NL2024323A - Sacrifical device for protecting an optical element in a path of a high-power laser beam - Google Patents

Abstract

A system is configured to form a path for laser radiation. The system has an optical element positioned in the path, and a sacrificial device also positioned in the path and configured to protect the optical element. 5 (Fig.6)

Description

SACRIFICAL DEVICE FOR PROTECTING AN OPTICAL ELEMENT IN A PATH OF A HIGH- POWER LASER BEAM

FIELD The invention relates to a system configured to form a path for laser radiation, wherein the system comprises an optical element positioned in the path. The invention also relates to an EUV radiation source including such system.

BACKGROUND ART High-power laser beams have many applications, from cutting steel to creating plasma that emits radiation of a desired wavelength to be used in lithography.

Optical components, such as mirrors and windows, may be present in the beam path between laser source and the beam’s destination. Mirrors are typically used to determine the course of the beam path by means of changing the direction of the incident beam or to adjust the beam’s wavefront. Windows are typically used to physically separate adjacent environments traversed by the beam path, e.g., environments of different gasses or held at different pressures.

The high-power laser beam is, in practice, reflected at a mirror not to the full 100%. As aresult, a fraction of the high-power beam gets absorbed by the mirror. The absorbed fraction of the laser beam is converted into heat. Accordingly, the mirror typically needs cooling to create a stable thermal state. The optical interaction of the mirror with the laser beam takes place at a surface of the mirror. This applied to mirrors having a reflective, electrically conductive coating on a substrate. This also applies to dielectric mirrors, also referred to as Bragg mirrors, having layers of different material, alternately arranged in a stack formed on a substrate. Cooling of a mirror can be accomplished by, for example, extracting the heat via cooling of the substrate. Preferably, the coating is thin enough so as to have the heat, generated by the absorption of the laser beam, diffusing from the coating into the cooled substrate fast enough in order to keep the coating’s temperature stable.

Similarly, a window does, in practice, not transmit the high-power laser beam to the full 100%. A fraction will get absorbed, and will generate heat, as a result of which the window needs cooling. A window will, in practice, be cooled via its surface. Therefore, it is advisable that the window be made of a material that has a suitably high thermal conductivity. A window may have be coating on one or more of its surfaces. Preferably, the coating is thin enough so to have the heat generated by the absorption of the laser beam, diffusing fast enough from the coating into the window’s material of high thermal conductivity, in order to keep the coating’s temperature stable.

An example of equipment using a high-power laser beam is an Extreme Ultraviolet (EUV) lithographic system wherein EUV radiation is produced by a so-called laser-produced plasma (LPP) source. In the LPP source, a high-power laser, e.g.. a CO:-laser, illuminates mass-limited targets, e.g., droplets of tin, one at a time. Each target is then converted into plasma that produces EUV radiation upon the plasma's free electrons recombining with the plasma’s ions. The EUV radiation thus produced is used in a lithographic apparatus to image a pattern via projection optics onto a semiconductor wafer.

For more information on some aspects of LPP EUV sources, please see, e.g., following publications: US patent application publication 2013/0321926, filed for Bergstedt et al., assigned to Cymer, now an ASML subsidiary, and incorporated herein by reference; US patent application publication 2014/0333915, filed for Hoogkamp et al, assigned to ASML and incorporated herein by reference; US patent application publication 2015/0156855, filed for Ershov et al., assigned to Cymer, now an ASML subsidiary, and incorporated herein by reference; US patent application publication 2018/0034235, filed for Sytina, assigned to ASML and incorporated herein by reference; US patent application publication 20180031979, filed for Bleeker et al., assigned to ASML and incorporated herein by reference.

At various locations in the EUV LPP source, diamond windows are being used in the beam path of the high-power laser beam.

For example, the laser beam produced by a CO: seed laser gets amplified in one or more power amplifiers using a combination of CO: gas and other gases as the amplifying medium. The beam input and beam output of such a power amplifier each have a diamond window. For some more background on optical amplifiers (power amplifiers) please see, e.g., US patent application publication 20140203195, filed for Fleurov et al., assigned to Cymer, now an ASML subsidiary, and incorporated herein by reference.

As another example, the high-power laser beam enters, via a diamond window, the chamber, wherein the EUV radiation is being produced.

