WO2011069881A1

WO2011069881A1 - Euv light source for an illumination system of a microlithographic projection exposure apparatus

Info

Publication number: WO2011069881A1
Application number: PCT/EP2010/068699
Authority: WO
Inventors: Damian Fiolka
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2009-12-09
Filing date: 2010-12-02
Publication date: 2011-06-16
Also published as: DE102009047712A1

Abstract

The invention concerns an EUV light source for an illumination system of a microlithographic projection exposure apparatus. According to an aspect of the invention, an EUV light source has a radiation source unit (105), at least one target source (135) for feeding target material (132) into the beam path of the radiation source unit (105) in such a way that said target material can be excited at at least one plasma ignition position (130) by the radiation (106) produced by the radiation source unit (105) into a plasma state with the emission of EUV radiation (138), a primary mirror which forms a collector mirror (110) and which at least partially reflects EUV radiation (138) emitted by the target material (132) in such a way that the reflected EUV radiation passes into the illumination system, and a secondary mirror (120) which reflects a beam produced by the radiation source unit (105) and focuses it on to the plasma ignition position (130), wherein the beam reflected at the secondary mirror (120) after reflection at the secondary mirror (120) has a numerical aperture greater than the numerical aperture of said beam prior to reflection at the secondary mirror (120) and wherein radiation (137) reflected at the target material (132) in the plasma ignition position after focusing by the secondary mirror (120) is at least predominantly not incident on the collector mirror (110).

Description

EUV light source for an illumination system of a microlithographic projection exposure apparatus

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application DE 10 2009 047 712.8, filed on December 9, 2009, and US Provisional Application No. 61 /267,948, filed on December 9, 2009. The content of these applications is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention concerns an EUV light source for an illumination system of a microlithographic projection exposure apparatus.

State of the art

Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (= reticle) illuminated by means of the illumination system is projected by means of the projection objective on to a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate. Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range, that is to say at wavelengths of for example about 13 nm or about 7 nm, due to the lack of availability of suitable translucent refractive materials. Production of the EUV light is effected by means of an EUV light source based on plasma excitation, in relation to which Figure 3 shows a conventional structure by way of example.

That EUV light source firstly has a C0₂ laser (not shown in Figure 3) to produce infrared radiation 306 of a wavelength of λ « 10.6 μιη which is focused by way of an optical focusing system (not shown in Figure 3), passes through an opening 31 1 in a collector mirror 310 in the form of an ellipsoid and is deflected on to a target material 332 (for example tin droplets) which are produced by means of a target source 335 and fed to a plasma ignition position 330.

The infrared radiation 306 heats the target material 332 in the plasma ignition position 330 in such a way that the material goes into a plasma state and produces EUV radiation. The spectral range used by the microlithographic projection exposure apparatus can be for example λ « 13.6 + 0.5 nm. That EUV radiation is focused by way of the collector mirror 310 on to an intermediate focus IF and passes through that intermediate focus IF into a subsequent illumination system, the boundary 340 of which is only indicated and which has a free opening 341 for the light entry. A light trap 320 serves to prevent the infrared radiation 306 from passing through into the illumination system directly (that is to say without previous reflection occurring at the collector mirror 310).

Now, the problem which arises in that respect is that the infrared radiation 306 incident on the target material 332 (in the example tin droplets) is partially reflected thereby. That reflection already occurs in the 'cold' state (that is to say before being converted into the plasma state), wherein the degree of reflection also increases in the conversion into the plasma state. In the diagrammatic view in Figure 4 the proportion of the infrared radiation reflected by the target material 332 in the plasma ignition position 330 is referenced '307'. As is also diagrammatically indicated a part of the infrared radiation incident on the target material 332 also penetrates into the target material 332. The radiation emitted by the target material 332 is referenced '308' and, besides the infrared radiation and the desired EUV radiation, also includes further wavelengths, for example in the VUV range (about 100 nm).

