KR20100049607A

KR20100049607A - Euv radiation source

Info

Publication number: KR20100049607A
Application number: KR1020107003754A
Authority: KR
Inventors: 바딤 예프겐예비치 바니네; 블라디미르 비탈에비치 이바노프
Original assignee: 에이에스엠엘 네델란즈 비.브이.
Priority date: 2007-08-23
Filing date: 2008-08-20
Publication date: 2010-05-12
Also published as: WO2009024860A3; JP5659015B2; WO2009024860A2; TWI448828B; CN101785368B; NL1035863A1; EP2191698B1; EP2191698A2; JP2010537377A; CN101785368A; TW200919112A; KR101528581B1; NL1035846A1

Abstract

A radiation source comprising a chamber and a supply of a plasma generating substance, the source having an interaction point at which the plasma generating substance introduced into the chamber may interact with a laser beam and thereby produce a radiation emitting plasma, wherein the source further comprises a conduit arranged to deliver a buffer gas into the chamber, the conduit having an outlet which is adjacent to the interaction point.

Description

EUV RADIATION SOURCE}

The present invention relates to a radiation source, a method of generating radiation, and a lithographic apparatus comprising a radiation source.

BACKGROUND A lithographic apparatus is a machine that applies a desired pattern onto a substrate, typically onto a target portion of the substrate. The lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). In that case, a patterning device, alternatively referred to as a mask or a reticle, can be used to create a circuit pattern to be formed on a separate layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically performed through imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus scans a pattern in a given direction ("scanning" -direction) through a radiation beam, and a so-called stepper through which each target portion is irradiated by exposing the entire pattern onto the target portion at one time, while in this direction And a so-called scanner to which each target portion is irradiated by synchronously scanning the substrate in a direction parallel to the same direction (direction parallel to the same direction) or anti-parallel direction (the direction parallel to the opposite 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 project even smaller structures onto the substrate, it has been proposed to use extreme ultraviolet radiation having a wavelength in the range of 10 to 20 nm, for example in the range of 13 to 14 nm. It has also been proposed that radiation having a wavelength of less than 10 nm, for example 6.7 nm or 6.8 nm, can be used. In the context of lithography, wavelengths below 10 nm are sometimes referred to as 'beyond EUV'.

Extreme ultraviolet radiation and Beyond EUV radiation can be generated using plasma. The plasma can be directed, for example, by directing the laser to particles of a suitable material (eg tin) or by using a suitable gas stream (eg Sn vapor, SnH ₄ , or Sn vapor with small nuclear charge). Can be produced by directing the laser to any mixture of gases (eg H ₂ to Ar). The resulting plasma emits extreme ultraviolet radiation (or Beyond EUV radiation), which can be collected using a collector mirror and focused on a focal point.

In addition to extreme ultraviolet radiation (or Beyond EUV radiation), plasma generates debris in the form of particles such as thermalized atoms, ions, nanoclusters, and / or microparticles. Debris can cause damage to the collector mirror (or other components). A buffer gas may be provided in the vicinity of the plasma. Particles produced by the plasma collide with molecules of the buffer gas, thereby losing energy. In this way, at least some of the particles can be slowed down so as not to reach the collector mirror. Damage caused to the collector mirror can thereby be reduced. However, even when a buffer gas is used, some particles may still reach the collector mirror and cause damage.

It is desirable to improve the effectiveness of the buffer gas.

According to a first embodiment of the present invention, there is provided a radiation source comprising a chamber and a supply of a plasma generating material, wherein the source is capable of interacting with a laser beam to produce a radiation emitting plasma. Having an interaction point, the source further comprises a conduit arranged to deliver a buffer gas into the chamber, the conduit having an outlet adjacent the interaction point.

According to a second embodiment of the present invention, a method of generating radiation is provided, which includes introducing a plasma generating material into a chamber and directing a laser beam to the plasma generating material to produce a radiation emitting plasma. The method further includes introducing a buffer gas at a location in the chamber adjacent to the point where the laser beam and the plasma generating material interact.

According to a third embodiment of the present invention, there is provided a source of radiation, an illumination system for conditioning a radiation, a support structure for supporting a patterning device capable of imparting a pattern to a cross section of the radiation beam, a substrate table for holding a substrate, And a projection system for projecting a patterned beam of radiation onto a target portion of a substrate, the radiation source comprising a chamber and a supply of plasma generating material, wherein the source is a laser generated plasma introduced into the chamber. Having an interaction point capable of interacting with the beam to produce a radiation emitting plasma, the source further comprises a conduit arranged to deliver a buffer gas into the chamber, the conduit having an outlet adjacent the interaction point.