As yet another example, coated diamond windows are being employed as dichroic mirrors in an EUV LPP source of the type using a laser pre-pulse (PP) and a laser main pulse (MP). The PP is contigured to condition the target for receipt of the MP, and the MP is configured to convert the conditioned target into plasma. The PP is produced by a PP seed laser, and the MP is produced by a MP seed laser. The PP and MP travel the same path through the power amplifiers and through the beam transport system towards a Final Focus Assembly (FFA). One of the functions of the FFA is create different paths for the PP and for the MP so as to be able to focus the PP and the MP at different focal points along the trajectory of a flying droplet of tin. The radiation of the PP and the radiation of the MP have slightly different wavelengths, and the different paths are created by means of dichroic mirrors. Adichroic mirror is formed by a coated diamond window. For more background information on the dichroic mirrors in the FFA, please see, e.g., US patent application publication 20130321926, referred to supra.

Industrial diamond can be grown from pure carbon under high pressures and high temperatures (HPHT diamond), or from hydrocarbon gas by chemical vapor deposition (CVD diamond). Diamond is known to have the highest thermal conductivity of any natural material.

SUMMARY OF THE INVENTION If a foreign particle gets stuck to the diamond window and within the area illuminated by the high-power laser beam, the foreign particle may absorb the laser beam’s radiation incident on it. As a result, the foreign particle will heat up rapidly, forming a hot spot on the window. The heat generated at the foreign particle will propagate into the window and disperse within the window’s material. The diamond window itself is cooled at its rim and if the heat absorbed by the window disperses rapidly enough, the window itself will not get damaged. For some background on conditions leading to damaging laser optics, please see, e.g., "Insensitivity of the catastrophic damage threshold of laser optics”, H.E.

Bennett, “Laser Induced Damage in Optical Materials: 1980”, STM STP 759, pp. 256 264.

As mentioned above, diamond has the highest thermal conductivity of any natural material. However, if the diamond window is coated as, e.g., in the dichroic mirror referred to above, the coating may be a barrier to the dispersal of heat that was generated at the foreign particle. In such case, the heat might accumulate locally, and eventually cause a burn-in in the window, leading to breakage of the window. The window is cooled at its rim, typically mounted so as to be contact with cooling water. Breakage of the diamond window may then also entail cooling water escaping and ending up where it is not wanted.

The inventor now proposes to protect the window from foreign particles getting stuck thereat using one or more sacrificial devices.

More particularly, the invention relates to a system configured to form a path for laser radiation. The system comprises an optical element and a sacrificial device, both positioned in the path. The sacrificial device is configured to protect the optical element.

It should be understood as sacrificial device, a device which it is could be easily attached and removed or detached and it could be discarded or replaced. Advantageously, a damaged sacrificial device is easier to replace without affecting the optical alignment of the optical element.

Preferably, the sacrificial device comprises a membrane, i.e, a device that has a thickness much smaller than its length and much smaller than its width. The length and width determine the membranes surface area. A magnitude of the surface area is larger than the portion of the membrane’s surface area illuminated by the high-power laser beam. Assume that the laser beam is incident on the membrane undera non-zero angle and assume that the membrane is located in a gaseous environment. Generally, the gaseous environment and the membrane have different magnitudes of the refractive index. However, as the thickness of the membrane is small, there will be only a negligible displacement, if any at all, between the propagation axis of the laser beam incident on the membrane and the propagation axis of the laser beam exiting the membrane. That is, the presence of the membrane will not affect in a substantial way the laser beam’s alignment through the system.

In an embodiment, a length of the path of the laser beam inside the membrane has a magnitude so that internal reflections of the laser radiation within the membrane get cancelled due to destructive interference. That is, the membrane optically serves as an etalon.

In a further embodiment, the membrane has an optical thickness of N times half the wavelength of the laser radiation, wherein N is an integer. Advantageously this embodiment minimizes the Fresnel losses.

In an embodiment, the sacrificial device comprises a diamond membrane. Diamond membranes can be made via CVD-technology. Such membranes, free-standing or mounted to a supporting frame, are relatively inexpensive, typically four orders of magnitude less expensive than a diamond window.

In an embodiment, the system comprises a monitoring system configured for monitoring a temperature of the sacrificial device. A change in temperature of the sacrificial device may indicate the development of a hot-spot due to the presence of a foreign particle. Upon detecting such temperature change, the laser system may be stopped and the membrane replaced or shifted so as to have the former hot-spot out of range of the laser beam.