As can now be seen from Figure 4 in conjunction with Figure 3 the target material 332 reflects the infrared radiation 306 at its surface precisely in that direction in which the collector mirror 310 is arranged in the Figure 3 structure. Now, besides the desired EUV radiation, the collector mirror 310 also collects the infrared radiation reflected by the target material 332 at the plasma ignition position 330 (referenced '307' in Figure 3) and combines it together at the intermediate focus (IF), from which the infrared radiation also passes into the subsequent illumination system.

In practice now the fact that that infrared radiation passes into the illumination system gives rise to the problem that, by virtue of the high levels of laser power which inter alia are required because of the limited conversion efficiency of the target material 332 or plasma droplet (of the order of magnitude for example about 1 - 10% of the total laser power employed is used for heating) the power produced by the infrared radiation in the intermediate focus IF is high (for example in the region of 4000 watts) and can result on the one hand in significant thermal stresses and possibly deformation of the optical elements and thus the need for suitably effective heat dissipation by cooling devices. On the other hand, by way of the EUV mirrors, in addition to the actual EUV illumination light, the infrared radiation is also deflected to the photoresist- coated wafer where the infrared radiation can lead to heating of the photoresist and thus to a disrupting background as far as destruction of the photoresist. WO 2004/092693 A2 discloses inter alia an EUV light source in which, in an embodiment, in addition to an elliptical collector mirror, there is a secondary mirror which reflects light beams which are focused on to a target droplet disposed at the position intended for ignition of the plasma and which are emitted by the target droplet in a direction away from the elliptical collector mirror, back through the focus of the elliptical collector mirror on to the elliptical collector mirror from where those light beams are also deflected on to the intermediate focus (IF).

SUMMARY OF THE I NVENTION

The object of the present invention is to provide an EUV light source for an illumination system of a microlithographic projection exposure apparatus, which permits a reduction in the stressing of the illumination system with radiation of an unwanted wavelength.

An EUV light source according to the invention for an illumination system of a microlithographic projection exposure apparatus comprises:

- a radiation source unit,

- at least one target source for feeding target material into the beam path of the radiation source unit in such a way that said target material can be excited at at least one plasma ignition position by radiation produced by the radiation source unit into a plasma state with the emission of EUV radiation,

- a primary mirror which forms a collector mirror and which at least partially reflects EUV radiation emitted by the target material in such a way that the reflected EUV radiation passes into the illumination system, and

- a secondary mirror which reflects a beam produced by the radiation source unit and focuses it on to the plasma ignition position, - wherein the beam reflected at the secondary mirror after reflection at the secondary mirror has a numerical aperture greater than the numerical aperture of said beam prior to reflection at the secondary mirror, and

- wherein radiation reflected at the target material in the plasma ignition position after focusing by the secondary mirror is at least predominantly not incident on the collector mirror.

The wording expressing that radiation reflected at the target material in the plasma ignition position after focusing by the secondary mirror is "at least predominantly" not incident on the collector mirror is used to denote that the intensity of irradiation which, after focusing by the secondary mirror and reflection at the target material, is not incident on the (primary) collector mirror is larger than the intensity of irradiation which, after focusing by the secondary mirror and reflection at the target material, is incident on the collector mirror.

According to embodiments of the invention, the intensity of irradiation which, after focusing by the secondary mirror and reflection at the target material, is incident on the collector mirror is not more than 30%, more particularly not more than 20%, still more particularly not more than 10%, and still more particularly not more than 5% of the total intensity of the irradiation reflected at the target material in the plasma ignition position after focusing by the secondary mirror. According to a further embodiment, the irradiation being reflected at the target material in the plasma ignition position after focusing by the secondary mirror is not incident at all on the collector mirror.

The numerical aperture of the beam is defined in the usual way as NA=n*sin(9) (that is to say as the product of the refractive index of the optical medium and the sine of the aperture angle), and therefore under vacuum conditions corresponds to the aperture angle of the beam. The invention is based in particular on the concept that the radiation produced by the radiation source unit and used for plasma excitation is firstly not - or at least not significantly - focused on the way to the plasma ignition position, but is guided at least predominantly past a target material in the plasma ignition position in the form of a substantially unfocused parallel beam, with the consequence that focusing on to the target material in the plasma ignition position occurs only after reflection at the further secondary mirror.