DETAILED DESCRIPTION Hereinafter, embodiments of the present invention will be described only by way of example, with reference to the accompanying schematic drawings in which corresponding reference numbers indicate corresponding parts:
1 shows a lithographic apparatus according to an embodiment of the invention;
2 shows a radiation source according to an embodiment of the invention; And
3 shows a radiation source according to an alternative embodiment of the invention.

Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention. The device is:

An illumination system (illuminator) IL configured to condition the radiation beam B (eg EUV radiation or beyond EUV radiation);

A support structure (eg mask) configured to support the patterning device (eg mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters. Table) (MT);

A substrate table (e.g., connected to a second positioner PW, configured to hold a substrate (e.g. a resist-coated wafer) W, and configured to accurately position the substrate according to certain parameters. Wafer table) (WT); And

A projection system (e.g., a projection system) configured to project a pattern imparted to the radiation beam B by a patterning device MA onto a target portion C (e.g. comprising one or more dies) For example, a refractive projection lens system (PS).

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other forms of optical components, or any combination thereof, to direct, shape, or control the radiation. .

The support structure supports the patterning device, i.e. bears its weight. This 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 for example whether or not the patterning device is maintained in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device. The support structure may be a frame or table, for example, which may be fixed or movable as required. The support structure can ensure that the patterning device is in 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” should be broadly interpreted to refer to any device that can be used to impart a pattern to a cross section of a radiation beam to create a pattern on a target portion of a substrate. The pattern imparted to the radiation beam may be precisely matched to the desired pattern in the target portion of the substrate, for example when the pattern comprises phase-shifting features or so-called assist features . Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in the device to be 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 the lithographic art and will typically be reflective in EUV or Beyond EUV lithographic apparatus. One example of a programmable mirror array employs a matrix configuration of small mirrors, each of which can be individually tilted to reflect the incident radiation beam in a different direction. Inclined mirrors impart a pattern to the beam of radiation reflected by the mirror matrix.

The term "projection system" as used herein is to be broadly interpreted as encompassing any type of projection system. Typically, the optical elements in an EUV or Beyond EUV lithographic apparatus will be reflective. However, other types of optical elements may be used. The optical elements 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 (e.g. 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 "multiple stage" machines additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and lithographic apparatus can be separate entities. In such a case, the source is not considered to form part of a lithographic apparatus, and the radiation beam is, for example, with the aid of a beam delivery system comprising a suitable directing mirror and / or beam expander. From the SO) to the illuminator IL. In other cases, the source may be an integral part of the lithographic apparatus. The source SO and the illuminator IL may be referred to as a radiation system together with a beam delivery system as necessary.

The illuminator IL may include an adjuster for adjusting the angular intensity distribution of the radiation beam. In general, at least the outer and / or inner radial extent (commonly referred to as -outer and -inner, respectively) of the intensity distribution in the pupil plane of the illuminator can be adjusted. The illuminator IL may also include various other components, such as an integrator and a capacitor. The illuminator can be used to condition the radiation beam B in order to have the desired uniformity and intensity distribution 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 on the target portion C of the substrate W. With the aid of the second positioner PW and the position sensor IF2 (eg interferometer device, linear encoder, or capacitive sensor), the substrate table WT is for example the path of the radiation beam B. It can be moved precisely to position different target portions C in it. Similarly, the first positioner PM and another position sensor IF1 are in the path of the radiation beam B, for example after mechanical retrieval from a mask library or during scanning. It can be used to accurately position the mask MA with respect to it. In general, the movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which 1 form part of the positioner PM. Similarly, the movement of the substrate table WT can be realized using a long-stroke module and a short-stroke module, which 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 or fixed only to the short-stroke actuator. The mask MA and the substrate W may be aligned using the mask alignment marks M1 and M2 and the substrate alignment marks P1 and P2. Although the illustrated substrate alignment marks occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations where more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The depicted apparatus may be used in at least one of the following modes:

1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while the entire pattern imparted to the radiation beam is projected onto the target portion C at one time (i.e., Single static exposure]. Thereafter, the substrate table WT is shifted in the X and / or Y direction so that different target portions 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 the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i. E., A single dynamic exposure )]. The speed and direction of the substrate table WT relative to the mask table MT may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In the scan mode, the maximum size of the exposure field limits the width (in the unscanned direction) of the target portion during a single dynamic exposure, while the length of the scanning operation determines the height (in the scanning direction) of the target portion.