In an embodiment, the monitoring system comprises a temperature sensor whose operation is based on Raman scattering. Such sensors are known in the art. Such sensor allows for contactless sensing and gives some freedom regarding the location at which to install the monitoring system.

In an embodiment, the system comprises a further sacrificial device positioned in the path and configured to protect the optical element. The sacrificial device is positioned between the further sacrificial device and the optical element. Having multiple sacrificial devices installed in series may allow for scheduling in advance the turning off of the laser beam as the optical component remains protected in case one sacrificial device fails as a result of the hot-spot.

In an embodiment of the system, the optical element has a surface that comprises a coating, and the sacrificial device is positioned in the path so as to face the coating. For example, the optical element comprises a diamond window with a coating so as to function as a dichroic mirror or as a polarizer. Typically, the diamond material itself has a higher thermal conductivity than the coating. A foreign particle lodged at the coating may therefore create a hot-spot whose heat-load is too high for the diamond window to spread out, leading to damage of the coating and of the diamond window proper.

In an embodiment, the membrane is uncoated but polished. Advantageously, the uncoated membrane do not absorbs light and as a consequence the power losses are reduced. It further advantageously allows the maintenance of the optical propagation axis upon transmission through the window.

5 In an embodiment, the further sacrificial device is a membrane comprising any features of the membrane described in any of the previous embodiments.

In an embodiment of the system, the system comprises a supporting frame configured to house the optical element and to attach and detach the sacrificial device. More particularly the supporting frame comprises a clipping system for attaching and detaching the sacrificial device.

The invention further relates to an EUV radiation source of the laser-produced plasma type and configured to supply EUV radiation to a lithographic apparatus. The EUV radiation source comprises a system as discussed supra.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in further detail, by way of example, and with reference to the accompanying drawings, wherein: Fig.l is a schematic view of a lithographic system comprising an LPP EUV light source; Fig.2 is a schematic view of an embodiment of a seed laser module; Figs 3, 4 and 5 are simplified schematics illustrating the use of dichroic mirrors; Figs. 6 and 7 are diagram illustrating the use of a sacrificial devices to protect a diamond window against foreign particles; and Fig.8 is a diagram illustrating the use of multiple sacrificial devices to protect a diamond window against foreign particles.

DETAILED EMBODIMENTS Fig.1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desiredcross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA.

As aresult of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Fig. 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on (he substrate W.

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

The radiation source SO shown in Fig. 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO: laser, is arranged to deposit energy via a high-power laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de- excitation and recombination of free electrons with ions of the plasma.

The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A firstone of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system | to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

As noted above, the laser system 1 may use one or more seed lasers to generate laser pulses, which may then be amplified to become the laser beam that irradiates the target material at the plasma formation site 4 to form a plasma that produces the EUV emission. As mentioned above, the laser beam enters the chamber 14 via a diamond window 26.

FIG. 2 is a simplified schematic view of one embodiment of a seed laser module 30 that may be used as part of the laser system 1 in the LPP EUV source SO. The seed laser module 30 is addressed here to provide some background on the use of dichroic mirrors in the EUV LPP source SO. Dichroic mirrors are discussed below in further detail with reference to FIGS. 3-5.

Seed laser module 30 is shown as having a folded arrangement rather than arranging the components in a straight line. In practice, such an arrangement is typical in order to limit the size of the module. To achieve this, the beams produced by the laser pulses of pre-pulse (PP) seed laser 32 and main pulse (MP) seed laser 34 are directed onto desired beam paths by a plurality of optical components 36. Depending upon the particular configuration desired, optical components 36 may include such elements as lenses, filters, prisms, mirrors or any other element which may be used to direct the beam in a desired direction. In some cases, optical components 36 may perform other functions as well, such as altering the polarization and or wavefront of the passing beam.