In other words the secondary mirror used in accordance with the invention does not serve for example for the reflection of (secondary) radiation already emitted by a target material in the plasma ignition position, but rather for reflection of the primary radiation which was still not at all focused on to the plasma ignition position but which has passed the carrier material in the plasma ignition position without being influenced.

The radiation which is focused on to the plasma ignition position in that way after reflection at the secondary mirror consequently is incident on the target material in the plasma ignition position from another and in particular opposite direction in comparison with the conventional structure described in the opening part of this specification and is thus also reflected again in a different direction - insofar as it is not absorbed and converted into EUV radiation - , more specifically in such a way that at any event it can no longer be predominantly incident on the (primary) collector mirror and can pass focused thereby by way of the intermediate focus into the illumination system.

In other words the principle according to the invention of focusing the infrared radiation produced by the radiation source unit only from a secondary mirror on to the target material in the plasma ignition position serves to heat up the target material in the plasma ignition position from a side which is in opposite relationship in comparison with the conventional arrangement in Figure 3. Consequently the beam path of any radiation still reflected at the target material in the plasma ignition position after focusing by the secondary mirror extends at least predominantly in such a way that the radiation reflected at the target material no longer is incident on the collector mirror and consequently cannot be collected at all by the collector mirror and fed into the illumination system.

In accordance with a further aspect the invention concerns an EUV light source for an illumination system of a microlithographic projection exposure apparatus comprising:

- a radiation source unit,

- a secondary mirror which reflects a beam produced by the radiation source unit and focuses it on to the plasma ignition position,

- wherein the beam path of said beam extends in such a way that focusing on to the target material in the plasma ignition position is effected only as a consequence of reflection at the secondary mirror, and

As to the wording is "at least predominantly", reference is made to the remarks above. According to embodiments of the invention, the collector mirror has an opening for the radiation produced by the radiation source unit to pass therethrough. This opening can be a central opening on said collector mirror. Furthermore, the opening can be located on a system axis of the EUV light source. The invention however is not limited thereto, so that in other embodiments the collector mirror can also have one or more openings at other positions (not corresponding to a central position) for enabling radiation produced by the radiation source unit to pass therethrough.

According to a further aspect, the invention also relates to an EUV light source for an illumination system of a microlithographic projection exposure apparatus comprising:

- a radiation source unit,

- wherein the beam reflected at the secondary mirror after reflection at the secondary mirror has a numerical aperture greater than the numerical aperture of said beam prior to reflection at the secondary mirror, and

- wherein the collector mirror has an opening for the radiation produced by the radiation source unit to pass therethrough. In an embodiment the plasma ignition position, the secondary mirror and the intermediate focus are arranged on a common axis. According to a further embodiment, the opening is also arranged on said common axis. In an embodiment the radiation produced by the radiation source unit has a propagation direction which is parallel to said common axis.

In an embodiment the radiation produced by the radiation source unit has, before reflection at the secondary mirror, a first average propagation direction, and the EUV radiation after reflection at the collector mirror has a second average propagation direction, wherein an angle between said first and second average propagation directions is less than 20°, more particular less than 10°, still more particularly less than 3°, and still more particularly 0°. The term "average propagation direction" is used to denote the propagation direction averaged (as arithmetic average or arithmetic mean) over all rays or beams contributing to the respective radiation.

In an embodiment the secondary mirror is an ellipsoidal mirror. The invention however is not restricted thereto but it is also possible to use another mirror depending on the respective specific arrangement. In particular in accordance with an embodiment it is also possible to use a plane mirror as the secondary mirror, in which case the beam produced by the radiation source unit must be only slightly focused initially or prior to reflection at the secondary mirror, wherein the secondary mirror also only effects a deflection effect without introducing additional refractive power.

As regards the infrared radiation which is unwanted in the illumination system as explained in the opening part of this specification, as a consequence of the arrangement according to the invention in the ideal case only such infrared radiation which is emitted by the target material can pass from the target material in the plasma ignition position on to the collector mirror. In that respect however the invention further makes use of the realisation that the radiation component emitted by the target material in the plasma state in the infrared range is comparatively slight in comparison with the component directly reflected by the plasma droplet in the infrared range.