3. In another mode, the mask table MT remains essentially stopped by holding the programmable patterning device, while the substrate table WT while the pattern imparted to the radiation beam is projected onto the target portion C. ) Is moved or scanned. In this mode, a pulsed radiation source is generally employed, and the programmable patterning device is updated as needed after each movement of the substrate table WT, or between successive radiation pulses during a scan . This mode of operation can be readily applied to maskless lithography using a programmable patterning device, such as a programmable mirror array of a type as mentioned above.

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

2 schematically shows a source SO according to an embodiment of the invention. 2a shows a cross-sectional view from one side of the source SO, and FIG. 2b shows a cross-sectional view from above the source.

The source SO comprises a chamber 1. The chamber 1 is defined by the walls 2 and the collector mirror 3. The collector mirror 3 has a reflective surface that is reflective to extreme ultraviolet radiation wavelengths.

The feeder 4 is arranged to feed drops of material (eg tin) into the chamber 1. The collector 5 is located below the feeder 4 at the bottom of the chamber 1 and is arranged to collect the material passing through the chamber 1.

The collector mirror 3 is arranged to focus the radiation at the focal point FP from which the radiation can pass into the illuminator IL (see FIG. 1) of the lithographic apparatus. The laser 6 is used to generate a radiation beam 7 which is directed into the chamber 1 via an aperture 8. The aperture 8 may comprise, for example, a window transparent to the wavelength of the laser beam 7. A beam dump 9 is located in the chamber 1 so that any portion of the laser beam 7 that does not interact with the material provided by the material feeder 4 is incident on the beam dump (also Thereby absorbed). The gas cooler 10 extends into the chamber 1 from the side walls of the chamber.

The buffer gas supply includes a conduit 11 extending from one sidewall of the chamber into the chamber 1, and at the interaction point 13 at which the laser beam 7 is incident on the material supplied from the material supply 4. Adjacent has an outlet 12 for delivering a buffer gas.

In use, the chamber 1 is filled with a suitable buffer gas (eg hydrogen). The laser 6 generates a laser beam 7 which passes through the aperture 8 in the collector mirror 3 into the chamber 1. The material feeder 4 produces a drop of material falling down through the chamber 1 towards the collector 5. When a drop of material passes through the interaction point 13, the interaction of the drop of material with the laser beam 7 causes at least a portion of the material to be converted into a plasma. The plasma emits extreme ultraviolet radiation which is collected by the collector mirror 3 and focused on the focal point FP. Extreme ultraviolet radiation passes from the focal point FP to the illuminator IL of the lithographic apparatus (see FIG. 1).

Some of the droplets of material that do not interact with the laser beam 7 continue to fall through the chamber 1 and are 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 will cause damage to the collector mirror 3. The buffer gas present in the chamber 1 is intended to slow the particles so that they do not reach the collector mirror 3. However, the violence of the interaction between the laser beam 7 and the tin particles at the interaction point 13 causes the buffer gas to be heated and pushed out of the interaction point when the laser beam interacts with droplets of material. . This will cause the buffer gas in the zone around the interaction point to have higher temperature and lower density.

In a conventional extreme ultraviolet radiation source, wherein the buffer gas is introduced from the side wall of the chamber, until the heated buffer gas deviates from the zone around the interaction point 13 (the heated buffer gas is for example gas cooled May move towards the force 10) Some time will elapse. The time taken for the heated buffer gas to exit the zone around the interaction point 13 may be, for example, about tens of milliseconds. The time between the delivery of successive drops of material to the interaction point 13 may be significantly shorter than this, for example 10 to 20 microseconds. This means that the heated buffer gas can remain in the region around the interaction point 13 during the successive pulses of EUV radiation.

The area around the interaction point 13 occupied by the heated buffer gas may comprise a substantial part of the volume between the interaction point 13 and the collector mirror 3. The heated buffer gas in this zone has a lower density than the unheated gas and as a result there is less interaction between the particles of the plasma and the buffer gas. As a result, the particles will be easier to reach the collector mirror 3. If this occurs, damage may occur to the collector mirror 3.

There are additional effects that can contribute to the problem described above. Many of the fast ions generated at the interaction point 13 are moving in the direction of the collector mirror 3. When these fast ions are stopped by the buffer gas, they transfer their momentum to the buffer gas, causing the buffer gas to flow in the direction of the collector mirror 3. This further reduces the density of the buffer gas in the zone around the interaction point.

The foregoing problem is solved or reduced in size by the conduit 11 shown in FIG. The conduit 11 has an outlet 12 located adjacent to the interaction point 13, thereby delivering an unheated buffer gas adjacent to the interaction point 13. Thus, only after the heated buffer gas has escaped from the zone, instead of the unheated buffer gas flowing into the zone around the interaction point 13, the outlet 12 of the conduit 11 is located around the interaction point 13. Immediately and directly delivers unheated buffer gas into the zone. As a result, when the next drop of material reaches the interaction point 13, there will be a freshly delivered buffer gas in the area around the interaction point 13.