A beam from a specific one of seed lasers 32, 34 is first passed through an electro-optic modulator 38 (EOM). The EOMs 38 are used with the seed lasers as pulse shaping units to trim the pulses generated by the seed lasers to pulses having shorter duration and faster fall-time. A shorter pulse duration and relatively fast fall-time may increase EUV output and light source efficiency because of a short interaction time between the pulse and a target, and because unneeded, and thus discarded portionsof the pulse do not deplete amplifier gain. While two separate pulse shaping units (EOMs 38) are shown, alternatively a common pulse shaping unit may be used to trim both PP and MP from the PP seed laser 32 and MP seed laser 34, respectively. A beam from a specific one of the seed lasers 32, 34 is then passed through acousto-optic modulators (AOMs) 40 and 42. The AOMs 40 and 42 act as switches or shutters, which operate to divert any reflections of the laser pulses from the target material from reaching the seed lasers 32 and 34. Seed lasers 32 and 34 typically contain sensitive optics, and the AOMs 40 and 42 thus prevent any reflections from causing damage to the seed lasers’ sensitive optics. In the embodiment shown here, the beams from the seed lasers 32, 34 each pass through two AOMs. In some other embodiments, the beams from seed lasers 32 and 34 may each be passed through only a single AOM on each path.

After passing through the AOMs 40 and 42, the two beams are combined by a beam combiner 44. Since the pulses from the seed laser 32 and the pulses from the seed laser 34 are generated at different times, this combining implies that the two temporally separated beams are placed on a common beam path 46 for further processing and use.

After being placed on the common beam path 46, the beam from one of the seed lasers (again, there will only be one active at a time in this example) passes through a beam delay unit 48 such as is known in the art. Next, the beam is directed through a pre-amplifier 50 and then through a beam expander

52. Following this, the beam passes through a polarizer 54, e.g., a thin film polarizer, and is then directed onward by optical device 56, which again is a device which directs the beam to the next stage in the LPP EUV system and may perform other functions as well. From optical device 56, the beam typically passes to one or more optical amplifiers (power amplifiers) and other components. For more information on optical amplifiers, please see US patent application publication 20140203195, referred to supra.

Accordingly, the beam from the PP seed laser 32 and the beam from the MP seed laser 34 are put on a common path in seed laser module 30. The PP is to condition the tin droplet for receipt of the MP.

As indicated above, the tin droplet is moving along a trajectory from the fuel emitter 3 to the plasma formation region 4. Typically, the PP will cause the tin droplet to expand, and will also impart a lateral velocity to the expanding tin droplet. As the PP and the MP occur one after the other, {he PP beam and the MP beam need to be focused at different locations. For this reason, the PP beam and the MP beam need to be separated again. As mentioned above, one of the functions of the Final Focus Assembly (FFA) — not shown- is to create different paths for the and for the MP so as to be able to focus the PP and the MP at different focal points along the trajectory of a flying droplet of tin. This separation of the PP beam and the MP beam is accomplished using dichroic mirrors, as will be explained with reference to FIGS 3 through

5.

FIGS. 3 through 5 are simplified schematics illustrating the use of dichroic mirrors in the laser system 1 in FIG. 1. More particularly, laser system 1 includes a dichroic beam splitter module 302 that has a first dichroic mirror 108, a first reflective element or mirror 110, a second reflective element or mirror 112 and a second dichroic mirror 114. As will now be explained, the first dichroic mirror 108 and the second dichroic mirror 114 are physically aligned along the beam path and are configured to allow a first beam at one wavelength, such as the PP beam, to pass through them yet reflect a second beam at another wavelength, such as the MP beam.

In FIGS. 3 through 5, the portion of the LPP EUV source SO upstream of the dichroic beam splitter module 302 is indicated by a reference numeral 104.

FIG. 3 illustrates the route of the (amplified) PP beam through the dichroic beam splitter module 302 upon exiting portion 104 on a beam path 106. The amplified PP beam passes through the dichroic beam splitter module 302 and subsequently interacts with a target material before the target material coming from the fuel emitter 3 reaches plasma formation site 4.

More particularly, the PP beam from on the beam path 106 enters dichroic beam splitter module 302 where it encounters the first dichroic mirror 108. Because first dichroic mirror 108 allows passage of laser light having the wavelength of the PP beam, the PP beam travels through the first dichroic mirror 108 until it encounters the second dichroic mirror 114. Because the second dichroic mirror 114 also allows passage of laser light having the wavelength of the PP beam, the PP beam travels through the second dichroic mirror 114. After exiting the dichroic beam splitter module 302 the PP beam then encounters one or more further mirrors (not shown) after which the PP beam interacts with the target material to condition the target for receipt of the MP beam.