The low proportion of the radiation emitted by the target material in the infrared range in comparison with the proportion in the infrared range that is reflected directly by the target material is based on the one hand on the limited conversion efficiency of the carrier material in the plasma state, which has the result that only a comparatively small radiation proportion is converted into plasma heat. A further cause or attenuation of the emitted radiation portion results from spectral distribution: the target material in the plasma state represents so-to-speak a black body emitting a multiplicity of wavelengths of a wavelength spectrum which for example besides infrared radiation also embraces wavelengths in the VUV range (of about 100 nm) so that the total energy of the emitted radiation is correspondingly distributed to those wavelengths.

For the aforementioned reasons the proportion of infrared radiation which is particularly problematical in regard to adversely affecting the lithography process as explained in the opening part of this specification, in the radiation proportion emitted by the plasma drop (and thus in part also going to the collector) is substantially less than in the reflected radiation proportion (completely in the infrared range or at the wavelength of the light incident on the plasma droplet).

According to the invention therefore an unwanted introduction of laser radiation which serves to excite the plasma in the EUV light source into the illumination system is reduced or even completely eliminated. Consequently the illumination system also does not require any filter for the wavelength of the radiation serving for plasma excitation, whereby structural complication and expenditure and the costs of the illumination device can be reduced. Insofar however as a filter should still be used for the wavelength serving for plasma excitation (that is to say in the foregoing example λ « 10.6 μιη) in order to filter out any residual radiation at that wavelength, which still passes from the target material to the collector mirror (for example as discussed hereinbefore as a consequence of emission of such radiation by the target material) in the illumination system, such a wavelength filter can at any event have a lower level of power density.

In accordance with a further aspect therefore the invention also concerns an illumination system of a microlithographic projection exposure apparatus, wherein the illumination system is designed for operation with EUV radiation produced by excitation of target material to a plasma state with radiation produced by a radiation source unit, wherein the illumination system does not have a filter for the wavelength of the radiation produced by the radiation source unit.

Any further wavelengths which are not in the infrared range, in the radiation component emitted by the target material, can admittedly also possibly go to the collector mirror, but they are less problematical than the infrared radiation as filter devices or compensation methods suitable for same (for example in the form of AR layers) are available.

In an embodiment the EUV radiation emitted by the target material is at least partially focused on to an intermediate focus by the collector mirror, wherein the secondary mirror is arranged geometrically between the plasma ignition position and the intermediate focus.

In an embodiment the radiation source unit has an infrared radiation source, in particular a C0₂ laser. In an embodiment the target source is designed for feeding target material in droplet form, in particular for feeding tin droplets, into the beam path of the radiation source unit. In an embodiment a target droplet of the target material is of a diameter in the ignition position which is less than 1 /100, in particular less than 1/120, further particularly less than 1/150, of the diameter of a beam which is incident directly on the plasma ignition position from the radiation source unit. That ensures that a significant or by far predominant proportion of the beam incident on the plasma ignition position from the radiation source unit directly (that is to say without previous reflection) passes substantially without impediment the target droplet disposed there so that that radiation proportion is only reflected at the secondary mirror. In an embodiment the secondary mirror has a cooling means in order in particular to effectively carry away heat dissipation produced by incident infrared radiation.

In an embodiment the collector mirror on its reflecting surface has an absorbent coating and/or a diffractive structure to reduce in particular radiation components in the VUV and DUV range which are also still emitted in the structure according to the invention by the target material which is in the plasma state and which by way of focusing at the collector mirror can pass to the intermediate focus and into the illumination system.

The absorbent coating or the diffractive structure can be designed in particular to absorb radiation, the wavelength of which is more than 150 nm, in particular more than 100 nm. In accordance with further embodiments measures are taken to prevent the radiation coming from the radiation source unit from being incident directly on the target material to eliminate reflections at the target material which can then pass to the collector mirror and thus also by way of the intermediate focus into the illumination system (therefore to provide that said radiation as desired is incident on the target material only after reflection at the additional mirror). For that purpose for example there can be provided a screening or an aperture member which at least partially screens radiation deflected by the radiation source unit directly in the direction of the plasma ignition position from a target material at the plasma ignition position. In addition radiation deflected by the radiation source unit in the direction of the plasma ignition position can also be divided up in such a way that it is guided past a target material at the plasma ignition position.