This newly delivered buffer gas is unheated and therefore more dense than the heated buffer gas. The buffer gas is thus more effective. Therefore, an embodiment of the present invention provides improved protection of the collector mirror 3 from particles generated during plasma formation. Therefore, this allows the collector mirror 3 to have a longer life before cleaning and / or replacement than otherwise.

The buffer gas can be delivered at a high speed (eg, 100-2000 m / s). This offers the advantage of quickly pushing the heated buffer gas out of the zone around the interaction point 13. The buffer gas may be delivered to a supersonic gas jet directed at or near the interaction point 13. Supersonic gas injection has the advantage that the density of the buffer gas in the injection is substantially greater than the average density of the buffer gas in the chamber, providing a buffer gas adjacent to the interaction point 13 for increased interaction of fast ions. Has

Since the conduit 11 introduces the buffer gas into the chamber 1, one or more vents (not shown) may be used to transport the buffer gas from the chamber 1 to regulate the pressure of the buffer gas in the chamber. have. The gas cooler 10 regulates the temperature of the buffer gas.

The conduit 11 is provided at a location where the extreme ultraviolet radiation obscure by the conduit 11 is selected to have been obscured by other elements of the device in the absence of the conduit 11. Thus, the conduit 11 is located in front of the gas cooler 10 that masks EUV radiation regardless of whether the conduit 11 is present. The conduit 11 is displaced perpendicularly to the laser beam 7 so that the laser beam does not pass into the conduit 11 but instead moves laterally and enters the beam dump 9.

As mentioned 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 zone where the heated buffer gas would be continuously present during operation of the EUV source if no buffer gas was supplied through the conduit 11.

The distance between the outlet 12 of the conduit 11 and the interaction point 13 is as follows: The closer the outlet 12 is to the interaction point 13, the more unheated the zone around the interaction point 13 is. It can be selected by considering that the delivery of the buffer gas is more effective. However, the closer the outlet 12 is to the interaction point 13, the more likely the conduit 11 is to undergo sputtering of ions to the conduit. In one example, outlet 12 may be 15 cm or less from the interaction point, or 10 cm or less from the interaction point. The outlet can be at least 3 cm from the point of interaction. The distance between the interaction point 13 and the collector mirror 3 may be 20 cm.

The rate at which buffer gas is provided through the outlet 12 may be sufficient to substantially remove the heated buffer gas from the zone around the interaction point 13. The rate may be sufficient to achieve this before the next laser and material drop interaction. To achieve this, the rate at which the buffer gas must be provided through the outlet 12 is determined by the volume of the buffer gas heated by the laser and material drop interactions, and the frequency at which the laser and material drop interactions occur (ie Frequency).

An alternative embodiment of the invention is shown schematically in FIG. 3. 3 shows the source SO from one side. Most of the elements of the source SO shown in FIG. 3 are identical to those shown in FIG. 2 and are not described herein again. However, the conduit 11 of FIG. 2 is not present in FIG. 3. Instead, the conduit 21 passes through the aperture 8 in the collector mirror 3 and runs parallel to the laser beam 7. The conduit 21 is provided with an outlet 22 adjacent the interaction point 13. Conduit 21 is used to introduce buffer gas adjacent to interaction point 13 in a manner equivalent to that described above with respect to FIG. 2. The conduit 21 is positioned so that it may mask some of the EUV radiation generated by the plasma in the chamber 1, but the amount of EUV radiation that is obscured is relatively small (eg, only the cross section rather than the length of the conduit Masking EUV radiation). The distance between the outlet 22 and the interaction point 13 can be selected using the criteria described above with respect to FIG. 2.

The advantage of the embodiment shown in FIG. 3 is that the flow of buffer gas provided by the conduit is away from it rather than towards the collector mirror 3 (this helps to push the heated buffer gas out of the collector mirror 3). .

In a modified version of the embodiment shown in FIG. 3, the conduit can consist of two tubes, one of which is inside the other tube. The laser beam may be arranged to guide along the interior of the two tubes, and the buffer gas may be arranged to follow along a channel formed between the two tubes. In this case, in order to allow the laser beam to move uninterrupted from the laser to the point of interaction, the corner shown in FIG. 3 may not be inside the two tubes.