FIG. 4 illustrates the route of the (amplified) MP beam through the dichroic beam splitter module 302 upon exiting the portion 104 on the beam path 106. Upon entering the dichroic beam splitter module 302 the MP beam encounters the first dichroic mirror 108. Because the first dichroic mirror 108 reflects laser light having the wavelength of the MP beam, the MP beam is reflected from the first dichroic mirror 108 along a new beam path to a first mirror 110 which reflects the MP beam to a second mirror 112 which, in turn, reflects the MP beam to the second dichroic mirror 114. The second dichroic mirror 114 also reflects laser light having the wavelength of the MP beam. After the second dichroic mirror element 114, the MP beam encounters one or more mirrors (not shown) before interacting with the target material, conditioned by the PP beam, at the plasma formation site 4 in order to create plasma.

FIG. 5 depicts the combined operations explained under FIG. 3 and FIG. 4. The PP and the MP, each arriving along the same beam path 106, are separated by the beam splitter module 302 to thereby facilitate the PP beam encountering the target material before, and in preparation for, the MP beam interacting with the target material at the plasma formation site 4. This angular beam separation results inthe PP beam encountering the target material at a location distant from where the MP beam encounters the irradiation site 116. As has been explained, the first dichroic mirror 108 and the second dichroic mirror 114 allow passage of radiation having one wavelength yet reflect radiation having another wavelength utilizing dichroic filter characteristics known in the art. In an embodiment, these dichroic mirrors each comprise a diamond window in a water-cooled housing, the diamond window beings coated to reflect radiation at one wavelength (e.g., the MP beam at 10.59 um) yet transmit radiation at a different wavelength {(e.g., the PP beam at 10.26 pm). The coating on the diamond window may be a multi-layer coating that functions as a Bragg reflector with regard to the radiation to be reflected. Coatings and materials to provide such dichroic filter characteristics are commercially available and known in the art.

FIG. 6 is a diagram schematically illustrating the use of a sacrificial device to protect a diamond window against foreign particles. More particularly, FIG. 6 illustrates a high-power laser beam 602 being perpendicularly incident on a diamond window 604 and traversing the diamond window 604. Examples of locations of diamond windows in an EUV LPP source have been addressed supra, such as the diamond window 26 through which the laser beam enters the chamber 14; the diamond windows forming the input and the output of an optical amplifier (not shown); and the diamond window carrying a coating so as to serve as a dichroic mirror, e.g., dichroic mirrors 108 and 114.

The diamond window 604 may have a coating 608 in some applications, e.g., if used as a dichroic mirror (shown in FIG. 6), and none or a negligibly thin one from a thermal perspective in another application, e.g., when used as a window 26 in the wall of the chamber 14. A foreign particle stuck on the coating 608 of the diamond window 604 is more likely to act as a hot-spot that will cause damage than a foreign particle stuck at an uncoated diamond window.

A sacrificial device 606, in the form of a membrane, is installed close to the diamond window 604, in the path of the laser beam 602. In the example shown, the sacrificial device 606 is located in front of the side of the diamond widow 604 that faces the incident laser beam 602. Alternatively (not shown) or in combination (not shown) with the sacrificial device 606 in front of the diamond window 604, a sacrificial device may be located (not shown) behind the diamond window 604, i.e, close to the side of the diamond window 604 through which the laser beam 602 exits the diamond window 604. The sacrificial device 606 in the form of a membrane, is mounted in a support frame 610 in the example shown. The support frame 610 facilitates the mounting and replacing of the sacrificial device 606.

A foreign particle traveling towards the diamond window 604 will now be intercepted by the sacrificial device 606 and will neither reach the diamond window 604, nor the coating 608. If the foreign particle gets stuck to the sacrificial device 606, the sacrificial device 606 may get damaged, but not the diamond window 604.

The sacrificial device 606 is formed so as to be transparent to the laser beam 602, and is preferably made from diamond as well, in view of transparency to CO: laser radiation and in view of diamond’s having a high thermal conductivity. Diamond membranes can be made through a CVD process and their thickness is well controllable. See, e.g., “The CVD diamond booklet”, made available on-line by Diamond Materials GmbH, a Spin-Off from Fraunhofer Institute IAF in Freiburg.