The invention however is not limited to such measures, that is to say it is also possible in accordance with the invention for infrared radiation to be incident from the radiation source directly on the target material as long as only a significant proportion of that radiation is deflected past the target material and is incident on the secondary mirror.

Further configurations of the invention are set forth in the description and the appendant claims. The invention is described in greater detail hereinafter by means of embodiments by way of example illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 shows a diagrammatic view of the structure of an EUV light source in accordance with an embodiment of the invention, Figures 2a-b show a portion (Figure 2a) on an enlarged scale and a front view respectively of a detail of the EUV light source in Figure 1 ,

Figure 2c shows a diagrammatic detail view to describe a further embodiment of the invention,

Figure 3 shows a diagrammatic view to describe the structure of a conventional EUV light source, and

Figure 4 shows a diagrammatic view to illustrate the behaviour of a target droplet in the plasma ignition position in the Figure 3 structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1 an EUV light source 100 according to the invention has a radiation source unit 105 which in the illustrated embodiment is in the form of a C0₂ laser for producing infrared radiation 106 at a wavelength λ « 10.6 μιη. That infrared radiation 106 serves for heating target material 132 which is produced by a target source 135 and fed to a plasma ignition position 130 in which, as indicated in Figure 2b, the respective target material 132 there is excited to a plasma state in which inter alia the desired EUV radiation (in the example at a wavelength λ « 13.6 nm + 0.5 nm) is emitted. In the illustrated embodiment the target source 135 is designed to feed target material 132 in droplet form, more precisely for feeding tin (Sn) droplets, into the beam path of the radiation source unit 105, the target source 135 being synchronised with the laser pulses generated by the C0₂ laser. The invention however is not limited thereto so that in other embodiments the target material 132 can also be produced for example in the form of a continuous target jet and/or from other target material by the target source 105 and can be fed to the plasma ignition position 130.

The EUV light source 100 further has a collector mirror 1 10 in the form of an ellipsoidal mirror and having an opening 1 1 1 for the radiation 106 coming from the radiation source unit 105 to pass therethrough. The plasma ignition position 130 is arranged in one of the foci of the ellipsoid formed by the collector mirror 1 10, whereas the other focus of the collector mirror 1 10 corresponds to an intermediate focus 140. From that intermediate focus 140 the radiation arriving here passes into a subsequent illumination system, the boundary 140 of which is indicated in Figure 1 and which has an opening 141 for the light to enter.

The dimensions of the collector mirror 1 10 can be suitably selected depending on the respective specific design factors, in which respect values of the curvature of the collector mirror 1 10 merely by way of example can be in the range of between 300 mm and 400 mm while the diameter of the collector mirror 1 10 can be for example between 600 mm and 800 mm. The opening 1 1 1 in the collector mirror 1 10 can be for example of a diameter in the range of between 50 mm and 100 mm.

As also indicated in Figure 2a the infrared radiation 106 produced by the radiation source unit 105 or the C0₂ laser from the direction of the radiation source unit 105 is not for example already focused, but rather is fed in the form of a parallel beam to the plasma ignition position 130. The consequence of that inter alia is that at least by far the predominant proportion of the infrared radiation 106 initially passes a target droplet which is in the plasma ignition position 130 (and which for example can be of a diameter of some 100 μιη) without it being possible for that infrared radiation 106 to be reflected by the target droplet (as for example in the conventional structure described with reference to Figure 3). At a position which is geometrically between the plasma ignition position 130 and the intermediate focus IF the EUV light source 100 now has a further secondary mirror 120. The curvature of the secondary mirror 120 depends on the spacing relative to the plasma or the target material in the plasma ignition position and approximately corresponds to that spacing. As indicated in Figure 2 the additional secondary mirror 120 reflects the incident infrared radiation 106 produced by the radiation source unit 105 and focuses it back on to the plasma ignition position 130. The resulting heating of the target material 132 in the plasma ignition position 130 causes a transition to the plasma state in which the desired EUV radiation is emitted (at a wavelength λ « 13.6 nm). That EUV radiation (identified by '138' in Figure 2a) is incident on the collector mirror 1 10 and is coupled thereby into the illumination system by way of the intermediate focus (IF). As regards in contrast that proportion of infrared radiation 106 which is not converted into heat energy for heating the target material 132 but is reflected by the target material in the plasma state, that reflected infrared radiation (identified by '137' in the portion on an enlarged scale in Figure 2a), unlike the situation in the conventional structure in Figure 3, is no longer incident on the collector mirror 1 10 and consequently can also no longer be collected by the collector mirror 1 10 and fed into the illumination system.

