , which is sufficient for the purpose. A radiation source;
A lighting system for adjusting the radiation;
A support structure for supporting a patterning device that serves to impart a pattern to a cross-section of the radiation beam;
A substrate table for holding the substrate;
A projection system for projecting a patterned beam of radiation onto a target portion of a substrate, comprising:
The radiation source includes a chamber, a collector mirror and a source of plasma generating material, wherein the radiation source interacts with the laser beam when the plasma generating material introduced into the chamber thereby generating a radiation emitting plasma. The radiation source further includes a conduit for delivering a buffer gas to the chamber and having an outlet adjacent to the interaction point, at least a portion of the conduit comprising the radiation An aperture in the collector mirror of the source and at least a portion of the conduit includes two tubes, one inside the other, the inner tube, the laser beam along the inner tube The channel between the two tubes is configured such that the buffer gas passes along the channel. Bets are configured to be, the lithographic apparatus.

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