The sacrificial device 606 is, therefore, a component optically open to the laser beam 602, but a physically closed barrier to foreign particles floating or traveling towards the relevant side of the diamond window 604. In the example shown, the sacrificial device 606 is a diamond membrane that is, here, uncoated. The lack of a coating on the membrane implies that heat generated by a foreign particle, that is stuck to the membrane and acts as a hot-spot, may then well be dispersed throughout the membrane itself and extracted via the frame 608, without damaging the membrane. Alternatively, the foreign particle may cause a burn-in of the sacrificial device 606. The sacrificial device 606 is typically much less expensive than the diamond window 604, and having the sacrificial device 606 replaced will also be less expensive than having to replace the diamond window 604.

In case of the laser beam 602 is produced by a CO: laser to form the high-power MP, the wavelength of the MP’s radiation is about 10.6 um. The sacrificial device 606 in the form of a membrane may then preferably have an optical thickness of N times half the wavelength (in the membrane’s material) of the radiation of the MP, N being an integer, so as to minimize Fresnel losses. That is, the membrane functions as an etalon if the length of the path of the laser beam 602 inside the membrane is given a magnitude so that internal reflections of the laser’s radiation within the membrane get cancelled due to destructive interference. If the membrane is a diamond membrane, the wavelength in vacuum of

10.6 um corresponds to a wavelength of 4.42 um in diamond by virtue of the refractive index of diamond being 2.4 for far-infrared. In FIG,6, the radiation of the laser beam 602 is incident on the diamond membrane under an angle of about 90°. Accordingly, the thickness of the membrane is preferably N times 221 pm.

Preferably, the surfaces of the diamond membrane are smooth enough so as to not scatter the radiation of the laser beam 602. One or both surfaces may therefore be polished so as to have a roughness in the order of, e.g., 10 nm.

Preferably, the sacrificial device 606, in the form of a membrane, is installed in such a way that the plane of the membrane makes a small angle (e.g., in the order of +/- 1 degree) with the plane of the surface of the diamond window 604 facing the membrane. Due to this non-zero angle, there will be no Fabry-Pérot effects, i.e., no interference pattern caused by reflections occurring between the diamond window 604 and the membrane. As the laser beam 602 is incident on the membrane under an angle, there will be a lateral shift between the laser beam incident on the membrane and the laser beam exiting themembrane. However, as the angle is small (in the order of +/- 1 degree) and as the membrane is thin (in the order of, say, 10 um), the lateral shift is negligible for all practical purposes related to aligning of the laser beam 602 with the target at the plasma formation region 4. The sacrificial device 606, installed for protection diamond window 604, may itself get damaged, as explained above. Therefore, it may be advisable to monitor the integrity of the sacrificial device, e.g., by monitoring a temperature of the sacrificial device. A rapid increase in temperature may be indicative of a developing hot-spot as a result of a foreign particle having arrived at the sacrificial device 606 and being illuminated by the laser beam 602. A monitoring system may therefore be employed to monitor the state of the sacrificial device 606. An example of such monitoring system is temperature sensor, that senses the temperature of the sacrificial device 606 based on Raman scattering. Raman scattering works as follows. If light of a specific frequency “v” {e.g., laser light) is incident on the sacrificial device 606 and is thereupon reflected or scattered off the sacrificial device 606, the reflected or scattered laser light contains radiation not only of the original, specific frequency *v”, but also radiation of a higher frequency “v + Av” and radiation of a lower frequency “v — Av”. The intensities of the reflected or scattered laser light at each of these shifted frequencies are generally different. The ratio of these intensities depends on the temperature. In the diagram of FIG. 6, a reference numeral 612 indicates the incident light of the specific frequency “v”, and a reference numeral 614 indicates the scattered light, containing the radiation of the higher frequency “v + Av” and the radiation of a lower frequency “v — Av”. In the diagram of FIG. 6, the monitoring system includes a (low-power) laser 616 configured to produce the incident light 612 of the specific frequency “v7, and a detector 618 configured to receive the scattered light 614 and to detect the shift in frequency “Av” that is indicative of the temperature of the area illuminated by the low-power laser light 612. FIG. 7 is a diagram schematically illustrating the use of a sacrificial device to protect a diamond window 704 against foreign particles in a scenario wherein a high-power laser beam 702 is incident on the diamond window 604 under an oblique angle. The diamond window 704 has a coating 708 and functions as a dichroic mirror. If the radiation of the incident laser beam 702 has a first wavelength, the incident laser beam 702 is reflected to form a reflected laser beam 720. If the radiation of the incident laser beam 702 has second wavelength, that differs from the first wavelength, the incident laser beam 702 is transmitted through the diamond window 704 to form: a transmitted laser beam 722. Within this context, reference is made to the diagrams of FIGS. 3- 5 and the accompanying description above.