Any wavelengths not in the infrared range in the radiation emitted by the target material (for example wavelengths in the VUV range or in the DUV range) can possibly go to the collector mirror 1 10, but are less problematical than the infrared radiation as suitable filter devices are available. For example in accordanc16e with an embodiment only diagrammatically indicated in Figure 2c the collector mirror 1 10, on its reflecting surface, may have an absorbent coating for the absorption of radiation at a wavelength of at least 100 nm and/or a diffractive structure 1 12 for diffracting radiation of a wavelength of at least 100 nm. In a further embodiment the radiation source unit 105 can also be arranged displaced in relation to the position which is shown in Figure 1 and which is symmetrical in relation to the system axis in order in that way to guide an increased proportion of radiation past the target material 132 towards the secondary mirror 120 and further reduce the amount of direct reflections of the infrared radiation 106 at the target material.

Accordingly in accordance with the invention an unwanted feed of laser radiation serving to excite the plasma in the EUV light source into the illumination system can be reduced or even completely eliminated. Consequently the illumination system also does not need any filter for the wavelength of the (infrared) radiation serving for plasma excitation or at most also a wavelength filter with a comparatively low level of power density, whereby structural complication and the costs of the illumination system are reduced.

Even if the invention has been described by reference to specific embodiments numerous variations and alternative embodiments will be apparent to the man skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly it will be appreciated by the man skilled in the art that such variations and alternative embodiments are also embraced by the present invention and the scope of the invention is limited only in the sense of the accompanying claims and equivalents thereof.

Claims

1 . An EUV light source for an illumination system of a microlithographic projection exposure apparatus comprising:

• a radiation source unit (105),

• at least one target source (135) for feeding target material (132) into the beam path of the radiation source unit (105) in such a way that said target material can be excited at at least one plasma ignition position (130) by radiation (106) produced by the radiation source unit (105) into a plasma state with the emission of EUV radiation (138),

• a primary mirror which forms a collector mirror (1 10) and which at least partially reflects EUV radiation (138) emitted by the target material (132) in such a way that the reflected EUV radiation (138) passes into the illumination system, and

• a secondary mirror (120) which reflects a beam produced by the radiation source unit (105) and focuses it on to the plasma ignition position (130),

• wherein the beam reflected at the secondary mirror (120) after reflection at the secondary mirror (120) has a numerical aperture greater than the numerical aperture of said beam prior to reflection at the secondary mirror (120), and

• wherein radiation (137) reflected at the target material (132) in the plasma ignition position after focusing by the secondary mirror (120) is at least predominantly not incident on the collector mirror (1 10).

2. An EUV light source for an illumination system of a microlithographic projection exposure apparatus comprising:

• a radiation source unit (105), • at least one target source (135) for feeding target material (132) into the beam path of the radiation source unit (105) in such a way that said target material can be excited at at least one plasma ignition position (130) by radiation (106) produced by the radiation source unit (105) into a plasma state with the emission of EUV radiation (138),

• a primary mirror which forms a collector mirror (1 10) and which at least partially reflects EUV radiation (138) emitted by the target material (132) in such a way that the reflected EUV radiation passes into the illumination system, and

• a secondary mirror (120) which reflects a beam produced by the radiation source unit (105),

• wherein the beam path of said beam extends in such a way that focusing on to the target material in the plasma ignition position is effected only as a consequence of reflection at the secondary mirror (120), and

An EUV light source as set forth in claim 1 or claim 2, characterised in that the beam produced by the radiation source unit (105) is an unfocused parallel beam prior to reflection at the secondary mirror (120).