Although conduits 11 and 21 with different positions and configurations are shown in FIGS. 2 and 3, other conduit positions and configurations may be used. The conduit location and configuration is preferably configured so that it does not cover any EUV radiation that would not be obscured by some other components of the source SO when the conduit is not used. In some cases, this may not be achievable, or it may be desirable to provide the conduits at some location where the conduits really cover some of the EUV radiation. In this case, it is desirable to minimize the amount of EUV radiation obscured by the conduits if possible. Appropriate locations and configurations for the conduit will depend on the particular configuration of the source from which the conduit is provided. One or more conduits may be provided (eg, the conduits shown in FIGS. 2 and 3 may all be provided from a single EUV source).

Although the foregoing description refers to the use of hydrogen as a buffer gas, other suitable gases may be used.

The foregoing description refers to drops of material that are tin, but other suitable materials may be used.

The invention is not limited to radiation sources using drops of material. Embodiments of the invention may generate a plasma from a gas rather than, for example, drops of material. Suitable gases include Sn vapor, SnH ₄ , or a mixture of Sn vapor and any gas having a small nuclear charge (eg from H ₂ to Ar). Drops of gases or material may be considered examples of plasma generating material.

The wavelength of EUV radiation mentioned in the foregoing description may be in the range of 10 to 20 nm, for example in the range of 13 to 14 nm.

While the foregoing description of embodiments of the present invention relates to a radiation source for generating EUV radiation, the present invention may be implemented in a 'beyond EUV', ie a radiation source for generating radiation having a wavelength of less than 10 nm. Beyond EUV radiation may for example have a wavelength of 6.7 nm or 6.8 nm. The radiation source that generates the Beyond EUV radiation may operate in the same manner as the radiation sources described above.

In the foregoing description, the term 'unheated buffer gas' refers to the buffer gas delivered from the outlets 12 and 22 after the interaction between the laser beam and the plasma generating material (also before the next interaction between the laser beam and the plasma generating material). It is intended to mean.

The foregoing description is for the purpose of illustration and not of limitation. 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

A radiation source comprising a chamber and a supply of plasma generating material,
The plasma generating material introduced into the chamber has an interaction point capable of interacting with a laser beam to produce a radiation emitting plasma,
The source further comprises a conduit arranged to deliver a buffer gas into the chamber, the conduit having an outlet adjacent the point of interaction.

The method of claim 1,
Wherein said outlet port is located within an outer boundary of the region where there would have been a continuous presence of heated buffer gas upon operation of said source if no buffer gas was supplied through said conduit.

The method according to claim 1 or 2,
The outlet of the conduit is no more than 15 cm from the point of interaction.

The method of claim 3, wherein
The outlet of the conduit is 10 cm or less from the point of interaction.

The method according to any one of claims 1 to 4,
The outlet of the conduit is at least 3 cm from the point of interaction.

6. The method according to any one of claims 1 to 5,
The conduit is positioned such that it does not obscure radiation that would not be obscured by some other component of the source if it was not located.

The method according to any one of claims 1 to 6,
All or part of the conduit running alongside the gas cooler of the source.

The method according to any one of claims 1 to 6,
All or part of the conduit passes through an aperture in the collector mirror of the source.

The method of claim 8,
All or part of the conduit comprises two tubes, one of which is inside the other tube, the inner tube being arranged so that the laser beam can follow it, and the channel between the two tubes being A radiation source disposed to cause the buffer gas to follow.

In the method of generating radiation,
Introducing a plasma generating material into the chamber, and directing a laser beam to the plasma generating material to produce a radiation emitting plasma;
The method further includes introducing a buffer gas at a location in the chamber adjacent to the point where the laser beam and the plasma generating material interact.

The method of claim 10,
The location at which the buffer gas is introduced is within the outer boundary of the zone where the heated buffer gas would have been continuously present during operation of the source if the buffer gas had not been supplied through the conduit.

The method of claim 10 or 11,
The buffer gas is introduced at a rate of 100 m / s or more.

The method according to any one of claims 10 to 12,
The buffer gas is introduced at a rate of 2000 m / s or less.

The method according to any one of claims 10 to 13,
The rate at which the buffer gas is introduced is sufficient to substantially remove the heated buffer gas from the area around the interaction point prior to subsequent interaction between the laser beam and the plasma generating material.

In a lithographic apparatus:
Source of radiation;
An illumination system to condition the radiation;
A support structure for supporting a patterning device capable of imparting a pattern to a cross section of the radiation beam;
A substrate table for holding a substrate; And
A projection system for projecting the patterned radiation beam onto a target portion of the substrate,
The radiation source includes a chamber and a supply of plasma generating material, the source having an interaction point at which the plasma generating material introduced into the chamber can interact with a laser beam to produce a radiation emitting plasma, the source And a conduit arranged to deliver buffer gas into the chamber, the conduit having an outlet adjacent the point of interaction.