The diamond window 704 is protected by a sacrificial device 706, here in the form of a membrane, e.g., a diamond membrane. The thickness of the membrane is preferably chosen so that the length of the path of the laser beam inside the membrane has a magnitude so that internal reflections of the laser radiation within the membrane get cancelled due to destructive interference. That is, themembrane optically serves as an etalon. In this case of an oblique angle of incidence of the laser beam 702, the optical path of the laser beam 702 within the membrane is slightly longer than in case the angle of incidence is about 909 as discussed above with reference to FIG. 6. This implies that the angle of incidence has then to be taken into account when determining the thickness, as is known in the art. In the scenario of the dichroic mirror discussed above with reference to FIGS. 3-5 the laser beam 702 may represent the PP of wavelength 10.59 um or the MP of wavelength 10.26 pm. This slight difference in wavelength may not be relevant to the optimal thickness to minimize internal reflections. A monitoring system may be installed, e.g., having a laser and a detector to detect the Raman shift, as discussed with reference to FIG.G. The incident laser light from the monitoring system is indicated with reference numeral 712 (only the outer rays are represented in the diagram) and the reflected laser light to the detector has been indicated by reference numeral 714 (only the outer rays are represented in the drawing). The laser and the detector have not been represented in the diagram order to not obscure the drawing. FIG. 8 is a diagram illustrating the use of multiple sacrificial devices 806, 816, 826 to protect a diamond window 804 against foreign particles. The diamond window 804 is located on the path of a high- power laser beam 802. The sacrificial devices 806, 816, 826, illustrated here in the form of membranes, are arranged on the path in series and upstream of the diamond window 804 that has a coating 808 facing the third sacrificial device 826. In case of damage to the first sacrificial device 806, due to a foreign particle causing a hot-spot at the sacrificial device 806, the second sacrificial device 816 and the third sacrificial device 826 will keep protecting the diamond window 804. Again, a monitoring system (not shown) may be installed to monitor the temperature of one or more of the sacrificial devices. The monitoring system may issue an alert to the system operator upon having detected a too high temperature. Having multiple sacrificial devise installed enables to schedule the turning off of the high-power laser in case a sacrificial device has been damaged due to foreign particles having been formed hot-spots. Other aspects of the invention are described as set out in the following numbered clauses:

1.A system configured to form a path for laser radiation and comprising: an optical element positioned in the path; and a sacrificial device positioned in the path and configured to protect the optical element.

2. The system of clause 1, wherein the sacrificial device comprises a membrane.

3. The system of clause 2, wherein a length of the path inside the membrane has a magnitude so that internal reflections of the laser radiation within the membrane get cancelled due to destructive interference.

4. The system of clause 3, wherein the membrane has an optical thickness of N times half the wavelength of the laser radiation, wherein N is an integer.

5. The system of clause 1, 2, 3 or 4, wherein the membrane is a diamond membrane,

6. The system of clause 1, 2, 3, 4 or 5, comprising a monitoring system configured for monitoring a temperature of the sacrificial device.

7. The system of clause 6, wherein the monitoring system comprises a temperature sensor, whose operation is based on Raman scattering.

8. The system of clause 1, 2, 3, 4, 5, 6 or 7, wherein: the system comprises a further sacrificial device positioned in the path and configured to protect the optical element; and the sacrificial device is positioned between the further sacrificial device and the optical element.

9. The system of clause 1, 2, 3, 4, 5, 6, 7 or 8, wherein: the optical element has a surface that comprises a coating; and the sacrificial device is positioned in the path so as to face the coating.

10. The system of clause 9, wherein the optical element includes one of: a dichroic mirror and a polarizer.

11. An EUV radiation source of the laser-produced plasma type and configured to supply EUV radiation toa lithographic apparatus, the EUV radiation source comprising a system of clause 1, 2, 3,4, 5,6, 7, 8,9 or 10.

Claims (1)

CONCLUSION A lithography apparatus comprising: an exposure apparatus adapted to provide a radiation beam; S a carrier constructed to carry a cartridge, which cartridge is capable of patterning a cross-section of the radiation beam to form a patterned radiation beam; a substrate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is arranged to position the target area of the substrate in a focal plane of the projection device.