An EUV light source as set forth in one of claims 1 through 3, characterised in that the EUV radiation (138) emitted by the target material (132) is at least partially focused on to an intermediate focus (IF) by the collector mirror (1 10), wherein the secondary mirror (120) is arranged geometrically between the plasma ignition position (130) and the intermediate focus (IF).

An EUV light source as set forth in one of the preceding claims, characterised in that the collector mirror (1 10) has an opening (1 1 1 ) for the radiation (106) produced by the radiation source unit (105) to pass therethrough.

An EUV light source for an illumination system of a microlithographic projection exposure apparatus comprising:

• a radiation source unit (105),

• wherein the collector mirror (1 10) has an opening (1 1 1 ) for the radiation (106) produced by the radiation source unit (105) to pass therethrough.

7. An EUV light source as set forth in one of the preceding claims, characterised in that the plasma ignition position (130), the secondary mirror (120) and the intermediate focus (IF) are arranged on a common axis.

8. An EUV light source as set forth in claim 5 or 6 and claim 7, characterised in that said opening (1 1 1 ) is also arranged on said common axis.

9. An EUV light source as set forth in claim 7 or 8, characterised in that the radiation (106) produced by the radiation source unit (105) has a propagation direction which is parallel to said common axis.

10. An EUV light source as set forth in one of the preceding claims, characterised in that the radiation (106) produced by the radiation source unit (105) has, before reflection at the secondary mirror (120), a first average propagation direction, and the EUV radiation (138) after reflection at the collector mirror (1 10) has a second average propagation direction, wherein an angle between said first and second average propagation directions is less than 20°, more particular less than 10°, still more particularly less than 3°, and still more particularly 0°.

1 1 . An EUV light source as set forth in one of the preceding claims, characterised in that the secondary mirror (120) is an ellipsoidal mirror.

12. An EUV light source as set forth in one of the preceding claims, characterised in that the plasma ignition position (130) is arranged in a focal point of the secondary mirror (120).

13. An EUV light source as set forth in one of the preceding claims, characterised in that the radiation source unit (105) has an infrared radiation source, in particular a C0₂ laser.

14. An EUV light source as set forth in one of the preceding claims, characterised in that the target source (135) is adapted to feed target material (132) in droplet form, in particular to feed tin droplets, into the beam path of the radiation source unit (105).

15. An EUV light source as set forth in claim 14, characterised in that a target droplet of the target material (132) in the ignition position (130) is of a diameter which is less than 1/100, in particular less than 1/120, further in particular less than 1/150, of the diameter of a beam incident on the plasma ignition position (130) directly from the radiation source unit (105).

16. An EUV light source as set forth in one of the preceding claims characterised in that on its reflecting surface the collector mirror (1 10) has an absorbent layer and/or a diffractive structure (1 12).

17. An EUV light source as set forth in claim 16, characterised in that the absorbent coating or the diffractive structure is adapted to absorb or diffract radiation, the wavelength of which is more than 150 nm, in particular more than 100 nm.

18. An EUV light source as set forth in one of the preceding claims, characterised in that there is provided a screening which at least partially screens radiation deflected from the radiation source unit (105) directly in the direction of the plasma ignition position (130) from a target droplet in the plasma ignition position (130).

19. An EUV light source as set forth in one of the preceding claims, characterised in that radiation deflected from the radiation source unit (105) in the direction of the plasma ignition position (130) is so divided up that it is guided past a target droplet in the plasma ignition position (130).

20. An EUV light source as set forth in one of the preceding claims, characterised in that the secondary mirror (120) has a cooling means.

21 . An illumination system of a microlithographic projection exposure apparatus,

• wherein the illumination system is designed for operation with EUV radiation (138) produced by excitation of target material (130) to a plasma state with a radiation (106) produced by a radiation source unit (105),

• wherein the illumination system does not have a filter for the wavelength of the radiation (106) produced by the radiation source unit (105).